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Long-term depression (LTD) of synaptic transmission can be induced by several mechanisms, one thought to involve Ca²⁺-dependent activation of postsynaptic nitric oxide (NO) synthase and subsequent diffusion of NO to the presynaptic terminal. We used the stable NO donorS-nitroso-N-acetylpenicillamine (SNAP) to study the NO-dependent form of LTD at Schaffer collateral-CA1 synapses in vitro. SNAP (100 μM) enhanced the induction of LTD via a cascade that was blocked by the N-methyl-d-aspartate receptor antagonist d-2-amino-5-phosphonopentanoic acid (50 μM), NO guanylyl cyclase inhibitor 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (10 μM), and the PKG inhibitor KT5823 (1 μM). We further show that LTD induced by low-frequency stimulation in the absence of SNAP also is blocked by KT5823 or Rp-8-(4-chlorophenylthio)-guanosine 3′,5′-cyclic monophosphorothioate (10 μM), cyclic guanosine …