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The Journal of Neuroscience, January 1, 2002, 22(1):29-37
Synaptic Potentials Mediated by  7 Nicotinic Acetylcholine
Receptors in Supraoptic Nucleus
Glenn I.
Hatton and
Qin Zhao
Yang
Department of Cell Biology and Neuroscience, University of
California, Riverside, California 92521
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ABSTRACT |
Brain slice preparations preserving projections from nearby
forebrain cholinergic neurons to the supraoptic nucleus (SON) were used
to study synaptic potentials mediated by nicotinic acetylcholine receptors (nAChRs) in the hypothalamus. Paired-pulse electrical stimulation in an area anterior to the SON that was rich in cholinergic cells confirmed the monosynaptic nature of the connections to putative oxytocin and vasopressin SON neurons. With ionotropic glutamate and GABAA transmission blocked, this stimulation
evoked fast, atropine-insensitive EPSPs that were sensitive to nAChR antagonists. Evoked EPSPs were blocked by methyllycaconitine and  -bungarotoxin, antagonists that are selective for nAChRs containing the 7 subunit, but not by dihydro- -erythroidine at concentrations known to antagonize 42 nAChRs. Although anatomical evidence exists for postsynaptic 42 nAChRs in the SON, these results indicate that postsynaptic 7 nAChRs are primarily responsible for
the cholinergically mediated EPSPs. Repetitive stimulation suggested
partial desensitization of the receptors. With ionotropic glutamate
transmission blocked, inhibition of AChE increased spontaneous EPSP
frequency and amplitude, suggesting spontaneous ACh release. ACh,
nicotine, and choline (a selective 7 nAChR agonist) were effective
in evoking action potentials and repetitive firing with synaptic
transmission blocked by low Ca2+, high
Mg2+ medium. These agonists were also effective in
evoking the type of phasic bursts characteristic of vasopressin
neurons, long thought to be completely dependent on activation of NMDA
receptors (NMDARs). Because phasic bursting is
Ca2+-dependent, the functional equivalence of 7
nAChR and NMDAR activation in this regard is likely attributable to
their large Ca2+ fluxing capacities. This is the
first demonstration that synaptically released ACh results in fast,
7 nAChR-mediated EPSPs in hypothalamic neurons.
Key words:
-bungarotoxin; choline; intracellular recording; magnocellular neuroendocrine neurons; methyllycaconitine; nicotine
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INTRODUCTION |
Nicotinic acetylcholine receptors
(nAChRs) have been documented only recently at postsynaptic sites in
the mammalian brain, and only in a few brain areas, including the
hippocampus (Alkondon et al., 1998 ; Frazier et al., 1998) and the
neocortex (Roerig et al., 1997). These discoveries have forced
reconsideration of the previous widely held view that all brain nAChRs
are presynaptic (Sargent, 1993). It is not surprising that the
hippocampus and the neocortex would be first in this regard, because
they have well-established sources of cholinergic inputs (e.g., the
septum and the basal forebrain). However, regions with less well
established sources of cholinergic afferents might be no less likely to
use postsynaptic nAChRs to control their complex functions. One such area for which there is long-standing functional evidence for involvement of cholinergic influence is the magnocellular
hypothalamo-neurohypophysial system, in particular the supraoptic
nucleus (SON) portion of that system (Hatton, 1990).
SON neurons of mammals synthesize either oxytocin (OX) or vasopressin
(VP) and project axons to terminate in the pituitary neural lobe.
Release of one or both of these peptides into the pituitary circulation
occurs under a variety of physiological conditions, including
dehydration, parturition, and lactation. Such complex stimuli
inevitably demand that peptide release, and therefore OX and VP
neuronal excitability, be subject to multiple controls, both synaptic
and nonsynaptic (Hatton, 1997, 1999). Acetylcholine (ACh) was among the
earliest neurotransmitters implicated in the control of water loss
because it produced antidiuresis, via VP release, when injected
systemically or directly into the SON (Pickford, 1939, 1947; Milton and
Paterson, 1974). Using explants of hypothalamus that included the SON
and the neural lobe, Sladek and Joynt (1979) showed that blockade of
nAChRs with compounds such as hexamethonium interfered with osmotic
release of VP. Electrophysiological experiments (Hatton et al., 1983;
Gribkoff et al., 1988) suggested that nAChRs mediated or modulated
synaptic responses to stimulation in an area near the SON that
contained cholinergic neurons (Mason et al., 1983). Although they
implicated nAChRs, these results did not define their presynaptic
versus postsynaptic location. A dearth of knowledge regarding the
characteristics and diversity of brain nAChRs forced an interruption of
research efforts in this important area.
The pioneering studies of Berg and colleagues (Jacob and Berg, 1983;
Zhang et al., 1994), demonstrating the presence of
-bungarotoxin-binding nAChRs at postsynaptic sites and suggesting
that their rapid desensitization may have clouded the previous
investigations, revived interest in pursuing this phenomenon in the
SON. At present, there is ultrastructural immunocytochemical evidence
for postsynaptic 4-containing nAChRs (Shioda et al., 1997),
electrophysiological evidence that pharmacologically applied agonists
activate postsynaptic 7 nAChRs (Zaninetti et al., 2000), and light
microscopic evidence for -bungarotoxin and 7 antibody binding
(Meeker et al., 1986; del Toro et al., 1994; Zaninetti et al., 2000).
Our horizontal hypothalamic slice preparation keeps intact a
cholinergic synaptic projection to the SON, allowing the first
demonstration that synaptically released ACh results in fast EPSPs in
SON neurons, and that these responses are blocked by 7-selective but
not by 4-selective or glutamate receptor antagonists.
