Inhibitory Inputs from Rostromedial Tegmental Neurons Regulate Spontaneous Activity of Midbrain Dopamine Cells and Their Responses to Drugs of Abuse

The rostromedial tegmental nucleus (RMTg), a structure located just posterior to the ventral tegmental area (VTA), is an important site involved in aversion processes. The RMTg contains γ-aminobutyric acid neurons responding to noxious stimuli, densely innervated by the lateral habenula and providing a major inhibitory projection to reward-encoding dopamine (DA) neurons in the VTA. Here, we studied how RMTg neurons regulate both spontaneous firing of DA cells and their response to the cannabinoid agonist WIN55212-2 (WIN), morphine, cocaine, and nicotine. We utilized single-unit extracellular recordings in anesthetized rats and whole-cell patch clamp recordings in brain slices to study RMTg-induced inhibition of DA cells and inhibitory postsynaptic currents (IPSCs) evoked by stimulation of caudal afferents, respectively. The electrical stimulation of the RMTg elicited a complete suppression of spontaneous activity in approximately half of the DA neurons examined. RMTg-induced inhibition correlated with firing rate and pattern of DA neurons and with their response to a noxious stimulus, highlighting that inhibitory inputs from the RMTg strongly control spontaneous activity of DA cells. Both morphine and WIN depressed RMTg-induced inhibition of DA neurons in vivo and IPSCs evoked by RMTg stimulation in brain slices with presynaptic mechanisms. Conversely, neither cocaine nor nicotine modulated DA neuron responses to RMTg stimulation. Our results further support the role of the RMTg as one of the main inhibitory afferents to DA cells and suggest that cannabinoids and opioids might disinhibit DA neurons by profoundly influencing synaptic responses evoked by RMTg activation.     

Nicotinic receptors promote susceptibility to social stress in female mice linked with neuroadaptations within VTA dopamine neurons

There are about twice as many women as men who experience depression during their lifetime. Although life circumstances and especially exposure to stressful situations constitute a major risk factor to develop depression, the underlying mechanisms have yet to be unraveled. We employed the chronic social defeat procedure to elicit depressive-like symptoms in females and ketamine to validate the model. We performed ex-vivo patch clamp recordings to assess cellular adaptations and used pharmacological agents to dissect these deregulations. Chronic social defeat exposure triggers a hyperactivity of VTA putative dopamine (DA) neurons in females susceptible to stress but not resilient ones. This hyperactivity was fully reversed by a single administration of ketamine. In virally-identified brain circuits of both susceptible and resilient females, we found a hypercholinergic tone to the VTA arising from the laterodorsal tegmentum. Application of puffs of nicotine revealed a decreased sensitivity of DA neurons in resilient mice when compared to naive or susceptible ones. The in vivo acute administration of the positive allosteric modulator for α7 nicotinic acetylcholine receptors (nAChRs) not only increased susceptibility to stress by enhancing activity of VTA DA neurons, but also triggered a switch in phenotype from resilient to susceptible. Our data unravel dysregulations of VTA DA neurons activity exclusively in females exhibiting depressive-like symptoms and identify VTA nAChRs as key molecular substrates that exacerbate susceptibility to stress.Subject terms: Stress and resilience, Reward     

咖啡因对急性分离的大鼠背根神经节细胞H-电流的影响

目的　探讨咖啡因在镇痛中的作用机制。方法　应用膜片钳技术研究了咖啡因对大鼠的 3种不同直径大小的背根神经节 (DRG)细胞H 电流特性的影响。结果　 5 .0mmol·L- 1咖啡因对DRG大细胞、中细胞和小细胞的H 电流分别降低了 (80±4 ) % ,(4 0± 3) %和 (11.0± 2 .3) %。结论　咖啡因使H 通道的电压敏感性下降 ,H 通道的激活电压升高。咖啡因对大型和中型DRG细胞H 电流的抑制作用 ,可能是发挥抗伤害性感受和用作镇痛佐剂的作用机制之一     

Functional impact of the hyperpolarization‐activated current on the excitability of myelinated A‐type vagal afferent neurons in the rat

