Dopamine modulates synaptic transmission between rat olfactory bulb neurons in culture

The glomerular layer of the olfactory bulb (OB) contains synaptic connections between olfactory sensory neurons and OB neurons as well as connections among OB neurons. A subpopulation of external tufted cells and periglomerular cells (juxtaglomerular neurons) expresses dopamine, and recent reports suggest that dopamine can inhibit olfactory sensory neuron activation of OB neurons. In this study, whole cell electrophysiological and primary culture techniques were employed to characterize the neuromodulatory properties of dopamine on glutamatergic transmission between rat OB mitral/tufted (M/T) cells and interneurons. Immunocytochemical analysis confirmed the expression of tyrosine hydroxylase, the rate-limiting enzyme for dopamine synthesis, in a subpopulation of cultured neurons. D2 receptor immunoreactivity was also observed in cultured M/T cells. Dopamine reduced spontaneous excitatory synaptic events recorded in interneurons. Although the D1 receptor agonist SKF38393 and the D2 receptor agonist bromocriptine mesylate mimicked this effect, evoked excitatory postsynaptic potentials (EPSPs) recorded from monosynaptically coupled neuron pairs were attenuated by dopamine and bromocriptine but not by SKF38393. Neither glutamate-evoked currents nor the membrane resistance of the postsynaptic interneuron were affected by dopamine. However, evoked calcium channel currents in the presynaptic M/T cell were diminished during the application of either dopamine or bromocriptine, but not SKF38393. Dopamine suppressed calcium channel currents even after nifedipine blockade of L-type channels, suggesting that inhibition of the dihydropyridine-resistant high-voltage activated calcium channels implicated in transmitter release may mediate dopamine's effects on spontaneous and evoked synaptic transmission. Together, these data suggest that dopamine inhibits excitatory neurotransmission between M/T cells and interneurons via a presynaptic mechanism.     

D1- and D2-like dopamine receptors are co-localized on the presynaptic varicosities of striatal and nucleus accumbens neurons in vitro

The neuromodulatory actions of dopamine in the striatum and nucleus accumbens are likely to depend on the distribution of dopamine receptors on individual postsynaptic cells. To address this, we have visualized D1- and D2-like receptors on living medium-spiny GABAergic neurons in cultures from the striatum and nucleus accumbens using receptor antagonist fluoroprobes. We labeled D1-like receptors with rhodamine-SCH23390, D2-like receptors with rhodamine-N-(p-aminophenethyl)spiperone and synaptic sites with K+-stimulated uptake of the activity-dependent endocytic tracer FM-143. The fluoroprobes were applied in sequence to assess co-localization. We found that D1- or D2-like receptors were present on about two-thirds of the cells, and co-localized on 22+/-3% (mean +/- S.E.M.) of striatal and 38+/-6% of nucleus accumbens cells. On either D1 or D2 labeled cells, postsynaptic labeling continuously outlined the cell body membrane and extended to proximal dendrites, but not axons. About two-thirds of synaptic varicosities showed D1 or D2 labeling. D1- and D2-like receptors were co-localized on 21+/-4% of striatal and 27+/-3% of nucleus accumbens varicosities. Presynaptic labeling was typically more intense than postsynaptic labeling. The distribution of presynaptic dopamine receptors contrasted with that of postsynaptic GABA(A) receptors, which were clustered in longer patches on neighboring postsynaptic membranes. The extensive presence of D1- and D2-like receptors on presynaptic varicosities of medium-spiny neurons suggests that the receptors are likely to play an important and interacting role in the presynaptic modulation of inhibitory synaptic transmission in the striatum and nucleus accumbens. The significant overlap in labeling suggests that D1-D2 interactions, which occur at the level of individual postsynaptic cells, the circuit level and the systems level, may also be mediated at the presynaptic level. Finally, the ability to visualize dopamine, as well as GABA(A), receptors on the individual synapses of living neurons now makes possible physiological studies of individual mesolimbic system synapses with known receptor expression.     

