Effect of oral contraceptives (Lyndiol) on rat brain gamma aminobutyric acid system.

(1) Virgin female albino rats were treated with the estrogenic substance mestranol, the progestogenic substance lynestrenol or with a combination of these two compounds. These two drugs are the components of the oral contraceptive pill Lyndiol®. (2) Mestranol pretreatment caused a significant increase in brain GABA level, lynestrenol pretreatment caused a significant decrease in brain glutamic acid level and pretreatment with a combination of the two drugs caused a significant rise in brain GABA content. Other changes were statistically insignificant. All three types of treatment, however, caused an identical and significant decrease in the glutamic acid: GABA ratio in rat brains. (3) The changes produced by oral contraceptive steroids on two brain enzymes, namely glutamate decarboxylase and aminobutyrate aminotransferase could not satisfactorily justify the assumption that the brain GABA concentration is controlled solely by the activities of such enzymes.