The anterior ethmoidal and nasopalatine nerves innervating the nasal passages are important for activating medullary dorsal horn neurons during repetitive diving in rats

The diving response, initiated when animals submerge underwater, is an autonomic reflex characterized by apnea, bradycardia and increased peripheral vascular resistance mediated by sympathetic vasoconstriction. To initiate this response, afferent stimulation of the nose and nasal mucosa produces activation of second-order neurons located within the brainstem, specifically the trigeminal medullary dorsal horn (MDH). Our objective was to determine whether the central projections from the nose, specifically the anterior ethmoidal nerve (AEN) and nasopalatine nerve (NPN), can provide the anatomic connection that activates MDH neurons during diving. Male Sprague-Dawley rats were trained to repetitively dive through an underwater maze. In these animals the anterograde tracer wheatgerm agglutinin (WGA) was injected into either the nasal passages, AEN or NPN to identify the central terminal projections from the nasal passages. These rats then repetitively dived underwater to activate MDH neurons which then produced the Fos protein. Other trained rats were used as non-diving controls. Brains from all animals were harvested, and 50 μm brainstem tissue sections were preserved. WGA immunochemistry confirmed nasal afferents terminate primarily ventrally within the superficial laminae of the MDH between the pyramidal decussation caudally and the area postrema rostrally. Within this WGA area of termination, immunohistochemistry further identified MDH neurons that express Fos and thus were activated by repetitive diving. In rats receiving WGA injection into the nasal passages (N=7), 32.3 ± 10.5 MDH neurons/hemisection (mean ± SE) were both Fos and WGA positive in repetitively diving animals (N=3), compared with 4.6 ± 2.1 MDH neurons/hemisection in non-diving control animals (N=4). In rats receiving WGA injection into the AEN (N=13), 29.0 ± 5.2 MDH neurons/hemisection were both Fos and WGA positive in repetitively diving animals (N=7), compared with 10.3 ± 3.2 MDH neurons/hemisection in non-diving control animals (N=6). In rats receiving WGA injection into the NPN (N=7), 26.1 ± 8.1 MDH neurons/hemisection were both Fos and WGA positive in repetitively diving animals (N=4), compared with 23.9 ± 16.1 MDH neurons/hemisection in non-diving control animals (N=3). Results indicate that repetitively diving rats express significantly more Fos positive MDH neurons within the superficial MDH compared with non-diving control rats. Additionally, the AEN constitutes the primary innervation of the nasal passages important for activation of MDH neurons during repetitive diving. However, the NPN also provides some innervation of the nasal passages to help activate MDH neurons during repetitive diving. We conclude that central projections from the nasal passages are important for activating MDH neurons during repetitive diving in rats. Research supported by MWU MBS Program (MM), CCOM Research Fellowships (KK, HN) and CGS Intramural funds (PFM). This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.