Distribution of cells expressing human renin-promoter activity in the brain of a transgenic mouse

Renin plays a critical role in fluid and electrolyte homeostasis by cleaving angiotensinogen to produce Ang peptides. Whilst it has been demonstrated that renin mRNA is expressed in the brain, the distribution of cells responsible for this expression remains uncertain. We have used a transgenic mouse approach in an attempt to address this question. A transgenic mouse, in which a 12.2 kb fragment of the human renin promoter was used to drive expression of Cre-recombinase, was crossed with the ROSA26-lac Z reporter mouse strain. Cre-recombinase mediated excision of the floxed stop cassette resulted in expression of the reporter protein, beta-galactosidase. This study describes the distribution of beta-galactosidase in the brain of the crossed transgenic mouse. In all cases where it was examined the reporter protein was co-localized with the neuronal marker NeuN. An extensive distribution was observed with numerous cells labeled in the somatosensory, insular, piriform and retrosplenial cortices. The motor cortex was devoid of labeled cells. Several other regions were labeled including the parts of the amygdala, periaqueductal gray, lateral parabrachial nucleus and deep cerebellar nuclei. Overall the distribution shows little overlap with those regions that are known to express receptors for the renin-angiotensin system in the adult brain. This transgenic approach, which demonstrates the distribution of cells which have activated the human renin promoter at any time throughout development, yields a unique and extensive distribution of putative renin-expressing neurons. Our observations suggest that renin may have broader actions in the brain and may indicate a potential for interaction with the (pro)renin receptor or production of a ligand for non-AT(1)/AT(2) receptors.

Diverse cell signalling pathways regulate pollen-stigma interactions: the search for consensus

Siphonogamy, the delivery of nonmotile sperm to the egg via a pollen tube, was a key innovation that allowed flowering plants (angiosperms) to carry out sexual reproduction on land without the need for water. This process begins with a pollen grain (male gametophyte) alighting on and adhering to the stigma of a flower. If conditions are right, the pollen grain germinates to produce a pollen tube. The pollen tube invades the stigma and grows through the style towards the ovary, where it enters an ovule, penetrates the embryo sac (female gametophyte) and releases two sperm cells, one of which fertilizes the egg, while the other fuses with the two polar nuclei of the central cell to form the triploid endosperm. The events before fertilization (pollen-pistil interactions) comprise a series of complex cellular interactions involving a continuous exchange of signals between the haploid pollen and the diploid maternal tissue of the pistil (sporophyte). In recent years, significant progress has been made in elucidating the molecular identity of these signals and the cellular interactions that they regulate. Here we review our current understanding of the cellular and molecular interactions that mediate the earliest of these interactions between the pollen and the pistil that occur on or within the stigma - the 'pollen-stigma interaction'.

Lateral hypothalamic 'command neurons' with axonal projections to regions involved in both feeding and thermogenesis

The concept of 'command neurons', whereby single neurons mediate complex and complementary motor functions to generate a stereotyped behaviour, is well developed in invertebrate physiology. The term has also been adopted more recently to explain the neural basis of 'fight or flight'. In this study we have investigated the possibility that single lateral hypothalamic neurons have the necessary neuroanatomical connections to coordinate two complementary limbs of body weight control, feeding and thermogenesis, thereby acting as 'command neurons'. The transynaptic retrograde transport of pseudorabies virus (Bartha) from a thermogenic endpoint in the brown adipose tissue of rats has been used in conjunction with other neuronal tracers, introduced into putative CNS feeding centres, to assess the potential for the involvement of command neurons in coordinating these processes. In discrete regions of the lateral hypothalamus, neurons have been identified which have the necessary complement of orexigenic peptides and collateral branching axons to both putative feeding sites and thermogenic sites in brown fat to qualify as candidate central command neurons controlling body weight.

Tyzzer's disease in free-living cottontail rabbits (Sylvilagus floridanus) in Maryland

Complement-fixing (CF) antibody to Bacillus piliformis antigen was found in 9 of 14 (64%) serum samples obtained from cottontail rabbits (Sylvilagus floridanus) killed in the wild. CF antibody was not present in the serum of 8 cottontail rabbits trapped as juveniles in the same geographic areas and held in captivity for 4 years. Sero-negative cottontail rabbits died acutely with lesions typical of Tyzzer's disease following the intragastric administration of 10(3.8) ELD50 of B. piliformis spores. The possible influence of Tyzzer's disease upon the cyclic population pattern of cottontail rabbits in the wild is discussed. A hypothesis is presented that B. piliformis spores passed in the feces of diseased wild animals could contaminate pastures, hay and grain, and thereby serve as sources of infection to other animals.

