Social defeat stress,sensitization, and intravenous cocaine self-administration in mice

Rationale Behavioral sensitization has been proposed as a process that is important in compulsive drug use and in psychotic disorders. Objective The present experiments examine the relationship between behavioral sensitization, induced by either social defeat or amphetamine, and intravenous cocaine self-administration in mice. Materials and methods Male CFW mice were exposed either to defeat experiences, amphetamine (2.5 mg/kg, i.p.) or saline (i.p.) every day for 10 days. Ten days after the last defeat or injection, mice were challenged with varying doses of amphetamine (1.0–2.5 mg/kg i.p). Mice were then trained to nose poke for intravenous cocaine (1.0 mg/kg/inf) during daily 3-h sessions. Following this acquisition phase, the animals self-administered varying doses of cocaine (0.3–1.8 mg/kg/inf) or were allowed to self-administer cocaine (0.3 mg/kg/inf) according to a progressive ratio schedule of reinforcement. Results Repeated social defeat produced a sensitized motor response to a single challenge of 1.5 mg/kg amphetamine and to a cumulative dosing of amphetamine. Amphetamine-pretreated mice exhibited increased cocaine self-administration during acquisition and elevated break points during performance on a progressive ratio schedule of reinforcement relative to stress-sensitized and control animals. Conclusions These data extend the evidence from rats to mice for the process of sensitization leading to more cocaine taking. Contrary to what is seen in rats, increased levels of cocaine self-administration were seen only in the amphetamine-pretreated mice and not after repeated defeat stress, suggesting that the sensitized response to defeat stress may not be as robust as it is in rats in this particular strain of mice.     

Behavioral sensitization to cocaine after a brief social defeat stress: c-fos expression in the PAG

The experiments explored the nature and time course of changes in behavior and Fos expression in the periaqueductal grey area (PAG) in response to an injection of cocaine that was given following a single episode of social defeat stress. Social defeat stress was defined as an intruder mouse’s response to an aggressive resident mouse. First, the intruder was briefly attacked, and secondly, it was threatened while protected by a perforated cage for 20 min. Plasma corticosterone levels rose after the beginning of the confrontation and remained elevated during the protected phase. In a first experiment, separate groups of intruder and control mice were challenged once with cocaine (20, 30, or 40 mg/kg) or saline. During tests for motor activity, behavioral measurements were obtained via (1) photobeam interruptions, (2) tracking of movements via image analysis, and (3) quantitative ethological analysis of postures and acts via videorecords. Several indices of ambulatory or horizontal forward locomotion confirmed the stimulant effects of cocaine. In a further experiment, separate groups of mice were challenged with 40 mg/kg cocaine at one time point, either during the social stress or 3, 5, 7 or 9 days thereafter. A cocaine challenge significantly increased locomotion 5 and 7 days after a brief social defeat stress, in excess of the level that is seen in non-stressed animals. Further experiments used immunohistochemical assays of sections through the caudal ventrolateral PAG and showed a significant increase in Fos-like immunoreactivity (Fos-LI) 1 h after the social stress experience or after cocaine. Importantly, concurrent administration of cocaine with social defeat stress produced inhibition of Fos expression throughout the PAG. A partial to complete recovery of cocaine-induced Fos expression was observed 5–7 days after social defeat stress. The results suggest that a single social stress episode is sufficient to engender a delayed sensitization of stimulant hyperactivity. The initial inhibition of Fos expression by concurrent social stress and cocaine may point to a relevant initiating event in the process of sensitization to stimulants. Received: 30 July 1997/Final version: 15 June 1998     

Development and expression of sensitization to cocaine's reinforcing properties: role of NMDA receptors

