Depressive effect of morphine on the sympathetic reflex elicited by stimulation of unmyelinated hindlimb afferent nerve fibers in anesthetized cats.

The effect of morphine on somatosympathetic reflexes was analyzed in central nervous system (CNS) intact, anesthetized cats. Sympathetic efferent nerve activity was recorded via an inferior cardiac nerve. Electrical train pulse stimulation of the superficial peroneal afferent nerve produced an A reflex of about 60 ms latency (evoked by myelinated fiber excitation) and a C reflex of about 360 ms latency (evoked by unmyelinated fiber excitation), both of which have central reflex pathways through the medulla oblongata. An intravenous injection of morphine (2 mg/kg), while not affecting the A reflex, markedly depressed the C reflex. The degree of this depressive effect was somewhat dependent on the stimulus intensity applied to the somatic afferent nerves. Naloxone (0.2 mg/kg, i.v.) antagonized the depressive effects of morphine on the C reflex. The present results suggest that the somatosympathetic C reflex can be a useful indicator of central morphine effects in anesthetized animals.