Ella M. Nikulina

Long-term behavioral and neuronal cross-sensitization to amphetamine induced by repeated brief social defeat stress: Fos in the ventral tegmental area and amygdala

Repeated exposure to stress induces cross-sensitization to psychostimulants. The present study assessed functional neural activation during social defeat stress-induced sensitization to a subsequent amphetamine challenge. Social defeat stress was induced in intruder rats during short confrontations with an aggressive resident rat once every third day during the course of 10 days. Rats received d-amphetamine injections (1 mg/kg, i.p.) 17 or 70 days after the first social defeat stress exposure. Amphetamine administration induced a significantly higher frequency of locomotor activity in stressed animals than in handled control rats, which was still evident 2 months after the last social stress exposure. Immunohistochemistry for Fos-like proteins was used to detect activated neural profiles in the striatum, nucleus accumbens (NAc), prefrontal cortex, amygdala, and ventral tegmental area (VTA). Repeated social defeat stress significantly increased Fos-like immunoreactive (Fos-LI) labeling 17 days after the start of stress exposure in the prelimbic and infralimbic cortical regions, NAc shell and core, medial, central and basolateral amygdala, and VTA, which probably represented the expression of chronic Fos-related antigens. Amphetamine augmented stress-induced Fos-LI labeling 17 days after the first stress episode in the dorsal striatum, NAc core, and medial amygdala, reflecting a cross-sensitization of Fos response. Amphetamine challenge 70 days after social stress exposures revealed sensitized Fos-LI labeling in the VTA and the amygdala. These data suggest that episodes of repeated social stress induce a long-lasting neural change that leads to an augmented functional activation in the VTA and amygdala, which might represent a neurobiological substrate for long-lasting cross-sensitization of repeated social defeat stress with psychostimulant drugs.

Brief social defeat stress: long lasting effects on cocaine taking during a binge and zif268 mRNA expression in the amygdala and prefrontal cortex.

Social stress can engender behavioral and neural sensitization and this process appears to enhance the transition to compulsive drug abuse. Exposures to brief social defeat stress in rats have significant consequences on cocaine-reinforced behavior and on the level of functional activation within regions of the mesocorticolimbic dopamine system. The objectives of the current study were to examine the enduring consequences of brief episodes of social defeat stress on cocaine bingeing (during 24 h of continuous access) and on the emergence of neural adaptations as revealed by zif268 immediate early gene expression. Adult, male Long–Evans rats were subjected to four 25 min episodes of social defeat (once every 72 h). After 2 months, cocaine binges or zif268 mRNA gene expression were studied after confirming behavioral cross-sensitization to stimulant challenge. Sensitization to social defeat increased cocaine intake during a 24 h binge, effectively abolishing the typical circadian pattern of intake. Furthermore, 60 days after exposure to the sensitizing regimen of social defeat, levels of functional activation, measured by zif268 mRNA expression, in the central and medial amygdala were increased, while levels of activation in the medial prefrontal cortex were decreased. Persistent stress-induced levels of zif268 in the central and medial amygdala were attenuated by an injection of amphetamine (1.0 mg/kg). Divergent changes in zif268 within the amygdala and cortex 2 months after social defeat stress indicate the vulnerability of distinct cellular populations in networks that modulate the behavioral actions of psychomotor stimulants.

Escalated or Suppressed Cocaine Reward, Tegmental BDNF, and Accumbal Dopamine Caused by Episodic versus Continuous Social Stress in Rats

The neural link between ostensibly aversive stress experiences and intensely rewarding drug taking remains to be delineated. Epidemiological data associate stress and the abuse of various drugs, and experimental data identify the conditions that determine how episodic social stress intensifies the motivation for cocaine and the actual self-administration of cocaine. Two types of social stress have been the focus of experimental study in Long–Evans rats, since they engender divergent changes in drug- or sugar-rewarded behavior and in neuroadaptation. Episodic social defeat stress consists of four brief confrontations between the experimental rat and an aggressive resident rat of the Long–Evans strain over the course of 10 d. Subordination stress involves the continuous exposure to an aggressive resident for 5 weeks, while living in a protective cage within the residents home cage with brief daily confrontations. These stress experiences result in (1) increased intravenous cocaine self-administration under a fixed ratio schedule with prolonged binge-like access in episodically defeated intruder rats but suppressed cocaine intake by continuously subordinate rats; (2) deteriorated sugar preference and intake and decreased exploratory behavior in subordinate, but not intermittently defeated, rats; and (3) a sensitized dopamine (DA) response in the nucleus accumbens via in vivo microdialysis and increased tegmental brain-derived neural growth factor (BDNF) in episodically defeated rats, whereas the continuously subordinate rats show suppression of the DA and BDNF responses. These divergent neuroadaptations to social stress may represent the substrates for the intensification of cocaine “bingeing” relative to the anhedonia-like deterioration of reward processes during subordination stress.

