Pre-exposed Rats Research Articles

RESEARCH FOCUS ON COMPULSIVE BEHAVIOUR IN ANIMALS: Pre‐exposure to environmental cues predictive of food availability elicits hypothalamic–pituitary–adrenal axis activation and increases operant responding for food in female rats

The present study was undertaken to develop an animal model exploiting food cue-induced increased motivation to obtain food under operant self-administration conditions. To demonstrate the predictive validity of the model, rimonabant, fluoxetine, sibutramine and topiramate, administered 1 hour before the experiment, were tested. For 5 days, female Wistar rats were trained to self-administer standard 45 mg food pellets in one daily session (30 minutes) under FR1 (fixed ratio 1) schedule of reinforcement. Rats were then trained to an FR3 schedule and finally divided into two groups. The first group (control) was subjected to a standard 30 minutes FR3 food self-administration session. The second group was exposed to five presentations of levers and light for 10 seconds each (every 3 minutes in 15 minutes total). At the completion of this pre-session phase, a normal 30-minute session (as in the control group) started. Results showed that pre-exposure to environmental stimuli associated to food deliveries increased response for food when the session started. Corticosterone and adrenocorticotropic hormone plasma levels, measured after the 15-minute pre-exposure, were also significantly increased. No changes were observed for the other measured hormones (growth hormone, prolactin, thyroid-stimulating hormone, luteinizing hormone, insulin, amylin, gastric inhibitor polypeptide, ghrelin, leptin, peptide YY and pancreatic polypeptide). Rimonabant, sibutramine and fluoxetine significantly reduced food intake in both animals pre-exposed and in those not pre-exposed to food-associated cues. Topiramate selectively reduced feeding only in pre-exposed rats. The present study describes the development of a new animal model to investigate cue-induced increased motivation to obtain food. This model shows face and predictive validity, thus, supporting its usefulness in the investigation of new potential treatments of binge-related eating disorders. In addition, the present findings confirm that topiramate may represent an important pharmacotherapeutic approach to binge-related eating.

Delayed Cardioprotective Effects of Hyperoxia Preconditioning Prolonged by Intermittent Exposure

In our previous study, it was indicated that pre-exposing rats to normobaric hyperoxia could induce a late preconditioning against infarction and arrhythmia. In this study, attempts were made to know whether the intermittent pre-exposure to the same environment could prolong the late phase of hyperoxia preconditioning. In the first series of experiments, rats were divided into five groups; group 1 was pre-exposed to normal air (NOR) and the other groups to hyperoxic air (O(2)>95%, 120 min once a d) 12, 24, 48, and 72 h (H12, H24, H48, and H72 groups) before 30 min ischemia. In the second series of experiments, rats were pre-exposed to intermittent hyperoxic air (1, 2, or 3 consecutive d) at different times before being subjected to ischemia (H48, H2-48, H2-72, H3-72, and H3-96 groups). The infarct size was measured by triphenyltetrazolium chloride staining, and lead II of electrocardiogram recorded to monitor ischemic-induced arrhythmia. Compared with NOR group, the infarct size and incidence of arrhythmia were reduced significantly in H24 and H48 groups. When the exposure periods were enhanced to 2 d, the infarct size did not decrease significantly, but the incidence of arrhythmia reduced. When the pre-exposure times were enhanced to 3 d, both the infarct size and incidence of arrhythmia decreased significantly in H3-72 group, but not in H3-96 group. These results show that the late phase of hyperoxia preconditioning may last for more than 48 h and prolong by intermittent per-exposure to the same environment.

