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Effects of central administration of arginine-vasopressin on aversive memory retrieval
BRAIN RESEARCH ELSEVIER
Brain Research 699 (1995) 293-296
Research report
Effects of central administration of arginine-vasopressin on aversive memory retrieval K.B. Kumar ~' *, K.S. Karanth b a Department of Psychiatry, Kasturba Hospital, Manipal-576 119, Karnataka, India b Department of Pharmacology, Kasturba Medical College, Manipal-576 119, Karnataka, India
Accepted 6 July 1995
Abstract
This study examined whether arginine-vasopressin (A-VP), given before the test would produce an improved retrieval of aversive memory, in the same way as pre-exposure to inescapable footshocks, in rats. For this purpose animals conditioned in a T-maze with appetitive (10% sucrose) and aversive (2.0 mA footshock) events were administered (intracerebroventricular) a single dose of 2.5, 5, 10 or 20 ng/rat of A-VP, 20-min before testing. In the retention test conducted with the same training apparatus 72 h after conditioning, the peptide treated rats showed a dose-dependent increase in latencies to enter the previously shocked goalarm, with the absence of such a difference in responding to the non-shocked goalarm. This differential response was not observed in saline treated rats. This effect of peptide on memory retrieval was similar to that seen following inescapable footshock in rats. These results suggest the possible involvement of central vasopressinergic mechanisms in the differential enhancement of memory of helplessness condition. Keywords: Learned helplessness; Arginine-vasopressin; Aversive memory
I. Introduction
Recent work has indicated that inescapable (but not escapable) footshocks in the 'learned helplessness' condition, enhance memory retrieval for earlier aversive learning in inhibitory and discriminatory avoidance task in rats [24,25]. These memory changes are considered to be qualitatively similar to the retrieval bias seen in humans in depressive and induced mood states [37]. However, the neurochemical substrates of this abnormality are yet to be identified. Since learned helplessness model is regarded as one of the most relevant models of clinical depression [37,38] and post-traumatic stress disorder [13,23,34], an inquiry into neuronal meclhanisms underlying this cognitive abnormality in helpless animals might be useful in understanding the psychobiological aspects of the observed 'emotive biasing' so characteristic of these human conditions. There is considerable evidence for the hypothesis that the neurohypophysial peptide vasopressin which is formed
* Corresponding author. Fax: (91) (8252) 70500 or (91) (8252) 70062; E-maih [email protected] 0006-8993/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0006-8993(95)00921-3
predominantly in certain cell bodies of the hypothalamus exerts, behavioral effects, particularly, facilitation of memory processes. Several studies have demonstrated that vasopressin and its analogs, both with peripheral and intracerebroventricular (i.c.v.) administration, have beneficial effects on a wide variety of tasks involving cognitive process, particularly memory consolidation and retrieval of already stored information [4-7,19,20]. Much of the evidence for the memory hypothesis however, has come from avoidance procedure (see refs. [8], [21] and [22] for a review) and work on appetitively learned responses has not yielded consistent results. Although some early studies have reported positive effects for sexually motivated behavior and water finding task [3,9], many recent studies that have used operant methods such as food motivated approach, matching or comparison tasks to measure mnemonic processes before and after vasopressin treatment have shown either no effect or ambiguous results depending on the procedures [1,14,18,32,33]. The bulk of these experimental data thus suggest that the beneficial effects of vasopressin on memory are more consistent with stress-related tasks than with appetitively-motivated tasks. In recent years, several studies have reported that inescapably stressed rats exhibit among other neuroen-
294
K.B. Kumar, K.S. Karanth /Brain Research 699 (1995) 293-296
docrine abnormalities, an increased release of argininevasopressin (A-VP) [35,39,41]. Since the acquisition of learned helplessness is blocked by intracerebroventricular (i.c.v.) administration of antivasopressin serum, vasopressinergic mechanism in the brain is thought to mediate the learned helplessness in rats [28]. Consequently, the present study examined whether exogenously supplied A-VP would exert similar effect on memory retrieval as pre-exposure to inescapable footshocks. The peptide was administered directly into one of the lateral brain ventricles and its effect on the aversive memory retrieval was examined employing the same paradigm as that used in an earlier study to understand the nature of aversive memory retrieval in the learned helplessness condition [25].
2. Materials and methods
first lick at the drinking spout. The rat was removed from the apparatus 15 s after the spout was withdrawn from the goalbox. Training sessions continued until a rat entered a goalbox within 20 s of the start of 80% of the trials in a session. Thirty min after meeting the learning criterion (a maximum of 3 sessions were required for all rats to meet the criterion) all animals were given three free-choice trials. In trial 1, immediately after the choice, animals were confined to either of the goalboxes and were exposed to a 3 s, 2.0 mA inescapable footshocks (the punished goalbox, hereafter referred to as GB + and the unpunished goalbox as GB - ). The rat was removed from the apparatus 15 s later. After a 5 min recovery period, two additional freechoice trials were conducted to assess the avoidance conditioning. Any rat failing to meet the criterion of two consecutive choices of GB - within 200 s of the start of each trial was eliminated from the study.
