The role of connexin-36 gap junctions in alcohol intoxication and consumption

Ventral tegmental area (VTA) GABA neurons appear to be critical substrates underlying the acute and chronic effects of ethanol on dopamine (DA) neurotransmission in the mesocorticolimbic system implicated in alcohol reward. The aim of this study was to examine the role of midbrain connexin‐36 (Cx36) gap junctions (GJs) in ethanol intoxication and consumption. Using behavioral, molecular, and electrophysiological methods, we compared the effects of ethanol in mature Cx36 knockout (KO) mice and age‐matched wild‐type (WT) controls. Compared to WT mice, Cx36 KO mice exhibited significantly more ethanol‐induced motor impairment in the open field test, but less disruption in motor coordination in the rotarod paradigm. Cx36 KO mice, and WT mice treated with the Cx36 antagonist mefloquine (MFQ), consumed significantly less ethanol than their WT controls in the drink‐in‐the‐dark procedure. The firing rate of VTA GABA neurons in WT mice was inhibited by ethanol with an IC₅₀ of 0.25 g/kg, while VTA GABA neurons in KO mice were significantly less sensitive to ethanol. Dopamine neuron GABA‐mediated sIPSC frequency was reduced by ethanol (30 mM) in WT mice, but not affected in KO mice. Cx36 KO mice evinced a significant up‐regulation in DAT and D2 receptors in the VTA, as assessed by quantitative RT‐PCR. These findings demonstrate the behavioral relevance of Cx36 GJ‐mediated electrical coupling between GABA neurons in mature animals, and suggest that loss of coupling between VTA GABA neurons results in disinhibition of DA neurons, a hyper‐DAergic state and lowered hedonic valence for ethanol consumption. Synapse, 2011. © 2010 Wiley‐Liss, Inc.     

Long-term reduction in ventral tegmental area dopamine neuron population activity following repeated stimulant or ethanol treatment.

BACKGROUND: Drugs of abuse exert profound effects on the mesolimbic/mesocortical dopaminergic (DA) systems. Few studies have investigated the long-term adaptations in ventral tegmental area (VTA) DA neuron activity after repeated exposure to drugs of abuse. We investigated changes in the electrical activity of VTA DA neurons after cessation from treatment with several stimulants and ethanol. METHODS: Adult rats were treated with stimulants (amphetamine: 2 mg/kg per day, 5 days/week, 2 weeks; cocaine: 15 mg/kg per day, 5 days/week, 2 weeks; nicotine: .5 mg/kg per day, 5 days; ethanol: 10 g/kg per day, 3 weeks) and the single-unit activity of VTA DA neurons was studied in vivo 3 to 6 weeks later. RESULTS: Stimulant and ethanol treatment decreased basal VTA DA neuron population activity but not firing rate or firing pattern. This effect was reversed by acute apomorphine, suggesting that the underlying mechanism for reduced population activity was depolarization inactivation. Anesthesia did not confound this result, as similar effects were observed in amphetamine-treated rats recorded in a conscious preparation. CONCLUSIONS: Reduced basal VTA DA neuron population activity presumably due to depolarization inactivation is a common and long-term neuroadaptation to repeated treatment with stimulants and ethanol. This change in VTA DA neuron activity could underlie the persistent nature of addiction-associated behaviors.     

Electrophysiological characterization of dopamine neuronal activity in the ventral tegmental area across the light–dark cycle

