Effects of chronic prenatal MK-801 treatment on object recognition, cognitive flexibility, and drug-induced locomotor activity in juvenile and adult rat offspring.

BACKGROUND: Patients with schizophrenia display impaired cognitive functioning and increased sensitivity to psychomimetic drugs. The neurodevelopmental hypothesis of schizophrenia posits that disruption of the developing brain predisposes neural networks to lasting structural and functional abnormalities resulting in the emergence of such symptoms in adulthood. Given the critical role of the glutamatergic system in early brain development, we investigated whether chronic prenatal exposure to the glutamate NMDA receptor antagonist, MK-801, induces schizophrenia-like behavioural and neurochemical changes in juvenile and adult rats. METHODS: Pregnant Long-Evans rats were administered saline or MK-801 (0.1mg/kg; s.c.) at gestation day 7-19. Object recognition memory and cognitive flexibility were assessed in the male offspring using a novel object preference task and a maze-based set-shifting procedure, respectively. Locomotor-activating effects of acute amphetamine and MK-801 were also assessed. RESULTS: Adult, but not juvenile, prenatally MK-801-treated rats failed to show novel object preference after a 90min delay, suggesting that object recognition memory may have been impaired. In addition, the set-shifting task revealed impaired acquisition of a new rule in adult prenatally MK-801-treated rats compared to controls. This deficit appeared to be driven by regression to the previously learned behaviour. There were no significant differences in drug-induced locomotor activity in juvenile offspring or in adult offspring following acute amphetamine challenges. Unexpectedly, MK-801-induced locomotor activity in adult prenatally MK-801-treated rats was lower compared to controls. CONCLUSIONS: Glutamate transmission dysfunction during early development may modify behavioural parameters in adulthood, though these parameters do not appear to model deficits observed in schizophrenia.

Gender-dependent differences in latent inhibition following prenatal stress and corticosterone administration.

Latent inhibition (LI) indexes an organisms' ability to ignore irrelevant stimuli. Its disruption in the rat is considered to provide an animal model of the impaired ability to ignore irrelevant stimuli in schizophrenia. Given the importance of neurodevelopmental factors in the pathophysiology of schizophrenia, the present experiments investigated the effects of restraint, exposure to inescapable footshock and corticosterone administration during the last trimester of pregnancy, on the development of LI in the adult male and female offspring. Prenatal restraint had no effect on LI in the adult offspring of both sexes. Inescapable footshock exposure and corticosterone administration led to LI disruption in the male, but not the female offspring. These gender-dependent effects of prenatal treatments on LI suggest that it may provide a neurodevelopmental model of at least a sub-group of schizophrenia, in which environmental factors and gender are considered to play a significant role.

Repeated lofexidine treatment attenuates stress-induced, but not drug cues-induced reinstatement of a heroin-cocaine mixture (speedball) seeking in rats.

Alpha-2 adrenoceptor agonists (lofexidine, clonidine) are used to alleviate short-term opioid withdrawal in humans. In rats, acute injections of these agents attenuate stress-induced reinstatement of heroin and cocaine seeking at time points that are beyond the acute drug withdrawal phase. Here, we studied whether exposure to lofexidine would attenuate reinstatement of a heroin-cocaine mixture (speedball) seeking induced by exposure to stress or to drug-associated cues. Rats were trained to lever press for speedball for 10 days, and the drug-reinforced behavior was then extinguished for 11 days in the presence (Experiment 1) or the absence (Experiment 2) of the drug cues. Subsequently, rats were tested for reinstatement of drug seeking after exposure to intermittent footshock stress (5-15 min; Experiment 1) or the drug cues (Experiment 2). Starting on day 7 of training, rats received daily injections of saline or lofexidine (0.1 or 0.2 mg/kg). Repeated lofexidine treatment significantly attenuated footshock-induced reinstatement, but did not alter drug cues-induced reinstatement of speedball seeking. In addition, lofexidine did not have a consistent effect on speedball self-administration and extinction behavior. Results extend previous reports with acute drug injections, indicating that lofexidine maintains its effect on stress-induced reinstatement after repeated treatment. The present data also suggest that the neurochemical events underlying stress- and drug cues-induced relapse are not identical.

Time-dependent changes in extinction behavior and stress-induced reinstatement of drug seeking following withdrawal from heroin in rats.

