Determining The Effects Of Castration And Estrogen Replacement On Habit Development In Adult Male Long Evans Rats

During instrumental learning, behavior transitions from goal-directed actions to habits. Goal-directed behavior is driven by the value of the outcome while habitual responding develops with increased training and is characterized by the automaticity with which it occurs independent of outcome value. The dorsal striatum, one of the many regions of the brain that has been implicated in the conversion of goal-directed actions to habits, is sexually dimorphic, and ovarian hormones are known to modulate the expression of habit in females. The present experiments were performed to begin deciphering how gonadal steroid hormones influence habit development in male rats. In each experiment, animals were trained to nose-poke for sucrose pellet reinforcers on a variable-interval (VI) schedule of reinforcement. In ½ of the males, the sucrose was then devalued through a conditioned taste aversion which was achieved by pairing the sucrose pellet with injections of lithium chloride (LiCl) to induce nausea. Classification of responding in each experiment occurred when animals were tested under an extinction condition. Devalued animals who remained goal-directed were expected to decrease responding for the devalued sucrose. Conversely, animals in habit, and therefore, insensitive to devaluation, were expected to respond at a similar rate as their non-devalued counterparts. Experiment 1 examined the role played by gonadal steroid hormones in the development of habit in male rats by removing circulating steroid hormones through castration. When trained to 160, 240, 320, and 400 reinforced exposures, castrated (CAST) male rats express habitual behavior with training to 240 reinforced exposures which is much earlier in instrumental learning than habitual responding is observed in intact males. These results indicate that male sex hormones play a role in delaying habit development in intact males. Results from Experiment 1 and the literature on high estrogenic states on behavior in female rodents motivated us to conduct Experiment 2 to begin studying the effects of estradiol replacement on habit development in CAST males. Following the same paradigm as was employed in Experiment 1 with training to 160 reinforcers, goal-directed behavior was observed in estradiol (E2)-replaced CAST males. These results alone cannot be used to conclude how E2 specifically is acting on behavior in intact males since this was a small amount of training and future research on responding following additional training would need to be analyzed to ascertain exactly how these hormones control behavior. Estrogens have been found to have opposite effects on behavior between males and females and may play a role in the sex difference observed in habit formation in intact male versus female rodents, however, intact males are also constantly exposed to androgens in addition to estrogens. Thus, studies using the same methodology on the effects of androgens on habit development are planned