Voltage‐Dependent Calcium Conductances in Mammalian Neurons

Abstract

Calcium plays several critical roles in the electrophysiology of mammalian central neurons. As a charge carrier, it is capable of generating either action potentials or graded voltage responses.' As a second messenger, calcium is involved in such events as transmitter release: the activation of ionic channels (e.g., the calcium-dependent potassium conductance'), and the phosphorylation of molecules, which in turn modulate ionic conductances.' Here we plan to review briefly some aspects of the voltagedependent calcium conductance in the neurons of the mammalian brain with particular emphasis on the conductance present in dendrites of cerebellar Purkinje cells.Before describing in detail the voltage-dependent channel properties of Purkinje cells, we will review in general terms the main calcium-dependent electroresponsiveness encountered in mammalian neurons. The first description of calcium-dependent spikes in vertebrate central nervous system (CNS) neurons was obtained by direct recordings from avian cerebellar Purkinje cell dendrites5 This was later confirmed in mammilary neurons.6 The existence of more than one voltage-dependent calcium conductance was originally encountered in the inferior olive (10). These cells demonstrated two types of responses, the high-and the low-threshold spikes (HTS and LTS, respectively).'.' Because the most common electroresponsiveness observed in intracellular recordings from different types of central neurons fall into these categories,' we will continue to describe them as stated above.By contrast the single channel responsible for the low-threshold calcium conductance was first described by Carbone and Lux? Later the calcium channels were grouped into three categories": (a) the T channels, which we now believe correspond to the low-threshold calcium conductance; (b) the N channels, which correspond to a certain extent to the high-threshold calcium conductance; and (c) the L calcium channel, which does not seem to be very commonly represented in the CNS, but which would also fall into this category of high-threshold calcium conductance. A simple incorporation of our nomenclature into the framework proposed above"' may prove to be problematic because the criteria for these two characterizations are so different. The single-channel criteria are based on direct measurements and do not consider cable properties. Ours was developed from the standpoint of neuronal integration, which takes into account parameters such as the spatial location of the 103

An Examination of Calcium Current Function on Heterotopic Neurons in Hippocampal Slices from Rats Exposed to Methylazoxymethanol

“…In some voltage-clamp experiments, pharmacologic agents were added to the perfusion medium: cadmium (CdCl 2 M) (from Alomone labs). In experiments using CdCl 2 , phosphate was omitted and MgSO 4 was substituted for MgCl 2 to prevent the precipitation of cadmium.…”

“…Several voltage-dependent calcium channel subtypes have been recognized in the central nervous system (CNS) (2)(3)(4)(5), each with characteristic voltage dependence, pharmacology, and single-channel kinetic properties. Often more than one voltage-dependent calcium current (VDCC) type coexists in the same neuron (6,7).…”

Migraine, ataxia and epilepsy: a challenging spectrum of genetically determined calcium channelopathies

“…24,28 The characteristics of the different calcium channel types are primarily correlated with the different α 1 isoforms. 29 The α 1A subunit encodes P and Q type calcium channels which were originally identified in cerebellar Purkinje cells 30 and granule cells. 31 P and Q type calcium channels differ in inactivation kinetics possibly due to α 1A subunit splice variants, 32 post-translational modification, or the influence of an auxiliary subunit.…”

Effects of hypomagnesia on transmitter actions in neocortical slices

“…The hyperpolarization during hypomagnesia may be attributed to an interruption of tonic, blocking actions of Mg on resting Ca-activated Kchannels, (cf. Llinas & Walton, 1980;Begenisich, 1988 (Heinonen & Akerman, 1986;Baker & Crawford, 1972).…”