The actions of philanthotoxin-343 on rat α 4β 2 neuronal nicotinic acetylcholine receptors

HA Kachel. University of Nottingham, Nottingham, UK

It is useful to understand the pharmacological and functional properties of the major hetero-oligomeric neuronal nicotinic acetylcholine receptor (N-nAChR), α4β2, in the mammalian central nervous system. Here, we studied the actions of philanthotoxin-343 (PhTX-343), a synthetic analogue of philanthotoxin-433, the active component of the Egyptian solitary digger wasp, Philanthus triangulum, venom, on rat α4β2 N-nAChR expressed in Xenopus oocytes. Whole-cell current elicited by application of ACh was measured electrophysiologically by using two-electrode voltage clamp at three different holding potentials (VH = -60 mV, -80 mV and -100 mV). PhTX-343 concentration-inhibition curves were constructed and IC50 values estimated for each holding potential. The IC50 value for PhTX-343 inhibition of peak α4β2 current at -100 mV was 0.18 µМ and this significantly increased to 1.41 µМ (p < 0.0001) and 1.87 µМ (p < 0.0001) at -80 mV and -60 mV, respectively. It is important to note that toxin potency was augmented by holding the cell at more negative VH indicating its voltage-dependent manner of action. This property supports the open channel blocker mechanism of N-nAChRs by PhTX-343 that requires gate opening first and then inhibition of ion flow through the nAChR channel. The IC50s were significantly lower than those obtained previously for muscle type nAChR. We conclude that PhTX-343 works as a potent open channel blocker of the α4β2 mammalian N-nAChR.