Acquisition of cocaine self-administration (0.125, 0.25 or 0.5 mg/kg/infusion) was assessed in rats that had received prior exposure to either saline or amphetamine (2.0 mg/kg). Acquisition of self-administration was dose-dependent, with the highest dose leading to the shortest latency to reliably discriminate between depression of a lever that resulted in drug delivery and an inactive lever. Latency to acquisition of the lever discrimination for rats that had received prior exposure to amphetamine was shorter than for the saline-pretreated counterparts in each cocaine dosage group. This suggests that repeated exposure to this drug prior to self-administration testing sensitized the rats to the reinforcing effects of cocaine. Co-administration of MK-801 (0.25 mg/kg, IP), a non-competitive NMDA antagonist, blocked the ability of chronic exposure to amphetamine to sensitize rats to cocaine. In experienced self-administering rats, acute pretreatment with MK-801 resulted in a loss of discriminative responding. The number of inactive lever responses was consistently higher than the number of active lever responses across all cocaine dosage groups. These data suggest that the NMDA receptor, possibly through interactions with dopamine systems, is critical for both the development and expression of sensitization to cocaine's reinforcing effects produced by intermittent preexposures to amphetamine.     

Nicotine-induced conditioned place preference in rats: Sex differences and the role of mGluR5 receptors

To elucidate sex differences in nicotine addiction and the underlying mechanisms of the conditioning aspects of nicotine, nicotine-induced conditioned place preference (CPP) was evaluated in male and female Sprague Dawley rats using a three-chambered CPP apparatus and a biased design. In a series of experiments, the dose-response curve was obtained, pairings between the drug and initially non-preferred versus preferred compartments were compared, and the involvement of mGluR5 receptors in nicotine-induced CPP was evaluated. Modulation of nicotine-induced CPP with mGluR5 inhibition was obtained by MPEP (2-methyl-6-(phenylethynyl)-pyridine hydrochloride). Our results show that nicotine induces CPP dose-dependently in male rats but not in female rats. The comparison of the biased protocol, pairing nicotine with the initially preferred and non-preferred chambers, indicated that nicotine-induced CPP in male rats under both conditions, but the effect was stronger when nicotine was paired with the initially non-preferred side. The selective mGluR5 antagonist MPEP inhibited nicotine-induced CPP in male rats. In conclusion, the results of the current study in rats demonstrate that the conditioning effect of nicotine is more important in males than in females. Furthermore, in line with reported findings, our results suggest that mGluR5 antagonism may be therapeutically useful in smoking cessation during the maintenance of smoking behavior when conditioning plays an important role, notwithstanding the fact that this effect is observed only in male rats, not in females.     

The mGluR5 antagonist MPEP selectively inhibits the onset and maintenance of ethanol self-administration in C57BL/6J mice

Rationale Many of the biochemical, physiological, and behavioral effects of ethanol are known to be mediated by ionotropic glutamate receptors. Emerging evidence implicates metabotropic glutamate receptors (mGluRs) in the biobehavioral effects of ethanol and other drugs of abuse, but there is little information regarding the role of mGluRs in the reinforcing effects of ethanol. Materials and methods Male C57BL/6J mice were trained to lever-press on a concurrent fixed ratio 1 schedule of ethanol (10% v/v) vs water reinforcement during 16-h sessions. Effects of mGluR1, mGluR2/3, and mGluR5 antagonists were then tested on parameters of ethanol self-administration behavior. Results The mGluR5 antagonist MPEP (1–10 mg/kg, i.p.) dose-dependently reduced ethanol-reinforced responding but had no effect on concurrent water-reinforced responding. Analysis of the temporal pattern of responding showed that MPEP reduced ethanol-reinforced responding during peak periods of behavior occurring during the early hours of the dark cycle. Further analysis showed that MPEP reduced the number of ethanol response bouts and bout-response rate. MPEP also produced a 13-fold delay in ethanol response onset (i.e., latency to the first response) with no corresponding effect on water response latency or locomotor activity. The mGluR1 antagonist CPCCOEt (1–10 mg/kg, i.p.) or the mGluR2/3 antagonist LY 341495 (1–30 mg/kg, i.p.) failed to alter ethanol- or water-reinforced responding. Conclusions These data indicate that mGlu5 receptors selectively regulate the onset and maintenance of ethanol self-administration in a manner that is consistent with reduction in ethanol’s reinforcement function.     

Intense cocaine self-administration after episodic social defeat stress,but not after aggressive behavior: dissociation from corticosterone activation

Rationale:

An intense stress response characterizes both the dominant and submissive individuals during an aggressive confrontation, and these stress responses have enduring neural and behavioral consequences.