1. The hyperpolarization‐induced, cation‐selective current I_h is widely observed in peripheral sensory neurons of the vagal and dorsal root ganglia, but the peak magnitude and voltage‐ and time‐dependent properties of this current vary widely across afferent fibre type. 2. Using patch clamp investigations of rat isolated vagal ganglion neurons (VGN) identified as myelinated A‐type afferents, we established a compendium of functional correlates between changes in membrane potential and the dynamic discharge properties of these sensory neurons as a result of the controlled recruitment of I_h using the current clamp technique. 3. Two robust reponses were observed in response to hyperpolarizing step currents: (i) upon initiation of the negative step current, there was a rapid hyperpolarization of membrane potential followed by a depolarizing voltage sag (DVS) towards a plateau in membrane potential as a result of steady state recruitment of I_h; and (ii) upon termination of the negative step current, there was a rapid return to the pretest resting membrane potential that often led to spontaneous action potential discharge. These data were strongly correlated (r² > 0.9) with a broad compendium of dynamic discharge characteristics in these A‐type VGN. 4. In response to depolarizing step currents of increasing magnitude, the discharge frequency of the A‐type VGN responded with increases in the rate of sustained repetitive discharge. Upon termination of the depolarizing step current, there was a post‐excitatory membrane hyperpolarization of a magnitude that was strongly correlated with action potential discharge rate (r² > 0.9). 5. Application of the selective hyperpolarization‐activated cyclic nucleotide gated (HCN) channel blockers ZD7288 (10 μmol/L) or CsCl (1.0 mmol/L) abolished I_h and all of the aforementioned functional correlates. In addition to reducing the excitability of the A‐type VGN to step depolarizing currents. 6. Because there is increasing evidence that the HCN channel current may represent a valid target for pharmacological intervention, the quantitative relationships described in the present study could potentially help guide the molecular and/or chemical modification of HCN channel gating properties to effect a particular outcome in VGN discharge properties, ideally well beyond merely selective blockade of a particular HCN channel subtype.     

Electrophysiological properties of dopamine neurons in the ventral tegmental area of Sardinian alcohol-preferring rats

Rationale  Sardinian alcohol-preferring (sP) or -nonpreferring (sNP) rats are one of the few pairs of lines of rats selectively bred for their voluntary alcohol preference or aversion, respectively. Ventral tegmental area (VTA) dopamine (DA) neurons have long been implicated in many drug-related behaviors, including alcohol self-administration. However, the electrophysiological properties of these cells in sP and sNP rats remain unknown. Objectives  This study was designed to examine the properties of posterior VTA DA neurons and to unveil functional differences between sP and sNP rats. Materials and methods  The electrophysiological properties of DA cells were examined performing either single-cell extracellular recordings in anesthetized rats or whole-cell patch-clamp recordings in slices. Results  Extracellular single-unit recordings revealed an increased spontaneous activity in sP rats. However, a corresponding difference was not found in vitro. Moreover, DA cells of sP and sNP rats showed similar intrinsic properties, suggesting changes at synaptic level. Therefore, inhibitory- and excitatory-mediated currents were studied. A decreased probability of GABA release was found in sP rats. Additionally, sP rats showed a reduced depolarization-induced suppression of inhibition, which is an endocannabinoid-mediated form of short-term plasticity. Additionally, the effect of cannabinoid-type 1 (CB1) receptor agonist WIN55,212-2 on GABA_A IPSCs was smaller in sP rats, suggesting either a reduced number or functionality of CB1 receptors in the VTA. Conclusions  Our findings suggest that both decreased GABA release and endocannabinoid transmission in the VTA play a role in the increased impulse activity of DA cells and, ultimately, in alcohol preference displayed by sP rats.     

Hyperpolarization-activated,cyclic nucleotide-gated (HCN) channels in the regulation of midbrain dopamine systems

Hyperpolarization-activated, cyclic nucleotide-gated channels (HCN channels) are expressed widely in the brain and invovled in various neuronal activities, including the control of neuronal rhythmic activity, setting the resting membrane potential, as well as dendritic integration. HCN channels also participate in the regulation of spontaneous activity of midbrain dopamine (DA) neurons to some extent. In slice preparations of midbrain, a hyperpolarization-activated non-selective cation current (Ih) mediated by the channels has been proposed as an electrophysiological marker to identify DA neurons. Recent evidence, however, shows that the functional roles of HCN channels in midbrain DA neurons are obviously underestimated. Here, we review the recent advances in the studies of the functional roles of Ih in midbrain DA neurons and further, their involvement in drug addiction and Parkinson''s disease.     