2.1. Animals
2.4. Drug administration
Inbred Wistar strain male rats, weighing 320 __+10 g and maintained in groups of two or three were used in the present study. The animal colony was housed under standard laboratory conditions on 12 h light/dark cycle (lights on 06.00-18.00 h).
Seventy-two hours after GB + conditioning animals (n = 25) were divided into groups of five each. The goalbox in which aversive stimulus was delivered was equated across groups. This was regarded necessary as an earlier study [18] tested for vasopressin effect on memory retrieval had found results to vary depending on whether the subjects were trained to approach a white or black goalarm. Each group of animals was given i.c.v, a single dose of 2.5, 5, 10 or 20 ng/rat of A-VP (Sigma Chemicals, St. Louis, USA) in a volume of 5/zl. The required concentration of A-VP was dissolved just before use in sterile 0.9% saline and administered at the rate of 1 /~1 every 30 s free-hand through Hamilton syringe via silastic tube (0.46) with a dead volume of about 30/zl, 20 min before testing. In the control condition, an equal volume of sterile saline was infused.
2.2. Cannula implantation One week before the behavioral testing all subjects under general anesthesia (sodium pentobarbital, 40 m g / k g , i.p.) were chronically implanted with a 7 mm, 23-gauge stainless steel guide cannula anchored over the right or left lateral ventricle (A-P - 0 . 8 mm from bregma, L +2.0 mm, D-V - 3 . 2 mm). A stainless steel stylet, cut the same length as the cannula, was inserted to keep the cannula patent. Following surgery the animals were housed separately in single cages until end of the experiment. At the end of the experiment cannula placements were verified by visual inspection, injecting 2% Fast green dye solution (5 /zl) through cannula system.
2.3. Conditioning The procedure for appetitive and avoidance conditioning was the same as described earlier [25]. In brief, all animals were given a single training session of six trials in T-maze everyday. In each session, an animal was subjected to one free-choice trial, followed by five forcedchoice trials. In free-choice trials, animals had free access to both the black and white goal arms of the maze; in forced-choice trials access to the goal arms was alternatively blocked so that the animal was denied access to the arm which it had entered in the immediately preceding trial. Thus, rats had equal exposure to both goal arms during training. Goalbox entries in all trials were reinforced with 15 s access to 10% sucrose, beginning with the
2.5. Retention test All rats were given a session consisting of one freechoice trial and five forced-choice trials in the T-maze as previously described for evaluation of retention performance. Entry or no entry within a maximum period of 200 s from the start of each trial, latency to enter and time spent in each goalbox (maximum 30 s) following the first lick at the drinking spout upon entry were recorded. An increased avoidance latency to GB + , and a decreased latency to enter G B - , in this test were taken, respectively, as measures of improved aversive and appetitive memories.
2.6. Statistical analysis One-way analysis of variance (ANOVA) and Dunnett's post-hoc comparisons (P(0.05) were performed on variable of latency to enter and time spent in the goalbox.
K.B. Kumar, K.S. Karanth / Brain Research 699 (1995) 293-296 200 175 -
Ga+
QB-
150 125 t
~EIO0 -
75-
;
25-
IS)
# # tal T iT)
0 0
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5
10
20
0
2.5
i
i
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10
20
A-VP (nglrat)
Fig. 1. Effect of A-VP on memory of shocked (GB +) and sucrose rewarded (GB-) goalboxes. Data are presented as mean + S.E.M. for 5 rats at each dose. Total number of no entries to the respective goalbox at each dose is given in the parentheses. Significant differences among the groups with respect to latency (open bars) to enter GB + (F4,62 = 15.678, P < 0.001) were observed. No d!ifferenceswere seen among the groups with respect to latency to enter GB- (F4,53= 0.868, N.S.), time spent upon entry (dotted bars) in GB+ (F4,39 = 1.29, N.S.) and in GB(F4,40 = 0.79, N.S.). * P < 0.05 (Dunnett's test) and # P < 0.05 (Fisher's exact test) compared with saline control group. Fisher's exact probability test was applied to analyze the number of no entries to the goalbox.
3. R e s u l t s A-VP produced a dose-dependent increase in latencies to enter GB + without altering the responses to GB - , as shown in Fig. 1. However, the effects were significant only with the higher doses of A-VP. Further, Fisher's exact test indicated that rats treated with the higher doses of A-VP failed to enter GB + on a significantly greater portion of the trials than saline control group, as given in the parentheses (Fig. 1). There was no significant change in the time spent upon entry in either of the goalboxes following drug administration (Fig. 1). Importantly, no difference was found with A-VP in the responsiveness to GB - .
