The effects of sexual experience and estrus on male-seeking motivated behavior in the female rat

Abstract

Ovariectomized (OVX) female rats were trained to traverse a straight alley and return to a goal box where they had previously encountered a male rat, a female rat or an empty goal box. The time required to run the alley was used as an index of the subjects' motivation to re-engage the goal box target. Subjects were tested in both estrus and non-estrus, first sexually naïve and then again after sexual experience. Female rats ran most quickly for a male target, most slowly for an empty goal box, and at intermediate speeds for a female target. Sexual experience tended to slow run times for all but male targets. Estrus enhanced approach behavior for males and an empty goal box, but tended to slow the approach toward females, both before and after sexual experience. This latter finding was further investigated in a second experiment in which sexually naïve OVX females were tested during estrus and non-estrus in a locomotor activity apparatus, a runway with an empty goal box, and an open field. Estrus produced no changes in spontaneous locomotion either in the activity box or the open field, but decreased run times in the alley and increased the number of center-square entries in the open-field. Thus, estrus produces increases in sexual motivation that selectively enhance exploratory, presumably male-seeking behavior, but not simple spontaneous locomotion.

Figure 1

A schematic representation of the runway apparatus. Note that the circular goal box is partitioned by a perforated Plexiglas wall that permits subjects who are placed on one side to see, hear and smell, but not come in direct contact with, target stimuli placed on the opposite side. Subjects are exposed to the target stimulus, then lifted and placed in the start box. The doors to the start box and goal box then open and the time required for the animal to return to the goal box (run time) provides an index of the subject’s motivation to seek the target stimulus.

Figure 2

Mean (+ SEM) run times of three groups of female rats returning to a goal box containing one of three different target stimuli: an empty goal box, a female rat, or a male rat. Each animal was tested under both estrus (dark bars) and non-estrus (open bars). Panel A depicts the run times of sexually naïve animals and Panel B represents the run times of the same animals a week after having experienced sexual behavior. A 3-factor ANOVA on the combined data (see Results for more details) confirmed a significant main effect of target (p<.05) (when averaged across all trials, females ran fastest for males and slowest for an empty goal box), a main effect of experience (p<.05) (overall animals were slower in the runway after sexual experience than before it), and a hormone × target interaction (p<.05) indicating that estrus produced different effects depending upon the goal box target (e.g., faster running for male but not female targets).

Figure 3

Mean (± SEM) locomotor activity (measured as distance traveled in inches) assessed every 5 min during a 60 min test session for two groups of female rats. One group was tested during estrus and the other during non-estrus. The inset represents the overall mean total activity (+ SEM) summed over the entire 60 min test. There was no statistically reliable difference in locomotor activity between the two groups.

Figure 4

Mean (+ SEM) run times in the alley for two groups of female rats returning to an empty goal box. These data were collected during a single test trial with half the animals in estrus and half non-estrus. There was a statistically modest (p=0.056) increase in run times in the estrous relative to the non-estrous group.

Figure 5

Mean (+ SEM) open-field behavior of two groups of female rats (one in estrus and the other non-estrus) during a single 5 min test. Panel A depicts the mean total number of squares entered (a measure of general locomotor activity) and panel B depicts the mean number of center squares entered (a measure of exploratory behavior). While there were no difference in total square entries, the estrous group exhibited more center square entries (less thigmotaxis or “wall-hugging” behavior) than the non-estrous group (Panel B, p<.05).