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 neurotrans mitters, modulators and other neuroactive substances in the extracellular s pace. The effects of nitric oxide synthase inhibition on extracellular spac e diffusion properties were studied using two different approaches, the his tological dextran method and the real-time iontophoretic tetramethylammoniu m method. The spread of biotinylated dextran (mol, wt 3000) in the extracel lular 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 a cetate (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 sp ecific neuronal nitric oxide synthase inhibitor, did not change the total v olume of spread of dextran. Using the tetramethylammonium method, the extra cellular space diffusion properties can be described by the volume fraction (alpha = extra-cellular space volume/total tissue volume), tortuosity lamb da (lambda(2) = free diffusion coefficient/apparent diffusion coefficient i n tissue), and non-specific uptake k' [Nicholson C. and Sykova E. (1998) Tr ends 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 f raction 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, b y 13% after N-G-nitro-L-arginine methyl ester and by 16% after N-G-monometh yl-L-arginine acetate, which correlates with the decreased total volume of spread of dextran observed with the dextran method. N-G-Nitro-L-arginine me thyl ester treatment (100 mg/kg) decreased striatal blood flow and increase d mean arterial blood pressure. The changes in dextran spread and non-speci fic uptake can be explained by an increased capillary clearance following t he inhibition of endothelial nitric oxide synthase, as neuronal nitric oxid e synthase inhibition had no effect. The observed changes after non-specific nitric oxide synthase inhibition ma y affect the extracellular space concentration of neurotransmitters and mod ulators, and influence volume transmission pathways in the central nervous system by increased capillary and/or cellular clearance rather than by chan ges in extracellular space diffusion. (C) 1999 IBRO. Published by Elsevier Science Ltd.