Topography of oxytocin and vasopressin neurons in the forebrain of Equus caballus: further support of proposed evolutionary relationships for proopiomelanocortin, oxytocin and vasopressin neurons.

The present study describes the topography of immunoreactive (ir) oxytocin (OXY) and vasopressin (AVP) neurons in the forebrain of Equus caballus and the coexistence of ir proopiomelanocortin (POMC)-derived peptides in the same cells. These data are compared to those for other mammalian species and the possible significance of species variations is considered. As expected, magnocellular neurons of the equine hypothalamus, which contain ir OXY or AVP, have prominent discernible projections to the neurohypophysis. Further, as in other mammalian species, the field of ir OXY perikarya generally extends rostral and dorsal to groups of ir AVP cell bodies, and caudal projections from OXY neurons appear to be more numerous than ir AVP projections to the brainstem and/or spinal cord. Interestingly, however, the brain of E. caballus also contains: (1) perikarya staining for OXY in the arcuate nucleus, (2) ir AVP and OXY cell bodies in the suprachiasmatic nucleus, and (3) neurons in the supraoptic and paraventricular nuclei that stained for beta-endorphin but not for other posttranslational products of POMC or dynorphin. These results give further credence to the proposal that there is an evolutionary relationship between OXY-, AVP- and POMC-producing hypothalamic neurons. Whether or not species differences in peptide coexistence reflect functional differences in neuronal populations or species differences in residual genomic expression by these neuroendocrine cells warrants further investigation.