Behavioral differences between male and female rats: effects of gonadal hormones on learning and memory

Effects of estradiol on radial arm maze performance of young and aged rats

Gonadectomized male and female Sprague-Dawley rats, given estradiol (E2) via sc Silastic capsules that generated proestrus levels of hormones, were tested for spatial memory performance on an 8-arm radial maze. Performance of males, with or without E2, exceeded that of females, with or without E2, for choice accuracy parameters over 20 trials. In addition, males reached criterion earlier than females (6 vs 11 trials). There were no significant effects of E2 on performance of either sex. When a 1-h delay was instituted between the 4th and 5th choices, the performance of males remained better than that of the females, and E2 administration was associated with a small, but significant, improvement in performance of the males but not the females. E2 administration to 25-month-old males also did not affect performance in regular trials, but performance was enhanced in trials with delays of 1-3 h after the 4th choice. These results show that estradiol can influence spatial memory performance and suggest that E2 may be beneficial for age and/or disease-related memory impairments.

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

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

Ovarian steroid deprivation results in a reversible learning impairment and compromised cholinergic function in female Sprague-Dawley rats

We hypothesized that estradiol (E2) serves as a neurotrophomodulatory substance for basal forebrain cholinergic neurons thought to be involved in learning and memory. Learning/memory was assessed using the two-way active avoidance paradigm and the Morris water task. Female Sprague-Dawley rats were either ovariectomized (OVX) or OVX for 3 weeks, followed by s.c. implantation of a Silastic pellet containing 17-beta E2 (E2 pellet), resulting in a replacement of E2 to physiological levels. Ovary-intact (INTACT) animals served as our positive control. Active avoidance behavior and choline acetyltransferase (ChAT) activity in the frontal cortex and hippocampus were assessed at 5 and 28 weeks postovariectomy while performance on the Morris water task and high-affinity choline uptake (HACU) were measured only at the 5-week time point. At the 5-week time point, E2 replacement caused a significant elevation in the level of active avoidance performance relative to OVX animals. At the 28-week time point, OVX animals demonstrated a significantly lower number of avoidances relative to controls (61%) whereas E2-pellet animals not only demonstrated superior performance relative to OVX animals but also showed an accelerated rate of learning. Morris water task performance, on the other hand, was not significantly affected by estrogenic milieu despite a trend towards better performance in the E2-pellet group. Neurochemical analyses revealed that 5 weeks of ovariectomy was sufficient to reduce HACU in both the frontal cortex and hippocampus by 24 and 34%, respectively, while E2 replacement was successful in elevating HACU relative to OVX animals in both regions.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Sex differences in schedule-induced alcohol consumption

Six female and five male Wistar rats obtained food by pressing a lever on a fixed-interval (FI) 60-s schedule in the presence of a sipper tube which allowed access to an alcohol solution. Systematic manipulation of the alcohol concentration revealed that male rats consumed higher alcohol concentrations (v/v) than female rats to obtain maximum alcohol intake (g ETOH/kg) during an experimental session. Males also reached higher blood alcohol levels (BALs) than females. Individual lick rates varied over a small range when the FI schedule parameter was manipulated (20 s and 180 s) during probe sessions. Very few licks were observed when all pellets were presented at the beginning of the session, during extinction, or during sessions in which only a few pellets were presented. Reduction of the solution's alcohol concentration to half the maximum concentration or presentation of distilled water resulted in increased lick rates at most values of the FI schedule in four of the five male subjects, but not in female subjects. Alcohol intake during sessions in which half the maximum alcohol concentration was available was lower than that observed during sessions in which subjects consumed the maximum alcohol concentration. The lick rate data in conjunction with those on alcohol intake suggest that male rats, but not female rats, maintained blood alcohol concentrations at levels which they had also reached during sessions in which the maximum alcohol concentration was available from the sipper tube.

Behavioral effects of long-term administration of an anabolic steroid in intact and castrated male Wistar rats

Once a week, intact and castrated male Wistar rats were intramuscularly injected with a 0.2 ml suspension of either 0, 10, or 50 mg nandrolone decanoate in cottonseed oil, for 8 consecutive weeks. After the sixth injection, locomotor activity was measured in an open-field and the acquisition of lever press behavior was assessed in an autoshaping procedure. Subsequently, all subjects were exposed to four sessions of continuous reinforcement prior to one session in which the effects of steroid administration on extinction were assessed. Locomotor activity decreased for all groups of rats with continued exposure to the open-field, differences between groups were not observed. Rats treated with the highest dose of nandrolone decanoate spent more time in the margin of the open-field. There were no significant differences between groups on any of the learning measures. Long-term, high-dose steroid administration in conjunction with mild food deprivation inhibited growth in intact and castrated rats, while low dose administration affected body weight in intact rats only. Steroid administration resulted in heavier and enlarged kidneys and lighter testes as well. These results suggest that the administration of anabolic steroids not only produces observable physiological changes, but that it may also affect spontaneous behavior. The failure to find differences in learning indices may have been a function of the particular paradigms used in the present experiment.

