 |
 Previous Article | Next Article 
The Journal of Neuroscience, April 15, 1998, 18(8):3035-3042
Adrenergic  2C-Receptors Modulate the Acoustic
Startle Reflex, Prepulse Inhibition, and Aggression in Mice
Jukka
Sallinen1,
Antti
Haapalinna2,
Timo
Viitamaa2,
Brian K.
Kobilka3, and
Mika
Scheinin1
1 Department of Pharmacology and Clinical Pharmacology,
University of Turku, FIN-20520 Turku, Finland, 2 Orion
Corporation, Orion Pharma, FIN-20101 Turku, Finland, and
3 Department of Molecular and Cellular Physiology, Howard
Hughes Medical Institute, and Division of Cardiovascular Medicine,
Stanford University, Stanford, California 94305
 |
ABSTRACT |
Studies on animal models of stress, anxiety, aggression, and
sensorimotor gating have linked specific monoamine neurotransmitter abnormalities to the cognitive and behavioral disturbances associated with many affective neuropsychiatric disorders. Although
 2-adrenoceptors (2-ARs) have been
suggested to have a modulatory role in these disorders, the specific
roles of each 2-AR subtype (2A,
2B, and 2C) are largely
unknown. The restricted availability of relevant animal models and the
lack of subtype-selective 2-AR drugs have precluded
detailed studies in this area. Therefore, transgenic mice were used to
study the possible role of the 2C-AR subtype in two well
established behavioral paradigms: prepulse inhibition (PPI) of the
startle reflex and isolation-induced aggression. The
2C-AR-altered mice appear grossly normal, but subtle
changes have been observed in their brain dopamine (DA) and serotonin (5-HT) metabolism. In this study, the mice with targeted inactivation of the gene encoding 2C-ARs (2C-KO) had
enhanced startle responses, diminished PPI, and shortened attack
latency in the isolation-aggression test, whereas tissue-specific
overexpression of 2C-ARs (2C-OE) was
associated with opposite effects. Correlation analyses suggested that
both the magnitude of the startle response and its relative PPI (PPI%)
were modulated by the mutations. In addition, the differences in PPI,
observed between drug-naive 2C-OE mice and their
wild-type controls, were abolished by treatment with a subtype
nonselective 2-agonist and antagonist. Thus, drugs
acting via 2C-ARs might have therapeutic value in
disorders associated with enhanced startle responses and sensorimotor
gating deficits, such as schizophrenia, attention deficit disorder,
post-traumatic stress disorder, and drug withdrawal.
Key words:
2C-adrenoceptor; gene-targeting; isolation-induced aggression; schizophrenia; sensorimotor gating; startle; transgenic mice
| |
INTRODUCTION |
The 2-adrenoceptors
(2-ARs), which include three subtypes
(2A, 2B, and
2C) encoded by three genes, mediate many of the CNS effects of norepinephrine (NE) and regulate the release of NE, but
also dopamine (DA), serotonin (5-HT), and other brain neurotransmitters
(Ruffolo et al., 1993 ; MacDonald et al., 1997). The expression of
2C-AR is distinct; it is restricted mainly to the CNS,
being prominent in the striatum and hippocampus (Nicholas et al., 1996;
MacDonald et al., 1997). The 2-ARs are known to have
modulatory roles in various neuropsychiatric disorders, such as
schizophrenia, post-traumatic stress disorder, depression, and various
cognitive abnormalities (Hornykiewicz, 1982; Coull, 1994; Nutt, 1994;
Ahmed and Takeshita, 1996; Arnsten et al., 1996), but the significance
of the 2-ARs, especially the role of each 2-AR subtype in these disorders, is poorly known.
However, the most evident pharmacological effects attributed to CNS
2-ARs, i.e., sedation, hypotension, hypothermia, and
analgesia, appear to be mediated via 2A-ARs, whereas the
role of 2C-ARs has remained obscure (MacMillan et al.,
1996; Hunter et al., 1997; Lakhlani et al., 1997; MacDonald et al.,
1997; Stone et al., 1997).
The startle reflex is a short-latency response of the skeletal
musculature elicited by a sudden auditory stimulus (Davis et al., 1982;
Yeomans and Frankland, 1995). The startle reflex can be modulated by
fear, stress, and other negative affective states (Davis, 1989; Howard
and Ford, 1992), and by different types of immediately preceding
stimuli. Prepulse inhibition (PPI), i.e., the attenuation of the
startle reflex response produced by a prepulse, is an important measure
of sensorimotor gating (Geyer et al., 1990). Like the startle reflex
itself, PPI is also a cross-species phenomenon. Deficits in PPI are
observed in schizophrenic patients (Braff et al., 1978), and deficits
in PPI can be induced in the rat with administrations of various
schizophrenomimetics, such as D-amphetamine
(D-Amph), 2,5-dimethoxy-4-iodoamphetamine (DOI), and
phencyclidine (PCP), and after social isolation (Geyer et al., 1993;
Bakshi et al., 1994; Swerdlow et al., 1994; Varty and Higgins, 1995).
Disrupted PPI can be normalized in rats by antipsychotics (Swerdlow et
al., 1994), and the PPI model is used in the development of new CNS
drugs.
Because subtype-selective 2-AR agonists or antagonists
are not available, we have investigated the role of the
2C-AR in the startle reflex and its PPI in two
genetically engineered mouse strains, one with targeted inactivation of
the 2C-AR gene (2C-KO) and the other with
tissue-specific overexpression of 2C-ARs
(2C-OE) (Link et al., 1995; Sallinen et al., 1997).
Because these mice have subtle alterations in their brain DA and 5-HT
metabolism that are specifically associated with the mutations, the
startle and PPI responses were first determined from drug-naive mice
with or without preceding isolation, and after D-Amph and
PCP. In addition, the effects of subtype-nonselective
2-AR drugs on startle and PPI were studied in
2C-OE mice and their wild-type controls. Finally,
because social isolation and changes in brain 5-HT metabolism are also
known to affect aggressive behavior, the mice were tested in the
isolation-induced aggression paradigm.
| |
MATERIALS AND METHODS |
Animals. A total of 449 8- to 17-week-old male mice
were used; 64 mice from the BALB/c strain (BOM, Bomholtgård, Denmark) were used as targets in isolation-aggression tests. All other mice
were from the breeding colony maintained in the Central Laboratory Animal Facility of the University of Turku, Finland. They represented two strains of genetically engineered mice and their wild-type controls, either littermates or those closely related to the mutants. The mutations were generated at Stanford University (Stanford, CA). One
mutant strain had a targeted disruption of the 2C-AR gene (2C-KO), and the other had tissue-specific
overexpression of 2C-ARs (2C-OE). The
wild-type controls are designated as 2C-KO-wt and
2C-OE-wt, respectively.
