INCREASED FOS EXPRESSION IN OXYTOCIN NEURONS FOLLOWING MASCULINE SEXUAL-BEHAVIOR

Induction of the c-fos protein product (Fos) was used to immunocytoche mically identity oxytocin (OT) neurons that may be activated during co pulatory interactions. Fos induction was quantified in sexually-experi enced male rats after either (a) exposure to a testing arena recently vacated by an estrous female, (b) copulatory interactions such as moun ting and intromission without ejaculation, or (c) mounting and intromi ssions culminating in ejaculation. In the parvocellular regions of the paraventricular nucleus of the hypothalamus (PVN), the number of neur ons expressing Fos increased following either intromission (53%) or ej aculation (124%). Significant, but less striking, increases in the num ber of cells expressing Fos were noted in magnocellular regions of the PVN where intromission resulted in a 13% increase and ejaculation in a 49% increase in Fos. The number of perikarya immunoreactive for OT a nd AVP did not differ as a function of increasing sexual contacts. In control (novel arena) males, 33-73% of the Fos labeling occurred in OT cells. Sexual interactions did not enhance the number of double-label ed cells in most parvocellular regions. However, in lateral parvocellu lar regions located in the most caudal aspects of the PVN, 31% of the Fos-positive cells occurred in OT neurons in ejaculated males, while i n control males none of the OT cells were double-labeled. This PVN sub division is known to consist of neurons that project to the brain stem and spinal cord at lumbar levels which contain motor neurons that reg ulate penile reflexes. The present data suggest a possible neurochemic al circuit which incorporates oxytocinergic neurons in the mediation o f masculine sexual responses.