Contribution of early environmental stress to alcoholism vulnerability

The most problematic aspects of alcohol abuse disorder are excessive alcohol consumption and the inability to refrain from alcohol consumption during attempted abstinence. The root causes that predispose certain individuals to these problems are poorly understood but are believed to be produced by a combination of genetic and environmental factors. Early environmental trauma alters neurodevelopmental trajectories that can predispose an individual to a number of neuropsychiatric disorders, including substance abuse. Prenatal stress (PNS) is a well-established protocol that produces perturbations in nervous system development, resulting in behavioral alterations that include hyperresponsiveness to stress, novelty, and psychomotor stimulant drugs (e.g., cocaine, amphetamine). Moreover, PNS animals exhibit enduring alterations in basal and cocaine-induced changes in dopamine and glutamate transmission within limbic structures, which exhibit pathology in drug addiction and alcoholism, suggesting that these alterations may contribute to an increased propensity to self-administer large amounts of drugs of abuse or to relapse after periods of drug withdrawal. Given that cocaine and alcohol have actions on common limbic neural substrates (albeit by different mechanisms), we hypothesized that PNS would elevate the motivation for, and consumption of, alcohol. Accordingly, we have found that male C57BL/6J mice subject to PNS exhibit higher operant responding and consume more alcohol during alcohol reinforcement as adults. Alterations in glutamate and dopamine neurotransmission within the forebrain structures appear to contribute to the PNS-induced predisposition to high alcohol intake and are induced by excessive alcohol intake. Accordingly, we are exploring the interactions between neurochemical changes produced by PNS and changes induced by consumption of alcohol in adulthood to model the biological bases of high vulnerability to alcohol abuse.

Accumbens Homer2 overexpression facilitates alcohol-induced neuroplasticity in C57BL/6J mice

Homer proteins are integral components of the postsynaptic density that are necessary for alcohol-induced neuroplasticity within the nucleus accumbens (NAC). In this report, we describe the effects of chronic alcohol consumption upon NAC Homer expression and investigate the functional consequences of mimicking the alcohol-induced changes in Homer expression vis-à-vis alcohol-induced changes in NAC neurochemistry and behavior. Chronic alcohol consumption under continuous access (3 months; daily intake approximately 11.2+/-1.5 g/kg/day) produced a robust increase in NAC Homer2 protein levels that was apparent at 2 days, 2 weeks, and 2 months following withdrawal from alcohol drinking. The increased Homer2 expression was accompanied by a less enduring elevation in total mGluR1 and NR2b levels that were evident at 2 days and 2 weeks but not at the 2-month time point. Mimicking the alcohol-induced increase in Homer2 levels by viral transfection of NAC neurons in alcohol-preferring C57BL/6J inbred mice enhanced behavioral output for alcohol reinforcement and increased alcohol intake under both preprandial and postprandial conditions. Moreover, NAC Homer2 overexpression facilitated the expression of an alcohol-conditioned place preference, as well as the development of motor tolerance. Finally, NAC Homer2 overexpression facilitated NAC glutamate and dopamine release following an acute alcohol injection and augmented alcohol-induced dopamine and glutamate sensitization, but did not affect NAC gamma-aminobutyric acid levels. Thus, an upregulation in NAC mGluR-Homer2-N-methyl-D-aspartic acid receptor signaling appears to be an important molecular adaptation to alcohol that promotes neuroplasticity facilitating motivational drive for alcohol and the development of alcoholism-related behaviors.

Prenatal stress enhances responsiveness to cocaine

Early environmental events have profound influences on a wide range of adult behavior. In the current study, we assessed the influence of maternal stress during gestation on psychostimulant and neurochemical responsiveness to cocaine, cocaine self-administration, and reinstatement of cocaine-seeking in adult offspring. Pregnant, female Sprague-Dawley rats were subjected to either no treatment or to restraint stress three times per day for the last 7 days of gestation and cocaine-related behavior was assessed in offspring at 10 weeks of age. Relative to controls, a noncontingent cocaine injection elevated locomotor activity as well as nucleus accumbens levels of extracellular dopamine and glutamate to a greater extent in both cocaine-naive and cocaine-experienced prenatal stress (PNS) rats and elevated prefrontal cortex dopamine in cocaine-experienced PNS rats. To assess the impact of PNS on cocaine addiction-related behavior, rats were trained to lever press for intravenous (i.v.) infusions of cocaine (0.25, 0.5, or 1 mg/kg/infusion), with each infusion paired with a light+tone-conditioned stimulus. Lever-pressing was extinguished and cocaine-seeking reinstated by re-exposure to the conditioned cues or by intraperitoneal cocaine-priming injections (5 or 10 mg/kg). PNS elevated active lever responding both during extinction and cocaine-primed reinstatement, but not during self-administration or conditioned-cued reinstatement. PNS also did not alter intake during self-administration. These findings demonstrate that PNS produces enduring nervous system alterations that increase the psychomotor stimulant, motivational, and neurochemical responsiveness to noncontingent cocaine. Thus, early environmental factors contribute to an individual's initial responsiveness to cocaine and propensity to relapse to cocaine-seeking.

