Annemieke van Hest

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

The organizational, activational and reorganizational effects of gonadal hormones have been extensively investigated with respect to sexual, aggressive and maternal behavior. It has thus been established that manipulations of gonadal hormones during critical periods in development functionally affect reproductive behavior. The effects of gonadal hormones on nonreproductive behavior are not immediately obvious because of the fact that the behavioral effects of gonadal hormones on learning and memory have been investigated in a large number of unrelated experimental procedures. The present paper provides an organized overview of these different experimental procedures, summarizes the most important findings and discusses some of the variables which determine the effects of manipulations in gonadal hormones on learning and memory in male and female rats.

Behavioral differences between male and female wistar rats on DRL schedules: Effect of stimuli promoting collateral activities☆

Female rats are more active than males in open-field procedures. They also make fewer unreinforced responses than males on schedules which maintain low rates of responding (differential reinforcement of low rate responding, DRL). The addition of objects to promote collateral activities facilitates the acquisition of efficient DRL behavior. It was hypothesized that differences in activity levels might underlie the observed sex differences in DRL learning. If females are more likely than males to engage in activities other than lever pressing, the addition of objects to the experimental environment should not as much influence the behavior of females than that of males. Male and female rats were exposed to a DRL 15 sec or a DRL 30 sec schedule, either with or without objects to promote collateral activities. General activity was tested twice in an open-field procedure. Females showed higher activity levels than males in the open-field. They also acquired a DRL task more efficiently than males. The addition of objects facilitated the acquisition of efficient DRL performance in males, but not in females. The results of the present experiments, therefore, suggest that differences in general activity between male and female rats may underlie sex differences in some experimental paradigms.

Acquisition of conditional associations and operant delayed spatial response alternation: Effects of lesions in the medial prefrontal cortex.

In this article 8 male Wistar rats received bilateral lesions of the medial prefrontal cortex, whereas another 8 rats were control operated. Three weeks after surgery, they were exposed to an autoshaping procedure in which the insertion of a lever into the experimental chamber (conditioned stimulus) always preceded the delivery of a response-independent food pellet (unconditioned stimulus). Subjects with lesions acquired this conditional association faster than control-operated subjects as evidenced by the fact that they were more likely than control-operated subjects to contact the conditioned stimulus at higher rates. Locomotor activity, observed in a standard open-field preceding autoshaping sessions, decreased for both groups of subjects with repeated exposure to the open-field, whereas differences between groups were not observed. The same subjects were also exposed to an operant delayed spatial response alternation procedure in which they were required to alternate responding between two levers that were inserted into the experimental chamber after delay intervals of either 5, 10, or 20 s had elapsed. Alternation response accuracy of both subjects with lesions and control subjects decreased as a function of the duration of the delay interval, but control-operated subjects responded more accurately than did lesion subjects at each interval duration. Response accuracy increased with prolonged training for both groups of subjects, but faster for control-operated than for subjects with lesions.(ABSTRACT TRUNCATED AT 250 WORDS)

The behavior of male and female Wistar rats pressing a lever for food is not affected by sex differences in food motivation.

Abstract Male rats both eat more and weigh more than females. Differences in food intake and body weight result, at least partly, from differences in gonadal hormone secretions. The present experiments were designed to investigate whether sex differences in food motivation might contribute to the behavioral differences observed when food-deprived male and female rats are exposed to appetitively motivated operant learning tasks. Male and female Wistar rats were exposed to different progressive ratio schedules of reinforcement which have been shown to generate reliable indices of ‘motivational’ conditions. In progressive ratio schedules, subjects are required to make a systematically increasing number of responses for each successive reinforcer, until the requirement becomes so large that the subjects stop responding. Expt. Ia was designed to investigate whether or not food-deprived males would be more motivated than food-deprived females to obtain food. Expt. Ib investigated whether gonadectomy might differentially affect food motivation of male and female rats exposed to a progressive ratio schedule of reinforcement. Motivational differences between males and females were not observed. Males and females obtained an equal number of reinforcers, while differences in the total number of responses and response rate were not observed. Gonadectomy did not affect the total number of responses in the final completed ratio. The results of the present experiments do therefore not support the hypothesis that sex differences in food motivation might underlie sex differences in behavior when food-deprived male and female rats are exposed to operant schedules of positive reinforcement.

Scopolamine differentially disrupts the behavior of male and female Wistar rats in a delayed nonmatching to position procedure

Evidence is available that pharmacological interference with the cholinergic system may disrupt behavior in experimental procedures designed to investigate learning and memory processes. Recently it has been suggested that the cholinergic system may be sexually dimorphic. The present experiment was designed to investigate whether or not manipulation of the cholinergic system differentially affected memory processes in both sexes. Male and female Wistar rats were exposed to a delayed nonmatching to position procedure and were challenged with increasing doses of scopolamine hydrobromide (a central and peripheral muscarinic receptor blocker) and scopolamine methyl bromide (which does not pass the blood-brain barrier). Response accuracy decreased in both sexes as the delay interval duration increased. Behavioral differences between saline-treated males and females were not observed. Response accuracy decreased dose-dependently after subjects were injected with scopolamine hydrobromide. Response accuracy also decreased after treatment with scopolamine methyl bromide, but to a smaller extent. Males showed less accurate responding after treatment with either drug than females. These results provide behavioral evidence for the hypothesis that cholinergic functioning may differ between the sexes.

