Alpha-1 Adrenergic Receptors Modulate Glutamate and GABA Neurotransmission onto Ventral Tegmental Dopamine Neurons during Cocaine Sensitization.

The ventral tegmental area (VTA) plays an important role in the reward and motivational processes that facilitate the development of drug addiction. Presynaptic α1-AR activation modulates glutamate and Gamma-aminobutyric acid (GABA) release. This work elucidates the role of VTA presynaptic α1-ARs and their modulation on glutamatergic and GABAergic neurotransmission during cocaine sensitization. Excitatory and inhibitory currents (EPSCs and IPSCs) measured by a whole cell voltage clamp show that α1-ARs activation increases EPSCs amplitude after 1 day of cocaine treatment but not after 5 days of cocaine injections. The absence of a pharmacological response to an α1-ARs agonist highlights the desensitization of the receptor after repeated cocaine administration. The desensitization of α1-ARs persists after a 7-day withdrawal period. In contrast, the modulation of α1-ARs on GABA neurotransmission, shown by decreases in IPSCs' amplitude, is not affected by acute or chronic cocaine injections. Taken together, these data suggest that α1-ARs may enhance DA neuronal excitability after repeated cocaine administration through the reduction of GABA inhibition onto VTA dopamine (DA) neurons even in the absence of α1-ARs' function on glutamate release and protein kinase C (PKC) activation. α1-AR modulatory changes in cocaine sensitization increase our knowledge of the role of the noradrenergic system in cocaine addiction and may provide possible avenues for therapeutics.

Putative dopamine neurons in the ventral tegmental area enhance information coding in the prefrontal cortex.

It has been proposed that neuronal populations in the prefrontal cortex (PFC) robustly encode task-relevant information through an interplay with the ventral tegmental area (VTA). Yet, the precise computation underlying such functional interaction remains elusive. Here, we conducted simultaneous recordings of single-unit activity in PFC and VTA of rats performing a GO/NoGO task. We found that mutual information between stimuli and neural activity increases in the PFC as soon as stimuli are presented. Notably, it is the activity of putative dopamine neurons in the VTA that contributes critically to enhance information coding in the PFC. The higher the activity of these VTA neurons, the better the conditioned stimuli are encoded in the PFC.

Lateral septum stimulation disinhibits dopaminergic neurons in the antero-ventral region of the ventral tegmental area: Role of GABA-A alpha 1 receptors.

The mechanisms commanding the activity of dopaminergic neurons of the ventral tegmental area (VTA) and the location of these neurons are relevant for the coding and expression of motivated behavior associated to reward-related signals. Anatomical evidence shows that several brain regions modulate VTA dopaminergic neurons activity via multiple mechanisms. However, there is still scarce knowledge of how the lateral septum (LS) modulates VTA activity. We performed in-vivo dual-probe microdialysis to measure VTA dopamine, glutamate and GABA extracellular levels after LS stimulation in the presence or absence of GABAergic antagonists. Anterograde tracing and immunohistochemical analysis was used to reveal the anatomical relationship between LS and VTA. LS stimulation significantly increased dopamine and GABA, but not glutamate, VTA extracellular levels. Intra VTA infusion of bicuculline, GABA-A receptor antagonist, inhibited the increase of dopamine but not of GABA VTA levels induced by LS stimulation. Intra VTA infusion of indiplon, selective positive allosteric modulator of GABA-A receptors containing alpha1 subunit, significantly increases VTA dopamine extracellular levels induced by LS. Combined c-Fos and tyrosine hydroxylase immunohistochemistry, revealed that LS stimulation increases the activity of dopaminergic neurons in the antero-ventral region of the VTA. Consistently, anterograde tracing with biotinylated dextran amine revealed the existence of fibers arising from the LS to the antero-ventral region of the VTA. Taken together, our results suggest that LS modulates dopaminergic activity in the antero-ventral region of VTA by inhibiting GABAergic interneurons bearing GABA-A receptors containing alpha1 subunit.

