Rhythmically Discharging Basal Forebrain Units Comprise Cholinergic, GABAergic, and Putative Glutamatergic Cells

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Abstract
The basal forebrain plays important roles in arousal, learning, and memory by stimulating cortical activation characterized by rhythmic slow theta and high-frequency beta-gamma activities. Although cholinergic neurons play a significant part in these roles, other, including GABAergic, neurons appear to contribute. Using juxtacellular labeling with neurobiotin of neurons recorded within the magnocellular preoptic-substantia innominata area in urethan-anesthetized rats, we show that in addition to cells that are cholinergic or GABAergic, other cells that are neither fire rhythmically in correlation with stimulation-induced rhythmic slow activity on the cortex. Neurons with the characteristics of the noncholinergic/nonGABAergic cells contain phosphate-activated glutaminase (PAG), the synthetic enzyme for transmitter glutamate and may thus be glutamatergic. Within their oscillatory spike trains, putative glutamatergic neurons fire at a lower frequency (∼20 Hz) than the GABAergic neurons (∼40 Hz) and the cholinergic neurons (average: 75 Hz), whose spike trains include high-frequency bursts. The three groups all discharge rhythmically at a slow frequency in correlation with rhythmic slow activity recorded on the prefrontal, entorhinal, piriform and olfactory bulb cortices. The predominant slow frequency corresponds to the respiratory-olfactory rhythm, which is commonly slower than, yet can be as fast as, the hippocampal theta rhythm during certain coordinated behaviors, such as sniffing-whisking. While stimulating higher frequency beta-gamma activities, putative glutamatergic together with GABAergic and cholinergic cells may thus collectively modulate rhythmic slow activity and thereby promote coherent processing and plasticity across distributed cortical networks during coordinated behaviors and states.

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