Analysis of the biphasic locomotor response to ethanol in high and low responders to novelty: a study in Nijmegen Wistar rats

Impact of a novel environment on alcohol-induced locomotor activity in Wistar rats

Clinical studies have shown a positive correlation between novelty-seeking behavior and the susceptibility to consume drugs of abuse. Although several animal studies have demonstrated this correlation with psychostimulants or morphine, studies with alcohol have shown conflicting results. The aim of this work was to investigate alcohol-induced motor effects in Wistar rats with different responses to novelty. Animals were classified as Low- (LR) or High-Responders (HR) to novelty, depending on their horizontal activity in an automated open field. Motor activity was recorded in naïve, saline, and alcohol-administered rats at different doses (0.1, 0.25, 0.5, 1.0, or 2.5 g/kg). Horizontal movements, rearings, and stereotyped behaviors were evaluated. After the behavioral test, animals were sacrificed and blood alcohol concentrations (BACs) were measured. Low (0.1 and 0.25 g/kg) and high (2.5 g/kg) alcohol doses decreased horizontal movements in LR animals, whereas 1.0 g/kg increased this parameter in HR rats. Rearings were increased by alcohol 1.0 g/kg in LR animals. In HR rats, alcohol doses of 0.5 and 1.0 g/kg also increased this parameter. Stereotyped behaviors were decreased by an alcohol dose of 2.5 g/kg in LR animals, but were increased by an intermediate dose (1.0 g/kg) in HR rats. Differences in horizontal movements and rearings were found between LR and HR animals at certain ethanol doses. Horizontal movements (0.25 g/kg) and rearings (0.5 g/kg) were lower in LR than HR rats; however, rearings were lower in HR than LR rats at 1.0 g/kg. BACs were similar between LR and HR rats at all ethanol doses. These findings suggest that HR rats are more responsive to the stimulant effects of intermediate alcohol doses, whereas LR animals are sensitive to low/high doses of the drug. Sensitivity to alcohol motor effects may substantially depend on the initial animal's response to a novel environment. The stimulant effects of alcohol may constitute important behavioral traits significantly associated with the rewarding properties of the drug.

Prenatal ethanol exposure modifies locomotor activity and induces selective changes in Met-enk expression in adolescent rats

Several studies suggest that prenatal ethanol exposure (PEE) facilitates ethanol intake. Opioid peptides play a main role in ethanol reinforcement during infancy and adulthood. However, PEE effects upon motor responsiveness elicited by an ethanol challenge and the participation of opioids in these actions remain to be understood. This work assessed the susceptibility of adolescent rats to prenatal and/or postnatal ethanol exposure in terms of behavioral responses, as well as alcohol effects on Met-enk expression in brain areas related to drug reinforcement. Motor parameters (horizontal locomotion, rearings and stereotyped behaviors) in pre- and postnatally ethanol-challenged adolescents were evaluated. Pregnant rats received ethanol (2g/kg) or water during gestational days 17-20. Adolescents at postnatal day 30 (PD30) were tested in a three-trial activity paradigm (habituation, vehicle and drug sessions). Met-enk content was quantitated by radioimmunoassay in several regions: ventral tegmental area [VTA], nucleus accumbens [NAcc], prefrontal cortex [PFC], substantia nigra [SN], caudate-putamen [CP], amygdala, hypothalamus and hippocampus. PEE significantly reduced rearing responses. Ethanol challenge at PD30 decreased horizontal locomotion and showed a tendency to reduce rearings and stereotyped behaviors. PEE increased Met-enk content in the PFC, CP, hypothalamus and hippocampus, but did not alter peptide levels in the amygdala, VTA and NAcc. These findings suggest that PEE selectively modifies behavioral parameters at PD30 and induces specific changes in Met-enk content in regions of the mesocortical and nigrostriatal pathways, the hypothalamus and hippocampus. Prenatal and postnatal ethanol actions on motor activity in adolescents could involve activation of specific neural enkephalinergic pathways.

Voluntary ethanol consumption changes anticipatory ultrasonic vocalizations but not novelty response

Novelty and sensation seeking (NSS) and affective disorders are correlated with earlier ethanol (ETOH) consumption, and sustained drinking into adulthood. Understanding the NSS response and affective response before and after voluntary ETOH consumption could elucidate important individual differences promoting sustained ETOH consumption. This study determined that NSS and affective response to rewarding stimulation-measured by ultrasonic vocalizations (USVs)-change after adolescent ETOH voluntary drinking. Rats were tested for their NSS response using the inescapable novelty test. Then rats were tested for their affective response to a natural reward and USVs were measured. The natural reward was experimenter-induced play behavior. Rats were exposed to ETOH for 8 weeks using an intermittent two bottle paradigm. After 8 weeks of voluntary consumption, rats were retested for their response to NSS and affective response to natural reward. Results indicate that voluntary ETOH consumption did not change the response to novelty. Control and ETOH exposed rats decreased their novelty response equally after ETOH consumption, suggesting the decrease was due to age. Importantly, voluntary ETOH consumption changed affective USVs. Compared to water-drinking control rats, ETOH-consuming rats elicited greater anticipatory trill USVs to a natural reward-associated context during a post-drinking probe test. Tickle-induced trill USVs did not change differently between ETOH and control rats. These results provide evidence that voluntary intermittent ETOH exposure increases the anticipation of reward and may represent a form of incentive salience. We postulate these diverging effects could be due to differences in incentive salience or reward processing. Together, these results suggest that voluntary ETOH consumption changes the affective response to conditioned and unconditioned natural rewards and offers a behavioral mechanism for studying affective reward processing after ETOH consumption.

