Psyehopharmaeologia (Berl.) 9, 1--16 (1966) Original Investigations 9 Originalarbeiten 9 Travaux originaux

The Effects of Pentylenetetrazole and Methylphenoxypropane on Discrimination Learning* EARL ItUNT and JARx I ~ I V ~ ] ~ K * * University of Sydney Received ~arch 13, i965 I t has been shown t h a t discrimination learning can be facilitated b y pro- and posttrial injections of subconvulsive amounts of certain convu]sant drugs. The experiments reported here are concerned primarily with the effects of 1 : 5-pcntamethylenetetrazole, a convulsant not previously studied as a facilitator of learning. Evidence will be presented to show t h a t pretrial injections of pentylenetetrazole facilitate discrimination learning much as other convulsants have been shown to do. I n addition, pentylenetetrazole appears to affect the behavior of the animals in the maze. Speed of running is markedly decreased, especially in the choice area of the maze. This is important because it provides a behavioral measure of drug action which can be observed on a single trial, while learning, by definition, can only be observed over trials. I t also raises the question of whether the effects on learning are due directly to the action of the drug or are a side effect of the drug's effect on performance. Some data will be reported indicating t h a t 3-(2-methyl-phenoxy) propane-1:2-diol has effects opposite to those of pentylenetetrazole. The speed of running is increased while learning deteriorates. Although the behavioral effects of the two drugs are opposed, their physiological effects need not of course be direct opposites.

ExperimentI The study is reported here because a) it contains certain data which, though inconclusive, will be important when taken in conjunction with the later findings, and b), it illustrates the general design and procedure used. * These experiments were mainly supported by the Psychology Department of the University of Sydney. Certain phases of data analysis and report preparation were also supported by the United States Public Health Service grant MH-07567 to the University of Sydney. Experiments I I I - - V I were done in conjunction with a B.A. (Hons) thesis submitted to the University of Sydney by J. KgIVANEKand supervised by E. Hu~T. ** The authors' names are Hsted alphabetically, without implications of seniority. Psychopharmacologia (BerI,), Bd. 9

1

Method Subjects. Sixteen male white rats, approximately 155 days old, were obtained from the University of Sydney animal breeding laboratory. Beyond obtaining all animals from this stock, no particular control was exerted over strain. The animals were fed a daffy ration of 14 g dry mash and 18 ml water on a 23 hr deprivation schedule for 1 week prior to the experiment. The animals were then weighed and placed in four comparable groups. Apparatus. A u maze with a mid gray run-up section was used. The two arms of the maze were about 10 cm wide and 76 cm long. One arm was painted black, the other white. The floor consisted of metal plates connected to a recording apparatus measuring the time spent in the various parts of the maze. A manually operated plastic starting gate separated the run-up section from the choice point. Apart from opening the starting gate, no manual intervention was required after the animal was placed in the maze. Two measures were taken: starting time, the time t a k e n to pass from the run-up section of the maze into the choice area once the gate had been lifted, and running time, the time taken to move from the choice area to the goal box. An error was defined as an entry of all four feet of the rat into the negative alley. Design. All animals were trained to make a black vs. white discrimination, with .5 g mash reward. A non-correction procedure was used, and the animals were run for three massed trials each day. Approximately 15 min before being run an animal would be removed from his home cage and injected intraperitoneally with 10 mg/kg methylphenoxypropane or 25 mg/kg pentylenetetrazole as appropriate. Injected animals were placed in a detention cage until run.

Rezults Animals receiving penty]enetetrazole learned the discrimination more rapidly in the sense t h a t b y the end of the experiment they had all made at least ten successive correct choices. Further, they began this run of ten trials earlier t h a n the animals receiving methylphenoxypropane. When the animals were ranked for number of trials and number of errors before reaching criterion, the Mann-Whitney U test (SIEGEL 1956, p. 116) indicated t h a t the two groups were significantly different at the .05 level. There was a clear effect on performance, the pentylenetetrazole group being markedly slower. This conclusion was confirmed b y switching the drugs. The performance of animals trained under pentylenetetrazole but tested under methylphenoxypropane resembled t h a t of the methylphenoxypropane group, and conversely. The pentylenetetrazole group seemed to be particularly slow in the choice area of the maze. However, the apparatus had not been designed to test this point explicitly.

