Abstract
In operant conditioning there are different schedules of reinforcements. One schedule of reinforcement, the differential reinforcement of low rates (DRL), is considered to be a specific one. On this schedule a response is reinforced only if it occurs after a specified time interval since the preceding response. Animals must refrain from responding for some time, and make good timing behaviors in order to get rewards effectively. Thus, this schedule have been thought to add some good informations to the psychopharmacological studies. In addition, the fixed interval (FI) schedule is another procedure of partial reinforcement, in which the condition of reinforcement is depended on the passed time after reinforced response, as well as in the case of DRL schedule. Drug's effects on the operant behavior under FI schedule also have been reported.
This study was designed to compare the influences of diazepam (DZP), chlorpromazine (CPZ) and amphetamine (AMP) on the performances of rats on FI and DRL schedules.
Seven male Wistar-Imamichi strain rats were trained to press the lever, protruded from the side wall of Skinner box (30×20×30 cm). Following the magazine training and the continuous reinforcement training, FI-60sec training (three rats) was conducted for about 15 sessions and DRL-20sec training (four rats) for 40-45 sessions. Each session lasted for 60 min a day. Reinforcement (reward) is a small food pellet weighing about 0.025g. Then the experimental (drug) sessions were performed to each rat.
The drugs tested were diazepam (p. o.), chlorpromazine (i. p.) and amphetamine (i. p.), which were administered 30 min before experiment. The order of treatments of the drugs was specified as randomly as possible and no less than three days separated successive drug administrations, in order to avoid the residual effects of drug.
The results obtained were as follows.
1. CPZ at all doses used here produced the decrease in average response, while DZP at low doses (2, 4mg/kg) increased them. The increase in the response rates was obtained with the treatment of low dose (1mg/kg) of AMP. These tendencies were almost equal in both FI-60sec and DRL-20sec schedules (Fig. 1). But all three drugs used were observed to produce profound decrease in the amount of lever pressings when treated with large doses. This fact might be connected with the “behavioral toxicity” as described by SIDMAN (14).
2. The relative frequency of short IRT (0-5sec on FI and O-16sec on DRL) was increased in comparison to that of Control by 4 and 8 mg/kg of DZP on the both schedules (Fig. 2). AMP also increased the relative frequency of short IRT on DRL schedule, with only a little changes in the distributions of IRT on FI schedule. CPZ produced only a slight changes in them on both of the schedules.
3. AMP (1-4mg/kg) produced the marked changes in the distribution of the duration of lever pressings (DLP) on FI-60sec schedule; the relative frequency of shorter DLP was distinctly increased. On the other hand, DZP and CPZ slightly prolonged DLP (Fig. 3). Therefore, DLP was considered to be a useful index, as well as IRT, for the evaluation of drug's effect on the operant behavior.
4. Mean response efficiency on DRL-20sec schedule, calculated from the data of IRT, was found to decrease with the treatments of all drugs used (Table 1).