Neurochemical factors underlying individual differences in locomotor activity and anxiety-like behavioral responses in zebrafish

Variation among individuals may arise for several reasons, and may have diverse underlying mechanisms. Individual differences have been studied in a variety of species, but recently a new model organism has emerged in this field that offers both sophistication in phenotypical characterization and powerful mechanistic analysis. Recently, zebrafish, one of the favorites of geneticists, have been shown to exhibit consistent individual differences in baseline locomotor activity. In the current study, we further explore this finding and examine whether individual differences in locomotor activity correlate with anxiety-like behavioral measures and with levels of dopamine, serotonin and the metabolites of these neurotransmitters. In addition, we examine whether individual differences in locomotor activity are also associated with reactivity to the locomotor stimulant effects of and neurochemical responses to acute ethanol exposure (30min long, 1% v/v ethanol bath application). Principal component analyses revealed a strong association among anxiety-like responses, locomotor activity, serotonin and dopamine levels. Furthermore, ethanol exposure was found to abolish the locomotion-dependent anxiety-like behavioral and serotonergic responses suggesting that this drug also engages a common underlying pathway. Overall, our results provide support for an important role of the serotonergic system in mediating individual differences in anxiety-like responses and locomotor activity in zebrafish and for a minor modulatory role of the dopaminergic system.

A novel cholinergic action of alcohol and the development of tolerance to that effect in Caenorhabditis elegans

Understanding the genes and mechanisms involved in acute alcohol responses has the potential to allow us to predict an individual's predisposition to developing an alcohol use disorder. To better understand the molecular pathways involved in the activating effects of alcohol and the acute functional tolerance that can develop to such effects, we characterized a novel ethanol-induced hypercontraction response displayed by Caenorhabditis elegans. We compared body size of animals prior to and during ethanol treatment and showed that acute exposure to ethanol produced a concentration-dependent decrease in size followed by recovery to their untreated size by 40 min despite continuous treatment. An increase in cholinergic signaling, leading to muscle hypercontraction, is implicated in this effect because pretreatment with mecamylamine, a nicotinic acetylcholine receptor (nAChR) antagonist, blocked ethanol-induced hypercontraction, as did mutations causing defects in cholinergic signaling (cha-1 and unc-17). Analysis of mutations affecting specific subunits of nAChRs excluded a role for the ACR-2R, the ACR-16R, and the levamisole-sensitive AChR and indicated that this excitation effect is dependent on an uncharacterized nAChR that contains the UNC-63 α-subunit. We performed a forward genetic screen and identified eg200, a mutation that affects a conserved glycine in EAT-6, the α-subunit of the Na(+)/K(+) ATPase. The eat-6(eg200) mutant fails to develop tolerance to ethanol-induced hypercontraction and remains contracted for at least 3 hr of continuous ethanol exposure. These data suggest that cholinergic signaling through a specific α-subunit-containing nAChR is involved in ethanol-induced excitation and that tolerance to this ethanol effect is modulated by Na(+)/K(+) ATPase function.

The amphetamine response moderates the relationship between negative emotionality and alcohol use

BACKGROUND

Considerable evidence suggests that sensitivity to the stimulant effects of alcohol and other drugs is a risk marker for heavy or problematic use of those substances. A separate body of research implicates negative emotionality. The goal of the present study was to evaluate the independent and interactive effects of the stimulant response, assessed with an amphetamine challenge, and negative emotionality on alcohol and drug use.

METHODS

Healthy young women and men completed the Multidimensional Personality Questionnaire (MPQ) and an inventory assessing alcohol and other drug use. Subsequently, the effects of 10-mg d-amphetamine were determined in the laboratory using the Stimulant scale of the Biphasic Alcohol Effects Scale. Hierarchical regression analyses evaluated the effects of amphetamine response and the MPQ factor Negative Emotionality on measures of substance use.

RESULTS

The amphetamine response moderated relationships between negative emotionality and alcohol use: in combination with a robust amphetamine response (i.e., enhanced stimulant effects as compared with baseline), negative emotionality predicted greater alcohol consumption, more episodes of binge drinking, and more frequent intoxication in regression models. A strong stimulant response independently predicted having used an illicit drug, and there was a trend for it to predict having used alcohol. Negative emotionality alone was not associated with any measure of alcohol or drug use.

CONCLUSIONS

Consistent with the idea that emotion-based behavioral dysregulation promotes reward seeking, a high level of negative emotionality was associated with maladaptive alcohol use when it co-occurred with sensitivity to drug-based reward. The findings contribute to our understanding of how differences in personality may interact with those in drug response to affect alcohol use.

Acute ethanol administration differentially alters enkephalinase and aminopeptidase N activity and mRNA levels in regions of the nigrostriatal pathway

Opioid peptides play a key role in ethanol reinforcement and may also represent important determinants in brain sensitivity to ethanol through modulation of nigrostriatal dopaminergic activity. Regulation of opioid levels by peptidase-degrading enzymes could be relevant in ethanol's actions. The aim of this work was to study the acute ethanol (2.5 g/kg) effects on the activity and mRNA expression of enkephalinase (NEP) and aminopeptidase N (APN) in the rat substantia nigra (SN) and the anterior-medial (amCP) and medial-posterior (mpCP) regions of the caudate-putamen (CP). Enzymatic activities were measured by fluorometric assays and mRNA expression by reverse transcriptase polymerase chain reaction. Acute ethanol administration differentially altered peptidase activities and mRNA expression with different kinetics. Ethanol increased and decreased NEP mRNA levels in the SN and amCP, respectively, but produced biphasic effects in the mpCP. APN mRNA levels were increased by ethanol in all brain regions. Ethanol induced a transient and long-lasting increase in NEP (mpCP) and APN (amCP) activities, respectively. Peptidase activities were not changed by ethanol in the SN. Our results indicate that striatal NEP and APN are important ethanol targets. Ethanol-induced changes in these neuropeptidases in the CP could contribute to the mechanisms involved in brain sensitivity to ethanol.