Objectives:

In spite of similar glucocorticoid and corticolimbic dopamine activation, dominant and defeated individuals appear to diverge in terms of their drug taking. Do rats that are intermittently subjected to defeat stress become more sensitized to cocaine taking relative to rats that engage in aggressive bouts?

Methods:

Separate groups of male Long-Evans rats were investigated after an initial 10-day period with four brief episodes of social defeat (intruders) or aggressive behavior (residents): (1) the corticosterone responses to the very first and the last confrontations were measured; (2) the locomotor response to an amphetamine (1 mg/kg) challenge 10 days after the last stress exposure served as an index of behavioral sensitization; (3) intravenous self-administration sessions assessed the reinforcing effects of 0.75 mg/kg/infusion cocaine when available after every fifth response (fixed ratio), when delivered after completing progressively more demanding response requirements (progressive ratio; 0.3 mg/kg/infusion), and when available during a 24-h binge of continuous access (0.3 mg/kg/infusion).

Results:

Both social defeat of the intruder rat and attack behavior by the resident rat rapidly increased plasma levels of corticosterone after the first and last aggressive confrontation, indicating no habituation to these types of stress. Intermittent social defeat engenders a sensitized locomotor response to a 1 mg/kg amphetamine challenge and increases cocaine self-administration as indicated by more behavioral effort to obtain cocaine infusions and by accumulating more cocaine during 24 h of continuous access (binge). By contrast, experiences with aggressive behavior do not impact on the motorically activating and reinforcing effects of stimulant administrations.

Conclusions:

The closely similar corticosterone activation in dominant and subordinate rats, followed by divergent patterns of cocaine self-administration indicates that different forms of social stress have dissociable effects on cocaine taking.

     

An animal model of copulatory disorder induced by social stress in male mice: effects of apomorphine and l-dopa

 We investigated the possible role of dopamine receptors in the mediation of copulatory disorder induced by defeat experience in male mice, using l-dopa and apomorphine. To generate the copulatory disorder, male mice were attacked 20 times daily for 5 consecutive days, as intruders in confrontation with an aggressive resident. Following the repeated exposure to defeat, virtually all intruder males failed to display copulatory behavior towards estrous females. Acute injection of apomorphine (25, 50, 75 μg/kg, SC) significantly increased both the incidence and the frequency of copulatory elements (mounting and intromission) in a dose-dependent manner. The combination of l-dopa with carbidopa, a dopa decarboxylase inhibitor, also increased significantly copulatory behavior, revealing an inverted U-shaped dose-effect curve. In both cases, locomotion and digging frequencies were significantly decreased. This evidence suggests that dopaminergic mechanisms are involved in the mediation of social stress-induced copulatory disorder. Received: 21 December 1996 / Final version: 25 April 1997     

NMDA receptors in the rat VTA: a critical site for social stress to intensify cocaine taking

Rationale Cocaine strengthens behaviors associated with its administration. The stress response by individuals that are defeated in a brief aggressive confrontation can also promote enduring behavioral consequences similar to those of stimulants. Objectives The study intends to find whether intermittent episodes of defeat promote cocaine’s reinforcing effects by triggering N-methyl-d-aspartic acid (NMDA)-receptor-mediated plasticity in the ventral tegmental area (VTA). Materials and methods Long–Evans rats were investigated after four social defeats in three experiments. Two experiments examined systemic or intra-VTA antagonism of NMDA receptors during stress on the later expression of behavioral sensitization and cocaine self-administration during fixed and progressive ratio (PR) schedules of reinforcement (0.3 mg/kg/infusion), including a novel 24-h variable-dose continuous access binge (0.2, 0.4, and 0.8 mg/kg/infusion, delivered in an irregular sequence). Third, the expression of receptor proteins NR1 (NMDA) and GluR1 [alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)] were examined in VTA and nucleus accumbens. Results Intermittent defeats augment locomotor responses to cocaine and increase cocaine taking. Rates of responding during binges are increased after defeat stress. These effects are prevented when NMDA or AMPA receptor antagonists are administered before defeats. VTA infusions of the NMDA antagonist AP-5 (5 nmol/side) before stress prevents locomotor sensitization to cocaine and intensified responding for cocaine during a PR schedule or binge. Episodic defeats increase GluR1 AMPA subunit protein expression in the VTA. Conclusions Social stress stimulates NMDA receptors in the VTA, and this neural action of defeat may be essential for prompting a later increase in cocaine intake during binges.     

