Characterization of nitric oxide generator-induced hippocampal [³H]norepinephrine release. II. The role of calcium, reverse norepinephrine transport and cyclic 3',5'-guanosine monophosphate

The mechanisms by which two nitrogen monoxide (NO) generators, hydroxylamine and S-nitroso-L-cysteine (NO-CYS), induce hippocampal [³H]norepinephrine ([³H]NE) release was investigated. Neither hydroxylamine- nor NO-CYS-induced release was affected by the guanylate cyclase inhibitors, methylene blue or LY 83,583. The effect of hydroxylamine was completely dependent on extracellular Ca⁺⁺ and reduced by 40% in the presence of ω-conotoxin GVIA, an N-type Ca⁺⁺channel antagonist; however it was unaffected by Ni⁺⁺, nifedipine, caffeine or thapsigargin. The stimulatory effect of hydroxylamine on hippocampal cyclic GMP formation was not significantly affected by removal of extracellular Ca⁺⁺, indicating that Ca⁺⁺-dependent release is not due to inhibition of NO formation from hydroxylamine. However, the response to NO-CYS was reduced by 35 to 50% in either nominally Ca⁺⁺-free or 10 mM MgSO₄-containing buffer. Interestingly, buffer containing ethylene glycol bis(β-aminoethyl ether)- N,N'-tetraacetic acid dramatically enhanced the formation of NO from NO-CYS and potentiated the NO-CYS response. Both NO-CYS- and hydroxylamine-induced [³H]NE release was inhibited by NE transport blockers, indicating a prominent role for reverse transport, NO-CYS completely inhibited synaptosomal uptake of [³H]NE (IC₅₀~, 300 μM). NO generator-induced [³H]NE release has a glutamate-dependent component (see accompanying article). Inhibition of glutamate-evoked [³H]NE release by mazindol, an inhibitor of NE transport, suggests that the glutamate-dependent component also involves reversal of the NE transporter. These data suggest that NO produced from hydroxylamine or NO-CYS evoke both vesicular and nonvesicular release of hippocampal [³H]NE. Putative NO target molecules and the role of extracellular Ca⁺⁺ are discussed.