Low and moderate doses of acute ethanol do not impair spatial cognition but facilitate accelerating rotarod performance in adolescent and adult rats

Adolescents and adult rodents have differing sensitivities to the acute effects of ethanol on a variety of behavioral and electrophysiological measures. Often, these differences are revealed using high ethanol doses and consequently little is known about these age-related effects using lower ethanol doses. We sought to determine if low-dose ethanol produces differential effects on cognition and motor behavior in adolescent and adult rats. Adolescent (postnatal day PD 30-32) and adult (PD 70-72) male Sprague Dawley rats were trained on the standard version of the Morris Water Maze (MWM) for 5 days or received 5 training trials on an accelerating rotarod (ARR). Adolescents learned the location of the submerged platform in the MWM significantly slower than adults during training and, acute ethanol administration (0.5 g/kg, 0.75 g/kg, or 1.0 g/kg) 30 min before testing did not impair spatial memory in either age group. On the ARR test, adolescent rats spent significantly more time on the rotarod compared to adults and, alcohol exposure (1.0 g/kg) significantly increased ARR performance 30 min following administration. Our findings address the utility of investigating low and moderate doses of ethanol during different developmental stages in rats.

Amphetamine modifies ethanol intake of psychosocially stressed male rats

Studies of socially housed rodents have provided significant information regarding the consequences of exposure to stressors. Psychosocial stressors are known to alter the ingestion of ethanol and the activity of the dopaminergic neuronal system. Since both stressors and ethanol are known to affect the function of dopaminergic neurons, we employed amphetamine to assess the role of this neural system on the ingestion of ethanol by psychosocially stressed male rats. Male rats housed two per cage were designated as dominant or subdominant rats based on evaluations of agonistic behavior and body weight changes. The dyad-housed rats and a group of single-housed rats were sequentially assessed for ethanol intake after injections of saline or amphetamine (0.3, 0.9 or 2.7 mg/kg i.p.) both prior to dyad housing and subsequently again during dyad-housing. Prior to dyad housing ethanol intake of future subdominant rats was higher than that of future dominant rats. Dyad-housing significantly increased ethanol intake of dominant rats. Pre-dyad the highest dose of amphetamine potently depressed ethanol ingestion. Sensitivity to amphetamine's depressant effect on ethanol intake was higher at the dyad test in all subjects, most prominently in single-housed rats. In contrast to the single-housed rats, the dyad-housed rats displayed saccharin anhedonia. It can be concluded that dopaminergic system modulates, at least partially, the psychosocial stress-induced changes in ethanol intake. Furthermore, the level of ethanol ingestion at the pre-dyad test was predictive of future hierarchical status.

Effects of acute ethanol administration on methionine-enkephalin expression and release in regions of the rat brain

The dopaminergic mesolimbic system plays a key role in mediating the reinforcing properties of ethanol and other drugs of abuse. Ethanol reinforcement and high alcohol drinking behaviour have been suggested to involve the ethanol-induced activation of endogenous opioid systems. Ethanol may alter opioidergic transmission at different levels, including opioid peptide biosynthesis and release, as well as binding to opioid receptors. The aim of this work was to investigate the effects of different ethanol doses on methionine-enkephalin (Met-enk) release from the rat nucleus accumbens (NAcc). Ethanol effects were also studied on Met-enk content in the NAcc, prefrontal cortex (PFC) and caudate-putamen (CP). Met-enk release was studied by microdialysis in Wistar anesthetized rats and peptide concentrations were quantitated by radioimmunoassay. Ethanol was administered by intraperitoneal injection after a 2-h basal release period. Ethanol doses of 0.5, 1 and 2.5 g/kg induced a 2.7-, 4.9- and 3.4-fold increase in Met-enk release from the NAcc. However, ethanol responses followed different kinetics, with earliest effects observed with the highest ethanol dose. In comparison, a 2.5-fold increase in peptide release was produced by 100 mM KCl. Ethanol, at a dose of 2.5 g/kg, induced a significant 66.7% decrease in Met-enk content in the NAcc, as well as a 76.4% reduction in peptide levels in the CP. Lower ethanol doses did not alter Met-enk content in these regions. On the other hand, an ethanol dose of 0.5 g/kg produced a non-significant decrease in Met-enk levels in the PFC. Our results suggest that ethanol-induced changes in enkephalin expression and release in regions of the mesocorticolimbic and nigrostriatal pathways could be involved in ethanol central effects. Released enkephalins by ethanol may modulate the dopaminergic activity of mesolimbic neurons and play a critical role in ethanol reinforcement mechanisms.

Predictors of ethanol consumption in adult Sprague-Dawley rats: relation to hypothalamic peptides that stimulate ethanol intake

To investigate mechanisms in outbred animals that increase the propensity to consume ethanol, it is important to identify and characterize these animals before or at early stages in their exposure to ethanol. In the present study, different measures were examined in adult Sprague-Dawley rats to determine whether they can predict long-term propensity to overconsume ethanol. Before consuming 9% ethanol with a two-bottle choice paradigm, rats were examined with the commonly used behavioral measures of novelty-induced locomotor activity and anxiety, as assessed during 15 min in an open-field activity chamber. Two additional measures, intake of a low 2% ethanol concentration or circulating triglyceride (TG) levels after a meal, were also examined with respect to their ability to predict chronic 9% ethanol consumption. The results revealed significant positive correlations across individual rats between the amount of 9% ethanol ultimately consumed and three of these different measures, with high scores for activity, 2% ethanol intake, and TGs identifying rats that consume 150% more ethanol than rats with low scores. Measurements of hypothalamic peptides that stimulate ethanol intake suggest that they contribute early to the greater ethanol consumption predicted by these high scores. Rats with high 2% ethanol intake or high TGs, two measures found to be closely related, had significantly elevated expression of enkephalin (ENK) and galanin (GAL) in the hypothalamic paraventricular nucleus (PVN) but no change in neuropeptide Y (NPY) in the arcuate nucleus (ARC). This is in contrast to rats with high activity scores, which in addition to elevated PVN ENK expression showed enhanced NPY in the ARC but no change in GAL. Elevated ENK is a common characteristic related to all three predictors of chronic ethanol intake, whereas the other peptides differentiate these predictors, with GAL enhanced with high 2% ethanol intake and TG measures but NPY related to activity.

