Reversible analgesia, atonia, and loss of consciousness on bilateral intracerebral microinjection of pentobarbital

Concussion, asphyxia, and systemically administered general anesthetics all induce reversible depression of the organism's response to noxious stimuli as one of the elements of loss of consciousness. This is so even for barbi turate anesthetics, which have only modest analgesic efficacy at subanesthe tic doses. Little is known about the neural circuits involved in this form of antinociception, although for anesthetic agents, at least, it is usually presumed that the drugs act in widely distributed regions of the nervous s ystem. We now report the discovery of a focal zone in the brainstem mesopon tine tegmentum in rats at which microinjection of minute quantities of pent obarbital induces a transient, reversible anesthetic-like state with non-re sponsiveness to noxious stimuli, flaccid atonia, and absence of the rightin g reflex. The behavioral suppression is accompanied by slow-wave EEG and, p resumably, loss of consciousness. This zone, which we refer to as the mesop ontine tegmental anesthesia locus (MPTA), apparently contains a barbiturate -sensitive 'switch' for both cortical and spinal activity. The very existen ce of the MPTA locus has implications for an understanding of the neural ci rcuits that control motor functions and pain sensation, and for the cerebra l representation of consciousness. (C) 2001 International Association for t he Study of Pain. Published by Elsevier Science BN. All rights reserved.