Interaction of dopamine D1 and NMDA receptors mediates acute clozapine potentiation of glutamate EPSPs in rat prefrontal cortex

The atypical antipsychotic drug clozapine effectively alleviates both negative and positive symptoms of schizophrenia via unclear cellular mechanisms. Clozapine may modulate both glutamatergic and dopaminergic transmission in the prefrontal cortex (PFC) to achieve part of its therapeutic actions. Using whole cell patch-clamp techniques, current-clamp recordings in layers V-VI pyramidal neurons from rat PFC slices showed that stimulation of local afferents (in 2 microM bicuculline) evoked mixed [AMPA/kainate and N-methyl-D-aspartate (NMDA) receptors] glutamate receptor-mediated excitatory postsynaptic potentials (EPSPs). Clozapine (1 microM) potentiated polysynaptically mediated evoked EPSPs (V(Hold) = -65 mV), or reversed EPSPs (rEPSP, V(Hold) = +20 mV) for >30 min. The potentiated EPSPs or rEPSPs were attenuated by elevating [Ca(2+)](O) (7 mM), by application of NMDA receptor antagonist 2-amino5-phosphonovaleric acid (50 microM), or by pretreatment with dopamine D1/D5 receptor antagonist SCH23390 (1 microM) but could be further enhanced by a dopamine reuptake inhibitor bupropion (1 microM). Clozapine had no significant effect on pharmacologically isolated evoked NMDA-rEPSP or AMPA-rEPSPs but increased spontaneous EPSPs without changing the steady-state resting membrane potential. Under voltage clamp, clozapine (1 microM) enhanced the frequency, and the number of low-amplitude (5-10 pA) AMPA receptor-mediated spontaneous EPSCs, while there was no such changes with the mini-EPSCs (in 1 microM TTX). Taken together these data suggest that acute clozapine can increase spike-dependent presynaptic release of glutamate and dopamine. The glutamate stimulates distal dendritic AMPA receptors to increase spontaneous EPSCs and enabled a voltage-dependent activation of neuronal NMDA receptors. The dopamine released stimulates postsynaptic D1 receptor to modulate a lasting potentiation of the NMDA receptor component of the glutamatergic synaptic responses in the PFC neuronal network. This sequence of early synaptic events induced by acute clozapine may comprise part of the activity that leads to later cognitive improvement in schizophrenia.     

Dopamine excites fast-spiking interneurons in the striatum

The striatum is the main recipient of dopaminergic innervation. Striatal projection neurons are controlled by cholinergic and GABAergic interneurons. The effects of dopamine on projection neurons and cholinergic interneurons have been described. Its action on GABAergic interneurons, however, is still unknown. We studied the effects of dopamine on fast-spiking (FS) GABAergic interneurons in vitro, with intracellular recordings. Bath application of dopamine elicited a depolarization accompanied by an increase in membrane input resistance (an effect that persisted in the presence of tetrodotoxin) and action-potential discharge. These effects were mimicked by the D1-like dopamine receptor agonist SKF38393 but not by the D2-like agonist quinpirole. Evoked corticostriatal glutamatergic synaptic currents were not affected by dopamine. Conversely, GABAergic currents evoked by intrastriatal stimulation were reversibly depressed by dopamine and D2-like, but not D1-like, agonists. Cocaine elicited effects similar to those of dopamine on membrane potential and synaptic currents. These results show that endogenous dopamine exerts a dual excitatory action on FS interneurons, by directly depolarizing them (through D1-like receptors) and by reducing their synaptic inhibition (through presynaptic D2-like receptors).     

Presynaptic dopamine D1 receptors attenuate excitatory and inhibitory limbic inputs to the shell region of the rat nucleus accumbens studied in vitro.

