Serotonin receptor mRNA expression in the hypoglossal motor nucleus

Brainstem serotonin (5-HT)-containing cells are remarkable for their widespread axonal projections and having their highest activity during wakefulness and lowest during rapid eye movement sleep. One important site of action of 5-HT is on upper airway motoneurons. However, which of the 14 known 5-HT receptors mediate the effects is uncertain. We used the reverse transcriptase/polymerase chain reaction to detect mRNA for six distinct 5-HT receptors (1A, 1B, 2A, 2C, 3 and 7) in 50 nl micro-punches collected from the hypoglossal (XII) motor nucleus and, for comparison, from the viscerosensory nucleus of the solitary tract (NTS) in adult rats. The relative abundance of the distinct mRNAs was characterized by the minimal number of amplification cycles (25-40) necessary to detect a given mRNA. In the XII nucleus, mRNA for type 1B, 2A and 2C receptors was detectable after 29-31 cycles, detection of type 3 and 7 receptor mRNA required 33-35 cycles; and type 1A receptor mRNA was not detected. In the NTS, detection of mRNA for type 1B, 2C and 7 receptors required 31-33 cycles; type 1A receptor mRNA required 39 cycles; and type 2A receptor mRNA was not detected. The data from the XII nucleus demonstrate that not only the previously recognized type 1B, 2A and 2C receptors, but also type 3 and 7 receptors have the potential to mediate serotonergic effects in XII motoneurons.