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MATERIALS AND METHODS |
Animals and procedures. Ninety-seven adult male
Sprague Dawley rats (Holtzman, Madison, WI), 55-70 d of age, were
housed three rats per cage with access to food and water ad
libitum and kept on a 12 hr light/dark cycle. At ~4-5 hr into
the light portion of the cycle, the rats were decapitated without
anesthesia. The brains were quickly removed, mounted cortex down on the
stage of a vibratome, and cut in a horizontal plane to 300-400 µm
thickness into room-temperature medium. Slices were then placed in
either a static bath (Hatton et al., 1980) or a perifusion chamber
(Hatton et al., 1983) for delivery of drugs and maintained at
34-36°C in a medium made up of 95% O2 and 5%
CO2. Control medium composition was (in
mM): NaCl 126, NaH2PO4 1.3, NaHCO3 26, KCl 5, CaCl2
2.4, MgSO4 1.3, glucose 10, and
3-[N-morpholino]propanesulfonic acid buffer 5, pH 7.4. The
combination of this organic buffer and NaHCO3 has
been found in previous studies to better stabilize the pH over
prolonged recording sessions than does the use of the bicarbonate buffer alone. In most of the experiments, the bathing medium also contained 6 µM
6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 20 or 50 µM
DL-2-amino-5-phosphonovaleric acid
(DL-APV); in many cases, 20 µM bicuculline methiodide (BIC) was also added.
The noncompetitive NMDA antagonist MK801 (40 µM) was also used. These concentrations have
been shown in a host of studies to be effective in blocking,
respectively, non-NMDA-, NMDA-, and
GABAA-mediated potentials and currents in the
SON. A solution shown previously to block synaptic transmission without
blocking Na+ channels was used; it
contained 9.3 mM
Mg2+ and 0.05 mM
Ca2+ but was otherwise the same as the
standard medium. This medium has been shown to block the Schaffer
collateral to CA1 synaptic transmission in hippocampal slices (Hatton,
1982).
Recording electrodes were glass micropipettes filled with 1 M K acetate, pH 7.3, with resistances of 80-120 M .
Measurements of input resistance were made by passing brief
hyperpolarizing current pulses (100-250 pA, 100 msec) through the
recording electrode at rates from 0.6 to 1 Hz. Recordings were made
using the bridge circuit of a Neurodata Dual Intracellular Amplifier
(Neurodata, New York, NY). Resting potentials were determined at
the times of both entering and exiting the cell. Only cells with
resting membrane potentials more negative than  55 mV and
overshooting action potentials were included in this study.
Extracellular electrical stimulation was delivered through concentric
bipolar electrodes (MCE 100; Rhodes Medical Instruments, Woodland
Hills, CA) using constant current (50-250 µA, 0.1 msec). Our
horizontal slices permitted placement of stimulating electrodes in an
area anterior to the SON (Fig. 1) that
was shown previously to be exceptionally rich in choline
acetyltransferase-immunoreactive neurons (Mason et al., 1983).
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Figure 1.
A, Micrograph of a transilluminated
hypothalamic slice in a static recording chamber, showing the plane of
the horizontal slice preparation used. CTX, Cortex;
HDB, horizontal limb of the diagonal band of Broca;
OC, optic chiasm; OT, optic tract;
REC, recording electrode (tip indicated by
arrow); STIM, stimulating
electrode; SON, supraoptic nucleus pars anterior;
3V, third ventricle. B, Diagram of
horizontal section on which cholinergic neurons near the SON are
plotted. (Modified from Mason et al., 1983.) Arrows
indicate the area stimulated in this study.
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The drugs used in the study were CNQX; DL-APV; MK801
(Tocris Cookson, Ballwin, MO); BIC; atropine sulfate (muscarinic
blocker); D-tubocurarine chloride, hexamethonium bromide,
and mecamylamine (three general nAChR antagonists); methyllycaconitine
(MLA) and -bungarotoxin (antagonists selective for nAChRs containing
the 7 subunit); dihydro--erythroidine (an antagonist selective
for 42 nAChRs), galanthamine, and tacrine (inhibitors of AChE); and nicotine, choline, and ACh. Except for MK801, all compounds were purchased from Sigma (St. Louis, MO).
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RESULTS |
Neuronal characteristics
A total of 109 SON neurons recorded from 97 male rats met the
criterion of being activated by electrical stimulation in the area rich
in cholinergic neurons (Fig. 1). The membrane characteristics of these
SON neurons were as follows (means ± SEM for each measure): resting membrane potentials, 58 ± 2 mV; action potentials,
64 ± 2 mV; and input resistances, 139 ± 7 M. These are
similar to those reported in the literature for SON neurons.
Synaptic responses
Figure 2 shows typical
(n = 10 cells) postsynaptic responses of two SON
neurons to extracellular electrical stimulation in the cholinergic cell
area. These traces were recorded in the presence of 6 µM CNQX, 50 µM
DL-APV, and 20 µM BIC,
thus eliminating or strongly reducing the possibility that recorded
EPSPs were mediated by glutamate. EPSPs were evoked by single, paired,
and repetitive stimulation. Cells followed paired-pulse frequencies of
50 and 100 Hz (Fig. 2B,C, left),
indicating that these connections are probably monosynaptic. Repetitive
stimulation frequencies of  33.33 Hz were relatively tetanizing in
maintaining the membrane potential in a depolarized state (Fig.
2B-D, right). The spontaneous activity of
phasic (putative VP) SON neurons was generally eliminated by medium
containing glutamate receptor antagonists, as reported previously (Hu
and Bourque, 1992). In three of three such cells tested in this manner,
stimulation of the cholinergic cell area was able to evoke single
prolonged bursts of firing (data not shown). Similar stimulation
occurring after the initial burst, when phasic cells would normally
tend to be refractory to burst firing, required several evoked EPSPs
before the occurrence of another prolonged burst. In this case, the
refractoriness may result partially from receptor desensitization.
Nonphasic (in many cases probably OX) cells also responded to this
stimulation with EPSPs (see Fig. 5B).
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Figure 2.
Recordings (averages of five traces) of two
cells maintained in medium containing (in µM): 6 CNQX, 50 DL-APV, and 20 BIC. Left, Synaptic responses
to single- and paired-pulse stimulation of a cholinergic cell area.
Right, Synaptic responses to repetitive stimulation of
the same area. Cells subjected to repetitive stimulation were
hyperpolarized by 2-5 mV.
