Orexin A and B, also known as hypocretin 1 and 2, are two recently isolated hypothalamic peptides. As orexin-containing neurons are strategically located in the lateral hypothalamus, which has long been suspected to play an important role in feeding behaviors, initial studies were focused on the involvement of orexins in positive food intake and energy metabolism. Recent studies implicate a more diverse biological role of orexins, which can be manifested at different level of the neuraxis. For example, canine narcolepsy, a disorder with close phenotypic similarity to human narcolepsy, is caused by a mutation of hypocretin receptor 2 gene. Results from our immunohistochemical and functional studies, which will be summarized here, suggest that the peptide acting on neurons in the rostral ventrolateral medulla augment sympathoexcitatory outflow to the spinal cord. This finding is discussed in the context of increased sympathetic activity frequently associated with obesity.

Cardiovascular disease (CVD) is the leading cause of death worldwide. The risks for women increase at the onset of menopause. A central feature in CVD patients is excessive sympathetic stimulation of beta-adrenergic receptors (β-ARs). Both clinical and animal studies show that estrogen loss and age exacerbate cardiac β-AR signaling and contractile function. We, therefore, examined the hypothesis that prolonged estrogen deficiency followed by chronic sympathetic injury worsens left ventricular cardiac function in the aged female heart. Bilateral ovariectomy (OVX) or SHAM surgery was performed in female mice at 2.5 months of age and infused with Isoproterenol (ISO; 400μg/kg/h) at 12 months (12M) post OVX for 3 days to induce chronic sympathetic stimulation. Transthoracic two-dimensional M-mode echocardiography was used to measure left ventricular (LV) wall thickness and left ventricular end-systolic diameter (LVESD) and left ventricular end-diastolic diameter (LVEDD), percent fractional shortening (%FS), and ejection fraction (EF). Animal body weight was measured to calculate the heart-body ratio, followed by the removal of the heart, left lung, and uterus during euthanasia. Tissue samples were treated with wheat germ agglutinin staining to measure cardiac myocyte cross-sectional area (hypertrophy). Results show that prolonged ovariectomy increased mortality in mice treated with ISO post-ovariectomy (OVX +ISO) compared to the SHAM+ISO group. Echocardiography imaging demonstrated a smaller systolic diameter and increased contractility in the ISO and ISO+OVX groups. OVX, ISO, and ISO+OVX treatment had a significant decrease in LVESD versus SHAM and OVX groups. The LVEDD resulted in a significant decrease with ISO treatment compared to the SHAM group, and no significant difference was observed between the OVX and ISO+OVX groups compared to the SHAM. Percent FS presented a significant increase in cardiac function in OVX, ISO, and ISO+OVX groups compared to the SHAM. There was an increased %FS in the ISO+OVX compared to the OVX group, and no significant difference between the ISO+OVX and ISO treatment groups. Percent EF significantly increased in the OVX, ISO, and ISO+OVX treatment groups from the SHAM and OVX group, and no significant difference between the ISO+OVX and ISO treatment groups. OVX increased left ventricular mass compared to SHAM. While ISO treatment did not increase LV mass ISO+OVX treatment group significantly increased in LV mass when compared to the ISO treatment group. There was no significant difference in the left ventricular mass between the ISO+OVX vs. OVX group. There was no significant difference in cardiac myocyte cross-sectional area in the SHAM, OVX vs ISO groups. There was however a significant increase in myocyte cross-sectional area in the ISO+OVX group compared to OVX treatment and ISO groups. The results presented here show that estrogen loss impairs left ventricular cardiac function and increases remodeling in response to β-AR stimulation and that prolonged estrogen loss may blunt the sympathetic response in the heart. These results highlight the importance of the long-term effects of estrogen loss during menopause in the treatment and management of heart disease.