Separate neurochemical classes of sympathetic postganglionic neurons project to the left ventricle of the rat heart

The sympathetic innervation of the rat heart was investigated by retrograde neuronal tracing and multiple label immunohistochemistry. Injections of Fast Blue made into the left ventricular wall labelled sympathetic neurons that were located along the medial border of both the left and right stellate ganglia. Cardiac projecting sympathetic postganglionic neurons could be grouped into one of four neurochemical populations, characterised by their content of calbindin and/or neuropeptide Y (NPY). The subpopulations of neurons contained immunoreactivity to both calbindin and NPY, immunoreactivity to calbindin only, immunoreactivity to NPY only and no immunoreactivity to calbindin or NPY. Sympathetic postganglionic neurons were also labelled in vitro with rhodamine dextran applied to the cut end of a cardiac nerve. The same neurochemical subpopulations of sympathetic neurons were identified by using this technique but in different proportions to those labelled from the left ventricle. Preganglionic terminals that were immunoreactive for another calcium-binding protein, calretinin, preferentially surrounded retrogradely labelled neurons that were immunoreactive for both calbindin and NPY. The separate sympathetic pathways projecting to the rat heart may control different cardiac functions.

Descending vasomotor pathways from the dorsomedial hypothalamic nucleus: role of medullary raphe and RVLM

The dorsomedial hypothalamic nucleus (DMH) is believed to play a key role in mediating vasomotor and cardiac responses evoked by an acute stress. Inhibition of neurons in the rostral ventrolateral medulla (RVLM) greatly reduces the increase in renal sympathetic nerve activity (RSNA) evoked by activation of the DMH, indicating that RVLM neurons mediate, at least in part, the vasomotor component of the DMH-evoked response. In this study, the first aim was to determine whether neurons in the medullary raphe pallidus (RP) region also contribute to the DMH-evoked vasomotor response, because it has been shown that the DMH-evoked tachycardia is mediated by the RP region. The second aim was to directly assess the effect of DMH activation on the firing rate of RVLM sympathetic premotor neurons. In urethane-anesthetized rats, injection of the GABA(A) receptor agonist muscimol (but not vehicle solution) in the RP region caused a modest ( approximately 25%) but significant reduction in the increase in RSNA evoked by DMH disinhibition (by microinjection of bicuculline). In other experiments, disinhibition of the DMH resulted in a powerful excitation (increase in firing rate of approximately 400%) of 5 out of 6 spinally projecting barosensitive neurons in the RVLM. The results indicate that neurons in the RP region make a modest contribution to the renal sympathoexcitatory response evoked from the DMH and also that sympathetic premotor neurons in the RVLM receive strong excitatory inputs from DMH neurons, consistent with the view that the RVLM plays a key role in mediating sympathetic vasomotor responses arising from the DMH.

Effect of I.C.V. injection of AT4 receptor ligands, NLE1-angiotensin IV and LVV-hemorphin 7, on spatial learning in rats

Central administration of angiotensin IV (Ang IV) or its analogues enhance performance of rats in passive avoidance and spatial memory paradigms. The purpose of this study was to examine the effect of a single bolus injection of two distinct AT4 ligands, Nle1-Ang IV or LVV-haemorphin-7, on spatial learning in the Barnes circular maze. Mean number of days for rats treated with either Nle1-Ang IV or LVV-haemorphin-7 to achieve learner criterion is significantly reduced compared with controls (P < 0.001 and P < 0.05 respectively). This is due to enhanced ability of the peptide-treated rats to adopt a spatial strategy for finding the escape hatch. In all three measures of learning performance, (1) the number of errors made, (2) the distance travelled and (3) the latency in finding the escape hatch, rats treated with either 100 pmol or 1 nmol of Nle1-Ang IV or 100 pmol LVV-haemorphin-7 performed significantly better than the control groups. As early as the first day of testing, the rats treated with the lower dose of Nle1-Ang IV or LVV-haemorphin-7 made fewer errors (P < 0.01 and P < 0.05 respectively) and travelled shorter distances (P < 0.05 for both groups) than the control animals. The enhanced spatial learning induced by Nle1-Ang IV (100 pmol) was attenuated by the co-administration of the AT4 receptor antagonist, divalinal-Ang IV (10 nmol). Thus, administration of AT4 ligands results in an immediate potentiation of learning, which may be associated with facilitation of synaptic transmission and/or enhancement of acetylcholine release.

