Evidence that a GABAergic mechanism influences the development of DOCA-salt hypertension in the rat

Sodium valproate stimulates potassium and chloride urinary excretion in rats: gender differences

BACKGROUND

The diuretic effect of valproates and its relation to urinary potassium (K+) and chloride (Cl-) excretion have not yet been investigated, so the aim of this study was to evaluate the influence of a single dose of sodium valproate (NaVPA) on 24-h urinary K+ and Cl- excretion in young adult Wistar rats of both genders. For measurement of K+ in urine, the same animals and samples as in our earlier publication were used (Pharmacology 2005 Nov, 75:111-115). The authors propose a new approach to the pathophysiological mechanisms of NaVPA effect on K+ and Cl- metabolism. Twenty six Wistar rats were examined after a single intragastric administration of 300 mg/kg NaVPA (13 NaVPA-male and 13 NaVPA-female), 28 control intact Wistar rats (14 males and 14 females) were studied as a control group. The 24-h urinary K+, Cl-, creatinine and pH levels were measured.

RESULTS

Total 24-h diuresis and 24-h diuresis per 100 g of body weight were found to be significantly higher in NaVPA-rats of both genders than in rats of the control group (p < 0.05). The data showed NaVPA to enhance 24-h K+ excretion in NaVPA-males and NaVPA-females with significant gender-related differences: 24-h K+ excretion in NaVPA-male rats was significantly higher than in control males (p = 0.003) and NaVPA-female rats (p < 0.001). Regarding the 24-h K+ excretion, NaVPA-female rats did not show a statistically significant difference versus females of the control group (p > 0.05). 24-h urinary K+ excretion per 100 g of body weight in NaVPA-male rats was significantly higher than in control males (p = 0.025). NaVPA enhanced Cl- urinary excretion: 24-h Cl- urinary excretion, 24-h urinary Cl- excretion per 100 g of body weight and the Cl-/creatinine ratio were significantly higher in NaVPA-male and NaVPA-female rats than in gender-matched controls (p < 0.05). 24-h chloriduretic response to NaVPA in male rats was significantly higher than in female rats (p < 0.05).

CONCLUSION

NaVPA causes kaliuretic and chloriduretic effects with gender-related differences in rats. Further investigations are necessary to elucidate the mechanism of such pharmacological effects of NaVPA.

Valproate: a reappraisal of its pharmacodynamic properties and mechanisms of action

Valproate is currently one of the major antiepileptic drugs with efficacy for the treatment of both generalized and partial seizures in adults and children. Furthermore, the drug is increasingly used for therapy of bipolar and schizoaffective disorders, neuropathic pain and for prophylactic treatment of migraine. These various therapeutic effects are reflected in preclinical models, including a variety of animal models of seizures or epilepsy. The incidence of toxicity associated with the clinical use of valproate is low, but two rare toxic effects, idiosyncratic fatal hepatotoxicity and teratogenicity, necessitate precautions in risk patient populations. Studies from animal models on structure-relationships indicate that the mechanisms leading to hepatotoxicity and teratogenicity are distinct and also differ from the mechanisms of anticonvulsant action of valproate. Because of its wide spectrum of anticonvulsant activity against different seizure types, it has repeatedly been suggested that valproate acts through a combination of several mechanisms. As shown in this review, there is substantial evidence that valproate increases GABA synthesis and release and thereby potentiates GABAergic functions in some specific brain regions, such as substantia nigra, thought to be involved in the control of seizure generation and propagation. Furthermore, valproate seems to reduce the release of the epileptogenic amino acid gamma-hydroxybutyric acid and to attenuate neuronal excitation induced by NMDA-type glutamate receptors. In addition to effects on amino acidergic neurotransmission, valproate exerts direct effects on excitable membranes, although the importance of this action is equivocal. Microdialysis data suggest that valproate alters dopaminergic and serotonergic functions. Valproate is metabolized to several pharmacologically active metabolites, but because of the low plasma and brain concentrations of these compounds it is not likely that they contribute significantly to the anticonvulsant and toxic effects of treatment with the parent drug. By the experimental observations summarized in this review, most clinical effects of valproate can be explained, although much remains to be learned at a number of different levels of valproate's mechanisms of action.

