Burst-firing activity of presumed 5-HT neurones of the rat dorsal raphe nucleus: electrophysiological analysis by antidromic stimulation.

We recently reported raphe neurones which frequently fired spikes in short bursts. However, the action potentials were broad and the neurones fired in a slow and regular pattern, suggesting they were an unusual type of 5-hydroxytryptamine (5-HT) neurone. In the present study, we investigated whether these putative burst-firing 5-HT neurones project to the forebrain and whether all spikes fired in bursts propagate along the axon. In anaesthetised rats, electrical stimulation of the medial forebrain bundle evoked antidromic spikes in both burst-firing neurones and in single-spiking, classical 5-HT neurones recorded in the dorsal raphe nucleus. Although the antidromic spike latency of the single-spiking and burst-firing neurones showed a clear overlap, burst-firing neurones had a significantly shorter latency than single-spiking neurones. For both burst-firing neurones and classical 5-HT neurones, antidromic spikes made collisions with spontaneously occurring spikes. Furthermore, in all burst-firing neurones tested, first, second and third order spikes in a burst could be made to collide with antidromic spike. Interestingly, in a small number of burst-firing neurones, antidromic stimulation evoked spike doublets, similar to those recorded spontaneously. From these data we conclude that burst-firing neurones in the dorsal raphe nucleus project to the forebrain, and each spike generated by the burst propagates along the axon and could thereby release transmitter (5-HT).