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Effects of post-trial injection of β-endorphin on shock-induced fighting are dependent on baseline of fighting
BEHAVIORALAND NEURALBIOLOGY43, 322-326 (1985)
BRIEF REPORT
Effects of Post-Trial Injection of,8-Endorphin on Shock-Induced Fighting Are Dependent on Baseline of Fighting A . TAZI, R . DANTZER, P . MORMEDE, AND M . L E MOAL I
Psychobiologie des Comportements Adaptatifs INSERM-INRA, U 259, Rue Camille St-Saens, 33077 Bordeaux Cedex, France The effects of repeated post-trial administration of 10/zg/kg/3-endorphin on the development of mutual fighting in pairs of rats submitted to various intensities of electric shock were investigated. /3-Endorphin blocked the development of fighting responses when a low footshock intensity was used, but facilitated it when a high shock intensity was delivered. A detailed analysis of the relationship between shock intensity, baseline of fighting, and effects of fl.endorphin showed that the effects of fl-endorphin were dependent on the behavioral baseline rather than on shock intensity per se. © 1985AcademicPress, Inc.
Several reports have shown that endogenous opioid systems may influence acquisition of new behavior and performance of learned behavior. Thus, when administered immediately after training of active or passive avoidance responding, opiate antagonists improved retention performance (GaUagher, 1982). In contrast, post-trial administration of an opiate agonist impaired retention (Martinez and Rigter, 1980). These effects are mediated via classical mu opiate receptors since simultaneous postacquisition administration of naloxone and morphine or /3-endorphin had no effect (Izquierdo et al., 1980). Using post-trial treatments, we have recently demonstrated the involvement of endogenous opioid systems in the development of mutual fighting between rats shocked in pairs (shock-induced fighting). The emergence of this behavior over repeated sessions of shock administration was facilitated by post-trial injection of 2 mg/kg naloxone and blocked by I0/zg/kg fl-endorphin (Tazi, Dantzer, Mormede, & Le Moal 1983). The incidence of shock-induced fighting is sensitive to environmental factors such as the size of the chamber and physical characteristics of electric shock. Concerning this last parameter, an inverted U-shape reAddress all correspondence and requests for reprints to Dr. A. Tazi. 322 0163-1047/85 $3.00 Copyright© 1985by AcademicPress, Inc. All rightsof reproductionin any formreserved.
fl-ENDORPHIN EFFECTS ON FIGHTING DEPEND ON BASELINE
323
lationship has been described between shock intensity and fighting, with an optimum around 2-2.5 mA (Ulrich and Azrin, 1962). Since the effects of post-training hormonal treatments have been found to depend on the intensity of electric shock used during conditioning (Gold and Van Burskirk, 1976), we decided to systematically assess the effects of a fixed dose of /3-endorphin (10/zg/kg) on the development of fighting induced by various shock intensities. One hundred and thirty four male Sprague-Dawley albino rats (IFFACREDO, Lyon), weighing about 250 g at the start of the experiment, were used. They were housed individually or two to four per cage, with water and food freely available. The colony room was maintained at 2223°C, with a 12-h on/off light cycle. All experiments were run between 9:00 and 12:00 AM. The methods have already been described (Tazi et al., 1983). Testing was carried out in a 30 × 30 × 40-cm opaque plastic chamber with a transparent cover to allow visual observation of the animals. This chamber was housed within a well-lit, sound-attenuated cabinet. Different intensities of shock were delivered by a scrambled constant current shocker (BRSForinger) programmed to administer 10 trains of 10 shocks (1, 1.6, 2. or 3 mA, 0.5 s on/1 s off) every 45 s to the stainless-steel-grid floor. Ten days after their arrival, animals were assigned to weight-matched pairs for testing, with the difference between members of the same pair not exceeding 10 g. When animals were housed two to four per cage, pairs were made up with animals from different cages. Test sessions lasted 10 min and were run daily for 10 consecutive days. During the test session, an observer sitting in the cabinet classified the behavior of the animals during each train of shocks (a "trial") into one of the three mutually exclusive categories (Stolk, Conner, Levine, & Barchas, 1974): no interaction (mainly escape attempts or freezing postures); upright position (mutual upright posture, without physical contact); or fighting (mutual upright posture accompanied by striking with forepaws). Only this last category was retained for subsequent analysis since it had been shown to be sensitive to fl-endorphin treatment in previous studies (Tazi et al., 1983). The reliability of behavioral scoring was regularly checked by a second observer. Treatments were administered at the end of the first nine sessions. /3-Endorphin (a gift from Dr. N. Ling, Salk Intitute) was dissolved in aliquots of 50/zg which were kept at - 80°C until use. Each aliquot was stored at -20°C on the day before use and was dissolved in saline on the day of use. Plastic tubes containing the solution were stored on ice during the course of the experiment and fresh solutions were prepared every day. All injections were given subcutaneously, in a volume of 2 ml/kg. Treatments were arbitrarily coded so that the observer was
324
TAZI ET AL.
