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

Discriminative-stimulus effects of cannabidiol oil in Sprague-Dawley rats

Cannabidiol (CBD) is one of the major centrally active phytocannabinoid components of cannabis, and has been approved by the FDA only for the treatment of seizures associated with three rare disorders. It has also been touted as a potential treatment for anxiety in place of more traditional treatments like benzodiazepines. Although there is some evidence of anxiolytic effects of CBD, its suitability as a substitute for benzodiazepines is unknown. This experiment was designed to assess the extent to which CBD shares interoceptive discriminative-stimulus properties with the anxiolytic drug chlordiazepoxide (CDP), a benzodiazepine. In the present experiment, a range of doses (0-1569 mg/kg) of over-the-counter CBD oil was administered (i.g.) in male Sprague-Dawley rats trained to discriminate 5.6 mg/kg CDP from saline. Due to the long time-course effects of CBD, generalization tests were conducted at 90 and 120 min post-CBD administration. The two highest doses of CBD tested (1064 and 1569 mg/kg) were found to partially substitute for 5.6 mg/kg CDP, with mean percent responding on the CDP-associated lever reaching above 20% at time 2 (120 min post-CBD administration), suggesting that high doses of the over-the-counter CBD oils used in this experiment share interoceptive discriminative-stimulus properties to some degree with CDP. These results are novel in comparison to existing research into stimulus effects of CBD, in which substitution for benzodiazepines has not previously been observed.

Behavioral evaluation of modafinil and the abuse-related effects of cocaine in rhesus monkeys

Modafinil is a central nervous system stimulant used to promote wakefulness, and it is being evaluated clinically as an agonist medication for treating stimulant abuse. This is the first report of the effects of modafinil on the abuse-related effects of cocaine in nonhuman primates. The behavioral effects of modafinil were examined in three studies. First, the discriminative stimulus effects of modafinil (3.2-32 mg/kg) were evaluated in rhesus monkeys (Macaca mulatta) trained to discriminate either low (0.18 mg/kg, IM) or high (0.4 mg/kg, IM) doses of cocaine from saline. Modafinil dose-dependently substituted for cocaine in 6 of 7 monkeys. In the second study, the effects of chronically administered modafinil (32-56 mg/kg/day, IV) on food- and cocaine-maintained (0.001-0.1 mg/kg/inj) operant responding were examined. Modafinil was administered 3 times/hr for 23 hr/day to ensure stable drug levels. Chronic treatment with 32 mg/kg/day modafinil selectively reduced responding maintained by intermediate and peak reinforcing doses of cocaine, but responding maintained by higher doses of cocaine was unaffected. Food-maintained behavior did not change during chronic modafinil treatment. In a third study, modafinil (32 and 56 mg/kg/day, IV) was examined in a reinstatement model. Modafinil transiently increased responding during extinction. These findings indicate that modafinil shares discriminative stimulus effects with cocaine and selectively reduces responding maintained by reinforcing doses of cocaine. In addition, modafinil reinstated cocaine-seeking behavior, which may reflect its cocaine-like discriminative stimulus effects. These data support clinical findings and indicate that these preclinical models may be useful for predicting the effectiveness of agonist medications for drug abuse treatment.

Discriminative-stimulus and time-course effects of kava-kava (Piper methysticum) in rats

Kava is a widely available and used herbal medicine that is not regulated in many countries. There are many questions concerning kava's stimulus properties, potential for therapeutic use, and potential for abuse. Although there is evidence that kava may possess some anxiolytic properties, kava's mechanism of action and the extent to which it may serve as an alternative to pharmaceutical anxiolytics are not fully known. The current study was designed to evaluate whether kava shares discriminative-stimulus properties with the anxiolytic chlordiazepoxide (CDP). Effects of different doses of kava extract were evaluated in two groups of rats trained to discriminate either a high or low training dose of CDP (i.p.). In order to assess time-course effects, two tests were conducted/session at 60 (Test One) and 90 (Test Two) min following oral administration of kava, CDP, or d-amphetamine. Dose-dependent substitution of CDP was found in both training groups in both tests. Kava (560 mg/kg, p.o.) occasioned responding indicative of partial substitution in both groups during Test One and only the low-dose group during Test Two. Partial substitution of kava extract for CDP suggests that the herbal compound may share a mechanism of action similar to CDP, but is less potent.

Sex differences in non-reinforced responding for cocaine

Prior studies report no sex differences in cocaine consumption during maintenance of self-administration. We find female rats show poorer lever discrimination during acquisition of self-administration. Now, we test whether female rats show greater non-reinforced or ineffective responding (presses during infusion and time-out periods as well as inactive lever presses) than male rats during maintenance of cocaine self-administration (.0625-1.0 mg/kg/infusion) in Experiment 1. Persistence of responding during extinction when saline-replaced cocaine was also examined. Whether response differences reflect sex differences in movements under a non-drug condition was tested in Experiment 2. Because cocaine may affect lever press rates differentially between sexes, we examined the effects of cocaine (.3-30 mg/kg; IP) on responding for food in Experiment 3. Cocaine consumption does not differ between female and male rats. However, females respond more during infusion and time-out periods and during extinction than males. There is no sex difference in movements and high cocaine doses decrease responding for food more in female vs. male rats. That females engage in more ineffective responding may represent heightened "craving" and cannot be explained by increased movements or cocaine-stimulated increases in lever pressing. In contrast, responding for cocaine in males appears driven by drug delivery.

