Increased dopamine D2 receptor activity in the striatum alters the firing pattern of dopamine neurons in the ventral tegmental area

There is strong evidence that the core deficits of schizophrenia result from dysfunction of the dopamine (DA) system, but details of this dysfunction remain unclear. We previously reported a model of transgenic mice that selectively and reversibly overexpress DA D2 receptors (D2Rs) in the striatum (D2R-OE mice). D2R-OE mice display deficits in cognition and motivation that are strikingly similar to the deficits in cognition and motivation observed in patients with schizophrenia. Here, we show that in vivo, both the firing rate (tonic activity) and burst firing (phasic activity) of identified midbrain DA neurons are impaired in the ventral tegmental area (VTA), but not in the substantia nigra (SN), of D2R-OE mice. Normalizing striatal D2R activity by switching off the transgene in adulthood recovered the reduction in tonic activity of VTA DA neurons, which is concordant with the rescue in motivation that we previously reported in our model. On the other hand, the reduction in burst activity was not rescued, which may be reflected in the observed persistence of cognitive deficits in D2R-OE mice. We have identified a potential molecular mechanism for the altered activity of DA VTA neurons in D2R-OE mice: a reduction in the expression of distinct NMDA receptor subunits selectively in identified mesolimbic DA VTA, but not nigrostriatal DA SN, neurons. These results suggest that functional deficits relevant for schizophrenia symptoms may involve differential regulation of selective DA pathways.Deficits in cognition and motivation are core features of schizophrenia (1, 2). These symptoms are listed in the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition as a diagnostic criterion for schizophrenia spectrum disorder (3) and have a significant impact on patients’ overall functioning and quality of life (4, 5). Currently, there are no effective treatments for these disabling aspects of the disease. Therefore, a high priority in the study of schizophrenia is to increase our understanding of the neurobiology of cognitive and motivational deficits. The midbrain dopamine (DA) system affects cognition and motivation in healthy subjects. It includes DA neurons of the ventral tegmental area (VTA), projecting to prefrontal cortex (PFC) and limbic areas (e.g., ventral striatum), and DA neurons of the substantia nigra (SN), projecting to the dorsal striatum (6). Involvement of the midbrain DA system is strongly implicated in both the cognitive and motivational deficits observed in schizophrenia (7, 8). Moreover, it is well documented that the DA system is altered in patients with schizophrenia (reviewed in refs. 9, 10).To model the increase in striatal DA D2 receptor (D2R) activity observed in patients with schizophrenia, we previously generated transgenic mice that selectively and reversibly overexpress D2Rs in the striatum (D2R-OE mice) (11). In this model, expression of the transgenic D2Rs is restricted to the postsynaptic medium spiny neurons in the striatum and can be temporally regulated. D2R-OE mice display phenotypes strikingly similar to the cognitive and negative symptoms of schizophrenia. Cognitive phenotypes of D2R-OE mice include deficits in working memory tasks, behavioral flexibility, conditional associative learning, and timing (11–14). D2R-OE mice also exhibit phenotypes similar to the negative symptoms of schizophrenia: a deficit in incentive motivation, without disruption of hedonic processes (13, 15–17). We previously reported that these behavioral deficits in striatal D2R-OE mice are accompanied by changes in cortical DA function (11, 18). D2R overexpression restricted to the striatum led to alterations in DA function in the PFC. These alterations include changes in both the amount and rate of turnover of DA in PFC tissue, as well as changes in the activation of D1 receptors in the PFC in vivo. We also identified changes in inhibitory transmission and DA sensitivity in the PFC of D2R-OE mice (18).The finding that increased D2R expression restricted to the striatum leads to changes in DA function in the cortex suggests that a central component of the DA midbrain system is perturbed in the D2R-OE mice. We therefore set out to determine whether these changes might be occurring at the level of presynaptic DA neuron activity. To determine if increased postsynaptic D2R activity in the striatum has an impact on the electrophysiological activity of DA midbrain neurons, we performed single-unit extracellular recordings and juxtacellular labeling of individual DA midbrain neurons in vivo from D2R-OE mice and their control littermates. We found that increased D2R activity in the striatum changed the electrophysiological properties of DA neurons in the VTA, whereas DA neurons in the SN remained unaffected. Specifically, we found that in the DA VTA neurons, both the firing frequency and burst activity were reduced in D2R-OE mice compared with controls. When we switched off the transgene in adulthood, the firing frequency was rescued but the decrease in burst activity was not. This dissociation of the two phenotypes may reflect potentially reversible and irreversible components of DA pathophysiology. In vivo burst activity of DA neurons is under powerful control of NMDA receptor currents (19, 20). To investigate a potential molecular mechanism for the observed alterations in the activity of DA neurons, we quantified NMDA receptor subunit mRNA levels in DA neurons of both the mesolimbic and nigrostriatal pathways. Consistent with the electrophysiological deficits, we found a specific reduction of NMDA receptor subunit 1 (NR1) and NR2B expression selectively in mesolimbic DA neurons of the VTA in D2R-OE mice.     

