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Operant conditioning in mice
Lüe Sciences Vol. 11, Part I, pp . 905-914, 1972 . Printed in Great Britain
Pergamon Press
OPERANT CONDITIONING IN MICE Gerhard Freund and Don W . Walker Veterans Administration Hospital and University of Florida Gainesville, Florida
(Received 16 May 1972; in final form 21 July 1972) Summary Only in recent years has the necessity to teat drugs and chemicals for behavioral toxicity been recognized . The mouse is the animal most commonly used for conventional toxicity screening, but it has rarely been used for toxicity studies using operant behavior . Thin investigation demonstrates that the mouse has a potential similar to that of the rat to learn operant tasks if lever press is substituted by lever touch and if the caloric value of reinforcement pellets is diluted by noncaloric bulk . Under these conditions mice can learn the following schedules of reinforcement : continuous, discriminated variable-interval, discriminated single alternation go, no-go, and differential reinforcement of low rate (DRL) . As in rata, performance is optimal within a range of 20 f 5 per cent weight reduction from the body weight under ad libitum feeding conditions . Toxic effects of chemicals may be detectable by behavioral methods before gross anatomical or biochemical damage is apparent .l -9
The increase of
potentially toxic chemicals in man's environment therefore requires the application of psychopharmacological methods to the toxicological evaluation of chemicals .
The growing importance of this relatively new field has been
recently recognized . 4 The laboratory mouse is the animal most widely used for conventional toxicity screening of drugs and chemicals, but little ie known about behavioral toxicity in mice as evaluated by standard operant conditioning proce dures .
These automated procedures appear to have great potential for the
detection of behavioral to:icity .
The purpose of this investigation is to
determine whether mice have the potential to perform as efficiently as rate in certain food-motivated, lever press paradigms if modifications of standard rat operant procedures are made .
905
908
Operant Condirioning in Mice
Vol . 11, No. 19
Materiale and Methods
Subjects .
C57BL/6J female, 3-month-old mice (Jackson Laboratory, Bar
Harbor, Maine), weighing between 21 and 22 g, were used as subjects .
They
were housed individually and were maintained on a 12-hour-light, 12-hour-dark cycle . Appca~atus .
One side of a mouse shuttle box (Lehigh Valley Electronics,
Fogelsville, Pa ., Model #146-021) was equipped with a mouse lever (Lehigh Valley Model #1535) connected to a contact relay that activated a pellet dis penser (Davia Scientific Instruments, North Hollywood, Calif .) .
The mouse
lever was made retractable by mounting it on the base of a retractable rat lever (Lehigh Valley Model #1405R) .
The electrical input from lever touch had
a 1 .5-sec delay between the initial touch and the earliest possible activation of the next lever touch signal to prevent release of multiple pellets by a single touch .
This 1 .5-sec delay was effective in all of the schedules re-
ported in this paper . divider .
The other aide of the shuttle box was closed off by a
The operant chamber was enclosed in a sound-attenuated cubicle and
was programmed by Grason Stadler 1200 series modules .
Procedure .
The mice were weighed daily immediately prior to being placed
in the operant chamber .
Immediately after being returned to their home cage,
they were fed their daily ration of Guinea Pig Chow pellets (Ralston Purina Co ., St . Louis, Mo .) ranging from 2 .2 to 3 .0 g/mouse/day .
Initial weight reduction
was accomplished by feeding 1 .5 g of Chow/mouse/day until body weight was between 16 .5 and 17 .0 g .
The wnighta of daily food rations were adjusted indi-
vidually to maintain each mouse at weights between 16 .5 and 17 .0 g .
Reinforce-
ments consisted of 20 mg pellets (P . J . Noyes Co ., Lancaster, N . H .) of various compositions, as described below .
Wastage was determined by weighing pellets
and their particles that dropped into the pan underneath the trough .
The per
cent wastage was calculated from the total weight of reinforcement pellets delivered on a CRF schedule . All sessions lasted 30 min except for the go, no-go sessions which
Operant Conditioning in Mice
Vol . il, No . 19
consisted of 60 trials .
907
During continuous reinforcement (CRF) a food pellet
was available upon lever touch every 1 .5 sec .
(Actually, the 1 .5-sec delay
used in all the schedules is more like a FI 1 .5-sec schedule, but for conven fence called CRF when .no other reinforcement contingencies are imposed .)
CRF
training was initiated by placing each mouse in the operant chamber, generally without shaping .
To determine the effect of various degrees of weight reduc-
tion on CRF performance, a group of eight mice was weight-reduced by decrements of 1 .0 g of body weight from 17 .0 to 13 .0 g (Table 2) .
