The purpose of the present study was to analyze the effect of activation of mu-opioid receptors (mu-OR) on the immune response under blockade of postsynaptic D1-and D2-receptors in mice of the C57BL/6J strain displaying either aggressive or depressive-like behaviors in the social conflict model. It is shown that activation of activation of mu-OR with a highly selective agonist DAGO (100 μg/kg) increased significantly IgM-immune response not only in C57BL/6J mice with an unchanged psychoemotional state but also in mice displaying aggressive or depressive-like behaviors in the social stress model (10 days of agonistic confrontations). Selective blockade of DA receptors of the D1-type with SCH-23390 (1.0 mg/kg with DAGO administration) caused a more pronounced elevation of IgM-immune response than DAGO alone while DAGO effect was completely blocked by prior administration of D2-receptor antagonist haloperidol (1.0 mg/kg). At the same time, both SCH-23390 and haloperidol prevented the immune response increase induced by DAGO injection in mice engaged in aggressive or depressive-like behaviors. Thus, in animals not subjected to social stress DAGO-induced immunostimulation is provided only by D2-receptors, whereas in animals with altered psychoemotional state mu-opioid immunostimulation is mediated by both types of DA receptors—D1 and D2. These data provide evidence for different impacts of the main subtypes of DA receptors in the mediation of immunomodulating effects of mu-opioid system under normal and stressful conditions.
At present there is strong evidence for the involvement of central mu-opioid system in immunomodulation [1-5]. A series of neurophysiological and neuropharmacological studies indicate that immunomodulatory effects of this system are mediated by the DAergic mechanisms [4-8] which are known to provide immunostimulation [9,10]. Our previous data have shown that the nigrostriatal (nucleus caudatus) and mesolimbic (nucleus accumbens) DAergic structures are playing an important role in immunostimulation [
These brain structures have also been found to contain significantly high amounts not only of DA D1- and D2- receptors [
Recent data have shown that psychoemotional state of animals and humans can significantly affect immune functions [16-19]. At the same time, mu-opioid and DA mechanisms have been found to be implicated in the regulation of psychosocial stress [17,20-22].
In this connection, a role for the main types of DA receptors in DAGO-induced immunostimulation needs to be examined not only in normal healthy animals but also in animals subjected to psychoemotional stress. Therefore, the purpose of the present study was to analyze the effect of activation of mu-OR on the immune response under blockade of postsynaptic D1- and D2-receptors in mice of the C57BL/6J strain displaying either aggressive or depressive-like behaviors in the social conflict model.
The experiments utilized 122 male mice of the C57BL/6J strain weighing 22 - 24 g. The animals were maintained at the State Research Institute of Physiology and Basic Medicine SB RAMS and were housed under standard vivarium conditions and a natural light regime. Food and water were available ad libitum.
The study was performed in compliance with principles of the declaration of Helsinki and was approved by the local Ethics Committee of the State Research Institute of Physiology and Basic Medicine SB RAMS.
To produce aggressive and submissive behaviors in C57BL/6J mice, the model of sensory contact was used [
The group with an unchanged psychoemotional state comprised the group-housed males, after 5 days of individual housing, since, in this case, the submissiveness of grouped C57BL/6J had already disappeared while the repeated experience of aggression had not yet been acquired [20,]">23].
Selective activation of mu-OR was performed by a structural analogue of enkephalin DAGO [D-Ala2, N-Me-Phe4, Gly5-ol]enkephalin (Sigma, Germany) at a dose of 100 µg/kg. To block postsynaptic DA D1- or D2-receptors their highly specific antagonists SCH-23390 [R(+)- 7-hloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro- 1H-3-benzazepinehydrochloride] (Sigma, Germany) and haloperidol (Gedeon Richter A.O., Hungary) respectively were used, both drugs at a dose of 1.0 mg/kg. Drugs were dissolved in saline and injected once intraperitoneally in a final volume of 0.2 ml. DAGO was administered 30 min prior to immunization, SCH-23390 or haloperidol— 5 - 10 min before DAGO. The doses for each drug and the routs of their administration used in the present study were chosen based on those previously reported to affect the corresponding receptor and immune reactivity [
Animals not subjected to social stress and mice engaged in aggression or depression were divided into groups receiving: 1) Vehicle (control); 2) DAGO alone; 3) Combination of SCH-23390 + DAGO; 4) Combination of haloperidol + DAGO.
All groups of mice were immunized with sheep red blood cells (SRBC), which were suspended in saline and injected in the tail vein at a dose of 5 × 108 cells in 0.5 ml.
The immune response was assessed by measuring the number of antibody-forming cells (IgM-AFC) [
The significance of the mean differences between experimental groups (set at p < 0.05) was first analyzed by one-way analysis of variance (ANOVA) with subsequent multiple comparisons by the Student t-test using Statistics for Windows ver.10.0. All data are expressed as the mean ± SEM.
