Anatomical, neurochemical and behavioural consequences of immunotoxin lesions of the basal forebrain cholinergic system

Abstract: The cholinergic hypothesis of geriatric memory dysfunction suggests that the learning and memory deficits representing the cardinal symptoms of dementia in the aged and in patients suffering from Alzheimer's disease may be attributable to a decline in the function of the cholinergic projection systems of the basal forebrain. Although many studies have addressed the issue of a causal relationship between cholinergic hypofunction and cognitive disturbance, analyses of the functional role of the basal forebrain cholinergic neuronal systems in the brain have so far been hampered by the lack of an efficient and specific cholinergic neurotoxin. The recent introduction of a novel toxin conjugate, 192 IgG-saporin, able to selectively target and efficiently lesion cholinergic neurons in the basal forebrain has provided an opportunity, not previously available, for a more direct evaluation of the assumptions stated in the original formulation of the cholinergic hypothesis. In present series of experiments, the effects and the selectivity of the 192 IgG-saporin immunotoxin on a variety of anatomical, biochemical and behavioural measures were investigated following intraventricular injections to neonatal and adult rats. Also, the same immunotoxin lesion model has been employed in neonatal and adult rats to explore respectively the functional integration and reinnervation capacity of embryonic septal grafts homotopically grafted into the neuron-depleted septum, as well as to investigate the ability of cholinergic-rich grafts, placed combinedly into the depleted hippocampus and cortex, to ameliorate the functional deficits induced by the immunotoxin treatment.The results show that severe learning and memory impairments, associated with a near total (“95%) loss of basal forebrain cholinergic neurons, can be induced in adult animals following intraventricular administration of the toxin conjugate, and that these effects are highly specific and of a magnitude higher than that reported for previous lesioning techniques producing less severe deficits. By contrast, none of the observed functional deficits were evident following neonatal immunotoxin lesions leaving substantial cholinergic neuronal sparing, but they manifested dramatically when the activity in the residual cholinergic neurons was either pharmacologically blocked or permanently disrupted by a second lesion. This suggests: (a) that a subthreshold (i.e. ¾80-85%) cholinergic loss may not suffice for functional deficits to become apparent, and (b) that compensatory changes and/or a higher degree of plasticity in the developing as compared to the mature cholinergic system may efficiently sustain functional sparing for extended periods after the lesion. The results of the transplantation studies show: in neonatal hosts, that cholinergic neuroblasts implanted into the neuron-depleted septum can survive, integrate and extend axons along major myelinated pathways which reinnervate the appropriate target territories; in adult hosts, that intrahippocampal and intracortical grafts of cholinergic tissue can efficiently reverse the severe functional deficits induced by a previous immunotoxin lesion. Taken together, these data therefore support the hypothesis for an important involvement of the basal forebrain cholinergic projection system in cognitive functions.