Alterations in a hypothalamic GABA system in the spontaneously hypertensive rat

Hematopoietic stem/progenitor involvement in retinal microvascular repair during diabetes: Implications for bone marrow rejuvenation

The widespread nature of diabetes affects all organ systems of an individual including the bone marrow. Long-term damage to the cellular and extracellular components of the bone marrow leads to a rapid decline in the bone marrow-hematopoietic stem/progenitor cells (HS/PCs) compartment. This review will highlight the importance of bone marrow microenvironment in maintaining bone marrow HS/PC populations and the contribution of these key populations in microvascular repair during the natural history of diabetes. The autonomic nervous system can initiate and propagate bone marrow dysfunction in diabetes. Systemic pharmacological strategies designed to protect the bone marrow-HS/PC population from diabetes induced-oxidative stress and advanced glycation end product accumulation represent a new approach to target diabetic retinopathy progression. Protecting HS/PCs ensures their participation in vascular repair and reduces the risk of vasogdegeneration occurring in the retina.

Brain angiotensin peptides regulate sympathetic tone and blood pressure

Brain angiotensin II (Ang II) induces tonic sympathoexcitatory effects through AT1 receptor stimulation of glutamatergic neurons and sympathoinhibitory effects via GABAergic neurons in the rostral ventrolateral medulla, the brainstem 'pressor area'. NADPH-derived superoxide production and reactive oxygen species signalling is critical in these actions, and AT2 receptors in the rostral ventrolateral medulla appear to mediate opposing effects on sympathetic outflow. In the hypothalamic paraventricular nucleus, Ang II has AT1 receptor-mediated sympathoexcitatory effects and enhances nitric oxide formation, which in turn inhibits the Ang II effects through a GABAergic mechanism. Ang II also decreases the tonic sympathoinhibitory effect of gamma amino butyric acid within the paraventricular nucleus. Angiotensin III and Angiotensin IV increase blood pressure via brain AT1 receptor stimulation. Angiotensin (1-7) influences cardiovascular function through a specific Mas-receptor. This review examines the evidence that brain angiotensin peptides, glutamate, gamma amino butyric acid and nitric oxide interact within the rostral ventrolateral medulla and paraventricular nucleus to control sympathetic tone and blood pressure.

Interaction between glutamate and GABA systems in the integration of sympathetic outflow by the paraventricular nucleus of the hypothalamus

The paraventricular nucleus (PVN) of the hypothalamus is a central site known to modulate sympathetic outflow. Excitatory and inhibitory neurotransmitters within the PVN dictate final outflow. The goal of the present study was to examine the role of the interaction between the excitatory neurotransmitter glutamate and the inhibitory neurotransmitter GABA in the regulation of sympathetic activity. In alpha-chloralose- and urethane-anesthetized rats, microinjection of glutamate and N-methyl-D-aspartate (NMDA; 50, 100, and 200 pmol) into the PVN produced dose-dependent increases in renal sympathetic nerve activity, blood pressure, and heart rate. These responses were blocked by the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP-5). Microinjection of bicuculline, a GABA(A) receptor antagonist, into the PVN (50, 100, and 200 pmol) also produced significant, dose-dependent increases in renal sympathetic nerve activity, blood pressure, and heart rate; AP-5 also blocked these responses. Using microdialysis and HPLC/electrochemical detection techniques, we observed that bicuculline infusion into the PVN increased glutamate release. Using an in vitro hypothalamic slice preparation, we found that bicuculline increased the frequency of glutamate-mediated excitatory postsynaptic currents in PVN-rostral ventrolateral medullary projecting neurons, supporting a GABA(A)-mediated tonic inhibition of this excitatory input into these neurons. Together, these data indicate that 1) glutamate, via NMDA receptors, excites the presympathetic neurons within the PVN and increases sympathetic outflow and 2) this glutamate excitatory input is tonically inhibited by a GABA(A)-mediated mechanism.

A comparative autoradiographic study of the density of [3H]SR95531, [3H]MK-801 and [3H]cGMP binding in the locus coeruleus and central pontine grey of spontaneously hypertensive and Wistar-Kyoto rats

The Spontaneously Hypertensive rat (SHR) has been previously shown to have a host of neurochemical differences compared with their normotensive counterpart, the Wistar-Kyoto (WKY) rat. Using quantitative receptor autoradiography, the density of GABA(A) and NMDA receptors and [3H]cGMP binding within the locus coeruleus (LC) and central pontine grey (CGPn) were compared in the SHR and WKY rat using the radioligands [3H]SR95531, [3H]MK-801 and [3H]cGMP respectively. It was found that [3H]SR95531 binding was significantly greater in both the LC and CGPn of the SHR compared with the WKY rat (unpaired t test; P < 0.05). Greater binding densities of [3H]MK-801 and [3H]cGMP were also observed in the LC of the SHR compared with the WKY rat; however, no differences in the binding density of these two ligands were observed in the CGPn. It is suggested that these neurochemical differences within the LC of the SHR may relate to phenotypic differences between SHR and WKY rats that have previously been reported.

