Vasoactive intestinal polypeptide induces analgesia and impairs the antinociceptive effect of morphine in mice

Vasoactive intestinal polypeptide (VIP) has numerous regulatory roles in pe ripheral, endocrine organs and in the central nervous system. The present s tudy related to the effects of centrally (intracerebroventricularly) admini stered VIP on pain sensitivity and on opiate tolerance and dependence in in tact male CFLP mice. VIP was analgesic when administered alone centrally. Naloxone treatment abo lished this analgesic effect. VIP decreased the analgesic effect of a singl e subcutaneous morphine injection and the development of chronic tolerance to morphine. Morphine withdrawal signs were not significantly affected afte r VIP pretreatment. These findings indicate that VIP may play a role in pain sensitivity and th at an opiate component may participate in this effect.