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

Abstract 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.

Short- and long-term effects of intermittent social defeat stress on brain-derived neurotrophic factor expression in mesocorticolimbic brain regions.

Social defeat stress is an ethologically salient stressor which activates dopaminergic areas and, when experienced repeatedly, has long-term effects on dopaminergic function and related behavior. The mechanism for these long-lasting consequences remains unclear. A potential candidate for mediating these effects is brain-derived neurotrophic factor (BDNF), a neurotrophin involved in synaptic plasticity and displaying alterations in dopaminergic regions in response to various types of stress. In this study, we sought to determine whether repeated social defeat stress altered BDNF mRNA and protein expression in dopaminergic brain regions either immediately after the last stress exposure or 4 weeks later. Male Sprague-Dawley rats were subjected to social defeat stress consisting of brief confrontation with an aggressive male rat every third day for 10 days; control rats were handled according to the same schedule. Animals were euthanized either 2 h or 28 days after the last stress or handling episode. Our results show that 2 h after stress, BDNF protein and mRNA expression increased in the medial prefrontal cortex. At this time-point, BDNF mRNA increased in the amygdala and protein expression increased in the substantia nigra. Twenty-eight days after stress, BDNF protein and mRNA expression were elevated in the medial amygdala and ventral tegmental area. Given the role of BDNF in neural plasticity, BDNF alterations that are long-lasting may be significant for neural adaptations to social stress. The dynamic nature of BDNF expression in dopaminergic brain regions in response to repeated social stress may therefore have implications for lasting neurochemical and behavioral changes related to dopaminergic function.

Long-lasting alteration in mesocorticolimbic structures after repeated social defeat stress in rats : time course of μ-opioid receptor mRNA and FosB/ΔFosB immunoreactivity

Social defeat stress is a salient stressor that induces neuroadaptive changes in the mesocorticolimbic dopaminergic system. Substantial evidence indicates that µ‐opioid receptors (MORs) modulate dopamine transmission in the ventral tegmental area (VTA). FosB/ΔFosB protein accumulation in dopaminergic projections during repeated treatments is thought to be involved in long‐term neuroplasticity. In this study we characterize the magnitude and time‐course of MOR mRNA expression and FosB/ΔFosB immunoreactivity in mesocorticolimbic regions following repeated social defeat stress. Effects of brief repeated social defeat stress or control handling procedures were studied in rats either 2 h after the last exposure, or 3, 7, 14, 21 and 28 days later. We found that MOR mRNA expression in the VTA doubled after the last stress compared with handling, and remained 30–70% higher until day 21. The number of FosB/ΔFosB‐labeled neurons in regions of the frontal cortex, nucleus accumbens (NAc) shell and core, and in the medial, central and basolateral amygdala increased significantly immediately after the last stress episode, and remained enhanced for 21 days. Another group of rats received bilateral intra‐VTA infusion of the MOR agonist, DAMGO, 7 days after the last stress. Prior social defeat stress augmented DAMGO‐induced Fos expression in the NAc shell, suggesting that Fos expression in this region might be the direct result of MOR activity in the VTA. Social defeat stress leads to an increased capacity for MOR activation in the VTA, which may be relevant to enduring FosB/ΔFosB expression in mesocorticolimbic areas and to the behaviorally sensitized response to psychostimulant drugs.

Role of genotype and dopamine receptors in behaviour of inbred mice in a forced swimming test

The role of genotype in the effects of selective D1 and D2 dopamine agonists and antagonists on behavioural despair (Porsolts test) was studied. Mice of nine inbred strains showed significant interstrain differences in duration of immobility. The influence of dopaminergic drugs was assessed in six strains characterized by different levels of swimming activity. SKF 38393 (10 mg/kg), an agonist at D1 dopamine receptors, increased swimming activity, while the D1 antagonist SCH 23390 (0.2 and 0.5 mg/kg) reduced it, the effects being genotype dependent. The involvement of D2 dopamine receptors in the regulation of mouse behaviour in the forced swimming test was not so evident; the D2 agonist bromocriptine (10 mg/kg) produced no significant effect. The D2 agonist quinpirole (2.5 mg/kg) increased immobility in the majority of the mouse strains studied, while in CBA mice it resulted in a marked reduction of immobility. The D2 antagonist sulpiride (20 mg/kg) decreased immobility and increased active swimming only in two strains. The present results suggest a different role for D1 and D2 dopamine receptors in the regulation of swimming in the mouse.