Effects of microinjections of apomorphine and haloperidol into the inferior colliculus on the latent inhibition of the conditioned emotional response

Electrical or chemical stimulation of the inferior colliculus (IC) induces fear-like behaviors. More recently, consistent evidence has shown that electrical stimulation of the central nucleus of the IC supports Pavlovian conditioning and latent inhibition (LI). LI is characterized by retardation in conditioning and also by an impaired ability to ignore irrelevant stimuli, after a non-reinforced pre-exposure to the conditioned stimulus. LI has been proposed as a behavioral model of cognitive abnormalities seen in schizophrenia. The aim of the present study was to determine whether dopaminergic mechanisms in the IC are involved in LI of the conditioned emotional response (CER). To induce LI, a group of rats was pre-exposed (PE) to six tones in two sessions, while rats that were not pre-exposed (NPE) had two sessions without tone presentations. The conditioning consisted of two tone presentations to the animal, followed immediately by a foot shock. PE and NPE rats received IC microinjections of physiological saline, the dopaminergic agonist apomorphine (9.0 μg/0.5 μL/side), or the dopaminergic antagonist haloperidol (0.5 μg/0.5 μL/side) before both pre-exposure and conditioning. During the test, the PE rats that received saline or haloperidol had a lower suppression of the licking response compared to NPE rats that received vehicle or haloperidol, indicating that latent inhibition was induced. There was no significant difference in the suppression ratio in rats that received apomorphine injections into the IC, indicating reduced latent inhibition. These results suggest that dopamine-mediated mechanisms of the IC are involved in the development of LI.

Delta-Opioid Receptor Antagonists Prevent Sensitization to the Conditioned Rewarding Effects of Morphine

Functional interactions between mu- and delta-opioid receptors (MOPr and DOPr, respectively) are implicated in morphine tolerance and dependence. The contribution of DOPr to the conditioned rewarding effects of morphine and the enhanced conditioned response that occurs after repeated morphine administration is unknown. This issue was addressed with the conditioned place preference procedure (CPP). Rats received home cage injections of saline or morphine (5.0 mg/kg/day x 5 days) before conditioning. For sensitization studies, DOPr antagonists (DOPr1/2: naltrindole, DOPr2: naltriben, DOPr1: 7-benzylidenenaltrexone) were administered before morphine injections. Conditioning sessions (2 morphine; 2 saline) commenced 3 days later. To assess the influence of acute DOPr blockade on the conditioning of morphine reward in naïve animals, 3 morphine and 3 saline conditioning sessions were employed. Antagonists were administered before morphine conditioning sessions. Morphine was ineffective as a conditioning stimulus after two conditioning sessions in naïve rats. However, doses > or = 3.0 mg/kg produced significant CPP in morphine pre-exposed rats, confirming that sensitization develops to the conditioned rewarding effects of morphine. In animals that received morphine pre-exposure with naltrindole or naltriben but not 7-benzylidenenaltrexone, sensitization was prevented. No attenuation of morphine CPP was observed in animals that received DOPr antagonists acutely, before conditioning sessions. These data indicate a critical role of DOPr systems in mediating sensitization to the conditioned rewarding effects of morphine. The efficacy of naltrindole and naltriben in preventing the enhanced response to morphine suggest the specific involvement of DOPr2 in the sensitization process.

Blockade of group II metabotropic glutamate receptors produces hyper-locomotion in cocaine pre-exposed rats by interactions with dopamine receptors

It was previously reported that blockade of group II metabotropic glutamate receptors (mGluRs) produces hyper-locomotion in rats previously exposed to amphetamine, indicating that group II mGluRs are well positioned to modulate the expression of behavioral sensitization by amphetamine. The present study further examined the locomotor activating effects of specific blockade of these receptors after cocaine pre-exposures. First, rats were pre-exposed to seven daily injections of cocaine (15 mg/kg, IP). When challenged the next day with an injection of either saline or the group II mGluR antagonist LY341495 (0.5, 1.0 or 2.5 mg/kg, IP), they produced hyper-locomotor activity, measured by infrared beam interruptions, to LY341495 compared to saline in a dose-dependent manner. Second, rats were pre-exposed to either saline or seven daily injections of cocaine (15 mg/kg, IP). Three weeks later, when they were challenged with an injection of either saline or LY341495 (1.0 mg/kg, IP), only rats pre-exposed to cocaine produced hyper-locomotor activity to LY341495 compared to saline. These effects, however, were not present when dopamine D1 (SCH23390; 5 or 10 μg/kg), but not D2 (eticlopride; 10 or 50 μg/kg), receptor antagonist was pre-injected, indicating that this cocaine-induced hyper-locomotor activity to LY341495 may be mediated in dopamine D1 receptor-dependent manner. These results suggest that group II mGluRs may be adapted to interact with dopaminergic neuronal signaling in mediating the sensitized locomotor activity produced by repeated cocaine pre-exposures.

