Physiology & Behavior, Vol. 22, pp. 851--854.PergamonPress and BrainResearch Publ., 1979. Printedin the U.S.A.
Forgetting of a Drug-Conditional Discrimination N O R M A N E. S P E A R , G R E G O R Y J. S M I T H , A L A N S H E R R A N D R I C H A R D G. B R Y A N
State University o f N e w York at Bingharnton, Binghamton N Y 13901 ( R e c e i v e d 13 July 1978) SPEAR, N. E., G. J. SMITH, A. SHERR AND R. G. BRYAN. Forgetting of a drug-conditional discrimination. PHYSIOL. BEHAV. 22(5)851--854, 1979.--Rats were trained on a drug-conditional discrimination to escape footshock in a T maze. Over a period of days rats learned to approach one goal box while under a pentobarbital drug state and another while under a saline drug state. Retention of this discrimination was then assessed with the rat under either a pentobarbital or saline state, after an interval of 24 hours, 7 days, 30 days or 60 days. The results indicated little forgetting overall, with better long-term retention of the drug discrimination when the rats were tested under the pentobarbital state than under the saline state. Forgetting
Drug-conditional discrimination
Long-term memory
THE puprose of this study was to assess the forgetting of a discrimination that is conditional on a drug state. A secondary purpose was to assess one possible consequence of a method conventionally used to study drug discrimination. This method basically is to make the correct choice in a T-maze spatial discrimination conditional on drug state and to alternate two drug states, and hence correct choice, on successive days (for a thorough and analytical review of such methods [8]). If the animal learns to choose correctly on the first trial of a daily session, before experiencing any reinforcement contingencies during that session, the discrimination between drug states is said to be acquired. It seems possible, perhaps only remotely, that the animal's first response of a daily session is controlled in part by its memory of the most recent training episode 24 hours earlier. In other words, the animal may learn to respond opposite to the requirements of the previous day, a sort of single-alternation learning. If this were so, any forgetting observed in this situation would not necessarily be that of the discrimination based on drug state alone. Forgetting of a discrimination that is conditional on a drug state (to avoid circumlocution, "drug-conditional discrimination") has not previously been examined systematically as a function of length of retention interval. It is useful to do so to complete our knowledge of functional similarities---or functional discontinuities---between state dependent learning and drug discrimination learning on the one hand, and learning controlled by nonpharmacological stimuli on the other. It has been recognized for some time that a key theoretical issue of state dependent learning is whether a drug state exerts its control over learned behavior through
Sodium pentobarbital
the same principles that govern any other discriminative stimulus, or if the drug state requires unique stimulus considerations. In many respects the most analytical work in this area has addressed this general question [5, 6, 7]; (for a review of general theoretical issues, see [1, 9, 12]). For the present experiment, separate groups of rats learned a spatial discrimination to escape footshock, with the correct choice conditional upon drug state. For half the animals in each condition the left alternative always was correct following an injection with pentobarbital and the right correct following an injection with saline; for the remaining animals, the opposite conditional discrimination prevailed. Following acquisition of the conditional discrimination to a common criterion, retention was tested after 24 hours, 7 days, 30 days or 60 days. To avoid a "ceiling" effect on measurement of retention, the retention test consisted of a discrimination that was the reverse of original training. With this particular negative transfer test, better retention of original training is evidenced by poorer performance on the reversal task [11]. METHOD
Animals The animals were 40 male albino rats of the SpragueDawley strain born and raised in our colony at the State University of New York at Binghamton. Animals were between 55-90 days of age at the start of the experiment and were randomly assigned, 8 per group, to each of 5 experimental conditions. Throughout the duration of the experi-
1Preparation of this article was supported by grants from the National Science Foundation (BNS 74-24194 and BNS 78-02360) to the first author. We would especially like to thank Norman Richter for technical advice and assistance and Teri Tanenhaus for preparation of the manuscript. Requests for reprints should be sent to: Dr. Norman E. Spear, Department of Psychology, State University of New York at Binghamton, Binghamton NY 13901.