1. Intracellular recordings were made from the shell region of the nucleus accumbens in an in vitro slice preparation. The mean resting membrane potential, input resistance, and action potential amplitude of these neurons were -76 +/- 1 mV, 87 +/- 5 M omega and 94 +/- 2 mV (N = 108), respectively. A sample of these neurons (N = 18) was identified as medium spiny neurons with the use of the biocytin-avidin labeling technique. 2. Electrical stimulation of the fornix, subcortical fibers, or neuropil within the nucleus accumbens shell itself elicited a depolarizing postsynaptic potential (PSP). Dopamine (10-100 microM) attenuated PSPs elicited by stimulation of all of these sites. In a paired-pulse stimulation protocol, dopamine was observed to enhance the facilitation of the test response with respect to the conditioning response. 3. The suppressive effect of dopamine was mimicked by the D1 receptor agonist SKF 82958 (10-30 microM), whereas the D2 receptor agonist quinpirole (10-30 microM) was ineffective. The action of dopamine was antagonized by the D1 receptor antagonist Sch 23390 (10-30 microM), but not by the D2 receptor antagonist sulpiride (10-50 microM) or various adrenergic receptor antagonists. 4. The PSP was usually composed of an excitatory postsynaptic potential (EPSP)-inhibitory postsynaptic potential (IPSP) sequence. Dopamine equally attenuated the excitatory and inhibitory component of the synaptic response. The attenuation of both EPSP and IPSP did not depend on membrane potential. 5. Dopamine effects on the resting membrane potential and input resistance were variable and did not correlate with changes in the PSP. Two further indications were found in favor of a presynaptic locus of dopaminergic modulation. First, the time course of the PSP was not altered during dopamine application. Second, dopamine did not attenuate depolarizations induced by bath-applied L-glutamate. In extracellular recordings, it was found that dopamine reduced the population spike but not the presynaptic fiber volley. 6. These findings strongly indicate that dopaminergic modulation of synaptic responses in neurons located in the accumbens shell region is mediated by presynaptic D1 receptors. Notably, dopamine does not exert a purely inhibitory effect on synaptic excitability in the nucleus accumbens, because it suppresses both the excitatory and inhibitory component of the synaptic response.     

DARPP-32 and regulation of the ethanol sensitivity of NMDA receptors in the nucleus accumbens

The medium spiny neurons of the nucleus accumbens receive both an excitatory glutamatergic input from forebrain and a dopaminergic input from the ventral tegmental area. This integration point may constitute a locus whereby the N-methyl-D-aspartate (NMDA)-subtype of glutamate receptors promotes drug reinforcement. Here we investigate how dopaminergic inputs alter the ethanol sensitivity of NMDA receptors in rats and mice and report that previous dopamine receptor-1 (D1) activation, culminating in dopamine and cAMP-regulated phosphoprotein-32 kD (DARPP-32) and NMDA receptor subunit-1 (NR1)-NMDA receptor phosphorylation, strongly decreases ethanol inhibition of NMDA responses. The regulation of ethanol sensitivity of NMDA receptors by D1 receptors was absent in DARPP-32 knockout mice. We propose that DARPP-32 mediated blunting of the response to ethanol subsequent to activation of ventral tegmental area dopaminergic neurons initiates molecular alterations that influence synaptic plasticity in this circuit, thereby promoting the development of ethanol reinforcement.     

Activity-dependent depression of medial prefrontal cortex inputs to accumbens neurons by the basolateral amygdala

The encoding of reward-predictive stimuli by neurons in the nucleus accumbens (NAcc) depends on integrated synaptic activity from the basolateral amygdala (BLA) and medial prefrontal cortex (mPFC) afferent inputs. In a previous study, we found that single electrical stimulation pulses applied to the BLA facilitate mPFC-evoked spiking in NAcc neurons in a timing-dependent manner, presumably by a fast glutamatergic mechanism. In the present study, the ability of repetitive BLA activation to modulate synaptic inputs to NAcc neurons through dopamine- or N-methyl-d-aspartate (NMDA)-dependent mechanisms is characterized. NAcc neurons receiving excitatory input from both mPFC and BLA were recorded in urethane-anesthetized rats. Train stimulation of the BLA depressed mPFC-evoked spiking in these neurons. This was not attributable to mechanisms involving NMDA or dopamine D1, D2, D3 or D5 receptors, since blockade of these receptors did not affect the BLA-mediated depression. BLA-mediated depression was only evident when the BLA stimulation evoked spikes in the recorded neuron; thus, depolarization of the recorded neuron may be critical for this effect. The ability of the BLA to suppress mPFC-to-NAcc signaling may be a mechanism by which normal or pathologically heightened emotional states disrupt goal-directed behavior in favor of emotionally-driven responses.     

