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The Journal of Neuroscience, July 1, 2002, 22(13):5705-5712
Functional Domains in Dorsal Striatum of the Nonhuman Primate Are
Defined by the Dynamic Behavior of Dopamine
Stephanie J.
Cragg,
Christopher J.
Hille, and
Susan A.
Greenfield
University Department of Pharmacology, Oxford OX1 3QT,
United Kingdom
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ABSTRACT |
The dorsal striatum comprises a continuum of distinct functional
domains, limbic, associative, and sensorimotor. In the primate it
exclusively subdivides further into two nuclei, the putamen and
caudate. Dopamine (DA) transmission is differentially affected between
these nuclei in neurodegenerative diseases such as Parkinson's and by
psychostimulants such as cocaine. Because rodent systems can offer only
limited insight into DA systems of the human brain, a fuller
appreciation of DA transmission and its role in dysfunction requires
direct study in primates.
DA behavior was explored in the major functional domains of the caudate
nucleus and compared with the putamen, using fast-scan cyclic
voltammetry in striatal sections from the marmoset (Callithrix jacchus). There was domain-specific variation in extracellular DA transients [i.e., concentration ([DA]o)
released by a single stimulus and the rate maximum of DA uptake,
Vmax]. Across nuclei, functional rather
than anatomical regions were differentiated by these dynamics. The
largest, fastest DA transients were at motor-associated loci. Evoked
[DA]o at physiological frequencies was differently
frequency-sensitive between functional domains but not between
anatomical nuclei. In contrast, presynaptic depression was not an index
of regional differentiation, recovering with similar kinetics at all loci.
Within a given functional domain of dorsal striatum, the dynamics of DA
release and uptake are similar for the putamen and the caudate nucleus.
Conversely, distinct functional domains are defined by these DA
dynamics, in a manner more marked in primates than in rodents. These
data from the primate brain highlight differences in DA availability
that may be central to DA function and dysfunction in the human.
Key words:
caudate; putamen; Parkinson's disease; basal ganglia; dopamine transporter; dopamine uptake; marmoset; voltammetry; corticostriatal; nigrostriatal; striatonigral; mesostriatal; cocaine; synaptic depression; primate; ventral striatum
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INTRODUCTION |
A distinction between putamen (Put)
and caudate (Cd) nuclei, divided by the internal capsule, is a hallmark
of the primate dorsal striatum. Interestingly, uneven patterns of
mesostriatal dopamine (DA) cell loss render the most dorsolateral (dl)
regions of the putamen most susceptible, whereas the caudate nucleus
remains less affected, in both idiopathic Parkinson's disease (PD) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)
parkinsonism in primate models (Hornykiewicz, 1966 ; Schoemaker et al.,
1985 ; Elsworth et al., 1987 ; Kish et al., 1988 ; Maloteaux et al., 1988 ; Seeman and Niznik, 1990 ; Kaufman and Madras, 1991 ; Moratalla et al.,
1992 ; Antonini et al., 1995 ). However, this anatomical subdivision is
only one way of analyzing the primate striatum. In particular, corticostriatal connectivity defines three distinct but interrelated "functional domains" that span the caudate and putamen:
ventromedial (vm)/limbic-, central/associative-, and
dorsolateral/motor-associated (Kunzle, 1975 , 1977 ; Selemon and
Goldman-Rakic, 1985 ; Haber and McFarland, 1999 ). Such topographic
segregation is maintained throughout the primate basal ganglia,
inter alia, thalamostriatal (Gimenez-Amaya et al., 1995 ;
McFarland and Haber, 2000 ), amygdalostriatal (Russchen et al., 1985 ),
striatopallidal (Hazrati and Parent, 1992 ), and striatonigral
projections (Lynd-Balta and Haber, 1994a ; Haber et al., 2000 ) and even
somatotropic organization (Crutcher and DeLong, 1984 ; Alexander and
DeLong, 1985 ). Furthermore, different functional domains receive inputs
from different mesostriatal DA neurons (Szabo, 1980 ; Parent et al.,
1983 ; Smith and Parent, 1986 ; Lynd-Balta and Haber, 1994b ; Haber et
al., 2000 ). Consequently, to understand striatal DA function and, in
turn, dysfunction, it is essential to explore these features in
functionally rather than anatomically (caudate/putamen) segregated domains.
We have shown previously that within the putamen of a nonhuman primate
there are significant differences in DA behavior that parallel the
functional organization (Cragg et al., 2000 ). Yet this same repertoire
of region-specific differences is not as apparent in the dorsal striata
of rodents (Cragg et al., 2000 ). In this study, using fast-scan cyclic
voltammetry, we have now determined the dynamic behavior of DA in the
primate caudate nucleus and compared these findings with those we
described previously for the putamen. In this manner, we have
determined in real time the dynamics of extracellular DA in limbic-
through to motor-associated functional domains throughout the dorsal
striatum. We have profiled DA dynamics by single-pulse availability,
uptake rate maximum (Vmax), frequency
sensitivity, and presynaptic depression. We address the hypotheses that
there are differences in DA dynamics within the caudate nucleus that
reflect functional subdivision, akin to that observed in the putamen,
rather than the anatomical segregation of the caudate versus the
putamen or other structural organization of the nuclei. In addition, we
discuss how these data provide additional support for the hypothesis
(Wu et al., 2001 ) that it is such dynamics that underlie the
paradoxical, documented preferential action of cocaine in the limbic-
rather than motor-associated striatum (Carboni et al., 1989 ; Cass et al., 1992 ; Kuczenski and Segal, 1992 ; Bradberry et al., 2000 ). These
data illustrate the function-associated repertoire of DA dynamics in
the striatum of a nonhuman primate of central relevance to basal
ganglia function in health and disease.