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In another three cells that were hyperpolarized (2-5 mV) from resting
potential, several seconds of high-frequency activation of the
cholinergic inputs resulted in maintained depolarizations that declined
in amplitude over time (Fig. 3,
top and middle). Cessation of repetitive
stimulation was followed by a period (~6-8 sec) of marked reduction
in the size of the evoked EPSPs (Fig. 3, middle), which then
recovered (Fig. 3, bottom). In these neurons, the EPSPs that
were evoked several seconds after recovery had enhanced amplitudes
compared with those before the high-frequency stimulation (Fig. 3,
bottom right). A paired t test analysis of EPSP
amplitudes for the three cells tested in this manner showed that these
changes were consistent across neurons. Mean EPSP amplitudes were 18.7 mV (range, 17-20 mV) before stimulation; 8.0 mV (range, 5-10 mV) at 1 sec after stimulation (p < 0.01 compared with
before stimulation); and 24.7 mV (range, 21-27 mV) at 10 sec after
stimulation (p < 0.03 compared with before
stimulation).
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Figure 3.
Continuous recording showing synaptic responses to
single and high-frequency repetitive stimulation of a cholinergic cell
area in medium containing (in µM): 6 CNQX, 50 DL-APV, and 20 BIC. Indicated EPSPs are shown at higher
sweep speed (insets below each trace). Subsequent single
stimuli evoked attenuated EPSPs, which then recovered to amplitudes at
or above those seen before stimulation. Cells were hyperpolarized by 5 mV from the resting state to enhance EPSP amplitude.
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The decline in response to repetitive stimulation may indicate
7-receptor desensitization or depletion of transmitter in the
presynaptic terminal. That the decline is incomplete may represent continued activation of the 42 receptor, which appears to be coexpressed on SON neurons and which may be otherwise masked by 7-receptor activation. Recovery occurs with the possibility of at
least short-term potentiation of this nAChR-mediated response. Using
this paradigm, we found no evidence for long-term potentiation when
test stimuli were applied after several minutes (data not shown).
Effects of general AChR antagonists
That these fast synaptic responses evoked by stimulation in the
cholinergic cell group were likely to be mediated by nAChRs was tested
in medium containing 6 µM CNQX and 20 µM
DL-APV. In five of five cells tested, the evoked EPSPs were
unaffected by adding 50 µM atropine sulfate to the bath
but were abolished in the presence of the nonselective nicotinic
receptor antagonist hexamethonium bromide (data not shown). Washout of
the antagonist restored the stimulus-evoked EPSPs. In another four
cells, the stimulus-evoked EPSPs were reversibly eliminated by bath
application of 20 µM D-tubocurarine chloride
(data not shown).
A dose-response relationship for blockade of the stimulus-evoked EPSPs
was established for a third general nAChR antagonist, mecamylamine.
Five cells were tested with this compound. In control medium containing
CNQX, DL-APV, and BIC, mecamylamine was ineffective at 5 µM, partially blocked the evoked EPSPs at 10 µM, and provided complete antagonism at 20 µM (data not shown). Blockade was reversed during
washout. Such results implicate nAChRs as mediators of these synaptic
potentials but provide no clues as to the identity of the receptor
subtype involved.
Effects of selective nAChR antagonists
To determine the receptor subtype(s) mediating these responses, a
series of experiments was performed using a variety of antagonists at
different concentrations. At concentrations of ~1 µM,
dihydro--erythroidine is a selective antagonist at the 42
nAChR subtype (Alkondon and Albuquerque, 1993; Buisson et al., 1996).
In medium containing CNQX and DL-APV, 1 µM
dihydro--erythroidine was ineffective in blocking the evoked EPSPs
in five of five cells tested (Fig.
4A,B). However, these
synaptic responses were reversibly abolished by the less selective,
10-fold higher concentration of dihydro--erythroidine (Fig.
4C,D).
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Figure 4.
Synaptic responses to cholinergic cell
stimulation (averages of five traces). Medium contained 6 µM CNQX and 50 µM DL-APV.
Responses were blocked by 10 µM but not 1 µM dihydro--erythroidine, which is an 42
nAChR-selective antagonist at concentrations up to 1 µM. Arrows indicate stimulus
artifacts.
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MLA and -bungarotoxin are two generally available antagonists that
are highly selective for the homomeric 7 nAChRs. Currents mediated
by 7 nAChRs are generally abolished by concentrations of MLA in the
nanomolar range (Alkondon and Albuquerque, 1993). We tested the ability
of this antagonist to eliminate the cholinergically mediated EPSPs in
both phasically and nonphasically firing SON neurons (Fig.
5A,B). Voltage traces in
Figure 5A1-A3 are from a cell that exhibited spontaneous
phasic firing in control medium, activity that was silenced in medium
containing (in µM): 20 BIC, 6 CNQX, and 50 DL-APV (data not shown). This cell was
hyperpolarized by 2 mV to suppress occasional spontaneous spikes. EPSPs
evoked by stimulation of the cholinergic cell area were effectively and reversibly eliminated by 5 nM MLA (Fig.
5A1-A3) in the cell shown and in the four other cells
tested. Effective blocking concentrations of MLA in all cells tested
(both phasic and nonphasic) ranged from 5 to 20 nM.
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Figure 5.
Reversible blockade of cholinergically mediated
synaptic responses in SON neurons with initial spontaneous firing
patterns that were either phasic (A) or
continuous (B) by the 7 nAChR-selective
antagonist MLA. MLA added to the medium in A1 or
B1 reversibly abolished the synaptic responses in both
cell types. Arrows indicate stimulus
artifacts.
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A nonphasic or continuous pattern of firing (putative OX) of a neuron
recorded in control medium is shown in Figure 5B. Such patterns were incompletely silenced in medium containing (in
µM): 6 CNQX, 20 DL-APV,
and 20 BIC (data not shown). Spontaneous firing was suppressed by 2 mV
of hyperpolarization. Reversible blockade of stimulus-evoked,
cholinergically mediated responses in this medium is shown in Figure
5B1-B3 (n = 5). Evoked EPSPs were blocked by the addition of MLA (20 nM) and restored by
washout with glutamate receptor-blocking medium (Fig.
