Experimental evidence that the serotonin transporter mediates serotonin accumulation in LSO neurons of the postnatal mouse

During the same postnatal period of development when their terminal projection patterns in the midbrain are maturing, lateral superior olivary (LSO) neurons are immunoreactive for serotonin (5-HT). As there is no evidence that LSO neurons synthesize 5-HT, it is likely that they accumulate 5-HT via the 5-HT transporter. To determine if the 5-HT transporter is responsible for 5-HT inside postnatal mouse LSO neurons, pups (postnatal ages 5–6) were treated with fluoxetine and LSO neurons examined for 5-HT. We also evaluated whether LSO neurons containing 5-HT expressed the 5-HT transporter. To further rule out any potential synthesis of 5-HT, brainstem sections of mice at postnatal ages when 5-HT staining is the most robust were stained for the rate-limiting enzyme in the synthesis of 5-HT, tryptophan hydroxylase. Fluoxetine treatment reduced or in most cases, completely eliminated the number of neurons in the LSO stained for 5-HT. Postnatal LSO neurons containing 5-HT were immunoreactive for the 5-HT transporter; in older animals in which 5-HT was no longer observed in the LSO neurons, 5-HT transporter expression was similarly absent. Further, LSO neurons in mice at any age did not stain for tryptophan hydroxylase. These results indicate that LSO neurons express the functional 5-HT transporter to internalize 5-HT; this mechanism may serve to regulate extracellular 5-HT levels during maturation of their terminal endings in the inferior colliculus.