The mice were main
tained at each weight level for 8 to 10 days and tested daily on a CRF schedule . The same procedure was then repeated in the reverse order, and the body weights were allowed to increase to 21 .0 g by 1 .0 g increments . Using a CRF schedule, various types of 20 mg pellets used for reinforcement (Table 1) were evaluated in a group of eight mice, maintained at 16 .5 to 17 .0 g Each type of pellet was fed to the group for 6 to 8 consecutive
body weight .
days after stable CRF responses had been established with pellets containing 10 per cent sucrose and 90 per cent noncaloric bulk for 14 days .
Cumulative
recorder tracings were obtained on all animals on Days 4 and 5 after they had been changed to a new type of reinforcement pellet . Various schedules of reinforcement were tested in groups of eight mice each after CRF training for at least two weeks and after the CRF response rate had reached asymptotic values for at least g days . was used for each schedule .
A different group of mice
All pellets for reinforcements consisted of 10 per
cent sucrose and 90 per cent bulk . The first teak was a discrete trial discrimination with trials presented on a VI schedule .
This task consisted of alternating 7 .5~min periods of cued dis-
criminations and 7 .5Jmin periods of time-out (S~)--no reinforcement or cue presentation .
The first 7 .5rmin period of each session consisted of the presenta-
tion of a compound SD (white noise and cue light above lever), eigaaling the availability of reinforcement .
The SD was presented on a VI-40-sec schedule .
As soon as the mouse touched the lever after the onset of SD, one food pellet
90 8
Operant Conditioning in Mice
was delivered and Sv was terminated .
Vol. 11, No . 19
Following 7 .5 min of this schedule a
7 .5-min, time-out period (S~) was initiated .
Each 30-min session consisted of
two 7 .5-min periods of VI discrimination and two 7 .5-min periods of time-out . The total number of responses during the SD (reinforced responses) and the total number of the responses in the absence of SD (nonreinforced responses) were recorded during each 7 .5-min period of discrimination .
The number of
responses (all nonreinforced) during each 7 .5-min, time-out period (S~) were recorded .
Immediately following training with SD presented on a VI-40-sec
schedule, the mice were trained with SD presented on a VI-20-sec schedule in a 2- to 38-sec range (Fig . 1) . A differential reinforcement of low rate (DRL) schedule was employed .s Reinforcement was contingent upon bar touch later than 20 sec after the previous reinforcement (DRL-20) .
Each daily session lasted for 30 min .
Inter
response times (IRT) and numbers of reinforced aad nonreinforced lever touches were recorded .
Efficiency ratios were calculated by dividing the number of
reinforced responses by the total number of responses (Fig . 2) . The final operant schedule tested was discriminated single alternation go, no-go task . 6
Each discrete trial consisted of the presentation of a retractable
mouse lever for 20 sec at a time followed by an intertrial interval (ITI) of 7 .5 sec .
On the odd-numbered (go) trials, food reinforcements were available
on a CRF schedule, and on the even-numbered (no-go) trials they were not . sessions consisted of 60 trials .
Da11y
White noise, from a noise generator (Graeon
Stadler Model ~9UlB), and a green light, 1 cm above the lever, served as conditioned stimuli .
For the go, no-go trials the conditioned stimuli were a low-
frequency sound presented simultaneously with a green light, 1 cm above the lever, for the go trials .
The no-go trials were indicated by a white light,
5 cm above and 3 cm to the left of the lever, associated with a high-frequency sound from an sudiogenerator (BRS Foringer Model AU902) .
The dependent vari-
abler were the latency to the first bar touch on each trial and the total number of touches on each trial (Fig . 3) .
Vol. 11, No . 19
Operant Conditioning in Mice
90 9
Results Direct observation and comparison with automatically recorded responses revealed that mice could not uniformly depress the mouse lever .
Only approxi-
mately 30 per cent of lever presses resulted in electrical contact closure . When lever touch was introduced, a single touch of the lever often released multiple pellets .
