The hippocampal formation, important for the process of laying down new memories, contains large amounts of zinc, located in synaptic vesicles and released with high-frequency stimulation. Furthermore, zinc exerts effects on many ion channels, some of which may be unique to the hippocampal region. Thus, it has been postulated, but not confirmed, that synaptic zinc and specialized zinc-sensitive ion channels act as a novel neuromodulatory system. As the next steps in testing this postulate, experiments are proposed here to test the following four hypotheses: Hypothesis A. The kinetics of zinc-induced block of voltage-dependent conductances are consistent with a neuromodulatory role. The kinetics of Zn2+-induced block of ion channels are crucial for determining whether this pharmacological effect has physiological significance. Using rapid solution-exchange and whole-cell patch-clamp techniques, these kinetics will be measured in dissociated neurons from the medial entorhinal cortex (MEC). Hypothesis B. Zinc-sensitive ionic conductances co-localize with Zn2+- positive nerve terminals in the hippocampal region. If specialized, Zn2+ -sensitive ion channels are the postsynaptic "receptors" of a novel neuromodulatory system, they are likely to be expressed in regions of the hippocampal formation other than the MEC. This hypothesis will be tested by recording from dissociated neurons from two additional Zn2+-rich regions: hippocampal region CA3 and the lateral entorhinal cortex (LEC). Hypothesis C. Zinc-sensitive Na+ channels of the MEC are structurally similar to cardiac Na+ channels. Demonstrating that Zn2+-sensitive ion channels are structurally related to cardiac Na+ channels, and hence different from known brain channels, would argue strongly that these channels are indeed "specialized." The structure of these channels will be probed using well-understood pharmacological manipulations in dissociated neurons. Hypothesis D. Endogenous, synaptically released Zn2+ modulates neuronal voltage-dependent conductances in the hippocampal region. To argue that Zn2+ modulates voltage-gated conductances in the MEC, LEC and region CA3, one must demonstrate that endogenous synaptically released Zn2+ reaches and affects these conductances. This demonstration will be attempted in recordings from hippocampal-entorhinal brain slices. The overall goal of this project - an understanding of the role zinc plays in the functioning of the hippocampal region - may prove crucial in understanding several devastating neurological disorders. Examples of brain disorders to which zinc metabolism in the hippocampal region has been linked include Alzheimer's disease, temporal lobe epilepsy, and stroke-related cell death.