Renal reflexes in the regulation of blood pressure and sodium excretion

The rich innervation of the kidney is distributed to all structures of renal parenchyma thus providing important anatomical support to the functional evidence that the renal nerves can control kidney functions and send signals on the kidney environment to the central nervous system. Efferent renal nerve fibres are known to influence renal haemodynamics by modifying arteriolar vascular tone, renin release by a direct action on juxtaglomerular cells, and the excretion of sodium and water by changing tubular reabsorption of sodium and water at the different tubular levels. Mechano- and chemo-receptors have been shown in the kidney. Afferent fibres connected with renal receptors convey signals to the central nervous system both at spinal and supraspinal levels. The central areas receiving inputs from the kidney are those involved in the control of cardiovascular homeostasis and fluid balance. Activation of renal receptors by the electrical stimulation of renal afferent fibres were found to elicit both excitatory and inhibitory sympathetic responses. Although the existence of excitatory renorenal reflexes has been suggested, electrophysiological and functional data demonstrate that neural renorenal reflexes exert a tonic inhibitory influence on the tubular sodium and water reabsorption and on the secretion of renin from the juxtaglomerular cells.     

The heart and control of renal excretion: neural and endocrine mechanisms.

There has been a great deal of research concerning the heart being an important regulator of renal fluid and electrolyte excretion. This cardiac-renal connection involves two different types of major mechanisms, both of which are covered in this review. The first of these to be discovered was neural reflex regulation. This type of control is due to the fact that the heart possesses nerve receptors whose activity is altered by changes in the degree of cardiac stretch that occur as a result of changes in blood volume. These receptors affect various humoral, neural, and perhaps hemodynamic mechanisms that modify renal excretion. A second, more recently discovered type of regulation is based on the concept that the heart is also an endocrine gland. Similar to neural receptor activity, cardiac hormone secretion is also linked to the degree of cardiac stretch or filling. These cardiac peptides have been shown to have a variety of physiologic effects, most of which directly or indirectly affect renal excretion. Both of the above cardiorenal control mechanisms, one neural and one humoral, may be important not only in maintaining normal fluid-electrolyte balance but may also have pathophysiologic relevance.     

Regulation of breathing during exercise

This paper reviewed in short neural and humoral factors which might be responsible for inducing exercise hyperpnea. As one of the neural factors afferent signals which arise in the exercising limbs and are transmitted via group III or IV high threshold sensory fibres were involved. The other neural factor is command signals originating in the central nervous system and being fed onto the respiratory center. Hypothalamic locomotor region is assumed to be a possible locus to integrate these peripheral and central neural signals. There are enough evidences to believe that humoral factors mediated via cardiac output is also essential for the hyperpnea. Changes in VCO2 is well correlated with those of VE in dynamic as well as in steady-state response. Oscillations in PaCO2 can be assumed to play a role to link metabolic CO2 changes to those in ventilation. Thus, no single factor can explain the whole process of exercise hyperpnea. Poon's optimization model may give a key to integrate complicated and coflicting experimental results in a unique concept.     

Involvement of a direct neural mechanism in the control of gonadal functions

Much time has been devoted to study of the hypothalamo-hypophyseal-gonadal axis. However, there is now evidence of a complementary control mechanism for the gonads, namely a pituitary-independent, direct neural link that exists between the central nervous system and the gonads. We investigated whether mediobasal temporal lobe structures could control gonadal functions by a purely neural mechanism or whether they acted through the classical hypothalamo-hypophyseal system. Right- or left-sided deafferentation of the temporal lobe was combined with right- or left-sided hemicastration in adult and prepubertal male and female rats. In adult females right-sided deafferentation, regardless of the side of hemiovariectomy significantly reduced the extent of compensatory ovarian hypertrophy. Similar lesions on the left side did not interfere with the usual compensatory ovarian growth. This difference in compensatory hypertrophy between right- and left-sided lesioned rats was observed even in the face of a significant drop in serum LH concentrations in both groups. In pre- and postpubertal females temporal lobe lesion in either side was unable to alter compensatory hypertrophy or serum LH or progesterone concentrations. In adult male rats only left-sided deafferentation combined with left orchidectomy resulted in decreased T production, while in prepubertal male rats, only right-sided brain surgery plus left orchidectomy resulted in a significant decrease in basal testosterone secretion of the remaining testis. These findings indicate that mediobasal temporolimbic structures are involved in the neural control of gonadal functions. It appears that this lateralized mechanism is age- and sex-dependent.     

