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Differential effects of morphine on active avoidance as a function of pre-drug performance
Pharmacological Research Communications, Vol. 5, No. I, 1973
47
DIFFERENTIAL EFFECTS OF MORPHINE ON ACTIVE AVOIDANCE 1 AS A FUNCTION OF PRE-DRUG PERFORMANCE W. M. Davis, J. M. Holbrook and M. Babbini 2 Department of Pharmacology, School of Pharmacy University of Mississippi, University, Mississippi 58677 (USA) Received 6 February 1973 SUMMARY
The effects of morphine sulfate (I0, 20, 40 mg/kg) on performance
of a one-way avoidance response were determined for drug-naive rats showing low, medium, and high levels of pre-drug performance after equal training. Performance was enhanced significantly from initially low levels and was depressed from initially high levels.
Log dose-effect relationships were
linear over the range tested, the former effect decreasing and the latter increasing. INTRODUCTION
The enhancement did not extend beyond the day of drug treatment. Pharmacological studies on conditioned avoidance in rats have
consisted mostly of testing various agents for interference with performance of well-trained subjects.
Incidentally to testing for inhibition by morphine
of avoidance in high performance subjects in a one-way avoidance apparatus, we tested for an effect of morphine on several rats which were "poor performers" despite training equal to that of the "high performers." low performance rats improved strikingly in avoidance scores.
These
Not being
aware of a previous study in which mo~hine was tested under such circumstances, we have exm,ined further the effects of morphine on active shockavoidance in groups of rats showing different performance levels after equal periods of training. iThis investigation was supported by NIMH research grant MH 11295 and by the NSF Undergraduate ResearchParticipation Program (J.M.H.). 2present address: Department of Pharmacology, University of Bologna, Bologna, Italy.
Pharmacological Research Communications, Vol. 5, No. 1, 1973
48 METHODS
Subjects were 135 male albino rats of the Holtzman [Sprague-Dawley
derived) strain between 4 and 6 months of age.
The apparatus was a four-
compartment one-way avoidance box described in detail previously (Davis et al., 1967).
Each trial was initiated with the sounding of a buzzer and
the raising of tile door to one adjacent compartment.
After 7 sec the grid
was ~l-ectrified through a shock scrambler concurrently with termination of the buzzer.
The door was lowered upon passage of the rat into the "safe"
compartment.
Thus, the subject was required to make a clockwise circuit
of the apparatus in the course of each 4 trials. recorded on an electric~timer.
Responso times were
Training and test sessions consisted of 30
trials giyen at 50-60 sec intervals.
Six training sessions (180 trials)
preceded a single test session under drug treatment.
Injections of 0.9%
saline were given i.p. 30 min before all training sessions.
Injections of
morphine sulfate solution in the drug test were of the same route, interval, and volume. Rats making less than 50% avoidances in the 6th training session were defined as "poor performers."
"High performers" were those rats making 90%
or more avoidances, and an additional intermediate group consisted of rats showing 60-80% avoidances in the~6th session.
Each performance group was
divided among treatment groups consisting of t h e 3 fate, I0, 20~ and 40 mg/kgo
doses of morphine sul-
Some poor performers were run on day 8 after
saline treatment to test whether improvement in avoidance under morphine was retained. The results under drug treatment (day 7) were evaluated relative to the 6th training day when high and medium performance subjects had reached a stable level according to observations on separate groups of 25 and II rats which received only saline through day 8.
At the same time, the poor per-
formers showed either stable or downward-trending performance.
For analysis
of variance (A NOVA) the percent avoidance scores were treated by the arcsin
Pharmacological Research Communications, VoL 5, No. 1, 1973
49
(angular) transformation, and regression curves for the dose-effect relationships were calculated.
Because the various cells had different numbers of
subjects, an unweighted analysis of cell means was used (Snedecor and Cochran, 1967).
As the high, medium and low performance groups were select-
ed non-randomly before morphine treatment, variances of these groups on day 6 did not represent variability due to random fluctations.
Therefore)
calculation of fiducial limits for day 6 means used instead the error mean square of the ANOVA from the scores of day 7.
Individual comparisons of
Table I were by use of non-parametric methods, the Wilcoxon Matched-pairs Signed Ranks Test and the Randomization Test (Siegel, 1956).
