Dopamine-dependent contralateral circling induced by neurotensin applied unilaterally to the ventral tegmental area in rats

Unilateral application of neurotensin (5, 10, and 15 micrograms) to the ventral tegmental area in rats caused contralateral circling. Pretreatment with pimozide (0.5 mg/kg) blocked the behavior. As is the case with circling elicited by unilateral ventral tegmental morphine, the diameter of the circles made by the animals was determined by environmental factors rather than by internal motor programming.     

Neurotensin self-injection in the ventral tegmental area

Earlier work with the conditioned place-preference paradigm suggested that neurotensin (NT) acts as a behavioral reinforcer when microinjected into the ventral tegmental area (VTA) of the midbrain. We report here that animals will perform an operant task to obtain microinfusions of NT into the VTA. Rats reliably pressed a lever to obtain NT infusions while neglecting an identical but inactive lever. Substitution of saline for NT initiated response extinction; following the reintroduction of NT, reliable responding resumed. These results extend earlier work suggesting that NT in the VTA can be a positive reinforcer.     

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.     

Neurotransmitter actions on oral pontine tegmental neurons of the rat: an in vitro study

The actions of neurotransmitters involved in the sleep-wakefulness cycle on neurons located in the ventral part of the oral pontine tegmentum were studied in a rat brain-slice preparation. Results show that glutamate and histamine evoke depolarizations and spike firing while serotonin and γ-aminobutyric acid evoke hyperpolarizations. The excitatory and inhibitory actions of these neurotransmitters increase pontine neuron activity during the conditions occurring during paradoxical sleep.     

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.     

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.     

Burst activity of ventral tegmental dopamine neurons is elicited by sensory stimuli in the awake cat

In light of evidence implicating dopamine in the pathophysiology of attention deficit disorder and schizophrenia, diseases involving attentional or sensory processing abnormalities, it was of interest to determine whether and how dopamine neurons in the ventral tegmental area respond to sensory stimuli. The single-unit responses of ventral tegmental dopamine neurons were recorded in freely-moving cats during the presentation of brief, non-conditioned auditory and visual stimuli. Both auditory and visual stimuli produced neuronal excitation, involving a greater than 5-fold increase in the probability of burst firing followed by a period of burst inhibition. The burst nature of the single-unit response suggests that sensory-induced dopamine release at target sites was disproportionally large relative to the discharge frequency. While characteristics of the dopaminergic sensory response were similar for auditory and visual stimuli, the response latency was longer for visual stimuli. The results demonstrate that dopamine neurons in the ventral tegmental area, the site of origin for mesolimbocortical dopamine neurons, are reliably activated by non-conditioned auditory and visual stimuli.     

Neurotensin and cholecystokinin microinjected into the ventral tegmental area modulate microdialysate concentrations of dopamine and metabolites in the posterior nucleus accumbens

Neurotensin and cholecystokinin, neuropeptides which coexist with dopamine in many ventral tegmental neurons, were microinjected into the ventral tegmental area during in vivo microdialysis in the posterior nucleus accumbens. Neurotensin significantly elevated concentrations of dopamine and its metabolites at doses of 10 pmol, 1 nmol, and 10 nmol, while cholecystokinin significantly elevated dopamine metabolite concentrations only at a dose of 10 nmol. These data suggest that neurotensin potently mediates the release of dopamine from the mesolimbic pathway via direct actions on the cell body.     

ALDH1 mRNA: presence in human dopamine neurons and decreases in substantia nigra in Parkinson's disease and in the ventral tegmental area in schizophrenia

Dopamine (DA) neurons degenerate in Parkinson's disease and dopamine neurotransmission may be affected in psychotic states seen in schizophrenia. Understanding the regulation of enzymes involved in DA metabolism may therefore lead to new treatment strategies for these severe conditions. We investigated mRNA expression of the cytosolic aldehyde dehydrogenase (ALDH1), presumably involved in DA degradation, by in situ hybridization in DA neurons of human postmortem material. Parallel labeling for GAPDH, neuron-specific enolase, tyrosine hydroxylase, dopamine transporter, and dopamine beta-hydroxylase was used to ensure suitability of tissue specimen and to identify all dopamine neurons. ALDH1 was found to be expressed highly and specifically in DA cells of both substantia nigra (SN) and the ventral tegmental area (VTA) of controls. A marked reduction of ALDH1 expression was seen in surviving neurons of SN pars compacta but not of those in the VTA in Parkinson's disease. In patients suffering from schizophrenia we found ALDH1 expression at normal levels in DA cells of SN but at significantly reduced levels in those of the VTA. We conclude that ALDH1 is strongly and specifically expressed in human mesencephalic dopamine neurons and that low levels of ALDH1 expression correlate with DA neuron dysfunction in the two investigated human conditions.     