Satellite Glial Cells and Astrocytes,a Comparative Review

Astroglia are neural cells, heterogeneous in form and function, which act as supportive elements of the central nervous system; astrocytes contribute to all aspects of neural functions in health and disease. Through their highly ramified processes, astrocytes form close physical contacts with synapses and blood vessels, and are integrated into functional syncytia by gap junctions. Astrocytes interact among themselves and with other cells types (e.g., neurons, microglia, blood vessel cells) by an elaborate repertoire of chemical messengers and receptors; astrocytes also influence neural plasticity and synaptic transmission through maintaining homeostasis of neurotransmitters, K⁺ buffering, synaptic isolation and control over synaptogenesis and synaptic elimination. Satellite glial cells (SGCs) are the most abundant glial cells in sensory ganglia, and are believed to play major roles in sensory functions, but so far research into SGCs attracted relatively little attention. In this review we compare SGCs to astrocytes with the purpose of using the vast knowledge on astrocytes to explore new aspects of SGCs. We survey the main properties of these two cells types and highlight similarities and differences between them. We conclude that despite the much greater diversity in morphology and signaling mechanisms of astrocytes, there are some parallels between them and SGCs. Both types serve as boundary cells, separating different compartments in the nervous system, but much more needs to be learned on this aspect of SGCs. Astrocytes and SGCs employ chemical messengers and calcium waves for intercellular signaling, but their significance is still poorly understood for both cell types. Both types undergo major changes under pathological conditions, which have a protective function, but an also contribute to disease, and chronic pain in particular. The knowledge obtained on astrocytes is likely to benefit future research on SGCs.

     

Central actions of angiotensin in cardiovascular control: multiple roles for a single peptide.

Angiotensin II (ANG II) acts peripherally as a hormone, with actions on the vasculature, adrenals, and kidney. In addition, certain actions of ANG II in the central nervous system are directed toward cardiovascular control and fluid volume homeostasis. Dense binding sites for ANG II are found at circumventricular organs, which apparently have the ability to relay information to cardiovascular centers via neural circuitry. Microinjection of ANG II into the subfornical organ (SFO) or area postrema (AP) produces site-specific increases in blood pressure. In addition, electrophysiological studies demonstrate profound effects of ANG II, acting at the SFO, on activity of neurohypophysial neurons and release of oxytocin and vasopressin, which can be antagonized by ANG II blockers or attenuated by SFO lesions. Evidence from microinjection, electrophysiological, and lesion studies indicate a complex interaction between central sites involved in mechanisms of cardiovascular control: the SFO, AP, organum vasculosum of the lamina terminalis, and paraventricular and supraoptic nuclei of the hypothalamus. Not only is ANG II a humoral messenger in this central scenario, but evidence suggests it acts as a neurotransmitter or neuroendocrine substance within specific CNS pathways, suggesting multiple roles for this peptide in central cardiovascular control.     

Glia as mediators of steroid hormone action on the nervous system: An overview.

This special issue on steroids and glia represents the intersection of two emerging themes in the neurosciences: (a) Glia actively modulate and participate in brain function throughout life, and (b) glia are sensitive to steroid hormones. This overview begins by reviewing some of the basic principles of steroid hormone action on the brain and introducing the various glia that inhabit the peripheral and central nervous system. A prominent theme among the articles that follow is that glia may be direct targets for steroid hormones since they possess steroid receptors and the promoter region of glial-specific genes such as glutamine synthetase contain hormone-responsive elements. The articles in this special issue discuss evidence that glia may mediate steroid action on the nervous system in the context of (a) steroid metabolism, which may control the hormonal microenvironment of neurons both in the normal and injured brain; (b) brain development including sexual differentiation; (c) synaptic plasticity which may underlie the cyclic release of luteinizing hormone releasing hormone in the female rodent brain; (d) neural repair and aging; and (e) brain immune function. Another theme among these articles is that glia influence neurons via specific secreted and cell-surface molecules, and that steroids affect this mode of communication by altering the level of glial production of these signaling molecules and/or the sensitivity of neurons to such signals.     

