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Sixty-three cortically projecting basal forebrain neurons were identified in chloral hydrate anesthetized rats by antidromic activation from the cerebral cortex. Two subpopulations were noted: type I neurons exhibited two antidromic action potentials of constant latency and identical waveform in response to double pulse cortical stimulation. In contrast, type II neurons exhibited two antidromic action potentials of constant latency but differing waveforms in response to the double pulse paradigm. The phenomenon exhibited by type II cortically projecting basal forebrain neurons is interpreted as evidence for loss of the somatodendritic portion of the antidromic action potential with high frequency stimulation. The median latency to antidromic activation of type II neurons (13.5 ms) was significantly longer than that of type I neurons (3.9 ms). Spontaneous firing rates varied over a wide range (0-49 Hz), and there was no significant difference between the rates of type I and type II neurons. These data underscore the physiological heterogeneity of this presumptive cholinergic cortical afferent system. Anatomical studies have shown that most, but possibly not all cortically projecting basal forebrain neurons are cholinergic. The relative proportions of type I (87%) and type II (13%) neurons encountered in this study suggest that type I neurons might be cholinergic and type II neurons non-cholinergic. If substantiated, this hypothesis would permit cholinergic and non-cholinergic cortically projecting basal forebrain neurons to be distinguished using a simple test of antidromicity.
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