Ethanol enhances GABAergic transmission onto dopamine neurons in the ventral tegmental area of the rat

Background:  Activation of the dopaminergic (DA) neurons of the ventral tegmental area (VTA) by ethanol has been implicated in its rewarding and reinforcing effects. At most central synapses, ethanol generally increases inhibitory synaptic transmission; however, no studies have explored the effect of acute ethanol on GABAergic transmission in the VTA.
Methods:  Whole-cell patch clamp recordings of inhibitory postsynaptic currents (IPSCs) from VTA-DA neurons in midbrain slices from young rats.
Results  Acute exposure of VTA-DA neurons to ethanol (25 to 50 mM) robustly enhanced GABAergic spontaneous and miniature IPSC frequency while inducing a slight enhancement of spontaneous IPSC (sIPSC) amplitude. Ethanol (50 mM) enhanced paired-pulse depression of evoked IPSCs, further suggesting enhanced GABA release onto VTA-DA neurons. The frequency of sIPSCs was suppressed by the GABA_B agonist, baclofen (1.25 μM) and enhanced by the antagonist, SCH50911 (20 μM); however, neither appeared to modulate or occlude the effects of ethanol on sIPSC frequency.
Conclusions:  The present results indicate that ethanol increases postsynaptic GABA_A receptor sensitivity, enhances action potential-independent GABA release onto VTA-DA neurons, and that this latter effect is independent of GABA_B auto-receptor inhibition of GABA release.     

Serotonin hyperpolarizes cholinergic low-threshold burst neurons in the rat laterodorsal tegmental nucleus in vitro.

Serotonergic suppression of cholinergic neuronal activity implicated in the regulation of rapid eye movement sleep and its associated phenomenon, pontogeniculooccipital waves, has long been postulated, but no direct proof has been available. In this study, intracellular and whole-cell patch-clamp recording techniques were combined with enzyme histochemistry to examine the intrinsic electrophysiological properties and response to serotonin (5-HT) of identified cholinergic rat laterodorsal tegmental nucleus neurons in vitro. Sixty-five percent of the recorded neurons demonstrated a prominent low-threshold burst, and of these, 83% were cholinergic. In current-clamp recordings 64% of the bursting cholinergic neurons tested responded to the application of 5-HT with a membrane hyperpolarization and decrease in input resistance. This effect was mimicked by application of the selective 5-HT type 1 receptor agonist carboxamidotryptamine maleate. Whole-cell patch-clamp recordings revealed that the hyperpolarizing response was mediated by an inwardly rectifying K+ current. Application of 5-HT decreased excitability and markedly modulated the discharge pattern of cholinergic bursting neurons: during a 5-HT-induced hyperpolarization these neurons exhibited no rebound burst after hyperpolarizing current input and a burst in response to depolarizing current input. In the absence of 5-HT, the relatively depolarized cholinergic bursting neurons responded to an identical hyperpolarizing current input with a burst and did not produce a burst after depolarizing current input. These data provide a cellular and molecular basis for the hypothesis that 5-HT modulates rapid eye movement sleep phenomenology by altering the firing pattern of bursting cholinergic neurons.     

Aversive behavior induced by optogenetic inactivation of ventral tegmental area dopamine neurons is mediated by dopamine D2 receptors in the nucleus accumbens