Differential effects of acute and chronic ethanol exposure on orexin expression in the perifornical lateral hypothalamus

BACKGROUND

Recent reports support the involvement of hypothalamic orexigenic peptides in stimulating ethanol intake. Our previous studies have examined the effects of ethanol on hypothalamic peptide systems of the paraventricular nucleus (PVN) and identified a positive feedback loop in which PVN peptides, such as enkephalin and galanin, stimulate ethanol intake and ethanol, in turn, stimulates the expression of these peptides. Recently, orexin (OX), a peptide produced mainly by cells in the perifornical lateral hypothalamus (PFLH), has been shown to play an important role in mediating the rewarding aspects of ethanol intake. However, there is little evidence showing the effects that ethanol itself may have on the OX peptide system. In order to understand the feedback relationship between ethanol and the OX system, the current investigation was designed to measure OX gene expression in the PFLH following acute as well as chronic ethanol intake.

METHODS

In the first experiment, Sprague-Dawley rats were trained to voluntarily consume a 2 or 9% concentration of ethanol, and the expression of OX mRNA in the PFLH was measured using quantitative real-time polymerase chain reaction (qRT-PCR). The second set of experiments tested the impact of acute oral gavage of 0.75 and 2.5 g/kg ethanol solution on OX expression in the PFLH using qRT-PCR, as well as radiolabeled in situ hybridization. Further tests using digoxigenin-labeled in situ hybridization and immunofluorescence histochemistry allowed us to more clearly distinguish the effects of acute ethanol on OX cells in the lateral hypothalamic (LH) versus perifornical (PF) regions.

RESULTS

The results showed chronic consumption of ethanol versus water to dose-dependently reduce OX mRNA in the PFLH, with a larger effect observed in rats consuming 2.5 g/kg/d (-70%) or 1.0 g/kg/d (-50%) compared to animals consuming 0.75 g/kg/d (-40%). In contrast to chronic intake, acute oral ethanol compared to water significantly enhanced OX expression in the PFLH, and this effect occurred at the lower (0.75 g/kg) but not higher (2.5 g/kg) dose of ethanol. Additional analyses of the OX cells in the LH versus PF regions identified the former as the primary site of ethanol's stimulatory effect on the OX system. In the LH but not the PF, acute ethanol increased the density of OX-expressing and OX-immunoreactive neurons. The increase in gene expression was detected only at the lower dose of ethanol (0.75 g/kg), whereas the increase in OX peptide was seen only at the higher dose of ethanol (2.5 g/kg).

CONCLUSION

These results lead us to propose that OX neurons, while responsive to negative feedback signals from chronic ethanol consumption, are stimulated by acute ethanol administration, most potently in the LH where OX may trigger central reward mechanisms that promote further ethanol consumption.

Ethanol-mediated aversive learning as a function of locomotor activity in a novel environment in infant Sprague-Dawley rats

Unlike adult heterogeneous rats, infant rats are sensitive to ethanol's locomotor stimulating effects. Susceptibility to this ethanol effect varies as a function of baseline locomotor activity levels. Infant rats with higher baseline activity levels are more sensitive to ethanol's stimulating effects than those with lower baseline activity levels. The present study was designed to analyze susceptibility to ethanol-induced motivational learning in subpopulations of infant heterogeneous rats that differ in baseline activity in a novel environment. On postnatal day 11 (PD 11) baseline locomotor activity was registered and infants were divided into high and low responders (HR, LR). In Experiment 1, pups were trained in a procedure of conditioned taste aversion employing ethanol (0.0, 0.5 or 2.5 g/kg) as unconditioned stimulus (US) and saccharin as conditioned stimulus. In Experiment 2 the same procedure was employed with LiCl (0.0, 0.25 or 0.5% of body weight of a 0.3 M LiCl solution) as US. HR were more resistant to the aversive effects of ethanol than LR while magnitude of LiCl-induced conditioned taste aversion was similar in HR and LR. These results suggest the possibility of early detection of subpopulations of rats with differential sensitivity to ethanol's effects.

Novelty modulates the stimulating motor effects of ethanol in preweanling rats

During early ontogeny heterogeneous rats are sensitive to ethanol's stimulating effects. In adulthood locomotor activity in a novel environment is a valuable predictor of acute sensitivity to the activating effects of various drugs, including ethanol. Environmental novelty modulates response to ethanol and other drugs in adult rats. The present study analyzed the role of novelty in the acute locomotor response induced by ethanol earlier in development, during the preweanling period, a stage characterized by enhanced sensitivity to ethanol's reinforcing effects. In Experiment 1 we evaluated the predictive value of baseline locomotor activity upon ethanol-induced locomotor effects in 12-day-old rats. In Experiment 2 we tested whether repeated familiarization with the testing environment would reduce the stimulating effects induced by ethanol on postnatal day 12. Individual differences in response to an inescapable novel environment significantly predicted the locomotor activating effects of ethanol, but not other acute effects of the drug, such as hypothermia, motor impairment or sedation. Behavioral activation induced by ethanol during the preweanling period was attenuated after familiarization with the testing environment, suggesting that environmental novelty is critical for activating effects of ethanol.