5B2,B3). Thus, both cell types in the SON displayed EPSPs
mediated by nAChRs that could be blocked by MLA in the low nanomolar
concentration range. To be certain that MLA was effective in
eliminating rather than just reducing the evoked responses, the EPSPs
of several cells were enhanced by hyperpolarization of the membrane
potential by up to 8 mV. MLA (50 nM) completely
and reversibly abolished the EPSPs (Fig. 6A1-A3).
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Figure 6.
Blockade of synaptic responses in two different
cells with 7-selective antagonists. A1, Cholinergic
synaptic responses at resting and hyperpolarized potentials remained in
the presence of 6 µM CNQX, 50 µM
DL-APV, and 20 µM BIC. A2, A3,
Recordings at hyperpolarized potentials show that these responses were
completely and reversibly blocked by 50 nM MLA.
B1, Synaptic responses that remained when glutamate
transmission was blocked were reversibly eliminated by medium
containing 9 mM Mg2+ and 0.05 mM Ca2+ (B2, B3).
Synaptic responses were partially (B4) and then
completely (B5) antagonized by 20 and 100 nM
-bungarotoxin. All records are the averages of five traces,
with five cells in each treatment. Arrows
indicate stimulus artifacts.
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Although its irreversibility makes it a less than perfect antagonist to
work with, -bungarotoxin is the sine qua non for demonstrating 7
nAChRs. In five of five cells tested, -bungarotoxin (50-100
nM) abolished the EPSPs evoked by stimulation in the
cholinergic cell-rich area (Fig. 6B). In medium
containing drug concentrations that block NMDA and non-NMDA glutamate
receptors, the responses to stimulation of the cholinergic cell group
were intact but were abolished when synaptic transmission was blocked
by a medium containing high Mg2+ and low
Ca2+ (Fig. 6B1,B2).
After washout with the blocking control medium (Fig.
6B3) had restored the synaptic responses, two
concentrations of -bungarotoxin were applied sequentially; the
higher concentration was effective in abolishing the EPSPs (Fig.
6B3-B5). This result shows that, in these cells,
-bungarotoxin blocks the postsynaptic response to ACh release.
Inhibition of AChE
Inhibition of the exquisitely rapid metabolism of ACh by AChE was
done to assess the effects of AChE inhibition on spontaneously occurring and evoked EPSPs. Spontaneous EPSPs, most if not all of which
were probably ACh-mediated, were readily observable in our
glutamate-GABAA receptor-blocking medium (Fig.
7A). Inhibition of AChE by 20 µM galanthamine produced an increase in the
EPSP frequency in three of three cells tested. In the control medium blocking glutamate and GABAA receptors, mean EPSP
frequency for these three cells was 2.73/sec, more than doubling to
7.72/sec when galanthamine was added, as shown in Figure 7B.
Washout with the blocking control medium returned the EPSP frequency to
the control value (Fig. 7C). At higher concentrations,
galanthamine application resulted in profound and prolonged increases
in excitability (Fig. 7D), here seen in a different cell
that was hyperpolarized to suppress spontaneous firing. Rising phase
and amplitudes, but not the decay times of EPSPs evoked by stimulation
of the cholinergic cell group, were enhanced in medium containing 20 µM galanthamine (Fig.
8A) in all eight cells
tested in this manner. Similar results were obtained using another AChE
inhibitor, tacrine (n = 3, data not shown). These
findings suggest that more transmitter was arriving at the receptor, as
would be expected during the inhibition of AChE, although it is not
possible here to rule out a direct effect of these inhibitors on the
7 receptors. In three cells, we observed similar effects on EPSP
enhancement in response to 1 mM choline (Fig.
8B), effects that were blocked by MLA. These results
are consistent with previous work showing that choline at this
concentration acts as a selective agonist at 7 receptors (Alkondon
et al., 1999) and as an inhibitor of AChE (Boyle et al., 1997; Stojan et al., 1999).
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Figure 7.
Effects of AChE inhibition by galanthamine.
A, Spontaneous EPSPs were observable with ionotropic
glutamate and GABAA receptors blocked. B,
Addition of galanthamine resulted in a doubling of EPSP frequency and
depolarization. C, Washout with the medium used in
A reduced EPSP frequency (n = 3).
D, At higher concentrations (different cell),
galanthamine induced larger depolarizations and prolonged excitability
(n = 3).
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Figure 8.
Enhancement of cholinergic EPSPs by AChE
inhibition. A, Addition of galanthamine to the medium
resulted in larger amplitudes and faster rising phases of evoked EPSPs.
B, Similar enhancements were seen with 1 mM
choline, which also may inhibit AChE (see Results). That choline
EPSPs are 7-mediated is suggested by their blockade with MLA.
Averages of five traces are shown; n = 3 in
each treatment. *Calibration pulse in A, 10 mV, 5 msec.
Arrows indicate stimulus artifacts.
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We also confirmed the blockade of the NMDARs by our
glutamate-GABAA-receptor blocking medium by
substituting the noncompetitive antagonist MK801 (40 µM)
for DL-APV (n = 3). In this medium, as in
the one containing DL-APV, synaptic responses to
stimulation of the cholinergic cell group were evoked and were blocked
by 20 nM MLA (data not shown).
Agonist effects on SON neurons
Acetylcholine
Both phasic and nonphasic types of SON neurons responded to bath
applications of ACh with excitation (Fig.
