Marcello Solinas

Effects of enriched environment on animal models of neurodegenerative diseases and psychiatric disorders.

Environmental stimulation throughout development adjusts the neurobehavioral systems involved in learning, memory and defensive responses. Environment-mediated phenotypic plasticity can be considered from two different, yet complementary, viewpoints. On one hand, the possibility that environmental interventions protect against the effects of genetic and/or acquired vulnerabilities, offers unprecedented avenues towards the elaboration and refinement of therapeutic strategies. On the other hand, an accurate understanding of the adaptive mechanisms regulating the interaction between an experimental subject and its environment may substantially benefit the quality of experimental data. Here we review experimental evidence showing that enriched environment can be beneficial in several psychiatric and neurodegenerative disorders implicating the monoamine systems where it can (i) compensate for impairments in animal models of schizophrenia, Huntingtons, and Parkinsons diseases; (ii) increase resistance to the addictive properties of psychostimulant drugs; (iii) level-out the consequences of prenatal stress in animal models of depression. Additionally we discuss why some of the effects of environmental enrichment question the validity of current animal models of mental disorders.

The endocannabinoid system in brain reward processes.

Food, drugs and brain stimulation can serve as strong rewarding stimuli and are all believed to activate common brain circuits that evolved in mammals to favour fitness and survival. For decades, endogenous dopaminergic and opioid systems have been considered the most important systems in mediating brain reward processes. Recent evidence suggests that the endogenous cannabinoid (endocannabinoid) system also has an important role in signalling of rewarding events. First, CB1 receptors are found in brain areas involved in reward processes, such as the dopaminergic mesolimbic system. Second, activation of CB1 receptors by plant‐derived, synthetic or endogenous CB1 receptor agonists stimulates dopaminergic neurotransmission, produces rewarding effects and increases rewarding effects of abused drugs and food. Third, pharmacological or genetic blockade of CB1 receptors prevents activation of dopaminergic neurotransmission by several addictive drugs and reduces rewarding effects of food and these drugs. Fourth, brain levels of the endocannabinoids anandamide and 2‐arachidonoylglycerol are altered by activation of reward processes. However, the intrinsic activity of the endocannabinoid system does not appear to play a facilitatory role in brain stimulation reward and some evidence suggests it may even oppose it. The influence of the endocannabinoid system on brain reward processes may depend on the degree of activation of the different brain areas involved and might represent a mechanism for fine‐tuning dopaminergic activity. Although involvement of the various components of the endocannabinoid system may differ depending on the type of rewarding event investigated, this system appears to play a major role in modulating reward processes.

Reversal of cocaine addiction by environmental enrichment

Environmental conditions can dramatically influence the behavioral and neurochemical effects of drugs of abuse. For example, stress increases the reinforcing effects of drugs and plays an important role in determining the vulnerability to develop drug addiction. On the other hand, positive conditions, such as environmental enrichment, can reduce the reinforcing effects of psychostimulants and may provide protection against the development of drug addiction. However, whether environmental enrichment can be used to “treat” drug addiction has not been investigated. In this study, we first exposed mice to drugs and induced addiction-related behaviors and only afterward exposed them to enriched environments. We found that 30 days of environmental enrichment completely eliminates behavioral sensitization and conditioned place preference to cocaine. In addition, housing mice in enriched environments after the development of conditioned place preference prevents cocaine-induced reinstatement of conditioned place preference and reduces activation of the brain circuitry involved in cocaine-induced reinstatement. Altogether, these results demonstrate that environmental enrichment can eliminate already established addiction-related behaviors in mice and suggest that environmental stimulation may be a fundamental factor in facilitating abstinence and preventing relapse to cocaine addiction.

Involvement of adenosine A1 and A2A receptors in the motor effects of caffeine after its acute and chronic administration.

The involvement of adenosine A1 and A2A receptors in the motor effects of caffeine is still a matter of debate. In the present study, counteraction of the motor-depressant effects of the selective A1 receptor agonist CPA and the A2A receptor agonist CGS 21680 by caffeine, the selective A1 receptor antagonist CPT, and the A2A receptor antagonist MSX-3 was compared. CPT and MSX-3 produced motor activation at the same doses that selectively counteracted motor depression induced by CPA and CGS 21680, respectively. Caffeine also counteracted motor depression induced by CPA and CGS 21680 at doses that produced motor activation. However, caffeine was less effective than CPT at counteracting CPA and even less effective than MSX-3 at counteracting CGS 21680. On the other hand, when administered alone in habituated animals, caffeine produced stronger motor activation than CPT or MSX-3. An additive effect on motor activation was obtained when CPT and MSX-3 were coadministered. Altogether, these results suggest that the motor-activating effects of acutely administered caffeine in rats involve the central blockade of both A1 and A2A receptors. Chronic exposure to caffeine in the drinking water (1.0 mg/ml) resulted in tolerance to the motor effects of an acute administration of caffeine, lack of tolerance to amphetamine, apparent tolerance to MSX-3 (shift to the left of its ‘bell-shaped’ dose–response curve), and true cross-tolerance to CPT. The present results suggest that development of tolerance to the effects of A1 receptor blockade might be mostly responsible for the tolerance to the motor-activating effects of caffeine and that the residual motor-activating effects of caffeine in tolerant individuals might be mostly because of A2A receptor blockade.

