Faculty Bibliography

The number of large intramembrane particles associated with sites of synaptic vesicle release at the squid giant synapse was determined and compared to the average maximal presynaptic calcium current in order to derive an estimate of the conductance each particle would have if it were a calcium channel. This value, 0.21 pS, compares favorably with conductances of calcium channels in other preparations, substantiating the idea that the large intramembrane particles, which are concentrated at 'active zones,' represent calcium channels.

The electrophysiological properties of the high- and low-threshold Ca spikes described in inferior olivary neurones were analysed in detail. 1. During hyperpolarization the low- and high-threshold Ca action potentials can coexist as two distinct spikes, demonstrating non-mutual exclusion. 2. The high-threshold Ca spike shows a lack of refractoriness, is generated remotely from the site of recording and is composed of several all-or-none components, the last two properties suggesting a dendritic origin. 3. Hyperpolarization of the neurones allows the activation of the low-threshold Ca spike, which has activation properties resembling those of the early K conductance described in invertebrates. This low-threshold Ca spike shows refractoriness. 4. The relation between membrane polarization and low-threshold Ca spike is S-shaped. Low-threshold Ca spikes become apparent at -70 mV and have a maximum rate of rise (saturation) at polarization levels more negative than -85 mV. Thus, hyperpolarization removes a voltage-dependent Ca inactivation which is present at normal resting membrane potential (-65 mV). 5. Replacement of extracellular Ca by Ba or addition of tetraethylammonium to the bath corroborates the lack of fast inactivation for the high-threshold Ca spike and the inactivation properties of the low-threshold Ca conductance. It also demonstrates that the duration of the after-depolarization is determined by an interplay between inward Ca current and both voltage-dependent and Ca-dependent K currents. 6. Extracellular recordings from single cells indicate that the Na-dependent spike and the low-threshold Ca action potential are somatic in origin, while the high-threshold Ca spike (after-depolarization) and the hyperpolarization that follows are apparently located in the dendrites. 7. The ionic conductances comprise the main components of the oscillatory behaviour of these cells. The sequence of events leading to oscillation entails initially a low-threshold Ca spike or Na spike, followed by an after-depolarization/after-hyperpolarization sequence and then a post-anodal exaltation product by a rebound low-threshold Ca spike.

A new technique is described that allows neurobiological research in mammalian brain in vitro. The approach utilizes the vascular system to irrigate portions of the brain-in this case the brain stem and cerebellum 'en block.' The preparation, which can survive for about 10 hours, demonstrates normal field potentials following stimulation of either the surface or the underlying white matter at both cerebellar and brain stem levels. Intracellular studies at both these levels indicate cellular activity in every way similar to the in vivo or the slice preparation from the same regions. This new technique offers potential for the study of ionic mechanisms underlying electrical activity as well as neurochemistry, neuroanatomy, neuropharmacology, and neuroendocrinology.