Cholecystokinin potentiates dopamine inhibition of mesencephalic dopamine neurons in vitro

Cholecystokinin octapeptide sulfate (CCK-S) is a neuropeptide that is co-localized with dopamine (DA) in some neurons of the ventral tegmental area (VTA). A functional role for this peptide/monoamine co-localization has not been firmly established; however, behavioral and in vivo electrophysiological studies indicate that CCK-S modifies the action of DA in some brain areas. A brain slice preparation of the rat VTA was developed in order to examine primary effects of CCK-S on DA-containing neurons, and to determine whether CCK-S modulates the inhibitory action of DA on these neurons. Spontaneously active DA neurons of the VTA were identified on the basis of their characteristic spike waveforms and firing rate as determined with extracellular recording techniques. These cells were inhibited by perfusion with DA in a dose-dependent, sulpiride-reversible manner. CCK-S produced brief excitatory increases in firing rate in 83% of these cells tested. This excitation was dose-dependent, and the excitatory responses frequently diminished even in the continued presence of CCK-S. Prior administration of CCK-S to these cells markedly potentiated DA-induced inhibition of spontaneous firing; the magnitude of this effect ranged from a 24 to 376% increase in the inhibitory response. This CCK-induced potentiation of DA inhibition was not blocked by low calcium, high magnesium superfusion medium, indicating that this effect is a direct consequence of a postsynaptic action on the VTA neurons from which recordings were made. These results suggest that co-localized CCK-S may significantly affect neuronal sensitivity to synaptically released DA.     

Group I metabotropic glutamate receptor-mediated enhancement of dopamine cell burst firing in rat ventral tegmental area in vitro

Metabotropic glutamate receptor (mGluR) agonist t-ACPD produced concentration-dependent enhancement of NMDA/apamin-induced burst firing and membrane oscillations in dopamine cells, recorded intracellularly from ventral tegmental area in the rat midbrain slice. Such effects were blocked reversibly by mGluR antagonist MCPG, and mimicked by the selective agonist for group I (but not group II and III) mGluRs. Our results point out a selective involvement of group I mGluRs in facilitating burst firing of midbrain dopamine cells.     

Sensitization occurs to the locomotor effects of morphine and the specific μ opioid receptor agonist, DAGO, administered repeatedly to the ventral tegmental area but not to the nucleus accumbens

Several recent reports have demonstrated that opiate action in both the ventral tegmental area (VTA) and the nucleus accumbens (N.Acc.) produces an increase in locomotor activity. In the present experiments, the effect of repeated bilateral injections into these sites of either morphine or the mu opioid receptor agonist Tyr-d-Ala-Gly-NMe-Phe-Gly-ol (DAGO) was investigated. As previously reported with morphine and other opioids, repeated injections of either morphine or DAGO into the VTA produced a progressive enhancement or sensitization of their locomotor activating effects. On the other hand, although both substances injected into the N.Acc. elicited increased locomotion, repeated injections did not lead to sensitization. It has been suggested that the increased locomotor activity produced by opiate injection into the VTA is dopamine-dependent while that produced by intra-N.Acc. injections is not¹⁵. The present findings provide neuroanatomical support for the view thatsensitization to the locomotor activating effects of opiates and opioids brought about by repeated drug exposure involves the mesolimbic dopamine system.     

