Morphine tolerance and withdrawal in capsaicin--pretreated rats.

Thermoregulatory reactions to warm exposure and naloxone treatment, as well as behavioral reactions to morphine withdrawal, were recorded in morphine tolerant rats after capsaicin pretreatment. Enhanced behavioural reactions to morphine withdrawal were found in capsaicin pretreated rats, although no differences in degree of withdrawal hypothermia were seen. Morphine tolerant rats displayed increased thermal tolerance; this was partially prevented by capsaicin pretreatment. The results provide further evidence concerning the interference of neural mechanisms sensitive to capsaicin and opiates.