Calcium Channels Coupled to Glutamate Release Identified by ω-Aga-IVA

Abstract

Presynaptic calcium channels are crucial elements of neuronal excitation-secretion coupling. In mammalian brain, they have been difficult to characterize because most presynaptic terminals are too small to probe with electrodes, and available pharmacological tools such as dihydropyridines and omega-conotoxin are largely ineffective. Subsecond measurements of synaptosomal glutamate release have now been used to assess presynaptic calcium channel activity in order to study the action of peptide toxins from the venom of the funnel web spider Agelenopsis aperta, which is known to inhibit dihydropyridine and omega-conotoxin-resistant neuronal calcium currents. A presynaptic calcium channel important in glutamate release is shown to be omega-Aga-IVA sensitive and omega-conotoxin resistant.

Phosphorylation of synapsin I and MARCKS in nerve terminals is mediated by Ca2+ entry via an Aga‐GI sensitive Ca2+ channel which is coupled to glutamate exocytosis

“…Under the present conditions of stimulation, no modulation of glutamate release is observed with L, N or P type voltage dependent Ca 2+ channel antagonists [4,7]. Thus the channel coupled to release and kinase activation is a specific subtype which can be blocked by Aga-GI.…”

“…We have isolated a novel spider toxin, Aga-GI, and established that the Ca 2÷ trigger mediating glutamate exocytosis from mammalian cerebrocortical synaptosomes occurs via voltage dependent Ca 2÷ channels sensitive to Aga-GI, and not via L-or N-type voltage dependent Ca 2÷ channels [4,5]. The action of Aga-GI is distinct from 09Aga-IVA in that Aga-GI can completely inhibit glutamate exocytosis under stimulatory conditions which optimize release [6,7]. However, at a concentration of Aga-GI which completely blocks KCl-evoked glutamate exocytosis, only a partial inhibition of KCl-evoked cytoplasmic free Ca 2÷ elevation, [Ca2+]c, is observed [4].…”

ω‐agatoxin IVA blocks nicotinic receptor channels in bovine chromaffin cells

“…This indicates that P channels are located in the vicinity of the nicotinic receptor and so would be preferentially activated by nicotinic stimulation. This suggests that P-type voltage-dependent Ca 2÷ channels may play a role in secretion in chromaffin cells similar to other systems [26,27]. However, since co-AgaTx did not affect high K+-induced secretion, an effect of the toxin on a target different from voltage-dependent P-type Ca 2÷ channels should be excluded.…”

Presynaptic plasticity: The regulation of Ca2+-dependent transmitter release

“…The agatoxin sensitive Ca 2 + channels may consist of several subtypes (Bowers et al, 1987;Llinas et al, 1989;Bindokas and Adams, 1989;Adams et al, 1989; see for a review, Jackson and Parks, 1989) and may be widely expressed in nerve terminals (Bowers et al, 1987: Llinas et al, 1989Lundy et al, 1992;. Recent evidence suggest that this toxin specifically blocks the secretion of fast-acting transmitters (Llinas et al, 1989;Bindokas et al, 1991;Pocock and Nicholls, 1992;Turner, 1992). From these data the picture emerges that co-conotoxin sensitive Ca 2 ~ channels (N-type) may be involved in secretion of monoamines, but not of the fast-acting amino acid transmitters.…”