Interactions between neuropeptides and dopamine neurons in the ventromedial mesencephalon

Cholecystokinin (CCK), enkephalin, neurotensin (NT), substance P (SP) and substance K (SK) are five neuropeptides that exist in neuronal perikarya or fibers in the vicinity of the A10 dopamine neurons in the ventromedial mesencephalon. Based upon this anatomical proximity, many investigators have been evaluating the possibility that these peptides may influence the function of the A10 dopamine neurons. A variety of experimental techniques have been employed in this regard, including anatomical, electrophysiological, neurochemical and behavioral methodologies. Measurement of immunoreactive peptide levels with radioimmunoassay, and visualization of peptidergic neurons and fibers with immunocytochemistry has demonstrated not only that peptides exist in the vicinity of A10 dopamine neurons, but using double labeling techniques NT and CCK have been found to coexist with dopamine in the same neuron. Further, by combining retrograde tracing techniques with immunocytochemistry, the origin of some peptidergic afferents to the ventromedial mesencephalon has been determined. With the exception of CCK-8, microinjection into the ventromedial mesencephalon of rats with all the peptides or potent analogues produces a dose-related increase in spontaneous motor activity. For SP, NT and enkephalin the motor response has been blocked by dopamine antagonists. Further, an increase in dopamine metabolism in mesolimbic dopamine terminal fields is produced concurrent with the behavioral hyperactivity. These data indicate that SP, SK, enkephalin and NT can activate dopamine neurons in the ventromedial mesencephalon. This postulate is supported by electrophysiological studies showing an excitatory action by iontophoretic administration of peptides onto dopamine neurons. However, in some studies, excitatory electrophysiological effects were not observed. While some observations are contradictory, sufficient data has accumulated that tentative postulates and conclusions can be made about how these peptides may influence the A10 dopamine neurons. Further, speculations are offered as to the role this modulatory action may play in the many behaviors and pathologies thought to involve these dopamine neurons.     

Ethanol acts via the ventral tegmental area to influence hippocampal physiology

Ethanol selectively alters hippocampal dentate physiology, in part by increasing recurrent inhibition and suppressing long-term potentiation (LTP), a result of ethanol modulation of subcortical inputs. One of these inputs includes the ventral tegmental area (VTA) in the midbrain, whose neurons have been shown to discharge faster following systemic ethanol. To further understand how subcortical inputs regulate hippocampal physiology and their modulation by ethanol, we studied the effects of acute intoxicating levels of ethanol on VTA facilitation of the perforant path to dentate (PPD) responses. Furthermore, to test the role of the VTA on known pharmacological effects of ethanol on hippocampal physiology, we studied the effects of disruption of the VTA-dentate inpute on ethanol actions on recurrent inhibition. Stimulation of the perforant path produced well-characterized evoked responses in the ipsilateral dentate gyrus. Whereas VTA stimulation had no effect on PPD population EPSPs, VTA conditioning markedly increased perforant path-evoked PS amplitudes (140%). The maximum facilitation was observed at VTA conditioning intervals of 30-40 ms. PS amplitudes returned to baseline levels immediately following cessation of VTA conditioning. Intraperitoneal injections of ethanol (1.2 g/kg) markedly decreased VTA facilitation of PPD PS amplitudes. Lesions of the VTA blocked the ethanol-mediated increase in PPD paired-pulse inhibition. These results demonstrate that, to a great extent, the effects of intoxicating doses of ethanol on hippocampal physiology are mediated by remote pharmacological effects on the ventral tegmental area, whose direct or indirect influences on dentate physiology are described. © 1994 Wiley-Liss, Inc.     

Effect of extracellular adenosine 5′-triphosphate on principal neurons of the rat ventral tegmental area

Intracellular recordings were made in a midbrain slice preparation of the rat brain containing the ventral tegmental area (VTA). Dopaminergic principal cells were identified by their electrophysiological properties and their hyperpolarizing responses to dopamine. Superfusion with dopamine (100 μM) caused hyperpolarization and a decrease of the apparent input resistance. By contrast, two structural analogues of ATP, 2-methylthio ATP (2-MeSATP; 10 μM) and α,β-methylene ATP (α,β-meATP; 30 μM) had no effect, when added to the superfusion medium. Pressure applied dopamine also hyperpolarized the membrane, while both 2-MeSATP and α,β-meATP were ineffective. Hence, dopaminergic principal neurons of the VTA do not possess somatic P2 purinoceptors present on peripheral and central noradrenergic neurons.     