Time-dependent change of ERK phosphorylation levels in the nucleus accumbens during withdrawals from repeated cocaine

Extracellular signal-regulated kinase (ERK) has been implicated in cocaine-induced behavioral sensitization. We sought to determine whether ERK activation in the nucleus accumbens (NAcc), the site mediating the expression of behavioral sensitization, shows time-dependent changes after cocaine withdrawals. The basal levels of ERK phosphorylation in the NAcc show no changes on withdrawal day 1, while they increase on day 7, then gradually lower down to reach the same level on day 21 in cocaine compared to saline pre-exposed rats. Either total ERK or both phosphorylated and total p38 protein levels are not different in any time-point measurements. These results suggest that such time-dependent ERK activation in the NAcc may contribute to neuronal plasticity leading to long-lasting behavioral changes such as drug craving.

Long-term increase in GAD67 mRNA expression in the central amygdala of rats sensitized by drugs and stress

Repeated administration of addictive drugs and prolonged exposure to stressful stimuli induce sensitization to their behavioural stimulant properties. In this study, male Sprague-Dawley rats were repeatedly exposed to morphine [twice a day for 3 days at increasing doses, 10, 20, 40 mg/kg subcutaneously (s.c)], amphetamine (1 mg/kg s.c., once a day for 10 days), nicotine (0.4 mg/kg s.c., once a day for 5 days) and stress (food restriction for 7 days). After an interval of 3-30 days, depending on the pretreatment, rats were challenged with vehicle, with the same drug received as pretreatment (5 mg/kg of morphine, 0.5 mg/kg of amphetamine or 0.4 mg/kg of nicotine, respectively) or, in the case of food-restricted rats, with 0.5 mg/kg of amphetamine. Thereafter, changes in the expression of glutamic acid decarboxylase (GAD)67 mRNA were estimated by in situ hybridization in the central nucleus of the amygdala (CeA), basolateral amygdala (BLA), dorsolateral striatum (dLStr), nucleus accumbens shell (AcS) and core (AcC). All sensitizing pretreatments increased GAD67 mRNA in the CeA. Drug challenge did not further affect GAD67 mRNA in the CeA of saline, drug and stress pre-exposed rats. As to the other areas, no differences were observed in drug pre-exposed compared with saline pre-exposed and fed ad libitum rats, except for amphetamine. Amphetamine pre-exposure decreased GAD67 mRNA levels in the dLStr and the AcC and AcS, and this effect was reversed by amphetamine challenge. The results show that different drugs and stress models of behavioural sensitization have in common an increase of GA67 in the CeA but not in the BLA, and suggest the changes of GAD67 in the CeA are a substrate of the sensitized response to drug challenge.

Expression of AP-1 family transcription factors in the amygdala during conditioned taste aversion learning: Role for Fra-2

