Plasma and brain catecholamines in experimental uremia: acute and chronic studies

Uremic encephalopathy

Uremic encephalopathy encompasses a wide range of central nervous system abnormalities associated with poor kidney function occurring with either progressive chronic kidney disease or acute kidney injury. The syndrome is likely caused by retention of uremic solutes, alterations in hormonal metabolism, changes in electrolyte and acid-base homeostasis, as well as changes in vascular reactivity, blood-brain barrier transport, and inflammation. There are no defining clinical, laboratory, or imaging findings, and the diagnosis is often made retrospectively when symptoms improve after dialysis or transplantation. The diagnosis is also made difficult because of the many confounding and overlapping conditions seen in patients with chronic kidney disease and acute kidney injury. Thus, institution of kidney replacement therapy should be considered as a trial to improve symptoms in the right clinical context. Neurological symptoms that do not improve after improvement in clearance should prompt a search for other explanations. Further knowledge linking possible uremic retention solutes with neurological symptoms is needed to better understand this syndrome as well as to develop more tailored treatments that aim to improve cognitive function.

Neurobehavioral effects of uremic toxin-indoxyl sulfate in the rat model

Chronic kidney disease (CKD) is deemed to be a worldwide health concern connected with neurological manifestations. The etiology of central nervous system (CNS) disorders in CKD is still not fully understood, however particular attention is currently being paid to the impact of accumulated toxins. Indoxyl sulfate (IS) is one of the most potent uremic toxins. The purpose of the present study was to assess IS concentrations in the cerebellum, brainstem, cortex, hypothalamus, and striatum with hippocampus of rats chronically exposed to IS. To evaluate IS impact on neurochemical and behavioral alterations, we examined its influence on brain levels of norepinephrine, epinephrine, dopamine, serotonin and their metabolites, as well as changes in behavioral tests (open field test, elevated plus maze test, chimney test, T maze test, and splash test). Our results show the highest IS accumulation in the brainstem. IS leads to behavioral alterations involving apathetic behavior, increased stress sensitivity, and reduced locomotor and exploratory activity. Besides, IS contributes to the impairment of spatial memory and motor coordination. Furthermore, we observed reduced levels of norepinephrine, dopamine or serotonin, mainly in the brainstem. Our findings indicate that IS can be one of the crucial uremic factors responsible for altered mental status in CKD.

Brain-kidney crosstalk

Encephalopathy and altered higher mental functions are common clinical complications of acute kidney injury. Although sepsis is a major triggering factor, acute kidney injury predisposes to confusion by causing generalised inflammation, leading to increased permeability of the blood–brain barrier, exacerbated by hyperosmolarity and metabolic acidosis due to the retention of products of nitrogen metabolism potentially resulting in increased brain water content. Downregulation of cell membrane transporters predisposes to alterations in neurotransmitter secretion and uptake, coupled with drug accumulation increasing the risk of encephalopathy. On the other hand, acute brain injury can induce a variety of changes in renal function ranging from altered function and electrolyte imbalances to inflammatory changes in brain death kidney donors.

Impaired Autofeedback Regulation of Hypothalamic Norepinephrine Release in Experimental Uremia

Chronic renal failure (CRF) is associated with multiple hypothalamic dysfunctions, including reduced secretion of gonadotropin-releasing hormone (GnRH). Because GnRH release is tightly controlled by sympathetic neuronal input, a possible alteration of local noradrenergic neurotransmission in experimental CRF was evaluated. Basal, stimulated, and autoinhibited norepinephrine (NE) release was assessed in hypothalamic and hippocampal tissue slices obtained from 5/6-nephrectomized and control rats. Autoinhibition-free NE release from brain slices, prelabeled with [3H]NE and superfused with physiologic buffer, was stimulated by six electrical pulses, 100 Hz (pseudo-one-pulse stimulation). Autoinhibited NE release was induced by 90 pulses at 3 Hz. The release of tritiated NE was measured upon addition of increasing concentrations of unlabeled NE to exogenously activate the inhibitory alpha2-autoreceptor. Although neither basal nor stimulated NE release differed between the groups, significantly lower pIC50 and Imax estimates of the concentration-response curves of exogenous NE on [3H]NE release were observed in CRF rats, suggesting a diminished autoinhibition of hypothalamic noradrenergic terminals in CRF. Western blotting of tissue homogenates disclosed a significantly reduced abundance of alpha2-autoreceptor protein in hypothalamic tissue from CRF rats. These abnormalities were selectively observed in the hypothalamus, whereas noradrenergic autoinhibition seemed unaltered in the hippocampus. The results suggest a diminished autoinhibition of hypothalamic NE release in CRF. Although impaired hypothalamic NE autoinhibition does not explain reduced GnRH secretion in CRF, it may be involved in the pathogenesis of sympathetic hyperactivity associated with this condition.