4. D i s c u s s i o n
In the present study i.cv. administration of A-VP produced a clear cut dissociation between an effect on the memory for unpleasant versus pleasant event. The peptide, when administered before the test, selectively enhanced the avoidance response to GB + without altering the approach response to G B - . This effect of A-VP on memory retrieval is similar to that seen following exposure to inescapable footshocks. The data appear to support the bulk of the work conducted by De Wied and associates and are
295
consistent with the view that mnemonic effects of A-VP are relatively more specific to stress-related experiences. It has been suggested that A-VP acts centrally via vasopressin receptors and affects directly the neural processes mediating memory consolidation and retrieval [7,19]. The posterior thalamic and dorsal hippocampal areas have been documented as possible sites of action for A-VP on learning and retention [4,20,36]. Recently some researchers have implicated hippocampal cholinergic arousal effect as one of the factors in the memory enhancing property of A-VP [10-12,17]. In view of this and the converging evidence that the septo-hippocampal cholinergic system, which plays a critical role in the memorization of aversively-motivated tasks [2], is involved in stress phenomena [15], it is plausible that central cholinergic neurons participate in the retrieval bias seen following inescapable footshock stress. Consistent with this suggestion is the finding that the retention of an avoidance behavior is enhanced following exposure to chronic unavoidable stress that induces a hypersensitivity of the central cholinergic system [27,40]. Besides, recent work in various models of depression has shown that a relationship exists between activation of cholinergic system and the retrieval of an avoidance memory [26,29,31]. To conclude, the outcome of this study provides the initial evidence for central vasopressinergic mechanism in the retrieval bias seen in helpless rats. The inescapable shock paradigm is considered as one of the potential models for understanding the biological substrates of depressive disorder and PTSD. Also, stress-related neuropeptides, or what have been termed 'stress-responsive neuromodulators' [16] have been suggested to underlie the intrusive recollection of traumatic memory of PTSD [30]. If so, the present data may provide an experimental basis for new avenues in the therapeutics of these conditions.
Acknowledgements
We thank Dr. N. Gopalan Kutty for his skillful help.
References
[1] Alliott, J. and Alexinsky, T., Effects of posttrial vasopressin injection on appetitively motivated learning in rats, Physiol. Behav., 28 (1982) 525-530. [2] Bartus, R.T., Dean, R.L. and Flicker, C., Cholinergicpsychopharmacology: An integration of human and animal research on memory. In H.Y. Meltzer (Ed.), Psychopharmacology: The Third Generation of Progress, Raven Press, New York, 1987, pp. 219-232. [3] Bohus, B., Effect of desglycinamide-lysine vasopressin (DG-LVP) on sexually-motivated T-maze behavior of the male rat, Horm. Behav., 8 (1977) 52-61. [4] Bohus, B., Urban, I., Van Wimersma Greidanus, T.B. and De Wied, D., Opposite effects of oxytocin and vasopressin on avoidance behavior and hippocampal theta rhythm in the rat, Neuropharmacology, 17 (1978) 239-247.
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K.B. Kumar, K.S. Karanth / Brain Research 699 (1995)293-296
[5] De Wied, D., Long-term effect of vasopressin on the maintenance of a conditioned avoidance response in rats, Nature, 232 (1971) 58-60. [6] De Wied, D., Behavioral effects of intraventricularly administered vasopressin and vasopressin fragments, LifeSci., 19 (1976) 685-690. [7] De Wied, D., Gaffori, O., Van Ree, J.M. and De Jong, W., Central target for the behavioral effects of vasopressin neuropeptides, Nature, 308 (1984) 276-278. [8] Doris, A.P., Vasopressin and central integrative processes, Neuroendocrinology, 38 (1984) 75-85. [9] Ettenberg, A., Le Moal, M., Koob, G.F. and Bloom, E., Vasopressin potentiation in the performance of a learned appetitive task: reversal by a pressor antagonist analog of vasopressin, Pharmacol. Biochem. Behav., 18 (1983) 645-647. [10] Faiman, C.P., De Erausquin, G.A. and Baratti, C.M., Vasopressin modulates the activity of nicotinic cholinergic mechanism during memory retrieval in mice, Behav. Neural. Biol., 50 (1988) 112-119. [11] Faiman, C.P., De Erausquin, G.A. and Baratti, C.M., The enhancement of retention induced by vasopressin in mice may be mediated by an activation of central nicotinic cholinergic mechanisms, Behav. Neural. Biol., 56 (1991) 183-189. [12] Faiman, C.P., De Erausquin, G.A. and Baratti, C.M., Modulation of memory retrieval of pre-testing vasopressin: involvement of a central cholinergic nicotinic mechanism, Methods Findings Exp. Clin. Pharmacol,, 14 (1992) 607-613. [13] Foa, E., Zinbarg, R. and Rothbaum, B., Uncontrollability and unpredictability in post traumatic disorder: an animal model, Psychol. Bull., 112 (1992) 218-237. [14] Garrud, P., Gray, J.A., Rickwood, L. and Coen, C., Pituitary-adrenal hormones and effects of partial reinforcement on appetitive behavior in the rat, Physiol. Behav., 18 (1974) 1-6. [15] Gilad, G.M., The stress-induced response of the septohippocampal cholinergic system. A vectorial outcome of psychoneuroendocrinological interactions, Psychoneuroendocrinology, 18 (1987) 167-184. [16] Gold, P.W., Stress-responsive neuromodulators, Biol. Psychiatry, 24 (1988) 371-374. [17] Horita, A. and Carino, M.A., Centrally administered vasopressin antagonists, pentobarbital-induced narcosis and depression of hippocampal cholinergic activity, Peptides, 11 (1990) 1021-1025. [18] Hostetter, G., Jubb, S.C. and Kozlowski, G.P., Vasopressin affects the behavior of rats in a positively reward discrimination task, Life Sci., 21 (1977) 1323-1328. [19] Koob, G.F., Dantzer, R., Bluthe, R.M., Le Brun, C., Bloom, F.E. and Le Moal, M., Central injection of arginine vasopressin prolong extinction of active avoidance, Peptides, 7 (1986) 213-218. [20] Kovacs, G.L., Bohus, B., Versteeg, D.H.G., De Kloet, R.E. and De Wied, D., Effeccts of oxytocin and vasopressin on memory consolidation - - sites of action and catecholaminergic correlates after local microinjections into limbic-midbrain structures, Brain Res., 175 (1979) 303-314. [21] Kovacs, G.L. and De Wied, D., Peptidergic modulation of learning and memory processes, Pharmacol. Rev., 46 (1994) 269-291. [22] Kovacs, G.L., Szabo, G., Sarnyai, J. and Telegdy, G., Neurohypophyseal hormones and behavior, Prog. Brain. Res., 72 (1987) 109-118. [23] Krystal, J.H., Kosten, T.R. and Perry, B.D., Neurobiological aspects of PTSD: review of clinical and preclinical studies, Behav. Then, 20 (1989) 177-198.