Dopamine D4 receptor activation inhibits presynaptically glutamatergic neurotransmission in the rat supraoptic nucleus

Oxytocin and vasopressin release from magnocellular neurons of the supraoptic nucleus is under the control of glutamate-dependent excitation. The supraoptic nucleus also receives a generalized dopaminergic input from hypothalamic sources. To determine if dopamine can influence this excitatory drive onto the magnocellular neurons, we used whole-cell patch clamp to record the effect of dopamine on evoked and miniature excitatory postsynaptic currents in rat hypothalamic slices. Dopamine exposure (30 microM to 1 mM) induced a large and reversible reduction in the amplitude of evoked excitatory postsynaptic current in nearly all magnocellular cells tested. D4 receptors appeared to mediate dopamine's activity, based on inhibition of the response with 50 microM clozapine, but not by SCH 23390 or sulpiride, and mimicry of dopamine's action with the D4 specific agonist, PD 168077. Analysis of paired-pulse experiments and miniature postsynaptic currents indicated that dopamine's action involved a presynaptic mechanism, since the frequency of miniature postsynaptic currents was reduced with dopamine exposure without any change in current kinetics or amplitude, while the paired-pulse ratio increased. We therefore have demonstrated for the first time a role for dopamine D4 receptors in the supraoptic nucleus in the presynaptic inhibition of glutamatergic neurotransmission onto magnocellular neurons.     

Dopamine depresses glutamatergic synaptic transmission in the rat parabrachial nucleus in vitro

Nystatin-perforated patch recordings were made from rat parabrachial neurons in an in vitro slice preparation to examine the effect of dopamine on parabrachial cells and on excitatory synaptic transmission in this nucleus. In current clamp mode, dopamine reduced the amplitude of the evoked excitatory postsynaptic potential without significant change in membrane potential. In cells voltage-clamped at -65 mV, dopamine dose dependently and reversibly decreased evoked, pharmacologically isolated, excitatory postsynaptic currents with an EC50 of 31 microM. The reduction in excitatory postsynaptic current was accompanied by an increase in paired pulse ratio (a protocol used to detect presynaptic site of action) with no change in the holding current or in the decay of the evoked excitatory postsynaptic currents. In addition, dopamine altered neither postsynaptic (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate-induced currents, nor steady-state current voltage curves. Miniature excitatory postsynaptic current analysis revealed that dopamine caused a rightward shift of the frequency-distribution curve with no change in the amplitude-distribution curve, which is consistent with a presynaptic mechanism. The dopamine-induced attenuation of the excitatory postsynaptic current was almost completely blocked by the D1-like receptor antagonist SCH23390 (10 microM), although the D2-like antagonist sulpiride (10 microM) also partially blocked it. Combined application of both antagonists blocked all dopamine-induced synaptic effects. The synaptic effect of dopamine was mimicked by the D1-like agonist SKF38393 (50 microM), but the D2-1ike agonist quinpirole (50 microM) also had a small effect. Combined application of both agonists did not produce potentiated responses. Dopamine's effect on the excitatory postsynaptic current was independent of serotonin, GABA and adenosine receptors, but may have some interactions with adrenergic receptors. These results suggest that dopamine directly modulates excitatory synaptic events in the parabrachial nucleus predominantly via presynaptic D1-like receptors.     

