The median raphe nucleus (MRN) is one of the main sources of serotonergic ascending projections. Previous electrophysiological studies have shown that serotonergic neurons spontaneously discharge single broad action potentials in a slow clocklike fi ring pattern. Subsequent studies have revealed another subpopulation of serotonin (5-HT) containing cells, which possess the same electrophysiological attributes of classical 5-HT neurons, simultaneously displaying bursting pattern of activity. More recently it has been shown additional class of 5-HT neurons, with relatively fast fi ring activity (more than 8 Hz), implying larger diversity of 5-HT cells populations. The present data confi rm formerly proposed supposition that individual 5-HT cells are able to switch between different activity pattern, earlier described as distinctive subpopulation of these cells. In the present study in vivo extracellular single unit recordings were made from the presumptively identifi ed 5-HT neurons within the MRN of the urethane anesthetized rat. The activity patterns were assessed by constructing interspike interval histograms. In the course of long lasting (up to 10 hours) recordings we have found, that in some cases individual 5-HT neurons have changed the pattern of activity from regular to less regular, bursting or nearly silent. This heterogeneity of fi ring modes is more suitable to an explanation of the role that serotonergic system plays in diverse physiological and pathological processes.
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