Sex differences in locomotor activity after acute and chronic cocaine administration

Cocaine-induced locomotor activity is increased by prior handling in adolescent but not adult female rats

Adolescence is a period of transition that is associated with increased levels of stress and a heightened propensity to initiate drug use. Neuronal development is still occurring during this transitional period, which includes the continued development of the dopamine system during the adolescent period. In the present study, the effects of pre-test handling on cocaine-induced locomotor activity were investigated among female adolescent and young adult rats upon presentation to a novel environment. On postnatal days (PND) 41-44 and 56-59 animals were handled (b.i.d.) in the colony room for 3 min. On PND 45 or PND 60, animals were removed from the colony room, weighed, and administered an acute injection of either cocaine or saline and presented to a novel environment where behavior was recorded for 30 min. Adolescent females (PND 45) that were handled prior to cocaine administration demonstrated elevated levels of cocaine-induced activity relative to their age-matched non-handled counterparts and also to their handled-adult counterparts. In contrast, among non-handled animals, young adults (PND 60) exhibited elevated drug-induced locomotion at several time points during the trial. Non-handled adolescent animals demonstrated the previously described "hyporesponsive" behavioral profile relative to their non-handled adult counterparts. The results from the present experiment indicate that adolescent animals may be more sensitive to basic laboratory manipulations such as pre-test handling, and care must be taken when utilizing adolescent animals in behavioral testing. Handling appears to be a sensitive manipulation in elucidating differences in cocaine-induced behavioral activation between ages.

Acute and repeated intravenous cocaine-induced locomotor activity is altered as a function of sex and gonadectomy

The present experiment examined the effects of sex and gonadectomy on cocaine-induced locomotor activity via intravenous (IV) cocaine. Male, female, castrated (CAST), and ovariectomized (OVX) rats received daily IV cocaine injections (3.0 mg/kg/injection) for 13 consecutive days. Locomotor activity was measured in automated activity chambers for 60 min following the baseline-saline administration and after the 1st and 13th cocaine injections. Observational time sampling was also performed, and the observational data were grouped into locomotor and orofacial composite incidence scores. Females exhibited more cocaine-induced locomotor activity, rearing, and locomotor incidence compared to males. The orofacial data revealed a sex difference in the expression of behavioral sensitization: females exhibited more orofacial behaviors than males after repeated, but not acute, cocaine injection. Females exhibited more cocaine-induced locomotor activity, rearing, and locomotor incidence compared to OVX rats, but exhibited less orofacial incidence following acute cocaine administration. There were no differences between male and CAST rats. CAST rats showed more locomotor incidence than OVX after repeated, but not acute, cocaine injection. CAST rats exhibited behavioral sensitization, whereas OVX rats' locomotor incidence did not change with repeated cocaine injection. CAST rats showed less orofacial incidence than OVX after acute, but not repeated, cocaine injection. These findings demonstrate sex differences in response to IV cocaine and replicate earlier findings which show that OVX attenuates increased locomotor activity in females. Furthermore, these findings suggest that IV cocaine administration produces behavioral differences between male and female rats in the absence of circulating gonadal hormones.

Plasma cocaine levels, metabolites, and locomotor activity after subcutaneous cocaine injection are stable across the postpartum period in rats

Plasma levels of cocaine (COC) and two of its principle metabolites, benzoylecgonine (BE) and ecgonine methyl ester (EME) were determined by liquid chromatography-tandem mass spectrometry (LC/MS/MS) in samples collected up to 3 h after a subcutaneous injection of cocaine (10 mg/kg) on six different days between days 4 and 24 postpartum, a period of dramatic change in the endocrine state of the female rat. Locomotor activity was measured in the same animals during this period using automated animal activity monitors. Additional measures in males provide a link to existing literature. We found that plasma levels of cocaine and its metabolites, as well as their respective time courses, are remarkably uniform across the postpartum period in female rats, as are the effects of cocaine on locomotor activity. Data from males show accord with prior published values. COC and BE, but not EME levels, were higher in males, and the time courses of COC and BE levels after injection varied somewhat between postpartum females and males; however, neither baseline nor cocaine-induced locomotor activity differed between postpartum females and males. We conclude that in the postpartum rat, there are no significant differences in the peripheral processing or general accessibility of cocaine to the brain to activate motor systems across the postpartum period. These data are critical to our understanding of differences in the reward salience of cocaine across the postpartum period and in other adult rat models [Mattson BJ, Williams S, Rosenblatt JS, Morrell JI. Comparison of two positive reinforcing stimuli: pups and cocaine throughout the postpartum period. Behav Neurosci 2001;115:683-94, Mattson BJ, Williams SE, Rosenblatt JS, Morrell JI. Preferences for cocaine- or pup-associated chambers differentiate otherwise behaviorally identical postpartum maternal rats. Psychopharmacology (Berl) 2003;167:1-8].

Influence of sex and estrous cyclicity on conditioned cue-induced reinstatement of cocaine-seeking behavior in rats

RATIONALE

Sex differences have been reported in physiological and behavioral responses to cocaine, but it is unclear whether sex differences exist in conditioned-cued relapse to cocaine seeking after prolonged abstinence. Furthermore, the role of estrous cyclicity in conditioned-cued relapse has not been investigated.

OBJECTIVE

We assessed the influence of sex and estrous cyclicity on conditioned-cued reinstatement of drug-seeking behavior in Sprague-Dawley rats.

METHODS

Rats were trained to self-administer intravenous cocaine (unconditioned stimulus, US; 0.25, 0.4, 0.5, 0.6, or 1.0 mg/kg per infusion) paired with light+tone conditioned stimuli (CSs) and were subsequently tested for the ability of the CSs to reinstate extinguished cocaine seeking (i.e., nonreinforced lever responding).

RESULTS

Females exhibited more responding on the cocaine-paired lever during self-administration and extinction than males. Subsequently, males exhibited equally robust conditioned-cued reinstatement of extinguished drug-seeking behavior independent of cocaine training dose. Males and females trained on 0.4-0.6 mg/kg cocaine reinstated to a similar extent. However, females trained on the lowest dose (0.25 mg/kg) exhibited less reinstatement than males, and the source of this effect was the absence of reinstatement in estrous females. In addition, independent of estrous state, females trained on the highest dose (1.0 mg/kg) exhibited less reinstatement than males.

CONCLUSIONS

While males and females are equally responsive to cocaine-paired CSs when the conditions for CS-US association are optimal, females appear to attribute less motivational significance to the CS when it presumably acquires weaker motivational salience because of (a) a low cocaine dose or (b) weaker CS-US contiguity due to the prolonged effects of a high cocaine dose.

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.