Reduced endogenous GABA-mediated inhibition in the PVN on renal nerve discharge in rats with heart failure

One characteristic of heart failure (HF) is increased sympathetic activation. The paraventricular nucleus (PVN) of the hypothalamus (involved in control of sympathetic outflow) has been shown to have increased neuronal activation during HF. This study examined the influence of endogenous GABA input (inhibitory in nature) into the PVN on renal sympathetic nerve discharge (RSND), arterial blood pressure (BP), and heart rate (HR) in rats with HF induced by coronary artery ligation. In alpha-chloralose- and urethane-anesthetized rats, microinjection of bicuculline (a GABA antagonist) into the PVN produced a dose-dependent increase in RSND, BP, and HR in both sham-operated control and HF rats. Bicuculline attenuated the increase in RSND and BP in HF rats compared with control rats. Alternatively, microinjection of the GABA agonist muscimol produced a dose-dependent decrease in RSND, BP, and HR in both control and HF rats. Muscimol was also less effective in decreasing RSND, BP, and HR in HF rats than in control rats. These results suggest that endogenous GABA-mediated input into the PVN of rats with HF is less effective in suppressing RSND and BP compared with control rats. This is partly due to the post-release actions of GABA, possibly caused by altered function of post-synaptic GABA receptors in the PVN of rats with HF. Reduced GABA-mediated inhibition in the PVN may contribute to increased sympathetic outflow, which is commonly observed during HF.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

Effects of six antihypertensive drugs on blood pressure and hypothalamic GABA content in spontaneously hypertensive rats

In order to investigate the effects of antihypertensive drugs on blood pressure and gamma-amino butyric acid (GABA) content in the hypothalamus and the possible relationship between blood pressure decrease and GABA content changes, blood pressure and GABA contents after chronic (20 weeks) treatments of nitrendipine, atenolol, captopril, hydrochlorothiazide, dihydralazine and prazosin were studied in spontaneously-hypertensive rats (SHR). The acute and subacute (1 week) effects of nitrendipine on GABA contents was also observed in SHR. It was found that 20 week treatments with six different antihypertensive agents produced a decrease in systolic blood pressure and an increase in GABA content. The blood pressure level was significantly correlated with GABA content in the hypothalamus, but not with that in the cortex. Acute treatment with a single dose of nitrendipine, did not alter GABA content. Bicuculline, a GABA receptor antagonist, did not attenuate the hypotensive effect of nitrendipine. In conclusion, chronic treatments by different antihypertensive agents produced an increase of hypothalamic GABA content and a decrease of blood pressure. The increase of GABA content induced by nitrendipine seems likely to be secondary to blood pressure decrease.

gamma-Aminobutyric acid (GABA)--A function and binding in the paraventricular nucleus of the hypothalamus in chronic renal-wrap hypertension

The goal of this study was to determine whether gamma-aminobutyric acid (GABA)ergic transmission and GABA binding are altered in chronic renal-wrap hypertension. Three groups of hypertensive and sham-operated rats were prepared for separate protocols. Four weeks later, the animals were prepared with femoral artery catheters for the measurement of mean arterial pressure. In all groups, blood pressure was significantly higher in the renal-wrapped animals. In the first study, bilateral microinjection of the GABA-A antagonist, bicuculline (50 pmol/site), into the paraventricular nucleus of the hypothalamus (PVN) caused a greater increase in arterial pressure (21.9+/-1.4 versus 16.7+/-1.8 mm Hg, P<0.05) and heart rate (135+/-15 versus 98+/-12 bpm, P=0.064) in hypertensive rats. [(3)H]Flunitrazepam was used to measure binding to the GABA-A receptor. Magnocellular neurons and the adjacent medial parvicellular neurons had more intense binding compared with the remainder of the PVN. B(max) was greater for the higher density binding area; the K(d) value was less in the high-density region. There were no differences in these parameters between normotensive and hypertensive animals. Competitive reverse transcription-polymerase chain reaction was used to measure the expression of mRNA for the alpha(1) subunit of the GABA-A receptor. No difference was observed in the mRNA between renal-wrapped and sham-operated rats. In summary, inhibition of GABA-A receptors in the PVN is augmented in the chronic phase of hypertension and is unrelated to a change in the expression of the number or affinity to the receptor. These findings suggest that the greater GABAergic activity is the result of an increase in GABA release in the PVN in chronic renal-wrap hypertension.

Chronic exercise alters caudal hypothalamic regulation of the cardiovascular system in hypertensive rats

Previous studies have documented a deficit in the GABA neurotransmitter system within the caudal hypothalamus (CH) of spontaneously hypertensive rats (SHR). The reduction in inhibitory influence on this cardiovascular excitatory brain region is associated with an increased neuronal activity and resting blood pressure. The purpose of this study was to determine if chronic treadmill and wheel-running activities alter the ability of the CH to regulate cardiovascular function. SHR were exercised on a treadmill (5 times/wk) at moderate intensity or allowed free access to running wheels (7 days/wk) for a period of 10 wk. Resting blood pressures were obtained before and after the exercise training periods. After the exercise period, rats were anesthetized and microinjection experiments were performed. Treadmill-trained SHR exhibited a significantly blunted developmental rise in resting blood pressure after 10 wk of exercise. A similar yet less marked effect was observed in wheel-run rats. Microinjection of the GABA synthesis inhibitor 3-mercaptopropionic acid (3-MP) into the CH of nonexercised SHR did not produce any change in arterial pressure. In contrast, microinjection of 3-MP into the CH produced significant increases in blood pressure and heart rate in exercised SHR. These results demonstrate that exercise training can alter CH cardiovascular regulation in hypertensive rats and therefore may play a role in increasing cardiovascular health.