Enhancing excitability of dopamine neurons promotes motivational behaviour through increased action initiation

Motivational deficits are a key symptom in multiple psychiatric disorders, including major depressive disorder, schizophrenia and addiction. A likely neural substrate for these motivational deficits is the brain dopamine (DA) system. In particular, DA signalling in the nucleus accumbens, which originates from DA neurons in the ventral tegmental area (VTA), has been identified as a crucial substrate for effort-related and activational aspects of motivation. Unravelling how VTA DA neuronal activity relates to motivational behaviours is required to understand how motivational deficits in psychiatry can be specifically targeted. In this study, we therefore used designer receptors exclusively activated by designer drugs (DREADD) in TH:Cre rats, in order to determine the effects of chemogenetic DA neuron activation on different aspects of motivational behaviour. We found that chemogenetic activation of DA neurons in the VTA, but not substantia nigra, significantly increased responding for sucrose under a progressive ratio schedule of reinforcement. More specifically, high effort exertion was characterized by increased initiations of reward-seeking actions. This effect was dependent on effort requirements and instrumental contingencies, but was not affected by sucrose pre-feeding. Together, these findings indicate that VTA DA neuronal activation drives motivational behaviour by facilitating action initiation. With this study, we show that enhancing excitability of VTA DA neurons is a viable strategy to improve motivational behaviour.     

Spontaneous firing properties of rat medial vestibular nucleus neurons in brain slices by infrared visual patch clamp technique

Domestic application of infrared patch clamp techniques on brain slices is limited. The key of the technique is to prepare high-quality brain slices. The present paper describes the preparation procedure of brainstem slices and the spontaneous firing properties of rat medial vestibular nucleus (MVN) neurons. By infrared differential interference contrast technique, neurons of rat MVN were visualized directly at the depth of 50–100 μm underneath the surface of slices. Firing activities of MVN neurons were recorded by the whole-cell patch clamp technique in artificial cerebrospinal fluid (ACSF) and low Ca²⁺-high Mg²⁺ fluid. The firing mode was more irregular and depressive in low Ca²⁺-high Mg²⁺ fluid than in ACSF. According to the averaged waveform of action potentials, cells were classified as the neurons with monophasic after-hyperpolarization potential (AHP), and the neurons with biphasic AHP. The resting membrane potential (RMP), input resistance (Rin) and membrane capacitance (Cm) of neurons were recorded and compared between groups. With infrared videomicroscopy, patch clamp recordings could be made under direct observation in freshly prepared brainstem slices. The discharge activities of MVN neurons were spontaneous and the firing mode was modulated by extracellular calcium concentration. The basic membrane properties of two types of neurons were not significantly different, while the differences in waveform might play a role in the segregation between tonic and kinetic cells.     

Decreased Cocaine Motor Sensitization and Self-Administration in Mice Overexpressing Cannabinoid CB2 Receptors

The potential involvement of the cannabinoid CB₂ receptors (CB₂r) in the adaptive responses induced by cocaine was studied in transgenic mice overexpressing the CB₂r (CB₂xP) and in wild-type (WT) littermates. For this purpose, the acute and sensitized locomotor responses to cocaine, conditioned place preference, and cocaine intravenous self-administration were evaluated. In addition, we assessed whether CB₂r were localized in neurons and/or astrocytes, and whether they colocalized with dopamine D1 and D2 receptors (D1Dr and D2Dr). Dopamine (DA) extracellular levels in the nucleus accumbens (NAcc), and gene expression of tyrosine hydroxylase (TH) and DA transporter (DAT) in the ventral tegmental area (VTA), and μ-opioid and cannabinoid CB₁ receptors in the NAcc were also studied in both genotypes. CB₂xP mice showed decreased motor response to acute administration of cocaine (10–20 mg/kg) and cocaine-induced motor sensitization compared with WT mice. CB₂xP mice presented cocaine-induced conditioned place aversion and self-administered less cocaine than WT mice. CB₂r were found in neurons and astrocytes and colocalized with D2Dr in the VTA and NAcc. No significant differences in extracellular DA levels in the NAcc were observed between genotypes after cocaine administration. Under baseline conditions, TH and DAT gene expression was higher and μ-opioid receptor gene expression was lower in CB₂xP than in WT mice. However, both genotypes showed similar changes in TH and μ-opioid receptor gene expression after cocaine challenge independently of the pretreatment received. Importantly, the cocaine challenge decreased DAT gene expression to a lesser extent in cocaine-pretreated CB₂xP than in cocaine-pretreated WT mice. These results revealed that CB₂r are involved in cocaine motor responses and cocaine self-administration, suggesting that this receptor could represent a promising target to develop novel treatments for cocaine addiction.     

