Influences of housing conditions and ethanol intake on binding characteristics of D2, 5-HT1A, and benzodiazepine receptors of rats

Early environmental enrichment and impoverishment differentially affect addiction-related behavioral traits, cocaine-taking, and dopamine D2/3 receptor signaling in a rat model of vulnerability to drug abuse

RATIONALE

Risk factors for drug addiction include genetics, environment, and behavioral traits such as impulsivity and novelty preference (NP), which have been related to deficits in striatal dopamine (DA) D_2/3-receptors (D_2/3R) and heightened amphetamine (AMPH)-induced DA release. However, the influence of the early rearing environment on these behavioral and neurochemical variables is not clear.

OBJECTIVES

We investigated the influence of early rearing environment on striatal D_2/3R availabilities and AMPH-induced DA release in relation to impulsivity, NP, and propensity to drug self-administration (SA) in "addiction-prone" Roman high- (RHA) and "addiction-resistant" Roman low-avoidance (RLA) rats.

METHODS

Animals were reared post-weaning in either environmental enrichment (EE) or impoverishment (EI) and were assessed at adulthood for impulsivity, NP, and propensity to cocaine SA. EE and EI rats were also scanned using single-photon emission computed tomography to concurrently measure in vivo striatal D_2/3R availability and AMPH-induced DA release.

RESULTS

EE vs. EI was associated with heightened impulsivity and a lack of NP in both rat lines. Higher dorsal striatal D_2/3R densities were found in RHA EE and higher AMPH-induced DA release in RLA EE. Both impulsivity and NP were negatively correlated to dorsal striatal D_2/3R availabilities and positively correlated with AMPH-induced DA release in EI but not in EE. EE vs. EI was related to a faster rate of cocaine intake and elevated active timeout responses in RHAs.

CONCLUSION

Our results suggest non-monotonic, environment-dependent, relationships between impulsivity, NP, and D_2/3R-mediated signaling, and suggest that EI vs. EE may decrease the reinforcing effects of psychostimulants in predisposed individuals.

The neuroscience of unmet social needs

John Cacioppo has compared loneliness to hunger or thirst in that it signals that one needs to act and repair what is lacking. This paper reviews Cacioppo's and others' contributions to our understanding of neural mechanisms underlying social motivation in humans and in other social species. We focus particularly on the dopaminergic reward system and try to integrate evidence from animal models and human research. In rodents, objective social isolation leads to increased social motivation, mediated by the brains' mesolimbic dopamine system. In humans, social rejection can lead to either increased or decreased social motivation, and is associated with activity in the insular cortex; while chronic loneliness is typically associated with decreased social motivation but has been associated with altered dopaminergic responses in the striatum. This mixed pattern of cross-species similarities and differences may arise from the substantially different methods used to study unmet social needs across species, and suggests the need for more direct and deliberate cross-species comparative research in this critically important domain.

Neural mechanisms of social homeostasis

Social connections are vital to survival throughout the animal kingdom and are dynamic across the life span. There are debilitating consequences of social isolation and loneliness, and social support is increasingly a primary consideration in health care, disease prevention, and recovery. Considering social connection as an "innate need," it is hypothesized that evolutionarily conserved neural systems underlie the maintenance of social connections: alerting the individual to their absence and coordinating effector mechanisms to restore social contact. This is reminiscent of a homeostatic system designed to maintain social connection. Here, we explore the identity of neural systems regulating "social homeostasis." We review findings from rodent studies evaluating the rapid response to social deficit (in the form of acute social isolation) and propose that parallel, overlapping circuits are engaged to adapt to the vulnerabilities of isolation and restore social connection. By considering the neural systems regulating other homeostatic needs, such as energy and fluid balance, we discuss the potential attributes of social homeostatic circuitry. We reason that uncovering the identity of these circuits/mechanisms will facilitate our understanding of how loneliness perpetuates long-term disease states, which we speculate may result from sustained recruitment of social homeostatic circuits.

The effects of buspirone on occupancy of dopamine receptors and the rat gambling task

BACKGROUND

The dopamine D3 receptor (DRD₃) has been proposed as a target for drug development for the treatment of addictive disorders. Recently, the anxiolytic buspirone has been shown to have affinity for DRD₃ and DRD₄, and interest in repurposing it for addictive disorders has grown.

METHODS

Binding of [³H]-(+)-PHNO in the rat cerebellum and striatum was used to measure occupancy by buspirone of DRD₃ or DRD₂, respectively. Effects of buspirone in the rat gambling task (rGT) and the five-choice serial reaction time task (5-CSRTT) were examined.