naive as to the treatment received by each pair of rats. The two members of each pair received the same treatment. Data, expressed as number of episodes of fighting per session, were submitted to a two-way analysis of variance with repeated measurements. Figure 1A represents fighting episodes in experiments conducted under various shock intensities. In saline-treated animals, the shock-response relationship displayed an inverted U shape, with maximal fighting occurring at 2 mA shock intensity. For the 1.6- and 2-mA shock intensities, posttraining injections of 10/zg/kg/3-endorphin resulted in an impaired acquisition of fighting responses, evidenced by a significant session x treatment interaction: F(9, 54) = 3.10, p < .01. and F(9, 207) = 7.20, p < .001, respectively. When rats were tested with 3 mA shock intensity, /3-endorphin facilitated the development of shock-induced fighting (F(9, 108) = 4.22, p < .001). In another stock of rats tested 4 months later, purchased from the same breeder, housed under the same conditions, and handled in the same way, exposure of pairs of animals to 2-mA shocks resulted in a lower baseline of SIF (Fig. I B). Under these conditions, /3-endorphin facilitated instead of impaired the development of SIF (F(8, 128) = 3.65, p < .001). The present results confirm that the relation between shock intensity and fighting responses has the shape of an inverted U curve (Ulrich and Azrin, 1962). Since the asymptote of the curve is reached at 2 mA, slight
[-~
Placebo p-endorphin
~ 25
, , ~ 1
1.6
2
3
Shock intensity (mA)
FIG. 1. Mean fighting responses (expressed as percentage of shock presentations) in saline- and/3-endorphin-treated rats, under various shock intensities. The number of pairs of animals used in each experiment is indicated at the top of each column, and the bars represent the standard error of the mean.
/3-ENDORPHIN EFFECTS ON FIGHTING DEPEND ON BASELINE
325
variations in sensitivity of rats to this shock level will result in large variations in the frequence of shock-induced fighting. This may explain the variability in SIF which was observed between different stocks provided by the same supplier at different intervals. Whatever the case, the effects of post-trial administration of/3-endorphin appear to depend mainly on baseline of SIF. In accordance with our previous results (Tazi et al., 1983), high levels of SIF are depressed by post-trial injection of/~-endorphin. In contrast, low levels of SIF are enhanced by the same treatment, but only if the behavior has a sufficient probability of appearance. The dependency of the effect of opiates on behavioral baseline has already been demonstrated (Classen & Mondadori, 1984). Several hypotheses may account for this phenomenon. One possibility is that /3-endorphin interacts with some hormonal and/or neurochemical system modulating the acquisition and retention of SIF. A likely candidate is the central catecholaminergic system, which has already been implicated in the effects of opiates on learning and memory (Izquierdo & Dias, 1984). According to this interpretation,/3-endorphin would act by inhibiting the modulatory effect of this system on SIF. Another possibility is that /3-endorphin is not active per se, but is active via metabolic fragments, such as ~- and y-endorphins, which have been found to exert opposite behavioral effects in aversively motivated tasks (Kovacs, Bohus, & De Wied, 1983). According to this interpretation, the differential effect of/3-endorphin would be mediated either by different biotransformation rates according to experimental conditions, or by differential effects of each metabolite according to behavioral baseline. Further experiments are therefore needed to elucidate the mechanisms of action of/3-endorphin on SIF and its site of action.
