A neuropharmacologically-relevant animal model of depression

Nuurophornumloqy. Vol. 18. pp. 891 10 893 Pergamon Press Ltd 1979. Printed in Great Britain

A NEUROPHARMACOLOGICALLY-RELEVANT MODEL OF DEPRESSION

ANIMAL

A. D. SHERMAN,G. L. ALLERS, F. PETTY and F. A. HENN Neurochemical Research Laboratories, Department of Psychiatry, University of Iowa, Iowa City, IA 52242, U.S.A. (Accepted 10 June 1979) Summary-The phenomenon of “learned helplessness” was evaluated as a potential animal model of depression. Imipramine, but not chlorpromazine or lorazepam, had a delayed protective effect on the development of this behavior although lorazepam was effective after a single dose. Dose-response curves related either to concentration of imipramine in drinking water or to free drug levels in brain were linear with higher drug levels associated with a lowered degree of learned helplessness following chronic administration. The effects of imipramine on the development of learned helplessness were seen following four days of access to drug in drinking water, but not after 1, 2 or 3 days. This model, although clearly not a perfect fit with depression in humans, has many characteristics which suggest its utility in further studies of the mechanism of action of antidepressants.

One of the major difficulties facing investigation of the mode of action of antidepressants is the lack of an appropriate animal model which would allow both neurochemical and behavioral assessment of drug action. The presence of a valid animal model would allow studies relating to the delayed onset of action of antidepressants, the anatomical site of drug action, and the molecular mechanism of action of the drug. The “learned helplessness” model of depression derives from the work of Seligman (Seligman and Maier, 1967; Overmier and Seligman, 1967). In this procedure, an animal is exposed to trauma (usually electric shock) of an uncontrollable nature. After this exposure, when the animal is later placed in a situation in which shock termination is contingent upon the animal’s behavior, the animal fails to behave in a manner which would reduce the amount of shock received, i.e. acquire the escape task. The controllable shock situation is often a one-way avoidance task. A naive (control) animal, when placed in this situation, rapidly acquires the appropriate escape response by moving across a barrier to produce termination of the shock. The experimental animals in the above studies, however, not only failed to acquire the escape response, but frequently made no effort to escape the shock at all. This failure to attempt to alter the receipt of painful shock was termed “learned helplessness” and was present in about 213 of the dogs previously exposed to unavoidable shock. Using rats, bar press acquisition for an appetitive reinforcer had previously been observed to be significantly retarded in animals with previous exposure to unavoidable trauma (Mowrer, 1940; Dinsmoor and Campbell, 1956; Dinsmoor, 1958). The finding of marked acquisition deficits has also been reported for escape responding

and avoidance

(Brown and Jacobs,

Key words: antidepressants, behavioral model. 891

1949; Mullin and Mogensen, 1963; Weiss, Kriekhaus and Conte, 1968). This inappropriate behavior (i.e. no attempt to escape trauma) seen when dogs were used as subjects has not usually been reported for rats, which usually show higher latencies of escape responding rather than totally maladaptive behavior. In analogy with human depression, learned helplessness dissipates with time provided that exposure to uncontrollable trauma is limited. In dogs, when testing in an escape situation takes place 24 hr after exposure to unavoidable shock, animals show behavior typical of learned helplessness. When tested at 48 hr, however, this behavior is essentially the same as controls, i.e. decreasing latency of escape responding with trials (Seligman, Maier and Geer, 1968; Seligman and Groves, 1970). In rats, this transience is also seen. On the other hand, weanling rats exposed to multiple instances of uncontrollable shocks showed significant avoidance acquisition deficits when tested as adults (Brookshire, Littman and Stewart, 1961; Levine, Chevalier and Korchin, 1956) suggesting that multiple exposure to unavoidable trauma produces essentially permanent changes in behavior. In spite of the numerous parallels between this behavior and depression, the pharmacological relevance of this model has not been evaluated. This was done in the following manner. METHODS

Learned helplessness was produced in male Sprague-Dawley rats (300g) by exposure to electric shock (0.7 mA) for 1 hr on a schedule of 10 set of consisted of a shock/min. The apparatus 12 x 18 x 12 inches box with a grid floor. At a height of 9 inches above the floor, a platform (3 x 3 inches) could be inserted through one side wall to allow a jump-up escape response to be performed. The plat-

A. D.

892

SHERMAN

form was unavailable during training. After the appropriate treatment, animals were tested for acquisition of a jump-up escape response in the same apparatus. At the beginning of a trial, the platform was pushed into the box and a 0.4 mA shock initiated. Shock was terminated in 10 set if the animal had not escaped onto the platform by this time. If an escape response occurred, the animal was allowed to remain on the platform for the duration of the lOsec, then returned to the grid floor. Ten such trials with an inter-trial interval of 20sec were given. In a naive control group of rats, this training resulted in 80% acquiring Experiment

learned helplessness

behavior.