Ethanol-induced changes in proenkephalin mRNA expression in the rat nigrostriatal pathway

Endogenous opioid systems have been suggested to play a key role in ethanol reinforcement mechanisms and alcohol-drinking behavior. Ethanol induces differential alterations in opioid peptide expression in brain areas of the reward circuits, which may be linked to the reinforcing effects of ethanol. In addition, ethanol-induced alterations in opioidergic nigrostriatal transmission could be involved in brain sensitivity to ethanol and play a role in addictive processes. The aim of this work was to study the effects of acute ethanol administration on proenkephalin (proenk) mRNA expression in the rat substantia nigra and caudate-putamen (CP) for up to 24 h post treatment. Male Wistar rats received ethanol (2.5 g/kg) or distilled water by intragastric administration, and proenk mRNA expression was studied by in situ hybridization and densitometry. Ethanol transiently increased proenk mRNA expression in the CP 1 h after drug administration. Proenk mRNA levels remained elevated 2 h post treatment in the anterior-medial and medial-posterior regions of the CP. In contrast, ethanol decreased proenk mRNA expression in the substantia nigra pars compacta and pars reticulata 2 h after drug exposure. Alterations in enkephalin expression in the substantia nigra and CP in response to ethanol exposure could be involved in the mechanisms underlying brain sensitivity to the drug.

Bupropion hydrochloride produces conditioned hyperactivity in rats

Bupropion is marketed as an antidepressant, Wellbutrin and smoking cessation aid, Zyban. Although the therapeutic neurological mechanisms of bupropion have not been fully elucidated, bupropion shares some behavioral similarities with classic psychomotor stimulants. The present study sought to further investigate these psychomotor stimulant effects of bupropion by assessing whether repeated administration of bupropion in a distinct environment produced conditioned hyperactivity. Paired rats received 10 daily i.p. injections of bupropion (2.5-30 mg/kg) before placement in locomotor chambers for 30 min. Bupropion (10-30 mg/kg) produced acute locomotor hyperactivity compared to Unpaired controls. After repeated administration, there was no progressive increase or decrease in bupropion-induced activity. In a subsequent drug-free session conditioned hyperactivity was observed at 5-30 mg/kg doses. In a follow-up experiment, we examined whether responsiveness to novelty predicted the subsequent unconditioned and conditioned locomotor effect of bupropion. Reactivity to inescapable novelty, novel environment approach, and novel-object interaction were measured before locomotor conditioning with 30 mg/kg bupropion. We replicated the previous experiment, but scores on the novelty screens did not predict locomotor response to bupropion. This study extends the literature by demonstrating that environmental cues repeatedly paired with the stimulant effects of bupropion come to evoke elevated activity in the absence of drug (i.e., conditioned hyperactivity). This finding is consistent with the literature suggesting that bupropion shares many behavioral similarities with other psychomotor stimulants which also produce conditioned hyperactivity. However, a predictive relation between reactivity to forced novelty and the subsequent locomotor effect of bupropion may not be one of these similarities.

Ethanol sensitization in a neurodevelopmental lesion model of schizophrenia in rats

Substance use disorder comorbidity in schizophrenia may reflect dysfunctional cortical-striatal-limbic circuitry commonly involved in the addiction process and the pathogenesis of schizophrenia. Rats with neonatal ventral hippocampal lesions (NVHL) demonstrate post-adolescent onset of schizophrenia-like symptoms and increased addiction vulnerability in paradigms using cocaine in adulthood. Here, we investigated response profiles of young adult NVHL vs. SHAM rats to ethanol, an addictive drug with many psychopharmacological effects divergent from those of cocaine, in a locomotor sensitization paradigm. Over 15 days of daily injections of saline, low (0.15 g/kg) or high (1.0 g/kg) doses of ethanol, NVHL rats showed stimulatory effects at the low dose compared to saline and high-dose conditions, while SHAM rats showed expected patterns of dose-dependent suppression of locomotor activity. In a challenge session 2 weeks later in which a moderate dose (0.25 g/kg) of ethanol was given to all subjects, NVHL rats with history of prior ethanol exposure showed greater locomotor activity consistent with installment of alcohol-induced sensitization not present in SHAMs. These findings provide further evidence of enhanced short- and long-term responsivity to abused drugs in a neurodevelopmental model of schizophrenia, and may reflect potentiation of common mechanisms of addiction shared between pharmacologically diverse addictive drugs.

Psychosocial stress alters ethanol's effect on open field behaviors

Psychosocial stress, including social rank status, has been shown to alter spontaneously occurring behaviors in rodents as well as the behavioral effects of drugs of abuse. In this study, rats were repeatedly evaluated in a modified open field following: their initial exposure, and after intraperitoneal injections of saline and 0.75 g/kg ethanol (EtOH). All subjects were first tested while under single housing conditions, then again following 35 days of differential housing (singly or 3 rats/cage) with social status determined by scoring agonistic behavior at triad formation. The data suggest that (1) future subordinate rats differed with respect to specific aspects of behavior displayed in a 'novel' open field arena, (2) future subordinate rats were more emotional since they showed greater "anxiety-like" behavior and less exploratory behavior, (3) subordinate rats were more impaired by the saline injection stress, (4) subordinate rats were more sensitive to the depressant effects of EtOH, (5) grooming behavior did not show habituation, in contrast to the other behaviors, but showed sensitization on the second test. Overall, subordinate rats may have differed from their cage mates in innate anxiety, and this may underlie their distinct response to both stressors and EtOH. Furthermore, while EtOH had mostly stimulant effects in naive rats, psychosocial stress and/or repeated testing resulted in enhancement of EtOH's depressant effects.