REFERENCES Classen, W., & Mondadori, C. (1984). Facilitation or inhibition of memory by morphine: A question of experimental parameters. Experientia, 40, 506-509. Gallagher, M. (1982). Naloxone enhancement of memory processes: Effects of other opiate antagonists. Behavioral and Neural Biology, 35, 375-382. Gold, P. E., & Van Burskirk, R. (1976). Enhancement and impairment of memory processes with post-trial injections of adrenocoticotrophic hormone. Behavioral Biology, 16, 387400. Izquierdo, I., & Dias, R. D. (1984). Involvement of a-adrenergic receptors in the amnestic and anti-amnestic action of ACTH, /~-endorphin and epinephrine. Psychoneuroendocrinology, 9, 77-81. Izquierdo, I., Dias, R. D., Souza, D. O., Carrasco, M. A., Elisabetsky, E., & Perry, M. L. (1980). The role of opioid peptides in memory and learning. Behavioral Brain Research, 1, 451-468. Kovacs, G. L., Bohus, B., & De Wied, D. (1983). Effects of/3-endorphin and its fragments on inhibitory avoidance behavior in rats. Psychoneuroendocrinology, 8, 411-419. Martinez, J. L., Jr., & Rigter, H. (1980). Endorphins alter acquisition and consolidation of an inhibitory avoidance response in rats. Neuroscience Letters, 18, 197-201.
326
TAZI ET AL.
Stolk, J. M., Conner, R. L., Levine, S., & Barchas, J. D. (1974). Brain norepinephrine metabolism and shock-induced fighting behavior in rats: Differential effects of shock and fighting on the neurochemical response to a common footshock stimulus. 1'he Journal of Pharmacology and Experimental Therapeutics. 190, 194-209. Tazi, A., Dantzer, R., Mormede, P., & Le Moal, M. (1983). Effects of post-trial administration of naloxone and/3-endorphin on shock-induced fighting in rats. Behavioral and Neural Biology, 39, 192-202. Ulrich, R. E., & Azrin, N. H. (1962). Reflexive fighting in response to aversive stimulation. Journal of Experimental Analysis of Behavior, 5, 511-520.
BRIEF REPORT
Effects of Post-Trial Injection of,8-Endorphin on Shock-Induced Fighting Are Dependent on Baseline of Fighting A . TAZI, R . DANTZER, P . MORMEDE, AND M . L E MOAL I
Psychobiologie des Comportements Adaptatifs INSERM-INRA, U 259, Rue Camille St-Saens, 33077 Bordeaux Cedex, France The effects of repeated post-trial administration of 10/zg/kg/3-endorphin on the development of mutual fighting in pairs of rats submitted to various intensities of electric shock were investigated. /3-Endorphin blocked the development of fighting responses when a low footshock intensity was used, but facilitated it when a high shock intensity was delivered. A detailed analysis of the relationship between shock intensity, baseline of fighting, and effects of fl.endorphin showed that the effects of fl-endorphin were dependent on the behavioral baseline rather than on shock intensity per se. © 1985AcademicPress, Inc.
Several reports have shown that endogenous opioid systems may influence acquisition of new behavior and performance of learned behavior. Thus, when administered immediately after training of active or passive avoidance responding, opiate antagonists improved retention performance (GaUagher, 1982). In contrast, post-trial administration of an opiate agonist impaired retention (Martinez and Rigter, 1980). These effects are mediated via classical mu opiate receptors since simultaneous postacquisition administration of naloxone and morphine or /3-endorphin had no effect (Izquierdo et al., 1980). Using post-trial treatments, we have recently demonstrated the involvement of endogenous opioid systems in the development of mutual fighting between rats shocked in pairs (shock-induced fighting). The emergence of this behavior over repeated sessions of shock administration was facilitated by post-trial injection of 2 mg/kg naloxone and blocked by I0/zg/kg fl-endorphin (Tazi, Dantzer, Mormede, & Le Moal 1983). The incidence of shock-induced fighting is sensitive to environmental factors such as the size of the chamber and physical characteristics of electric shock. Concerning this last parameter, an inverted U-shape reAddress all correspondence and requests for reprints to Dr. A. Tazi. 322 0163-1047/85 $3.00 Copyright© 1985by AcademicPress, Inc. All rightsof reproductionin any formreserved.
