Effects of some narcotic analgesics upon the monosynaptic reflex inhibition from muscular and cutaneous afferents in spinal cord of the cat.

It has been know that, at a small dose, narcotic analgesics depress polysynaptic reflex activity in the spinal cats (1) and nociccptive reflexes in the spinal rats (2), as well as in spinal cats and dogs (3-6), where they do not depress, or slightly enhace the monosynaptic reflex (3). In order to explain this depression induced by narcotic analgesics, it has been suggested that the interneurons responsible for mediation of the polysynaptic reflex are suppressed (3). Further, for a remarkable depression of nociceptive reflex discharges from high threshold cutaneous afferents by morphine and pethidine, it has been suggest ed that some interneurons activated by such small fibers of the cutaneous nerve might be inhibited (7). On the other hand, large doses of morphine (15-20 mg/kg) has been re ported to potentiate the Group la inhibition and depress the recurrent inhibition via Ren shaw cell arcs in decerebrate cats (8). However, it remains still unknown whether narcotic analgesics may depress spinal interneurons generally or special interneurons specifically. Therefore, it was attempted in the present study to find either one of these. As a test system, the effects of some nar cotic analgesics upon the inhibitory influences to the extensor monosynaptic reflex from flexor reflex afferents were studied using unanesthetized low spinal cats.