Extracellular dopamine in the rat ventral tegmental area and nucleus accumbens following ventral tegmental infusion of cocaine

Rats were implanted with dual dialysis probes, one in the ventral tegmental area, and another one ipsilateral in the nucleus accumbens. Infusion of cocaine (10, 100, 1000 mM) into the ventral tegmental area gradually increased extracellular dopamine to 164, 329 and 991% of baseline in the ventral tegmental area, but reduced dopamine to 76, 47 and 38% of baseline in the nucleus acumbens. These results are consistent with cocaine-induced feedback regulation of dopamine cell activity involving somatodendritec impulse-regulating dopamine D₂ autoreceptors.     

Fischer and Lewis rat strains differ in basal levels of neurofilament proteins and their regulation by chronic morphine in the mesolimbic dopamine system.

We studied levels of neurofilament (NF) proteins in the ventral tegmental area (VTA), and other regions of the central nervous system, of two genetically inbred rat strains, Lewis (LEW) and Fischer (F344) rats. These strains represent genetically divergent populations of rats that have been used to study possible genetic factors involved in a variety of biological processes, including drug addiction: compared to F344 rats, LEW rats show a much higher preference for several classes of drugs of abuse. We found 30-50% lower levels of three NF proteins, NF-200 (NF-H), NF-160 (NF-M), and NF-68 (NF-L), in the VTA of LEW compared to F344 rats by use of immunolabeling and Coomassie blue staining. These strain differences were highly specific to this brain region, with no differences observed elsewhere in brain or spinal cord. Interestingly, chronic treatment of F344 rats with morphine decreased levels of these three NF proteins in the VTA, as found previously in outbred Sprague-Dawley rats (Beitner-Johnson, D., Guitart, X., and Nestler, E.J.:J. Neurosci., 12:2165-2176, 1992), whereas morphine had no effect on NF levels in the VTA of LEW rats. A similar strain difference was observed in chronic morphine regulation of tyrosine hydroxylase, with morphine increasing enzyme immunoreactivity in the VTA of F344 rats (as has been observed previously in Sprague-Dawley rats [Beitner-Johnson, D., and Nestler, E.J.:J. Neurochem., 57:344-347, 1991]), but not in LEW rats. In view of the observations that LEW and F344 rats show different levels of preference for several types of drugs of abuse, and of the evidence supporting a central role of the mesolimbic dopamine system in drug reward mechanisms, the results of the current study suggest the possibility that levels of NFs and tyrosine hydroxylase may mediate some aspects of drug reinforcement and contribute to individual genetic differences in vulnerability to drug addiction.     

Electrophysiological effects of cocaine in the mesoaccumbens dopamine system: studies in the ventral tegmental area

Extracellular single-cell recording and microiontophoretic techniques were used to characterize the effects of cocaine on the activity of mesoaccumbens A10 dopamine (DA) neurons in the rat ventral tegmental area (VTA), which have been implicated in the rewarding effects of this and other drugs of abuse. Because cocaine inhibits the reuptake of DA, norepinephrine (NE), and serotonin (5-HT), and exerts local anesthetic actions, the possible involvement of each of these various mechanisms in the effects of cocaine on A10 DA neurons was investigated. Intravenous administration of cocaine caused a significant, dose-dependent, partial inhibition (50-70%) of the firing of antidromically identified mesoaccumbens DA neurons. Similar partial inhibition of A10 neurons was observed following intravenous administration of nomifensine, GBR-12909, and norcocaine, all of which inhibit DA reuptake. Neither the selective 5-HT uptake inhibitor fluoxetine nor the selective NE uptake inhibitor desmethylimipramine (DMI) inhibited the firing of A10 DA neurons. The local anesthetic agent procaine, which lacks DA uptake blocking efficacy, caused a slight, transient increase in firing rate. These results suggest that the effects of cocaine on A10 DA neurons are due to inhibition of DA reuptake, a conclusion that has been supported by several other findings. Pretreatment with reserpine to deplete vesicular stores of DA significantly reduced the ability of intravenous cocaine to suppress A10 DA neuronal activity. Microiontophoretic administration of cocaine caused only a weak (15-20%) inhibition of the activity of A10 DA neurons, but significantly increased and prolonged the inhibition produced by iontophoretic DA. This effect was not observed with iontophoretically administered procaine iontophoresis of cocaine also significantly potentiated the inhibition of A10 DA activity caused by electrical stimulation of the nucleus accumbens (NAc). Both unilateral ibotenic acid lesions of the NAc and hemitransections of the brain rostral to the VTA significantly reduced the inhibitory effects of intravenous cocaine on A10 DA neurons, suggesting that both somatodendritic impulse-regulating DA autoreceptors and inhibitory NAc-VTA feedback processes are involved in the effects of intravenous cocaine on A10 DA neurons. Therefore, it is hypothesized that the relatively weak inhibitory effects of cocaine on A10 DA neurons may represent a poor compensatory response to enhanced DA neurotransmission within the NAc, and may help to explain the extremely potent rewarding effects of this important drug of abuse.     

