The suprachiasmatic nucleus of the mink (Mustela vison): apparent absence of vasopressin-immunoreactive neurons.

The hypothalamic suprachiasmatic nucleus is centrally involved in generation of several circadian rhythms. Neurons of the mammalian suprachiasmatic nucleus express a number of neuropeptides including vasopressin. The suprachiasmatic nucleus of the mink (Mustela vison) is easily distinguished from neighbouring hypothalamic areas and the underlying optic chiasm as a small nucleus containing densely packed parvocellular neurons. A dorsal and ventral subdivision were clearly recognized within the midportion and caudal part of the nucleus. Using immunohistochemistry, we have identified vasopressin-, neurophysin-, and vasoactive intestinal peptide-immunoreactive neuronal elements in the hypothalamus of the mink. Vasoactive intestinal peptide-immunoreactive neurons can be observed in the ventral aspect of the suprachiasmatic nucleus, but to our surprise, no vasopressin immunoreactive perikarya are found within the suprachiasmatic nucleus, this absence being independent of the experienced annual cycle. The hypothalamic paraventricular and supraoptic nuclei contain large numbers of vasopressin-, neurophysin- and vasoactive intestinal peptide-immunoreactive magnocellular neurons with extensive projections towards the infundibulum and neurohypophysis. A comparative analysis of the distribution of vasopressin-immunoreactive elements in a number of conventional laboratory animals has demonstrated that, in contrast to the rat, golden hamster and Mongolian gerbil, neither vasopressin-containing perikarya in the suprachiasmatic nucleus nor fine calibered immunoreactive fibres entering the adjacent subparaventricular zone are present in the mink. The mink is a photodependent seasonal breeder, and thus vasopressin-immunoreactive neurons in the suprachiasmatic nuclei may not be essential for the photoperiodic regulation of reproduction and seasonal events experienced by this species.