LOCALIZATION AND AXOTOMY-INDUCED REGULATION OF THE PEPTIDE SECRETONEURIN IN THE RAT SUPERIOR CERVICAL-GANGLION

This study demonstrates the localization and regulation of a novel neu ropeptide of 33 amino acids, secretoneurin (SN), in the rat superior c ervical ganglion. Gel filtration chromatography of ganglion proteins f ollowed by a specific radioimmunoassay revealed that SN is the predomi nant cleavage product of secretogranin II, a member of the chromograni n/secretogranin protein family, in adult ganglia. SN was detected with in the majority of nerve endings surrounding postganglionic neurons th at were identified by the presence of synaptophysin and, in part, colo calized leu-encephalin. Applying immuno-electronmicroscopy, SN was loc alized to large dense core vesicles of neuronal and small intensely fl uorescent (SIF) cells. In situ hybridization revealed the presence of secretogranin II mRNA in postganglionic neurons and, to a lesser exten t, in SIF cells. One week after transection of the postganglionic bran ches SN levels were not significantly altered; however, a decrease of secretogranin II mRNA was observed in postganglionic neurons but not i n SIF cells. After decentralization of the ganglion, SN-immunoreactive nerve terminals disappeared and intraganglionic SN levels were reduce d by 70%, indicating the preganglionic origin of SN-positive nerve fib res and varicosities. Secretogranin II mRNA was slightly reduced under this condition. Combined axotomy and decentralization further diminis hed intraganglionic secretogranin ii mRNA, although peptide levels inc reased significantly above control values under these conditions. Doub le-labelling immunofluorescence with antibodies against the somatodend ritic marker microtubule-associated protein 2 (MAP2) revealed that the increase in SN immunoreactivity was due to an accumulation of SN in a xonal processes of postganglionic neurons. SN immunoreactivity was als o detected in dissociated neonatal superior cervical ganglion cultures and increased significantly upon treatment with nerve growth factor, the survival and differentiation factor of sympathetic neurons during perinatal development. Coculture with non-neuronal cells or addition o f leukaemia inhibitory factor, a cytokine known to stimulate synthesis of various peptides after nerve transection, did not influence SN imm unoreactivity. Therefore, since no fixed relationship between SN and a ny of the known neuropeptides or neurotransmitters expressed in sympat hetic neurons was observed, the expression of this novel peptide appea rs to be independently regulated.