By delaying the availability of the next pellet by 1 .5 sec,
only single pellets were released with each touch . When standard 20 mg Noyas pellets were used for reinforcement (Table 1) on a CRF schedule, the total number of bar touches during a 30-min period was low . TABLE 1
The effect of various response during daily are shown on the left aide are expressed se
Food in pellets
X Added noncaloric bulk
Noyer Stockt
Cal/g
types of reinforcement pellets * upon rate of CRF 30-min sessions . The properties of the pellets aide of the table and the results on the right the mean number of responses/animal/session,
X Wastage
_ R Cal/ 30-min Session
Weight change after session (g)
R Reinforcemeats CRF/ 30 min
S .E .R Reinforcemeats CRF/ 30 min
0
4 .3
32
2 .2
+ 1 .3
82
4 .6
Noges Stock 20X
80
0 .8
83
0 .3
+ 0 .1
106
4 .4
Noges Stock 10X#
90
0 .4
--
---
0
115
8 .1
Sucrose 60X
40
2 .4
68
1 .1
+ 0 .6
88
4 .3
Sucrose 40X
60
1 .6
66
1 .0
+ 0 .6
88
4 .8
Sucrose 20X
80
0 .8
81
0 .3
0
107
6 .3
Sucrose lOX
90
0 .4
88
0 .15
+ 0 .1
144
6 .8
Sucrose lOX~
90
0 .4
88
0 .16
0
150
8 .2
Sucrose lOX~
90
0 .4
90
0 .15
0
140
8 .8
* 20 mg pellets, P . J . Noyas Co ., Lancaster, N . H . t Staadard formula of laboratory animal food #Bacon flavor added §Peanut flavor added
91 0
Operant Conditioning in Mice
Vol. il, No . 19
Inspection of cumulative recordings revealed a fast rate of response during the first 5-10 min and a marked decline thereafter .
A similar pattern of responses
was observed with pellets containing 60 per cent sucrose and 40 per cent noncaloric bulk .
The use of pellets containing 40 per cent and 20 per cent
sucrose resulted in a decline of bar touching rates after 15-20 min .
In exper-
iments with 10 per cent sucrose pellets, bar touching was maintained at a uniform rate during the entire 30-min period .
The addition of peanut and bacon
flavors to 10 per cent sucrose pellets had no significant effect on bar touching rates (Table 1) .
The effect of varying decrements of body weight
on CRF performance using 10 per cent sucrose pellets as reinforcement is shown in Table 2 .
Optimal performance was obtained with a reduction of body weights
from 21 to 17 g (approximately 20 per cent), and performance declined with greater weight reductions . TABLE 2 Effect of weight reduction on rate of lever touch . CRF Responses
R S .E .S t
Body Weight (g) <-14
14-15
15-16
16-17
17-18
18-19
19-20
20-21
70 .3
122 .0
127 .0
137 .3
130 .8
117 .3
101 .E
80 .8
7 .3
6 .9
7 .9
8 .3
11 .0
3 .8
10 .5
6 .8
*Mean n~ber of responses/30 min daily CRF sessions . t Standard errors of the mean . Initial CRF training of mice, using 10 per cent sucrose pellets as reinforcement, was accomplished in 8-10 daily 30-min sessions in 32 of 34 mice . The responses of only two mice required shaping by induced pellet release upon approaching the lever .
Of 34 mice trained on CRF schedule of reinforcement,
only two did not maintain a response rate of more than 120/30-min sessions, left pellets in the trough by the end of the session, and had to be omitted from the study . The acquisition rates of a discrete trial discrimination with SD
Vol . 11, No. 19
Operant Conditioning in Mice
911
presented on VI-40 and VI-20 schedules under alternating 7 .5rmin periods of time-in and time-out conditions are shown in Figure 1 .
The mice learned not to
respond during the 7 .5~min, time-out intervals, when no cues were presented .
r
~ Reinforcements (CRF, S~)
o Total Responses (S o) e Responses(S°)
FIG . 1 Acquisition rate of a discrete trial discrimination with SD presented on a VI schedule (7 .5 min) alternating with 7 .5~min periods of absence of stimuli and einforcemen~ts (time-out, Sa) . Each daily 30~min session consisted of two S~ and two S periods in alternating sequence . Results are expressed as mean number of responses/mouse/session of a group of eight mice . Also, there was a decline of nonreinforced responses during the discrimination periods is the absence of SD .
The total daily number of reinforcements was rel-
atively constant because each lever touch resulted in the delivery of only one pellet and in the restarting of the VI-timer .
A repeated measures, one-way
analysis of variance comparing the total responses during SD with the total responses during S~ confirmed that the mice learned to respond leas during S~ than during SD (P < .05, F ~ 11 .1) on the VI-40-sec schedule .
A similar analysis com-
paring SD and S~ responses during the subsequent VI-20-sec schedule also resulted in a statistically significant effect (P < .001, F
m
35 .5) .
The standard error
of the means (S .E .a) ranged from 2 .4 to 10 .5 on all of the points on Figure 1 . The results of performance on a DRL schedule of reinforcement are shown
Operant Conditioning in Mice
912
Vol. 11, No . 19
0.40
0.20 ~
5. E . U W
2
E
10
14
2
6
10
Cue Light Added
Days FIG . 2
Acquisition rate of a differential reinforcement of low rate (DRL) schedule of reinforcement . The availability of the next reinforcement was delayed by 20 sec (DRL-20) from the preceding lever touch . Each daily session lasted 30 min . Results are expressed as mean number of reinforcements/response/session of a group of eight mice . in Figure 2 .
