Long-lasting activity-dependent alterations in the strength of synaptic connections are thought to be essential substrates of memory. Long-term potentiation (LTP) of synaptic transmission has been actively studied, while long-term depression (LTD) of synapse strength much less so. Prolonged low-frequency synaptic stimulation can elicit robust sLTD of synaptic strength. Our previous studies suggest that sLTD consists of multiple, distinct cascades, one dependent on the actions of the intercellular messenger nitric oxide (NO) that can act on presynaptic terminals, and a second due to postsynaptic, NO-independent mechanisms. We have discovered a chemical means of inducing LTD (cLTD) that appears to isolate presynaptic LTD mechanisms. cLTD requires the simultaneous production of cyclic GMP and inhibition of cyclic AMP-dependent protein kinase, and activates a NO-dependent, presynaptic cascade contribution to stimulus-induced sLTD. The proposed studies will employ electrophysiological recording, biochemical and two-photon confocal fluorescence imaging techniques in in vitro hippocampal slices to answer the following questions: (1) Are sLTD and cLTD associated with presynaptic alterations in vesicular transmitter release? (2) What biochemical cascades underlie the presynaptic portion of stimulus-induced sLTD? (3) Does cLTD share mechanisms with NMDA or group I mGluR receptor-dependent LTD?, and (4) Are presynaptic calcium channels and/or vesicle release apparatus proteins targets for phosphorylation or dephosphorylation underlying sLTD and cLTD? Better understanding of LTD impacts many clinical issues. There is a hypothetical role for LTD in memory processing, and impairments in LTD may contribute to pathologies of memory storage such as Alzheimer's Disease. LTD-like dampening of neuronal excitation could be effective in preventing epileptic seizures and reduce excitotoxic neuronal injury. Such potential applications await a detailed understanding of the cellular neurochemistry underlying LTD.