Inhibition of efferent sympathetic nerve activity by centrally administered paraoxon in the cat

We recently found that central administration of the cholinesterase inhibitor paraoxon lowered blood pressure substantially. It was postulated that the decrease in pressure was mediated by a reduction of sympathetic outflow. In the present study, efferent splanchnic nerve activity in anaesthetized and paralysed cats was recorded, and quantified by measuring the variance of signal amplitude. After administration of 8 micrograms paraoxon into the vertebral arteries, blood pressure and splanchnic nerve activity decreased simultaneously. A mean fall of 46 +/- 6% and 45 +/- 13% (mean +/- S.E.M.) respectively was reached within 12 min and was maintained during the period studied (30 min). When the effect of paraoxon was antagonized by dexetimide, both blood pressure and splanchnic nerve activity returned to control values. Since previous work has shown that the depressor action could not be prevented by efferent vagal blockade it seems likely that the fall in blood pressure after paraoxon was mainly caused by a decreased sympathetic outflow. In addition, we varied the amplifier band width in recording splanchnic nerve activity. The measurement of frequencies between 10 and 225 Hz appeared to be sufficient for studying the change in activity after paraoxon.