Michael J. McKinley

The brain renin-angiotensin system: location and physiological roles

Angiotensinogen, the precursor molecule for angiotensins I, II and III, and the enzymes renin, angiotensin-converting enzyme (ACE), and aminopeptidases A and N may all be synthesised within the brain. Angiotensin (Ang) AT(1), AT(2) and AT(4) receptors are also plentiful in the brain. AT(1) receptors are found in several brain regions, such as the hypothalamic paraventricular and supraoptic nuclei, the lamina terminalis, lateral parabrachial nucleus, ventrolateral medulla and nucleus of the solitary tract (NTS), which are known to have roles in the regulation of the cardiovascular system and/or body fluid and electrolyte balance. Immunohistochemical and neuropharmacological studies suggest that angiotensinergic neural pathways utilise Ang II and/or Ang III as a neurotransmitter or neuromodulator in the aforementioned brain regions. Angiotensinogen is synthesised predominantly in astrocytes, but the processes by which Ang II is generated or incorporated in neurons for utilisation as a neurotransmitter is unknown. Centrally administered AT(1) receptor antagonists or angiotensinogen antisense oligonucleotides inhibit sympathetic activity and reduce arterial blood pressure in certain physiological or pathophysiological conditions, as well as disrupting water drinking and sodium appetite, vasopressin secretion, sodium excretion, renin release and thermoregulation. The AT(4) receptor is identical to insulin-regulated aminopeptidase (IRAP) and plays a role in memory mechanisms. In conclusion, angiotensinergic neural pathways and angiotensin peptides are important in neural function and may have important homeostatic roles, particularly related to cardiovascular function, osmoregulation and thermoregulation.

The neurochemical characterisation of hypothalamic pathways projecting polysynaptically to brown adipose tissue in the rat.

The identification of leptin and a range of novel anorectic and orexigenic peptides has focussed attention on the neural circuitry involved in the genesis of food intake and the reflex control of thermogenesis. Here, the neurotropic virus pseudorabies has been utilised in conjunction with the immunocytochemical localisation of a variety of neuroactive peptides and receptors to better define the pathways in the rat hypothalamus directed polysynaptically to the major thermogenic endpoint, brown adipose tissue. Infected neurones were detected initially in the stellate ganglion, then in the spinal cord followed by the appearance of third-order premotor neurones in the brainstem and hypothalamus. Within the hypothalamus these were present in the paraventricular nucleus, lateral hypothalamus, perifornical region, and retrochiasmatic nucleus. At slightly longer survival times virus-infected neurones appeared in the arcuate nucleus and dorsomedial hypothalamus. Neurones in the retrochiasmatic nucleus and in the adjacent lateral arcuate nucleus which project to the brown adipose tissue express cocaine- and amphetamine-regulated transcript, pro-opiomelanocortin and leptin receptors. Neurones in the lateral hypothalamus, a site traditionally associated with the promotion of feeding, project to brown adipose tissue and large numbers of these contained melanin-concentrating hormone and orexin A and B. These data provide part of an anatomical framework which subserves the regulation of energy expenditure.

Sensors for antidiuresis and thirst—osmoreceptors or CSF sodium detectors?

Change in sodium concentration of lateral ventricle CSF caused by intracarotid infusion of hypertonic solutions was measured in conscious sheep. Intracarotid infusion (1.6 ml/min) of 1 M NaCl, 2 M sucrose or 4.6 M or 2 M urea caused progressive increase of CSF sodium concentration, whereas 2 M glucose, 2 M galactose or 0.15 M NaCl did not. Of these solutions, only intracarotid 1 M NaCl or 2 M sucrose caused rapid water intake or rapid decrease in free water clearance. 2 M urea caused relatively slow antidiuresis and no water intake. 4.6 M urea which produced the largest rise of CSF[Na] caused slow antidiuresis and inconsistent small water intake. Infusion into a lateral ventricle (0.05 ml/min) of 0.35 M NaCl or 0.7 M sucrose, or fructose, made up in artificial CSF (0.15 M Na) or 0.5 M NaCl alone, all rapidly elicited an antidiuresis and water drinking, whereas intraventricular infusion of pure non-saline 1 M sucrose of 0.7 M urea in CSF was ineffective. Intraventricular 0.35 M NaCl in CSF caused greater antidiuretic and dipsogenic effects than intraventricular 0.7 M sucrose or fructose in CSF. It is postulated that a dual osmoreceptor-sodium sensor system may participate in regulating antidiuretic hormone secretion and thirst, and that the osmoreceptor system mediates the rapid antidiuresis and water drinking caused by intracarotid 1 M NaCl or 2 M sucrose, and is probably located in a brain region without a blood-brain barrier.