RESULTS

Buspirone occupied both the DRD₂ and DRD₃ at high doses and the DRD₃, but not the DRD₂, in the narrow dose range of 3 mg/kg. At 10 mg/kg, a disruption of performance on rGT was observed. All measures of performance on the rGT, except for perseverations, were affected at 3 mg/kg. On the 5-CSRTT, omissions were increased. Impairments in the rGT were not mimicked by the effects induced by satiation. Further, buspirone did not impair food-maintained responding under a progressive ratio schedule of reinforcement at any dose, suggesting that the effects of buspirone on the rGT cannot be explained by non-selective actions.

CONCLUSIONS

Although buspirone had effects on the rGT at the dose that selectively occupied the DRD₃, the effects found do not parallel those found in previous studies of the effects of selective DRD₃ antagonists on the rGT. Thus, buspirone may impair performance on the rGT through actions at multiple receptor sites.

Contribution of non-genetic factors to dopamine and serotonin receptor availability in the adult human brain

The dopamine (DA) and serotonin (5-HT) neurotransmission systems are of fundamental importance for normal brain function and serve as targets for treatment of major neuropsychiatric disorders. Despite central interest for these neurotransmission systems in psychiatry research, little is known about the regulation of receptor and transporter density levels. This lack of knowledge obscures interpretation of differences in protein availability reported in psychiatric patients. In this study, we used positron emission tomography (PET) in a twin design to estimate the relative contribution of genetic and environmental factors, respectively, on dopaminergic and serotonergic markers in the living human brain. Eleven monozygotic and 10 dizygotic healthy male twin pairs were examined with PET and [(11)C]raclopride binding to the D2- and D3-dopamine receptor and [(11)C]WAY100635 binding to the serotonin 5-HT1A receptor. Heritability, shared environmental effects and individual-specific non-shared effects were estimated for regional D2/3 and 5-HT1A receptor availability in projection areas. We found a major contribution of genetic factors (0.67) on individual variability in striatal D2/3 receptor binding and a major contribution of environmental factors (pairwise shared and unique individual; 0.70-0.75) on neocortical 5-HT1A receptor binding. Our findings indicate that individual variation in neuroreceptor availability in the adult brain is the end point of a nature-nurture interplay, and call for increased efforts to identify not only the genetic but also the environmental factors that influence neurotransmission in health and disease.

On the interaction between drugs of abuse and adolescent social behavior

RATIONALE

Social factors influence drug abuse. Conversely, drugs of abuse alter social behavior. This is especially pertinent during post-weaning development, when there are profound changes in the social repertoire, and the sensitivity to the positive and negative effects of drugs of abuse is altered.

OBJECTIVES

This study aimed to provide an overview of our current understanding of the interaction between drugs of abuse and juvenile/adolescent social behavior.

METHODS

We first provide evidence that a characteristic form of juvenile and adolescent social behavior, i.e., social play behavior, has reinforcing properties and is affected by drugs of abuse. Next, social risk factors for drug use and addiction are described, including antisocial personality traits and early social insults. Last, we discuss research that investigates social influences on drug use, as well as the consequences of perinatal drug exposure on later social interactions.

RESULTS

Social play behavior is highly rewarding in laboratory animals, and it is affected by low doses of opioids, cannabinoids, ethanol, nicotine, and psychostimulants. In humans, antisocial personality traits, most prominently in the form of conduct disorder, are a prominent risk factor for drug addiction. Preclinical studies have consistently shown altered sensitivity to drugs as a result of social isolation during post-weaning development. The social environment of an individual has a profound, but complex, influence on drug use, and perinatal drug exposure markedly alters later social interactions.

CONCLUSIONS

The studies reviewed here provide a framework to understand the interaction between drugs of abuse and adolescent social interaction, at the preclinical and the clinical level.

Initial d2 dopamine receptor sensitivity predicts cocaine sensitivity and reward in rats

The activation of dopamine receptors within the mesolimbic dopamine system is known to be involved in the initiation and maintenance of cocaine use. Expression of the D₂ dopamine receptor subtype has been implicated as both a predisposing factor and consequence of chronic cocaine use. It is unclear whether there is a predictive relationship between D₂ dopamine receptor function and cocaine sensitivity that would enable cocaine abuse. Therefore, we exploited individual differences in behavioral responses to D₂ dopamine receptor stimulation to test its relationship with cocaine-mediated behaviors. Outbred, male Sprague-Dawley rats were initially characterized by their locomotor responsiveness to the D₂ dopamine receptor agonist, quinpirole, in a within-session ascending dose-response regimen (0, 0.1, 0.3 & 1.0 mg/kg, sc). Rats were classified as high or low quinpirole responders (HD₂ and LD₂, respectively) by a median split of their quinpirole-induced locomotor activity. Rats were subsequently tested for differences in the psychostimulant effects of cocaine by measuring changes in cocaine-induced locomotor activity (5 and 15 mg/kg, ip). Rats were also tested for differences in the development of conditioned place preference to a low dose of cocaine (7.5 mg/kg, ip) that does not reliably produce a cocaine conditioned place preference. Finally, rats were tested for acquisition of cocaine self-administration and maintenance responding on fixed ratio 1 and 5 schedules of reinforcement, respectively. Results demonstrate that HD₂ rats have enhanced sensitivity to the locomotor stimulating properties of cocaine, display greater cocaine conditioned place preference, and self-administer more cocaine compared to LD₂ animals. These findings suggest that individual differences in D₂ dopamine receptor sensitivity may be predictive of cocaine sensitivity and reward.

