Differential responses of serotonergic and non-serotonergic neurons in nucleus raphe magnus to systemic morphine in rats.

Forty-eight raphe-spinal units in nucleus raphe magnus were identified by antidromic stimulation and further classified into serotonergic and non-serotonergic populations according to their conduction velocity and spontaneous discharge rate. Following morphine administration (5 mg/kg, i.p.) they showed different responses: excitatory, depressive or non-responsive. It was found that 6 of 7 non-responsive, 3 of 20 depression-responsive and only 1 of 21 excitation-responsive units proved to be serotonergic neurons, indicating that the involvement of serotonergic neurons in morphine analgesia is probably insignificant. It is suggested that the raphe-spinal fiber systems, both excitatory and inhibitory are originated mainly from the non-serotonergic neurons.