Neurotransmitter regulation of dopamine neurons in the ventral tegmental area

Over the last 10 years there has been important progress towards understanding how neurotransmitters regulate dopaminergic output. Reasonable estimates can be made of the synaptic arrangement of afferents to dopamine and non-dopamine cells in the ventral tegmental area (VTA). These models are derived from correlative findings using a variety of techniques. In addition to improved lesioning and pathway-tracing techniques, the capacity to measure mRNA in situ allows the localization of transmitters and receptors to neurons and/or axon terminals in the VTA. The application of intracellular electrophysiology to VTA tissue slices has permitted great strides towards understanding the influence of transmitters on dopamine cell function, as well as towards elucidating relative synaptic organization. Finally, the advent of in vivo dialysis has verified the effects of transmitters on dopamine and γ-aminobutyric acid transmission in the VTA. Although reasonable estimates can be made of a single transmitter's actions under largely pharmacological conditions, our knowledge of how transmitters work in concert in the VTA to regulate the functional state of dopamine cells is only just emerging. The fact that individual transmitters can have seemingly opposite effects on dopaminergic function demonstrates that the actions of neurotransmitters in the VTA are, to some extent, state-dependent. Thus, different transmitters perform similar functions or the same transmitter may perform opposing functions when environmental circumstances are altered. Understanding the dynamic range of a transmitter's action and how this couples in concert with other transmitters to modulate dopamine neurons in the VTA is essential to defining the role of dopamine cells in the etiology and maintenance of neuropsychiatrie disorders. Further, it will permit a more rational exploration of drugs possessing utility in treating disorders involving dopamine transmission.     

Somatodendritic targeting of M5 muscarinic receptor in the rat ventral tegmental area: Implications for mesolimbic dopamine transmission

Muscarinic modulation of mesolimbic dopaminergic neurons in the ventral tegmental area (VTA) plays an important role in reward, potentially mediated through the M5 muscarinic acetylcholine receptor (M5R). However, the key sites for M5R‐mediated control of dopamine neurons within this region are still unknown. To address this question we examined the electron microscopic immunocytochemical localization of antipeptide antisera against M5R and the plasmalemmal dopamine transporter (DAT) in single sections through the rat VTA. M5R was located mainly to VTA somatodendritic profiles (71%; n = 627), at least one‐third (33.2%; n = 208) of which also contained DAT. The M5R immunoreactivity was distributed along cytoplasmic tubulovesicular endomembrane systems in somata and large dendrites, but was more often located at plasmalemmal sites in small dendrites, the majority of which did not express DAT. The M5R‐immunoreactive dendrites received a balanced input from unlabeled terminals forming either asymmetric or symmetric synapses. Compared with dendrites, M5R was less often seen in axon terminals, comprising only 10.8% (n = 102) of the total M5R‐labeled profiles. These terminals were usually presynaptic to unlabeled dendrites, suggesting that M5R activation can indirectly modulate non–DAT‐containing dendrites through presynaptic mechanisms. Our results provide the first ultrastructural evidence that in the VTA, M5R has a subcellular location conducive to major involvement in postsynaptic signaling in many dendrites, only some of which express DAT. These findings suggest that cognitive and rewarding effects ascribed to muscarinic activation in the VTA can primarily be credited to M5R activation at postsynaptic plasma membranes distinct from dopamine transport. J. Comp. Neurol. 521: 2927–2946, 2013. © 2013 Wiley Periodicals, Inc.     

