Relationship between the central alpha-adrenoceptors and central effects of morphine

In conscious rats, noradrenaline given intracerebroventricularly(icv) exhibited marked antidiuretic effect and antagonized the antidiuretic effect of morphine administered subcutaneously(sc). Although icv clonidine had no apparent-effect on the urine volume, it antagonized the antidiuretic effect of morphine. Both phentolamine and phenoxybenzamine given icv showed antidiuretic effect and enhanced the morphine-induced antidiuretic action.Phenoxybenzamine,yohimbine or phentolamine given by icv route showed antinociceptive effect in mice using the hot-plate method. The sc ED50 for morphine analgesia was significantly reduced when icv ED20 analgesic doses of phenoxybenzamine. These results suggested that the functional state of central alpha-adrenoceptors could modify the central effects of morphine, and that the morphine analgesia was more closely related with the attenuation of central adrenoceptor activity in the post-synaptic alpha-adrenoceptive sites.