Melatonin receptors in the suprachiasmatic nuclei and pars tuberalis of testosterone induced photoresponsive rats Joaquı ´n Recio a,b, *, Paul Pe ´vet a , Jesu ´s M. Mı ´guez a , Mireille Masson-Pe ´vet a a Neurobiologie des Fonctions Rythmiques et Saisonnie `res, URA-CNRS 1332, Universite ´ Louis Pasteur, 12, rue de l’Universite ´, 67000 Strasbourg, France b Departamento de Fisiologı ´a y Farmacologı ´a, Facultad de Medicina, Universidad de Cantabria, Santander, Spain Received 25 January 1996; revised version received 3 July 1996; accepted 3 July 1996 Abstract The sexual axis of rats can be rendered photoresponsive by testosterone implants. We have studied in these conditions whether rat pars tuberalis (PT) melatonin receptor density would be decreased after exposure to short photoperiod (SP) like as observed in long day seasonal breeders. The answer is no, but we observed that testosterone induced a photoperiod independent decrease in PT melatonin receptor density. These results show that testosterone-induced photosensitivity in rat is not linked to an SP-induced decrease in PT melatonin receptor density. However, testosterone regulates PT melatonin receptors independently of the photoperiod. Keywords: Melatonin receptor; Photoperiodism; Rat; Pars tuberalis; Suprachiasmatic nuclei; Testosterone The pineal hormone melatonin is known to play a major role in the synchronization of reproduction with seasons [4,6,11,16]. Among the many brain structures displaying melatonin receptors [8], the pars tuberalis (PT) of the ante- rior pituitary is the area consistently and most intensely labeled with 2-[ 125 I]melatonin in mammals [7]. A seasonal variation in PT melatonin receptors has been reported in several photoperiodic mammalian species showing a decrease in receptor density associated to short photoper- iod (SP) exposure and gonadal involution [2,3,8,14,15,18]. On the contrary, in the rat, a non-photoperiodic species, no change in PT 2-[ 125 I]melatonin specific binding was found between rats exposed to either SP or long photoperiod (LP) [3]. Rats miss thus both gonadal regression and downre- gulation of PT melatonin receptors induced by exposure to SP in seasonal breeders. These results indicate that PT melatonin receptors could be implicated in the regulation of seasonal breeding in mammals [9,22]. The sexual axis of laboratory rats can be rendered photoresponsive by chronic exposure to exogenous testos- terone, this effect depending on the presence of a func- tional pineal gland, and likely on melatonin secretion [20,21]. We hypothesized that testosterone could enable photosensitivity in rats by restoring the sensitivity of PT melatonin receptors to endogenous melatonin. Restored receptor sensitivity would lead to downregulation after chronic exposure to longer melatonin levels, as occurs in seasonal breeders exposed to SP. Thereby, the aim of the present study was to determine whether there was a link between the photosensitivity state and the photoperiodic regulation of PT melatonin receptors by using the testos- terone treated rat as a model. Melatonin receptors in the suprachiasmatic nuclei (SCN), the circadian pacemaker, have been shown not to vary with seasons in two photoperiodic species, the hedge- hog [2] and the Siberian hamster [12]. Specificity of the action on PT melatonin receptors was tested by studying also 2-[ 125 I]melatonin binding to the SCN in the same animals. Male Wistar rats weighing 200–250 g (Iffa Credo, France) were maintained under a 12:12 h light/dark photo- period (12L:12D, lights on at 0700 h), for 2 weeks prior to experimentation. A dim red light was continuously pre- sent, and food and water were provided ad libitum. Two groups of 14 rats were lightly anesthetized with ether and subcutaneously implanted with either empty (control; C) Neuroscience Letters 214 (1996) 53–56 0304-3940/96/$12.00  1996 Elsevier Science Ireland Ltd. All rights reserved PII S0304-3940(96)12903-7 * Corresponding author. Tel.: +33 88358504; fax: +33 88240461.