The distribution of cocaine in mice differs by age and strain

Few studies have examined the influence of the age and the strain of mouse on the pharmacokinetics of psychostimulants, or the role of pharmacokinetics in age-related differences in drug responses. The present study compared concentrations of cocaine, and its metabolite, benzoylecgonine (BZE), in the blood and brain of early (P35) and later (P42) periadolescent and adult (P63) CD-1 and C57BL/6 male mice 15 min after acute intraperitoneal injection of cocaine (20 mg/kg). Brain levels of cocaine and BZE after seven daily cocaine injections in CD-1 and C57BL/6 mice beginning on P35 and on P63 were also measured. P35 periadolescents of both strains had lower blood cocaine levels than did the adults, but only C57BL/6 periadolescents had lower brain cocaine levels than the adults. C57BL/6 mice of both ages had higher blood cocaine levels than did the corresponding CD-1 mice. Concomitant with lower cocaine levels, periadolescent CD-1 mice had higher blood BZE levels than the adults, suggesting that periadolescents may metabolize cocaine faster. Brain cocaine levels in P42 C57BL/6 mice were similar to those of adults. Cocaine-induced activity did not differ between periadolescent and adult CD-1 mice after a single injection of cocaine, whereas periadolescent C57BL/6 mice had lower activity levels than did the adults after a single cocaine injection. Periadolescent CD-1 mice exhibited higher levels of locomotor activity following cocaine injection than did periadolescent C57BL/6 mice. Following chronic cocaine administration, cocaine and BZE levels in the brains of periadolescent and adult mice did not differ from each other in either strain. However, brain cocaine levels at both ages were lower in CD-1 mice than in C57BL/6 mice. In conclusion, the age and the strain of mouse significantly affect the levels of cocaine obtained in brain and blood following acute administration. Our data are consistent with the notion that CD-1 and C57BL/6 mice metabolize cocaine faster during the early periadolescent period than as adults. Furthermore, potentially important strain differences between CD-1 and C57BL/6 mice were noted in cocaine levels following acute and chronic cocaine administration, and in locomotor activity following acute cocaine administration.

Estrous cycle influence on individual differences in the response to novelty and cocaine in female rats

In rats, individual differences in vulnerability to self-administration of drugs of abuse can be predicted by individual locomotor responses to a novel environment. This phenomenon has been well described for male rats, however very little information is available with regard to female rats and the added complication of estrous cycle hormone changes influencing activity levels. This study was designed to explore the relationship between individual responsiveness to exposure to a novel environment, the estrous cycle, and the response to cocaine in intact, cycling female rats. Locomotor activity of naive female rats was measured upon exposure to a novel environment followed by determination of estrous cycle stage and level of circulating estradiol. Rats were identified as high-responder (HR; 15% most active) or low-responder (LR; 15% least active) rats based on the locomotor response. Hyperactivity in response to cocaine was greater in HR than in LR rats. These data in combination with evaluation of the stage of estrous suggest that the estrous cycle interacts with individual phenotypic characteristics to modify the sensitivity to cocaine in female animals.

Sex differences and repeated intravenous nicotine: behavioral sensitization and dopamine receptors

The present study examined the sex-dependent expression of behavioral sensitization as well as changes of dopamine (DA) transporters and D1, D2, and D3 receptors following repeated intravenous nicotine administration. Male and female Sprague-Dawley rats were implanted with indwelling jugular catheters, equipped with subcutaneous intravenous injection ports. Rats were habituated to activity chambers for 3 days and were subsequently administered 15-s bolus injections of intravenous nicotine (50 microg/kg/ml) 1/day for 21 days. Animals were placed in activity chambers for 60 min immediately after the 1st and 21st nicotine injection. Observational time sampling was also performed. Brains were subsequently removed and frozen for autoradiographic DA transporter/DA receptor analysis on the afternoon females were in proestrus. With one exception, no robust sex differences were observed for locomotor activity or any rearing measures either during baseline or after initial nicotine injection. Females exhibited markedly more behavioral sensitization of locomotor activity, rearing, duration of rearing, and incidence of observed rearing. There were no sex differences in the number of D1 or D2 receptors. Females exhibited an increased number of DA transporters and decreased D3 receptors in the NAcc, relative to males. Multiple regression analyses suggest that D3 receptors and DA transporters in various striatal and NAcc subregions differentially predicted nicotine-induced behaviors for males and females. Collectively, these findings demonstrate that repeated intravenous nicotine produces sex differences in the expression of behavioral sensitization, and suggest that nicotine-induced changes of DA transporters and D3 receptors are partly responsible for increased behavioral sensitization in female rats.

Gonadal hormones provide the biological basis for sex differences in behavioral responses to cocaine

Both clinical and rodent studies show sexually dimorphic patterns in the behavioral response to cocaine in all phases of the addiction process (induction, maintenance, and relapse). Clinical and rodent studies also indicate that hormonal fluctuations during the menstrual/estrous cycle modulate cocaine-induced subjective effects in women and locomotor activity in female rats. Evidence suggests that gonadal hormones underlie these observed differences and could be the biological basis of sex-specific differences in cocaine addiction. To study the effects of gonadal hormones on cocaine-induced activity, two approaches have been used. First, studies have examined the role of endogenous hormones through gonadectomy (GDX) and side-by-side comparisons with intact rats. Second, the individual contributions of testosterone, progesterone, and estrogen have been determined by hormone replacement in GDX rats. In this review, we discuss gonadal hormones as the biological basis for the behavioral responses to cocaine, and the clinical implications of these findings.

Sex differences in cocaine-induced behavioral responses, pharmacokinetics, and monoamine levels

Female rats display a more robust behavioral response to acute cocaine administration than do male rats. However, a clear understanding of the biological mechanisms underlying these differences remains elusive. The present study investigated whether sexual dimorphisms in cocaine-induced motor behavior might be based on monoaminergic levels and/or cocaine pharmacokinetics. An acute injection of cocaine (5, 15, 20 or 30 mg/kg) or saline was administered to male and female rats, and behavioral activity was monitored for 3 h. Following acute cocaine or saline administration motor behavior varied according to dose and sex; overall, female rats displayed greater rearing counts and stereotypic scores, greater total locomotor counts at 15, 20, and 30 mg/kg of cocaine, and greater ambulatory counts at 20 and 30 mg/kg of cocaine than did male rats. Neurochemical determinations in post-mortem tissue showed that both male and female rats had increases in total dopamine (DA) in the caudate putamen (CPu) 15 min following cocaine administration. Additionally, male rats had a decrease in dihydroxyphenylacetic acid (DOPAC)/DA turnover. Female rats showed significant reductions in total levels of DA, DOPAC, HVA, serotonin (5-HT), 5-hydroxyindole acetic acid (5-HIAA), and DOPAC/DA turnover in the nucleus accumbens (NAc). Male rats displayed a reduction only in DOPAC/DA turnover and increases in 5-HT in the NAc following cocaine administration. Furthermore, sex differences in cocaine metabolism were observed where females had greater brain/blood levels of norcocaine and ecgonine methyl ester while male rats had higher blood levels of benzoylecgonine. These results suggest that sex differences in the behavioral responses to cocaine administration could be explained in part by intrinsic differences in both monoaminergic levels and metabolic processes.

