Exposure to the scent of male mice infected with the protozoan parasite, Eimeria vermiformis, induces opioid- and nonopioid-mediated analgesia in female mice.

The present study examined the nociceptive responses of female mice exposed to the scent (soiled cage bedding) of male mice infected with the protozoan parasite, Eimeria vermiformis. A 30-min exposure to the odors of a parasitized male induced naloxone (1.0 mg/kg)-sensitive opioid-mediated analgesia in female mice, whereas a brief 1-min exposure to these odors resulted in a lower amplitude, relatively short, nonopioid analgesia that was insensitive to naloxone and blocked by the serotonin-1A (5-HT1A), agonist, 8-OH-DPAT. Exposure to the odors of nonparasitized males had no significant effects on the nociceptive responses of female mice. These results indicate that female mice are able to distinguish between the odors of parasitized and nonparasitized male mice, and that female mice display both opioid- and nonopioid-mediated aversive responses to the odor cues associated with the parasitized males. The implications of these findings for parasite-based mate choice are discussed.