VIP-like immunoreactive and vasopressin-like immunoreactive neuronal elements in the suprachiasmatic nucleus of Japanese monkey (Macaca fuscata): Light and electron microscopic immunocytochemical study

VIP-like immunoreactive (VIP-LIR) and vasopressin-like immunoreactive (VP-LIR) neuronal elements located in the monkey suprachiasmatic nucleus (SCN) were investigated by light and electron microscopic immunocytochemistry. Both populations of neurons in the monkey SCN show a similar distribution to those of the rat except for some subtle differences. In addition to the ordinary distribution of VIP-LIR and VP-LIR neurons, VIP-LIR neurons of similar or some smaller size to those in the ventrolateral portion of the nucleus were also observed in the periventricular nucleus adjacent to the third ventricle. VP-LIR neurons with similar size and either strong or weak immunoreactivity compared with those located in the dorsomedial area were detected in the periventricular nucleus in which magnocellular VP-LIR neurons with strong immunoreactivity were distributed. Both VIP-LIR and VP-LIR neuronal perikarya were found to contain well developed cell organelles such as rER and mitochondria as well as immunoreactive dense granules. VIP-LIR neuronal perikarya were generally surrounded by astroglial processes. However, non-immunoreactive axons were frequently found to make synaptic contact with VIP-LIR dendrites or dendritic spines. Thus the VIP-LIR neurons are thought to receive neuronal inputs from other parts of CNS mainly from the dendrites or dendritic spines. VIP-LIR and VP-LIR axons were seen to form synapses on non-immunoreactive dendrites. VIP-LIR axons were also found to form axo-axonic synapses on non-immunoreactive axons. Synaptic contacts between VIP-LIR neuronal elements were also occasionally observed. The present study has demonstrated that both the VIP-LIR and VP-LIR neurons are at least partly intrinsic neurons in the SCN, and also projecting elements to the other neurons of the CNS.