The generation of both mutant strains has been described previously
(Link et al., 1995; Sallinen et al., 1997). Briefly, the 2C-AR gene was inactivated in 129/Sv embryonic stem
cells (Nagy et al., 1993), which were injected into C57BL/6J
blastocysts, and the resulting chimeric mice were bred to
F1 (C57BL/6J × DBA/2J) animals. These animals were
back-crossed for several generations to C57BL/6J mice and then
intercrossed, and the tested 2C-KO mice were offspring
of closely related F11-12 pairs, which were combinations
of mice with wild-type, heterozygous, or homozygous genotypes for the
2C-AR mutation. All tested 2C-KO mice
were homozygous for the mutation. The germline transmission of the mutation was monitored from mouse tail biopsies by Southern (DNA) analysis.
The 2C-OE mice were generated by pronuclear
microinjection to one-cell fertilized eggs from the FVB/N strain and
are thus congenic. A tyrosinase minigene construct was coinjected for
visual identification of the transgenic progeny on the basis of coat color (Overbeek et al., 1991). The tested 2C-OE mice
were heterozygous. According to brain in situ mRNA
hybridization results and receptor autoradiograms, the overexpression
of 2C-ARs is approximately threefold in
2C-OE mice in areas that normally express the receptor (Sallinen et al., 1997). Correspondingly, 2C-AR binding
is absent in the brains of 2C-KO mice (Link et al.,
1995).
The mice of both mutant strains are viable and fertile and appear
grossly normal, and their diurnal patterns of locomotor activity do not
differ from their wild-type littermates, although the background
strains show different behavior. Compared with 2C-KO-wt
mice, 2C-OE-wt mice have been shown to be more active, and the potency of 2-AR agonists to induce locomotor
inhibition is different between these strains of wild-type mice but not
between respective mutant and wild-type mice. However, both mutant
strains have subtle alterations in brain monoamine metabolism that are specifically associated with the mutations (Sallinen et al., 1997).
The mice were housed in groups of 7-10 in standard polypropylene cages
(38 × 22 × 15 cm) at 22 ± 1°C and kept on a 12 hr
light/dark cycle with light onset at 6 A.M. Some of the tested mice,
however, were isolated for the experiments at the age of 6-10 weeks;
these mice were subsequently housed alone in smaller cages (22 × 16 × 13 cm). Their bedding was changed once a week, but otherwise they were not disturbed. Experiments were conducted between 8:30 A.M.
and 4 P.M.
The animal care was in accordance with the regulations of the
International Council for Laboratory Animal Science, and the experiments had approval of the local committee for laboratory animal
welfare.
Drugs. The following drugs were used: dexmedetomidine
hydrochloride (3 or 10 µg/kg) (Orion Corporation, Orion Pharma,
Turku, Finland), atipamezole hydrochloride (100 µg/kg) (Orion
Pharma), D-amphetamine sulfate (0.25, 0.5, or 1.0 mg/kg)
(Sigma, St. Louis, MO), and phencyclidine hydrochloride (0.3, 1.0, or
3.0 mg/kg) (RBI, Natick, MA). All drugs were dissolved in distilled
water, and the injection volume was 5 ml/kg (s.c.).
Startle apparatus and experimental design. The startle
responses were measured with four identical ventilated and illuminated startle chambers [39 × 38 × 58 cm (length × width × height)] (SR-LAB system, San Diego Instruments, San
Diego, CA). Each chamber consisted of a Plexiglas cylinder (3.9 cm in
diameter) mounted on a removable frame on a base unit. Movement of the
mouse within the cylinder was detected by a piezoelectric accelerometer
attached below the frame. A loudspeaker (Radio Shack Supertweeter, San
Diego, CA) mounted 25 cm above the cylinder, provided the background
white noise and the acoustic stimuli. Presentation of acoustic stimuli and the piezoelectric responses from the accelerometer were controlled and digitized by the SR-LAB software and interface system. Sensitivity of the chambers was adjusted at average readings of 250 using the
standardization unit from San Diego Instruments. Sound levels within
each chamber were measured repeatedly using the A weighting scale
(Radio Shack Sound Level Meter, Fort Worth, TX) and were found to
remain constant.
At the beginning of each startle session, the mice were placed in the
startle chambers and exposed to 5 min of 72 dB background noise, which
continued for the remainder of the session. The PULSE intensity was 118 dB, and the PREPULSE intensity was 3, 6, 9, or 15 dB above the 72 dB
background level. The duration of both prepulses and pulses was 40 msec, and the prepulse-pulse interval was 100 msec. The allocation of
different trial types and the intertrial intervals (7-30 sec) was
pseudorandomized and kept unchanged within a study. The startle
amplitudes from PULSE ALONE, PREPULSE ALONE, and PREPULSE + PULSE
stimuli were determined by averaging 100 readings of 1 msec each taken
from the beginning of the PULSE stimulus onset in PULSE and PREPULSE + PULSE trials, or from the beginning of the PREPULSE in PREPULSE ALONE
trials.
Three startle experiments were conducted with separate groups of mice.
In the first experiment, 73 male 2C-OE and 68 male 2C-OE-wt mice were divided randomly into four groups and
administered either the subtype nonselective 2-agonist
dexmedetomidine hydrochloride (Dex) (3 or 10 µg/kg), the subtype
nonselective 2-antagonist atipamezole (Ati) (100 µg/kg), or vehicle (distilled water) subcutaneously 20 min before the
test session (Haapalinna et al., 1997). In this first drug challenge,
the startle session consisted of 30 trials with 10 PULSE ALONE, 10 PREPULSE + PULSE, and 10 PREPULSE ALONE stimuli during a period of 11 min, and the prepulse intensity was set at 15 dB above background.
These mice were not used in subsequent experiments.
In the second experiment, groups of 2C-KO
(n = 49), 2C-KO-wt (n = 49), 2C-OE (n = 42), and
2C-OE-wt (n = 40) mice underwent three
separate startle challenges as follows. In the first step, drug-naive
mice were exposed to a 5 min startle session with 10 PULSE and 6 PREPULSE + PULSE trials, with prepulses 9 dB above background to
determine the average baseline startle and PPI levels of each genotype
group. Results from this experiment were then used to establish four
matched treatment groups within each genotype group according to their
responses to PULSE ALONE stimuli. In the second step on the next day,
the same mice were given 0.25, 0.5, 1.0 mg/kg of D-Amph or
vehicle 30 min before a startle session, which consisted of 30 PULSE
ALONE, 10 PREPULSE ALONE (15 dB above background), and 40 PREPULSE + PULSE trials (the prepulse level was 3, 6, 9, or 15 dB above
background, 10 repetitions of each); these 80 stimuli were presented
during a period of 18 min. In the third step, after at least 10 d,
the same 18 min startle session was presented to the same mice starting
30 min after PCP injections (0.3, 1.0, or 3.0 mg/kg or vehicle). In
this third challenge, the mice were rearranged to new matched treatment
groups according to their initial startle responses: those mice that
had received the same D-Amph doses in the previous
experiment were put into different dose groups.
In the third startle experiment, the effect of isolation on startle
responses and PPI was investigated in individually housed mice (see
below) that had been studied once in the isolation-aggression test at
least 1 week earlier. These mice were exposed to the same 5 min startle
session that was used in the study with drug-naive group-housed mice in
the first step of the second experiment.
