Acetylcholine release from synaptosomes and phenytoin action

Antioxidants in peripheral nerve

Oxidative stress and antioxidants have been related in a wide variety of ways with nervous tissue. This review attempts to gather the most relevant information related to a) the antioxidant status in non pathologic nervous tissue; b) the hypothesis and evidence for oxidative stress (considered as the disequilibrium between prooxidants and antioxidants in the cell) as the responsible mechanism of diverse neurological diseases; and c) the correlation between antioxidant alterations and neural function, in different experimental neuropathies. Decreased antioxidant availability has been observed in different neurological disorders in the central nervous system, for example, Parkinson's disease, Alzheimer's disease, epilepsy, amyotrophic lateral sclerosis, cerebral ischaemia, etc. Moreover, the experimental manipulation of the antioxidant defense has led in some cases to interesting experimental models in which electrophysiological alterations are associated with the metabolic modifications induced. In view of the electrophysiological and biochemical effects of some protein kinase C inhibitors on different neural experimental models, special attention is dedicated to the role of this kinase in peripheral nervous tissue. The nervous tissue, central as well as peripheral, has two main special features that are certainly related to its antioxidant metabolism: the lipid-enriched membrane and myelin sheaths, and cellular excitability. The former explains the importance of the glutathione (GSH)-conjugating activity towards 4-hydroxy-nonenal, a biologically active product of lipid peroxidation, present in nervous tissue and in charge of its inactivation. The impairment of the latter by oxidative damage or experimental manipulation of antioxidant metabolism is discussed. Work on different experimental neuropathies from author's laboratory has been primarily used to provide information about the involvement of free radical damage and antioxidants in peripheral nerve metabolic and functional impairment.

Severe phenytoin-induced bone marrow depression and agranulocytosis treated with human recombinant granulocyte-macrophage colony-stimulating factor. Case report

An unusual instance of severe and potentially lethal depression of the bone marrow is described as a result of the administration of phenytoin for seizure prophylaxis. The patient was treated successfully by prompt cessation of phenytoin and intravenous administration of human recombinant granulocyte-macrophage colony-stimulating factor.

Prevention of the acute neurotoxic effects of phenytoin on rat peripheral nerve by H7, an inhibitor of protein kinase C

The neurotoxic effects of a single dose of phenytoin (150 mg/kg body weight) alone or 30 min after H7 (a protein kinase C inhibitor) injection (20 mg/kg body weight) were investigated in terms of peripheral neuromuscular function and Na+,K(+)-ATPase activity of the sciatic nerve. This intraperitoneal injection of phenytoin induced complete blockade of muscle action potentials in the dorsal segmental muscles of the rat tail evoked by electric stimulation of the caudal nerve and a 40% decrease in the Na+,K(+)-ATPase activity of the rat sciatic nerve when compared with control values, measured as the difference between total and ouabain-insensitive ATPase activity. Prior administration of H7 resulted in the complete prevention of both effects. Implications of protein kinase C inhibition in phenytoin neurotoxicity are discussed.

Modulation of release of acetylcholine from the striatum by a proposed excitatory amino acid antagonist U-54494A: Comparison with known antagonists, diazepam and phenytoin

The effect of (U-54494A) cis-3,4-dichloro-N-methyl-N-[2-(1-Pyrrolidinyl)- cyclohexyl] benzamide monohydrochloride, an excitatory amino acid antagonist, on N-methyl-D-aspartic acid (NMDA)- and K(+)-evoked release of [3H]acetylcholine [( 3H]ACh) from slices of striatum was investigated. For the purpose of comparison, MK 801, PCP, CGP 37849, CPP, phenytoin and diazepam were investigated under identical conditions. Both U-54494A and the excitatory amino acid antagonists blocked NMDA-evoked release of [3H]ACh but these compounds failed to inhibit K(+)-evoked release of this neurotransmitter. Phenytoin blocked both NMDA and K(+)-evoked release of [3H]ACh, whereas diazepam was ineffective under similar conditions. These observations indicate that excitatory amino acid antagonists, including U-54494A, may mediate their anticonvulsant effect by blocking the activity of NMDA receptors, diazepam by activating the benzodiazepine receptors and phenytoin by inhibiting the activity of various depolarizing agents.

Effect of phenytoin sodium in the management of poorly controlled bronchial asthma at a rural health center in Phalodi, Rajasthan, India

The efficacy of phenytoin (PHT) for the relief of chronic asthma was evaluated in an open trial in 190 intractable rural asthmatics. Our data indicate that phenytoin (100 mg po bid for adults and 50 mg po bid for children) is a useful antiasthmatic agent used either alone, or as adjuvant therapy. Phenytoin significantly reduced the frequency and severity of asthma attacks, coughing, nocturnal awakenings, and work absenteeism, and improved effort tolerance and subjective sense of well being. Phenytoin also reduces corticosteroid requirements of steroid-dependent patients. Not only were these results sustained over the course of 12-month treatment, but more than 60% of phenytoin-treated patients could be discontinued without recurrence of symptoms.

