Central cholinergic depression reduces MAC for isoflurane in rats.

The role of central nervous system cholinergic neuromechanisms during inhalation (isoflurane) anesthesia was evaluated by measuring the minimum alveolar concentration (MAC) in rats before and after selective modulation of cholinergic neurotransmission in the brain. Cholinergic neurotransmission was depressed by reducing synaptic levels of acetylcholine with intracerebroventricular (ICV) injection of the selective anticholinergic hemicholinium-3 and with ICV injection of the presynaptic inhibitory autoreceptor agonist oxotremorine. Hemicholinium-3 (20 micrograms) decreased MAC by 18% (P less than 0.001), and oxotremorine (20 micrograms) decreased MAC by 29% (P less than 0.001). Conversely, elevating synaptic levels of acetylcholine and facilitation of cholinergic neurotransmission by treatment with physostigmine significantly increased MAC. Antagonism of muscarinic receptors with ICV injection of atropine (30 micrograms) failed to alter MAC, but antagonism of nicotinic receptors with ICV injection of pancuronium significantly decreased MAC in a dose-dependent fashion. The results support the hypothesis that depressed cholinergic neurotransmission in the brain plays a role in the mediation of the anesthetized state. The consequences of depressed synaptic levels of acetylcholine during anesthesia may be mediated through a previously unidentified postsynaptic nicotinic neuromechanism.