Prenatal stress alters limbo-corticostriatal Homer protein expression

Early environmental stress influences developmental processes resulting in alterations in behavior and brain function, including abnormalities in glutamate neurotransmission. Here, we assessed the influence of prenatal stress on limbo-corticostriatal expression of Homer proteins that are critical elements in glutamatergic signaling. Pregnant, female Sprague-Dawley rats were subjected to either no treatment or to restraint stress 3 times per day for the last 7 days of gestation. At 21 d of age, offspring were sacrificed and brain tissue was rapidly extracted. Immunoblotting revealed regionally specific increases in certain Homer protein isoforms within prefrontal cortex and limbic structures, whereas the striatum exhibited a reduction in Homer 1a levels. These findings indicate that stress during development can produce enduring perturbations in Homer protein expression that likely contribute to glutamatergic and behavioral abnormalities observed following early environmental stress.

A progressive and cell non-autonomous increase in striatal neural stem cells in the Huntington's disease R6/2 mouse

Neural stem and progenitor cells are located in the subependyma of the adult forebrain. An increase in adult subependymal cell proliferation is reported after various kinds of brain injury. We demonstrate an expansion of neural precursor cells in the postnatal subependyma in a murine genetic disease model of Huntington's disease (HD), the R6/2 mouse. We used the in vitro neurosphere assay as an index of the number of neural stem cells in vivo and to assess proliferation kinetics in vitro and in vivo bromodeoxyuridine labeling to assess the progenitor cell population and their fates. Disease progression in this model leads to an increase in the numbers of neural stem cells in the adult striatal subependyma. This increase is produced cell non-autonomously by events in the R6/2 brains as the mice become increasingly symptomatic. Once the neural stem cell increase is induced in vivo, it is maintained during in vitro passaging of neural stem cells, but the neural stem cell increase is not reproduced during in vitro passaging of neural stem cells from presymptomatic R6/2 mice. In addition, we show that some of the R6/2 neural progenitor cells show a change from their normal migration destiny toward the olfactory bulb. Instead, some of these cells migrate into the striatum, one of the main affected areas in HD. Our findings demonstrate that HD damage recruits precursor cells in two ways: expansion of neural stem cells and altered migration of progenitor cells.

Contributions of prolonged contingent and noncontingent cocaine exposure to enhanced reinstatement of cocaine seeking in rats

RATIONALE

Recent evidence suggests that prolonged cocaine self-administration produces escalation in drug-seeking behavior in rats analogous to the increased intake patterns observed in cocaine-dependent individuals. However, the contributions of prolonged access to cocaine taking vs the pharmacologic effects of the consequent increased cocaine exposure on escalation of drug-seeking behaviors have not been investigated.

OBJECTIVE

The present study assessed the effects of these two factors on maintenance of cocaine self-administration and reinstatement of cocaine seeking.

METHODS

Male, Sprague-Dawley rats were trained to self-administer cocaine (0.2 mg/i.v. infusion; FR1) for 1 h per day for 10 sessions followed by short access (1 h/day), contingent long access (6 h/day), or noncontingent long access (1 h contingent + 5 h of yoked cocaine infusions/day; i.e., short access + yoked) to cocaine for 14 daily sessions. All rats underwent extinction training and were subsequently tested for the ability of cocaine-paired cues or a cocaine-priming injection (7.5 mg/kg i.p.) to reinstate extinguished cocaine seeking.

RESULTS

Rats in all groups maintained stable responding for cocaine reinforcement and subsequently showed significant reinstatement of cocaine-seeking behavior. Conditioned-cued reinstatement was enhanced after the contingent long access and short access + yoked cocaine exposure relative to short access cocaine exposure. Conversely, cocaine-primed reinstatement was enhanced after contingent long-access cocaine exposure relative to the other two conditions.

CONCLUSIONS

Enhanced drug seeking produced by prolonged daily cocaine self-administration is due to a combination of behavioral and pharmacological factors. Specifically, conditioned-cued reinstatement is enhanced by increased cocaine intake and cocaine-primed reinstatement is enhanced by increased cocaine taking.

