Opioid analgesia in the mouse: evidence for multiple receptors using beta-FNA

The icv injection of morphine or DADLE ED50 a few min before the alkylating agent beta-FNA resulted in complete protection of their respective analgesic effects when evaluated 24h later, although a little cross-protection could be observed. The analgesia evoked by DADLE was partially protected using higher doses of morphine before beta-FNA. However, higher doses of DADLE did not protect the analgesia induced by morphine. On the other hand, the antagonistic action of KCl on opioid analgesia was found to be dependent on the opioid utilized to protect the opioid receptor against the effect of beta-FNA. These results are discussed in terms of multiple receptors mediating opioid analgesia at supraspinal level in the mouse.