NO-ergic neurons of the cervical nucleus of the rat brain in normal conditions and after administration of opiates.

Histochemical reactions for NADPH-diaphorase (NADPH-d) were used to study the dynamics of NO synthesis activity in the cervical nuclei of the rat brain after i.v. administration of morphine hydrochloride. In normal conditions, NADPH-d activity was found in neurons in all the cervical nuclei. Acute and chronic administration of morphine at different doses (0.5 and 5 mg/kg) suppressed the NO-ergic activity of most cervical neurons. Decreases in the level of NADPH-d activity were different in different nuclei. NO-ergic changes were due to activation of opiate receptors, as they were dependent on the dose of morphine used, and treatment with the opiate antagonist naloxone restored the NO-ergic function of cervical neurons. Formation of tolerance to the analgesic effect of opiates was accompanied by significant but short-lived increases in NO synthesis activity. It is suggested that changes in the NO-ergic functions of cervical neurons may affect the balance of serotonin in the brain during opiate treatment.