Involvement of the parafascicular nucleus of the thalamus and the cholinoreactive system of the neostriatum in controlling a food-procuring reflex in rats at different stages of learning.

A model of a Skinner box food-procuring reflex in rats was used to study the relationship between the strength applied to a pedal and disruption of the parafascicular nucleus of the thalamus and microinjections of the cholinolytic scopolamine and the cholinomimetic carbachol into the neostriatum at different stages of learning. In untrained rats at the stage of learning to press strongly on the pedal without the conditioned signal being switched on (i.e., every strong press was rewarded) showed (a) a decrease in the rate of learning to press strongly and an increase in the number of weak pedal presses after bilateral lesioning of the parafascicular nucleus of the thalamus; (b) that rats with bilateral lesions of this nucleus responded to microinjections of scopolamine into the neostriatum with increases in the number of strong presses, with no change in the number of weak pedal presses, while microinjections of carbachol decreased the number of strong and increased the number of weak presses as compared with the pre-microinjection baseline. In trained rats at the stage of recovery the reflex (i.e., strong pedal presses were reinforced only during exposure to the conditioned signal), lesioning of the parafascicular nucleus of the thalamus had the effect that the time required for recovery of the reflex became dependent on the level of pre-operative training; scopolamine microinjections into the neostriatum of rats achieving high levels of correct performances of the reflex only after surgery led to sharp degradation in performance of the reflex on the day of microinjections; microinjection of carbachol into the neostriatum of rats with low post-operative levels of performance of the reflex had no effect on this measure.