Raphe serotonergic neurons projecting to the olfactory bulb contain galanin or somatostatin but not neurotensin

Abstract

Retrograde axonal transport with [3H]5-HT has been developed as a specific tracing technique to identify serotonergic projections. This method, in combination with immunocytochemistry, offers considerable advantage of specificity and sensitivity to study pathways of multitransmitter-containing neurons. In this work, we studied the presence of galanin, somatostatin, and neurotensin in serotonergic neurons of dorsal and median raphe, which project to the olfactory bulbs. After [3H]5-HT injections into the rat olfactory bulbs, double galanin-immunoreactive and [3H]5-HT radiolabelled cells were located in the dorsal, lateral, and ventral region of dorsal raphe, but they were never seen in the median raphe. In the dorsal raphe, galanin-radiolabelled neurons represented 28% of the total number of radiolabelled cells. Double somatostatin-immunoreactive and radiolabelled neurons were located in the dorsal and median raphe. In the dorsal raphe, double somatostatin-radiolabelled neurons represented only 11% of the radiolabelled cells and they were mainly located ventral to the aqueduct. In the median raphe, 15% of radiolabelled cells were also immunopositive for somatostatin. In contrast, neurotensin-immunoreactive cells in the dorsal and median raphe were distributed among [3H]5-HT radiolabelled neurons but they were never radiolabelled. Our results show subpopulations of serotonergic raphe-olfactory bulb projection neurons containing either galanin or somatostatin, but not neurotensin.