In this study, we examined the effect of 5-HT depletion produced by the acute administration of para-chlorophenylalanine (PCPA) on the number of spontaneously active dopamine (DA) cells in the ventral tegmental area (VTA or A10) and substantia nigra pars compacta (SNC or A9) in the rat. We also determined the effect of PCPA administration on the spike discharge pattern of midbrain DA cells. This was accomplished using standard extracellular single cell recording techniques. The administration of PCPA (400 mgkg, i.p., 24 h before the experiment) produced a significant decrease in the number of spontaneously active DA cells in both the A9 (52%) and A10 (63%) areas compared to controls. The burst firing analysis indicated that there was a significant increase in the mean interspike interval of A9 and A10 DA neurons in PCPA treated animals compared to controls. Furthermore, a decrease in the percentage of A10 DA neurons exhibiting a burst firing pattern and the number of bursts was observed in the PCPA treated animals compared to controls. The intravenous (i.v.) administration of 5-hydroxytryptophan (40 mgkg) and the peripheral aromatic acid decarboxylase inhibitor benserazide (10 mgkg) which restores 5-HT content, reversed the decrease in the number of spontaneously active A9 and A10 DA neurons, as well as the decrease in the percentage of A10 DA neurons exhibiting a bursting pattern. In contrast, the i.v. administration of benserazide (10 mgkg) and L-DOPA (40 mgkg) did not reverse the decrease in the number of spontaneously active midbrain DA neurons produced by PCPA treatment. The pretreatment of animals with PCPA did not alter the sensitivity of spontaneously active A9 or A10 DA cells to the intravenous administration of (+)apomorphine (1-32 pgkg) compared to controls. Overall, our results indicate that the depletion of brain 5-HT by PCPA produces a decrease in the activity of midbrain DA cells, suggesting that endogenous 5-HT is required to maintain DA tone. o 1996 Wiley-Liss, Inc.