Direct evidence that dopamine (DA) neurotransmission varies during the 24 h of the day is lacking. Here, we have characterized the firing activity of DA neurons located in the ventral tegmental area (VTA) using single‐unit extracellular recordings in anesthetized rats kept on a standard light–dark cycle. DA neuronal firing activity was measured under basal conditions and in response to intravenous administration of increasing doses of amphetamine (AMPH: 0.5, 1, 2, 5 mg/kg), apomorphine (APO: 25, 50, 100, 200 µg/kg) and melatonin (MLT: 0.1, 1, 10 mg/kg) at different time intervals of the light–dark cycle. DA firing activity peaked between 07:00 and 11:00 h (3.5 ± 0.3 Hz) and between 19:00 and 23:00 h (4.1 ± 0.7 Hz), with lowest activity occurring between 11:00 and 15:00 h (2.4 ± 0.2 Hz) and between 23:00 and 03:00 h (2.6 ± 0.2 Hz). The highest number of spontaneously active neurons was observed between 03:00 and 06:00 h (2.5 ± 0.3 neurons/track), whereas the lowest was between 19:00 and 23:00 h (1.5 ± 0.2 neurons/track). The inhibitory effect of AMPH on DA firing rate was similar in both phases. The inhibitory effect of low dose of APO (25 μg/kg, dose selective for D₂ autoreceptor) was more potent in the dark phase, whereas APO effects at higher doses were similar in both phases. Finally, MLT administration (1 mg/kg) produced a moderate inhibition of DA cell firing in both phases. These experiments demonstrate the existence of an intradiurnal rhythmic pattern of VTA DA neuronal firing activity and a higher pharmacological response of D₂ autoreceptors in the dark phase. Synapse 68:454–467, 2014 . © 2014 Wiley Periodicals, Inc.     

Mefloquine effects on ventral tegmental area dopamine and GABA neuron inhibition: a physiologic role for connexin-36 GAP junctions

Connexin‐36 (Cx36) gap junctions (GJs) appear to be involved in the synchronization of GABA interneurons in many brain areas. We have previously identified a population of Cx36‐connected ventral tegmental area (VTA) GABA neurons that may regulate mesolimbic dopamine (DA) neurotransmission, a system implicated in reward from both natural behaviors and drugs of abuse. The aim of this study was to determine the effect mefloquine (MFQ) has on midbrain DA and GABA neuron inhibition, and the role Cx36 GJs play in regulating midbrain VTA DA neuron activity in mice. In brain slices from adolescent wild‐type (WT) mice the Cx36‐selective GJ blocker mefloquine (MFQ, 25 μM) increased VTA DA neuron sIPSC frequency sixfold, and mIPSC frequency threefold. However, in Cx36 KO mice, MFQ only increased sIPSC and mIPSC frequency threefold. The nonselective GJ blocker carbenoxolone (CBX, 100 μM) increased DA neuron sIPSC frequency twofold in WT mice, did not affect Cx36 KO mouse sIPSCs, and did not affect mIPSCs in WT or Cx36 KO mice. Interestingly, MFQ had no effect on VTA GABA neuron sIPSC frequency. We also examined MFQ effects on VTA DA neuron firing rate and current‐evoked spiking in WT and Cx36 KO mice, and found that MFQ decreased WT DA neuron firing rate and current‐evoked spiking, but did not alter these measures in Cx36 KO mice. Taken together these findings suggest that blocking Cx36 GJs increases VTA DA neuron inhibition, and that GJs play in key role in regulating inhibition of VTA DA neurons. Synapse, 2011. © 2011 Wiley‐Liss, Inc.     

Novel in vivo electrophysiological assay for the effects of cocaine and putative "cocaine antagonists" on dopamine transporter activity of substantia nigra and ventral tegmental area dopamine neurons

The aim of these studies was to establish a rapid in vivo assay for evaluating potential "cocaine antagonists," i.e., drugs postulated to block cocaine binding to the dopamine transporter (DAT) without corresponding blockade of dopamine reuptake. The assay is based on the ability of dopamine, and drugs that elevate synaptic dopamine levels, to inhibit the extracellular single unit activities of midbrain dopamine neurons in chloral hydrate-anesthetized rats. As expected, cocaine itself (0.06-16 mg/kg, i.v.) caused a dose-dependent inhibition of firing of both substantia nigra and ventral tegmental area (VTA) dopamine neurons, but had a significantly higher potency on VTA than nigral dopamine cells (ED(50)'s 1.2 and 8.8 mg/kg, respectively). VTA cells were also inhibited to a greater extent (to 4.7 +/- 4.5% vs. 41.3 +/- 6.3% of baseline rates at 16 mg/kg, respectively). We next evaluated GBR12909, a piperazine analog promoted as a "cocaine antagonist" because of its ability to bind with high affinity to the DAT, while only modestly elevating extracellular dopamine levels. The agonist- and antagonist-like properties of GBR12909 were evaluated on only VTA dopamine cells since these neurons were more fully inhibited by cocaine and have been implicated in its rewarding effects. Given alone, GBR12909 exhibited modest "cocaine-like" activity insofar as it partially inhibited VTA dopamine neurons (to 59.0 +/- 4.6% of baseline at 8 mg/kg). However, consistent with an antagonist profile, pretreatment with a low (0.5 mg/kg) dose of GBR12909, which depressed firing only slightly, resulted in a >2-fold rightward shift in the dose-response curve to cocaine (ED(50) 2.6 mg/kg). We conclude that electrophysiological testing of putative "anti-cocaine" drugs for their abilities to inhibit the firing of VTA dopamine neurons, and to block their inhibitory responses to cocaine, may provide a rapid in vivo screen for compounds expected to behave as functional cocaine antagonists in the dopamine reward system.     