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MATERIALS AND METHODS |
Brain slice preparation. Male marmosets (1-2 years)
from an established colony were killed with an overdose of
pentobarbitone (i.p.). Striatal slices (400 µm) were prepared in
ice-cold, HEPES-buffered physiological saline saturated with 95%
O2 and 5% CO2, as
described previously (Rice et al., 1997 ; Cragg et al., 2000 ) using a
Vibratome (Lancer Series 1000; Lancer, St. Louis, MO). Striatal
coordinates were midcommissural to rostral [anteroposterior (AP),
8.5-11.5 mm] (Stephan et al., 1980 ). All recordings were made at
32°C in bicarbonate-buffered artificial CSF (aCSF; 95%
O2 and 5% CO2) as
described previously (Cragg et al., 2000 ).
Anatomical definitions. By dorsal striatum, we mean the
striatum excluding the nucleus accumbens. According to corticostriatal connectivity (Selemon and Goldman-Rakic, 1985 ; Haber and McFarland, 1999 ), three major functional domains of the striatum can be delineated along a ventromedial to dorsolateral axis: ventromedial, central, and
dorsolateral. Each domain is associated, respectively, with one of the
following functions (and distinct frontal inputs): limbic
(orbital and medial prefrontal cortex), cognitive/associative (dorsolateral prefrontal cortex), and sensorimotor (premotor and motor cortex). In both the caudate and the putamen, we defined three
recording sites for comparison along the ventromedial-dorsolateral axis: vm, central (or mid), and dl, respectively (Fig.
1b). These regions correspond
approximately to the three functionally defined territories, with the
exception of the caudate nucleus, in which both the central and
dorsolateral regions sampled are predominantly cognitive-associated,
central striatum. Therefore, note that the dorsolateral loci in the
caudate sampled in this study, unlike dorsolateral loci in putamen, do
not constitute a component of the dorsolateral striatum (DLS) as
defined by Haber et al. (2000) .

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Figure 1.
Regional variation in single
pulse-evoked [DA]o between domains of the dorsal
striatum. a, c, Mean [DA]o versus time
evoked by a single pulse (arrows) at varying vm to dl
coordinates in the Cd and Put at two ranges of AP coordinates of
striatum: rostral-central, 11.9-10.5 mm (a) and
central-postcommissural, 10.25-8.5 mm (c).
Inset, Cyclic voltammogram indicating oxidation and
reduction peaks for DA. Typical vm-dl recording sites are illustrated
by the circles in b, a schematic
illustration of a coronal striatal section, 10.5 mm. IC,
Internal capsule; Dors, dorsal; Lat,
lateral. At both coordinate ranges in a and
c, there is significant regional variation in
[DA]o along this ventromedial-dorsolateral axis in both
the caudate (one-way ANOVA; p < 0.001) and the
putamen (p < 0.001). Post
hoc t tests: *p < 0.05;
**p < 0.01; ***p < 0.001 versus ventromedial; n = 4-25. In the putamen,
there is lateral variation between each of the three territories
( p < 0.05,   p < 0.001 vs central).
d, Geographic representation of overall mean peak
[DA]o evoked from the caudate and the putamen along
vm-dl axes. ***p < 0.001 versus ventromedial;
  p < 0.001 versus central;
post hoc t tests. Only at the
dorsolateral coordinate is [DA]o significantly greater in
the putamen than in the caudate nuclei (***p < 0.001; n = 11-41). [DA]o follows the
following hierarchy: dorsolateral putamen > dorsolateral
caudate central caudate central putamen > ventromedial caudate = ventromedial putamen. All data are
means ± SEM.
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Voltammetry and microelectrodes.
[DA]o was measured using fast-scan cyclic
voltammetry at a 6 µm carbon-fiber microelectrode beveled to a point
(tip length, ~30 µm; MPB Electrodes, London, UK) using a Millar
Voltammeter (PD Systems, West Moseley, UK) as described
previously (Cragg and Greenfield, 1997 ; Cragg et al., 2000 ). The scan
rate was 800 V/sec, from 0.7 to 1.3 V to 0.7 V versus
Ag/AgCl, and the sampling frequency was 4-8 Hz. Illustrated
voltammograms are faradic currents obtained by subtraction of
background current. Identification of the released substance as DA was
confirmed by comparison in situ and in DA calibration of
oxidation and reduction potentials (+540 and 180 mV vs Ag/AgCl, respectively). Contributions from the monoamine oxidase
(MAO)-metabolite DOPAC or norepinephrine (NE) were determined as
minimal from experiments including MAO inhibition (pargyline) and NE
uptake inhibition (desipramine). Electrodes were calibrated in 1-2
µM DA in aCSF. Sensitivity to DA is linear at
these concentrations and typically 2-7 nA/µM.