The efficiency ratio improved and reached an asymptotic level of
performance after approximately 12 days . efficient response interval distribution .
This improvement was due to more As training progressed, responses at
short intervals decreased and responses at the longer intervals increased in frequency resulting in a more efficient DRL performance . The improvement of performance of a discriminated single alternation go, no-go schedule is shown in Figure 3 .
A change in latency measure was a more
sensitive indicator of learning than the response measure . A repeated measures, one-way analysis of variance over the means of five blocks of trials (Fig . 3) for the latency measure confirmed that the mice
Operant Conditioning in Mice
Vol. 11, No . 19
913
12 .0 Il .o W
~ r
o.o
I
z
9.0
W H
â
8.0
W
7.0 ~ 6D
8.0
O NO-ß0
6.0
i
i-
z
O 70
~
v
~+ 8 0
m
50
Ô 40 n. a~ ~ 30F Y
m
I
6
o
15 DAYS
IO
.
20
26
.
30
FIG . 3 Acquisition rate of a discriminated, single alternation go, no-go schedule of reinforcement . The touch of a single retractable lever on alternate trials was reinforced by 20 mg pellets that were availResults able for 20 sec . Each daily session consisted of 60 trials . are expressed as mean number of responses/mouse/session and mean latencies from the beginning of the trial to the first response in a group of eight mice . waited longer to respond on the no-go trials than on the go trials F ~ 18 .2) .
(P < .01,
A similar analysis of variance on the response measure failed .t o
find statistically a significant effect on the response measure, confirming the impression that the latency measure is a more sensitive measure of learning than the response measure on this task .
The S .E .x ranged from 1.3 to 1 .6 on
the latency measure and from 4 .6 to 7 .5 on the response measure . Discussion Von Boaberger~ demonstrated that mice can learn complex maze discrimination tasks as efficiently ss rats .
Anliker and MayerB had previously developed a
sensitive lever for the delivery of food pellets for adult Swiss mice . hands of the authors,
In the
this type of lever performed satisfactorily for the
heavier Swiss mice, but not for the smaller C-57 mice used in these experiments .
914
Operant Conditioning in Mice
Therefore, lever touch was substituted for lever press .
Vol . 11, No . 19 Touch of a drinking
tube for release of a drop of milk was previously used by Sidman et al 9 to establish the ability of C-57 mice to discriminate between auditory and visual stimuli .
However, the relative paucity of studies of operant behavior using
mice is probably attributable to the methodological difficulties examined in this report .
It is probable that mice can successfully learn operant schedules
if the following modifications of standard procedures are made :
substituting
lever touch for lever press to initiate reinforcement, and by diluting the caloric value of food reinforcement by noncaloric bulk to prevent rapid satiation . The degree of food deprivation markedly affects performance in mice (Table 2), similar to the previous demonatratioa in rats . l ~
Mice can learn a dis-
Crete trial discrimination when SD is presented on a variable-interval schedule (Fig . 1) alternating with periods when no cues are presented (time-out) for the evaluation of random activity . 2
They can also learn DRL (Fig . 2) and diecrimi-
nâted single alternation go, no-go (Fig . 3) schedules of reinforcement . Acknowledgements--The authors thank Mrs . N . Pitzer, Mrs . C . Williams, Mra . F . James, and Mr . J . B . Welcome, Jr . for their expert technical assistance and Mr . George Cave and Mr . William Dean for illustrations . The work was supported by U . S . Public Health Service grants, MH-12574, MH-10320, and MH-21748, aad by the Veterans Administration . References 1.
L . I . MEDVED, E . I . SPYNU and Iu . S . KAGAN, Res . Rev . 6, 42 (1964) .
2.
T . THOMPSON and C . R . SCHUSTER, Behavioral Pharmacology . Englewood Cliffs (1968) .
3.
G . FREUND, Science 168, 1599 (1970) .
4.
B . WEISS and V . G . LATIES, Ana . Rev . Pharmacol . 9, 927 (1969) .
5.
B . WEISS and V . G . LATIES, Fed . Proc . 26, 1146 (1967) .
6.
L . W . MEANS, D . W . WALKER and R . L . ISSACSON, J . Comp . Physiol . Peychol . 72, 278 (1910) .
7.
F . VON BOXBERGER, Z . Tierpsychol . 9, 433 (1952) .
Prentice Hall,
8.
J . ANLIKER and J . MAYER, J . App . Phyaiol . 8, 667 (1956) .
9.
M . SIDMAN, B . A . RAY, R . L . SIDMAN and J . M . KLINGER, Expl . Neurol . 16, 377
l0 .
C . B . FERSTER and B . F . SKINNER, Schedules of Reinforcement . Century-Crofts, New York (1957) .