Activation of mu-OR with a highly selective agonist DAGO (100 µg/kg) produced a significant increase of IgM-immune response (F(1,12) = 79.99; p < 0.001) in C57BL/6J mice having no experience of social encounters when compared to the vehicle-injected group (
As is seen in
DAGO at 100 µg/kg administered alone have been also found to increase IgM-immune response in mice conditioned to display aggressive behavior during 10 tests of daily confrontations (F(1,13) = 28.22; p < 0.001) compared to aggressive animals that did not receive the drug (control) (
Mu-OR activation with DAGO (100 µg/kg) also led to immunostimulation in mice with experience of 10 defeats in daily confrontations compared to depressive animals not treated with DAGO (control) (F(1,13) = 124.65; p < 0.001) (
According to the present data, C57BL/6J mice at early stage of depression (10 days of confrontations) showed increased IgM-immune response when DAGO was combined with one of the DA antagonists: (F(1,12) = 38.4; p < 0.001) for SCH-23390 and (F(1,12) = 19.75; p < 0.001) for haloperidol compared to the control (
It is well established that neuromediator/neuromodulator mu-opioidergic system is playing an important role in psychoneuroimmunomodulation. There is increasing evidence that activation of mu-OR by agonists of different
origin may produce a wide variety of effects on immune parameters [2,3,25-7]">27]. It has been shown earlier that administration of the most widely used agonist of mu-OR DAGO significantly increased the intensity of the immune response in mice of the CBA strain and Wistar rats with an unchanged psychoemotional state [1,4,9,26]. According to our previously reported data, the immunostimulatory effect of DAGO is mediated by central mechanisms via the hypothalamus-hypophysis complex [1,4].
In the present study DAGO-induced immunostimulation has been found in C57BL/6J mice having no experience of social confrontations or conditioned to display aggressive or depressive-like behavior during 10 tests of daily social encounters. This effect was more pronounced in depressive mice than that of unconditioned (control) or aggressive animals.
To date, the immunomodulatory effects of mu-agonists are known to be mediated by the DAergic mechanisms with the involvement of brain D1- and D2 receptors [4,5,7,8]. It should be noted that close mu-opioid/DA interconnections in the brain structures have been shown to be particularly important for the regulation of different physiological functions including behavioral [28,9]">29] and immune responses [4,5,7-9].
Our earlier data and results presented here demonstrate a significant role for the DA D2-receptors in mediating DAGO-induced activation of immune responsiveness in mice of the CBA [4,7] and C57BL/6J strains, which were not subjected to social stress. This conclusion is confirmed by the finding that the immunostimulatory effect of DAGO was completely blocked by D2-receptor antagonist haloperidol (1.0 mg/kg) while it remained unaffected after the blockade of D1-receptors with SCH- 23390 (1.0 mg/kg). At the same time, previous animal studies have indicated that both antagonists, which are known to block central DA receptors [
Unlike mice having no experience of social confrontations, the effect of DAGO on immunity was prevented by antagonists for the two receptor types in C57BL/6J mice showing either aggressive or depressive-like behaviors. Although, co-administration of DAGO with either D1- or D2-receptors antagonist (SCH-23390 and haloperidol, respectively) did not completely block the effect of activation of mu-OR on the immune response in animals with aggressive and depressive behaviors, these results indicate the requirement for D1- and D2-receptors in the mediation of DAGO-induced immunostimulation under psychoemotional stress.
Aggressive and depressive-like behaviors are known to be associated with changes in the level and distribution of serotonin (5-HT) and DA and their metabolites over brain structures [17,20,30-32], which appeared to be involved in the mechanisms of immunomodulation [9,10, 17]. There is also evidence for significant differences in immune reactivity of C57BL/6J mice engaged in aggression or depressive-like behavior [17,33]. Numerous data indicate that aggression is characterized by increasing activity of the DAergic system, known to stimulate immune functions, while depressive behavior is accompanied with changes in activity of the 5-HTergic system providing an inhibitory mechanism of immunomodulation [17,31,32].
Consistent with previous results [17,21,33], the present study has demonstrated that aggressive animals showed a higher immune responsiveness compared to animals with depressive-like behavior. At the same time depressivelike behavior is associated with the decreased immune function relative to that of controls and aggressive mice.
Despite the existing facts on the differences of the neurochemical pattern of the brain as well as the immune system functioning in animals displaying aggressive or depressive-like behaviors, our data indicate that the activation of mu-OR produced similar immunostimulatory effect in C57BL/6J mice with opposite types of behavior.
The enhanced immune response induced by the muOR agonist in aggressive animals is likely to be associated with further activation of the DAergic system. Immunostimulation found in mice with depressive-like behavior after DAGO injection seems to be also DAdependent due to the changes in the balance between functionally linked 5-HTand DAergic systems with the domination of the latter. Moreover, as is shown in the present study, DAGO-induced stimulation of immune response observed in mice with the opposite types of behavior is mediated by DA receptors of the D1- and D2-types.
Thus, our results provide evidence for different impact of the main subtypes of DA receptors in the mediation of immunomodulating effects of mu-opioid system under normal and stressful conditions. In animals not subjected to social stress DAGO-induced immunostimulation is provided only by D2-receptors, whereas in animals with altered psychoemotional state mu-opioid immunostimulation is mediated by both types of DA receptors—D1 and D2. These data give a new insight in the receptor mechanisms of the interactions between mu-opioid and DA systems, in which changing activity may contribute to a variety of neurological and psychiatric diseases [34,35], known to be associated with immune dysfunctions [36,37].