Cocaine‐induced locomotor activity and Fos expression in nucleus accumbens are sensitized for 6 months after repeated cocaine administration outside the home cage

DeltaFosB accumulation in ventro-medial caudate underlies the induction but not the expression of behavioral sensitization by both repeated amphetamine and stress

Both repeated psychostimulants and stress have the ability to promote behavioral sensitization, i.e. enhanced behavioral response to drug challenge. To test whether the behavioral phenotype is also accompanied by similar neuroplastic adaptations, the present study evaluated changes in Fos and FosB/DeltaFosB transcription factors induced in the brain of C57BL/6J mice behaviorally sensitized by repeated amphetamine or repeated restraint stress. Groups of mice received repeated injections of D-amphetamine or saline in group-specific environments. Different groups of mice experienced 2 h of restraint daily for 10 consecutive days. Amphetamine- pre-treated mice, drug-challenged in the environment in which they received drug treatments (Paired), as well as repeatedly stressed mice expressed robust sensitization to the locomotor effects of amphetamine. Both stress- and amphetamine-pre-treated groups showed changes in amphetamine-induced Fos expression; however, none of these changes was shared by the two sensitizing treatments. Instead, accumulation of FosB/DeltaFosB immunoreactivity in the ventro-medial caudate was common to both pre-treatments. These results support the hypothesis that a common neuroadaptive process involving DeltaFosB accumulation in the ventro-medial caudate underlies the induction but not the expression of behavioral sensitization by different conditions.

Repeated amphetamine administration outside the home cage enhances drug-induced Fos expression in rat nucleus accumbens

Induction of the immediate early gene protein product Fos has been used extensively to assess neural activation in the striatum after repeated amphetamine administration to rats in their home cages. However, this technique has not been used to examine striatal activation after repeated administration outside the home cage, an environment where repeated drug administration produces more robust psychomotor sensitization. We determined the dose-response relationship for amphetamine-induced psychomotor activity and Fos expression in nucleus accumbens and caudate-putamen 1 week after repeated administration of amphetamine or saline in locomotor activity chambers. Repeated administration of amphetamine enhanced amphetamine-induced locomotor activity and stereotypy and Fos expression in nucleus accumbens, but not in caudate-putamen. In comparison, levels of Fos expression induced by 1mg/kg amphetamine were not altered in nucleus accumbens or caudate-putamen by repeated amphetamine administration in the home cage. Double-labeling of Fos protein and enkephalin mRNA indicates that Fos is expressed in approximately equal numbers of enkephalin-negative and enkephalin-positive neurons in nucleus accumbens and caudate-putamen following injections outside the home cage. Furthermore, repeated amphetamine administration increased drug-induced Fos expression in enkephalin-positive, but not enkephalin-negative, neurons in nucleus accumbens. We conclude that repeated amphetamine administration outside the home cage recruits the activation of enkephalin-containing nucleus accumbens neurons during sensitized amphetamine-induced psychomotor activity.

Context-specific sensitization of cocaine-induced locomotor activity and associated neuronal ensembles in rat nucleus accumbens

Repeated cocaine administration to rats outside their home cage induces behavioral sensitization that is strongly modulated by the drug administration environment. We hypothesized that stimuli in the drug administration environment activate specific sets of striatal neurons, called neuronal ensembles, for further cocaine-enhanced activation, and that repeated activation of these neuronal ensembles underlies context-specific sensitization. In the present study, we repeatedly administered cocaine or saline to rats on alternate days in two distinct environments outside the home cage, one paired with cocaine and the other with saline. On test day, cocaine challenge injections in the cocaine-paired environment produced strongly enhanced levels of locomotor activity, while cocaine challenge injections in the saline-paired environment did not. The corresponding record of past neuronal activation in nucleus accumbens and caudate-putamen during repeated drug administration was assessed using FosB immunohistochemistry, while acute neuronal activation on test day was assessed using c-fos in situ hybridization. Although only 2% of striatal neurons were FosB labeled, 87% of these FosB-labeled neurons were co-labeled with c-fos when cocaine was injected in the cocaine-paired environment. The degree of co-labeling was significantly less following cocaine or saline challenge injections in the saline-paired environment. Furthermore, the total number of c-fos-labeled neurons was greater with either cocaine or saline challenge injections in the cocaine-paired environment than in the saline-paired environment. These findings demonstrate that the drug administration environment partly determines which striatal neuronal ensembles are activated, and to what extent, following context-specific sensitization to cocaine.

