CENTRAL CARDIOVASCULAR ACTIONS OF GAMMA-AMINOBUTYRIC ACID

Antihypertensive Effect of γ-Aminobutyric Acid-Enriched Brown Rice on Spontaneously Hypertensive Rats

'Haiibuki' is a giant embryo rice cultivar that contains abundant γ-aminobutyric acid (GABA) compared with conventional rice cultivars. Here, we performed a functional evaluation of 'GABA-enriched brown rice' (GEBR) prepared by modifying the 'Haiibuki' cultivar to contain more GABA. Study 1: Spontaneously hypertensive rats were divided into three groups [control (cornstarch), normal brown rice, and GEBR] and fed an orally administered diet for 4 wk. A significant blood pressure elevation-inhibitory effect was observed in the GEBR group as compared with the other groups. Study 2: Rats were divided into two groups and fed ad libitum for 12 wk. The two groups were control (commercial feed with 5% cornstarch) and GEBR (commercial feed with 5% GEBR). Body weight, blood pressure, food consumption, and water intake were measured during the study period, and blood chemistry was analyzed after the study. Plasma 8-hydroxy-2'-deoxyguanosine (8-OHdG) and urinary isoprostane were measured 12 and 10 wk after the start of the study, respectively. A significant blood pressure elevation-inhibitory effect was observed in the GEBR group. The 8-OHdG and isoprostane levels were significantly lower in the GEBR group than in the control group, demonstrating an oxidative stress-reducing effect. Therefore, GEBR exhibited a blood pressure elevation-inhibitory effect under the conditions of this study. The antioxidative action may occur secondarily to the antihypertensive action of GABA, suggesting that the long-term ad libitum ingestion of GEBR prevents hypertension. A reduction in oxidative stress could reduce the chances of complications in cardiovascular diseases.

Gamma-aminobutyric acid production using immobilized glutamate decarboxylase followed by downstream processing with cation exchange chromatography

We have developed a gamma-aminobutyric acid (GABA) production technique using his-tag mediated immobilization of Escherichia coli-derived glutamate decarboxylase (GAD), an enzyme that catalyzes the conversion of glutamate to GABA. The GAD was obtained at 1.43 g/L from GAD-overexpressed E. coli fermentation and consisted of 59.7% monomer, 29.2% dimer and 2.3% tetramer with a 97.6% soluble form of the total GAD. The harvested GAD was immobilized to metal affinity gel with an immobilization yield of 92%. Based on an investigation of specific enzyme activity and reaction characteristics, glutamic acid (GA) was chosen over monosodium glutamate (MSG) as a substrate for immobilized GAD, resulting in conversion of 2.17 M GABA in a 1 L reactor within 100 min. The immobilized enzymes retained 58.1% of their initial activities after ten consecutive uses. By using cation exchange chromatography followed by enzymatic conversion, GABA was separated from the residual substrate and leached GAD. As a consequence, the glutamic acid was mostly removed with no detectable GAD, while 91.2% of GABA was yielded in the purification step.

Evidence for a spinal origin of the effect of baclofen on the myocardial oxygen demand indexes

In a previous study in anaesthetized rabbits we observed that electrical stimulation of the hypothalamic paraventricular nucleus (PVN) elicited substantial rises in the maximum rate of change of left ventricular pressure (dP/dtmax) and in myocardial oxygen demand indexes (rate-pressure product and triple product), similar to the changes observed during stress or physical effort. Baclofen, a selective GABA(B) receptor agonist, injected intravenously prevented these responses. In the present study, we show that low doses of baclofen (0.1, 0.3 and 1 microgram/kg), injected intrathecally (i.t.) at the T9 level, reduced the myocardial oxygen demand during PVN stimulation. After 0.3 microgram/kg baclofen i.t., the peak value of the triple product during stimulation was 140 +/- 20 compared with 193 +/- 20 before treatment. An i.t. injection (500 micrograms/kg), of saclofen a selective GABA(B) receptor antagonist, did not modify the resting haemodynamics significantly but attenuated the inhibitory effects of baclofen (3 mg/kg i.v.). These results suggest that the main site of the effects of baclofen is located within the spinal cord and that GABA(B) receptors probably mediate these effects by modulating the central control of cardiac function. In conclusion, baclofen might be a useful tool to prevent the centrally evoked increases of myocardial oxygen demand.

Age-dependent changes in cardiovascular responses induced by muscimol infused into the nucleus tractus solitarii and nucleus parabrachialis medialis in rats

1. The effects of muscimol and bicuculline on mean arterial blood pressure (MAP) and heart rate (HR) were studied after their microinfusion into the nucleus tractus solitarii (NTS) and into the nucleus parabrachialis medialis (NPBmed) in 3 and 24 month old rats. 2. In 3 month old rats a dose of 0.01 microgram of muscimol given into the NTS increased MAP, whereas higher doses (0.05, 0.1 and 0.25 microgram) produced dose-dependent bradycardia with either no change (0.05 and 0.1 microgram) or a decrease (0.25 microgram) in MAP. 3. On the other hand, in 24 month old rats, the same doses of muscimol given into the NTS failed to change MAP, whereas in comparison to 3 month old rats they produced a significantly lesser bradycardia. 4. The cardiovascular changes elicited by infusion of muscimol into the NTS in 3 and 24 month old rats were prevented by prior microinfusion of bicuculline into the same site. 5. Muscimol given into the NPBmed in doses from 0.05 to 0.25 microgram produced, in 3 month old rats dose-dependent decreases in MAP and HR which were prevented by prior administration of bicuculline. These effects were significantly reduced in 24 month old rats. 6. The present experiments show that with aging there is impairment of GABA-ergic mechanisms involved in the regulation of cardiovascular activity in the NTS and NPBmed.

Influence of gamma-aminobutyric acid on the changes of blood pressure in rats

This is an attempt to investigate the effect of gamma-aminobutyric acid (GABA), a well-known major inhibitory neurotransmitter in the central nervous system, on the blood pressure response in rats and to elucidate the mechanism of its action. GABA injected into a femoral vein of the rat produced a dose-related fall in blood pressure followed by a secondary pressor response. The depressor response evoked by GABA was clearly blocked by pretreatment with chlorisondamine, diazepam and picrotoxin but was unaffected by atropine, prazosin and debrisoquin. GABA-induced pressor responses were significantly attenuated by pretreatment with prazosin or picrotoxin, while not affected by atropine, diazepam, debrisoquin and chlorisondamine. These experimental data suggest that GABA causes biphasically depressor and pressor responses in rats, and that the hypotensive activity evoked by GABA may be exerted through activation of GABAergic receptors and hypertensive activity due to stimulation of the adrenergic alpha-receptors, which appears to be associated with GABAergic receptors.