[24] Kumar, K.B. and Karanth, K.S., Enhanced retrieval of unpleasant memory in helpless rats, Biol. Psychiatry, 30 (1991) 493-501. [25] Kumar, K.B. and Karanth, K.S., Enhanced processing of an aversive memory following inescapable shock in rats, Biol. Psychiatry, 33 (1993) 169-172. [26] Kumar, K.B. and Karanth, K.S., An improved avoidance memory following forced swimming, (submitted). [27] Laborit, H. and Zerbib, R., Different environmental situations modify scopolamine-induced amnesia in mice assessed with a passive avoidance test, Res. Commun. Psychol. Psychiatr. Behav., 14 (1989) 143-156. [28] Leshner, A.I., Hofstein, F. and Samuel, D., Intraventricular injection of antivasopressin serum blocks learned helplessness in rats, Pharmacol. Biochem. Behav., 9 (1978) 887-897. [29] Overstreet, D.H., Janowsky, D.S., Gillin, J.C., Shiromani, P.J. and Sutin, E.L., Stress-induced immobility in rats with cholinergic supersensitivity, Biol. Psychiatry, 21 (1986) 657-664. [30] Pitman, R.K., Post-traumatic stress disorder, hormones and memory, Biol. Psychiatry, 26 (1989) 221-223. [31] Prathiba, J., Kumar, K.B. and Karanth, K.S., Effects of neonatal clomipramine on cholinergic receptor sensitivity and passive avoidance behavior in adult rats, J. Neural Transm., (in press). [32] Sahagal, A., Keith, A.B. and Lloyd, S., Opposing effects of vasopressin on matching versus non-matching to position: further evidence for response, not memory, modulation, Psychopharmacology, 102 (1990) 130-135. [33] Sara, S.J., Barnett, J. and Toussant, P., Vasopressin accelerates appetitive discrimination learning and impairs its reversal, Behav. Processes, 7 (1982) 157-167. [34] Van Der Kolk, B., Greenberg, M., Boyd, H. and Krystral, J., Inescapable shock, neurotransmitters and addiction to trauma: toward a psychobiology of post traumatic stress, Biol. Psychiatry, 20 (1985) 314-325. [35] Van Dijken, H.H., De Goeij, D.C., Sutanto, W., Mos, J., De Kloet, E.R. and Tilders, F.J., Short inescapable stress producess long-lasting changes in the brain-pituitary-adrenal axis of adult male rats, Neuroendocrinology, 58 (1993) 57-64. [36] Van Wimersma Greidanus, T.B., Bohus, B. and De Wied, D., Differential localization of the influence of lysine vasopressin and of ACTH(4-10) on avoidance behavior: a study in rats bearing lesions in the parafascicular nuclei, Neuroendocrinology, 4 (1974) 280-288. [37] Willner, P., Depression: a Psychobiological Synthesis, Wiley, New York, 1985, pp. 101-143. [38] Willner, P., Validation criteria for animal models of human mental disorders: learned helplessness as a paradigm case, Prog. Neuropsychopharmacol. Biol. Psychiatry, 10 (1986) 677-690. [39] Young, E. and Akil, H., Changes in releasability of ACTH and fl-endorphin with chronic stress, Neuropeptides, 5 (1985) 545-548. [40] Zerbib, R. and Laborit, H., Chronic stress and memory: Implication of the central cholinergic system, Pharmacol. Biochem. Behav., 36 (1990) 897-900. [41] Zhukov, D.A., An increased vasopressin level in the blood plasma a possible cause of dexamethasone resistance in rats following stress, F iziol. Zh. Im. 1. M. Sechenova, 79 (1993) 35-38. -
-
Brain Research 699 (1995) 293-296
Research report
Effects of central administration of arginine-vasopressin on aversive memory retrieval K.B. Kumar ~' *, K.S. Karanth b a Department of Psychiatry, Kasturba Hospital, Manipal-576 119, Karnataka, India b Department of Pharmacology, Kasturba Medical College, Manipal-576 119, Karnataka, India
Accepted 6 July 1995
Abstract
This study examined whether arginine-vasopressin (A-VP), given before the test would produce an improved retrieval of aversive memory, in the same way as pre-exposure to inescapable footshocks, in rats. For this purpose animals conditioned in a T-maze with appetitive (10% sucrose) and aversive (2.0 mA footshock) events were administered (intracerebroventricular) a single dose of 2.5, 5, 10 or 20 ng/rat of A-VP, 20-min before testing. In the retention test conducted with the same training apparatus 72 h after conditioning, the peptide treated rats showed a dose-dependent increase in latencies to enter the previously shocked goalarm, with the absence of such a difference in responding to the non-shocked goalarm. This differential response was not observed in saline treated rats. This effect of peptide on memory retrieval was similar to that seen following inescapable footshock in rats. These results suggest the possible involvement of central vasopressinergic mechanisms in the differential enhancement of memory of helplessness condition. Keywords: Learned helplessness; Arginine-vasopressin; Aversive memory
