The effects of morphine administered intrathecally on the somatosympathetic reflex discharges in anesthetized cats

The effect of intrathecally applied morphine or naloxone on the sympathetic A and C reflexes evoked by stimulation of somatic myelinated A and unmyelinated C afferent nerves was investigated in anesthetized cats. Intrathecal application of morphine in the lumbar spinal region depressed the C reflex elicited by stimulation of a hindlimb nerve (superficial peroneal, SP) without affecting the C reflex elicited by stimulation of a forelimb nerve (radial, RA). Also, intrathecal application of naloxone in the lumbar spinal region was found to reverse the intravenous morphine-induced depression of the C reflex elicited by stimulation of the SP nerve. The present results suggest that selective depression of morphine on the somatosympathetic C reflex is mediated at the spinal cord level.