Secretoneurin: a new neuropeptide in the rodent pineal gland.

The pineal gland of mammals is richly innervated by nerve fibres containing various peptides of central or sympathetic origin. In this study, we have investigated the presence, regulation and effect of secretoneurin (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 immunoreactivity (SN-IR) is the free peptide. SN-IR is localised in only a few nerve fibres found in the parenchyma of the gland and along the pineal stalk. In the rat pineal, SN-IR shows no midday/midnight differences but is significantly decreased (55%-65%) after superior cervical ganglionectomy or exposure to constant light. These observations suggest that, in the rat pineal, SN-IR originates partly from central areas and partly from the superior cervical ganglia. The pineal of the Syrian hamster contains much more SN-IR: 120 fmol SN-IR/pineal (about 20% being the 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 are of central origin, although a sympathetic origin cannot be ruled out. Pineal SN-IR is below detectable levels in the Siberian hamster. In cultured rat pinealocytes, SN significantly inhibits serotonin release and, to a lower extent, melatonin release.