RATIONALE AND OBJECTIVES: Footshock stress reliably reinstates heroin seeking in rats, but the time course of the development of this effect following drug withdrawal is not known. Here we studied the effect of intermittent footshock stress on reinstatement of heroin seeking following different withdrawal periods (1-66 days). We also studied whether changes in corticotropin-releasing factor (CRF) mRNA in the central nucleus of the amygdala (CeA) and the bed nucleus of the stria terminalis (BNST) are correlated with this reinstatement after 1 day and 6 days of heroin withdrawal. METHODS: Rats were trained to self-administer heroin (9 h/day; 0.1 mg/kg per infusion) for 10 days. Tests for extinction behavior and footshock-induced reinstatement of heroin seeking were then conducted after 1, 6, 12, 25, or 66 days of heroin withdrawal. On the test day, rats were given five to ten 60-min extinction sessions until they reached the extinction criterion of less than 15 responses per 60 min on the lever previously associated with heroin. Rats were then exposed to intermittent foot-shock (0.8 mA; 10 min), and lever-pressing behavior was recorded for 120 min. RESULTS: Reinstatement of lever-pressing behavior by footshock followed an inverted U-shaped curve with maximal responding after 6 days and 12 days of heroin withdrawal. Surprisingly, foot-shock did not reinstate lever-pressing behavior on day 1 of withdrawal. Lever pressing during extinction, prior to exposure to footshock, also followed an inverted U-shaped curve, with higher responding after 6, 12, and 25 days of heroin withdrawal. Finally, compared with control groups not exposed to shock, CRF mRNA levels in response to footshock were increased in the CeA (day 1 of withdrawal) and the dorsal BNST (day 1 and day 6), but not in the ventral BNST. CONCLUSIONS: The duration of the heroin withdrawal period is an important factor in the manifestation of (1) footshock stress-induced reinstatement of heroin seeking and (2) extinction of the heroin-reinforced behavior. Finally, the time-dependent changes in footshock stress-induced reinstatement following withdrawal from heroin were not correlated with alterations in CRF mRNA in the CeA and BNST.

Leptin attenuates acute food deprivation-induced relapse to heroin seeking.

Studies in rats have shown that intermittent footshock stress reinstates drug seeking after prolonged drug-free periods. Recently, we found that another environmental stressor, acute 1 d food deprivation, potently reinstates heroin seeking in rats. Here we report that this effect of food deprivation can be blocked by leptin, a hormone involved in the regulation of energy balance and food intake. Rats were trained to self-administer heroin (0.05-0.1 mg/kg, i.v., per infusion, three 3 hr sessions per day) for 8-10 d. The heroin-reinforced behavior was then extinguished for 10-13 d, during which lever presses had no reinforced consequences. Subsequently, rats were tested for reinstatement after 1 d of food deprivation (experiment 1) or exposure to intermittent footshock (15 min, 0.6 mA) and heroin priming injections (0.25 mg/kg, s.c.) (experiment 2). Acute food deprivation reinstated heroin seeking, an effect that was attenuated by leptin (2 or 4 microgram/rat, i.c.v.; two infusions, given 21 hr and 20-30 min before the start of the test sessions). In contrast, leptin had no effect on reinstatement of heroin seeking induced by intermittent footshock or priming injections of heroin. These data indicate that food deprivation can provoke relapse to heroin seeking via a leptin-dependent mechanism, which is not involved in relapse induced by footshock stress or reexposure to heroin.

The role of corticotrophin-releasing factor in stress-induced relapse to alcohol-seeking behavior in rats.

RATIONALE: Intermittent footshock stress reliably reinstates extinguished alcohol-taking behavior in drug-free rats, but the neurochemical events involved in this effect are not known. OBJECTIVE: We studied here whether two main modulators of stress responses, corticotropin-releasing factor (CRF) and corticosterone, are involved in reinstatement of alcohol seeking induced by the intermittent footshock stressor. METHDOS: Rats were given alcohol in a two-bottle choice procedure (water versus alcohol) for 30 days and were then trained for 60 min per day to press a lever for alcohol (12% w/v) for 24-30 days in operant conditioning chambers. After stable drug-taking behavior was obtained, lever pressing for alcohol was extinguished by terminating drug delivery for 5-8 days. Reinstatement of alcohol seeking was then determined after exposure to intermittent footshock (0.8 mA; 10 min) in different groups of rats that were pretreated with CRF receptor antagonists or underwent adrenalectomy (ADX) to remove endogenous corticosterone from the body. RESULTS: The CRF receptor antagonists, d-phe-CRF (0.3 or 1.0 microg; ICV) and CP-154,526 (15, 30 or 45 mg/kg; IP) attenuated footshock-induced reinstatement of alcohol seeking in a dose dependent manner. In contrast, the removal of circulating corticosterone by ADX had no effect on footshock stress-induced reinstatement of alcohol-taking behavior. In addition, the prevention of the footshock-induced rise in corticosterone while maintaining basal levels of the hormone by providing adrenalectomized rats with corticosterone pellets (50 mg/kg per day), had no effect on stress-induced reinstatement. CONCLUSIONS: These data suggest that CRF contributes to stress-induced relapse to alcohol seeking via its actions on extra-hypothalamic sites. The present data, and previous data with heroin- and cocaine-trained rats, point to a general role of CRF in relapse to drugs induced by stressors.

