Modulation of cellular activity and synaptic transmission in the ventral tegmental area

Orexin receptors in the hippocampal dentate gyrus modulated the restraint stress-induced analgesia in the animal model of chronic pain

Previous studies have shown that stressful stimuli induced an adaptive response of reduced nociception, known as stress-induced analgesia (SIA). Since orexin neuropeptides are involved in pain modulation, and orexin neurons, primarily located in the lateral hypothalamus (LH), project to various hippocampal regions, such as the dentate gyrus (DG), the current study aimed to examine the role of orexin receptors within the DG region in the restraint SIA in the animal model of chronic pain. One hundred-thirty adult male Wistar rats (230-250 g) were unilaterally implanted with a cannula above the DG region. Animals were given SB334867 or TCS OX2 29 (1, 3, 10, and 30 nmol, 0.5 µl/rat) into the DG region as orexin-1 receptor (OX1r) and orexin-2 receptor (OX2r) antagonists, respectively, five min before exposure to a 3-hour restraint stress (RS) period. Animals were then undergone the formalin test to assess pain-related behaviors as the animal model of chronic pain. The results showed that RS produces an analgesic response during the early and late phases of the formalin test. However, intra-DG microinjection of OX1r and OX2r antagonists attenuated the restraint SIA. OX2r antagonist was more potent than OX1r antagonist in the early phase of the formalin test, while OX1r antagonist was little more effective in the late phase. Predominantly, it could be concluded that the orexinergic system in the DG region might act as a potential endogenous pain control system and a novel target for treating stress-related disorders.

The Neuroanatomy of the Habenular Complex and Its Role in the Regulation of Affective Behaviors

The habenular complex is a diencephalic structure divided into the medial and lateral divisions that lie within the epithalamus of most vertebrates. This brain structure, whose activities are mainly regulated via inputs/outputs from and to the stria medullaris and the fasciculus retroflexus, plays a significant role in the modulation of anti-reward behaviors in both the rodent and human brain. Such anti-reward circuits are regulated by dopaminergic and serotonergic projections with several other subcortical and cortical regions; therefore, it is plausible that impairment to this key subcortical structure or its connections contributes to the pathogenesis of affective disorders. Current literature reveals the existence of structural changes in the habenula complex in individuals afflicted by such disorders; however, there is a need for more comprehensive investigations to elucidate the underlying neuroanatomical connections that underpin disease development. In this review article, we aim to provide a comprehensive view of the neuroanatomical differences between the rodent and human habenular complex, the main circuitries, and provide an update on the emerging roles of this understudied subcortical structure in the control of affective behaviors, with special emphasis to morbid conditions of the affective sphere.

The role of enkephalinergic systems in substance use disorders

Enkephalin, an endogenous opioid peptide, is highly expressed in the reward pathway and may modulate neurotransmission to regulate reward-related behaviors, such as drug-taking and drug-seeking behaviors. Drugs of abuse also directly increase enkephalin in this pathway, yet it is unknown whether or not changes in the enkephalinergic system after drug administration mediate any specific behaviors. The use of animal models of substance use disorders (SUDs) concurrently with pharmacological, genetic, and molecular tools has allowed researchers to directly investigate the role of enkephalin in promoting these behaviors. In this review, we explore neurochemical mechanisms by which enkephalin levels and enkephalin-mediated signaling are altered by drug administration and interrogate the contribution of enkephalin systems to SUDs. Studies manipulating the receptors that enkephalin targets (e.g., mu and delta opioid receptors mainly) implicate the endogenous opioid peptide in drug-induced neuroadaptations and reward-related behaviors; however, further studies will need to confirm the role of enkephalin directly. Overall, these findings suggest that the enkephalinergic system is involved in multiple aspects of SUDs, such as the primary reinforcing properties of drugs, conditioned reinforcing effects, and sensitization. The idea of dopaminergic-opioidergic interactions in these behaviors remains relatively novel and warrants further research. Continuing work to elucidate the role of enkephalin in mediating neurotransmission in reward circuitry driving behaviors related to SUDs remains crucial.

Modulatory role of the orexin system in stress-induced analgesia: Involvement of the ventral tegmental area

BACKGROUND

Exposure to stressful experiences is often accompanied by suppressing pain perception, referred to as stress-induced analgesia. The neuropeptides orexins are essential in regulating the mechanism that responds to stressful and painful stimuli. Meanwhile, the ventral tegmental area (VTA), as a part of descending pain inhibitory system, responds to noxious stimuli. This study aimed to investigate the role of intra-VTA administration of orexin receptor antagonists on stress-induced antinociceptive responses in the animal model of acute pain.

METHOD

Ninety-three adult Wistar rats weighing 230-250 g were unilaterally implanted by a cannulae above the VTA. Animals were pretreated with different doses (1, 3, 10 and 30 nM/0.3 μl) of SB334867 as the orexin-1 receptor antagonist and TCS OX2 29 as the orexin-2 receptor antagonist into the VTA, just 5 min before 6 min exposure to forced swim stress (FSS). Nociceptive threshold was measured using the tail-flick test as a model of acute pain.

