Extracellular and intracellular actions of azadirachtin on the electrophysiological properties of cultured rat DRG neurones

Abstract

The distinct electrophysiological actions of extracellular and intracellular azadirachtin (type A) were investigated using cultured dorsal root ganglion neurones from neonatal rats and the whole cell variant of the patch clamp technique. The cultured mammalian neurones were relatively insensitive to azadirachtin compared with some invertebrate preparations, such as insect chemosensory systems. Insect preparations have been found to respond to 1–100 nM azadirachtin. However, at concentrations of azadirachtin between 10 and 100 μM significant changes in the electrophysiological properties of cultured DRG neurones were seen. Extracellular application of azadirachtin prolonged the late repolarization phase of evoked action potentials and consistent with this produced modulation of voltage-activated K + currents. This modulation involved an initial transient increase in K + current followed by reversible inhibition when azadirachtin was applied at a concentration of 10 μM. Higher concentrations of azadirachtin (50 and 100 μM), had only inhibitory effects on voltage-activated K + currents. Azadirachtin produced a degree of voltage-dependent inhibition, with a greater level of K + current inhibition observed when neurones were held at −90 mV compared with −40 mV. Prolonged application of azadirachtin for 24 h reversibly reduced action potential amplitude. In contrast intracellular application of 100 μM azadirachtin via the patch pipette solution had no significant effects on action potentials but produced an increase in conductance. Intracellular azadirachtin activated several conductances including a TEA-sensitive K + current and an inward current that had an estimated reversal potential close to 0 mV. In conclusion, data showed that azadirachtin had different effects when it was applied inside and outside the neurones and that azadirachtin at high concentrations reversibly altered neuronal excitability mainly by modulating potassium conductances. It appears that rat cultured DRG neurones are less affected by azadirachtin than some insect sensory systems.