Abstract:

The overall purpose of this thesis is to investigate the physiological role of the hormone melatonin in the physiology of the Xenopus tectum. For this purpose, calcium dynamics of retinotectal axon terminals and tectal cells have been monitored by the fluorescent indicator Fluo-4. The main finding of this study is that melatonin decreases calcium rises evoked by depolarization in tectal cells and retinotectal axons by a receptor-mediated mechanism.

This thesis first examines the functional role of melatonin on tectal cells. Bath application of KCl elicited fluorescence increases that were reduced by melatonin. This effect was blocked by incubation of tectal slices with pertussis toxin and by application of 4-phenyl-2-propionamidotetralin (4-P-PDOT; 1 ?M) a melatonin receptor antagonist with a 300-fold higher selectivity far the MT2 receptor over the MT1 receptor. The effect of melatonin in tectal cells was stronger at the end of the light period than at the end of the dark period.

The role of GABA receptors in the effect of melatonin was also investigated. It was found that melatonin increases GABAC receptor activity, as demonstrated by the ability of the GABAC receptor antagonists picrotoxin and [1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid] (TPMPA), to abolish the effects of melatonin. In contrast, neither the GABAA receptor antagonist bicuculline nor the GABAB receptor antagonist 3-amino-propyl (diethoxy-methyl) phosphinic acid (CGP 35348) diminished the effects of melatonin.

This study also indicates that MT2 receptor is expressed in the optic tectum of Xenopus primarily as a dimeric complex and is glycosylated. The level of expression of the MT2 receptor is influenced by the light/dark cycle, with higher amounts at the end of the light period than at the end of the dark period, in a pattern that correlates with the effect of melatonin in tectal cells. These results implicate MT2 receptors as modulators of GABA C receptor activity in the optic tectum and raise the possibility that the role played by GABAC receptors under the influence of melatonin is influenced by the light/dark cycle. In retinotectal axons however, a potential contribution of the Mel1c receptor to the effect of melatonin was not eliminated. (Abstract shortened by UMI.)