Date of Award

8-2021

Degree Name

Doctor of Philosophy

Department

Biological Sciences

First Advisor

Dr. John Spitsbergen

Second Advisor

Dr. Cindy Linn

Third Advisor

Dr. Pamela Hoppe

Fourth Advisor

Dr. James Springstead

Keywords

Exercise, aging, sedentary, innervation, heart, neurotrophic factor

Abstract

Neurotrophic factors (NFs) are important molecules responsible for development, differentiation, regeneration, and maintenance of new and mature neurons. Neurotrophic factors act as neurocytokines and may assist with the regulation of axonal and dendritic arrangements and synaptic plasticity between neurons themselves or with other non-neural target tissues. In this study, we analyze the levels of two NFs: glial cell line-derived neurotrophic factor (GDNF) and nerve growth factor (NGF). Cardiomyocytes produce these neurotrophic factors which assist with the innervation pattern of the heart. The heart is innervated by the two branches of the autonomic nervous system; namely the sympathetic nervous system and the parasympathetic nervous system, as well as the sensory nervous system. Changes in neurotrophic factor protein content in the heart may cause modifications in structural plasticity of all branches of the nervous system, which may contribute to, or prevent development of, cardiac diseases. Aging, sedentarism, and exercise are factors known to contribute to changes in neurotrophic factor levels in a diversity of tissues throughout the body, including the heart. The goal of this research was to investigate the impact of aging, sedentarism, and exercise on NFs levels and to examine structural plasticity of all branches of the nervous system that innervate the heart throughout the entire lifetime of rats.

Calcitonin gene-related peptide (CGRP) plays an important role in physiology as a potent vasodilator, which may help prevent cardiac and pulmonary hypertension, ischemia, migraine, and ultimately, improve blood flow distribution, and wound healing. It has been suggested that CGRP may play a role in cardiovascular regulation; however, the effects of exogenous CGRP on cardiac physiology has not been adequately investigated. An additional goal of this research was to investigate the effects of exogenous αCGRP on heart function.

Sprague-Dawley rats were used for the aging/exercise studies in this thesis. NGF and GDNF levels in the heart, innervation pattern of the heart, blood pressure (BP) and heart rate (HR) were examined throughout the entire animals’ lifetime. For the CGRP studies, adult bullfrogs (Lithobates catesbeianus) were used. Following CGRP treatment alone or in combination with autonomic antagonists, the force of contraction (FOC) and HR were examined in the heart.

Resting HR and BP were significantly increased in young Sprague Dawley rats compared to older rats. Specifically, eighteen months old and 24 months old sedentary animals had significantly higher HR and BP when compared to 6mo-sed and 12mo-sed. In addition, the mean arterial blood pressures in older sedentary Sprague Dawley rats were greater than 100mmHg, which is characterized as hypertension. At 18mo-sed and 24mo-sed, NGF levels were significantly lower when compared to all younger ages. Voluntary exercise significantly increased NGF and GDNF levels in all heart chambers when compared to the age-matching sedentary groups. From 4wk-sed to 14wk-sed, GDNF protein levels significantly increased in all heart chambers. From 6mo-sed and older groups, GDNF protein levels progressively and significantly decreased in all heart chambers. Our data demonstrates that, throughout the animal’s lifespan, aging combined with sedentary behavior can lead to an increase in sympathetic nerve density and a decrease in parasympathetic and sensory nerve densities in the heart. Our results suggest that exercise may significantly increase parasympathetic nerve density in the heart and reduce resting BP and HR.

The data collected by this thesis suggests that neurotrophic factor content in the heart peaks in young animals and declines with aging. GDNF content declines with aging earlier and more drastically than NGF content. These results support the hypothesis that NGF primarily supports sympathetic nervous system, which does not seem to change much with age, while GDNF supports the parasympathetic system, which does decline with age. NGF supports the sensory nervous system. Therefore, the changes in NGF content that were observed in these studies may be linked to the changes in sensory nerve density. Density of parasympathetic and sensory innervation decline with aging, while sympathetic innervation does not decline as much with aging, which may be the cause for an increase in BP and HR. Therefore, both BP and HR increase with age, as balance between sympathetic and parasympathetic innervation is impaired. With exercise, GDNF content increases, parasympathetic innervation increases, and BP and HR decrease. The effects of exercise on neurotrophic factor expression may be a possible mechanism by which exercise exerts positive effects on cardiac innervation, promoting the prevention and treatment of cardiovascular diseases.

All bullfrogs had lower FOC and lower HR within 5-10 minutes after CGRP treatment when compared to the untreated controls. Our results demonstrate that combined treatments using atropine, a non-selective muscarinic acetylcholine receptor antagonist, and CGRP, promoted negative inotropic and chronotropic effects in the heart. Evidence obtained from immunocytochemical studies suggest that CGRP is available in nerve fibers in the wall of the frog heart. CGRP could be released by these nerve fibers, possibly through varicosities, to cause the effects observed in these studies.

This data suggests that exogenous CGRP treatment significantly reduces heart rate and force of contraction in the heart of frogs, even when the parasympathetic nervous system was blocked, and can influence cardiac physiology. CGRP and the sensory nervous system may actively play additional and important roles in the heart and other organs and systems.

Access Setting

Dissertation-Open Access

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