Early postnatal estrogen organizes sex differences in the extinction of a CRF running response

In the present study the organizational effects of sex steroids on the sexually dimorphic extinction of a continuously food-rewarded running response were investigated. Gonadally intact female rats neonatally treated from day 1 to day 8 of the postnatal life with estradiol benzoate (EB), dihydrotestosterone (DHT) or vehicle, and males treated in the same period with the antiandrogen ciproterone acetate (AC), the estrogen antagonist tamoxifen (TX) or vehicle were studied in adulthood during the acquisition and extinction phases of the response in a short and narrow runway. No difference in performance between groups was obtained in the response acquisition. However, during extinction control males extinguished faster than control females. DHT treatment to females and neonatal CA administration to males had no effect on the expression of sexual dimorphism. Conversely, TX administration to the males increased male's resistance to extinction at the levels shown by control or DHT females, whereas the females treated with EB exhibited similar extinction rates to those observed in nonhormonal treated or CA males. This finding suggests that the organizational effect of testosterone on the sexually dimorphic behavior studied in the present report are mediated by testosterone conversion to estradiol throughout the aromatization pathway in the brain.

Gender differences in escape-avoidance behavior of mice after haloperidol administration

Gender differences in the disruptive effects of haloperidol on some reinforced behaviors have been observed in different species. However, the inhibitory action of haloperidol on the acquisition and performance of escape-avoidance behavior has only been investigated in male subjects. The present experiment was designed to investigate possible gender differences in the effects of haloperidol on the initial phase of an escape-avoidance learning task. Male and female mice of the OF1 strain were given a single training session in a shuttle-box. Thirty minutes prior to the behavioral test, mice were injected IP with haloperidol (0.25 mg/kg) or physiological saline (10 ml/kg). Latencies of escape and avoidance responses and the number of nonresponses, escapes, avoidances, pseudoavoidances, crossings during the adaptation period, and crossings during intertrial intervals (ITIs) were evaluated. The disruptive action of haloperidol on the escape-avoidance behavior of the mice was greater in males than in females. The number of nonresponses were higher and the number of escapes lower in treated males than in their female counterparts. These gender differences were not found in control subjects. Activity measures of spontaneous motor behavior (crossings in the adaptation period and during ITIs) did not present gender differences either. Several possible mechanisms responsible for this greater susceptibility of males to the inhibitory effects of haloperidol on escape-avoidance learning are discussed, especially the modulating role of female hormones on dopaminergic activity.

Testosterone as appetitive and discriminative stimulus in rats: sex- and dose-dependent effects

Stimulus properties of subcutaneously injected testosterone were studied in male and female rats. In a conditioned place preference procedure, dose-dependent effects (doses: 0, 0.5, and 1 mg/kg) were observed in males. In females, no place preference could be established (doses: 0, 1, and 3 mg/kg). In addition, 1 mg/kg testosterone acquired discriminative stimulus control in male rats in a taste aversion procedure. Animals injected with this hormone prior to saccharin-LiCl pairings and with its vehicle prior to saccharin-NaCl pairings suppressed fluid intake following the administration of testosterone and not following the administration of the vehicle. Subsequent generalization tests revealed dose-dependent stimulus control of this hormone (range of substitution doses: 0.125-2 mg/kg). It is concluded from the results that at least pharmacological (supraphysiological) doses of testosterone may act as appetitive stimuli in male rats, but not in female rats. Furthermore, in male rats (pharmacological doses of) testosterone also possess discriminative stimulus properties.

Response acquisition with fixed and variable resetting delays of reinforcement in male and female Wistar rats

Experimentally naive, food-deprived male and female Wistar rats were exposed to different experimental procedures in which response-independent food pellets were delivered as long as the rats did not press either one of the two levers in the experimental chamber. Once a lever was pressed, the presentation of response-independent food was suspended for the remainder of that session and food pellets were delivered only after an unsignalled, resetting period of time from each lever press had elapsed. For five different groups of subjects there was either no delay between a lever press and food presentation, a fixed delay of 10 s or 30 s, or a variable delay of 10 s or 30 s. Response acquisition occurred most rapidly when a lever press was immediately followed by food presentation. Responding was also acquired when variable resetting delays of 10 s or 30 s or a fixed resetting delay of 10 s was inserted between a lever press and food presentation. When a fixed resetting delay of 30 s followed a lever press before food presentation some responding was observed, but not as much as in the other experimental conditions. The present results show that naive male and female Wistar rats will acquire a lever-press response when food presentation is contingent upon, but not immediately contiguous with a lever press. Consistent differences between the sexes were not observed.