Experiment II This experiment was designed to confirm and extend the results obtained in Experiment I. A control group injected with normal saline was included.

Method Subjects. Nineteen male white rats, approximately 55 days old, were accustomed to the deprivation schedule as before, then divided into three groups. 7 animals received pentylenetetrazole throughout then experiment (group p)l, 4 received saline (group S), and 8 received methylphenoxypropane (group M). Procedure. The same apparatus and procedure were used. Starting time was replaced b y decision time, the time taken to pass from the gate into one arm of the alley. Running time became the time taken from the beginning of the alley until the goal box. Training was to a color discrimination with food reward, with white positive for half the animals in each group, black for the other half. The positive color was changed from left to right in random sequence throughout the experiment. Before the actual experiment began, three " d r y " trials, without injection or food reward, were given to determine position preferences as far as possible. During the experiment itself animals received daffy intraperitoneal injections of .15 cm 3 normal saline, 10 mg/kg methylphenoxypropane, or 20 mg/kg pentylenetetrazole. The pentylenetetrazole dose was reduced because too m a n y convulsions had occurred at the 25 mg/kg dose. Under the new dose convulsions were rare and not very severe. The experiment was performed in two sections, each consisting of a learning and a criterion phase. I n section (1) the learning phase was defined as all trials up to and including the last error prior to a run of twelve successive correct trials ; the first criterion phase consisted of these twelve trials. I n section (2) the animals were given a new problem. For half the animals in each group the previously positive color became negative, for the other half a position, evenly distributed between preferred and non-preferred positions, became positive. The reversal phase in section (2) was defined as the trials up to and including the last error, and the second criterion phase again consisted of twelve successive correct trials.

Results During the learning phase, the order of mean speeds of both decision and running was M, S, P. Fig. 1 shows the speeds during the first criterion phase. Note t h a t the speed of the pentylenetetrazole group continues to increase during the criterion period, while those of the saline and methyl1 An additional animal in group P died during the experiment, apparently as a result of infection. 1"

Table 1. Analyses o/ Variance o/the Speed Data/or the First Criterion Period Decision Time Source

df

Total C (residual) A (trials) B (drugs) AB

75 64 3 2 6

Total C (residual) A (trials) B (drugs) AB

75 64 3 2 6

Sz

124.115 276.312 2,923.300 453.072 ~unning

/ oo~

1.353 12.230 2.696

n.s. 5o/0 n.s.

Total Errors

Total Trials

8 9 16 18

34 34 35 39

Pentylenetetrazole

3 4 5 6 7 9 15

1O 17 22 24 33 34 35

Methylphenoxypropane

8 10 15 16 18 21 24 26

30 43 45 48 52 55 55 58

Drug

I/

/

I

Saline

/, /

t,

/

/

/

n.s. 50/0 n.s.

Rats Ranked/or Total Errors and Total Trials During the Learning Period

1I1

/

2.154 23.560 3.652

Table 2

'------<---......

n%,

P

Time

39.633 53.627 484.494 106.770

~o/o

F

~

3

r

0

I

2

3

8lucksof/hfee/r/zls a

r

b Fig. i a and b

~0./0" I 84

t

'

I

t

Fig. 1 a and b, Experiment II: Speeds during the first criterion period, a decision; b running. - lYleth:dphenoxypropane; -- 9 -Saline; --Pentylenetetrazolc

"~ 005

5-

/0

Blocks of/hree/rlzls :Fig. 2

/:7

F i g . 2. E x p e r i m e n t I I : D e c i s i o n s p e e d s during the reversal period. - Methylphenoxypropane; ----Saline; - - - Pentylenetetrt~zole

5 Table 3. Analyses o/ Variance el the Speed Data/or the Second Criterion Period Decision Time Source

df

Total C (residual) A (trials) AB

47 36 3 2 6

Total C (residual) A (trials) B (drugs) AB

47 36 3 2 6

B (drugs)

S2

F

P

112.861

230.619 816.980 1,558.750

2.004 7.239 13.810

n.s.

