The opioid system is involved in infant-mother bonds and adult-adult bonds in many species. We have previously shown that μ opioid receptors (MORs) and κ opioid receptors (KORs) are involved in regulating the adult attachment of the monogamous titi monkey. The present study sought to determine the distribution of MOR and KOR in the titi monkey brain using receptor autoradiography. We used [(3)H][D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) to label MORs and [(3)H]U69,593 to label KORs. MOR binding was heterogeneous throughout the titi monkey brain. Specifically, MOR binding was observed in the cingulate gyrus (CG), striatum, septal regions, diagonal band, amygdala, hypothalamus, hippocampus, and thalamus. Binding was particularly dense in the septum, medial amygdala, paraventricular nucleus of the hypothalamus, mediodorsal thalamus with moderate binding in the nucleus accumbens. Consistent with other primate species, MOR were also observed in "neurochemically unique domains of the accumbens and putamen" (NUDAPs). In general KOR binding was more homogenous. KORs were primarily found in the CG, striatum, amygdala and hippocampus. Dense KOR binding was observed in the claustrum. Relative MOR and KOR binding in the titi monkey striatum was similar to other humans and primates, but was much lower compared to rodents. Relative MOR binding in the titi monkey hypothalamus was much greater than that found in rodents. This study was the first to examine MOR and KOR binding in a monogamous primate. The location of these receptors gives insight into where ligands may be acting to regulate social behavior and endocrine function.

Vasopressin signaling has important effects on the regulation of social behaviors and stress responses, and is considered a promising pathway to target for new therapeutics of stress-induced psychiatric disorders. Although there is evidence for sex differences in the behavioral effects of arginine vasopressin (AVP), few data have directly compared the effects of stress on endogenous AVP signaling in males and females. We used California mice (Peromyscus californicus) to study the short and long term effects of social defeat stress on AVP immunoreactive cells in the paraventricular nucleus (PVN) and the posteromedial bed nucleus of the stria terminalis (BNSTmp). Acute exposure to defeat increased AVP/c-fos cells in the PVN and SON of both males and females. In contrast, there were sex differences in the long term effects of defeat. Males but not females exposed to defeat had less avp mRNA in the PVN, and in two experiments defeat reduced the number of AVP positive cells in the caudal PVN of males but not females. Interestingly, during relatively benign social encounters with a target mouse, there was a rapid decrease in AVP percent staining (including cell bodies and fibers) in the PVN of males but not females. Defeat reduced AVP percent staining in males, but did not block the socially induced decrease in percent staining. When mice were tested in resident-intruder tests, males exposed to defeat were no less aggressive than control males whereas aggression was abolished in females. However, bouts of aggression were positively correlated with the number of AVP neurons in the BNSTmp of control males but not stressed males, suggesting that different mechanisms mediate aggression in control and stressed males. These data show that while acute AVP responses to defeat are similar in males and females, the long term effects of defeat on AVP are stronger in males.