Reversal of inhibition of putative dopaminergic neurons of the ventral tegmental area: interaction of GABA(B) and D2 receptors.

Neurons of the ventral tegmental area (VTA) are critical in the rewarding and reinforcing properties of drugs of abuse. Desensitization of VTA neurons to moderate extracellular concentrations of dopamine (DA) is dependent on protein kinase C (PKC) and intracellular calcium levels. This desensitization is called DA inhibition reversal, as it requires concurrent activation of D2 and D1-like receptors; activation of D2 receptors alone does not result in desensitization. Activation of other G-protein-linked receptors can substitute for D1 activation. Like D2 receptors, GABA(B) receptors in the VTA are coupled to G-protein-linked potassium channels. In the present study, we examined interactions between a GABA(B) agonist, baclofen, and dopamine agonists, dopamine and quinpirole, to determine whether there was some interaction in the processes of desensitization of GABA(B) and D2 responses. Long-duration administration of baclofen alone produced reversal of the baclofen-induced inhibition indicative of desensitization, and this desensitization persisted for at least 60 min after baclofen washout. Desensitization to baclofen was dependent on PKC. Dopamine inhibition was reduced for 30 min after baclofen-induced desensitization and conversely, the magnitude of baclofen inhibition was reduced for 30 min by long-duration application of dopamine, but not quinpirole. These results indicate that D2 and GABA(B) receptors share some PKC-dependent mechanisms of receptor desensitization.

Pharmacological reduction of small conductance calcium-activated potassium current (SK) potentiates the excitatory effect of ethanol on ventral tegmental area dopamine neurons.

Dopaminergic neurons in the ventral tegmental area (VTA) are important for the rewarding properties of drugs of abuse, including ethanol. We previously demonstrated that ethanol excites VTA neurons and that ethanol reduces the amplitude of the after hyperpolarization (AHP) that follows spontaneous action potentials. Because the small conductance calcium-activated potassium current (SK) is a component of the AHP of VTA neurons, we assessed the effect of the SK blockers apamin and d-tubocurarine (d-TC) on the action of ethanol on dopaminergic VTA neurons with intracellular and extracellular recording in rat brain slices. Apamin (1-200 nM) and d-TC (100 and 400 microM) caused concentration-dependent reductions in the AHP amplitude. Ethanol (80 mM) caused a small reduction in the AHP. In the presence of apamin (40 nM), ethanol (80 mM) caused a much larger reduction in AHP amplitude. Extracellular studies showed that apamin (20, 40, and 100 nM) and d-TC (400 microM) had no significant effect on the spontaneous firing rate of dopaminergic VTA neurons but enhanced the potency of ethanol to increase their firing rate. These results indicate that the ethanol-induced reduction of the AHP and excitation of VTA neurons is not due to a reduction in SK current. Furthermore, blockade of SK current by apamin or d-TC enhances the excitatory effect of ethanol on dopaminergic VTA neurons. These data suggest that the amount of SK current present affects the sensitivity of dopaminergic VTA neurons to ethanol excitation and that neurotransmitters that reduce SK current may increase the reward potency of ethanol.