Presynaptic regulation of glutamate release in the ventral tegmental area during morphine withdrawal.

The regulation of glutamate (Glu) release from the excitatory input to dopamine cells in the ventral tegmental area (VTA) during acute withdrawal from morphine was studied in slices from animals treated for 6-7 d with morphine. EPSCs were inhibited by opioid agonists acting at micro-subtype receptors but not by selective delta- or kappa-subtype agonists. The opioid inhibition was reduced by 65% with the potassium channel blocker 4-aminopyridine (4-AP; 100 microM) and a 12-lipoxygenase inhibitor, baicalein (5 microM), suggesting that opioids acted via a transduction pathway involving activation of a voltage-dependent potassium conductance by lipoxygenase metabolites as has been shown in the periaqueductal gray (). During withdrawal, neither the potency nor the efficacy of D-Ala-Met-enkephalin-Gly-ol (DAMGO) were changed; however, the blockade of micro-opioid inhibition by both 4-AP and baicalein was reduced. In addition, the potency of baclofen to depress EPSCs by GABA-B receptors and the effects of the GABA-uptake inhibitor NO-711 (10 microM) were increased in withdrawn rats. Finally, group 2 (but not group 4 or 1) metabotropic glutamate receptor-mediated presynaptic inhibition was also enhanced in morphine-withdrawn rats. These results suggest that one of the consequences of withdrawal from chronic morphine is an enhanced presynaptic inhibition of the excitatory inputs to the dopamine cells of the VTA. Inhibition of glutamate release during acute withdrawal would add to the inhibition of dopamine cells that is mediated by an augmented release of GABA ().     

Selective activation of mesolimbic and mesocortical dopamine metabolism in rat brain by infusion of a stable substance P analogue into the ventral tegmental area

Microinfusion of the metabolically stable substance P (SP) agonist, [pGlu5,MePhe8,Sar9]-SP5-11 (DiMe-C7), into the ventral tegmental area (VTA) of rat brain increased levels of the dopamine (DA) metabolite dihydroxyphenylacetic acid in the prefrontal cortex (+ 120%) and nucleus accumbens (+30%) but not in other regions of forebrain. In contrast, infusions of DiMe-C7 or SP into the lateral ventricles or microinfusions of SP into VTA failed to elicit increases in DOPAC levels in forebrain. DA levels were unaffected by SP or DiMe-C7 regardless of the route of administration. These data and previous studies suggest a role for endogenous SP in the modulation of mesocortical and mesolimbic DA neurones.     

Repeated cocaine administration into the rat ventral tegmental area produces behavioral sensitization to a systemic cocaine challenge.

The aim of the present study was to investigate the capacity of repeated administration of cocaine (5 nmol/side) or the selective dopamine re-uptake inhibitor GBR 12909 (15 nmol/side) into the ventral tegmental area (VTA) to initiate behavioral sensitization to systemically administered cocaine (15 mg/kg, intraperitoneally). Following 1 week of withdrawal from intra-VTA treatment, cocaine or GBR 12909 pretreated animals displayed sensitized locomotor and rearing behavior to acute systemic cocaine administration. These data support the possibility that increased dopamine transmission in the VTA is involved in the cellular events that determine the initiation of behavioral sensitization to cocaine.     