The minimum detection limit for [DA]o (~2×
currents for noise) was roughly 20-40 nM.
Electrical stimulation. Electrical stimulation was at local,
surface bipolar electrodes (50 µm diameter), as described previously (Cragg et al., 2000 ). Stimulus pulses (0.1 msec pulse width,
half-maximal at 10 V) were applied in one of three paradigms: singly;
in trains at 1-20 Hz for 3 sec; or in pairs with interstimulus
intervals ranging from 50 msec to 40 sec (and interpair intervals of 5 min). Evoked release was TTX-sensitive and
Ca2+-dependent (data not illustrated).
Analysis of data and dopamine kinetics. Voltammetric data
were acquired and analyzed on a personal computer running the
Strathclyde Whole Cell Program (Dr. J. Dempster, University of
Strathclyde, Strathclyde, UK). Data simulations for analysis of
Michaelis-Menten kinetics used software provided by Dr. R. M. Wightman (University of North Carolina, Chapel Hill, NC) to fit
each experimental data curve (concentration vs time) with theoretical
curves. The kinetic analysis is based on the assumption that DA
released into the extracellular space after a single pulse appears as
an instantaneous concentration increase, [DA]p,
followed by a rapid decline that is predominantly attributable to
transporter-mediated uptake (Giros et al., 1996 ), operating with
Michaelis-Menten kinetics. Concentration-time (t)
profiles can be described by the following equation:
d[DA]/dt = [DA]p Vmax/{[Km/(DA)] + 1}, where Km is the affinity
constant of the dopamine transporter (DAT) and
Vmax is the maximum velocity of
uptake. This method of analysis has been described in detail previously (Wightman and Zimmerman, 1990 ; Kawagoe et al., 1992 ; Jones
et al., 1995 ) and incorporates a factor d that introduces a
delay to compensate for electrode response time attributable to surface
coatings or other interactions at uncoated electrodes. In the present
studies, d (140-220 nm) was comparable with that reported
previously (Cragg et al., 2000 ). Km
was set at a constant value of 210 nM (Ross,
1991 ; for review, see Cragg et al., 2000 ). For each accepted
simulation, R2 was >0.9. In
cases in which small changes in variable parameter values could
sometimes give equal degrees of fit (equivalent
R2), both sets were included
for analysis (n = number of simulations). Data
simulations were undertaken blind with respect to recording site.
Linear regressions, Pearson correlations, and slope comparisons used
GraphPad software (GraphPad Software Inc., San Diego, CA).
Paired-pulse data were used to determine time constants for recovery of
release at a given site using a double exponential curve fit (Kennedy
et al., 1992 ) of the form y = y0 + a(1 exp bx) + a(1 exp cx), where x
is time and a, b, and c are
variables. We include a y-intercept term,
y0, because this relationship may not
necessarily approach zero at time 0 in a simple manner (our unpublished
observation). We constrained a and
ymax to a relationship in which
ymax = y0 + 2 × a = 1. Time
constants were [ln2]/b and
[ln2]/c.
Data are means ± SEM and the sample size is the number of
recording sites, unless otherwise stated. Some data from the putamen have been published in part previously (Cragg et al., 2000 ); we have
included expanded data about the putamen, where appropriate, from sets
that match those for the caudate to make comparisons with the caudate.
The number of animals in each experiment is 3-12. Comparisons for
differences in means were assessed by one- or two-way ANOVA and
post hoc multiple comparison t tests
(Newman-Keuls). In pulse train experiments, "net" or
"steady-state" refers to [DA]o at a period
during the stimulation during which [DA]o
approaches an apparent steady state (2-3 sec). Note that the steady
state may be a complex state and is not necessarily seen at high frequencies.
Solutions. All drugs and solutions were obtained from
Sigma (Poole, UK).
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RESULTS |
Single-pulse evoked [DA]o within and between
striatal domains
A single, electrical stimulus pulse in marmoset striatum evoked
the rapid release and removal of the electroactive substance, DA. The
substance was identified as DA by a characteristic voltammogram with
peak potentials for oxidation (~500 mV) and reduction currents (~-200 mV vs Ag/AgCl) (Fig. 1a, inset) that are
identical for exogenously applied DA and DA signals obtained
previously, including in marmoset putamen (Cragg et al., 2000 ). The
metabolite DOPAC did not contribute significantly to evoked signals in
the putamen (Cragg et al., 2000 ) or in the caudate nucleus: inhibition
of monoamine oxidases A and B for up to 5 hr (pargyline, 20 µM), had no effect on the magnitude of the
signal observed (n = 4; data not shown).
There was significant regional variation in evoked
[DA]o within both the caudate and the putamen
attributable to vm-dl coordinates at both central-rostral locations
(Fig. 1a,b) (one-way ANOVAs; p < 0.001;
caudate, n = 9-16; putamen, n = 7-25)
and central-postcommissural locations (Fig. 1c) (one-way
ANOVAs; p < 0.001; caudate, n = 6-12; putamen, n = 4-15). Throughout the AP range of the
striatum investigated, evoked [DA]o in
dorsolateral positions was significantly greater than that released
ventromedially, by more than twofold in the caudate and, most markedly,
by threefold in the putamen. The more marked regional variation within
the putamen than within the caudate was attributable to the most
elevated [DA]o of all of those occurring in the
dorsolateral putamen.
