Cocaine sensitivity in Roman High and Low Avoidance rats is modulated by sex and gonadal hormone status

Cocaine sensitization in male rats requires activation of estrogen receptors

Gonadal steroids play a modulatory role in cocaine use disorders, and are responsible for many sex differences observed in the behavioral response to cocaine. In females, it is well established that estradiol enhances the behavioral response to cocaine. In males, we have recently shown that testosterone enhances sensitization to cocaine but its mechanism of action remains to be elucidated. The current study investigated the contribution of DHT, a non-aromatizable androgen, and of estradiol, in regulating cocaine-induced sensitization in male rats. Gonadectomized (GDX) male rats treated with estradiol sensitized to repeated cocaine administration, while GDX rats treated with DHT did not, implicating estradiol in cocaine sensitization. Furthermore, intact male rats treated with the antiestrogen ICI 182,780 did not show sensitization to repeated cocaine. This study demonstrates the pivotal role of estradiol in cocaine-induced neuroplasticity and neuroadaptations in the rodent brain.

The influence of sex and reproductive cycle on cocaine-induced behavioral and neurobiological alterations: a review

This systematic review (SR) was aimed at answering two questions: (1) how sex and ovarian hormones alter behavior associated with cocaine use; (2) which possible neurobiological mechanisms explain behavioral differences. Three different researchers conducted a search in PUBMED for all kinds of articles published between the years of 1991 to 2021 on the theme "reproductive cycle and cocaine", "estrous cycle and cocaine", "menstrual cycle and cocaine", "fluctuation of ovarian hormones and cocaine", "estrogen and cocaine" and "progesterone and cocaine". Sixty original studies were identified and subdivided into experimental rodent studies and clinical trials. Experimental studies were characterized by author/year, species/strain, sex/number, age/weight, dose/route/time of administration, hormonal assessment, or administration. Clinical trials were characterized by author/year, sex/number, age, exclusion criterion, dose/route of administration/time of cocaine, and hormonal assessment. Results gathered showed that rodent females develop increased consumption, seeking behavior, craving, relapse, locomotion, increases in stress and anxiety, among other behavioral alterations during peaks of estrogen. These observations are related to the direct effects played by ovarian hormones (in particularly estradiol), in dopamine, but also in serotonin neurons, and in brain regions such as the tegmental area, the nucleus accumbens, the hypothalamus, the amygdala and the prefrontal cortex. Increased sensitization to cocaine presented by high estradiol females was linked to the activation of a CBR1-mediated mechanism and GABA-A-dependent suppression of inhibitory synaptic activity of the prelimbic prefrontal cortex. Estradiol facilitation of cocaine-increased locomotion and self-administration was shown to require the release of glutamate and the activation of metabotropic glutamate receptors subtype 5. Clinical studies also tend to point to a stimulatory effect of estradiol on cocaine sensitization and a neuroprotective effect of progesterone. In conclusion, the results of the present review indicate a need for further preclinical and clinical trials and neurobiological studies to better understand the relationship between sex and ovarian hormones on cocaine sensitization.

Neurobehavioral and neurodevelopmental profiles of a heuristic genetic model of differential schizophrenia- and addiction-relevant features: The RHA vs. RLA rats

The Roman High- (RHA) and Low-(RLA) avoidance rat lines/strains were generated through bidirectional selective breeding for rapid (RHA) vs. extremely poor (RLA) two-way active avoidance acquisition. Compared with RLAs and other rat strains/stocks, RHAs are characterized by increased impulsivity, deficits in social behavior, novelty-induced hyper-locomotion, impaired attentional/cognitive abilities, vulnerability to psychostimulant sensitization and drug addiction. RHA rats also exhibit decreased function of the prefrontal cortex (PFC) and hippocampus, increased functional activity of the mesolimbic dopamine system and a dramatic deficit of central metabotropic glutamate-2 (mGlu2) receptors (due to a stop codon mutation at cysteine 407 in Grm2 -cys407*-), along with increased density of 5-HT2A receptors in the PFC, alterations of several synaptic markers and increased density of pyramidal "thin" (immature) dendrític spines in the PFC. These characteristics suggest an immature brain of RHA rats, and are reminiscent of schizophrenia features like hypofrontality and disruption of the excitation/inhibition cortical balance. RHA rats represent a promising heuristic model of neurodevelopmental schizophrenia-relevant features and comorbidity with drug addiction vulnerability.

Assessment of aversive effects of methylone in male and female Sprague-Dawley rats: Conditioned taste avoidance, body temperature and activity/stereotypies

Methylone's rewarding effects have been well characterized; however, little is known about its aversive effects and how such effects may be impacted by sex. In this context, the present study investigated the aversive effects of methylone (vehicle, 5.6, 10 or 18 mg/kg, IP) in 35 male and 31 female Sprague-Dawley rats assessed by conditioned taste avoidance and changes in body temperature and activity/stereotypies. Methylone induced significant taste avoidance, changes in temperature and increased activity and stereotypies in both males and females. Similar to work with other synthetic cathinones, methylone has aversive effects as indexed by significant taste avoidance and changes in temperature and activity (two characteristics of methylone overdose in humans). The only endpoint for which there were significant sex differences was in general activity with males displaying a faster onset and females displaying a longer duration. Although sex was not a factor with taste avoidance and temperature, separate analyses for males and females revealed different patterns, e.g., males displayed a more rapid acquisition of taste avoidance and females displayed changes in temperature at lower doses. Males displayed a faster onset and females displayed a longer duration of activity (consistent with the analyses considering sex as a factor), while time- and dose-dependent stereotypies did not show consistent pattern differences. Although sex differences were relatively limited when sex was specifically assessed as a factor (or only evident when sex comparisons were made in the patterns of effects), sex as a biological variable in the study of drugs should be made to determine if differences exist and, if evident, the basis for these differences.