Midbrain muscarinic receptors modulate morphine-induced accumbal and striatal dopamine efflux in the rat

Midbrain dopamine neurons are critical in mediating the rewarding effects of opiates in dependent rats, as well as modulating some manifestations of opiate withdrawal. Morphine is known to excite dopamine neurons and thereby facilitate forebrain dopamine transmission through inhibition of GABA neurons. Cholinergic neurons in the mesopontine laterodorsal and pedunculopontine tegmental nuclei provide the principal source of excitatory cholinergic input to ventral tegmental area and substantia nigra pars compacta dopamine-containing neurons, via actions on midbrain muscarinic and nicotinic acetylcholine receptors. The present study hypothesized that a reduction in tonic cholinergic input via blockade of midbrain muscarinic receptors would reduce the pharmacological effects of morphine on forebrain dopamine release. Using in vivo chronoamperometry, alterations in morphine-evoked dopamine efflux were monitored at stearate-graphite paste electrodes implanted unilaterally in the nucleus accumbens and striatum of urethane (1.5 g/kg) anesthetized rats, following the pharmacological inhibition of ventral tegmental area/substantia nigra pars compacta muscarinic receptors. The facilitatory effects of morphine (2.0 mg/kg, i.v.) on accumbens and striatal dopamine efflux were markedly reduced by prior infusion of the non-selective muscarinic receptor antagonist scopolamine (200 microg/microl) into the ventral tegmental area or substantia nigra pars compacta, respectively. These findings demonstrate that decreased activation of midbrain muscarinic receptors attenuates the excitatory effects of morphine on mesoaccumbens and nigrostriatal dopaminergic transmission.     

D2-like dopamine receptor-mediated modulation of activity-dependent plasticity at GABAergic synapses in the subthalamic nucleus

Reciprocally connected glutamatergic subthalamic nucleus (STN) and GABAergic external globus pallidus (GP) neurons normally exhibit weakly correlated, irregular activity but following the depletion of dopamine in Parkinson's disease they express more highly correlated, rhythmic bursting activity. Patch clamp recording was used to test the hypothesis that dopaminergic modulation reduces the capability of GABAergic inputs to pattern 'pathological' activity in STN neurons. Electrically evoked GABA(A) receptor-mediated IPSCs exhibited activity-dependent plasticity in STN neurons, i.e. IPSCs evoked at frequencies between 1 and 50 Hz exhibited depression that increased with the frequency of activity. Dopamine, the D(2)-like dopamine receptor agonist quinpirole and external media containing a low [Ca(2+)] reduced both the magnitude of IPSCs evoked at 1-50 Hz and synaptic depression at 10-50 Hz. Dopamine/quinpirole also reduced the frequency but not the amplitude of miniature IPSCs recorded in the presence of tetrodotoxin. D(1)-like and D(4) agonists were ineffective and D(2/3) but not D4 receptor antagonists reversed the effects of dopamine or quinpirole. Together these data suggest that presynaptic D(2/3) dopamine receptors modulate the short-term dynamics of GABAergic transmission in the STN by lowering the initial probability of transmitter release. Simulated GABA(A) receptor-mediated synaptic conductances representative of control or modulated transmission were then generated in STN neurons using the dynamic clamp technique. Dopamine-modulated transmission was less effective at resetting autonomous activity or generating rebound burst firing than control transmission. The data therefore support the conclusion that dopamine acting at presynaptic D(2)-like receptors reduces the propensity for GABAergic transmission to generate correlated, bursting activity in STN neurons.     

Selective reduction by dopamine of excitatory synaptic inputs to pyramidal neurons in primate prefrontal cortex

We have employed in vitro physiological methods to investigate dopaminergic modulation of excitatory synaptic transmission in monkey prefrontal cortex (PFC) circuits. We show that combined activation of D1-like and D2-like dopamine receptors results in the reduction of extracellular stimulation-evoked isolated EPSCs in layer 3 pyramidal neurons. Using paired recordings from synaptically connected pyramidal neurons we have determined the basic properties of unitary synaptic connections between layer 3 pyramids in the primate PFC and, interestingly, we found that dopamine does not reduce synaptic transmission between nearby pairs of synaptically coupled PFC pyramidal neurons. This input specificity may be a critical aspect of the dopaminergic regulation of recurrent excitatory circuits in the PFC.     

Converging and diverging cholinergic inputs from submucosal neurons amplify activity of secretomotor neurons in guinea-pig ileal submucosa.

