Non-synaptic and dendritic exocytosis from dense-cored vesicles in the suprachiasmatic nucleus

THE sites of exocytosis by dense-cored vesicles (DCVs) from neurones in the rat suprachiasmatic nucleus (SCN) were studied at the ultrastructural level. The tannic acid procedure, which stabilizes extruded proteinaceous substances, was used to demonstrate exocytosis of DCV cores. Fresh brain slices containing the SCN were incubated in media containing high levels of potassium (56 mM) or glutamate (10 mM) in the presence of tannic acid. Long-term slice explant cultures of the SCN were similarly treated. Exocytosis from DCVs occurred from axonal terminals, from dendrites, and occasionally from somata. The sites of DCV exocytosis were generally nonsynaptic or para-synaptic, including release immediately adjacent to axo-spinous synaptic densities. These observations show that, in the suprachiasmatic nucleus, neuroactive substances contained in DCVs do not necessarily function at synaptic contact zones, but could also act as neuromodulators at non-synaptic sites.