Chronic experiments performed on 32 Sprague-Dawley rats using a movement-feeding operant reflex (Skinner box) model showed that microinjection of scopolamine into the neostriatum had effects on this reflex which depended on the stage of learning. In animals with weakly fixed reflexes (prior to reaching the stage of memory consolidation), bilateral microinjection of 0.3 microgram of scopolamine into the caudate nucleus completely inhibited the reflex for a prolonged period of time. When the operant habit was well fixed, bilateral microinjection of the same doses of scopolamine into the neostriatum had no effect on the reflex. These results suggest that the neostriatum cholinergic system is critically involved in forming the motor engram. The cholinergic system of the caudate nucleus either takes no part in realizing the well-fixed conditioned reflex movement response and/or other forebrain structures are involved in the reflex, compensating for the disturbance in neostriatal cholinergic function.
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.
Studies have demonstrated that lesioning of the parafascicular nucleus, a member of the caudal group of intralaminar nuclei, worsens performance of defensive reflexes [ 1, 2]. Degradation of the parafascicular nucleus has been shown [2] to have no effect on simple types of food-procuring contextual reflexes, while complex types of these reflexes (selection tasks, complex mazes) are irreversibly damaged. Thus, the question of the possible mechanisms by which the parafascicular nuclei of the thalamus are involved in organizing and performing food behavior remains open.The aim of the present work was to study the effects of lesioning of the parafascicular nucleus of the thalamus on the formation and performance of an operant food-procuring reflex in untrained rats and in rats trained to conditioned reflex responses.Studies were carried out on 33 male Sprague-Dawley rats. Animals were trained in a Skinner box. The conditioned signal consisted of rhythmic flashes of a white lamp. Reinforcement consisted of pieces of cheese (25-30 rag). Animals were kept on a limited diet (8 g of dried concentrated food per day per rat) from training day 3. Training was performed in three stages. In the first stage, rats were trained to associate a click accompanied by delivery of food into the feeder with reinforcement, the aim being to correct the pedal-pressing force for subsequent experiments. Training was continued until rats received reinforcement 20 times during ten-minute periods in the chamber, during which time 20 clicks were presented. Clicks were presented every 30 see. At the second stage of training, rats were trained to press the pedal to obtain food. Weak actions were initially reinforced, after which only strong pedal presses, opening the feeder, were reinforced. The second stage of training was continued until rats pressed the pedal 20 times during 10-min periods in the chamber. The conditioned signal (10 sec with a 20-sec interval) was introduced at the third stage of training. Pedal presses without the conditioned signal were not reinforced and were regarded as interstimulus responses. Each experiment consisted of 10 presentations of the conditioned signal. Rats were considered trained when they produced correct responses to at least 70% of conditioned signal presentations.High levels (70-90%) of reflex performance were stabilized in intact animals by repeated testing for an average of 18 days, after which surgery was performed. The effects of lesions to the parafascicular nucleus on the initial stage of learning were studied by operating on a second group of rats before training to the food-procuring reflex. Surgical procedures were performed under ketamine anesthesia (250 mg/kg). A stereotaxic atlas [4] was used to determine the coordinates for electrolytic lesioning (5 /zA, 20 sec) of the parafascicular nucleus of the thalamus. Experiments were restarted 6 days after surgery. Animals were sacrificed by decapitation under ketamine anesthesia when experiments were finished. Histological sections were prepared on a ...
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