Raphe neurons and barbiturate induced rhythmic activity in the subthalamic nucleus and ventral tegmental area

A continuous rhythmic discharge in the subthalamic nucleus (STN), lateral habenular nucleus, and ventral tegmental area (VTA) is associated with tremor in cats lightly anesthetized with barbiturate. The pontine raphe nuclei centralis superior and inferior also show barbiturate dependent slow wave and unitary activity. Stimulation of the raphe region produces evoked potentials and synchronization of the oscillatory activity in STN and VTA. The latency of the evoked response is shortest in STN. Extensive raphe lesions abolish the rhythmic activity in STN and VTA, but lesions which do not completely destroy nuclei centralis superior and inferior are not effective. In single cell recordings 81% of 142 cells in the pontine raphe region and medial reticular formation are inhibited following VTA stimulation. Fifty-four percent of 129 cells in the same region were inhibited when tested with STN stimulation. Forty-seven raphe paramedian reticular formation cells showed convergence of inhibitory input from STN, VTA, and contralateral sciatic nerve (Group II and III fibers). Depletion of serotonin with p-chlorophenylalanine did not affect the appearance of barbiturate rhythmic activity. It is concluded that the pontine raphe region is not essential for maintenance of the oscillatory activity in STN and VTA, and that the raphe region is on the output side of the system.     

Responses of ventral tegmental area GABA neurons to brain stimulation reward

Dopamine neurons in the ventral tegmental area (VTA) have been implicated in rewarded behaviors, including intracranial self-stimulation (ICSS). We demonstrate, in unrestrained rats, that the discharge activity of a homogeneous population of presumed VTA GABA neurons, implicated in cortical arousal, increases before ICSS of the medial forebrain bundle (MFB). These findings suggest that VTA GABA neurons may be involved in the attentive processes related to brain stimulation reward (BSR).     

Selective serotonin reuptake inhibitors reduce the spontaneous activity of dopaminergic neurons in the ventral tegmental area

Electrophysiological techniques were used to study the effects of paroxetine, sertraline, and fluvoxamine on the basal activity of dopaminergic neurons in the ventral tegmental area (VTA) of rats. Acute i.v. administrations of paroxetine (20–1280 μg/kg), sertraline (20–1280 μg/kg), and fluvoxamine (20–1280 μg/kg) caused a slight but significant reduction in the firing rate of the VTA dopaminergic cells studied. Paroxetine produced a maximal inhibitory effect of 10 ± 11% at the cumulative dose of 160 μg/kg. Sertraline induced a dose-related inhibition of VTA dopaminergic neurons, which reached its maximum (10 ± 7%) at the cumulative dose of 1280 μg/kg. The effect of fluvoxamine on the basal firing rate of VTA dopaminergic neurons was more pronounced as compared to that of paroxetine and sertraline, in that it produced a maximal inhibition of 17 ± 12% at the cumulative dose of 1280 μg/kg. Acute i.v. injections of paroxetine (20–1280 μg/kg), sertraline (20–1280 μg/kg), and fluvoxamine (20–5120 μg/kg) caused a dose-dependent decrease in the basal firing rate of serotonergic neurons in the dorsal raphé nucleus (DRN). Paroxetine and sertraline stopped the spontaneous firing of serotonergic neurons at the cumulative dose of 1280 μg/kg, whereas fluvoxamine reached the same effect only at the cumulative dose of 5120 μg/kg. Pretreatment with the 5-HT_1A receptor antagonist tertatolol (1 mg/kg, i.v.) reduced the inhibitory effects of paroxetine, fluvoxamine, and sertraline on the basal activity of serotonergic neurons in the DRN. Administration of tertatolol induced a 15-fold increase in the ED₅₀ for fluvoxamine. The antagonistic effect of tertatolol was much less evident in blocking the inhibitory action exerted by paroxetine and sertraline on the activity of serotonergic neurons. Pretreatment with tertatolol (1 mg/kg, i.v.) potentiated the inhibitory effect of fluvoxamine on the basal activity of VTA dopaminergic neurons. Tertatolol did not affect the inhibitory action exerted by paroxetine and sertraline on these neurons. It is concluded that inhibition of the basal firing rate of dopaminergic neurons in the VTA is a common characteristic of selective serotonin reuptake inhibitors (SSRIs). The effects of SSRIs on VTA dopaminergic cell activity might be relevant for their therapeutic action and may explain the origin of the reported cases of akathisia.     

