The exercise pressor reflex activates nuclear c-Fos translocation within insulin receptor positive neurons of the nucleus tractus solitarius in Sprague-Dawley rats
Authors: Estrada Juan , Hotta Norio , Kim Han-Kyul , Fukazawa Ayumi , Ishizawa Rie , Iwamoto Gary , Smith Scott , Vongpatanasin Wanpen , Mizuno Masaki 
Published: May 2023

Insulin signaling in the brain is capable of modulating neuronal excitability. The nucleus tractus solitarius (NTS) receives and integrates sensory input from working skeletal muscle during physical exertion. The neural input arising from skeletal muscle reflexively mediates, in part, the cardiovascular response to exercise and is termed the exercise pressor reflex (EPR). Thus, it is hypothesized that insulin signaling may modulate EPR function if EPR-sensitive neurons in the NTS are co-expressed with insulin receptors (IR). Purpose: The current study was conducted to investigate the extent to which 1) IRs are expressed on neurons in the NTS, and 2) IR positive neurons in the NTS are stimulated by ascending afferent input generated from hindlimb muscle contractions, as evidenced by enhanced nuclear translocation of c-Fos. Methods: A precollicular decerebration was performed on adult male Sprague-Dawley rats. Following a 1hr rest period after surgery, L4 and L5 ventral roots were electrically stimulated to evoke hindlimb muscle contraction activating the EPR (n=7). Hindlimb muscle contraction was intermittently evoked at 1m intervals, 30s contraction followed by 30s rest, over a period of 1hr. As a control (n=5), a surgical sham operation was performed with the ventral roots not being stimulated during the experimental contraction period. Afterwards, brainstem tissue was fixed, then sections (bregma -15.4 to -13.5 mm) were cut (35mm) and probed with fluorescent antibodies and dyes for photomicrograph analysis. Triple fluorescence overlap areas of nuclei (DAPI), IRs, and c-Fos in the NTS were evaluated using NIH software. Results: Blood pressure was significantly increased during muscle contraction induced by ventral root stimulation in EPR but not in sham animals, undergoing no ventral stimulation. Fluorescence overlap analysis demonstrated an increase in c-Fos nuclear translocation within IR positive neurons in the rostral regions of the NTS (bregma -13.5 mm, 4794±1460 pixel2) relative to the most caudal regions of the NTS (bregma -15.5 mm, 1245±994 pixel2) (within-group, P<0.0001) in rats undergoing contraction whereas no differences were observed in sham animals. Conclusion: These observations are consistent with previous work demonstrating nuclear translocation of c-Fos in the NTS following repeated contraction of hindlimb muscle in cats and rats. The current findings extend these observations to IR positive neurons in the NTS, which appear to be stimulated by ascending afferent input from contracting hindlimb muscles. The data suggests that IR signaling within the NTS may modulate neuronal excitability, thereby potentially altering EPR function in both health and disease.

Supported by NIH-NHLBI, Lawson & Rogers Lacy Research Fund

This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.