Isolation-aggression tests. A total of 64 drug-naive male
mice (17 2C-KO, 18 2C-KO-wt, 14 2C-OE, and 15 2C-OE-wt) were analyzed for
aggression after 6 weeks of isolation. On the basis of our preliminary
tests, the aggressiveness of 2C-OE and
2C-OE-wt mice remaining in their home cages after
isolation was extremely high, and the rating of attacks was difficult
because of almost continuous fighting. To decrease the aggression level
and to increase the sensitivity of the experiment, the tests were
subsequently performed in a fresh cage identical to the home cages.
Each isolated mouse was put into the test cage 1 min before the target
(BALB/c) mouse. The attack latency and number of attacks on the target mouse were measured during a 10 min session, starting from the introduction of the target mouse. An attack was given a score of
one point when the isolated mouse bit the target mouse. If fighting
continued for at least 10 sec, the same attack was scored as 2 points.
The mice were analyzed for aggression only once. The observer was
unaware of the expected results, but 2C-OE and 2C-OE-wt mice have different coat color and were tested
first. 2C-KO and 2C-KO-wt mice could not
be identified by visual inspection, and the observer was unaware of the
randomized order of the mutant and wild-type mice.
Data analysis. The results are presented as mean ± SEM. Statistical analysis was performed using STATISTICA 4.5 computer
software (StatSoft, Tulsa, OK). ANOVA for repeated measurements showed that statistically significant habituation effects were occasionally present only in the second and third step of the second startle experiment (D-Amph and PCP challenges). These effects,
however, were minor: reductions of the startle amplitude in the course of the session were between 10 and 20% in the vehicle group. In addition, alterations in responses during a session could have arisen
from differences in drug action, in habituation, or in their
interaction. Therefore, and because the startle sessions were not
designed to explore habituation, the habituation effects were not
evaluated further. The startle responses of each animal were summed to
give one integrated startle response value for each animal and trial
type; these were then used in subsequent analyses. The PREPULSE ALONE
responses were barely measurable and were omitted from further
analyses.
Pearson product-moment correlation analysis was performed for each
genotype group of drug-naive mice to assess the relationship between
the startle response magnitude and PPI. The extent of PPI is usually
determined as PPI%, according to the formula [100  (mean
startle amplitude on PREPULSE + PULSE trials/mean startle amplitude on PULSE ALONE trials) × 100]. Because both the startle amplitudes and the PPI% appeared to be altered by the mutations, and
because startle magnitude after PULSE ALONE is used in the denominator
of PPI%, the relationship between the startle reflex and PPI phenomena
was examined further. The correlation coefficients were calculated for
relationships between absolute startle magnitudes after PULSE ALONE and
PREPULSE + PULSE as well as the absolute PPI (= startle magnitude after
PULSE ALONE startle magnitude after PREPULSE + PULSE). It
should also be stressed that the wild-type control mice representing
two different background strains had different startle and PPI levels,
as expected (Logue et al., 1997; Paylor and Crawley, 1997), and that
statistical comparisons were made only between the appropriate mutant
and wild-type groups. Before the calculation of linear correlation, the
data were plotted on a scatter plot to detect possible nonlinear
associations; such associations were not found.
Results from isolation-aggression tests and the startle experiments
with drug-naive mice were analyzed with Mann-Whitney U tests between mutant and respective wild-type groups, because startle
responses and attack latencies were not normally distributed. However,
nonparametric statistical methods for multivariate analysis are not
available, and results from drug challenges were thus analyzed with
parametric statistics using two- or three-way ANOVA. Three-way ANOVA
for repeated measurements revealed that in the second experiment, in
which multiple prepulse intensities (3, 6, 9, and 15 dB above
background) were used, the PPI was dependent on prepulse intensity, as
expected (prepulse intensity × PPI% interaction:
p < 0.0001). However, the different prepulse
intensities had no effect on the differences between drug doses or
genotypes. Therefore, for clarity, only the PPI results after the 9 dB
above background prepulses are shown from these experiments.
| |
RESULTS |
Startle responses and PPI in drug-naive mice with altered
2C-adrenoceptor expression
The average startle amplitudes to PULSE ALONE stimuli were greater
in 2C-KO mice compared with 2C-KO-wt mice
(Z = 2.1; p = 0.034), whereas
2C-OE and 2C-OE-wt mice had similar
startle responses (Fig.
1A,B).
The prepulses inhibited startle responses less in 2C-KO
mice (Z = 2.6; p = 0.011) and more in
2C-OE mice (Z = 3.0; p = 0.0028), when compared with 2C-KO-wt and
2C-OE-wt mice (Fig. 1C,D).
View larger version (39K):
[in this window]
[in a new window]
|
Figure 1.
A-D, Startle responses without
prepulses (A, B) and prepulse inhibition (PPI) of the
startle reflex (C, D) in drug-naive mice with altered
2C-AR expression. Data are presented as mean ± SEM
(n = 42-49). The mean startle response amplitude
of mice with targeted disruption of the 2C-AR
(2C-KO) (hatched bars) were
higher than in their respective wild-type control group (open
bars). In addition, 2C-KO mice had diminished
PPI, and mice overexpressing the 2C-ARs
(2C-OE mice) (closed bars) had
increased PPI compared with respective wild-type controls
(Mann-Whitney U test). *p < 0.05;
**p < 0.01.
|
|
Effects of subtype nonselective 2-adrenoceptor drugs
on startle responses and PPI
The specific, subtype nonselective 2-antagonist Ati
increased the startle response in 2C-OE and
2C-OE-wt mice compared with respective vehicle-treated
mice (mean change: +22%; two-way ANOVA, dose effect:
F(1,67) = 5.0; p = 0.028) (Fig.
2A). A low dose of the
subtype nonselective 2-AR agonist Dex also slightly increased the startle response (+24%; F(1,65) = 6.0; p = 0.017), but a larger, slightly sedative dose
of Dex clearly decreased the startle response (55%;
p < 0.0001) (Fig. 2A). Although the drug effects seemed to be slightly different between the genotypes (Fig. 2A), two-way ANOVA did not indicate any
significant drug × genotype interactions between
2C-OE and 2C-OE-wt mice in the startle
responses.
View larger version (24K):
[in this window]
[in a new window]
|
Figure 2.
A, B, The effects (mean ± SEM; n = 14-20) of vehicle, the
2-antagonist atipamezole (Ati), and the
2-subtype-nonselective agonist dexmedetomidine
(Dex) on startle responses without prepulses
(A) and prepulse inhibition (PPI)
(B) in mice overexpressing the
2C-ARs (filled bars) and their
wild-type controls (open bars). Two-way ANOVA revealed
significant genotype × dose interactions between doses of Ati 100 and vehicle and Dex 3.0 and vehicle; these are indicated as  (p < 0.05). The difference in PPI between
vehicle-treated 2C-OE and 2C-OE-wt mice
was also significant, which is indicated as ** (Z = 2.9; p = 0.0035; Mann-Whitney U
test).
|
|
Ati abolished the PPI differences between 2C-OE and
2C-OE-wt mice that were observed after vehicle
injections and in the drug-naive mice (two-way ANOVA, drug × genotype interaction: F(1,67) = 5.5;
p = 0.022) (Fig. 2B). Also the small
dose of Dex (3 µg/kg) abolished the PPI difference between the
genotypes, enhancing only the PPI of 2C-OE-wt mice
(dose × genotype: F(1,65) = 4.8; p = 0.033). The larger dose of Dex (10 µg/kg)
increased PPI in both 2C-OE and 2C-OE-wt
mice (dose effect: F(1,67) = 16;
p = 0.0001; dose × genotype:
F(1,67) = 0.57; p = 0.44).