Prevention of the analgesic consequences of social defeat in male mice by 5-HT1A anxiolytics,buspirone, gepirone and ipsapirone

Behavioural and pharmacological studies have suggested that anxiety may be an important factor in the initiation of non-opioid analgesia in defeated male mice. In the present study, the effects of three 5-HT_1A anxiolytics (buspirone, ipsapirone and gepirone) on basal nociception and defeat analgesia were examined. Results show that the analgesic consequences of social defeat were potently blocked by all three compounds, with a rank-order potency (minimum effective doses) of ipsapirone (0.05 mg/kg) > gepirone (0.1 mg/kg) > buspirone (0.5 mg/kg). These inhibitory effects on defeat analgesia were observed in the absence of intrinsic activity on basal nociception (tail-flick assay). When administered alone, (-)pindolol produced biphasic effects on defeat analgesia with enhancement at 0.5 mg/kg and inhibition at 5.0 mg/kg. Lower doses of (-)pindolol (0.05 and 0.25 mg/kg) which did not affect defeat analgesia when administered alone, totally blocked the inhibitory effects of ipsapirone (0.5 mg/kg). Data are discussed in relation to the involvement of 5-HT_1A receptor mechanisms in this adaptive form of pain inhibition.     

Selective blockade of mGlu5 metabotropic glutamate receptors is hepatoprotective against fulminant hepatic failure induced by lipopolysaccharide and d‐galactosamine in mice

This study was designed to investigate the influence of 2‐methyl‐6‐phenylethynyl pyridine hydrochloride (MPEP), an antagonist of metabotropic glutamate receptor subtype 5, in lipopolysaccharide (LPS) and d‐ galactosamine (d‐ GalN)‐induced fulminant hepatic failure in mice. Mice were given an intraperitoneal injection of 50 µg kg^?1 LPS and 500 mg kg^?1 d‐ GalN. MPEP (1, 5 and 25 mg kg^?1) was administered intraperitoneally 1 h before LPS/d‐ GalN injection. Twenty‐four hours after administration of LPS/d‐ GalN, plasma was collected and used for biochemical assays. Mice were euthanized and histological analysis and toxicological parameters were carried out in the liver. MPEP, at all doses tested, protected against the increase in aspartate and alanine aminotransferase activities induced by LPS/d‐ GalN exposure. Ascorbic acid levels were not altered in all experimental groups. Glutathione S‐transferase activity was increased by administration of LPS/d‐ GalN and MPEP did not modify the enzyme activity in mice. MPEP, at the doses of 5 and 25 mg kg^?1, was effective in protecting against the decrease in catalase activity caused by LPS/d‐ GalN administration in mice. The histological data showed that sections of liver from LPS/d‐ GalN‐exposed mice presented extensive injuries. MPEP, at all doses tested, reduced the scores of liver damage and markedly ameliorated the degree of liver damage. The hepatoprotective effect of MPEP on fulminant hepatic failure induced by LPS and d‐ GalN in mice was demonstrated. Copyright © 2009 John Wiley & Sons, Ltd.     

Social instigation and aggressive behavior in mice: role of 5-HT1A and 5-HT1B receptors in the prefrontal cortex