Contingent and non-contingent effects of heroin on mu-opioid receptor-containing ventral tegmental area GABA neurons

Opiate activation of mu-opioid receptors (muORs) in the ventral tegmental area (VTA) modulates gamma-aminobutyric acid (GABA) neurotransmission within the mesocorticolimbic dopamine (DA) reward system. We combined in vivo extracellular electrophysiological recordings in anesthetized and freely behaving rats with intracellular Neurobiotin filling and immunocytochemistry to characterize the effects of opiates on VTA GABA neurons, evaluate their discharge activity during opiate self-administration, and identify the cellular sites for opiate activation. We identified a subpopulation of VTA GABA neurons that was characterized by location, spike discharge profile, activation by microelectrophoretic DA, and response to internal capsule (IC) stimulation. Systemic administration of heroin or microelectrophoretic application of the selective muOR agonist [d-Ala2, N-Me-Phe4, Gly-ol]-Enkephalin (DAMGO) reduced VTA GABA neuron firing rate (heroin IC(50) = 0.35 mg/kg) and was blocked by the muOR antagonist naloxone. Heroin also reduced IC-evoked post-stimulus spike discharges, a manifestation of gap-junction-mediated electrical coupling between VTA GABA neurons. The baseline firing rate of VTA GABA neurons significantly increased (239%) following the acquisition of heroin self-administration behavior and transiently increased during each response for heroin (105%), but decreased (49%) following heroin, similar to non-contingent heroin. Electrophysiologically characterized VTA GABA neurons were filled with Neurobiotin and labeled dendrites contained plasmalemmal muOR immunoreactivity. Dually labeled muOR dendrites contained dendrodendritic appositions characteristic of gap junctions. These findings indicate that inhibition of this population of GABAergic neurons by opiates acting on dendritic muORs has implications for modulation of electrical coupling between VTA GABA neurons and dopamine (DA) neurotransmission in the VTA and terminal field regions.     

D1 dopamine receptor stimulation inhibits firing activity of midbrain dopamine neurons in reserpine treated rats: An effect eliminated after hemitransection of diencephalon

It has been reported that systemic administration of the D₁ dopamine (DA) receptor agonist SKF 38393 inhibits the firing rate of substantia nigra pars compacta (SNC, A9) DA neurons after repeated reserpine treatment in locally anesthetized rats, although SKF 38393 induces little effect on the firing of midbrain DA neurons in normal rats. The present study found that local pressure microejection of SKF 38393 (10⁻² M, 20–100 nl) to SNC or substantia nigra pars reticulata (SNR) failed to influence the firing of SNC DA neurons in reserpinized rats (reserpine 1 mg/kg × 6 days, s.c.); subsequent intravenous (i.v.) injection of SKF 38393 (4 mg/kg), however, inhibited their firing and the inhibition was reversed by the D₁ receptor antagonist SCH 23390. Similarly, systemic administration of SKF 38393 (4 mg/kg, i.v.) inhibited the firing of ventral tegmental area (VTA, A10) DA cells in reserpinized rats, while local microejection of SKF 38393 (10⁻² M, 30–60 nl) did not affect their firing. Furthermore, the inhibitory effect of systemic SKF 38393 on firing rate of either SNC or VTA DA neurons in reserpinized rats was eliminated after hemitransection of diencephalon. These results suggest that repeated reserpine treatment renders midbrain DA neurons responsive to D₁ receptor stimulation and that D₁ receptor agonist-induced inhibition of midbrain DA cell firing in reserpinized rats may require the involvement of long-loop feedback pathways. © 1996 Wiley-Liss, Inc.     

