Inositol 1,4,5-trisphosphate (IP3)-induced calcium release from intracellular stores is a regulator of cytosolic-free calcium levels. The subsecond kinetics and regulation of IP3-induced calcium-45 release from synaptosome-derived microsomal vesicles were resolved by rapid superfusion. Extravesicular calcium acted as a coagonist, potentiating the transient IP3-induced release of calcium-45. Thus, rapid elevation of cytosolic calcium levels may trigger IP3-induced calcium release in vivo. Extravesicular calcium also produced a more slowly developing, reversible inhibition of IP3-induced calcium-45 release. Sequential positive and negative feedback regulation by calcium of IP3-induced calcium release may contribute to transients and oscillations of cytosolic-free calcium in vivo.

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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.

The regulation of excitation-secretion coupling by Ca2+ channels is a fundamental property of the nerve terminal. Peptide toxins that block specific Ca2+ channel types have been used to identify which channels participate in neurotransmitter release. Ca2+ channels in the nerve terminal regulate excitationsecretion coupling by controlling the entry of Ca2+ necessary for exocytosis (1). Multiple Ca2+ channel types in mammalian central neuron somata have been described (2-4), and several types can be defined based on their sensitivity to specific antagonists. One such antagonist, w-conotoxin GVIA (eoCgTx), was originally identified as an irreversible blocker of presynaptic release at the frog neuromuscular junction (5) and has been shown to specifically block N-type Ca2+ channels (6, 7). Subsequent work showed that neurotransmitter release from peripheral neurons (8, 9) and nerve terminal preparations (synaptosomes) from rat brain (10-14) was partially blocked by co-CgTx but not by 1,4-dihydropyridine antagonists that are specific for L channels. These results led to the widely accepted notion that neurosecretion is regulated primarily if not exclusively by N channels (15). co-CgTx can also block synaptic transmission in brain slice preparations (16)(17)(18), but the block is incomplete and in one case (18) was overcome by increased stimulus intensity. The partial block of neurotransmitter release as well as synaptic transmission suggested that o-CgTx-resistant channels may mediate excitation-secretion coupling at central synapses in some cases.More recently, co-CgTx-resistant Ca2+ channels have been described in mammalian brain. One such channel, the P type, was first characterized in Purkinje neurons, where it is the predominant Ca2+ channel (19). P channels, which have subsequently been found in many other regions, are specifThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.ically blocked by w-Aga-IVA, a peptide purified from the venom of Agelenopsis aperta (20). We demonstrated previously (21) that synaptosomal [3H]glutamate release was resistant to co-CgTx and partially but potently blocked by w-Aga-IVA, providing evidence for a role for P channels at glutamatergic synapses. We now report that [3H]dopamine release evoked by low levels of depolarization is partially blocked by w-CgTx as well as by w-Aga-IVA. However, under conditions of strong depolarization where neither toxin is very effective alone, a combination of nanomolar concentrations of both toxins is synergistic, producing substantial block of dopamine release. This.observation suggests that in striatum, P-type and N-type Ca2+ channels coexist and regulate secretion from dopaminergic nerve terminals, while P-type and a toxin-resistant Ca2+ channel type coexist to regulate glutamate release. This arrangement has significant implications for the regulation of excitation-secretion coupling...