2

Groups of 5 animals each were maintained on imipramine in drinking water at 0, 10, 20, 50, 100 or 200 mg/l. After 6 days, they were run on the training and testing schedule as above. After testing, wholebrain free drug leveis were determined by homogenization in 5% tri~hloroa~tic acid followed by the gaschromatographic method of Cooper, AIlen and Simpson (1975). Experiment

3

Groups of animals were maintained on imipramine at 100 mg/l in drinking water and were tested for learned helplessness after 1, 2, 3 or 4 days. All drugs were administered as hydrochloride salts with doses calculated as free bases. RESULTS

Experiment

1

After a single dose (Table I), imipramine failed to prevent the devefopment of learned helplessness as did chlorpromazine. Lorzepam, however was effective, as seen by the low number of failures to escape the shock within 10sec. On the other hand, when tested after 6 days of exposure to imipramine, its protective effects were evident. Lorazepam, which was protective after a single dose remained protective, while chlorpromazine remained ineffective. Experiment

Table 1. Effect of chronic and acute drug schedules on learned helplessness Day I

Drug

Day 2 7.4 f 2.3 6.7 + 1.4 1.2 + 0.6+ 1.5 _+0.6f 1.4 + 0.3*

5.8 i: 4.9 4.2 & 2.6 1.5 _i 1.0* 4.3 * 1.3 1.2 & 0.6

None Chlorpromazine Lorazepam Imipramine Control

Animals received the indicated drugs intraperitoneally for 1 or 7 days. Controls received no training shock, and were tested in the escape situation. Data represent the mean escape failures on all 10 trials 2 SD. * Not different from controls by Mann-Whitney U-test (P < 0.05).

I

Groups of 5 rats were given chlorpromazine (10 mg/kg), lorazepam (0.5 mg/kg) or imipramine (10 mg/kg) intraperitoneahy. Four hours later, they were exposed to the training procedure described. Two hours after the end of training, all animals were tested on the escape acquisi~on task which is the measure of the development of learned helptessness. The procedure was repeated in animals which were maintained on the above drug schedule for 6 days before testing. Experiment

et al.

2

The dose-effiect curve (Table 2) for imipramine shows a decreasing number of failures to escape relating to increasing dose of drug in drinking water. Ten

mg/l. (about 1 mg/kg per day) had no effect on the development of learned helplessness while the doses above 100 mgi’l. were no more effective than 100 mg/l. Analysis of total drug level (imipramine plus desipramine) in brain revealed the same levels in animals Table 2. Dose-response Concentration in drinking water (mgP)

curve for. imipramine

Total escape failures 5.0 5.1 3.4 2.0 0.5 0.5 0.7

0 10 20 50 100 200 Control

+ f + + + f *

2.0 2.2 2.8 0.0 1.0* 1.0* 0.4

Animals received imipramine in drinking water for 6 days before behavioral testing. Data represent mean escape failures f SD. * P > 0.05 vs non-shocked controls. drinking 200 mg/l. water as opposed to those drinking water at lOOmg/l., probably due to decreased water consumption at the higher concentration. The relationship between free drug leveb in brain and behavior in the test situation (Table 3} was essentially linear, with a rank-order

corretation

of -0.97.

Table 3. Brain levels of imipramine plus desipramine and escape failures Escape failures

Drug level (pg/g)

0

3.89 + 0.40 (2) 3.08 + 0.23 (4) 3.03 k 0.24 (5) 2.61 + 0.22 (3) 2.09 +_0.12 (7) 1.71 + 0.15 (2)

1 2 3 4 5 6 7 8 9 10

1.00 + 0.21 (3) 0.93 + 0.17 (3)

Conditions as in Table 2.

0.27

(1)

Model of depression Table 4. Time-course of imipramines effects on learned helplessness Drug/days

Escape failures

None/l Imipramine/l Imipramine/Z Imipramine/3 Imipramine/4 Imipramine/S Control

2.4 + 2.9’+ 1.6 + 2.5 + 0.6 + 0.4 + 0.4 +

2.0 (10) 2.5 (10) 1.8 (10) 2.5 (10) 0.9* (10) 0.5* (10) 0.5 (10)

Animals drank imipramine in water (lOOg/l) for the number of days indicated before tests. * P > 0.05 vs non-shocked control Experiment

3

On the first three days of drug exposure (Table 4), imipramine had no effect on learned helplessness, while on the fourth and subsequent days it prevented the development of this behavior. This delayed action parallels the development of antidepressant activity in humans, but it occurs faster in the model system than it does clinically.