Hypocretin /orexin preferentially activates caudomedial ventral tegmental area dopamine neurons

The hypocretin/orexin (HCRT) neuropeptide system modulates behavioral state and state-dependent processes via actions on multiple neuromodulatory transmitter systems. Recent studies indicate that HCRT selectively increases dopamine (DA) neurotransmission within the prefrontal cortex (PFC) and the shell subregion of the nucleus accumbens (NAs), but not the core subregion of the nucleus accumbens (NAc). The circuitry underlying the differential actions of HCRT across distinct DA systems is unclear. The current study examined whether HCRT preferentially activates PFC- and NAs-projecting relative to NAc-projecting DA neurons within the VTA. One week after infusion of the retrograde tracer fluorogold (FG) into the medial PFC, NAc or NAs, animals received a ventricular infusion of HCRT-1. Subsequent analyses conducted across the rostral-caudal extent of the VTA determined the degree to which: (i) Fos-immunoreactivity (ir) was observed within tyrosine hydroxylase (TH)-ir neurons; (ii) TH-ir was observed within FG-ir neurons; and (iii) Fos-ir was observed within FG-ir neurons. HCRT significantly increased Fos-ir in VTA DA (TH-ir) neurons, primarily in a restricted population of small-to-medium-sized DA neurons located within the caudomedial VTA. Furthermore, within this region of the VTA, PFC- and NAs-projecting TH-ir neurons were more likely to contain Fos-ir than were NAc-projecting TH-ir neurons. These results provide novel evidence that HCRT selectively activates PFC- and NAs-projecting DA neurons within the VTA, and suggest a potential role for HCRT in PFC- and NAs-dependent cognitive and/or affective processes. Moreover, these and other observations suggest that the dysregulation of HCRT-DA interactions could contribute to cognitive/affective dysfunction associated with a variety of behavioral disorders.     

Nicotine injections into the ventral tegmental area increase locomotion and Fos-like immunoreactivity in the nucleus accumbens of the rat

Systemic administration of nicotine has been shown to increase locomotor activity in rats, an effect which is enhanced by chronic pretreatment with the drug. Furthermore, administration of nicotine either systemically, or locally within the ventral tegmental area (VTA), increases extracellular levels of dopamine (DA) in the nucleus accumbens (NAc). In the present study, we examined the effect of local, bilateral injections into the VTA of nicotine (0.02, 0.2, 2.0 and 8.0 μg/0.5μl/side) on locomotor activity of rats in an open field. Nicotine (8.0 μg/side) significantly increased forward locomotion within 20 min after injection, whereas rearing was not affected. The stimulatory effect of locally applied nicotine was completely blocked by pretreatment with mecamylamine (1.0 mg/kg, s.c.). Repeated intra-tegmental injections of a subthreshold dose of nicotine (2.0 μg/side every 2 days), gradually increased locomotion, compared to the effect of acute intra-tegmental administration or control injections of saline, after the fifth and sixth injection. The effects of intra-tegmental injections of nicotine were further investigated on cells in several target areas for the VTA-DA neurons through determination of c-fos expression by means of Fos immunohistochemistry. Intra-tegmental injections of nicotine (8.0 μg/side) increased Fos-like immunoreactivity in the NAc, but did not affect the number of Fos-positive nuclei in the medial prefrontal cortex or in the dorsolateral striatum. The increase in accumbal Fos-like immunoreactivity was attenuated by pretreatment with mecamylamine (1.0 mg/kg, s.c.). Our data demonstrate that locomotor activating effects similar to those evoked by systemically administered nicotine, including behavioral sensitization, can be produced by intra-tegmental nicotine administration. Moreover, such local VTA administration of the drug was found to significantly affect neurons within DA target areas. Our findings support the notion that the effects of systemically administered nicotine in mesolimbic target areas are largely dependent on stimulation of nicotini receptors in the VTA.