Evidence for a repetitive (burst) firing pattern in a sub-population of 5-hydroxytryptamine neurons in the dorsal and median raphe nuclei of the rat.

Previous electrophysiological studies have shown that spontaneously active mesencephalic 5-hydroxytryptaminergic neurons of anaesthetized or freely moving animals fire solitary spikes in a slow, regular pattern. In the present study, using extracellular single unit recordings from dorsal and median raphe neurons of the anaesthetized rat, an additional electrophysiological property of a sub-population of presumed 5-hydroxytryptaminergic neurons was observed. These neurons, during their otherwise regular firing pattern, repeatedly fired two (or occasionally three or even four) spikes where only one was expected. Spikes in this burst-like repetitive firing mode (spikes in doublets or triplets) occurred in a short time interval (range: 2.4-11.5 ms), and with a diminishing spike amplitude. Cross-correlation analysis of spikes in doublets revealed a very high interdependency between them. The proportion of spikes in doublets to solitary spikes showed great variation between different neurons, ranging from 5 to 95% of the total spikes displayed. However, for each neuron the proportion of spikes in doublets to solitary spikes, and the time interval between the spikes in doublets, remained constant during control recordings. All these features are characteristic of single neurons firing in a repetitive firing pattern rather than simultaneous recordings of two separate 5-hydroxytryptaminergic neurons. Repetitive firing neurons were recorded with a similar frequency in both chloral hydrate and Saffan anaesthetized rats, and were detected using both glass and metal electrodes. Furthermore, neurons with a repetitive firing pattern were inhibited by intravenous administration of a selective 5-hydroxytryptamine1A receptor agonist and a 5-hydroxytryptamine reuptake inhibitor, thus displaying responses typical of 5-hydroxytryptaminergic neurons. Repetitive firing neurons occurred in both the dorsal and median raphe nuclei, although they were much more frequent in the dorsal raphe nucleus (91 of 332 neurons). The occurrence of repetitive firing neurons in the midbrain raphe nuclei is a newly described phenomenon which may indicate unique properties of a sub-population of 5-hydroxytryptaminergic neurons. In functional terms, it could modify both axonal and dendritic 5-hydroxytryptamine release, and provide an additional option for neuronal information signalling.