RESULTS

The results showed that exposure to FSS could significantly increase analgesic responses. Moreover, intra-VTA administration of SB334768 and TCS OX2 29 blocked the antinociceptive effect of FSS in the tail-flick test.

CONCLUSION

The findings suggest that OX1 and OX2 receptors in the VTA might modulate the antinociceptive behaviours induced by FSS in part.

SIGNIFICANCE

Acute exposure to physical stress suppresses pain-related behaviors in the animal model of acute pain. Blockade of the OX1 and OX2 receptors in the VTA attenuates antinociceptive responses induced by FSS. The contribution of the OX2 receptors in the VTA is more predominant than OX1 receptors in stress-induced analgesia.

Nilotinib restores memory function by preventing dopaminergic neuron degeneration in a mouse model of Alzheimer's Disease

What happens precociously to the brain destined to develop Alzheimer's Disease (AD) still remains to be elucidated and this is one reason why effective AD treatments are missing. Recent experimental and clinical studies indicate that the degeneration of the dopaminergic (DA) neurons in the Ventral Tegmental Area (VTA) could be one of the first events occurring in AD. However, the causes of the increased vulnerability of DA neurons in AD are missing. Here, we deeply investigate the physiology of DA neurons in the VTA before, at the onset, and after onset of VTA neurodegeneration. We use the Tg2576 mouse model of AD, overexpressing a mutated form of the human APP, to identify molecular targets that can be manipulated pharmacologically. We show that in Tg2576 mice, DA neurons of the VTA at the onset of degeneration undergo slight but functionally relevant changes in their electrophysiological properties and cell morphology. Importantly, these changes are associated with accumulation of autophagosomes, suggestive of a dysfunctional autophagy, and with enhanced activation of c-Abl, a tyrosine kinase previously implicated in the pathogenesis of neurodegenerative diseases. Chronic treatment of Tg2576 mice with Nilotinib, a validated c-Abl inhibitor, reduces c-Abl phosphorylation, improves autophagy, reduces Aβ levels and - more importantly - prevents degeneration as well as functional and morphological alterations in DA neurons of the VTA. Interestingly, the drug prevents the reduction of DA outflow to the hippocampus and ameliorates hippocampal-related cognitive functions. Our results strive to identify early pathological brain changes in AD, to provide a rational basis for new therapeutic interventions able to slow down the disease progression.

Functional crosstalk of nucleus accumbens CB1 and OX2 receptors in response to nicotine-induced place preference

In the present study, we have evaluated the existence of functional interaction between orexin-2 receptor (OX2R) and cannabinoid-1 receptor (CB1R) in the nucleus accumbens core (NAcc), in nicotine-induced conditioned place preference (CPP) of Wistar male rat. Nicotine (0.5 mg/kg; s.c.) in the course of conditioning, produced a significant place preference, without any effect on the locomotor activity. Intra-NAcc administration of ineffective and effective doses of TCS-OX2-29 (2 and 6 ng/rat), a selective OX2R antagonist and AM251 (10 and 50 ng/rat), a selective CB1R antagonist, showed a significant interaction between OX2R and CB1R in the acquisition of nicotine-induced CPP (p < 0.01), and the locomotor activity (p < 0.05). No significant interaction was observed between these two receptors in the expression of nicotine-induced CPP. Our findings provide insight into the possible interaction of OX2R and CB1R of the NAcc in nicotine addiction. We propose a potential interaction between cannabinoid and orexinergic systems within the NAcc, in producing the rewarding effects.

Orexin type-2 receptor blockade prevents the nicotine-induced excitation of nucleus accumbens core neurons in rats: An electrophysiological perspective

BACKGROUND

The nucleus accumbens core (NAcc) expresses both orexin and nicotinic acetylcholine receptors (nAChRs). Orexin is among important neurotransmitters, which regulates addictive properties of drugs of abuse including nicotine. The role of orexin-2 receptor (OX2R) in the regulation of NAcc neural activity in response to nicotine has not yet been studied. Hence, in this study, we examined whether the OX2R antagonist (TCS-OX2-29) can adjust the effects of nicotine on electrical activity of NAcc neurons, in urethane-anesthetized rats, using the single unit recording.

METHODS

Neuronal firing of NAcc was recorded for 15 min, then TCS-OX2-29 (OX2R-antagonist; 1, 3 and 10 ng/rat) or DMSO were microinjected into NAcc, just 5 min before subcutaneous (sc) administration of nicotine (0.5 mg/kg) or saline. The spontaneous firing activity was recorded for 70 min, after nicotine injection.