Effects of early postnatal sex steroids on acquisition and extinction of a continuously reinforced lever-pressing response

The effects of early postnatal dihydrotestosterone (DHT) and estradiol on the sexually dimorphic continuously reinforced lever-pressing response were investigated. 90-day-old male rats postnatally treated (during the first eight days of postnatal life) with cyproterone acetate (CA), tamoxifen (TX) or vehicle, and 90-day-old females treated with estradiol benzoate (EB), DHT or vehicle in the same postnatal period, were studied during the acquisition and extinction of the continuously reinforced lever-pressing response using a free-operant procedure. During acquisition, the control males made more responses per minute than the control females, and also reached the extinction criterion significantly sooner than the females. CA treatment impaired the male's performance at the levels of that shown by females, whereas TX treatment affected neither acquisition nor extinction. Inversely, in both experimental phases females treated with DHT performed like control females, whereas the acquisition and extinction performances of the EB-females were similar to those obtained in the control or TX male groups.

The relation of body height to handedness in male and female right- and left-handed human subjects

The association between body size and the degree of hand preference was studied in male and female right- and left-handed human subjects. Hand preference was assessed by the Edinburgh Handedness Inventory. In left-handers (total sample), there was a significant, negative linear correlation between the degree of left-hand preference and body height. There was no significant correlation between these parameters if the males and females were considered separately. In right-handers (total sample), body height was found to be significantly and negatively linearly correlated with the degree of right-hand preference. A similar correlation was observed in males without familial sinistrality (FS-) and in FS+ females. There was no significant correlation between these parameters in FS+ right-handed males and FS- right-handed females. It was suggested that hormones affecting body height and brain size such as human growth hormone and testosterone would also contribute to the development of cerebral lateralization.

Visual habit formation in 3-month-old monkeys (Macaca mulatta): reversal of sex difference following neonatal manipulations of androgens

The ability to perform on a concurrent visual discrimination task with 24-h intertrial intervals (24-h ITI task) develops a few weeks earlier in female than in male infant monkeys. To test whether this sex difference was related to the presence of perinatal androgens, plasma testosterone levels were reduced in male infant monkeys by neonatal orchiectomy and increased in neonatally ovariectomized female infant monkeys by treatment with either testosterone propionate (TP) or its reduced metabolite, dihydrotestosterone (DHT). At 3 months of age, the animals were tested on the 24-h ITI task and their performance compared with that of age-matched intact male and female monkeys. Orchiectomy which was followed by a slight but visible atrophy of the external genitalia, hastened performance of male infant monkeys to the level of intact infant females. Conversely, androgenization of ovariectomized female infant monkeys given DHT, which had only a slight virilizing effect on the external genitalia, showed the learning of these female infants to the rate of intact male infant monkeys. Curiously, although TP treatment in ovariectomized female infant monkeys was more effective than DHT in virilizing the external genitalia, it failed to slow the rate of learning. This dissociation between the effects of TP and DHT on external genital organs and learning abilities is discussed in terms of possible differences in dose-dependent, time-dependent, and receptor-binding mechanisms of the two androgens. The present study provides further evidence that early androgen secretions affect the organization not only of brain structures related to primary sexual characteristics but also of those related to learning abilities.

Sex differences in locomotor activity after acute and chronic cocaine administration

Adult, intact and gonadectomized male and female Wistar rats (n = 9) were exposed to an automated open field to assess the behavioral effects of acute cocaine administration (saline, 1.0 and 10.0 mg/kg subcutaneous). The subjects were exposed to the open field for 10 min, removed to be injected and returned to the open field for another 30 min. Three saline and two drug sessions were run in counterbalanced order. Locomotor activity in intact and castrated male rats and ovariectomized female rats decreased following injection, irrespective of the dose of cocaine. The locomotor activity of intact female rats was higher than that of any other group of subjects. It decreased during the session after saline and 1.0 mg/kg cocaine, but increased towards the end of the 30 min session after 10.0 mg/kg. Rearing measures paralleled the observations on locomotor activity. To determine the effects of chronic, home-cage, cocaine administration, five of the subjects in each group were injected with 10.0 mg/kg cocaine for 9 consecutive days. The remaining four subjects received saline injections. On day 10, all subjects were re-exposed to the open-field for 10 min, removed, injected with 10.0 mg/kg cocaine and returned to the open field for another 30 min. Chronic home cage cocaine administration produced an increase in cocaine's effects on locomotor activity and rearing in intact female rats only. However, behavioral sensitization was also observed in intact female rats who had been treated with saline for 9 consecutive days, suggesting that behavioral sensitization to cocaine in intact female rats may develop very rapidly and independent of environmental context.