Fos production in retrogradely labelled neurons of the lamina terminalis following intravenous infusion of either hypertonic saline or angiotensin II

The lamina terminalis consists of neurons which are activated by both osmotic and angiotensinergic stimuli and which project axons to many sites including regions of the hypothalamus responsible for vasopressin production. Combination of retrograde neuronal tracing procedures with the identification of Fos protein following discrete stimuli shows populations of neurons, projecting to the supraoptic nuclei, which are preferentially activated by intravenous infusion of either hypertonic saline or angiotensin II. Following infusion of hypertonic saline, the greatest percentage of neurons both labelled with cholera toxin-gold and having elevated levels of Fos protein occurred in that part of the lamina terminalis called the organum vasculosum lamina terminalis. Conversely, angiotensin infusion resulted in greatest numbers of Fos and cholera toxin-gold-labelled neurons in the subfornical organ with fewer double-labelled cells represented in the other components of the lamina terminalis, the median preoptic nucleus and the organum vasculosum lamina terminalis. While these data do not support more than a general separation of the functions examined among neurons of the lamina terminalis, they do highlight a discrete group of osmoresponsive neurons in the dorsal cap of the organum vasculosum lamina terminalis. These cells, by virtue of their response to infusions of hypertonic saline and their axonal connections to regions of the hypothalamus responsible for vasopressin production, are likely candidates for cerebral osmoreceptors.

The sensory circumventricular organs of the mammalian brain.

The brains three sensory circumventricular organs, the subfornical organ, organum vasculosum of the lamina terminalis and the area postrema lack a blood brain barrier and are the only regions in the brain in which neurons are exposed to the chemical environment of the systemic circulation. Therefore they are ideally placed to monitor the changes in osmotic, ionic and hormonal composition of the blood. This book describes their. General structure and relationship to the cerebral ventricles Regional subdivisions Vasculature and barrier properties Neurons, glia and ependymal cells Receptors, neurotransmitters, neuropeptides and enzymes Neuroanatomical connections Functions.

Vasopressin secretion: osmotic and hormonal regulation by the lamina terminalis.

The lamina terminalis, located in the anterior wall of the third ventricle, is comprised of the subfornical organ, median preoptic nucleus (MnPO) and organum vasculosum of the lamina terminalis (OVLT). The subfornical organ and OVLT are two of the brains circumventricular organs that lack the blood–brain barrier, and are therefore exposed to the ionic and hormonal environment of the systemic circulation. Previous investigations in sheep and rats show that this region of the brain has a crucial role in osmoregulatory vasopressin secretion and thirst. The effects of lesions of the lamina terminalis, studies of immediate–early gene expression and electrophysiological data show that all three regions of the lamina terminalis are involved in osmoregulation. There is considerable evidence that physiological osmoreceptors subserving vasopressin release are located in the dorsal cap region of the OVLT and possibly also around the periphery of the subfornical organ and in the MnPO. The circulating peptide hormones angiotensin II and relaxin also have access to peptide specific receptors (AT1 and LGR7 receptors, respectively) in the subfornical organ and OVLT, and both angiotensin II and relaxin act on the subfornical organ to stimulate water drinking in the rat. Studies that combined neuroanatomical tracing and detection of c‐fos expression in response to angiotensin II or relaxin suggest that both of these circulating peptides act on neurones within the dorsal cap of the OVLT and the periphery of the subfornical organ to stimulate vasopressin release.

Intravenous hypertonic saline induces Fos immunoreactivity in neurons throughout the lamina terminalis.