Reduced GABA inhibition of sympathetic function in renal-wrapped hypertensive rats

Animals with bilateral cannulas in the paraventricular nucleus were made hypertensive by a one-kidney, figure eight renal wrap procedure or sham operated. Femoral artery and vein catheters were inserted for arterial pressure measurement and plasma catecholamine determination. After recovery and 4 days after hypertension surgery, bicuculline methiodide or muscimol was microinjected into the paraventricular nucleus. In some rats, nitroprusside was infused intravenously to reflexly stimulate the sympathetic nervous system. In control rats, bicuculline increased blood pressure, heart rate, and plasma norepinephrine and epinephrine concentrations. In contrast, in hypertensive rats blood pressure did not change while the heart rate response was maintained. Plasma norepinephrine and epinephrine responses were reduced 75 and 68%, respectively. Muscimol injections decreased arterial pressure in the hypertensive rats. Heart rate responses to nitroprusside were similar in the two groups of rats, while the plasma catecholamine responses were attenuated in the hypertensive animals. These data suggest that GABA function in the paraventricular nucleus is reduced in renal wrap hypertension.

Effects of the antiepileptic drug valproate on metabolism and function of inhibitory and excitatory amino acids in the brain

Valproate is currently one of the major antiepileptic drugs in clinical use. Because of its wide spectrum of anticonvulsant activity against different seizure types, it has repeatedly been suggested that valproate acts through a combination of several mechanisms. As shown in this review, there is substantial evidence that valproate increases GABA turnover and thereby potentiates GABAergic functions in some specific brain regions, such as substantia nigra, thought to be involved in the control of seizure generation and propagation. Furthermore, valproate seems to reduce the release of the epileptogenic amino acid gamma-hydroxybutyric acid and to block cell firing induced by NMDA-type glutamate receptors. In addition to effects on amino acidergic neurotransmission, valproate presumably exerts a direct action on ion channels, thereby limiting sustained repetitive neuronal firing. Recent microdialysis data suggest that valproate also alters dopaminergic and serotonergic functions. These diverse effects of valproate might explain why the drug not only exerts anticonvulsant activity but also other pharmacodynamic and pharmacotherapeutic actions, such as antipsychotic and antidystonic efficacy.

Cardiovascular responses to bicuculline in the paraventricular nucleus of the rat

The present study was undertaken to determine whether gamma-aminobutyric acid in the paraventricular nucleus contributes to the regulation of cardiovascular function. Blood pressure and heart rate were recorded and plasma catecholamines were measured in conscious rats receiving microinfusions of either artificial cerebrospinal fluid or a gamma-aminobutyric acid antagonist, bicuculline methiodide, bilaterally into the paraventricular nucleus. Artificial cerebrospinal fluid had no effect on any of the recorded variables. In contrast, infusion of bicuculline into the region of the paraventricular nucleus produced increases in blood pressure (20 +/- 2 mm Hg), heart rate (110 +/- 11 beats/min), and plasma concentrations of norepinephrine (640 +/- 107 pg/ml) and epinephrine (1,266 +/- 267 pg/ml). Pretreatment with a ganglionic blocking agent abolished both the blood pressure (-1 +/- 2 mm Hg) and heart rate (5 +/- 18 beats/min) effects. Bilateral adrenal medullectomy reduced the changes in plasma norepinephrine concentrations (81 +/- 14 pg/ml) significantly and abolished the changes in plasma epinephrine concentrations (5 +/- 4 pg/ml). Conversely, adrenal medullectomy reduced the pressor effects (18 +/- 2 mm Hg) only slightly while the heart rate responses were attenuated (42 +/- 9 beats/min) by approximately 50%. These results suggest that an endogenous gamma-aminobutyric acid system exerts a tonic inhibitory effect on the sympathetic nervous system at the level of the paraventricular nucleus of the hypothalamus.

Effect of clonidine on blood pressure and GABAergic mechanisms in spontaneously hypertensive rats

The action of clonidine on blood pressure and on the functional state of the gamma-aminobutyric acid-ergic (GABAergic) system was studied. A single injection of clonidine (1, 5, 10, 20 micrograms.kg-1) induced a dose-dependent decrease of blood pressure. Chronic administration of clonidine, 10 micrograms.kg-1, produced the maximum effect after the third injection. The effect was maintained for the duration of the study. Single or chronic clonidine injections, at the dose of 10 micrograms.kg-1 enhanced the GABA content in the brain and hypothalamus. This effect was less pronounced in the hippocampus. The drug administered according to the same regimen stimulated glutamic acid decarboxylase activity only in the hypothalamus. Clonidine caused a marked enhancement of specific [3H]GABA binding in the hypothalamus. These data suggest that the hypotensive action of clonidine is related to stimulation of the GABAergic system.