Clonidine action in spontaneously hypertensive rats (SHR) depends on the GABAergic system function

The effect of gamma-aminobutyric acid (GABA)A antagonists (bicuculline, picrotoxin) on clonidine hypotension in spontaneously hypertensive (SHR) and Wistar Kyoto (WKY) rats were examined. The GABA turnover changes after clonidine injection in both strains were also studied. Administration of clonidine alone induced the stronger decrease of systolic blood pressure (SBP) in SHR. Co-dosage of clonidine with these agents reduced its hypotensive effect in dose dependent manner and the effectiveness of both antagonists was higher in SHR. We find that clonidine stimulates GABA synthesis in the hypothalamus and the pons-medulla in both strains but the GABA turnover rate is significantly slower in SHR. Therefore, the differences in inhibitory action of GABAA receptor antagonists between WKY and SHR rats may be explained by central GABAergic system dysfunction in the hypertension. Our results indicate that the down regulation of the GABAergic system observed in hypertension may be compensated by the action of clonidine.

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.

Pharmacology and function of imidazole 4-acetic acid in brain

1. Imidazole 4-acetic acid (IMA) is a naturally occurring metabolite in brain, although it is unclear what biochemical pathways are involved in its biosynthesis and breakdown. Some evidence, however, suggests that IMA is an oxidation product of histamine. 2. The compound has pronounced neuropharmacological properties, many of which are consistent with an activation of GABA(A) receptors. Indeed, IMA is able to displace [3H]GABA from GABA(A) sites in a potent manner. 3. IMA displays definite partial agonist characteristics as an enhancer of benzodiazepine binding to the GABA(A) receptor complex in membrane preparations. In addition, it has an affinity for GABA(C) receptors, where it seems to act as an antagonist, and perhaps as a weak partial agonist. A third recognition site for IMA in brain is the I1-imidazoline receptor. 4. Parenteral administration to experimental animals leads to a sleep-like state which can often be accompanied by seizures. In addition, central application of IMA has been associated with a dose-related reduction in arterial pressure and sympathetic nervous discharge. 5. No specific receptor site or uptake system for IMA has yet been discovered, adding uncertainty to its role in central nervous system function. Yet the possibility cannot be overlooked that IMA plays a role in regulating blood pressure.

Decrease in glutamic acid decarboxylase level in the hypothalamus of spontaneously hypertensive rats

BACKGROUND

A reduction in gamma-aminobutyric (GABA)-mediated inhibition of pressor sites in the caudal hypothalamus of spontaneously hypertensive rats compared with that of normotensive Wistar-Kyoto rats has recently been demonstrated.

OBJECTIVE

To determine whether the reduction in GABA-mediated inhibition of the caudal hypothalamus of the spontaneously hypertensive rats results from reductions both in the number of GABA-synthesizing neurons and in the amount of the GABA-synthesizing enzyme, glutamic acid decarboxylase messenger RNA (mRNA).

DESIGN AND METHODS

A polyclonal antibody (Chemicon) for the 67 kDa isoform of glutamic acid decarboxylase (GAD67) was used to immunocytochemically label GABAergic neurons in the caudal hypothalamus of spontaneously hypertensive and Wistar-Kyoto rats that had been treated beforehand with colchicine. The labeled cells were counted for both strains by a blinded analysis and compared. Caudal hypothalamic tissues from spontaneously hypertensive and Wistar-Kyoto rats were analysed for GAD67 mRNA by Northern blotting. The signal intensities of the radioactive probe specific for GAD67 for the two strains were analyzed by using a phosphorimager and compared. Control areas for the immunocytochemical (zona incerta) and Northern blotting (cortex, midbrain, cerebellum, and brain stem) experiments were used to determine regional differences in expression of GAD67.

RESULTS

Both the hypothalamus and cerebellum of spontaneously hypertensive and Wistar-Kyoto rats contained GAD67-immunoreactive neurons; however, there were 42% fewer GAD67 neurons in the caudal hypothalamus of spontaneously hypertensive rats than there were in that of Wistar-Kyoto rats. Furthermore, a 33% reduction in the amount of GAD67 messenger RNA in the caudal hypothalamus of spontaneously hypertensive rats compared with that for Wistar-Kyoto rats was demonstrated. Analysis of the expression of GAD67 in the cortex, midbrain, cerebellum, brain stem, and total brain revealed no difference between spontaneously hypertensive and Wistar-Kyoto rats.