893

dosage level. For example, some animals in the group drinking lOmg/l had higher brain levels than animals in the group receiving 20 mg/l and also showed fewer escape failures. The ED,, calculated from concentrations of drug in drinking water was 38 mg/l or about 3.6 mg/kg per day. This would correspond to 280mgJday for an “average” (77 kg) human, while doses given clinically rarely exceed 2OOmg/day, thus demonstrating one major difference between the model and the clinical situation. In this light, however, it should be noted that the effective doses in human depression and learned helplessness in rats are reasonably close. To summarize, the data presented here demonstrate the potential utility of this model in future studies on antidepressant activity. Although no exact parallels with human situation should be expected, the behavior used here shows appropriate characteristics for a model and may prove to be useful in evaluating several parameters of the mechanism of action of antidepressants. Acknowledgement-This work was supported from the Iowa Mental Health Research Fund.

by funds

DISCUSSION REFERENCES

In order for a model of depression to be considered appropriate for studies on the mechanism of action of antidepressants, it is necessary for several conditions to be met. First, the behavior should be altered by chronic, but not acute administration of antidepressants. Second, a dose-response relationship should hold and thus the amount of drug present in brain should be inversely related to be measure of learned helplessness. The first of these conditions is demonstrated by the data in Tables 1, 2 and 4. The first dose of imipramine was ineffective in preventing the development of learned helplessness, while continuation on the same dosage schedule for 45 days produced a clear intervention into establishing this behavior. Chlorpromazine, either on acute or chronic schedules, had no effect on the development of learned helplessness. On the other hand, lorazepam protected the animals both acutely and after chronic administration. This finding is not unexpected in light of previous studies (Dantzer, Mormede and Favre, 1976) which have demonstrated an effect of diazepam-based drugs on other shock-motivated behaviors. It does, however, suggest that learned helplessness contains a fear conditioning component which is necessary for the development of the full spectrum of this behavior. The second condition necessary for an animal model to be considered valid is seen from the remaining data. In these, a clear dose-effect relationship can be observed. Combined with knowledge of the level of drug in brain, the dose-effect curve becomes essentially linear. As expected, the presence of greater amounts of drug in brain lead to fewer escape failures. The importance of pharmacokinetic variables could be seen by the large behavioral variability at each

Brookshire, K. H., Littmann, R. A. and Stewart, C. N. (1961). Residue of shock trauma in the white rat: a threefactor theory. Psychol. Monogr. 75(10). Brown, J. and Jacobs, A. (1949). The role of fear in the motivation and acquisition of response. J. exp. Psychol. 39: 747-759. Cooper, T. B., Allen, D. and Simpson, G. M. (1975). A sensitive GLC method for the determination of Imipramine and Desmethylimipramine using a nitrogen detector. Psychopharmac. Comm. 1: 445454. Dantzer, R., Mormede, P. and Favre, B. (1976). Feardependent variations in continuous avoidance behavior of pigs--II. Effects of diazepam on acquisition and performance of Pavlovian fear conditioning and plasma corticosteroid levels. Psychopharmacology 49: 75-78. Dinsmoor, J. (1958). Pulse duration and food deprivation in escape from shock training. Psychol. Rep. 4: 531-534. Dinsmoor, J. and Campbell, S. L. (1956). Escape-fromshock training following exposure to inescapable shock. Psychol. Rep. 2: 3449. Levine, S., Chevalier, J. and Korchin, S. (1956). The effects of early shock and handling on later avoidance learning. J. Personality 24: 475493. Mowrer, 0. H. (1940). An experimental analysis of “regression” with incidental observations on “reaction formation”. J. abnorm. Psychol. 35: 5687. Mullin, A. D. and Mogenson, G. J. (1963). Effects of fear conditioning on avoidance learning. Psycho/. Rep. 13. 707-710. Overmier, J. B. and Seligman, M. E. P. (1967) Effects of inescapable shock upon subsequent escape and avoidance learning. J. camp. physiol. Psychol. 63: 23-33. Seligman, M. E. P. and Groves, D. (1970). Non-transient learned helplessness. Psychonom. Sri. 19: 191-192. Seligman, M. E. P. and Maier, S. F. (1967). Failure to escape traumatic shock. J. exp. Psycho/. 74: l-9. Seligman, M. E. P., Maier, S. F. and Geer, J. (1968). The alleviation of learned helplessness in the dog. J. abnorm. Psychol. 73: 256-262.

Weiss, J. M., Krieckhaus, E. E. and Conte, R. (1968). Alfects of fear conditioning on subsequent avoidance behavior. J. camp. physiol. Psychol. 65: 413421.