Karie E. Scrogin, Ph.D. - US grants

Following a critical amount of blood loss, compensatory neural responses that normally help maintain blood pressure suddenly fail, causing vasodilation, profound bradycardia and life-threatening hypotension. The central nervous system mechanisms that mediate this sympatholytic response are unknown. A more comprehensive understanding of such mechanisms could lead to novel treatments for circulatory shock and other disorders with aberrant activation of sympatholytic reflexes such as myocardial infarct of the inferoposterior wall of the heart, exertional syncope associated with aortic stenosis or neurogenic syncope. In vivo models devised to examine these responses have been limited due to the effects of anesthesia on autonomic function. The objective of this proposal is to elucidate the role of hindbrain serotonergic cellular- and receptor mediated mechanisms in the sudden loss of sympathetic activity that accompanies severe blood loss in the conscious rat. Specifically, studies have been designed to identify the source of serotonin and the receptor populations that mediate sympathetic withdrawal during hemorrhage. The proposed experiments involve novel uses of classic physiological models to study the role of discrete hindbrain neuronal and receptor populations in the regulation of sympathetic function in unanesthetized rats. Anatomical studies that combine neuronal tract tracing with immunhistochemical markers of neuronal phenotype and function will be used to determine the source and projection site of serotonin involved in sympathetic reflexes. The novel techniques outlined in this proposal will help to determine hindbrain serotonergic cellular and receptor function on sympathetic regulation in general and on hemorrhage responses in particular, a goal which has, until now, been hampered by the confounding influence of anesthesia on hemorrhage responses and serotonergic function.

DESCRIPTION (provided by applicant): Trauma is the leading cause of death of young people in the U.S. (150,000/year). Most trauma deaths result either from insufficient tissue perfusion, due to excessive blood loss, or the development of inflammation, infection and end organ damage following resuscitation. Current treatment plans for hypovolemic shock rely on massive and rapid infusion of crystalloid fluids to raise cardiac output. It is now recognized that excessive volume resuscitation may increase blood loss and the reperfusion injury that contributes to the morbidity of hypovolemic shock. As such, the development of alternative strategies for the treatment of traumatic blood loss will be critical for the improvement of patient outcomes following hypovolemic shock; progressive hemorrhage produces a biphasic response. Increases in heart rate and sympathetic activity maintain blood pressure in the initial compensatory phase. These compensatory responses suddenly abate after significant blood loss (20-30%), resulting in hypotension, bradycardia and sympathoinhibition. We recently discovered that drugs that activate serotonin 5-HTIA receptors rapidly reverse the hypotensive and sympathoinhibitory responses to hemorrhage in conscious rats. Both central and systemic administration of 5-HTIA receptor agonists effectively raises blood pressure after either acute or sustained blood loss. These results indicate that 5-HTIA agonists could provide a promising therapy for hypovolemic shock. However, it is not known how 5-HTIA receptor activation increases arterial pressure or if the hemodynamic responses to agonist administration provide a beneficial effect on tissue perfusion. More importantly, it is not known if activation of 5-HTIA receptors can delay the transition from hypovolemic shock to circulatory collapse. Studies proposed in this project will determine the autonomic and hemodynamic effects of selective 5-HTIA agonists following sustained hypotensive hemorrhage to assess their impact on perfusion. Additional studies will assess the central nervous system mechanisms responsible for pressor effects of selective 5-HTIA agonists during hemorrhage. We will also assess the ability of clinically available 5-HT1A agonists to delay the transition from hypovolemic shock to circulatory collapse.

DESCRIPTION (provided by applicant): This project will help to characterize the central nervous system mechanisms that regulate autonomic and respiratory compensation following hypovolemic hypotension and circulatory shock. Experiments will be conducted to test the hypothesis that caudal hindbrain serotonergic neurons activated by acidosis, stimulate 5-HT1A receptors to promote sympathetic-mediated venoconstriction of the splanchnic vascular bed during hypovolemia. It is further proposed that the preferential constriction of the venous vasculature induced by 5-HT1A receptor activation will produce less reperfusion injury during resuscitation from hypovolemic shock than clinically used vasoconstrictor agents which tend to constrict arterial vascular beds. Aim 1 will determine whether caudal hindbrain serotonin is critical for maintenance or recovery of sympathetic-mediated whole body venous tone and venous return following severe blood loss. Aim 2 will determine whether the acidemia associated with hypovolemia or respiratory and metabolic acidosis per se contribute to activation of caudal hindbrain serotonin neural activation and subsequent respiratory and autonomic responses. Further studies will assess with acidosis contributes to the maintenance of blood pressure through a preferential venoconstriction. Aim 3 will determine whether serotonin acts on 5-HT1A receptors to mediate compensatory responses to hypovolemia and whether this endogenous pathway can be exploited to produce a more favorable hemodynamic response during resuscitation from hypovolemic shock. These studies will rely heavily on a carefully developed in vivo rat and mouse models of hypotensive hemorrhage and hypovolemic shock. State of the art techniques for continuous monitoring of hemodynamic parameters, sympathetic nerve activity and central respiratory drive in unanesthetized animals will be used to assess cardiovascular parameters after pharmacological and molecular manipulation of serotonin and serotonin receptor levels. In addition, newly developed techniques for the recording of sympathetic activity in the unanesthetized mouse will enable use of genetically altered mice for investigation of the receptors involved in the compensatory responses to blood loss. Furthermore, novel molecular techniques to more acutely alter serotonin levels in discrete brain regions will be utilized to dissect regions important in the neural control of the circulatory responses to blood loss. Finally, pre-clinical, translational studies will address the potential utility of using 5-HT1A receptor agonists as adjuvants in resuscitation from circulatory shock. PUBLIC HEALTH RELEVANCE: Despite recent advances in emergency medicine, traumatic blood loss is currently one of the leading causes of death of individuals under 40 in the US. Patients typically succumb to severe blood loss either because of too little tissue perfusion or because of tissue injury incurred during the resuscitation process. Our studies will attempt to validate a new, promising therapy that may help patients recover from circulatory shock without further injuring tissue during the resuscitation process.