The cytoarchitectonic and TH-immunohistochemical characterization of the dopamine cell groups in the substantia nigra, ventral tegmental area and retrorubral field in a bat (Artibeus planirostris).

The dopamine (DA) neurons of the retrorubral field (RRF - A8), the substantia nigra (SN - A9), and the ventral tegmental area (VTA - A10) have been implicated in motor regulation, reward, aversion, cognition, and several neuropsychiatric disorders. A series of studies have identified subdivisions of these cell groups in rodents, but these cell groups have not been well described in bats. An understanding of the motor system organization in bats would provide a context for comparing motor systems across rodent, primate, and bat phylogenies. The aim of this work was to determine whether typical subdivisions of RRF, SN, and VTA are present in Artibeus planirostris, a common frugivorous bat species found throughout South America. Coronal and sagittal sections of bat brain were subjected to Nissl staining and TH immunohistochemistry. The organizational pattern of the nuclei in A. planirostris showed a conspicuous tail in the SN, which has been not described in bats to date, and also contained a well-defined substantia nigra reticulata (SNR) not previously reported in microbats. This work provides for the first time a morphometric analysis of DA neurons in a microchiropteran species, enabling a comparative investigation of vertebrates. Our analysis revealed an apparent phylogenetic stability in these structures, although the SN tail might represent a functional specialization in this species.

Escalation in high fat intake in a binge eating model differentially engages dopamine neurons of the ventral tegmental area and requires ghrelin signaling.

Binge eating is a behavior observed in a variety of human eating disorders. Ad libitum fed rodents daily and time-limited exposed to a high-fat diet (HFD) display robust binge eating events that gradually escalate over the initial accesses. Intake escalation is proposed to be part of the transition from a controlled to a compulsive or loss of control behavior. Here, we used a combination of behavioral and neuroanatomical studies in mice daily and time-limited exposed to HFD to determine the neuronal brain targets that are activated--as indicated by the marker of cellular activation c-Fos--under these circumstances. Also, we used pharmacologically or genetically manipulated mice to study the role of orexin or ghrelin signaling, respectively, in the modulation of this behavior. We found that four daily and time-limited accesses to HFD induce: (i) a robust hyperphagia with an escalating profile, (ii) an activation of different sub-populations of the ventral tegmental area dopamine neurons and accumbens neurons that is, in general, more pronounced than the activation observed after a single HFD consumption event, and (iii) an activation of the hypothalamic orexin neurons, although orexin signaling blockage fails to affect escalation of HFD intake. In addition, we found that ghrelin receptor-deficient mice fail to both escalate the HFD consumption over the successive days of exposure and fully induce activation of the mesolimbic pathway in response to HFD consumption. Current data suggest that the escalation in high fat intake during repeated accesses differentially engages dopamine neurons of the ventral tegmental area and requires ghrelin signaling.

Differential projections from the lateral habenula to the rostromedial tegmental nucleus and ventral tegmental area in the rat.

The mesopontine rostromedial tegmental nucleus (RMTg) is a mostly γ-aminobutyric acid (GABA)ergic structure believed to be a node for signaling aversive events to dopamine (DA) neurons in the ventral tegmental area (VTA). The RMTg receives glutamatergic inputs from the lateral habenula (LHb) and sends substantial GABAergic projections to the VTA, which also receives direct projections from the LHb. To further specify the topography of LHb projections to the RMTg and VTA, small focal injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin were aimed at different subdivisions of the LHb. The subnuclear origin of LHb inputs to the VTA and RMTg was then confirmed by injections of the retrograde tracer cholera toxin subunit b into the VTA or RMTg. Furthermore, we compared the topographic position of retrogradely labeled neurons in the RMTg resulting from VTA injections with that of anterogradely labeled axons emerging from the LHb. As revealed by anterograde and retrograde tracing, LHb projections were organized in a strikingly topographic manner, with inputs to the RMTg mostly arising from the lateral division of the LHb (LHbL), whereas inputs to the VTA mainly emerged from the medial division of the LHb (LHbM). In the RMTg, profusely branched LHb axons were found in close register with VTA projecting neurons and were frequently apposed to the latter. Overall, our findings demonstrate that LHb inputs to the RMTg and VTA arise from different divisions of the LHb and provide direct evidence for a disynaptic pathway that links the LHbL to the VTA via the RMTg.