C o p y r i g h t © 1979 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/79/050851-04502.00/0
852 ment all animals were housed individually in laboratory steel mesh cages and maintained on a 0700 (lights on)-2300 (lights off) cycle. All training and testing took place between 1000 and 1500 hours. Apparatus The apparatus was a T maze (stem, 26.5 cm longx 10 cm w i d e x 13 cm high; each arm, 18 cm longx 10 cm w i d e x 13 cm high; each goal box attached perpendicular to the arm, 26.5 cm Iongx 10 cm w i d e x 13 cm high) made of clear Plexiglas and mounted on a grid floor (0.4 cm diameter grid bars spaced 1.5 cm apart). The grid floor could be electrified by a Lehigh Valley Electronic shock generator and scrambler (Model 26861) with power supplied by a transformer from the Scientific Prototype Manufacturing Co. (Model 4026J). The stem, choice section, and arms were separated by clear Plexiglas guillotine doors. The maze was constantly illuminated from below by four 6 W incandescent bulbs. Procedure and Design Each rat was carried individually in a holding cage to a room adjacent to the experimental room. Here it was weighed and administered an intraperitoneal injection of either sodium pentobarbital (Nembutal, 15 mg/kg/ml) or an equal volume injection of saline (0.9% saline solution), fifteen minutes prior to training or testing. Preexperimental treatment. On the two days immediately prior to initiation of training, each rat was injected daily with either Nembutal or saline in a counter-balanced manner. This meant that on Day 1 half the rats were injected with Nembutal and half with saline; on Day 2 this order was reversed. After receiving the daily injection, each rat was allowed to explore the T maze for 6 min during which time each rat was picked up and placed back in the start box on five occasions. After this 6 min period each rat was returned to its home cage. Training. In this stage, rats learned to escape a footshock (1.0 mA) by entering a particular arm of the T maze when the pentobarbital state was present and the opposite arm when the saline state was present. On the first day of training, each rat was randomly assigned to one of two initial drugs (pentobarbital drug state vs saline drug state on Day 1) and correct choice in the maze (left vs right). Fifteen minutes after the drug injection, each rat was given ten training trials using a correction training procedure. For each trial, the rat was placed in the start box facing away from the guillotine door that separated the choice point from the start box. Raising the guillotine door initiated the footshock, which was terminated when the rat placed four paws onto a wood platform located inside the goal box of the correct alternative (the platform was not visible from the choice point). The animal was left in the correct arm of the maze for 20 sec before removal to a holding cage for 30 sec to await the next training trial. If the rat failed to choose the correct alternative within 60 sec, the shock was terminated and the rat placed directly into the holding cage for the intertrial interval. On Day 2 each rat was trained under the drug state opposite that experienced on Day 1. Again, each rat was required to choose the correct alternative to escape the footshock. The correct response on Day 2 was to select the arm that had been incorrect on Day 1. Training continued, alternating drug states and correct choice every other day. The criterion of learning required
SPEAR, SMITH, SHERR A N D BRYAN that on the first trial of each of four consecutive days, the rat choose the correct alternative associated with that day's drug state. Our previous studies with these procedures had indicated that rats seldom made an error after attaining this criterion. After a rat attained the learning criterion, it was randomly assigned to one of the five retention test conditions. Retention test. Five groups of rats were tested after one of four retention intervals, 24 hr, 7 days, 30 days, or 60 days. Testing consisted of 10 trials identical to training, except that the rats were required to learn a reversal task. That is, if a right choice was correct under the pentobarbital drug state, the test required that the rat learn to choose the left alternative to escape footshock when under pentobarbital or the right alternative when under saline. Rats tested after 24 hr, 7 days, 30 days or 60 days (Groups 24a, 7d, 30d, and 60d, respectively) were tested under the drug state opposite that imposed on the final day of training. Group 24b was tested 24 hr after attaining criterion, under the same drug state present on the last day of training. Unlike the other groups for which the same choice was correct for the (drug reversal) test as for the last acquisition day, the correct choice for Group 24b was opposite that of the last acquisition day but the drug remained the same. To the extent that rats learn to alternate responding from one day to the next with the present procedure, it would be expected that this group would be superior to Group 24a in manifesting the reversal response. RESULTS