CONCLUSIONS

Our findings demonstrate that the spontaneously hypertensive rat has fewer neurons synthesizing GABA and less GAD67 mRNA in the caudal hypothalamus than do Wistar-Kyoto rats. This deficit in the GABAergic system in the caudal hypothalamus, a well-known cardiovascular regulatory site, could contribute to the essential hypertension in this animal model.

Strain, age and sex differences in the release of vasopressin from the pituitary: a study in the spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rat

The basal and depolarization-induced arginine-vasopressin (AVP) release from intact pituitaries of SHR and WKY rats was studied in vitro in a perfusion chamber. Differences associated to strain, sex and two age periods (pre-adult: 25-30 days of age; adult: 60-70 days of age) were assessed. The results show an enhanced AVP release in the adult male as well as female SHR compared to the WKY rat. The stimulated AVP release was also significantly higher in the preadult male SHR and indicated in preadult females SHR. No differences associated to strain in basal AVP release were detected at the age interval 25-30 days. The response to muscimol was increased in preadult female and male SHR rats compared to the WKY animals. It is concluded that the augmented depolarization-induced AVP release and sensitivity to muscimol in the SHR is not related to sex, and no apparent change in this pattern was associated to the transition between the juvenile and adult condition.

Effects of gamma-aminobutyric acid on regional vascular resistances of conscious spontaneously hypertensive rats

1. The regional vascular effects of a central injection of gamma-aminobutyric acid (GABA) in conscious spontaneously hypertensive rats (SHR) were compared with normotensive control rats (NCR), including Wistar-Kyoto rats (male, 10-20 weeks of age). One week or more after insertion of a cannula into the cisterna magna, an electromagnetic flow probe was implanted around one of five arteries, and a catheter for measurement of blood pressure and heart rate was inserted into the terminal aorta or common carotid artery. Cardiovascular changes were observed in the conscious animal after recovery from the surgery. Peripheral resistance was calculated as pressure divided by flow. 2. Ten min following intracisternal injection of 10 mu mol of GABA, mean arterial pressure and heart rate were significantly lowered (P < 0.05; t-test): - 29 and -16% (70 d.f.) in SHR versus -23 (116 d.f.) and -19% (112 d.f.) in NCR. Hindquarters, carotid, superior mesenteric and renal, but not coeliac, vascular resistances were significantly lowered (P < 0.05; t-test, 21, 9, 18 d.f.) in SHR. However, when comparing the effect of GABA injection in SHR and NCR, there were no significant differences in coeliac, carotid and hindquarters vascular resistances, but there were significant decreases in superior mesenteric and renal vascular resistances in SHR. 3. These results indicate that: (i) a cisternal injection of GABA in SHR decreases both peripheral vascular resistance and heart rate, and thereby lowers blood pressure; (ii) the vascular resistance of the superior mesenteric and renal vascular beds was more decreased in SHR than in NCR. This suggests that the GABAergic control of regional vascular resistance differs in the SHR and the NCR.

Functional alteration of the GABAB receptor in the brain of spontaneously hypertensive rats

1. We studied the ontogenetic development of GABAB receptors and their coupling to cyclic AMP formation in spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rat brains. GABAB receptor binding to various brain regions was compared in age-matched SHR and WKY rats. 2. The specific [3H]-GABA binding to the posterior hypothalamus (PH) was significantly lower in not only 4 week old (normotensive) but also 11 week old (hypertensive) SHR, when compared with age-matched WKY. 3. Moreover, GABAB receptor agonists (baclofen and DN-2327)-induced suppression of adenylate cyclase activity also showed a decrease in 4 week old and 11 week old SHR PH. 4. We concluded that the number and the functional responsiveness of GABAB receptors in rat brains decrease in SHR, preceding blood pressure elevation. The activity of GABABergic mechanisms may be different in the brains of SHR and WKY rats.

Lower GABAA receptor binding in the amygdala and hypothalamus of spontaneously hypertensive rats

The central GABAergic system is associated with normal blood pressure regulation, but the role of GABA receptors in genetic hypertension remains unclear. This study was conducted to investigate GABAA receptor binding in several brain regions of spontaneously hypertensive (SHR) rats during development of hypertension. GABAA receptor binding was labeled with [35S]TBPS and was assessed by quantitative autoradiography with the aid of a computer-assisted image analysis system. Densities of GABAA receptor binding sites were significantly lower in all hypothalamic and amygdaloid nuclei evaluated in 4-week-old SHR rats, when compared with their age-matched normotensive Wistar-Kyoto rats. At 12 weeks of age, GABAA receptor binding remained significantly lower in the central amygdaloid nucleus and paraventricular hypothalamic nucleus of SHR rats. Collectively, the results suggest that GABAA receptors in these nuclei are likely to be involved in the initiation and maintenance of hypertension. In conclusion, this study supports a notion that downregulation of GABAA receptor binding occurs in the hypothalamus and amygdala of SHR rats and may play a role in genetic hypertension.