Dopamine specifically inhibits forebrain neural stem cell proliferation, suggesting a novel effect of antipsychotic drugs

Neurogenesis has been implicated in antidepressant drug action and animal models of depression, suggesting that proliferating cells play a role in psychiatric disorders. Similar studies using antipsychotic drugs have yielded conflicting results, perhaps because of the lack of focus on specific cell types. We examine the effect of haloperidol on neural stem cells (NSCs), the ultimate precursors for adult cell genesis. We show that haloperidol increases NSC numbers, resulting in more progenitors and more new neurons and glia in the adult rat brain. The increase in NSCs by haloperidol is dependent on central dopamine D2 receptors, and these receptors are expressed by NSCs. D2 receptor stimulation in vitro inhibits NSC proliferation, which is reversed by haloperidol. Thus, haloperidol increases adult mammalian brain proliferation by antagonizing dopamine at D2 receptors on NSCs. These findings demonstrate a direct link between neural activity and NSC proliferation and implicate cell genesis in antipsychotic drug effects.

Potentiation of cocaine-primed reinstatement of drug seeking in female rats during estrus

RATIONALE

Gender differences exist in the patterns of drug taking in cocaine addiction, suggesting that the propensity to relapse varies between men and women. Previous reports have shown sex differences in both cocaine-primed and conditioned-cued reinstatement of cocaine-seeking behavior, including recent evidence that the estrous cycle influences the level of conditioned-cued reinstatement. However, the influence of the estrous cycle on cocaine-primed reinstatement has not been examined.

OBJECTIVE

Accordingly, we assessed the influence of sex and estrous cycle status on cocaine-primed reinstatement of drug-seeking behavior in Sprague-Dawley rats.

METHODS

Intact male and female rats were trained to lever press to self-administer intravenous cocaine (0.5 mg/kg every infusion; fixed ratio 1, 20-s time-out following each infusion), followed by prolonged extinction training, and subsequently tested for the ability of a cocaine-priming injection (0, 5, or 10 mg/kg i.p.) to reinstate extinguished cocaine seeking (i.e., nonreinforced lever responding).

RESULTS

Despite no differences in cocaine intake between male and female rats, females responded more on the cocaine-paired lever during self-administration and extinction relative to males. Subsequently, both males and females exhibited a dose-dependent cocaine-primed reinstatement of extinguished drug-seeking behavior. Moreover, females in estrus exhibited significantly higher reinstatement than either males or non-estrus females, following a high-dose (10 mg/kg) cocaine prime.

CONCLUSIONS

Estrus females display heightened sensitivity to the motivational and/or stimulant effects of cocaine, suggesting that hormonal state modulates drug craving and propensity for drug relapse in cocaine addicts.

p21 loss compromises the relative quiescence of forebrain stem cell proliferation leading to exhaustion of their proliferation capacity

Adult stem cells in various tissues are relatively quiescent. The cell cycle inhibitor p21cip1/waf1 (p21) has been shown to be important for maintaining hematopoietic stem cell quiescence and self-renewal. We examined the role of p21 in the regulation of adult mammalian forebrain neural stem cells (NSCs). We found that p21-/- mice between post-natal age 60-240 d have more NSCs than wild-type (+/+) controls due to higher proliferation rates of p21-/- NSCs. Thereafter, NSCs in p21-/- mice decline and are reduced in number at 16 mo relative to p21+/+ mice. Similarly, both p21-/- and p21+/+ NSCs display self-renewal in vitro; however, p21-/- NSCs display limited in vitro self-renewal (surviving a few passages, then exhausting). Thus, p21 contributes to adult NSC relative quiescence, which we propose is necessary for the life-long maintenance of NSC self-renewal because NSCs may be limited to a finite number of divisions.

Neural stem cells show bidirectional experience-dependent plasticity in the perinatal mammalian brain

Many of the effects of prenatal stress on the endocrine function, brain morphology, and behavior in mammals can be reversed by brief sessions of postnatal separation and handling. We have tested the hypothesis that the effects of both the prenatal and postnatal experiences are mediated by negative and positive regulation of neural stem cell (NSC) number during critical stages in neurodevelopment. We used the in vitro clonal neurosphere assay to quantify NSCs in hamsters that had experienced prenatal stress (maternal restraint stress for 2 hr per day, for the last 7 d of gestation), postnatal handling (maternal-offspring separation for 15 min per day during postnatal days 1-21), orboth. Prenatal stress reduced the number of NSCs derived from the subependyma of the lateral ventricle. The effect was already present at postnatal day 1 and persisted into adulthood (at least 14 months of age). Similarly, prenatal stress reduced in vivo proliferation in the adult subependyma of the lateral ventricle. Conversely, postnatal handling increased NSC number and reversed the effect of prenatal stress. The effects of prenatal stress on NSCs and proliferation and the effect of postnatal handling on NSCs did not differ between male and females. The findings demonstrate that environmental factors can produce changes in NSC number that are present at birth and endure into late adulthood. These changes may underlie some of the behavioral effects produced by prenatal stress and postnatal handling.

