EVIDENCE FOR A REPETITIVE (BURST) FIRING PATTERN IN A SUBPOPULATION OF 5-HYDROXYTRYPTAMINE NEURONS IN THE DORSAL AND MEDIAN RAPHE NUCLEI OFTHE RAT

Previous electrophysiological studies have shown that spontaneously ac tive 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 fro m dorsal and median raphe neurons of the anaesthetized rat, an additio nal electrophysiological property of a sub-population of presumed 5-hy droxytryptaminergic neurons was observed. These neurons, during their otherwise regular firing pattern, repeatedly fired two (or occasionall y three or even four) spikes where only one was expected. Spikes in th is 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 dim inishing spike amplitude. Cross-correlation analysis of spikes in doub lets revealed a very high interdependency between them. The proportion of spikes in doublers to solitary spikes showed great variation betwe en different neurons, ranging from 5 to 95% of the total spikes displa yed. 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 c haracteristic of single neurons firing in a repetitive firing pattern rather than simultaneous recordings of two separate 5-hydroxytryptamin ergic neurons. Repetitive firing neurons were recorded with a similar frequency in both chloral hydrate and Saffan anaesthetized rats, and w ere detected using both glass and metal electrodes. Furthermore, neuro ns with a repetitive firing pattern were inhibited by intravenous admi nistration of a selective 5-hydroxytryptamine(1A) receptor agonist and a 5-hydroxytryptamine reuptake inhibitor, thus displaying responses t ypical of 5-hydroxytryptaminergic neurons. Repetitive firing neurons o ccurred in both the dorsal and median raphe nuclei, although they were much more Frequent in the dorsal raphe nucleus (91 of 332 neurons).Th e 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 the axonal and dendritic 5-hydroxytryptami ne release, and provide an additional option For neuronal information signalling.