Transient expression of a functional serotonin transporter in Merkel cellsduring late gestation and early postnatal rat development

We and others have previously identified serotonin transporter mRNA through out the trigeminal system in the whisker region, trigeminal ganglion, trige minal nucleus and thalamic relay stations. In order to further implicate a role for the serotonin transporter in this sensory system, we have now char acterized serotonin transporter gene expression and function in primary cul tures from the rat snout, at several stages of gestation. In this study, we have demonstrated a transient expression of serotonin transporter mRNA in quinacrine-positive Merkel cells between embryonic day 16 and postnatal day 5. Peak levels of mRNA occurred at embryonic day 20 and postnatal day 1. M erkel cells in culture exhibited a transient, antidepressant-sensitive [H-3 ]-serotonin uptake, which was maximal at a time in culture corresponding to embryonic day 22 (day of birth). This transient uptake of serotonin sugges ts a role for this monoamine during a critical time period of the developin g trigeminal sensory system. Regulation of extracellular serotonin levels b y transporter activity may reflect the specific formation of the merkel cel l-sensory neuron complex in an analogous mechanism by which serotonin modul ates synaptogenesis in the central nervous system.