Lesions of the diagonal band of broca enhance drinking in the rat

This study examined the role of the diagonal band of Broca (DBB) in drinking behaviour and vasopressin release. Adult male rats were anaesthetized (pentobarbital 50 mg/kg) and received DBB injections of either ibotenic acid (0.5 microl of 5 micro g/ microl) or vehicle (0.5 microl of phosphate-buffered saline). Although baseline drinking and urine output were not affected, drinking to 30% polyethylene glycol (MW 8000; 1 ml/100 g s.c.) and angiotensin II (0, 1.5 and 3.0 mg/kg s.c.) were significantly increased in ibotenic acid in phosphate-buffered saline (DBBX) rats. Drinking to hypertonic saline (0.9, 4 and 6%; 1 ml/100 g), and water deprivation were not significantly affected. DBBX rats had significantly lower basal heart rates than controls but the cardiovascular responses to infusions of angiotensin II (100 ng/kg/min i.v. for 45 min) were not affected. DBBX rats had significantly higher basal vasopressin, but angiotensin-stimulated vasopressin release was not significantly different. Although the DBB is not involved in basal water intake, it is involved in dipsogenic responses to hypovolemic stimuli and possibly basal autonomic function and basal vasopressin release.

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.

Do dose-volume metrics predict pulmonary function changes in lung irradiation?

PURPOSE

To examine the ability of standard dose-volume metrics to predict pulmonary function changes as measured by pulmonary function tests (PFTs) in a group of patients with non-small-cell lung cancer treated with nonconventional beam arrangements on a Phase I dose-escalation study. In addition, we wanted to examine the correlation between these metrics.

MATERIALS AND METHODS

Forty-three patients received a median treatment dose of 76.9 Gy (range 63-102.9). Eight patients also received induction chemotherapy with cisplatin and vinorelbine. They all had pre- and posttreatment PFTs >/=3 months (median 6.2) after treatment. The volume of normal lung treated to >20 Gy, effective volume, and mean lung dose were calculated for both lungs for all patients. Linear regression analysis was performed to determine whether correlations existed between the metrics and changes in the PFTs. Additionally, the three metrics were compared with each other to assess the degree of intermetric correlation.

RESULTS

No correlation was found between the volume of normal lung treated to >20 Gy, effective volume, and mean lung dose and changes in the PFTs. Subgroup analyses of patients without atelectasis before irradiation, Stage I and II disease, or treatment without induction chemotherapy were also performed. Again, no correlation was found between the dose-volume metrics and the PFT changes. The intermetric correlation was good among all three dose-volume metrics.

CONCLUSIONS

In this relatively small series of patients, dose-volume metrics that correlate with the risk of pneumonitis did not provide a good model to predict early changes in pulmonary function as measured with PFTs.

Neural pathways from the lamina terminalis influencing cardiovascular and body fluid homeostasis

1. The lamina terminalis, a region of the brain with a high concentration of angiotensin AT1 receptors, consists of three distinct nuclei, the median preoptic nucleus, the subfornical organ and organum vasculosum of the lamina terminalis (OVLT). These latter two regions lack a blood-brain and detect changes in plasma angiotensin (Ang) II concentration and osmolality. 2. Efferent neural pathways from the lamina terminalis to the hypothalamic paraventricular and supraoptic nuclei mediate vasopressin secretion in response to plasma hypertonicity and increased circulating levels of AngII. 3. Studies using the neurotropic virus pseudorabies, which undergoes retrograde transynaptic neuronal transport following injection into peripheral sites, show that neurons in the lamina terminalis have efferent polysynaptic neural connections to the peripheral sympathetic nervous system. Some of these neurons have been shown to have polysynaptic connections to the kidney and to express AT1 receptor mRNA. We propose that circulating AngII acts at AT1 receptors in the subfornical organ and OVLT to influence the sympathetic nervous system. It is likely that the neural pathway subserving this influence involves a synapse in the hypothalamic paraventricular nucleus. 4. The lamina terminalis may exert an inhibitory osmoregulatory influence on renin secretion by the kidney. This osmoregulatory influence may be mediated by inhibition of renal sympathetic nerve activity and appears to involve a central angiotensinergic synapse. 5. The lamina terminalis exerts an osmoregulatory influence on renal sodium excretion that is independent of the renal nerves and is probably hormonally mediated.