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.

The role of exogenous testosterone in cocaine-induced behavioral sensitization and plasmalemmal or vesicular dopamine uptake in castrated rats

The possible interaction between testosterone and cocaine on behavioral and neurochemical alterations was investigated. In castrated (CAST) rats, chronic administration of testosterone propionate (TP, 2 mg/kg, s.c.) delayed and reduced chronic cocaine-induced (24 mg/kg, i.p.) focused stereotypy sensitization measured by a force-plate actometer, while the absence of TP was associated with robust behavioral sensitization to cocaine. TP itself did not produce focused stereotypy. Postmortem assays revealed that TP administration to CAST rats partially, but significantly, restored the cocaine-induced reduction of striatal plasmalemmal dopamine (DA) uptake compared to CAST rats without TP. In contrast, TP treatment did not significantly differ from the oil treatment of CAST rats in cocaine-associated enhancement of vesicular DA uptake. Thus, testosterone may modulate cocaine-induced alteration of homeostasis between extracellular and cytosolic DA pools, which may play a role in behavioral sensitization.

Estrogen-dependent alterations in D2/D3-induced G protein activation in cocaine-sensitized female rats

Estrogen potentiates behavioral sensitization to cocaine in the female rat by mechanisms that remain undetermined. In this study, functional receptor autoradiography was used to investigate estrogen modulation of D2/D3 receptor-induced G protein activation in components of the reward pathway of female rats treated acutely and repeatedly with cocaine. Rats were ovariectomized and given an empty (OVX group) or estradiol benzoate-filled (OVX-EB group) implant. After a week, animals received a daily saline or cocaine injection (15 mg/kg, i.p.) for 5 days, and again on day 13. Animals were killed, and brains were removed and cryosectioned. D2/D3-stimulated [35S]guanosine 5'-(gamma-thio) triphosphate (GTPgammaS) binding was assessed in the cingulate cortex area 2 (Cg2), striatum (STR), nucleus accumbens (NAc) and ventral tegmental area (VTA). OVX-EB rats showed more [35S]GTPgammaS binding in the Cg2 and lower binding in the VTA than OVX rats; in the VTA this effect was reversed by a single cocaine injection. Repeated cocaine administration had opposite effects in OVX and OVX-EB rats. [35S]GTPgammaS binding was decreased in the Cg2, NAc and STR of OVX-EB rats, and increased in OVX rats. The present results support the hypothesis that cocaine-induced changes in D2/D3 receptor activation are regulated by estrogen. These data suggest that changes in D2/D3 receptor function represent one mechanism by which estrogen regulates behavioral sensitization to cocaine.

Chronic neonatal isolation stress enhances cocaine-induced increases in ventral striatal dopamine levels in rat pups

Cocaine-induced increases in ventral striatal dopamine levels are enhanced in adult rats previously exposed to chronic stress. In neonatal rats, isolation from dam, nest, and siblings is stressful as evidenced by elevated corticosterone levels, an effect that increases with chronic isolation. Whether chronic neonatal isolation cross-sensitizes to cocaine leading to greater increases in ventral striatal dopamine levels was assessed in this study. Litters were assigned to isolate or non-handled conditions. After culling on postnatal Day 1 (PN1), pups in the neonatal isolation condition were isolated individually for 1 h/day on PN days 2-9 while pups in the non-handled condition were left undisturbed. On PN10, pups were implanted with probes aimed at the ventral striatum. Baseline measures of dopamine and its metabolite, DOPAC, were obtained. Separate groups of male and female pups were then administered 0, 2.5, 5.0, or 10 mg/kg cocaine and samples were collected for 2-h. Isolate pups showed greater cocaine-induced increases in ventral striatal dopamine levels than non-handled pups. However, DOPAC levels did not differ by isolation condition or gender. Neonatal isolation-induced increases in the effects of cocaine on ventral striatal dopamine levels are consistent with our previous study using amphetamine in 10-day-old pups and show that chronic stress sensitizes the dopamine response to psychostimulants in infant rats.

Cocaine-conditioned behavioral effects: a role for habituation processes

Cocaine has potent locomotor stimulant effects in rodents, which seemingly can become conditioned to test environment cues. In two experimental protocols, we measured the effects of cocaine on locomotor activity and grooming behavior, and subsequently tested whether these cocaine effects became conditioned to contextual cues. In the first experiment, three groups of rats received 14 injections of either saline or cocaine (10 mg/kg) paired or unpaired to the test environment. Cocaine increased locomotion and decreased grooming during treatment and on the conditioning test. Over the course of the treatment phase, however, the saline- and cocaine-unpaired groups but not the cocaine paired group developed progressively lower locomotion and higher grooming scores indicative of substantial habituation effects. To examine whether the cocaine may have impaired the acquisition of habituation effects rather than induce a Pavlovian cocaine conditioned response, an additional experiment was conducted in which two additional non-habituation saline and cocaine control groups were added to the experimental design. On a conditioning test, the two non-habituation control groups were equivalent in activity and grooming behavior to the cocaine-paired group. The findings were consistent with a failure by cocaine-paired animals to acquire habituation effects, which could transfer to the non-cocaine state. The connection between cocaine and novelty/habituation may have substantial importance for understanding cocaine effects.

Estrogen effects on the hyperactivity induced by (+)-MDMA and cocaine in female rats

This study compared the effects of estrogen (E) on the hyperactivity induced by (+)-3,4-methylenedioxymethamphetamine (MDMA) with E effects on cocaine-evoked hyperactivity in female rats. Sprague-Dawley rats were ovariectomized (OVX); half of them received a 17beta-estradiol (E2) implant (OVX + E). Three weeks later, rats received saline, (+)-MDMA (1, 2, or 4 mg/kg) or cocaine (5, 10, or 20 mg/kg), and locomotor activity was monitored. OVX + E rats exhibited greater locomotor hyperactivity in response to both psychostimulants than did OVX rats. The enhanced response to cocaine appeared within 5 min following drug injection whereas the enhanced response to (+)-MDMA was delayed for approximately 30 min. The differential effects of E on hyperactivity may be due to the unique profiles of DA and 5-HT in response to (+)-MDMA and cocaine.