RESULTS

The results demonstrated that nicotine significantly excites the NAcc neurons and interestingly, the administration of TCS-OX2-29 (3 and 10 ng/rat) into the NAcc, inhibited nicotine-induced increases of NAcc neuronal responses. Furthermore, administration of TCS-OX2-29 (10 ng/rat), just 5 min before sc administration of saline instead of nicotine, did not significantly alter the neuronal responses, compared to the saline-control group.

CONCLUSION

Our results showed that, although OX2R blockade alone did not affect neuronal activity in the NAcc, it was able to prevent the exciting effects of nicotine on NAcc neuronal activity. Therefore, we proposed that orexin has a potential modulator effect, in response to nicotine.

Altered brainstem responses to modafinil in schizophrenia: implications for adjunctive treatment of cognition

Candidate pro-cognitive drugs for schizophrenia targeting several neurochemical systems have consistently failed to demonstrate robust efficacy. It remains untested whether concurrent antipsychotic medications exert pharmacodynamic interactions that mitigate pro-cognitive action in patients. We used functional MRI (fMRI) in a randomized, double-blind, placebo-controlled within-subject crossover test of single-dose modafinil effects in 27 medicated schizophrenia patients, interrogating brainstem regions where catecholamine systems arise to innervate the cortex, to link cellular and systems-level models of cognitive control. Modafinil effects were evaluated both within this patient group and compared to a healthy subject group. Modafinil modulated activity in the locus coeruleus (LC) and ventral tegmental area (VTA) in the patient group. However, compared to the healthy comparison group, these effects were altered as a function of task demands: the control-independent drug effect on deactivation was relatively attenuated (shallower) in the LC and exaggerated (deeper) in the VTA; in contrast, again compared to the comparison group, the control-related drug effects on positive activation were attenuated in LC, VTA and the cortical cognitive control network. These altered effects in the LC and VTA were significantly and specifically associated with the degree of antagonism of alpha-2 adrenergic and dopamine-2 receptors, respectively, by concurrently prescribed antipsychotics. These sources of evidence suggest interacting effects on catecholamine neurons of chronic antipsychotic treatment, which respectively increase and decrease sustained neuronal activity in LC and VTA. This is the first direct evidence in a clinical population to suggest that antipsychotic medications alter catecholamine neuronal activity to mitigate pro-cognitive drug action on cortical circuits.

Brainstem system of hippocampal theta induction: The role of the ventral tegmental area

This article summarizes the results of studies concerning the influence of the ventral tegmental area (VTA) on the hippocampal theta rhythm. Temporary VTA inactivation resulted in transient loss of the hippocampal theta. Permanent destruction of the VTA caused a long-lasting depression of the power of the theta and it also had some influence on the frequency of the rhythm. Activation of glutamate (GLU) receptors or decrease of GABAergic tonus in the VTA led to enhancement of dopamine release and increased hippocampal theta power. High time and frequency cross-correlation was detected for the theta band between the VTA and hippocampus during paradoxical sleep and active waking. Thus, the VTA may belong to the broad network involved in theta rhythm regulation. This article also presents a model of brainstem-VTA-hippocampal interactions in the induction of the hippocampal theta rhythm. The projections from the VTA which enhance theta rhythm are incorporated into the main theta generation pathway, in which the septum acts as the central node. The neuronal activity that may be responsible for the ability of the VTA to regulate theta probably derives from the structures associated with rapid eye movement (sleep) (REM) sleep or with sensorimotor activity (i.e., mainly from the pedunculopontine and laterodorsal tegmental nuclei and also from the raphe).

Dorsal medial prefrontal cortex (MPFC) circuitry in rodent models of cocaine use: implications for drug addiction therapies

Although the importance of the medial prefrontal cortex (MPFC) in cocaine addiction is well established, its precise contribution to cocaine seeking, taking and relapse remains incompletely understood. In particular, across two different models of cocaine self-administration, pharmacological or optogenetic activation of the dorsal MPFC has been reported to sometimes promote and sometimes inhibit cocaine seeking. We highlight important methodological differences between the two experimental paradigms and propose a framework to potentially reconcile the apparent discrepancy. We also draw parallels between these pre-clinical models of cocaine self-administration and human neuro-imaging studies in cocaine users, and argue that both lines of evidence point to dynamic interactions between cue-reactivity processes and control processes within the dorsal MPFC circuitry. From a translational perspective, these findings underscore the importance of interventions and therapeutics targeting not just a brain region, but a specific computational process within that brain region, and may have implications for the design and implementation of more effective treatments for human cocaine addiction.