The regulation of striatal tyrosine hydroxylase

Summary Gamma-hydroxybutyric acid (GHBA) in doses that increased the striatal dopamine (DA) content of rat brain failed to increase the affinity of striatal tyrosine hydroxylase (TH) for its pterdine cofactor or to change the sensitivity of the enzyme to the inhibition by DA. Haloperidol (1 mg/kg) decreased the apparent K _mof striatal TH for the pteridine cofactor. However, when GHBA was injected before haloperidol it prevented the decrease in the apparent K _mof TH, in a dose related manner. In vitro GHBA (10^–4 M) neither changed the stimulation of the striatal adenylyl cyclase by DA nor its inhibition by haloperidol.These results suggest that in striatal dopaminergic terminals the K _mof TH for the pteridine cofactor is regulated by an molecular mechanism which requires that the impulse flow in the DA neurons is unimpaired.     

Differential glutamate AMPA-receptor plasticity in subpopulations of VTA neurons in the presence or absence of residual cocaine: implications for the development of addiction

Cocaine-induced plasticity of mesocorticolimbic dopamine (DA) neurons, originating in the ventral tegmental area (VTA), persists in the absence of cocaine and may contribute to both drug-craving and relapse. Glutamate AMPA receptors (AMPARs) in these neurons are implicated in this plasticity. However, there is no ultrastructural evidence that the absence of cocaine following repeated administrations affects the critical surface/synaptic availability of AMPAR GluR1 subunits in either DA or non-DA, putative GABAergic neurons within the VTA. To assess this, we used electron microscopic immunolabeling in the VTA of adult male mice sacrificed at 30 min or 72 h after receiving the final of six (15 mg/kg) cocaine injections, a dosing paradigm that resulted in development of locomotor sensitization. At each time point, both cocaine- and saline-injected mice showed AMPAR GluR1 immunogold labeling in somatodendritic profiles, many of which contained immunoperoxidase labeling for the DA-synthesizing enzyme, tyrosine hydroxylase (TH). At 30 min after the last injection, when cocaine was systemically present, only the non-TH labeled dendrites showed a significant increase in the synaptic/plasmalemmal density of GluR1 immunogold particles. At 72 h, when systemic cocaine was depleted, synaptic GluR1 labeling was greatly enhanced in TH-containing dendrites throughout the VTA and in non-TH dendrites of the limbic-associated paranigral VTA. Our results demonstrate that systemic cocaine produces GluR1 trafficking specifically in non-DA neurons of the VTA, which may subsequently contribute to the abstinent-induced enhancement of AMPA receptor synaptic transmission in mesocorticolimbic DA neurons leading to heightened drug seeking behavior.This article is part of a Special Issue entitled ‘Synaptic Plasticity and Addiction’.     

Dopamine-related drugs act presynaptically to potentiate GABA(A) receptor currents in VTA dopamine neurons

Electrical activity of ventral tegmental area (VTA) dopamine (DA) neurons is immediately inhibited following in?vivo administration of cocaine and other DA-related drugs. While various forms of synaptic modulation were demonstrated in the VTA following exposure to DA-related drugs, comprehensive understanding of their ability to inhibit the activity of DA neurons, however, is still lacking. In this study, using whole-cell patch-clamp recordings from rat brain slices, a novel form of synaptic modulation induced by DA-related drugs was isolated. DA exposure was shown to cause potentiation of γ-amino-butyric acid (GABA) receptor type A (GABA(A)R)-mediated evoked inhibitory postsynaptic currents (eIPSCs), recorded from VTA DA neurons, under conditions of potassium channels blockade. The potentiation of these eIPSCs lasted for more than twenty minutes, could be mimicked by activation of D2-like but not D1-like DA receptors, and was accompanied by an increase in the frequency of GABA(A)R-mediated spontaneous miniature inhibitory postsynaptic currents (mIPSCs). Furthermore, exposure to inhibitors of DA transporter (DAT) led to potentiation of GABA(A) currents in a manner similar to the DA-mediated potentiation. Finally, a prolonged presence of l-NAME, an inhibitor of nitric-oxide (NO) signaling was found to conceal the potentiation of GABA(A) currents induced by the DA-related drugs. Taken together, this study demonstrates a new modulatory form of VTA GABA(A) neurotransmission mediated by DA-related drugs. These results also suggest better understanding of the initial inhibitory action of DA-related drugs on the activity of DA neurons in the VTA.     