Motor stimulant effects of ethanol injected into the substantia nigra pars reticulata: importance of catalase-mediated metabolism and the role of acetaldehyde

A series of experiments was conducted to investigate the locomotor effects of local injections of ethanol and the ethanol metabolite, acetaldehyde, into substantia nigra pars reticulata (SNr). Infusions of ethanol into SNr resulted in a dose-related increase in locomotor activity, with maximal effects at a dose of 1.4 micromol. Ethanol injected into a control site dorsal to substantia nigra failed to stimulate locomotion, and another inactive site was identified in brainstem areas posterior to substantia nigra. The locomotor effects of intranigral ethanol (1.4 micromol) were reduced by coadministration of 10 mg/kg sodium azide, a catalase inhibitor that acts to reduce the metabolism of ethanol into acetaldehyde in the brain. SNr infusions of acetaldehyde, which is the first metabolite of ethanol, also increased locomotion. Taken together, these results indicate that SNr is one of the sites at which ethanol and acetaldehyde may be acting to induce locomotor activity. These results are consistent with the hypothesis that acetaldehyde is a centrally active metabolite of ethanol, and provide further support for the idea that catalase activity is a critical step in the regulation of ethanol-induced motor activity. These studies have implications for understanding the brain mechanisms involved in mediating the ascending limb of the biphasic dose-response curve for the effect of ethanol on locomotor activity.

Housing and rank status of male Long-Evans rats modify ethanol's effect on open-field behaviors

RATIONALE

Psychosocial stress is known to alter behavior of rodents. While psychosocial stress may alter the response to some drugs, the response to ethanol (EtOH) has not been evaluated.

OBJECTIVE

To examine open-field behaviors of triad- and singly housed rats treated acutely or voluntarily ingesting EtOH.

METHOD

Triad-housed rats were categorized as dominant, subdominant, or subordinate based on assessments of offensive and defensive behaviors. Open-field behaviors were monitored during a 10-min test in rats voluntarily ingesting a 6% solution of EtOH for 2 weeks (1), and after an i.p. injection of saline, 0.5 or 1.0 g kg(-1) of EtOH (2).

RESULTS

Daily intake of EtOH was highest in subdominant and lowest in dominant rats. Overall, open-field behaviors did not differ between water- and EtOH-consuming triad- or singly housed rats. The 0.5-g kg(-1) dose of EtOH enhanced locomotor activity only in triad-housed rats, center entries primarily in singly housed rats, and head-poke behavior in dominant and singly housed rats. Rearing behavior was not altered by the 0.5-g kg(-1) dose, but in singly housed rats, rearing behavior was depressed by the 1.0-g kg(-1) dose. This larger dose of EtOH had no effect on the other behaviors.

CONCLUSIONS

EtOH's effects on open-field behaviors show behavioral specificity and vary with the subject's housing and/or rank status. EtOH's acute anxiolytic-like effect was primarily evident in singly housed rats.

[3H]DPDPE binding to delta opioid receptors in the rat mesocorticolimbic and nigrostriatal pathways is transiently increased by acute ethanol administration

Dopaminergic transmission in the mesolimbic and nigrostriatal pathways plays a key role in the reinforcement mechanisms and brain sensitivity to ethanol, respectively. Ethanol reinforcement and high alcohol drinking behaviour have been postulated to be partially mediated by a neurobiological mechanism involving the ethanol-induced activation of the endogenous opioid system. Activation of opioid neural pathways by ethanol may include alterations in the processing, release and/or the receptor binding of opioid peptides. The aim of this work was to investigate the effects of acute ethanol administration on delta opioid receptors in the rat mesocortical, meso-accumbens and nigrostriatal pathways by quantitative receptor autoradiography, using [(3)H] (2-D-penicillamine, 5-D-penicillamine)-enkephalin as radioligand. A significant increase in [(3)H] (2-D-penicillamine, 5-D-penicillamine)-enkephalin binding was observed in the substantia nigra pars reticulata 1 h after ethanol treatment. Two hours after drug exposure, ligand binding was significantly increased in the frontal and prefrontal cortices, the core and shell regions of the nucleus accumbens, and in the anterior-medial and medial-posterior regions of the caudate-putamen. In contrast, ligand binding was significantly decreased in the posterior region of the caudate-putamen 30 min after ethanol administration. The observed effects may reflect ethanol-induced changes in ligand binding affinity and/or in receptor density. Our results suggest that transitory changes in delta opioid receptors with different kinetic patterns may be involved in ethanol reinforcement and brain sensitivity to the drug. Ethanol-induced delta receptor up- and down-regulation mechanisms may participate in modulation of dopaminergic transmission in the mesocorticolimbic and nigrostriatal pathways.

Hormesis and high-risk groups

The concept of hormesis (i.e., biological phenomena characterized by dose-response relationships displaying low-dose stimulation and high-dose inhibition) has important implications for current risk assessment practices because of its generalizability with respect to experimental model, agent, and endpoint measured. This paper addresses the question of whether hormesis is present in high-risk subpopulations and highly susceptible species. Evaluation of published data revealed that hormetic dose-response relationships occur with similar quantitative characteristics among species and individuals that display widely differing susceptibility to various toxicants. This observation suggests that the cause of the differential susceptibility in the more susceptible organisms is not due to the absence of the hormetic response but to some other factor(s). However, despite the recognition that hormetic responses are common and similar in susceptible and resistant organisms there are sufficient examples indicating that some strains/individuals may lack the capacity to produce the low-dose stimulatory response. Thus, the capacity to display hormetic effects is one of a variety of factors affecting differential susceptibility to xenobiotics and needs to be addressed within the hazard assessment process.

Behavioral sensitization to ethanol in rats: evidence from the Sprague-Dawley strain

Although it has been shown with other drugs of abuse, behavioral sensitization has not been shown with ethanol in rats. One possible reason for the negative previous findings may be due to the doses of ethanol employed in the different phases of sensitization. In the current experiment, outbred Sprague-Dawley rats were divided into either high or low responders to novelty. They were pretreated for 15 days with intraperitoneal injections of either saline or 1.0 g/kg ethanol, and then given a challenge dose of 0.25 g/kg ethanol after a 3-week period. During the first 10 min after the challenge dose, rats high in response to novelty pretreated with ethanol displayed higher locomotor activity scores relative to the other three groups. These data demonstrated evidence for behavioral sensitization with ethanol in outbred rats.