Ibudilast attenuates expression of behavioral sensitization to cocaine in male and female rats

There are no FDA-approved pharmacotherapies for cocaine use disorder, indicating a need to identify novel reagents with therapeutic potential. Ibudilast is an anti-inflammatory glial attenuator and non-selective phosphodiesterase inhibitor currently undergoing clinical evaluations for methamphetamine, opiate, and alcohol abuse disorders. We previously showed that twice daily (b.i.d.) ibudilast reduces the development of methamphetamine sensitization in male mice. However, nothing is known about the ability of ibudilast to modulate the expression of sensitization that occurs after drug re-exposure during abstinence, effects on cocaine-mediated behaviors, or potentially sexually dimorphic effects. Male and female rats were administered cocaine for 7 days and expression of sensitization was assessed by cocaine challenge after 21 days abstinence. On test days, 15 mg/kg i. p. cocaine was evaluated, whereas 30 mg/kg was administered on intervening days. Lower test doses avoid competition of non-motor behaviors with locomotion. In all measures where sensitization was expressed, ibudilast (7.5 and 10 mg/kg, i. p., b. i.d. for 3 days and once on test day) reversed this behavior to levels seen after acute exposure, but not below. There were some intriguing sexually dimorphic effects that were not a function of estrous cycle. Specifically, distance travelled in the center of the test arena and rearing only sensitized in male rats, and ibudilast reversed these behaviors to levels seen after acute cocaine exposure. In females, center distance travelled was reduced below acute cocaine levels by 7.5 mg/kg ibudilast. Increased distance travelled in the center versus periphery is thought to model anxiolytic-like behavior due to increased predation risk. Taken together, these data suggest that the clinical evaluation of ibudilast could be extended to cocaine use disorder.

Sex-specific tonic 2-arachidonoylglycerol signaling at inhibitory inputs onto dopamine neurons of Lister Hooded rats

Addiction as a psychiatric disorder involves interaction of inherited predispositions and environmental factors. Similarly to humans, laboratory animals self-administer addictive drugs, whose appetitive properties result from activation and suppression of brain reward and aversive pathways, respectively. The ventral tegmental area (VTA) where dopamine (DA) cells are located is a key component of brain reward circuitry, whereas the rostromedial tegmental nucleus (RMTg) critically regulates aversive behaviors. Reduced responses to either aversive intrinsic components of addictive drugs or to negative consequences of compulsive drug taking might contribute to vulnerability to addiction. In this regard, female Lister Hooded (LH) rats are more vulnerable than male counterparts to cannabinoid self-administration. We, therefore, took advantage of sex differences displayed by LH rats, and studied VTA DA neuronal properties to unveil functional differences. Electrophysiological properties of DA cells were examined performing either single cell extracellular recordings in anesthetized rats or whole-cell patch-clamp recordings in slices. In vivo, DA cell spontaneous activity was similar, though sex differences were observed in RMTg-induced inhibition of DA neurons. In vitro, DA cells showed similar intrinsic and synaptic properties. However, females displayed larger depolarization-induced suppression of inhibition (DSI) than male LH rats. DSI, an endocannabinoid-mediated form of short term plasticity, was mediated by 2-arachidonoylglycerol (2-AG) activating type 1-cannabinoid (CB1) receptors. We found that sex-dependent differences in DSI magnitude were not ascribed to CB1 number and/or function, but rather to a tonic 2-AG signaling. We suggest that sex specific tonic 2-AG signaling might contribute to regulate responses to aversive intrinsic properties to cannabinoids, thus resulting in faster acquisition/initiation of cannabinoid taking and, eventually, in progression to addiction.

Corticotrophin releasing factor (CRF) induced reinstatement of cocaine seeking in male and female rats

Significant sex differences have been demonstrated in clinical and preclinical studies of cocaine addiction, with some of the most consistent differences noted in regard to the role of stress and craving. The current study examined stress-induced reinstatement of cocaine seeking in male and female rats in an animal model of relapse using corticotropin-releasing factor (CRF) administration. Both male and female rats demonstrated increased cocaine seeking in response to CRF. CRF-induced reinstatement was highly variable across both male and female rats, and further analysis revealed a subpopulation that was particularly sensitive to CRF (high responders). Female high responders displayed significantly increased responding to CRF compared to males. Individual differences in stress responsivity could thus contribute to the likelihood of relapse, with females showing greater heterogeneity to stress-induced relapse.

Functional somatic syndromes, anxiety disorders and the upper airway: a matter of paradigms

The relationship between the functional somatic syndromes, anxiety disorders and the upper airway (particularly, sleep disordered breathing) remains ambiguous. This ambiguity, despite a growing body of research supporting a relationship, may result from the absence of a paradigm to explain how upper airway dysfunction can promote disorders commonly associated with one's mental health. This review models the functional somatic syndromes and anxiety disorders as consequences of chronically increased hypothalamic-pituitary-adrenal axis activity. It then examines the literature supporting a relationship between these disorders and upper airway dysfunction during wakefulness and sleep. Finally, building upon an existing paradigm of neural sensitization, sleep disordered breathing is linked to functional somatic syndromes and anxiety disorders through chronic activation of the hypothalamic-pituitary-adrenal axis.

