Takahiko Sumi

Amitriptyline inhibits striatal efflux of neurotransmitters via blockade of voltage-dependent Na+ channels

Amitriptyline, a tricyclic antidepressant, almost completely inhibited veratridine- or scorpion toxin-evoked efflux of endogenous dopamine (DA) and gamma-aminobutyric acid (GABA) from rat striatal slices without any effects of 30 mM K+ or 4-aminopyridine on basal and evoked efflux. The effects of amitriptyline on glutamate efflux were comparable to those on DA or GABA efflux. These effects of amitriptyline can be well explained by its blockade of voltage-dependent Na+ channels and may be independent of other activities of this drug such as re-uptake inhibition of monoamines and blockade of K+ channels.

Evoked release of endogenous amino acids from rat striatal slices and its modulation

The release of endogenous gamma-aminobutyric acid (GABA), glutamate and aspartate stimulated by high K+ was studied by superfusing rat striatal slices. Stimulation with 5 min of 30 mM K+ was applied twice (S1 and S2) at a 20 min interval. The maximum release of GABA following stimulation was 40 (at S1) and 26 (at S2) times greater than the basal release. S1 and S2 each produced a maximum release of almost same magnitude for both glutamate and aspartate (about 2.5 times basal release). The removal of Ca2+ from the perfusion medium reduced the maximum release of these amino acids by more than 80% without affecting basal release significantly. Striatal slices were next stimulated in the same way after the addition of apomorphine or haloperidol to the perfusion medium. Apomorphine, 10 or 100 microM, reduced the K+-evoked release of GABA (by 24% at S1 and 35% at S2 with 10 microM; by 37% at S1 and 47% at S2 with 100 microM) but failed to affect the simultaneous release of glutamate and aspartate. Haloperidol (1 microM) reduced the S1-induced release of GABA but had no significant effect on the subsequent S2-induced release. The evoked release of aspartate or glutamate did not respond significantly to haloperidol, but there was a tendency to a decrease in aspartate release similar to the decrease seen for GABA release, particularly with S1. Based on these results, we discuss the property of GABA, aspartate and glutamate as neurotransmitters and the possible dopaminergic regulation of the release of these amino acids.

A spectrofluorometric method for determining plasma allantoin based on the glyoxylate reductase reaction

Abstract A sensitive and specific assay method for allantoin is described together with its application to rat plasma. The method consists of two main successive steps: hydrolytic formation of glyoxylic acid from allantoin, and fluorometric measurement of NAD produced from the reaction of glyoxylic acid and NADH, catalyzed by glyoxylate reductase. As little as 0.1 μg of allantoin can be determined. For the determination of endogeneous allantoin levels in rat plasma, 50 μl of the plasma is sufficient. Due to the higher sensitivity and specificity, the proposed method has advantages over the colorimetric method which has long been used for the determination of allantoin.

Release of endogenous acetylcholine from rat brain slices with or without cholinesterase inhibition and its potentiation by hemicholinium-3

The direct measurement of basal and high K(+)-stimulated release of endogenous acetylcholine from striatum and hippocampus slices was achieved without as well as with a cholinesterase inhibitor, physostigmine. Hemicholinium-3, opposite to its well-known activity as an inhibitor of endogenous acetylcholine release, significantly potentiated both the basal and stimulated release, in particular in the absence of physostigmine, suggesting an involvement of some unknown activity stimulating ACh release in hemicholinium-3.

Decrease in the evoked release of endogenous dopamine and dihydroxyphenyiacetic acid from rat striatal slices after withdrawal from repeated haloperidol