9A-C). In control medium
containing no antagonists (Fig. 9A,B), bath application of
ACh depolarized by several millivolts and enhanced the firing rate of
cells that had been firing continuously for several minutes before
treatment (Fig. 9A). This cell, and others treated
similarly, slowly returned to the control firing rate. Phasically
firing cells (Fig. 9B) often showed larger depolarizations
and an enhancement of fluctuations in membrane potential in response to
ACh. However, we did not systematically study these effects on the
cells exhibiting the two firing patterns. To further establish these
responses to ACh as being attributable to activation of postsynaptic
nAChRs, ACh application was performed in high
Mg2+ (9.3 mM), low
Ca2+ (0.05 mM)
medium (n = 3). ACh was effective in inducing
excitation in all three neurons treated in this manner. Figure
9C shows the response of a continuously firing neuron to ACh
application in the absence of normal synaptic activity. As is typical
for such neurons, this medium tends to stabilize the membrane
potential, allowing a somewhat smaller membrane depolarization in
response to ACh than in control medium (compare Fig.
9A).
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Figure 9.
Effects of ACh on SON neurons showing two
spontaneous firing patterns. A, ACh application
increased the firing rates of continuously firing cells with slight
membrane depolarization (n = 5). B,
Firing of phasic cells was increased by ACh application, which usually
also led to membrane potential oscillations (n = 5). C, The postsynaptic site of ACh action is suggested
by its efficacy with synaptic transmission blocked
(n = 3) .
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Nicotine
Responses of cells to nicotine, applied by either bath or nanodrop
to the surface of the slice, revealed a general sensitivity to nicotine
in SON neurons. For four cells tested with bath applications of
nicotine at relatively low concentrations (5-20 µM), the
minimal effective dose was 10 µM (data not shown). At the
higher concentrations tested on phasic neurons in control medium,
nicotine tended to produce prolonged bursts superimposed on plateau
potentials. This is seen in a cell that spontaneously displayed classic
phasic activity (Fig.
10A-C). Nicotine at
10 and 20 µM but not at 5 µM prolonged burst lengths for several minutes
after bath application. High Mg2+, low
Ca2+ blockade of synaptic transmission did
not interfere with the enhancement of neuronal excitability by nicotine
(Fig. 10C,D).
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Figure 10.
Effect of nicotine on phasically firing neurons.
Although ineffective at 5 µM (A),
nicotine prolonged phasic bursts at 10 µM
(B) and 20 µM
(C). D, Blocking synaptic
transmission typically eliminates phasic bursts in the SON, but
nicotine application, presumably acting postsynaptically during this
blockade, was able to evoke such bursts (n = 5).
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Choline
A main metabolite of ACh, choline has been shown to be an agonist
at 7 nAChRs (Papke et al., 1996; Alkondon et al., 1997). Typically,
spontaneous phasic firing of VP neurons is eliminated by the blockade
of glutamate transmission, in particular that attributable to NMDARs
(Hu and Bourque, 1992). This is because plateau potentials and phasic
firing are generated by Ca2+-dependent
depolarizing afterpotentials (Bourque, 1986; Li et al., 1995; Li and
Hatton, 1997), facilitated by Ca2+ entry
through NMDAR-gated channels. However, blocking glutamate transmission
did not eliminate the effectiveness of choline to induce bursts. As
shown in Figure 11A,
1 mM choline induced phasic bursts in a putative
VP neuron for which spontaneous phasic activity had been eliminated by
blockade of the glutamate receptors. It seems from the data shown for
Figure 11A,C that activation of postsynaptic 7
nAChRs, with their ability to flux large amounts of
Ca2+, can also trigger similar bursts
independently of NMDAR activation. The selective 7 nAChR antagonist
MLA was effective in blocking the choline-induced bursts (Fig.
11B).
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Figure 11.
Effects of choline on phasically and continuously
firing neurons. A, Choline applied by bath evoked bursts
in neurons for which spontaneous phasic activity was silenced by
glutamate receptor antagonists. B, MLA blocked these
choline-induced responses, and washout reinstated them
(C). D, Continuous firing persists
with glutamate receptors antagonized, and choline application was
effective in enhancing firing rates. E, F, MLA
reversibly blocked the choline-induced effect.
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Continuous firing patterns, typical of OX cells, are not dependent on
ionotropic glutamate transmission in the slice (Fig. 11D). Seconds after 1 mM
choline was applied by bath, continuously firing (putative OX) neurons
showed membrane depolarizations with increased firing, but still in the
continuous pattern. Similar responses to 1 mM
choline were seen in all six cells tested, and in each case MLA blocked
the effect, which was at least partially reversible (Fig.
11E,F). Again, the possibility that this
excitation was at least in part attributable to activation of
presynaptic 7 receptors cannot be ruled out.
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DISCUSSION |
This study is the first to demonstrate local circuit cholinergic
activation of postsynaptic nAChRs in the hypothalamus. Both VP and OX
neurons of the SON receive monosynaptic cholinergic inputs. Synaptic
release of ACh from these inputs activates postsynaptic 7 nAChRs and
possibly 42 nAChRs as well. Normally, this ACh release would also
be expected to activate any presynaptic nAChRs in the area. Olfactory
bulb inputs constitute one known source of glutamatergic presynaptic
terminals here (Hatton and Yang, 1989) that might express 7 nAChRs.
Selective activation of the postsynaptic 7 nAChRs, either by
selective agonists or with the 42 nAChRs antagonized, was
sufficient to produce prolonged bursts of action potentials
superimposed on plateau potentials in putative VP neurons. These
potentials were similar in frequency and pattern to the potentials
commonly produced by activation of NMDARs but occurred when NMDARs were
blocked. Inhibition of both ionotropic glutamate transmission and AChE
resulted in an increased frequency of EPSPs, suggesting that there was
spontaneous presynaptic release of ACh in our horizontal slice preparations.
That these cholinergic inputs are monosynaptic, and not relayed to SON
neurons via perinuclear interneurons, is evident from their
consistently following high-frequency stimulation. Although the exact
locations of the nicotinic cholinergic synapses on SON neurons are
unknown at present, they are likely to be found on the distal
dendrites, as is the case for most excitatory inputs. This would either
be in the deep dendritic zone near the ventral glial lamina or in the
more dorsal SON, where dendrites giving rise to axons are found
(Hatton, 1990). It is in these regions that both cholinergic terminals
and a high density of -bungarotoxin binding sites are seen (Meeker
et al., 1988). Careful analyses using specific antibodies against the
7- and 4-containing nAChRs are needed to localize these synapses further.