Alpha-2 noradrenergic receptor activation inhibits the hyperpolarization-activated cation current (Ih) in neurons of the ventral tegmental area.

The ventral tegmental area (VTA) is the source of dopaminergic projections innervating cortical structures and ventral forebrain. Dysfunction of this mesocorticolimbic system is critically involved in psychiatric disorders such as addiction and schizophrenia. Changes in VTA dopamine (DA) neuronal activity can alter neurotransmitter release at target regions which modify information processing in the reward circuit. Here we studied the effect of alpha-2 noradrenergic receptor activation on the hyperpolarization-activated cation current (I(h)) in DA neurons of the rat VTA. Brain slice preparations using whole-cell current and voltage-clamp techniques were employed. Clonidine and UK14304 (alpha-2 receptor selective agonists) were found to decrease I(h) amplitude and to slow its rate of activation indicating a negative shift in the current's voltage dependence. Two non-subtype-selective alpha-2 receptor antagonists, yohimbine and RS79948, prevented the effects of alpha-2 receptor activation. RX821002, a noradrenergic antagonist specific for alpha-2A and alpha-2D did not prevent I(h) inhibition. This result suggests that clonidine might be acting via an alpha-2C subtype since this receptor is the most abundant variant in the VTA. Analysis of a second messenger system associated with the alpha-2 receptor revealed that I(h) inhibition is independent of cyclic AMP (cAMP) and resulted from the activation of protein kinase C. It is suggested that the alpha-2 mediated hyperpolarizing shift in I(h) voltage dependence can facilitate the transition from pacemaker firing to afferent-driven burst activity. This transition may play a key role on the changes in synaptic plasticity that occurs in the mesocorticolimbic system under pathological conditions.

Noradrenergic modulation of the hyperpolarization-activated cation current (Ih) in dopamine neurons of the ventral tegmental area.

Alterations in the state of excitability of midbrain dopamine (DA) neurons from the ventral tegmental area (VTA) may underlie changes in the synaptic plasticity of the mesocorticolimbic system. Here, we investigated norepinephrine's (NE) regulation of VTA DA cell excitability by modulation of the hyperpolarization-activated cation current, Ih, with whole cell recordings in rat brain slices. Current clamp recordings show that NE (40 microM) hyperpolarizes spontaneously firing VTA DA cells (11.23+/-4 mV; n=8). In a voltage clamp, NE (40 microM) induces an outward current (100+/-24 pA; n=8) at -60 mV that reverses at about the Nernst potential for potassium (-106 mV). In addition, NE (40 microM) increases the membrane cord conductance (179+/-42%; n=10) and reduces Ih amplitude (68+/-3% of control at -120 mV; n=10). The noradrenergic alpha-1 antagonist prazosin (40 microM; n=5) or the alpha-2 antagonist yohimbine (40 microM; n=5) did not block NE effects. All NE-evoked events were blocked by the D2 antagonists sulpiride (1 microM) and eticlopride (100 nM) and no significant reduction of Ih took place in the presence of the potassium channel blocker BaCl2 (300 microM). Therefore, it is concluded that NE inhibition of Ih was due to an increase in membrane conductance by a nonspecific activation of D2 receptors that induce an outward potassium current and is not a result of a second messenger system acting on h-channels. The results also suggest that Ih channels are mainly located at dendrites of VTA DA cells and, thus, their inhibition may facilitate the transition from single-spike firing to burst firing and vice versa.

Astroglial and microglial activation in the wistar rat ventral tegmental area after a single striatal injection of 6-hydroxydopamine.