Similar discriminative-stimulus effects of D-amphetamine in women and men

The results of controlled non-human animal and human laboratory studies are mixed regarding whether women and men respond differently to stimulant drugs. In order to assess potential gender differences in the effects of D-amphetamine, we conducted a retrospective analysis of six studies conducted in our laboratory that used identical procedures and measures. Thirteen women and fourteen men learned to discriminate 15 mg oral D-amphetamine. After acquiring the discrimination (i.e., >or=80% correct responding on 4 consecutive sessions), the effects of a range of doses of D-amphetamine (0, 2.5, 5, 10, and 15 mg) alone and in combination with other drugs, were assessed. Only data from sessions in which D-amphetamine was administered alone were included in this analysis. D-amphetamine functioned as a discriminative stimulus and dose-dependently increased drug-appropriate responding. Women and men did not differ in their ability to discriminate D-amphetamine. Women and men differed on participant-ratings of high (women<men), nausea (women>men) and sluggish (women<men), women also experienced greater increases in diastolic pressure than men. Because the results of this study may have been confounded by the training procedures, future research should use other behavioral arrangements (e.g. drug self-administration) to determine if women and men respond differently to the effects of D-amphetamine.

Reinforcement schedule effects in rats trained to discriminate 3,4-methylenedioxymethamphetamine (MDMA) or cocaine

RATIONALE

Relatively few studies have compared the discriminative stimulus effects of 3,4-methylenedioxymethamphetamine (MDMA) and cocaine, and findings from different laboratories are somewhat inconsistent. One possible reason for discrepant results may be the use of different reinforcement schedules during discrimination training.

OBJECTIVE

The present study compared fixed ratio (FR) 20 and variable interval (VI) 15-s reinforcement schedules to determine their influence on discrimination acquisition, response rates, frequency of reinforcements, and stimulus generalization in rats trained to discriminate cocaine or MDMA.

MATERIALS AND METHODS

Thirty-two male Sprague-Dawley rats were trained to discriminate cocaine (10 mg/kg; n=16) or MDMA (1.5 mg/kg; n=16) from saline under either a FR 20 or a VI 15-s schedule of food reinforcement. Stimulus generalization tests were conducted with a range of doses of cocaine, MDMA, d-amphetamine, and lysergic acid diethylamide in all four training groups.

RESULTS

The FR 20 schedule facilitated more rapid discrimination acquisition compared to the VI 15-s schedule and established differential response rates and frequency of reinforcement under drug and vehicle conditions. However, reinforcement schedule had little influence on stimulus generalization between MDMA and cocaine. Cocaine produced partial substitution for MDMA in both training groups (FR 20, 51%; VI 15-s, 58%). Likewise, MDMA produced only partial substitution for cocaine in both training groups (FR 20, 40%; VI 15-s, 72%).

CONCLUSIONS

The present findings suggest that the number of sessions required to establish discriminative stimulus control varies with different reinforcement schedules. Nevertheless, training schedules alone do not appear to have significant effects on stimulus generalization between MDMA and cocaine.

Discriminative-stimulus effects of triazolam in women and men

Benzodiazepines are among the most commonly prescribed therapeutics. Women seem to be more likely than men to be prescribed a benzodiazepine and to use benzodiazepines for nonmedical reasons; they also appear to be at higher risk for benzodiazepine dependence. The aim of the present investigation was to assess the acute behavioral effects of a benzodiazepine in women and men. To accomplish this, 13 volunteers (6 women, 7 men) first learned to discriminate 0.375-mg triazolam, a triazolobenzodiazepine hypnotic. After acquiring the discrimination, (i.e., >80% correct responding on 4 consecutive sessions) a range of doses of triazolam (0, 0.0625, 0.125, 0.25, and 0.375 mg) were tested in each participant. Triazolam dose dependently increased drug-appropriate responding and subject ratings of sedation and impaired performance (i.e., significant effect of dose). The women and men did not differ significantly on any measure. The results of the present experiment suggest that women and men are not differentially sensitive to the behavioral effects of triazolam.

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.

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.

Effects of dopamine agonists and antagonists on locomotor activity in male and female rats

Male and female Sprague-Dawley rats were treated with cocaine, the specific dopamine uptake inhibitor GBR 12909, the dopamine D1 agonist SKF 82958 or the dopamine D2 agonist quinpirole. After treatment, the rats were placed in an activity chamber for 30 min and locomotor activity was monitored. Cocaine, GBR 12909 and SKF 82958 all increased locomotor activity in both males and females, but greater increases were observed in females. In contrast, quinpirole produced decreases in activity, with males showing greater decreases than females. Separate groups of animals were given SCH 23390 or eticlopride prior to cocaine. The D1 antagonist SCH 23390 reduced the locomotor activating effects of cocaine in both males and females, with females showing greater sensitivity to SCH 23390. The D2 antagonist eticlopride also reduced the locomotor activating effects of cocaine, with no clear differences between males and females. These results suggest that the differences between males and females in their locomotor response to cocaine can be at least partially attributed to differences in the function of dopamine D1 and D2 receptors.

Gender differences in the behavioral effects of methamphetamine

The effects of methamphetamine were tested in male and female rats on two different behavioral tasks. Following habituation to a locomotor activity chamber, female rats were more sensitive to the locomotor activating effect of i.p. methamphetamine (0.1-3.0 mg/kg) than were male rats. A similar effect has been observed for other psychomotor stimulants, including cocaine and amphetamine. However, males and females did not differ on methamphetamine-induced place preference following eight conditioning trials with a wide range of doses (0.1-5.6 mg/kg). These results suggest that males and females differ in their response to methamphetamine for only some behavioral tasks.

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.