50/0 50/0

R u n n i n g Time

46.680 77.770 146.840 288.490

p h e n o x y p r o p a n e groups a p p e a r to h a v e stabilised. T h e a n a l y s e s of v a r i a n c e of t h e speed d a t a for t h e first c r i t e r i o n p e r i o d are shown in T a b l e 1. T a b l e 2 shows t h e n u m b e r of trials a n d t h e n u m b e r of errors before r e a c h i n g criterion. T h e results confirmed E x p e r i m e n t I w h e r e v e r d i r e c t c o m p a r i s o n s could be m a d e . F i g . 2 shows t h e effect u p o n decision speed o f switching t h e a n i m a l s to t h e new p r o b l e m . D u r i n g learning, t h e o r d e r of t h e g r o u p s was a g a i n M, S, P, a n d was m a i n t a i n e d d u r i n g t h e criterion phase. T h e d a t a for r u n n i n g s p e e d is e s s e n t i a l l y similar. The pentylenetetrazole group s h o w e d a m a r k e d decrease in decision speed as soon as t h e n e w p r o b l e m was i n t r o d u c e d . N o c o m p a r a b l e effect was o b s e r v e d in t h e saline a n d methylphenoxypropane groups. T a b l e 3 shows t h e a n a l y s e s of v a r i a n c e of t h e speed d a t a for t h e second c r i t e r i o n period. T a b l e 4 shows t h e errors a n d t r i a l s to criterion on t h e second p r o b l e m . T h e p e n t y l e n e t e t r a zole g r o u p was s u p e r i o r t o t h e saline

1.666 3.264 6.181

n.s.

5~ 5o/o

Table 4. Rats Ranked ]or Total Errors and Total Trials During the Reversal Period Total Errors

Total Trials

Saline

9 12 15 28

23 25 25 26 5

Pentylene tetrazole

2 5 5 6 11 12 14

Methylphenoxypropane

l0 (ll) (15) (17) (28) (30) (33)

24 (33) (33) (36) (39)

(34)

(46)

Drug

6 9 9

12 16 20

(42) (42)

Only one rat in the methylphenoxypropane group learned the reversal task. For the rest of the group, it is assumed that the last error trial marks the end of the reversal phase. These figures are shown in parentheses.

group, while the methylphenoxypropane group was markedly inferior to both. All but one of the animals receiving methylphenoxypropane failed to solve the second problem before the experiment was terminated.

ExperimentIII Both the first two experiments involved ~_sual discrimination. In Experiment I I I animals were required to learn a position discrimination. Only pentylenetetrazole-saline comparisons were made in this and subsequent studies, as it was felt that enhancement was a more interesting phenomenon.

Method Subjects. Following habituation to the deprivation schedule, 24 male rats, 150 days old, were divided into two comparable groups. They received daffy injections of pentylenetetrazole or saline as described for Experiment II. Procedure. Six " d r y " trials were given in an attempt to determine position preferences more accurately. The preferred side became positive for all animals. The training procedure was the same as that used in Experiment I I except that the criterion period was extended from 12 to 16 successive errorless trials. /.3

J :~ 0.s i ~ ~.~.__...o..--"~" ~

S a

I

/

.-.o---~---~--" r'#'Cr 0

I0

S

/5

Trials Fig. 3 E x p e r i m e n t I I I : Decision speed d u r i n g the c r ite r ion period, - -

