REGIONAL DISTRIBUTION OF IODOMELATONIN BINDING-SITES WITHIN THE SUPRACHIASMATIC NUCLEUS OF THE SYRIAN-HAMSTER AND THE SIBERIAN HAMSTER

The pineal hormone melatonin is a potent regulator of seasonal and cir cadian rhythms in vertebrates. In order to characterize potential targ et tissues of melatonin, the distribution of iodomelatonin (IMEL)-bind ing sites was examined within neurochemically and anatomically defined subdivisions of the suprachiasmatic nucleus (SCN), a structure necess ary for seasonal and circadian rhythms in mammals. Studies were carrie d out in both the adult Syrian (Mesocricetus auratus) and Siberian (Ph odopus sungorus) hamster. The retinoreceptive zone of the SCN was iden tified anatomically by immunocytochemical (ICC) visualization of chole ra toxin B subunit tracer (ChTB-ir) following its intra-ocular injecti on. Photically-responsive SCN cells were identified by immunostaining for the protein product of the immediate-early gene c-fos (Fos-ir) fol lowing exposure of the animal to light. The non-photoresponsive zone o f the SCN was identified using in situ hybridization (ISH) for arginin e vasopressin (AVP) mRNA, whilst sites of IMEL-binding in the SCN were identified by in vitro film autoradiography using the specific ligand 2-[I-125]-iodomelatonin. To compare directly the distribution of IMEL -binding sites and one of the functional zones of the nucleus, alterna te serial coronal sections through the SCN were processed for autoradi ography for IMEL and one of the following: ICC for ChTB-ir or Fos-ir, or ISH for AVP mRNA. Overall, the regional distribution of the Various markers within the SCN was comparable in the two species. The retinor ecipient (ChTB-ir) and photically-responsive (Fos-ir) zones of the SCN mapped together to the middle and caudal thirds of the nucleus, predo minantly in its ventro-lateral division. IMEL-binding was present thro ughout the full rostro-caudal extent of the SCN, but by far the most e xtensive area of IMEL-binding was in the rostral half of the nucleus, leading to a clear dissociation along the rostro-caudal axis of the pr incipal zone of IMEL-binding and the retinorecipient zone of the nucle us. In the Syrian hamster, in coronal sections of the caudal SCN which did contain significant amounts of both IMEL-binding and Fos-ir, IMEL -binding was confined to the medial zone, distinct from the Fos-ir reg ion of the ventro-lateral SCN. The segregation was less clear-cut in t he Siberian hamster where the area of IMEL-binding was more extensive. The dissociation of IMEL-binding and photically-responsive cells in t he Syrian hamster was confirmed in a series of sagittal sections which were processed alternately for Fos-ir and IMEL-binding. Whereas fos-i r was confined to the ventro-lateral SCN, IMEL-binding was concentrate d in the medial zone of the nucleus. In both species, mRNA for AVP was found throughout the rostro-caudal extent of the SCN, but the peak ar ea was located in the rostral half, and so was segregated from the pri ncipal retinorecipient zone. The distribution of mRNA for AVP along th e rostro-caudal and medio-lateral axes was in direct register with the IMEL-binding in both species. These studies suggest that melatonin ac ts upon pathways within the SCN different to those addressed by light, and that it may influence directly the efferent activity of the nucle us, possibly via an effect on vasopressinergic cells.