Developmental expression of the glycine transporter GLYT2 in the auditory system of rats suggests involvement in synapse maturation.

The synaptic action of many neurotransmitters is terminated by specific transporters that remove the molecules from the synaptic cleft and help to replenish the transmitter supply. Here, we have investigated the spatiotemporal distribution of the glycine transporter GLYT2 in the central auditory system of rats, where glycinergic synapses are abundant. In adult rats, GLYT2 immunoreactivity was found at all relay stations, except the auditory cortex. Many immunoreactive puncta surrounded the neuronal somata in the cochlear nuclear complex, the superior olivary complex, and the nuclei of the lateral lemniscus. In contrast, diffuse neuropil labeling was seen in the inferior colliculus and the medial geniculate body. The punctate perisomatic labeling and the diffuse neuropil labeling were very similar to the staining pattern described previously with glycine antibodies in the auditory system, suggesting that GLYT2 is a reliable marker for glycinergic synapses. However, there was a discrepancy between cytoplasmic GLYT2 and glycine labeling, as not all neuron types previously identified with glycine antibodies displayed somatic GLYT2 immunoreactivity. During development, GLYT2 immunoreactivity appeared between embryonic days 18 and 20, i.e., shortly after the time when the earliest functional synapses have been established in the auditory system. Labeling turned from a diffuse pattern to a clustered, punctate appearance. The development was also characterized by an increase of the signal intensity, which generally lasted until about postnatal day 10. Thereafter, a decrease occurred until about postnatal day 21, when the mature pattern was established in most nuclei. Because of the perinatal onset of GLYT2 immunoreactivity, we speculate that the transporter molecules participate in the process of early synapse maturation.