Opposing roles of the nucleus accumbens and anterior lateral hypothalamic area in the control of sexual behaviour in the male rat

Opposing roles have been implicated for the nucleus accumbens (NAc) and anterior portion of the lateral hypothalamic area (aLHA) in the regulation of sexual behaviour in male rats based on in vivo neurochemical correlates. The present study provides functional evidence supporting this hypothesis by examining the effects of lesions to these structures on copulation, noncontact erection and receptive female preference. Sexually naïve male Long-Evans rats received either bilateral 1.0- micro L injections of NMDA (10 micro g/ micro L/side) or vehicle (shams) into either the aLHA or the NAc. During repeated tests of copulation most of the sham-lesioned males, but few of the aLHA-lesioned and NAc-lesioned males, copulated to ejaculation. Most of the NAc-lesioned males also failed to intromit, whereas the majority of the aLHA-lesioned males intromitted repeatedly. During exposure to an inaccessible receptive female behind a wire-mesh screen, aLHA-lesioned males displayed facilitation of noncontact erections, whereas NAc-lesioned males displayed impaired noncontact erections. Conversely, during simultaneous exposure to inaccessible receptive and nonreceptive females in different compartments, all males spent more time in the proximity of the receptive female. These findings indicate that the aLHA plays an inhibitory role in the regulation of sexual arousal and an excitatory role in the regulation of ejaculation. Conversely, the NAc plays an excitatory role in the regulation in sexual arousal.

Excitotoxic lesions of the tegmental pedunculopontine nucleus impair copulation in naive male rats and block the rewarding effects of copulation in experienced male rats

The tegmental pedunculopontine nucleus (TPP) of the brainstem mediates food reward in food-sated animals and opiate reward in drug-naive animals. In the present study, we examine the effect of excitotoxic lesions of the TPP on sexual behaviour in naive and experienced male rats. Male, Long-Evans rats received either 0.25 micro L injections of NMDA (4.2 micro g/side) or vehicle (shams) into the TPP. In sexually naive males, complete bilateral TPP lesions decreased all measure of copulation (i.e. mounts, intromissions and ejaculations), prevented acquisition of conditioned sexual excitement, decreased approach preference for a receptive female over a non-receptive one, and decreased non-contact erections; unilateral or bilateral posterior-sparing TPP lesions did not affect any of these measures. Conversely, in sexually experienced males, lesions not only failed to disrupt copulation, but also increased conditioned sexual excitement, decreased post-ejaculatory interval and blocked the effect of prolonged copulation on conditioned sexual excitement. Following differential pairing of distinctive environments with and without copulation, sham males with sexual experience displayed a significant preference for the environment paired with copulation, whereas the lesion males with sexual experience displayed a significant aversion for the environment paired with copulation. These findings indicate that the TPP is critical for the acquisition of copulation in naive males and mediates the rewarding consequences of copulation in experienced males. Together these findings demonstrate that the TPP mediates sexual reward, but that sexual experience is not sufficient to produce a deprivation state.

Estrous odors and sexually conditioned neutral odors activate separate neural pathways in the male rat

Olfactory stimuli play important roles in sexual behavior. Previous studies have demonstrated that both estrous odors and initially neutral odors paired with copulation influence the sexual behavior of male rats. The present study examines the pattern of neural activation as revealed by Fos immunoreactivity (Fos-IR) following exposure to bedding scented with either a neutral odor (almond) paired previously with copulation, estrous odors or no odor. Following exposure to estrous odors Fos-IR increased in the accessory olfactory bulb, medial amygdala, medial bed nucleus of the stria terminalis, medial preoptic area, ventromedial hypothalamus, ventral tegmental area, and both the nucleus accumbens core and shell. Conversely, following exposure to the sexually conditioned odor Fos-IR increased in the piriform cortex, basolateral amygdala, nucleus accumbens core, and the anterior portion of the lateral hypothalamic area. In addition, following exposure to almond odor Fos-IR increased in the main olfactory bulb independent of its pairing with copulation. These patterns of Fos-IR following exposure to estrous or sexually conditioned odors were not influenced by either the addition or omission of the other type of odor. These findings demonstrate that estrous and sexually conditioned odors are processed by distinct neural pathways and converge in the nucleus accumbens core, suggesting that this structure has a unique role in processing sexual stimuli of both pheromonal and olfactory natures.

What can animal models tell us about human sexual response?

In all species, sexual behavior is directed by a complex interplay between steroid hormone actions in the brain that give rise to sexual arousability and experience with sexual reward that gives rise to expectations of competent sexual activity, including sexual arousal, desire, and performance. Sexual experience allows animals to form instrumental and Pavlovian associations that predict sexual outcome and thereby directs the strength of sexual responding. Although the study of animal sexual behavior by neuroendocrinologists has traditionally been concerned with mechanisms of copulatory responding, more recent use of conditioning and preference paradigms, and a focus on environmental circumstances and experience, has revealed behaviors and processes that resemble human sexual responses. In this paper, we review behavioral paradigms used with rodents and other species that are analogous or homologous to human sexual arousal, desire, reward, and inhibition. The extent to which these behavioral paradigms offer predictive validity and practicality as preclinical tools and models is discussed. Identification of common neurochemical and neuroanatomical substrates of sexual responding between animals and humans suggests that the evolution of sexual behavior has been highly conserved and indicates that animal models of human sexual response can be used successfully as preclinical tools.