Anandamide administration alone and after inhibition of fatty acid amide hydrolase (FAAH) increases dopamine levels in the nucleus accumbens shell in rats

Although endogenous cannabinoid systems have been implicated in the modulation of the rewarding effects of abused drugs and food, little is known about the direct effects of endogenous ligands for cannabinoid receptors on brain reward processes. Here we show for the first time that the intravenous administration of anandamide, an endogenous ligand for cannabinoid receptors, and its longer‐lasting synthetic analog methanandamide, increase the extracellular dopamine levels in the nucleus accumbens shell of awake, freely moving rats, an effect characteristic of most drugs abused by humans. Anandamide produced two distinctly different effects on dopamine levels: (1) a rapid, transient increase that was blocked by the cannabinoid CB1 receptor antagonist rimonabant, but not by the vanilloid VR1 receptor antagonist capsazepine, and was magnified and prolonged by the fatty acid amide hydrolase (FAAH) enzyme inhibitor, URB597; (2) a smaller delayed and long‐lasting increase, not sensitive to CB1, VR1 or FAAH blockade. Both effects were blocked by infusing either tetrodotoxin (TTX, 1 µm) or calcium‐free Ringers solution through the microdialysis probe, demonstrating that they were dependent on the physiologic activation of dopaminergic neurotransmission. Thus, these results indicate that anandamide, through the activation of the mesolimbic dopaminergic system, participates in the signaling of brain reward processes.

Two Brain Sites for Cannabinoid Reward

The recent findings that Δ9tetrahydrocannabinol (Δ9THC), the active agent in marijuana and hashish, (1) is self-administered intravenously, (2) potentiates the rewarding effects of electrical brain stimulation, and (3) can establish conditioned place preferences in laboratory animals, suggest that these drugs activate biologically primitive brain reward mechanisms. Here, we identify two chemical trigger zones for stimulant and rewarding actions of Δ9THC. Microinjections of Δ9THC into the posterior ventral tegmental area (VTA) or into the shell of the nucleus accumbens (NAS) increased locomotion, and rats learned to lever-press for injections of Δ9THC into each of these regions. Substitution of vehicle for drug or treatment with a cannabinoid CB1 receptor antagonist caused response cessation. Microinjections of Δ9THC into the posterior VTA and into the posterior shell of NAS established conditioned place preferences. Injections into the core of the NAS, the anterior VTA, or dorsal to the VTA were ineffective. These findings link the sites of rewarding action of Δ9THC to brain regions where such drugs as amphetamines, cocaine, heroin, and nicotine are also thought to have their sites of rewarding action.

Motivational Effects of Cannabinoids and Opioids on Food Reinforcement Depend on Simultaneous Activation of Cannabinoid and Opioid Systems

Strong functional interactions exist between endogenous cannabinoid and opioid systems. Here, we investigated whether cannabinoid–opioid interactions modulate motivational effects of food reinforcement. In rats responding for food under a progressive-ratio schedule, the maximal effort (break point) expended to obtain 45 mg pellets depended on the level of food deprivation, with free-feeding reducing break points and food-deprivation increasing break points. Delta-9-tetrahydrocannabinol (THC; 0.3–5.6 mg/kg intrapeitoneally (i.p.)) and morphine (1–10 mg/kg i.p.) dose-dependently increased break points for food reinforcement, while the cannabinoid CB1 receptor antagonist rimonabant (SR-141716A; 0.3–3 mg/kg i.p.) and the preferential mu-opioid receptor antagonist naloxone (0.3–3 mg/kg i.p.) dose-dependently decreased break points. THC and morphine only increased break points when food was delivered during testing, suggesting that these treatments directly influenced reinforcing effects of food, rather than increasing behavior in a nonspecific manner. Effects of THC were blocked by rimonabant and effects of morphine were blocked by naloxone, demonstrating that THCs effects depended on cannabinoid CB1 receptor activation and morphines effects depended on opioid-receptor activation. Furthermore, THCs effects were blocked by naloxone and morphines effects were blocked by rimonabant, demonstrating that mu-opioid receptors were involved in the effects of THC and cannabinoid CB1 receptors were involved in the effects of morphine on food-reinforced behavior. Thus, activation of both endogenous cannabinoid and opioid systems appears to jointly facilitate motivational effects of food measured under progressive-ratio schedules of reinforcement and this facilitatory modulation appears to critically depend on interactions between these two systems. These findings support the proposed therapeutic utility of cannabinoid agonists and antagonists in eating disorders.

