The neglected constituent of the basal forebrain corticopetal projection system: GABAergic projections.

At least half of the basal forebrain neurons which project to the cortex are GABAergic. Whilst hypotheses about the attentional functions mediated by the cholinergic component of this corticopetal projection system have been substantiated in recent years, knowledge about the functional contributions of its GABAergic branch has remained extremely scarce. The possibility that basal forebrain GABAergic neurons that project to the cortex are selectively contacted by corticofugal projections suggests that the functions of the GABAergic branch can be conceptualized in terms of mediating executive aspects of cognitive performance, including the switching between multiple input sources and response rules. Such speculations gain preliminary support from the effects of excitotoxic lesions that preferentially, but not selectively, target the noncholinergic component of the basal forebrain corticopetal system, on performance in tasks involving demands on cognitive flexibility. Progress in understanding the cognitive functions of the basal forebrain system depends on evidence regarding its main noncholinergic components, and the generation of such evidence is contingent on the development of methods to manipulate and monitor selectively the activity of the GABAergic corticopetal projections.