Conditioned taste aversion (CTA) learning occurs after the pairing of a novel taste with a toxin (e.g. sucrose with LiCl). The immediate early gene c-Fos is necessary for CTA learning, but c-Fos alone cannot be sufficient for consolidation. The expression of other AP-1 proteins from the Fos- and Jun-families may also be required shortly after conditioning for CTA consolidation. To screen for the expression of AP-1 transcription factors within small subregions, RT-PCR analysis was used after laser capture microdissection of the amygdala. Rats were infused intraorally with 5% sucrose (6 ml/6 min) or injected with LiCl (12 ml/kg, 0.15 M, i.p.) or given sucrose paired with LiCl (sucrose/LiCl), or not treated; 1 h later their brains were dissected. The lateral (LA), basolateral (BLA), and central (CeA) subnuclei of the amgydala of single 5 μm sections from individual rats were dissected using the Arcturus PixCell II system. Semi-quantitative RT-PCR showed the consistent presence of c-Fos, Fra-2, c-Jun, and JunD in the amygdala. In situ hybridization confirmed that c-Fos and Fra-2 mRNA expression was increased in the CeA after LiCl and sucrose/LiCl treatment. Immunohistochemistry for Fra-2 revealed high baseline levels of Fra-2 protein in the BLA and CeA, but also an increase in Fra-2 in the BLA and CeA after LiCl and sucrose/LiCl treatment. The similarity of response in LiCl and sucrose/LiCl treated groups might reflect activation by LiCl in both groups. To control for the effects of LiCl, rats were tested in a learned safety experiment. Fra-2 and c-Fos were examined in response to sucrose/LiCl in rats with prior familiarity with sucrose compared to rats without prior exposure to sucrose. The familiar (pre-exposure) group showed a significantly decreased number of Fra-2-positive cells compared with the novel group in the BLA, but not in the CeA. Because pre-exposure to sucrose attenuates CTA learning, a decreased cellular response in pre-exposed rats suggests a specific correlation with CTA learning. Changes in Fra-2 and c-Fos expression in the BLA and CeA at the time of conditioning, together with constitutive expression of c-Jun and JunD, may contribute to CTA learning.

Reduced sensitivity to MDMA-induced facilitation of social behaviour in MDMA pre-exposed rats

The acute effects of the party drug 3,4-methylenedioxymethamphetamine (MDMA, “Ecstasy”) in humans include feelings of love, closeness towards other people and an increased acceptance of others views and feelings. Some evidence suggests that regular MDMA users develop a subsensitivity to the positive effects of the drug and escalate their intake of the drug over time as a result. The current study investigated whether brief exposure to relatively high doses of MDMA in rats produces a subsequent attenuation in the ability of MDMA to enhance social interaction. Male Wistar rats were exposed to either MDMA (4 × 5 mg/kg over 4 h) or vehicle on two consecutive days. Twelve weeks later, MDMA pre-exposed rats displayed a significantly shorter period of time spent in social interaction than controls when tested in the drug-free state. MDMA pre-exposed rats also showed a blunted prosocial response to MDMA (2.5 mg/kg) relative to controls. This difference was overcome by increasing the MDMA dose to 5 mg/kg. The 5-HT 1A agonist 8-OH-DPAT (250 µg/kg but not 125 µg/kg) increased social interaction and this effect did not differ in MDMA and vehicle pre-exposed rats. HPLC analysis showed a small but significant depletion of prefrontal 5-HT and 5-HIAA in MDMA pre-exposed rats. Prefrontal 5-HIAA concentrations were also reduced in the subset of vehicle and MDMA pre-exposed rats that received additional testing with MDMA. These results indicate that treatment with MDMA not only causes lasting reductions in social interaction in rats but causes an attenuation of the prosocial effects of subsequent MDMA administration. The lack of a differential response to 8-OH-DPAT agrees with other findings that the 5-HT 1A receptor system remains functionally intact following MDMA pre-exposure and suggests that other neuroadaptations may underlie the lasting social deficits caused by MDMA.

Beneficial effects of Centella asiatica aqueous extract against arsenic‐induced oxidative stress and essential metal status in rats