Expression of Fos, the protein product of c-fos, was studied immunohistochemically in the forebrain of rats infused intravenously with hypertonic solutions. Intravenous 1.5 or 0.75 mol/l NaCl or 1.2 mol/l sucrose in 0.15 mol/l NaCl, but not isotonic 0.15 mol/l NaCl, caused increased Fos expression in the hypothalamic paraventricular and supraoptic nuclei and throughout the lamina terminalis (organum vasculosum laminae terminalis, median preoptic nucleus and subfornical organ). These results show that neurons in the lamina terminalis are activated by physiological increases in plasma tonicity and support an involvement of the lamina terminalis in osmoregulation.

Intravenous angiotensin II induces Fos-immunoreactivity in circumventricular organs of the lamina terminalis.

Conscious rats were infused intravenously with either angiotensin II (30-55 pmol/kg/min), isotonic saline or phenylephrine for 2 h, then killed. Fos was identified by immunohistochemistry in the brains. Fos expression occurred in many neurons of the subfornical organ and organum vasculosum of the lamina terminalis (OVLT) with angiotensin infusion but not with isotonic NaCl or phenylephrine. Fos immunoreactivity was induced in cells in several medullary, hypothalamic and limbic structures with infusions of angiotensin II or phenylephrine at pressor doses. The results suggest that blood-borne angiotensin II at physiological levels causes angiotensin receptive neurons in the subfornical organ and OVLT to express Fos. Activation of baroreceptor pathways may also induce Fos expression at several other sites.

Mice lacking angiotensin-converting enzyme have increased energy expenditure, with reduced fat mass and improved glucose clearance

In addition to its role in the storage of fat, adipose tissue acts as an endocrine organ, and it contains a functional renin-angiotensin system (RAS). Angiotensin-converting enzyme (ACE) plays a key role in the RAS by converting angiotensin I to the bioactive peptide angiotensin II (Ang II). In the present study, the effect of targeting the RAS in body energy homeostasis and glucose tolerance was determined in homozygous mice in which the gene for ACE had been deleted (ACE−/−) and compared with wild-type littermates. Compared with wild-type littermates, ACE−/− mice had lower body weight and a lower proportion of body fat, especially in the abdomen. ACE−/− mice had greater fed-state total energy expenditure (TEE) and resting energy expenditure (REE) than wild-type littermates. There were pronounced increases in gene expression of enzymes related to lipolysis and fatty acid oxidation (lipoprotein lipase, carnitine palmitoyl transferase, long-chain acetyl CoA dehydrogenase) in the liver of ACE−/− mice and also lower plasma leptin. In contrast, no differences were detected in daily food intake, activity, fed-state plasma lipids, or proportion of fat excreted in fecal matter. In conclusion, the reduction in ACE activity is associated with a decreased accumulation of body fat, especially in abdominal fat depots. The decreased body fat in ACE−/− mice is independent of food intake and appears to be due to a high energy expenditure related to increased metabolism of fatty acids in the liver, with the additional effect of increased glucose tolerance.

The cholinergic anti-inflammatory pathway: A critical review

From a critical review of the evidence on the cholinergic anti-inflammatory pathway and its mode of action, the following conclusions were reached. (1) Both local and systemic inflammation may be suppressed by electrical stimulation of the peripheral cut end of either vagus. (2) The spleen mediates most of the systemic inflammatory response (measured by TNF-α production) to systemic endotoxin and is also the site where that response is suppressed by vagal stimulation. (3) The anti-inflammatory effect of vagal stimulation depends on the presence of noradrenaline-containing nerve terminals in the spleen. (4) There is no disynaptic connection from the vagus to the spleen via the splenic sympathetic nerve: vagal stimulation does not drive action potentials in the splenic nerve. (5) Acetylcholine-synthesizing T lymphocytes provide an essential non-neural link in the anti-inflammatory pathway from vagus to spleen. (6) Alpha-7 subunit-containing nicotinic receptors are essential for the vagal anti-inflammatory action: their critical location is uncertain, but is suggested here to be on splenic sympathetic nerve terminals. (7) The vagal anti-inflammatory pathway can be activated electrically or pharmacologically, but it is not the efferent arm of the inflammatory reflex response to endotoxemia.