5-HT3A receptor subunit is expressed in a subpopulation of GABAergic and enkephalinergic neurons in the mouse dorsal spinal cord

5-Hydroxytryptamine (5-HT)3 receptors have been proposed to modulate nociception and pain responses at the spinal level. To gain insight into the cellular mechanism of 5-HT3 receptors, we examined their expression in GABAergic and enkephalinergic (ENKergic) neurons in the spinal dorsal horn (SDH) using single-cell reverse transcription-polymerase chain reaction (RT-PCR). The glutamic acid decarboxylase (GAD)67-green fluorescent protein (GFP) knock-in mouse was used in which all GABAergic neurons were fluorescent. The general tissue RT-PCR results showed that 5-HT3A receptor subunit mRNA was present in the mouse SDH, while 5-HT3B receptor subunit was absent. Single-cell RT-PCR results showed that 76.2% (16/21) and 33.3% (7/21) of the total 5-HT3A-expressing neurons were positive for GAD67 and preproenkephalin (PPE, a precursor of ENK), respectively. 5-HT3A receptor subunit was detected in 28.1% (16/57) of GABAergic neurons and 22.6% (7/31) of ENKergic neurons. About 40.4% (23/57) of GABAergic neurons expressed PPE mRNA. Of the neurons that co-express GAD67 mRNA and PPE mRNA, about 22% expressed 5-HT3A mRNA. These observations indicate that 5-HT3A receptor co-localizes with GABA and ENK in the SDH, suggesting that serotonin may activate GABAergic and ENKergic neurons via 5-HT3A receptor subunit and therefore affect the release of GABA and ENK. The different cellular localization of 5-HT3A receptor subunit suggest the complex participation of this receptor in the inhibitory neuronal circuits of the SDH neurons.