Effects of lesioning of the parafascicular nucleus of the thalamus on an operant food-procuring reflex in rats.

Abstract

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 cryomicrotome and were stained by the Nissl method. Data were analyzed statistically using Student's t test. Parafascicular Nucleus Lesions in Trained Rats. Before surgery, all rats of this group (n = 21) were trained to associate a click with feed for 6-7 days, and to apply strong pressure to the pedal for a further 1-2 days. Animals reached high levels of correct performances after 1-2 days of training after introduction of the conditioned signal. After bilateral lesioning of the parafascicular nucleus (destruction of more than 40% of the structure on both sides), animals (n = 6) showed significant reductions in the levels of intersignal pedal pressings in the first 1-6 days of testing, as compared with the pre-operative level (4.17 + 0.56). The number of correct performances was significantly reduced on post-operative days 1-3, 5, and 6. On other days, the level of performances was not significantly different from baseline (79.33 + 7.81) (Fig. 1A). After unilateral lesions of the parafascicular nucleus, rats (n = 8) showed a significant reduction in the level of intersignal pressings on post-operative