Dopamine (DA) transmission from the ventral tegmental area (VTA) is critical for controlling both rewarding and aversive behaviors. The transient silencing of DA neurons is one of the responses to aversive stimuli, but its consequences and neural mechanisms regarding aversive responses and learning have largely remained elusive. Here, we report that optogenetic inactivation of VTA DA neurons promptly down-regulated DA levels and induced up-regulation of the neural activity in the nucleus accumbens (NAc) as evaluated by Fos expression. This optogenetic suppression of DA neuron firing immediately evoked aversive responses to the previously preferred dark room and led to aversive learning toward the optogenetically conditioned place. Importantly, this place aversion was abolished by knockdown of dopamine D2 receptors but not by that of D1 receptors in the NAc. Silencing of DA neurons in the VTA was thus indispensable for inducing aversive responses and learning through dopamine D2 receptors in the NAc.The mesolimbic dopaminergic system not only plays a pivotal role in a wide range of motivation and learning (1–3), but its dysfunction has also been implicated in severe neuropsychiatric disorders as exemplified in Parkinson disease, schizophrenia, and drug addiction. Dopamine (DA) neurons in the ventral tegmental area (VTA) react to rewarding stimuli by phasic firing, and the main function of this firing is theorized to encode “the reward prediction error,” the difference in the value between the predicted reward and the actual reward (4). In contrast to the response to rewarding stimuli, their reactions to aversive stimuli are far from homologous; i.e., some DA neurons are activated in response to aversive stimuli, whereas most others react by transiently suppressing their firings (5–9). In fact, recent studies have revealed that optogenetic activation of GABAergic neurons and resultant inactivation of DA neurons suppress reward consumption and induce an aversive response (10, 11). However, it has largely remained elusive as to which mechanisms in the neural circuits are essential for the acquisition of aversive learning following the inactivation of DA neurons in the VTA and as to how behavioral responses are controlled toward suppressing reward consumption and inducing aversive behaviors.Accumulated evidence has revealed that the motivational and cognitive learning in response to positive and negative stimuli is largely regulated by the neural circuits including the basal ganglia (12), which receive a large amount of the dopaminergic projection from the midbrain. In the striatum, two fundamental neural circuits are constituted by specified medium-sized spiny neurons (MSNs), each expressing a distinct type of DA receptor (13). One circuit is the direct pathway, consisting of the MSNs directly projecting to the output nuclei of the basal ganglia, substantia nigra pars reticulata (SNr), and predominantly expressing dopamine D1 receptors (D1Rs). The other is the indirect pathway, consisting of the MSNs that project indirectly through the globus pallidus to the SNr and primarily express dopamine D2 receptors (D2Rs). DA signals from the midbrain dynamically modulate these two parallel pathways in the opposite manner via D1Rs and D2Rs, and this modulation is supposed to facilitate motivational learning (3, 14). As for the rewarding stimuli, up-regulated DA levels induced by rewarding signals are considered to activate the D1Rs and thus predominantly facilitate the direct pathway in the nucleus accumbens (NAc). On the other hand, the suppression of DA neuron firings in response to aversive stimuli decreases DA levels in the NAc; and this reaction is supposed to specifically promote the signal transmission in the indirect pathway through activated D2Rs.Although studies using the pharmacological strategies and reversible neurotransmission blocking (RNB) method have supported this mechanism of regulation in the NAc (15, 16), it has remained unknown whether the suppression of DA neuron firing is sufficient to promote the activity of the indirect pathway and subsequently induce the avoidance behavior. In this present study, we addressed this issue by selectively inactivating DA neurons in the VTA by optogenetically manipulating membrane-hyperpolarizing Arch protein (17) and explicitly demonstrated that the suppression of DA neurons in the VTA subsequently decreased DA levels in the NAc and induced aversive reaction and learning. Furthermore, we investigated the mechanisms of the regulation of this reaction and disclosed that this aversive reaction was specifically controlled by D2Rs in the NAc.     

Ethanol directly excites dopaminergic ventral tegmental area reward neurons

BACKGROUND: The mesolimbic/mesocortical dopamine pathway mediates the rewarding effects of ethanol and other drugs of abuse like cocaine and opiates. Dopaminergic neurons of the ventral tegmental area (VTA) are the cells of origin of the mesolimbic/mesocortical dopamine pathway. Ethanol's rewarding properties result from its ability to excite dopaminergic cell bodies in the VTA which results in increased dopamine release in the nucleus accumbens. Many recent papers have speculated that ethanol excitation of dopaminergic VTA neurons is indirect, either that ethanol acts on GABAergic or other interneurons, which in turn modulate the activity of dopaminergic VTA neurons, or that ethanol modulates the action of neurotransmitter-gated ion channels in the VTA. METHODS: VTA neurons were acutely dissociated and plated onto a cover slip in an electrophysiological recording chamber. These neurons generated spontaneous action potentials which could be measured with cell attached loose patch recording. The dissociation procedure truncated the dendritic trees, severed synaptic contacts and widely dispersed these neurons. Dopamine (10-50 nM) and ethanol (20-120 mM) were bath applied and their effects on firing rate were measured. After some experiments, plated cells were fixed and processed for immunostaining of tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis. RESULTS: All neurons met electrophysiological criteria previously established for dopaminergic VTA neurons. Dopamine inhibited all VTA neurons tested, indicating the presence of dopamine autoreceptors. All neurons identified as dopaminergic by these electrophysiological and pharmacological criteria, and that were processed for immunohistochemistry, stained positive for tyrosine hydroxylase immunoreactivity. All acutely dissociated VTA neurons, identified as dopaminergic by electrophysiological, pharmacological and immunohistochemical criteria, were robustly excited by behaviorally relevant concentrations of ethanol. The ethanol-induced excitation was concentration-dependent. CONCLUSIONS: These data provide strong evidence that ethanol directly excites dopaminergic VTA neurons, as this excitation still occurs in the absence of input from surrounding neurons.     