RESULTS
The mean avoidance scores (untransformed) in control and drug
sessions for the three performance classes and three dosage groups are shown in Table I.
Analysis of variance performed after arcsin transformation of
the data showed that the effects of pre-drug (day 6) performance level and of drug dosage'both were significant (p<0.01).
However, there was not a signi-
ficant interaction between doses and performance levels, i.e., the dose-effect relationship
was the same for all three performance groups considered.
Analysis of the response curves shows that the dose-effect relationship was linear on the log of dosage.
As performance level did contribute a signifi-
cant effect) we analyzed further the differences between performance groups under morphine using Duncan's Multiple Range Test.
This revealed a signifi-
cant difference between the high performance group and the other two groups) but not between the latter two,
Therefore, the results could be represented
by the two regression lines (derived from the itransformed data) shown in Fig. I with their 95~fiducial zones and with the transformed means of the day 6 saline sessions and their 954 fiducial limits. F r o m this figure it is evident how morphine differentially affected the avoidance response according to the pre-drug performance level.
For the high performance group,
Pharmacological Research Communications, Vol. 5, No. 1, 7973
50
TABLE 1,
Mean number of avoidance responses in 30-trial sessions after pretreatment with i.p. saline (days 1-6) or morphine sulfate (day 7).
Groups by Performance and Dosage
N
1
2
3
4
S
6
7
P*
i0 mg/kg
7
6.0
11.0
12.4
13.3
12,0
10.6
21.1
0,06
20 mg/kg
8
5.6
6,4
8.6
10.S
8.1
9.2
18.9
<0.01
40 mg/kg
6
5.2
8.2
9.5
10.S
12,5
8.5
13.5
NS
Medium i0 mg/kg
12
6.8
16.0
19.1
22.3
19,4
21.1
23.8
NS
20 mg/kg
10
II.0
18.S
21.3
22.9
22.3
21.5
19.8
NS
40 mg/kg
7
8.0
14.3
13.7
17.6
17,9
22.0
Iio0
<0.05
i0 mg/kg
21
9.9
19.0
23.6
24.3
26.9
29.0
28.6
--
20 mg/kg
23
8.4
15.7
22.4
22.5
25.3
28,7
24.0
--
40 mg/kg
18
8.4
17.2
22.7
23.9
24,8
28.9
17.1
--
Low
High
*P value for comparison of days 6 and 7; tests not applicable for highs,
~E
80
~
70
~
60
~
50
•~
40
'-
30
~.
20
~
10
MORPHIHE HIGH PERFORMERS •
SALINE A
MEDIUM PERFORMERS
•
o
LOW PERFORMERS
g
"
10
20
40
OOSAGE (tool kO)
FIGURE I. Regression lines for dose-effect relationship of morphine on avoidance response according to pre-drug performance. Bottom line represents combined medium and low groups which did not differ significantly after morphine, Bars show 95% fiducial limits of control means. Dashed lines show 95% fiducial zones of regression lines.
Pharmacological Research Communications, VoL 5, No. 1, 1973
51
avoidance was inhibited at 20 mg/kg, while only the 40 mg/kg dose was able to inhibit avoidance for the medium group.
When the pre-drug performance was
very low, morphine caused it to improve even at a dose of 20 mg/kg, although
not so greatly as at i0 mg/kg. The improved avoidance of poor performers on day 7 after morphine cannot be attributed evCeh in p art to improvement with practice.
The data
of Table I show that there had been no tendency for improvement in the previous several days, but rather a slight downward trend from maxima reached 2 or 3 days earlier.
Also, the impro%~ment in avoidance ~under morphine was ~
,,~
related to the actual presence of the drug, as rats returned to their previous poor performance on the day following the drug.
Poor. 1 5 additional rats
receiving saline on day 6, I0 mg/kg morphine on day 7 and saline on day 8, the avoidances averaged 5.1, 21.5 and 6.9, respectively, out of 30 trials.
DISCUSSION
As the best known behavioral effects of morphine in rats are
inhibitory ones, this facilitatory effect on an active avoidance response initially seems~paradoxical.