Brain angiotensin and body fluid homeostasis

Angiotensinogen, the precursor molecule of the peptides angiotensin I, II, and III, is synthesized in the brain and the liver. Evidence is reviewed that angiotensin II, and possibly angiotensin III, that are generated within the brain act within neural circuits of the central nervous system to regulate body fluid balance. Immunohistochemical studies in the rat brain have provided evidence of angiotensin-containing neurons, especially in the hypothalamic paraventricular nucleus, subfornical organ, periventricular region, and nucleus of the solitary tract, as well as in extensive angiotensin-containing fiber pathways. Angiotensin immunoreactivity is observed by electron microscope in synaptic vesicles in several brain regions, the most prominent of these being the central nucleus of the amygdala. Neurons in many parts of the brain (lamina terminalis, paraventricular and parabrachial nuclei, ventrolateral medulla, and nucleus of the solitary tract) known to be involved in the regulation of body fluid homeostasis exhibit angiotensin receptors of the AT(1) subtype. Pharmacological studies in several species show that intracerebroventricular administration of AT(1) receptor antagonist drugs inhibit homeostatic responses to the central administration of hypertonic saline, intravenous infusion of the hormone relaxin, or thermal dehydration. Responses affected by centrally administered AT(1) antagonists are water drinking, vasopressin secretion, natriuresis, increased arterial pressure, reduced renal renin release, salt hunger, and thermoregulatory adjustments. We conclude that angiotensinergic neural pathways in the brain probably have an important homeostatic function, especially in regard to osmoregulation and thermoregulation, and the maintenance of arterial pressure.

Baroreflex inhibition of cardiac sympathetic outflow is attenuated by angiotensin II in the nucleus of the solitary tract

Homeostatic regulation of arterial pressure is maintained by arterial baroreceptors. Activation of these receptors results in an inhibition of sympathetic activity to the heart. It is known that angiotensin II in the nucleus tractus solitarii attenuates the baroreceptor reflex-evoked vagal bradycardia. Here, we determined whether the cardiac sympathetic component of the baroreceptor reflex could be modulated by angiotensin II in the nucleus of the solitary tract. An in situ, arterially perfused working heart--brainstem preparation of rat was employed and the sympathetic inferior cardiac nerve recorded. Increases in perfusion pressure caused a reflex bradycardia and inhibition of inferior cardiac nerve activity. Microinjection of angiotensin II (500 fmol) in the nucleus of the solitary tract attenuated significantly both the reflex bradycardia and inhibition of inferior cardiac nerve activity (P<0.01). The latter was reversible and sensitive to losartan, an angiotensin II type 1 receptor antagonist. In contrast, the peripheral chemoreceptor reflex evoked an increase in inferior cardiac nerve activity that was not affected by angiotensin II applied exogenously in the nucleus of the solitary tract. We conclude that within the nucleus of the solitary tract angiotensin II exerts a powerful and specific inhibitory modulation of the baroreceptor reflex control of sympathetic nerve activity destined for the heart. We suggest that our data may have clinical implications relating to hypertension, a condition when angiotensin II activity is heightened in the brain and the efficacy of the baroreflex is reduced.

