Neuronal voltage-activated calcium channels: on the roles of the alpha 1E and beta 3 subunits

Calcium regulation of spontaneous and asynchronous neurotransmitter release

The molecular machinery underlying action potential-evoked, synchronous neurotransmitter release, has been intensely studied. It was presumed that two other forms of exocytosis, delayed (asynchronous) and spontaneous transmission, were mediated by the same voltage-activated Ca(2+) channels (VACCs), intracellular Ca(2+) sensors and vesicle pools. However, a recent explosion in the study of spontaneous and asynchronous release has shown these presumptions to be incorrect. Furthermore, the finding that different forms of synaptic transmission may mediate distinct physiological functions emphasizes the importance of identifying the mechanisms by which Ca(2+) regulates spontaneous and asynchronous release. In this article, we will briefly summarize new and published data on the role of Ca(2+) in regulating spontaneous and asynchronous release at a number of different synapses. We will discuss how an increase of extracellular [Ca(2+)] increases spontaneous and asynchronous release, show that VACCs are involved at only some synapses, and identify regulatory roles for other ion channels and G protein-coupled receptors. In particular, we will focus on two novel pathways that play important roles in the regulation of non-synchronous release at two exemplary synapses: one modulated by the Ca(2+)-sensing receptor and the other by transient receptor potential cation channel sub-family V member 1.

Modulation of neuronal voltage-activated calcium and sodium channels by polyamines and pH

The endogenous polyamines spermine, spermidine and putrescine are present at high concentrations inside neurons and can be released into the extracellular space where they have been shown to modulate ion channels. Here, we have examined polyamine modulation of voltage-activated Ca(2+) channels (VACCs) and voltage-activated Na(+) channels (VANCs) in rat superior cervical ganglion neurons using whole-cell voltage-clamp at physiological divalent concentrations. Polyamines inhibited VACCs in a concentration-dependent manner with IC(50)s for spermine, spermidine, and putrescine of 4.7 +/- 0.7, 11.2 +/- 1.4 and 90 +/- 36 mM, respectively. Polyamines caused inhibition by shifting the VACC half-activation voltage (V(0.5)) to depolarized potentials and by reducing total VACC permeability. The shift was described by Gouy-Chapman-Stern theory with a surface charge density of 0.120 +/- 0.005 e(-) nm(-2) and a surface potential of -19 mV. Attenuation of spermidine and spermine inhibition of VACC at decreased pH was explained by H(+) titration of surface charge. Polyamine-mediated effects also decreased at elevated pH due to the inhibitors having lower valence and being less effective at screening surface charge. Polyamines affected VANC currents indirectly by reducing TTX inhibition of VANCs at high pH. This may reflect surface charge induced decreases in the local TTX concentration or polyamine-TTX interactions. In conclusion, polyamines inhibit neuronal VACCs via complex interactions with extracellular H(+) and Ca. Many of the observed effects can be explained by a model incorporating polyamine binding, H(+) binding and surface charge screening.

Molecular physiology and pathology of Ca2+-conducting channels in the plasma membrane of mammalian sperm

Current evidence indicates that mechanisms controlling the intracellular Ca2+concentration play pivotal roles in determining sperm fertilizing ability. Multiple Ca2+-permeable channels have been identified and characterized in the plasma membrane and in the acrosome membrane of mammalian sperm. This review summarizes the recent findings and assesses the evidence suggesting that these channels play roles in controlling a host of sperm functions ranging from motility to the acrosome reaction, and describes recent advances in the identification of the underlying gene defects of inherited sperm Ca2+channelopathies.

Alpha1E-containing Ca2+ channels are involved in synaptic plasticity

Long-term potentiation (LTP) is the most prominent model for the molecular and cellular mechanisms of learning and memory. Two main forms of LTP have been distinguished. The N-methyl-D-aspartate-receptor-dependent forms of LTP have been studied most extensively, whereas much less is known about N-methyl-D-aspartate-receptor-independent forms of LTP. This latter type of LTP was first described at the mossy fiber synapses in the hippocampus and subsequently at parallel fiber synapses in the cerebellum as well as at corticothalamic synapses. These presynaptic forms of LTP require a rise in the intraterminal calcium concentration, but the channel through which calcium passes has not been identified. By using pharmacological tools as well as genetic deletion, we demonstrate here that alpha1E-containing voltage-dependent calcium channels (VDCCs) shift the threshold for mossy fiber LTP. The channel is not involved in the expression mechanism, but it contributes to the calcium influx during the induction phase. Indeed, optical recordings directly show the presence and the function of alpha1E-containing VDCCs at mossy fiber terminals. Hence, a previously undescribed role for alpha1E-containing VDCCs is suggested by these results.

Calcium current components in intact and dissociated adult mouse sympathetic neurons

We examined which types of high threshold Ca(2+) channels are activated by depolarization in intact and dissociated sympathetic neurons from adult mouse superior cervical ganglia (SCG). Ba(2+) currents were recorded with microelectrodes and discontinuous voltage clamp from neurons in intact ganglia, and using the perforated patch clamp technique in dissociated cells. Peak current was larger in intact neurons, although the voltage dependence was similar. Successive application of omega-conotoxin GVIA, omega-conotoxin MVIIC and nifedipine revealed that the total current in intact cells was composed by 29% N-type, 13% P/Q-type, 32% L-type and 26% resistant to blockade (R-type). In dissociated cells, the N component was larger and the L component smaller, whereas P/Q-type and R-type were similar. Peak currents evoked with an action potential waveform instead of a square pulse were larger in both preparations but the proportions of each component were similar. We conclude that dissociating and culturing somata results in data that only partially reflect the situation in intact neurons. Assuming that the main effect of dissociation is the removal of mature dendritic membrane, the data suggest that L channels are more abundant on dendrites and N channels on the soma of intact sympathetic neurons, whereas P/Q and R channels may be uniformly distributed over the cell surface. Finally, in intact SCG neurons from rats, the proportions of current evoked by a pulse were: 49% N-type, 11% P/Q-type, 21% L-type and 20% R-type when nifedipine was applied last, suggesting that there are species differences in the expression of L and N channels.