Melatonin Receptors and Signal Transduction in Photorefractory Siberian Hamsters (Phodopus sungorus)*

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Abstract
In seasonally breeding mammals, seasonal alterations in day length are perceived via the pineal hormone melatonin. When exposed to short day lengths, hamsters and other long-day breeders undergo gonadal regression. With chronic exposure (greater than 20 weeks) to short days, however, the animals become photorefractory, or insensitive to the inhibitory influence of short-day melatonin patterns, and gonadal recrudescence occurs. In this report, we examined photorefractory Siberian hamsters and long-day housed control hamsters to examine whether this apparent insensitivity to melatonin is due to alterations in melatonin receptors or signal transduction. In vitro autoradiographic assessment of melatonin receptors using 125I-labeled 2-iodomelatonin (I-MEL) revealed that melatonin receptor distribution, affinity, density, and G protein coupling are unaltered in photorefractory animals. In each animal, high-affinity (dissociation constant approximately 40 pM) 2-iodomelatonin binding sites were observed in the hypophysial pars tuberalis, in the suprachiasmatic nucleus of the hypothalamus, and in the thalamus (paraventricular nucleus, reuniens nucleus, and nucleus of the stria medullaris). The nonhydrolyzable GTP analog, GTP gamma S (100 microM) caused a 10-fold reduction in melatonin receptor affinity in the pars tuberalis in both photorefractory and control hamsters, demonstrating receptor-G protein coupling in both groups. Furthermore, melatonin (10 nM) inhibited forskolin-stimulated cAMP accumulation in median eminence/pars tuberalis explants in photorefractory animals, just as previously observed in explants from long-day hamsters. These results suggest that melatonin receptors, receptor-G protein coupling, and inhibition of adenylyl cyclase by melatonin are not altered in photorefractory hamsters.

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