Social defeat stress increases expression of μ-opioid receptor mRNA in rat ventral tegmental area

Prompted by previous studies linking social defeat stress to changes in opioid antinociception, we evaluated the expression of mu-opioid receptor (MOR)-encoding mRNA in selected rat brain areas as a function of this type of stress. Because opioids mediate significant regulatory activities of brain dopamine neurons, dopaminergic loci in the ventral tegmental area (VTA) and substantia nigra (SN) were selected for analysis. Within 30 min after social defeat stress, the level of MOR-encoding mRNA, as detected and quantified by in situ hybridization histochemistry, increased in the lateral VTA and this increase was present for at least 6 h. In contrast, defeat stress was without effect on the expression of MOR-encoding mRNA in the SN. These data suggest that stress-induced alteration of MOR-encoding mRNA expression in the VTA may be involved in the consequences of social defeat stress.

Behavioral sensitization to cocaine after a brief social stress is accompanied by changes in fos expression in the murine brainstem.

The objective of the present study was to determine how c-fos gene expression in brainstem structures after a brief episode of social defeat stress is related to behavioral sensitization to cocaine challenge. Social stress was defined as defeat in a brief confrontation with an aggressive resident mouse and subsequent 20-min exposure to the residents threats behind a protective screen. Mice were treated with cocaine (40 mg/kg, i.p.) immediately or 1 week after social defeat stress. Fos-like immunoreactive (Fos-LI) cell nuclei were analyzed in the ventral tegmental area (VTA), dorsal raphe nucleus (DR), periaqueductal grey area (PAG) and locus coeruleus (LC). One episode of social stress induced behavioral sensitization to cocaine as indicated by an augmented locomotor response to a challenge injection 7 days after a single defeat. In naive mice, social stress markedly increased the number of Fos-LI nuclei in the DR, PAG and LC, but not in the VTA. Similarly, cocaine administration resulted in a significantly increased number of Fos-LI nuclei in the same areas. Administration of cocaine immediately following social defeat significantly reduced the number of Fos-LI nuclei in the DR, PAG and LC. Cocaine-induced Fos expression returned in the PAG and DR, but not in the LC, 1 week after social stress. In conclusion, the present results suggest that the presence of brainstem Fos be related to the ability to express stress-induced behavioral sensitization to cocaine.

Prolonged effects of repeated social defeat stress on mRNA expression and function of μ-opioid receptors in the ventral tegmental area of rats

Social defeat stress alters the activity of mesocorticolimbic dopamine projections from the ventral tegmental area (VTA), a process that has been implicated in the development of sensitization and drug-seeking behavior. We showed previously that acute brief social defeat stress increased short-term expression of μ-opioid receptor mRNA in the VTA. The present study assessed the presence and functional significance of μ-opioid receptor mRNA expression 1 week after the last episode of social defeat stress. Social defeat stress was induced in intruder rats during short confrontations with an aggressive resident rat, and subsequent exposures behind a protective screen once a day for 5 days. Regional μ-receptor mRNA levels were assessed by in situ hybridization histochemistry, and the amount of mRNA labeling was measured in the VTA and the substantia nigra (SN). Expression of μ-opioid receptor mRNA was significantly higher in defeated rats relative to handled control animals in the VTA, but not in the SN. In an additional group of rats, bilateral local intra-VTA injection of the selective μ-opioid receptor agonist DAMGO (1.0 μg per side) was performed 7–10 days after the last defeat stress or handling control procedure. Baseline motor activity did not differ between control and stressed rats. Intra-VTA DAMGO significantly increased locomotor activity in stressed rats compared to handled control rats. These results suggest that repeated social stress upregulates VTA μ-opioid receptors and can produce locomotor activation via stimulation of these receptors. This locomotor effect is probably the consequence of enhanced disinhibition of mesolimbic dopamine neurons.