Hypocretin (orexin) in the rat pineal gland: a central transmitter with effects on noradrenaline-induced release of melatonin

Hypocretin-1 (HCRT-1) and hypocretin 2 (HCRT-2), also known as orexin-A and orexin-B, are two neuropeptides derived from the same precursor. Hypocreti nergic neurons have been found exclusively in the hypothalamic dorsolateral area. These neurons are implicated in sleep and feeding through activation of specific G-protein-coupled orexin-1 and orexin-2 receptor (OR-RI and OR -R2). The purpose of this study was to determine the existence of the HCRT peptides in the central input of the rat pineal gland. Further, OR-R1 and O R-R2 expression was determined in the pineal gland and the effect of HCRT-2 on melatonin synthesis and secretion was analysed in dissociated rat pinea locytes. A large contingent of HCRT-positive nerve fibres and terminals wer e observed in the epithalamus, many of which entered into the pineal parenc hyma. A significant number of nerve fibres endowed with positive boutons we re identified in the pineal stalk, though the number of positive fibres dec reased along the extension of the stalk. So far, no positive fibres have be en found in the superficial pineal gland. RT-PCR analysis revealed the expr ession of OR-R2 mRNA, whereas OR-R1-receptor mRNA was not detected. When te sted alone, HCRT-2 had no effect on secretion of melatonin from cultured ra t pinealocytes. However, HCRT-2 partially inhibited (by a maximum of 30%) t he beta -adrenergic-induced melatonin secretion. The same effect was seen o n activation of N-acetyltransferase activity. The distribution and the larg e number of HCRT-positive fibres together with the effect on noradrenaline- mediated melatonin release through specific receptors suggests that these p eptides may be significant central transmitters in pineal function, probabl y mediating homeostatic signals to the pineal gland.