Sex differences in the expression and antagonism of swim stress-induced analgesia in deer mice vary with the breeding season

Swim stress-induced analgesia (SSIA) was examined in photoperiodically induced breeding (reproductive) and 'non-breeding' (non-reproductive) adult male and female deer mice, Peromyscus maniculatus. Nociceptive responses (50 degrees C, hot-plate) of breeding and non-breeding deer mice were determined after either a 1-or 3-min swim in 20 degrees C water. The 1-min swim induced an immediate and relatively short-lasting naloxone (1.0 mg/kg) insensitive 'non-opioid'-mediated SSIA that was antagonized by the N-methyl-D-aspartate (NMDA) antagonist, MK-801 (0.10 mg/kg) in all of the groups of mice except the breeding (reproductive) females. Breeding females displayed a non-opioid analgesia that was insensitive to MK-801. The 3-min swim induced a relatively more prolonged mixed opioid and 'non-opioid' SSIA of which the initial portion was sensitive to antagonism by MK-801 in all groups of the mice except the breeding females, while the latter portion (15 min after swim) was reduced by naloxone in all of the groups of mice. Overall, the breeding males displayed greater levels of SSIA than the breeding females, with no consistent sex differences in the non-breeding mice. Within sexes, the breeding males displayed greater levels of opioid and non-opioid SSIA than the non-breeding males, while the non-breeding females displayed greater levels of SSIA than the breeding females. These results show that both sex and reproductive status affect the expression and neurochemical mediation of non-opioid SSIA. These findings also suggest that reproductive females may have an unique or novel hormonally (estrogen) dependent mechanism associated with the expression of SSIA.