SEASONAL BREEDING IN SHEEP - MECHANISM OF ACTION OF MELATONIN

Melatonin, through its duration of nocturnal secretion, transduces the effects of daylength on the reproductive axis by modifying the pulsat ile secretion of luteinising hormone-releasing hormone (LHRH). It is n ot yet known where or how this pineal indoleamine acts to exert this e ffect. Although melatonin binding sites are preferentially localised i n the pars tuberalis (PT) of the adenohypophysis, which suggests that this may be the site of action, the direct delivery of melatonin into the PT does not influence LH secretion, In contrast, melatonin deliver ed in the mediobasal hypothalamus mimics the effects on LH secretion o f peripherally administered melatonin. These results suggest that, des pite the presence of high melatonin binding in the PT, the mediobasal hypothalamus contains the physiological target site of melatonin. The action of melatonin on LHRH secretion does not seem to be directly on LHRH neurones; rather, it appears to involve a complex circuit of inte rneurones. Both the A15 dopaminergic cell group and dopaminergic neuro nal terminals of the median eminence are parts of that network but the ir anatomical and functional relationships are still unknown. Serotoni n and excitatory amino acids also appear to be involved in the seasona l regulation of LHRH secretion, Characterisation of this neuronal circ uit is a key to understanding photoperiodic regulation of reproduction .