Appleton-
Pergamon Press
OPERANT CONDITIONING IN MICE Gerhard Freund and Don W . Walker Veterans Administration Hospital and University of Florida Gainesville, Florida
(Received 16 May 1972; in final form 21 July 1972) Summary Only in recent years has the necessity to teat drugs and chemicals for behavioral toxicity been recognized . The mouse is the animal most commonly used for conventional toxicity screening, but it has rarely been used for toxicity studies using operant behavior . Thin investigation demonstrates that the mouse has a potential similar to that of the rat to learn operant tasks if lever press is substituted by lever touch and if the caloric value of reinforcement pellets is diluted by noncaloric bulk . Under these conditions mice can learn the following schedules of reinforcement : continuous, discriminated variable-interval, discriminated single alternation go, no-go, and differential reinforcement of low rate (DRL) . As in rata, performance is optimal within a range of 20 f 5 per cent weight reduction from the body weight under ad libitum feeding conditions . Toxic effects of chemicals may be detectable by behavioral methods before gross anatomical or biochemical damage is apparent .l -9
The increase of
potentially toxic chemicals in man's environment therefore requires the application of psychopharmacological methods to the toxicological evaluation of chemicals .
The growing importance of this relatively new field has been
recently recognized . 4 The laboratory mouse is the animal most widely used for conventional toxicity screening of drugs and chemicals, but little ie known about behavioral toxicity in mice as evaluated by standard operant conditioning proce dures .
These automated procedures appear to have great potential for the
detection of behavioral to:icity .
The purpose of this investigation is to
determine whether mice have the potential to perform as efficiently as rate in certain food-motivated, lever press paradigms if modifications of standard rat operant procedures are made .
905
908
Operant Condirioning in Mice
Vol . 11, No. 19
Materiale and Methods
Subjects .
C57BL/6J female, 3-month-old mice (Jackson Laboratory, Bar
Harbor, Maine), weighing between 21 and 22 g, were used as subjects .
They
were housed individually and were maintained on a 12-hour-light, 12-hour-dark cycle . Appca~atus .
One side of a mouse shuttle box (Lehigh Valley Electronics,
Fogelsville, Pa ., Model #146-021) was equipped with a mouse lever (Lehigh Valley Model #1535) connected to a contact relay that activated a pellet dis penser (Davia Scientific Instruments, North Hollywood, Calif .) .
The mouse
lever was made retractable by mounting it on the base of a retractable rat lever (Lehigh Valley Model #1405R) .
The electrical input from lever touch had
a 1 .5-sec delay between the initial touch and the earliest possible activation of the next lever touch signal to prevent release of multiple pellets by a single touch .
This 1 .5-sec delay was effective in all of the schedules re-
ported in this paper . divider .
The other aide of the shuttle box was closed off by a
The operant chamber was enclosed in a sound-attenuated cubicle and
was programmed by Grason Stadler 1200 series modules .
Procedure .
The mice were weighed daily immediately prior to being placed
in the operant chamber .
Immediately after being returned to their home cage,
they were fed their daily ration of Guinea Pig Chow pellets (Ralston Purina Co ., St . Louis, Mo .) ranging from 2 .2 to 3 .0 g/mouse/day .
Initial weight reduction
was accomplished by feeding 1 .5 g of Chow/mouse/day until body weight was between 16 .5 and 17 .0 g .
The wnighta of daily food rations were adjusted indi-
vidually to maintain each mouse at weights between 16 .5 and 17 .0 g .
Reinforce-
ments consisted of 20 mg pellets (P . J . Noyes Co ., Lancaster, N . H .) of various compositions, as described below .
Wastage was determined by weighing pellets
and their particles that dropped into the pan underneath the trough .
The per
cent wastage was calculated from the total weight of reinforcement pellets delivered on a CRF schedule . All sessions lasted 30 min except for the go, no-go sessions which
Operant Conditioning in Mice
Vol . il, No . 19
consisted of 60 trials .
907
During continuous reinforcement (CRF) a food pellet
was available upon lever touch every 1 .5 sec .
(Actually, the 1 .5-sec delay
used in all the schedules is more like a FI 1 .5-sec schedule, but for conven fence called CRF when .no other reinforcement contingencies are imposed .)
CRF
training was initiated by placing each mouse in the operant chamber, generally without shaping .
To determine the effect of various degrees of weight reduc-
tion on CRF performance, a group of eight mice was weight-reduced by decrements of 1 .0 g of body weight from 17 .0 to 13 .0 g (Table 2) .
