Tsuneichi Hashimoto

In vivo evidence that GABAB receptors are negatively coupled to adenylate cyclase in rat striatum

Abstract: The characteristics of the cerebral GABAB receptor/cyclic AMP (cAMP)‐generating system were investigated using the in vivo microdialysis technique in freely moving rats. Addition of forskolin, an activator of adenylate cyclase, to perfusate for 20 min resulted in a dose‐dependent increase of cAMP efflux from the striatum. Pre‐ and coinfusions of baclofen for 80 min had no effect on the basal efflux of cAMP from the striatum but induced a significant decrease of forskolin (10 µM)‐stimulated cAMP efflux from the striatum in a dose‐dependent manner. SKF 97541 (100 µM), a GABAB receptor agonist, and GABA (50 µM) also decreased forskolin‐induced cAMP efflux from the striatum. Coinfusion of CGP 54626A (100 µM), a GABAB receptor antagonist, counteracted the effect of baclofen on the forskolin‐stimulated cAMP efflux. In contrast, the isoproterenol (5 mM)‐induced increase of cAMP efflux from the striatum was significantly enhanced by pre‐ and coinfusions with baclofen. These results suggest that this test system using in vivo microdialysis may be useful for examining the effect of drugs on the GABAB receptor‐linked cAMP‐generating system in vivo.

Interrelationship between Taurine and GABA

β-Aminoethanesulfonic acid (taurine) and γ-aminobutyric acid (GABA) are recognized as major inhibitory neuroactive amino acids distributed in large quantities in various central nervous system (CNS) areas. The role of GABA as an inhibitory neurotransmitter has been well established, whereas that of taurine is still unclear. In the case of GABA, the structure and function of two types of the GABA receptor, GABAA and GABAB receptors, have been identified, while the taurine receptor is not yet fully understood. However, some very convincing evidence concerning the functional interrelationships between taurine and GABA in the brain has accumulated.

Muscarinic M1 receptor mediated inhibition of GABA release from rat cerebral cortex

Presynaptic modulation of [3H]GABA release was examined using rat cerebral cortical slices. In vitro addition of carbachol, a muscarinic receptor agonist, resulted in a significant suppression of the release of [3H]GABA evoked by high potassium (50 mM) stimulation in a dose dependent manner, while noradrenaline, isoproterenol, dopamine, 5-hydroxytryptamine, histamine and glutamic acid had no significant effect on the evoked release of [3H]GABA. This suppressive effect of carbachol was antagonized invariably by atropine. Furthermore, it was found that the suppressive action of carbachol could be antagonized by pirenzepine, a selective M1 muscarinic receptor antagonist, but not by AF-DX 116 and 4-DAMP, M2 and M3 receptor antagonists, respectively. These results suggest that the release of GABA from cerebral cortical GABA neurons may be modulated by presynaptic M1 muscarinic receptor.

Alcohol, acetaldehyde and salsolinol-induced alterations in functions of cerebral GABA/benzodiazepine receptor complex.

Effects of alcohol (ethanol) and acetaldehyde on the metabolism and function of primary cultured GABAergic neurons and that of salsolinol, a condensation product of acetaldehyde with dopamine, on cerebral GABA/benzodiazepine (BZP) receptor complex were investigated. In vitro addition of acetaldehyde induced a significant reduction not only on the content of GABA but also on the basal and GABA-activated [3H]flunitrazepam ([3H]FLN) bindings in primary cultured neurons. In contrast, alcohol exhibited only suppressive effects on [3H]FLN binding as well as on the enhancement of [3H]FLN binding by GABA. On the other hand, salsolinol showed a significant stimulatory effect on [3H]FLN binding to cerebral particulate fractions obtained from alcohol-untreated mice in the presence of NaCl. Although a similar activation of cerebral [3H]FLN binding by salsolinol was found in alcohol-treated mice, this activation was significantly weaker in alcohol-withdrawn mice than those found in alcohol-untreated as well as alcohol-inhaled mice. These results strongly suggest that acetaldehyde may have more important pathophysiological roles than those of alcohol in the exhibition of neurotoxicity during alcohol intake. The present results also suggest that salsolinol may have a modulatory role on cerebral benzodiazepine receptor and the decreased capacity of such a modulating mechanism may be involved in the exhibition of alcohol withdrawal syndrome, possibly by decreasing the function of endogenous ligands for benzodiazepine receptor in the brain.

Possible involvement of supersensitivity in cerebral β-adrenergic and dopaminergic receptors in the occurrence of sensitization to d-methamphetamine in mice

Behavioral sensitization of mice (male STD-ddy) to d-methamphetamine (MAP) and its correlation with the neurochemical alterations in cerebral catecholaminergic systems were investigated following the repeated administration of MAP (5 mg/kg, i.p. x 2/day, 5 days). MAP-pretreated mice showed an enhancement in MAP-stimulated locomotor activity following the readministration of MAP (0.625 mg/kg). The metabolic turnovers of cerebral norepinephrine (NE) and dopamine (DA) showed a significant decrease in these sensitized mice. The activities of tyrosine hydroxylase. DOPA decarboxylase, monoamine oxidase and catechol-O-methyltransferase, however, showed no change under the same experimental conditions. On the other hand, the repeated MAP pretreatment induced the enhancement of the specific bindings of [(3)H]dihydroalprenolol and [(3)H]spiperone to cerebral synaptic membrane, which resulted from the increase of B(max) values. Furthermore, pretreatments of mice with propranolol (10 mg/kg) and haloperidol (0.1 mg/kg) inhibited the development of behavioral sensitization to MAP. These results suggest that the supersensitivity in cerebral ?-adrenergic and dopaminergic receptors may be involved in behavioral sensitization to MAP.