Environmental manipulations alter age differences in attribution of incentive salience to reward-paired cues

Cues repeatedly paired with rewards often themselves become imbued with enhanced motivational value, or incentive salience. During Pavlovian conditioned approach procedures, a cue repeatedly preceding reward delivery often elicits conditioned responses at either the reward delivery location ("goal-tracking") or the cue itself ("sign-tracking"). Sign-tracking behavior is thought to reflect the individual differences in attribution of incentive salience to reward-paired cues that may contribute to addiction vulnerability. Adolescent rats typically demonstrate less sign-tracking behavior than adult rats, a surprising finding given that adolescence is hypothesized to be a time of heightened addiction vulnerability. Given evidence that adult sign-tracking behavior can be influenced by environmental conditions, the present study compared the effects of isolate housing and food deprivation on expression of sign-tacking and goal-tracking behavior in adolescent and adult male rats across eight days of a Pavlovian conditioned approach procedure. Pair-housed adults exhibited more sign-tracking behavior than pair-housed adolescents; however, this age difference was not apparent in isolate-housed subjects. Adolescents often appeared more sensitive than adults to both food restriction- and isolate housing-induced changes in behavior, with food restriction promoting an increase in sign-tracking among isolate-housed adolescents and an increase in goal-tracking among pair-housed adolescents. For adults, food restriction resulted in a modest increase in overall expression of both sign- and goal-tracking behavior. To the extent that sign-tracking behavior reflects attribution of incentive salience to reward-paired cues, results from the present study provide evidence that reactivity to rewards during adolescence is strongly related to the nature of the surrounding environment.

Individual differences in the effects of environmental stimuli on cocaine choice in socially housed male cynomolgus monkeys

RATIONALE

Studies in laboratory animals have demonstrated an influence of environmentally derived stress and enrichment on the reinforcing effects of stimulants.

OBJECTIVE

To characterize the effects of acute exposure to ethologically valid environmental stimuli on the reinforcing strength of cocaine relative to food in socially housed monkeys.

MATERIALS AND METHODS

Choice between cocaine and food was assessed in subsets of 16 socially housed (4/pen) male cynomolgus monkeys immediately after the following manipulations: (1) treats placed in home cage, (2) a 10-min exposure to a rubber snake, or (3) 3 to 7 days of living in a larger environment without cage mates.

RESULTS

Placing treats in the home cage shifted the cocaine dose-response curve to the left in five monkeys tested and to the right in 4 of 12 animals. The rubber snake significantly shifted the cocaine choice curve to the left in dominant monkeys. Exposure to an enlarged environment decreased cocaine choice in 9 of 15 monkeys; this effect was transient and not related to social rank. Repeated testing did not affect cocaine choice.

CONCLUSIONS

Brief exposure to environmental events hypothesized to be stressors or enrichment altered cocaine choice, although not all individuals were affected and the effects were transient. Importantly, the data suggest that implementing positive changes in the environment produced effects that are clinically desirable. Understanding the behavioral and neurobiological mechanisms mediating sensitivity to environmental events in socially housed animals will lead to better treatment strategies for drug addiction.

Biological contribution to social influences on alcohol drinking: evidence from animal models

Social factors have a tremendous influence on instances of heavy drinking and in turn impact public health. However, it is extremely difficult to assess whether this influence is only a cultural phenomenon or has biological underpinnings. Research in non-human primates demonstrates that the way individuals are brought up during early development affects their future predisposition for heavy drinking, and research in rats demonstrates that social isolation, crowding or low social ranking can lead to increased alcohol intake, while social defeat can decrease drinking. Neurotransmitter mechanisms contributing to these effects (i.e., serotonin, GABA, dopamine) have begun to be elucidated. However, these studies do not exclude the possibility that social effects on drinking occur through generalized stress responses to negative social environments. Alcohol intake can also be elevated in positive social situations, for example, in rats following an interaction with an intoxicated peer. Recent studies have also begun to adapt a new rodent species, the prairie vole, to study the role of social environment in alcohol drinking. Prairie voles demonstrate a high degree of social affiliation between individuals, and many of the neurochemical mechanisms involved in regulation of these social behaviors (for example, dopamine, central vasopressin and the corticotropin releasing factor system) are also known to be involved in regulation of alcohol intake. Naltrexone, an opioid receptor antagonist approved as a pharmacotherapy for alcoholic patients, has recently been shown to decrease both partner preference and alcohol preference in voles. These findings strongly suggest that mechanisms by which social factors influence drinking have biological roots, and can be studied using rapidly developing new animal models.