Testosterone is essential for cocaine sensitization in male rats

Most studies agree that males and females respond differently to drugs of abuse. In females, estradiol enhances the behavioral response to cocaine. However, studies on the role of testosterone and the locomotor response to psychostimulants in the male rat are inconclusive. Our study was designed to determine the behavioral effects of testosterone on the development and persistence of cocaine sensitization in male rats. We tested different doses of cocaine (10, 15 and 30mg/kg) to determine which dose induced locomotor sensitization in intact (INT) and gonadectomized (GDX) animals. We also investigated if GDX males with testosterone replacement (GDX-T) showed a similar locomotor response to cocaine as INT males. Our data showed that gonadectomy enhanced the locomotor response to a single cocaine injection. This effect was not observed in gonadectomized rats that received testosterone replacement. However, GDX rats did not show a progressive increase in their locomotor response to repeated cocaine administration (15 and 30mg/kg) (sensitization) as did INT and GDX-T animals. It is possible that in GDX males, the initial high locomotor response to cocaine creates a ceiling effect that limits further increase in cocaine-induced hyperactivity. These findings indicate that testosterone not only modulates the behavioral response to a single and to repeated cocaine injections, but is also essential for male rats to become sensitized to cocaine.

U-69593, a kappa opioid receptor agonist, decreases cocaine-induced behavioral sensitization in female rats

This study was designed to investigate if the kappa opioid system regulates the locomotor response to cocaine in the female rat and to determine if the effect is dependent on estradiol treatment. Adult rats were ovariectomized (OVX) and half received an estradiol (OVX-EB) implant. After a week, rats were injected for 5 consecutive days with vehicle or with the kappa opioid receptor (KOPr) agonist U-69593 (0.16, 0.32, and 0.64 mg/kg) 15 min prior to cocaine injection (15 mg/kg). Following a 7-day drug-free period, rats were challenged with cocaine (Day 13). The locomotor response to cocaine was measured on Days 1, 5, and 13. U-69593 (0.32 mg/kg) decreased cocaine-induced locomotor activity in drug-naïve OVX rats and in those that received the OVX-EB implant. These results indicate that the acute effects of U-69593 are independent of estradiol treatment. Repeated exposure to U-69593 (0.32 mg/kg) prior to cocaine decreased the development of behavioral sensitization in OVX-EB-implanted rats. This decrease in cocaine-induced hyperlocomotion persisted after 1 week of cocaine withdrawal. These data indicate that the KOPr system participates in estradiol modulation of cocaine-induced behavioral sensitization in the female rat.

Methylphenidate improves performance on the radial arm maze in periadolescent rats

Methylphenidate (Ritalin; MPD) is one of the most commonly prescribed drugs in childhood and adolescence and many clinical studies have documented its efficacy. Due to the limitations of conducting invasive research in humans, animal models can be beneficial for studying drug effects. However, few animal studies have demonstrated the effects of methylphenidate on cognitive processes. The objective of this study was to find a dose of methylphenidate that was effective in improving performance on a spatial working memory cognitive task when administered orally to periadolescent rats. Therefore, we dosed subjects with methylphenidate at 1 or 3 mg/kg/day via gastric intubation from postnatal day 22 to 59 and assessed the effects of the drug on performance on the radial arm maze each day. To enhance performance overall, a second experiment was conducted where the subjects were moderately food restricted (to 90% of the free-feeding weight). Results of Experiment 1 show that during the first week of testing only the 3 mg/kg MPD-treated males showed improved performance (entries prior to repeated entry) when ad lib fed and housed in pairs while the same dose significantly improved performance in both males and females under conditions of food-restriction and individual housing in Experiment 2. MPD also produced a pattern of increased errors and arms entered during the first week, especially in Experiment 2. MPD increased locomotor activity when tested at postnatal day 60 in both experiments. The data suggest that 3 mg/kg oral methylphenidate improves performance on a spatial cognitive task only early in treatment in the rat. While males show improvement under conditions of both high and low motivation, females only show MPD effects when highly motivated. Hypothetically, methylphenidate may improve radial arm maze performance through increased attention and improved spatial working memory and/or alterations in locomotion, reactivity to novelty or anxiety. Regardless, the study supports the utility of the rat as a suitable model to examine the effects of low dose oral MPD.

Estrogen-modulated frontal cortical CaMKII activity and behavioral supersensitization induced by prolonged cocaine treatment in female rats

RATIONALE

Females have been demonstrated repeatedly to be more sensitive to cocaine. The role of the frontal cortex (FCX) in mediating behavioral sensitization and the underlying signaling pathways are unclear.

OBJECTIVE

The study was designed to characterize the role of FCX calcium/calmodulin-dependent protein kinase II (CaMKII) activity in the behavioral supersensitization observed in female rats after prolonged cocaine exposure.

MATERIALS AND METHODS

Intact female rats that received cocaine for 9 days followed by 7 days of drug withdrawal constituted the model used for studying the mechanism of supersensitization.

RESULTS

This cocaine withdrawal treatment resulted in behavioral supersensitization in intact female rats as indicated by an enhanced behavioral response to cocaine challenge assessed on day 16 (7-day withdrawal) and compared to the response on day 9 of cocaine treatment. This treatment regimen did not lead to supersensitization in male or in ovariectomized (OVX) rats. Administration of estrogen to OVX rats restored behavioral supersensitivity to repeated cocaine. FCX CaMKII activity was significantly altered by cocaine in females, and this effect was related to estrogen's presence; cocaine-induced changes in striatal CaMKII activity were, however, less estrogen-sensitive. Furthermore, estrogen-modulated FCX CaMKII activity in cocaine-supersensitized rats was dependent on D(1) dopamine receptor activation.

