Faculty Bibliography

Glutamate application at synapses between hippocampal neurons in culture produces long-term potentiation of the frequency of spontaneous miniature synaptic currents, together with long-term potentiation of evoked synaptic currents. The mini frequency potentiation is initiated postsynaptically and requires activity of NMDA receptors. Although the frequency of unitary quantal responses increases strongly, their amplitude remains little changed with potentiation. Tests of postsynaptic responsiveness rule out recruitment of latent glutamate receptor clusters. Thus, postsynaptic induction can lead to enhancement of presynaptic transmitter release. The sustained potentiation of mini frequency is expressed even in the absence of Ca2+ entry into presynaptic terminals

Sympathetic neurons display robust [Ca2+]i oscillations in response to caffeine and mild depolarization. Oscillations occur at constant membrane potential, ruling out voltage-dependent changes in plasma membrane conductance. They are terminated by ryanodine, implicating Ca(2+)-induced Ca2+ release. Ca2+ entry is necessary for sustained oscillatory activity, but its importance varies within the oscillatory cycle: the slow interspike rise in [Ca2+]i requires Ca2+ entry, but the rapid upstroke does not, indicating that it reflects internal Ca2+ release. Sudden alterations in [Ca2+]o, [K+]o, or [caffeine]o produce immediate changes in d[Ca2+]i/dt and provide information about the relative rates of surface membrane Ca2+ transport as well as uptake and release by internal stores. Based on our results, [Ca2+]i oscillations can be explained in terms of coordinated changes in Ca2+ fluxes across surface and store membranes

Influx of Ca2+ through NMDA channels may initiate the stabilization of coactive synapses during development of the retinotectal projection in frogs. Ca2+ imaging techniques were applied to cultured tectal cells to investigate whether excitatory amino acids cause a rise in [Ca2+]i. High [K+], NMDA, and glutamate increase [Ca2+]i in about 75% of the cells. NMDA and glutamate responses were completely blocked in the absence of extracellular Ca2+ and by the NMDA receptor or channel blockers APV and MK-801. The NMDA response was also blocked by Mg2+. Quisqualate and kainate produced little or no rise in [Ca2+]i. These studies indicate that when tectal cells are exposed to the retinal ganglion cell transmitter glutamate, the predominant means of Ca2+ entry is through NMDA channels

A peptide toxin from a Conus marine snail, omega-conotoxin GVIA (omega-CgTx) has been used extensively as a probe for certain types of neuronal calcium channels. It is often assumed that omega-CgTx interacts with Ca2+ channels exclusively. We have tested this assumption in a study of omega-CgTx-binding sites in the electric organ of Discopyge ommata. Synaptosomal membranes from this tissue contain low affinity omega-CgTx receptor sites (Kd = 0.6 microM) in great abundance (280 pmol/mg of protein), as first reported by Ahmad and Miljanich (Ahmad, S. N., and Miljanich, G.P. (1988) Brain Res. 453, 247-256). However, we find that a large majority of these omega-CgTx-binding sites co-purify with the nicotinic acetylcholine receptor (nAChR) and can be immunoprecipitated by monoclonal antibodies generated against the nAChR of Torpedo. Cross-linking experiments with radiolabeled omega-CgTx show pronounced specific labeling of the alpha-subunit of the nAChR but not other subunits. Specific omega-CgTx binding to the nAChR is reduced by millimolar Ca2+ but not by alpha- or kappa-bungarotoxin, alpha-conotoxin, or carbamylcholine. Cross-linking experiments also reveal omega-CgTx-binding proteins of 170 and 60 kDa. The characteristics of the 170-kDa protein make it a likely candidate for the alpha 1-subunit of an N-type Ca2+ channel

Voltage-dependent Ca2+ channels regulate Ca2+ entry and thereby contribute to Ca2+ signalling in many cells. Functional studies have uncovered several types of Ca2+ channel, distinguished by pharmacology, electrophysiology and tissue localization. More recently, molecular cloning has revealed an even greater diversity among Ca2+ channels, arising from multiple genes and alternative splicing. L-type, dihydropyridine-sensitive Ca2+ channels have been the most extensively characterized to date. Recently, Numa's group has reported the cloning and expression of a dihydropyridine-insensitive Ca2+ channel from brain that most closely resembles the P-type channel described by Llinas and colleagues. These results contribute to rapidly growing knowledge about molecular determinants of Ca2+ channel diversity