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. Of these substances, picrotoxin was the most effective. The hyper-irritability elicited by picrotoxin or penicillin G-K was associated with a relatively localized trigger zone in the face, while that elicited by strychnine or brucine spread beyond the trigeminal region to the first cervical dermatome. D,L-homocysteic acid evoked only spontaneous scratching and vocalization. Picrotoxin-induced hyper-irritability involved facilitation of impulse transmission from cutaneous afferent terminals to second-order neurons in the trigeminal nucleus caudalis.