CONCLUSION

Estrogen-modulated D(1) dopamine receptor activity mediates the effects of prolonged cocaine exposure on FCX CaMKII, and this, in turn, may contribute to the development of behavioral supersensitivity to repeated cocaine treatment in intact female rats.

Exogenous progesterone attenuates the subjective effects of smoked cocaine in women, but not in men

In a previous study, we showed that the positive subjective effects of cocaine were higher during the follicular phase compared to the luteal phase of the menstrual cycle. The purpose of the present study was to determine if exogenously administered progesterone during the follicular phase in females would attenuate the response to cocaine compared to the normal follicular phase, thus making the response to cocaine similar to the luteal phase. To address the role of sex differences, males were also administered exogenous progesterone during one inpatient stay. In all, 11 female and 10 male non-treatment-seeking cocaine smokers participated. Females had three inpatient stays: one during a normal follicular phase, one during a normal luteal phase, and one during a follicular phase when exogenous progesterone was administered. Males had two inpatient stays: one when exogenous progesterone was administered and the other when placebo was administered. During each inpatient admission, there were four smoked cocaine administration sessions: participants were administered six doses of cocaine (0, 6, 12, or 25 mg cocaine base) at 14 min intervals. Smoked cocaine increased heart rate, blood pressure and several subjective effects such as 'good drug effect' and 'drug quality' cluster scores. Administration of progesterone during the follicular phase in women attenuated the positive subjective effects of cocaine, whereas only minimal changes were observed in men. These results indicate that progesterone modulates the response to cocaine in women and suggests that fluctuations in endogenous progesterone levels account for some of the sex differences observed in humans.

The role of age, genotype, sex, and route of acute and chronic administration of methylphenidate: A review of its locomotor effects

Children with attention deficit hyperactivity disorder (ADHD) are treated for extended periods of time with the psychostimulant methylphenidate (MPD). The psychostimulants cocaine, amphetamine, and MPD exhibit similar structural configuration and pharmacological profile. The consequence of the long-term use of psychostimulants such as MPD as treatment for ADHD in the developing brain of children is unknown. Repeated treatment with psychostimulants has been shown to elicit adverse effects in behavior, such as dependence, paranoia, schizophrenia, and behavioral sensitization. Behavioral sensitization and cross-sensitization between two drugs are used as experimental markers to determine the potential of a drug to develop dependence/addiction. Although there are many reviews written about behavioral sensitization involving psychostimulants, scarcely any have focused specifically on MPD-elicited behavioral sensitization and cross-sensitization with other psychostimulants. Moreover, the response to MPD and the expression of ADHD vary among females and males and among different populations due to genetic variability. Since the interpretation and synthesis of the data reported are controversial, this review focuses on the adverse effects of MPD and the role of age, sex, and genetic composition on the acute and chronic effects of MPD, such as MPD-elicited behavioral sensitization and cross-sensitization with amphetamine in animal models. Animal models of drug-induced locomotor stimulation, particularly locomotor sensitization, can be used to understand the mechanisms underlying human drug-induced dependence.

Effect of short- vs. long-term estrogen on reinstatement of cocaine-seeking behavior in female rats

Estrogen effects on cocaine-induced reinstatement of lever responding were examined in sham-operated, vehicle-treated (SH+VEH), ovariectomized (OVX+VEH), and OVX female Wistar rats with estrogen replacement (OVX+EB). The effect of long- (64+/-1.56 days) and short-term (9 days) EB treatment on reinstatement of cocaine-seeking behavior was compared in Experiment 1 and 2, respectively, in order to compare the effect of EB when it was present during the development vs. expression of reinstatement of cocaine-seeking behavior. Rats were trained to self-administer 0.4 mg/kg/inf cocaine. After the acquisition criteria were met, rats continued to respond for cocaine for 2 h/day for a 14-day maintenance period. Cocaine was then replaced with saline and the 21-day extinction period commenced. Subsequently, rats were tested for reinstatement of lever responding on the previously drug-paired lever after alternating daily injections of saline or cocaine. In both experiments, there were no differences between groups in self-administration behavior during training, maintenance, or extinction. In Experiment 1, SH+VEH and chronically treated OVX+EB rats had greater cocaine-induced reinstatement than OVX+VEH rats. In Experiment 2, short-term treated OVX+EB rats also showed enhanced cocaine-induced reinstatement compared to OVX+VEH rats. The results indicate that EB-mediated enhancement of cocaine-induced reinstatement is dependent on EB presence during the expression of reinstatement but not during the formation of stimulus-reward associations during the development of cocaine-reinforced behavior.

Repeated maternal separation: differences in cocaine-induced behavioral sensitization in adult male and female mice

RATIONALE

Repeated maternal separations profoundly alter the adult stress response, the development of the hypothalamic-pituitary-adrenal axis, and prominently, the GABAergic and monoaminergic systems. These neural changes are postulated to influence the vulnerability to drugs of abuse implicating glucocortocoids in the behavioral responses to psychomotor stimulants.

OBJECTIVE

To investigate whether repeated brief maternal separation stress increases behavioral sensitization to cocaine in adult male and female mice, and to assess any concurrent changes in hippocampal glucocorticoid receptors and accumbal dopamine transporters.

METHODS

Half of the litters were separated from the nest for 1 h/day from post-natal days 1 to 13. Starting on post-natal day 50, all mice were injected with either cocaine (10.0 mg/kg) or saline for 10 consecutive days. Locomotor activity was assessed in an open field on days 50, 54 and 59 via a tracking system. Approximately 10 and 40 days later, all mice were challenged with 7.5 mg/kg cocaine.

