A synthetic small-molecule Isoxazole-9 protects against methamphetamine relapse.

Adult neurogenesis in the dentate gyrus (DG) is strongly influenced by drug-taking behavior and may have a role in the etiology of drug-seeking behavior. However, mechanistic studies on the relationship of neurogenesis on drug seeking are limited. Outbred Wistar rats experienced extended access methamphetamine self-administration and individual differences in drug taking defined animals with higher preferred and lower preferred levels of drug intake. Forced abstinence from higher preferred levels of drug taking enhanced neurogenesis and neuronal activation of granule cell neurons (GCNs) in the DG and produced compulsive-like drug reinstatement. Systemic treatment with the drug Isoxazole-9 (a synthetic small molecule known to modulate neurogenesis in the adult rodent brain) during abstinence blocked compulsive-like context-driven methamphetamine reinstatement. Isoxazole-9 modulated neurogenesis, neuronal activation and structural plasticity of GCNs, and expression of synaptic proteins associated with learning and memory in the DG. These findings identify a subset of newly born GCNs within the DG that could directly contribute to drug-seeking behavior. Taken together, these results support a direct role for the importance of adult neurogenesis during abstinence in compulsive-like drug reinstatement.

Chronic acamprosate eliminates the alcohol deprivation effect while having limited effects on baseline responding for ethanol in rats.

Acamprosate (calcium-acetyl homotaurinate) is a relatively new compound developed for the treatment of alcoholism and has been shown to be effective in attenuating relapse in human alcoholics. In the current study, the effects of this drug were further examined using an animal model of oral ethanol self-administration in a limited access paradigm. Male Wistar rats were trained to respond for ethanol (10% w/v) or water in a two-lever free-choice operant condition. Acute administration of acamprosate (400 mg/kg) reduced ethanol consumption and increased responding for water. Chronic administration of lower daily doses of acamprosate (100 and 200 mg/kg) blocked the increased ethanol consumption typically observed in rats after an imposed abstinence period. This effect of acamprosate was selective for ethanol, as responding for water was unaffected at any dose tested. These results with rats suggest a model by which to explore the mechanisms for anti-relapse effects of acamprosate.

Excessive ethanol drinking following a history of dependence: animal model of allostasis.

Alcohol withdrawal symptoms, particularly negative emotional states, can persist for months following the removal of alcohol. These protracted withdrawal symptoms have been implicated as an important trigger of relapse to excessive drinking in alcoholics and may represent a long lasting shift in affective tone as a result of chronic alcohol exposure. It was shown previously that ethanol-dependent rats increased their operant responding for ethanol when tested during the first 12 hr after withdrawal. The purpose of the present experiments was to determine the persistence of this finding by examining operant oral ethanol self-administration in rats with a history of physical dependence upon ethanol, detoxified and then allowed a two week period of protracted abstinence. The results of these experiments indicate that operant responding for ethanol was enhanced during protracted abstinence by 30-100% and remained elevated for 4-8 weeks post acute withdrawal. These results have important implications for understanding the characteristics and mechanisms underlying vulnerability to relapse.

Behavioral effects of central administration of the novel CRF antagonist astressin in rats.

Astressin, a novel corticotropin releasing factor (CRF) antagonist, has been found to be particularly potent at inhibiting the hypothalamo-pituitary-adrenal axis. The aim of the present study was to determine the effects in rats of astressin in attenuating the anxiogenic-like response produced by social stress and intracerebroventricular (ICV) CRF administration on the elevated plus-maze, and ICV CRF-induced locomotor activation in the rat. Astressin significantly reversed the anxiogenic-like response induced by both social stress and ICV rat/humanCRF (r/hCRF) on the elevated plus-maze, but failed to block the effects of r/hCRF-induced locomotor activity in a familiar environment. When these results were compared to previous studies performed with the same paradigms using other CRF antagonists, astressin showed effects similar to those of D-PheCRF(12-41) on plus-maze performance. However, contrary to alpha-helicalCRF(9-41) and D-PheCRF(12-41), astressin had no effect on CRF-induced locomotor activity. These results suggest that astressin may have a unique anti-CRF profile compared to previously tested antagonists.

Effects of prenatal exposure to cocaine on conditional discrimination learning in adult rats.

