Expression of glycine receptor alpha 3 in the rat trigeminal neurons and central boutons in the brainstem

Abstract

Increasing evidence shows that the homomeric glycine receptor is expressed in axon terminals and is involved in the presynaptic modulation of transmitter release. However, little is known about the expression of the glycine receptor, implicated in the presynaptic modulation of sensory transmission in the primary somatosensory neurons and their central boutons. To address this, we investigated the expression of glycine receptor subunit alpha 3 (GlyR alpha 3) in the neurons in the trigeminal ganglion and axon terminals in the 1st relay nucleus of the brainstem by light- and electron-microscopic immunohistochemistry. Trigeminal primary sensory neurons were GlyR alpha 3-immunopositive/gephyrin-immunonegative (indicating homomeric GlyR), whereas GlyR alpha 3/gephyrin immunoreactivity (indicating heteromeric GlyR) was observed in dendrites. GlyR alpha 3 immunoreactivity was also found in the central boutons of primary afferents but far from the presynaptic site and in dendrites at subsynaptic sites. Boutons expressing GlyR alpha 3 contained small round vesicles, formed asymmetric synapses with dendrites and were immunoreactive for glutamate. These findings suggest that trigeminal primary afferent boutons receive presynaptic modulation via homomeric, extrasynaptic GlyR alpha 3, and that different subtypes of GlyR may be involved in pre- and postsynaptic inhibition.