Opposite modulatory roles for adenosine A1 and A2A receptors on glutamate and dopamine release in the shell of the nucleus accumbens. Effects of chronic caffeine exposure

Previous studies have demonstrated opposing roles for adenosine A1 and A2A receptors in the modulation of extracellular levels of glutamate and dopamine in the striatum. In the present study, acute systemic administration of motor‐activating doses of the A2A receptor antagonist MSX‐3 significantly decreased extracellular levels of dopamine and glutamate in the shell of the rat nucleus accumbens (NAc) and counteracted both dopamine and glutamate release induced by systemic administration of motor‐activating doses of either the A1 receptor antagonist CPT or caffeine. Furthermore, exposure to caffeine in the drinking water (1 mg/mL, 14 days) resulted in tolerance to the effects of systemic injection of CPT or caffeine, but not MSX‐3, on extracellular levels of dopamine and glutamate in the NAc shell. The present results show: first, the existence of opposite tonic effects of adenosine on extracellular levels of dopamine and glutamate in the shell of the NAc mediated by A1 and A2A receptors; second, that complete tolerance to caffeines dopamine‐ and glutamate‐releasing effects which develops after chronic caffeine exposure is attributable to an A1 receptor‐mediated mechanism. Development of tolerance to the dopamine‐releasing effects of caffeine in the shell of the NAc may explain its weak addictive properties and atypical psychostimulant profile.

Environmental Enrichment During Early Stages of Life Reduces the Behavioral, Neurochemical, and Molecular Effects of Cocaine

It is known that negative environmental conditions increase vulnerability to drugs, whereas little is known on whether positive environmental conditions such as enriched environments (EE) have protective effects against addiction. We have previously found that EE consisting of bigger cages containing several toys that were changed once per week reduce cocaine-induced increases in locomotor activity. Here, we also show that the rewarding effects of cocaine are blunted in mice reared from weaning to adulthood in EE compared to mice reared in standard environments (SE). In addition, although both EE and SE mice develop behavioral sensitization to cocaine, EE mice show less activation in response to repeated administration of cocaine injections and reduced responses to cocaine challenges. In vivo microdialysis experiments demonstrate that the protective effects of EE do not depend on reduced cocaine-induced increases in the dopamine levels in the ventral or dorsal striatum. On the other hand, they were associated with reduced cocaine-induced expression of the immediate early gene zif-268 in the nucleus accumbens (shell and core) of EE mice. Finally, basal levels of Delta-Fos B, a transcription factor known to be increased by sustained activation of striatal neurons, are higher in the striatum of EE compared to SE mice and repeated administration of cocaine increases Delta-Fos B levels in SE mice but decreases them in EE mice. Altogether our results demonstrate that exposure to complex environments during early stages of life produce dramatic changes in the striatum that result in reduced reactivity to drugs of abuse.

Environmental Enrichment Reduces Cocaine Seeking and Reinstatement Induced by Cues and Stress but Not by Cocaine

Whereas earlier studies have focused on the preventive effects of enriched environments (EE) in drug addiction, in a recent study we suggested that EE can also have ‘curative’ effects. In fact, we found that cocaine addiction-related behaviors can be eliminated by housing cocaine-treated mice in EE during periods of forced abstinence. However, those results were obtained with two simple models of addiction, conditioned place preference (CPP), and behavioral sensitization. In this study, we used intravenous drug self-administration procedures in rats to further investigate the beneficial effects of EE on cocaine addiction in a reinstatement model of relapse. Singly housed rats learned to self-administer cocaine during 10 consecutive daily sessions (0.6 mg/injection, 6 h/day). They were then housed three per cage in either standard environments (SE) or EE and were kept abstinent in the animal facility until testing for extinction and reinstatement. We found that 30 days of EE significantly and consistently reduced cocaine seeking during a 6-h extinction session. In addition, EE significantly reduced cue- and stress-induced reinstatement. Surprisingly, given our earlier results in mice with CPP, EE did not reduce cocaine-induced reinstatement regardless of the level of exposure to cocaine and the duration of the period of abstinence and exposure to EE. Altogether, these results support the hypothesis that EE can reduce cocaine-induced craving and highlight the importance of positive life conditions in facilitating abstinence and preventing relapse to cocaine addiction.