Effect of insulin on excitatory synaptic transmission onto dopamine neurons of the ventral tegmental area in a mouse model of hyperinsulinemia

Obesity has drastically increased over the last few decades. Obesity is associated with elevated insulin levels, which can gain access to the brain, including into dopamine neurons of the ventral tegmental area (VTA), a brain region critical for mediating reward-seeking behavior. Synaptic plasticity of VTA dopamine neurons is associated with altered motivation to obtain reinforcing substances such as food and drugs of abuse. Under physiological circumstances, insulin in the VTA can suppress excitatory synaptic transmission onto VTA dopamine neurons and reduce aspects of palatable feeding behavior. However, it is unknown how insulin modulates excitatory synaptic transmission in pathological circumstances such as hyperinsulinemia. Using patch-clamp electrophysiology, we demonstrate that, in a hyperinsulinemic mouse model, insulin has reduced capacity to cause a synaptic depression of VTA dopamine neurons, although both low-frequency stimulation-induced long-term depression and cannabinoid-induced depression were normal. These results suggest that insulin action in the VTA during pathological hyperinsulinemia is disrupted and may lead to increased feeding behavior.     

Increased ethanol excitation of dopaminergic neurons of the ventral tegmental area after chronic ethanol treatment

BACKGROUND: The mesolimbic dopamine pathway, which originates in the ventral tegmental area (VTA), is important for the rewarding effects of ethanol. Acute administration of ethanol has been shown to excite dopaminergic neurons of the VTA. Chronic ethanol treatment has been reported to alter the in vitro response of dopaminergic neurons to NMDA and dopamine. The present electrophysiological study tested the hypothesis that the effect of ethanol, gamma-aminobutyric acid (GABA), and NMDA on individual dopaminergic VTA (DA-VTA) neurons from C57BL/6J mice would be changed by chronic treatment with ethanol. METHODS: C57BL/6J mice were injected intraperitoneally twice daily with either saline or ethanol in saline (3.5 g/kg) for at least 21 days. Extracellular single unit recordings of spontaneous action potentials were made from DA-VTA neurons in brain slices from these mice. Ethanol (20-120 mM), GABA (50-500 microM), or NMDA (2-20 microM) was administered in the superfusate, and the resulting change in firing rate was measured. RESULTS: There was no significant difference in mean basal spontaneous firing rate of DA-VTA neurons between saline-treated and ethanol-treated mice. The DA-VTA neurons from ethanol-treated mice were excited by ethanol more potently than those from saline-treated mice. Dopaminergic VTA neurons from ethanol-treated mice were inhibited less potently by GABA than those from saline-treated mice. There was no difference in the potency of NMDA to excite DA-VTA neurons from saline-treated and ethanol-treated mice. CONCLUSIONS: Chronic treatment of C57BL/6J mice with ethanol injections sensitizes DA-VTA neurons to ethanol excitation and also decreases the inhibitory potency of GABA. The increase in sensitivity to ethanol excitation of dopaminergic VTA neurons after chronic ethanol treatment may increase the reward value of ethanol. This sensitization to ethanol activation may be an important change in reward area neurons and may contribute to the development of alcoholism.     

The reinforcing properties of salsolinol in the ventral tegmental area: evidence for regional heterogeneity and the involvement of serotonin and dopamine

Background:  Salsolinol (SAL), the condensation product of acetaldehyde and dopamine, may be a factor contributing to alcohol abuse. Previous research indicated that both ethanol and acetaldehyde are self-administered into the posterior ventral tegmental area (VTA). The current study examined SAL self-infusions into the VTA, and determined the involvement of dopamine neurons and 5-HT₃ receptors in this process.
Methods:  The intracranial self-administration technique was used to determine the self-infusion of SAL into the VTA of adult, male Wistar rats. The rats were placed in 2-lever (active and inactive) experimental chambers, and allowed to respond for the self-infusion of 0, 0.03, 0.1, 0.3, 1.0 or 3.0 μM SAL into the posterior or anterior VTA. In a second experiment, rats self-administered 0.3 μM SAL for the initial 4 sessions, co-administered SAL with ICS-205,930 (a 5-HT₃ receptor antagonist) or quinpirole (a D_2,3 receptor agonist) for sessions 5 and 6, and then only 0.3 μM SAL for session 7.
Results:  Wistar rats, given 0.03 to 0.3 μM SAL, received more infusions per session than did the group given artificial cerebrospinal fluid (aCSF) alone (e.g., 41 infusions for 0.1 μM SAL versus 9 infusions for the aCSF group), and responded more on the active than inactive lever. These effects were observed in the posterior but not in anterior VTA. Co-infusion of 100 μM ICS-205,930, or quinpirole significantly reduced self-infusions and active lever responding.
Conclusions:  SAL produces reinforcing effects in the posterior VTA of Wistar rats, and these effects are mediated by activation of DA neurons and local 5-HT₃ receptors.     