The efficacy of an aqueous extract of Centella asiatica (100, 200 and 500 mg/kg for 5 consecutive days) was studied in the depletion of arsenic and in the recovery of a few altered biochemical variables in arsenic pre-exposed rats (20 ppm in drinking water for 5 weeks). Exposure to arsenic significantly depleted delta-aminolevulinic acid dehydratase (ALAD) activity, reduced glutathione (GSH) level, superoxide dismutase (SOD) and increased thiobarbituric acid reactive substance (TBARS) activity in red blood cells. Significant depletion of ALAD activity, GSH level, glutathione peroxidase (GPx), SOD and catalase (CAT) activities and an increase in TBARS levels in liver tissues was also noted. There was a significant depletion of SOD, CAT and GPx activities in kidneys and an increased TBARS levels in kidney and brain accompanied by increased arsenic concentration in blood and soft tissues. Treatment with aqueous extract of Centella asiatica provided significant protection against ALAD, GSH and TBARS levels, particularly at doses of 200 and 500 mg. Centella asiatica also provided significant recovery in the inhibited liver ALAD and G6PD activities. Arsenic concentration in blood and soft tissues remained uninfluenced after Centella asiatica administration. The present study thus suggests a beneficial effect of Centella asiatica against arsenic-induced oxidative stress but possesses no chelating property.

Increased locomotor response to amphetamine induced by the repeated administration of the selective kappa‐opioid receptor agonist U‐69593

Acute administration of kappa opioid receptor (KOR) agonists decreases both dopamine (DA) extracellular levels in the nucleus accumbens (NAc) and locomotor activity. Opposing to its acute effects, recent studies show that chronic administration of KOR agonists potentiates both stimulated DA release and induced locomotor activity. Since KOR agonists have been considered as potential treatment for stimulant dependence, the effects of their repeated administration on psychostimulant actions are of major concern. The present study was undertaken to investigate the in vivo effect of repeated administration of the KOR agonist U-69593 on DA extracellular levels in the NAc and on the locomotor activity challenged with amphetamine. Rats were injected once daily with the selective KOR agonist U-69593 or vehicle for four consecutive days. One-day after the last U-69593 injection, microdialysis studies assessing extracellular DA levels in the NAc and locomotor activity challenged with amphetamine were conducted. Microdialysis studies revealed that preexposure to U-69593 had no effect on basal DA levels but significantly augmented amphetamine-induced DA extracellular levels. Accordingly, amphetamine-induced locomotor activity was also significantly potentiated in U-69593 preexposed rats. These results suggest that long-term effect of KOR activation results in facilitation of amphetamine-induced DA extracellular levels in the NAc accompanied by sensitization of amphetamine-induced increase in locomotor activity.

Previous Exposure to Cocaine Enhances Cocaine Self-Administration in an Alpha 1-Adrenergic Receptor Dependent Manner

Noradrenergic transmission is implicated in the biochemical and behavioral effects of cocaine. Recently, we demonstrated that the alpha 1-adrenergic receptor antagonist prazosin attenuates cocaine-induced reinstatement of drug-seeking behavior. We now assessed whether prazosin could counter the effect of previous exposure to cocaine to enhance subsequent self-administration behavior. Rats were pre-exposed to systemic injections of either saline, prazosin (0.3 mg/kg), saline+cocaine (10 mg/kg), or prazosin+cocaine for 5 days. Starting 15-18 days after the last pre-exposure injection, rats were trained to self-administer cocaine (0.5 mg/kg/infusion) under a fixed ratio 3 (FR3) schedule of reinforcement. Several tests were conducted. First, responding for cocaine under an FR3 schedule was assessed across several doses (0.125-1.0 mg/kg/infusion). Second, responding for cocaine (0.5 mg/kg/infusion) under a progressive-ratio (PR) schedule was examined for 6 consecutive days. Finally, responding for cocaine (0, 0.5, and 1.0 mg/kg/infusion) was determined under the PR schedule of reinforcement. Results showed that cocaine pre-exposed rats self-administer more cocaine compared to saline pre-exposed rats when tested under both the FR and PR schedules. Rats pre-exposed to cocaine plus prazosin did not show enhanced cocaine self-administration. These rats, as well those pre-exposed to prazosin alone, showed levels of cocaine self-administration similar to saline pre-exposed rats. Thus, previous exposure to cocaine enhanced cocaine self-administration, an effect that appears to involve activation of alpha 1-adrenergic receptors. These data, along with several recent studies, show further support for the contribution of noradrenergic transmission in the behavioral effects of cocaine.

