Studies on the innervation of the ovine pineal and the regulation of melatonin secretion : a thesis in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Physiology at Massey University
The studies described in this thesis were designed to investigate the neural regulation of melatonin secretion from the ovine pineal gland. Initial studies sought to establish the effects of a range of anaesthetics on the nocturnal rise in plasma melatonin levels in Romney rams and to identify the anaesthetic treatment most suitable for use in future acute studies; this was found to be halothane induction and maintenance. To investigate the neural mediation of melatonin secretion from the ram pineal, a series of experiments was performed in which the pre-ganglionic sympathetic innervation was stimulated electrically. Acute stimulation during the night or day resulted in an immediate, sustained rise in plasma melatonin levels, with nighttime responses being significantly greater than daytime responses (P<0.05). Similar studies in conscious rams were subsequently made possible by the development of a cuff electrode which could be implanted around the CST's and remain functional for at least six weeks. In these studies it was demonstrated that photoperiod (16L:8D of 8L:16D) did not influence melatonin output, while responsiveness to stimulation was highest during the middle, rather than at the beginning or end, of the photoperiod. The second study employing chronically implanted electrodes was designed to evaluate the influence of various parameters of continuously applied CST stimulation on the pineal's melatonin secretory response. Unexpectedly, it was found that only small rises in plasma melatonin levels resulted from stimulation with any of the combinations of stimulus parameters tested and that no one combination was significantly more effective than any other in promoting increases in plasma melatonin levels. These findings appear to have resulted from inadequate performance of the electrodes and/or nerve damage. The final study undertaken in this thesis was designed to identify the innervation of ovine pineal by immunocytochemically localizing the neuropeptides NPY & VIP, and the enzymes NSE & PNMT. All four antigens were observed in intrapineal nerve fibres, while NSE was also present in pinealocytes. SCGX reduced, but did not eliminate, this immunoreactivity suggesting that both central and peripheral regions innervate the pineal and that partial denervation initiates a neural compensatory mechanism.