Repetitive Diving in Trained Rats Still Increases Fos Production in Brainstem Neurons after Bilateral Sectioning of the Anterior Ethmoidal Nerve

Abstract

This research was designed to investigate the role of the anterior ethmoidal nerve (AEN) during repetitive trained diving in rats, with specific attention to activation of afferent and efferent brainstem nuclei that are part of this reflexive response. The AEN innervates the nose and nasal passages and is thought to be an important component of the afferent limb of the diving response. Male Sprague-Dawley rats (N = 24) were trained to swim and dive through a 5 m underwater maze. Some rats (N = 12) had bilateral sectioning of the AEN, others a Sham surgery (N = 12). Twelve rats (6 AEN cut and 6 Sham) had 24 post-surgical dive trials over 2 h to activate brainstem neurons to produce Fos, a neuronal activation marker. Remaining rats were non-diving controls. Diving animals had significantly more Fos-positive neurons than non-diving animals in the caudal pressor area, ventral medullary dorsal horn, ventral paratrigeminal nucleus, nucleus tractus solitarius, rostral ventrolateral medulla, Raphe nuclei, A5, Locus Coeruleus, and Kölliker-Fuse area. There were no significant differences in brainstem Fos labeling in rats diving with and without intact AENs. Thus, the AENs are not required for initiation of the diving response. Other nerve(s) that innervate the nose and nasal passages, and/or suprabulbar activation of brainstem neurons, may be responsible for the pattern of neuronal activation observed during repetitive trained diving in rats. These results help define the central neuronal circuitry of the mammalian diving response.

Innervation of the Nose and Nasal Region of the Rat: Implications for Initiating the Mammalian Diving Response

“…At the first node in this pathway, the afferent information produces activation of secondary neurons within the trigeminal nucleus (Hollandsworth et al, 2009 ), as determined by their production of Fos protein (Coggeshall, 2005 ). These secondary neurons are located within the ventral tip of MDH, between pyramidal decussation and obex, peaking just rostral to calamus scriptorius with the appearance of Sp5I (Figures 13A1,A2 ; McCulloch, 2005 ; Panneton et al, 2010a , 2012 ; McCulloch et al, 2016 ). An important aim of the present study was to determine whether the afferent sensory information that initiates activation of these secondary neurons originates from the internal nasal passages or from more external nasal regions.…”

“…These secondary trigeminal neurons become activated, as indicated by the production of Fos protein (Coggeshall, 2005 ), during voluntary diving in rats. After repetitive voluntary diving Fos-positive neurons are located within the ventral tip of MDH, between pyramidal decussation and obex, peaking just rostral to calamus scriptorius (McCulloch, 2005 ; Panneton et al, 2012 ; McCulloch et al, 2016 ). Thus the ventral tip of MDH serves as the initial afferent node in the neural circuitry of the diving response.…”

“…This indicates loss of AEN functioning does not eliminate the cardiorespiratory responses occurring during voluntary diving. There are no differences in Fos expression within the ventral MDH during voluntarily diving in rats with or without intact AENs (McCulloch et al, 2016 ). In addition, after the AENs are cut bilaterally, the number of Fos-positive neurons within the ventral MDH and paratrigeminal nucleus increases significantly in voluntarily diving rats compared with non-diving control rats (McCulloch et al, 2016 ).…”

“…There are no differences in Fos expression within the ventral MDH during voluntarily diving in rats with or without intact AENs (McCulloch et al, 2016 ). In addition, after the AENs are cut bilaterally, the number of Fos-positive neurons within the ventral MDH and paratrigeminal nucleus increases significantly in voluntarily diving rats compared with non-diving control rats (McCulloch et al, 2016 ). Furthermore, there are no significant differences in activation of brainstem areas involved in cardiorespiratory functioning in rats diving with or without intact AENs (McCulloch et al, 2016 ).…”

“…In addition, after the AENs are cut bilaterally, the number of Fos-positive neurons within the ventral MDH and paratrigeminal nucleus increases significantly in voluntarily diving rats compared with non-diving control rats (McCulloch et al, 2016 ). Furthermore, there are no significant differences in activation of brainstem areas involved in cardiorespiratory functioning in rats diving with or without intact AENs (McCulloch et al, 2016 ). These findings indicate the brainstem areas responsible for both afferent and efferent aspects of the diving response are still activated even when they do not receive sensory afferent information via the AEN during submersion under water.…”