Results of activating these postsynaptic 7 nAChRs in the SON are
generally consistent with those previously reported in hypothalamus (Zaninetti et al., 2000) and in hippocampal interneurons (Alkondon et
al., 1998; Frazier et al., 1998). In all of these studies, including
the present one, the nicotinic cholinergic responses were blocked by
the selective 7 nAChR antagonists MLA and -bungarotoxin but not
by the 4 nAChR-selective antagonist dihydro--erythroidine.
Zaninetti et al. (2000) found evidence for 7 nAChRs in all parts of
the SON, suggesting that these receptors were on both OX and VP
neurons. We have confirmed this, showing that continuously firing
neurons for which spontaneous activity was not altered by blockade of
ionotropic glutamate receptors (i.e., OX cells), as well as those for
which spontaneous phasic patterns were blocked by the glutamate
receptor antagonists (VP cells), were activated by synaptically
released ACh. In both neuron types, exogenously applied ACh or its
agonist, nicotine, were also effective in evoking the appropriate
patterns of action potentials. Furthermore, we showed that these
agonists were effective in the absence of synaptic transmission,
confirming a postsynaptic site of action. EC50
values for ACh and nicotine obtained by Zaninetti et al. (2000) led
them to suggest that there might be another subunit expressed together with the 7 in SON neurons. Our finding of incomplete desensitization to high-frequency activation of the cholinergic synapses is consistent with that suggestion. It seems to us, however, that both findings could
be explained by the coexpression of 7- and 4-containing receptors
on these cells, because there is immunocytochemical evidence for both
receptor types in the SON (del Toro et al., 1994; Shioda et al.,
1997).
Like Zaninetti et al. (2000), we observed that ACh and nicotine
required relatively high concentrations to evoke responses. This is
consistent with the reported relatively low affinity of 7 nAChRs for
these compounds. In contrast to the hippocampal 7 nAChRs studied by
Frazier et al. (1998), who reported that 1 µM
nicotine desensitized the receptors and diminished the synaptic response, we did not observe desensitization to even 10 times higher
concentrations of nicotine. Perhaps this is attributable to the lack of
coexpression in the hippocampus of two receptor subtypes.
At low concentrations (EC50, 1.6 mM),
choline has been shown to be a full agonist at 7- but not at
4-containing receptors (Alkondon et al., 1997). In agreement with
this finding, our results showed that 1 mM choline
applications were able to induce phasic firing bursts in the absence of
synaptic transmission or to enhance the activity of continuously firing
neurons. In addition, choline acted to increase the rate of the rising
phase and amplitudes of evoked EPSPs in a manner and magnitude similar
to that observed with inhibition of AChE by galanthamine. The former
finding is consistent with the agonist role of choline, whereas the
latter is consistent with findings showing that choline can inhibit
AChE (Boyle et al., 1997; Stojan et al., 1999). Because choline is a
metabolic product of ACh hydrolysis by AChE, its selective action at
7 nAChRs suggests a rather complex interplay between these receptors
and their transmitter system. This interpretation is based on the
reasonable assumption that millimolar concentrations are attained in
the synaptic region.
When contrasted with reports of certain previous studies, some of our
findings present an apparent paradox. For example, it was found that
antagonizing AMPA and GABAA receptors eliminated all stimulus-evoked (Gribkoff and Dudek, 1990) and miniature (Wuarin and Dudek, 1993) EPSPs and IPSPs recorded in SON slices. These authors concluded that, therefore, no fast synaptic responses could be
mediated by ACh. The results presented here are antithetical to that
conclusion. Indeed, we showed that fast EPSPs are mediated by
synaptically released ACh, and that blocking AMPA, NMDA, and GABAA receptor-mediated responses did not
eliminate all spontaneously occurring EPSPs. In addition, additional
blockade of AChE, along with glutamate receptors, enhanced the
frequency and amplitudes of these EPSPs. In our view, this discrepancy
has a straightforward explanation. Those previous studies were done
using coronal slices of hypothalamus containing the SON. Slices in this
plane sever the axonal projections between the SON and areas that lie
directly anterior (e.g., cholinergic neurons) or posterior (e.g.,
histaminergic neurons) to it. Whether the terminals of ACh axons that
are severed from their cell bodies spontaneously release transmitter is
unknown, but possibly they do not. Moreover, slicing in the coronal
plane often damages the SON dendritic zone and ventral glial lamina, which are at the edge of the brain, where many cholinergic terminals and the most dense -bungarotoxin binding sites are found (Meeker et
al., 1986, 1988). Therefore, spontaneous ACh-mediated EPSPs may not
have been observable in coronal slices, although they were readily seen
in ours, which were cut in a horizontal plane that left the cholinergic
neurons and their axonal projections intact and did not damage the SON
dendritic zone (Yang and Hatton, 1987).
What functional significance can we attach to these cholinergic inputs
to OX and VP neurons in the SON, in particular, to their activation of
nAChRs? The postsynaptic nAChRs studied here are likely to be among
those involved in the long-recognized cholinergic control of VP release
in response to dehydration (Pickford, 1939, 1947; Milton and Paterson,
1974; Sladek and Joynt, 1979). More recent advances in SON physiology,
and the observation that both OX and VP neurons show 7
nAChR-mediated postsynaptic responses, present several interesting
possibilities. One of these, as conjectured by Zaninetti et al. (2000),
is that the huge Ca2+ influx allowed by
7 nAChR activation could participate in the now well-documented
dendritic release of peptide within the SON (Moos et al., 1984; Pow and
Morris, 1989). It has also been shown that dendritic release of peptide
can be functionally divorced from systemic release into the
neurohypophysial blood circulation (Neumann et al., 1995), suggesting
that SON neurons might be partitioned by different transmitter systems
synapsing on different portions of the cell (e.g., ventrally vs
dorsally projecting dendrites).