I. Introduction
Recent work has indicated that inescapable (but not escapable) footshocks in the 'learned helplessness' condition, enhance memory retrieval for earlier aversive learning in inhibitory and discriminatory avoidance task in rats [24,25]. These memory changes are considered to be qualitatively similar to the retrieval bias seen in humans in depressive and induced mood states [37]. However, the neurochemical substrates of this abnormality are yet to be identified. Since learned helplessness model is regarded as one of the most relevant models of clinical depression [37,38] and post-traumatic stress disorder [13,23,34], an inquiry into neuronal meclhanisms underlying this cognitive abnormality in helpless animals might be useful in understanding the psychobiological aspects of the observed 'emotive biasing' so characteristic of these human conditions. There is considerable evidence for the hypothesis that the neurohypophysial peptide vasopressin which is formed
* Corresponding author. Fax: (91) (8252) 70500 or (91) (8252) 70062; E-maih [email protected] 0006-8993/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0006-8993(95)00921-3
predominantly in certain cell bodies of the hypothalamus exerts, behavioral effects, particularly, facilitation of memory processes. Several studies have demonstrated that vasopressin and its analogs, both with peripheral and intracerebroventricular (i.c.v.) administration, have beneficial effects on a wide variety of tasks involving cognitive process, particularly memory consolidation and retrieval of already stored information [4-7,19,20]. Much of the evidence for the memory hypothesis however, has come from avoidance procedure (see refs. [8], [21] and [22] for a review) and work on appetitively learned responses has not yielded consistent results. Although some early studies have reported positive effects for sexually motivated behavior and water finding task [3,9], many recent studies that have used operant methods such as food motivated approach, matching or comparison tasks to measure mnemonic processes before and after vasopressin treatment have shown either no effect or ambiguous results depending on the procedures [1,14,18,32,33]. The bulk of these experimental data thus suggest that the beneficial effects of vasopressin on memory are more consistent with stress-related tasks than with appetitively-motivated tasks. In recent years, several studies have reported that inescapably stressed rats exhibit among other neuroen-
294
K.B. Kumar, K.S. Karanth /Brain Research 699 (1995) 293-296
docrine abnormalities, an increased release of argininevasopressin (A-VP) [35,39,41]. Since the acquisition of learned helplessness is blocked by intracerebroventricular (i.c.v.) administration of antivasopressin serum, vasopressinergic mechanism in the brain is thought to mediate the learned helplessness in rats [28]. Consequently, the present study examined whether exogenously supplied A-VP would exert similar effect on memory retrieval as pre-exposure to inescapable footshocks. The peptide was administered directly into one of the lateral brain ventricles and its effect on the aversive memory retrieval was examined employing the same paradigm as that used in an earlier study to understand the nature of aversive memory retrieval in the learned helplessness condition [25].
2. Materials and methods
first lick at the drinking spout. The rat was removed from the apparatus 15 s after the spout was withdrawn from the goalbox. Training sessions continued until a rat entered a goalbox within 20 s of the start of 80% of the trials in a session. Thirty min after meeting the learning criterion (a maximum of 3 sessions were required for all rats to meet the criterion) all animals were given three free-choice trials. In trial 1, immediately after the choice, animals were confined to either of the goalboxes and were exposed to a 3 s, 2.0 mA inescapable footshocks (the punished goalbox, hereafter referred to as GB + and the unpunished goalbox as GB - ). The rat was removed from the apparatus 15 s later. After a 5 min recovery period, two additional freechoice trials were conducted to assess the avoidance conditioning. Any rat failing to meet the criterion of two consecutive choices of GB - within 200 s of the start of each trial was eliminated from the study.
