Intricacies of brain angiotensin II signalling pathways

It is widely accepted that reactive oxygen species (ROS) play a pathophysiological role in oxidative stress in many organs, including the vasculature, heart, kidney and, more recently, the brain ^[1–3]. However, it is becoming increasingly evident that ROS and oxidative state play integral roles in physiological signal transduction pathways that are important in maintaining cellular homeostasis in the neuron, rather than just being the culprits in disease states ^[1]. Currently, there is intense interest in the role of ROS as a mediator of angiotensin II actions in cardiovascular regulatory nuclei of the midbrain and hypothalamus. The recent melding of new molecular and imaging technologies with integrative physiological approaches in the whole animal has significantly advanced our understanding in this area.

The sympathetic nervous system plays an essential part in the regulation of arterial blood pressure. A wide range of inputs from peripheral and central sensory afferent pathways are reconciled at the level of the pre-motor neurons in the midbrain and hypothalamus to achieve a sympathetic outflow and an appropriate functional response ^[4]. There are five major cardiovascular regulatory pre-motor nuclei: (i) the rostral ventrolateral medulla (RVLM), which plays a principal role in determining basal sympathetic nerve activity by balancing both inhibitory and excitatory inputs; (ii) the rostral ventromedial medulla; (iii) the medullary raphe; (iv) the A5 area in the pons; and (v) the paraventricular nucleus (PVN). The hypothalamic PVN contributes to sympathetic outflow through projections to the RVLM and the intermediolateral cell column of the spinal cord ^[4,5]. The PVN exerts a tonic excitatory signal on the RVLM ^[5]. These complex pathways for the control of sympathetic outflow open the door for potential mechanisms whereby differential control of sympathetic flow to organs may be mediated.

Angiotensin II enhances sympathetic activity both centrally and peripherally, although the exact mechanisms are not well established particularly in the central nervous system ^[5,6]. Within the brain, angiotensin II receptors are widely expressed in areas crucial for the control of cardiovascular function. The subtype predominately expressed is the angiotensin type 1 (AT₁) receptor ^[7]. An intriguing situation occurs in the brain whereby renin is found in neurons and angiotensinogen is restricted primarily to astrocytes ^[8]. The close association of astrocytes with neurons suggests that angiotensin II formation proceeds when these products are secreted into the extracellular fluid. However, angiotensin II can also be produced intracellularly. This has led to the hypothesis that angiotensin II can also act as a classical neurotransmitter in the central nervous system. For example, the subfornical organ manufactures angiotensin II in vesicles that are transported some distance to act at the PVN ^[5,8].

It is firmly established that micro-injection of angiotensin II into the RVLM causes an increase in arterial pressure and sympathetic activity ^[9]. Similarly, local application of angiotensin II in the PVN causes an increase in the excitability of the PVN in central angiotensin II, increasing sympathetic outflow via the RVLM and blunting the baroreflex response in normal animals ^[10]. Nicotinamide adenosine dinucleotide (phosphate) [NAD(P)H] oxidase is a major source of superoxide anion production ^[11]. In the heart, kidney and peripheral vasculature, angiotensin II-dependent production of superoxide is predominantly mediated via NAD(P)H oxidation ^[1–3,11]. Accumulating evidence indicates that redox signalling is also critical in the central cardiovascular effects of angiotensin II ^[12,13]. However, the intracellular mechanisms by which ROS mediates neuronal angiotensin II actions remain to be revealed. Studies in isolated primary cell cultures of the hypothalamus and brainstem have demonstrated that superoxide production via angiotensin II requires NADPH oxidase ^[14]. These data have fuelled the hypothesis that, in vivo, NAD(P)H oxidase mediates superoxide production in response to angiotensin II in cardiovascular brain nuclei.

Most recently, several studies have demonstrated an association between NAD(P)H activation, superoxide production and sympatho-excitation in response to exogenous or endogenous angiotensin II in the RVLM ^[15–17]. In the current issue of the journal, Erdos et al. ^[18] extend this finding to include angiotensin II-mediated NAD(P)H coupling of superoxide production in the hypothalamic areas of the median and medial preoptic nuclei, PVN and subfornical organ (SFO). Importantly, measurements of NAD(P)H oxidase activity and superoxide production were carried out in vivo, using confocal microscopy in anaesthetized animals. This signalling appears to be critical in the response to angiotensin II because the pressor response to central administration of angiotensin II was abolished by the central application of the NAD(P)H inhibitor apocynin. Furthermore, the authors show that the response was selective for angiotensin II because another central pressor agent, carbachol, failed to stimulate NAD(P)H activity or superoxide production.

The central actions of angiotensin II occur via activation of AT₁ receptors ^[19]. Thus, the studies by Erdos et al. ^[18] and others ^[15–17] are the first to provide direct evidence that superoxide production via NAD(P)H oxidase mediates the enhancement of sympathetic outflow in response to central angiotensin II via AT₁ receptors in the RVLM, PVN and SFO in the whole animal. However, NAD(P)H oxidase-derived superoxide is but one step in the intracellular signal transduction pathways mediating the central actions of angiotensin II. It has become apparent that a wide variety of AT₁ receptor signalling transduction pathways exist in neurons ^[17,20,21]. Moreover, these signalling pathways may mediate different functional effects of angiotensin II within the neuron. These include known acute rapid actions of angiotensin II on neuronal activity via changes in membrane ionic currents and firing rate and chronic actions of angiotensin II on noradrenaline synthesis and synaptic channels ^[5,8,22].

There is speculation that the response to angiotensin II in the cardiovascular regulatory nuclei may be altered depending on the background physiological or pathophysiological condition. This concept is based on recent evidence that angiotensin II can have a tonic sympatho-inhibitory effect on the RVLM under conditions of hypoxia ^[23]. Thus, the effect of angiotensin II in cardiovascular regions will depend on the balance of excitatory and inhibitory signals. Further fuel for such speculation comes from a study demonstrating that, in PVN neurons, angiotensin-(1-7) and AT₂ receptors contribute to tonic maintenance of renal sympathetic activity ^[24].

Short-term control of arterial blood pressure by the sympathetic nerves is well accepted ^[25,26]. However, the possible contribution of the sympathetic nerves to the long-term control of blood pressure, with its implications for the mechanisms underlying the development of hypertension, has recently become a matter of debate ^[25,27]. One possibility, among many, is that tonic PVN sympatho-excitatory activity may increase if there is a chronic up-regulation of AT₁ receptors or a reduction in neuronal nitric oxide synthase (a counter-regulatory mechanism) under pathophysiological conditions ^[25].

In summary, recent evidence suggests that angiotensin II-mediated oxidant regulation plays a role in the central neural control of blood pressure. Understanding the signalling pathways that contribute to the balance of inhibitory and excitatory signals from cardiovascular nuclei and play a part in sympathetic outflow may offer insight into disease states.