Inhibition of non-dopamine cells in the ventral tegmental area by benzodiazepines: relationship to A10 dopamine cell activity.

Previous electrophysiological studies have demonstrated that non-dopaminergic (non-DA) neurons within the substantia nigra pars reticulata (SNR) are extremely sensitive to the inhibitory effects of GABA and GABA-mimetic drugs, including benzodiazepines, whereas dopaminergic (DA) neurons in the substantia nigra pars compacta (SNC) are less sensitive to these compounds and may be influenced indirectly by SNR neurons. The interactions between A10 DA and non-DA neurons within the adjacent ventral tegmental area (VTA) are not as well characterized. In the present experiments, single unit recording and microiontophoretic techniques were used to determine the effects of benzodiazepines on DA and non-DA neurons in the VTA of chloral hydrate anesthetized rats. Diazepam, administered intravenously (i.v.), potently inhibited non-DA, SNR-like cells within the VTA. The effects of diazepam on A10 DA cells were more variable than those observed on non-DA, SNR-like cells in this region, but 77% of such cells showed moderate to marked excitation. Both of these effects were reversed by the benzodiazepine antagonist Ro 15-1788; on many cells, this agent produced marked rebound effects beyond the original basal firing rates. However, when administered alone, Ro 15-1788 exerted no effect on either cell population. Microiontophoretic administration of the benzodiazepines chlordiazepoxide and flurazepam resulted in marked inhibition of non-DA SNR-like cells, but produced either mild inhibition or no effect on A10 DA cells; excitation of DA cells was never observed even though the same neuron was excited by i.v. diazepam. These findings suggest that benzodiazepines act directly upon non-DA, SNR-like cells in the VTA to produce inhibition of activity and a disinhibition of A10 DA cells. This relationship makes it unlikely that benzodiazepines would enhance feedback inhibition of DA cells following neuroleptic administration. In fact, when administered following haloperidol, i.v. diazepam failed to reverse haloperidol-induced increases of A10 DA cell firing; if anything, diazepam further depolarized the cell. If antipsychotic drugs produce their clinical effects, in part, by inducing depolarization inactivation of DA cells, then benzodiazepines might be a useful adjunctive therapy in the treatment of schizophrenia.