ROLE OF DOPAMINE RECEPTORS IN THE REGULATION OF AGGRESSION IN MICE; RELATIONSHIP TO GENOTYPE 1~. M. Nikulina and N. S. Kapralova

UDC 591.513+612.822.3+615.78

The interline differences in the manifestation of aggression evoked by stimulation was studied in mice of eight inbred lines, and the role of different types of dopamine (DA) receptors in its manifestation was investigated. Aggression was assessed in a test involving the effect of a weak electrical stimulation through the floor of the cage. A significant relationship to the animals' genotype was demonstrated, and low-aggression (C3h/He, DD, BALB/c, and AKR) and high-aggression (CBA, DBA/2, and CC57Br) lines could be distinguished on the basis of the level of aggressivity. The mixed agonist of DA receptors, apomorphine, in a one-time administration activated aggressivity in the low-aggression mice. The selective stimulation of D2-receptors with bromocriptine substantially increased the evoked aggressivity in the low-aggression mice; the blockade of D 2receptors by sulpiride decreased or prevented the manifestation of aggressivity in the high-aggression lines. At the same time, the selective Dl_agonist SKF 38393 and the selective Dl-antagonist SCH 23390 did not exert a substantial influence on evoked aggressivity. Evidently the D2-receptors play a key role in the control of aggression evoked by stimulation, which constitutes a model of affective aggression.

Aggression is exhibited in rodents in the presence of a weak painful electrical stimulation, when previously unaggressive animals assume a fighting posture in the presence of a partner [6]. Aggression evoked by stimulation is the most frequently used model of affective aggression in psychopharmacological investigations. The study of the mechanisms of stimulationevoked aggression has revealed the participation of the brain dopaminergic system and of its regulation [1, 14]. The possibility of identifying the role of the principal types of receptors in affective aggression, the Dl-receptors which are associated with adenylate cyclase activity, and the D2-receptors, which are not associated with adenylate cyclase or inhibit it [12], has come to pass with the appearance of highly specific ligands for different types of dopamine receptors. At the same time, the study of the role of the different types of receptors in the manifestation of stimulation-evoked aggression has not been carried out. Since it has been demonstrated that the functioning of the dopaminergic system may be governed by genotype [5, 18], it seemed of interest to carry out such an investigation in animals of different lines. The objective of the present study was the investigation of the interline differences in the manifestation of stimulation-evoked aggression in inbred mice, and the elucidation of the type of dopamine receptors through which its control is realized. The pharmacological stimulation or blockade of the D1- or the D2-receptors by specific preparations was employed for this purpose. METHODS The experiments were carried out on sexually mature mice of the C3h/He, DD, BALB/c, AKR, C57BL/6, CBA, DBA/2, and CC57Br lines, three to four months of age, weighing 20-32 g. The mice were kept in groups of eight animals each in a cage, with free access to water and food. Two to three days before testing the animals were placed in individual cages measuring 26 x 14 x 10 cm to eliminate zoosocial influences. The mice were used in the experiment once. The testing of the mice both for initial aggressivity and during the action of the preparations was done in blocks, in which the animals of several, as a rule three, lines were brought simultaneously into the experiment. Each group of animals included 8-16 pairs of mice. The experiments were carried out at the same time, in the morning hours.

Institute of Cytology and Genetics, Siberian Division, Russian Academy of Sciences, Novosibirsk. Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 41, No. 4, pp. 734-740, July-August, 1991. Original article submitted April 28, 1990; revision submitted October 31, 1990. 364

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Fig. l. Manifestation of aggressivity induced by painful electrical stimulation in mice of inbred lines. The number of pairs of tested animals are designated here and the other figures by numbers; along the vertical, the number of attacks during the test. Fig. 2. Influence of apomorphine (2.5 mg/kg) on aggressivity induced by painful electrical stimulation in mice of different inbred lines. Here and in Figs. 3 and 4, *, p < 0.05; **, p < 0.01; ***, p < 0.001, as compared with the adminislration of physiologica! solution (white columns). Fig. 3. Activating effect of bromocriptine (10 mg/kg) on evoked aggressivity in mice of low-aggression lines. Fig. 4. Changes in aggressivity induced by painful electrical stimulation in highly-aggressive mice of different lines, following the blockade of D2-receptors by sulpiride (20 mg/kg).