RESULTS

Repeated maternal separation increased the hyperlocomotor response to 10.0 mg/kg cocaine regardless of gender. During expression tests (days 69/71, 99), male, but not female, mice with a history of maternal separation exhibited significant sensitized hyperactivity in response to cocaine. Male mice that were maternally separated and had no history of cocaine sensitization, demonstrated cross-sensitization to 7.5 mg/kg cocaine. Immunohistochemical analysis revealed that the hippocampal CA1 glucocorticoid receptor and nucleus accumbens dopamine transporter proteins were expressed more in females than in males, regardless of maternal separation experience.

CONCLUSIONS

Repeated maternal separation is a stressor that can induce heightened sensitivity to low doses of cocaine, as expressed by hyperactivity. Furthermore, sex differences in glucocorticoid receptor and dopamine transporter expression may be responsible for the sexual dimorphic expression of behavioral sensitization to cocaine.

Inter-individual vs line/strain differences in psychogenetically selected Roman High-(RHA) and Low-(RLA) Avoidance rats: neuroendocrine and behavioural aspects

Inter-individual differences in neuroendocrine and behavioural responses to environmental challenges will be considered within the context of psychogenetic selection, using the Roman High-(RHA) and Low-(RLA) Avoidance rat lines as an example. We assume that the selected genotypes, by interacting with environmental factors, determine specific 'biobehavioural profiles'. Practical and theoretical problems regarding the measurement of inter-individual vs line/strain differences, the definition of 'traits' vs experimental variables, and possible correlations between physiological and behavioural parameters will be discussed. We will argue that environmental influences are the main cause of inter-individual variability, and that the genotype only constitutes a 'blueprint' from which typical biobehavioural profiles are established, notably under the influence of early environmental factors. These biobehavioural profiles may correspond in part to human categories known as 'types', 'temperaments' or 'personality traits'. Within each category (including those which can be obtained by psychogenetic selection), more individual personality traits can evolve, notably as a result of social interactions and particular life events.

Neonatal isolation stress potentiates cocaine seeking behavior in adult male and female rats

Little is known with regard to how sex and stress might interact as vulnerability factors in cocaine abuse. In this study, we compared the effects of neonatal isolation stress on cocaine self-administration under extended access conditions and on subsequent responding in a cue-induced reinstatement paradigm in adult male and female rats. Pups from each litter were subjected to either neonatal isolation (1 h/day) or brief daily handling from postnatal day 2 through 12. Adults rats were then trained to self-administer cocaine, and once they acquired lever responding for cocaine under a fixed ratio 1 schedule, they were given 24-h access to intravenous cocaine infusions (1.5 mg/kg) that were available in discrete trials (4, 10 min trials/h) for 7 consecutive days. At 10 days after the last discrete trial session, responding was assessed during six to eight 1-h extinction sessions that were followed by a 1-h cue-induced reinstatement session. Results revealed that females took more cocaine than did males over the 7-day discrete trial self-administration period and tended to respond at higher levels during the initial extinction sessions. Although intake did not differ between handled control rats and isolated rats under extended access conditions, stress effects were observed under subsequent extinction and cue-induced reinstatement testing conditions with isolated rats responding at higher levels during both phases. Notably, stress seemed to obscure sex differences in extinction responding such both isolated males and females responded at high levels. These findings demonstrate robust and enduring effects of neonatal isolation stress on cocaine seeking behavior in adult male and female rats.

Sex differences in the behavioral effects of 24-h/day access to cocaine under a discrete trial procedure

Although more men than women are addicted to cocaine, it has been suggested that women may have an accelerated transition to addiction, and that once addicted they may be more vulnerable to relapse. Here we investigate the effects of extended access to cocaine under a 24-h/day discrete trial procedure on patterns of intake and subsequent motivation to use cocaine as assessed by responding under a progressive-ratio schedule in male and female rats. Rats were initially trained to self-administer cocaine (1.5 mg/kg/infusion) under a fixed-ratio 1 schedule until acquisition occurred, and then responding was assessed under a progressive schedule for three sessions. Subsequently, rats had 24-h access to intravenous cocaine infusions (1.5 mg/kg) that were available in discrete trials (4, 10 min trials/h) for 7 consecutive days. At 10 days after the last discrete trial session, responding was reassessed under a progressive-ratio schedule for three additional sessions to investigate changes in motivation to obtain cocaine. Prior to cocaine self-administration under the 24-h access discrete trial procedure, males and females did not differ on cocaine self-administration under the fixed-ratio or progressive-ratio schedules. However, sex differences emerged under the 24-h access discrete trial procedure with females self-administering higher levels of cocaine, for longer initial periods of time, and showing a greater disruption in the diurnal control over intake than did males. Additionally, following a 10-day forced abstinence period, females responded at higher levels under the progressive-ratio schedule to obtain cocaine infusions than did males. These findings suggest that extended access to cocaine under the discrete trial cocaine self-administration procedure produces sex-dependent patterns of intake and sex-specific changes in motivation to obtain cocaine as measured by progressive-ratio responding.