Genetically altered brain amino acid metabolism in spontaneously hypertensive rats: a study by using young spontaneously hypertensive rats and renal hypertensive rats

Previously we demonstrated altered amino acid levels in brainstem regions of adult spontaneously hypertensive rats (SHR). For comparison, in this study, we determined amino acid concentrations in discrete brainstem regions in young prehypertensive SHR and renal hypertensive rats. In prehypertensive SHR, the content of glutamate was increased in the rostral ventrolateral medulla and the caudal ventrolateral medulla, and the content of beta-alanine was decreased in the nucleus tractus solitarii. In renal hypertensive rats, there was no change in glutamate and beta-alanine contents in all the regions. The profiles of contents of glutamate and beta-alanine in the brainstem regions in young SHR but not in renal hypertensive rats are the same as those found previously in adult SHR. Thus, the results of the present study suggest that the altered amino acid metabolism in the brainstem of SHR may be genetically inherent.

Hypothalamic GABA and sympathetic regulation in spontaneously hypertensive rats

The posterior hypothalamus contains a sympathoexcitatory system that can be modulated by changes in GABAergic tone. We tested the hypothesis that the GABAergic mechanism in the posterior hypothalamus is altered in spontaneously hypertensive rats (SHR) compared with the Wistar-Kyoto (WKY) control rats. Blood pressure and heart rate were continuously measured in the conscious state; blood samples were obtained for determination of plasma catecholamine concentrations. Bilateral microinjections of the GABAA receptor antagonist bicuculline methiodide into the posterior hypothalamus increased heart rate and blood pressure in a dose-related fashion and increased plasma catecholamine concentrations in both SHR and WKY rats. The responses were not significantly different between the two strains of rats. Microinjections of the GABAA receptor agonist muscimol in this same region caused dose-related decreases in both heart rate and blood pressure in SHR and WKY rats. Although the decreases in heart rate caused by muscimol were not significantly different between the SHR and WKY rats, the decreases in blood pressure were significantly greater in SHR compared with WKY rats. Further, microinjection of muscimol caused a significant decrease in plasma catecholamines in SHR but not in WKY rats. These data indicate that in SHR and WKY rats the posterior hypothalamus contains a sympathoexcitatory mechanism that is tonically inhibited by GABA. The ability of muscimol to decrease plasma catecholamines selectively in SHR and to cause greater decreases in blood pressure, suggests that the GABAergic mechanisms in the posterior hypothalamus of the SHR and WKY rats may differ.

Altered amino acid levels in brainstem regions of spontaneously hypertensive rats

The contents of glutamate, gamma-aminobutyric acid (GABA), glycine, beta-alanine and taurine in brainstem regions responsible for blood pressure regulation, the rostral ventrolateral medulla (RVL), the nucleus tractus solitarii (NTS) and the caudal ventrolateral medulla (CVL) regions, were determined in spontaneously hypertensive rats (SHR) as compared with Wistar Kyoto rats. In SHR, the content of glutamate was increased in the RVL, NTS and CVL regions whereas GABA and beta-alanine contents were decreased in the RVL and all the regions studied, respectively. There was no difference between WKY and SHR in glycine and taurine contents in all the regions studied. These results provide evidence suggesting alterations of brainstem amino acid metabolism in SHR.

Is the hypotension of cirrhosis a GABA-mediated process?