Protective effect of WEB 1881 FU on AF64A (ethylcholine aziridinium ion)-induced impairment of hippocampal cholinergic neurons and learning acquisition

The effect of continuous administration of 4-aminomethyl-1-benzylpyrrolidin-2-one-hemifumarate (WEB 1881 FU) on cerebral cholinergic neurons was studied using rats treated with ethylcholine aziridinium ion (AF64A), a neurotoxic choline analog. AF64A (2.0 nmol, administered i.c.v.) caused a significant decrease in the hippocampal acetylcholine (ACh) content. This decrease in hippocampal ACh content was accompanied by a reduction of choline acetyltransferase (CAT) activity. Under these experimental conditions, the latency of the passive avoidance response of rats, determined with a step-through method, was strongly decreased as compared with that of sham-operated rats. Although treatment with WEB 1881 FU (50 mg/kg per day, administered orally for 7 days) from immediately after the administration of AF64A did not affect the AF64A-induced decrease of ACh in the hippocampus, 100 mg/kg per day of WEB 1881 FU (orally for 7 days) significantly suppressed the AF64A-induced declines in hippocampal ACh content and CAT activity. The AF64A-induced reduction in latency of the passive avoidance response was also significantly antagonized by the treatment with 100 mg/kg per day of WEB 1881 FU (administered orally for 7 days) from immediately after the administration of AF64A. Continuous administration of WEB 1881 FU (100 mg/kg per day, orally for 7 days) from 7 days after the treatment with AF64A also had a significant inhibitory effect on the AF64A-induced decrease in ACh content in the hippocampus. These results suggest that WEB 1881 FU may have protective actions on the destruction of hippocampal cholingergic neurons as well as memory impairment induced by AF64A administration.

Taurine and Receptor Mechanisms in the Heart: Possible Correlates with the Occurrence of Ischemic Myocardial Damages

Cardiac tissue contains a high concentration of taurine (2-amino-ethanesulfonic acid), which represents approximately 50% of the free amino acid pool [1–3] in the mammalian heart, but its functional role has not been clearly defined. In cardiac tissue, proposed actions of taurine include positive inotropic [4–6], anti-arrhythmic [7,8], osmoregulatory [9,10], and neuromodulatory effects [11,12]. The modulation of ion fluxes and membrane stabilization by taurine have been well documented. In addition, protective effects of taurine on Ca++-paradox [13], isoproterenol (ISP)-induced heart failure [14], and hypertension in spontaneously hypertensive rats [15] have also been reported. On the other hand, it has been reported that hypoxia [16], cardiac ischemia [17,18], and cardiac surgery [19] induce a decrease of taurine in the heart, whereas congestive cardiac failure is accompanied with an increase of taurine [2,20]. Similarly, it has been reported that taurine has different effects such as stimulating myocardial contraction induced by ISP, while inhibiting the elevation of myocardial Ca++ [21] in the perfused chick heart. It is not clear, however, whether or not the changes in cardiac taurine content are directly related to the pathogenesis of the cardiac disorders.

Oxidative metabolite of dopamine, 3,4-dihydroxyphenylacetaldehyde, induces dopamine release from PC12 cells by a Ca2+-independent mechanism

3,4-dihydroxyphenylacetaldehyde (DOPALD), an oxidative metabolite of dopamine (DA), induced dose-dependent DA release from pheochromocytoma (PC12) cells without affecting leakage of lactate dehydrogenase from the cells. DOPALD-induced DA release was independent of extracellular Ca2+ concentration and was not blocked by nifedipine, an L-type Ca2+ channel antagonist. These results indicated a novel intrinsic role of DOPALD in dopaminergic nerve terminals that may take part in the activation of dopamine neurons.

GABAA receptor-mediated K+-evoked GABA release from globus pallidus : Analysis using microdialysis

Presynaptic modulation of gamma-aminobutyric acid (GABA) release in the globus pallidus of rat was examined using in vivo microdialysis procedures. The addition of nipecotic acid (0.5 mM), a neuronal GABA uptake inhibitor, into perfusate, resulted in an increase in the basal GABA released from the globus pallidus. GABA release from the globus pallidus was also augmented dose-dependently by the addition of KCI. Muscimol, a GABAA receptor agonist, caused a significant suppression of the high potassium (100 mM)-evoked release of GABA, and this suppressive effect of muscimol was antagonized invariably by bicuculline, a GABAA receptor antagonist. On the other hand, baclofen, a GABAB receptor agonist, did not induce any significant changes in the 100 mM KCl-evoked GABA release. Similarly, 3-aminopropylphosphonous acid, a GABAB receptor agonist, failed to suppress the GABA release induced by high (100 mM) and low (50 mM) concentrations of KCl from the globus pallidus. Furthermore, CGP 54626A,-a GABAB receptor antagonist, had no significant effect on these KCl-evoked GABA releases. These results suggest that presynaptic modulation of GABA release in the globus pallidus may be mediated by the GABAA autoreceptor.

Age-related decrease of γ-aminobutyric acid (GABA) release in brain of spontaneously hypertensive rats

We studied the developmental alteration of GABA release in spontaneous ly hypertensive(SHR) and normotensive Wistar-Kyoto rat(WKY) brain. The release of [3H]GABA observed under high potassium(30 mM) in eleven-week-old(hypertensive) SHR hippocampus and the spontaneous release of [3H]GABA in the same aged SHR medulla oblongata were lower than those of age-matched WKY. We conclude that the GABAergic mechanisms may be different in SHR and WKY brain and may be associated with the development of hypertension.