No significant effect of the AP coordinate was detected in either the
caudate or the putamen (two-way ANOVAs). There was a trend in
ventromedial putaminal regions toward higher evoked
[DA]o more caudally. At these loci, the most
medial coordinate of the putamen is more lateral than in rostral
regions (Stephan et al., 1980 ). For all subsequent data analysis, AP
data were pooled. Within the caudate nucleus, the mean evoked
[DA]o in central and dorsolateral regions was
twice that seen in ventromedial areas (Fig. 1d) (193 ± 17% and 197 ± 15% of ventromedial, respectively; p < 0.001; n = 17-28). In contrast,
within the putamen, there was significant variation between each of the
three subregions measured, such that there was more than a threefold
vm-dl variation in evoked [DA]o (Fig.
1d) (dorsolateral 313 ± 18% > central 181 ± 8% of ventromedial; p < 0.001; n = 11-41).
A comparison of putamen versus caudate nuclei reveals that internucleus
variation (20-30%) is a far smaller source of variation than
intranucleus variation (300%). At similar vm-dl coordinates in the
caudate and the putamen, [DA]o is similar in
all but the most lateral regions: The only significant source of
variation between the caudate and the putamen (two-way ANOVA;
p < 0.05) is at this dorsolateral coordinate, where
the mean [DA]o in the putamen (1.83 ± 0.11 µM; n = 41) exceeds that
in the caudate by 33% (1.38 ± 0.11 µM;
p < 0.001; n = 28). This
region-specific availability of DA parallels the segregation of the
functional organization of the striatum, as delineated by
corticostriatal topography.
Dopamine uptake pharmacology
Single-pulse evoked [DA]o and
extracellular lifetime were unaltered by application of desipramine
(300 nM), a competitive inhibitor of the NE transporter in
the caudate nucleus (Fig. 2a) (n = 7; three applications) as well as the
putamen (Cragg et al., 2000 ). In contrast, uptake by the DAT avidly
regulated the extracellular concentration and lifetime of DA (Fig.
2b): during competitive inhibition of the DAT by GBR 12909 (500 nM), maximum [DA]o,
the clearance rate, and thus the extracellular lifetime of single-pulse evoked [DA]o throughout the caudate were
significantly elevated (one-way ANOVA; p < 0.001;
n = 19; four applications), as seen in the putamen
previously (Cragg et al., 2000 ). Together, these data also verify that
the substance measured is not NE but DA.

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Figure 2.
DA uptake: pharmacology and
Vmax. a, b, Mean
[DA]o versus time after a single pulse
(arrow) in the caudate nucleus in the control
(circles) and with an uptake transporter
inhibitor for norepinephrine (a; desipramine, 300 nM; triangles) or DA (b; GBR
12909; 500 nM; squares). a,
Desipramine had no effect on the peak [DA]o or the
removal rate (n = 7). b, GBR 12909 slowed the rate of removal and significantly enhanced
[DA]o (n = 19;
***p < 0.001). c, Data points of a
typical observation in the central caudate (Mid Cd)
after a single pulse (arrow). Superimposed is a
simulated data curve that describes the [DA]o detected
after release and uptake with Michaelis-Menten kinetics. In this
example, [DA]p = 2.15 µM,
Vmax = 3.1 µM/sec,
R2 = 0.99, Km = 210 nM, and
d = 180 nm. d,
Vmax versus [DA]p from all
simulations in the caudate (triangles;
n = 67) and the putamen (circles;
n = 91). Positive correlations (dotted
lines) within the putamen (r = 0.68) and
the caudate (r = 0.5) are significantly different
from zero (p < 0.001). The mean
[DA]p (e; ***) and mean
Vmax within the caudate
(f; bars) compared with the
putamen (dashed and/or dotted lines;
means ± SEM) across vm-dl axes are shown.
[DA]p and Vmax increase
significantly with an increasing dorsolateral coordinate in both nuclei
(one-way ANOVAs; p < 0.001; post
hoc t tests; ***p < 0.001 vs ventromedial) but only in the putamen is there a lateral variation
between each of the three territories (Cragg et al., 2000 ). Only at the
dorsolateral coordinate are either [DA]p or
Vmax significantly greater in the putamen
than in the caudate ( p < 0.01;
  p < 0.001; n = 16-38). All data are means ± SEM.
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Regional variation in Vmax of dopamine
uptake between striatal domains
Each experimental observation of [DA]o
versus time was modeled with simulations of Michaelis-Menten kinetics
to evaluate the maximal uptake rate,
Vmax, via the DAT, and the underlying
release per pulse, [DA]p (Fig.
2c-f). Vmax and
[DA]p varied in a manner that was positively
correlated (Fig. 2d) in the caudate (Pearson test;
p < 0.001; r = 0.50; n = 67) and in the putamen (p < 0.001; r = 0.68; n = 91).
Analysis of the distribution of the values of
[DA]p and Vmax
within a vm-dl axis in each nuclei revealed significant effects of
coordinate (Fig. 2e,f), as seen for
[DA]o (Fig. 1d).
