SECRETONEURIN - A NEW NEUROPEPTIDE IN THE RODENT PINEAL-GLAND

The pineal gland of mammals is richly innervated by nerve fibres conta ining various peptides of central or sympathetic origin. In this study , we have investigated the presence, regulation and effect of secreton eurin (SN), a 33-amino-acid peptide discovered recently in the central and sympathetic nervous systems. The rat pineal gland contains about 34 fmol SN/pineal, although only 20% of the secretoneurin immunoreacti vity (SN-IR) is the free peptide. SN-IR is localised in only a few ner ve fibres found in the parenchyma of the gland and along the pineal st alk. In the rat pineal, SN-IR shows no midday/midnight differences but is significantly decreased (55%-65%) after superior cervical ganglion ectomy or exposure to constant light. These observations suggest that, in the rat pineal, SN-IR originates partly from central areas and par tly from the superior cervical ganglia. The pineal of the Syrian hamst er contains much more SN-IR: 120 fmol SN-IR/pineal (about 20% being th e free peptide). SN-IR is found both in nerve fibres and pineal cells. Some of the fibres run along the pineal stalk, indicating that they a re of central origin, although a sympathetic origin cannot be ruled ou t. Pineal SN-IK is below detectable levels in the Siberian hamster. In cultured rat pinealocytes, SN significantly inhibits serotonin releas e and, to a lower extent, melatonin release.