Food restriction increases glutamate receptor-mediated burst firing of dopamine neurons

Restriction of food intake increases the acquisition of drug abuse behavior and enhances the reinforcing efficacy of those drugs. However, the neurophysiological mechanisms responsible for the interactions between feeding state and drug use are largely unknown. Here we show that chronic mild food restriction increases the burst firing of dopamine neurons in the substantia nigra. Dopamine neurons from food-restricted mice exhibited increased burst firing in vivo, an effect that was enhanced by an injection of the psychomotor stimulant cocaine (10 mg/kg, i.p.). Food restriction also enhanced aspartic acid-induced burst firing of dopamine neurons in an ex vivo brain slice preparation, consistent with an adaptation occurring in the somatodendritic compartment and independent of a circuit mechanism. Enhanced burst firing persisted after 10 d of free feeding following chronic food restriction but was not observed following a single overnight fast. Whole-cell patch-clamp recordings indicated that food restriction also increased electrically evoked AMPAR/NMDAR ratios and increased D2 autoreceptor-mediated desensitization in dopamine neurons. These results identify dopamine neurons in the substantia nigra as a convergence point for the interactions between feeding state and drugs of abuse. Furthermore, increased glutamate transmission combined with decreased autoreceptor inhibition could work in concert to enhance drug efficacy in response to food restriction.

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.

Morphine withdrawal enhances constitutive μ-opioid receptor activity in the ventral tegmental area

μ-Opioid receptors (MORs) in the ventral tegmental area (VTA) are pivotally involved in addictive behavior. While MORs are typically activated by opioids, they can also become constitutively active in the absence of any agonist. In the current study, we present evidence that MOR constitutive activity is highly relevant in the mouse VTA, as it regulates GABAergic input to dopamine neurons. Specifically, suppression of MOR constitutive activity with the inverse agonist KC-2-009 enhanced GABAergic neurotransmission onto VTA dopamine neurons. This inverse agonistic effect was fully blocked by the specific MOR neutral antagonist CTOP, which had no effect on GABAergic transmission itself. We next show that withdrawal from chronic morphine further increases the magnitude of inverse agonistic effects at the MOR, suggesting enhanced MOR constitutive activity. We demonstrate that this increase can be an adaptive response to the detrimental elevation in cAMP levels known to occur during morphine withdrawal. These findings offer important insights in the physiological occurrence and function of MOR constitutive activity, and have important implications for therapeutic strategies aimed at normalizing MOR signaling during addiction and opioid overdose.

Cooperative interaction between the basolateral amygdala and ventral tegmental area modulates the consolidation of inhibitory avoidance memory

The aim of the current study was to examine the existence of a cooperative interaction between the basolateral nucleus of amygdala (BLA) and the ventral tegmental area (VTA) in inhibitory avoidance task. The BLA and the VTA regions of adult male Wistar rats were simultaneously cannulated and memory consolidation was measured in a step-through type inhibitory avoidance apparatus. Post-training microinjection of muscimol, a potent GABA-A receptor agonist (0.01-0.02 μg/rat), into the VTA impaired memory in a dose-dependent manner. Post-training intra-BLA microinjection of NMDA (0.02-0.04 μg/rat), 5 min before the intra-VTA injection of muscimol (0.02 μg/rat), attenuated muscimol-induced memory impairment. Microinjection of a NMDA receptor antagonist, D-AP5 (0.02-0.06 μg/rat) into the BLA inhibited NMDA effect on the memory impairment induced by intra-VTA microinjection of muscimol. On the other hand, post-training intra-BLA microinjection of muscimol (0.02-0.04 μg/rat) dose-dependently decreased step-through latency, indicating an impairing effect on memory. This impairing effect was however significantly attenuated by intra-VTA microinjection of NMDA (0.01-0.03 μg/rat). Intra-VTA microinjection of D-AP5 (0.02-0.08 μg/rat), 5 min prior to NMDA injection, inhibited NMDA response on the impairing effect induced by intra-BLA microinjection of muscimol. It should be considered that post-training microinjection of the same doses of NMDA or D-AP5 into the BLA or the VTA alone had no effect on memory consolidation. The data suggest that the relationship between the BLA and the VTA in mediating memory consolidation in inhibitory avoidance learning may be dependent on a cooperative interaction between the glutamatergic and GABAergic systems via NMDA and GABA-A receptors.

Impact of prefrontal cortex in nicotine-induced excitation of ventral tegmental area dopamine neurons in anesthetized rats

Systemic administration of nicotine increases dopaminergic (DA) neuron firing in the ventral tegmental area (VTA), which is thought to underlie nicotine reward. Here, we report that the medial prefrontal cortex (mPFC) plays a critical role in nicotine-induced excitation of VTA DA neurons. In chloral hydrate-anesthetized rats, extracellular single-unit recordings showed that VTA DA neurons exhibited two types of firing responses to systemic nicotine. After nicotine injection, the neurons with type-I response showed a biphasic early inhibition and later excitation, whereas the neurons with type-II response showed a monophasic excitation. The neurons with type-I, but not type-II, response exhibited pronounced slow oscillations (SOs) in firing. Pharmacological or structural mPFC inactivation abolished SOs and prevented systemic nicotine-induced excitation in the neurons with type-I, but not type-II, response, suggesting that these VTA DA neurons are functionally coupled to the mPFC and nicotine increases firing rate in these neurons in part through the mPFC. Systemic nicotine also increased the firing rate and SOs in mPFC pyramidal neurons. mPFC infusion of a non-α7 nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine blocked the excitatory effect of systemic nicotine on the VTA DA neurons with type-I response, but mPFC infusion of nicotine failed to excite these neurons. These results suggest that nAChR activation in the mPFC is necessary, but not sufficient, for systemic nicotine-induced excitation of VTA neurons. Finally, systemic injection of bicuculline prevented nicotine-induced firing alterations in the neurons with type-I response. We propose that the mPFC plays a critical role in systemic nicotine-induced excitation of VTA DA neurons.