Saline; - - - P e n t y l e n e t e t r a z o l e

Results The error data are summarised in Table 5. Animals receiving pentylenetetrazole were again superior to those in the saline group in terms of both speed and accuracy of learning. Decision and running speeds closely resembled those obtained in Experiment II, the pentylenetetrazole group being the slower in both cases. The decision speed data for the criterion

period is shown i n Fig. 3. The d a t a for r u n n i n g speed are almost identical. The results are t h u s essentially the same as those o b t a i n e d with visual tasks. E x p e r i m e n t IV The relation b e t w e e n the behavioral measures used a n d variable levels of dosage was examined. Method Subjects. The 24 rats from E x p e r i m e n t I I I were used, a n d the s t u d y was b e g u n i m m e d i a t e l y after all a n i m a l s h~d reached criterioa i n t h e Table 5. Rats Ranked ]or Total Errors and Total Trials During the Learning Period 1Jentylenetetrazole Total Total Errors Trials

Table 6. Total Errors and Total Trials over Dose Levels (Two Animals at each Dose Level)

Saline

Total Errors

Total Trials

Dose (mg/kg)

0 0

3 4

0

1 3

5 6

6 6 i0 11 11 14 14 16

6 6

8 10 11 20 21 22 23 24 25 29 32 33

9

10 10 12 17 17

2 4 6 8 i0

12

Total Errors

Total Trials 43 39 34 34 43 31 34 37 34 35 21 27 17 21 18 15 13 18 16 21 10 12

position l e a r n i n g experiment. All 14 a n i m a l s were r u n u n d e r saline for 16 8 three trials. T h e a n i m a l s were t h e n i0 r a n k e d for r u n n i n g speed a n d the two 18 9 groups were c o m b i n e d i n t o twelve 15 pairs i n such a w a y t h a t one m e m b e r 4 20 5 of each pair came from the p e n t y l e n e tetrazole a n d one from the saline group, while one was a fast r u n n e r a n d the other a slow r u n n e r . The pairs were t h e n assigned to eleven dose levels, v a r y i n g from 0 - - 2 0 mg/kg i n steps of 2 mg/kg. Three a n i m a l s were used a t the 20 mg/kg dose. Procedure. T h e procedure was the same as t h a t used i n E x p e r i m e n t I I : a black-white d i s c r i m i n a t i o n with a criterion of 16 successive correct trials. On the first trial all a n i m a l s were rewarded, i.e. t h e positive color was placed on the previously positive side. After the first trial the positive color was a l t e r n a t e d r a n d o m l y from side to side. W h i t e was positive for half the animals, black for the other half.

Results

Table 6 summarises the relation between errors, trials to criterion, and dose level. Considering the very small number of animals in each group, the data are surprisingly regular. There seems to be little, ff any, effect on accuracy of discrimination below the 8 mg/kg dose. Thereafter both errors and trials to criterion decrease regularly as dose increases.

"~ ~3~

~2/g I

0

3

r

I

I

I

G 8 /0 /3 /l/ /~ /8 30

Dose in r~g/kg

.,.7

3

o"

8

/2

Bloek~ of tkree trials

Fig. 4 :Fig, 5 Fig.4. Experiment I V : Average decision speed for the ten trials following the last error over dose levels Fig.5. Experiment I V : Decision speeds over dose levels

Note that the highest dose used in this experiment, 20 mg/kg, is just below the convulsive levels used in Experiment I. This suggests that we have explored the effective range of the drug. Both decision and running speeds are continuously affected by the drug. The relevant data is illustrated by Fig. 4, which shows the decision speed immediately after the last error as a function of dose. The speed data for the criterion period were tested for linearity by means of the analysis of variance method described by M c N ~ A ~ (1962). The results are shown in Table 7. A linear relationship between dose and

Table 7 Analyses o] Variance/or Linear Trend o/the Speed Data During the Criterion Period Decision Time Source

df

Between Levels Linear Trend Deviation Within Levels Total

S~

10

214.64 1,532.73 68.17 98.03 153.56

1 9

11 21

F

F

2.19 15.64 0,69

n.s.

2.40 9.87 0.24

n.s,

.01 n.s.

R u n n i n g Time

Between Levels Linear Deviation Within Levels

10

45.79 188.05 4.64 19.06 20.88

1 9

11 21

TotM

.01 n.s.

