Passive membrane properties of neurones in the dorsal raphe and periaqueductal grey recorded in vitro.

A slice preparation of the rat mesencephalon containing the dorsal and medial raphe nucleus, the periaqueductal grey, the superior colliculi and the reticular formation is described. Intracellular recordings showed marked differences in the passive membrane properties of neurones of the dorsal raphe. Serotonin-containing neurons were characterized by a high membrane input resistance, a very long time constant and by the presence of membrane rectification only in a very hyperpolarized (less than -120 mV) region of their voltage-current relationship. In most of the neurones in and around the dorsal raphe area a brief pulse of depolarizing current was followed by a pronounced after-hyperpolarization, which appeared to be mediated by the activation of a Ca2+ dependent-K+ conductance.