Food-entrained circadian rhythms in rats are insensitive to deuterium oxide

Rats anticipate a scheduled daily meal by entrainment of a circadian pacemaker separate from the light-entrainable circadian pacemaker located in the suprachiasmatic nuclei (SCN). The site and molecular mechanisms of the food-entrainable pacemaker are unknown. The intrinsic period (tau) of the SCN pacemaker is significantly lengthened by deuteriation. Sensitivity of food-entrained circadian rhythms to D(2)O (25% in drinking water) was evaluated in intact and SCN-ablated rats entrained to daily feeding schedules. In intact rats fed ad-libitum, D(2)O lengthened tau sufficiently to drive activity rhythms out of entrainment to the light-dark cycle. By contrast, food-entrained rhythms were surprisingly resistant to modulation by D(2)O. The mean daily onset time of food anticipatory activity in rats with complete SCN-ablations was not affected by up to 28 days of D(2)O intake. Transient delays and disruption of anticipatory activity were evident in intact and one partial SCN-ablated rat during D(2)O treatment, but these are interpretable as effects of coupling and/or masking interactions between a D(2)O-sensitive light-entrainable pacemaker, and a D(2)O-resistant food-entrained pacemaker. Differential sensitivity to D(2)O suggests diversity in the molecular mechanisms of food- and light-entrainable circadian pacemakers in mammals. D(2)O may have utility as a screening test to identify putative food-entrainable pacemakers from among those central and peripheral tissues that can express circadian oscillations of clock genes independent of the SCN.

Conditioning and sexual behavior: a review

Sexual behavior is directed by a sophisticated interplay between steroid hormone actions in the brain that give rise to sexual arousability and experience with sexual reward that gives rise to expectations of competent sexual activity, sexual desire, arousal, and performance. Sexual experience allows animals to form instrumental associations between internal or external stimuli and behaviors that lead to different sexual rewards. Furthermore, Pavlovian associations between internal and external stimuli allow animals to predict sexual outcomes. These two types of learning build upon instinctual mechanisms to create distinctive, and seemingly "automated," patterns of sexual response. This article reviews the literature on conditioning and sexual behavior with a particular emphasis on incentive sequences of sexual behavior that move animals from distal to proximal with regard to sexual stimuli during appetitive phases of behavior and ultimately result in copulatory interaction and mating during consummatory phases of behavior. Accordingly, the role of learning in sexual excitement, in behaviors that bring about the opportunity to mate, in courtship and solicitation displays, in sexual arousal and copulatory behaviors, in sexual partner preferences, and the short- and long-term influence of copulatory experience on sexual and reproductive function is examined. Although hormone actions set the stage for sexual activity by generating the ability of animals to become sexually excited and aroused, it is each animal's unique experience with sexual behavior and sexual reward that molds the strength of responses made toward sexual incentives.

The nature of the conditioned response mediating olfactory conditioned ejaculatory preference in the male rat

We have developed a model to study the influence of conditioning on sexual partner preference in the rat. In this model, pairing a neutral odor (almond) with copulation to ejaculation produces a subsequent preference to ejaculate with females bearing that odor. We refer to this phenomenon as a conditioned ejaculatory preference (CEP). The present study investigates the nature of the conditioned response that mediates CEP. Given equal mount and intromission distributions, but an unequal ejaculation distribution, we hypothesized that two mechanisms could account for CEP: facilitated ejaculation or selective ejaculation. To test these hypotheses, we examined the effect of omitting the olfactory conditioned stimulus (CS) or applying it to a nonreceptive female. Males trained with the CS paired with copulation (paired-trained males) failed to display evidence of delayed ejaculation when copulating with unscented females. Conversely, paired-trained males displayed CS-elicited copulatory behavior with CS-bearing nonreceptive females. In addition, we re-analyzed the data from the copulatory preference tests of previous experiments for CEP-displaying males. Again, we failed to find evidence of facilitated ejaculation with the scented female. However, the time between the last mount or intromission and ejaculation was increased if either occurred with the scented female. Furthermore, more mounts were directed towards the scented female near the end, but not at other points, of an ejaculatory series. These findings suggest that the paired-trained males attend to the scented female near the point of ejaculation, and are consistent with the hypothesis that CEP is mediated by selective ejaculation.