2.1. Animals
2.4. Drug administration
Inbred Wistar strain male rats, weighing 320 __+10 g and maintained in groups of two or three were used in the present study. The animal colony was housed under standard laboratory conditions on 12 h light/dark cycle (lights on 06.00-18.00 h).
Seventy-two hours after GB + conditioning animals (n = 25) were divided into groups of five each. The goalbox in which aversive stimulus was delivered was equated across groups. This was regarded necessary as an earlier study [18] tested for vasopressin effect on memory retrieval had found results to vary depending on whether the subjects were trained to approach a white or black goalarm. Each group of animals was given i.c.v, a single dose of 2.5, 5, 10 or 20 ng/rat of A-VP (Sigma Chemicals, St. Louis, USA) in a volume of 5/zl. The required concentration of A-VP was dissolved just before use in sterile 0.9% saline and administered at the rate of 1 /~1 every 30 s free-hand through Hamilton syringe via silastic tube (0.46) with a dead volume of about 30/zl, 20 min before testing. In the control condition, an equal volume of sterile saline was infused.
2.2. Cannula implantation One week before the behavioral testing all subjects under general anesthesia (sodium pentobarbital, 40 m g / k g , i.p.) were chronically implanted with a 7 mm, 23-gauge stainless steel guide cannula anchored over the right or left lateral ventricle (A-P - 0 . 8 mm from bregma, L +2.0 mm, D-V - 3 . 2 mm). A stainless steel stylet, cut the same length as the cannula, was inserted to keep the cannula patent. Following surgery the animals were housed separately in single cages until end of the experiment. At the end of the experiment cannula placements were verified by visual inspection, injecting 2% Fast green dye solution (5 /zl) through cannula system.
2.3. Conditioning The procedure for appetitive and avoidance conditioning was the same as described earlier [25]. In brief, all animals were given a single training session of six trials in T-maze everyday. In each session, an animal was subjected to one free-choice trial, followed by five forcedchoice trials. In free-choice trials, animals had free access to both the black and white goal arms of the maze; in forced-choice trials access to the goal arms was alternatively blocked so that the animal was denied access to the arm which it had entered in the immediately preceding trial. Thus, rats had equal exposure to both goal arms during training. Goalbox entries in all trials were reinforced with 15 s access to 10% sucrose, beginning with the
2.5. Retention test All rats were given a session consisting of one freechoice trial and five forced-choice trials in the T-maze as previously described for evaluation of retention performance. Entry or no entry within a maximum period of 200 s from the start of each trial, latency to enter and time spent in each goalbox (maximum 30 s) following the first lick at the drinking spout upon entry were recorded. An increased avoidance latency to GB + , and a decreased latency to enter G B - , in this test were taken, respectively, as measures of improved aversive and appetitive memories.
2.6. Statistical analysis One-way analysis of variance (ANOVA) and Dunnett's post-hoc comparisons (P(0.05) were performed on variable of latency to enter and time spent in the goalbox.
K.B. Kumar, K.S. Karanth / Brain Research 699 (1995) 293-296 200 175 -
Ga+
QB-
150 125 t
~EIO0 -
75-
;
25-
IS)
# # tal T iT)
0 0
2.5
5
10
20
0
2.5
i
i
5
10
20
A-VP (nglrat)
Fig. 1. Effect of A-VP on memory of shocked (GB +) and sucrose rewarded (GB-) goalboxes. Data are presented as mean + S.E.M. for 5 rats at each dose. Total number of no entries to the respective goalbox at each dose is given in the parentheses. Significant differences among the groups with respect to latency (open bars) to enter GB + (F4,62 = 15.678, P < 0.001) were observed. No d!ifferenceswere seen among the groups with respect to latency to enter GB- (F4,53= 0.868, N.S.), time spent upon entry (dotted bars) in GB+ (F4,39 = 1.29, N.S.) and in GB(F4,40 = 0.79, N.S.). * P < 0.05 (Dunnett's test) and # P < 0.05 (Fisher's exact test) compared with saline control group. Fisher's exact probability test was applied to analyze the number of no entries to the goalbox.
3. R e s u l t s A-VP produced a dose-dependent increase in latencies to enter GB + without altering the responses to GB - , as shown in Fig. 1. However, the effects were significant only with the higher doses of A-VP. Further, Fisher's exact test indicated that rats treated with the higher doses of A-VP failed to enter GB + on a significantly greater portion of the trials than saline control group, as given in the parentheses (Fig. 1). There was no significant change in the time spent upon entry in either of the goalboxes following drug administration (Fig. 1). Importantly, no difference was found with A-VP in the responsiveness to GB - .
