Microinjection of
pentobarbital into a restricted region of rat brainstem, the mesopontine tegmental
anesthesia area (
MPTA), induces a reversible
anesthesia-like state characterized by loss of the righting reflex, atonia, antinociception, and
loss of consciousness as assessed by electroencephalogram synchronization. We examined cerebral activity during this state using FOS expression as a marker. Animals were anesthetized for 50 min with a series of intracerebral microinjections of
pentobarbital or with systemic
pentobarbital and intracerebral microinjections of vehicle. FOS expression was compared with that in awake animals microinjected with vehicle. Neural activity was suppressed throughout the cortex whether
anesthesia was induced by systemic or
MPTA routes. Changes were less consistent subcortically. In the zona incerta and the nucleus raphe pallidus, expression was strongly suppressed during systemic
anesthesia, but only mildly during
MPTA-induced
anesthesia. Dissociation was seen in the tuberomammillary nucleus where suppression occurred during systemic-induced
anesthesia only, and in the lateral habenular nucleus where activity was markedly increased during systemic-induced
anesthesia but not following intracerebral microinjection. Several subcortical nuclei previously associated with cerebral arousal were not affected. In the
MPTA itself FOS expression was suppressed during systemic
anesthesia. Differences in the pattern of brain activity in the two modes of
anesthesia are consistent with the possibility that
anesthetic endpoints might be achieved by alternative mechanisms: direct
drug action for systemic
anesthesia or via ascending pathways for
MPTA-induced
anesthesia. However, it is also possible that systemically administered agents induce
anesthesia, at least in part, by a primary action in the
MPTA with cortical inhibition occurring secondarily.