The effect of the acute and chronic administration of CP 96,345, a selective neurokinin1 receptor antagonist,on midbrain dopamine neurons in the rat: A single unit,extracellular recording study

In this study, we examined the effect of acute and chronic administration of the selective neurokinin₁ receptor antagonist CP 96,345 on the basal activity of spontaneously active dopamine (DA) neurons in the substantia nigra pars compacta (SNC) and the ventral tegmental area (VTA). This was accomplished using the technique of in vivo, extracellular single unit recording in anesthetized rats. The intravenous (i.v.) administration of CP 96,345 (0.01–1.28 mg/kg) did not significantly alter the firing rate of spontaneously active DA neurons in the SNC and VTA areas. The acute administration of 5 or 10 mg/kg, i.p., of CP 96,345 produced a significant decrease in the number of spontaneously active SNC and VTA dopamine cells compared to vehicle-treated rats. In contrast to its effect on the number of spontaneously active DA neurons, the administration of 5 mg/kg, i.p., of CP 96,345 did not significantly alter the basal firing pattern of either SNC or VTA DA neurons. The acute administration of CP 96,345 (10 mg/kg, i.p.) significantly potentiated the suppressant action of (+)-apomorphine on the basal firing rate of spontaneously active SNC and VTA DA cells. The chronic administration of CP 96,345 (5 or 10 mg/kg, i.p.) for 21 days also produced a significant decrease in the number of spontaneously active SNC and VTA DA cells compared to vehicle controls. This effect was not reversed by the systemic administration of (+)-apomorphine (50 μg/kg, i.v.), suggesting that the reduction in the number of spontaneously active DA cells produced by CP 96,345 is probably not the result of depolarization inactivation. Overall, our results indicate that the tonic activation of NK₁ receptors by substance P may be necessary to maintain the spontaneous activity of a proportion of midbrain DA neurons. © 1996 Wiley-Liss, Inc.     

GSK‐3β inhibitors reverse cocaine‐induced synaptic transmission dysfunction in the nucleus accumbens

Nucleus accumbens receives glutamatergic projection from the prefrontal cortex (PFC) and dopaminergic input from the Ventral tegmental area (VTA). Recent studies have suggested a critical role for serine/threonine kinase glycogen synthase kinase 3β (GSK3β) in cocaine‐induced hyperactivity; however, the effect of GSK3β on the modulation of glutamatergic and dopaminergic afferents is unclear. In this study, we found that the GSK3 inhibitors, LiCl (100 mg/kg, i.p.) or SB216763 (2.5 mg/kg, i.p.), blocked the cocaine‐induced hyperlocomotor activity in rats. By employing single‐unit recordings in vivo, we found that pretreatment with either SB216763 or LiCl for 15 min reversed the cocaine‐inhibited firing frequency of medium spiny neuron (MSN) in the nucleus accumbens (NAc). Preperfusion of SB216763 (5 μM) ameliorated the inhibitory effect of cocaine on both the α‐Amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA) (up to 99 ± 6.8% inhibition) and N‐methyl‐D‐aspartic acid receptor (NMDAR)‐mediate EPSC (up to 73 ± 9.7% inhibition) in the NAc in brain slices. The effect of cocaine on AMPA and NMDA receptor‐mediate excitatory postsynaptic current (EPSC) were mimicked by the D₁‐like receptor agonist SKF 38393 and blocked by the D₁‐like receptor antagonist SCH 23390, whereas D₂‐like receptor agonist or antagonist failed to mimic or to block the action of cocaine. Preperfusion of SB216763 for 5 min also ameliorated the inhibitory effect of SKF38393 on both AMPA and NMDA receptor‐mediated components of EPSC, indicate the effect of SB216763 on cocaine was via the D₁‐like receptor. Moreover, cocaine inhibited the presynaptic release of glutamate in the NAc, and SB216763 reversed this effect. In conclusion, D₁ receptor–GSK3β pathway, which mediates glutamatergic transmission in the NAc core through a presynaptic mechanism, plays an important role in acute cocaine‐induced hyperlocomotion.     

