CHARACTERIZING THE LEPTIN‐INDUCED ACTIVATION OF PRE‐SYMPATHETIC CARDIOVASCULAR AND METABOLIC‐RELATED HYPOTHALAMIC NEURONS IN MICE

Abstract

Obesity has risen 75% in the United States since 1980 and an estimated 80 million American adults are considered obese. Obesity activates the sympathetic nervous system and is associated with neurogenic hypertension. Leptin is an obesity‐related neuropeptide released from fat cells which reduces appetite and increases metabolism. Leptin activates metabolic and cardiovascular responsive pre‐sympathetic neurons within the hypothalamus. Although leptin increases metabolism and curbs appetite, it is also reported to increase blood pressure and heart rate. Considering that the common goal of obesity treatments is to diminish the cardiovascular‐related co‐morbidities increases in blood pressure and heart rate are unacceptable side effects for any potential treatment. Thus, a better understanding of the role hypothalamic sites involved in obesity‐related hypertension is necessary for the development of successful treatments. In this study, we tested the hypothesis that leptin activates pre‐sympathetic hypothalamic neurons that control either metabolic (raphe pallidus‐projecting) or cardiovascular activity (RVLM, rostroventrolateral medulla‐projecting). First, to identify leptin receptor expressing neurons we created a line of transgenic reporter mice using the cre‐lox recombination system to express tdTomato under the control of the leptin (ObRb) receptor gene. Then, to determine if exogenous leptin treatment activates pre‐sympathetic metabolic or cardiovascular‐related hypothalamic neurons we performed neuroanatomical tracer studies in these mice. Metabolically‐related neurons were identified by microinjection (50 nl) of green FluoSpheres™ (505/515) into the raphe nucleus and cardiovascular‐related neurons were identified by microinjection (50 nl) of magenta FluoSpheres™ (660/680) into the rostroventrolateral medulla (RVLM). We then treated these mice with an acute dose of leptin (25 μg/mouse IP) and then performed fluorescence immunohistochemical labelling to identify leptin‐induced neuronal activation of cFos, a marker of neuronal activation. Using this approach we identified both a population of RVLM‐projecting and a population of raphe‐projecting leptin expressing neurons. These data provide additional information to suggest that the leptin‐induced increases in cardiovascular and metabolic activity is mediated in part by activation of pre‐sympathetic hypothalamic neurons.Support or Funding InformationETSU RDC 19‐030 (Zahner MR) NHLBI R15 HL145645‐01 (Zahner MR)