Increased dopamine D2High receptors in rats reared in social isolation

Postweaning social isolation in the rat induces lasting alterations that parallel several of the core symptoms seen in human schizophrenics, including hyperreactivity to novel environments, cognitive impairment, and deficits in sensorimotor gating. The current study determined whether these changes are accompanied by any elevation in the proportion of striatal dopamine receptors in the functional high affinity state (D(2) (High)), as observed in other preclinical models of psychosis. Male Lister hooded rats (20-24 days) were housed in groups of three or alone. On Day 36 postweaning locomotor activity was monitored for 60 min in a novel arena, and on Day 37 novel object discrimination was assessed using a 2 h intertrial interval. Three days later striata were collected, homogenized, washed three times to remove endogenous dopamine, and the proportion of D(2) (High) determined by competition between dopamine and 2.27 nM [(3)H]domperidone. Isolates were significantly more active than group housed controls for both ambulation and rears. Although both groups exhibited comparable levels of familiarization trial object exploration, group housed animals were able to discriminate between novel and familiar objects during the choice trial while isolates were not. Social isolation was associated with a highly significant elevation in the proportion of striatal D(2) (High), equivalent to a 3.3-fold increase (group 15.2% +/- 1.4%, isolate 49.8% +/- 4.8%; P < 0.0001, Student's unpaired t-test). These findings support both the hypothesis that elevated D(2) (High) is a common feature of multiple animal models of psychosis, and the validity of isolation rearing as a neurodevelopmental model of a "schizophrenia-like" state.

Review. Positron emission tomography imaging studies of dopamine receptors in primate models of addiction

Animal models have provided valuable information related to trait and state variables associated with vulnerability to drug addiction. Our brain imaging studies in monkeys have implicated D2 receptors in cocaine addiction. For example, an inverse relationship between D2 receptor availability and rates of cocaine self-administration has been documented. Moreover, environmental variables, such as those associated with formation of the social hierarchy, can impact receptor availability and sensitivity to the abuse-related effects of cocaine. Similarly, both D2 receptor availability and cocaine self-administration can be altered by chronic drug administration and fluctuations in hormone levels. In addition, cocaine self-administration can be altered in an orderly fashion by presentation of an acute stressor, such as acting as an intruder into an unfamiliar social group, which can shift the cocaine dose-response curve to the left in subordinate monkeys and to the right in dominant animals, suggesting an interaction between social variables and acute stressors. Conversely, irrespective of social rank, acute environmental enrichment, such as increasing the size of the living space, shifts the cocaine dose-response curve to the right. These findings highlight a pervasive influence of the environment in modifying the reinforcing effects of cocaine and strongly implicate brain D2 receptors.

Long-term voluntary consumption of MDMA and THC in rats is modified by individual and situational factors

Factors influencing long-term voluntary intake of 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) and Delta9-tetrahydrocannabinol (THC; cannabis) were studied in a rat model. Sixty-four male Wistar rats were given free choice for 49 weeks between (1) water and MDMA solution, (2) water and THC solution or (3) water, MDMA solution and THC solution. After the first experiences with the drugs, animals of both monodrug and polydrug group developed an individually stable pattern of MDMA consumption, whereas the individual predictability of THC consumption remained poor. While THC consumption was maintained for the whole experimental period, MDMA consumption decreased with time and was nearly ceased after 3-7 months. Intake of both drugs was adapted to social changes, with THC consumption being more sensitive to social changes than MDMA consumption. In the re-test after 4 months of abstinence, all animals ceased drug consumption when the drug solutions were adulterated with bitter tasting quinine. The results show that the rats had maintained a flexible mode of drug consumption and had not become addicted. Response to novelty of the rats in test trials before the start of drug supply correlated with later MDMA intake. In conclusion, although very low amounts of MDMA and/or THC were consumed, the findings that drug-experienced animals responded differentially to a stressor and that housing conditions influenced drug intake suggests that MDMA and THC can induce psychopharmacological effects in our long-term voluntary consumption paradigm.

The importance of housing conditions on behavioral sensitization and tolerance to ethanol

The differential outcomes of social isolation and crowding environment on the effects of single or repeated administration of ethanol on open-field behavior were examined in female mice. Whereas housing conditions did not alter the increase in locomotor activity induced by ethanol single administration, behavioral sensitization (a progressive increase of a drug effect following repeated drug administration) to the locomotor activating effect of ethanol was significantly greater in crowded mice as compared to isolated and control groups. Single administration of ethanol significantly decreased rearing frequency and increased immobility duration, there being tolerance to these ethanol behavior effects after repeated treatment. Social isolation attenuated the increase in immobility behavior induced by single administration of ethanol and potentiated the tolerance of ethanol-induced rearing decrease, verified after repeated treatment. These results point out that both sensitization and tolerance to the behavioral effects of ethanol can be critically influenced by housing conditions.