Cocaine self-administration in rats with histories of cocaine exposure and discrimination

Interrelationships between the discriminative stimulus and reinforcing properties of psychoactive drugs and the way in which they may interact to control drug intake are unclear. Studies have shown that drug history can influence the expression of drug-produced behavioral effects. The present study examined the acquisition and maintenance of intravenous cocaine self-administration in rats with a history of drug discrimination. Two groups of male hooded rats (n=12 each) were successfully trained in a single-lever food-reinforced procedure to discriminate cocaine (10 mg/kg) from saline. Control groups (n=12 each) received drug injections and/or saline injections only and lever-pressed for food reinforcers with no discrimination training. Subsequently, all subjects were implanted with chronic jugular catheters and allowed to nose-poke for infusions of cocaine (0.2 mg/kg per infusion). Initial rates of responding were similar for all groups. Acquisition of self-administration on a FR-10 schedule of drug delivery was significantly faster for cocaine-exposed rats in comparison to all other groups (P<0.02). There were no differences between groups in the breaking points of cocaine and saline on a progressive ratio schedule of self-administration. Dose-response functions were obtained by two methods and were similar for all groups. These results are consistent with earlier studies demonstrating weakly sensitized primary reinforcing properties of cocaine in preexposed rats. Previous learning to discriminate cocaine impaired this sensitization.

Nicotine pre-exposure does not potentiate the locomotor or rewarding effects of Δ-9-tetrahydrocannabinol in rats

This study assessed the effects of nicotine pre-exposure on subsequent locomotor and rewarding effects of repeated Delta-9-tetrahydrocannabinol administration in Sprague-Dawley rats. Repeated administration of the same dose of Delta-9-tetrahydrocannabinol (0.01-2 mg/kg) did not produce significant tolerance or behavioral sensitization to Delta-9-tetrahydrocannabinol's locomotor effects. An unbiased place conditioning paradigm was then used to obtain a measure of the rewarding effects of Delta-9-tetrahydrocannabinol. Rats received an injection of either Delta-9-tetrahydrocannabinol (0.01-2 mg/kg) before being placed in one compartment (three trials) or saline before being placed in the other compartment (three trials) of a two-compartment apparatus. Control rats received saline injections associated with both compartments. Significant conditioned place preferences developed with 0.1 mg/kg Delta-9-tetrahydrocannabinol in control rats, but not in nicotine pre-exposed rats. Surprisingly, significant place aversions developed at higher 1 and 2 mg/kg doses of Delta-9-tetrahydrocannabinol in nicotine pre-exposed rats. To the extent that behavioral sensitization may reflect reward processes in drug dependence, the lack of behavioral sensitization on repeated Delta-9-tetrahydrocannabinol administration is consistent with the difficulties usually encountered in demonstrating rewarding or reinforcing effects of Delta-9-tetrahydrocannabinol in rats. The present findings suggest, moreover, that nicotine pre-exposure alters the qualitative nature of rewarding effects and accentuates aversive effects of Delta-9-tetrahydrocannabinol.

Repeated administration of the selective kappa-opioid receptor agonist U-69593 increases stimulated dopamine extracellular levels in the rat nucleus accumbens

Reinforcing properties of drugs of abuse are reduced by the coadministration of kappa opioid receptor (KOR) agonists. This effect is related to the inhibition of dopamine (DA) release in the nucleus accumbens (NAc) produced by the acute administration of KOR agonists. The present study was undertaken to investigate the in vivo effect of the repeated administration of KOR agonist on extracellular DA levels in the NAc. Rats were injected once daily with the selective KOR agonist U-69593 (0.16-0.32 mg/kg) or vehicle for 4 days. Microdialysis studies assessing extracellular concentration of DA in the NAc under basal and K(+)-stimulatory conditions were conducted 1 day later. The microdialysis studies revealed that preexposure to U-69593 had no effect on basal extracellular DA levels but significantly augmented the amount of extracellular DA induced by high K(+) compared with vehicle pretreated rats. The D2 receptor agonist quinpirole perfused through the dialysis probe in the NAc, although it produced a significant decrease on basal and K(+)-stimulated DA levels in control rats, it did not decrease significantly either basal or K(+)-stimulated DA levels in U-69593 preexposed rats. Preexposure to U-69593 did not alter the expression of tyrosine hydroxylase or dopamine transporter in the ventral tegmental area. These results show that repeated administration of U-696593 increases the amount of extracellular DA induced by high K in the NAc, an effect that may be related to decreased D2 autoreceptor function. It is suggested that repeated activation of KOR changes the response status of dopaminergic neurons in the NAc.