Effects of D-amphetamine and phencyclidine on startle
responses and PPI
The results from D-Amph and PCP challenges are
presented in Figures 3A-D and
4A-D, respectively. A significant genotype
difference was observed between 2C-KO and
2C-KO-wt mice in their average startle responses to
pulse alone stimuli after D-Amph
(F(1,90) = 9.0; p = 0.0034); the
responses differed between genotypes, especially at the 0.5 mg/kg dose
(Fig. 3A) (Tukey's post hoc test: p = 0.018). Startle amplitudes were similar in
2C-OE and 2C-OE-wt mice and were not
influenced by D-Amph. There was also a significant difference in startle amplitudes between 2C-KO and
2C-KO-wt mice after PCP (F(1,90) = 8.3; p = 0.0049) (Fig.
4A). PCP clearly increased the startle amplitudes of 2C-OE-wt mice, but
had a smaller effect in 2C-OE mice (Fig.
4B) (dose effect: F(3,77) = 3.2; p = 0.027; genotype difference:
F(1,77) = 5.1; p = 0.027). Interestingly, this difference in startle responses between
2C-OE mice and their wild-type controls was opposite to
the difference observed between 2C-KO mice and their
controls.
View larger version (55K):
[in this window]
[in a new window]
|
Figure 3.
A-D, Startle responses (A,
B) and prepulse inhibition (PPI) (C, D) in mice
with altered 2C-AR expression after different doses of
D-amphetamine (D-Amph). Error bars
represent mean ± SEM (n = 10-14) results
from mice with targeted disruption of 2C-ARs
(2C-KO) (hatched bars), overexpression of
the 2C-AR (2C-OE) (closed
bars), and corresponding wild-type mice
(2C-KO-wt or 2C-OE-wt) (open
bars).
|
|
View larger version (55K):
[in this window]
[in a new window]
|
Figure 4.
A-D, The effects of different
doses of phencyclidine (PCP) on startle responses and
PPI in mice with altered 2C-AR expression.
Symbols are as described in Figure
3A-D.
|
|
PPI was not statistically significantly different between
2C-KO and 2C-KO-wt mice after
D-Amph in overall ANOVA. PCP potently disrupted PPI in both
2C-KO and 2C-KO-wt mice (dose effect: F(3,90) = 4.9; p = 0.0035)
without significant differences in responses between the genotypes
(Fig. 4C).
In 2C-OE and 2C-OE-wt mice,
D-Amph did not have any detectable effect on the PPI. The
difference in PPI between genotypes was perhaps even more prominent
than earlier without drug (Figs. 1D, 3D)
(overall genotype difference: F(1,83) = 29;
p < 0.0001). PCP diminished the PPI also in
2C-OE-mice, but not in 2C-OE-wt mice
(dose effect: F(3,77) = 3.1; p = 0.034; dose × genotype interaction: F(3,77) = 2.8; p = 0.048) (Fig.
4D).
The effect of isolation on startle and prepulse inhibition
After isolation, the average startle responses to PULSE ALONE
stimuli were larger in 2C-KO mice than in
2C-KO-wt mice, as observed earlier in group-housed mice
(Figs. 1A,
5A). The startle responses of
isolated 2C-OE mice were slightly attenuated compared with isolated 2C-OE-wt mice (Fig. 5B)
(Z = 2.0; p = 0.049) in contrast to
group-housed 2C-OE and 2C-OE-wt mice, in
which no differences between genotype groups were seen in their PULSE
ALONE responses (Fig. 1B).
View larger version (41K):
[in this window]
[in a new window]
|
Figure 5.
A-D, Prepulse inhibition (PPI) of
the startle reflex in drug-naive mice with altered 2C-AR
expression after 7-8 weeks of isolation (n = 14-18). Compared with group-housed mice (Fig.
1A-D), isolation had no clear effect in mice
with targeted disruption of the 2C-AR gene
(2C-KO) (hatched bars) or in
their controls (2C-KO-wt) (open
bars), but it reduced startle responsiveness in mice
overexpressing the 2C-AR
(2C-OE) (closed bars) and
increased the PPI in both 2C-OE mice and in their
wild-type controls (2C-OE-wt) (open
bars).
|
|
The differences in PPI, previously observed between genotypes, were now
evident only as trends without significance. This probably resulted
from lack of statistical power, because the differences in the average
responses were similar to those observed earlier in group-housed mice;
the group sizes of the studied isolated mice were considerably smaller
(n = 14-18) than those of the group-housed mice
(n = 40-49). Somewhat surprisingly, compared with
group-housed mice, PPI was increased in both 2C-OE and
2C-OE-wt mice after isolation (51-56% vs 23-38%,
respectively; two-way ANOVA, effect of isolation:
F(1,117) = 32; p < 0.0001)
(Figs. 1D, 5D). In addition, startle
responses to PULSE ALONE stimuli were decreased by isolation (effect of
isolation: F(1,117) = 6.0; p = 0.016).
Association of startle amplitude and prepulse inhibition
Linear regression analysis of the results from group-housed
drug-naive mice revealed substantial negative correlation between individual mean startle responses to PULSE ALONE stimuli and PPI% values in the genotype groups of 2C-KO-wt
(r = 0.45; p = 0.0014) and
2C-OE mice (r = 0.54;
p = 0.0002), but these variables were independent of
each other in the groups of 2C-KO (r = 0.078; p = 0.82) and 2C-OE-wt mice
(r = + 0.028; p = 0.61) (Fig.
6, Table
1). The relationship between absolute
startle magnitudes after PULSE ALONE and PREPULSE and PULSE trials was
highly dependent (r = 0.82 in 2C-OE-wt
mice and r > 0.93 in other genotype groups; p < 0.001), and a weaker but substantial positive
correlation was observed also between the PULSE ALONE and absolute PPI
responses (r = 0.46-0.62; p < 0.01)
(Table 1).
View larger version (26K):
[in this window]
[in a new window]
|
Figure 6.
Scatter plot of associations between individual
mean startle responses to PULSE ALONE stimuli and the percentage of
prepulse inhibition (PPI%) of all studied group-housed drug-naive mice
from each of the four 2C-AR genotype groups. A
statistically significant negative correlation was observed in groups
of 2C-KO-wt and 2C-OE mice
(p < 0.0014), but not in
2C-KO and 2C-OE-wt mice
(p > 0.61) (Pearson product-moment
analysis).
|
|
View this table:
[in this window]
[in a new window]
|
Table 1.