Central nervous dysfunction in uremia

The mechanisms of central nervous system dysfunction in uremia are multifactorial and only partially characterized. Studies using sealed presynaptic nerve terminals (synaptosomes) for in vitro ion transport and metabolism of neurotransmitter in chronic renal failure (CRF) neuronal cell culture and in vivo brain structure microdialysis generated significant new information. An increase in total calcium content of the cerebral cortex accompanied by increased levels of cytosolic calcium ([Ca(2+)]i) in synaptosomes are common findings in rats with CRF. Mechanisms leading to the increase in [Ca(2+)]i include increased calcium uptake mediated by parathyroid hormone and decreased activity of Na(+),K(+)-adenosine triphosphatase (ATPase) and Ca(2+)-ATPase of synaptosomes in CRF rats. Moreover, these synaptosomes respond inappropriately to depolarization, which can impair neurotransmitter metabolism. Brain gamma-aminobutyric acid content, norepinephrine, and acetylcholine release uptake and degradation are affected by uremia. These may lead to certain somatic, behavioral, and motor dysfunctions in uremia. Many derangements of the central nervous system in uremia appear to be mediated by secondary hyperparathyroidism of CRF because parathyroidectomy of animals with CRF prevented the increase in basal levels of [Ca(2+)]i and derangements in neurotransmitter metabolism. The role of other neurotoxins, such as guanidinosuccinic acid, are also reviewed.

Behavioural deficits during the acute phase of mild renal failure in mice

Partially nephrectomized (NX) and sham-operated mice were biochemically and behaviourally compared, 10 days, 1 month and 1 year post-surgery. Plasma urea and creatinine concentrations were mildly increased in all NX groups, but creatinine clearance was significantly decreased, 10 days post-surgery only. NX mice showed lower body weights and reduced growth. Wire suspension and rotarod indicated unaffected motor functions, but NX mice did show reduced ambulation and swimming velocity, 10 days post-surgery. Hidden-platform water maze indicated a spatial learning impairment in NX mice, 10 days post-surgery, which could not be entirely reduced to motor incapacity. The acute behavioural deficits in these mildly uremic mice may relate to analogous symptoms in uraemic encephalopathy, a poorly understood brain syndrome occurring in uraemic patients.

Renal afferent denervation prevents hypertension in rats with chronic renal failure

Increased activity of the sympathetic nervous system has been described in chronic renal failure, but its role in the genesis and maintenance of hypertension associated with this condition has not been established. The kidney has an intense network of chemoreceptors and baroreceptors that send impulses to the brain. To what extent activation of these receptors by the scarred kidney or the uremic milieu may contribute to this model of hypertension is unknown. In the present study, we evaluated the effect of bilateral dorsal rhizotomy on the development of hypertension and neuroadrenergic activity in the anterior, lateral, and posterior hypothalamic nuclei, in the locus ceruleus, and in the nucleus tractus solitarius of Sprague-Dawley rats that underwent 5/6 nephrectomy or were sham operated. Neuroadrenergic activity was determined by calculating norepinephrine turnover rate after inhibition of norepinephrine synthesis with alpha-methyl-DL-p-tyrosine methyl ester hydrochloride. The endogenous norepinephrine concentration was significantly greater in the posterior and lateral hypothalamic nuclei and the locus ceruleus, but not in the nucleus tractus solitarius, and the anterior hypothalamic nuclei of uremic rats compared with control rats. In rats with chronic renal failure and sham rhizotomy, the turnover rate of norepinephrine in the posterior (15.3 +/- 1.61 nmol.g-1.h-1) and lateral hypothalamic nuclei (11.7 +/- 2.12 nmol.g-1.h-1) and in the locus ceruleus (26.6 +/- 2.42 nmol.g-1.h-1) was significantly faster (P < .01) than in rats with renal failure and dorsal rhizotomy (4.1 +/- 0.51, 4.7 +/- 0.77, and 5.1 +/- 1.13 nmol.g-1.h-1, respectively) or control animals with or without rhizotomy.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma catecholamines and alpha 1-adrenoceptor function in hemodialysis-associated hypotension

