β-Endorphin levels in the cerebrospinal fluid of male talapoin monkeys in social groups related to dominance status and the luteinizing hormone response to naloxone

Abstract

β-Endorphin-like immunoreactivity was measured in the cerebrospinal fluid of 20 male talapoin monkeys living in mixed-sex social groups. It was shown that β-endorphin (1–31) was the major immunoreactive peptide; there was no evidence for high molecular weight precursors, or for either N-acetyl or C-shortened metabolites. Dominant males (those at the top of the social hierarchy) had lower levels of β-endorphin than those of intermediate rank; subordinate males had higher levels than either of the other two ranks—about three times those measured in dominants. There were significant negative correlations between β-endorphin in cerebrospinal fluid and both the amount of aggression given and sexual behaviour shown towards females. The response of the hypothalamo-pituitary system to opiate blockade was tested by giving the males naloxone in doses of 0.125, 0.25, 0.5, 1.0 and 5.0 mg/kg and assaying serum levels of luteinizing hormone 20min later. Dominant males released significant amounts of luteinizing hormone at doses of 0.25 and higher; there was no release in either intermediate or subordinate monkeys at any dose. These findings show that an animal's rank in the social group in which it lives is strongly correlated with β-endorphin levels in the cerebrospinal fluid, and with changes in the neuroendocrine response to opiate blockade. Altered opiate neural activity may be responsible for the depressed levels of sexual behaviour and gonadal function observed in monkeys at the bottom of the hierarchy.