The organization of synaptic connections between guinea-pig ileal submucosal neurons was examined using intracellular recordings from single or pairs of submucosal neurons. Synaptic inputs were elicited by stimulating cholinergic neurons using pressure-pulse application of 5-hydroxytryptamine (5-HT) in ganglia adjacent to those where intracellular recordings were obtained. In addition, when pairs of intracellular recordings were obtained, one neuron was activated by intracellular stimulation and synaptic responses were recorded in the other neuron. Neurobiotin-filled microelectrodes were employed to characterize cells electrophysiologically and immunohistochemically. Recordings were obtained from 176 (173 S-type and three AH-type) neurons; 81% of cells were classified as vasoactive intestinal peptide (VIP) neurons. No fast excitatory postsynaptic potentials and only rare slow excitatory postsynaptic potentials were recorded following intracellular stimulation of paired S-type neurons. However, when paired intracellular recordings were obtained from neurons within the same ganglion and 5-HT was applied to an adjacent ganglion, this stimulation evoked synchronized fast excitatory postsynaptic potentials in 94% of pairs. In contrast, when cell bodies of VIP-VIP pairs were located in different ganglia, fast synaptic activation evoked by 5-HT stimulation was not synchronized in 87% of pairs. When intracellular recordings were obtained from a single neuron and two separate ganglia were stimulated by 5-HT pressure-pulse activation, fast excitatory postsynaptic potentials originating from both sources were recorded in the same VIP neuron. Morphological study of 34 S-type and three AH-type horseradish peroxidase-labeled neurons was conducted. AH-type neurons had multiple axonal branches with dense arborization of collaterals containing numerous varicosities in three to nine ganglia, whereas axons of S-type neurons exhibited relatively rare collaterals and varicosities within adjacent ganglia.These results demonstrate that cholinergic neurons provide both diverging and converging inputs to VIP neurons, providing a mechanism to enhance activation of VIP secretomotor neurons. The axonal projections of AH-type neurons suggest they are likely candidates to provide diverging inputs to multiple VIP neurons.     

Dopamine prevents muscarinic-induced decrease of glutamate release in the auditory cortex

Acetylcholine and dopamine are simultaneously released in the cortex at the occurrence of novel stimuli. In addition to a series of excitatory effects, acetylcholine decreases the release of glutamate acting on presynaptic muscarinic receptors. By recording evoked excitatory postsynaptic currents in layers II/III neurons of the auditory cortex, we found that activation of muscarinic receptors by oxotremorine reduces the amplitude of glutamatergic current (A(oxo)/A(ctr) = 0.53 +/- 0.17) in the absence but not in the presence of dopamine (A(oxo)/A(ctr) = 0.89 +/- 0.12 in 20 microM dopamine). These data suggested that an excessive sensitivity to dopamine, such as postulated in schizophrenia, could prevent the decrease of glutamate release associated with the activation of cholinergic corticopetal nuclei. Thus, a possible mechanism of action of antipsychotic drugs could be through a depression of the glutamatergic signal in the auditory cortex. We tested the capability of haloperidol, clozapine and lamotrigine to affect glutamatergic synaptic currents and their muscarinic modulation. We found that antipsychotics not only work as dopamine receptor antagonists in re-establishing muscarinic modulation, but also directly depress glutamatergic currents. These results suggest that presynaptic modulation of glutamate release can account for a dual route of action of antipsychotic drugs.     

Functional characterization of kainate receptors in the rat nucleus accumbens core region

The nucleus accumbens, a brain region involved in motivation, attention, and reward, receives substantial glutamatergic innervation from many limbic structures. This excitatory glutamatergic input plays an integral role in both normal and pathophysiological states. Despite the importance of glutamatergic transmission in the nucleus accumbens, the specific receptor subtypes that mediate glutamatergic signaling in this brain region have not been fully characterized. The current study sought to examine the possible role of the kainate subclass of glutamate receptor in the nucleus accumbens. Kainate receptors are relatively poorly understood members of the ionotropic glutamate receptor family and are highly expressed in the nucleus accumbens. Recent studies have highlighted a number of novel pre- and postsynaptic functions of kainate receptors in several other brain regions. Using the whole cell patch-clamp technique, we report the first demonstration of functional kainate receptors on neurons within the core region of the nucleus accumbens. In addition, we present evidence that activation of kainate receptors in this brain region inhibits excitatory synaptic transmission via a presynaptic mechanism.     