Astroglial and microglial activation were analyzed in the ventral tegmental area (VTA) in adult male Wistar rats, after an unilateral striatal 6-hydroxydopamine (6-OHDA) injection. Different doses (8, 4, and 1 microg) of 6-OHDA were injected in the left side of the neostriatum; animals were sacrificed 22 days later. Control animals received an injection of the same volume of the solvent. The tyrosine hydroxylase (TH) positive dopamine cells, the glial fibrillary acidic protein (GFAP) immuno -labeled astrocytes, and the OX42 immunoreactive microglia were visualized by means of immunohistochemistry and quantified by stereologic methods employing the optical dissector and the point intercepts. The number and the density of TH immunoreactive cell bodies were decreased by 45% and 46%, respectively, in the sampled field of the ipsilateral VTA of 8 microg 6-OHDA injected rats. The GFAP immunohistochemistry revealed in the ipsilateral VTA increases the number and density of astroglial cells (154% and 166% of control, respectively) in the rats with a higher dose of the 6-OHDA, and also in the volume fraction of the astroglial processes after 8 microg (41% of control) and 4 microg (24% of control) of 6-OHDA. Increased number (76% of control) and density (77% of control) of OX42 microglial labeled profiles and microglial processes (51% of control) were found in the ipsilateral VTA of the 8 microg 6-OHDA injected animals. These results suggest that the retrograde degeneration of the mesostriatal dopamine pathways, induced by a striatal injection of 6-OHDA, leads to astroglial and microglial reactions in the VTA. The interaction between activated glial cells may be involved in the wounding and repair events in the partial lesioned system, and also in the trophic paracrine responses in the surviving VTA dopamine neurons.

Dopaminergic ventral tegmental neurons modulated by methylphenidate.

Treatment of psychostimulants leads to the development of behavioral sensitization, an augmented behavioral response to drug re-administration. The induction of behavioral sensitization to psychostimulants such as amphetamine and cocaine occurs at the ventral tegmental area's dopaminergic neurons (VTA-DA). Currently, there is limited experimental data about the physiological properties of methylphenidate (MPD) on VTA-DA neurons. Behavioral and electrophysiological experiments using male rats were performed before and after MPD treatment. The behavioral experiment included dose-response (0.6, 2.5, and 10.0 mg/kg MPD) study to select the most effective dose for the electrophysiological study. Methylphenidate increased locomotion in typical dose response characteristics. Based on this experiment, the 10.0 mg/kg MPD was used in two types of electrophysiological recordings: 1) intracellular recording of neuronal activity performed on horizontal 275-300 microm brain slices and 2) whole-cell patch clamping before and after electrical stimulation to study post-synaptic currents on neurophysiologically identified VTA-DA neurons. Methylphenidate suppressed the neuronal activity of these neurons for 210 +/- 30 sec. Stimulation of the prefrontal cortex afferent fibers to these VTA-DA neurons in the presence of TTX, saclofen, and picrotoxin led to the conclusion that this input is mediated via NMDA and kainate/AMPA receptors and may participate to induce behavioral sensitization to psychostimulants.

Origin of the dopaminergic innervation of the central extended amygdala and accumbens shell: a combined retrograde tracing and immunohistochemical study in the rat.

