Abstract
In a preceding paper we described that small doses of fentanyl and morphine depressed the late phase of the inhibition of extensor spinal monosynaptic reflex (MSR) induced by the stimulation of either Aδ fibers of the cutaneous nerve or high threshold muscle afferents (1).
Since it has already been reported that pain impulses may he transmitted through Group III muscle and cutaneous Ad and C afferent fibers (2), it was considered that this depressive action of morphine and fentanyl might be related to their analgesic action. In order to test this possibility, an attempt was made to study and compare, as the first step, effects on this neuronal system of some other narcotic analgesics and those of non-narcotic analgesics, narcotic antagonists and some compounds with similar chemical structures to morphine but without analgesic activity. The following drugs were tested on this inhibitory transmission from flexor reflex afferents. Group 1, narcotic analgesics; oxymorphone, dimethylthiarnbutene and azabicyclane, group 2, non-narcotic analgesics; pentazocine and Aspirin, group 3, narcotic antagonists; nalorphine and levallorphan, and group 4, compounds with similar chemical structures to morphine but without analgesic activity; 14-hydroxy-dihydro-6α-thebainol (3) and its 6β isomer (3).
It was previously reported (7) that the analgesic potency of azabicyclane, a newly synthetized compound, was about 3-6 times greater than that of meperidine, and in spite that this drug induced no significant loss of body weight in rats during the withdrawal periods after 4 weeks, azabicyclane had some narcotic properties such as follows; 1) the antagonism to the analgesic effect by nalorphine, 2) rapid development of torelance to analgesic effect, 3) Straub tail reaction in mice, etc. Therefore, in this paper azabicyclane was included in narcotic analgesic group.
The results will clearly show that the depressant action on this inhibitory system in the spinal cord is inherent among narcotic analgesics (including azabicyclane), but is not shared by drugs of the other categories.