fl-ENDORPHIN EFFECTS ON FIGHTING DEPEND ON BASELINE
323
lationship has been described between shock intensity and fighting, with an optimum around 2-2.5 mA (Ulrich and Azrin, 1962). Since the effects of post-training hormonal treatments have been found to depend on the intensity of electric shock used during conditioning (Gold and Van Burskirk, 1976), we decided to systematically assess the effects of a fixed dose of /3-endorphin (10/zg/kg) on the development of fighting induced by various shock intensities. One hundred and thirty four male Sprague-Dawley albino rats (IFFACREDO, Lyon), weighing about 250 g at the start of the experiment, were used. They were housed individually or two to four per cage, with water and food freely available. The colony room was maintained at 2223°C, with a 12-h on/off light cycle. All experiments were run between 9:00 and 12:00 AM. The methods have already been described (Tazi et al., 1983). Testing was carried out in a 30 × 30 × 40-cm opaque plastic chamber with a transparent cover to allow visual observation of the animals. This chamber was housed within a well-lit, sound-attenuated cabinet. Different intensities of shock were delivered by a scrambled constant current shocker (BRSForinger) programmed to administer 10 trains of 10 shocks (1, 1.6, 2. or 3 mA, 0.5 s on/1 s off) every 45 s to the stainless-steel-grid floor. Ten days after their arrival, animals were assigned to weight-matched pairs for testing, with the difference between members of the same pair not exceeding 10 g. When animals were housed two to four per cage, pairs were made up with animals from different cages. Test sessions lasted 10 min and were run daily for 10 consecutive days. During the test session, an observer sitting in the cabinet classified the behavior of the animals during each train of shocks (a "trial") into one of the three mutually exclusive categories (Stolk, Conner, Levine, & Barchas, 1974): no interaction (mainly escape attempts or freezing postures); upright position (mutual upright posture, without physical contact); or fighting (mutual upright posture accompanied by striking with forepaws). Only this last category was retained for subsequent analysis since it had been shown to be sensitive to fl-endorphin treatment in previous studies (Tazi et al., 1983). The reliability of behavioral scoring was regularly checked by a second observer. Treatments were administered at the end of the first nine sessions. /3-Endorphin (a gift from Dr. N. Ling, Salk Intitute) was dissolved in aliquots of 50/zg which were kept at - 80°C until use. Each aliquot was stored at -20°C on the day before use and was dissolved in saline on the day of use. Plastic tubes containing the solution were stored on ice during the course of the experiment and fresh solutions were prepared every day. All injections were given subcutaneously, in a volume of 2 ml/kg. Treatments were arbitrarily coded so that the observer was
324
TAZI ET AL.
naive as to the treatment received by each pair of rats. The two members of each pair received the same treatment. Data, expressed as number of episodes of fighting per session, were submitted to a two-way analysis of variance with repeated measurements. Figure 1A represents fighting episodes in experiments conducted under various shock intensities. In saline-treated animals, the shock-response relationship displayed an inverted U shape, with maximal fighting occurring at 2 mA shock intensity. For the 1.6- and 2-mA shock intensities, posttraining injections of 10/zg/kg/3-endorphin resulted in an impaired acquisition of fighting responses, evidenced by a significant session x treatment interaction: F(9, 54) = 3.10, p < .01. and F(9, 207) = 7.20, p < .001, respectively. When rats were tested with 3 mA shock intensity, /3-endorphin facilitated the development of shock-induced fighting (F(9, 108) = 4.22, p < .001). In another stock of rats tested 4 months later, purchased from the same breeder, housed under the same conditions, and handled in the same way, exposure of pairs of animals to 2-mA shocks resulted in a lower baseline of SIF (Fig. I B). Under these conditions, /3-endorphin facilitated instead of impaired the development of SIF (F(8, 128) = 3.65, p < .001). The present results confirm that the relation between shock intensity and fighting responses has the shape of an inverted U curve (Ulrich and Azrin, 1962). Since the asymptote of the curve is reached at 2 mA, slight
[-~
Placebo p-endorphin
~ 25
, , ~ 1
1.6
2
3
Shock intensity (mA)
FIG. 1. Mean fighting responses (expressed as percentage of shock presentations) in saline- and/3-endorphin-treated rats, under various shock intensities. The number of pairs of animals used in each experiment is indicated at the top of each column, and the bars represent the standard error of the mean.
