Repeated administration of psychostimulant drugs or stress can elicit a sensitized response to the stimulating and reinforcing properties of the drug. Here we explore the mechanisms in the nucleus accumbens (NAc) whereby an acute restraint stress augments the acute locomotor response to cocaine. This was accomplished by a combination of behavioral pharmacology, microdialysis measures of extracellular dopamine and glutamate, and Western blotting for GluR1 subunit of the AMPA glutamate receptor (AMPAR). A single exposure to restraint stress 3 weeks before testing revealed that enduring locomotor sensitization to cocaine was paralleled by an increase in extracellular dopamine in the core, but not the shell subcompartment of the NAc. Wistar rats pre-exposed to acute stress showed increased basal levels of glutamate in the core but the increase in glutamate by acute cocaine was blunted. The alterations in extracellular glutamate seem to be relevant, since blocking AMPAR by CNQX microinjection into the core prevented both the behavioral cross-sensitization and the augmented increase in cocaine-induced extracellular dopamine. Further implicating glutamate, the locomotor response to AMPAR stimulation in the core was potentiated, but not in the shell of pre-stressed animals, and this was accompanied by an increase in NAc GluR1 surface expression. This study provides evidence that the long-term expression of restraint stress-induced behavioral cross-sensitization to cocaine recapitulates some mechanisms thought to underpin the sensitization induced by daily cocaine administration, and shows that long-term neurobiological changes induced in the NAc by acute stress are consequential in the expression of cross-sensitization to cocaine.

The administration of psychostimulant drugs or stress can elicit a sensitized response to the stimulating and reinforcing properties of the drug. We previously demonstrated that a single restraint stress session enhanced d-amphetamine (d-AMPH)-induced locomotion the day after the stress session, which lasted up to 8 days. The present experiments were designed to identify the contribution of major dopamine (DA) brain areas in the short-and long-lasting enhancement of d-AMPH-induced locomotion following a single stress, and to test the involvement of N-methyl-D-aspartate (NMDA) receptors in that phenomena. To achieve our goal, 24 h and 8 days after a 2-h restraint stress session either with or without a NMDA receptor blockade, we measured locomotor activity and DA overflow in nucleus accumbens (NAcc) core and shell and caudate putamen (CPu) following a d-AMPH injection (0.5 mg/kg i.p.). The stimulant effect of d-AMPH on DA overflow was enhanced in all nuclei at 24 h after a single stress, while at 8 days the enhanced responsiveness was maintained only in the NAcc core. When the rats were administered with MK-801 (0.1 mg/kg i.p.) 30 min before restraint stress, the d-AMPH-induced enhancement on locomotor activity and DA neurotransmission was prevented in all studied brain areas at both times. These findings show that a glutamate-dopamine link is underlying the short-and long-term d-AMPH-induced enhancement on DA and locomotor activity following stress. The persistent glutamate-dependent DA enhancement in NAcc core highlights the relevance of this region in the long-term proactive effects of stress on vulnerability to drug abuse.

Evidence indicates that repeated exposure to stressful events sensitizes the motor and addictive effects of drugs of abuse in rats. Regarding a single exposure to one restraint stress, previous findings have shown that it is sufficient to induce behavioral sensitization to stimulating and reinforcing properties of abuse drugs (e.g., amphetamine and morphine), as measured by locomotor activity and conditioned place preference, respectively. It is well known that enhanced dopaminergic neurotransmission in the nucleus accumbens and striatum plays a critical role in the development and/or expression of repeated stress‐induced or drug‐induced sensitization. In addition, involvement of NMDA receptors has been implicated in its development. However, whether sensitization induced by a single restraint stress exposure represents the same neurobiologic phenomenon is unknown. We studied the following issues: (a) influence of a single restraint exposure on the stimulating effects of amphetamine on dopamine release by microdialysis from striatum and (b) involvement of glutamatergic pathways, specifically those innervating striatum, on stress‐induced sensitization to amphetamine, by administering MK‐801 ip (0.1 mg/kg) or intrastriatally (1 μg/0.5 μL) previous to an acute restraint stress. For microdialysis studies (a) or intrastriatal administration of MK‐801 (b), Wistar rats (250‐330 g) were implanted stereotactically under anesthesia with a guide cannula in the striatum. After 2 days, animals were immobilized for 2 hours in a Plexiglas device. Control animals remained in their home cages. The following day we evaluated the stimulating effect of amphetamine on (a) dopamine release from striatum or (b) locomotor activity. In studies (a), dialysis probes were inserted into the guide cannula, and baseline dopamine levels were collected for 2 hours before a challenge of amphetamine (1.5 mg/kg ip). Dialysates were then collected by 3 hours. Amphetamine challenge induced a significantly higher increase in dopamine release and locomotor activity in animals previously subjected to one restraint stress exposure, relative to that observed in the no‐restraint stress group. MK‐801 administered ip or intrastriatally blocked the restraint stress‐induced sensitization to amphetamine. First, our results point out that a single restraint stress exposure is a pertinent stimulus to induce sensitization of amphetamine's stimulating effects on dopaminergic neurotransmission in the striatum. Secondly, NMDA‐glutamatergic receptors, specifically those placed in the striatum, are implicated in the development of stress restraint‐induced sensitization.

Behavioral sensitization to cocaine is associated to neuroadaptations that contribute to addiction. Enkephalin is highly expressed in mesocorticolimbic areas associated with cocaine-induced sensitization; however, their influence on cocaine-dependent behavioral and neuronal plasticity has not been explained. In this study, we employed a knockout (KO) model to investigate the contribution of enkephalin in cocaine-induced behavioral sensitization. Wild-type (WT) and proenkephalin KO mice were treated with cocaine once daily for 9 days to induce sensitization. Additionally, to clarify the observations in KO mice, the same procedure was applied in C57BL/6 mice, except that naloxone was administered before each cocaine injection. All animals received a cocaine challenge on days 15 and 21 of the treatment to evaluate the expression of locomotor sensitization. On day 21, microdialysis measures of accumbal extracellular dopamine, Western blotting for GluR1 AMPA receptor (AMPAR), phosphorylated ERK2 (pERK2), CREB (pCREB), TrKB (pTrkB) were performed in brain areas relevant for sensitization from KO and WT and/or naloxone- and vehicle pre-treated animals. We found that KO mice do not develop sensitization to the stimulating properties of cocaine on locomotor activity and on dopamine release in the nucleus accumbens (NAc). Furthermore, pivotal neuroadaptations such as the increase in pTrkB receptor, pERK/CREB and AMPAR related to sensitized responses were absent in the NAc from KO mice. Consistently, full abrogation of cocaine-induced behavioral and neuronal plasticity after naloxone pre-treatment was observed. We show for first time that the proenkephalin system is essential in regulating long-lasting pivotal neuroadaptations in the NAc underlying behavioral sensitization to cocaine.