Environmental enrichment decreases cell surface expression of the dopamine transporter in rat medial prefrontal cortex

Rats raised in an enriched environmental condition (EC) exhibit a decreased (35%) maximal velocity (V(max)) of [3H]dopamine (DA) uptake in medial prefrontal cortex (mPFC) compared with rats raised in an impoverished condition (IC); however, no differences between EC and IC groups in V(max) for [3H]DA uptake were found in nucleus accumbens and striatum. Using biotinylation and immunoblotting techniques, the present study examined whether the brain region-specific decrease in DA transporter (DAT) function is the result of a reduction in DAT cell surface expression. In mPFC, nucleus accumbens and striatum, total DAT immunoreactivity was not different between EC and IC groups. Whereas no differences in cell surface expression of DAT were found in nucleus accumbens and striatum, DAT immunoreactivity in the biotinylated cell surface fraction of mPFC was decreased (39%) in EC compared with IC rats, consistent with the magnitude of the previously observed decrease in V(max) for [3H]DA uptake in mPFC in EC rats. These results suggest that the decrease in DAT cell surface expression in the mPFC may be responsible for decreased DAT function in the mPFC of EC compared with IC rats, and that there is plasticity in the regulatory mechanisms mediating DAT trafficking and function.

Effect of housing conditions on sulpiride-induced increases in extracellular dopamine (DA) levels in the nucleus accumbens of alcohol-preferring (P) rats

The effect of housing conditions on sulpiride-induced increases in extracellular dopamine (DA) levels was investigated in the nucleus accumbens (NAC) of P rats. Rats were double-housed (DH) in plastic tubs, or single-housed (SH) in hanging wire cages for 12 weeks. Microdialysis in the NAC showed greater sulpiride-induced DA increases in the NAC of SH vs. DH rats, with no difference in basal levels. The data indicate that housing conditions can alter DA D(2) receptor function in the NAC.

Characterization of dopamine D1 and D2 receptor function in socially housed cynomolgus monkeys self-administering cocaine

RATIONALE

Social rank has been shown to influence dopamine (DA) D(2) receptor function and vulnerability to cocaine self-administration in cynomolgus monkeys. The present studies were designed to extend these findings to maintenance of cocaine reinforcement and to DA D(1) receptors.

OBJECTIVE

Examine the effects of a high-efficacy D(1) agonist on an unconditioned behavior (eyeblinking) and a low-efficacy D(1) agonist on cocaine self-administration, as well as the effects of cocaine exposure on D(2) receptor function across social ranks, as determined by positron emission tomography (PET).

METHODS

Effects of the high-efficacy D(1) agonist SKF 81297 and cocaine (0.3-3.0 mg/kg) on spontaneous blinking were characterized in eight monkeys during 15-min observation periods. Next, the ability of the low-efficacy D(1) agonist SKF 38393 (0.1-17 mg/kg) to decrease cocaine self-administration (0.003-0.1 mg/kg per injection, IV) was assessed in 11 monkeys responding under a fixed-ratio 50 schedule. Finally, D(2) receptor levels in the caudate and putamen were assessed in nineteen monkeys using PET.

RESULTS

SKF 81297, but not cocaine, significantly increased blinking in all monkeys, with slightly greater potency in dominant monkeys. SKF 38393 dose-dependently decreased cocaine-maintained response rates with similar behavioral potency and efficacy across social rank. After an extensive cocaine self-administration history, D(2) receptor levels did not differ across social ranks.

CONCLUSIONS

These results suggest that D(1) receptor function is not substantially influenced by social rank in monkeys from well-established social groups. While an earlier study showed that dominant monkeys had higher D(2) receptor levels and were less sensitive to the reinforcing effects of cocaine during initial exposure, the present findings indicate that long-term cocaine use changed D(2) receptor levels such that D(2) receptor function and cocaine reinforcement were not different between social ranks. These findings suggest that cocaine exposure attenuated the impact of social housing on DA receptor function.