Analysis of firing correlations between sympathetic premotor neuron pairs in anesthetized cats

The activity of sympathetic premotor neurons in the rostral ventrolateral medulla (subretrofacial nucleus) supports sympathetic vasomotor tone, but the factors that drive these premotor neurons' activity have not been determined. This study examines whether either direct interconnections between subretrofacial neurons or synchronizing common inputs to them are important for generating their tonic activity. Simultaneous extracellular single-unit recordings were made from 32 pairs of sympathetic premotor neurons in the subretrofacial nucleus of chloralose-anesthetized cats. Paired spike trains were either separated by spike shape from a single-electrode recording (14 pairs) or recorded from two electrodes less than 250 microm apart (18 pairs). All neurons were inhibited by carotid baroreceptor stimulation and most had a spinal axon proven by antidromic stimulation from the spinal cord. Autocorrelation, inter-spike interval, and cardiac cycle-triggered histograms were constructed from the spontaneous activity of each neuron, and cross-correlation histograms covering several time scales were generated for each neuron pair. No significant peaks or troughs were found in short-term cross-correlation histograms (2 ms bins, +/-100 ms range), providing no support for important local synaptic interactions. On an intermediate time scale (20 ms bins, +/-1 s range), cross-correlation revealed two patterns indicating shared, synchronizing inputs. Repeating peaks and troughs (19/32 pairs) were due to the two neurons' common cardiac rhythmicity, of presumed baroreceptor origin. Single, zero time-spanning peaks of 40--180 ms width were seen in 5/32 cases. Calculations based on the prevalence and strength of these synchronizing inputs indicate that most of the ensemble spike activity of the subretrofacial neuron population is derived from asynchronous sources (be they intrinsic or extrinsic). If synchronizing sources such as neuronal oscillators were responsible for more than a minor part of the drive, they would be multiple, dispersed, and weak.

Potentiation of cholinergic transmission in the rat hippocampus by angiotensin IV and LVV-hemorphin-7

Recent evidence demonstrates that the fragment of angiotensin II, angiotensin II (3-8) termed angiotensin IV, binds with high affinity to a specific binding site, the AT(4) receptor. Intracerebroventricular injection of AT(4) receptor agonists improves the performance of rats in passive avoidance and spatial learning paradigms. AT(4) receptors and cholinergic neurons are closely associated in regions involved in cognitive processing, such as the hippocampus and neocortex. We therefore postulated that AT(4) receptors affect cognitive processing by modulating cholinergic neurotransmission. To test this, we examined the effect of AT(4) receptor ligands, angiotensin IV and LVV-hemorphin-7, on potassium-evoked [(3)H]acetylcholine ([(3)H]ACh) release from rat hippocampal slices. Hippocampal slices from male Sprague--Dawley rats were incubated with [(3)H]choline chloride, perfused with Krebs--Henseleit solution and [(3)H]ACh release was determined. Angiotensin IV and LVV-hemorphin-7 both potentiated depolarisation-induced [(3)H]ACh release from the rat hippocampus in a concentration-dependent manner with the maximal dose (10(-7)M) of each inducing an increase of 45+/-7.5% (P<0.01) and 95.8+/-19% (P<0.01) above control, respectively. Potentiation of release by both agonists was attenuated by the AT(4) receptor antagonist, divalinal-Ang IV. Angiotensin IV-induced potentiation was not affected by AT(1) and AT(2) receptor antagonists. These results indicate that stimulation of AT(4) receptors can potentiate depolarisation-induced release of ACh from hippocampal slices and suggest that potentiation of cholinergic transmission may be a mechanism by which AT(4) receptor ligands enhance cognition.

Efferent neural projections of angiotensin receptor (AT1) expressing neurones in the hypothalamic paraventricular nucleus of the rat

Angiotensin II acts within the hypothalamic paraventricular nucleus (PVN) to help mediate a number of autonomic and endocrine responses. Evidence is sparse in regard to the particular neuronal cell groups that exhibit angiotensin II type 1 receptors within the PVN, and does not exist in relation to specified efferent neuronal populations in the nucleus. In the present experiments, retrogradely transported neuronal tracers were utilized in conjunction with immunohistochemistry using a well characterized polyclonal antibody raised against a decapeptide sequence at the carboxy terminus of the AT1 receptor, to determine whether it is preferentially distributed amongst different efferent populations within the PVN. The AT1 receptor is not associated with neurones in the PVN that project axons to the spinal cord, dorsomedial or ventrolateral medulla but coexists strongly with neurones in the anterior parvocellular division of the nucleus which direct axons to the median eminence. Such neurones often contain corticotropin releasing factor. These findings highlight the role that angiotensin II and AT1 receptors in the PVN may play in the mediation of responses to stress.

