The diving response is a cardiorespiratory reflex characterized by apnea, bradycardia and increased peripheral vascular resistance when animals submerge underwater. Afferent information initiating this reflex response projects from the nasal mucosa via trigeminal nerves to activate second-order afferent neurons within the medullary dorsal horn (MDH). The present study investigated the synapse between these trigeminal nerves and MDH neurons, specifically what neurotransmitter receptors are expressed by MDH neurons activated during diving. Male Sprague-Dawley rats were trained to dive repetitively through an underwater maze, and Fos was used as a measure of neuronal activation. Immunohistochemistry combined with epifluorescent microscopy was used to determine which ionotropic glutamate receptor subunits, or if the Substance P receptor NK-1, were co-expressed by activated MDH neurons. Results indicate repetitive diving primarily activates small oval-shaped MDH neurons, and these Fos-positive MDH neurons co-express AMPA glutamate receptor subunits GluA1 and GluA2/3, KA glutamate receptor subunit GluK5, and NMDA glutamate receptor subunit GluN1. Since these subunits are constituent parts of their respective receptor structure, these findings reveal the formation of AMPA, KA, and NMDA glutamate receptors on the cellular membrane of these activated MDH neurons. In addition, MDH neurons activated by repetitive diving co-express the Substance P receptor NK-1. This research indicates that MDH neurons activated by diving express neurotransmitter receptors for both glutamate and SP. This supports the assertion that trigeminal nerves innervating the nasal passages release glutamate and SP within their central synapses in the brainstem and can function as part of the afferent limb of a cardiorespiratory reflex.
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