Co-expression in Xenopus neurons and neuroendocrine cells of messenger RNAhomologues of exocytosis proteins DOC2 and munc18-1

The proteins munc 18-1 and DOC2 are assumed to play a role in docking of sy naptic vesicles in neurotransmitter exocytosis at the presynaptic junction. As the proteins are known to interact, they should co-exist within neurons . We have tested this hypothesis for exocytosis of both classical and pepti dergic messengers, by investigating the distribution of the messenger RNAs of munc 18-1 and DOC2 homologues in the brain and pituitary gland of the cl awed toad Xenopus laevis, using in situ hybridization. For this purpose we cloned a partial complementary DNA encoding Xenopus unc18 (xunc18) and used a corresponding RNA probe, together with an RNA probe for Xenopus DOC2. At the messenger RNA level DOC2 and xunc18 were found to be expressed through out the Xenopus brain. All brain nuclei expressing DOC2-messenger RNA showe d xunc18-messenger RNA expression as well. Co-expression was shown at the i ndividual cell level in consecutive sections of large-sized neurons. A stro ng expression was demonstrated in the suprachiasmatic and magnocellular nuc lei and in peptidergic endocrine cells in the intermediate and anterior lob es of the pituitary gland, suggesting roles of DOC2 and xunc18 in messenger release from peptidergic secretory systems. Combined in situ hybridization and immunocytochemical analyses show that neuropeptide Y-containing cells in the suprachiasmatic nucleus also express DOC2 and xunc18 messenger RNAs, Since these cells have a high secretory activity, controlling the activity of the pituitary pars intermedia, the levels of expression of DOC2 and xun c18 may be indicators for neuronal secretory activity. The present data represent the first evidence for the co-existence of DOC2 and munc 18-1 and suggest co-ordinate action of these proteins at the level of brain nuclei, individual neurons and endocrine cells. (C) 1999 IBRO. Pu blished by Elsevier Science Ltd.