Involvement of the cholinergic system and the basal midbrain in the organization of tonic immobility in the toad Bufo paracnemis.

Microinjection of 200 ng carbachol into the midbrain tegmentum of awake toads increases the duration of tonic immobility episodes induced by postural inversion maneuvers and by movement restriction. These increases were statistically significant when compared to those induced in untreated animals and in animals microinjected with an equivalent volume (0.2 microliter) of saline at the same site, and were partially blocked by pretreatment with atropine. The potentiation of tonic immobility episodes was not due to cardiovascular, respiratory, or motor changes caused by carbachol microinjection because these changes had a mean duration of 30 min and the immobility episodes induced both before and after carbachol injection did not present predictable durations but lasted either less or more than 30 min.