Rebound burst firing in the reticular thalamus is not essential for pharmacological absence seizures in mice

Intrinsic burst and rhythmic burst discharges (RBDs) are elicited by activation of T-type Ca²⁺ channels in the thalamic reticular nucleus (TRN). TRN bursts are believed to be critical for generation and maintenance of thalamocortical oscillations, leading to the spike-and-wave discharges (SWDs), which are the hallmarks of absence seizures. We observed that the RBDs were completely abolished, whereas tonic firing was significantly increased, in TRN neurons from mice in which the gene for the T-type Ca²⁺ channel, Ca_V3.3, was deleted (Ca_V3.3^−/−). Contrary to expectations, there was an increased susceptibility to drug-induced SWDs both in Ca_V3.3^−/− mice and in mice in which the Ca_V3.3 gene was silenced predominantly in the TRN. Ca_V3.3^−/− mice also showed enhanced inhibitory synaptic drive onto TC neurons. Finally, a double knockout of both Ca_V3.3 and Ca_V3.2, which showed complete elimination of burst firing and RBDs in TRN neurons, also displayed enhanced drug-induced SWDs and absence seizures. On the other hand, tonic firing in the TRN was increased in these mice, suggesting that increased tonic firing in the TRN may be sufficient for drug-induced SWD generation in the absence of burst firing. These results call into question the role of burst firing in TRN neurons in the genesis of SWDs, calling for a rethinking of the mechanism for absence seizure induction.Absence seizures are generalized, nonconvulsive seizures characterized by the appearance of bilaterally synchronous spike-and-wave discharges (SWDs) on the electroencephalogram (EEG). The frequency of the SWDs is variable among different models and is usually higher (4–12 Hz) in rodents than in humans (3 Hz) (1). SWDs represent synchronized oscillations of the thalamocortical network (2–4), a network that includes neurons of the cerebral cortex, thalamocortical nucleus (TC), and thalamic reticular nucleus (TRN) (5). This thalamocortical circuitry is a key CNS structure for gating the flow of sensory information from the periphery to the cortex (6, 7). Both thalamocortical and corticothalamic connections are mainly glutamatergic (8). The TRN is a shell-like structure that covers most of the rostral, lateral, and ventral parts of the thalamus (5) and is composed exclusively of GABAergic interneurons that provide massive inhibitory input to TC neurons (9). The most distinctive feature of thalamocortical circuitry is its intrinsic ability to generate oscillations via the reciprocal circuits between TC and TRN neurons (10–12).Both TC and TRN neurons are able to generate two distinctive patterns of action potential firing: tonic and burst (13, 14). Burst firing is mediated by low-voltage–activated (LVA) T-type Ca²⁺ channels (15). There are three subtypes of T-type Ca²⁺channels, called Ca_V3.1, Ca_V3.2, and Ca_V3.3, each with distinctive expression patterns and kinetic properties (16). Within the thalamocortical circuit, Ca_V3.1 channels are predominantly expressed in TC neurons, whereas Ca_V3.2 and Ca_V3.3 channels are expressed only in TRN neurons (17). Unlike high-voltage–activated Ca²⁺ channels, T-type Ca²⁺ channels are inactivated at membrane potentials around −60 to −50 mV (15). However, when the membrane potential is hyperpolarized for longer than 100 ms, these channels are de-inactivated and can then initiate a burst of action potentials once the membrane potential is repolarized (18). TC neurons with a relatively depolarized resting membrane potential exhibit a tonic mode of firing associated with conventional, Na/K-dependent action potentials upon receiving excitatory inputs. However, when the neurons are hyperpolarized by inhibitory inputs, high-frequency burst firing can be triggered by Ca_V3.1 channels. TRN neurons, on the other hand, have a relatively hyperpolarized resting potential around −70 mV and most of their T-type Ca²⁺ channels are available for activation. These channels, especially Ca_V3.3, enable TRN neurons to generate rhythmic bursts, even in response to excitatory inputs (12, 19). These oscillatory bursts of action potentials in the TRN provide a barrage of hyperpolarizing inhibitory postsynaptic potentials (IPSPs) to the TC neurons, which in turn respond with action potential bursts due to the activation of Ca_V3.1. These TC bursts provide rhythmic excitatory drive to the cortex, eventually resulting in SWDs.Several models have been proposed to account for the mechanism of SWD generation in the thalamocortical circuit. One well-known model argues that intrinsic cortical oscillations drive the synchronized activity of the entire thalamocortical circuit to cause SWDs (20). The most widely accepted model suggests that rhythmic bursting activity in TRN neurons is a key element for initiation and maintenance of SWDs (4, 10, 12). This model has been indirectly supported by previous observations that lesions or blockade of voltage-gated Ca²⁺ channels in TRN disrupt SWDs (21), leading to the idea that the degree of the synchrony between TC and TRN is regulated by the burst firing of TRN neurons (10). However, there have been no direct experimental tests of the hypothesis that TRN bursts are indeed essential for SWDs in absence seizures. Here we have tested the role of TRN bursts in absence epilepsy and have surprisingly found that mice with complete genetic deletion of TRN burst activity exhibited enhanced SWDs with higher susceptibility to γ-butyrolactone (GBL), a widely used seizure-inducing drug. Apparently that enhanced TRN tonic firing is observed in the absence of bursts is sufficient to support SWDs.     