Autoradiographic localization and quantification of components of the Renin-Angiotensin system in tissues

In situ radioligand binding with autoradiography allows localization and quantification of bound radiolabeled ligands in tissues. This is a very sensitive technique that enables the characterization of binding kinetics and ligand specificity and the quantification of the amount of radioligand bound in different structures within the tissue. This technique is complementary to the higher resolution of immunohistochemical localization of proteins or binding sites on fixed tissue sections and in situ hybridization histochemical localization of mRNA.

Does angiotensin II have a significant tonic action on cardiovascular neurons in the rostral and caudal VLM?

The peptidic ANG II receptor antagonists [Sar(1),Ile(8)]ANG II (sarile) or [Sar(1),Thr(8)]ANG II (sarthran) are known to decrease arterial pressure and sympathetic activity when injected into the rostral part of the ventrolateral medulla (VLM). In anesthetized rabbits and rats, the profound depressor and sympathoinhibitory response after bilateral microinjections of sarile or sarthran into the rostral VLM was unchanged after prior selective blockade of angiotensin type 1 (AT(1)) and ANG-(1---7) receptors, although this abolished the effects of exogenous ANG II. Unlike the neuroinhibitory compounds muscimol or lignocaine, microinjections of sarile in the rostral VLM did not affect respiratory activity. Sarile or sarthran in the caudal VLM resulted in a large pressor and sympathoexcitatory response, which was also unaffected by prior blockade of AT(1) and ANG-(1---7) receptors. The results indicate that the peptidic ANG receptor antagonists profoundly inhibit the tonic activity of cardiovascular but not respiratory neurons in the VLM and that these effects are independent of ANG II or ANG-(1---7) receptors.

The physiological role of AT1 receptors in the ventrolateral medulla

Neurons in the rostral and caudal parts of the ventrolateral medulla (VLM) play a pivotal role in the regulation of sympathetic vasomotor activity and blood pressure. Studies in several species, including humans, have shown that these regions contain a high density of AT1 receptors specifically associated with neurons that regulate the sympathetic vasomotor outflow, or the secretion of vasopressin from the hypothalamus. It is well established that specific activation of AT1 receptors by application of exogenous angiotensin II in the rostral and caudal VLM excites sympathoexcitatory and sympathoinhibitory neurons, respectively, but the physiological role of these receptors in the normal synaptic regulation of VLM neurons is not known. In this paper we review studies which have defined the effects of specific activation or blockade of these receptors on cardiovascular function, and discuss what these findings tell us with regard to the physiological role of AT1 receptors in the VLM in the tonic and phasic regulation of sympathetic vasomotor activity and blood pressure.

Localization and function of angiotensin AT1 receptors

The distributions of angiotensin AT1 and AT2 receptors have been mapped by in vitro autoradiography throughout most tissues of many mammals, including humans. In addition to confirming that AT1 receptors occur in sites known to be targets for the physiologic actions of angiotensin, such as the adrenal cortex and medulla, renal glomeruli and proximal tubules, vascular and cardiac muscle and brain circumventricular organs, many new sites of action have been demonstrated. In the kidney, AT1 receptors occur in high density in renal medullary interstitial cells. The function of these cells, which span the interstitial space between the tubules and the vasa rectae, remains to be determined. Renal medullary interstitial cells possess receptors for a number of vasoactive hormones in addition to AT1 receptors and this, in concert with their anatomic location, suggests they may be important for the regulation of fluid reabsorption or renal medullary blood flow. In the heart, the highest densities of AT1 receptors occur in association with the conduction system and vagal ganglia. In the central nervous system, high AT1 receptor densities occur in many regions behind the blood-brain barrier, supporting a role for neurally derived angiotensin as a neuromodulator. The physiologic role of angiotensin in many of these brain sites remains to be determined. The AT2 receptor also has a characteristic distribution in several tissues including the adrenal gland, heart, and brain. The role of this receptor in physiology is being elucidated, but it appears to inhibit proliferation and to participate in development. Thus, receptor-binding studies, localizing the distribution of AT1 and AT2 receptors, provide many insights into novel physiologic roles of angiotensin.

