OXYTOCIN AND PROLACTIN-RELEASE AFTER HYPERTONIC SALINE ADMINISTRATIONIN MELATONIN-TREATED MALE SYRIAN-HAMSTERS

The aim of the present investigations was to examine the effects of me latonin (Mel) on oxytocin (OT) release under conditions of osmotic sti mulation, brought about by hypertonic saline administration, as well a s to determine whether osmotically stimulated OT release in Mel-treate d Syrian hamster is associated with alterations in the release of prol actin (PRL) and in norepinephrine (NE) and dopamine (DA) content in th e hypothalamus. In both Mel- and vehicle-treated hamsters, injection o f hypertonic saline was followed by a significant decrease in OT conte nt in the pituitary neurointermediate lobe (NIL) and elevation of plas ma OT and PRL levels. Melatonin injections had no significant affect o n NIL OT content in either isotonic-or hypertonic-saline treated anima ls. Pretreatment with Mel did not alter plasma OT or PRL levels in iso tonic saline-injected animals. However, Mel facilitated the release of OT, but prevented the release of PRL after hypertonic saline administ ration. Melatonin treatment reduced hypothalamic NE content (but not t hat of DA) in isotonic-saline treated animals. After osmotic stimulati on, hypothalamic content of NE and DA was significantly lower in Mel-t reated than in vehicle-treated animals. Data from the present study su ggest that the osmotically-stimulated release of OT and PRL seems to b e related to the activation of noradrenergic rather than dopaminergic transmission. Both dopaminergic and noradrenergic transmission may be, however, involved in mediating the effects of Mel on the osmotically- activated OT and PRL release.