Formalin-induced changes in adrenocorticotropic hormone and corticosterone plasma levels and hippocampal choline acetyltransferase activity in male and female rats.

Formalin (10%) induces higher levels of licking and flexing in female than in male rats, as shown in the present study. In order to ascertain the neural and hormonal modifications that accompany these behavioural differences, we determined the activity of choline acetyltransferase in the hippocampus and the levels of adrenocorticotropic hormone and corticosterone in the plasma. Two concentrations of formalin were used (50 microliters; 0.1% or 10%). Formalin was injected subcutaneously in the dorsal part of the hindpaw, and the animal's behaviour was then recorded for 60 min in a familiar open-field apparatus. Hippocampal choline acetyltransferase activity did not differ between the two genders in controls, while a significant gender difference was present in both formalin-injected groups, with higher levels in females than in males. This was the result of a decrease in males but not in females. In contrast, adrenocorticotropic hormone was increased by both formalin concentrations in females; corticosterone was not affected by treatment in either gender. Results are discussed in the light of the morphological and functional differences between the two genders in the hippocampus and in the hypothalamo-pituitary-adrenal axis.