alpha 1-Adrenoceptor function and plasma catecholamine levels were investigated in chronically hemodialyzed patients with and without hemodialysis-induced hypotension. In the interdialytic period blood pressure responses as well as the mydriasis after topical application of the alpha 1-selective agonist phenylephrine were not significantly different in patients with or without hypotension during dialysis, although patients with hypotension were more susceptible to miosis induced by the muscarinergic agonist carbachol. In the normotensive group the blood pressure increasing effect of phenylephrine was attentuated after 120 min of hemodialysis therapy. Plasma noradrenaline levels were not significantly different in both groups and did not change significantly during hemodialysis, while plasma dopamine was significantly increased in the hypotensives. Thus, evidence for a pathophysiological role of a postsynaptic alpha-adrenoceptor dysfunction in hemodialysis-induced hypotension was lacking.

Abnormal norepinephrine uptake and release in brain synaptosomes in chronic renal failure

Abnormalities in the function of the central nervous system exist in chronic renal failure (CRF) and some of these derangements may be related to excess parathyroid hormone (PTH) which causes a rise in brain calcium. The latter may affect metabolism of neurotransmitters such as norepinephrine (NE) in brain synaptosomes. We measured NE content, uptake and release in brain synaptosomes of CRF rats and studied whether excess PTH affects these parameters. Synaptosomes from rats with 21 days of CRF compared to those from normal animals have higher calcium content (11.4 +/- 0.92 vs. 7.1 +/- 0.50 nmol/mg protein, P less than 0.01) and lower Na-K ATPase activity (6.5 +/- 0.81 vs. 11.4 +/- 0.76 mumol Pi/mg protein/hr, P less than 0.01). NE content (11.0 +/- 0.60 vs. 13.6 +/- 0.55 pmol/mg protein/hr, P less than 0.01), uptake (46 +/- 4.5 vs. 110 +/- 5.9 pmol/mg protein times 50 min, P less than 0.01) and release (2.0 +/- 0.2 vs. 5.1 +/- 0.47 pmol/mg protein times 10 min, P less than 0.01). Parathyroidectomy (PTX) in CRF rats kept normocalcemic reversed these abnormalities in brain synaptosomes; indeed calcium content, Na-K ATPase activity and NE content, uptake and release in synaptosomes from PTX-CRF rats were not different from those seen in normal rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Plasma and urinary catecholamines as related to renal function in man

To assess the relationship between renal plasma flow (ERPF) or glomerular filtration rate (GFR) and the levels of norepinephrine (NE) or epinephrine (E) in plasma or urine in the presence of progressive degrees of non-oliguric renal functional impairment, these variables were assessed simultaneously in 18 normal subjects, 72 with parenchymal kidney disease and 14 with essential hypertension. ERPF and GFR were lower (P less than 0.01 to 0.001) in the groups with renal disease (mean +/- SD, 340 +/- 230 and 68 +/- 43 ml/min/1.73 m2, respectively) or essential hypertension (434 +/- 101 and 97 +/- 25 ml/min/1.73 m2) than normal subjects (597 +/- 133 and 118 +/- 14 ml/min/1.73 m2). Plasma and urinary NE and E did not differ significantly among groups and were unrelated with ERPF or GFR (range 4 to 160 ml/min/1.73 m2), except for reduced (P less than 0.001) urinary NE and E excretion in the presence of a GFR less than 20 ml/min. Subgroups with renal disease and a normal (N = 39) or high blood pressure (N = 33) also were comparable in their plasma and urinary NE and E, while ERPF and GFR tended to be lower in hypertensive patients. It is concluded that a chronic reduction in excretory kidney function may have no relevant impact on circulating levels of NE and E per se, although their urinary excretion falls distinctly at the stage of advanced renal failure. These aspects deserve consideration when pathogenetic or diagnostic studies of catecholamines are performed in normotensive or hypertensive patients with impaired kidney function.