NEUROANATOMICAL ASPECTS OF MYDRIATIC ACTION OF MORPHINE IN RATS

Morphine causes mydriasis in rats. In order to investigate whether thi s effect is due to direct inhibition of preganglionic pupilloconstrict or neurons in the Edinger-Westphal nucleus (EWN), we injected opiate a gonists into the EWN in male albino Charles River rats. Bilateral ster eotactic microinjections of morphine (10, 20, 30, 40 mu g/side) inhibi ted spinal nociceptive reflexes and caused pronounced catalepsy, but h ad no effect on pupillary size. The powerful opiate agonist, fentanyl, also elicited analgesia and catalepsy, when given in doses of 5 and 1 0 mu g/side, but no dose of fentanyl up to 10 mu g/side induced mydria sis. Naloxone (10 mu g/side), given into the EWN, effectively antagoni zed inhibition of the tail-flick response induced by subcutaneously ad ministered morphine (30 mg/kg), but had no effect on the cataleptic an d mydriatic actions of systemic morphine. These results indicate that, in the rat, morphine-induced mydriasis is not accounted for by a dire ct action on the EWN.