Dual Labeling of Neural Crest Cells and Blood Vessels Within Chicken Embryos Using ChickGFP Neural Tube Grafting and Carbocyanine Dye DiI Injection

All developing organs need to be connected to both the nervous system (for sensory and motor control) as well as the vascular system (for gas exchange, fluid and nutrient supply). Consequently both the nervous and vascular systems develop alongside each other and share striking similarities in their branching architecture. Here we report embryonic manipulations that allow us to study the simultaneous development of neural crest-derived nervous tissue (in this case the enteric nervous system), and the vascular system. This is achieved by generating chicken chimeras via transplantation of discrete segments of the neural tube, and associated neural crest, combined with vascular DiI injection in the same embryo. Our method uses transgenic chick^GFP embryos for intraspecies grafting, making the transplant technique more powerful than the classical quail-chick interspecies grafting protocol used with great effect since the 1970s. Chick^GFP-chick intraspecies grafting facilitates imaging of transplanted cells and their projections in intact tissues, and eliminates any potential bias in cell development linked to species differences. This method takes full advantage of the ease of access of the avian embryo (compared with other vertebrate embryos) to study the co-development of the enteric nervous system and the vascular system.     

Central nervous system mechanisms of arterial pressure regulation

This paper provides a review of recent developments in the field of neural and humoral control of the cardiovascular system mediated through the central nervous system. The areas covered include central mechanism of baroreceptor reflex control, sites of origin of tonic vasomotor activity, interactions between forebrain and brain stem, central actions of humoral factors, the role of visceral and somatic afferents, and the potential for central selectivity of vasomotor control.     

Regression of cardiac hypertrophy with drug treatment in spontaneously hypertensive rats

Left ventricular hypertrophy is an important complication of essential hypertension. Some antihypertensive drugs have been shown to allow regression of cardiac hypertrophy, both in spontaneously hypertensive rats and in hypertensive patients. Recent results show that the agents which interfere with the functions of the sympathetic nervous system, converting enzyme inhibitors and calcium antagonists are effective in reducing arterial blood pressure and regression of left ventricular hypertrophy. The use of vasodilators and diuretics may under certain circumstances, however, even exacerbate cardiac hypertrophy. Regression of left ventricular hypertrophy in hypertension does not appear to depend solely on reduction of arterial blood pressure. Other factors seem to modulate the myocardial response to antihypertensive treatment. Included among these mechanisms are neural, humoral, haemodynamic and biochemical factors. The available experimental data further suggest that some functional derangements and biochemical changes associated with hypertrophy may be reversed by antihypertensive treatment. There is, however, insufficient experience with human subjects to determine whether a reduction in left ventricular mass is associated with lower incidences of heart failure or mortality than may be achieved by adequate blood pressure control alone.     

Dual vascular effects of leptin via endothelium: hypothesis and perspective

Secreted by adipocytes, leptin is a hormone which regulates appetite and metabolism. Leptin secretion is proportional to the fat mass, and thus leptin concentration is raised in most obese subjects. In recent years, more and more biological effects have been attributed to leptin; one of the most well-known effects is the effect of leptin on the vascular tone. Obesity is very often associated with hypertension, and it has been known that leptin affects the blood pressure by activating the sympathetic nervous system and causing endothelial cell (EC) dysfunction. However, there has been strong evidence that leptin is able to dilate blood vessels. Such vasodilation has been shown to be EC-dependent and EC-independent. Further, both nitric oxide-dependent and nitric oxide-independent mechanisms have been reported. In this mini-review, we summarize the heterogeneous mechanisms by which leptin causes relaxation of vascular smooth muscle. We also argue that while leptin may act as a direct dilator on the vasculature in healthy subjects, hyperleptinemia in obese subjects gradually dysregulates blood pressure control by deteriorating EC functions. How these dual effects of leptin on EC might be related to EC ionic channels is also discussed.     

Electromyography of pelvic floor muscles

Pelvic floor muscles (PFM) are intimately involved in function of lower urinary tract, the anorectum and sexual functions, therefore their neural control transcends the primarily important somatic innervation of striated muscle, as they are directly involved in “visceral activity”. Neural control of pelvic organs is affected by a unique co-ordination of somatic and autonomic motor nervous systems. Visceral and somatic sensory fibres supply sensory information from pelvic organs; their input influences through central integrative mechanisms also pelvic floor muscle activity. Anatomically, somatic afferent and efferent nerves of the sacral cord segments, reflexly integrated at the spinal cord and brainstem level, conduct neural control of PFM. The inputs from several higher centres influence the complex reflex control and are decisive for voluntary control, and for socially adapted behaviour related to excretory functions.     