Systolic and diastolic blood pressures were recorded in 176 ambulant patients with chronic liver disease, including 36 patients with compensated cirrhosis (Group I), 119 patients with noncirrhotic chronic liver disease (Group II) and 21 patients with benign structural or functional liver disease (Group III). Group I patients had significantly lower systolic (113.0 +/- 2.2 mm Hg, mean +/- S.E.) and diastolic (65.3 +/- 1.7 mm Hg) pressures than Group II patients (125.8 +/- 3.5 and 76.6 +/- 1.5 mm Hg, respectively (p less than 0.0001) or Group III patients (125.1 +/- 3.4 and 77.5 +/- 2.4 mm Hg, respectively) (p less than 0.0001). Serum levels of GABA, a potent amino acid neurotransmitter with known vasodilatory effects in vitro, were higher in Group I patients (1.12 +/- 0.26 microM, mean +/- S.E.) than in Group II patients (0.41 +/- 0.05 microM) (p less than 0.005) or Group III patients (0.34 +/- 0.03 mM) (p less than 0.05). A constant infusion of GABA into the systemic circulation of six adult dogs, at rates required to achieve serum GABA levels within one order of magnitude of those observed in humans with cirrhosis, resulted in a 17.0 +/- 4.3 mm Hg decrease in systolic pressure (p less than 0.05) and a 10.8 +/- 3.7 mm Hg decrease in diastolic pressure (p less than 0.05). Control amino acids were not vasoactive. The results of this study suggest that, in addition to other vasoactive compounds, a GABA-mediated process might contribute to the hypotension observed in patients with compensated cirrhosis.

GABAergic mechanisms in nucleus tractus solitarius alter blood pressure and vasopressin release

GABAergic agents microinjected into the nucleus tractus solitarius (NTS) influence blood pressure (BP) and plasma vasopressin (VP) levels. The direct-acting GABAergic agonist muscimol (3.75-160 pmol) microinjected bilaterally into the NTS of chloralose-anesthetized, paralyzed, ventilated rats increased BP without significantly altering heart rate. Similar results were obtained using the indirect GABA agonist nipecotic acid (10 nmol), a GABA uptake blocker. In contrast, blocking the action of GABA with bicuculline (5 pmol) elicited a small but consistent decrease in BP. Injections of the vehicle (artificial cerebrospinal fluid) into the NTS or GABAergic agents in an area lateral to the NTS did not alter BP. Unilateral injection of muscimol into the NTS did not elicit immediate pressor responses as did bilateral injections; unilateral muscimol injections following destruction of the contralateral NTS produced a pressor response similar to that seen following bilateral muscimol administration. Stimulation of GABA receptors within the NTS by either muscimol or nipecotic acid resulted in an increase in plasma VP levels. The elevated levels of plasma VP contributed to the pressor response elicited by stimulation of GABAergic receptors as evidenced by the decrease in BP observed following the intravenous administration of a VP pressor antagonist during the pressor response. These studies indicate that tonically active GABAergic mechanisms within the NTS influence BP and VP release, and provide further evidence that VP can be involved in cardiovascular responses elicited from the NTS.

Central receptor binding and cardiovascular effects of GABA analogues in the cat

Several structural analogues of GABA were shown to be inhibitors of GABAA receptor binding in membranes from cat cerebral cortex. These compounds were 3-aminopropanesulfonic acid (APS; IC50 = 0.04 microM), imidazoleacetic acid (IMA; IC50 = 0.4 microM), morpholinopropanesulfonic acid (MOPS; IC50 = 1.6 microM), 5-phenylpyrrolepropionic acid (PPP; IC50 = 15 microM), aminoethanethiosulfonic acid (AETS; IC50 = 22 microM), 3-amino-3-phenylpropionic acid (APP; IC50 = 35 microM), meta-aminobenzoic acid (MABA; IC50 = 58 microM) and urocanic acid (UCA; IC50 = 354 microM). The IC50 value for GABA was 0.03 microM. GABA, PPP, AETS, MABA and UCA were previously shown to reduce arterial pressure in the cat after intracerebroventricular infusion. In the present study MOPS (ED50 = 0.26 nmol/kg), APS (ED50 = 4.7 nmol/kg), APP (ED50 = 49 nmol/kg), and IMA (ED50 = 350 nmol/kg) were also found to be able to decrease blood pressure when infused into the fourth ventricle. All nine compounds reduced blood pressure to the same extent, but in some cases their relative potencies (ED50 values) exhibited significant differences. When the IC50 values for receptor binding were plotted against the ED50 values for the cardiovascular effects, no significant correlation emerged. This lack of a correlation does not necessarily imply that the reductions in blood pressure elicited by the drugs are not related to an activation of central GABAA receptors. Instead, it highlights the difficulties that are sometimes encountered in attempting to obtain quantitative measurements after intracerebroventricular infusion.

