OPIOID AND NONOPIOID NMDA-MEDIATED PREDATOR-INDUCED ANALGESIA IN MICEAND THE EFFECTS OF PARASITIC INFECTION

The present study examined the nociceptive responses (50 degrees C, ho t-plate) of uninfected and subclinically parasitized male mice exposed to the odor of a predator, an ecologically relevant threatening stimu lus. In uninfected mice a 15-min exposure to 2-propylthietane, the maj or component of weasel odor, induced a naloxone-reversible opioid anal gesia. A 30-s exposure elicited a shorter duration and lower amplitude 'non-opioid' analgesia that was insensitive to naloxone, partially se nsitive to either the serotonin-1A (5-HT1A) agonist, 8-OH-DPAT, or the GABA(A) antagonist, bicuculline, and blocked by the competitive N-met hyl-D-aspartate (NMDA) antagonist, NPC 12626. In contrast, mice chroni cally (25 days) and subclinically infected with the murine nematode, H eligmosomoides polygyrus, failed to show a significant non-opioid anal gesia and displayed a markedly lower level of opioid analgesia than un infected mice. These results suggest that NMDA receptor mechanisms are potently associated with the expression of the analgesia arising from exposure to the naturally aversive stimulus of predator odor. These f indings also demonstrate that parasites, and likely other subchronic i nfections, can have a significant impact on the display of opioid and non-opioid stress-induced analgesia arising from exposure to the ethol ogically relevant stimulus of predator odor. (C) 1997 Elsevier Science B.V.