Dopamine neurons in the ventral tegmental area fire faster in adolescent rats than in adults

Adolescence may be a period of vulnerability to drug addiction. In rats, elevated firing activity of ventral tegmental area (VTA) dopamine neurons predicts enhanced addiction liability. Our aim was to determine if dopamine neurons are more active in adolescents than in adults and to examine mechanisms underlying any age-related difference. VTA dopamine neurons fired faster in adolescents than in adults as measured with in vivo extracellular recordings. Dopamine neuron firing can be divided into nonbursting (single spikes) and bursting activity (clusters of high-frequency spikes). Nonbursting activity was higher in adolescents compared with adults. Frequency of burst events did not differ between ages, but bursts were longer in adolescents than in adults. Elevated dopamine neuron firing in adolescent rats was also observed in cell-attached recordings in ex vivo brain slices. Using whole cell recordings, we found that passive and active membrane properties were similar across ages. Hyperpolarization-activated cation currents and small-conductance calcium-activated potassium channel currents were also comparable across ages. We found no difference in dopamine D2-class autoreceptor function across ages, although the high baseline firing in adolescents resulted in autoreceptor activation being less effective at silencing neurons. Finally, AMPA receptor-mediated spontaneous excitatory postsynaptic currents occurred at lower frequency in adolescents; GABA(A) receptor-mediated spontaneous inhibitory postsynaptic currents occurred at both lower frequency and smaller amplitude in adolescents. In conclusion, VTA dopamine neurons fire faster in adolescence, potentially because GABA tone increases as rats reach adulthood. This elevation of firing rate during adolescence is consistent with it representing a vulnerable period for developing drug addiction.

Differential effects of cocaine on dopamine neuron firing in awake and anesthetized rats

Cocaine (benzoylmethylecgonine), a natural alkaloid, is a powerful psychostimulant and a highly addictive drug. Unfortunately, the relationships between its behavioral and electrophysiological effects are not clear. We investigated the effects of cocaine on the firing of midbrain dopaminergic (DA) neurons, both in anesthetized and awake rats, using pre-implanted multielectrode arrays and a recently developed telemetric recording system. In anesthetized animals, cocaine (10 mg/kg, intraperitoneally) produced a general decrease of the firing rate and bursting of DA neurons, sometimes preceded by a transient increase in both parameters, as previously reported by others. In awake rats, however, injection of cocaine led to a very different pattern of changes in firing. A decrease in firing rate and bursting was observed in only 14% of DA neurons. Most of the other DA neurons underwent increases in firing rate and bursting: these changes were correlated with locomotor activity in 52% of the neurons, but were uncorrelated in 29% of them. Drug concentration measurements indicated that the observed differences between the two conditions did not have a pharmacokinetic origin. Taken together, our results demonstrate that cocaine injection differentially affects the electrical activity of DA neurons in awake and anesthetized states. The observed increases in neuronal activity may in part reflect the cocaine-induced synaptic potentiation found ex vivo in these neurons. Our observations also show that electrophysiological recordings in awake animals can uncover drug effects, which are masked by general anesthesia.

A role for hypocretin/orexin in motivation

Obesity and drug addiction, both a result of aberrant motivated behavior, are growing problems in western society. Increased dopamine neurotransmission occurs with both drug-seeking and ingestive behaviors and has been linked to effort related functions. Hypocretin/orexin (Hcrt/ox) neurons have long been known to mediate arousal and feeding. Over the last 5 years, hcrt/ox has been demonstrated to play a novel role in mediating a variety of reward-seeking behaviors and can modulate the activity and output of dopamine neurons. Here, we propose that hcrt/ox action on mesolimbic dopamine circuitry serves to promote effort to obtain highly salient natural or drug rewards.