It has been noted (Broide and Leslie, 1999) that 7 nAChRs are
inwardly rectifying, allowing cholinergically stimulated
Ca2+ influx to occur through these
postsynaptic receptors at resting or hyperpolarized membrane
potentials. This would mean that
Ca2+-dependent dendritic release of
peptide in the SON could be evoked without the depolarization that
accompanies activation of the outwardly rectifying NMDARs. Conversely,
cholinergic activation of 7 nAChRs located presynaptically on nearby
glutamate terminals could result in enhanced NMDAR activation. This
reciprocal interplay between these two high
Ca2+-fluxing receptors also involves their
differential control by Mg2+.
Extracellular Mg2+ blocks NMDAR-gated
currents, whereas 7 nAChR-gated currents are blocked by
intracellular Mg2+. Because NMDAR
activation has been shown to result in elevated intracellular
Mg2+, it appears that the presence of the
two receptors in close proximity may be a mechanism for fine control of
both Ca2+ influx and neuronal excitability.
| |
FOOTNOTES |
Received July 16, 2001; revised Oct. 5, 2001; accepted Oct. 11, 2001.
This work was supported by National Institutes of Health Grants R01
NS09140 and R01 NS16942 from the National Institute of Neurological
Disorders and Stroke. We thank B. Cohen, E. R. Gillard, and
T. A. Ponzio for helpful comments on an earlier draft of the manuscript. We also thank J. Kitasako for technical assistance.
Correspondence should be addressed to Glenn I. Hatton, Department of
Cell Biology and Neuroscience, University of California, Riverside, CA
92521. E-mail: glenn.hatton{at}ucr.edu.
| |
REFERENCES |
-
Alkondon M,
Albuquerque E
(1993)
Diversity of nicotinic acetylcholine receptors in rat hippocampal neurons: I. Pharmacological and functional evidence for distinct structural subtypes.
J Pharmacol Exp Ther
265:1455-1473[Abstract/Free Full Text].
-
Alkondon M,
Reriera E,
Cortes W,
Maelicke A,
Albuquerque E
(1997)
Choline is a selective agonist of alpha 7 nicotinic receptors in rat brain neurons.
Eur J Neurosci
9:2734-2742[Web of Science][Medline].
-
Alkondon M,
Pereira E,
Albuquerque E
(1998)
-Bungarotoxin- and methllycaconitine-sensitive nicotinic receptors mediate fast synaptic transmission in interneurons of rat hippocampal slices.
Brain Res
810:257-263[Web of Science][Medline].
-
Alkondon M,
Pereira E,
Eisenberg H,
Albuquerque E
(1999)
Choline and selective antagonists identify two types of nicotinic acetylcholine receptors that modulate GABA release from CA1 interneurons in rat hippocampal slices.
J Neurosci
19:2693-2705[Abstract/Free Full Text].
-
Bourque CW
(1986)
Calcium-dependent spike after-current induces burst firing in magnocellular neurosecretory cells.
Neurosci Lett
70:204-209[Web of Science][Medline].
-
Boyle N,
Talesa V,
Giovannini E,
Rosi G,
Norton S
(1997)
Synthesis and study of thiocarbonate derivatives of choline as potential inhibitors of acetylcholinesterase.
J Med Chem
40:3009-3013[Web of Science][Medline].
-
Broide R,
Leslie F
(1999)
The 7 nicotinic receptor in neuronal plasticity.
Mol Neurobiol
20:1-16[Web of Science][Medline].
-
Buisson B,
Gopalakrishnan M,
Arneric S,
Sullivan J,
Bertrand D
(1996)
Human 42 neuronal nicotinic acetylcholine receptor in HEK 293 cells: a patch-clamp study.
J Neurosci
16:7880-7891[Abstract/Free Full Text].
-
del Toro E,
Juiz J,
Peng X,
Lindstrom J,
Criado M
(1994)
Immunocytochemical localization of the a7 subunit of the nicotinic acetylcholine receptor in the rat central nervous system.
J Comp Neurol
349:325-342[Web of Science][Medline].
-
Frazier CJ,
Buhler AV,
Weiner JL,
Dunwiddie TV
(1998)
Synaptic potentials mediated via -bungarotoxin-sensitive nicotinic acetylcholine receptors in rat hippocampal interneurons.
J Neurosci
18:8228-8235[Abstract/Free Full Text].
-
Gribkoff VK,
Dudek FE
(1990)
Effects of excitatory amino acid antagonists on synaptic responses of supraoptic neurons in slices of rat hypothalamus.
J Neurophysiol
63:60-71[Abstract/Free Full Text].
-
Gribkoff VK,
Christian EP,
Robinson JH,
Deadwyler SA,
Dudek FE
(1988)
Cholinergic excitation of supraoptic neurons in hypothalamic slices of rat.
Neuropharmacology
27:721-727[Web of Science][Medline].
-
Hatton GI
(1982)
Phasic bursting activity of rat paraventricular neurones in the absence of synaptic transmission.
J Physiol (Lond)
327:273-284[Abstract/Free Full Text].
-
Hatton GI
(1990)
Emerging concepts of structure-function dynamics in adult brain: the hypothalamo-neurohypophysial system.
Prog Neurobiol
34:437-504[Web of Science][Medline].
-
Hatton GI
(1997)
Function-related plasticity in hypothalamus.
Annu Rev Neurosci
20:375-397[Web of Science][Medline].
-
Hatton GI
(1999)
Astroglial modulation of neurotransmitter/peptide release from the neurohypophysis: present status.
J Chem Neuroanat
16:203-222[Web of Science][Medline].
-
Hatton GI,
Yang QZ
(1989)
Supraoptic nucleus afferents from the main olfactory bulb. II. Intracellularly recorded responses to lateral olfactory tract stimulation on rat brain slices.
Neuroscience
31:289-297[Web of Science][Medline].
-
Hatton GI,
Doran AD,
Salm AK,
Tweedle CD
(1980)
Brain slice preparation: hypothalamus.
Brain Res Bull
5:405-414[Web of Science][Medline].