Striatal dopaminergic responses observed in latent inhibition are dependent on the hippocampal ventral subicular region

We showed recently that behavioural and striatal dopaminergic (DA) responses obtained in latent inhibition are crucially dependent on the parahippocampal region, the entorhinal cortex. In the present study, we investigated the influence exerted by the hippocampal ventral subicular region (SUB) on the DA responses in the anterior part of the dorsal striatum using in vivo voltammetry in freely moving rats and the same latent inhibition paradigm. To that end, the left SUB was temporarily blocked with tetrodotoxin (TTX) during pre-exposure to a new olfactory stimulus (banana odour). During the second session the animals were aversively conditioned to banana odour. With respect to the results obtained during the test session (third presentation of banana odour), similar changes in behaviour and DA levels were obtained in control and conditioned rats microinjected with the solvent, phosphate-buffered saline (PBS), in the SUB, consistently with a latent inhibition phenomenon. In contrast, after reversible inactivation of the SUB during the pre-exposure session, TTX-pre-exposed conditioned animals displayed aversive behaviour in the test session, and anterior striatal DA variations in these animals differed significantly from those obtained in pre-exposed rats injected locally with PBS. Striatal DA variations obtained in conditioned animals microinjected with TTX were also significantly different from those observed in conditioned non-pre-exposed animals. The present data suggest that, in parallel to the entorhinal cortex, the SUB regulates the latent inhibition-related behavioural and DA responses in the anterior part of the dorsal striatum. These data may provide new insight into the pathophysiology of schizophrenic psychoses.

Increased susceptibility to transcriptional changes with novel stressor in adrenal medulla of rats exposed to prolonged cold stress

The response to stress is influenced by prior experience with the same or different stressor. For example, exposure of cold pre-stressed rats to heterotypic (novel) stressors, such as immobilization (IMO), triggers an exaggerated release of catecholamines and increase in gene expression for adrenomedullary tyrosine hydroxylase (TH), the rate limiting catecholamine biosynthetic enzyme. To study the mechanism, we examined induction or phosphorylation of several transcription factors, which are implicated in IMO-triggered regulation of TH transcription, in rats exposed to cold (4 °C) for up to 28 days and then subjected to IMO. Levels of c- fos increased transiently after 2–6 h and returned to basal levels after 1–28 days cold stress. Fra-2, was unaffected by short term cold, but was induced about 2-fold by 28 days continual cold. In contrast, there were no significant changes in CREB phosphorylation or Egr1 induction. Rats, with and without pre-exposure to 28 days cold, were subjected to single IMO for up to 2 h. Phosphorylation of CREB after 30 min IMO was greater in cold pre-exposed rats. Induction of Egr1 was three times higher in cold pre-exposed rats and remained significantly elevated even 3 h after cessation of IMO. Exposure to IMO triggered a 10–20-fold elevation in Fra-2 in both groups, which was even higher 3 h after the IMO. However, Fra-2 was more heavily phosphorylated following IMO stress in cold pre-exposed animals. The results reveal that sensitization to novel stress in cold pre-exposed animals is manifested by exaggerated response of several transcription factors.