Rationale  Social instigation is used in rodents to induce high levels of aggression, a pattern of behavior with certain parallels to that of violent individuals. This procedure consists of a brief exposure to a provocative stimulus male, before direct confrontation with an intruder. Studies using 5-HT_1A and 5-HT_1B receptor agonists show an effective reduction in aggressive behavior. An important site of action for these drugs is the ventral orbitofrontal cortex (VO PFC), an area of the brain which is particularly relevant in the inhibitory control of aggressive and impulsive behavior. Objectives  The objectives of the study are to assess the anti-aggressive effects of 5-HT_1A and 5-HT_1B agonist receptors [8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT) and CP-93,129] in the VO PFC of socially provoked male mice. To confirm the specificity of the receptor, 5-HT_1A and 5-HT_1B antagonist receptors (WAY-100,635 and SB-224,289) were microinjected into the same area, in order to reverse the agonist effects. Results  8-OH-DPAT (0.56 and 1.0 μg) reduced the frequency of attack bites. The lowest dose of CP-93,129 (0.1 μg) also decreased the number of attack bites and lateral threats. 5-HT1A and 5-HT1B receptor agonists differed in their effects on non-aggressive activities, the former decreasing rearing and grooming, and the latter, increasing these acts. Specific participation of the 1A and 1B receptors was verified by reversal of anti-aggressive effects using selective antagonists WAY-100,635 (10.0 μg) and SB-224,289 (1.0 μg). Conclusions  The decrease in aggressiveness observed with microinjections of 5-HT_1A and 5-HT_1B receptor agonists into the VO PFC of socially provoked mice, supports the hypothesis that activation of these receptors modulates high levels of aggression in a behaviorally specific manner.     

Behavioral and autonomic responses to intermittent social stress: differential protection by clonidine and metoprolol

The present study investigated physiological and pharmacological characteristics of socially stressed animals. Specifically, we examined (1) to what degree autonomic and behavioral stress reactions during intermittent confrontations between an intruder male adult Long-Evans rat with an aggressive resident undergo habituation, and (2) to what extent the defeat-experienced animal can be protected against these stress reactions with clonidine or metoprolol, two adrenergic agents with clinical anxiolytic effects. We developed an acute social stress situation that consisted of initially placing an experimental rat as an intruder into the homecage of a resident while the resident was not present, thereafter permitting brief physical agonistic interactions with the reintroduced resident until the intruder was forced into a submissive supine posture and emitted ultrasonic vocalizations (USV), and eventually exposing the intruder to the resident's threats for one hour, while being shielded from potentially injurious attacks (threat encounter). Over the course of the initial 4-weekly threat encounters the acute tachycardia but not the hyperthermic stress responses decreased in magnitude. Following the first three threat encounters core temperature (T_c) was significantly elevated for at least 3 h. The T_c was already elevated when the repeatedly defeated intruder was confronted with the olfactory cues of the resident's cage. This conditioned anticipatory hyperthermia developed in the course of the first three confrontations and was paralleled by a decrease in exploratory and motor behavior and by an increase in defensive behaviors and in both types of USV emitted in the low (20–30 kHz) and the high (31–70 kHz) frequency range. Clonidine (0.01–0.1 mg/kg, IP), an ₂-adrenergic agonist and metoprolol, a -adrenergic blocker (1.0–10.0 mg/kg, IP), dose-dependently prevented the tachycardic response to stress. Only clonidine, but not metoprolol, also attenuated the rise in T₀ during the 1-h agonistic interaction. Clonidine decreased those aspects of motor behavior (e.g. rearing, walking) that are of lesser cost for the individual but maintained high levels of defensive reactions and increased the duration of low USV. The high doses of clonidine (0.06, 0.1 mg/kg) attenuated the homeostatic regulation and sedated the intruder while exposed to threats during a social confrontation. The absence of attenuation of the high level of defensive behavior and the prolonged low USV suggest a stress intensification by the higher doses of clonidine. In conclusion, after the fourth encounter, the autonomic, behavioral and vocal response pattern prior to and during repeated weekly confrontations show no evidence for habituation for the following 6 weeks. Moreover, adrenergic therapeutic agents that are applied to treat symptoms of anxiety block the tachycardic response but may actually intensify defensive behavior and certain stress vocalizations.     

Effect of the mGluR5 antagonist 6-methyl-2-(phenylethynyl)pyridine (MPEP) on the acute locomotor stimulant properties of cocaine, <Emphasis Type="SmallCaps">d</Emphasis>-amphetamine,and the dopamine reuptake inhibitor GBR12909 in mice