Effects of sex and estrogen on behavioral sensitization to cocaine in rats

Estrogen rapidly enhances dopamine (DA) activity in the striatum and nucleus accumbens as well as behavioral responses to psychomotor stimulants in female rats but not males. This experiment was conducted to investigate the role of pulsatile estrogen treatment on and sex differences in the development and expression of sensitization of cocaine-induced rotational behavior in rats with unilateral striatal DA denervation. Four groups were tested: ovariectomized (OVX) females treated with 5 microg of estradiol benzoate (OVX+E), OVX females, castrated (CAST) males, and intact males. Animals received estrogen or vehicle 30 min before cocaine (0, 5, 10, or 20 mg/kg, i.p.) on 4 consecutive days, followed by 3 d without treatment for 3 weeks. At the conclusion of the experiment, animals were withdrawn from hormone and/or cocaine for 10 d, and all groups underwent a challenge test with 10 mg/kg cocaine. We report here that OVX+E females exhibit significantly greater sensitization of rotational behavior with a faster rate of sensitization than the three other groups. There is also a sex difference independent of gonadal hormones: OVX females exhibit a greater magnitude of sensitization of rotational behavior than do CAST males at 20 mg/kg cocaine. Furthermore, on the challenge test, OVX+E animals tested without estrogen treatment continue to exhibit greater rotational behavior than do all other groups. Thus, estrogen enhances sensitization to cocaine, there are sex differences in behavioral sensitization, and sensitization that develops under conditions with estrogen persists even when estrogen levels are low.

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.

Influence of estrous cycle and estradiol on behavioral sensitization to cocaine in female rats

The hypotheses that the estrous cycle and estradiol modulate behavioral sensitization to cocaine in female rats were assessed. In an analysis of sensitization across the estrous cycle, female rats were administered saline or cocaine (15 mg/kg) twice daily for 5 days. Sensitization developed in the intact female rats as measured by the significant increase in stimulant behaviors seen between day 1 and day 5 of treatment. Rats were challenged with cocaine (5 mg/kg) at 3 days following discontinuation of drug treatment. The expression of sensitization as measured between cocaine and saline-treated rats was evident only in female rats in diestus at the time of the challenge test with cocaine. To explore the role of estradiol in sensitization, female rats were ovariectomized or ovariectomized and implanted with estradiol for two weeks prior to treatment with cocaine (15 mg/kg) twice daily for 5 days. Sensitization developed in both ovariectomized and ovariectomized+estradiol rats treated with cocaine as measured by the significant increase in stimulant-like behaviors seen between day 1 and day 5 of treatment. Rats were challenged with 5 mg/kg of cocaine at 3, 13 and 34 days following discontinuation of drug treatment. While neither hormone treatment group exposed to the cocaine regimen expressed sensitization at 3 days of withdrawal, both groups exhibited sensitization at 13 and 34 days following discontinuation of cocaine treatment. The estradiol-treated groups exhibited higher levels of activity relative to their untreated cohorts in both saline or cocaine treatment groups. These results suggest that detection of sensitization in female rats is not only influenced by injection regimen and length of abstinence but also by the presence of estrogens which effectively enhance the response to an acute cocaine challenge in the presence or absence of prior cocaine exposure.

Endogenous gonadal hormones modulate behavioral and neurochemical responses to acute and chronic cocaine administration

Recent evidence demonstrates that there are sex differences in behavioral responses to cocaine. Further, reproductive gonadal hormones (estrogen, progesterone and testosterone) have been further implicated in mediating some of the cocaine-induced alterations. To better understand sex differences and the role of gonadal hormones in cocaine-induced locomotor and stereotypic behavior, intact and gonadectomized male and female Fischer rats were randomly assigned to either chronic cocaine (15 mg/kg) or saline treatments for 14 days followed by a challenge administration (7 days after the last cocaine/saline administration). Locomotor (ambulatory and rearing) and stereotypic activities were measured on days 1, 7 and 14 as well as after withdrawal/challenge with cocaine. Overall, intact female rats consistently showed a rapid (acquired by day 7) and longer lasting (persistent through the challenge dose) sensitization for all locomotor behaviors than any of the other groups. In contrast, intact males developed sensitization of these locomotor activities only in response to chronic cocaine administration, and after withdrawal and drug challenge the sensitization to cocaine-induced locomotor activity was no longer present. In female rats, gonadectomy affected ambulatory activity but not total and rearing responses after acute, sub-acute, chronic and challenge response to cocaine. On the other hand, castrated male rats were affected in cocaine-induced ambulatory activity but not rearing activity. In intact male rats, cocaine-induced stereotypic activity was rapidly and persistently sensitized after 7 days of cocaine administration, where gonadectomized male rats developed sensitization to cocaine-induced stereotypic activity only after a challenge cocaine administration. Although cocaine induced stereotypic activity, no statistically significant differences were observed between intact and ovariectomized female rats or throughout the different lengths of cocaine administration. After a challenge of cocaine, corticosterone levels were induced in all experimental groups. Moreover, no differences in levels of benzoylecgonine, a cocaine metabolite, were observed. Similar to our previous observations after acute cocaine administration, after challenge of cocaine, an increase in progesterone and a decrease in testosterone levels were observed in intact females and males, respectively. In summary, endogenous hormones seem to be involved in the behavioral activation and development of sensitization to cocaine.

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.

Effects of cocaine on c-fos and preprodynorphin mRNA levels in intact and ovariectomized Fischer rats

Psychostimulants such as cocaine have been shown to regulate c-fos and opioid gene expression in male rats. However, little information is available on cocaine effects in female rats or how the ovarian hormones, estrogen and progesterone, modulate these effects. In this study we used quantitative solution hybridization assays to measure c-fos and preprodynorphin (PDYN) mRNA levels after cocaine administration in the caudate/putamen of intact male and female rats or ovariectomized (OVX) female rats that were pretreated with vehicle, estrogen and/or progesterone. The c-fos mRNA levels were increased in intact male and female rats after 30min or 3h of one single cocaine injection and after 14 days of single daily cocaine injections. The c-fos mRNA levels were also increased after 30min of a single cocaine injection in OVX female rats that were treated with vehicle, estrogen and/or progesterone. The PDYN mRNA levels did not change after 30min, 3h or 14 days in intact male or female rats. However, PDYN mRNA levels were increased in the caudate/putamen of OVX female rats pretreated with vehicle or a combination of estrogen and progesterone but not in OVX female rats that were pretreated with either estrogen or progesterone alone. Our data suggest hormonal regulation of cocaine effects on PDYN mRNA levels which may modulate cocaine-induced behaviors in female rats.

