Effects of nitric oxide inhibition on the spread of biotinylated dextran and on extracellular space parameters in the neostriatum of the male rat

Volume transmission in the brain is mediated by the diffusion of neurotransmitters, modulators and other neuroactive substances in the extracellular space. The effects of nitric oxide synthase inhibition on extracellular space diffusion properties were studied using two different approaches, the histological dextran method and the real-time iontophoretic tetramethylammonium method. The spread of biotinylated dextran (mol, wt 3000) in the extracellular space was measured morphometrically following microinjection into the neostriatum of male rats. Two parameters were used to describe the spread of biotinylated dextran in brain tissue, namely, total volume of spread and the mean grey value. The nonspecific nitric oxide synthase inhibitors N-G-nitro-L-aginine methyl ester (10-100 mg/kg) and N-G-monomethyl-L-arginine acetate (30-200 mg/kg) decreased the total volume of spread of dextran in a dose dependent manner. 7-Nitroindazole monosodium salt (50-100 mg/kg), a specific neuronal nitric oxide synthase inhibitor, did not change the total volume of spread of dextran. Using the tetramethylammonium method, the extracellular space diffusion properties can be described by the volume fraction (alpha = extra-cellular space volume/total tissue volume), tortuosity lambda (lambda(2) = free diffusion coefficient/apparent diffusion coefficient in tissue), and non-specific uptake k´ [Nicholson C. and Sykova E. (1998) Trends Neurosci. 21, 207-215]. Nitric oxide synthase inhibition by N-G-nitro-L-arginine methyl ester (50 mg/kg) had relatively little effect on volume fraction and tortuosity, and no changes were observed after N-G-monomethyl-L-arginine acetate (20 mg/kg) or 7-nitroindazole monosodium salt (100 mg/kg) treatment. A substantial increase was found only in non-specific uptake, by 13% after N-G-nitro-L-arginine methyl ester and by 16% after N-G-monomethyl-L-arginine acetate, which correlates with the decreased total volume of spread of dextran observed with the dextran method. N-G-Nitro-L-arginine methyl ester treatment (100 mg/kg) decreased striatal blood flow and increased mean arterial blood pressure. The changes in dextran spread and non-specific uptake can be explained by an increased capillary clearance following the inhibition of endothelial nitric oxide synthase, as neuronal nitric oxide synthase inhibition had no effect. The observed changes after non-specific nitric oxide synthase inhibition may affect the extracellular space concentration of neurotransmitters and modulators, and influence volume transmission pathways in the central nervous system by increased capillary and/or cellular clearance rather than by changes in extracellular space diffusion.