Kindling of the ventral tegmental area induces supersensitivity in the central dopamine system

Kindling of the ventral tegmental area (VTA), a major source of the mesolimbic dopamine pathway, was examined in rats. We applied two quantitative measurements of dopamine sensitivity before and 2 weeks after VTA kindling (20 times electrical stimulations (100 microA at 1 min intervals) delivered once per day for 14 consecutive days): behavioral responses induced by test VTA stimulation and methamphetamine (MAP)-induced locomotor activity. The total amount of MAP-induced locomotor activity was significantly increased after VTA kindling, while the responses to electrical stimulation were unchanged. These results indicate that repeated activation of the mesolimbic dopamine system can produce a neuroplastic change, which results in dopamine supersensitivity.     

The somatodendritic release of dopamine in the ventral tegmental area and its regulation by afferent transmitter systems

The release of dopamine in the ventral tegmental area (VTA) plays an important role in the autoinhibition of the dopamine neurons of the mesocorticolimbic system through the activation of somatodendritic dopamine D2 autoreceptors. Accordingly, the intra-VTA application of dopamine D2 receptor agonists reduces the firing rate and release of dopamine in the VTA, and this control appears to possess a tonic nature because the corresponding antagonists enhance the somatodendritic release of the transmitter. In addition, the release of dopamine in the VTA is increased by potassium or veratridine depolarization and abolished by tetrodotoxin and calcium omission. Overall, it appears that the somatodendritic release of dopamine is consistently lower than that in nerve endings. Apart from intrinsic dopaminergic mechanisms, other transmitter systems such as serotonin, noradrenaline, acetylcholine, GABA and glutamate play a role in the control of the activity of dopaminergic neurons of the VTA, although the final action depends on the particular receptor involved as well as the neuronal type where it is localized. Given the involvement of the mesocorticolimbic dopaminergic systems in the pathogenesis of severe neuropsychiatric disorders such as schizophrenia, the knowledge of the factors that regulate the release of dopamine in the VTA could provide new insight into the ethiogenesis of the disease as well as its implication on the mechanisms of action of therapeutic drugs.     

Lateral habenula projections to dopamine and GABA neurons in the rat ventral tegmental area

Ventral tegmental area (VTA) dopamine (DA) neurons and their forebrain projections are critically involved in reward processing and cognitive functions. Descending projections from the lateral habenula (LHb) play a central role in inhibiting DA cell activity in response to the absence of expected rewards. As LHb efferents are reportedly glutamatergic, their ability to inhibit DA cells would theoretically require a disynaptic connection involving VTA GABA neurons and their local collateral inputs to DA cells. We therefore used anterograde tract-tracing from the LHb to investigate the relative selectivity of LHb synapses onto GABA versus DA VTA neurons. LHb axons were visualized using immunoperoxidase, and DA and GABA cells were marked by immunogold-silver labeling for tyrosine hydroxylase (TH) or GABA, respectively. By ultrastructural analysis, 16% of LHb axons were observed to form synaptic contacts in the VTA, and most of these were of an intermediate morphological type that did not exhibit definitive asymmetric or symmetric character. LHb axons synaptically targeted TH- and GABA-labeled dendrites to a comparable extent (45 and 52% observed incidence, respectively). Pre-embedding immunogold labeling for the vesicular glutamate transporter type 2 and post-embedding immunogold staining for GABA confirmed that approximately 85% of LHb terminals were glutamatergic and not GABAergic. These results suggest that the robust inhibition of DA cells evoked by the LHb is unlikely to arise from a selective innervation of VTA GABA neurons. Moreover, the LHb may mediate a direct excitation of DA cells that is over-ridden by indirect inhibition originating from an extrinsic source.     

