The effects of specific opiate receptor antagonists on the habituation of novelty-induced analgesia /

Animals exposed to nociceptive stimulation for the first time display a novelty induced hypoalgesia (NIH) that habituates with repeated exposure to the same stimuli. The non-selective opiate receptor antagonist naloxone, has been shown to attenuate this habituation. Antagonists selective for the $ mu, partial$ and $ kappa$ receptors were used in order to elucidate the opiate receptor subtype that is involved in this effect. Animals were exposed to a hot-plate apparatus set at 48.5$ sp circ$C once per day, for 8 days. The latency to lick the hind paw was used as an index of pain sensitivity. CTOP ($ mu$), naltrindole ($ partial$), nor-binaltorphimine ($ kappa$) (0.5, 1.0 or 2.0 nM), or vehicle were injected ICV 30 min. prior to each plate exposure. Only CTOP (1.0; 2.0 nM) and naltrindole (2.0 nM) prevented the habituation of NIH. These results suggest that the specific receptor subtypes $ mu$ and $ partial$, are involved in the habituation of NIH