Table 8. Lowest Mean Decision Times over Dose Levels, and the Comparison o/Each Mean with that o/the Zero Dose Group Dose (mg/kg)

Mean Decision Time Variance t (against 0 dose) df P

0

2--4

6--8

10--12

14--20

2.32 0.04

2.83 0.28 2.83 2

3.45 0.08 10.27 2 .01

5.31 0.97 9.65 2 .02

8.21 8.14 3.53 4 .02

n.s.

speed is indicated, the only departures from linearity being found at extreme dose levels. These departures could well be chance fluctuations, as there is no evidence for significant non-linear effects. I t should be remembered, however, that because of the very small number of animals in each group the experiment is not powerful in the statistical sense. Further studies might reveal deviation from linearity at extreme dosages. Since this experiment required that subjects learn a new discrimination after having reached criterion on a previous learning task, it can be regarded as a reversal learning study. I t was noted in Experiment i I that in a reversal situation animals given pentylenetetrazole showed increased hesitation in the choice area immediately after the switch. No such effects were noted in the saline and methylphenoxypropane groups. The renewed hesitation effect in second problem learning can be examined in more detail with variable levels of pentylenetetrazole. Table 8 shows the longest decision time on any trial for the different dose levels. Lowest speed is an inverse function of dosage. I n addition, animals receiving higher doses reach their slowest speeds sooner in second problem learning. This is shown in Fig. 5, which plots decision speeds as a function of trials, beginning with S, the average decision speed on the three trials on which M1 animals received saline.

10

ExperimentV The effects of pentylenetetrazole in successive discrimination and transfer situations were studied.

Method Subjects. Twelve male rats, 100 days old, were divided into two groups which received pretrial injections of 20 mg/kg pentylenetetrazole and .15 cm a normal saline respectively. H a l f the animals in each group were always rewarded in the black alley and never in the white, with reward conditions reversed for the other animals. Procedure. The animals were run for three massed trials a day in a black or white straight alley, the black and white forced trials being alternated randomly. The apparatus, running and recording procedures were those already described. I n this experiment, decision time becomes the time spent in the first 20 cm of the straight alley. Occasionally an animal refused to run to the negative color. A refusal was defined as the rat's backing out of the choice point after the gate had been lifted, and refusing to re-enter for 90 sec. Refusals were scored as 90 sec time for both decision and running. All the animals were rewarded on the first trial--i.e, each animal was first run to its positive color. Colors were then alternated randomly, so t h a t each rat ran eleven white and eleven black trials. The first choice trial was given in the Y maze on trial 24. Thereafter each day's testing consisted of one black and one white trial, in random order, followed by one choice trial, until a total of 17 choice trials had been run. At t h a t time all animals in the pentylenetetrazole group had made 16 successive correct choices. Results As the animals learn the successive discrimination task, they should start more rapidly under the positive condition t h a n under the negative. I f we define (decision time in kth negative trial) Yk ~ (decision time on kth positive trial) a graph of y~ as a function of k provides a picture of learning. This is shown in Fig.6. Evidence for discrimination learning is obtained b y noting the point at which y~ rises above 1, since this indicates faster running on the positive trials. Animals given pentylenetetrazole reach this point earlier in the experiment. The animals receiving pentylenetetrazo]e performed more accurately when transferred to the simultaneous discrimination situation. I n the simultaneous discrimination task five of the six rats given pentylenetetrazole made no errors, and the sixth made only one, before reaching

11 c r i t e r i o n o f 16 s u c c e s s i v e c o r r e c t choices. All a n i m a l s in t h e saline g r o u p m a d e a t l e a s t one error. S p e e d d a t a for t h e s t r a i g h t a l l e y is s h o w n i n F i g . 7. <1~' r e p r e s e n t s t h e a v e r a g e s p e e d on t h e first trial, o n w h i c h all a n i m a l s w e r e r e w a r d e d . O n t h e 3o

2#

i /0 /

/ ,i

,/

/

i

.---...o-----

3

# LTIocks oPtkree lrials

I

I

Fig.6. Experiment V: ~atio of positive to negative decision times in the straight alley. Pentylenet~trazole; - - Saline -