Stress and relapse to drug seeking in rats: studies on the generality of the effect.

RATIONALE: Intermittent footshock reinstates drug-taking behavior in rats, but not behaviors previously maintained by food reinforcers. Here we tested further the generality of this phenomenon by determining whether restraint and food deprivation stressors would reinstate heroin seeking, whether the environment in which footshock is given modulates footshock-induced reinstatement, and whether footshock would reinstate operant responding previously maintained by brain stimulation reward (BSR). METHODS: Groups of rats were trained to self-administer for 10 days either heroin (0.05-0.1 mg/kg/infusion, IV, three 3-h sessions/day) or brain stimulation into the septal area (trains of monopolar cathodal pulses of 100 micros for 500 ms, one 60-min session/day). After extinction of the heroin-reinforced behavior (10-13 days), the rats were tested for reinstatement after exposure to food deprivation (1 and 21 h), restraint given outside the self-administration environment (5, 15 and 30 min), or intermittent footshock (0.8 mA, 15 min) given in the self-administration environment or in a novel (non-drug) environment. For BSR-trained rats, the effect of footshock on reinstatement after extinction (6-10 days) was compared with that induced by noncontingent brain stimulation (three or six discrete stimulations at the start of the test sessions). RESULTS: Food deprivation reinstated heroin seeking. Footshock reliably reinstated heroin seeking when given in the drug environment, but not when given in a non-drug environment. Similarly, restraint given outside the self-administration environment failed to reinstate heroin seeking. In addition, footshock was as effective as priming brain stimulation in reinstating operant responding previously maintained by BSR. CONCLUSIONS: The effect of footshock on reinstatement of heroin seeking generalizes to food deprivation, and appears to be dependent on the environment in which the stressor is given. The data with BSR indicate that the phenomenon of footshock-induced reinstatement is not selective for drug reinforcers.

Involvement of the medial septum in stress-induced relapse to heroin seeking in rats.

Intermittent footshock stress has been shown to reinstate extinguished drug-taking behaviour in rats, but the brain areas involved in this effect are to a large degree unknown. Here we studied the role of the septum in stress-induced reinstatement of heroin seeking. Rats were trained to self-administer heroin for 9-10 days (three 3-h sessions per day, 0.1 mg/kg per infusion). Following training, extinction sessions were given for 8-13 days by substituting saline for heroin, and then tests for reinstatement of heroin seeking were carried out. Reversible inactivation of the medial septum with tetrodotoxin (TTX; 1-5 ng, infused 25-40 min before the test sessions) reliably reinstated heroin seeking, mimicking the effect of 15 min of intermittent footshock. This effect of TTX was not observed after infusions made 1.5 mm dorsally into the lateral septum. In other experiments, it was found that infusions of a low, subthreshold dose of TTX (0.5 ng) into the medial septum, when combined with 2 min of footshock that in itself was ineffective, reinstated heroin seeking. Furthermore, electrical stimulation (400 microA pulses, 100 micros duration, 100 Hz frequency) of the medial septum during exposure to 10 min of intermittent footshock attenuated footshock-induced reinstatement of heroin seeking. These data suggest a role for the medial septum in stress-induced relapse to drug seeking. The septum is thought to be involved in neuronal processes underlying behavioural inhibition, thus we speculate that stressors provoke relapse by interfering with these processes.

Gender- and age-dependent differences in latent inhibition following pre-weaning non-handling: implications for a neurodevelopmental animal model of schizophrenia.