From the Cover: Disruption of NMDAR-dependent burst firing by dopamine neurons provides selective assessment of phasic dopamine-dependent behavior

Midbrain dopamine (DA) neurons fire in 2 characteristic modes, tonic and phasic, which are thought to modulate distinct aspects of behavior. However, the inability to selectively disrupt these patterns of activity has hampered the precise definition of the function of these modes of signaling. Here, we addressed the role of phasic DA in learning and other DA-dependent behaviors by attenuating DA neuron burst firing and subsequent DA release, without altering tonic neural activity. Disruption of phasic DA was achieved by selective genetic inactivation of NMDA-type, ionotropic glutamate receptors in DA neurons. Disruption of phasic DA neuron activity impaired the acquisition of numerous conditioned behavioral responses, and dramatically attenuated learning about cues that predicted rewarding and aversive events while leaving many other DA-dependent behaviors unaffected.     

Effects of microinjection of the D2 dopamine antagonist raclopride into the ventral tegmental area on ethanol and sucrose self-administration

From previous microinjection studies, a reciprocal feedback between the nucleus accumbens and the ventral tegmental area (VTA) has been implicated in the reinforcing stimulus actions of ethanol and sucrose. In these studies, the effects of self administration of ethanol or sucrose solutions on maintained responding were similar when a dopamine antagonist was injected in the nucleus accumbens or a dopamine agonist was injected into the VTA. Our study was performed to determine if the effects on responding that had been observed when a dopamine agonist was injected into the nucleus accumbens would occur after an injection of a dopamine antagonist into the VTA. Male, Long-Evans rats were initially trained to lever press using either 10% ethanol or 75% sucrose solutions as the reinforcers. Bilateral guide cannulae were implanted to allow microinjection into the VTA of differing doses of the dopamine D2 antagonist, raclopride. Only at the highest dose tested (10 microg) was any effect observed on responding maintained by either reinforcer. The effect was minimal and different from that observed after the microinjection of a dopamine agonist into the nucleus accumbens. This suggests that either the actions of the nucleus accumbens agonist manipulation involved other processes or that the level of enhanced dopamine release in the nucleus accumbens from the VTA antagonist injection was not sufficient to mimic the effect of the nucleus accumbens agonist injections.     

Lordosis of rats is modified by neurosteroidogenic effects of membrane benzodiazepine receptors in the ventral tegmental area

Progestins modulate lordosis through actions in the ventral tegmental area (VTA). Whether neurosteroidogenesis of 5alpha-pregnan-3alpha-ol-20-one (3alpha,5alpha-THP), involving mitochondrial benzodiazepine receptors (MBR), is important for lordosis was investigated. Ovariectomized (Ovx), hormone-primed rats (experiments 1, 3, 5, 6) and rats in behavioral estrus (experiments 2 and 4) were unilaterally infused via chronic guide cannula to the VTA with a MBR agonist, N,N-dihexyl-2-(4-fluorophenyl) indole-30-acetamide (FGIN 1-27) or antagonist 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboximide (PK-11195). Experiment 1: Estradiol benzoate (EB)-primed (25 microg) rats administered 0 or 25 microg progesterone (P4) SC showed increased lordosis when infused with 5.0 microg FGIN 1-27 to the VTA; those administered 100 or 200 microg P4 SC exhibited greater lordosis when infused with 2.5 or 5.0 microg FGIN, relative to saline-infused rats. Experiment 2: Rats, near the termination of behavioral estrus, infused with 2.5 or 5.0 microg of FGIN 1-27 to the VTA, showed increased lordosis compared to that seen following vehicle administration. Experiment 3: EB-primed rats administered 200 or 500 microg P4 SC showed decreased lordosis when infused with 100, 200, or 400 ng PK-11195, relative to saline-infused rats. Experiment 4: Rats infused at the peak of behavioral estrus with 100, 200, or 400 ng PK-11195 to the VTA exhibited reduced lordosis compared to that seen following vehicle administration. Experiment 5: 3alpha,5alpha-THP (100 ng) infusions to the VTA reinstated lordosis of hormone-primed rats infused with PK-11195 (100 ng) to the VTA. Experiment 6: FGIN 1-27 (5.0 microg) and PK-11195 (100 ng) infusions aimed at the VTA respectively increased and decreased midbrain levels of 3alpha,5alpha-THP compared to vehicle. Notably, the specific effects observed with infusions to the VTA were not seen with infusions to the control site, the substantia nigra. These data suggest that neurosteroidogenesis involving MBRs in the VTA mediates lordosis of hormone-primed or behavioral estrous rats.     

