Responses of presumed cholinergic mesopontine tegmental neurons to carbachol microinjections in freely moving cats.

The effects of microinjections of a cholinergic agonist, carbachol (0.2 microgram/0.2 microliter), were examined on three different types of rostrally projecting tonic neurons that we have reported previously in the dorsal part of the pontomesencephalic tegmentum known to contain numerous cholinergic cell bodies: 1) tonic type I slow (Type I-S); 2) tonic type I rapid (Type I-R); and 3) tonic type II (Type II) (El Mansari et al. 1989). Microinjections of carbachol near unit recording sites in freely moving cats induced within a few minutes a complete suppression of the spontaneous activity and a marked reduction in orthodromic excitation of identified and non-identified type I-S neurons. These effect lasted for approximately 90-120 min and were reversed by local (0.4 microgram/0.2 microliter) or systemic (0.1-0.2 mg/kg, i.m.) administration of atropine sulfate. In contrast, the cholinergic agonist had no consistent effects on tonic type II nor on tonic type I-R neurons. In the light of these and other recent findings, we suggested the direct inhibition of central cholinergic neurons via muscarinic receptors, on the one hand, and the cholinergic nature of type I-S, but not type I-R nor type II neurons, on the other.