Rationale Recent evidence suggests that, in addition to ascending monoaminergic systems, glutamate systems also play a role in psychostimulant-induced locomotor activity. The present study was conducted to examine the effects of the selective type-5 metabotropic glutamate receptor (mGluR5) antagonist 6-methyl-2-(phenylethynyl)pyridine (MPEP) on the acute locomotor stimulant effects of cocaine, d-amphetamine, and the dopamine reuptake inhibitor GBR12909.Methods Male DBA/2J mice were treated with saline or MPEP (1, 5, 20 or 30 mg/kg i.p.) 10 min prior to the administration of cocaine (15 mg/kg or 30 mg/kg i.p.), d-amphetamine (3 mg/kg or 5 mg/kg i.p.) or GBR12909 (10 mg/kg or 20 mg/kg i.p.). Locomotor activity was then monitored in an open-field environment for 30 min. The effects of MPEP alone (1, 5, 20 and 30 mg/kg i.p.) on locomotor activity were also examined.Results MPEP dose dependently inhibited the acute locomotor stimulant effects of cocaine, d-amphetamine, and the 10-mg/kg dose of GBR12909. However, MPEP had no effect on the locomotor stimulant effects of the higher (20 mg/kg) dose of GBR12909. When tested alone, MPEP increased locomotor activity at doses of 5 mg/kg and 20 mg/kg.Conclusions Our data suggest that mGluR5 receptors not only mediate spontaneous locomotor activity in DBA/2J mice but also the acute locomotor stimulant effects of cocaine, d-amphetamine and lower doses of GBR12909. However, the fact that MPEP did not attenuate the locomotor stimulant effects of the high (20 mg/kg) dose of GBR12909 suggests complex interactions between metabotropic glutamate receptors, dopamine transporters and possibly other monoamines in the regulation of psychostimulant-induced locomotor activity.     

Brain responses to chronic social defeat stress: Effects on regional oxidative metabolism as a function of a hedonic trait, and gene expression in susceptible and resilient rats

Chronic social defeat stress, a depression model in rats, reduced struggling in the forced swimming test dependent on a hedonic trait-stressed rats with high sucrose intake struggled less. Social defeat reduced brain regional energy metabolism, and this effect was also more pronounced in rats with high sucrose intake. A number of changes in gene expression were identified after social defeat stress, most notably the down-regulation of Gsk3b and Map1b. The majority of differences were between stress-susceptible and resilient rats. Conclusively, correlates of inter-individual differences in stress resilience can be identified both at gene expression and oxidative metabolism levels.     

Blockade of serotonin 5-HT1B and 5-HT2A receptors suppresses the induction of locomotor activity by 5-HT reuptake inhibitors,citalopram and fluvoxamine,in NMRI mice exposed to a novel environment: a comparison to other 5-HT receptor subtypes

Rationale Though 5-HT plays an important role in the modulation of motor function, which is perturbed in depressive states, little is known concerning the influence of serotonin reuptake inhibitors (SSRIs) on locomotor activity (LA). Recently, we demonstrated that SSRIs, such as citalopram, enhance LA in mice exposed to a novel environment. Objectives This study examined the role of multiple classes of 5-HT receptor in citalopram-induced LA. Methods The most selective antagonists currently available were used. Results Citalopram-induced LA was dose-dependently attenuated by the 5-HT_1B/1D receptor antagonists, S18127, GR125,743 and GR127,935, and by the selective 5-HT_1B antagonist, SB224,289, but unaffected by the selective 5-HT_1A antagonist, WAY100,635. The selective antagonists at 5-HT_2A receptors, MDL100,907 and SR46,349 also dose-dependently attenuated induction of locomotion by citalopram, whereas the 5-HT_2B antagonist, SB204,741, and the 5-HT_2B/2C antagonist, SB206,553 were ineffective. Further, the selective 5-HT_2C antagonist, SB242,084, potentiated the response to citalopram. Selective antagonists at 5-HT₃ (ondansetron), 5-HT₄ (GR125,487), 5-HT₆ (SB271,046) and 5-HT₇ (SB269,970) receptors did not significantly modify the action of citalopram. Underpinning these findings, SB224,289, GR125,743, MDL100,907 and SR46,349 likewise attenuated induction of locomotion by a further SSRI, fluvoxamine. Conclusions The locomotor response to SSRIs of mice exposed to a novel environment is mediated via 5-HT_1B and 5-HT_2A receptors. In view of the importance of motor function to the etiology and treatment of depression, the significance of these observations to the clinical actions of SSRIs will be of interest to elucidate.