Inhibitory mechanisms of the hyper-irritability caused by picrotoxin in the rat.

Topically applied norepinephrine, dopamine, serotonin, GABA and glycine, and systemically administered clonidine, L-DOPA (plus carbidopa) and 5-hydroxytryptophan completely suppressed the cutaneous hyper-irritability produced in the trigeminal sensory distribution by picrotoxin overlying the caudal medulla. Cholinergic agents and apomorphine were ineffective. Of the positive compounds, norepinephrine, serotonin and GABA showed the shortest latencies and norepinephrine and serotonin required the lowest concentrations in order to inhibit the hyper-irritability.

An experimental model of hyper-irritability in the trigeminal skin field of the rat.

Cutaneous hyper-irritability was produced in the trigeminal sensory distribution of chronically prepared rats by injecting various central stimulants into the dorsal subarachnoid space overlying caudal medulla. Effective substances were classified into three groups according to the behavioral patterns induced. These were (1) picrotoxin and penicillin G-K, (2) strychnine and brucine, and (3) D,L-homocysteic acid.