(-N-ALLYLNORMETAZOCINE ENHANCES N-ACETYLTRANSFERASE ACTIVITY AND MELATONIN SYNTHESIS - PRELIMINARY EVIDENCE FOR A FUNCTIONAL-ROLE OF SIGMA-RECEPTORS IN THE RAT PINEAL-GLAND())

In the present study, to evaluate the role that sigma receptors play i n the physiology of the pineal gland, we assessed the effects of the s igma receptor ligand (+)-N-allylnormetazocine on the gland activity du ring either the day or the night. As compared to saline, (+)-N-allylno rmetazocine enhanced the physiological increases in both pineal N-acet yltransferase (NAT) activity and melatonin content at night, but it di d not affect the biosynthetic activity of the gland during the day. Mo reover, (+)-N-allylnormetazocine potentiated the enhancement of NAT ac tivity and pineal melatonin content induced by isoproterenol administr ation during the day. The nocturnal stimulation of pineal NAT activity and melatonin levels by (+)-N-allylnormetazocine was prevented by pre treatment with rimcazole, a specific sigma receptor antagonist. These results demonstrate that sigma receptor activation by (+)-N-allylnorme tazocine is not able, by itself, to stimulate pineal melatonin product ion, whereas it potentiates the biosynthetic activity of the pineal gl and when this is stimulated noradrenergically.