Blockade of mGLUR5 receptors differentially alters amphetamine-induced enhancement of locomotor activity and of brain stimulation reward

This study was aimed at determining the role of mGLUR5 glutamate receptors on amphetamine-induced enhancement of locomotion and of brain stimulation reward (BSR). The effect of different doses of the mGLUR5 antagonist, MPEP (0, 1, 3 and 9 mg/kg, i.p.), was assessed on reward induced by electrical stimulation of the lateral hypothalamus, and on the enhancement of reward by amphetamine (1 mg/kg, i.p.) in adult male Long Evans rats. The effect of a single dose of MPEP (0 and 9 mg/kg) on amphetamine-induced increase in locomotor activity was also assessed. Systemic injection of MPEP alone did not alter reward threshold and maximum rate of responding. Amphetamine produced a 25-30% decrease in reward threshold, an effect not altered by the highest dose of MPEP. At this dose, MPEP produced a weak inhibition of spontaneous locomotion and a significant attenuation of the enhanced locomotor activity induced by amphetamine. These findings show that mGLUR5 glutamate receptors are unlikely to constitute important elements of the reward-relevant pathway, and do not intervene in the enhancement effect of amphetamine. They also show, however, that these glutamate receptors play a key role in amphetamine-induced increased locomotor activity, providing additional evidence for a dissociation between the substrates that mediate these two behaviours.

AMPA receptor synaptic plasticity induced by psychostimulants: the past, present, and therapeutic future

Experience-dependent plasticity at excitatory synapses of the mesocorticolimbic system is a fundamental brain mechanism that enables adaptation to an ever-changing environment. These synaptic responses are critical for the planning and execution of adaptive behaviors that maximize survival. The mesocorticolimbic system mediates procurement of positive reinforcers such as food and sex; however, drugs of abuse resculpt this crucial circuitry to promote compulsive drug-seeking behavior. This review will discuss the long-term changes in glutamatergic neurotransmission that occur within the mesolimbic system following cocaine exposure. In addition, we will examine how these long-lasting neuroadaptations may drive the pathology of psychostimulant addiction. Finally, we review clinical trials that highlight antagonists at excitatory AMPA receptors as promising targets against cocaine abuse.

How conditioned stimuli acquire the ability to activate VTA dopamine cells: a proposed neurobiological component of reward-related learning

The ability to learn about conditioned stimuli (CS) associated with rewards is a crucial adaptive mechanism. Activity in the mesocorticolimbic dopamine (DA) system, as well as in the ventral tegmental area (VTA), is correlated with responding to and learning about CSs. The mechanism by which VTA neurons become activated by signals associated with conditioned stimuli is not fully understood. Our model suggests that NMDA receptor stimulation in the VTA allows originally weak glutamate signals carrying information about environmental stimuli, coincident with strong excitation correlated with primary rewards, to be strengthened and thereby acquire the ability to activate VTA neurons in themselves, producing approach. Furthermore, once synaptic strengthening occurs, the model suggests that NMDA receptor stimulation in VTA is not necessary for the expression of reward-related learning. In this review we survey evidence that VTA cells respond to cues associated with primary rewards, that this responding is acquired, and that the VTA possesses the attributes to function as a site of integration of signals of primary and conditioned stimuli.

Prominent activation of brainstem and pallidal afferents of the ventral tegmental area by cocaine

Blockade of monoamine transporters by cocaine should not necessarily lead to certain observed consequences of cocaine administration, including increased firing of ventral mesencephalic dopamine (DA) neurons and accompanying impulse-stimulated release of DA in the forebrain and cortex. Accordingly, we hypothesize that the dopaminergic-activating effect of cocaine requires stimulation of the dopaminergic neurons by afferents of the ventral tegmental area (VTA). We sought to determine if afferents of the VTA are activated following cocaine administration. Rats were injected in the VTA with retrogradely transported Fluoro-Gold and, after 1 week, were allowed to self-administer cocaine or saline via jugular catheters for 2 h on 6 consecutive days. Other rats received a similar amount of investigator-administered cocaine through jugular catheters. Afterward, the rats were killed and the brains processed immunohistochemically for retrogradely transported tracer and Fos, the protein product of the neuronal activation-associated immediate early gene, c-fos. Forebrain neurons exhibiting both Fos and tracer immunoreactivity were enriched in both cocaine groups relative to the controls only in the globus pallidus and ventral pallidum, which, together, represented a minor part of total forebrain retrogradely labeled neurons. In contrast, both modes of cocaine administration strongly increased double-labeling relative to the controls in the brainstem, specifically in the caudal ventromedial mesencephalon and rostromedial pontine tegmentum. It is concluded that a previously unappreciated activation of pallidal and brainstem afferents may contribute to the modulation of dopaminergic neuronal activity following cocaine administration.

Cocaine but not natural reward self-administration nor passive cocaine infusion produces persistent LTP in the VTA

Persistent drug-seeking behavior is hypothesized to co-opt the brain's natural reward-motivational system. Although ventral tegmental area (VTA) dopamine (DA) neurons represent a crucial component of this system, the synaptic adaptations underlying natural rewards and drug-related motivation have not been fully elucidated. Here, we show that self-administration of cocaine, but not passive cocaine infusions, produced a persistent potentiation of VTA excitatory synapses, which was still present after 3 months abstinence. Further, enhanced synaptic function in VTA was evident even after 3 weeks of extinction training. Food or sucrose self-administration induced only a transient potentiation of VTA glutamatergic signaling. Our data show that synaptic function in VTA DA neurons is readily but reversibly enhanced by natural reward-seeking behavior, while voluntary cocaine self-administration induced a persistent synaptic enhancement that is resistant to behavioral extinction. Such persistent synaptic potentiation in VTA DA neurons may represent a fundamental cellular phenomenon driving pathological drug-seeking behavior.