The testing for stimulation-evoked aggression was carried out in accordance with the methods of [2], with certain modifications. A pair of mice of the same line of approximately identical weight were placed in a special cage (9 • 9 • 14 cm); a current (0.2-0.4 mA) was passed through its floor; the current was delivered in pulses, 0.1 sec in duration, every 0.15 sec. Following three minutes of adaptation in the test cage, the test, which lasted 3 min, was carried out; the number of attacks during this time was counted. Contacts which were manifested in the form of vertical "boxer's" stances and protection of the flanks were considered aggressive. 365

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Fig. 5. Aggression induced by painful electrical stimulation following the administration of the agonist of Dl-receptors (A) SKF 38393 (10 mg/kg) and of antagonists of Dl-receptors (B) cis-flupentixol (0.2 mg/kg, horizontal hatching) and SCH 23390 (0.2 mg/kg, double hatching) in mice of different lines. White columns, control. TABLE 1. Effects of Agent on D2-Receptors in the Manifestation of Aggression Induced by Painful Electrical Stimulation in Mice of Different Lines Lines of mice

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*, p < 0.001, as compared with the administration of physiological solution. The following preparations were used in the experiments: apomorphine hydrochloride, bromocriptine, sulpiride (all Sigma, USA); SKF 38393 (Smith, Kline & French Labs., Belgium); SCH 23390 (Schering, USA); and cis-flupentixol (Lundbeck, Denmark). All the preparations, with the exception of sulpiride, were dissolved in distilled water immediately prior to administration. The solutions of the preparations were administered in a volume of 1 ml per 100 g of weight. The sulpiride was dissolved in a drop of glacial acetic acid; the solution was brought up to the necessary volume with distilled water; 5N NaOH was used to neutralize the acid. The testing was carried out 10 min after the administration of the apomorphine; 20 min after the administration of the SKF 38393, SCH 23390, and cis-flupentixol; and 90 min after the administration of the bromocriptine, since the maximum of its action falls within that time period [11]. The control mice were administered physiological solution in the same volume as the preparations. Statistical analysis results were obtained by using variance analysis and Fisher's and Student's tests. INVESTIGATION RESULTS The manifestation of aggression evoked by painful electrical stimulation in the mice of the eight inbred lines depended on genotype. A univariate analysis of variance (source of variation, the genotype) demonstrated high significance of the influ-

366

ence of the genotype on evoked aggression (Fisher's test F(7.81) = 16.59, p < 0.001). All the mice investigated with respect to the level of current-evoked aggression could be divided into low-aggression (C3h/He, DD, BALB/c, and AKR), which are characterized by a small number of attacks during the test (from 0.2 _+0.1 to 1.9 + 0.5) and high-aggression (CBA, DBA/2, and CC57Br) (from 6.3 + 1.1 to 10.1 + 1.0) lines; and the C57BL/6 line occupies an intermediate position between the two extreme groups (Fig. 1). The administration of physiological solution, which was used as the control for the administration of the preparations did not altered the current-induced aggressivity, and the number of attacks did not differ significantly from their number when the intact animals were tested. When administered in a dose of 2.5 mg/kg, apomorphine, an agonist of dopamine receptors, increased the manifestation of aggressivity in mice of the DD, AKR, and C3h/He lines as compared with the control. The increase in the number of attacks in mice of the BALB/c line following the administration of apomorphine was found to be insignificant; consequently, the effect of apomorphine depends on the line of animals tested (Fig. 2). The administration of the selective agonist of D2-receptors, bromocriptine, in a dose of 10 mg/kg, increased the number of attacks in mice of the low-aggression lines (C3h/He, DD, BALB/c, and AKR) with high degree of significance (Fig. 3). In mice of the C57BL/6 line, which is characterized by a medium level of aggressivity, bromocriptine did not alter the number of aggressive attacks as compared with the control. This probably attests to differences in the sensitivity of the D2-receptors in mice of different genotypes. The administration of the D2-receptor antagonist, sulpiride, in a dose of 20 mg/kg, exerted the opposite effect on evoked aggressivity. Sulpiride substantially reduced the number of aggressive confrontations in the high-aggression CBA, DBA/2, and CC57Br mice and in the C57BL/6 mice with medium aggression (Fig. 4). The stimulation of the D2-receptors against the background of the preliminary administration of sulpiride to low-aggression DD and BALB/c mice, which demonstrate the most intense (by a factor of 9) increase in the number of attacks following the administration of bromocriptine alone, did not reveal a significant influence of bromocriptine on the aggressivity of the mice of this line. Sulpiride removes the activating effect of bromocriptine; this reflects the influence of bromocriptine on aggressivity through the D2-receptors (Table I), In the case of the pharmacological stimulation of the Dl-receptors by SKF 38393 in a dose of 10 rag&g, a significant increase in aggressive confrontations was not observed in the low-aggression in the animals of the DD and BALB/c lines. The blockade of the Dl-receptors by antagonists cis-flupentixol (0.2 mg/kg) and SCH 23390 (0.2 mg/kg) in the high-aggression CBA and DBA/2 lines also did not lead to a change in evoked aggressivity (Fig. 5). DISCUSSION OF RESULTS The results of these experiments showed that mice of different genotypes clearly differed with respect to the expressivity of aggressivity evoked by painful electrical stimulation. While in the mice of some lines aggressive contacts were practically not observed with weak shocks of current, other lines manifested quite intense aggression. The differences in behavior within each line were significantly less than between lines, and the high value of Fisher's test confirms the substantial contribution of genotype to the manifestation of stimulation-evoked aggression. The existence of interline differences in mice has been demonstrated for various types of aggressivity, predatory [3, 71, intermale [2], evoked by a large dose of clonidine [4]. A dependence on genotype has also been demonstrated for stimulation-evoked aggression in an investigation carried out on three lines of mice [15]. The interline differences in stimulation-evoked aggression, as in other types of aggressivity, are apparently governed by a genetically determined different state of the neuromediator systems which regulate affective aggression. The mixed agonist of dopamine receptors, apomorphine, intensified current-evoked aggression in the mice and this effect depended on the animal's genotype. The activation by apomorphine of aggression evoked by painful electrical stimulation has been previously demonstrated in rats [1] and mice [15]. Genotypic differences in sensitivity to apomorphine were found in mice when locomotion was recorded [21], and the results we have obtained confirm these data. It should be noted that the use of lineal mice with a varied level of aggressivity provides the opportunity of investigating preparations which both increase and inhibit aggressivity; this was in fact done in the study of the participation of the different types of dopamine receptors in aggressivity. The selective analysis of the role of the dopamine receptors with the use of specific preparations showed that the selective stimulation of D2-receptors by bromocriptine in low-aggression animals leads to a sharp increase in the number of attacks. Since the effect of bromocriptine was to a large degree similar to the influence of apomorphine, which was administered in our experiments in a dose acting on postsynaptic receptors [8], it can be assumed that in increasing aggressivity bromocrip367