Estrogen regulation of gene expression in the brain: a possible mechanism altering the response to psychostimulants in female rats

Acute behavioral responses to cocaine are more pronounced in female than in male rats. We have shown that 3 weeks of treatment with 17beta-estradiol (E(2)) implants significantly enhanced the hyperactivity induced by cocaine in ovariectomized (OVX) rats. The ligand-bound estrogen receptor (ER) functions as a transcription factor to regulate the expression of E-responsive genes. Thus, we hypothesized that estrogen (E) modulates the behavioral response to cocaine via regulation of expression of components of dopamine (DA) and serotonin (5-HT) systems in mesolimbic nuclei important in the response to cocaine as well as the hypothalamus, a brain area known to be E-responsive. Adult female Sprague-Dawley rats were OVX; half of them then received E(2) implant (OVX+E) (n=8/group, two groups). Twenty-seven days later, brain tissue was collected to study E(2) effects on mRNA expression for DA D(1)-like (D(1)) and D(2)-like (D(2S), D(2L), D(3)) receptors, DA transporter (DAT), 5-HT(1A), 5-HT(1B), 5-HT(2A), 5-HT(2C) receptors, and 5-HT transporter (SERT) as well as ERalpha and ERbeta in amygdala, hypothalamus, nucleus accumbens, midbrain, and ventral tegmental area (VTA). We found that E(2) implants in OVX rats increased mRNA levels for D(1) receptor in hypothalamus, D(2L) receptor in midbrain, and D(3) receptor in VTA, and decreased D(3) receptor mRNA levels in midbrain relative to OVX controls. E(2) also increased 5-HT(2C) receptor mRNA levels in midbrain and hypothalamus. In addition, E(2) decreased mRNA levels for ERalpha in amygdala and hypothalamus and ERbeta in amygdala. The present study demonstrates that E can regulate mRNA expression for specific DA and 5-HT receptors in a region-specific manner and suggests that such modifications may contribute to the behavioral response to cocaine.

HPA axis function and drug addictive behaviors: insights from studies with Lewis and Fischer 344 inbred rats

Much research supports a link between stress and its concomitant hypothalamic-pituitary-adrenal (HPA) axis responses with behavioral sensitivity to psychoactive drugs. Our research demonstrates that Lewis inbred rats more readily acquire drug self-administration than Fischer 344 (F344) inbred rats and, compared to this strain, Lewis rats have hyporesponsive HPA axis responses to stress exposure. This association appears to conflict with investigations using outbred rats and suggests that the relationship between drug sensitivity and HPA axis responsiveness is more complicated than originally thought. It is essential to better understand this relationship because of its relevance to vulnerability and relapse to drug abuse. Thus, this paper reviews the literature in which these two inbred strains have been compared. We discuss strain differences in HPA axis function, in characteristics of the mesolimbic dopamine system, and in behaviors thought to reflect emotionality. Strain differences in unconditioned and conditioned effects of psychoactive drugs are then reviewed. Next, we discuss the possible role of sex and gonadal hormones on responsiveness to psychoactive drugs in these strains. Finally, a comparison of results obtained from these strains to three other comparator groups (e.g., high and low responders) suggests that a non-monotonic relationship between behavioral sensitivity to drugs and HPA axis responsiveness can explain much of the discrepancies in the literature.

Individual differences in behavioral responses to novelty and amphetamine self-administration in male and female rats

Previous work has shown that individual differences in locomotor activity in an inescapable novel environment can predict acquisition of amphetamine self-administration. The current study examined whether individual differences in approach to novelty in a free choice test could also predict amphetamine self-administration. Further, the current study examined whether individual differences in either free choice or inescapable novelty tests could predict responding for a nondrug reinforcer (sucrose) in the presence and absence of amphetamine. Male and female rats were first tested for their response to free choice novelty (playground maze and novelty-induced place preference tests) and inescapable novelty. They were then tested for acquisition of sucrose-reinforced responding, amphetamine-induced changes in maintenance of sucrose-reinforced responding, and amphetamine self-administration. Based on the inescapable novelty test, acquisition of sucrose-reinforced responding was more rapid in male high responders (HR) compared to low responders (LR). This effect in males did not generalize to females. None of the novelty tests predicted the ability of amphetamine to decrease sucrose-maintained responding. However, using the inescapable novelty test, both male and female HRs self-administered more amphetamine than LRs within the dose range tested (0.03-0.16 mg/kg/infusion). Neither the playground maze nor the novelty-induced place preference test predicted amphetamine self-administration. These results indicate that responses to free choice novelty and inescapable novelty predict different components of amphetamine-induced behavior.

Sex, steroids, and stimulant sensitivity

The current study investigated ovarian modulation of the locomotor response to cocaine in rats. Ovariectomy in females lowered the response to cocaine (10 mg/kg i.p.), whereas castration did not change that of males. The locomotor responses of prepubertal males and females to cocaine were similar. However, the postpubertal sex difference resulted from a fall in cocaine-stimulated locomotion in males rather than a rise in females. Neonatal testosterone treatment of female rat pups decreased the response to cocaine in adulthood. These findings suggest that both the activational and organizational effects of gonadal steroids contribute to the greater response of females to cocaine.

Endocrinological basis of sex differences in cocaine-induced behavioral responses

Currently, 1.8 million Americans use cocaine, 30% of whom are females. Sex differences in the pattern of cocaine abuse may reside in neuroendocrinological modulations that affect the use of and/or dependence on cocaine. This review discusses sex differences in cocaine-induced behavioral and molecular alterations in the central nervous system, with emphasis on the role of endocrine responses in the neuronal modulations of this drug. Mechanisms and data supporting the role of the hypothalamic-gonadal axis in the modulation of cocaine-induced behavioral and molecular alterations are also provided.

Gender differences in the behavioral responses to cocaine and amphetamine. Implications for mechanisms mediating gender differences in drug abuse

When ovariectomized female rats receive estrogen, the response to the psychomotor stimulants amphetamine or cocaine is enhanced. Estrous cycle-dependent differences in amphetamine-stimulated behaviors and striatal dopamine release are also noted. Intact female rats exhibit a greater behavioral response to amphetamine on estrus than they do on other days of the cycle. Ovariectomy results in attenuation of amphetamine-induced behavior and the striatal dopamine response to amphetamine. Physiological doses of estrogen given to ovariectomized rats reinstate both of these responses to a level comparable to that in estrous females. Furthermore, a sex difference is noted, in that females tend to exhibit a greater behavioral response to the psychomotor stimulants, and estrogen enhances this sex difference. Repeated treatment with amphetamine or cocaine produces a progressive increase in behavioral responsiveness with subsequent drug administration, a process known as sensitization. In rodents, behavioral sensitization results in increases in both frequency and duration of psychomotor behaviors such as rotational behavior, stereotyped grooming, headbobs, and forelimb movements. Interestingly, females display greater sensitization of behaviors in response to psychomotor stimulants than do males. Previous research results are summarized, and new results are presented, demonstrating that estrogen selectively enhances components of behavior that exhibit sensitization in female rats. Results also indicate gender differences in sensitization independent of gonadal hormones, suggesting that the neural systems that undergo sensitization are sexually dimorphic.