Adult male rats that were gestationally exposed to cocaine and control offspring were trained on an instrumental conditioning task for assessment of the acquisition and reversal of an appetitive conditional discrimination based on olfactory cues. Offspring were derived from Sprague-Dawley dams that had received subcutaneous injections of 40 mg/kg/3 cc cocaine hydrochloride (C40) daily on Gestational Days 8-20, pair-fed (PF) dams that were injected with saline, nutritional control dams (NC) that had received saline injections, and nontreated control dams (LC). There were no differences among the prenatal treatment groups in acquisition of the barpress response or response rate throughout all phases of training. All prenatal treatment groups required approximately the same number of sessions to criterion on the initial odor discrimination. In contrast, adult C40 offspring required more sessions to acquire the reversal of the conditional discrimination than did animals from the other treatment groups (PF, NC, and LC). In addition, even at criterion performance for acquisition of the reversal discrimination, C40 animals exhibited lower accuracy on the first 10 responses and made significantly more errors before the first reward. Taken together with previous results, these findings suggest that gestational exposure to cocaine results in long-lasting alterations in performance on conditioning tasks that are evident early in life and that persist into adulthood.

Effects of prenatal exposure to cocaine on Morris water maze performance in adult rats.

The spatial memory of adult rats prenatally exposed to cocaine and that of control offspring was assessed using the Morris water maze. Offspring were derived from Sprague Dawley dams that received subcutaneous injection of 40 mg/kg/3 cc cocaine hydrochloride (C40) daily on gestational Days 8-20, pair-fed dams injected with saline, or nontreated control dams. After acquisition, the platform was moved to a new location (reversal phase). Probe trials were conducted at the end of acquisition and reversal training. On the 1st acquisition day, adult male and female offspring prenatally exposed to cocaine required significantly more time and traversed a greater distance to find the hidden platform than did control offspring. Despite these initial differences observed in C40 offspring performance, all of the rats were performing at equivalent levels at the time probe trials were conducted.

Hippocampal cell firing correlates of delayed-match-to-sample performance in the rat.

Hippocampal CA1 and CA3 neurons were recorded in rats performing a delayed-match-to-sample (DMTS) task. Complex spike cells showed significant firing peaks following sample and match responses and during delivery of water reward. Individual cells were classified into 4 subtypes according to the presence or absence of firing in each of these 3 phases. There were significant differences in delay interval firing among the 4 subtypes, but firing during the delay did not predict the correct response: 34% of the cells showed a linear change in firing during the delay. Further analyses revealed significant lever position firing biases in approximately 70% of the cells tested irrespective of subtype. The complexity of firing correlates of the neurons recorded in this DMTS task suggests that the hippocampus divides specific aspects of the performance demands of the task across different cell subtypes, which together provide sufficient information to resolve the matching-to-sample problem on any given trial.

Prenatal cocaine exposure induces deficits in Pavlovian conditioning and sensory preconditioning among infant rat pups.

Offspring derived from Sprague-Dawley dams that received daily subcutaneous injection of 40 mg/kg.3 cc-1 cocaine hydrochloride (C40) or saline (LC) from Gestational Days 8-20 were tested for first-order Pavlovian conditioning and sensory preconditioning at Postnatal Days 8 (P8), P12, and P21. Although C40 dams gained significantly less weight than LC dams, pup body weights did not differ between the two groups. Significant sensory preconditioning was obtained at P8 and P12 (but not at P21) in LC offspring, confirming previous reports of decline in performance in this task during ontogeny. In contrast, C40 offspring failed to exhibit sensory preconditioning at any test age. In addition, C40 pups tested at P8 did not display significant first-order conditioning. Taken together these results suggest a more general deficit in cognitive functioning rather than a delay in cognitive development in prenatally cocaine-exposed offspring.

Strain distribution of mice in discriminated Y-maze avoidance learning: genetic and procedural differences.

The current study was conducted to characterize discriminated avoidance learning in mice by using a Y-maze task. In Experiment 1, the task parameters were manipulated, including the amount of time spent in the start arm, the amount of time to make the avoidance response, and the intertrial interval (ITI) using C57 x SJL F1 hybrid mice. Avoidance performance was significantly improved with longer times to avoid the shock and longer ITIs. In Experiment 2, mice from 4 inbred strains (BALB/cByJ, DBA/2J, C57BL/6J, and SJL/J), an F1 hybrid (C57 x SJL), and 1 outbred strain (CD1) were tested with various ITIs. Strain differences were observed in avoidance learning, with BALB, DBA, C57 x SJL and CD1 mice showing significantly better avoidance learning than C57 mice, which were better than SJL mice. These data demonstrate that Y-maze performance is significantly influenced by the genetic background of the mouse and the parameters of the task.

Opiate withdrawal signs precipitated by naloxone following a single exposure to morphine: potentiation with a second morphine exposure.

