DYE-COUPLING BETWEEN DORSAL RAPHE NEURONS

Neurones in the dorsal raphe nucleus (DRN) were impaled and filled wit h biocytin in coronal slices of midbrain taken from young adult rats. The electrophysiological properties and gross morphology of the cells were similar to those reported previously for serotonergic neurones in the DRN. Of 27 cases in which filled neurones were recovered in histo logical material, almost half (48%) showed labelling of two or three c ells, although only one cell had been recorded from. Coupled cells wer e identified as close or distantly coupled, depending on the distance from the soma of the presumed impaled cell (23.5+/-15 mu m, n=7 and 15 0+/-26.5 mu m, n=10 respectively). Whereas close-coupled cells may hav e been artefactually ''coupled'' by the penetrating electrode, couplin g between distant cells is most likely to be a result of transfer of b iocytin through gap junctions. Camera lucida reconstructions of pairs of labelled cells revealed extensive overlap of dendritic fields and n umerous crossings between dendrites. When examined at high magnificati on under a light microscope, many of the crossing dendrites were found to travel in different focal planes. Nevertheless, for each pair of c ells, at least one point of close apposition was observed between den drites or between the axon and a dendrite of the presumed impaled and coupled cell. The incidence of dye coupling between neurones in the DR N may reflect a relatively high level of electrotonic coupling between the neurones. This form of coupling may be important in determining t he synchronous nature of firing of neurones in the DRN.