Postnatal development of serotonin 1B, 2 A and 2C receptors in brainstem motoneurons.

The effects of serotonin (5-HT) on motoneurons are mediated via multiple receptor subtypes. In hypoglossal (XII) motoneurons, the prototypic brainstem motoneurons whose functions change during the postnatal period, 5-HT effects evolve from inhibitory to excitatory, probably in association with changes in receptor expression. We studied 5-HT1B, 5-HT2A and 5-HT2C receptor mRNA in 414 dissociated XII motoneurons and 5-HT2A protein in the XII, facial and spinal cervical (C2-3) motor nuclei. The percentage of motoneurons expressing distinct mRNAs varied with the postnatal age (P3-33 days) and receptor subtype. Initially, 5-HT1B mRNA was present in 50-85% of cells, but on P14 its expression transiently decreased below 35%. 5-HT2A mRNA was present in nearly all cells after P6, but in less than 65% on P3-5. Normal and/or short splice variants of the 5-HT2C mRNA were expressed in less than 20% of motoneurons on P3-9, and in approximately 35% thereafter. 5-HT1B and 5-HT2A mRNAs often were expressed in different cells during early and intermediate postnatal periods, whereas 5-HT2C mRNA never occurred alone. The 5-HT2A receptor protein level gradually increased through P15 in the XII and facial nuclei, with dendritic labelling appearing in XII motoneurons only after P12. In spinal motoneurons, both somatic and dendritic labelling was strongest on P5 and then decreased. The development of 5-HT receptors in XII motoneurons may be related to changes in feeding behaviour, whereas different cues regulate 5-HT receptor expression in upper spinal motoneurons.