4. D i s c u s s i o n
In the present study i.cv. administration of A-VP produced a clear cut dissociation between an effect on the memory for unpleasant versus pleasant event. The peptide, when administered before the test, selectively enhanced the avoidance response to GB + without altering the approach response to G B - . This effect of A-VP on memory retrieval is similar to that seen following exposure to inescapable footshocks. The data appear to support the bulk of the work conducted by De Wied and associates and are
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consistent with the view that mnemonic effects of A-VP are relatively more specific to stress-related experiences. It has been suggested that A-VP acts centrally via vasopressin receptors and affects directly the neural processes mediating memory consolidation and retrieval [7,19]. The posterior thalamic and dorsal hippocampal areas have been documented as possible sites of action for A-VP on learning and retention [4,20,36]. Recently some researchers have implicated hippocampal cholinergic arousal effect as one of the factors in the memory enhancing property of A-VP [10-12,17]. In view of this and the converging evidence that the septo-hippocampal cholinergic system, which plays a critical role in the memorization of aversively-motivated tasks [2], is involved in stress phenomena [15], it is plausible that central cholinergic neurons participate in the retrieval bias seen following inescapable footshock stress. Consistent with this suggestion is the finding that the retention of an avoidance behavior is enhanced following exposure to chronic unavoidable stress that induces a hypersensitivity of the central cholinergic system [27,40]. Besides, recent work in various models of depression has shown that a relationship exists between activation of cholinergic system and the retrieval of an avoidance memory [26,29,31]. To conclude, the outcome of this study provides the initial evidence for central vasopressinergic mechanism in the retrieval bias seen in helpless rats. The inescapable shock paradigm is considered as one of the potential models for understanding the biological substrates of depressive disorder and PTSD. Also, stress-related neuropeptides, or what have been termed 'stress-responsive neuromodulators' [16] have been suggested to underlie the intrusive recollection of traumatic memory of PTSD [30]. If so, the present data may provide an experimental basis for new avenues in the therapeutics of these conditions.
Acknowledgements
We thank Dr. N. Gopalan Kutty for his skillful help.
References
[1] Alliott, J. and Alexinsky, T., Effects of posttrial vasopressin injection on appetitively motivated learning in rats, Physiol. Behav., 28 (1982) 525-530. [2] Bartus, R.T., Dean, R.L. and Flicker, C., Cholinergicpsychopharmacology: An integration of human and animal research on memory. In H.Y. Meltzer (Ed.), Psychopharmacology: The Third Generation of Progress, Raven Press, New York, 1987, pp. 219-232. [3] Bohus, B., Effect of desglycinamide-lysine vasopressin (DG-LVP) on sexually-motivated T-maze behavior of the male rat, Horm. Behav., 8 (1977) 52-61. [4] Bohus, B., Urban, I., Van Wimersma Greidanus, T.B. and De Wied, D., Opposite effects of oxytocin and vasopressin on avoidance behavior and hippocampal theta rhythm in the rat, Neuropharmacology, 17 (1978) 239-247.
296
K.B. Kumar, K.S. Karanth / Brain Research 699 (1995)293-296
[5] De Wied, D., Long-term effect of vasopressin on the maintenance of a conditioned avoidance response in rats, Nature, 232 (1971) 58-60. [6] De Wied, D., Behavioral effects of intraventricularly administered vasopressin and vasopressin fragments, LifeSci., 19 (1976) 685-690. [7] De Wied, D., Gaffori, O., Van Ree, J.M. and De Jong, W., Central target for the behavioral effects of vasopressin neuropeptides, Nature, 308 (1984) 276-278. [8] Doris, A.P., Vasopressin and central integrative processes, Neuroendocrinology, 38 (1984) 75-85. [9] Ettenberg, A., Le Moal, M., Koob, G.F. and Bloom, E., Vasopressin potentiation in the performance of a learned appetitive task: reversal by a pressor antagonist analog of vasopressin, Pharmacol. Biochem. Behav., 18 (1983) 645-647. [10] Faiman, C.P., De Erausquin, G.A. and Baratti, C.M., Vasopressin modulates the activity of nicotinic cholinergic mechanism during memory retrieval in mice, Behav. Neural. Biol., 50 (1988) 112-119. [11] Faiman, C.P., De Erausquin, G.A. and Baratti, C.M., The enhancement of retention induced by vasopressin in mice may be mediated by an activation of central nicotinic cholinergic mechanisms, Behav. Neural. Biol., 56 (1991) 183-189. [12] Faiman, C.P., De Erausquin, G.A. and Baratti, C.M., Modulation of memory retrieval of pre-testing vasopressin: involvement of a central cholinergic nicotinic mechanism, Methods Findings Exp. Clin. Pharmacol,, 14 (1992) 607-613. [13] Foa, E., Zinbarg, R. and Rothbaum, B., Uncontrollability and unpredictability in post traumatic disorder: an animal model, Psychol. Bull., 112 (1992) 218-237. [14] Garrud, P., Gray, J.A., Rickwood, L. and Coen, C., Pituitary-adrenal hormones and effects of partial reinforcement on appetitive behavior in the rat, Physiol. Behav., 18 (1974) 1-6. [15] Gilad, G.M., The stress-induced response of the septohippocampal cholinergic system. A vectorial outcome of psychoneuroendocrinological interactions, Psychoneuroendocrinology, 18 (1987) 167-184. [16] Gold, P.W., Stress-responsive neuromodulators, Biol. Psychiatry, 24 (1988) 371-374. [17] Horita, A. and Carino, M.A., Centrally administered vasopressin antagonists, pentobarbital-induced narcosis and depression of hippocampal cholinergic activity, Peptides, 11 (1990) 1021-1025. [18] Hostetter, G., Jubb, S.C. and Kozlowski, G.P., Vasopressin affects the behavior of rats in a positively reward discrimination task, Life Sci., 21 (1977) 1323-1328. [19] Koob, G.F., Dantzer, R., Bluthe, R.M., Le Brun, C., Bloom, F.E. and Le Moal, M., Central injection of arginine vasopressin prolong extinction of active avoidance, Peptides, 7 (1986) 213-218. [20] Kovacs, G.L., Bohus, B., Versteeg, D.H.G., De Kloet, R.E. and De Wied, D., Effeccts of oxytocin and vasopressin on memory consolidation - - sites of action and catecholaminergic correlates after local microinjections into limbic-midbrain structures, Brain Res., 175 (1979) 303-314. [21] Kovacs, G.L. and De Wied, D., Peptidergic modulation of learning and memory processes, Pharmacol. Rev., 46 (1994) 269-291. [22] Kovacs, G.L., Szabo, G., Sarnyai, J. and Telegdy, G., Neurohypophyseal hormones and behavior, Prog. Brain. Res., 72 (1987) 109-118. [23] Krystal, J.H., Kosten, T.R. and Perry, B.D., Neurobiological aspects of PTSD: review of clinical and preclinical studies, Behav. Then, 20 (1989) 177-198.