-

-

i

r --~ Penty/enetetrazole; rewardedtri'a/s fanty elnatatrozole; non-rewai'ded frh,ls - - o-- - ~ ~7oh'ne." r e w a r ~ d trial~ I~.~. ~-'-~ Saline: non-rewarded/Nob I

2r

~ 2.0o

I

IGO

'/

<~ 0.80

I"

t /

I

9

"

"" i

i,~

#

.~.d._<>....=....-~ ~

QVO

9

",, t

R

/"

Ii I

IA\\

0.01

l-"

r

"'----~ ~ Blocks o f tkree trialm i

2

I

3

Fig.7. Experiment V: Decision speeds on rewarded and non-rewarded trials in &he straight alley a v e r a g e , t h e a n i m a l s in t h e p e n t y l e n e t e t r a z o l e g r o u p r a n m o r e r a p i d l y t h a n a n i m a l s i n t h e saline g r o u p o n t h e r e w a r d e d trials, a n d less r a p i d l y on t h e n o n r e w a r d e d trials. T h i s is f u r t h e r e v i d e n c e for a f a c i l i t a t i v e effect

12 of pentylenetetrazole on the learning of a successive discrimination. Note t h a t there is no conflict between the finding of faster running b y the pentylenetetrazole group in this particular situation, as against slower running in the other situations studied, because here speed of running reflects in part the stage of discrimination learning. Experiment VI A n operant discrimination situation was used in order to study the effect of pentylenetctrazole upon the generalisation gradient. Animals were placed in a single lever Skinner box with either a positive, negative or test signal in view. When the positive signal was present bar pressing was rewarded on a 100 ~ schedule 2. When the negative signal was present bar pressing was never rewarded. After the animals had learned to respond differentially to the presence of the two signals a test signal, physically intermediate between the positive and negative signals, was placed in the box. The response to this signal provided a measure of generalisation.

Method Subjects. Two groups of six animals each were used, receiving saline or pentylenetetrazole at the same dose as in Experiment II. The animals were about 100 days old. Procedure. The rats were pretrained in the Skinner box for 12 min a day for 3 days without drugs. On the 4th day the animals were injected, placed in a detention cage for 15 man, and then trained in the Skinner box for 15 rain. This procedure was followed for the next 6 days. During this period the animals were "shaped" to press the bar. The black and white discrimination "signals" were first introduced on the 7th day. These were black or white plastic rectangles, about 22.5 b y 25 cm, which covered the entire front portion of the box above the level of the bar. H a l f the animals in each group were rewarded when the white plaque was present and never rewarded in the presence of the black plaque. These conditions were reversed for the other animals. The training period for each day consisted of four consecutive 4 rain sessions, broken only b y the insertion of a new plaque. Black and white plaques were alternated randomly, subject to the restriction t h a t over the training period each animal had an equal amount of experience with both signals. A test session was given on the day following the 1st day on which an animal gave at least three times as m a n y responses to the positive as to the negative signal. The test trial consisted of three 4 rain sessions: the first with the positive signal present, the second with the negative There were occasional apparatus failures, so that a few bar presses to the positive stimulus were not rewarded.

13 signal, and the third with a mid gray test signal. For half the animals in each group bar pressing in the presence of the mid gray signal was rewarded on a 100~ schedule; for the other animals bar pressing was never rewarded. Results

Table 9 shows the number of responses made by each rat in each period on the test day. All the animals in the pentylenetetrazole group gave a response to the test signal that was intermediate between those given to the positive and negative signals preceding it. All the animals receiving saline either a), gave more responses to the test signal than Table 9. Average Number o/Responses on the Test Trial in the Skinner Box