8-OH DPAT can restore the locomotor stimulant effects of cocaine blocked by haloperidol

In the first experiment, separate groups of rats (n = 7) were treated with either saline, cocaine (10 mg/kg), haloperidol (0.1 mg/kg), or cocaine (10 mg/kg) plus haloperidol (0.1 mg/kg). Locomotor behavior was measured in an open-field environment, and cocaine induced a reliable locomotor stimulant effect compared to saline-treated animals. Haloperidol produced a progressive decline in locomotion over the 5 test days. Haloperidol also blocked cocaine stimulant effects compared to cocaine-treated animals. In the second experiment, five groups (n = 7) of animals were treated either with saline, cocaine (10 mg/kg), 8-OH DPAT (0.2 mg/kg), 8-OH DPAT (0.2 mg/kg) plus haloperidol (0.1 mg/kg), or 8-OH DPAT (0.2 mg/kg) plus haloperidol 0.1 mg/kg plus cocaine (10 mg/kg). Over the course of 5 days of treatment, cocaine induced a locomotor stimulant effect. Saline and 8-OH DPAT animals did not differ in terms of locomotion. The 0.1 mg/kg haloperidol plus 0.2 mg/kg 8-OH DPAT treatment decreased locomotion compared to the saline group, but the group given 0.2 mg/kg 8-OH DPAT plus 0.1 mg/kg haloperidol plus cocaine (10 mg/kg) exhibited a locomotor stimulant effect equivalent to the cocaine group. In a third experiment, it was found that the 0.2 mg/kg 8-OH DPAT treatment did not enhance the locomotor stimulant effect of cocaine. Thus, the 8-OH DPAT treatment was able to restore a cocaine locomotor stimulant effect in animals treated with haloperidol without directly enhancing the locomotor stimulant effects of cocaine. In Experiments 2 and 3, entries into the central zone of the open field were measured. Cocaine reliably increased central zone entries. The 8-OH DPAT treatment, however, selectively blocked this behavioral effect of cocaine suggesting a qualitative influence of 5-HT(1A) receptors upon cocaine, independent of locomotion activation by cocaine. Ex vivo measurements of dopamine and 5-hydroxytryptamine metabolism in limbic tissue were consistent with the established effects of cocaine, haloperidol, and 8-OH DPAT upon dopamine and 5-hydroxytryptamine neurotransmission. In addition, measurement of cocaine brain concentration indicated that neither haloperidol or 8-OH DPAT affected cocaine concentration in brain.

Effects of SCH-23390 and raclopride on cocaine discrimination in male and female Wistar rats

Male and female rats were trained to discriminate 10.0 mg/kg cocaine from saline in a two-lever discrimination task. Injection-appropriate responding was reinforced by food pellet presentation on a tandem random-interval 30-s fixed-ratio 10 schedule. Generalization testing was conducted in extinction 10 min following an injection of saline, 1.0, 3.0, 5.6, or 10.0 mg/kg cocaine. No differences in the generalization gradients and ED(50)s were observed between male and female rats. Following the determination of the cocaine generalization gradient, the dopamine D(1) antagonist SCH-23390 (0.01-0.10 mg/kg) and the dopamine D(2) antagonist raclopride (0.1-1.6 mg/kg) were administered (independently) prior to the injection of the training dose of cocaine (10.0 mg/kg). Cocaine-antagonism tests were conducted in extinction. It was found, for each dopamine antagonist, that as the dose increased, the percentage of cocaine-appropriate responding decreased. No sex differences were observed between these generalization gradients.

Decreased accumbens dopamine release after cocaine challenge in behaviorally sensitized female rats

The effects of the competitive NMDA receptor antagonist CPP on the initiation of behavioral sensitization to acute cocaine and basal and acute cocaine-induced dopamine (DA) release in the nucleus accumbens (NAC) were assessed in female Sprague-Dawley rats. Cocaine pretreated rats (30 mg/kg IP, once daily for 7 days) challenged with cocaine (10 mg/kg) on day 8 displayed increased motor activity relative to controls challenged with cocaine on day 8. This effect was blocked in rats receiving CPP (2 mg/kg) 15 min prior to all cocaine pretreatments. Basal DA levels in the NAC of both cocaine-pretreated and CPP plus cocaine-pretreated rats were higher on day 8 compared to controls. Acute cocaine challenge on day 8 resulted in increased extracellular DA concentrations in the NAC in control rats, no increase in rats pretreated with CPP plus cocaine, and a decrease in rats pretreated with cocaine only. These data demonstrate that development of behavioral sensitization to cocaine in female Sprague-Dawley rats can be completely blocked by a peripherally administered competitive NMDA receptor antagonist and that an increase in DA release in the NAC after a cocaine challenge is not an absolute requirement for expression of motor sensitization to cocaine in female rats.

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.

Gender differences in dopaminergic function in striatum and nucleus accumbens

In female rats the gonadal hormones estrogen and progesterone modulate dopamine (DA) activity in the striatum and nucleus accumbens. For example, there is estrous cycle-dependent variation in basal extracellular concentration of striatal DA, in amphetamine (AMPH)-stimulated DA release, and in striatal DA-mediated behaviors. Ovariectomy attenuates basal extracellular DA, AMPH-induced striatal DA release, and behaviors mediated by the striatal DA system. Estrogen rapidly and directly acts on the striatum and accumbens, via a G-protein-coupled external membrane receptor, to enhance DA release and DA-mediated behaviors. In male rats, estrogen does not affect striatal DA release, and removal of testicular hormones is without effect. These effects of estrogen also result in gender differences in sensitization to psychomotor stimulants. The effects of the gonadal hormones on the striatum and ascending DA systems projecting to the striatum and nucleus accumbens are hypothesized to occur as follows: estrogen induces a rapid change in neuronal excitability by acting on membrane receptors located in intrinsic striatal GABAergic neurons and on DA terminals. The effect of these two actions results in enhanced stimulated DA release through modulation of terminal excitability. These effects of gonadal hormones are postulated to have important implications for gender differences in susceptibility to addiction to the psychomotor stimulants. It is suggested that hormonal modulation of the striatum may have evolved to facilitate reproductive success in female rats by enhancing pacing behavior.

Mesolimbic gender differences in peptide CART mRNA expression: effects of cocaine

The putative neurotransmitter peptide CART has been suggested to be involved in the actions of psychostimulants. We analyzed the CART mRNA expression in mesolimbic brain areas of male and ovariectomized 17beta-estradiol- (30 microg) and vehicle-treated female rats. A gender difference was noted in the accumbens shell during basal conditions; male rats expressed higher levels of CART mRNA than both female groups. Following binge cocaine injections (3 x 15 mg/kg), elevated levels were found in the central amygdala of male but not female rats. In the medial accumbens shell CART mRNA was elevated after cocaine, but only in the non-treated females. The present results support a role of mesolimbic CART in psychostimulant drug action.