The origin of the dopaminergic innervation of the central extended amygdala (EAc; i.e., the lateral bed nucleus of the stria terminalis [BSTl]-central amygdaloid nucleus [Ce] continuum) and accumbens shell (AcSh) was studied in the rat by combining retrograde transport of Fluoro-Gold (FG) with tyrosine hydroxylase (TH) immunofluorescence. Perikaryal profiles (PP) immunoreactive to FG and to both FG and TH were counted in A8-A14 dopaminergic districts. Our results suggest that dopaminergic inputs to the EAc and AcSh arise from the ventral tegmental area-A10, substantia nigra, pars compacta-A9, and retrorubral nucleus-A8 groups as well as from the dorsal raphe nucleus and periaqueductal gray substance, housing the dorsocaudal part of A10 group (A10dc). Quantitative estimates reveal that the A10dc group contains approximately half of the total number of FG/TH double-labeled PP projecting to Ce and BSTl. By using an anti-dopamine serum, DR/PAG projections to Ce were confirmed to be in part dopaminergic. In contrast, modest numbers of FG/TH double-labeled PP were seen in the A10dc group after injections in the sublenticular extended amygdala, interstitial nucleus of the posterior limb of the anterior commissure or AcSh. Ventral mesencephalic projections to the EAc display a crude mediolateral topographic organization, whereas those to the AcSh are topographically organized along a mediolateral and an inverted dorsoventral dimension. The diencephalic dopaminergic groups do not innervate the EAc or AcSh, except for the periventricular gray-A11 which sends light dopaminergic projections to Ce and BSTl. Overall, the present results provide additional details on the organization of the mesolimbic dopaminergic system that critically controls behavioral responsiveness to salient environmental stimuli.

Acute ethanol administration differentially modulates mu opioid receptors in the rat meso-accumbens and mesocortical pathways.

Biochemical and pharmacological evidence suggest that the dopaminergic mesolimbic system plays a key role in mediating the reinforcing properties of alcohol and other drugs of abuse. Alcohol reinforcement and high alcohol drinking behavior have been postulated to be partially mediated by a neurobiological mechanism involving the alcohol-induced activation of the endogenous opioid system. The aim of this work was to study the effect of the in vivo acute administration of ethanol on mu (mu) opioid receptors in the rat dopaminergic meso-accumbens and mesocortical pathways by quantitative receptor autoradiography. [(3)H]DAMGO binding was significantly decreased in the ventral tegmental area (VTA) 30 min after ethanol administration. A small ethanol-induced reduction was observed in the shell region of the nucleus accumbens 1 h after exposure. In contrast, 2 h after ethanol administration, [(3)H]DAMGO binding was significantly increased in the frontal and prefrontal cortices. The observed changes correlated well with high ethanol plasma levels. Our results suggest that the reinforcing properties of ethanol may be partially mediated by mechanisms involving the ethanol-induced down- and up-regulation of mu receptors in the dopaminergic mesolimbic system. Mu receptors in the VTA and the frontal and prefrontal cortices may be involved in the in vivo acute responses to ethanol and could play a key role in modulating the dopaminergic activity of the mesocortical pathway in response to the drug. In contrast, the contribution of both mu and delta receptors in the nucleus accumbens might be relevant in these processes.

Lesion and electrical stimulation of the ventral tegmental area modify persistent nociceptive behavior in the rat.

The ventral tegmental area (VTA) has been traditionally related with the control of motor responses. However, some studies show that this area is also involved in the processing of nociceptive information. It has been reported that this nucleus participates in the dissociative analgesia phenomenon. In the few works where electrical stimulation and lesion of the VTA have been performed, evaluated with persistent or chronic pain related behaviors, contradictory results have been obtained. Thus, a more detailed analysis of the role of the VTA in persistent pain is needed. Two series of experiments were performed: lesions of this nucleus were done with radiofrequency, (bilaterally at two points per side using a temperature range from 50 to 80 degrees C), and the VTA was electrically stimulated (10 min daily over 5 days, 2 ms rectangular pulses at 100 Hz during 1 s every 5 s) using two different schemes:10 min before the induction of the nociceptive stimulus and 90 min after the induction of the nociceptive stimulus. The latter allowed us to distinguish if the VTA electrical stimulation had a distinctive antinociceptive effect when applied before or after the induction of the nociceptive stimulus on a persistent pain related behavioral response in the rat, the self injury behavior (SIB). Our results showed that VTA lesions enhanced the occurrence of SIB; while activation of this same nucleus by electrical stimulation after the nociceptive stimulus, but not before, facilitates the analgesic process, expressed as a 1 day delay in SIB onset. These results indicate that the VTA is a brain structure that plays a key role in the processing and modulation of persistent pain information. Data are discussed in terms of the relationship of the VTA with the affective component of pain.