Impact of the whole-cell patch-clamp configuration on the pharmacological assessment of the hERG channel: Trazodone as a case example

IntroductionVoltage- and state-dependent blocks are important mechanisms by which drugs affect voltage-gated ionic channels. However, spontaneous (i.e. drug-free) time-dependent changes in the activation and inactivation of hERG and Na⁺ channels have been reported when using conventional whole-cell patch-clamp in HEK-293 cells.MethodshERG channels were heterologously expressed in HEK-293 cells and in Xenopus laevis oocytes. hERG current (I_hERG) was recorded using both conventional and perforated whole-cell patch-clamp (HEK-293 cells), and two microelectrode voltage-clamp (Xenopus oocytes) in drug-free solution, and in the presence of the drug trazodone.ResultsIn conventional whole-cell setup, we observed a spontaneous time-dependent hyperpolarizing shift in the activation curve of I_hERG. Conversely, in perforated patch whole-cell (HEK-293 cells) or in two microelectrode voltage-clamp (Xenopus oocytes) activation curves of I_hERG were very stable for periods ~50 min. Voltage-dependent inactivation of I_hERG was not significantly altered in the three voltage clamp configurations tested. When comparing voltage- and state-dependent effects of the antidepressant drug trazodone on I_hERG, similar changes between the three voltage clamp configurations were observed as under drug-free conditions.DiscussionThe comparative analysis performed in this work showed that only under conventional whole-cell voltage-clamp conditions, a leftward shift in the activation curve of I_hERG occurred, both in the presence and absence of drugs. These spontaneous time-dependent changes in the voltage activation gate of I_hERG are a potential confounder in pharmacological studies on hERG channels expressed in HEK-293 cells.     

Cortical control of VTA function and influence on nicotine reward

Tobacco use is a major public health problem. Nicotine acts on widely distributed nicotinic acetylcholine receptors (nAChRs) in the brain and excites dopamine (DA) neurons in the ventral tegmental area (VTA). The elicited increase of DA neuronal activity is thought to be an important mechanism for nicotine reward and subsequently the transition to addiction. However, the current understanding of nicotine reward is based predominantly on the data accumulated from in vitro studies, often from VTA slices. Isolated VTA slices artificially terminate communications between neurons in the VTA and other brain regions that may significantly alter nicotinic effects. Consequently, the mechanisms of nicotinic excitation of VTA DA neurons under in vivo conditions have received only limited attention. Building upon the existing knowledge acquired in vitro, it is now time to elucidate the integrated mechanisms of nicotinic reward on intact systems that are more relevant to understanding the action of nicotine or other addictive drugs. In this review, we summarize recent studies that demonstrate the impact of prefrontal cortex (PFC) on the modulation of VTA DA neuronal function and nicotine reward. Based on existing evidence, we propose a new hypothesis that PFC–VTA functional coupling serves as an integration mechanism for nicotine reward. Moreover, addiction may develop due to nicotine perturbing the PFC–VTA coupling and thereby eliminating the PFC-dependent cognitive control over behavior.     

Effects of ticagrelor pretreatment on electrophysiological properties of stellate ganglion neurons following myocardial infarction

