Suppression of juvenile social behavior requires antagonism of central opioid systems

Pairs of male and female rats were injected with either tertiary naltrexone (NTX) which readily crosses the blood-brain barrier, or quaternary naltrexone (QNTX) which does not, to determine the importance of central opioid systems in the elaboration of juvenile social behavior. In the first experiment, only intraperitoneal injections of NTX (1.0 mg/kg) suppressed the frequency of wrestling pins. Peripheral injections of QNTX (10.0 mg/kg) were without effect. In a second experiment, QNTX (2.0, 4.0, or 8.0 micrograms/4.0 microliters) was injected directly into the lateral ventricles. Intracerebroventricular injection of the moderate dose reliably reduced frequency of pinning while the higher dose was severely incapacitating and the low dose was without effect. The results of these two experiments confirm an important role for brain opioid systems in the control of juvenile social interaction.