Evolutionary and comparative aspects of nociception

Morphine induces aggression but not analgesia in the naked mole-rat (Heterocephalus glaber)

1. The antinociceptive effect in the mole-rat of morphine (1, 10, 20 or 30 mg/kg) and nefopam (10 or 20 mg/kg) was studied. 2. In the hotplate test, morphine had no analgesic effect. A reduced response latency after morphine (10 and 20 mg/kg) could possibly be explained by hyperactivity and excited behaviour. 3. After morphine (10, 20 and 30 mg/kg) most of the animals died after fighting when kept in colony cages. Aggressive behaviour and death was prevented by naloxone, or by keeping the animals in single cages. 4. Nefopam (20 mg/kg) significantly increased the latency for the nociceptive response. 5. It was concluded that in the mole-rat, opioid systems in the CNS may not be involved in the regulation of nociception, but in the regulation of agonistic and motor behaviour.

Beta-funaltrexamine disrupts the day-night rhythm of nociception in mice

Determinations were made of the effects of beta-funaltrexamine (beta-FNA), an irreversible mu-opioid receptor antagonist, on the day-night rhythm of nociception in male mice. Peripheral administration of beta-FNA (20 and 40 mg/kg) disrupted the day-night rhythm of foot-licking response to aversive thermal (50 degrees C) stimulation. The peak nocturnal response latency was attenuated and the marked increases and decreases in response latency present at the light-dark and dark-light transitions, respectively, were suppressed. These results suggest that mu opioids are associated with the generation and expression of the day-night rhythm of this particular measure of nociception in mice.

Exposure to novel odors induces opioid-mediated analgesia in the land snail, Cepaea nemoralis

Land snails, Cepaea nemoralis, that were exposed for 1-30 min to a novel odor of either peppermint extract or vegetable juice concentrate displayed an increase in the latency of their nociceptive response to an aversive thermal stimulus (40 degrees C, hot-plate). This "analgesic" response, which entailed the elevation of the fully extended foot in hydrated snails, was evident directly after exposure to the novel chemostimuli and lasted for 15-30 min. This novelty-induced analgesia was blocked by the exogenous opiate antagonist naloxone. Analgesia was not observed in snails that were exposed to the same olfactory cue 4 or 24 h later, but was evident when the alternate novel odor (peppermint or vegetable juice) was presented. However, a significant analgesia was displayed by snails that were reexposed to their initial olfactory stimulus after 48-72 h. These findings indicate that exposure to a novel olfactory stimulus can activate endogenous opioid systems and induce an analgesic response in mollusks.