Role of phenytoin in healing of large abscess cavities

The promotion of healing of large abscess cavities attained with topical phenytoin was evaluated in controlled studies of clinical and experimental wounds. In the clinical abscess cavities, phenytoin application in 20 patients compared with conventional treatment in 20 patients resulted in earlier separation of slough, decrease in oedema, control of pain and overall enhanced healing. The mean(s.d.) rate of reduction of wound area was 2.02(0.48) cm2/day in the phenytoin group versus 1.58(0.51) cm2/day in controls (P less than 0.05) on day 10, and 1.8(0.32) cm2/day versus 1.19(0.21) cm2/day (P less than 0.01) on day 20. The mean volume reduction rates at both the 10th and 20th day were 0.48(0.01) cm3/day for phenytoin versus 0.32(0.04) cm3/day for controls; (P less than 0.005). By day 20, 17 of the patients treated with phenytoin were rated as having healed completely, compared with only one of the controls. In a standardized guinea-pig model of the clinical abscess cavity, which included inoculation of the wound with Bacillus proteus and Klebsiella pneumoniae, an enhanced healing rate was also observed (at 7 days 0.40(0.05) cm2/day with phenytoin versus 0.21(0.08) cm2/day in controls; P less than 0.005). All eight of the animals treated with phenytoin healed by day 21, compared with one of the eight controls. Biopsies of wounds treated with phenytoin showed less inflammation, no necrosis, and enhanced neovascularization, collagen deposition and fibroblast proliferation compared to controls. Bacterial colonies also decreased more rapidly with the use of phenytoin.

Inhibition by phenytoin of in vitro secretion of calcitonin from rat thyroid glands and cultured rat C cells

Baby rat thyroid glands and cultured rat medullary carcinoma C cells were incubated acutely with phenytoin (38-100 microM), and the calcitonin (CT) secreted into the serum-free medium was measured by radioimmunoassay (RIA). Phenytoin did not alter CT release from glands or C cells incubated in 1 mM Ca, but, when Ca was raised to 1.75 or 2.5 mM, a marked inhibitory effect of phenytoin was apparent. The inhibitory effect could be negated by including 10 microM BAY-K-8644 in the medium. Inhibitory effects on CT release also were obtained with 100 microM trifluoperazine or 100 microM nitrendipine, and these inhibitory effects also were counteracted by 10 microM BAY-K-8644. The results show that clinically relevant amounts of phenytoin can inhibit CT release, perhaps by interfering with C-cell Ca channels or by inhibiting calmodulin-dependent processes.

Phenytoin reduces early acetylcholine release after depolarization

Phenytoin 10 microM inhibits the K-evoked release of acetylcholine (ACh) from synaptosomes, a process which is biphasic. Phenytoin acts only on the early phase of release. Replacement of external Na with Li does not modify phenytoin's effect. Phenytoin augments the spontaneous release of ACh from resting synaptosomes but this effect is eliminated in Li media. It is likely that phenytoin reduces K-evoked Ca uptake and the Na/Ca exchange by separate mechanisms.

Phenytoin: mechanisms of its anticonvulsant action

Phenytoin is a major anticonvulsant drug that is very effective in controlling a wide variety of seizure disorders while impairing neurological function little, if at all. Early work suggested the hypothesis that the drug's effects were due to a selective block of high-frequency neuronal activity. This theory is reevaluated in the light of accumulated observations on the effects of phenytoin in many neuronal and synaptic preparations. Most of these observations can be explained by a use- and frequency-dependent suppression of the sodium action potential by phenytoin, with a consequent filtering out of sustained high-frequency neuronal discharges and synaptic activity. The molecular mechanism for this is a voltage-dependent blockade of membrane sodium channels responsible for the action potential. Through this action, phenytoin obstructs the positive feedback that underlies the development of maximal seizure activity, while normal brain activity, proceeding at lower neuronal firing rates, is spared its depressant action. Other mechanisms of action that may contribute to the drug's efficacy and selectivity are also discussed.

Effect of phenytoin on [3H]norepinephrine release from synaptosomes

Phenytoin reduces depolarization-linked [3H]norepinephrine release from rat brain synaptosomes. When choline chloride was substituted for NaCl the phenytoin effect was attenuated but still significant. This is consistent with the theory that phenytoin reduces both Na and Ca influx during depolarization.

Calmodulin, synchronous and asynchronous release of neurotransmitter

Evidence collected from studies on a wide range of secretory cells suggests that calmodulin may play an important role in stimulus-secretion coupling. Work on synaptosomes, central synaptic preparations and chromaffin cell preparations indicates that calmodulin probably also acts as the intracellular Ca2+-receptor for secretion in neuronal cells, Ca2+-binding resulting in activation of protein kinases and phosphorylation of certain secretory vesicle proteins. Studies on the effects of calmodulin-binding drugs at peripheral synapses have given surprising results, particularly the finding that evoked (synchronous) transmitter release is not suppressed by calmodulin inhibition, though asynchronous release can be markedly inhibited. It is suggested that the insensitivity of synchronous release to drug treatment is due to the fact that only vesicle-bound calmodulin is involved in this form of transmitter secretion. Asynchronous release, however, involves recruitment of cytosolic calmodulin and can therefore be inhibited by calmodulin-binding drugs.