Latent inhibition (LI) refers to retarded conditioning to a stimulus as a consequence of its prior nonreinforced pre-exposure, and is considered to index the capacity of an organism to ignore irrelevant stimuli. LI disruption has received increasing attention as an animal model of the widely described attentional deficit of schizophrenia, consisting of an inability to ignore irrelevant stimuli. The present experiments investigated the effects of infantile manipulations on the development of LI. Male and female rats handled or nonhandled in infancy (days 1-22), were tested at 3 and 16 months. Young handled animals had lower emotional reactivity than nonhandled, and this difference persisted in females at 16 months. At 3 months. LI, poorer conditioning of stimulus pre-exposed as compared to nonpre-exposed rats, was obtained in handled and nonhandled females, as well as in handled males, but was absent in nonhandled males. This pattern changed at 16 months: both nonhandled males and females failed to show LI. These gender- and age-dependent effects of pre-weaning manipulations on LI loss may provide an animal parallel to the susceptibility of young adult males to schizophrenia and the attenuation of gender differences in long-term outcome schizophrenia.

Latent inhibition is disrupted by acute and repeated administration of corticosterone.

Latent inhibition (LI), namely, a retardation in conditioning to a stimulus, as a consequence of its prior non- reinforced pre-exposure, is disrupted in amphetamine-treated rats and humans and in some subsets of schizophrenic patients. One factor that has been repeatedly implicated in precipitating and/or exacerbating psychotic episodes is stress. Since a principal biological response to stress is the activation of the hypothalamic-pituitary-adrenocortical (HPA) axis, leading, as its end product, to the secretion of corticosterone, the present experiments tested whether increase in corticosterone levels following exogenous corticosterone administration would disrupt LI. Both repeated (Experiment 1) and acute (Experiment 2) administration of corticosterone led to LI disruption, providing evidence for the involvement of the HPA axis alterations in LI and further supporting the viability of disrupted LI as an animal model of psychosis. Both regimens also increased amphetamine-induced activity. We suggest that disrupted LI may reflect a cognitive mechanism whereby prolonged periods of increased corticosterone levels can lead to 'sensory flooding' characteristic of psychosis.

Neonatal nonhandling and in utero prenatal stress reduce the density of NADPH-diaphorase-reactive neurons in the fascia dentata and Ammon's horn of rats.

The density of nitric oxide (NO)-producing neurons in the fascia dentata and Ammon's horn was assessed in 6-month-old male rats using NADPH-diaphorase (NADPH-d) histochemistry. Two separate experiments investigated whether (1) the complete absence of neonatal handling or (2) the administration of periodic prenatal stress could affect the expression and distribution of NADPH-d reactivity in the hippocampus, when compared with rats raised in normal standard laboratory conditions. Experiment 1 demonstrated that adult rats that received no handling during neonatal development (from birth to postnatal day 22) showed a very substantial reduction in NADPH-d-positive neurons per unit area throughout the entire hippocampus when compared with rats that received regular daily handling in this period. Quantitative analysis further revealed that this effect was significantly more pronounced in Ammon's horn than in the fascia dentata, and within Ammon's horn the dorsal region was selectively more affected. Experiment 2 showed that prenatal stress, which involved the administration of daily restraint stress to pregnant dams throughout the gestation period, also led to a reduction in NADPH-d reactivity in the hippocampus of the offspring of these dam when they reached adulthood. The present results suggest that behavioral manipulations in the early neonatal or prenatal period can significantly alter the neurodevelopment of the hippocampal NO system and these changes might be related to some of the behavioral abnormalities that emerge later in adulthood.

Intermingling heavy and light strain chickens may cause social stress.

The present study examines the effects of intermingling day-old broiler chicks with 2-wk-old light strain chicks, on body weight, general activity, feeding behavior, feather condition, and heart size of the broilers. Fifteen groups of chicks (five for each treatment) were raised until 6 wk of age in 1 x 2 m experimental pens. Each group consisted of 10 broiler chicks in Treatment 1 (control pens), 8 broiler, and 2 light strain chicks in Treatment 2 (8:2), and 5 broiler with 5 light strain chicks in Treatment 3 (5:5). The feeding program consisted of a commercial starter ration (21% protein, 3,000 kcal ME/kg) for the first 4 wk followed by a finishing ration (18% protein, 3,150 kcal ME/kg). Feed and water were provided for ad libitum consumption. Body weight did not differ between Treatment 1 (controls) and the "intermingled" broilers. Feather condition was significantly poorer among the 5: 5 groups than in the controls. Heart weight (a possible indicator of chronic stress) in broilers of the 5:5 groups was significantly heavier than in the controls. No differences in the general activity parameters were found between broilers of the various treatments. Pecking at feed among broilers was significantly greater (P < 0.05) in the 8:2 groups than in both control and 5:5 groups. During the first 6 wk after hatching, intermingling broiler chicks with equal numbers of light strain chicks that are 2 wk older may lead to long-term chronic stress for the broilers.