Neurochemical and Behavioural Effects Following Lesions of the Nucleus Basalis in the Rat

Immunohistochemical and histochemical evidence demonstrates that the nucleus basalis of Meynert (Nb) is the major source of cholinergic innervation to the frontal and parietal cortex in all mammalian species so far investigated (Pepeu et al. , 1985a). The Nb is part of a complex system of magnocellular neuclei of the basal forebrain characterised by intense staining for cholineacetyltransferase (ChAT) and acetylcholinesterase (AChE) (Bigl et al ., 1982). The various cholinergic neurons of the basal forebrain form a continuum along the ventral portion of the telencephalon and are not always aggregated into discrete nuclei: in some instances they are admixed with ChAT negative cells. The rostral components of this system consist of the medial septal nucleus, nuclei of the diagonal band of Broca and magnocellular preoptic nucleus. These nuclei have been designated Ch1 and Ch2 by Mesulam et al. (1983) in an attempt to provide a simpler classification. Ch3 includes the cholinergic neurons of the horizontal limb of the diagonal band of Broca. Ch4 is the larger and more loosely aggregated of these nuclei, including the nucleus basalis of Meynert, which partly overlaps the so called area innominata (Lamour et al ., 1982a), the nucleus of the ansa peduncolaris, and the nucleus of the ansa lenticularis. The most constant feature of the Ch4 neurons is their closeness to the medioventral portion of the globus pallidus.