Correlations between individual means of startle magnitudes
after PULSE alone and PREPULSE (PP) + PULSE stimuli, relative prepulse
inhibition (PPI%), and the absolute prepulse inhibition (absolute
PPI = startle amplitude after PULSE alone minus startle amplitude
after PP + PULSE stimuli)
|
|
Isolation-induced aggression tests
The onset of fighting was clearly dependent on the expression of
2C-ARs. 2C-AR overexpression was
associated with increased latency to attack (Z = 2.7;
p = 0.0078), and the lack of 2C-AR expression was associated with reduced latency to attack
(Z = 2.7; p = 0.0070), when compared
with the respective wild-type control mice (Fig.
7). All mice attacked the BALB/c target
mice within the first 4.5 min, except one 2C-OE mouse
that did not attack at all (the latency was scored as 600 sec). In two
cases, the BALB/c mice also attacked, but otherwise they performed only defensive behavior. The number of attacks during the 10 min observation period, however, showed no statistically significant differences between the genotype pairs (Table 2).
View larger version (18K):
[in this window]
[in a new window]
|
Figure 7.
Isolation-induced aggression test: scatter plot of
attack latencies. Horizontal lines indicate medians of
each genotype group. All studied mice attacked during the first 270 sec, except one 2C-OE mouse that did not attack at all.
The attack latency was shortened in 2C-KO mice and
prolonged in 2C-OE mice (p < 0.01) (Mann-Whitney U test).
|
|
| |
DISCUSSION |
The lack of 2C-AR expression was associated with
increased startle reactivity, reduced PPI, and reduced attack latency.
The overexpression of 2C-AR resulted in opposite
changes. These contrasting findings strongly support the significance
of the observed differences and their causal relationship to altered
2C-AR expression, because the two pairs of mutant and
wild-type mouse strains are independent of each other. In addition, the
2-AR agonist Dex and the antagonist Ati were capable of
abolishing the PPI difference between 2C-OE and
2C-OE-wt mice. Taken together, these findings suggest
that activation of 2C-ARs reduces the hyper-reactivity
and impulsivity of mice, which are common features of the studied
paradigms.
Current opinion generally assumes that the startle reflex amplitude and
its prepulse inhibition, determined usually as PPI%, are independent
variables and that startle amplitude and PPI are not necessarily
correlated. In the present study, negative correlations were observed
between the startle amplitude and the PPI%, i.e., the greater the
reactivity to the PULSE ALONE stimuli, the smaller the relative
inhibition in the startle magnitude by prepulse. These correlations
were present only in those genotype groups with marked PPI efficacy
(mean PPI% >35; 2C-KO-wt and 2C-OE), but not when PPI was weak (mean PPI% <24; 2C-KO and
2C-OE-wt). This suggests that the extent of PPI may be
responsible for these now observed negative correlations, but it
simultaneously supports the notion that 2C-ARs are
involved in the modulation of both the startle reflex and its PPI. Also
two recent studies with mice have explored differences in the startle
and PPI of multiple inbred strains. In one study, which compared 12 different mouse strains, the average startle magnitude and PPI% were
not correlated over the strains (Paylor and Crawley, 1997), whereas in
another study in 20 strains a positive correlation (r = 0.57; p < 0.01) was observed between the average
startle amplitude evoked by a weak auditory stimulus (90 dB) and PPI%
(Logue et al., 1997). However, in these studies the correlations were
analyzed using group means of the markedly different mouse strains,
whereas in the present study the analyzed values were individual means
of mice having similar strain backgrounds within a group, which
obviously can explain, at least partly, the discrepancies. The results
support the possibility that calculation of PPI% is the best simple
way to control the relationships between startle magnitude and its PPI.
Nevertheless, the association between startle and PPI is currently not
clear, and the now observed negative correlations may raise the
question of whether more sophisticated mathematical methods should be
developed to explore the startle plasticity.
The interpretation of the results from D-Amph tests is
limited by the failure of D-Amph to produce clear effects
on startle responses or PPI% in any of the studied mouse strains. This
was probably because of the mild dosage (Dulawa and Geyer, 1996). However, the startle amplitude difference between 2C-KO
and 2C-KO-wt genotypes was pronounced after
D-Amph. After PCP stimulations, the startle responses of
2C-KO mice were again larger than those of their
controls, whereas 2C-OE mice had smaller startle
amplitudes than 2C-OE-wt mice. This suggests that the
inhibitory contribution of 2C-ARs on the startle
response may increase in stimulated conditions. In contrast to the
D-Amph doses that were used, the PCP doses were sufficient
to reduce PPI as expected. In conclusion, the stimulation experiments
with D-Amph and PCP were in line with the results from
drug-naive mice, but they provided only limited additional information
on the mechanisms involved in the modulation of the startle response
and PPI by 2C-ARs.
These results and current knowledge do not allow us to identify the
probable multiple neural structures that are involved in the inhibitory
role of 2C-ARs in CNS reactivity. The startle reflex
itself may be partly spinally modulated by 2-ARs (Davis et al., 1989), but otherwise the mechanisms are obviously supraspinal, involving the cortico-striato-pallido-pontine circuitry (Geyer et
al., 1990). According to in situ mRNA and
immunohistochemical studies, 2A-ARs are expressed widely
throughout the CNS and in peripheral tissues, 2B-ARs are
present mainly in the periphery, and 2C-ARs have a
distinct expression pattern in the brain (Nicholas et al., 1993;
Scheinin et al., 1994; Nicholas et al., 1996; Rosin et al., 1996;
Talley et al., 1996; Wang et al., 1996). Compared with
2A-ARs, the expression of 2C-ARs in the
CNS is generally less abundant and their distribution is more
concentrated into distinct structures, such as some hippocampal and
cortical regions and the nucleus accumbens and other striatal nuclei,
which are important structures in the pathophysiology of impaired PPI.
Nevertheless, it is unclear in which types of neurons each of the
2-ARs are expressed. It is generally agreed that
2-ARs couple to adenylyl cyclases and ion channel
regulation through inhibitory Gi/0-type proteins, which
also supports the observed inhibitory characteristics of
2C-ARs in this study.
The results from previous aggression studies with
2-AR-subtype-nonselective drugs in rodents are
inconsistent and difficult to interpret. One possible explanation for
this is the complexity of adrenoceptor interactions when both pre- and
postsynaptic 2-ARs are affected with drugs lacking
subtype selectivity. In addition, it has been postulated that the drug
injection per se may mask the possible aggression-heightening effects
of small doses of 2-AR antagonists in mice (Haller et
al., 1996); the genetic approach used in this study circumvents this
problem. Targeted disruption of the mouse gene encoding neuronal nitric
oxide synthase has recently been reported to enhance aggression without
preceding isolation (Nelson et al., 1995). Of other targeted gene
disruptions, those leading to lack of 5-HT1B receptors and
the neuropeptide precursor pre-proenkephalin also enhance aggression,
manifesting especially as shortened attack latency after isolation
(Sadou et al., 1994; König et al., 1996). The mice with altered
2C-AR expression, when normally housed in groups, do not
differ markedly from their wild-type cage mates in their
aggressiveness. Therefore, NE and 2C-ARs may not have a
specific role in the development of enhanced aggression. Rather, the
isolation possibly induces a complex neural process in which
2C-ARs may subserve an inhibitory function when a
strange stimulus is introduced (Bell and Hepper, 1987). On the other
hand, 5-HT is known to be an important modulator of aggression;
therefore, 2C-AR-dependent modulation of brain 5-HT
systems may also explain these findings. Opposite to the now observed
relationships between startle amplitude, PPI, and aggression is the
finding that mice with targeted disruption of the 5-HT1B
receptor gene that were more aggressive than their wild-type controls
(Sadou et al., 1994) had reduced startle amplitude and increased PPI
(Dulawa et al., 1997). Another contradictory finding in this study was
that the isolation increased or did not have an effect on PPI, whereas
isolation has clearly disrupted PPI in rats (Geyer et al., 1993; Varty
and Higgins, 1995). It is possible that mice and rats have significant
differences in their startle and PPI responsiveness. This possibility
is supported by a recent report in which the 5-HT1A agonist
8-OH-DPAT increased PPI in mice (Dulawa et al., 1997), whereas in
previous studies with rats 8-OH-DPAT disrupted PPI (Rigdon and
Weatherspoon, 1992; Sipes and Geyer, 1995).