Heart rate during exercise with leg vascular occlusion in spinal cord-injured humans.

Feed-forward and feedback mechanisms are both important for control of the heart rate response to muscular exercise, but their origin and relative importance remain inadequately understood. To evaluate whether humoral mechanisms are of importance, the heart rate response to electrically induced cycling was studied in participants with spinal cord injury (SCI) and compared with that elicited during volitional cycling in able-bodied persons (C). During voluntary exercise at an oxygen uptake of approximately 1 l/min, heart rate increased from 66 +/- 4 to 86 +/- 4 (SE) beats/min in seven C, and during electrically induced exercise at a similar oxygen uptake in SCI it increased from 73 +/- 3 to 110 +/- 8 beats/min. In contrast, blood pressure increased only in C (from 88 +/- 3 to 99 +/- 4 mmHg), confirming that, during exercise, blood pressure control is dominated by peripheral neural feedback mechanisms. With vascular occlusion of the legs, the exercise-induced increase in heart rate was reduced or even eliminated in the electrically stimulated SCI. For C, heart rate tended to be lower than during exercise with free circulation to the legs. Release of the cuff elevated heart rate only in SCI. These data suggest that humoral feedback is of importance for the heart rate response to exercise and especially so when influence from the central nervous system and peripheral neural feedback from the working muscles are impaired or eliminated during electrically induced exercise in individuals with SCI.     

Central nervous system and mechanisms of hypertension

There are several mechanisms by which the central nervous system participates in the neural and humoral alterations associated with various forms of experimental hypertension. Structures in forebrain with multiple integrative roles in neuroendocrine control of the circulation are involved. Tissue surrounding the anteroventral region of the third cerebral ventricle (AV3V region) is involved physiologically in thirst, sodium homeostasis, osmoreception, secretion of vasopressin and natriuretic factor and sympathetic discharge to blood vessels. Destruction of this tissue prevents or reverses many forms of hypertension. In genetically based spontaneous hypertension, limbic structures such as the central nucleus of the amygdala rather than the AV3V region are the necessary neuroanatomic substrate. Recent evidence suggests that a circumventricular organ in brain stem, the area postrema, is also involved in the mediation of several forms of experimental hypertension. In renin- and nonrenin-dependent forms of renal hypertension, two major factors activate central mechanisms. First, direct central actions of angiotensin, acting through receptors in the subfornical organ and organum vasculosum of the lamina terminalis, increase sympathetic discharge and secretion of vasopressin through mechanisms integrated at the level of the AV3V region. Second, sensory systems originating in the kidney can activate increased sympathetic discharge through complex projection pathways involving forebrain systems. Mineralocorticoid hypertension appears to involve enhanced secretion of vasopressin and central vasopressinergic mechanisms also dependent on the AV3V region. Reciprocal connections between key central areas involved in control of arterial pressure provide the neuroanatomical basis for central nervous system participation in hypertension.     

Temperature-sensitive mechanism regulating diapause in Heliothis zea

Pupal diapause in Heliothis zea is regulated by a temperature-sensitive mechanism which prevents ecdysone production despite the release of prothoracicotropic hormone. To determine how this mechanism functioned, donor prothoracic glands were implanted into prothoracic gland-ablated hosts to test their ability to produce ecdysone in a diapause-sustaining temperature of 19°C. Results of these experiments ruled out the possibility that ecdysis production was regulated by the nervous system or by a mechanism intrinsic to the prothoracic glands, and suggested that a humoral factor was required for diapause termination.Haemolymph injection experiments supported this humoral factor hypothesis, i.e. haemolymph from non-diapausing donor pupae terminated diapause in hosts maintained at 19°C, whereas haemolymph from diapausing donor pupae had no such effect. These findings indicate that the temperature-sensitive mechanism regulating H. zea diapause functions by controlling the availability of a humoral factor necessary for ecdysone production by the prothoracic glands.     

