The effect of morphine on the activity evoked in ventrolateral tract axons of the cat spinal cord.

The effect of morphine on the activity in ventrolateral tract axons was studied in intercollicularly decerebrate cats with and without spinal section. Activity was elicited by electrical stimulation of Adelta- and C-fibres in the sural nerves. In spinal animals, morphine injected intravenously in a dose as low as 0.5 mg/kg reduced the post-stimulus discharge of impulses recorded in ventrolateral tract axons below the site of transection. The depression was not only abolished but reversed by levallorphan and naloxone. Pretreatment with reserpine did not diminish the effect of morphine. The effect of morphine was considerably weaker in decerebrate cats. Reversible block of the spinal cord produced by cold revealed that morphine reduced inhibition from the brain stem controlling the impulse transmission to ventrolateral tract axons. It is concluded that a spinal effect contributes to the analgesic action of morphine.