Sex-dependent effects of aversive stimulation on holeboard and elevated plus-maze behavior

The behavioral effects of exposure to inescapable shocks (IS) were studied in both the holeboard and the elevated plus-maze, 24 and 72 h after IS in male and female Wistar rats. The following effects were observed at the 24-h interval. In both sexes, head-dipping in the holeboard was reduced by IS, whereas general activity (ambulation and rearing) was reduced in males and not in females. Furthermore, the results of a correlation analysis indicate that previous exposure to IS disrupts the dissociation observed in control groups between exploratory activity directed at the holes (head-dipping) and general activity in the holeboard (ambulation and rearing). Effects of IS on plus-maze behavior could be observed in a clear suppression of rearing in males and not in females. IS did not affect time spent on the open arms. At the 72-h interval, IS affected head-dipping in the holeboard only in males and not in females. The present findings show that the effect of IS on specified behavioral elements is sex-dependent, with stronger and longer-lasting effects in males than in females.

Electrophysiological evidence for a rapid membrane action of the gonadal steroid, 17 beta-estradiol, on CA1 pyramidal neurons of the rat hippocampus

The rapid electrophysiological effects of 17 beta-estradiol on CA1 pyramidal neurons (n = 86) were investigated utilizing intracellular recording from the rat hippocampal slice preparation. Bath application of 17 beta-estradiol, but not 17 alpha-estradiol, caused a reversible depolarization and increased input resistance with a latency of less than 1 min in 19.8% of CA1 neurons tested. There was no significant difference in the percentage of estradiol-responsive cells between male and female rats. Estradiol-responsive cells were identified from prepubertal female rats, as well as females in all stages of the estrous cycle. 17 beta-estradiol had no effect on the slow afterhyperpolarization or accommodative properties of CA1 neurons. In 2 out of 4 cells tested, the specific antiestrogen, tamoxifen, blocked the excitatory response to 17 beta-estradiol.

Cocaine-induced conditioned taste aversions in male and female Wistar rats

After an initial period of adaptation to 20 min per day of limited water availability, male and female Wistar rats were allowed access to water or a 0.1% sodium-saccharin solution. Saccharin exposures were followed by the subcutaneous administration of 0, 5, 10 or 20 mg/kg cocaine for different groups of rats. Four pairings of the saccharin solution with cocaine administration resulted in a consistent decrease in saccharin consumption only in female subjects injected with the largest dose of cocaine (20 mg/kg). Choice testing in which subjects could choose between two drinking tubes, one containing water, the other one containing the saccharin solution, was then conducted during extinction. During four of such experimental sessions, subjects which had previously been injected with vehicle mostly consumed the saccharin solution or showed a position bias. Conditioned taste aversions were not only observed in the group of female subjects injected with 20 mg/kg cocaine, but also in males previously treated with 20 mg/kg cocaine. In addition, compared to vehicle control groups, males and females injected with 5 and 10 mg/kg cocaine tended to avoid the saccharin solution in favor of regular water. It is suggested that previous failures to obtain consistent cocaine-mediated taste aversions may have been a function of the experimental procedures used to assess cocaine's efficacy in inducing conditioned taste aversions.

Fixed-consecutive-number performance in male and female Wistar rats

The present experiment was designed to investigate whether or not response rate differences between male and female Wistar rats observed in many different experimental procedures could be attributed to sex differences in behavioral perseverance, as has been suggested by the results of previous experiments. Male and female Wistar rats were thus exposed to different fixed-consecutive-number schedules of reinforcement. Fixed-consecutive-number schedules require subjects to emit a specified number of responses on one (work) lever, before a response on another (food) lever results in the presentation of reinforcement. The response requirement on the work lever was manipulated in different experimental conditions. Subjects had to emit between 3 and 7, 8 and 12 or 13 and 17 responses on the work lever before a response on the food lever produced reinforcement. When subjects emitted fewer or more than the required number of responses on the work lever, a 5-s time-out period was presented. Males responded at higher rates than females during all experimental conditions; response rates of males and females increased as the response requirement on the work lever was increased. Sex differences in response efficiency were not observed, but males seemed to reach final response efficiency faster than females. Response efficiency decreased as the response requirement on the work lever was increased. Error analysis showed that both males and females made more errors by not producing enough responses on the work lever than by producing too many. However, males were more likely than females to emit more responses than the requirement on the work lever, while females were more likely than males not to produce enough responses on the work lever.