[DA]p varied significantly within the caudate
(Fig. 2) [1.67 ± 0.10 µM (vm) to
2.70 ± 0.14 µM (dl); one-way ANOVA;
p < 0.001; n = 16-30] and within the
putamen (Fig. 2e) [1.45 ± 0.07 µM (vm) to 3.35 ± 0.16 µM (dl); one-way ANOVA; p < 0.001; n = 23-38]. The central and dorsolateral
caudate did not differ. The source of variation between the caudate and
the putamen, when compared at paired coordinates (two-way ANOVA;
p < 0.05), was attributable to 25% greater
[DA]p in the dorsolateral putamen than in the
caudate. [DA]p in the dorsolateral putamen
significantly exceeded release in all other regions.
Vmax varied significantly within the
caudate (Fig. 2f) [3.51 ± 0.15 µM/sec (vm) to 4.20 ± 0.14 µM/sec (dl); one-way ANOVA; p < 0.001; n = 16-30] and within the putamen (Fig.
2f) [3.27 ± 0.13 µM/sec (vm) to 4.94 ± 0.13 µM/sec (dl); one-way ANOVA; p < 0.001; n = 23-38]. The central and dorsolateral
caudate did not differ. The source of variation between the caudate and
the putamen, when compared at paired coordinates (two-way ANOVA;
p < 0.05), was attributable to ~20% greater
Vmax in the putamen than in the
caudate at dorsolateral regions. Indeed, the highest Vmax observed was in the dorsolateral
putamen, where Vmax was significantly
greater than in all other regions.
It should be noted that the values of [DA]p and
Vmax obtained using this approach were
validated by an alternative means of estimating
Vmax (Cragg et al.,
2000 ). After an intense stimulation (e.g., 100 Hz, 0.5 sec train) that generates sufficiently high [DA]o ( Km
of the DAT), the clearance rate of DA approaches a constant,
Vmax (i.e., zero-order kinetics). The
values of Vmax obtained using this
method were not different from those obtained by the data simulations
and, furthermore, demonstrated regional heterogeneity (data not shown).
Frequency dependence of dopamineo during
pulse trains
Steady-state [DA]o was compared in
ventromedial versus dorsolateral regions of both the caudate and the
putamen during pulse trains, over a range of frequencies (2-20 Hz) at
which DA neurons may fire action potentials in situ (Grace
and Bunney, 1983 , 1984a ,b ; Schultz et al., 1983 ; Schultz, 1984 , 1986 ).
These data have been reported in part previously for the putamen (Cragg
et al., 2000 ). As for the putamen, evoked [DA]o
in the caudate nucleus peaked initially (t 0.5 sec)
after the start of the stimulus (Fig. 3a,b) to a concentration
resembling that observed after a single pulse (Fig. 1). Subsequently,
[DA]o either declined, remained constant at, or
increased to an approximately steady-state concentration (at
t = 2-3 sec) as a function both of the frequency of
stimulation (Fig. 3a-c) (two-way ANOVA; p < 0.001; n = 4-10) and of the region (p < 0.001). In the ventromedial,
limbic-associated caudate, each increase in the stimulation frequency
increased [DA]o at steady-state times (compared
with previous frequency) (e.g., Newman-Keuls multiple comparison
t tests; p < 0.01 for 20 Hz vs 10 or 5 Hz)
with a facilitative (nonlinear) relationship, as in the
ventromedial putamen (Fig. 3c). In the dorsolateral caudate,
each increase in the stimulation frequency increased the steady-state
[DA]o (compared with the previous frequency) up
to 10 Hz (e.g., Newman-Keuls multiple comparison t tests;
p < 0.05 for 2 Hz vs 10 or 20 Hz): at 20 Hz, no
significant increase in [DA]o compared with 10 Hz was observed, as in the dorsolateral putamen (Fig.
3c).

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Figure 3.
Frequency sensitivity of [DA]o
during pulse trains. a, b, [DA]o versus
time during 3 sec stimulation trains (stim, solid
bar) over a range of frequencies (2-20 Hz) in the vm
(a) and dl (b) caudate
nucleus, 3 < n < 10. SEMs are excluded for
clarity. The steady-state [DA]o, between
dotted lines, varies differently with the frequency in
each region. c, Steady-state [DA]o versus
frequency in ventromedial (open symbols) and
dorsolateral (filled symbols) regions of both the
caudate (triangles) and, for comparison, putamen
(circles). [DA]o is a function of the
frequency of stimulation (two-way ANOVA; p < 0.001) and subregion (p < 0.01). As in the
putamen (dotted lines), in the dorsolateral caudate each
increase in frequency up to 10 Hz increased [DA]o
(post hoc t tests; p < 0.001-0.05; n = 5-8); in the ventromedial
caudate, [DA]o increased supralinearly with the frequency
(p < 0.01-0.05; n = 4-10). [DA]o in the ventromedial regions significantly
exceeded that in the dorsolateral regions at frequencies of >10 Hz (20 Hz; **p < 0.01). There was no significant
difference between the caudate and the putamen (two-way ANOVA). Data
are means ± SEM.