However, it is well known that "performance
always underestimates the level of learning to a degree which varies directly with the extent to which the training conditions depart from the ideal... The mere act of responding is knows to create an inhibitory state which interwith performance"
(Kimble, 1961).
In an avoidance situation this inhibitory
state may be particularly strong, being reinforced by many cues associated with shock.
It may lead to a conflict behavior that hinders avoidance.
If this condition is assumed to be the case for our "poor performers," morphine could be regarded as having two effects upon avoidance:
a direct
inhibitory effect upon avoidance performance that was recognized much earlier (Cook and Weidley, 1957), and an inhibitory effect upon the conflict behavior.
The end result then would depend not only on the inhibitorystrength
of the conflict behavior, but also on The relative sensitivity to morphine
Pharmacological Research Communications, VoL 5, No. 1, 1973
52
of the avoidance behavior and the conflict behavior.
Following this inter-
pretation, our results would suggest that the former is less sensitive to morphine than the latter.. That morphine might have improved performance by temporarily reducing an "inhibitory state" may be supported also by the fact that improvement is lost completely by the subsequent day, suggesting a conflict behavior that recovers full strength when morphine is no longer present. The above interpretation emphasizes the familiar, i.e., inhibitory effects of morphine upon the CNS.
However, an alternative explanation may
be suggested by the fact that morphine also may exert an excitatory influence on behavior, for example, locomotor activity (Babbini and Davis, 1972). Because the locomotor excitatory component of morphine action has amphetaminelike features (Davis et al., 1972), morphine may improve shock-avoidance of poor performers by the same mechanism as does d-amphetamine (Rech, 1966). Further studies are called for to discriminate between these two possible mechanisms for enhancement of avoidance. Finally, these findings are pertinent to the general relationship between performance level and drug effects.
Manocha (1968) emphasized this problem
in regard to the effect of chlorpromazine on performance of rats leverpressing for water reinforcement.
He found that the decrement in r( ,pond-
ing was greater for a high performance group than for his poor perf. vmers. Unfortunately, the study was limited to only one dose of the drug, en
most
papers dealing with this subject suffer the same limitation (Powell et 'i., 1965; Kamano et al., 1967; Bindra and Mendelson, 1965; Singh, 1964).
If
our study had been limited to the high and medium perf o rmance groups and to the 20 mg/kg dosage, our results would have been much like those of Manocha.
It is likely that performance level frequently represents an
algebraic sum of two "opposing" behaviors~ each of which may be influenced at the same time by a drug.
if-so, only with a multiple dose and multiple
Pharmacological Research Communications, Vol. 5, No. 1, 1973 performance level analysis, as used in this study, can we hope to cla:rify the relationship between performance level and drug effects. REFERENCES i
_
Babbini, M. and Davis, W. M.
Br. J. Pharmac. 46,
Bindra, D. and Mendelson, J.
J. comp. physiol. Psychol. 56__j 183-189 (1965).
Cook, L. and Weidley, E.
213-224 (1972).
Ann. N. Y. Acad. Sci. 66,
Davis, W. M., Babbini, M. and Khalsa, J . H . Pharmac. 4_.) 267-279 (1972). Davis, W. M., Huneycutt, B. D. and Babbini, M. (1967}. Kamano, D. K., Powell, B. J. and Martin, L.K.
740-752 (1957).
Res. Commun. chem. Pathol. Psychon. Sci. 8._,185-186 Psychon. Sci. 8, 119-120
{1967). Kimble, G.A. Hilgard and Marquis' Conditioning and Learning, 2nd ed., Appleton-Century-Crofts) New York, 1961. Manocha, S, N.
Psychopharmacologia
[Berl.) 12, 123-126 (1968).
Powell, B.J., Martin, L.K., and Kamano, D.K. Rech) R.H. Singh, S.D.
Psychopharmacologia
Psychol. Rep. 17, 330 (1965).
(Berl.) ~, 110-117 (1966).
J. comp. physiol. Psychol. 58, 468-469 (1964).
Siegel, S. Nonparametric Statistics for the Behavioral Sciences, pp. 75-83,88-92. McGraw-Hill, New York, 1956. Snedecor, G.W. and Cochran, W.G. Statistical Methods, 6th ed.) pp. 475-477. Iowa University Press~ Ames, 1967.
