Discrimination of ovarian steroids by rats

Rats' ability to discriminate their hormonal states was examined by observing the effects of ovarian steroids on state-dependent learning using a drug discrimination task. A rat's entry into the correct arm (left or right) of a Y-maze terminated mild foot shock. The arm designated as correct was alternated daily, but was consistently paired with a pretrial injection of hormone or vehicle. In Experiment 1, ovariectomized rats successfully discriminated pretreatment with 4.0 or 8.0 mg/kg progesterone vs. the oil vehicle but not 2.0 or 0.5 mg/kg progesterone vs. oil, or the daily alternation schedule alone (oil vs. oil). In Experiment 2a, ovariectomized rats correctly discriminated pretreatment with progesterone (4 mg/kg) vs. oil or dihydroprogesterone (4 mg/kg) vs. oil, and the results of Experiment 2b, appear consistent with the possibility that the rats discriminated pretreatment with progesterone vs. dihydroprogesterone. In Experiment 3a, ovariectomized rats discriminated on the basis of progesterone (4.0 mg/kg) vs. oil, estrogen (6.4 micrograms/kg estradiol benzoate) vs. oil, and estrogen vs. progesterone. In Experiment 3b, the rats were adrenalectomized, and the procedure from Experiment 3a repeated. The removal of the adrenal glands failed to abolish hormonal discriminations, suggesting that the effect is not adrenally mediated. The results of these studies show that rats can discriminate relatively low doses of ovarian steroids.