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Consistent with the regional differences in
[DA]o observed after a single pulse, net
[DA]o at low frequencies (<10 Hz) was significantly greater (approximately twofold) in the dorsolateral than
in the ventromedial caudate (t tests; p < 0.01; n = 4-5). However, at higher frequencies,
steady-state [DA]o in the ventromedial caudate
becomes equal to (10 Hz) and eventually significantly greater than (20 Hz; more than twofold) [DA]o in the
dorsolateral regions (Fig. 3c) (p < 0.01; n = 6-10). An identical effect has been
described previously for the putamen (Cragg et al., 2000 ), as
illustrated for the comparison in Figure 3c.
Comparison of net, steady-state [DA]o in the
caudate with the putamen at paired coordinates indicates no significant
difference between the two nuclei (two-way ANOVAs for frequency and
nucleus; p < 0.001 and p > 0.05, respectively) at either of the two coordinates.
Recovery from presynaptic depression
To characterize further the presynaptic properties of DA neurons
innervating the striatal region most vulnerable to parkinsonian degeneration (i.e., the dorsolateral putamen), we evaluated and compared the recovery of the releasability of DA after presynaptic depression at the dorsolateral loci of the putamen and the caudate. At
a given dorsolateral site in both the putamen and the caudate nuclei in
each of four different animals, paired pulses of varying interpulse
intervals (50 msec to 40 sec) were administered repeatedly and in
random order, 5 min apart (Fig.
4a). The recordings sites were
typical for each nucleus: Mean peak
[DA]o evoked by a single pulse was 2.08 ± 0.03 µM in the putamen, which more importantly was consistently greater than evoked [DA]o in
the caudate (1.64 ± 0.23 µM;
p < 0.05; n = 4). In both nuclei, a
strong depression of release was observed at the second, paired pulse,
which was inversely related to the interpulse interval (Fig.
4a,b). Full recovery of [DA]o occurs
only after ~30 sec (Fig. 4a,b). The relationships between
the paired-pulse ratio of [DA]o [pulse 2 (P2)/pulse 1 (P1)] and interpulse interval in both the caudate
and the putamen could be fitted well with double exponentials
(R2 > 0.99), as described
previously for rodent striata (Kennedy et al., 1992 ; Abeliovich et al.,
2000 ). The time constants derived from both fast and slow components
were: caudate, 2.7 and 9.4 sec; putamen, 2.8 and 7.7 sec, shorter than
those described in rodents. Although a two-way ANOVA (for time and
region) indicates significant variation in
[DA]o attributable to interpulse interval (p < 0.001), there is no significant difference
in relative recovery between the caudate and the putamen
(n = 4), despite the difference in absolute
[DA]o.

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Figure 4.
Recovery from presynaptic depression.
a, A typical series for [DA]o versus time
after paired stimulus pulses (arrows) delivered at
interpulse intervals ranging from 2 to 20 sec (dotted
lines), recorded at a single site in the dorsolateral caudate.
Plots are standardized to 100% of the release by the first pulse.
Release is restored only after at least 20 sec.
b, Mean [DA]o released by the second,
paired pulse as a fraction of the first (P2/P1) versus
the interpulse interval for the dorsolateral caudate
(triangles) and the putamen (circles).
Curve fits (solid lines) are double exponentials of the
form y = y0 + a(1 e bx) + a(1 e cx),
where y0 + 2 × a = 1. Time constants of fast and slow components of illustrated fits are,
respectively: Cd, 2.7 and 9.4 sec; Put, 2.8 and 7.7 sec. The two-way
ANOVA indicates variation attributable to time
(p < 0.001) but not region
(n = 4).
|
|
 |
DISCUSSION |
These data indicate region-specific heterogeneity in the dynamic
regulation of [DA]o throughout the dorsal
striatum of a nonhuman primate. Different functional domains are
defined by corresponding differences in DA dynamics. Within a given
functional domain, the anatomically segregated putamen and caudate
nuclei nonetheless share similar DA dynamics.
Single-pulse evoked dopamineo and striatal domain
[DA]o evoked locally by a single pulse was
a function of the vm-dl coordinate in the caudate nucleus, as seen
previously in the putamen (Cragg et al., 2000 ). From the three-site
analysis of [DA]o (and
[DA]p) within each nucleus, a marked gradient
in DA availability is apparent (Fig. 5):
evoked [DA]o increases from the ventromedial
striatum through the central striatum to the DLS by twofold (caudate)
to threefold (putamen).

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|
Figure 5.
Summary of regional availability of DA. The
gradient fill within the dorsal striatum enclosed by the dashed
lines illustrates the relative regional availability of DA as
determined by mean [DA]o evoked by a single pulse in the
striatum, here indicated by a key, in micrometers.