53
47
DIFFERENTIAL EFFECTS OF MORPHINE ON ACTIVE AVOIDANCE 1 AS A FUNCTION OF PRE-DRUG PERFORMANCE W. M. Davis, J. M. Holbrook and M. Babbini 2 Department of Pharmacology, School of Pharmacy University of Mississippi, University, Mississippi 58677 (USA) Received 6 February 1973 SUMMARY
The effects of morphine sulfate (I0, 20, 40 mg/kg) on performance
of a one-way avoidance response were determined for drug-naive rats showing low, medium, and high levels of pre-drug performance after equal training. Performance was enhanced significantly from initially low levels and was depressed from initially high levels.
Log dose-effect relationships were
linear over the range tested, the former effect decreasing and the latter increasing. INTRODUCTION
The enhancement did not extend beyond the day of drug treatment. Pharmacological studies on conditioned avoidance in rats have
consisted mostly of testing various agents for interference with performance of well-trained subjects.
Incidentally to testing for inhibition by morphine
of avoidance in high performance subjects in a one-way avoidance apparatus, we tested for an effect of morphine on several rats which were "poor performers" despite training equal to that of the "high performers." low performance rats improved strikingly in avoidance scores.
These
Not being
aware of a previous study in which mo~hine was tested under such circumstances, we have exm,ined further the effects of morphine on active shockavoidance in groups of rats showing different performance levels after equal periods of training. iThis investigation was supported by NIMH research grant MH 11295 and by the NSF Undergraduate ResearchParticipation Program (J.M.H.). 2present address: Department of Pharmacology, University of Bologna, Bologna, Italy.
Pharmacological Research Communications, Vol. 5, No. 1, 1973
48 METHODS
Subjects were 135 male albino rats of the Holtzman [Sprague-Dawley
derived) strain between 4 and 6 months of age.
The apparatus was a four-
compartment one-way avoidance box described in detail previously (Davis et al., 1967).
Each trial was initiated with the sounding of a buzzer and
the raising of tile door to one adjacent compartment.
After 7 sec the grid
was ~l-ectrified through a shock scrambler concurrently with termination of the buzzer.
The door was lowered upon passage of the rat into the "safe"
compartment.
Thus, the subject was required to make a clockwise circuit
of the apparatus in the course of each 4 trials. recorded on an electric~timer.
Responso times were
Training and test sessions consisted of 30
trials giyen at 50-60 sec intervals.
Six training sessions (180 trials)
preceded a single test session under drug treatment.
Injections of 0.9%
saline were given i.p. 30 min before all training sessions.
Injections of
morphine sulfate solution in the drug test were of the same route, interval, and volume. Rats making less than 50% avoidances in the 6th training session were defined as "poor performers."
"High performers" were those rats making 90%
or more avoidances, and an additional intermediate group consisted of rats showing 60-80% avoidances in the~6th session.
Each performance group was
divided among treatment groups consisting of t h e 3 fate, I0, 20~ and 40 mg/kgo
doses of morphine sul-
Some poor performers were run on day 8 after
saline treatment to test whether improvement in avoidance under morphine was retained. The results under drug treatment (day 7) were evaluated relative to the 6th training day when high and medium performance subjects had reached a stable level according to observations on separate groups of 25 and II rats which received only saline through day 8.
At the same time, the poor per-
formers showed either stable or downward-trending performance.
For analysis
of variance (A NOVA) the percent avoidance scores were treated by the arcsin
Pharmacological Research Communications, VoL 5, No. 1, 1973
49
(angular) transformation, and regression curves for the dose-effect relationships were calculated.
Because the various cells had different numbers of
subjects, an unweighted analysis of cell means was used (Snedecor and Cochran, 1967).
As the high, medium and low performance groups were select-
ed non-randomly before morphine treatment, variances of these groups on day 6 did not represent variability due to random fluctations.
Therefore)
calculation of fiducial limits for day 6 means used instead the error mean square of the ANOVA from the scores of day 7.
Individual comparisons of
Table I were by use of non-parametric methods, the Wilcoxon Matched-pairs Signed Ranks Test and the Randomization Test (Siegel, 1956).