Kappa opioid receptor inhibition of glutamatergic transmission in the nucleus accumbens shell

Microinjection of kappa-opioid receptor agonists into the nucleus accumbens produces conditioned place aversion. While attention has focused primarily on the inhibition of dopamine release by kappa-receptor agonists as the synaptic mechanism underlying this effect, recent anatomical studies have raised the possibility that regulation of noncatecholaminergic transmission also contribute. We have investigated the effects of kappa-receptor activation on fast excitatory synaptic transmission in an in vitro slice preparation using whole cell voltage-clamp or extracellular field recordings in the shell region of the nucleus accumbens. The kappa-receptor agonist U69593 produces a pronounced, dose-dependent inhibition of glutamatergic excitatory postsynaptic currents (EPSCs) that can be reversed by 100 nM nor-BNI. Furthermore, U69593 causes an increase in the paired-pulse ratio as well as a decrease in the frequency of spontaneous miniature events, suggesting a presynaptic site of action. Despite anatomical evidence for kappa-receptor localization on dendritic spines of nucleus accumbens neurons, no electrophysiological evidence of a postsynaptic effect was found. This presynaptic inhibition of excitatory synaptic transmission in the nucleus accumbens shell provides a novel mechanism that may contribute to the kappa-receptor-mediated aversion observed in intact animals.     

Synaptic heterogeneity between mouse paracapsular intercalated neurons of the amygdala

GABAergic medial paracapsular intercalated (Imp) neurons of amygdala are thought of as playing a central role in fear learning and extinction. We report here that the synaptic network formed by these neurons exhibits distinct short-term plastic synaptic responses. The success rate of synaptic events evoked at a frequency range of 0.1–10 Hz varied dramatically between different connected cell pairs. Upon enhancing the frequency of stimulation, the success rate increased, decreased or remained constant, in a similar number of cell pairs. Such synaptic heterogeneity resulted in inhibition of the firing of the postsynaptic neurons with different efficacies. Moreover, we found that the different synaptic weights were mainly determined by diversity in presynaptic release probabilities rather than postsynaptic changes. Sequential paired recording experiments demonstrated that the same presynaptic neuron established the same type of synaptic connections with different postsynaptic neurons, suggesting the absence of target-cell specificity. Conversely, the same postsynaptic neuron was contacted by different types of synaptic connections formed by different presynaptic neurons. A detailed anatomical analysis of the recorded neurons revealed discrete and unexpected peculiarities in the dendritic and axonal patterns of different cell pairs. In contrast, several intrinsic electrophysiological responses were homogeneous among neurons, and synaptic failure counts were not affected by presynaptic cannabinoid 1 or GABA_B receptors. We propose that the heterogeneous functional connectivity of Imp neurons, demonstrated by this study, is required to maintain the stability of firing patterns which is critical for the computational role of the amygdala in fear learning and extinction.     

Dopamine neuron glutamate cotransmission: frequency-dependent modulation in the mesoventromedial projection