Recent studies in humans with no prior history of opiate abuse indicated that naloxone-precipitated signs of opiate withdrawal could be observed after a single exposure to morphine, and that the severity of withdrawal was enhanced following a second morphine exposure 24 h later. The current study was conducted to establish a paradigm in rodents that resembled these conditions described in humans. To that end, naloxone-precipitated (0.03-3.0 mg/kg) suppression of operant response rates and somatic signs of withdrawal following single or repeated treatments with morphine (5.0 mg/kg) were assessed in previously opiate-naive rats. In one group of rats, naloxone was administered 4 h after both the first and second morphine pretreatment, while in a separate group of rats naloxone was administered 4 h after the second morphine pretreatment only. A single morphine pretreatment significantly increased naloxone's potency to suppress operant response rates, and resulted in the precipitation by naloxone of certain somatic signs of withdrawal. The effects of naloxone on both dependent measures (operant response rates and somatic signs) were potentiated following a second morphine pretreatment, regardless of whether naloxone was administered following both morphine exposures or only following the second morphine exposure. Thus, repeated morphine administration appears to be the critical factor underlying the progressive increase in antagonist potency, whereas prior experience with naloxone is not a necessary factor. The results provide additional support for the hypothesis that the development of dependence on opiates is a progressive phenomenon that may begin with a single dosing.

Chronic variable stress or chronic morphine facilitates immobility in a forced swim test: reversal by naloxone.

The behaviors displayed in a forced swim test were investigated in rats previously exposed to a chronic variable stress treatment or chronic administration of morphine. In addition, to further explore the participation of an endogenous opiate mechanism in these behavioral effects, naloxone was either administered during the chronic treatment (prior to each stress or morphine exposure) or immediately prior to the forced swim test. Animals were submitted daily to a different stressor for 1 week or injected with morphine (10 mg/kg, IP) for 6 days, whereas controls were unmanipulated except for the injection process. On the day following the last stressor, control and stressed animals were administered saline or naloxone (2 mg/kg, IP) 15 min prior to the forced swim test. Morphine treated animals were similarly tested on the third day following the last morphine injection. In a separate group of rats, naloxone (2 mg/kg, IP) was administered daily 10 min prior to each stressor of the chronic stress regime or each daily morphine injection. A significant increase in the time spent in immobility was observed in stressed animals as well as in rats chronically treated with morphine. In both groups, this potentiated immobility was attenuated by naloxone pretreatment prior to the forced swim test or when given before each daily stressor or morphine injection. In addition, the concurrent exposure to stress or morphine along with naloxone administration enhanced struggling in the first 5 min of the forced swim test.(ABSTRACT TRUNCATED AT 250 WORDS)

Responsiveness to cocaine challenge in adult rats following prenatal exposure to cocaine.

Adult rats that were gestationally exposed to cocaine and control offspring were examined for their sensitivity to challenge doses of cocaine. Offspring were derived from Sprague-Dawley dams that had received subcutaneous injections of 40 mg/kg per 3 cc cocaine hydrochloride daily on gestational days 8-20, pair-fed dams that were injected with saline, and nontreated control dams. In order to investigate the sensitivity to challenge doses of cocaine, offspring were assessed in adulthood for locomotor activity, cocaine drug discrimination, and the time course of cocaine in brain tissue following acute cocaine challenge. Adult offspring prenatally exposed to cocaine were observed to exhibit a reduced sensitivity to the discriminative stimulus effects of cocaine as evidenced by a significant shift to the right in the dose-response curve of cocaine discrimination. No prenatal treatment effects were observed in terms of the temporal patterns of cocaine discrimination or with regard to brain levels of cocaine. In addition, baseline locomotor activity and locomotor responses to challenge doses of cocaine were comparable across the prenatal treatment groups. Thus, prenatal cocaine exposure reduced sensitivity of offspring to the discriminative stimulus properties of cocaine without altering either the distribution of cocaine to the brain or the sensitivity of the offspring to the locomotor stimulant effects of cocaine.

The effects of cocaine on operant responding for food in several strains of mice.

