First evidence of melatonin receptors distribution in the suprachiasmatic nucleus of tree shrew brain.

OBJECTIVE: In this study we investigated the distribution of melatonergic receptors in the tree shrew brain. The psychosocial stress in the tree shrew is a validated model of depression with disturbance of circadian rhythms.

METHODS: Given the role of melatonin in the modulation of circadian rhythms, we determined by autoradiography the distribution of binding sites of 2-[125I]-MLT, a ligand for the two melatonergic receptors MT1 and MT2, in the tree shrew brain focusing mostly on the suprachiasmatic nucleus (SCN), the biological clock involved in resynchronization of circadian rhythms. We also analyzed the distribution of 2-[125I]-MLT in other brain areas involved in regulation of mood.

RESULTS: Specific binding of 2-[125I]-MLT was found in the SCN. In addition, several structures in the tree shrew brain were labeled, among them the pars tuberalis, the cerebellum, structures of the hippocampal formation and dopaminergic areas such as the caudate putamen and nucleus accumbens, providing other potential targets of psychosocial stress in the tree shrew, in addition to the SCN.

CONCLUSIONS: The demonstration of melatonergic receptors in these brain areas supports their probable involvement in the behavioural, neuroendocrine and circadian rhythms disturbances observed in the psychosocial stress model of depression in the tree shrew.

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