Blockade of mesolimbic dopamine transmission dramatically increases sensitivity to the rewarding effects of nicotine in the ventral tegmental area

Nicotine produces rewarding and aversive motivational effects in humans and other animal species. Here, we report that the mammalian ventral tegmental area (VTA) represents a critical neural substrate for the mediation of both the rewarding and aversive properties of nicotine. We demonstrate that direct infusions of nicotine into the VTA can produce both rewarding and aversive motivational effects. While the rewarding effects of higher doses of nicotine were not attenuated by dopamine (DA) receptor blockade, blockade of mesolimbic DA signalling with either systemic or intra-nucleus accumbens (NAc) neuroleptic pretreatment potentiated the sensitivity to nicotine's rewarding properties over a three-order-of-magnitude dose range. Furthermore, the behavioural effects of lower doses of intra-VTA nicotine were reversed, switching the motivational valence of nicotine from aversive to rewarding. Our results suggest that blockade of mesolimbic DA signalling induced by neuroleptic medications may block selectively the aversive properties of nicotine, thus increasing the vulnerability to nicotine's rewarding and addictive properties by inducing a unique, drug-vulnerable phenotype.     

Cholinergic axons in the rat ventral tegmental area synapse preferentially onto mesoaccumbens dopamine neurons

Cholinergic afferents to the ventral tegmental area (VTA) contribute substantially to the regulation of motivated behaviors and the rewarding properties of nicotine. These actions are believed to involve connections with dopamine (DA) neurons projecting to the nucleus accumbens (NAc). However, this direct synaptic link has never been investigated, nor is it known whether cholinergic inputs innervate other populations of DA and gamma-aminobutyric acid (GABA) neurons, including those projecting to the prefrontal cortex (PFC). We addressed these questions by using electron microscopic analysis of retrograde tract-tracing and immunocytochemistry for the vesicular acetylcholine transporter (VAChT) and for tyrosine hydroxylase (TH) and GABA. In tissue labeled for TH, VAChT(+) terminals frequently synapsed onto DA mesoaccumbens neurons but only seldom contacted DA mesoprefrontal cells. In tissue labeled for GABA, one-third of VAChT(+) terminals innervated GABA-labeled dendrites, including both mesoaccumbens and mesoprefrontal populations. VAChT(+) synapses onto DA and mesoaccumbens neurons were more commonly of the asymmetric (presumed excitatory) morphological type, whereas VAChT(+) synapses onto GABA cells were more frequently symmetric (presumed inhibitory or modulatory). These findings suggest that cholinergic inputs to the VTA mediate complex synaptic actions, with a major portion of this effect likely to involve an excitatory influence on DA mesoaccumbens neurons. As such, the results suggest that natural and drug rewards operating through cholinergic afferents to the VTA have a direct synaptic link to the mesoaccumbens DA neurons that modulate approach behaviors.     

Periaqueductal gray afferents synapse onto dopamine and GABA neurons in the rat ventral tegmental area

The midbrain central gray (periaqueductal gray; PAG) mediates defensive behaviors and is implicated in the rewarding effects of opiate drugs. Projections from the PAG to the ventral tegmental area (VTA) suggest that this region might also regulate behaviors involving motivation and cognition. However, studies have not yet examined the morphological features of PAG axons in the VTA or whether they synapse onto dopamine (DA) or GABA neurons. In this study, we injected anterograde tracers into the rat PAG and used immunoperoxidase to visualize the projections to the VTA. Immunogold‐silver labeling for tyrosine hydroxylase (TH) or GABA was then used to identify the phenotype of innervated cells. Electron microscopic examination of the VTA revealed axons labeled anterogradely from the PAG, including myelinated and unmyelinated fibers and axon varicosities, some of which formed identifiable synapses. Approximately 55% of these synaptic contacts were of the symmetric (presumably inhibitory) type; the rest were asymmetric (presumably excitatory). These findings are consistent with the presence of both GABA and glutamate projection neurons in the PAG. Some PAG axons contained dense‐cored vesicles indicating the presence of neuropeptides in addition to classical neurotransmitters. PAG projections synapsed onto both DA and GABA cells with no obvious selectivity, providing the first anatomical evidence for these direct connections. The results suggest a diverse nature of PAG physiological actions on midbrain neurons. Moreover, as both the VTA and PAG are implicated in the reinforcing actions of opiates, our findings provide a potential substrate for some of the rewarding effects of these drugs. © 2009 Wiley‐Liss, Inc.     