Previous results from startle experiments in humans are in good
agreement with the current results, because the classical subtype
nonselective 2-AR agonist clonidine has been shown to reduce and the 2-AR antagonist idazoxan to facilitate
the acoustic startle response (Morgan et al., 1993; Kumari et al.,
1996), but the role of 2-ARs in PPI is not clear.
Interestingly, clonidine has been suggested to be beneficial in the
treatment of various neuropsychiatric disorders, although its clinical
usefulness has not been proven (Ahmed and Takeshita, 1996). In recent
studies performed with gene-targeted mice with dysfunctional
2A-ARs, the sedative and hypotensive effects of
nonselective 2-AR agonists were blocked by the
2A-AR disruption (MacMillan et al., 1996; Hunter et al.,
1997; Lakhlani et al., 1997), whereas the hypotensive or sedative
responses to 2-AR agonists are not significantly altered
in 2C-KO mice (Link et al., 1995; Sallinen et al.,
1997). Thus, it is possible that the therapeutic benefit and clinical acceptance of clonidine in neuropsychiatric disorders might have been
restricted by the adverse effects of hypotension and sedation, which
seem to be mediated solely by 2A-ARs. Furthermore, some of the atypical neuroleptics, such as clozapine and risperidone, have
been reported to have moderate affinity to 2-ARs
in vivo and especially to the human 2C-AR
subtype in vitro (Schotte et al., 1996). In light of the
present results, it would be interesting to compare the functional
effects of various antipsychotics at different 2-AR
subtypes, and to screen for polymorphisms of the 2C-AR
gene among various psychiatric patients.
In summary, 2C-ARs modulate the startle reflex and its
prepulse inhibition and aggressive behavior in mice. Because
symptomatology related to the now studied paradigms is present in
several patient categories, it is possible that yet to be developed
subtype-selective 2C-AR drugs might have therapeutic
value in the treatment of schizophrenia and also other neuropsychiatric
disorders with pathological hyper-reactivity and affective
disturbances, such as schizophrenia, attention deficit hyperactivity
disorder, post-traumatic stress disorder, and drug withdrawal
symptoms.
| |
FOOTNOTES |
Received Nov. 24, 1997; revised Jan. 22, 1998; accepted Jan. 27, 1998.
We thank Dr. Richard Link for generating the strains of mice used in
these studies, and Ms. Päivi Saikkonen for skillful technical
assistance.
Correspondence should be addressed to Dr. Jukka Sallinen, Department of
Pharmacology and Clinical Pharmacology, University of Turku,
Kiinamyllynkatu 10, FIN-20520 Turku, Finland.
| |
REFERENCES |
-
Ahmed I,
Takeshita J
(1996)
Clonidine: a critical review of its role in the treatment of psychiatric disorders.
CNS Drugs
6:53-70.
-
Arnsten AFT,
Steere JC,
Hunt RD
(1996)
The contribution of 2-noradrenergic mechanisms to prefrontal cortical cognitive function.
Arch Gen Psychiatry
53:448-455[Abstract/Free Full Text].
-
Bakshi VP,
Swerdlow NR,
Geyer MA
(1994)
Clozapine antagonizes phencyclidine-induced deficits in sensorimotor gating of the startle response.
J Pharmacol Exp Ther
271:787-794[Abstract/Free Full Text].
-
Bell R,
Hepper PG
(1987)
Catecholamines and aggression in animals.
Behav Brain Res
23:1-21[Web of Science][Medline].
-
Braff D,
Grillon C,
Gallaway E,
Geyer M,
Bali L
(1978)
Prestimulus effects on human startle reflex in normals and schizophrenics.
Psychophysiology
15:339-343[Web of Science][Medline].
-
Coull JT
(1994)
Pharmacological manipulations of the 2-noradrenergic system: effects on cognition.
Drugs Aging
5:116-126[Web of Science][Medline].
-
Davis M
(1989)
Neural systems involved in fear-potentiated startle.
Ann NY Acad Sci
563:165-183[Web of Science][Medline].
-
Davis M,
Gendelman DS,
Tischler MD,
Gendelman PM
(1982)
A primary acoustic startle circuit: lesion and stimulation studies.
J Neurosci
2:791-805[Abstract].
-
Davis M,
Commissaris RL,
Yang S,
Wagner KR,
Kehne JH,
Cassella JV,
Boulis NM
(1989)
Spinal vs. supraspinal sites of action of the 2-adrenergic agonists clonidine and ST-91 on the acoustic startle reflex.
Pharmacol Biochem Behav
33:233-240[Web of Science][Medline].
-
Dulawa SC,
Geyer MA
(1996)
Psychopharmacology of prepulse inhibition in mice.
Clin J Physiol
39:139-146.
-
Dulawa SC,
Hen R,
Scearce-Levie K,
Geyer MA
(1997)
Serotonin1B receptor modulation of startle reactivity, habituation, and prepulse inhibition in wild-type and serotonin1B knockout mice.
Psychopharmacology
132:125-134[Medline].
-
Geyer MA,
Swerdlow NR,
Mansbach RS,
Braff DL
(1990)
Startle response models of sensorimotor gating and habituation deficits in schizophrenia.
Brain Res Bull
25:485-498[Web of Science][Medline].
-
Geyer MA,
Wilkinson LS,
Humby T,
Robbins TW
(1993)
Isolation rearing of rats produces a deficit in prepulse inhibition of acoustic startle similar to that in schizophrenia.
Biol Psychiatry
34:361-372[Web of Science][Medline].
-
Haapalinna A,
Viitamaa T,
MacDonald E,
Savola J-M,
Tuomisto L,
Virtanen R,
Heinonen E
(1997)
Evaluation of the effects of a specific 2-adrenoceptor antagonist, atipamezole, on 1-and 2-adrenoceptor subtype binding, brain neurochemistry and behaviour in comparison with yohimbine.
Naunyn Schmiedebergs Arch Pharmacol
356:570-582[Web of Science][Medline].
