GluN2A- and GluN2B-immunoreactive type I cells attached to vesicular glutamate transporter 2-immunoreactive afferent nerve terminals of the rat carotid body.

We previously reported the immunoreactivity for the vesicular glutamate transporter 2 (VGLUT2) in afferent nerve terminals attached to chemoreceptor type I cells of the carotid body (CB), suggesting that glutamate is released from afferent terminals to stimulate these cells. In the present study, we examined the immunoreactivity for the glutamate-binding subunits of N-methyl-D-aspartate (NMDA) receptors, GluN2A and GluN2B in the rat CB, and the immunohistochemical relationships between these subunits and VGLUT2. Immunoreactivities for GluN2A and GluN2B were predominant in a subpopulation of tyrosine hydroxylase-immunoreactive type I cells rather than those of dopamine beta-hydroxylase-immunoreactive cells. Punctate VGLUT2-immunoreactive products were attached to GluN2A- and GluN2B-immunoreactive type I cells. Bassoon-immunoreactive products were localized between VGLUT2-immunoreactive puncta and type I cells immunoreactive for GluN2A and GluN2B. These results suggest that afferent nerve terminals release glutamate by exocytosis to modulate chemosensory activity of a subpopulation of type I cells via GluN2A- and GluN2B subunits-containing NMDA receptors.