Relative contributions and mapping of ventral tegmental area dopamine and GABA neurons by projection target in the rat

The ventral tegmental area (VTA) is a heterogeneous midbrain structure that contains dopamine (DA), GABA, and glutamate neurons that project to many different brain regions. Here, we combined retrograde tracing with immunocytochemistry against tyrosine hydroxylase (TH) or glutamate decarboxylase (GAD) to systematically compare the proportion of dopaminergic and GABAergic VTA projections to 10 target nuclei: anterior cingulate, prelimbic, and infralimbic cortex; nucleus accumbens core, medial shell, and lateral shell; anterior and posterior basolateral amygdala; ventral pallidum; and periaqueductal gray. Overall, the non-dopaminergic component predominated VTA efferents, accounting for more than 50% of all projecting neurons to each region except the nucleus accumbens core. In addition, GABA neurons contributed no more than 20% to each projection, with the exception of the projection to the ventrolateral periaqueductal gray, where the GABAergic contribution approached 50%. Therefore, there is likely a significant glutamatergic component to many of the VTA's projections. We also found that VTA cell bodies retrogradely labeled from the various target brain regions had distinct distribution patterns within the VTA, including in the locations of DA and GABA neurons. Despite this patterned organization, VTA neurons comprising these different projections were intermingled and never limited to any one subregion. These anatomical results are consistent with the idea that VTA neurons participate in multiple distinct, parallel circuits that differentially contribute to motivation and reward. While attention has largely focused on VTA DA neurons, a better understanding of VTA subpopulations, especially the contribution of non-DA neurons to projections, will be critical for future work.     

Laterodorsal tegmental stimulation elicits dopamine efflux in the rat nucleus accumbens by activation of acetylcholine and glutamate receptors in the ventral tegmental area

Cholinergic and glutamatergic neurons in the laterodorsal tegmentum (LDT) and neighbouring mesopontine nuclei are thought to influence mesolimbic dopaminergic neuronal activity involved in goal-directed behaviours. We measured the changes in dopamine oxidation current (corresponding with dopamine efflux) in the nucleus accumbens (NAc) in response to electrical stimulation of the LDT using in vivo chronoamperometry in urethane-anaesthetized rats. LDT stimulation (35 Hz pulse trains for 60 s, 1 s intertrain interval) evoked a three-component change in dopamine efflux in the NAc: (i) an initial stimulation time-locked increase in the dopamine signal above baseline, followed by (ii) an immediate decrease below baseline, and thereafter by (iii) a prolonged increase in the dopamine signal above baseline. Intra-VTA infusion of the nicotinic receptor antagonist mecamylamine (5 microg/0.5 microL) or the ionotropic glutamate receptor antagonist kynurenate (10 microg/microL) attenuated the first LDT-elicited component. The second suppressive component was abolished by intra-LDT infusions of either the nonselective or the M2-selective muscarinic receptor antagonists scopolamine (100 microg/microL) and methoctramine (50 microg/microL), respectively. In contrast, intra-VTA infusions of scopolamine (200 microg/microL) resulted in a selective attenuation of the third facilitatory component, whereas both second and third components were abolished by systemic injections of scopolamine (5 mg/kg). These results suggest that the initial increase, subsequent decrease, and final prolonged increase in extracellular dopamine levels in the NAc are selectively mediated by LDT-elicited activation of (i) nicotinic and glutamatergic receptors in the VTA, (ii) muscarinic M2 autoreceptors on LDT cell bodies, and (iii) muscarinic receptors in the VTA, respectively.     

