The effect of prefrontal stimulation on the firing of basal forebrain neurons in urethane anesthetized rat.

The basal forebrain (BF) contains a heterogeneous population of cholinergic and non-cholinergic corticopetal neurons and interneurons. Neurons firing at a higher rate during fast cortical EEG activity (f>16Hz) were called F cells, while neurons that increase their firing rate during high-amplitude slow-cortical waves (f<4Hz) were categorized as S-cells. The prefrontal cortex (PFC) projects heavily to the BF, although little is known how it affects the firing of BF units. In this study, we investigated the effect of stimulation of the medial PFC on the firing rate of BF neurons (n=57) that were subsequently labeled by biocytin using juxtacellular filling (n=22). BF units were categorized in relation to tail-pinch induced EEG changes. Electrical stimulation of the medial PFC led to responses in 28 out of 41 F cells and in 8 out of 9 S cells. Within the sample of responsive F cells, 57% showed excitation (n=8) or excitation followed by inhibitory period (n=8). The remaining F cells expressed a short (n=6) or long inhibitory (n=6) response. In contrast, 6 out of the 8 responsive S cells reduced their firing after prefrontal stimulation. Among the F cells, we recovered one cholinergic neuron and one parvalbumin-containing (PV) neuron using juxtacellular filling and subsequent immunocytochemistry. While the PV cell displayed short latency facilitation, the cholinergic cell showed significant inhibition with much longer latency in response to the prefrontal stimulus. This is in agreement with previous anatomical data showing that prefrontal projections directly target mostly non-cholinergic cells, including GABAergic neurons.