INTERACTIONS BETWEEN THE EFFECTS OF BASAL FOREBRAIN LESIONS AND CHRONIC TREATMENT WITH MDL 26,479 ON LEARNING AND MARKERS OF CHOLINERGIC TRANSMISSION

The effects of ibotenic acid-induced basal forebrain lesions and treatment with the triazole MDL 26,479 on the acquisition of an operant visual conditional discrimination task and on [H-3]hemicholinium-3 and [H-3]vesamicol binding were examined. Lesioned animals required more training sessions to acquire the stimulus-response rules of this task. They also showed longer response latencies throughout the experiment. The effects of the treatment with MDL 26,479 (5 mg/kg; i.p. 60 min before each training session) interacted with the effects of the lesion, producing a decrease in the number of sessions required to perform above chance-level in lesioned but not in control animals. MDL 26,479 did not seem to produce immediate performance effects but interacted with the learning process. The lesions destroyed the cell bodies in the area of the substantia innominata, basal nucleus of Meynert, and the globus pallidus. The number of frontocortical cholinergic terminals as primarily indicated by hemicholinium-3 binding was reduced in lesioned animals; however, another measure of cholinergic terminals, vesamicol binding, was unchanged. Behavioral performance of animals correlated significantly with hemicholinium binding in the frontal cortex of the right hemisphere. The fact that the lesion delayed but did not block the acquisition of the task may have been a result of compensatory mechanisms in remaining cholinergic terminals as indicated by stable vesamicol binding. These data allow assumptions about the conditions for the demonstration of beneficial behavioral effects of MDL 26,479. They also suggest that the long-term effects of basal forebrain lesions on cortical cholinergic transmission remain unsettled.