Development of the sympathetic nervous system (SNS) tone onto peripheral organs has been shown to be susceptible to a wide range of external factors, such as temperature. Although it was initially postulated that the sympathetic signal is uniform across the body, there is growing evidence that there can be target-specific sympathetic signals. To date, evidence for a relationship between developmental influences on SNS tone and organ function is purely correlational. An obstacle to investigating the programming of SNS permanently altering physiology is that experimental manipulations of SNS activity during development would impact multiple organ functions simultaneously, which could affect the overall health of the animal and therefore confound interpretation of the results. Here we used brown adipose tissue (BAT) as a platform to define a critical period and identify molecules that contribute to the development of SNS outflow to peripheral organs. In addition, we explored the molecular target-specificity of sympathetic neurons by performing a single-cell RNA sequencing transcriptomic analysis of adult mouse stellate ganglion (SG) in conjunction with retrograde tracing from two of its targets, brown adipose tissue and forelimb. We discovered four molecularly distinct populations of SG neurons that express unique combinations of neuropeptides and receptors, but we did not find evidence of target specificity. The four distinct SG neuronal populations had marker genes that showed unique expression in each population, including genes encoding secreted peptides and receptors of circulating factors. Also, we found that the expression of some of the marker genes differs across the sympathetic chain, which could provide a means for coordinated regulation of SNS responses to specific types of homeostatic challenges.

ABSTRACT Chronic activation of the sympathetic nervous system, via β-adrenoreceptor (AR) stimulation, contributes toward progressive heart failure. However, in this regard there are some outstanding issues which require clarity. First, in addition to contributing toward progressive heart failure, it is not clear whether chronic β-AR activation can also initiate cardiac decompensation. If so, the mechanisms of this effect also need to be determined. Second, the role of functional variants of β-AR genes as determinants of either the development or progression of heart failure requires elucidation. Moreover, whether there is any practical value in genotyping of patients for these variants has yet to be determined. These questions were addressed in the present thesis. With respect to the question of whether chronic β-AR activation initiates cardiac decompensation, the mechanisms responsible for the transition from compensated cardiac hypertrophy to heart failure in pressure overload states, such as hypertension, are uncertain. In this thesis I explored whether chronic sympathetic nervous system activation, produced by daily administration of a β-AR agonist, could promote the transition to cardiac pump failure in spontaneously hypertensive rats (SHR) with compensated cardiac hypertrophy. After 5 months of daily administration of a β-AR agonist, SHR developed marked left ventricular pump dysfunction, whereas normotensive control rats maintained pump function. The pump dysfunction noted in SHR was attributed to marked chamber dilatation with wall thinning, whilst myocardial contractile function appeared to be intact. The changes in cardiac structure and function noted after chronic β-AR activation in SHR were similar to those noted in SHR with advanced heart failure. These data provided the first evidence to indicate that chronic β- AR activation can promote the transition to decompensated cardiac hypertrophy in pressure overload states, and that this effect is principally mediated by adverse structural remodeling of the cardiac chamber. iii The mechanisms responsible for the effect of chronic β-AR activation on cardiac chamber dilatation were subsequently studied. The identified mechanisms included activation of an enzyme that degrades myocardial collagen (matrix metalloproteinase 2) and an increase of myocardial collagen of the type that is susceptible to collagen degradation (non-cross-linked collagen). I also excluded alternative potential mechanisms such as necrosis, apoptosis and an accumulation of type III collagen. However, previous studies have indicated that increases in myocardial collagen concentrations determine myocardial stiffness and not cardiac chamber dilatation. Hence, I performed a study to examine whether the impact of increases in myocardial collagen concentrations on cardiac structure and function depends on the qualitative changes in myocardial collagen. Indeed, using a variety of models of pressure overload hypertrophy associated with increases in myocardial collagen concentrations, I was able to provide evidence to support the theory that increases in myocardial collagen of the cross-linked phenotype will promote myocardial stiffness, whereas increase in myocardial collagen of the non-cross-linked phenotype promotes cardiac dilatation. With respect to the question of whether functional variants of β-AR genes contribute toward either the development or progression of heart failure, I studied the role of both functional β1-AR and β2-AR (together with a α2C-AR) gene variants in black South Africans with idiopathic dilated cardiomyopathy (IDC). In a prospective study I obtained data to indicate that the relationship between functional β2-AR genotypes and the progression to hospitalization, death or transplantation; a reduced exercise capacity, and left ventricular functional responses to b-blocker therapy, as described by other groups, is unlikely to be attributed to an independent effect of genotype on cardiac chamber dimensions and pump function. Moreover, I was able to show that contrary to what had previously been suggested, genotyping black subjects for functional α2C-AR iv and β1-AR gene variants is of little use when predicting the development or severity of IDC in this population group.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.