Latency of neuroactivity and optimum period of treatment with evening primrose oil in diabetic rats.

The interval between oral intake of evening primrose oil and its effect in peripheral nerves of rats was studied 35 days after induction of streptozotocin diabetes. Myelinated and unmyelinated sensory nerve conduction velocities were measured in the saphenous nerve and motor nerve conduction velocity was measured in the sciatic nerve to assess neuroactivity of primrose oil. The severity of diabetes was assessed by levels of HbA1 and plasma glucose. The diabetic state remained constant between 35 and 70 days post induction. Plasma glucose increased by 133%, and HbA1 by 122% in diabetic rats. Myelinated sensory and motor nerves had conduction deficits of 13 and 16%, respectively, 35 days post induction. Unmyelinated nerves had no conduction deficit until 45 days post induction. Primrose oil did not affect nerve conduction velocity in the first 12 h of its administration, but significantly increased it 24 h later (p < 0.001). Daily treatment with primrose oil caused oscillation of nerve conduction velocity in diabetic rats needing 10 days to stabilise. The latency suggests that neuroactivity of primrose oil may be mediated by its metabolic products synthesised in the body, and not by ready made constituents of the oil.