The release of endogenous DA (dopamine) and DOPAC (3,4-dihydroxyphenylacetic acid) from rat striatal slices was measured after withdrawal from a prior long-term treatment of the rat with haloperidol to investigate adaptive changes in striatal DA and DOPAC release induced by chronic haloperidol treatment. Striatal slices prepared 24 h after the last injection of daily treatment with haloperidol for up to 14 days (2.5 mg/kg per day) were superfused and stimulated for 5 min with K+ (50 mM). Haloperidol treatment for 3 or 7 days decreased K(+)-stimulated DA release by maximally 35%, but a 14-day treatment was not effective. The K(+)-stimulated release of DOPAC, which occurred after the change in DA release, was reduced significantly by the treatment for 7 or 14 days. A higher daily dose of haloperidol (10 mg/kg per day) produced a more pronounced decrease in stimulated DA release after a 14-day treatment without having an effect after 3 days. However, the stimulated release of DOPAC decreased markedly after both 3 and 14 days of haloperidol treatment. The slight reduction in the DA content of the slices after K+ stimulation was seen in some haloperidol-treated tissues, although this change did not always parallel the simultaneous decrease in DA release. These results indicate that the K(+)-induced stimulation of endogenous DA release and the synthesis of DA are impaired after withdrawal from repeated haloperidol treatment.

Hyperuricemia in the rat following hypothalamic stimulation

Abstract Ventromedial hypothalamic electrical stimulation elicited a marked elevation of plasma uric acid with a rise in plasma allantoin in the rat. The magnitude of this hyperuricemia was greater than that of the hyperglycemia which was also produced by the ventromedial stimulation. On the other hand, lateral hypothalamic stimulation did not significantly affect the plasma levels of either of the purine metabolites. These results strongly indicate that the ventromedial hypothalamus is specifically very active in producing hyperuricemia in the rat.

Adrenergic regulation of the plasma levels of purine metabolites in the rat

The effects of i.v. infusions of catecholamines on the plasma levels of uric acid and allantoin were investigated in the conscious rat. Isoproterenol and epinephrine, in this decreasing order of potency, remarkably elevated both uric and allantoin levels. Norepinephrine did not affect these purine metabolite levels. These effects of isoproterenol and epinephrine were completely abolished by propranolol. Phentolamine, on the contrary, slightly potentiated the effects of epinephrine. In contrast to i.v. fusion, s.c. injection of epinephrine had no effect, but, after prior treatment of animals with phentolamine, elicited striking increases in the plasma purine metabolite levels. Catecholamines thus caused hyperuricemia with a rise in plasma allantoin, both effects being mediated via beta-adrenoceptors and alpha-adrenergic mechanism sometimes functions in such a manner as to mask or inhibit this beta-mediated response.

Epinephrine-independent production of hyperuricemia by means of hypothalamic stimulation in the conscious rat

Abstract The electrical stimulation of the ventromedial hypothalamus with 0.6 mA current produced an acute hyperuricemia associated with a rise in plasma allantoin in the conscious rat. Adrenal demedullation did not depress the hyperuricemic response. Even after the demedullation, neither propranolol nor phentolamine had any significant effects as well. These results indicate that endogeneous catecholamines are not involved in the production of the hyperuricemia; this is in sharp contrast with our previous result demonstrating the development of hyperuricemia via adrenal epinephrine by stimulating the ventromedial nucleus with 0.2 mA current. It is concluded therefore that ventromedial hypothalamic stimulation is capable of producing two different types of hyperuricemia, i.e. adrenal-epinephrine-dependent and epinephrine-independent, by varying the intensity of the stimulation current.

A change in the spontaneous release of endogenous acetylcholine from rat striatal slices after repeated injection of haloperidol

The spontaneous and 50 mM K(+)-stimulated release of endogenous acetylcholine (ACh) from rat striatal slices was measured to investigate an adaptive change of striatal ACh after a withdrawal period for 24 h from chronic treatment of the rat with haloperidol. The haloperidol injections (2.5 mg/kg/day) for a period of 3, 7 or 14 days all reduced the spontaneous release of ACh significantly by 20-50% without changing tissue levels of ACh. On the contrary, these treatments produced a very small (by about 10%) but significant increase in K(+)-stimulated release except for the treatment of 7 days. These results suggest that the spontaneous ACh release is under the control of a dopaminergic mechanism, which might be different from the mechanism controlling the evoked release of ACh and emerges following withdrawal from chronic haloperidol treatment.