Effects of combined systemic alcohol and central nicotine administration into ventral tegmental area on dopamine release in the nucleus accumbens

BACKGROUND: Alcoholism is associated with a higher incidence of smoking. The mesolimbic dopaminergic pathway is believed to play an important role in the reinforcing effects of both ethanol and nicotine. This study was undertaken to determine whether simultaneous administration of systemic ethanol and microinjection of nicotine into the ventral tegmental area (VTA) would result in exaggerated release of dopamine (DA) in the shell region of nucleus accumbens. METHODS: Microdialysis was applied in awake, freely moving adult male Wistar rats, and DA concentration in the dialysate was measured by HPLC-electrochemical detectors. RESULTS: Systemic administration of ethanol or microinjection of nicotine into VTA resulted in a dose-dependent increase in DA release (extracellular DA concentration). Simultaneous administration of lower doses of nicotine and ethanol resulted in an additive effect on the released DA. This additive effect was not observed with higher doses of nicotine and ethanol. Administration of the nicotinic antagonist mecamylamine into VTA completely blocked ethanol-induced DA release. CONCLUSIONS: These data support the hypothesis that the reinforcing effects of ethanol are at least partially mediated through the nicotinic receptors in the VTA. Furthermore, administration of selective nicotinic antagonists may be of therapeutic potential in reducing the rewarding effects of ethanol. The data also suggest that the combined effects of ethanol and nicotine on the "reward pathway" may be a contributing factor to the high incidence of smoking in alcoholics.     

Involvement of dopamine D2 autoreceptors in the ventral tegmental area on alcohol and saccharin intake of the alcohol-preferring P rat

BACKGROUND: The ventral tegmental area (VTA) dopamine (DA) system is considered to be involved in mediating the actions of ethanol (EtOH). The objective of the present study was to examine the role of VTA DA D2 receptors in regulating EtOH intake of alcohol-preferring P rats. METHODS: EtOH (10% v/v) and saccharin (SACC, 0.0125% g/v) intake during 2 hr of limited access was assessed after microinjections of the D2 agonist quinpirole and the D2 antagonist sulpiride into the anterior VTA (AVTA) of female P rats. Both EtOH-SACC alternate-day-access conditions and daily availability of EtOH and SACC solutions to separate groups of subjects were used. A second D2 agonist, quinelorane, and coadministration of 2.0 microg sulpiride with 2.0 microg quinpirole were tested in animals given limited access to EtOH. Finally, the effects of quinpirole injected 2 mm dorsal to the VTA and within the posterior VTA (PVTA) were assessed under EtOH-SACC alternate-day-access conditions. RESULTS: Microinjections of 2.0-6.0 microg quinpirole into the AVTA dose dependently decreased EtOH intake 40-80% during the first 30 min of the limited access sessions but did not alter SACC intake. Injections of 2.0-4.0 microg quinelorane into the AVTA also reduced EtOH intake in the first 30 min. Administration of 0.5-2.0 microg sulpiride into the AVTA had no effect on either EtOH or SACC intakes but did attenuate the effects of quinpirole on reducing EtOH intake. Injections of 2.0-4.0 quinpirole 2 mm dorsal to the VTA did not alter EtOH or SACC intakes. Posterior VTA injections of quinpirole decreased EtOH and SACC intakes approximately 25-30% and 60-70%, respectively, in the first 30 min. None of the treatments altered intakes during the 30-120 min period. CONCLUSIONS: The data suggest that DA neuronal activity within the AVTA may be important for maintaining EtOH drinking in P rats, whereas DA neuronal activity within the PVTA may be involved in regulating general drinking and/or motivational behaviors. Overall, the results confirm the involvement of mesolimbic DA in EtOH self-administration and suggest that there is functional heterogeneity within the VTA regulating drinking behavior of the P rat.     

