Head shaking in the forced swim test: A robust but unexplored sex difference

Preclinical psychopharmacology research needs novel behavioral indices and improved animal models for both sexes. The forced swim test (FST) is the most popular test for screening antidepressant potential. Sex differences in FST behaviors, such as immobility and swimming, are not consistent among laboratories. Reliable indices, sensitive to sex differences, are required. We identified a robust sex difference in the frequency of headshakes during the standard two session FST, with male rats exhibiting higher number of head shakes than females. Furthermore, we explored whether strain, ageing, sex- and stress-hormone levels influence this sex difference. Experiments in middle-aged and senescent Wistar rats, as well as in gonadectomized and adrenalectomized with corticosterone replacement young adult males and females, revealed that sex differences in headshakes during FST are not influenced by age or corticosterone, but are abolished following castration of male rats. Interestingly, headshake frequency correlated positively with testosterone, but not corticosterone levels. Finally, testing of Flinders Sensitive Line (FSL) and Sprague-Dawley (SD) rats in a single 5min FST session revealed that headshake frequency is sensitive to antidepressant treatment with female rats exhibiting opposite responses to treatment than male FSL rats. Mirtazapine, a 5-HT2 antagonist, enhanced headshakes in females and decreased them in male FSL rats. Based on current data and the available literature, sex differences in headshake frequency should be linked to analogous sex differences in serotonin receptors. Headshake frequency during the FST is an additional valuable behavioral index, sensitive to sex differences, gonadal hormones and antidepressants modulating serotonin receptors.