Effects of Septal Lesion on Operant Response and Collateral Behavior in DT-DRL of the Rat

Abstract

The present study was designed to investigate the effect of septal lesion on operant response and collateral behavior in DT-DRL schedule of the rat. After the feeding schedule of 7 days in which food intake was limited to about 4% of ad lib body weight, 8 male rats of Wister strain were adapted to a Skinner box having a retractable (response) lever and a drinking spout for water intake. Next day, they were shaped to leverpress for a 45-mg food pellet every lever pressing. Then they were trained on a typical CRF schedule of 2 days. After then, the training procedure was changed to a DT-CRF of 5 days. In the first 2 days, DT time was started at 0.5 sec and gradually increased to 2 sec, and in the last 3 days DT time was fixed at 2 sec. According to the (operant) response results of each subjects in these stages, all rats were divided into 2 conditions; one of them was DT 2 DRL 6/LH 36 condition (N=4) and the other was DT-0 DRL 6/LH 36 condition (N=4). This stage was maintained for 3 consecutive days, and then only DRL time was increased to 20 seconds in each condition. The preoperative training procedure of DT-2 DRL 20/LH 36 or DT-0 DRL 20/LH 36 was carried out for 30 consecutive days. At the end of these training sessions, septal lesions were nitroduced. All surgery was performed under sodium pentobarbital anesthesia (Nembutal, 45mg/kg). In the half of subjects in each condition, septal lesions were made by passing 2.0 mA anodal d. c. for 30 sec, at the uninsulated tip of a stainless steel eelctrode inserted stereotaxically into the brain. An anal cathode completed the circuit. For septal lesions, the stereotaxical coordinates based on the rat brain atlas of PELLEGRINO & CUSHMAN were 2.0 mm anterior to bregma, and 5. 0 mm below the skull. Control lesions for remaining half of subjects in each condition, were made by only inserting the electrode to the corpus callosum at the same coordinates used for the septal lesions and withdrawing it for 30 sec without passing current. Following surgery, animals were allowed 3 days of recovery time with ad lib food and water intake, and the same feeding schedule used before surgery was applyed for 4 days. The postoperative retraining procedure was the same as the preoperative one, and the retraining sessions were carried out for 14 consecutive days. Following these behavioral testing, all subjects were sacrificed with overdose of anesthetic. The lesioned brain was perfused with 10 % formalin, and the block of a tissue containing the lesion was cut from the brain, dehydrated, embedded in paraffin, and sectioned at 20 p. Every fifth section was stained with hematoxylin-eosin for subsequent analysis of lesioned placement. The main behavioral findings were as follows. In the pre- and postoperative training sessions, the rats that belong to DT-2 condition showed more adaptive operant response than the rats in DT-0. After surgery, septal rats exhibited a great increase of lever pressings and poor reinforcements compared with those in the control rats or those of their own control in the last preoperative training session. But in the present experiment, it was very much important that the considerable recovery of DRL behavior was clear at the end of the retraining sessions, especially in the septal rats of DT-2 condition (Fig. 1 and Fig. 2). The analysis concerning to IRT distributions suggested that the effect of septal lesion in DT-2 was fairy different from that in DT-0. The mode of IRT distributions in DT-2 was slightly moved to sub-reinforcible IRT class just after the surgery, but in several days the mode of IRT distributions was returned to the level of the last preoperative session which was considered very excellent in DRL 20/LH 36 situation.