Saline Pentylenete~razole

Grey Test Stimulus

l~ositive Stimulus

Negative Stimulus

~ewarded

~Not l~ewarded

32.8 32.2

13.0 6.0

37.3 21.3

9.0 23.3

to the preceding positive signal if bar presses to the test signal were rewarded, or b), gave fewer responses to the test signal than to the preceding negative signal ff bar presses were not rewarded during the test period. These results are consistent with the hypothesis that for the saline group the controlling stimulus elements were those stimuli received from the delivery or nondelivery of food following a bar press, while the animals receiving pentylenetetrazole were responding to a wider range of cues, including those from the plaque introduced as a signal. Discussion Facilitative effects similar to those reported here for pentylenetetrazoie have also been found for strychnine sulphate and several other convnlsant drugs (LAsgL]~u 1917; McGAuGg and t)nT~INOWC~ 1959; MCGAgGg, T~oMso~, W]~ST]~oo~ and HUDSP]~T~ 1962; McGAuGg, WESTS~ooK and T x o ~ s o ~ 1962; P~T~rtCOVlCg 1963; STlgATTON and PnTm-~OVlC~ 1963). Apparently the only property these have in common with each other and with pentylenetetrazole is that they are convulsants given in subconvulsive doses. I t is possible that this convulsant property itself is somehow connected with their facilitation of learning, and that the mechanism by which all these drugs act is the same. McGAucE and PnT~r~OVlC~ (1959) also report that animals given pretrial injections of strychnine ran a complex maze more slowly than animals given saline. A similar effect was very pronounced in our work with pentylenetetrazole.

14 The results obtained in previous studies with drugs have been somewhat variable. The statistical reliability of the work of M c G A v ~ , PET•I~OVICH and their collaborators is not in question. However, we have obtained comparable levels of statistical significance with smaller groups and less training. This could indicate that pentylenetetrazole has a more powerful effect on learning, or it could be due to differences in experimental situations. The studies cited have typically involved very complex situations (e.g. a 14-unit T maze), whereas the problem used in our work was a relatively very simple one. Two effects were found with pentylenetetrazole and methylphenoxypropane: an alteration in the rate of discrimination learning, and an alteration in the speed of running, especially in the choice area of the maze. The question is whether the two effects are both produced by the drugs independently, or whether they are produced "in series", hesitation at the choice point resulting in more rapid learning. Studies of vicarious trial and error (VTE) by Mvv~NzrsG~, TOLMA~ and their associates have shown that hesitation at the choice point in discrimination learning situations will lead to better learning, no matter how such hesitation is produced (WooDWO~TH and SCHLOSB~G 1954). Direct comparison between the VTE studies and our work is difficult. Hesitation would have to be produced not only without drugs, but also in a way that would obviate such side effects as fear and frustration. This last point appears to us to be a weakness of the earlier work on VTE. Alternately, the drugs would have to be introduced without affecting hesitation. Our experiments therefore do not directly test the VTE hypothesis. :Nevertheless, some of our results are difficult to reconcile with the position that pentylenetetrazole acts by increasing VTE. The conventional definition of VTE -- alternately making partial responses to one side or the other in a simultaneous discrimination situation--does not apply to the successive discriminations of Experiments V and V$, in which pentylenetetrazole was found to have a facilitative effect. I t has been suggested that the facilitative effects observed with pretrial injections of convulsants are the result of the potentiating effects these drugs have on the consolidation of the memory trace in the nervous system. As B~v~EN and McGAvG~ (1961) point out, there are some objections to the use of this hypothesis in the case of pretrial drug injection. Stronger support comes from studies in which posttrial injections of convnlsants have also been shown to facilitate learning (B~EE~ and MCGAVG~ 1961; MCGAVG~, T~oMso~ and W~STB~OOK 1962; McGAuc~, W~STB~OOX and TgoMsox 1962; P ~ m ~ o w c g 1963; ST~ATTO~ and Pv~T~r~OWC~ 1963). I t may well be that the effects obtained in our experiments are posttrial rather than pretrial ones. Given intraperitoneally, pentylenetetrazole begins to be effective in about 15 rain, and

