Serotonergic excitation of sympathetic preganglionic neurons: a microiontophoretic study

The effects of microiontophoretically applied serotonin on the extracellularly recorded discharges of sympathetic preganglionic neurons (SPNs) were studied in anesthetized cats. Thoracic SPNs were identified on the basis of constancy of antidromic activation and collision. Low ejecting currents of serotonin (5-30 nA) invariably excited spontaneously active SPNs. Serotonin also excited the vast majority of quiescent SPNs, as well as neurons brought to discharge threshold by the excitatory amino acid L-glutamate. A population of SPNs was identified which was insensitive to the excitatory effects of both serotonin and L-glutamate. Iontophoretic or intravenous administration of the putative serotonin antagonists methysergide and metergoline blocked the excitatory effects of serotonin on SPNs. The blockade of the serotonin-induced excitation was not associated with a local anesthetic action of methysergide or metergoline. Methysergide and metergoline also reduced the firing rate of SPNs in intact but not in spinal animals. These data provide strong evidence to support the contention that serotonergic neurons provide a tonic excitatory input to SPNs.