Cocaine discrimination and time-course effects in male and female Wistar rats

Previously, sex differences have been observed in the behavioral effects of acute and chronic cocaine administration. In the present experiment, male and female rats were trained to discriminate intraperitoneal injections of 10.0 mg/kg cocaine from its vehicle. It was hypothesized that the subjective effects of cocaine might differ between male and female rats. It was further hypothesized that generalization gradients between male and female rats might differ as a function of the time since cocaine administration. In addition, we were interested to see whether multiple generalization gradients could be determined within the same experimental session. For that purpose, two different types of generalization tests were conducted in extinction, one in which subjects were tested both 10 min and 30 min following cocaine administration (vehicle, 1.0, 3.0, 5.6, 10 or 17 mg/kg) and one in which subjects were only tested 30 min after cocaine administration. The generalization gradients obtained 30 min following drug administration were shifted to the right of the gradient obtained 10 min following drug administration. The two 30-min gradients were not different from one another, showing that multiple generalization gradients can be obtained within the same experimental session.

Ontogeny of ethanol-induced locomotor activity and hypothermia differences in selectively bred FAST and SLOW mice

The replicate lines of selectively bred FAST and SLOW mice differ in locomotor response to 2 g/kg ethanol (EtOH). FAST mice show enhanced locomotion; SLOW mice exhibit no change or locomotor depression. Little is known about the responses of FAST and SLOW mice to EtOH during development. We assessed the locomotor responses of FAST and SLOW mice at postnatal days (P) 10, 15, 30, and 60. A genetically correlated response, EtOH-induced hypothermia, was also investigated. Although all animals demonstrated their respective selection phenotypes in adulthood, developing FAST mice exhibited ethanol stimulation by P15 (replicate 1) or P30 (replicate 2). At these ages, responses of FAST mice differed from those of SLOW. The stimulant response in FAST mice was adult-like at P30. EtOH-induced hypothermia was seen in SLOW mice by P15. These data suggest that sensitivity to the locomotor stimulant effects of EtOH changes during postnatal development, and may mirror developmental profiles for certain neurotransmitter systems.

Influence of sex and female hormones on nicotine-induced changes in locomotor activity in rats

The acute and chronic effects of nicotine (0.4 mg/kg s.c.) on locomotor activity in photocell cages have been compared in male, female, and ovariectomized hooded rats. In Experiment 1, female rats displayed higher locomotion than males (n = 12); acutely, nicotine-reduced locomotion, and this effect was slightly larger in females than males. Daily administration of nicotine for 21 days produced a similar, gradual increase in activity in both sexes. Tests then confirmed greater activity in females than males and as a function of previous chronic exposure to nicotine (n = 6); there was an activating effect of nicotine challenge but no interaction of nicotine effects with sex. In Experiment 2, ovariectomized rats were primed with 17-beta-estradiol (50 microg/kg s.c.) and progesterone (2.5 mg/kg s.c.) or vehicle only. Acute administration of nicotine reduced activity in both groups similarly (n = 12). After nicotine daily for 21 days, there was increased activity as a function of both chronic nicotine and hormonal priming, and challenge with nicotine increased activity (n = 6). The effects of these challenges with nicotine were also slightly greater, as a function of previous nicotine exposure and priming. As a whole, these experiments showed robust effects of acute and chronic nicotine administration, sex, and hormonal priming; neither sex nor gonadal hormones had marked influences on changes in locomotor activity produced by nicotine.

Sex differences in discriminative stimulus and diuretic effects of the kappa opioid agonist U69,593 in the rat

Female and male rats were trained to discriminate the kappa opioid agonist (5alpha,7alpha,8beta)-(-)-N-methyl-[7-(1-pyrrolidinyl) -1-oxaspiro(4,5)dec-8-yl]benzeneacetamide (U69,593, 0.13 mg/kg SC) from vehicle using a FR-10 schedule of food reinforcement. Female rats took significantly longer than males to acquire the discrimination (66.9 vs. 44.1 sessions, respectively), and the ED50 for U69,593 discrimination was significantly higher in females than in males (0.074 vs. 0.025 mg/kg). The time course of U69,593 discrimination also differed between the sexes: peak and offset occurred earlier in females than in males. The ED50 for bremazocine substitution was significantly higher in females than in males (0.0039 vs. 0.0006 mg/kg), whereas ethylketazocine substituted for U69,593 in all males and five of seven females, with no sex difference in substitution ED50. Morphine and BW373U86 did not substitute for U69,593 in a majority of rats of either sex. U69,593 also produced significantly less urine output/dose in females compared to males (e.g., 5.92 vs. 14.83 ml urine/kg body weight after 1.0 mg/kg U69,593), but was equipotent between the sexes in producing hot-plate antinociception. There was no sex difference in response rate-decreasing effect of any opioid agonist tested, and no sex difference in brain/blood ratio of [3H]U69,593 measured in a separate group of rats, suggesting that sex differences observed in some effects of U69,593 probably are not due to sex differences in U69,593 pharmacokinetics. When retested at the end of the study, U69,593 and bremazocine were no longer differentially potent as discriminative stimuli in females and males, suggesting that factors that change over time (e.g., additional training, age, hormonal status) may contribute to initial sex differences in discriminability of U69,593.

Intracranial self-stimulation in female and male rats: no sex differences using a rate-independent procedure

Given gender differences in human drug use and dependence, this study examined sex differences in reinforcement processes that may underlie such behavior. A psychophysical determination of reinforcement threshold was made using an intracranial self-stimulation (ICSS) paradigm, electrically activating the medial forebrain bundle (MFB) as it passes through the lateral hypothalamus (LH). Using this response rate-independent procedure, basal reinforcement thresholds were not significantly different in male vs. female rats (119.4 +/- 3.3 microA vs. 110.8 +/- 4.0 microA, respectively; N = 8/sex). Further, baseline reinforcement threshold did not fluctuate systematically across stages of the estrous cycle in female rats. The psychostimulants D-amphetamine (0.056-0.56 mg/kg s.c.) and cocaine (1.8-18.0 mg/kg i.p.) dose-dependently lowered reinforcement threshold, with no significant sex difference. The opioid morphine (0.56-5.6 mg/kg s.c.) did not significantly lower reinforcement threshold in either sex. These results contrast those of some previous studies that have used response rate-dependent measures of reinforcement threshold; procedures which are less rate-dependent may be more appropriate when examining subject variables such as sex and stage of estrous.