In vitro effects of GABA and hypoxia on posterior hypothalamic neurons from spontaneously hypertensive and Wistar-Kyoto rats

Recent studies from this laboratory have shown that neurons in this hypothalamic region are stimulated by hypoxia in vivo and in vitro. In addition, GABAergic activity is depressed in the posterior hypothalamus of the spontaneously hypertensive rat compared to the normotensive rat. The major purposes of the present study were to: a) evaluate if posterior hypothalamic neurons respond differently to GABA in the hypertensive rat compared to the normotensive rat; and b) examine the possibility that hypothalamic neurons from spontaneously hypertensive rats respond differently to hypoxia than those from normotensive rats. In addition, the effects of GABA on hypoxia-sensitive neurons was recorded. Extracellular single unit recordings of hypothalamic neurons were performed in a rat brain slice preparation. Neuronal responses to hypoxia (10% O2/5% CO2/85% N2) and to GABA were recorded from slices taken from both Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Administration of three different concentrations of GABA evoked a dose-related decrease in discharge rate in similar percentages of neurons from both SHR and WKY rats. The magnitude of the depression elicited by GABA did not differ significantly between the neurons from SHR and WKY rats. Hypoxia increased the firing rate of 75% and 69% of the SHR and WKY neurons, respectively; no differences (p > 0.05) were noted in the magnitude of the response or in the percentage of neurons responding to hypoxia between the two strains of rats. The discharge rate of most of these neurons fell to below control level following removal of the hypoxic stimulus. A significant percentage of SHR (75%) and WKY (75%) neurons that were stimulated by hypoxia were inhibited by exogenously applied GABA. These results indicate that a) an altered sensitivity of hypothalamic neurons to GABA does not contribute to hypertension in the SHR and b) the depressed respiratory response to hypoxia in the SHR is not due to a decreased responsiveness of hypothalamic neurons to hypoxia.

Effect of pentobarbital and chlordiazepoxide on the central pressor action of angiotensins in normo- and hypertensive rats

Pentobarbital and chlordiazepoxide administered intracerebroventricularly (i.c.v.) attenuated dose dependently the pressor action of i.c.v. angiotensin II and angiotensin III in the conscious spontaneously hypertensive rat (SHR) and the normotensive control Wistar Kyoto rat (WKY). Attenuation of the pressor action by the two drugs was more marked in the WKY than in the SHR. Based on the fact that both drugs act via the gamma-aminobutyric acid (GABA) receptors, the data support the suggestions (i) that a central GABAergic system regulates blood pressure, (ii) that there is probable dysfunction of the GABA receptors in SHR.

Enhancement of GABAergic system activity by metoprolol in spontaneously hypertensive rats

The relevance of the GABAergic system for the antihypertensive action of metoprolol in spontaneously hypertensive rats was studied by comparing the effect of metoprolol with the effect of dihydralazine. Chronic oral treatment with metoprolol produced the maximum effect after 49 days (-delta 34 mm Hg). This effect persisted on the same level for up to 55 days. The measurements of gamma-aminobutyric acid (GABA) synthesis and specific [3H]GABA binding were performed in the hypothalamus, the pons-medulla, the hippocampus and the striatum. Significant stimulation of GABA synthesis and turnover appeared in the hypothalamus and the pons medulla. In contrast, chronic administration of dihydralazine had no influence on GABA synthesis rate. It was also shown that metoprolol elevated significantly (P < 0.01) specific [3H]GABA binding in the hypothalamus and the pons-medulla. In the striatum this effect of metoprolol was less pronounced. Binding constant analysis revealed changes in both the receptor density and affinity. Our results suggest that the hypotensive response to chronic treatment with metoprolol might be attributed to an enhancement of GABAergic system activity.

Visual dysfunction in the spontaneously hypertensive rat

We report two forms of visual defect in the spontaneously hypertensive rat (SHR) compared to its normotensive control strain (WKY). Ten-month-old male SHR and WKY were tested for intensity discrimination ability using a Y-maze and food reward. The SHR took more trials in the acquisition phase of the task using white light. In the test phase, in which the white light intensity difference was reduced, there was no significant difference in performance between SHR and WKY. Also, there was no significant difference between the strains when red light was used. The defect in SHR occurred in the blue range of the spectrum. Using blue light, the performance of SHR declined significantly at an intensity difference of 1.15 lux, whereas the performance of WKY did not decline significantly until the difference reached 0.01 lux. The defect in SHR for intensity discrimination was not related to cataract formation, but we detected a strain difference in the nuclei of the photoreceptor cells. Thus, this visual dysfunction may involve the rod photoreceptors. Further testing revealed a deficit in visual acuity in SHR aged 40-66 days, prior to the development of hypertension. Thus, the visual dysfunction of SHR is not secondary to their development of hypertension, but rather a discrete characteristic of the strain.

