Ionic mechanisms underlying excitatory effects of serotonin on embryonic rat motoneurons in long-term culture

The actions of serotonin were investigated on motoneurons isolated from emb ryonic day 14 rat spinal cord and enriched by metrizamide density gradient centrifugation. Trophic support was provided by a spinal cord glial monolay er, ciliary neurotrophic factor and heat-inactivated serum. Cultures were m aintained for 17-83 days and investigated using whole-cell patch-clamp reco rding. Serotonin evoked slow depolarizations (6.2 +/- 0.7 or 9.3 +/- 1.3 mV in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione and strychnine, EC 50 8.2 nM), which were reversibly blocked by 0.1 mu M ketanserin. Serotonin generated synaptic potentials in motoneurons, lowered the threshold for re petitive firing and changed the slope of the current intensity-firing frequ ency relationship. The inward current evoked by serotonin (-147 +/- 15.2 pA ) was ascribed to a complex ionic mechanism, which varied amongst neurons i n the sampled population. It was due to closure of barium-sensitive potassi um channels, effects on Ih and increase in a separate mixed cation current which comprised both transient voltage-sensitive and sustained components. We conclude that serotonergic responses develop in motoneurons cultured und er these conditions in the absence of serotonergic input, sensory neurons o r many interneurons. (C) 1999 IBRO. Published by Elsevier Science Ltd.