Perseverative responding in male and female Wistar rats: Effects of gonadal hormones

Response perseveration was investigated in an experimental procedure which has previously been shown to be sensitive to pharmacologically induced behavioral perseveration and response stereotypy. Different groups of intact, gonadectomized, and gonadectomized plus chronically testosterone-treated male and female Wistar rats were exposed to this procedure in which reinforcers were randomly assigned to one of two levers in an operant chamber. One response on the lever to which the reinforcer was assigned was sufficient to produce a food pellet. Response perseveration, defined as the percentage of trials on which more than one response on the lever not selected for reinforcement was made prior to switching to the selected lever was highest in testosterone-treated subjects. Females made more responses on the lever which had been selected for food on the preceding trial, suggesting that females may be more sensitive than males to the consequences of their behavior. This behavioral difference between the sexes may be mediated by the male hormone testosterone.

The effects of scopolamine and methylscopolamine on visual and auditory discriminations in male and female Wistar rats

Abstract The present experiment was designed to investigate whether or not the administration of scopolamine hydrobromide would differentially disrupt auditory or visual discrimination performance in male and female Wistar rats. Two groups of male and female Wistar rats were trained to discriminate between a continuous and intermittent visual stimulus, while two other groups were trained to discrinate between a continuous or intermittent auditory stimulus in a discrete-trial discrimination procedure. Once discrimination performance had stabilized, subjects were treated with different doses (0.125, 0.25, 0.50 or 1.0) of scopolamine hydrobromide or scopolamine methylbromide. Treatment effects were assessed with respect to discrimination performance, as well as with respect to the number of trials which were not completed. Scopolamine hydrobromide, but not scopolamine methylbromide, disrupted visual and auditory discrimination performance. The auditory discrimination was more seriously disrupted. However, both the administration of scopolamine hydrobromide and of scopolamine methylbromide increased the number of trials which were not completed suggesting that the accuracy of visual and auditory discriminations after drug treatment may have been influenced by other variables than drug effects on memory processes. Sex differences were not observed, neither with respect to discrimination performance, nor with respect to the number of trials which were not completed.

Behavioral differences between male and female wistar rats in food rewarded lever holding

Female rats are more active and explore more than males. In addition, females make fewer unreinforced responses on schedules which maintain low rates of responding (DRL). When general activity is high, subjects are more likely to engage in activities other than lever pressing. To test this hypothesis, male and female Wistar rats were trained to hold a response lever depressed for a minimum duration of 10 seconds. Reinforcement was delivered upon release of the lever. Females spent a significant smaller proportion of the total session time depressing the response lever than males. Differences between males and females in the number of unreinforced responses were not observed. Providing subjects with objects to promote collateral activities decreased the proportion of total session time subjects spent depressing the response lever for females, but not for males. An effect on the number of unreinforced responses was not observed.

Acquisition and reversal of a discriminated autoshaped response in male and female rats: Effects of long or short and fixed or variable intertrial interval durations☆

Abstract Groups of male and female Wistar rats were exposed to autoshaping procedures in which the duration (50 vs 20 s) and the variablity (fixed vs variable) of the intertrial interval were manipulated. Pellet delivery occurred after one of the levers (CS+) had been presented for 10 s. Presentation of the other lever (CS−) was not associated with pellet delivery. CS+ contacts occurred more frequently and at higher rates when the intertrial interval was 50 s than when it was 20 s, while intertrial interval variability did not play an important role. Males contacted CS+ and CS− more frequently and at a higher rate than did females. Goal-directed activity (nose pokes in feeder tray) occurred more often when the intertrial interval was short than when it was long. Females showed higher goal-directed activity than did males, both during CS presentation and during the intertrial interval. When experimental contingencies were reversed, CS+ contacts decreased and CS− contacts increased for males and females, but the decrease in CS+ contacts occurred more slowly for males than for females. The results of these experiments add to a growing body of literature suggesting the existence of fundamental behavioral differences between the sexes. These differences are partly attributable to the influence of gonadal hormones during development and maturation and partly attributable to the behavioral requirements of the experimental procedures.

Memory in male and female Wistar rats: Effects of gonadectomy, and stimulus presentations during the delay interval

Abstract The present experiment was designed to investigate the effects of gonadectomy on the response accuracy of male and female Wistar rats in an operant delayed spatial response alternation procedure. Subjects were exposed to 3 randomly presented delay intervals during each experimental session (15, 30 and 60 s). Response accuracy decreased as a function of the duration of the delay interval. Prolonged exposure to the experimental procedure differentially affected the behavior of intact and gonadectomized males and females. Intact males showed a more rapid increase in response accuracy as compared to any of the other groups of subjects, but differences in steady-state behavior were not observed. The effect of stimulus presentations during the delay interval on the alternation accuracy of intact and gonadectomized male and female Wistar rats was studied in a second experimental condition. The presentation of stimuli during the delay interval equally affected response accuracy of intact and gonadectomized subjects at all delay interval durations. Differences between the sexes, or between intact and gonadectomized subjects were not observed in the second experimental condition. The results of the present experiments thus suggest that intact and gonadectomized male and female Wistar rats do not differ with respect to memory as measured in operant delayed spatial alternation tasks, since both males and females were equally susceptible to the presentation of stimuli meant to disrupt ongoing memory processes.