Modulatory effect of environmental context and drug history on heroin-induced psychomotor activity and fos protein expression in the rat brain

The goal of the present study was to investigate the role of environmental context and drug history in modulating the effects of heroin on locomotor activity and Fos protein expression in the neocortex and striatal complex of the rat. It was found that (1) repeated i.p. administrations of a relatively low dose of heroin (1 mg/kg, i.p.) induced psychomotor sensitization only when the treatment was administered in a relatively 'novel' environment (ie, a unique test environment distinct from the home cage) but not when the same treatment was administered in the home cage; (2) environmental novelty facilitated heroin-induced Fos expression in the caudate, particularly in its most caudal regions; (3) environmental context also modulated heroin-induced Fos expression in the nucleus accumbens and in the neocortex; (4) repeated exposures to heroin dramatically altered its effects on Fos expression in the caudate and in the neocortex; and (5) Fos protein levels in the postero-dorsal caudate, in the shell of the nucleus accumbens, and in the barrel field cortex predicted most of the variance in heroin-induced activity scores, as shown by multiple regression analysis. The present report demonstrates that environment and drug history powerfully interact in shaping the neurobehavioral response to heroin, as previously shown for amphetamine and cocaine. Thus, a full understanding of the mechanisms responsible for the neurobehavioral adaptations produced by addictive drugs will also require taking into due consideration the environment in which drugs are experienced.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Modeling the role of environment in addiction

The aim of this review is to provide an overview of the main types of animal models used to investigate the modulatory role of environment on drug addiction. The environment can alter the responsiveness to addictive drugs in at least three major ways. First, adverse life experiences can make an individual more vulnerable to develop drug addiction or to relapse into drug seeking. Second, neutral environmental cues can acquire, through Pavlovian conditioning, the ability to trigger drug seeking even after long periods of abstinence. Third, the environment immediately surrounding drug taking can alter the behavioral, subjective, and rewarding effects of a given drug, thus influencing the propensity to use the same drug again. We have focused in particular on the results obtained using an animal model we have developed to study the latter type of drug-environment interaction.

Behavioral expression of cocaine sensitization in rats is accompanied by a distinct pattern of modifications in the PKA/DARPP-32 signaling pathway

Repeated cocaine administration induces behavioral sensitization and modifications in the phosphorylation pattern of dopamine and cAMP-regulated phosphoprotein of Mr 32,000 (DARPP-32), characterized by a tonic increase in the Thr75 phosphorylated form, and a decrease in the Thr34 phosphorylated form. This study further investigated the correlations between cocaine sensitization and modifications in the DARPP-32 phosphorylation pattern, cAMP-dependent protein kinase (PKA) activity, and mGluR5 tone in the medial prefrontal cortex and nucleus accumbens. Behavioral sensitization and modifications in these neurochemical markers followed a similar temporal pattern. Moreover, in sensitized rats acute cocaine administration modified phosphorylation levels of Thr75- and Thr34-DARPP-32, GluR1, and NR1 subunits in the nucleus accumbens only at a dose double the efficacious dose in control rats. These results suggest that the high levels of phospho-Thr75 DARPP-32 maintain PKA in a prevalent inhibited state. Furthermore, in sensitized rats the acute administration of 6-methyl-2-(phenylethynyl)-pyridine, a mGluR5 antagonist, reinstated the phosphorylation levels of Thr75- and Thr34-DARPP-32, GluR1, and NR1 to control values, and a subsequent cocaine challenge did not elicit a sensitized response. These data suggest that a tonic increase in mGluR5 transmission in cocaine-sensitized rats sustains both the increase in phospho-Thr75 DARPP-32 levels and the expression of behavioral sensitization.

