The effect of dopaminergic stimulation and blockade on the nociceptive and antinociceptive responses of mice.

Agents which stimulate dopaminergic receptors directly or indirectly such as apomorphine and L-dopa, increased the reactivity of mice to a nociceptive stimulus. The increased reactivity was pharmacologically quantitated by estimating the hyperalgesic ED50 to be 4.4 and 115 mg/kg for apomorphine and L-dopa, respectively. This hyperalgesia was blocked by the dopamine receptor blocking agents, haloperidol and pimozide, but not by the narcotic antagonist, naloxone. Apomorphine antagonizes morphine analgesia. However the induced hyperalgesia only accounts for part of the antagonistic activity of apomorphine. The majority of the antagonistic activity of apomorphine appears to be by means other than action on dopaminergic receptors.