Effect of gonadectomy and gonadal hormone replacement on cocaine self-administration in female and male rats

Both sex and gonadal steroid hormones may influence the abuse-related behavioral effects of cocaine under some conditions, but there is considerable inconsistency in the literature. In the present study, rats were trained under a fixed ratio (FR) 5 schedule of food presentation and were then allowed to self-administer cocaine (1.0 mg/kg/injection) until behavior stabilized. Subsequently, complete dose-effect functions for cocaine self-administration (0.032-3.2 mg/kg/injection) were determined in female and male rats before and after gonadectomy, and in gonadectomized female and male rats before and during chronic treatment with estradiol or testosterone, respectively. Sex, gonadectomy, and gonadal hormones did not alter the shape or position of dose-effect functions for cocaine self-administration. These results suggest that sex, estrogen, and testosterone levels are not critical determinants of cocaine's reinforcing effects in rats under these conditions. This study differed from earlier studies in that complete dose-effect functions for cocaine were determined. These findings suggest that the behavioral training history, the unit dose of cocaine, and the schedule of reinforcement are important variables in studies of sex and gonadal hormone effects on cocaine self-administration.

The role of D1 and D2 receptors in the cocaine conditioned place preference of male and female rats

The rewarding effects of cocaine have been shown to be sexually dimorphic; female rats develop cocaine conditioned place preference at lower doses and with fewer cocaine pairings than male rats. The present study was conducted to determine whether D1 and D2 receptors contribute to sex differences in cocaine conditioned place preference using a 4-day paradigm. Fifteen minutes prior to receiving saline or cocaine (5mg/kg for females and 20mg/kg for males), rats were pretreated with either SCH 23390, a D1 receptor antagonist, (0.10, 0.25, or 0.50mg/kg), eticlopride, a D2 receptor antagonist, (0.05, 0.10, or 0.25mg/kg), or vehicle (saline). Antagonism of D1 receptors by SCH 23390 fully blocked cocaine conditioned place preference in male rats, while only the two lower doses of SCH 23390 blocked cocaine conditioned place preference in female rats. Conversely, antagonism of D2 receptors using eticlopride had no effect on cocaine conditioned place preference in male or female rats. Due to the known role of D1 receptors in cocaine conditioned place preference, sex differences in D1 receptor sensitivity may explain the differences observed in cocaine reward between male and female rats.

Vaginal lavage attenuates cocaine-stimulated activity and establishes place preference in rats

Sex and estrous cycle stage affect psychostimulant responses in animals. Cycle stage is typically monitored by vaginal lavage. The present studies tested the hypothesis that vaginal lavage modifies behavioral responses to acute cocaine. Female rats were restrained by briefly holding the tail for either vaginal lavage or touching the thigh, or were undisturbed, for 7-10 days prior to testing. Although habituation to the open-field test chamber was equal in each group, repeated lavage decreased horizontal activity relative to naive rats following acute cocaine (10 mg/kg ip). Lavage and touch attenuated cocaine-stimulated vertical activity. A single lavage prior to testing did not affect cocaine-stimulated motor behavior. Estrous cycle influenced motor activity only in nonlavaged rats. The high cocaine-induced responding observed in proestrous and estrous nonlavaged rats was completely blocked by vaginal lavage. A separate experiment tested the ability of vaginal lavage to establish a conditioned place preference. Vaginal lavage immediately prior to the conditioning session, but neither lavage after conditioning nor touch before, induced a significant preference. These results suggest that vaginal lavage serves as a reinforcing stimulus and interacts with a neural substrate that mediates enhanced locomotor responses to cocaine during proestrus and estrus.

Estrogen and opioids interact to modulate the locomotor response to cocaine in the female rat

Estrogen is known to modulate the behavioral response to cocaine; however the mechanisms by which this is accomplished is unknown. In this study we examine one possible candidate, the endogenous opioid system. Adult Sprague-Dawley rats were ovariectomized (OVX), half received Silastic implants with estradiol benzoate (OVX-EB), the other half received empty implants (OVX). After 1 week, spontaneous locomotor and stereotyped activity was measured for 60 min using an automated system. On day 2, locomotor activity was recorded for 30 min. Rats were injected with saline (SAL) or naloxone (NAL) (2 mg/kg, i.p.) and activity measured for the next 20 min. Each of these groups were further subdivided, one that received a saline injection (SAL) and another that received a cocaine injection (COC) (15 mg/kg, i.p.). Locomotor and stereotyped activities were recorded for 60 min. This resulted in the following injection groups: SAL-SAL, NAL-SAL, SAL-COC and NAL-COC. During habituation, OVX rats displayed an overall higher level of activity than OVX-EB rats. Similar to what is observed in males, naloxone significantly reduced locomotion and stereotyped behavior but only in OVX rats. Estrogen administration to OVX rats abolished the effect of naloxone. Surprisingly, when naloxone was administered prior to cocaine, an increase in cocaine-induced locomotor and stereotyped activity was observed, but only in OVX-EB rats. These results indicate that opioid modulation of cocaine-induced locomotor and stereotype activity in the female differs from that reported in the male. In addition in the female, the effect of opioids on cocaine-induced locomotor behavior is dependent on plasma levels of estrogen.

Early social isolation, but not maternal separation, affects behavioral sensitization to amphetamine in male and female adult rats

Early life stressful manipulations, such as maternal separation (MS) or social isolation (SI), can influence the neurobiological development of rats and alter the response of adult animals to drugs of abuse. The present study examined the acute and sensitized behavioral responses (locomotor activity (LMA) and stereotypy) induced by amphetamine after MS or SI in male and female rats. In addition, the hypothesis that the combination of SI and MS could lead to additional effects on the behavioral response to amphetamine was tested. After the repetitive, intermittent administration of 1.5 mg/kg D-amphetamine over five consecutive days, the behavioral expression of sensitization to a challenge injection was assessed following a 2-day withdrawal period. In both sexes, MS and SI did not alter the acute locomotor activating effects of amphetamine as measured in the open-field environment after the first administration of the drug. Whereas SI altered the expression of sensitization to amphetamine in both sexes, MS did not affect it. Finally, in none of the behavioral variables measured did MS and SI interact to further modify the behavioral profile of the animals. The present results suggest that a postweaning manipulation of the environment (SI) is more effective than a preweaning manipulation (MS) in modifying the expression of sensitization to amphetamine.