The mice were main
tained at each weight level for 8 to 10 days and tested daily on a CRF schedule . The same procedure was then repeated in the reverse order, and the body weights were allowed to increase to 21 .0 g by 1 .0 g increments . Using a CRF schedule, various types of 20 mg pellets used for reinforcement (Table 1) were evaluated in a group of eight mice, maintained at 16 .5 to 17 .0 g Each type of pellet was fed to the group for 6 to 8 consecutive
body weight .
days after stable CRF responses had been established with pellets containing 10 per cent sucrose and 90 per cent noncaloric bulk for 14 days .
Cumulative
recorder tracings were obtained on all animals on Days 4 and 5 after they had been changed to a new type of reinforcement pellet . Various schedules of reinforcement were tested in groups of eight mice each after CRF training for at least two weeks and after the CRF response rate had reached asymptotic values for at least g days . was used for each schedule .
A different group of mice
All pellets for reinforcements consisted of 10 per
cent sucrose and 90 per cent bulk . The first teak was a discrete trial discrimination with trials presented on a VI schedule .
This task consisted of alternating 7 .5~min periods of cued dis-
criminations and 7 .5Jmin periods of time-out (S~)--no reinforcement or cue presentation .
The first 7 .5rmin period of each session consisted of the presenta-
tion of a compound SD (white noise and cue light above lever), eigaaling the availability of reinforcement .
The SD was presented on a VI-40-sec schedule .
As soon as the mouse touched the lever after the onset of SD, one food pellet
90 8
Operant Conditioning in Mice
was delivered and Sv was terminated .
Vol. 11, No . 19
Following 7 .5 min of this schedule a
7 .5-min, time-out period (S~) was initiated .
Each 30-min session consisted of
two 7 .5-min periods of VI discrimination and two 7 .5-min periods of time-out . The total number of responses during the SD (reinforced responses) and the total number of the responses in the absence of SD (nonreinforced responses) were recorded during each 7 .5-min period of discrimination .
The number of
responses (all nonreinforced) during each 7 .5-min, time-out period (S~) were recorded .
Immediately following training with SD presented on a VI-40-sec
schedule, the mice were trained with SD presented on a VI-20-sec schedule in a 2- to 38-sec range (Fig . 1) . A differential reinforcement of low rate (DRL) schedule was employed .s Reinforcement was contingent upon bar touch later than 20 sec after the previous reinforcement (DRL-20) .
Each daily session lasted for 30 min .
Inter
response times (IRT) and numbers of reinforced aad nonreinforced lever touches were recorded .
Efficiency ratios were calculated by dividing the number of
reinforced responses by the total number of responses (Fig . 2) . The final operant schedule tested was discriminated single alternation go, no-go task . 6
Each discrete trial consisted of the presentation of a retractable
mouse lever for 20 sec at a time followed by an intertrial interval (ITI) of 7 .5 sec .
On the odd-numbered (go) trials, food reinforcements were available
on a CRF schedule, and on the even-numbered (no-go) trials they were not . sessions consisted of 60 trials .
Da11y
White noise, from a noise generator (Graeon
Stadler Model ~9UlB), and a green light, 1 cm above the lever, served as conditioned stimuli .
For the go, no-go trials the conditioned stimuli were a low-
frequency sound presented simultaneously with a green light, 1 cm above the lever, for the go trials .
The no-go trials were indicated by a white light,
5 cm above and 3 cm to the left of the lever, associated with a high-frequency sound from an sudiogenerator (BRS Foringer Model AU902) .
The dependent vari-
abler were the latency to the first bar touch on each trial and the total number of touches on each trial (Fig . 3) .
Vol. 11, No . 19
Operant Conditioning in Mice
90 9
Results Direct observation and comparison with automatically recorded responses revealed that mice could not uniformly depress the mouse lever .
Only approxi-
mately 30 per cent of lever presses resulted in electrical contact closure . When lever touch was introduced, a single touch of the lever often released multiple pellets .
