The detection of a nicotinic receptor antagonist in V. dubius venom extends the range of theraphosid venoms known to affect vertebrate neurotransmission in vitro. Previous work has shown that Selenocosmia huwena venom contains a toxin (HWTX-I) that causes irreversible (by washing) neuromuscular blockade in chick biventer cervicis preparations by interacting with AZD2281 nicotinic receptors but has no effect on the responses to direct muscle stimulation, i.e., HWTX-I does not affect the muscle contractile mechanisms ( Liang et al., 1993; Zhou et al., 1997).

The venom of the giant Amazonian spider Theraphosa blondii produces fast, potent neuromuscular blockade in mouse phrenic nerve-diaphragm preparations at a concentration of 7.5 μg/mL ( Fontana et al., 2002). The authors attributed this activity to the presence of a nicotinic receptor blocker since the venom abolished miniature end-plate potentials but did not interfere

with the muscle responses to direct stimulation. More recently, Herzig and Hodgson (2009) reported that the venom of the Australian theraphosid Coremiocnemis tropix produced complete neuromuscular blockade in chick biventer cervicis preparations at a concentration of 10 μg/mL. In contrast to V. dubius venom, that of C. tropix caused a significant decrease in the baseline tension of the preparations, indicating relaxation of skeletal muscle. Together, these studies indicate that while theraphosid venoms contain compounds Unoprostone capable of interacting with post-synaptic nicotinic receptors SB203580 in vivo they differ in their ability to directly affect the contractility of skeletal muscle, i.e., S. huwena and T. blondi venom have no effect, C. tropix venom produces relaxation and V. dubius venom produces contracture. Such divergent effects show

that these venoms may contain more than one component capable of affecting vertebrate neurotransmission and muscle contractility. Spider venoms contain a variety of low molecular mass compounds, particularly peptides, capable of interacting with ion channels and receptors (Escoubas and Rash, 2004; Escoubas, 2006). In addition, (acyl)polyamines have also been identified in these venoms (Skinner et al., 1990). The photosensitivity of VdTX-1, which was similar to that of polyamines described by Choi et al. (1995) in venoms of Argiopidae spiders, and its low molecular mass suggest that this toxin may be an (acyl)polyamine. Although the molecular mass of (acyl)polyamines is generally 300–450 Da, Palma et al. (1997) described polyamines of up to 744 Da in the venom of the araneomorph spider Nephilengys cruentata; the mass of VdTX-1 (728 Da) is within this range. Wasp polyamines, such as the philanthotoxins from the digger wasp P. triangulum ( Rozental et al., 1989), are well-known nicotinic non-competitive blockers.