Age-related differences in the chronic and acute response to cocaine in the rat

Behavioral sensitization is known to occur in adult animals after the chronic intermittent administration of cocaine. Dopaminergic pathways in the brain, such as the nigrostriatal and mesoaccumbens projections play a vital role in this phenomenon. These pathways are rudimentary in the 1st week of life, indicating that the developing animal may be unable to respond to cocaine in the same manner as an adult. In the present study, we report that the acute response to cocaine is remarkably similar between week-old and adult rats. Pups do not, however, show locomotor sensitization to acute cocaine after chronic cocaine-administration as adults.

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.

Effects of acute and chronic cocaine administration on EEG and behaviour in intact and castrated male and intact and ovariectomized female rats

Intact and gonadectomized male and female WAG/ Rij rats were used to study the effects of gender and gonadal hormones on the development of sensitization and tolerance to cocaine-induced changes in EEG and behaviour. The four groups of WAG/Rij rats differed in the number of spontaneously occurring spike-wave discharges: ovariectomy decreased and castration increased the number of spike-wave discharges. This confirms that testosterone has antiabsence effects and that female gonadal hormones may promote the occurrence of spike-wave discharges. Cocaine [10 and 20 mg/kg, intraperitoneally (IP)] was administered before and after chronic cocaine administration (9 days, one daily injection with 10 mg/kg) and EEG and behaviour were monitored. Cocaine strongly suppressed the occurrence of spike-wave discharges before and after chronic administration in all four groups, although the decrease was less in the intact males. Sensitization or tolerance induced by cocaine on EEG could not be established. Acute cocaine administration eliminated explorative, automatic, and passive behaviour, whereas various stereotypical activities such as uncoordinated head and body movements and head swaying emerged. Differences between groups were observed as intact males were less likely than subjects in the three other groups to engage in intense stereotyped behaviour. These data suggest that testosterone inhibits EEG and behavioural effects of acute cocaine administration. All four groups displayed less head swaying and more uncoordinated head and body movements after chronic cocaine administration, suggesting that behavioural sensitization had occurred. Differences between the four groups had faded away. Although pharmacokinetic differences in levels of cocaine and benzoylecgonine between the four groups were found, they could not easily be related to the behavioural differences between groups.

Time course effects of MK-801: the relationship between brain neurochemistry and behavior

Separate groups of rats were given saline or MK-801 treatments (0.3 mg/kg) and tested for locomotion activity levels for 10 min at 30, 60, and 120 min postinjection. At each postinjection time interval the MK-801 rats exhibited a marked hyperactivity that was unchanged across the three postinjection intervals. Ex vivo biochemical assays were performed to assess the neurochemical effects of MK-801 at each injection interval. In the striatum, a marked increase in dopamine metabolism was observed in the 120 injection group, but, otherwise, no other changes in striatum were detected. In contrast, a significant increase in dopamine metabolism was observed after 30 min in the medial prefrontal cortex, and this effect persisted across all postinjection intervals. At 120 min, however, the biochemical impact of the MK-801 treatment on medial prefrontal cortex broadened to include a decrease in purine metabolism and norepinephrine. Serotonin metabolism was unaffected in striatum or medial prefrontal cortex across all injection intervals, and there was no effect of MK-801 on plasma corticosterone levels.

The effects of acute and chronic cocaine administration on paced responding in intact and gonadectomized male and female Wistar rats

Intact and gonadectomized male and female Wistar rats pressed a lever on a multiple (tandem Random-Interval 30-s, Differential Reinforcement of High Rate 0.5-s) (tandem Random-Interval 30-s, Differential Reinforcement of Low-Rate 5.0-s) schedule of reinforcement. The pacing requirements maintained high and low response rates under equal reinforcement frequencies. Low doses of cocaine (1 and 3 mg/kg) either did not affect or slightly increased high and low response rates of intact and gonadectomized female rats, while they did not affect or decrease high and low response rates of intact and castrated male rats. Higher doses of cocaine (up to 30 mg/kg) dose-dependently decreased both high and low response rates for all subjects. Intact male rats were less sensitive to the rate-decreasing effects of these doses of cocaine than castrated male rats or intact and ovariectomized female rats. Chronic cocaine administration consistently resulted in behavioral sensitization only in intact male subjects. The results of this experiment provide further support for the notion that the behavioral effects of psychomotor stimulant drugs are not necessarily rate dependent but may depend upon the extent to which schedule contingencies allow for behavioral variability without negatively affecting behavioral outcomes.

Effects of cocaine on fixed-interval behavior and schedule-induced alcohol consumption in male and female rats

Three male and three female Wistar rats pressed a lever on a fixed-interval 60-s schedule of food reinforcement while they had simultaneous access to an alcohol solution. They were challenged with different doses of cocaine hydrochloride (vehicle, 1, 3, 10, and 30 mg/kg) once lever press rates and lick rates had stabilized. Low doses of cocaine (1 and 3 mg/kg) did not systematically affect lever press rates or lick rates. The administration of 10 and 30 mg/kg cocaine dose-dependently decreased lever press rates and schedule-induced licking to a greater extent in female than in male rats. Lick rates decreased even when cocaine administration did not affect the number of pellets obtained during an experimental session. Lever press rates accelerated throughout the interreinforcement interval during control sessions. Licking was mostly limited to the first 10 s (males) or 20 s (females) after pellet presentation. Cocaine administration did not affect the distribution of lever presses and licks during the interreinforcement interval. The results of the present experiment extend previous observations that cocaine's rate-dependent effects on lever press rates may be limited to situations in which changes in lever press frequency and/or distribution negatively affect reinforcement frequency and/or the physiological consequences of schedule-induced behavior.

Neurotensin increases extracellular striatal dopamine levels in vivo

This study employed intracranial microdialysis to assess the effects of neurotensin (NT) infusion on extracellular dopamine (DA) and DA metabolite concentrations in the rat striatum and nucleus accumbens, and the effects of NT on alterations in extracellular DA levels induced by cocaine and the DA D-2 receptor agonist, quinpirole. Direct NT infusion (.10, 1.0, 10.0 microM) did not significantly affect extracellular DA in the nucleus accumbens, but did produce a significant increase in the DA metabolite homovanillic acid (HVA). In contrast, direct NT infusion produced an increase in striatal DA levels, without altering DA metabolites. Neurotensin infusion (.10 microM) into the striatum significantly attenuated the peak DA increase induced by an intraperitoneal (IP) injection of a low dose (10.0 mg/kg) but not a high dose (30.0 mg/kg) of cocaine. Neurotensin infusion (.10 microM) did not affect the decrease in DA and its metabolites induced by an IP injection of a low dose of quinpirole (.03 mg/kg), but did alter the decrease in HVA induced by a high dose of quinpirole (.10 mg/kg). These results suggest that NT differentially affects in vivo DA release in the striatum and nucleus accumbens, and further strengthens the assertion that NT is an important modulator of dopaminergic function.