-
Hatton GI,
Ho YW,
Mason WT
(1983)
Synaptic activation of phasic bursting in rat supraoptic nucleus neurones recorded in hypothalamic slices.
J Physiol (Lond)
345:297-317[Abstract/Free Full Text].
-
Hu B,
Bourque CW
(1992)
NMDA receptor-mediated rhythmic bursting activity in rat supraoptic nucleus neurones in vitro.
J Physiol (Lond)
458:667-687[Abstract/Free Full Text].
-
Jacob M,
Berg D
(1983)
The ultrastructural localization of -bungarotoxin binding sites in relation to synapses on chick ciliary ganglion neurons.
J Neurosci
3:260-271[Abstract].
-
Li ZH,
Hatton GI
(1997)
Ca2+ release from internal stores: role in generating depolarizing after-potentials in rat supraoptic neurones.
J Physiol (Lond)
498:339-350[Abstract/Free Full Text].
-
Li ZH,
Decavel C,
Hatton GI
(1995)
Calbindin-D28k: role in determining intrinsically generated firing patterns in rat supraoptic neurones.
J Physiol (Lond)
488:601-608[Abstract/Free Full Text].
-
Mason WT,
Ho YW,
Eckenstein F,
Hatton GI
(1983)
Mapping of cholinergic neurons associated with rat supraoptic nucleus: combined immunocytochemical and histochemical identification.
Brain Res Bull
11:617-626[Web of Science][Medline].
-
Meeker RB,
Michels KM,
Libber MT,
Hayward JN
(1986)
Characteristics and distribution of high- and low-affinity alpha bungarotoxin binding sites in the rat hypothalamus.
J Neurosci
6:1866-1875[Abstract].
-
Meeker RB,
Swanson DJ,
Hayward JN
(1988)
Local synaptic organization of cholinergic neurons in the basolateral hypothalamus.
J Comp Neurol
276:157-168[Web of Science][Medline].
-
Milton AS,
Paterson AT
(1974)
A microinjection study of the control of antidiuretic hormone release by the supraoptic nucleus of the hypothalamus in the cat.
J Physiol (Lond)
241:607-628[Abstract/Free Full Text].
-
Moos F,
Freund-Mercier MJ,
Guerne Y,
Guerne JM,
Stoeckel ME,
Richard P
(1984)
Release of oxytocin and vasopressin by magnocellular nuclei in vitro: specific facilitatory effect of oxytocin on its own release.
J Endocrinol
102:63-72[Abstract/Free Full Text].
-
Neumann I,
Landgraf R,
Bauce L,
Pittman QJ
(1995)
Osmotic responsiveness and cross talk involving oxytocin, but not vasopressin or amino acids, between the supraoptic nuclei in virgin and lactating rats.
J Neurosci
15:3408-3417[Abstract].
-
Papke R,
Bencherif M,
Lippiello P
(1996)
An evaluation of neuronal nicotinic acetylcholine receptor activation by quaternary nitrogen compounds indicates that choline is selective for the alpha-7 subtype.
Neurosci Lett
213:201-204[Web of Science][Medline].
-
Pickford M
(1939)
The inhibitory effects of acetylcholine on water diuresis in the dog, and its pituitary transmission.
J Physiol (Lond)
95:226-238[Web of Science][Medline].
-
Pickford M
(1947)
The action of acetylcholine in the supraoptic nucleus of the chlorolosed dog.
J Physiol (Lond)
106:264-270[Web of Science][Medline].
-
Pow DV,
Morris JF
(1989)
Dendrites of magnocellular hypothalamic neurons release neurohypophysial peptides by exocytosis.
Neuroscience
32:435-439[Web of Science][Medline].
-
Roerig B,
Nelson D,
Katz L
(1997)
Fast synaptic signaling by nicotinic acetylcholine and serotonin 5-HT3 receptors in developing visual cortex.
J Neurosci
17:8353-8362[Abstract/Free Full Text].
-
Sargent PB
(1993)
The diversity of nicotinic acetylcholine receptors.
Annu Rev Neurosci
16:403-443[Web of Science][Medline].
-
Shioda S,
Nakajo S,
Hirabayashi T,
Nakayama H,
Matsuda K,
Nakai Y
(1997)
Neuronal nicotinic acetylcholine receptor in the hypothalamus: morphological diversity and neuroendocrine regulations.
Brain Res Mol Brain Res
47:45-54.
-
Sladek CD,
Joynt RJ
(1979)
Cholinergic involvement in osmotic control of vasopressin release by the organ-cultured rat hypothalamo-neurohypophyseal system.
Endocrinology
105:367-371[Abstract/Free Full Text].
-
Stojan J,
Marcel V,
Fournier D
(1999)
Effect of tetramethylammonium, choline and adrophonium on insect acetylcholinesterase: test of a kinetic model.
Chem Biol Interact
119-120:137-146.
-
Wuarin J-P,
Dudek FE
(1993)
Patch-clamp analysis of spontaneous synaptic currents in supraoptic neuroendocrine cells of the rat hypothalamus.
J Neurosci
13:2323-2331[Abstract].
-
Yang QZ,
Hatton GI
(1987)
Dye coupling among supraoptic nucleus neurons without dendritic damage: differential incidence in nursing mother and virgin rats.
Brain Res Bull
19:559-565[Web of Science][Medline].
-
Zaninetti M,
Blanchet C,
Tribollet E,
Bertrand D,
Raggenbass M
(2000)
Magnocellular neurons of the rat supraoptic nucleus are endowed with functional nicotinic acetylcholine receptors.
Neuroscience
95:319-323[Web of Science][Medline].
-
Zhang Z-W,
Vijayaraghavan S,
Berg DK
(1994)
Neuronal acetylcholine receptors that bind -bungarotoxin with high affinity function as ligand-gated ion channels.
Neuron
12:167-177[Web of Science][Medline].
Copyright © 2002 Society for Neuroscience 0270-6474/02/22129-09$05.00/0
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TOP
ABSTRACT
INTRODUCTION
