Effects of clonidine on the lumbar sympathetic pathways to the large intestine and urinary bladder of the cat.

The effects of clonidine on lumbar sympathetic outflow to the colon and urinary bladder were examined by recording drug-induced changes in spontaneous postganglionic firing and spinal viscero-sympathetic reflexes in the lumbar colonic and hypogastric nerves of the cat. Clonidine in doses between 2--40 micrograms/kg produced a dose dependent depression of spontaneous and evoked discharges in efferents of the hypogastric nerve but did not alter activity in the lumbar colonic nerves. Clonidine also reduced arterial blood pressure and spontaneous firing in the renal nerves. These data coupled with previous observations in this laboratory indicate that spinal sympathetic pathways can exhibit a considerable variation in the sensitivity to the depressant actions of clonidine. This variation in sensitivity may reflect the relative importance of noradrenergic inhibitory mechanisms in the respective pathways.