Regional adaptations in PSD-95, NGFI-A and secretogranin gene transcripts related to vulnerability to behavioral sensitization to amphetamine in the Roman rat strains

Genetically selected for high or low two-way active avoidance, Roman high-avoidance (RHA) and Roman low-avoidance (RLA) rats differ in their central dopaminergic activity, sensation/novelty- and substance-seeking profiles. These animals are, therefore, well suited to identify anatomical and neurochemical concomitants of behavioral sensitization, a phenomenon linked to addictive liability. We submitted inbred RHA (RHA-I), inbred RLA (RLA-I) and Sprague-Dawley-OFA (SD-OFA) rats to a sensitization regimen with amphetamine and studied the behavioral response to an amphetamine challenge after a 2-week withdrawal period. The expression patterns of nerve growth factor inducible clone A (NGFI-A), secretogranin, post-synaptic density protein of 95 Kd (PSD-95), prodynorphin and proenkephalin mRNA were also analyzed using in situ hybridization, after the challenge with amphetamine. RHA-I rats showed stronger sensitization than SD-OFA rats. RLA-I rats did not show sensitization but were hyper-reactive to amphetamine. Expression of behavioral sensitization in RHA-I rats activated secretogranin and PSD-95 mRNA in the nucleus accumbens core. On the other hand, high induction of NGFI-A mRNA in the central amygdala was observed in RLA-I rats when they experienced amphetamine for the first time in the challenge. Our results reveal that 1) the acute locomotor response to amphetamine does not predict vulnerability to behavioral sensitization and 2) differences in vulnerability to sensitization may involve distinctive cellular adaptations at particular brain locations which may be related to addictive vulnerability.

Dysregulation of gene induction in corticostriatal circuits after repeated methylphenidate treatment in adolescent rats: differential effects on zif 268 and homer 1a

Psychostimulants and other dopamine agonists produce molecular changes in neurons of cortico-basal ganglia-cortical circuits, and such neuronal changes are implicated in behavioural disorders. Methylphenidate, a psychostimulant that causes dopamine overflow (among other effects), alters gene regulation in neurons of the striatum. The present study compared the effects of acute and repeated methylphenidate treatment on cortical and striatal gene regulation in adolescent rats. Changes in the expression of the immediate-early genes zif 268 and homer 1a were mapped in 23 striatal sectors and 22 cortical areas that provide input to these striatal sectors, in order to determine whether specific corticostriatal circuits were affected by these treatments. Acute administration of methylphenidate (5 mg/kg, i.p.) produced modest zif 268 induction in cortical areas. These cortical zif 268 responses were correlated in magnitude with zif 268 induction in functionally related striatal sectors. In contrast, after repeated methylphenidate treatment (10 mg/kg, 7 days), cortical and striatal gene induction were dissociated. In these animals, the methylphenidate challenge (5 mg/kg) produced significantly greater gene induction (zif 268 and homer 1a) in the cortex. This enhanced response was widespread but regionally selective, as it occurred predominantly in premotor, motor and somatosensory cortical areas. At the same time, striatal gene induction was partly suppressed (zif 268) or unchanged (homer 1a). Thus, repeated methylphenidate treatment disrupted the normally coordinated gene activation patterns in cortical and striatal nodes of corticostriatal circuits. This drug-induced dissociation in cortical and striatal functioning was associated with enhanced levels of behavioural stereotypies, suggesting disrupted motor switching function.

Environmental modulation of cocaine self-administration in the rat

RATIONALE

Previous studies have shown that environmental context can powerfully modulate the induction of psychomotor sensitization to cocaine in the rat. Rats that receive repeated administrations of cocaine in association with environmental novelty exhibit greater psychomotor sensitization than animals that receive the same treatments in their home cages.

OBJECTIVES

The goal of the present study was to investigate whether environmental context can exert its modulatory influence also on cocaine self-administration.

MATERIALS AND METHODS

Independent groups of rats with intravenous catheters were given the possibility to self-administer different doses of cocaine (0.0, 0.2, 0.4, and 0.8 mg/kg per infusion) under two environmental conditions. Some animals were housed in the self-administration cages (home groups), whereas other rats were transported to the self-administration cages only for the test sessions (novelty groups).

RESULTS

Environmental "novelty" facilitated the acquisition of cocaine self-administration at the doses of 0.2 and 0.4 mg/kg per infusion. When rats were given access to a higher dose of cocaine (0.8 mg/kg per infusion), there were no significant group differences in drug taking. Environmental context had no effect on the self-administration of the vehicle. Thus, it appears that environmental "novelty" produced a shift to the left in the dose-effect curve for cocaine self-administration. Furthermore, "novelty" enhanced the motivation of the rats to work for cocaine, as indicated by the results of a progressive ratio procedure.

CONCLUSIONS

The present findings demonstrate for the first time that the environment surrounding drug taking can alter both the intake of and motivation for cocaine.