Sex differences in the olfactory stimulation-induced effects on the formalin test

Aromatic plants and their oils have long been known to exert various psychological and physiological effects on the human mind and body. The ability of olfactory input to stimulate, suppress or modulate various types of behavior in mammalian species has repeatedly been demonstrated. However, no data are available concerning tonic long-lasting pain. The formalin test is a well-characterized method to induce tonic long-lasting pain that shows several features of clinical pain. In the present experiment, male and female rats were subcutaneously injected with 50 μ1 of dilute formalin (5%) and introduced to a hole-board apparatus. Under the bottom of the hole-board, half of the animals received lemon essence in water and half only water. During the test, the formalin-induced responses (licking duration, flexing duration and jerking frequency of the injected limb) and other spontaneous behaviors (rearing, grooming) were recorded. Sixty minutes later, the animals were killed and blood was collected from the abdominal aorta to determine corticosterone plasma levels. The results showed a clear reduction of licking in the lemon essence group during the first 25 min of the test; the effect was greater in female than male rats. There were no differences between the groups in flexing and jerking during the first phase (0-15 min); however, the levels were lower in the lemon-treated group throughout the second phase (16-60 min), the greatest reduction occurring at 16-30 min of the formalin test. Rearing and grooming were increased by the lemon odor only in sham-treated animals, while in formalin-treated ones there were no differences. The present results clearly indicate the ability of lemon odor to modulate persistent pain in male and female rats and support further studies to better define the neuronal circuits involved.