-
Haller J,
Makara GB,
Kovács JL
(1996)
The effect of 2 adrenoceptor blockers on aggressive behavior in mice: implications for the actions of adrenoceptor agents.
Psychopharmacology
126:345-350[Medline].
-
Hornykiewicz O
(1982)
Brain catecholamines in schizophrenia: a good case for noradrenaline.
Nature
299:484-486[Medline].
-
Howard R,
Ford R
(1992)
From the jumping Frenchmen of Maine to post-traumatic stress disorder: the startle response in neuropsychiatry.
Psychol Med
22:695-707[Web of Science][Medline].
-
Hunter JC,
Fontana DJ,
Hedley LR,
Jasper JR,
Kassotakis L,
Lewis R,
Eglen RM
(1997)
The relative contribution of 2-adrenoceptor subtypes to the antinociceptive action of dexmedetomidine and clonidine in rodent models of acute and chronic pain.
Br J Pharmacol
120:229P.
-
König M,
Zimmer AM,
Steiner H,
Holmes PV,
Crawley JN,
Brownstein MJ,
Zimmer A
(1996)
Pain responses, anxiety and aggression in mice deficient in pre-proenkephalin.
Nature
383:535-537[Medline].
-
Kumari V,
Cotter P,
Corr PJ,
Gray JA,
Checkley SA
(1996)
Effect of clonidine on the human acoustic startle reflex.
Psychopharmacology
123:353-360[Medline].
-
Lakhlani PP,
MacMillan LB,
Guo TZ,
McCool BA,
Lovinger DM,
Maze M,
Limbird LE
(1997)
Substitution of a mutant 2a-adrenergic receptor via "hit and run" gene targeting reveals the role of this subtype in sedative, analgesic, and anesthetic-sparing responses in vivo.
Proc Natl Acad Sci USA
94:9950-9955[Abstract/Free Full Text].
-
Link RE,
Stevens MS,
Kulatunga M,
Scheinin M,
Barsh GS,
Kobilka BK
(1995)
Targeted inactivation of the gene encoding the mouse 2C-adrenoceptor homolog.
Mol Pharmacol
48:48-55[Abstract].
-
Logue SF,
Owen EH,
Rasmussen DL,
Wehner JM
(1997)
Assessment of locomotor activity, acoustic and tactile startle, and prepulse inhibition of startle in inbred mouse strains and F1 hybrids: implications of genetic background for single gene and quantitative trait loci analyses.
Neuroscience
80:1075-1086[Web of Science][Medline].
-
MacDonald E,
Kobilka BK,
Scheinin M
(1997)
Gene targeting: homing in on 2-adrenoceptor-subtype function.
Trends Pharmacol Sci
18:211-219[Medline].
-
MacMillan LB,
Hein L,
Smith MS,
Piascik MT,
Limbird LE
(1996)
Central hypotensive effects of the 2a-adrenergic receptor subtype.
Science
273:801-803[Abstract].
-
Morgan CA,
Southwick SM,
Grillon C,
Davis M,
Krystal JH,
Charney DS
(1993)
Yohimbine: facilitated acoustic startle reflex in humans.
Psychopharmacology
110:342-346[Medline].
-
Nagy A,
Rossant J,
Nagy R,
Abramow-Newerly W,
Roder JC
(1993)
Derivation of completely cell culture-derived mice from early-passage embryonic stem cells.
Proc Natl Acad Sci USA
90:8424-8428[Abstract/Free Full Text].
-
Nelson RJ,
Demas GE,
Huang PL,
Fishman LC,
Dawson VL,
Dawson TM,
Snyder SH
(1995)
Behavioural abnormalities in male mice lacking neuronal nitric oxide synthase.
Nature
378:383-386[Medline].
-
Nicholas AP,
Pieribone V,
Hökfelt T
(1993)
Distributions of mRNA for alpha-2 adrenergic receptor subtypes in rat brain: an in situ hybridization study.
J Comp Neurol
328:575-594[Web of Science][Medline].
-
Nicholas AP,
Hökfelt T,
Pieribone VA
(1996)
The distribution and significance of CNS adrenoceptors examined with in situ hybridization.
Trends Pharmacol Sci
17:245-255[Medline].
-
Nutt DJ
(1994)
Putting the "A" in atypical: does 2-adrenoceptor antagonism account for the therapeutic advantage of new antipsychotics?
J Psychopharmacol
8:193-195.
-
Overbeek PA,
Aguilar-Cordova E,
Hanten G,
Schaffner DL,
Patel P,
Lebovitz RM,
Lieberman MW
(1991)
Coinjection strategy for visual identification of transgenic mice.
Transgenic Res
1:31-37[Medline].
-
Paylor R,
Crawley JN
(1997)
Inbred strain differences in prepulse inhibition of the mouse startle response.
Psychopharmacology
132:169-180[Medline].
-
Rigdon GC,
Weatherspoon J
(1992)
5HT1A receptor agonists block prepulse inhibition of the acoustic startle reflex.
J Pharmacol Exp Ther
263:486-493[Abstract/Free Full Text].
-
Rosin DL,
Talley EM,
Lee A,
Stornetta RL,
Gaylinn BD,
Guyenet PG,
Lynch KR
(1996)
Distribution of 2C-adrenergic receptor-like immunoreactivity in the rat central nervous system.
J Comp Neurol
372:135-165[Web of Science][Medline].
-
Ruffolo RRJ,
Nichols AJ,
Stadel JM,
Hieble PJ
(1993)
Pharmacologic and therapeutic applications of 2-adrenoceptor subtypes.
Annu Rev Pharmacol Toxicol
32:243-279.
-
Sadou F,
Amara DA,
Dierich A,
LeMeur M,
Ramboz S,
Segu L,
Buhot MC,
Hen R
(1994)
Enhanced aggressive behavior in mice lacking 5-HT1B receptor.
Science
265:1875-1878[Abstract/Free Full Text].
-
Sallinen J,
Link RE,
Haapalinna A,
Viitamaa T,
Kulatunga M,
Sjöholm B,
MacDonald E,
Pelto-Huikko M,
Leino T,
Barsh GS,
Kobilka BK,
Scheinin M
(1997)
Genetic alteration of 2C-adrenoceptor expression in mice: influence on locomotor, hypothermic, and neurochemical effects of dexmedetomidine, a subtype-nonselective 2-adrenoceptor agonist.
Mol Pharmacol
51:36-46[Abstract/Free Full Text].
-
Scheinin M,
Lomasney JW,
Hayden-Hixson DM,
Schambra UB,
Caron MG,
Lefkowitz RJ,
Fremeau Jr RT
(1994)
Distribution of 2-adrenergic receptor subtype gene expression in rat brain.
Brain Res Mol Brain Res
21:133-149[Medline].
-
Schotte A,
Janssen PFM,
Gommeren W,
Luyten WHML,
Van Gompel P,
Lesage AS,
De Loore K,
Leysen JE
(1996)
Risperidone compared with new and reference antipsychotic drugs: in vitro and in vivo receptor binding.
Psychopharmacology
124:57-73[Medline].