/3-ENDORPHIN EFFECTS ON FIGHTING DEPEND ON BASELINE
325
variations in sensitivity of rats to this shock level will result in large variations in the frequence of shock-induced fighting. This may explain the variability in SIF which was observed between different stocks provided by the same supplier at different intervals. Whatever the case, the effects of post-trial administration of/3-endorphin appear to depend mainly on baseline of SIF. In accordance with our previous results (Tazi et al., 1983), high levels of SIF are depressed by post-trial injection of/~-endorphin. In contrast, low levels of SIF are enhanced by the same treatment, but only if the behavior has a sufficient probability of appearance. The dependency of the effect of opiates on behavioral baseline has already been demonstrated (Classen & Mondadori, 1984). Several hypotheses may account for this phenomenon. One possibility is that /3-endorphin interacts with some hormonal and/or neurochemical system modulating the acquisition and retention of SIF. A likely candidate is the central catecholaminergic system, which has already been implicated in the effects of opiates on learning and memory (Izquierdo & Dias, 1984). According to this interpretation,/3-endorphin would act by inhibiting the modulatory effect of this system on SIF. Another possibility is that /3-endorphin is not active per se, but is active via metabolic fragments, such as ~- and y-endorphins, which have been found to exert opposite behavioral effects in aversively motivated tasks (Kovacs, Bohus, & De Wied, 1983). According to this interpretation, the differential effect of/3-endorphin would be mediated either by different biotransformation rates according to experimental conditions, or by differential effects of each metabolite according to behavioral baseline. Further experiments are therefore needed to elucidate the mechanisms of action of/3-endorphin on SIF and its site of action.
REFERENCES Classen, W., & Mondadori, C. (1984). Facilitation or inhibition of memory by morphine: A question of experimental parameters. Experientia, 40, 506-509. Gallagher, M. (1982). Naloxone enhancement of memory processes: Effects of other opiate antagonists. Behavioral and Neural Biology, 35, 375-382. Gold, P. E., & Van Burskirk, R. (1976). Enhancement and impairment of memory processes with post-trial injections of adrenocoticotrophic hormone. Behavioral Biology, 16, 387400. Izquierdo, I., & Dias, R. D. (1984). Involvement of a-adrenergic receptors in the amnestic and anti-amnestic action of ACTH, /~-endorphin and epinephrine. Psychoneuroendocrinology, 9, 77-81. Izquierdo, I., Dias, R. D., Souza, D. O., Carrasco, M. A., Elisabetsky, E., & Perry, M. L. (1980). The role of opioid peptides in memory and learning. Behavioral Brain Research, 1, 451-468. Kovacs, G. L., Bohus, B., & De Wied, D. (1983). Effects of/3-endorphin and its fragments on inhibitory avoidance behavior in rats. Psychoneuroendocrinology, 8, 411-419. Martinez, J. L., Jr., & Rigter, H. (1980). Endorphins alter acquisition and consolidation of an inhibitory avoidance response in rats. Neuroscience Letters, 18, 197-201.
326
TAZI ET AL.
Stolk, J. M., Conner, R. L., Levine, S., & Barchas, J. D. (1974). Brain norepinephrine metabolism and shock-induced fighting behavior in rats: Differential effects of shock and fighting on the neurochemical response to a common footshock stimulus. 1'he Journal of Pharmacology and Experimental Therapeutics. 190, 194-209. Tazi, A., Dantzer, R., Mormede, P., & Le Moal, M. (1983). Effects of post-trial administration of naloxone and/3-endorphin on shock-induced fighting in rats. Behavioral and Neural Biology, 39, 192-202. Ulrich, R. E., & Azrin, N. H. (1962). Reflexive fighting in response to aversive stimulation. Journal of Experimental Analysis of Behavior, 5, 511-520.