Fluvoxamine, a selective serotonin reuptake inhibitor, suppresses tetrahydrobiopterin in the mouse hippocampus

In the present study, we investigated the effects of fluvoxamine, a selective serotonin reuptake inhibitor, on brain tetrahydrobiopterin (BH4) levels. We directly measured levels of BH4 by Tani and Ohno's direct method as well as the serotonin (5-HT) turnover ratio, i.e. 5-hydroxyindoleacetic acid (5-HIAA)/5-HT, after sub-acute s.c. injection of fluvoxamine in the hippocampus of mice. Our animal model incorporated two risk factors of depression, social isolation and acute environmental change. Male ddY mice (6W) were housed in isolation (1 per cage; 35 days), injected with fluvoxamine (20 or 40 mg/kg; days 29-35), and exposed to novelty stress (20 min; day 35). In the stress session, behavioral parameters, i.e. total distance and rearing behavior, were measured. Isolation housing increased both behaviors. Fluvoxamine attenuated rearing behavior, but did not influence total distance. Isolation housing increased BH4 levels. Novelty stress increased BH4 levels in group housing, although it did not change them in isolation housing. Fluvoxamine suppressed BH4 levels. In isolation housing, fluvoxamine increased 5-HT turnover ratios, while it decreased them in group housing. In conclusion, fluvoxamine, housing condition, and novelty stress regulated BH4 levels. Fluvoxamine may have changed behavior and 5-HT turnover by suppressing BH4 levels as well as by inhibiting 5-HT reuptake.

Very low doses of ethanol induce behavioral changes involving dopamine D2 receptors

In male rats, pretreatment for 20 days with very low (0.5, 1%, v/v) but not with high (5, 10%, v/v) oral doses of ethanol delayed the initiation and reduced the duration of narcosis induced by an acute high intraperitoneal (i.p.) dose of the drug (3 g/kg in 25% saline solution). Furthermore, the treatment improved the acquisition of shuttle-box active avoidance response but did not affect the emission of ultrasonic calls, an index of emotional state of animals. These effects were inhibited by peripheral administration of the dopamine D2 receptor antagonist, sulpiride (1 mg/kg). A higher dose of sulpiride (10 mg/kg) prolonged the duration of narcosis in rats pretreated with high-dose ethanol and reduced the number of conditioned avoidance responses in the shuttle-box paradigm. The pretreatment with the dopamine D2 receptor agonist, (+/-)-2-(N-phenethyl-N-propylamino)-5-hydroxytetralin (PPHT, 0.1 mg/kg), enhanced the effects of ethanol very low doses in delaying the initiation and reducing the duration of narcosis induced by an acute i.p. dose of the drug. A pharmacokinetic study in ethanol-pretreated animals revealed that administration of 0.5% or 1% ethanol for 20 days did not modify significantly the bioavailability of acute ethanol administered i.p. in a dose of 3 g/kg in 25% saline solution. Thus, repeated administration of ethanol very low doses may have affected the sensitivity of presynaptic dopamine D2 receptors. The influence on dopamine release through an action on presynaptic receptors may be involved in these effects of small doses of ethanol.

Differential effects of intraspecific interactions on the striatal dopamine system in social and non-social voles

We used in vivo microdialysis to examine the responses to intraspecific social interactions in the striatal dopamine systems of females of two vole species displaying vastly different social structures. Both highly social prairie voles and asocial meadow voles had similar increases in extracellular dopamine associated with mating. There was a species-specific effect of social condition on extracellular dihydroxyphenylacetic acid (DOPAC). Exposure to a conspecific male significantly decreased extracellular DOPAC in female prairie voles isolated for approximately 18 h during surgical recovery. Such decrease in DOPAC was not seen if females experienced continued isolation or if they were housed with a sibling during surgical recovery. No changes in extracellular DOPAC were seen in meadow voles after manipulations of social environment. Together, our data indicate that mating-associated dopamine release is independent from mating systems. However, species-specific patterns of extracellular DOPAC suggest that social isolation may be a more stressful stimulus for the social prairie vole than for the asocial meadow vole.

Effect of one-week ethanol treatment on monoamine levels and dopaminergic receptors in rat striatum

We studied the effects of ethanol on the levels of norepinephrine, dopamine, serotonin (5-HT) and their metabolites as well as on D1- and D2-like receptors in the rat striatum. Ethanol (2 or 4 g/kg, po) was administered daily by gavage to male Wistar rats and on the 7th day, 30 min or 48 h after drug administration, the striatum was dissected for biochemical assays. Monoamine and metabolite concentrations were measured by HPLC and D1- and D2-like receptor densities were determined by binding assays. Scatchard analyses showed decreases of 30 and 43%, respectively, in D1- and D2-like receptor densities and no change in dissociation constants (Kd) 48 h after the withdrawal of the dose of 4 g/kg. Ethanol, 2 g/kg, was effective only on the density of D2-like receptors but not on Kd of either receptor. Thirty minutes after the last ethanol injection (4 g/kg), decreases of D2 receptor density (45%) as well as of Kd values (34%) were detected. However, there was no significant effect on D1-like receptor density and a 46% decrease was observed in Kd. An increase in dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC), a decrease in norepinephrine, and no alteration in 5-HT levels were demonstrated after 48-h withdrawal of 4 g/kg ethanol. Similar effects were observed in dopamine and DOPAC levels 30 min after drug administration. No alteration in norepinephrine concentration and a decrease in 5-HT levels were seen 30 min after ethanol (4 g/kg) administration. Our findings indicate the involvement of the monoaminergic system in the responses to ethanol.