The availability of numerous genetically homogenous mouse strains permits the analysis of genetic influences on behavior and also behavioral sensitivity (responsivity) to drugs of abuse. The current study was conducted to characterize discriminated operant responding for food in four inbred strains (Balb/cByJ, DBA/2J, C57BL/6J, SJL/J), an F1 Hybrid (C57BL/6xSJL), and one outbred strain (CD1) of mouse. The effect of cocaine on this operant behavior was also examined. Initially, all animals were trained to nosepoke for food on a continuous reinforcement schedule. The minimum response requirement for reinforcement was increased every 5 days until the animals were responding on an FR-15 schedule of reinforcement. All strains increased operant responding as the schedule of reinforcement was raised. However, significant differences in response rate and discrimination learning were observed among the various strains of mice. Cocaine administration reduced operant responding for food in Balb/cByJ, C57BL/6J, C57BL/6xSJL/J and CD1 mice at a dose of 15.0 mg/kg, whereas higher doses were required in DBA/2J mice (30.0 mg/kg) and SJL/J mice (56.0 mg/kg). These results suggest that operant performance and the effect of cocaine on this behavior is differentially influenced by genetic make-up.

Evidence for behavioral sensitization to cocaine in preweanling rat pups.

While chronic intermittent administration of stimulants often induces behavioral sensitization in adulthood, stimulant sensitization has rarely been reported prior to weaning [around postnatal day (P) 21]. Consistent pairing of drug administration with the test context often facilitates sensitization in adults, yet young animals have been typically returned to the home cage immediately post-injection. To determine whether promoting context-dependent sensitization might facilitate expression of sensitization in preweanlings, Sprague-Dawley rats were injected daily from P14 to P20 with 0, 5, 15, or 30 mg/kg cocaine HCl and placed for 30 min in either the experimental chamber or home cage. On P21 (test day), subjects were challenged with either 15 mg/kg cocaine or saline prior to placement in the experimental chamber. Significant sensitization of cocaine-induced stereotyped head movements was evident in animals given 15 or 30 mg/kg chronically in the experimental chamber, but not when these same doses were given in the home cage. Less consistent evidence for cocaine-induced sensitization was seen when examining locomotion, although trends for sensitization of this behavior were seen in animals chronically injected in either the test chamber or home cage. Thus, preweanlings can exhibit cocaine sensitization, particularly in terms of stereotypy, when tested shortly after the chronic exposure period, with expression of this sensitization being facilitated by pairing the chronic injections with the test context.

DARPP-32 knockout mice exhibit impaired reversal learning in a discriminated operant task.

The current study was conducted to examine the performance of mice with a targeted deletion of the gene for DARPP-32 in a discriminated operant task using food reinforcement. DARPP-32 plays a central role in regulating the efficacy of dopaminergic neurotransmission. Initially, wild-type and DARPP-32 knockout mice were trained to nose-poke for food on a continuous reinforcement schedule. The minimum response requirement was increased every 5 days until the animals were responding on an FR-15 schedule of reinforcement. At the completion of extensive operant training, reversal learning was assessed. Wild-type and DARPP-32 knockout mice exhibited equivalent performance during acquisition of this task, with both groups increasing operant responding as the schedule of reinforcement was raised. However, significant differences in discrimination learning were observed during the reversal phase, with DARPP-32 knockout mice requiring significantly more trials to reach criterion than wild-type controls. These results provide evidence for a functional role of DARPP-32 in the mediation of processes underlying learning and memory.

The effects of prenatal cocaine exposure on spontaneously active midbrain dopamine neurons in adult male offspring: an electrophysiological study.

In this study, the technique of extracellular single unit recording was used to examine the effect of prenatal cocaine exposure on the number of spontaneously active dopamine (DA) cells in the substantia nigra pars compacta (SNC or A9) and the ventral tegmental area (VTA or A10) of male Sprague-Dawley rats on postnatal days 56-68. In addition, the effect of the direct DA receptor agonist (+/-)-apomorphine (APO) on the basal firing rate of A10 DA cells was also determined. A significant decrease in the number of spontaneously active A10 DA cells was observed in offspring whose dams were treated with 40 mg/kg/day of cocaine s.c. from gestational days 8-20 when compared with offspring of pair-fed and non-treated control dams. The number of spontaneously active A9 DA cells was significantly decreased in the offspring of cocaine-exposed dams when compared to pair-fed offspring. In contrast, there were no significant differences among the three prenatal groups regarding the sensitivity of spontaneously active A10 DA cells to APO (2-64 micrograms/kg, i.v.). Overall, our results suggest that in utero cocaine exposure may alter presynaptic DA activity in offspring long after their exposure has been terminated.

Chronic variable stress enhances the stimulatory action of a low dose of morphine: reversal by desipramine.