-
Sipes TA,
Geyer MA
(1995)
8-OH-DPAT disruption of prepulse inhibition in rats: reversal with (+) WAY 100,135 and localization of site of action.
Psychopharmacology
117:41-48[Medline].
-
Stone LS,
MacMillan LB,
Kitto KF,
Limbird LE,
Wilcox GL
(1997)
The 2a adrenergic receptor subtype mediates spinal analgesia evoked by 2 agonists and is necessary for spinal adrenergic-opioid synergy.
J Neurosci
17:7157-7165[Abstract/Free Full Text].
-
Swerdlow NR,
Braff DL,
Taaid N,
Geyer MA
(1994)
Assessing the validity of an animal model of deficient sensorimotor gating in schizophrenic patients.
Arch Gen Psychiatry
51:139-154[Abstract/Free Full Text].
-
Talley EM,
Rosin DL,
Lee A,
Guyenet PG,
Lynch KR
(1996)
Distribution of 2A-adrenergic receptor-like immunoreactivity in the rat central nervous system.
J Comp Neurol
372:111-134[Web of Science][Medline].
-
Varty GB,
Higgins GA
(1995)
Examination of drug-induced and isolation-induced disruptions of prepulse inhibition as models to screen antipsychotic drugs.
Psychopharmacology
122:15-26[Medline].
-
Wang R,
MacMillan LB,
Fremeau Jr RT,
Magnuson MA,
Lindner J,
Limbird LE
(1996)
Expression of 2-adrenergic receptor subtypes in the mouse brain: evaluation of spatial and temporal information imparted by 3 kb of 5' regulatory sequence for the 2AAR-receptor gene in transgenic animals.
Neuroscience
74:199-218[Web of Science][Medline].
-
Yeomans JS,
Frankland PW
(1995)
The acoustic startle reflex: neurons and connections.
Brain Res Brain Res Rev
21:301-314[Medline].
Copyright © 1998 Society for Neuroscience 0270-6474/98/1883035-08$05.00/0
This article has been cited by other articles:
|
|
|
|
 
M. M. Solis and D. J. Perkel
Noradrenergic Modulation of Activity in a Vocal Control Nucleus In Vitro
J Neurophysiol,
April 1, 2006;
95(4):
2265 - 2276.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. M. Stein, W. Bergman, Y. Fang, L. Davison, C. Brensinger, M. B. Robinson, N. B. Hecht, and T. Abel
Behavioral and neurochemical alterations in mice lacking the RNA-binding protein translin.
J. Neurosci.,
February 22, 2006;
26(8):
2184 - 2196.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
W. F. Schwindinger, K. S. Betz, K. E. Giger, A. Sabol, S. K. Bronson, and J. D. Robishaw
Loss of G Protein gamma 7 Alters Behavior and Reduces Striatal alpha olf Level and cAMP Production
J. Biol. Chem.,
February 14, 2003;
278(8):
6575 - 6579.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. Philipp, M. Brede, and L. Hein
Physiological significance of alpha 2-adrenergic receptor subtype diversity: one receptor is not enough
Am J Physiol Regulatory Integrative Comp Physiol,
August 1, 2002;
283(2):
R287 - R295.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
C. A. Fairbanks, L. S. Stone, K. F. Kitto, H. O. Nguyen, I. J. Posthumus, and G. L. Wilcox
alpha 2C-Adrenergic Receptors Mediate Spinal Analgesia and Adrenergic-Opioid Synergy
J. Pharmacol. Exp. Ther.,
January 1, 2002;
300(1):
282 - 290.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
B. E. Viramontes, A. Malcolm, M. Camilleri, L. A. Szarka, S. McKinzie, D. D. Burton, and A. R. Zinsmeister
Effects of an alpha 2-adrenergic agonist on gastrointestinal transit, colonic motility, and sensation in humans
Am J Physiol Gastrointest Liver Physiol,
December 1, 2001;
281(6):
G1468 - G1476.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. J. Millan, F. Lejeune, A. Gobert, M. Brocco, A. Auclair, C. Bosc, J.-M. Rivet, J.-M. Lacoste, A. Cordi, and A. Dekeyne
S18616, a Highly Potent Spiroimidazoline Agonist at alpha 2-Adrenoceptors: II. Influence on Monoaminergic Transmission, Motor Function, and Anxiety in Comparison with Dexmedetomidine and Clonidine
J. Pharmacol. Exp. Ther.,
December 1, 2000;
295(3):
1206 - 1222.
[Abstract]
[Full Text]
|
|
|
|
|
|
|
J. W. Kable, L. C. Murrin, and D. B. Bylund
In Vivo Gene Modification Elucidates Subtype-Specific Functions of alpha 2-Adrenergic Receptors
J. Pharmacol. Exp. Ther.,
April 1, 2000;
293(1):
1 - 7.
[Abstract]
[Full Text]
|
|
|
|
|
|
|
T. Olli-Lahdesmaki, J. Kallio, and M. Scheinin
Receptor Subtype-Induced Targeting and Subtype-Specific Internalization of Human alpha 2-Adrenoceptors in PC12 Cells
J. Neurosci.,
November 1, 1999;
19(21):
9281 - 9288.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
J. D. Altman, A. U. Trendelenburg, L. MacMillan, D. Bernstein, L. Limbird, K. Starke, B. K. Kobilka, and L. Hein
Abnormal Regulation of the Sympathetic Nervous System in alpha 2A-Adrenergic Receptor Knockout Mice
Mol. Pharmacol.,
July 1, 1999;
56(1):
154 - 161.
[Abstract]
[Full Text]
|
|
|
|
|
|
|
W. Zhang, V. Klimek, J. T. Farley, M.-Y. Zhu, and G. A. Ordway
alpha 2C Adrenoceptors Inhibit Adenylyl Cyclase in Mouse Striatum: Potential Activation by Dopamine
J. Pharmacol. Exp. Ther.,
June 1, 1999;
289(3):
1286 - 1292.
[Abstract]
[Full Text]
|
|
|
|
|
|
|
K. M. Small, S. L. Forbes, F. F. Rahman, K. M. Bridges, and S. B. Liggett
A Four Amino Acid Deletion Polymorphism in the Third Intracellular Loop of the Human alpha 2C-Adrenergic Receptor Confers Impaired Coupling to Multiple Effectors
J. Biol. Chem.,
July 21, 2000;
275(30):
23059 - 23064.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
C. M. Hurt, F. Y. Feng, and B. Kobilka
Cell-type Specific Targeting of the alpha 2c-Adrenoceptor. EVIDENCE FOR THE ORGANIZATION OF RECEPTOR MICRODOMAINS DURING NEURONAL DIFFERENTIATION OF PC12 CELLS
J. Biol. Chem.,
November 3, 2000;
275(45):
35424 - 35431.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
K. M. Small, K. M. Brown, S. L. Forbes, and S. B. Liggett
Polymorphic Deletion of Three Intracellular Acidic Residues of the alpha 2B-Adrenergic Receptor Decreases G Protein-coupled Receptor Kinase-mediated Phosphorylation and Desensitization
J. Biol. Chem.,
February 9, 2001;
276(7):
4917 - 4922.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
Top
Abstract
Introduction