Orexin axons in the rat ventral tegmental area synapse infrequently onto dopamine and gamma-aminobutyric acid neurons

Cells in the ventral tegmental area (VTA) facilitate motivated behaviors, and the activity of VTA neurons is regulated by dense projections from the lateral hypothalamic area (LHA). Orexin (Orx) neurons in the lateral and perifornical hypothalamus play important roles in arousal, feeding, and energy metabolism. Orx cells contribute substantially to the LHA projection to the rat midbrain. However, the morphological features of Orx fibers in the VTA and whether they synapse onto dopamine (DA) or gamma-aminobutyric acid (GABA) neurons have not yet been investigated. We utilized immunoperoxidase and immunogold-silver staining to examine the morphological features and synaptic incidence of Orx-labeled axons in the VTA. We then combined immunoperoxidase labeling for Orx with immunogold-silver labeling for GABA or for tyrosine hydroxylase (TH) in DA neurons. Electron microscopic analysis revealed that most Orx-labeled axons in the VTA were passing fibers. The less common Orx varicosities were occasionally apposed to TH- or GABA-labeled dendrites without synapsing. Only a small proportion of Orx-positive axons synapsed onto dendrites or soma. The synapses included both asymmetric and symmetric types and targeted TH- and GABA-labeled profiles with equal frequency. These findings suggest that most Orx fibers in the VTA are axons passing to caudal brainstem structures. However, Orx does mediate some direct synaptic influence on VTA DA and GABA neurons. Additional nonsynaptic effects are suggested by the presence of numerous dense-cored vesicles. These studies have important implications for understanding the mechanisms whereby Orx can alter behavior through regulating VTA DA and GABA cell activity.     

Glycine receptor-mediated inhibition of dopamine and non-dopamine neurons of the rat ventral tegmental area in vitro

Dopaminergic and non-dopaminergic neurons of the ventral tegmental area (VTA) were recorded intracellularly in slices of rat midbrain. Glycine (0.1-3 mM) caused a strychnine-sensitive and chloride-dependent reduction in membrane input resistance in both types of neuron. However, glycine also reduced the frequency of spontaneous bicuculline-sensitive inhibitory postsynaptic potentials (IPSPs) when recorded in dopaminergic cells. We conclude that glycine inhibits both types of VTA neuron by activating a strychnine-sensitive chloride conductance. Our data also raise the possibility that glycine could increase dopamine output from the VTA by a mechanism of disinhibition.     

Ethanol increases the firing rate of dopamine neurons of the rat ventral tegmental area in vitro

The ventral tegmental area (VTA) is a brain region rich in dopamine-containing neurons. Since most agents which act as substrates for self-administration increase dopaminergic outflow in the mesolimbic or mesocortical areas, the VTA slice preparation may be useful for identifying drugs with potential for abuse. While ethanol (EtOH) is a drug of abuse which has been widely studied, the properties of ethanol which contribute to its abuse potential are not known. We have developed a brain slice preparation of the VTA in order to study the action of EtOH on putative dopamine neurons. Concentrations of EtOH from 20 to 320 mM produce a dose-dependent excitation of the dopamine-type neurons of the VTA. About 89% of neurons which have electrophysiological characteristics established for presumed dopamine-containing neurons were excited by ethanol in the pharmacologically relevant concentration range. This excitation persists in low-calcium, high-magnesium medium, which suggests a direct excitatory action of EtOH on dopamine-type cells in the VTA slice.     

Circling from intracranial morphine applied to the ventral tegmental area in rats

Crystalline morphine applied unilaterally to the ventral tegmental area in rats caused circling away from the side of application. This circling was reversed by naloxone (3 mg/kg) and blocked by pretreatment with pimozide (0.5 mg/kg). When tested in an open field the animals followed the perimeter of the enlcosure; thus the radius of the circles described was determined by environmental rather than central factors. Morphine induced forward locomotion in all four limbs; there were no major signs of postural asymmetry noted in the longitudinal axis of the animal's body. This study suggests that morphine activates a population of A-10 dopamine cells known to be involved in locomotion.