RESULTS
The mean avoidance scores (untransformed) in control and drug
sessions for the three performance classes and three dosage groups are shown in Table I.
Analysis of variance performed after arcsin transformation of
the data showed that the effects of pre-drug (day 6) performance level and of drug dosage'both were significant (p<0.01).
However, there was not a signi-
ficant interaction between doses and performance levels, i.e., the dose-effect relationship
was the same for all three performance groups considered.
Analysis of the response curves shows that the dose-effect relationship was linear on the log of dosage.
As performance level did contribute a signifi-
cant effect) we analyzed further the differences between performance groups under morphine using Duncan's Multiple Range Test.
This revealed a signifi-
cant difference between the high performance group and the other two groups) but not between the latter two,
Therefore, the results could be represented
by the two regression lines (derived from the itransformed data) shown in Fig. I with their 95~fiducial zones and with the transformed means of the day 6 saline sessions and their 954 fiducial limits. F r o m this figure it is evident how morphine differentially affected the avoidance response according to the pre-drug performance level.
For the high performance group,
Pharmacological Research Communications, Vol. 5, No. 1, 7973
50
TABLE 1,
Mean number of avoidance responses in 30-trial sessions after pretreatment with i.p. saline (days 1-6) or morphine sulfate (day 7).
Groups by Performance and Dosage
N
1
2
3
4
S
6
7
P*
i0 mg/kg
7
6.0
11.0
12.4
13.3
12,0
10.6
21.1
0,06
20 mg/kg
8
5.6
6,4
8.6
10.S
8.1
9.2
18.9
<0.01
40 mg/kg
6
5.2
8.2
9.5
10.S
12,5
8.5
13.5
NS
Medium i0 mg/kg
12
6.8
16.0
19.1
22.3
19,4
21.1
23.8
NS
20 mg/kg
10
II.0
18.S
21.3
22.9
22.3
21.5
19.8
NS
40 mg/kg
7
8.0
14.3
13.7
17.6
17,9
22.0
Iio0
<0.05
i0 mg/kg
21
9.9
19.0
23.6
24.3
26.9
29.0
28.6
--
20 mg/kg
23
8.4
15.7
22.4
22.5
25.3
28,7
24.0
--
40 mg/kg
18
8.4
17.2
22.7
23.9
24,8
28.9
17.1
--
Low
High
*P value for comparison of days 6 and 7; tests not applicable for highs,
~E
80
~
70
~
60
~
50
•~
40
'-
30
~.
20
~
10
MORPHIHE HIGH PERFORMERS •
SALINE A
MEDIUM PERFORMERS
•
o
LOW PERFORMERS
g
"
10
20
40
OOSAGE (tool kO)
FIGURE I. Regression lines for dose-effect relationship of morphine on avoidance response according to pre-drug performance. Bottom line represents combined medium and low groups which did not differ significantly after morphine, Bars show 95% fiducial limits of control means. Dashed lines show 95% fiducial zones of regression lines.
Pharmacological Research Communications, VoL 5, No. 1, 1973
51
avoidance was inhibited at 20 mg/kg, while only the 40 mg/kg dose was able to inhibit avoidance for the medium group.
When the pre-drug performance was
very low, morphine caused it to improve even at a dose of 20 mg/kg, although
not so greatly as at i0 mg/kg. The improved avoidance of poor performers on day 7 after morphine cannot be attributed evCeh in p art to improvement with practice.
The data
of Table I show that there had been no tendency for improvement in the previous several days, but rather a slight downward trend from maxima reached 2 or 3 days earlier.
Also, the impro%~ment in avoidance ~under morphine was ~
,,~
related to the actual presence of the drug, as rats returned to their previous poor performance on the day following the drug.
Poor. 1 5 additional rats
receiving saline on day 6, I0 mg/kg morphine on day 7 and saline on day 8, the avoidances averaged 5.1, 21.5 and 6.9, respectively, out of 30 trials.
DISCUSSION
As the best known behavioral effects of morphine in rats are
inhibitory ones, this facilitatory effect on an active avoidance response initially seems~paradoxical.