Augmented neuronal activity in the hypothalamus of spontaneously hypertensive rats

Previous studies indicate a tonic GABAergic inhibitory mechanism in the posterior hypothalamus (PH) contributes to modulating cardiovascular activity. Blockade of GABA receptors on neurons in this area elicits an increase in sympathetic discharge, arterial pressure, and heart rate. It has been proposed that a deficit in this inhibitory system may be responsible for the elevated pressure in the spontaneously hypertensive rat (SHR). The purpose of this study was to determine if the spontaneous neuronal activity in the posterior hypothalamus of spontaneously hypertensive rats differs from that of age-matched normotensive Wistar-Kyoto rats (WKY). Single unit, extracellular recordings of posterior hypothalamic neurons were performed on both in vivo and in vitro preparations. The spontaneous firing rate of posterior hypothalamic neurons in the anesthetized adult SHR was significantly higher (3.66 +/- 0.55 Hz) compared to that of the anesthetized adult WKY rat (2.11 +/- 0.29 Hz). Moreover, more of the neurons in the anesthetized SHR (38%) had a bursting discharge pattern than in the WKY (16%). In order to exclude inputs from peripheral receptors or other brain areas, an in vitro preparation was used. Neurons from both young and adult SHRs also had an increased spontaneous discharge rate and higher percentage of burster-type cells in the posterior hypothalamus compared to neurons from age-matched WKYs in the brain slice preparation. Both the in vivo and in vitro findings support the possibility that an elevated neuronal activity in the posterior hypothalamus, a known pressor area, of the SHR contributes to the development and/or maintenance of hypertension in this animal model.

Cardiovascular responses to blockade of GABA synthesis in the hypothalamus of the spontaneously hypertensive rat

Previous studies have suggested that a decreased inhibitory input onto neurons within the posterior hypothalamus (PH), a known pressor area, may contribute to hypertension in the spontaneously hypertensive rat (SHR). Recent experiments from this laboratory have shown that neurons in the PH of the SHR have an altered and elevated discharge frequency compared to those in the normotensive rat. In addition, biochemical studies have reported that there is a decreased concentration of the inhibitory neurotransmitter, GABA, in the hypothalamus of the SHR. The objective of the present study was to assess any variations in GABAergic modulation of cardiovascular activity in SHRs compared to normotensive Wistar-Kyoto (WKY) rats and Sprague-Dawley (SD) rats. Arterial pressure and heart rate responses to microinjections of the GABA synthesis inhibitor 3-mercaptopropionic acid (3-MP) into the posterior hypothalamic area of anesthetized young (6-8 weeks) and mature (11-16 weeks) hypertensive and normotensive rats were recorded. Microinjection of 3-MP elicited increases in arterial pressure of 17.4 +/- 3.9 mmHg, 18.1 +/- 7.8 mmHg, 16.9 +/- 6.4 mmHg, and 10.4 +/- 3.5 mmHg in the mature WKY, mature SD, young WKY, and young SHR, respectively. In addition, heart rate was elevated by 33.2 +/- 21.9 beats/min, 70.0 +/- 25.3 beats/min, 56.3 +/- 15.0 beats/min and, 45.9 +/- 10 beats/min in the mature WKY, adult SD, young WKY, and young SHR groups, respectively. In contrast, microinjection of 3-MP into the posterior hypothalamus of adult SHRs produced no significant change in arterial pressure (-5.0 +/- 1.8 mmHg) or heart rate (+5.3 +/- 6.1 beats/min).(ABSTRACT TRUNCATED AT 250 WORDS)

Regulatory role of brain angiotensins in the control of physiological and behavioral responses

Considerable evidence now indicates that a separate and distinct renin-angiotensin system (RAS) is present within the brain. The necessary precursors and enzymes required for the formation and degradation of the biologically active forms of angiotensins have been identified in brain tissues as have angiotensin binding sites. Although this brain RAS appears to be regulated independently from the peripheral RAS, circulating angiotensins do exert a portion of their actions via stimulation of brain angiotensin receptors located in circumventricular organs. These circumventricular organs are located in the proximity of brain ventricles, are richly vascularized and possess a reduced blood-brain barrier thus permitting accessibility by peptides. In this way the brain RAS interacts with other neurotransmitter and neuromodulator systems and contributes to the regulation of blood pressure, body fluid homeostasis, cyclicity of reproductive hormones and sexual behavior, and perhaps plays a role in other functions such as memory acquisition and recall, sensory acuity including pain perception and exploratory behavior. An overactive brain RAS has been identified as one of the factors contributing to the pathogenesis and maintenance of hypertension in the spontaneously hypertensive rat (SHR) model of human essential hypertension. Oral treatment with angiotensin-converting enzyme inhibitors, which interfere with the formation of angiotensin II, prevents the development of hypertension in young SHR by acting, at least in part, upon the brain RAS. Delivery of converting enzyme inhibitors or specific angiotensin receptor antagonists into the brain significantly reduces blood pressure in adult SHR. Thus, if the SHR is an appropriate model of human essential hypertension (there is controversy concerning its usefulness), the potential contribution of the brain RAS to this dysfunction must be considered during the development of future antihypertensive compounds.