By delaying the availability of the next pellet by 1 .5 sec,
only single pellets were released with each touch . When standard 20 mg Noyas pellets were used for reinforcement (Table 1) on a CRF schedule, the total number of bar touches during a 30-min period was low . TABLE 1
The effect of various response during daily are shown on the left aide are expressed se
Food in pellets
X Added noncaloric bulk
Noyer Stockt
Cal/g
types of reinforcement pellets * upon rate of CRF 30-min sessions . The properties of the pellets aide of the table and the results on the right the mean number of responses/animal/session,
X Wastage
_ R Cal/ 30-min Session
Weight change after session (g)
R Reinforcemeats CRF/ 30 min
S .E .R Reinforcemeats CRF/ 30 min
0
4 .3
32
2 .2
+ 1 .3
82
4 .6
Noges Stock 20X
80
0 .8
83
0 .3
+ 0 .1
106
4 .4
Noges Stock 10X#
90
0 .4
--
---
0
115
8 .1
Sucrose 60X
40
2 .4
68
1 .1
+ 0 .6
88
4 .3
Sucrose 40X
60
1 .6
66
1 .0
+ 0 .6
88
4 .8
Sucrose 20X
80
0 .8
81
0 .3
0
107
6 .3
Sucrose lOX
90
0 .4
88
0 .15
+ 0 .1
144
6 .8
Sucrose lOX~
90
0 .4
88
0 .16
0
150
8 .2
Sucrose lOX~
90
0 .4
90
0 .15
0
140
8 .8
* 20 mg pellets, P . J . Noyas Co ., Lancaster, N . H . t Staadard formula of laboratory animal food #Bacon flavor added §Peanut flavor added
91 0
Operant Conditioning in Mice
Vol. il, No . 19
Inspection of cumulative recordings revealed a fast rate of response during the first 5-10 min and a marked decline thereafter .
A similar pattern of responses
was observed with pellets containing 60 per cent sucrose and 40 per cent noncaloric bulk .
The use of pellets containing 40 per cent and 20 per cent
sucrose resulted in a decline of bar touching rates after 15-20 min .
In exper-
iments with 10 per cent sucrose pellets, bar touching was maintained at a uniform rate during the entire 30-min period .
The addition of peanut and bacon
flavors to 10 per cent sucrose pellets had no significant effect on bar touching rates (Table 1) .
The effect of varying decrements of body weight
on CRF performance using 10 per cent sucrose pellets as reinforcement is shown in Table 2 .
Optimal performance was obtained with a reduction of body weights
from 21 to 17 g (approximately 20 per cent), and performance declined with greater weight reductions . TABLE 2 Effect of weight reduction on rate of lever touch . CRF Responses
R S .E .S t
Body Weight (g) <-14
14-15
15-16
16-17
17-18
18-19
19-20
20-21
70 .3
122 .0
127 .0
137 .3
130 .8
117 .3
101 .E
80 .8
7 .3
6 .9
7 .9
8 .3
11 .0
3 .8
10 .5
6 .8
*Mean n~ber of responses/30 min daily CRF sessions . t Standard errors of the mean . Initial CRF training of mice, using 10 per cent sucrose pellets as reinforcement, was accomplished in 8-10 daily 30-min sessions in 32 of 34 mice . The responses of only two mice required shaping by induced pellet release upon approaching the lever .
Of 34 mice trained on CRF schedule of reinforcement,
only two did not maintain a response rate of more than 120/30-min sessions, left pellets in the trough by the end of the session, and had to be omitted from the study . The acquisition rates of a discrete trial discrimination with SD
Vol . 11, No. 19
Operant Conditioning in Mice
911
presented on VI-40 and VI-20 schedules under alternating 7 .5rmin periods of time-in and time-out conditions are shown in Figure 1 .
The mice learned not to
respond during the 7 .5~min, time-out intervals, when no cues were presented .
r
~ Reinforcements (CRF, S~)
o Total Responses (S o) e Responses(S°)
FIG . 1 Acquisition rate of a discrete trial discrimination with SD presented on a VI schedule (7 .5 min) alternating with 7 .5~min periods of absence of stimuli and einforcemen~ts (time-out, Sa) . Each daily 30~min session consisted of two S~ and two S periods in alternating sequence . Results are expressed as mean number of responses/mouse/session of a group of eight mice . Also, there was a decline of nonreinforced responses during the discrimination periods is the absence of SD .
The total daily number of reinforcements was rel-
atively constant because each lever touch resulted in the delivery of only one pellet and in the restarting of the VI-timer .
A repeated measures, one-way
analysis of variance comparing the total responses during SD with the total responses during S~ confirmed that the mice learned to respond leas during S~ than during SD (P < .05, F ~ 11 .1) on the VI-40-sec schedule .
A similar analysis com-
paring SD and S~ responses during the subsequent VI-20-sec schedule also resulted in a statistically significant effect (P < .001, F
m
35 .5) .
The standard error
of the means (S .E .a) ranged from 2 .4 to 10 .5 on all of the points on Figure 1 . The results of performance on a DRL schedule of reinforcement are shown
Operant Conditioning in Mice
912
Vol. 11, No . 19
0.40
0.20 ~
5. E . U W
2
E
10
14
2
6
10
Cue Light Added
Days FIG . 2
Acquisition rate of a differential reinforcement of low rate (DRL) schedule of reinforcement . The availability of the next reinforcement was delayed by 20 sec (DRL-20) from the preceding lever touch . Each daily session lasted 30 min . Results are expressed as mean number of reinforcements/response/session of a group of eight mice . in Figure 2 .
