The role of cyclo-oxygenase and or its substrate(s) on the neuroactivity of evening primrose oil was investigated on the basis that a blockade of cyclo-oxygenase activity using aspirin would inhibit neuroactivity of primrose oil if this effect was mediated by prostanoids. Streptozotocin diabetic rats and controls were all given large doses of aspirin, but only subgroups of them received primrose oil. Saphenous sensory A- and C-fibre, and sciatic motor conduction velocities were measured to assess neuroactivity of primrose oil. Aspirin enhanced the neuroactivity of primrose oil thus indicating that prostanoids are unlikely to mediate this neuroactivity, but also suggesting that substrates of cyclo-oxygenase are neuroactive. It is likely that cyclo-oxygenase antagonises neuroactivity of primrose oil by competing with the process for substrates. Thickly myelinated sensory A-fibres were most affected by primrose oil suggesting that the predominant sensory symptoms in diabetic neuropathy could be due to the sensitivity of sensory nerves to the metabolic aberration in diabetes. Normal nerve function is probably preserved by cyclo-oxygenase during an influx of neuroactive fatty acids from the gut, since inhibition of the enzyme rendered non-diabetic nerves vulnerable to dietary primrose oil.