The development of olfactory conditioned ejaculatory preferences in the male rat. I. Nature of the unconditioned stimulus

We have demonstrated previously that repeated pairing of a neutral odor with copulation produces a subsequent conditioned ejaculatory preference (CEP) for a female bearing that odor. Here we examine the copulatory components that comprise the unconditioned stimulus (UCS). In Experiment 1, male Long-Evans rats were allowed to copulate with scented females for nine sessions in which they achieved two ejaculations, one ejaculation plus the first intromission following the postejaculatory interval (PEI), one ejaculation without a PEI, or five intromissions without ejaculation. Only the males that achieved two ejaculations or one ejaculation plus the PEI displayed significant CEP. In Experiment 2, males were allowed to remain in the presence of the scented female without access to her after different amounts of copulatory stimulation. Under these conditions, both one and two ejaculations, but not five intromissions, supported the development of CEPs. In Experiment 3, males were allowed to copulate to ejaculation with an unscented female followed by exposure without access to a scented female. This treatment also supported the development of CEP. These results indicate that ejaculation plus a PEI are necessary for the development of CEPs and that the female must be present during the PEI for this to occur. These findings indicate that events during the PEI are the critical components of the UCS for CEP development.

The development of olfactory conditioned ejaculatory preferences in the male rat. II. Parametric manipulation of conditioning session number and duration

We have previously demonstrated that repeated pairing of a neutral odor with copulation produces a subsequent conditioned ejaculatory preference (CEP) for females bearing that odor. The present study examines the course of CEP development. In Experiment 1, Long-Evans male rats were allowed access to almond-scented, sexually receptive females for either one, five, or nine conditioning sessions that were 30 min in duration. Males given five or nine sessions displayed significant CEPs. In Experiment 2, male rats were given a single conditioning session with multiple almond-scented females until either a duration (60, 120, 180, or 240 min) or copulatory criterion (two, four, or six ejaculatory series) was satisfied. Males that received 120-, 180-, or 240-min sessions or four ejaculations displayed significant CEPs; males that received two or six ejaculations displayed a trend for CEPs. Analysis of effect size estimates revealed that the strongest CEPs were produced by 120 min of copulation or four ejaculations. In Experiment 3, males receiving nine conditioning sessions each 30 min in duration displayed a more enduring CEP than did males receiving a single conditioning session 240 min in duration. These data suggest that early sexual experiences have particularly powerful influences on subsequent sexual preferences and that the development of sexual preferences are influenced by interactions between CS-UCS pairings and motivational variables.

Basal forebrain damage and object-recognition in rats

Damage to the basal forebrain (BF) produces permanent learning and memory impairments in humans. Most efforts to model these deficits in rats have focused on spatial memory dysfunction; this study was the first to assess the effects of BF damage in rats on the performance of a battery of object-memory tasks commonly employed to assess brain damage-produced amnesia in primates. The performance of rats with bilateral electrolytic lesions of the medial septum and diagonal band (MS/NDB) region of the BF was assessed on three object-memory tasks: nonrecurring items delayed nonmatching-to-sample (DNMS), simple object discrimination, and eight-pair concurrent object discrimination. Lesioned rats and sham-surgery controls were tested on the DNMS task at retention delays of 4, 15, 30, 60 and 120 s both before and after surgery. After surgery, the rats with MS/NDB lesions required significantly more trials than controls to relearn the nonmatching rule; and, once they relearned the rule, they were significantly and comparably impaired at all delays. This impairment did not diminish with either the passage of time or additional practice. In contrast, there were no significant differences between the MS/NDB-lesioned and control groups in the performance of either simple or concurrent object-discrimination tasks. The delay-independent nature of the DNMS deficit and the lack of deficits on the other two object-memory tasks suggest that the effect of the lesion is not the result of an impairment in retention.

Contingent tolerance, compensatory responses, and physical dependence in diazepam-treated amygdala-kindled rats

Three groups of amygdala-kindled rats received 10 bidaily treatment trials: On each trial, the drug-before group received a diazepam (2.5 mg/kg i.p.) injection 1 hr before a convulsive stimulation, the drug-after group received a diazepam injection 1 hr after a stimulation, and the vehicle control group received a vehicle injection either 1 hr before or 1 hr after a stimulation. After treatment, only the drug-before group displayed significantly longer forelimb clonus under the influence of diazepam (that is, they displayed contingent tolerance to diazepam's anticonvulsant effect) and significantly longer forelimb clonus while drug free. Following a 14-day retention period, the rats in the drug-before group retained significant levels of contingent tolerance but did not display significant increases when tested drug free. These data suggest that compensatory responses do not play a causal role in the expression of contingent tolerance.