Acute and repeated administration of the selective 5-HT(2A) receptor antagonist M100907 significantly alters the activity of midbrain dopamine neurons: an in vivo electrophysiological study

We examined the effect of the acute and repeated administration of M100907 (formerly MDL 100907), a selective 5-HT(2A) receptor antagonist, on spontaneously active dopamine (DA) neurons in the substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) of rats. This was accomplished using in vivo, extracellular single unit recording. The i.v. administration of M100907 (0.01-0.64 mg/kg) did not significantly alter the basal firing rate or pattern of spontaneously active SNC and VTA DA neurons. A single injection of either 0.01 or 0.03 mg/kg i.p. of M100907 did not significantly alter the number of spontaneously active DA neurons in either the SNC or VTA areas. However, 0.1 mg/kg i.p. of M100907 significantly increased the number of spontaneously active SNC and VTA DA neurons compared to vehicle-treated animals. A single injection of all doses of M100907 significantly decreased the degree of bursting in VTA DA neurons, whereas the 0.1 mg/kg dose increased the degree of bursting in SNC DA neurons. The repeated administration (one injection per day for 21 days) of 0.03 and 0.1 mg/kg i.p. of M100907 produced a significant decrease in the number of spontaneously active SNC and VTA DA neurons compared to vehicle-treated animals. The repeated administration of M100907 did not significantly alter the firing pattern of VTA DA neurons but significantly altered the firing pattern of SNC DA neurons. The results of this study indicate that M100907 administration alters the activity of midbrain DA neurons in anesthetized rats.     

Corticotropin‐releasing factor type‐2 receptor and corticotropin‐releasing factor‐binding protein coexist in rat ventral tegmental area nerve terminals originated in the lateral hypothalamic area

There is significant functional evidence showing that corticotropin‐releasing factor type‐2 receptor (CRF₂R) and corticotropin‐releasing factor‐binding protein (CRF‐BP) regulate glutamatergic synapses onto ventral tegmental area (VTA) dopaminergic neurons. It has been shown that CRF requires CRF‐BP to potentiate N‐methyl‐d ‐aspartate receptors in dopaminergic neurons through CRF₂R, and that increases glutamate release in cocaine‐treated rats through the activation of CRF₂R only by agonists with high affinity to CRF‐BP. Furthermore, this CRF‐mediated increase in VTA glutamate is responsible for stress‐induced relapse to cocaine‐seeking behaviour. However, there is a lack of anatomical evidence to explain the mechanisms of CRF actions in VTA. Thus, it was studied whether CRF₂R and CRF‐BP are expressed in VTA nerve terminals, using a synaptosomal preparation devoid of postsynaptic elements. The current results show that both proteins are co‐expressed in glutamatergic and γ‐aminobutyric acid (GABA)ergic VTA synaptosomes. A main glutamatergic input to the VTA that has been associated to addictive behaviour is originated in the lateral hypothalamic area (LHA). Thus, this study was focused in the LHA–VTA input using orexin as a marker of this input. The results show that CRF₂R and CRF‐BP mRNA and protein are expressed in the LHA, and that both proteins are present in orexin‐positive VTA synaptosomes. The results showing that CRF₂R and CRF‐BP are expressed in the LHA–VTA input give anatomical support to suggest that this input plays a role in stress‐induced relapse to cocaine‐seeking behaviour.     

Effect of acute and chronic administration of the selective 5-HT2C receptor antagonist SB-243213 on midbrain dopamine neurons in the rat: an in vivo extracellular single cell study

