Nicotine stimulation of dorsal raphe neurons: effects on laterodorsal and pedunculopontine neurons.

Previous studies showed that nicotine suppresses the ponto-geniculo-occipital (PGO) spikes of rapid eye movement (REM) sleep in cats. This effect may depend on stimulation of dorsal raphe nucleus (DRN) serotoninergic neurons that inhibit the pedunculopontine (PPT) and laterodorsal tegmental (LDT) cholinergic neurons, generators of PGO spikes. For testing this hypothesis 37 experiments were performed in rat midbrain slices. Nicotine (2 mM), administered locally into DRN, significantly increased the firing rate of 81.1% DRN neurons and serotonin release while simultaneously and significantly decreasing the firing rate of 80.8% LDT neurons and of 81.8% PPT neurons. The inhibition of LDT neurons by nicotine administered into DRN was blocked by the 5-HT1A receptor antagonist WAY-100635 (140 nM) administered into LDT. These results indicate that nicotine inhibits the activity of LDT and PPT neurons and consequently the generation of PGO spikes through stimulation of DRN serotoninergic neurons.