Different effects of chronic exposure to ELF magnetic field on spontaneous and amphetamine-induced locomotor and stereotypic activities in rats

The effects of chronic (7 days) exposure to an extremely low frequency magnetic field (ELF-MF, 50 Hz, 0.5 mT) on spontaneous and amphetamine-induced (1.5 mg/kg, i.p.) locomotor and stereotypic activities in adult rats were examined by open field test for 2 h on exposure days 1, 3, and 7. After 1 day of exposure to ELF-MF, the spontaneous locomotor activity was increased clearly at the first hour of observation and significantly at the second one as compared to the corresponding values in other series with ELF-MF and sham-exposed animals. After 7 days of exposure to ELF-MF, an amphetamine enhancing effect on the locomotor activity was significantly reduced at the second hour of observation as compared to that in 1-day- and sham-exposed rats treated with amphetamine. In contrast to the locomotor activity, the amphetamine-induced stereotypic behaviour in 7-day pre-exposed rats was significantly reduced at the first hour versus sham-exposed rats. While at the second hour of observation this effect was significant as compared to 1- and 3-day exposed animals (but not sham-exposed rats). Our results indicate that an extremely low frequency magnetic field is able to affect differently two types of behaviour, which are dependent on both the time course of exposure and the imbalance in the brain mediatory systems.

Evidence for Pavlovian conditioning of cocaine-induced responses linked to emotional behavioral effects

The pairing of cocaine treatments with a specific test environment typically leads to cocaine-conditioned drug effects. In this study, we first pre-exposed rats 10 times to an open-field environment to establish an habituation asymptote in locomotor activity prior to the initiation of cocaine treatments. Two groups ( N=10) equated for locomotion, grooming, central zone penetrations and rearing behavior were used. One group received five pairings of cocaine (10.0 mg/kg) and the second group five pairings of saline injections with placements in the open-field environment. Subsequently, both groups received a saline test to detect possible cocaine-conditioned behavioral effects. During the cocaine treatment phase, cocaine enhanced locomotion and central zone penetrations but decreased rearing and grooming. On the conditioning test, the cocaine group exhibited enhanced central zone penetrations and decreased grooming as compared to the saline group. There were no group differences in locomotion or rearing. When within group comparisons were performed between behavioral responses on the pre-conditioning test vs. the conditioning test, the saline group scores were essentially unchanged. In contrast, the cocaine group exhibited higher central zone penetrations and decreased grooming without changes in locomotion or rearing. In that a cocaine conditioning test can also be viewed as a cocaine withdrawal test, two additional experiments were conducted using an unpaired conditioning protocol to test for withdrawal effects without conditioning. These results indicated that the central zone and grooming effects observed in the conditioning protocol were not withdrawal effects. Altogether, these findings provide support for Pavlovian conditioning of cocaine-induced changes in emotion-related behavioral responses.

Behavioural and cardiovascular responses during latent inhibition of conditioned fear: measurement by telemetry and conditioned freezing

This study assessed freezing behaviour and cardiovascular responses during the expression of latent inhibition of conditioned fear. Animals that were either repeatedly preexposed (PE) to a tone conditioned stimulus (CS) or naive to the tone (non-preexposed; NPE) subsequently experienced three presentations of the tone paired with footshock. Animals were tested 24 h later in the context of the footshock chamber, and on the following day, in the presence of the tone CS. Changes in heart rate and blood pressure were recorded by radio-telemetry. The PE rats spent more time freezing to the conditioned contextual cues and exhibited higher blood pressures during the last half of the context test session than did the NPE animals. During the tone test, the PE rats exhibited less conditioned freezing to the tone CS compared with the NPE animals, i.e. expression of the latent inhibition. This behavioural effect was associated with a significant increase in heart rate, but not blood pressure, in the PE but not the NPE animals. Our results suggest that the increased blood pressures of the PE rats during the context test directly reflect their greater fear of the conditioning context. In contrast, the increased heart rate response but decreased freezing shown by PE rats in response to the tone CS may be due to the fact that lower stress levels (e.g. PE condition) elicit sympathetically-mediated increases in heart rate, whereas higher stress levels (e.g. NPE condition) activate both sympathetic and parasympathetic systems, thus eliminating any CS-induced increase in heart rate in the NPE rats.