In this study, we examined the effect of the acute and chronic administration of the selective 5-HT2C receptor antagonist SB-243213 (SB) on the activity of spontaneously active dopamine (DA) cells in the substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) in anesthetized, albino, male Sprague-Dawley rats. This was accomplished using the technique of in vivo extracellular single cell recording. The acute i.v. administration of SB-243213 (0.025-3.2 mg/kg) did not significantly alter the basal firing rate or pattern of either spontaneously active SNC or VTA DA neurons compared to vehicle-treated controls. The acute i.p. administration of either 1 or 10 mg/kg of SB-243213 did not significantly alter the number of spontaneously active DA cells in the SNC or VTA compared to vehicle-treated controls, whereas the 3 mg/kg dose only significantly decreased the number of spontaneously active VTA DA neurons. Overall, the 1 mg/kg dose of SB-243213 did not significantly alter the firing pattern of either SNC or VTA DA neurons compared to vehicle-treated controls. In contrast, the 3 mg/kg dose significantly altered the firing pattern of SNC DA neurons, whereas the 10 mg/kg dose altered the firing pattern of DA neurons in both the SNC and VTA. The repeated i.p. administration (21 days) of 1, 3, and 10 mg/kg of SB-243213 or 20 mg/kg of clozapine produced a significant decrease in the number of spontaneously active DA cells in the VTA compared to vehicle-treated controls. The decrease in the number of spontaneously active VTA DA cells was not reversed by the i.v. administration of (+)-apomorphine (50 microg/kg). The repeated administration of either 1 or 3 mg/kg of SB-243213 had minimal effects on the firing pattern of either SNC or VTA DA neurons. In contrast, the firing pattern of VTA DA neurons was significantly altered by 10 mg/kg dose of SB-243213. Overall, our results indicate that antagonism of the 5-HT2C receptor alters the activity of midbrain DA neurons in anesthetized rats and suggest that SB-243213 has an atypical antipsychotic profile following chronic administration.     

The acute and chronic administration of (+/-)-8-hydroxy-2-(Di-n-propylamino)tetralin significantly alters the activity of spontaneously active midbrain dopamine neurons in rats: an in vivo electrophysiological study

This study examined the effect of the acute and chronic systemic administration of (+/-)-8-Hydroxy-2-(Di-n-propylamino)Tetralin(8-OH-DPAT) on the number and firing pattern of spontaneously active dopamine (DA) neurons in the ventral tegmental area (VTA or A10) and substantia nigra pars compacta (SNC or A9) in anesthetized male rats. These parameters were measured using extracellular in vivo electrophysiology. A single s.c. injection of 0.01, 0.1, or 1 mg/kg of 8-OH-DPAT did not significantly alter the number of spontaneously active SNC DA neurons compared to vehicle-treated animals (controls). The acute administration of 0.01 or 0.1 mg/kg of 8-OH-DPAT did not significantly alter, whereas the 1 mg/kg dose significantly decreased the number of spontaneously active VTA DA neurons compared to controls. The acute administration of 8-OH-DPAT significantly increased the percentage of VTA DA neurons firing in a bursting pattern. In contrast, there was a significant decrease in the percentage of SNC DA neurons firing in a bursting pattern following the acute administration of 8-OH-DPAT. The number of spontaneously active SNC DA neurons was not significantly altered by the chronic s.c. administration of 8-OH-DPAT (0.01, 0.1, or 1 mg/kg s.c.) as compared to controls. However, the chronic s.c. administration of all doses of 8-OH-DPAT significantly decreased the number of spontaneously active VTA DA neurons compared to controls. The i.v. administration of (+)-apomorphine (50 microg/kg) did not reverse the 8-OH-DPAT-induced decrease in the number of spontaneously active VTA DA neurons, suggesting that this effect is unlikely due to depolarization blockade. The percentage of VTA DA neurons exhibiting burst firing was significantly increased by 0.01 and 0.1 mg/kg, but significantly decreased by 1 mg/kg of 8-OH-DPAT. Overall, the systemic administration of 8-OH-DPAT preferentially affects the activity of spontaneously active A10 DA neurons in rats.     

Increased burst‐firing of ventral tegmental area dopaminergic neurons in d‐amino acid oxidase knockout mice in vivo