[24] Kumar, K.B. and Karanth, K.S., Enhanced retrieval of unpleasant memory in helpless rats, Biol. Psychiatry, 30 (1991) 493-501. [25] Kumar, K.B. and Karanth, K.S., Enhanced processing of an aversive memory following inescapable shock in rats, Biol. Psychiatry, 33 (1993) 169-172. [26] Kumar, K.B. and Karanth, K.S., An improved avoidance memory following forced swimming, (submitted). [27] Laborit, H. and Zerbib, R., Different environmental situations modify scopolamine-induced amnesia in mice assessed with a passive avoidance test, Res. Commun. Psychol. Psychiatr. Behav., 14 (1989) 143-156. [28] Leshner, A.I., Hofstein, F. and Samuel, D., Intraventricular injection of antivasopressin serum blocks learned helplessness in rats, Pharmacol. Biochem. Behav., 9 (1978) 887-897. [29] Overstreet, D.H., Janowsky, D.S., Gillin, J.C., Shiromani, P.J. and Sutin, E.L., Stress-induced immobility in rats with cholinergic supersensitivity, Biol. Psychiatry, 21 (1986) 657-664. [30] Pitman, R.K., Post-traumatic stress disorder, hormones and memory, Biol. Psychiatry, 26 (1989) 221-223. [31] Prathiba, J., Kumar, K.B. and Karanth, K.S., Effects of neonatal clomipramine on cholinergic receptor sensitivity and passive avoidance behavior in adult rats, J. Neural Transm., (in press). [32] Sahagal, A., Keith, A.B. and Lloyd, S., Opposing effects of vasopressin on matching versus non-matching to position: further evidence for response, not memory, modulation, Psychopharmacology, 102 (1990) 130-135. [33] Sara, S.J., Barnett, J. and Toussant, P., Vasopressin accelerates appetitive discrimination learning and impairs its reversal, Behav. Processes, 7 (1982) 157-167. [34] Van Der Kolk, B., Greenberg, M., Boyd, H. and Krystral, J., Inescapable shock, neurotransmitters and addiction to trauma: toward a psychobiology of post traumatic stress, Biol. Psychiatry, 20 (1985) 314-325. [35] Van Dijken, H.H., De Goeij, D.C., Sutanto, W., Mos, J., De Kloet, E.R. and Tilders, F.J., Short inescapable stress producess long-lasting changes in the brain-pituitary-adrenal axis of adult male rats, Neuroendocrinology, 58 (1993) 57-64. [36] Van Wimersma Greidanus, T.B., Bohus, B. and De Wied, D., Differential localization of the influence of lysine vasopressin and of ACTH(4-10) on avoidance behavior: a study in rats bearing lesions in the parafascicular nuclei, Neuroendocrinology, 4 (1974) 280-288. [37] Willner, P., Depression: a Psychobiological Synthesis, Wiley, New York, 1985, pp. 101-143. [38] Willner, P., Validation criteria for animal models of human mental disorders: learned helplessness as a paradigm case, Prog. Neuropsychopharmacol. Biol. Psychiatry, 10 (1986) 677-690. [39] Young, E. and Akil, H., Changes in releasability of ACTH and fl-endorphin with chronic stress, Neuropeptides, 5 (1985) 545-548. [40] Zerbib, R. and Laborit, H., Chronic stress and memory: Implication of the central cholinergic system, Pharmacol. Biochem. Behav., 36 (1990) 897-900. [41] Zhukov, D.A., An increased vasopressin level in the blood plasma a possible cause of dexamethasone resistance in rats following stress, F iziol. Zh. Im. 1. M. Sechenova, 79 (1993) 35-38. -
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