Blockade of acetylcholine release at the motor endplate by a polypeptide from the venom of Phoneutria nigriventer

Abstract

1 The mechanisms underlying the muscle relaxation effect of a fraction (PF3) isolated from the Phoneutria nigriventer spider venom were assessed on mouse diaphragm and chick biventer cervicis muscle preparations. 2 PF3 (0.25-4 ug mli') produced a concentration-dependent blockade of the nerve-elicited muscle twitch of the mouse diaphragm (IC5o = 0.8 yg ml-') without affecting the directly induced muscle twitch.In similar preparations, the crude venom (1-10 yg ml-') produced muscle contracture and blocked both the direct and indirectly induced muscle twitches. 3 In the chick biventer cervicis muscle, PF3 (1-5 jg mli) blocked the nerve stimulated muscle twitch (IC50 = 1.26 yg ml-'), but did not alter the postjunctional response to exogenous acetylcholine (ACh, 10 PM-10 mM).

Promiscuous and Reversible Blocker of Presynaptic Calcium Channels in Frog and Crayfish Neuromuscular Junctions FromPhoneutria nigriventerSpider Venom

“…The lack of effect of -Ptx-IIA on MEPPs is similar to that observed with -CgTx-GVIA (personal observations) and suggests that -Ptx-IIA does not affect the postsynaptic muscle cells and does not by itself alter intracellular [Ca 2ϩ ]. This observation contrasts with previous work on mouse NMJs by Souccar et al (1995) showing that -Ptx-IIA reduced both amplitude and frequency of MEPPs. The difference in the effect on MEPP frequency may be due to differences in the membrane potential of the presynaptic terminals as discussed in Hua et al (1998).…”

“…For instance intraperitoneal injection of 3 g of -Ptx-IIA in 25 g mice causes flaccid paralysis and death (Rezende et al 1991). At mouse phrenic-hemidiaphragm NMJ (Souccar et al 1995) and rat brain striatal tissue (Troncone et al 1995), -Ptx-IIA caused strong blockade of neurotransmitter release. Both of these effects are likely to be due to blockade of P-type Ca 2ϩ channels that normally cause transmitter release at the mammalian NMJ (Katz et al 1997;Meir et al 1999;Protti et al 1996;Sugiura et al 1995) as well as in brain tissue (Carvalho et al 1995;Kimura et al 1995).…”

“…The venom of P. nigriventer contains many peptides with distinct pharmacological actions that can block Na ϩ , K ϩ , and Ca 2ϩ channels (Araujo et al 1993;Figueiredo et al 1995;and reviewed in Gomez et al 2002). A fraction of the venom called PF3 blocked brain dopamine release (Troncone et al 1995) and blocked acetylcholine (ACh) release at the mouse NMJ (Souccar et al 1995). The effect of PF3 at mouse NMJs was antagonized by increased concentration of extracellular Ca 2ϩ , and it was hypothesized that PF3 blocks presynaptic Ca 2ϩ channels.…”

Envenomation by Wandering Spiders (Genus Phoneutria)

“…In rat isolated phrenic nerve-diaphragm preparations, PNV causes muscle membrane depolarization with no change in the duration of muscle action potentials and an increase in the rate of spontaneous acetylcholine release (= increased amplitude and frequency of MEPPs); these responses are mediated by the activation of sodium channels since they are abolished by TTX (Fontana and Vital-Brazil 1985). Subsequent investigation of the PhTx3 (=PF3) group of toxins demonstrated neuromuscular blockade through the inhibition of acetylcholine release in mouse diaphragm, without affecting muscle responses to direct stimulation, in contrast to PNV that affected the responses to both direct and indirect stimulation; PhTx3 reduced the frequency and amplitude of MEPPs and also reduced the quantal content in mouse diaphragm muscle, effects that were considered to reflect diminished entry of extracellular Ca 2+ into the nerve terminals (Souccar et al 1995). In contrast to these findings with the PhTx3 fraction, individual toxins from this group, e.g., PhKv (PnTx3-1), can stimulate acetylcholine release from nerve terminals in rat phrenic nerve-diaphragm preparations (Almeida et al 2011).…”