Single cholinergic mesopontine tegmental neurons project to both the pontine reticular formation and the thalamus in the rat.

Microinjections of the cholinergic agonist carbachol into a caudal part of the pontine reticular formation of the rat induce a rapid eye movement sleep-like state. This carbachol-sensitive region of the pontine reticular formation is innervated by cholinergic neurons in the pedunculopontine and laterodorsol tegmental nuclei. The same population of cholinergic neurons also project heavily to the thalamus, where there is good evidence that acetylcholine facilitates sensory transmission and blocks rhythmic thalamocortical activity. The present study was undertaken to examine the degree to which single cholinergic neurons in the mesopontine tegmentum project to both the carbachol-sensitive region of the pontine reticular formation and the thalamus, by combining double fluorescent retrograde tracing and immunofluorescence with a monoclonal antibody to choline acetyltransferase in the rat. The results indicated that a subpopulation (5-21% ipsilaterally) of cholinergic neurons in the mesopontine tegmentum projects to both the thalamus and the carbachol-sensitive site of the pontine reticular formation, and these neurons represented the majority (45-88%) of cholinergic neurons projecting to the pontine reticular formation site. The percentage of cholinergic neurons with dual projections was higher in the pedunculopontine tegmental nucleus (6-27%) than in the laterodorsal tegmental nucleus (4-11%). In addition, mixed with cholinergic neurons in the mesopontine tegmentum, there was a small population of dually projecting neurons that did not appear to be cholinergic. Mesopontine cholinergic neurons with dual projections may simultaneously modulate neuronal activity in the pontine reticular formation and the thalamus, and thereby have the potential of concurrently regulating different aspects of rapid eye movement sleep.