Increased activity of the GABAergic system in selected brain areas after chronic propranolol treatment in spontaneously hypertensive rats

The influence of chronically administered propranolol on the functional state of the gamma-aminobutyric acid-ergic (GABAergic) system in spontaneously hypertensive rats was studied and compared with the effect of dihydralazine. GABA content, synthesis and turnover rate in selected brain areas were assessed. Hypotensive activity of propranolol and dihydralazine after injection of GABA antagonist pictrotoxin was examined in acute experiment. Prolonged administration of propranolol increased GABA content, synthesis and turnover rate in the hypothalamus and the pons-medulla. After chronic injections of dihydralazine there was no change in GABA indices. Antihypertensive activity of dihydralazine in picrotoxin-treated animals remained unchanged. On the contrary, picrotoxin suppressed the propranolol-induced decrease in blood pressure. Our results indicate that propranolol increases GABAergic system activity. Therefore, we conclude that down-regulation of the GABAergic system in hypertension may be compensated by the regulatory action of propranolol.

Effects of gamma-vinyl GABA (vigabatrin) on blood pressure and body weight of hypertensive and normotensive rats

Inactivation of GABA was inhibited by gamma-vinyl GABA (GVG) and the effects of the increased GABA level in the brain on blood pressure and body weight of spontaneously hypertensive rats (SHR) and normotensive rats (WKY) were investigated. When started at the age of 8 weeks or 5 weeks, treatment of SHR and WKY with GVG (150 mg/kg, s.c.) for several weeks did not influence systolic blood pressure. In 1-week old SHR, treatment with GVG (up to 150 mg/kg, s.c.) abolished the rise in blood pressure until animals were 8 weeks old. Thereafter, arterial blood pressure started to increase but it remained distinctly lower than that in untreated animals. When started at the age of 1 week, treatment with GVG for 7 weeks did not influence arterial blood pressure in WKY. GVG delayed increase in body weight in SHR and WKY, irrespective of their age. GVG greatly increased GABA levels in the hypothalamus, frontal cortex, brainstem and rest of the brain in both WKY and SHR. It is concluded that an increase in the GABA level in the brain leads to a delay in the development of hypertension in young SHR. Hence, development of genetic hypertension seems to be susceptible to activation of the GABAergic system in a very early critical phase only.

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.

Neonatal stress and long-term modulation of GABA receptors in rat brain

Neonatal handling, known to have long-term effects on behaviour and neuroendocrine responses to acute stress, has been found to produce a long-term change in gamma-aminobutyric acid (GABA) receptor binding (Bmax) in whole-brain membranes of spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. A significant 10% increase in the binding of [3H] GABA to receptors was evident more than 100 days after handling. There were no differential effects of handling between these two strains, but there was a whole-brain deficit in GABA receptor binding in SHR as compared to WKY animals. Adult corticosterone levels did not correlate with GABA receptor binding.

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.

Comparative studies of muscarinic and dopamine receptors in three strains of rat

Cholinesterase activities and characteristics of muscarinic and dopamine receptors from 9 week old male Sprague-Dawley (SD), Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) were studied. Plasma cholinesterase activity in WKY was significantly lower (50%) than activity in the other strains. In studies of muscarinic receptors, the number of [3H]QNB binding sites in striata from SD rats was lower (18%) than those from WKY and SHR. However, muscarinic receptor properties (Kd and Bmax) were the same in hypothalami. Studies of dopamine receptors revealed that the densities of both D-1 and D-2 receptors in both striata and hypothalami were significantly higher in SHR than in other strains. However, there were no differences in the affinity constant (Kd). The higher densities in hypothalami from SHR were mainly due to the high population of D-1 and D-2 receptors in the posterior hypothalamus. In the anterior hypothalamus, there was no difference in the population of D-2 receptors. These results provide a substantive basis, i.e. demonstration of alterations in drug metabolizing enzymes and receptor populations, on which to build an understanding of the genetic predisposition to the actions of xenobiotic agents.

Microinjection of GABA antagonists into the posterior hypothalamus elicits locomotor activity and a cardiorespiratory activation

It is well known that electrical stimulation of an area (subthalamic locomotor region, STLR) of the posterior hypothalamus evokes locomotion as well as increases in cardiorespiratory activity. Uncertainty exists over whether these responses are due to stimulation of STLR neurons or to activation of fibers of passage originating outside this area. The purpose of the present study was to determine if stimulation of STLR neurons alone would elicit the cardiorespiratory and locomotor responses. Neurons were stimulated by microinjections of gamma-aminobutyric acid (GABA) antagonists (picrotoxin and bicuculline) into the posterior hypothalamus of anesthetized cats. Both picrotoxin and bicuculline produced increases in arterial pressure, heart rate and minute ventilation which were accompanied by locomotor movements of the limbs. Increases in arterial pressure, heart rate and phrenic nerve activity were also caused by picrotoxin microinjections in paralyzed, ventilated cats. Microinjections of GABA or a GABA agonist (muscimol) reversed all of these responses. In contrast, microinjection of GABA or muscimol into the STLR without a prior antagonist injection had only small, transient effects upon cardiorespiratory activity. However, microinjection of muscimol prevented the responses to a subsequent injection of bicuculline. These results indicate that: (1) stimulation of cell bodies alone in the subthalamic locomotor region of the hypothalamus produces all the cardiorespiratory and locomotor responses evoked by electrical stimulation; (2) the responses evoked by picrotoxin and bicuculline are due to an interaction with GABA receptors and (3) a GABAergic mechanism exerts a tonic depressive influence over the cardiorespiratory and locomotor systems by an action in the posterior hypothalamus.