Identification of Sox8 as a modifier gene in a mouse model of Hirschsprung disease reveals underlying molecular defect

Mice carrying heterozygous mutations in the Sox10 gene display aganglionosis of the colon and represent a model for human Hirschsprung disease. Here, we show that the closely related Sox8 functions as a modifier gene for Sox10-dependent enteric nervous system defects as it increases both penetrance and severity of the defect in Sox10 heterozygous mice despite having no detectable influence on enteric nervous system development on its own. Sox8 exhibits an expression pattern very similar to Sox10 with occurrence in vagal and enteric neural crest cells and later confinement to enteric glia. Loss of Sox8 alleles in Sox10 heterozygous mice impaired colonization of the gut by enteric neural crest cells already at early times. Whereas proliferation, apoptosis, and neuronal differentiation were normal for enteric neural crest cells in the gut of mutant mice, apoptosis was dramatically increased in vagal neural crest cells outside the gut. The defects in enteric nervous system development of mice with Sox10 and Sox8 mutations are therefore likely caused by a reduction of the pool of undifferentiated vagal neural crest cells. Our study suggests that Sox8 and Sox10 are jointly required for the maintenance of these vagal neural crest stem cells.     

Central and peripheral mechanisms of arterial pressure lability following baroreceptor denervation

Deafferentation of sinoaortic baroreceptors produces a marked increase in the lability of arterial pressure that is sustained chronically. Studies reviewed in this paper were designed to determine the mechanisms responsible for generating arterial pressure lability. Pharmacological interruption of the humoral vasopressin and angiotensin systems failed to alter arterial pressure lability. In contrast, blockade of sympathetic nervous system transmission at both ganglionic and alpha-adrenergic receptor levels significantly attenuated lability. A similar effect was observed with the peripheral neurotoxin, 6-hydroxydopamine. After blockade of sympathetic transmission, a further reduction in lability was produced by blocking the renin-angiotensin or vasopressin systems. The dissociation of the level of arterial pressure from lability was achieved with parachloroamphetamine which raised arterial pressure but reduced lability. A substantial peripheral contribution to lability was obtained in experiments in which the alpha-adrenergic agonist, phenylephrine, produced a marked increase in lability in both normal and baroreceptor-denervated animals in which humoral and neural transmission were blocked. These data demonstrate that following baroreceptor deafferentation, arterial pressure lability is produced primarily by the sympathetic nervous system and secondarily by circulating humoral factors that appear to act on vascular smooth muscle to induce fluctuations in the level of arterial pressure.     

Effect of lithium on diffusion chamber granulopoiesis

We studied the effect of lithium on diffusion chamber (DC) granulopoiesis. When DC loaded with bone marrow cells were implanted into the peritoneal cavity of mice previously injected with lithium carbonate, more proliferative and nonproliferative granulocytes were produced as compared to DC implanted into control hosts. The number of DC CFU-c was increased significantly in the lithium-treated group, but there was no difference in the number of DC CFU-s. Levels of DC fluid CSF showed no evident correlation with DC myelopoiesis. These data suggest that a humoral factor other than CSF mediates the action of lithium in DC granulopoiesis, and that lithium's influence on DC hematopoietic stem cell proliferation occurs mainly at the CFU-c level.     

Granulopoiesis inhibition in acute inflammation: comparative studies in healthy and leukaemic mice

It was demonstrated previously that mice undergoing an inflammatory reaction induced by subcutaneous (SC) implantation of copper rods, produce humoral factors that initially enhance, but subsequently inhibit, diffusion chamber (DC) granulopoiesis. This provided evidence that granulopoiesis is under the control of both humoral stimulators and inhibitors. In order to test the granulopoietic regulatory mechanism in leukaemic mice, we investigated the regulatory role of granulopoietic humoral inhibitors during in vivo granulopoiesis. We noticed that mice suffering from acute myeloid leukaemia (AML) are unable to augment the production of these humoral inhibitory factors when acute inflammation is induced, since no change in DC cell content was observed with or without prior inflammation. Moreover, unlike healthy mice, the serum of leukaemic mice withdrawn during the inhibition phase of acute inflammation did not show any inhibitory activity toward granulocyte-monocyte (GM) colony growth in vitro. Our results also show that increased levels of normal humoral inhibitors do not influence the proliferation and/or differentiation of leukaemic cells implanted in diffusion chamber cultures.