Mesoventromedial dopamine neurons projecting from the medial ventral tegmental area to the ventromedial shell of the nucleus accumbens play a role in attributing incentive salience to environmental stimuli that predict important events, and appear to be particularly sensitive to the effects of psychostimulant drugs. Despite the observation that these dopamine neurons make up almost the entire complement of neurons in the projection, stimulating their cell bodies evokes a fast glutamatergic response in accumbens neurons. This is apparently due to dopamine neuron glutamate cotransmission, suggested by the extensive coexpression of vesicular glutamate transporter 2 (VGLUT2) in the neurons. To examine the interplay between the dopamine and glutamate signals, we used acute quasi-horizontal brain slices made from DAT-YFP mice in which the intact mesoventromedial projection can be visualized. Under current clamp, when dopamine neurons were stimulated repeatedly, dopamine neuron glutamate transmission showed dopamine-mediated facilitation, solely at higher, burst-firing frequencies. Facilitation was diminished under voltage clamp and flipped to inhibition by intracellular Cs⁺ or GDPβS, indicating that it was mediated postsynaptically. Postsynaptic facilitation was D1 mediated, required activation of NMDA receptors and closure of voltage gated K⁺-channels. When postsynaptic facilitation was blocked, D2-mediated presynaptic inhibition became apparent. These counterbalanced pre- and postsynaptic actions determine the frequency dependence of dopamine modulation; at lower firing frequencies dopamine modulation is not apparent, while at burst firing frequency postsynaptic facilitation dominates and dopamine becomes facilitatory. Dopamine neuron glutamate cotransmission may play an important role in encoding the incentive salience value of conditioned stimuli that activate goal-directed behaviors, and may be an important subtract for enduring drug-seeking behaviors.     

Dopamine D1-receptors modulate lateral inhibition between principal cells of the nucleus accumbens

One of the current hypotheses on dopamine in the physiology of motivation posits that this neurotransmitter regulates filtering and selection of inputs to the nucleus accumbens. The effects of dopamine (100 microM) and the D1-receptor agonist SKF 38393 (20-50 microM) on GABAergic synaptic transmission between pairs of principal cells of rat nucleus accumbens were studied by using simultaneous dual patch-clamp recordings in acutely prepared brain slices. Both compounds attenuated postsynaptic responses induced by presynaptic firing and this effect was reversed by the D1-receptor antagonist SCH 23390 (25 microM). This attenuating effect of dopamine D1-receptors may act to diminish competitive interactions between single projection neurons or ensembles in the nucleus accumbens.     

Subthalamic stimulation-induced synaptic responses in substantia nigra pars compacta dopaminergic neurons in vitro.

The subthalamic nucleus (STN) is one of the principal sources of excitatory glutamatergic input to dopaminergic neurons of the substantia nigra, yet stimulation of the STN produces both excitatory and inhibitory effects on nigral dopaminergic neurons recorded extracellularly in vivo. The present experiments were designed to determine the sources of the excitatory and inhibitory effects. Synaptic potentials were recorded intracellularly from substantia nigra pars compacta dopaminergic neurons in parasagittal slices in response to stimulation of the STN. Synaptic potentials were analyzed for onset latency, amplitude, duration, and reversal potential in the presence and absence of GABA and glutamate receptor antagonists. STN-evoked depolarizing synaptic responses in dopaminergic neurons reversed at approximately -31 mV, intermediate between the expected reversal potential for an excitatory and an inhibitory postsynaptic potential (EPSP and IPSP). Blockade of GABA(A) receptors with bicuculline caused a positive shift in the reversal potential to near 0 mV, suggesting that STN stimulation evoked a near simultaneous EPSP and IPSP. Both synaptic responses were blocked by application of the glutamate receptor antagonist, 6-cyano-7-nitroquinoxalene-2,3-dione. The confounding influence of inhibitory fibers of passage from globus pallidus and/or striatum by STN stimulation was eliminated by unilaterally transecting striatonigral and pallidonigral fibers 3 days before recording. The reversal potential of STN-evoked synaptic responses in dopaminergic neurons in slices from transected animals was approximately -30 mV. Bath application of bicuculline shifted the reversal potential to approximately 5 mV as it did in intact animals, suggesting that the source of the IPSP was within substantia nigra. These data indicate that electrical stimulation of the STN elicits a mixed EPSP-IPSP in nigral dopaminergic neurons due to the coactivation of an excitatory monosynaptic and an inhibitory polysynaptic connection between the STN and the dopaminergic neurons of substantia nigra pars compacta. The EPSP arises from a direct monosynaptic excitatory glutamatergic input from the STN. The IPSP arises polysynaptically, most likely through STN-evoked excitation of GABAergic neurons in substantia nigra pars reticulata, which produces feed-forward GABA(A)-mediated inhibition of dopaminergic neurons through inhibitory intranigral axon collaterals.