Dopamine release and uptake are greater in female than male rat striatum as measured by fast cyclic voltammetry

The present studies investigated sexual dimorphisms in dopamine release and uptake using fast-scan cyclic voltammetry in anesthetized rats and in brain slices. Electrical stimulation of the medial forebrain bundle of anesthetized rats at high frequency (60 Hz) elicited significantly more extracellular dopamine in the caudate nucleus of females than males. This sex difference was apparent over a range of current intensities applied to the stimulating electrode. Local electrical stimulation of brain slices in vitro verified in vivo results as more extracellular dopamine was elicited by single and 10 pulse stimulations in the caudate nucleus of females. Kinetic analysis of in vivo and in vitro dopamine overflow data indicated that dopamine release (the concentration of dopamine released per stimulus pulse) and the maximal velocity of dopamine uptake are greater in female rats, but the affinity of the transporter for dopamine was the same in males and females. None of these three parameters varied across the female estrous cycle. Linear regression analysis of dopamine release versus maximal uptake velocity data indicated a significant association of release and uptake sites in each sex and regression lines for males and females virtually overlapped. One explanation for these results is greater dopamine neuron terminal density in female caudate nucleus. These sexual dimorphisms in dopaminergic neurotransmission provide a novel, plausible mechanism to explain robust sex differences in behavioral responses of rats to psychostimulant drugs and may have implications for human neurological disorders and drug abuse.

The role of endogenous sensitization in the pathophysiology of schizophrenia: implications from recent brain imaging studies

Long-term sensitization is a process whereby exposure to a given stimulus such as a drug or a stressor results in an enhanced response at subsequent exposures. Sensitization of mesolimbic dopamine systems has been postulated by several authors to underlie the development of dopaminergic abnormalities associated with schizophrenia. In this review, core features of stimulant-induced sensitization of dopamine systems in rodents are briefly reviewed, as well as the behavioral and clinical evidence suggesting the relevance of this process to drug-induced psychosis and schizophrenia. Results of recent brain imaging studies relevant to the question of sensitization in schizophrenia are then discussed. These studies indicate that schizophrenia is associated with increased amphetamine-induced dopamine release. This exaggerated response was detected in patients experiencing an episode of clinical deterioration but not in clinically stable patients. Since increased stimulant-induced dopamine release is a hallmark of sensitization, these results support the view that schizophrenia is associated with a process of endogenous sensitization. Based on the preclinical evidence that dopamine projection to the prefrontal cortex acts as a buffer that oppose the development of sensitization in subcortical dopamine projections, we propose that, in schizophrenia, neurodevelopmental abnormalities of prefrontal dopaminergic systems might result in a state of enhanced vulnerability to sensitization during late adolescence and early adulthood. It is also proposed that D(2) receptor blockade, if sustained, might allow for an extinction of this sensitization process, with possible re-emergence upon treatment discontinuation. A better understanding of the neurocircuitry associated with endogenous sensitization and its consequence in schizophrenia might be important for the development of better treatment and relapse prevention strategies.

Strain and sex differences in the locomotor response and behavioral sensitization to cocaine in hyperactive rats

Individual variability in the behavioral responsiveness to psychostimulant drugs is due, in part, to genetic factors. The present study investigated the effects of acute and repeated administrations of cocaine (0, 5, 10 or 20 mg/kg, i.p.) on locomotor activity in male and female rats from genetically distinct strains often used as a model of human childhood hyperactivity/attentional deficit disorder: Wistar Kyoto Hyperactive (WKHA) rats, Spontaneous Hypertensive rats (SHR) and their control Wistar Kyoto (WKY). The results, expressed as percent change in locomotor activity relative to respective control groups, showed that cocaine elicits a dose-dependent hyperactivity in all strains and revealed neither strain nor sex differences in acute sensitivity to moderate doses of the drug. Nevertheless, across repeated administrations, strain and sex differences appeared: WKHA rats displayed a moderate extent of sensitization to psychomotor stimulant effects of cocaine and female rats showed more robust sensitization than males, whatever the strain. These findings support the genotype-dependence in the development of behavioral sensitization to cocaine and confirm the robustness of the sexual dimorphism across different inbred rat strains. Interestingly, the present results demonstrate that sensitization to psychostimulant drugs occurs in genetically hyperactive strains as well as in their normoactive control strain.

Stress and emotionality: a multidimensional and genetic approach

The use of behavioural tests aiming to assess the psychological components of stress in animals has led to divergent and sometimes arbitrary interpretations of animal behaviour. This paper presents a critical evaluation of behavioural methods currently used to investigate stress and emotionality. One of its main goals is to demonstrate, through experimental evidence, that emotionality may no longer be seen as a unidimensional construct. Accordingly, following a discussion about concepts, we propose a multiple-testing approach, paralleled by factor analyses, as a tool to dissociate and study the different dimensions of emotionality. Within this multidimensional context, genetic studies (illustrated here by different rat models) are shown to be particularly useful to investigate the neurobiology of stress/emotionality. A genetic approach can be used (i) to broaden and dissect the variability of responses within and between populations and (ii) to search for the molecular bases (i.e. genes and gene products) which underlie such a variability.

