It is well known that melatonin provokes reproductive alterations in response to changes in hours of daylight in seasonally breeding mammals, exerting a regulatory role at different levels of the hypothalamic–pituitary–gonadal axis. Although it has also been demonstrated that melatonin may affect testicular activity in vertebrates, until now, very few data support the hypothesis of a local action of melatonin in the male gonads. The aim of this study was to investigate whether MT1, MT2 melatonin receptors and the H9 melatonin-related receptor, are expressed in the adult rat testes and during development. A semi-quantitative RT-PCR method was used to analyse the expression of MT1, MT2 and H9 receptors mRNAs in several rat tissues, mainly focusing on testes during development and adult life. Our results provide molecular evidences of the presence of both MT1 and, for the first time, MT2 melatonin receptors as well as of the H9 melatonin-related receptor in the examined tissues, including adult testes. During development MT1 and MT2 transcripts are expressed at lower levels in testes of rats from 1 day to 1 week of age, lightly increased at 2 weeks of age and remained permanently expressed throughout development until 6 months. These data strongly support the hypothesis that melatonin acts directly in male vertebrate gonads suggesting that rat testes may be a suitable model to verify the role of indolamine in vertebrate testicular activity.

Cloning and sequencing of the cDNA for avian melatonin (MEL) receptors have made it possible to investigate the expression of these receptors in different animal tissues and organs by reverse transcription polymerase chain reaction. Our study demonstrates for the first time, the presence of MEL receptor transcripts in maternal RNA from Japanese quail oocytes and in RNA from the early embryos of the laid eggs. Specific primers permitted discrimination between mel-1a, mel-1b and mel-1c receptor sequences, and special techniques used to obtain the biological material made it possible to avoid accidental contamination with cells of somatic origin. Mel-1c transcript was the main one found in the oocytes and was constantly present in the maternal RNA from individual ovulated oocytes, while mel-1a and mel-1b transcripts were only sporadically present. All three MEL receptor transcripts appeared in the RNA from blastoderms of laid eggs, pointing to their de novo synthesis during uterine embryo development. The relative amount of the MEL receptor transcripts was low in relation to that of β-actin, roughly 3 × 10-4 the amount of control β-actin transcript. The presence of MEL receptor transcripts and their de novo synthesis in the 24-h-old bird embryos point to their probable wider distribution and earlier appearance in development than originally thought; the existence of the corresponding functional receptor proteins in such early embryos has not previously been reported.