Objectives: The route of administration is an important factor in determining the action of some drugs. We previously demonstrated that subcutaneous monosodium glutamate (MSG) accelerated cortical spreading depression (CSD) in the rat and that treadmill exercise attenuated this effect. This study evaluated whether other routes of administration exert the same action by testing orogastric (gavage) and topical cortical MSG administration in treadmill-exercised and sedentary rats. Additionally, in the orogastric treatment we tested anxiety-like behavior.
Methods: Exercised and sedentary rats received per gavage water or MSG (1 or 2 g/kg) daily from postnatal (P) day 7 to 27. Behavioral tests (open field and elevated plus-maze) occurred at P53 ± 3. At P56 ± 3, we analyzed CSD parameters (velocity, amplitude, and duration of the negative potential change). Other three groups of rats received an MSG solution (25, 50 or 75 mg/ml) topically to the intact dura mater during CSD recording.
Results: MSG-gavage increased anxiety-like behavior and the CSD velocities compared with water-treated controls (P < 0.05). Exercise decelerated CSD. In contrast to gavage, which accelerated CSD, topical MSG dose-dependently and reversibly impaired CSD propagation, reduced CSD amplitude and increased CSD duration (P < 0.05).
Conclusions: The exercise-dependent attenuation of the effects of MSG confirms our previous results in rats treated subcutaneously with MSG. CSD results suggest two distinct mechanisms for gavage and topical MSG administration. Additionally, data suggest that exercise can help protect the developing and adult brain against the deleterious actions of MSG.
Keywords: Anxiety; Brain electrophysiology; Cortical spreading depression; Monosodium glutamate; Physical exercise.