Presynaptic metabotropic glutamate receptors regulate glutamatergic input to dopamine neurons in the ventral tegmental area

The ventral tegmental area is part of the midbrain dopamine system and is crucially involved in reward, motivation and drug abuse. The activity of dopamine neurons within this region is controlled by synaptic input. In particular, excitatory glutamatergic inputs are important for the switch from regular firing into burst firing. In the present manuscript we determined the role of presynaptic metabotropic glutamate receptors (mGluRs) in the regulation of spontaneous glutamate release of terminals projecting to dopamine cells in the ventral tegmental area of mice. We show that group III mGluRs regulate spontaneous glutamate release and this effect is most likely mediated by mGluR7. The presynaptic dampening of glutamatergic input might open new perspectives in the treatment of drug addiction.

Effects of Ethanol on Midbrain Neurons: Role of Opioid Receptors

BACKGROUND

Although ethanol addiction is believed to be mediated by the mesolimbic dopamine system, originating from the ventral tegmental area (VTA), how acute ethanol increases the activity of VTA dopaminergic (DA) neurons remains unclear.

METHOD

Patch-clamp recordings of spontaneous firings of DA and GABAergic neurons in the VTA in acute midbrain slices from rats.

RESULTS

Ethanol (20-80 mM) excites DA neurons, and more potently depresses firing of local GABAergic neurons. The ethanol-induced excitation of DA neurons is considerably attenuated by DAMGO (Tyr-d-Ala-Gly-N-Me-Phe-Gly-ol enkephalin), a mu-opioid agonist that suppresses firing of GABAergic neurons, or by naloxone, a general opioid antagonist. The ongoing opioid-induced facilitation of DA cell firing (revealed by naloxone) is enhanced by ethanol, probably by an increase in opioid release or action.

CONCLUSION

Ethanol excites VTA DA neurons at least partly by increasing ongoing opioid-mediated suppression of local GABAergic inhibition. This indirect mechanism may contribute significantly to the positively reinforcing properties of ethanol.

Linkage disequilibrium, haplotype and association studies of a chromosome 4 GABA receptor gene cluster: candidate gene variants for addictions

Strong genetic contributions to individual differences in vulnerability to addictions are well supported by classical genetic studies. Linkage and association genome scans for addiction vulnerability have provided converging evidence for several chromosomal regions which are likely to harbor allelic variants that contribute to such vulnerability. We and others have delineated a candidate addiction-associated chromosome 4p12 "rSA3" region based on convergent data from association genome scanning studies in polysubstance abusers [Uhl et al. (2001); Am J Hum Genet 69(6):1290-1300], linkage-based studies in alcoholism [Long et al. (1998); Am J Med Genet 81(3):216-221; Reich et al. (1998); Am J Med Genet 81(3):207-215] and association-based studies for alcoholism and association-based studies for individual differences in electroencephalographic (EEG) spectral power phenotypes [Porjesz et al. (2002); Proc Natl Acad Sci USA 99(6):3729-3733; Edenberg et al. (2004); Am J Hum Genet 74(4):705-714]. The rSA3 region contains interesting candidate genes that encode the alpha 2, alpha 4, beta 1, and gamma 1 receptor subunits for the principal brain inhibitory neurotransmitter, gamma-aminobutyric acid (GABA) [Covault et al. (2004); Am J Med Genet Part B 129B:104-109; Edenberg et al. (2004); Am J Hum Genet 74(4):705-714; Lappalainen et al. (2005); Alcohol Clin Exp Res 29(4):493-498]. We now report assessment of single nucleotide polymorphism (SNP) genotypes in this region in three samples of substance abusers and controls. These results delineate the haplotypes and patterns of linkage disequilibrium in this region, focus attention of the GABRA2 gene and identify modest associations between GABRA2 genotypes and addiction phenotypes. These results are consistent with modest roles for GABRA2 variants in addiction vulnerabilities.

Reduced psychostimulant effects on dopamine dynamics in the nucleus accumbens of mu-opioid receptor knockout mice

Dopamine neurotransmission in the nucleus accumbens plays a pivotal role in the reinforcing properties of drugs of abuse. Two interacting processes regulate nucleus accumbens dopamine overflow: release of dopamine from presynaptic terminals and the subsequent reuptake by dopamine transporters. Opioid neurotransmission, primarily through mu-opioid receptors has also been strongly implicated in drug reward. We have previously shown that mice lacking the mu-opioid receptor display decreased cocaine self-administration. In addition, we found decreased impulse activity of midbrain dopaminergic neurons and an increased GABAergic input to these neurons in mu-opioid receptor knockout mice. In the present study we investigated whether these changes in dopaminergic cell bodies are accompanied by altered dopamine dynamics at the terminal level. To that aim, we measured nucleus accumbens dopamine overflow using fast scan cyclic voltammetry. Our data demonstrate that in mu-opioid receptor knockout mice 1) the reuptake of dopamine in the nucleus accumbens is slower, and 2) the relative effect of cocaine and amphetamine on the reuptake of dopamine is smaller compared with wild type mice. These data provide a mechanism for the decreased reinforcing properties of cocaine observed in mu-opioid receptor knockout mice.

