NMDA receptor subunit NR1-immunoreactivity in the rat pons and brainstem and colocalization with Fos induced by nasal stimulation

In the present study, we examined the distribution of neurons in the parabrachial nucleus (PB), the Kölliker–Fuse nucleus (KF), the spinal trigeminal nucleus caudalis (Sp5C), the nucleus of the solitary tract (NTS) and the ventrolateral medulla (VLM), which are activated by evoking the nasotrigeminal reflex and which exhibit immunoreactivity for the N-methyl-d-aspartate (NMDA) receptor subunit NR1. By stimulating the nasal mucosa with saline, we induced the expression of the immediate early gene c-fos and combined the immunocytochemical detection of the Fos protein with the detection of the NR1 subunit. Cell counts revealed that nasal stimulation, compared to anesthesia controls, resulted in highly significant increases (p≤0.001) of Fos-immunoreactive (-ir) neurons in the midlevel KF, the external lateral PB, and the Sp5C. In the central lateral PB, the rostral ventrolateral medulla including the Bötzinger/pre-Bötzinger complex, and in the ventrolateral and commissural NTS the increases were only moderately significant (p≤0.05). With respect to the numbers of NR1-/Fos-ir double-labeled neurons, significant increases were only observed in a subset of these pontomedullary nuclei. Increases were highly significant in the Sp5C (p≤0.001) and the midlevel KF (p≤0.01) and moderately significant (p≤0.05) in the external lateral PB, Bötzinger/pre-Bötzinger complex, and ventrolateral NTS. The present study revealed that nasotrigeminally activated neurons in mandatory and potential relay sites of the nasotrigeminal reflex circuit express the NR1 subunit. This finding strongly suggests that NMDA-type glutamate receptors are involved in the mediation of the nasotrigeminally evoked cardiovascular and respiratory responses.