Beta-endorphin levels in longtailed and pigtailed macaques vary by abnormal behavior rating and sex.

Frequent or severe abnormal behavior may be associated with the release of endorphins that positively reinforce the behavior with an opiate euphoria or analgesia. One line of research exploring this association involves the superhormone, proopiomelanocortin (POMC). The products of POMC appear to be dysregulated in some human subjects who exhibit self-injurious behavior (SIB). Macaque monkeys have POMC very similar to humans, and some laboratory macaques display SIB or frequent stereotypies. We investigated associations between plasma levels of three immunoreactive POMC fragments with possible opioid action and abnormal behavior ratings in macaques. In 58 adult male and female macaques (24 Macaca fascicularis and 34 Macaca nemestrina), plasma levels of intact beta-endorphin (betaE) and the N-terminal fragment (BEN) were significantly higher in animals with higher levels of abnormal behavior. The C-terminal fragment (BEC) was significantly higher in males but unrelated to ratings of abnormal behavior. Levels of ACTH, cortisol, and (betaE-ACTH)/betaE dysregulation index were unrelated to abnormal behavior. None of the POMC products differed significantly by subjects' species, age, or weight. The finding that intact beta-endorphin is positively related to abnormal behavior in two species of macaque is consistent with some previous research on human subjects and nonprimates. The positive relation of the N-terminal fragment of betaE to abnormal behavior is a new finding.

Personality characteristics and basal cortisol concentrations in adult male rhesus macaques (Macaca mulatta).

Although data show that psychosocial factors can regulate physiological processes, few data have been collected on normative populations. Studies in humans have suggested that personality characteristics might be related to regulation of the hypothalamic--pituitary--adrenal (HPA) axis. We explored the relationship between personality characteristics and plasma cortisol concentrations in adult male rhesus macaques. Two sets of blood samples were obtained from monkeys using a procedure with which they were very familiar; thus, cortisol concentrations reflected basal values. Analyses indicated high-excitable animals had lower basal cortisol concentrations during the afternoon period, and that low-confidence was associated with lower cortisol in the morning period, and lack of a circadian decline in the afternoon period. Sociability and equability were unrelated to cortisol levels. Our data confirm and extend some results found in human studies, and suggest that even in normal populations, personality characteristics are related to measures of HPA function. We propose that comparative studies of personality in nonhuman primates that parallel studies in humans can increase our understanding of mechanisms whereby personality may relate to mental and physical health outcomes.