Effect of isolation rearing on pre- and post-synaptic serotonergic function in the rat dorsal hippocampus

Several behavioural, neurochemical, and structural alterations found in isolation-reared rats are similar to those in human schizophrenia. This study investigated changes in cholinergic and serotonergic function in the hippocampus following isolation rearing. Rats were reared in social isolation from weaning for 6 weeks before study and compared to group-reared rats. An in vitro electrophysiological study investigated the effect of isolation rearing on postsynaptic 5-HT(1A) function on CA1 hippocampal neurones activated with the muscarinic agonist carbachol and found no change in the sensitivity of these postsynaptic receptors between the groups. However, a change in presynaptic function was identified, as there was a significant reduction in the time taken for neuronal firing to recover to 50% of the original rate following 5-HT (10 microM) application, in isolation compared to group-reared rats. These data suggest a possible change in reuptake following isolation. Uptake studies using (3)[H]5-HT, however, found no change in the inhibition of uptake produced by either fluoxetine or paroxetine in isolation compared to group-reared rats. The selective 5-HT(1B) antagonist CP-294253 (1 microM), increased endogenous 5-HT release from hippocampal slices in vitro and this effect was greater (P < 0.001) in group compared to isolation-reared rats. These results indicate that the change in presynaptic 5-HT neuronal function was due to impaired autoreceptor responsiveness. Carbachol (1 microM) increased the firing rate of all neurones recorded but only a proportion of these showed a concentration-related increase. Isolation rearing increased the sensitivity of neurones, showing a concentration-related increase in firing in response to carbachol, but had no effect on the other neurones. In summary, the present study showed that isolation rearing alters presynaptic 5-HT(1B) but not postsynaptic 5-HT(1A) receptor activity in the hippocampus. Isolation rearing in the rat results in hippocampal dysfunction, including reduced serotonergic and enhanced muscarinic activity of some neurones. These effects may in part underlie the behavioural consequences of isolation relevant to human developmental disorders.

Influence of aging and social isolation on changes in brain monoamine turnover and biosynthesis of rats elicited by novelty stress

Aging is a risk factor of human depression. Middle-aged or older men are vulnerable to adverse life events and an absence of social contact and easily become depressed. In the present study, we investigated the influence of aging on responses to life events in socially isolated conditions. We applied isolation-rearing (4 W) to two age groups, older (18 M) and younger (11 W), of male F344 rats that had been reared in a group and then examined responses to novelty stress (20 min). Changes in brain monoamines and their metabolites such as dopamine (DA), serotonin (5-HT), dihydroxyphenylacetic acid (DOPAC), homovanilic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) were measured in six regions: the prefrontal cortex, nucleus accumbens, hippocampus, amygdala, midbrain, and raphe nuclei. MANOVA was carried out for rearing condition, age, and novelty stress. Isolation significantly changed monoamines and their metabolites, except in amygdala and raphe nuclei. Aging significantly altered them in all regions, although novelty stress did not. In the amygdala and midbrain, isolation significantly changed monoamine biosynthesis, with monoamine turnover remaining unchanged. In the prefrontal cortex and nucleus accumbens, aging significantly altered turnover, while biosynthesis remained unchanged. Novelty stress significantly varied only the turnover in the prefrontal cortex. The interaction between isolation and aging indicated that aging influences changes in turnover and biosynthesis elicited by isolation primarily at the center of the mesolymbic DA system, the midbrain, and in raphe nuclei of the 5-HT system. In peripheral regions of the mesolymbic system, aging primarily affects changes in turnover induced by isolation.

Effects of isolation-rearing on serotonin-1A and M1-muscarinic receptor messenger RNA expression in the hipocampal formation of rats

The aim of the present work was to investigate if isolation rearing could change 5-HT1A or M1 muscarinic receptors messenger RNA (mRNA) expression in the hippocampal formation. Male Wistar rats were isolated either in single cages or in groups of six per cage soon after wearing during 30 days. After this period they were sacrificed and their brains removed for 'in situ' hybridization study using 32P-labeled oligonucleotide probes complementary to 5-HT1A or M1 muscarinic receptor mRNA. The results were analyzed by computerized densitometry. They showed a significant (P < 0.05, Mann-Whitney test) serotonin 1A (5-HT1A) mRNA expression increase in the dentate gyrus and CA3 areas of isolated animals. The signal also tended to be higher (P < 0.10) in CA1 and CA4 regions. No significant change on M1 mRNA expression was found. These results may reflect up-regulation of 5-HT1A gene transcription in response to deficits in hippocampal serotonin neurotransmission induced by social isolation.