Noncontingent drug exposure facilitates the development of contingent tolerance to the anticonvulsant effects of ethanol and diazepam in kindled rats

Tolerance to anticonvulsant drug effects on kindled convulsions can result from drug exposure alone, but convulsive activity during drug exposure has a substantial facilitatory effect on tolerance development. Tolerance produced by drug exposure in the absence of a criterion response (in this case convulsions) has been termed pharmacologic tolerance (10); tolerance produced by drug exposure with concomitant performance of the criterion response has been termed contingent tolerance (1). The present study examines whether noncontingent drug exposure facilitates the development of contingent tolerance to the anticonvulsant effects of ethanol and diazepam. Amygdala-kindled, Long-Evans rats were treated with either ethanol (5.0 g/kg once daily for 21 days) or diazepam (5.0 mg/kg three times daily for 10 days) in the absence of convulsive stimulation to produce pharmacologic tolerance--control rats received treatments of vehicle. Then, all of the rats were rendered contingently tolerant by a series of "bidaily" (once every 2 days) injections (ethanol 2.0 g/kg or diazepam 2.0 mg/kg), each 1 h prior to a kindled convulsion. The rats that had received noncontingent exposure to ethanol or diazepam developed contingent tolerance significantly faster than the control rats. These results suggest that the mechanisms underlying pharmacologic and contingent tolerance to anticonvulsant drug effects are additive.

Persistence of the interictal emotionality produced by long-term amygdala kindling in rats

Long-term amygdala kindling in rats results in large and reliable increases in emotional behaviour that model the interictal emotionality often observed in temporal lobe epileptics [Kalynchuk L. E. et al. (1997) Biol. Psychiat. 41, 438-451; Pinel J. P. J. et al. (1977) Science 197, 1088-1089]. These experiments investigated the persistence of these kindling-induced increases in emotional behaviour after the cessation of the kindling stimulations. In Experiment 1, rats received 99 amygdala or sham stimulations. Then, they were tested on three tests of emotionality (i.e. activity in an unfamiliar open field, resistance to capture from the open field, and activity in an elevated-plus maze) either one day, one week, or one month after the final stimulation. The rats tested one day after the last stimulation displayed substantial decreases in open-field activity, increases in resistance to capture and increases in open-arm activity on the elevated-plus maze; these effects decreased, but not to control levels, in the rats tested one month after the final stimulation. In Experiment 2, rats received 99 amygdala or sham stimulations, and their resistance to capture was assessed one day later. Then, after a 60-day stimulation-free period, the rats received another zero, one, 10, or 30 amygdala stimulations and their resistance to capture was reassessed one day later. The high levels of resistance to capture observed in the rats tested one day after the 99 stimulations declined significantly during the 60-day stimulation-free period, but it remained significantly above control levels. However, the administration of 30 additional stimulations reinstated asymptotic levels of resistance to capture. These results provide the first systematic evidence that kindling-induced increases in emotional behaviour persist at significant levels for at least two months following the termination of kindling stimulations. Thus, they suggest that the neural changes underlying the genesis of interictal emotionality may be closely related to those mediating epileptogenesis itself.

Changes in emotional behavior produced by long-term amygdala kindling in rats

The effects of long-term amygdala kindling on emotional behavior were investigated. In Experiment 1, rats received 99 basolateral amygdala, central amygdala, or sham stimulations. The rats in both kindled groups displayed more resistance to capture from an open field and more open-arm activity on an elevated plus maze than did the sham control rats. In Experiment 2, rats received either 20, 60, or 100 amygdala stimulations or sham stimulations. Compared to the sham controls, the kindled rats explored less during the first 30s in a novel open field, avoided the central area of the open field, resisted being captured from the open field, and engaged in more open-arm activity on the elevated plus maze. The magnitude of these effects was greatest in the 100-stim rats and least in the 20-stim rats. Together, these results suggest that long-term amygdala kindling in rats is a useful model for studying the emotionality associated with temporal lobe epilepsy.

Tolerance to the anticonvulsant and ataxic effects of pentobarbital: effect of an ascending-dose regimen

We assessed the effect of an ascending-dose regimen on the development of tolerance to the anticonvulsant and ataxic effects of pentobarbital in four groups of amygdala-kindled rats. Each rat received 20 bidaily (one every 48 h) trials in which an intraperitoneal (IP) pentobarbital or vehicle injection was delivered 1 h before a convulsive amygdala stimulation. On each trial, the rats in the three pentobarbital groups received either a high dose (50 mg/kg), a low dose (10mg/kg), or ascending doses of pentobarbital that began at 10 mg/kg and increased to as high as 26 mg/kg by 1 mg/kg increments as tolerance developed to its anticonvulsant effect; the rats in the vehicle group received saline. The rats in the ascending-dose condition displayed significantly more tolerance to the anticonvulsant effect of pentobarbital than did the other rats; in contrast, the high-dose rats displayed more tolerance to the ataxic effect of pentobarbital than did the other rats. These findings extend previous reports of the facilitatory effect of ascending-dose regimens on the development of tolerance to the anticonvulsant effect of benzodiazepines, and show that the facilitatory effect of ascending-dose regimens does not extend to all drug effects.

