Extracellular or intracellular application of argiotoxin-636 has inhibitory actions on membrane excitability and voltage-activated currents in cultured rat sensory neurones

The whole cell variant of the patch clamp technique was used to investigate the actions of the polyamine amide spider toxin, argiotoxin-636, on the ex citability of cultured dorsal root ganglion neurones. Synthesized argiotoxi n-636 (0.1 - 100 mu M) reduced neuronal excitability when applied to the ex tracellular environment by low pressure ejection or to the intracellular en vironment via the patch pipette solution. The toxin prolonged the duration of evoked action potentials and reduced the peak amplitude of action potent ials. Intracellular and extracellular application of argiotoxin-636 also de creased the number of action potentials evoked in response to 800-ms depola rizing current commands. This action of the toxin was mimicked by 100 mu M tetraethylammonium. Extracellular application of argiotoxin-636 inhibited v oltage-activated K+ currents ill a dose-dependant manner over the complete voltage range. This inhibition occurred without any significant changes in the voltage dependence of activation or inactivation. Intracellular applica tion of argiotoxin-636, during 5-10 min of whole cell recording, also inhib ited voltage-activated K+ currents without changing the voltage dependence of activation or steady-state inactivation. Extracellular or intracellular spermidine (250 mu M) reversibly attenuated the inhibitory actions of extra cellular argiotoxin-636. Argiotoxin-636 also inhibited voltage-activated Na + currents: this effect was dependent on repeated activation of the current s and the period during which the neurones were ill culture. We conclude th at application of argiotoxin-636 to either the extracellular or intracellul ar environment reduced excitability of cultured sensory neurones from neona tal rats and that this involved inhibition of both voltage-activated K+ and Na+ currents. The data suggest that the toxin was more effective at attenu ating action potentials when neurones were repeatedly excited. and that acc ess to inhibitory sites of action oil the voltage-activated ion channels ca n be achieved from the inside of the neurone, (C) 1998 Elsevier Science Ltd . All rights reserved.