Attenuating effects of the isolated rearing condition on increased brain serotonin and dopamine turnover elicited by novelty stress

Isolation and acute environmental change are risk factors in human depression. In the present study, we investigated the differences in the brain monoamine activity of rats between two rearing conditions, isolated and group. Moreover, we examined the responses to novelty stress. Male F344 rats aged 11 weeks were divided into the above two groups. Four weeks later they were further divided into non-stress and stress groups. The latter received 20 min exposure to novelty stress. Isolation significantly changed brain monoamine levels, with the levels of dopamine (DA) in the nucleus accumbens and midbrain, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the midbrain, and 5-hydroxyindoleacetic acid (5-HIAA) in the hippocampus increasing. Serotonin (5-HT) levels also increased in all brain areas except the raphe nuclei. HVA levels in the raphe nuclei decreased. Novelty stress significantly altered brain monoamine levels. DA, DOPAC, and HVA levels in the prefrontal cortex decreased, as did those of 5-HT in the prefrontal cortex and hippocampus. DA levels in the nucleus accumbens increased. Isolation attenuated the enhanced brain monoamine turnover elicited by novelty stress. The enhanced DA turnover ratio in the prefrontal cortex of the group-reared group was attenuated in the isolated-reared group, and the unchanged DA turnover ratio in the nucleus accumbens of the group-reared group declined in the isolated-reared group. The enhanced 5-HT turnover ratio in the prefrontal cortex, nucleus accumbens, and hippocampus of the group-reared group was attenuated in the isolated-reared group. Isolation may exacerbate adaptation to stress, and be related to the etiology of human depression.

Social dominance in monkeys: dopamine D2 receptors and cocaine self-administration

Disruption of the dopaminergic system has been implicated in the etiology of many pathological conditions, including drug addiction. Here we used positron emission tomography (PET) imaging to study brain dopaminergic function in individually housed and in socially housed cynomolgus macaques (n = 20). Whereas the monkeys did not differ during individual housing, social housing increased the amount or availability of dopamine D2 receptors in dominant monkeys and produced no change in subordinate monkeys. These neurobiological changes had an important behavioral influence as demonstrated by the finding that cocaine functioned as a reinforcer in subordinate but not dominant monkeys. These data demonstrate that alterations in an organism's environment can produce profound biological changes that have important behavioral associations, including vulnerability to cocaine addiction.

Dopamine and alcohol relapse: D1 and D2 antagonists increase relapse rates in animal studies and in clinical trials

A considerable number of animal studies on the effects of dopaminergic agents on alcohol intake behavior have been performed. Acute alcohol administration in rats induces dopamine release in the caudate nucleus and in the nucleus accumbens, an effect related among others to reinforcement. It has been repeatedly suggested that D1 and D2 receptor activation mediates reward. As alcohol consumption and dopaminergic transmission seem to have a close relationship, all kinds of dopaminergic agents may be regarded as putative therapeutics for preventing relapse. In a prospective European double-blind multicenter clinical trial, comparing the D1, D2, D3 antagonist flupenthixol and placebo in 281 chronic alcohol-dependent patients (27.4% women), the application of the Lesch typology made an outcome differentiation possible. It could be shown in which patients flupenthixol administration was followed by a significantly higher relapse rate and in which patient groups no differences were found when compared to placebo.

Regional variations in the effects of chronic ethanol administration on GABA(A) receptor expression: potential mechanisms

Gamma-aminobutyric acid type A (GABA(A)) receptors in brain adapt to chronic ethanol exposure via changes in receptor function and subunit expression. The present review summarizes currently available data regarding changes in GABA(A) receptor subunit mRNA and peptide expression. Data are presented from various different brain regions and the variations between specific brain regions used to draw conclusions about mechanisms that may underlie GABA(A) receptor adaptations during chronic ethanol exposure. In the whole cerebral cortex, chronic ethanol exposure leads to a reduction of GABA(A) receptor alpha1 subunit mRNA and peptide levels and a near equivalent increase in alpha4 subunit mRNA and peptide levels. This observation is the primary support for the hypothesis that altered receptor composition is a mechanism for GABA(A) receptor adaptation produced by chronic ethanol exposure. However, other brain regions do not display similar patterns of subunit changes. Moreover, subregions within cortex (prefrontal, cingulate, parietal, motor, and piriform) exhibit patterns of changes in subunit expression that differ from whole cortex. Therefore, regional differences in GABA(A) receptor subunit expression are evident following chronic ethanol administration, thus suggesting that multiple mechanisms contribute to the regulation of GABA(A) receptor expression. These mechanisms may include the involvement of other neurotransmitter systems, endogenous steroids and second or third messenger cross-talk.