Blood pressure development in SHR and WKY rats: effects of neonatal monosodium glutamate treatment and evidence for transient hypertension in WKY rats

Neonatal administration of monosodium glutamate (MSG) failed to alter blood pressure (BP) development in either SHR or WKY rats. Hypothalamic disturbance relevant to hypertension in the SHR is not contributed to by neonatal MSG treatment. All normotensive WKY animals show a transient rise in BP corresponding to the time at which BP peaks in SHR.

Defense reaction elicited by injection of GABA antagonists and synthesis inhibitors into the posterior hypothalamus in rats

Blockade of gamma-aminobutyric acid (GABA) in the posterior hypothalamic nucleus elicits cardiorespiratory stimulation in anesthetized rats. The present study was conducted to test the hypothesis that blockade of GABA in this cardiostimulatory area of the posterior hypothalamus in conscious animals would elicit a defense reaction characterised by a "fight or flight" response. Blockade of GABA was achieved by injecting bicuculline methiodide (BMI 1-25 ng) and picrotoxin (4-100 ng), two post-synaptic GABA antagonists and isoniazid (INH 35 and 70 micrograms), an inhibitor of the synthesis of GABA, bilaterally into the posterior hypothalamus through chronically implanted microinjection cannulae. All three drugs produced dose-dependent increases in locomotor activity, suggesting an "escape" reaction which was quantified as number of crossings and rearings. The effects of bicuculline and picrotoxin appeared immediately after the injection while those of isoniazid appeared much more slowly, attaining peak effects 24 +/- 1 min after injection. Injection of either strychnine (38 ng) into the posterior hypothalamus or bicuculline into the lateral hypothalamic area (LHA) or the dorso-medial/ventro-medial hypothalamus (DMH/VMH) did not elicit a significant increase in locomotor behavior. These results suggest that both the physiological and locomotor components of the hypothalamic defense reaction may be under tonic GABAergic inhibition in the region of the posterior hypothalamus.

Evidence for GABAergic inhibition of a hypothalamic sympathoexcitatory mechanism in anesthetized rats

The hypothesis that endogenous gamma-aminobutyric acid (GABA) suppresses the activity of a latent hypothalamic sympathoexcitatory mechanism was tested by examining the effects of stereotaxic intrahypothalamic microinjection of drugs influencing GABAergic inhibition in anesthetized rats. Bicuculline methiodide (BMI) 1-25 ng, a competitive antagonist at post-synaptic GABA receptors, as well as isoniazid (INH) 35 and 70 micrograms and 3-mercaptopropionic acid (3MP) 0.02 microliter, inhibitors of GABA synthesis, all evoked marked increases in heart rate and modest pressor responses. However, while the effects of BMI appeared almost immediately and peaked within 10 min of injection, changes caused by INH or 3MP developed much more slowly, attaining a maximum 35-40 and 19 min after injection, respectively. The effects of BMI on heart rate were blocked by pretreatment with propranolol 2 mg/kg i.v. or hexamethonium 20 mg/kg i.v. plus atropine 2 mg/kg i.v. and were shown to be highly localized to the posterior hypothalamic nucleus and the adjacent lateral hypothalamus. In addition to the cardiovascular effects, BMI also elicited dose-related increases in respiratory rate which were independent of the heart rate changes although they followed a similar time course. The results support the notion that hypothalamic GABA inhibits a local mechanism capable of generating cardiorespiratory arousal.

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.

Baroreceptor reflex effect on sympathetic nerve activity in stroke-prone spontaneously hypertensive rats

To elucidate the relationship between responses in sympathetic outflow to incremental changes in blood pressure, we performed baroreflex activation experiments in conscious, stroke-prone, spontaneously hypertensive rats (SHRSP) and Wistar-Kyoto (WKY) rats. We gave incremental bolus injections of nitroprusside and methoxamine and measured mean blood pressure (MAP), heart rate, and splanchnic nerve activity (SpNA). Change in heart rate per change in blood pressure was greater in WKY rats than in SHRSP indicating the decrease in baroreceptor sensitivity displayed in the latter strain. In contrast, change in SpNA per change in blood pressure was greater in SHRSP than in WKY rats. These findings suggest that sympathetic outflow in SHRSP is more reactive to blood pressure changes than in WKY rats. The discordance in heart rate and SpNA changes supports the notion that central neural integration of autonomic function in SHRSP is impaired.

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.

Microinjection of GABA antagonists into posterior hypothalamus elevates heart rate in anesthetized rats

Microinjection of the GABA antagonist bicuculline methiodide 1-25 ng into the posterior hypothalamus of urethane-anesthetized rats evoked sympathetically-mediated increases in heart rate of up to 150 beats/min and modest increases in blood pressure which could be prevented by prior local microinjection of muscimol 50 ng. Microinjection of picrotoxin but not strychnine produced similar effects. These results suggest that a latent sympathoexcitatory mechanism in this region is tonically inhibited by endogenous GABA.