d ‐Amino acid oxidase (DAO) degrades the N‐methyl‐d ‐aspartate (NMDA) receptor co‐agonist d ‐serine, and is implicated in schizophrenia as a risk gene and therapeutic target. In schizophrenia, the critical neurochemical abnormality affects dopamine, but to date there is little evidence that DAO impacts on the dopamine system. To address this issue, we measured the electrophysiological properties of dopaminergic (DA) and non‐DA neurons in the ventral tegmental area (VTA) of anaesthetised DAO knockout (DAO^?/?) and DAO heterozygote (DAO^+/?) mice as compared with their wild‐type (DAO^+/+) littermates. Genotype was confirmed at the protein level by western blotting and immunohistochemistry. One hundred and thirty‐nine VTA neurons were recorded in total, and juxtacellular labelling of a subset revealed that neurons immunopositive for tyrosine hydroxylase had DA‐like electrophysiological properties that were distinct from those of neurons that were tyrosine hydroxylase‐immunonegative. In DAO^?/? mice, approximately twice as many DA‐like neurons fired in a bursting pattern than in DAO^+/? or DAO^+/+ mice, but other electrophysiological properties did not differ between genotypes. In contrast, non‐DA‐like neurons had a lower firing rate in DAO^?/? mice than in DAO^+/? or DAO^+/+ mice. These data provide the first direct evidence that DAO modulates VTA DA neuron activity, which is of interest for understanding both the glutamatergic regulation of dopamine function and the therapeutic potential of DAO inhibitors. The increased DA neuron burst‐firing probably reflects increased availability of d ‐serine at VTA NMDA receptors, but the site, mechanism and mediation of the effect requires further investigation, and may include both direct and indirect processes.     

Effect of the acute and chronic administration of the selective neurokinin2 receptor antagonist SR 48968 on midbrain dopamine neurons in the rat: An in vivo extracellular single cell study

In this study, we examined the effect of the acute and chronic administration of the selective neurokinin₂ (NK₂) receptor antagonist SR 48968 on the activity of spontaneously active dopamine (DA) cells in the substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) in anesthetized, male rats. This was accomplished using the technique of in vivo, extracellular single cell recording. The intravenous (i.v.) administration of SR 48968 (10–1280 μg/kg) did not significantly alter the basal firing rate or pattern of either spontaneously active SNC or VTA DA neurons compared to control. However, the acute administration of 1 mg/kg, i.p., of SR 48968, but not its inactive enantiomer SR 48965, produced a significant increase in the number of spontaneously active DA cells in the SNC (48%) and VTA (28%) compared to vehicle controls. The i.p. administration of SR 48968 did not alter the basal firing pattern of either SNC or VTA DA neurons compared to vehicle controls. The pretreatment of animals with 1 mg/kg, i.p., of SR 48968 significantly potentiated the suppressant action of (+)-apomorphine on spontaneously active SNC and VTA DA cells. In contrast to its acute effects, the administration of 1 mg/kg, i.p., of SR 48968 once daily for 21 days produced a significant decrease in the number of spontaneously active DA cells in the SNC and VTA. The decrease in the number of spontaneously active VTA DA cells was not reversed by (+)-apomorphine administration; in fact, a further decrease in the number of VTA DA cells was observed. This suggests that the SR 48968-induced decrease in the number of spontaneously active DA neurons may not be the result of depolarization block. Overall, these results suggest that the acute and chronic administration of SR 48968 alters the number of spontaneously active midbrain DA neurons in anesthetized rats. Synapse 25:196–204, 1997. © 1997 Wiley-Liss, Inc.     

l‐Stepholidine‐induced excitation of dopamine neurons in rat ventral tegmental area is associated with its 5‐HT1A receptor partial agonistic activity

Rationale: l‐Stepholidine (l‐SPD), a tetrahydroprotoberberine alkaloid, possesses a pharmacological profile of a D₁/5‐HT_1A agonist and a D₂ antagonist. This unique pharmacological profile makes it a promising novel antipsychotic candidate. Preliminary clinical trials and animal experiments suggest that l‐SPD improves both positive and negative symptoms of schizophrenia without producing significant extrapyramidal side effects. To further explore the antipsychotic mechanisms of the drug, we studied the effects of l‐SPD on the activity of dopamine (DA) neurons in the ventral tegmental area (VTA) using in vivo single‐unit recording technique in rats. Result: We found that l‐SPD increased VTA DA neurons firing rate and induced slow oscillation in firing pattern. Moreover, l‐SPD, not clozapine, reversed d‐amphetamine‐induced inhibition which induced an excitation of VTA DA neurons. Furthermore, our data indicated that the excitatory effect of l‐SPD is associated with its partial agonistic action for the 5‐HT_1A receptor since the 5‐HT_1A receptor antagonist WAY100635 could block the l‐SPD‐induced excitatory effect. However, activation of 5‐HT_1A receptor alone by specific agonist (±)‐8‐Hydroxy‐2‐(dipropylamino) tetralin (8‐OH‐DPAT) was insufficient to elicit excitation of VTA DA neurons, but the excitation of 8‐OH‐DPAT on VTA DA neurons was elicited in the presence of D₂‐like receptors antagonist raclopride. Collectively, these results indicate that l‐SPD excited VTA DA neurons requiring its D₂‐like receptors antagonistic activity and 5‐HT_1A receptor agonistic activity. Conclusion: The present data demonstrate that D₂ receptor antagonist/5‐HT_1A receptor agonistic dual properties modulate dopaminergic transmission in a unique pattern that may underlie the different therapeutic responses between l‐SPD and other atypical antipsychotic drugs. Synapse, 2010. © 2010 Wiley‐Liss, Inc.     

