Different effects on learning ability after injection of the cholinergic immunotoxin ME20.4IgG-saporin into the diagonal band of Broca, basal nucleusof Meynert, or both in monkeys

Immunotoxic lesions of the diagonal band of Broca (VDB) in monkeys disrupte d cholinergic input to the hippocampus, producing impaired learning of visu ospatial conditional discriminations but not simple visual discriminations. Immunotoxic lesions of the basal nucleus of Meynert (NBM) deprived the cor tex of most of its cholinergic input, producing impaired learning of simple visual discriminations but not visuospatial conditional discriminations. C ombined lesions of the NBM + VDB resulted in impaired learning of both type s of task. The impairment after NBM lesions ameliorated with time but could be reinstated by a low dose of the glutamate blocking drug MK801, which, a t this dose, did not impair simple visual discrimination learning in normal monkeys. The cholinergic projections from the NBM and VDB may sustain the function of the glutamatergic pyramidal cell pathways within the cortex and hippocampus, respectively.