Individual differences in rat locomotor activity are diminished by nicotine through stimulation of central nicotinic acetylcholine receptors

An increasing body of research has focused on isolating factors that predict or alter individual differences in the behavioral and neural processes mediating the effects of abused drugs. Within this framework, the current report assessed individual differences and the locomotor effect of nicotine. Rats were screened for activity induced by a novel environment. Rats, which were more active to initial environment exposure, remained more active even after seven additional 30-min exposures to the same environment. Treatment with nicotine-di-D tartrate (1 mg/kg, sc) disrupted this effect. This nicotine disruption of individual differences occurred whether nicotine suppressed locomotor activity (initial administration) or stimulated locomotor activity (seventh and eighth administration). Mecamylamine (1 mg/kg), but not hexamethonium (10 mg/kg), completely blocked the suppressant and stimulant effects of nicotine. Further, mecamylamine restored the nicotine-induced disruption of individual differences; hexamethonium had no effect. This data pattern suggests that the disruptive effects of acute and chronic nicotine on individual differences were mediated by neural nicotinic acetylcholine (nACh) receptors.

Nicotine enhances acquisition of a T-maze visual discrimination: assessment of individual differences

In the present report, rats' performance was assessed in five tasks designed to measure behavioral response to different novel stimuli under different experimental situations. Daily nicotine treatment (0, 0.3 or 1.0 mg / kg) began after the conclusion of the behavioral tasks and continued throughout the experiment. Training of a T-maze visual discrimination task commenced after 11 days of nicotine pretreatment. As a group, rats treated with the higher dose of nicotine (1.0 mg / kg) made fewer errors to acquire the initial T-maze discrimination than saline-treated controls. Activity induced by an inescapable novel environment (i.e. first behavioral screen) was positively correlated with the number of errors to acquire the initial discrimination in the T-maze for the two nicotine-treated groups (0.3 and 1.0 mg / kg). To examine this positive correlation further, a median split analysis was conducted on the novelty-induced activity for each group. Nicotine, especially the high dose (1.0 mg / kg), enhanced performance in rats that were less active in the inescapable novel environment. Nicotine treatment did not affect the performance of rats that were more active in that environment. After the initial visual discrimination was acquired, the reverse discrimination was trained. Nicotine treatment did not affect performance; the number of errors to acquire the reversal for nicotine- and saline-treated rats did not differ. Overall a nicotine-induced improvement in performance is demonstrated which can be predicted by a rat's reactivity to environmental novelty.

Basal levels and alcohol-induced changes in nociceptin/orphanin FQ, dynorphin, and enkephalin levels in C57BL/6J mice

In order to investigate the involvement of the opioid and nociceptin/orphanin FQ (N/OFQ) system in alcohol drinking behaviour, N/OFQ and the opioid peptides dynorphin B (DYNB) and Met-enkephalin-Arg(6) Phe(7) (MEAP) were examined in the alcohol-preferring C57BL/6J mice. Basal peptide levels were compared in the brain and the pituitary gland with basal levels in the alcohol-avoiding DBA/2J mice. Furthermore, the effects of chronic alcohol self-administration on peptides were studied in the C57BL/6J mice. Compared to the DBA/2J mice, C57BL/6J mice had low immunoreactive (ir) levels of DYNB and MEAP in the nucleus accumbens, the hippocampus, and the substantia nigra, low ir-DYNB levels in the striatum and low ir-MEAP levels in the frontal cortex. Higher ir-DYNB levels in the pituitary gland and in the periaqueductal gray (PAG) and higher ir-N/OFQ levels in the frontal cortex and in the hippocampus were detected in C57BL/6J mice compared to the DBA/2J mice. After 4 weeks of voluntary alcohol consumption, only minor changes in steady-state peptide levels were identified. However, 5 days after the alcohol-drinking period, lower levels of all peptides were detected in the ventral tegmental area and ir-DYNB levels were also lower in the amygdala and in the substantia nigra. Twenty-one days after cessation of alcohol self-administration, the opioid peptides in alcohol-consuming C57BL/6J mice were lower in the PAG, the N/OFQ was lower in the frontal cortex and DYNB was higher in the amygdala and substantia nigra as compared to control C57BL/6J mice. This study demonstrates strain differences between C57BL/6J mice and DBA/2J mice that could contribute to divergent drug-taking behaviour, and it also demonstrates time- and structure-specific changes in neuropeptide levels after alcohol self-administration in the C57BL/6J mice.

High and low responders to novelty: effects of a catecholamine synthesis inhibitor on novelty-induced changes in behaviour and release of accumbal dopamine

The purpose of the present study was two-fold: (i) to investigate to what extent novelty, i.e. a novel cage with slightly larger dimensions than the home cage and lacking the floor covering that was originally present, produced behavioural effects in high responders to novelty and low responders to novelty that could be correlated with the extracellular amount of accumbal dopamine, using the microdialysis technique, and (ii) to establish the ability of the catecholamine synthesis inhibitor alpha-methyl-p-tyrosine to inhibit the novelty-induced responses in high and low responders. The difference in the behavioural response to novelty between the high and low responders was limited to walking, which lasted significantly longer in high responders than in low responders. The novelty-induced increase in extracellular concentration of accumbal dopamine was significantly greater in high responders than in low responders; moreover, the shape of the growth curves differed between high and low responders. The behavioural changes did not correlate with the neurochemical effects, which outlasted the duration of the novelty-induced behavioural arousal. It is hypothesized that this long-lasting increase in accumbal dopamine produces "adaptive changes" that help and/or allow the animal to incorporate knowledge about the condition that it experienced. When the nucleus accumbens was perfused with alpha-methyl-p-tyrosine for a period of 40 min, given at the same time as the transfer of the rat to the novel cage, it reduced the novelty-induced increase in walking in the high responders, but did not alter the novelty-induced behaviour of low responders. Finally, alpha-methyl-p-tyrosine reduced the novelty-induced increase in the release of accumbal dopamine in high responders, but enhanced it in low responders. The present neurochemical data are discussed in view of the outcome of earlier reported pharmacobehavioural studies on the neurochemical state of the nucleus accumbens of non-challenged versus challenged high and low responders. It is hypothesized that, in the high responder, exposure to novelty enhances the release of accumbal dopamine from reserpine-resistant, alpha-methyl-p-tyrosine-sensitive pools that are under the stimulatory control of beta-adrenergic receptors in the nucleus accumbens, and that, in the low responder, exposure to novelty enhances the release of accumbal dopamine from reserpine-sensitive, alpha-methyl-p-tyrosine-resistant pools that are under the inhibitory control of alpha-adrenergic receptors in the nucleus accumbens.

