Vasoactive Intestinal Polypeptide in the Suprachiasmatic Nucleus of the Mink (Mustela vison) Could Play a Key Role in Photic Induction

The present study was conducted to visualize neuropeptides in the SCN of a mustelid, the American mink in which seasonal cycles of reproduction rely totally on the annual changes in day length. At this time, data in mustelids are lacking. Results were obtained with in situ hybridization (ISH) using synthetic oligonucleotide vasopressin (AVP) and somatostatin (SOM) and with single and dual immunohistochemistry (IHC) performed with antisera against AVP, SOM, vasoactive intestinal polypeptide (VIP), gastrin releasing peptide (GRP) and met‐enkephalin (Met‐ENK) in untreated (AVP and VIP) or colchicine (SOM, Met‐ENK and GRP) treated adult male and female mink. The most striking result, evidenced by ISH as well as IHC was the lack of AVP, SOM and Met‐ENK immunoreactive (ir)‐neurons in the SCN. In contrast, strongly VIP ir‐perikarya were widely distributed within the SCN and gave rise to a dense network of fibres extending within the periventricular (peVA) and subparaventricular (subPVA) areas. Weakly GRP ir‐perikarya were also observed in the median part of the SCN. Dual IHC revealed that the magnocellular neurons located just dorsal to the SCN, in the peVA and subPVA co‐stored AVP with VIP, SOM or Met‐ENK. The lack of SCN AVP and SOM ir‐neurons, reported for the first time in a mammalian species, raises the question of their implication in the functions of the circadian pacemaker and its entrainment by the light/dark cycle in other species. The significance of the large neurons co‐storing peptides in the terminal field of VlPergic fibres originating in the SCN has also to be determined. These results suggest that VIP could be of major importance in processing photic information mediating circadian entrainment and consequently annual rhythms.