Re-examining the role of ventral tegmental area dopaminergic neurons in motor activity and reinforcement by chemogenetic and optogenetic manipulation in mice

Metabolic Brain Disease - The precise contributions of ventral tegmental area (VTA) dopaminergic (DAergic) neurons to reward-related behaviors are a longstanding hot topic of debate. Whether the...     

D2 receptor regulation of synaptic burst firing in prefrontal cortical pyramidal neurons

The efficacy of antipsychotics in the treatment of schizophrenia depends on their ability to block dopamine (DA) D2 receptors. D2 receptor excitatory mediation of glutamatergic receptors has been implicated in in vivo studies. However, D2 receptor enhancement of glutamatergic transmission has rarely been reported in slice recordings. Instead, D2 receptor depression of both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) action was obtained in previous slice studies. To obtain insight into this paradox, we examined DA's actions on synaptic responses of layer V pyramidal cells to minimal extracellular stimulation in layer III of ferret prefrontal cortical slices under NMDA and gamma-aminobutyric acid type A blockade. This experimental design models the proposed hypofunction of NMDA receptor and gamma-aminobutyric acid type A deficiency in schizophrenia. We found that DA and D2 receptor agonists promoted burst firing in a subset of pyramidal cells, which was reversed by haloperidol, a D2 antagonist and a D3 agonist, compounds having antipsychotic efficacy. In contrast, a D4 antagonist, which has not proven clinically effective, was not effective in blocking DA-promoted bursts. These results revealed excitatory effects of DA mediated mainly via D2 receptors, potentially providing a cellular mechanism for the D2 antagonism in treating schizophrenia.     

GLP-1 neurons in the nucleus of the solitary tract project directly to the ventral tegmental area and nucleus accumbens to control for food intake

Central glucagon-like-peptide-1 (GLP-1) receptor activation reduces food intake; however, brain nuclei and mechanism(s) mediating this effect remain poorly understood. Although central nervous system GLP-1 is produced almost exclusively in the nucleus of the solitary tract in the hindbrain, GLP-1 receptors (GLP-1R) are expressed throughout the brain, including nuclei in the mesolimbic reward system (MRS), e.g. the ventral tegmental area (VTA) and the nucleus accumbens (NAc). Here, we examine the MRS as a potential site of action for GLP-1-mediated control of food intake and body weight. Double immunohistochemistry for Fluorogold (monosynaptic retrograde tracer) and GLP-1 neuron immunoreactivity indicated that GLP-1-producing nucleus tractus solitarius neurons project directly to the VTA, the NAc core, and the NAc shell. Pharmacological data showed that GLP-1R activation in the VTA, NAc core, and NAc shell decreased food intake, especially of highly-palatable foods, and body weight. Moreover, blockade of endogenous GLP-1R signaling in the VTA and NAc core resulted in a significant increase in food intake, establishing a physiological relevance for GLP-1 signaling in the MRS. Current data highlight these nuclei within the MRS as novel sites for GLP-1R-mediated control of food intake and body weight.     

Ethanol increases glutamate neurotransmission in the posterior ventral tegmental area of female wistar rats

Background: The posterior ventral tegmental area (pVTA) mediates the reinforcing and stimulating effects of ethanol (EtOH). Electrophysiological studies indicated that exposure to EtOH increased glutamate synaptic function in the VTA. This study determined the neurochemical effects of both acute and repeated EtOH exposure on glutamate neurotransmission in the pVTA. Methods: Adult female Wistar rats were implanted with microdialysis probes in the pVTA. During microdialysis, rats received acute intraperitoneal (i.p.) injection of saline or EtOH (0.5, 1.0, or 2.0 g/kg), and extracellular glutamate levels were measured in the pVTA. The effects of repeated daily injections of EtOH (0.5, 1.0, or 2.0 g/kg) on basal extracellular glutamate concentrations in the pVTA and on glutamate response to a subsequent EtOH challenge were also examined. Results: The injection of 0.5 g/kg EtOH significantly increased (120 to 125% of baseline), whereas injection of 2.0 g/kg EtOH significantly decreased (80% of baseline) extracellular glutamate levels in the pVTA. The dose of 1.0 g/kg EtOH did not alter extracellular glutamate levels. Seven repeated daily injections of each dose of EtOH increased basal extracellular glutamate concentrations (from 4.1 ± 0.5 to 9.2 ± 0.5 μM) and reduced glutamate clearance in the pVTA (from 30 ± 2 to 17 ± 2%), but failed to alter glutamate response to a 2.0 g/kg EtOH challenge. Conclusions: The results suggest that the low dose of EtOH can stimulate the release of glutamate in the pVTA, and repeated EtOH administration increased basal glutamate transmission in the pVTA, as a result of reduced glutamate clearance.