Cholinergic neurons of the feline pontomesencephalon. I. Essential role in 'Wave A' generation.

Wave A in the cat appears to be analogous to P1 in the human. Both are positive middle-latency auditory-evoked potentials, present at slow click rates during wakefulness and REM sleep but absent during slow-wave sleep. Wave A has been recorded in the parabrachial and medial tegmental areas of the midbrain and in thalamic target projections of the reticular activating system. Two nuclei in this system, the pedunculopontine tegmental (PPT) and laterodorsal tegmental (LDT) nuclei, contain cholinergic cells; the cholinergic antagonist scopolamine eliminates Wave A. To test whether PPT and LDT were important in Wave A generation, we attempted to lesion these nuclei bilaterally in 11 cats. Wave A was markedly diminished or absent in all but 2 cats, in which the lesions did not include PPT. Loss of choline acetyltransferase-positive cells in PPT, but not LDT, was correlated with effects on Wave A, i.e. greatest cell loss occurred in cats in which Wave A disappeared, and least cell loss in cats with no change in Wave A. We conclude that the PPT nucleus, and particularly its cholinergic cell component, is essential for Wave A generation and suggests that a similar substrate may be significant for generation of the human P1.