Angiotensin II receptors in the human brain

The distribution of angiotensin AT1 and AT2 receptors in the human central nervous system has been mapped and is reviewed here. The results discussed provide the anatomical basis for inferences regarding the physiological role of angiotensin in the human brain. The distribution of the AT2 receptor is very restricted in the human brain and shows a high degree of variability across species. The physiological role of this receptor in the adult central nervous system is not clear. In contrast, a high correlation exists between the distributions of AT1 receptors in the human and other mammalian brains studied. This pattern of distribution suggests that angiotensin, acting through the AT1 receptor, would act as a neuromodulator or neurotransmitter in the human central nervous system to influence fluid and electrolyte homeostasis, pituitary hormone release and autonomic control of cardiovascular function.

Localization of angiotensin AT1 and AT2 receptors

The distributions of angiotensin AT1 and AT2 receptors have been mapped by in vitro autoradiography throughout most tissues of many mammals, including humans. In addition to confirming that AT1 receptors occur in sites known to be targets for the physiologic actions of angiotensin, such as the adrenal cortex and medulla, renal glomeruli and proximal tubules, vascular and cardiac muscle, and brain circumventricular organs, many new sites of action have been demonstrated. In the kidney, AT1 receptors occur in high density in renal medullary interstitial cells. The function of these cells, which span the interstitial space between the tubules and the vasa rectae, remains to be determined. Renal medullary interstitial cells possess receptors for a number of vasoactive hormones in addition to AT1 receptors and this, in concert with their anatomical location, suggest that they may be important for the regulation of fluid reabsorption or renal medullary blood flow. In the heart, the highest densities of AT1 receptors occur in association with the conduction system and vagal ganglia. In the central nervous system, high AT1 receptor densities occur in many regions behind the blood-brain barrier, supporting a role for neurally derived angiotensin as a neuromodulator. The physiologic role of angiotensin in many of these brain sites remains to be determined. The AT2 receptor also has a characteristic distribution in several tissues including the adrenal gland, heart, and brain. The role of this receptor in physiology is being elucidated, but it appears to participate in development. Thus, receptor binding studies, localizing the distribution of AT1 and AT2 receptors, outline a number of regions where the actions of angiotensin are known but also provide many insights into novel physiologic roles of this peptide.

Mapping tissue angiotensin-converting enzyme and angiotensin AT1, AT2 and AT4 receptors

BACKGROUND

The renin-angiotensin system (RAS) functions as both a circulating endocrine system and a tissue paracrine/autocrine system. As a circulating peptide, angiotensin II (Ang II) plays a prominent role in blood-pressure control and body fluid and electrolyte balance by acting on the AT1 receptor in the brain and peripheral tissues. As a paracrine/autocrine peptide, locally formed Ang II also plays additional roles in tissues involving the regulation of regional haemodynamics, cell growth and remodelling, and neurotransmitter release. Evidence is emerging that Ang II is not the only active peptide of the RAS, and other Ang II fragments may also have important biological activities.

OBJECTIVES

To provide a morphological basis for understanding novel actions of angiotensin-converting enzyme (ACE), Ang II and related peptides in tissues, this article will review the localization of ACE and AT1, AT2 and AT4 receptors in the central nervous system, blood vessels and kidney.

RESULTS AND CONCLUSION

Autoradiographic mapping of the major components of the RAS has proved a valuable strategy to reveal, or suggest, cellular sites of novel actions for Ang II and related peptides in tissues. First, colocalization of ACE and AT1 receptors in the substantia nigra, the caudate nucleus and putamen of human and rat brain, which contain the dopamine-synthesizing neurons, suggests that the central RAS may be important in modulating central dopamine release. Secondly, the distribution of AT4 receptors with a striking association with cholinergic neurons, motor and sensory nuclei in the brain reveals that Ang IV may modulate central motor and sensory activities and memory. Thirdly, the occurrence of high levels of ACE and AT1 and/or AT2 receptors in the adventitia of blood vessels suggests important paracrine roles of the vascular RAS. Finally, the identification of abundant AT1 receptor and elucidation of its roles in the renomedullary interstitial cells of the kidney may provide a new impetus to study further the role of Ang II in the regulation of renal medullary function and blood pressure. Overall, circulating and locally produced Ang II and related peptides may exert a remarkable range of actions in the brain, kidney and cardiovascular system through multiple angiotensin receptors.