Acquisition The number of days required to attain the learning criterion was compared by analysis of variance (p
Forgetting of a Drug-Conditional Discrimination N O R M A N E. S P E A R , G R E G O R Y J. S M I T H , A L A N S H E R R A N D R I C H A R D G. B R Y A N
State University o f N e w York at Bingharnton, Binghamton N Y 13901 ( R e c e i v e d 13 July 1978) SPEAR, N. E., G. J. SMITH, A. SHERR AND R. G. BRYAN. Forgetting of a drug-conditional discrimination. PHYSIOL. BEHAV. 22(5)851--854, 1979.--Rats were trained on a drug-conditional discrimination to escape footshock in a T maze. Over a period of days rats learned to approach one goal box while under a pentobarbital drug state and another while under a saline drug state. Retention of this discrimination was then assessed with the rat under either a pentobarbital or saline state, after an interval of 24 hours, 7 days, 30 days or 60 days. The results indicated little forgetting overall, with better long-term retention of the drug discrimination when the rats were tested under the pentobarbital state than under the saline state. Forgetting
Drug-conditional discrimination
Long-term memory
THE puprose of this study was to assess the forgetting of a discrimination that is conditional on a drug state. A secondary purpose was to assess one possible consequence of a method conventionally used to study drug discrimination. This method basically is to make the correct choice in a T-maze spatial discrimination conditional on drug state and to alternate two drug states, and hence correct choice, on successive days (for a thorough and analytical review of such methods [8]). If the animal learns to choose correctly on the first trial of a daily session, before experiencing any reinforcement contingencies during that session, the discrimination between drug states is said to be acquired. It seems possible, perhaps only remotely, that the animal's first response of a daily session is controlled in part by its memory of the most recent training episode 24 hours earlier. In other words, the animal may learn to respond opposite to the requirements of the previous day, a sort of single-alternation learning. If this were so, any forgetting observed in this situation would not necessarily be that of the discrimination based on drug state alone. Forgetting of a discrimination that is conditional on a drug state (to avoid circumlocution, "drug-conditional discrimination") has not previously been examined systematically as a function of length of retention interval. It is useful to do so to complete our knowledge of functional similarities---or functional discontinuities---between state dependent learning and drug discrimination learning on the one hand, and learning controlled by nonpharmacological stimuli on the other. It has been recognized for some time that a key theoretical issue of state dependent learning is whether a drug state exerts its control over learned behavior through
Sodium pentobarbital
the same principles that govern any other discriminative stimulus, or if the drug state requires unique stimulus considerations. In many respects the most analytical work in this area has addressed this general question [5, 6, 7]; (for a review of general theoretical issues, see [1, 9, 12]). For the present experiment, separate groups of rats learned a spatial discrimination to escape footshock, with the correct choice conditional upon drug state. For half the animals in each condition the left alternative always was correct following an injection with pentobarbital and the right correct following an injection with saline; for the remaining animals, the opposite conditional discrimination prevailed. Following acquisition of the conditional discrimination to a common criterion, retention was tested after 24 hours, 7 days, 30 days or 60 days. To avoid a "ceiling" effect on measurement of retention, the retention test consisted of a discrimination that was the reverse of original training. With this particular negative transfer test, better retention of original training is evidenced by poorer performance on the reversal task [11]. METHOD
Animals The animals were 40 male albino rats of the SpragueDawley strain born and raised in our colony at the State University of New York at Binghamton. Animals were between 55-90 days of age at the start of the experiment and were randomly assigned, 8 per group, to each of 5 experimental conditions. Throughout the duration of the experi-
1Preparation of this article was supported by grants from the National Science Foundation (BNS 74-24194 and BNS 78-02360) to the first author. We would especially like to thank Norman Richter for technical advice and assistance and Teri Tanenhaus for preparation of the manuscript. Requests for reprints should be sent to: Dr. Norman E. Spear, Department of Psychology, State University of New York at Binghamton, Binghamton NY 13901.