Representative recording locations along the vm-dl axes used to define
this gradient in both the Cd and the Put are indicated
(circles). Regional differences in DA availability obey
a vm-dl organization that spans the two nuclei. This territorial
delineation parallels the known functional domains of the striatum
defined by corticostriatal connectivity (for review, see Haber and
McFarland, 1999 ; Haber et al., 2000 ). IC, Internal
capsule.
|
|
Notably, intranucleus variation in [DA]o
reflected the functional domain. For example, within the caudate
nucleus, evoked [DA]o was lower in the
ventromedial compared with the central sites, yet it was similar in the
central and the dorsolateral sites: this heterogeneity parallels the
segregation of these three loci into primarily two functional domains,
the ventromedial/limbic Cd (vm Cd) and the central/associative
(mid and dl Cd) (Haber et al., 2000 ). Interestingly, this finding is
mirrored in basal [DA]o seen within the caudate
nucleus in rhesus monkeys (Bradberry et al., 2000 ). In contrast, in the
putamen the significant difference in evoked
[DA]o between each of the three loci in turn
parallels the division into three functional domains, including the
dorsolateral/motor Put (dl Put). Moreover, mean evoked
[DA]o within each functional domain was common
to the putamen and the caudate. In other words, functional domains,
which span anatomical segregation into the caudate and the putamen, can
be delineated by DA availability.
These data suggest that functions assigned to a striatal subregion are
more accurately described by mediolateral coordinates than by the
nucleus. In particular, mesostriatal features assigned preferentially
to the putamen and not the caudate may reside in dorsolateral
projections (e.g., onset of parkinsonian degeneration), whereas
features assigned preferentially to the caudate may result from a
greater component of central/associative functions. The greatest
[DA]o is evoked in the dorsolateral putamen,
the only motor domain sampled, and notably, the region most susceptible to parkinsonian degeneration. Greater apparent DA availability/turnover in dorsolateral regions may, in turn, generate a greater accumulation of potentially toxic DA metabolites. Given the concordance between the
degree of degeneration associated with each region in PD (Kish et al.,
1988 ; Antonini et al., 1995 ) and DA availability, our observations
indirectly support DA autotoxicity as a contributing factor in PD (for
review, see Jenner and Olanow, 1996 ; Olanow and Tatton, 1999 ).
Dopamine uptake and striatal domain
The DAT was the dominant uptake mechanism governing
[DA]o and lifetime in the caudate, as observed
in the putamen (Cragg et al., 2000 ). Like [DA]o
and [DA]p, uptake
Vmax varied in a region-specific manner that paralleled the segregation of the caudate and the putamen
into, respectively, two versus three functional domains: Vmax in dorsolateral locations was
greater than that in ventromedial locations by 20% in the caudate and
50% in the putamen.
Within each functional domain, the mean
Vmax was similar in the caudate and
the putamen. Vmax was greatest in the
dorsolateral putamen, the only motor domain sampled, and exceeded
Vmax in the dorsolateral caudate by
20%. Thus, uptake, as for release, reflects the functional striatal
domain, as defined by corticostriatal connectivity, primarily
independently of anatomical segregation into the caudate versus the putamen.
Previous findings from techniques with less-defined spatial resolution
show that overall DAT protein levels or ligand-binding density,
Bmax
([125I]altropane,
[3H]mazindol,
[3H]CFT, and
[125I]RTI-55), in primate,
including human, striatum are highest in the putamen (Donnan et al.,
1991 ; Kaufman and Madras, 1992 ; Miller et al., 1997 ; Madras et al.,
1998 ). These data can now be reinterpreted as a specific consequence of
the greater motor component of the putamen compared with the caudate,
and its associated greater Vmax.
Moreover, the regional heterogeneity in uptake will be an additional
contributing factor in neurodegeneration; it explains the greater
accumulation and regional neurotoxicity of the MPTP metabolite and DAT
substrate, 1-methyl-4-phenylpyridinium (Elsworth et al., 1987 ;
Moratalla et al., 1992 ), and may be a component of regional
methamphetamine-induced toxicity (Harvey et al., 2000 ).
However, the distribution of DAT activity is inversely correlated with
the effect of the DAT antagonist cocaine. Paradoxically, cocaine
preferentially elevates [DA]o in limbic rather
than motor-associated striata (Carboni et al., 1989 ; Cass et al., 1992 ;
Kuczenski and Segal, 1992 ; Bradberry et al., 2000 ). The competitive
antagonistic action of cocaine will be differently apparent when the
DAT is differently saturated by endogenous DA. Wu et al. (2001) have suggested that lower rates for both dopamine release and uptake together underlie the preferential increase in
[DA]o in the nucleus accumbens rather than the
caudatoputamen of rats after the systemic injection of cocaine.
According to this hypothesis, the greater action of cocaine in the
ventromedial striatum rather than in the DLS documented in the nonhuman
primate (Bradberry et al., 2000 ) could also be explained by our
findings that both [DA]p and
Vmax are lowest in limbic-associated regions.
The positive correlations observed between
[DA]p and Vmax
in both the caudate and the putamen, together with the regional variation in DA content (Cragg et al., 2000 ), suggest that the largest
and fastest dopamine transients at dorsolateral coordinates result
primarily from the densest dopaminergic innervation (Bjorklund and
Lindvall, 1984 ). However, the faster
Vmax is not necessarily wholly
attributable to fiber packing density, because dorsolaterally projecting mesostriatal DA neurons have the greatest DAT expression levels (Shimada et al., 1992 ; Blanchard et al., 1994 ; Hurd et al.,
1994 ; Sanghera et al., 1994 ; Haber et al., 1995 ). Moreover, a
dissociation is apparent between the parameters
[DA]p and
Vmax, where
[DA]p tends toward zero before
Vmax. Unless there is increased DAT
density per releasing bouton with the ventromedial coordinate [a
hypothesis contradictory to both expression studies (above) and
ultrastructural findings in rodents (Nirenberg et al., 1997 ; Sesack et
al., 1998 )], this dissociation suggests that
[DA]p is governed by factors beyond packing
density. Other differences between DA neurons [e.g., membrane ionic
conductances (for review, see Meir et al., 1999 ; Wolfart et al., 2001 ;
Liss et al., 2001 ) or calcium-buffering properties (Gerfen et al.,
1987 ; Haber et al., 1995 )] may generate regional variation in release probability.