Nijmegen high and low responders to novelty: a new tool in the search after the neurobiology of drug abuse liability

Knowledge about the differences in structure, function, and reactivity of the brain and body between Nijmegen high responders to novelty and Nijmegen low responders to novelty may help us to understand which factors give rise to the vulnerability and/or susceptibility to drugs of abuse. For that purpose, this contribution provides a short overview of the outcome of the available studies on Nijmegen high responders to novelty and Nijmegen low responders to novelty. These animals can be selected using three major behavioral paradigms: (a) the open-field test (which allows the separation of high and low responders to novelty); (n) the intruder test (which allows the separation of fleeing and nonfleeing rats); (c) the apomorphine test (which allows the separation of apomorphine-susceptible and apomorphine-unsusceptible rats). Data to date suggest that the same traits have been selected by all three paradigms, and point to the hypothesis that the neurochemical state of the nucleus accumbens directs the sensitivity to drugs of abuse. In addition, recent evidence suggests that the sensitivity to the psychostimulant and/or reinforcing effects of dexamphetamine and ethanol is smaller in HR than in LR under certain experimental conditions, whereas the reverse is found when different experimental conditions are chosen. The data all together lay the foundation for the overall hypothesis that there are three factors ultimately determining the individual-specific sensitivity to drug of abuse: (a) the genetic background that predisposes an individual to become a HR or a LR, (b) early postnatal factors that direct the phenotypic expression of a particular genotype at adult age, and (c) the degree of stress during exposure to the drug of abuse. Further testing of this hypothesis may provide important information about the factors that contribute to individual differences in vulnerability to drugs of abuse.

No major differences in locomotor responses to dexamphetamine in high and low responders to novelty: a study in Wistar rats

The aim of the study was to compare locomotor responses to acute and sub-chronic dexamphetamine in two distinct types of Wistar rats, namely the Nijmegen high responders to novelty (HR) and Nijmegen low responders to novelty (LR). HR and LR were chosen because they differ in neurochemical processes relevant to the control of the locomotor effects of dexamphetamine, such as the dopaminergic and adrenergic activity in the nucleus accumbens. In experiment 1, a dexamphetamine dose-response curve (0.0-2.0 mg/kg/i.p.) was established using standard activity boxes. The dose-response curve slightly, but significantly, differed between HR and LR: especially the increase elicited by 1.5 mg/kg dexamphetamine was significantly greater in HR than in LR. In experiment 2, locomotor effects of sub-chronic administration of dexamphetamine (1.0 mg/kg/i.p.) were analyzed in HR and LR for 5 consecutive days. HR showed a higher locomotor response to dexamphetamine than LR; however, the two groups did not differ in their sensitization rate. It is concluded that there are neither major HR-LR differences in the locomotor response to acute administration of various doses of dexamphetamine nor HR-LR differences in the rate of sensitization of this locomotor response to sub-chronic administration of dexamphetamine. Type-specific differences in the mutual interaction between corticosteroids and dexamphetamine as well as the nature of the chosen dependent variable, namely locomotor activity, are hypothesized to underlie the results of the present study.

Evidence for the involvement of dopamine receptors in ethanol-induced hyperactivity in mice

Based on the hypothesis that drugs of abuse increase locomotor activity through mechanisms related to reinforcement, i.e. the mesolimbic dopamine (DA) system, ethanol-induced hyperactivity might provide a screening model to investigate the effect of ethanol on reward pathways. In the present study, ethanol had bidirectional effects on locomotion in mice: hyperactivity at low doses (2-3 g/kg) and sedation at high doses (4-5 g/kg). Such high doses induced a loss of righting reflex (LRR). The stimulant effect of ethanol was blocked by the D2/D3 antagonists, haloperidol (0.2 mg/kg) and tiapride (30-60 mg/kg), and by the D1 antagonist, SCH 23390 (0.03 mg/kg) whereas the non selective DA antagonist, clozapine decreased ethanol-induced hyperactivity at a dose (1 mg/kg) which also decreased activity in control animals. Unlike haloperidol and clozapine which potentiated LRR induced by ethanol, the selective DA antagonists, tiapride and SCH 23390, had no effect. Pretreatment with the D2/D3 agonist, quinpirole (0.1-0.3 mg/kg), reduced hyperactivity induced by ethanol presumably by stimulation of pre-synaptic receptors but did not change LRR. The D1 full agonist, SKF 81297 which produced hyperactivity by itself and the D1 partial agonist, SKF 38393, did not specifically affect ethanol-induced activities. The results indicate that activation of D1 and D2/D3 DA receptors is implicated in ethanol-induced hyperactivity whereas other mechanisms might mediate the sedative effects of ethanol. Tiapride and haloperidol, both used in the management of alcohol dependence, might exert beneficial effects by counteracting the reinforcing effects of ethanol. Tiapride's lack of interaction with the depressant effects of ethanol may account for its better tolerance in alcoholic patients.