Decreased firing frequency of midbrain dopamine neurons in mice lacking mu opioid receptors

Dopamine neurons originating in the midbrain and projecting to cortico-limbic and motor structures are one of the major neuronal substrates implicated in the reinforcing properties of drugs of abuse. The output of this system is largely determined by its impulse activity (amount and pattern of firing activity). Several intrinsic and synaptic factors can influence dopamine neuronal activity and, consequently, addiction liability. Pharmacological studies indicate that mu-opioid receptors and their activation by endogenous opioids may play an important role. In the present study, we use a genetic approach to better understand the role of mu-opioid receptors in modulating dopamine neuronal activity in vivo. Using in vivo extracellular single-unit recordings, we show that mice lacking mu-opioid receptors exhibit lower firing rates of dopamine neurons compared with their wild-type littermates. Although we observed no overall changes in bursting activity compared with wild-type mice, animals lacking mu-opioid receptors exhibited a higher proportion of regular-spiking cells that lacked bursting activity. These findings are the first to emphasize the critical role of mu-opioid receptors in modulating action potential output of dopamine neurons in vivo using a genetic approach. They also provide a possible underlying mechanism for the decreased reinforcing properties of drugs of abuse that was previously observed in mice lacking mu-opioid receptors.

Increased gabaergic input to ventral tegmental area dopaminergic neurons associated with decreased cocaine reinforcement in mu-opioid receptor knockout mice

There is general agreement that dopaminergic neurons projecting from the ventral tegmental area (VTA) to the nucleus accumbens and prefrontal cortex play a key role in drug reinforcement. The activity of these neurons is strongly modulated by the inhibitory and excitatory input they receive. Activation of mu-opioid receptors, located on GABAergic neurons in the VTA, causes hyperpolarization of these GABAergic neurons, thereby causing a disinhibition of VTA dopaminergic neurons. This effect of mu-opioid receptors upon GABA neurotransmission is a likely mechanism for mu-opioid receptor modulation of drug reinforcement. We studied mu-opioid receptor signaling in relation to cocaine reinforcement in wild-type and mu-opioid receptor knockout mice using a cocaine self-administration paradigm and in vitro electrophysiology. Cocaine self-administration was reduced in mu-opioid receptor knockout mice, suggesting a critical role of mu-opioid receptors in cocaine reinforcement. The frequency of spontaneous inhibitory post-synaptic currents onto dopaminergic neurons in the ventral tegmental area was increased in mu-opioid receptor knockout mice compared with wild-type controls, while the frequency of spontaneous excitatory post-synaptic currents was unaltered. The reduced cocaine self-administration and increased GABAergic input to VTA dopaminergic neurons in mu-opioid receptor knockout mice supports the notion that suppression of GABAergic input onto dopaminergic neurons in the VTA contributes to mu-opioid receptor modulation of cocaine reinforcement.

Acute morphine administration increases extracellular DA levels in the rat lateral septum by decreasing the GABAergic inhibitory tone in the ventral tegmental area

We studied the effect of an acute systemic administration of morphine and of a local intra-ventral tegmental area (VTA) infusion of the same drug on extracellular levels of dopamine (DA) in the lateral septum (LS) by in vivo microdialysis in anesthetized rats. The extracellular levels of 5-hydroxytryptamine (5-HT) were also measured in all dialysate samples. The acute systemic administration of morphine dose-dependently increased extracellular levels of DA but not of 5-HT in the LS, in the absence or presence of fluoxetine. This morphine effect was antagonized by the previous administration of naloxone, a specific opioid antagonist. The local infusion of morphine in the VTA also induced a significant increase of the extracellular levels of DA in the LS, concomitantly with a decrease of gamma-aminobutyric acid (GABA) extracellular levels in the VTA itself. Intriguingly, the LS extracellular levels of DA returned to basal values before the VTA GABA extracellular levels recovered. Our results show for the first time that an acute administration of morphine increases DA extracellular levels in the LS. The results also suggest that DA cells in the VTA and innervating the LS are under an inhibitory GABAergic tone sensitive to morphine. Taken together, our neurochemical data and previous studies involving LS DA in stress-related behavior support the hypothesis that DA in the LS plays a significant role in addictive behavior. The participation of LS DA and 5-HT systems in stress-induced relapse to drug seeking should be studied further.