Effects of antihistamines on the spinal reflex in rats.
The effect of antihistamines on the spinal reflex in rats was studied. When H1-blocking agents such as pyrilamine, promethazine and diphenhydramine were given intravenously, both monosynaptic reflex (MSR) and polysynaptic reflex potentials (PSR) were inhibited in a dose-dependent fashion without affecting the dorsal root reflex and dorsal root-dorsal root potential. The extent of suppression of the MSR and PSR measured in spinal rats was smaller than that of intact rats. When H1-blocking agents (0.25-5.0 nmol) were applied by intraspinal microinjection close to the motoneuron, the MSR and PSR were depressed dose-dependently. Simultaneous application of carbachol partially blocked the inhibitory effects of H1-blocking agents induced by intraspinal microinjection. Also, the motoneuron discharges evoked by microinjection of glutamic acid or acetylcholine were depressed by simultaneous administration of any of H1-blocking agents tested. However, H2-blocking agents, such as cimetidine and ranitidine, given either intravenously or intraspinally had a scarcely measurable effect on the spinal reflex. The depression of H1-blocking agents on the spinal reflex is due to a direct inhibition of the motoneuron on the one hand. On the other hand, there is also an involvement of higher centers in the central nervous system increasing potential depression.
D000818:Animals; D002217:Carbachol / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005071:Evoked Potentials / Q000187:drug effects; D006634:Histamine H1 Antagonists / Q000494:pharmacology*; D008297:Male; D011738:Pyrilamine / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D012018:Reflex / Q000187:drug effects*; D012023:Reflex, Monosynaptic / Q000187:drug effects; D013116:Spinal Cord / Q000187:drug effects* / Q000502:physiology
D006634:Histamine H1 Antagonists; D002217:Carbachol; D011738:Pyrilamine