Higher sympathetic activity predisposes to malignant ventricular arrhythmias in the context of myocardial infarction (MI). This is, in part, mediated by the electrical activity of the stellate ganglion (SG). The aim of this study is to examine the effects of ticagrelor pretreatment on the electrophysiological properties of SG neurons following MI in rabbits. MI was induced by isoproterenol (ISO) of 150 mg kg^-1 d^-1 (twice at an interval of 24 hours). Ticagrelor pretreatment was administered at low- (10 mg kg^-1 d^-1) or high-dose (20 mg kg^-1 d^-1). Protein and RNA expression were determined by immunohistochemical analysis and real-time PCR, respectively. The activity of sodium channel current (I_Na), delayed rectifier potassium current (I_KDR), M-type potassium current (I_KM) as well as action potentials (APs) from SG neurons were measured by whole-cell patch-clamp. Intracellular calcium concentrations were measured by confocal microscopy. Compared with the control group, the MI group exhibited a greater amplitude of I_Na, I_KDR and I_KM, significantly altered activation and inactivation characteristics of I_Na, no significant alterations in protein or mRNA expression of sodium and M-type potassium channels, along with higher AP amplitude and frequency and intracellular calcium concentrations. Most of these abnormalities were prevented by pretreatment with low- or high-dose ticagrelor. Our data suggest that ticagrelor exerts cardioprotective effects, potentially through modulating the activity of different ion channels in SG neurons.     

铅对大鼠海马神经元钠电流的抑制作用

目的　铅对神经系统有损害作用 ,钠通道是神经元产生和传递电信号的重要枢纽 ,故研究铅对大鼠海马CA1区神经元钠电流 (INa)的影响。方法全细胞膜片钳技术。结果　醋酸铅可浓度依赖地抑制INa,1 ,1 0 ,50和 1 0 0 μmol·L-1 醋酸铅对INa的抑制率分别为 (8.2± 0 .8) % ,(2 0 .9± 2 .6) % ,(51 .8±4.8) %和 (66 .4±5 .7) %。此外 ,它还与电压呈依赖关系 ,50 μmol·L-1 醋酸铅可使INa的激活曲线显著右移 ,但不改变斜率因子 ,还可使INa的失活曲线显著左移。结论　铅可抑制INa的激活过程 ,可促进INa的失活过程。铅改变了细胞膜的电压感应 ,这可能是铅损伤海马神经元的作用机制之一     

Effects of carbamazepine and amitriptyline on tetrodotoxin-resistant Na<Superscript>+</Superscript> channels in immature rat trigeminal ganglion neurons

Although anticonvulsant drugs that block voltage-dependent Na⁺ channels have been widely used for neuropathic pain, including peripheral nerve injury-induced pain, much less is known about the actions of these drugs on immature trigeminal ganglion (TG) neurons. Here we report the effects of carbamazepine (CBZ) and amitriptyline (ATL) on tetrodotoxin-resistant (TTX-R) Na⁺ channels expressed on immature rat TG neurons. TTX-R Na⁺ currents (I_Na) were recorded in the presence of 300 nM TTX by use of a conventional whole-cell patch clamp method. Both CBZ and ATL inhibited TTX-R I_Na in a concentration-dependent manner, but ATL was more potent. While CBZ and ATL did not affect the overall voltage-activation relationship of TTX-R Na⁺ channels, both drugs shifted the voltage-activation relationship to the left, indicating that they inhibited TTX-R Na⁺ channels more efficiently at depolarized membrane potentials. ATL showed a profound use-dependent blockade of TTX-R I_Na, but CBZ had little effect. The present results suggest that both CBZ and ATL, common drugs used for treating neuropathic pain, efficiently inhibit TTX-R Na⁺ channels expressed on immature TG neurons, and that these drugs might be useful for the treatment of trigeminal nerve injury-induced neuropathic pain, as well as the inhibition of ongoing central sensitization, even during immature periods.     

Morphine-induced modulation of LTD at GABAergic synapses in the ventral tegmental area

