Differential autoinhibition of 5-hydroxytryptamine neurons by 5-hydroxytryptamine in the dorsal raphe nucleus

5-Hydroxytryptamine neurons in the dorsal raphe nucleus are under autoinhibitory control by endogenous 5-hydroxytryptamine. Tonic activation of 5-hydroxytryptamine 1A autoreceptors was demonstrated in awake animals, but was inconsistently observed in anaesthetized animals and slice preparations, leading to questioning of its physiological significance. We re-evaluated autoinhibition in single-unit recordings from deeply seated 5-hydroxytryptamine neurons in slices in which endogenous 5-hydroxytryptamine bioavailability was restored by supplementing its precursor L-tryptophan. In these conditions, the application of the neutral 5-hydroxytryptamine 1A receptor antagonist WAY-100635 markedly increased 5-hydroxytryptamine neuron firing. Responses to WAY-100635 in single experiments ranged from a lack of effect to a several-fold increase in firing rate, suggesting that 5-hydroxytryptamine neurons in the dorsal raphe nucleus represent a heterogeneous population regarding their susceptibility to autoinhibition by endogenous 5-hydroxytryptamine.