Clinically relevant EEG studies and psychophysiological findings: possible neural mechanisms for multiple chemical sensitivity

This paper addresses the evidence for the face, construct, and criterion-related validity of the olfactory-limbic/neural sensitization model for multiple chemical sensitivity (MCS). MCS is a poorly-understood, controversial condition in which low levels of environmental chemicals are reported to trigger disabling levels of illness in certain individuals. Neural sensitization processes could generate an endogenous amplification of responsivity to exogenous substances, thereby providing a plausible explanation for the apparent lack of a classical toxicological dose-response relationship in MCS. Convergent data from both survey and psychophysiological studies of MCS patients and of persons from the community without MCS, but who report elevated frequency of illness from chemical odors (cacosmics), support the involvement of the limbic system and the sensitizability of cacosmics, as predicted by the model. Recent studies show that cacosmics do sensitize their heart rate, blood pressure, and plasma beta-endorphin responses to repeated exposures to a novel laboratory procedure involving dietary manipulations over time. Cacosmia may represent a pathological form of neural plasticity. Taken together, the model and the available evidence suggest the need for more intensive investigation of MCS from the standpoint of possible neurobiological mechanisms affecting cognitive, emotional, and somatic functions.

Neuropsychiatric and somatic characteristics of young adults with and without self-reported chemical odor intolerance and chemical sensitivity

The psychological, neuropsychiatric, and somatic characteristics of young adults who have different degrees of cacosmia (i.e., feeling "ill" from the odor of xenobiotic chemicals) and who have self-described "chemical sensitivity" were examined. A total of 800 college students completed the following: a self-rating scale for frequency of odor intolerance for 10 common substances, Simon Environmental Illness Symptom Survey, the SCL-90-R, Barsky Amplification Scale, Pearlin-Schooler Mastery Scale, Cheek-Buss and Kagan Shyness scales, Marlowe-Crowne Social Desirability Scale, and a health-symptom and physician-diagnosed checklist. Two pairs of groups were compared: (1) subjects in the top 16% (i.e., cacosmics) and bottom 15% (noncacosmics) of the sample with respect to odor intolerance scale scores; and (2) subjects from the entire sample who did (28%) or did not (72%) consider themselves to be "especially sensitive to certain chemicals.¿ Cacosmics and the chemically sensitive subjects scored significantly higher on measures of psychological distress and amplification of somatic symptoms, but there was little evidence of lifestyle change, as assessed by the Simon Survey. Compared with their respective comparison groups, cacosmic and chemically sensitive groups had significantly higher incidences of illnesses associated with chemicals, alcohol intake, opiate drug use, and caffeine use, even after controlling for the psychological measures and histories of atopic allergy. Subjects with and without neuropsychiatric symptoms were differentiated with respect to chemical odor intolerance, but subjects with and without atopic allergies and possible autoimmune diseases were differentiated with respect to chemical sensitivity. Females were more cacosmic than males. Cacosmia is defined by a population subset, with or without occupational xenobiotic exposures or disability, that has distress and symptom amplification and neuropsychiatric and somatic symptoms, none of which are explained fully by psychological measures. Prospective clinical studies are possible with such individuals. The data are also consistent with a time-dependent sensitization model for illness from low-level chemical exposures.

Smoked cocaine self-administration in females

Although approximately 32% of all smoked cocaine ("crack") users are women, most studies investigating cocaine use have recruited only male subjects. Therefore, this study examined the smoked cocaine self-administration patterns of female crack abusers. A work requirement, where up to five tokens were earned by subjects, was followed by the administration of a sample delivery of one of three dose sizes [5.0 mg (placebo), 0.2 mg/kg, 0.4 mg/kg] of cocaine. The three dose sizes of cocaine were administered in counterbalanced order across subjects, with each subject receiving one dose size within a session and all dose sizes across the three experimental sessions. A self-administration phase followed the sample delivery, during which a token could be exchanged every 30 min for monetary reimbursement or a delivery of cocaine in the same dose size as the sample delivery. The results show that females' patterns of behavioral and subjective responding to smoked cocaine exhibit clear dose-related effects, thus affirming this self-administration model as safe and appropriate for use with women.

Social deprivation enhances the vulnerability of male Wistar rats to stressor- and amphetamine-induced behavioral sensitization

The aim of the present study was to assess the influence of experiential factors on the vulnerability of rats to develop amphetamine (AMPH)- and stressor-induced behavioral sensitization. Young male Wistar rats with previous social experience were isolated from their peers for 2 weeks. 1) The effect of this short-lasting social deprivation were: a) a reduced tendency to explore a fearful environment; b) a prolonged exploratory activity in response to a novel but little fearful environment; and c) a dose-dependent increase in the psychomotor stimulation induced by systemic AMPH injection. 2) After repeated AMPH injections (injection every other day for 10 days), isolated rats exhibited behavioral sensitization at lower doses (0.5 and 0.75 mg/kg) than those required for group-housed rats (1 mg/kg). 3) After being submitted to a repeated stressor (3, 7 or 14 footshock sessions, with 2 days between sessions), the isolated rats exhibited a greater increase in the behavioral responsivity to a subsequent AMPH challenge (1 mg/kg) than did the group-housed rats regardless of the number of stress sessions. In conclusion, these results suggest that experiential factors such as privation of contact with peers (social isolation) may make rats more vulnerable to the long-term repercussions of chronic environmental and pharmacological challenges.