Novel GABA analogues as hypotensive agents

The actions of four analogues of gamma-aminobutyric acid (GABA) on blood pressure and heart rate were measured in the cat after intracerebroventricular administration. These compounds were previously found to inhibit binding to GABAA receptors of neuronal membranes from the CNS of the rat. Each of the drugs, together with GABA, produced an average maximum reduction in blood pressure of 27.63% +/- 12.5. However, aminoethanethiosulfonic acid (AETS) was the most potent (ED50 = 2.24 X 10(-10) mol/kg) of the drugs, followed by 5-phenyl-2-pyrrole propionic acid (PPP), urocanic acid (UCA), m-aminobenzoic acid (MABA) and GABA. None of the compounds produced a significant effect on heart rate. The fact that these analogues mimicked the action of GABA on the cardiovascular system of the cat and that they were able to inhibit binding to GABAA receptors, indicates that they may be GABA agonists.

gamma-Aminobutyric acid in peripheral tissues

Significant amounts of gamma-aminobutyric acid (GABA), an endogenous amino acid, are present in mammalian peripheral tissues. This finding led to the suggestion that GABA may act as a neurotransmitter in the peripheral nervous system as it does in the central nervous system. This review deals with recent identification of GABA in the autonomic nervous system and the possible functional role of GABA in neuronal and non-neuronal tissues. The identification of GABA in the autonomic nervous system has paved the way for new approaches in pharmacological investigations.

Benzodiazepine-GABA receptor-ionophore complex. Current concepts

The benzodiazepine--gamma-aminobutyric acid (GABA) receptor--ionophore system is an oligomeric complex, composed of at least three interacting components. These three components have been well characterized in vitro by radioreceptor binding assays. A variety of centrally acting anxiolytic, depressant, anticonvulsant and convulsant drugs, which affect GABAergic transmission, bind to one of the sites and modulate the binding of ligands at the other sites. Thus, depressant barbiturates, nonbarbiturate hypnotics (like etomidate) and pyrazolopyridines (like etazolate), while inhibiting the binding of alpha-dihydropicrotoxinin (DHP), enhance the binding of GABA and benzodiazepines. These enhancing effects are blocked by convulsant drugs that inhibit the binding of dihydropicrotoxinin and also by bicuculline. These interactions involving barbiturates and other modulatory drugs, exhibit stereoselectivity, anion dependence and brain regional selectivity. Several classes of drugs which facilitate GABAergic transmission appear to interact with the sites for GABA and benzodiazepines allosterically via the dihydropicrotoxinin site of the oligomeric complex. The GABA system has also been implicated in a variety of pathological conditions, including anxiety, seizure activity, movement disorders, cardiovascular control, pain and in drug dependence. Since most of the GABA agonists do not pass the blood-brain barrier, future trends in the pharmacology of GABA may be the development of drugs that will activate the GABA receptor system via picrotoxinin or benzodiazepine sites.

GABA and benzodiazepine binding sites in spontaneously hypertensive rat

The binding of GABA to its central receptor sites and of benzodiazepines to central and peripheral types of binding sites was compared in age-matched spontaneously hypertensive rat (SHR) and normotensive (WKY) rat. The KD and Bmax values of GABA agonist binding were not significantly different between SHR and WKY rat in the forebrain, cerebellum or pons-medulla. The binding of [3H]flunitrazepam to central types of binding sites was also not different in 12 week-old SHR and WKY rat. However, the binding of [3H]flunitrazepam and [3H]RO5-4864 was significantly lower in the SHR kidney, as compared to WKY rat kidney. This decrease was due to a lower Bmax in SHR kidney. In contrast, the binding of these radioligands in the hearts of the two strains was not different. These results indicate that the SHR has lower peripheral benzodiazepine binding sites in the kidney as compared to age-matched normotensive WKY rat kidney.

gamma-Aminobutyric acid and cardiovascular function

Evidence supporting the hypothesis that GABA-ergic mechanisms are involved in controlling mammalian cardiovascular function has been reviewed and analyzed. In vivo and in vitro studies with GABA-agonists and GABA-antagonists have revealed that activation of GABA-receptors is involved in the control of blood pressure and heart rate. Further studies conducted with agents that modify central and/or peripheral GABA-ergic systems could lead to the discovery of drugs that might be useful for treating certain cardiovascular disorders in man, such as hypertension and stroke, and should increase our understanding of the pathophysiological bases of such disorders.