Frequency sensitivity
Net [DA]o during pulse trains reflects the
release per pulse ([DA]p) minus the reuptake
(Wightman et al., 1988 ; Wightman and Zimmerman, 1990 ; Limberger et al.,
1991 ; Kawagoe et al., 1992 ). At frequencies observed in vivo
(e.g., 1-20 Hz) (Bunney et al., 1973 ; Grace and Bunney, 1983 , 1984a ;
Schultz et al., 1983 ), the frequency sensitivity of
[DA]o varied significantly within both nuclei.
There was a supralinear effect of the frequency in the ventromedial
caudate but a frequency insensitivity in the dorsolateral regions of
>10 Hz. These two effects strongly resemble those seen previously
within the putamen (cf. Cragg et al., 2000 ). We have already
demonstrated that because of weaker uptake and presynaptic autoinhibition of release by D2-like receptors,
levels of evoked [DA]o in the
ventromedial putamen can exceed those dorsolaterally during pulse
trains despite the lower levels of DA available for release (Cragg et
al., 2000 ): it is possible that a similar scenario operates within the
caudate nucleus. The similarity in net [DA]o between related domains of the caudate versus the putamen could have
been predicted given the approximately similar
[DA]p and Vmax
of uptake. By related mechanisms, similar net
[DA]o could be generated in dorsolateral
regions of the caudate and the putamen (central vs dorsolateral
territories), despite the greater release and uptake during any single
release transient. These differences in homeostatic control of
[DA]o between domains in both the putamen and
the caudate are consistent with a different temporal precision of DA
signaling (low vs high) that has been proposed to be appropriate for
limbic- versus motor-associated functions, respectively (Horvitz, 2000 ).
Recovery from presynaptic depression
After a single stimulus, DA release exhibited a fast depression.
Slow recovery was described by a double exponential function, which (in
rodents) results from, among others (Kennedy et al., 1992 ), DA
autoreceptors (Kennedy et al., 1992 ; Benoit-Marand et al., 2001 ),
[Ca2+], -synuclein (Abeliovich et
al., 2000 ), and by analogy to other neurotransmitters, rates of vesicle
refill/redock/priming (for review, see Thomson, 2000 ). Recovery time
constants are faster than those reported previously in rodents;
however, it is unclear whether this discrepancy reflects experimental
or important species differences. According to these time constants,
the net constraints on presynaptic recovery in the caudate and the
putamen were equivalent. Therefore, greater initial DA availability and
reuptake do not confer greater apparent presynaptic
re-releasability.
Conclusions
We have described distinct differences within the primate dorsal
striatum in DA dynamics, in particular, in availability for release,
avidity of uptake, and their interactions. Consequently, striatal
functional domains are differentiated by this dynamic availability of
DA, independently of caudate/putamen anatomical segregation.
Caudate/putamen differences (e.g., susceptibility to
degeneration) are probably attributable to their mediolateral organization and consequent limbic versus motor dopaminergic components.
These data indicate how DA may be used in different manners,
quantitatively and qualitatively, for different functions within the
striatum. Interestingly, DA behavior may be necessarily correlated with
properties of sites of action: for example, the density of striatonigral projection neurons varies in parallel along the same axis
(ventromedial > dorsolateral) (Haber et al., 2000 ). The
heterogeneity in the dynamic behavior of DA surpasses that seen in the
rodent caudatoputamen (Cragg et al., 2000 ). This unparalleled heterogeneity may reflect the increased functional specialization of
the primate brain, in particular the greater repertoire of functions
modulated by DA, including movements and affect. Furthermore, these
data may underlie the regional specificity of the primate striatum to
psychostimulant drugs and neurodegenerative disease.
 |
FOOTNOTES |
Received Feb. 27, 2002; revised April 9, 2002; accepted April 11, 2002.
This work was supported by an E. P. Abraham Research Fellowship
(Keble College, Oxford, UK), by Novartis Pharma (S.J.C.), and by
Synaptica Ltd. (C.J.H.). S.J.C. is a Beit Memorial Research Fellow. We
thank Dr. S. Judge, Dr. A. Whatham, and P. W. Tynan for their contributions.
Correspondence should be addressed to Stephanie J. Cragg, University
Department of Pharmacology, Oxford OX1 3QT, UK. E-mail: stephanie.cragg{at}pharm.ox.ac.uk.
Dr. Hille's present address: Department of Migraine and Stroke,
Neurology Centre of Excellence for Drug Discovery, New Frontiers Science Park, Harlow, Essex CM19 5AW, UK.
 |
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