However, it is well known that "performance
always underestimates the level of learning to a degree which varies directly with the extent to which the training conditions depart from the ideal... The mere act of responding is knows to create an inhibitory state which interwith performance"
(Kimble, 1961).
In an avoidance situation this inhibitory
state may be particularly strong, being reinforced by many cues associated with shock.
It may lead to a conflict behavior that hinders avoidance.
If this condition is assumed to be the case for our "poor performers," morphine could be regarded as having two effects upon avoidance:
a direct
inhibitory effect upon avoidance performance that was recognized much earlier (Cook and Weidley, 1957), and an inhibitory effect upon the conflict behavior.
The end result then would depend not only on the inhibitorystrength
of the conflict behavior, but also on The relative sensitivity to morphine
Pharmacological Research Communications, VoL 5, No. 1, 1973
52
of the avoidance behavior and the conflict behavior.
Following this inter-
pretation, our results would suggest that the former is less sensitive to morphine than the latter.. That morphine might have improved performance by temporarily reducing an "inhibitory state" may be supported also by the fact that improvement is lost completely by the subsequent day, suggesting a conflict behavior that recovers full strength when morphine is no longer present. The above interpretation emphasizes the familiar, i.e., inhibitory effects of morphine upon the CNS.
However, an alternative explanation may
be suggested by the fact that morphine also may exert an excitatory influence on behavior, for example, locomotor activity (Babbini and Davis, 1972). Because the locomotor excitatory component of morphine action has amphetaminelike features (Davis et al., 1972), morphine may improve shock-avoidance of poor performers by the same mechanism as does d-amphetamine (Rech, 1966). Further studies are called for to discriminate between these two possible mechanisms for enhancement of avoidance. Finally, these findings are pertinent to the general relationship between performance level and drug effects.
Manocha (1968) emphasized this problem
in regard to the effect of chlorpromazine on performance of rats leverpressing for water reinforcement.
He found that the decrement in r( ,pond-
ing was greater for a high performance group than for his poor perf. vmers. Unfortunately, the study was limited to only one dose of the drug, en
most
papers dealing with this subject suffer the same limitation (Powell et 'i., 1965; Kamano et al., 1967; Bindra and Mendelson, 1965; Singh, 1964).
If
our study had been limited to the high and medium perf o rmance groups and to the 20 mg/kg dosage, our results would have been much like those of Manocha.
It is likely that performance level frequently represents an
algebraic sum of two "opposing" behaviors~ each of which may be influenced at the same time by a drug.
if-so, only with a multiple dose and multiple
Pharmacological Research Communications, Vol. 5, No. 1, 1973 performance level analysis, as used in this study, can we hope to cla:rify the relationship between performance level and drug effects. REFERENCES i
_
Babbini, M. and Davis, W. M.
Br. J. Pharmac. 46,
Bindra, D. and Mendelson, J.
J. comp. physiol. Psychol. 56__j 183-189 (1965).
Cook, L. and Weidley, E.
213-224 (1972).
Ann. N. Y. Acad. Sci. 66,
Davis, W. M., Babbini, M. and Khalsa, J . H . Pharmac. 4_.) 267-279 (1972). Davis, W. M., Huneycutt, B. D. and Babbini, M. (1967}. Kamano, D. K., Powell, B. J. and Martin, L.K.
740-752 (1957).
Res. Commun. chem. Pathol. Psychon. Sci. 8._,185-186 Psychon. Sci. 8, 119-120
{1967). Kimble, G.A. Hilgard and Marquis' Conditioning and Learning, 2nd ed., Appleton-Century-Crofts) New York, 1961. Manocha, S, N.
Psychopharmacologia
[Berl.) 12, 123-126 (1968).
Powell, B.J., Martin, L.K., and Kamano, D.K. Rech) R.H. Singh, S.D.
Psychopharmacologia
Psychol. Rep. 17, 330 (1965).
(Berl.) ~, 110-117 (1966).
J. comp. physiol. Psychol. 58, 468-469 (1964).
Siegel, S. Nonparametric Statistics for the Behavioral Sciences, pp. 75-83,88-92. McGraw-Hill, New York, 1956. Snedecor, G.W. and Cochran, W.G. Statistical Methods, 6th ed.) pp. 475-477. Iowa University Press~ Ames, 1967.
53