The strength of synapses in many areas of the nervous system is not fixed. When activated repetitively their strength can be altered for prolonged periods of time. The use of dependent plasticity, referred to as long-term potentiation (LTP) is thought to play a role in certain forms of learning and memory. There are clearly at least two distinct forms of LTP; an N-methyl-D-aspartate (NMDA)-dependent form as expressed in the CA1 region of the hippocampus and an NMDA-independent form of LTP as expressed at mossy fiber synapses in the CA3 region. The studies proposed here will focus on aspects of the induction and expression in both forms of LTP. NMDA-dependent LTP. We will carry out experiments to determine if Ca++ entry via the NMDA receptor is necessary for LTP. We will use whole- cell voltage clamp to determine if a suppression potential can be found for LTP. Ca++ imaging will be performed with Dr. John Connor to measure directly Ca++ transients at various positive holding potentials. We will also address whether a rise in Ca++ is sufficient to generate stable potentiation. The Ca=== rise will be generated by activating voltage-dependent Ca++ channels which ca, under certain conditions, cause a transient potentiation. We will maximize conditions for Ca++ entry (e.g., raised extracellular Ca++) to see if Ca++ alone can evoke stable potentiation. If not, voltage-dependent Ca++ entry will be paired with synaptic stimulation in the presence of an NMDA antagonist in an attempt to reconstitute stable potentiation. If successful we will identify the necessary component provided by synaptic stimulation. Three types of experiment will address the expression of LTP. 1.) Whole-cell recording to reexamine the relative effect of LTP on the NMDA and non-NMDA component of the evoked and miniature EPSC. 2.) The sensitivity of the postsynaptic membrane to the glutamate agonist AMPA will be tested before and after NMDA application. 3.) We will use outside-out membrane patches from the soma to monitor the synaptic release of glutamate as detected by NMDA channel activity in the patch. Mossy fiber LTP. We will examine the action of a number of drugs which affect transmitter release and/or various signaling pathways in an attempt to localize the site of induction and expression of mossy fiber LTP. The manipulations will include changing Ca++/Mg++ ratio, phorbol esters, forskolin, kinase inhibitors, 4 amino-pyridine, CNQX and 2- amino-4-phosphonobutyrate (AP-4). A second series of experiments will examine mechanism underlying the modulation of mossy fiber LTP by various neurotransmitters known to alter this form of LTP, e.g., opioid peptides, norepinephrine and acetylcholine. The proposed studies will greatly advance our understanding of the cellular and molecular events underlying synaptic plasticity in the CNS. It can be anticipated that this knowledge will lead to the design of drugs that will have therapeutic value in such devastating conditions as Alzheimer's Disease.