C o p y r i g h t © 1979 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/79/050851-04502.00/0
852 ment all animals were housed individually in laboratory steel mesh cages and maintained on a 0700 (lights on)-2300 (lights off) cycle. All training and testing took place between 1000 and 1500 hours. Apparatus The apparatus was a T maze (stem, 26.5 cm longx 10 cm w i d e x 13 cm high; each arm, 18 cm longx 10 cm w i d e x 13 cm high; each goal box attached perpendicular to the arm, 26.5 cm Iongx 10 cm w i d e x 13 cm high) made of clear Plexiglas and mounted on a grid floor (0.4 cm diameter grid bars spaced 1.5 cm apart). The grid floor could be electrified by a Lehigh Valley Electronic shock generator and scrambler (Model 26861) with power supplied by a transformer from the Scientific Prototype Manufacturing Co. (Model 4026J). The stem, choice section, and arms were separated by clear Plexiglas guillotine doors. The maze was constantly illuminated from below by four 6 W incandescent bulbs. Procedure and Design Each rat was carried individually in a holding cage to a room adjacent to the experimental room. Here it was weighed and administered an intraperitoneal injection of either sodium pentobarbital (Nembutal, 15 mg/kg/ml) or an equal volume injection of saline (0.9% saline solution), fifteen minutes prior to training or testing. Preexperimental treatment. On the two days immediately prior to initiation of training, each rat was injected daily with either Nembutal or saline in a counter-balanced manner. This meant that on Day 1 half the rats were injected with Nembutal and half with saline; on Day 2 this order was reversed. After receiving the daily injection, each rat was allowed to explore the T maze for 6 min during which time each rat was picked up and placed back in the start box on five occasions. After this 6 min period each rat was returned to its home cage. Training. In this stage, rats learned to escape a footshock (1.0 mA) by entering a particular arm of the T maze when the pentobarbital state was present and the opposite arm when the saline state was present. On the first day of training, each rat was randomly assigned to one of two initial drugs (pentobarbital drug state vs saline drug state on Day 1) and correct choice in the maze (left vs right). Fifteen minutes after the drug injection, each rat was given ten training trials using a correction training procedure. For each trial, the rat was placed in the start box facing away from the guillotine door that separated the choice point from the start box. Raising the guillotine door initiated the footshock, which was terminated when the rat placed four paws onto a wood platform located inside the goal box of the correct alternative (the platform was not visible from the choice point). The animal was left in the correct arm of the maze for 20 sec before removal to a holding cage for 30 sec to await the next training trial. If the rat failed to choose the correct alternative within 60 sec, the shock was terminated and the rat placed directly into the holding cage for the intertrial interval. On Day 2 each rat was trained under the drug state opposite that experienced on Day 1. Again, each rat was required to choose the correct alternative to escape the footshock. The correct response on Day 2 was to select the arm that had been incorrect on Day 1. Training continued, alternating drug states and correct choice every other day. The criterion of learning required
SPEAR, SMITH, SHERR A N D BRYAN that on the first trial of each of four consecutive days, the rat choose the correct alternative associated with that day's drug state. Our previous studies with these procedures had indicated that rats seldom made an error after attaining this criterion. After a rat attained the learning criterion, it was randomly assigned to one of the five retention test conditions. Retention test. Five groups of rats were tested after one of four retention intervals, 24 hr, 7 days, 30 days, or 60 days. Testing consisted of 10 trials identical to training, except that the rats were required to learn a reversal task. That is, if a right choice was correct under the pentobarbital drug state, the test required that the rat learn to choose the left alternative to escape footshock when under pentobarbital or the right alternative when under saline. Rats tested after 24 hr, 7 days, 30 days or 60 days (Groups 24a, 7d, 30d, and 60d, respectively) were tested under the drug state opposite that imposed on the final day of training. Group 24b was tested 24 hr after attaining criterion, under the same drug state present on the last day of training. Unlike the other groups for which the same choice was correct for the (drug reversal) test as for the last acquisition day, the correct choice for Group 24b was opposite that of the last acquisition day but the drug remained the same. To the extent that rats learn to alternate responding from one day to the next with the present procedure, it would be expected that this group would be superior to Group 24a in manifesting the reversal response. RESULTS
Acquisition The number of days required to attain the learning criterion was compared by analysis of variance (p