The efficiency ratio improved and reached an asymptotic level of
performance after approximately 12 days . efficient response interval distribution .
This improvement was due to more As training progressed, responses at
short intervals decreased and responses at the longer intervals increased in frequency resulting in a more efficient DRL performance . The improvement of performance of a discriminated single alternation go, no-go schedule is shown in Figure 3 .
A change in latency measure was a more
sensitive indicator of learning than the response measure . A repeated measures, one-way analysis of variance over the means of five blocks of trials (Fig . 3) for the latency measure confirmed that the mice
Operant Conditioning in Mice
Vol. 11, No . 19
913
12 .0 Il .o W
~ r
o.o
I
z
9.0
W H
â
8.0
W
7.0 ~ 6D
8.0
O NO-ß0
6.0
i
i-
z
O 70
~
v
~+ 8 0
m
50
Ô 40 n. a~ ~ 30F Y
m
I
6
o
15 DAYS
IO
.
20
26
.
30
FIG . 3 Acquisition rate of a discriminated, single alternation go, no-go schedule of reinforcement . The touch of a single retractable lever on alternate trials was reinforced by 20 mg pellets that were availResults able for 20 sec . Each daily session consisted of 60 trials . are expressed as mean number of responses/mouse/session and mean latencies from the beginning of the trial to the first response in a group of eight mice . waited longer to respond on the no-go trials than on the go trials F ~ 18 .2) .
(P < .01,
A similar analysis of variance on the response measure failed .t o
find statistically a significant effect on the response measure, confirming the impression that the latency measure is a more sensitive measure of learning than the response measure on this task .
The S .E .x ranged from 1.3 to 1 .6 on
the latency measure and from 4 .6 to 7 .5 on the response measure . Discussion Von Boaberger~ demonstrated that mice can learn complex maze discrimination tasks as efficiently ss rats .
Anliker and MayerB had previously developed a
sensitive lever for the delivery of food pellets for adult Swiss mice . hands of the authors,
In the
this type of lever performed satisfactorily for the
heavier Swiss mice, but not for the smaller C-57 mice used in these experiments .
914
Operant Conditioning in Mice
Therefore, lever touch was substituted for lever press .
Vol . 11, No . 19 Touch of a drinking
tube for release of a drop of milk was previously used by Sidman et al 9 to establish the ability of C-57 mice to discriminate between auditory and visual stimuli .
However, the relative paucity of studies of operant behavior using
mice is probably attributable to the methodological difficulties examined in this report .
It is probable that mice can successfully learn operant schedules
if the following modifications of standard procedures are made :
substituting
lever touch for lever press to initiate reinforcement, and by diluting the caloric value of food reinforcement by noncaloric bulk to prevent rapid satiation . The degree of food deprivation markedly affects performance in mice (Table 2), similar to the previous demonatratioa in rats . l ~
Mice can learn a dis-
Crete trial discrimination when SD is presented on a variable-interval schedule (Fig . 1) alternating with periods when no cues are presented (time-out) for the evaluation of random activity . 2
They can also learn DRL (Fig . 2) and diecrimi-
nâted single alternation go, no-go (Fig . 3) schedules of reinforcement . Acknowledgements--The authors thank Mrs . N . Pitzer, Mrs . C . Williams, Mra . F . James, and Mr . J . B . Welcome, Jr . for their expert technical assistance and Mr . George Cave and Mr . William Dean for illustrations . The work was supported by U . S . Public Health Service grants, MH-12574, MH-10320, and MH-21748, aad by the Veterans Administration . References 1.
L . I . MEDVED, E . I . SPYNU and Iu . S . KAGAN, Res . Rev . 6, 42 (1964) .
2.
T . THOMPSON and C . R . SCHUSTER, Behavioral Pharmacology . Englewood Cliffs (1968) .
3.
G . FREUND, Science 168, 1599 (1970) .
4.
B . WEISS and V . G . LATIES, Ana . Rev . Pharmacol . 9, 927 (1969) .
5.
B . WEISS and V . G . LATIES, Fed . Proc . 26, 1146 (1967) .
6.
L . W . MEANS, D . W . WALKER and R . L . ISSACSON, J . Comp . Physiol . Peychol . 72, 278 (1910) .
7.
F . VON BOXBERGER, Z . Tierpsychol . 9, 433 (1952) .
Prentice Hall,
8.
J . ANLIKER and J . MAYER, J . App . Phyaiol . 8, 667 (1956) .
9.
M . SIDMAN, B . A . RAY, R . L . SIDMAN and J . M . KLINGER, Expl . Neurol . 16, 377
l0 .
C . B . FERSTER and B . F . SKINNER, Schedules of Reinforcement . Century-Crofts, New York (1957) .
Appleton-