Coma induced by cholinergic activation of a restricted region in the pontine reticular formation--a model of reversible forms of coma.

We previously demonstrated evidence suggesting increased neural activity in the pontine reticular formation ventromedial to the principal nucleus of locus coeruleus after experimental concussive head injury in the cat. We report here that microinjections of small doses of the cholinergic agonist carbachol (0.4 μg in 0.2 μl, bilaterally) into this restricted brain stem region of the awake cat produces a reversible, flaccid, comatose state analogous to that of the lowest score on the Glasgow Coma Scale, the grading system commonly employed in assessments of human coma. Microinjection of the local anesthetic tetracaine into the same region failed to produce detectable effects, and therefore the carbachol effects were probably due to activation of pathways within the injection sites that actively inhibit various aspects of the animals' responsiveness to the external environment. The flaccid, comatose state produced by carbachol was not the result of epileptic processes, since neither epileptic electroencephalographic discharges nor convulsive movements were observed. The muscarinic antagonist atropine reversed the carbachol effects. We shall also discuss possible implications of these data in terms of mechanisms of flaccid coma associated with good recovery in humans.