GABAB-Receptor activation alters the firing pattern of dopamine neurons in the rat substantia nigra

Previous electrophysiological experiments have emphasized the importance of the firing pattern for the functioning of midbrain dopamine (DA) neurons. In this regard, excitatory amino acid receptors appear to constitute an important modulatory control mechanism. In the present study, extracellular recording techniques were used to investigate the significance of GABA_B-receptor activation for the firing properties of DA neurons in the substantia nigra (SN) in the rat. Intravenous administration of the GABA_B-receptor agonist baclofen (1–16 mg/kg) was associated with a dose-dependent regularisation of the firing pattern, concomitant with a reduction in burst firing. At higher doses (16–32 mg/kg), the firing rate of the DA neurons was dose-dependently decreased. Also, microiontophoretic application of baclofen regularized the firing pattern of nigral DA neurons, including a reduction of burst firing. Both the regularisation of the firing pattern and inhibition of firing rate produced by systemic baclofen administration was antagonized by the GABA_B-receptor antagonist CGP 35348 (200 mg/kg, l.v.). The GABA_A-receptor agonist muscimol produced effects on the firing properties of DA neurons that were opposite to those observed following baclofen, i.e., an increase in firing rate accompanied by a Cecreased regularity. The NMDA receptor antagonist MK 801 (0.4–3.2 mg/kg, i.v.) produced a moderate, dose-dependent increase in the firing rate of the nigral DA neurons as well as a slightly regularized firing pattern. Pretreatment with MK 801 (3.2 mg/kg, i.v., 3–10 min) did neither promote nor prevent the regularisation of the firing pattern or inhibition of firing rate on the nigral DA neurons produced by baclofen. The present results clearly show that GABA_B-receptors can alter the firing pattern of nigral DA neurons, hereby counterbalancing the previously described ability of glutamate to induce burst Firing activity on these neurons. © 1993 Wiley-Liss, Inc.     

Species‐specific diversity in the anatomical and physiological organisation of the BNST‐VTA pathway

The anteromedial part of the bed nucleus of the stria terminalis (amBNST) is a limbic structure innervating the ventral tegmental area (VTA) that is remarkably constant across species. The amBNST modulates fear and anxiety, and activation of VTA dopamine (DA) neurons by amBNST afferents seems to be the way by which stress controls motivational states associated with reward or aversion. Because fear learning and anxiety states can be expressed differently between rats and mice, we compared the functional connectivity between amBNST and the VTA‐DA neurons in both species using consistent methodological approaches. Using a combination of in vivo electrophysiological, neuroanatomical tracing and laser capture approaches we explored the BNST influences on VTA‐DA neuron activity. First, we characterised in rats the molecular phenotype of the amBNST neurons projecting to the VTA. We found that this projection is complex, including both GABAergic and glutamatergic neurons. Then, VTA injections of a conventional retrograde tracer, the β‐sub‐unit of the cholera toxin (CTB), revealed a stronger BNST‐VTA projection in mice than in rats. Finally, electrical stimulations of the BNST during VTA‐DA neuron recording demonstrated a more potent excitatory influence of the amBNST on VTA‐DA neuron activity in rats than in mice. These data illustrate anatomically, but also functionally, a significant difference between rats and mice in the amBNST‐VTA pathway. More generally, together with previous findings, our research highlights the importance of species differences for the interpretation and the generalisation of research data.