Approaches to establishing angiotensin II as a neurotransmitter revisited.

During the past 3 to 4 decades, there has been debate on the presence as well as the role of local tissue renin–angiotensin systems (RAS) in cardiovascular physiology and pathophysiology. Although it is now accepted that all components of the RAS are present in a variety of extrarenal tissues and their regulation may be independent of the circulating hormonal system, the factors contributing to this regulation are still not well understood. Also apparent is that our understanding of the relationships among various cell types expressing individual RAS components and production of specific angiotensin peptides in each tissue is still lacking. This is particularly true of the brain RAS, where even after 20 years the question of whether there are angiotensinergic versus reninergic neurons or a complete functioning RAS in cerebrospinal fluid (CSF) or extracellular fluid or glia remains in question.1 Ever since early reports provided biochemical evidence of RAS components in brain, controversy regarding their cellular localization, independence from the circulating system, and authenticity of the proteins and peptides persists. It is well accepted that angiotensinogen is present in CSF/interstitial fluid and localization via immunocytochemistry and in situ hybridization histochemistry reveals that production of the precursor protein is primarily in glia, but also in neurons, within key cardiovascular nuclei. Lingering questions remain concerning local expression of authentic renin in tissues, especially given that prorenin or active renin can be sequestered from the circulation2 and other enzymes can exhibit similar proteolytic profiles under certain conditions. However, there is unequivocal evidence of discrete cells within the pituitary, choroid plexus, medulla oblongata, and hypothalamus that are positive for renin immunoreactivity colocalizing mainly with neurons, but in the medulla oblongata and subfornical organ, in glial elements as well. Evidence of renin mRNA in brain tissue provides a mechanism for local synthesis of the protein. Using green fluorescent protein driven by the renin promoter,3 studies revealed predominant but not exclusive presence of renin mRNA in neurons. All enzymes required for subsequent processing of angiotensin I into active peptides, including converting enzyme (ACE) for angiotensin II, ACE2 and neprilysin for angiotensin-(1-7), and aminopetidases for angiotensins III and IV, are present on the basis of immunocytochemical or molecular approaches. The predominant localization of these secondary enzymes on plasma membranes is interpreted as evidence for extracellular formation of the final bioactive peptide products. Finally, evidence of receptors defined by molecular, functional, or binding methods for angiotensins II, III, IV, and (1-7) complete the brain RAS. The receptors are widely distributed throughout cardiovascular and neuroendocrine control centers as well as in areas of the brain more typically considered as part of motor and sensory processing and memory and affective behaviors. However, this is consistent with the widespread actions of the angiotensin peptides in brain. Molecular approaches developed in the past 15 to 20 years yielded substantial progress in establishing firmly the presence and, to some extent, the cellular localization of processing enzymes and precursor substrate of the brain RAS. A significant insight with respect to the importance of renin, specifically in blood pressure control and hypertension, arose when intraventricular administration of renin mRNA antisense oligodeoxynucleotides lowered blood pressure in spontaneously hypertensive rats (SHRs).4 These findings are consistent with many other studies using a variety of RAS inhibitors (molecular or pharmacological) administered into brain ventricles or specific brain nuclei to reveal major contributions of brain RAS to hypertension in SHRs. Because the SHR is a genetic form of hypertension exhibiting overactivity of one or more components of the brain RAS, amelioration of hypertension by blockade of brain renin clearly supports the concept that upregulation of the brain RAS participates in hypertension. The chronic phase of renal hypertension is another example where an activated central RAS plays a role in hypertension.5 The powerful influence of brain angiotensin II on cardiovascular and renal function was elegantly demonstrated in studies where an angiotensin II– generating construct was inserted behind a glial fibrillary acidic protein promoter in angiotensinogen deficient mice.6 This replacement of angiotensin II rescued the renal defects characteristic of systemic deficiency of angiotensinogen. Despite the use of molecular tools to demonstrate the presence and function of the brain RAS, we do not have a clear concept of the configuration of the brain RAS for processing of angiotensinogen by renin or any other potential candidate proteolytic enzyme for that matter. Processing may occur in the extracellular spaces, including CSF, as a result of transfer of precursor enzymes and substrates between cells or within a single cell. In part, this question arises because of the limited localization in brain tissue of cells expressing renin The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Hypertension & Vascular Disease Center, Wake Forest University School of Medicine, Winston-Salem, NC. Correspondence to Debra I. Diz, PhD, Hypertension & Vascular Disease Center, Wake Forest University School of Medicine, Medical Center Blvd, Winston-Salem, NC 27157. E-mail [email protected] (Hypertension. 2006;47:334-336.) © 2006 American Heart Association, Inc.