Contingent drug tolerance: differential tolerance to the anticonvulsant, hypothermic, and ataxic effects of ethanol

The kindled-convulsion model of epilepsy was used to study contingent tolerance to ethanol's (1.5 g/kg; IP) anticonvulsant, hypothermic, and ataxic effects in adult male rats. In the present experiments, three groups of amygdala-kindled rats received a series of bidaily (one every 48 h) convulsive stimulations: one group received ethanol 1 h before each stimulation; one group received ethanol 1 h after each stimulation; and another group served as the saline control. Tolerance to ethanol's anticonvulsant effect (Experiments 1 and 2) was greatest in those rats that received ethanol before each convulsive stimulation; whereas, tolerance to ethanol's hypothermic (Experiments 1 and 2) and ataxic (Experiments 2) effects developed in both groups that received ethanol. These results were predicted on the basis of the drug-effect theory of drug tolerance: the theory that functional drug tolerance is an adaptation to the disruptive effects of drugs on concurrent patterns of neural activity, not to drug exposure per se.

Dissipation of contingent tolerance to the anticonvulsant effect of diazepam: effect of the criterion response

The effect of convulsive stimulations on the dissipation of tolerance to the anticonvulsant effect of diazepam was investigated using the kindled-convulsion model. Amygdala-kindled rats were rendered tolerant to diazepam's anticonvulsant effect by 25 "bidaily" (one/48 h) diazepam injections (2.5 mg/kg), each followed 1 h later by a convulsive stimulation. They were then divided into nine groups for the tolerance-dissipation phase of the experiment. Of the nine groups, three received bidaily control handling for one trial, three trials, or seven trials; three received bidaily saline injections, each 1 h before a convulsive stimulation, for one, three, or seven trials; and three received bidaily diazepam injections, each 1 h after a convulsive stimulation, for one, three, or seven trials. Finally, each rat received a tolerance-retention test (i.e., a diazepam injection followed 1 h later by a convulsive stimulation) 48 h after its last tolerance-dissipation trial. The tolerance dissipated gradually but completely over the 4-, 8-, and 16-day test intervals in the rats that received a convulsive stimulation before each injection during the tolerance-dissipation phase, whether they were injected with saline or diazepam; in contrast, tolerance did not dissipate in the rats that received saline injections but no stimulations. Remarkably, the discontinuance of the bidaily diazepam injections, even for 16 days, was not sufficient to dissipate the tolerance that had developed to diazepam's anticonvulsant effect; nor was the continuation of the bidaily diazepam injections sufficient to keep tolerance from dissipating.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of an ascending dose regimen on the development of tolerance to the anticonvulsant effect of diazepam

The effect of an ascending dose regimen on the development of tolerance to diazepam's anticonvulsant effect was assessed. During the 22 trials of the tolerance development phase, amygdala-kindled rats received either a series of dosage injections ranging from high (10 mg/kg), to low (1.0 mg/kg), and ascending (1.0 mg/kg and increased by 0.2-mg/kg increments to 3.0 mg/kg) or saline injections. Diazepam was administered by ip injection once every 48 hr, and each injection was followed 1 hr later by a convulsive stimulation. The ascending dose rats displayed significantly more tolerance to the anticonvulsant effect of diazepam than did the high dose, low dose, or saline rats. By contrast, both the ascending and high dose rats displayed a significant withdrawal effect (i.e., increased duration of convulsions) after the cessation of diazepam injections. Results demonstrate that administration of ascending dosages can facilitate the development of tolerance to anticonvulsant drug effects and that tolerance and withdrawal are not necessarily inextricably related.

Effect of an ascending dose regimen on the development of tolerance to the anticonvulsant effect of diazepam

The effect of an ascending dose regimen on the development of tolerance to diazepam's anticonvulsant effect was assessed. During the 22 trials of the tolerance development phase, amygdala-kindled rats received either a series of dosage injections ranging from high (10 mg/kg), to low (1.0 mg/kg), and ascending (1.0 mg/kg and increased by 0.2-mg/kg increments to 3.0 mg/kg) or saline injections. Diazepam was administered by ip injection once every 48 hr, and each injection was followed 1 hr later by a convulsive stimulation. The ascending dose rats displayed significantly more tolerance to the anticonvulsant effect of diazepam than did the high dose, low dose, or saline rats. By contrast, both the ascending and high dose rats displayed a significant withdrawal effect (i.e., increased duration of convulsions) after the cessation of diazepam injections. Results demonstrate that administration of ascending dosages can facilitate the development of tolerance to anticonvulsant drug effects and that tolerance and withdrawal are not necessarily inextricably related.