15 remains active for some 30 rain. I t would therefore have been active for some time after the daffy trials. On the other hand, some doubt has recently been cast on the posttrial facilitation effect itself. C o o P ~ and K~Ass (1963) report data indicating that strychnine facilitates learning when injected as long as 72 hrs before testing. I f this is the case, virtually all the posttrial facilitation studies would need to be re-examined. The results of Experiment VI indicated that pentylenetetrazole affects a generalisation gradient. The "generalisation-diserimination" hypothesis being proposed is that the process of discrimination learning involves orientation towards a "correct" set of cues among those presented b y t h e stimulus situation. Discrimination becomes the failure of the response appropriate to the positive stimulus to generalise to the negative stimuli and conversely. O n this hypothesis, one way in which pentylenetetrazole might facilitate learning would be by increasing the animal's sensitivity to changes in the environment. In terms of stimulus sampling theory (ATKI~SO~ and E s ~ s 1963), this could be done by increasing the size of the stimulus sample taken at each trial. I f a larger sample of stimulus cues is taken in at each trial, fewer trials will be required to locate relevant cues. I f we assume that time is required to sample each cue, more time ~ill be required for each trial. While we cannot discriminate between the memory trace and stimulus sampling explanations on the basis of this data, subsequent experimental work (to be reported later) indicates that our results are at least in part due to the action of pentylenetetrazole on the memory trace. We have been able to enhance discrimination learning by posttrial injection. We point out that it is possible that pretrial injections affect both lore- and posttrial processes.

Summary Pretrial injections of pentylenetetrazole had two major effects on discrimination learning: they slowed the speed of response, whether running down an alley or bar pressing in a Skinner box, and at the same time enhanced the ability to learn both successive and simultaneous discriminations. In simultaneous discrimination situations the effects of pretrial injections of methy]phenoxypropane were essentially opposite to those of pentylenetetrazole. The effects of pentylenetetrazole appear to be dosedependent, and it is suggested that it might act by broadening the range of stimulus situation cues to which the animal will respond at any one time.

References A T ~ s o ~ , R., and W. ESTES: Stimulus sampling Cheery. In: Handbook of Mathematical Psychology, Vol. II (Eds., R. D. Lvcv., R. Bvs~, and E. G~A~TER). New York: John Wiley and Sons 1963. B~Ev,N, R. A., and J. L. McG~v(~: Facilitation of maze learning with posttrial injections of picrotoxin. J. comp. physiol. Psychol. 54, 498--501 (1961).

16 CooPmr R. M., and M. KI~Ass: Strychnine: duration of the effects on maze learning. Psychopharmacologia (Berl.) 4, 472--475 (i963). LASaI~Y, K. S. : The effect of strychnine and caffeine upon rate of learning. Psychobiol. 1, 141--170 (1917). McG&tlG~, J. L., and L. PETRI~IOVlCH: The effect of strychnine sulphate on maze learning. Amer. J. Psychol. 72, 99--102 (1959). -- C.W. T~o~soN, W. H. WESTBROOK, and W. J. HUDSPETE: A further study of learning facilitation with strychnine sulphate. Psychopharmacologia (Berl.) B, 352--360 (1962). W. H. WESTBROOK, and C. W. T~o~Isox: Facilitation of maze learning with posttrial injections of 5-7-diphenyl-1-3-diazadamantan-6-ol (1757 I.S.) J. comp. physiol. Psycho]. 55, 710--713 (1962). MoNE~AI~, Q. : Psychological Statistics. Third edit. New York: John Wiley and Sons 1962. PETI~I~OVIC~, L.: Facilitation of successive discrimination learning by strychnine sulphate. Psychopharmacologia (Berl.) 4, 103--113 (1963). SIEGEL, S.: Non-parametric Statistics. New York: McGraw-Hill 1956. STtCATTON,L. 0., and L. PETRIN0VlOH: Posttrial injections of an anticholinesterase drug and maze learning in two strains of rats. Psychopharmacologia (Berl.) 5, 47--54 (1963). W0ODWOl~, R. S., and H. Sc~mos~I~G: Experimental psychology. Revised edit. New York: Henry Holt and Co. 1954. -

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JARA A. KRIVANEK Dept. of Psychobiology University of California Irvine, California (USA)