Dual projections of single cholinergic and aminergic brainstem neurons to the thalamus and basal forebrain in the rat

Compelling evidence indicates that cholinergic basal forebrain neurons are strongly activated during waking, and concurrently thalamic spindle activity is suppressed and thalamocortical sensory transmission is facilitated. Both thalamus and basal forebrain are known to receive projections from brainstem cholinergic and aminergic neuronal pools that are involved in wake/sleep regulation. The present study addressed the question of whether single cholinergic and aminergic neurons contributed to both of these ascending projections, by using two fluorescent retrograde tracers combined with immunofluorescence. Cholinergic neurons projecting to both the basal forebrain and thalamus were found in the pedunculopontine and laterodorsal tegmental nuclei, representing an average of 8.0% of the total cholinergic cell population in these nuclei. Serotonergic neurons with dual projections were observed in the dorsal, median and caudal linear raphe nuclei, accounting for a mean of 4.7% of total serotonergic neurons in these nuclei. Relatively few noradrenergic neurons (2.0%) in the locus ceruleus projected to both target structures, and a very small subpopulation of histaminergic neurons (1.5%) in the tuberomammillary hypothalamic nucleus had dual projections. Of all brainstem neurons with dual projections, cholinergic and serotonergic neurons accounted for an overwhelming majority, with noradrenergic followed by histaminergic neurons representing the remaining minority. These data suggest that through dual projections, cholinergic and aminergic brainstem neurons can concurrently modulate the activity of neurons in the thalamus and basal forebrain during cortical arousal.