Adaptive behaviors often require the learning of appropriate responses to rewarding stimuli, yet aberrant learning processes can lead to serious diseases such as addiction. Dopamine (DA) neurons of the ventral tegmental area (VTA) play an essential role in the treatment of rewarding stimuli, and they exhibit plasticity in response to such stimuli, but also to drugs of abuse. Previously we discovered a form of presynaptic nitric oxide (NO)-mediated long-term potentiation (LTP_GABA) at GABAergic synapses onto VTA DA neurons that is prevented with morphine in vivo 24 h after exposure. Here we investigated whether the same GABAergic synapses are capable of exhibiting long-term depression (LTD in addition to LTP_GABA) and its possible modulation by morphine in vivo. We found that indeed the efficacy of VTA GABAergic synapses can be down-regulated through induction of a novel form of LTD (i.e., LTD_GABA) in response to synaptic stimulation. Paired pulse ratio (PPR) and coefficient of variance (CV) analyses of evoked IPSCs confirmed that this plasticity may be postsynaptic. Consistently, LTD_GABA did not involve presynaptic cannabinoid CB₁receptors (CB₁Rs). Moreover, NMDAR activation was not necessary for LTD_GABA. However, blockade of D₂ dopamine receptors (D₂R) significantly attenuated LTD_GABA proposing a novel synaptic mechanism for the regulation of excitability of DA neurons by endogenous DA and D₂R activation. Interestingly, 24 h after a single in vivo exposure to morphine, LTD_GABA was absent in slices from morphine-treated rats but unaffected in slices from saline-treated rats, confirming a bidirectional impact of morphine on GABAergic synaptic plasticity in the VTA. The control of bidirectional GABAergic plasticity by morphine in the VTA may represent the neural correlates necessary for the addictive properties of opiates.This article is part of a Special Issue entitled ‘Synaptic Plasticity and Addiction’.     

Adaptations in the mesoaccumbens dopamine system resulting from repeated administration of dopamine D1 and D2 receptor-selective agonists: relevance to cocaine sensitization

The mesoaccumbens dopamine (DA) system is intricately involved in sensitization to the locomotor stimulant effects of cocaine. Among the adaptations implicated in cocaine sensitization are transient subsensitivity of impulse-regulating DA D₂ autoreceptors on ventral tegmental area (VTA) DA neurons leading to hyperactivity of the mesoaccumbens DA pathway, and persistently enhanced DA D₁ receptor responses of nucleus accumbens (NAc) neurons. We have tested the hypothesis that both of these adaptations are necessary to produce cocaine sensitization. We injected rats twice daily for 2 weeks with the selective DA D₁ class receptor agonist SKF 38393, the DA D₂ class receptor agonist quinpirole, or both. We then used single-cell recording procedures to determine possible alterations in VTA DA autoreceptor sensitivity and NAc D₁ receptor sensitivity at three withdrawal times: 1 day, 1 week and 1 month. We also tested whether these treatments produced cross-sensitization to cocaine at each withdrawal time. Repeated quinpirole treatment produced a reduction in VTA autoreceptor sensitivity and cross-sensitization to cocaine, but these effects lasted for less than 1 week. Repeated SKF 38393 treatment produced enhanced NAc D₁ responses which lasted for 1 week and cross-sensitization to cocaine which was only evident after 1 week of withdrawal. Repeated treatment with the combination of the two agonists transiently down-regulated autoreceptor sensitivity, enhanced and prolonged D₁ receptor supersensitivity (lasting 1 month), and produced enduring cross-sensitization to cocaine. These results suggest that neuroadaptations within both the VTA and NAc may be necessary for the induction of enduring cocaine sensitization. Received: 23 February 1998/Final version: 2 April 1998     

Prenatal Stress Exposure Increases the Excitation of Dopamine Neurons in the Ventral Tegmental Area and Alters Their Reponses to Psychostimulants

Prenatal stress exposure (PSE) is known to increase addiction risk. Dopamine (DA) neurons in the ventral tegmental area (VTA) play an important role in addiction. In order to understand the cellular mechanisms underlying PSE-induced increase in addiction risk, we examined the effects of PSE on the electrical impulse activity of VTA DA neurons using the in vivo extracellular single-unit recording technique. Amphetamine self-administration was also conducted to confirm increased addiction risk after PSE. The PSE was carried out by restraining pregnant dams from GD 11 to 20. Adult male offspring (3–6 months old) were used in the experiments. Animals with PSE showed enhanced amphetamine self-administration compared with controls when amphetamine dose was reduced after acquisition. The number of spontaneously active VTA DA neurons was also reduced in PSE rats. The reduction was reversed by acute apomorphine that normally inhibits the impulse activity of DA neurons. The reversal effect suggests that PSE-induced reduction in the number of spontaneously active VTA DA neurons is caused by overexcitation to the extent of depolarization block. Furthermore, the reduced number of spontaneously active VTA DA neurons was also reversed by acute psychostimulants (eg, amphetamine; cocaine), which in control rats inhibited the activity of VTA DA neurons. The reversal effect on VTA DA neuron in PSE animals represents an actual increase in the impulse activity. This effect might contribute to increased responding to psychostimulants and mediate increased addiction risk after PSE.