tine stimulates receptors located postsynaptically. The D2-receptor antagonist sulpiride exerted a distinctly expressed opposite effect on evoked aggressivity; it decreased or prevented its manifestation of aggressivity in mice of the high-aggression lines, The series of experiments involving the preliminary blockade of the D2-receptors is yet another convincing evidence of the participation of the D2-receptors in evoked aggression. Bromocriptine did not induce a sharp increase in aggressivity against the background of sulpiride in low-aggression animals in whom the administration of the agonist alone led to the aggression which was observed only in high-aggression mice. At the same time, the effects on the Dl-receptors, both with the administration of the agonist SKF 38393 and the administration of the selective antagonist SCH 23390 and the mixed antagonist with a primary influence on the Dl-receptors, cisflupentixol [10] did not influence the manifestation of affective aggression. The stimulation of the Dl-receptors did not induce aggression in low-aggression mice, while the blockade of the Dl-receptors did not exert an effect on the behavior of the high-aggression animals. In this connection it is pertinent to note that doses of the Dl-preparations we used were chosen as effectively influencing other types of behavior, such as locomotor and stereotypic activity [9, 13, 19]. Thus, the D2-receptors play a key role in stimulation-evoked aggression in mice, which is a model of affective aggressivity. These results are of special interest since it is known that the effectiveness of neuroleptics as antipsychotic substances is associated with their effect on the D2-receptors [17]. An important role of the D2-receptors was identified in a recently published paper in which intraspecies aggression, which has a defensive component, was studied [16]. A pharmacological analysis which was carried out on C57BL/6 mice revealed the involvement of the D2-receptors in the expression of defense behavior in the mice. It should be noted that the defensive behavior has a different adaptive aspect than stimulation-evoked aggression. The state of affect was dramatically expressed in the type of behavior we studied. Evidently the increased functional sensitivity of the D2-receptors induces the manifestation of aggression both in defensive behavior and in the presence of painful stimulation. CONCLUSIONS 1. Significant differences, governed by genotype, in the manifestation of aggression evoked by painful electrical stimulation were found in mice of inbred lines. 2. Selective analysis of the participation of the dopamine receptors in the control of stimulation-evoked aggressivity revealed the important role of the D2-receptors, since the administration of the D2-agonist bromocriptine activated it, while the antagonist sulpiride significantly decreased the number of aggressive contacts. 3. Pharmacological analysis did not reveal the participation of the Dl-receptors in stimulation-evoked aggression, since neither the Dl-agonist SKF 38393 nor the selective Dl-antagonist SCH 23390 of the Dl-receptors exerted an influence on its manifestation. LITERATURE CITED 1.

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