Phospholipases A2 represent the most abundant family of snake venom proteins. They manifest an array of biological activities, which is constantly expanding. We have recently shown that a protein bitanarin, isolated from the venom of the puff adder Bitis arietans and possessing high phospholipolytic activity, interacts with different types of nicotinic acetylcholine receptors and with the acetylcholine-binding protein. To check if this property is characteristic to all venom phospholipases A2, we have studied the capability of these enzymes from other snakes to block the responses of Lymnaea stagnalis neurons to acetylcholine or cytisine and to inhibit α-bungarotoxin binding to nicotinic acetylcholine receptors and acetylcholine-binding proteins. Here we present the evidence that phospholipases A2 from venoms of vipers Vipera ursinii and V. nikolskii, cobra Naja kaouthia, and krait Bungarus fasciatus from different snake families suppress the acetylcholine- or cytisine-elicited currents in L. stagnalis neurons and compete with α-bungarotoxin for binding to muscle- and neuronal α7-types of nicotinic acetylcholine receptor, as well as to acetylcholine-binding proteins. As the phospholipase A2 content in venoms is quite high, under some conditions the activity found may contribute to the deleterious venom effects. The results obtained suggest that the ability to interact with nicotinic acetylcholine receptors may be a general property of snake venom phospholipases A2, which add a new target to the numerous activities of these enzymes.

a-Conotoxins are a group of relatively short peptides (12-19 amino acid residues, two disulfide bridges) from the venom of poisonous marine snails of the Conus

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In certain Aplysia neurons, glutamate, GABA, and acetylcholine (ACh) all elicit desensitizing Cl-dependent responses. This fact and the finding that the glutamate and GABA responses "crossdesensitize" led to the suggestion (Swann and Carpenter, 1975;King and Carpenter, 1987) that the responses to these transmitters were mediated by the same receptor-channel complex.This hypothesis is incompatible with the demonstration given here that the GABA-and glutamate-gated channels are clearly distinct; the GABA channel, but not the glutamate channel, shows outward rectification (Matsumoto, 1982; Carpenter, 1987, 1989) and is selectively blocked by intracellular sulfate. Exploiting these distinctive characteristics and the independent expression of the receptors in some cells, we have been able to reevaluate the so-called cross-desensitization by analyzing the ability of GABA, glutamate, and other agonists to interact with each of the receptor molecules.The cross-desensitization was found to be exclusively attributable to the ability of GABA to interact with the glutamate receptor (Oyama et al., 1990). The GABA receptor is unaffected by glutamate. Nevertheless, in cells expressing both receptors, glutamate can reduce the GABA response by auto-desensitizing the part of the response that is mediated by the glutamate receptor. No interactions were observed between ACh-induced responses and either of the responses elicited by the amino acids.The invertebrate glutamate-gated Cl channels that have been cloned resemble the vertebrate glycine receptor (Vassilatis et al., 1997). Our pharmacological evaluation of the molluscan glutamate receptor points in the same direction.

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Nicotinic acetylcholine receptors (nAChRs) fulfill a variety of functions making identification and analysis of nAChR subtypes a challenging task. Traditional instruments for nAChR research are d-tubocurarine, snake venom protein α-bungarotoxin (α-Bgt), and α-conotoxins, neurotoxic peptides from Conus snails. Various new compounds of different structural classes also interacting with nAChRs have been recently identified. Among the low-molecular weight compounds are alkaloids pibocin, varacin and makaluvamines C and G. 6-Bromohypaphorine from the mollusk Hermissenda crassicornis does not bind to Torpedo nAChR but behaves as an agonist on human α7 nAChR. To get more selective α-conotoxins, computer modeling of their complexes with acetylcholine-binding proteins and distinct nAChRs was used. Several novel three-finger neurotoxins targeting nAChRs were described and α-Bgt inhibition of GABA-A receptors was discovered. Information on the mechanisms of nAChR interactions with the three-finger proteins of the Ly6 family was found. Snake venom phospholipases A2 were recently found to inhibit different nAChR subtypes. Blocking of nAChRs in Lymnaea stagnalis neurons was shown for venom C-type lectin-like proteins, appearing to be the largest molecules capable to interact with the receptor. A huge nAChR molecule sensible to conformational rearrangements accommodates diverse binding sites recognizable by structurally very different compounds.

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