| Abstract: | Lactating mice respond differently to male and female conspecific intruders, displaying defensive attack towards the former and offensive attack towards the latter. Two studies are reported in which the effects of naloxone on this differential response pattern were assessed. In the first study, lactating residents were pretreated with saline or naloxone (0.5, 2.5 mg/kg) and consecutively confronted with intruders of differing sex. Results suggested that attack (offensive) against female conspecifics is more sensitive to the inhibitory effects of opiate-receptor blockade than attack (defensive) on males. This conclusion was strengthened in a follow-up study, in which independent groups of lactating residents were used to further examine the effects of naloxone (0.5 mg/kg) on response to male and female intrusion. Analysis indicated that this low dose of naloxone significantly inhibited attack on female, but not on male, intruders. Indeed, in response to male, but not to female, intrusion, naloxone-treated residents showed a significant increase in fear-related behaviour. Findings are discussed in relation to the motivational differences in the response of lactating mice to intruders of differing sex and to possible mechanisms underlying the reported differential effects of opiate-receptor blockade. |
