Monoamines increase the excitability of spinal neurones in the neonatal rat by hyperpolarizing the threshold for action potential production

During fictive locomotion in the adult decerebrate cat, motoneurone excitability is increased by a hyperpolarization of the threshold potential at which an action potential is elicited (V_th). This lowering of V_th occurs at the onset of fictive locomotion, is evident for the first action potential elicited and is presumably caused by a neuromodulatory process. The present study tests the hypothesis that the monoamines serotonin (5-HT) and noradrenaline (NA) can hyperpolarize neuronal V_th. The neonatal rat isolated spinal cord preparation and whole-cell recording techniques were used to examine the effects of bath-applied 5-HT and NA on the V_th of spinal ventral horn neurones. In the majority of lumbar ventral horn neurones, 5-HT (13/26) and NA (10/16) induced a hyperpolarization of V_th ranging from -2 to -8 mV. 5-HT and NA had similar effects on V_th for individual neurones. This hyperpolarization of V_th was not due to a reduction of an accommodative process, and could be seen without changes in membrane potential or membrane resistence. These data reveal a previously unknown action of 5-HT and NA, hyperpolarization of V_th of spinal neurones, a process that would facilitate both neuronal recruitment and firing.