Adult male rats were exposed to a chronic variable stress treatment, an animal model of depression, with or without concurrent daily administration of desipramine. Animals given chronic and variable stress were submitted daily to a different stressor following an injection of either saline or desipramine (5 mg/kg, i.p.), whereas control animals were unmanipulated except for the injection process. One day after the last event of the chronic procedure, control and stressed animals were administered saline or morphine (0.75 or 1.5 mg/kg, i.p.) and their locomotion assessed for 90 min. In an additional experiment, 24 h after the last stressor, stressed and control rats were challenged with either saline or one of two higher doses (behaviorally suppressant) of morphine (5 and 10 mg/kg, i.p.). A significantly greater increase in locomotor activity following a low dose (1.5 mg/kg) of morphine was observed in chronically stressed rats as compared to control rats. This potentiated locomotor response to morphine in stressed rats was prevented by desipramine pretreatment. The chronic and variable stress treatment did not modify the sedative response to the high doses of morphine. These data support the suggestion that a chronic and variable stress procedure results in sensitization to the stimulant effect of opioid stimulation, and that pretreatment with the antidepressant agent desipramine blocks the development of this sensitization.

Coloboma hyperactive mutant exhibits delayed neurobehavioral developmental milestones.

The coloboma mutation (Cm) is a neutron-irradiation induced gene deletion located on the distal portion of mouse chromosome 2. This deletion region includes a gene encoding the synaptic vesicle docking fusion protein, synaptosomal-associated protein of 25 kDa (SNAP-25). The resulting mutation is semi-dominant with heterozygote mice exhibiting a triad of phenotypic abnormalities that comprise profound spontaneous hyperactivity, head bobbing and a prominent eye dysmorphology. Because the expression pattern of two SNAP-25 isoforms begins to change during the first postnatal week, neurobehavioral developmental milestones were examined in order to determine if the expression of the coloboma behavioral phenotype could be detected during this period of postnatal development. The early classification of coloboma mutant offspring may help to further describe the penetrance of this mutation as well as the contribution of developmental changes to the adult behavioral phenotype. The coloboma mutation resulted in delays in some tests of complex motor skills including righting reflex and bar holding. In addition, coloboma mutants were characterized by body weight differences (first appearance day 7) and hyperreactivity to touch (day 11) and head bobbing (day 14). These data demonstrate disruptions in the time course of attaining developmental milestones in coloboma mutants and provide further evidence supporting the hypotheses that alterations in Snap gene expression are associated with functional behavioral consequences in developing offspring.

Substance dependence as a compulsive behavior.

A compulsion to take a drug combined with a loss of control in limiting intake is the defining feature of substance dependence or addiction, and is the conceptual framework for the criteria of substance dependence or addiction outlined by the World Health Organization and the American Psychiatric Association. However, defining exactly what constitutes loss of control and compulsive drug taking at the level of animal models is a daunting task, and it is clear that no validated animal model exists for the whole syndrome of addiction. The present discussion redefines loss of control as a narrowing of the behavioral repertoire toward drug-seeking behavior and suggests that there are many sources of reinforcement that contribute to this behavioral focus on drug seeking. Evidence is presented demonstrating separate animal models for many of these sources of reinforcement as well as for most of the criteria for substance dependence. Evidence is also presented showing that the brain neurochemical systems involved in processing drug reward are altered by chronic drug exposure to contribute additional sources of reinforcement. Challenges for the future involve not only elucidation of the neurobiological substrates of the different behavioral components of addiction, but better animal models of these components with which to effect such studies.

Prenatal cocaine exposure attenuates cocaine-induced odor preference in infant rats.

In order to further examine whether prenatal cocaine exposure alters the later reward efficacy of cocaine, exposed offspring were tested for cocaine-induced odor preference early in life. Test offspring were derived from Sprague-Dawley dams that received daily SC injections of 40 mg/kg/3 cc cocaine hydrochloride (C40) from gestational day 8-20, nutritional control dams receiving daily SC saline injections (NC), and nontreated control dams (LC). At testing on postnatal day 8 (P8), both LC and NC offspring were observed to exhibit a preference for the odor that had been paired on P7 with 2.0, 5.0, or 10.0 mg/kg cocaine. In contrast, C40 offspring exhibited a significant odor preference only when the odor had been previously paired with 5.0 or 10.0 mg/kg cocaine. These results, combined with previous work from our laboratory showing that adult offspring exposed gestationally to cocaine did not exhibit a cocaine-induced conditioned place preference, provide evidence that offspring exposed prenatally to cocaine are less likely to develop a preference for stimuli associated with cocaine. Further studies are needed to determine whether these alterations in cocaine preference reflect a learning deficit, pharmacokinetics factors, or an attenuation in the rewarding properties of cocaine.