Hansotia, Mazza and Gatlin, American journal of Clinical Pathology (1975), ¹⁸⁸¹ studied the effects of PHT on membrane function and fragility of red blood cells in normal subjects and in patients with hereditary spherocytic anemia. Red blood cells from five normal individuals and from three patients with congenital spherocytic hemolytic anemia were incubated with PHT and osmotic fragility was measured. It was shown that PHT reduces the osmotic fragility of normal and spherocytic erythrocytes. In the presence of ouabain, which may induce hemolysis, PHT still had a protective effect on red blood cells.

1881. Hansotia, P., Mazza, J. J. and Gatlin, P., Diphenylhydantoin and fragility of erythrocytes in normal subjects and in patients with hereditary spherocytic anemia. Amer. J. Clin. Path., 64(1): 75-9, 1975.

Richelson and Tuttle, Brain Research (1975), ²⁰⁴⁵ found that PHT (20 µM) specifically inhibits the repetitive action potential discharge induced in single mouse neuroblastoma cells by low calcium and potassium medium. PHT's action was rapid (10 sec.) and reversible. Other compounds tested had no effect, even at high concentrations.

2045. Richelson, E. and Tuttle, J. B., Diphenylhydantoin inhibits ionic excitation of mouse neuroblastoma cells, Brain Res., 99:209-12, 1975.

Rosen, Danilo, Alonso and Pippenge, The Journal of Pharmacology and Experimental Therapies (1976), ²⁰⁵⁵ using intracellular microelectrode techniques, studied the effects of therapeutic concentrations of PHT on transmembrane potentials of normal and depressed canine Purkinje fibers. The authors state that their findings show that the electrophysiologic effects of PHT are determined by the condition of the cardiac fiber. Depending on the condition of the cardiac fiber, PHT either enhances or depresses conduction, in the direction of the normal.

2055. Rosen, M. R., Danilo, P., Alonso, M. and Pippenger, C. E., Effects of therapeutic concentrations of diphenylhydantoin on transmembrane potentials of normal and depressed Purkinje fibers, J. Pharmacol. Exp. Ther., 197(3): 594-604, 1976.

Ayala, Johnston, Lin and Dichter, Brain Research (1977), ¹⁷³² studied the effects of PHT (25-200 µM) on certain synapses of neurons in the abdominal ganglia of Aplysia and on the GABA-mediated inhibitory synapse of the crayfish stretch receptor neuron. PHT decreased the amplitude of the excitatory postsynaptic potential and enhanced the "long" acetylcholine-mediated, potassium-dependent inhibitory postsynaptic potential (IPSP) at Aplysia synapses. PHT prolonged the stretch receptor IPSP up to ten times control values. The authors suggest that these selective synaptic actions of PHT contribute to its regulation of neuronal hyperexcitability.

1732. Ayala, G. F., Johnson, D., Lin, S., and Dichter, H. N., The mechanism of action of diphenylhydantoin on invertebrate neurons: II. Effects on synaptic mechanisms, Brain Res., 121: 259-70, 1977.

Ayala, Lin and Johnston, Brain Research (1977), ¹⁷³³ studied PHT's effects on the membrane properties of crayfish stretch receptor neurons and Aplysia abdominal and buccal ganglia neurons. PHT (100-300 µM) decreased abnormal repetitive firing and membrane resistance, with little or no change in the resting potential. Effects on the action potential, which varied with the type of neuron being studied, included decreased overshoot and prolonged falling phase. Potassium conductance and post-tetanic hyperpolarization were also decreased. The authors comment that the observed changes in membrane parameters are consistent with PHT’s ability to reduce repetitive firing and the spread of abnormal electrical activity.

1733. Ayala, G. F., Lin, S. and Johnston, D., The mechanism of action of diphenylhydantoin on invertebrate neurons: I. Effects on basic membrane properties, Brain Res., 121: 245-58, 1977.

Deisz and Lux, Neuroscience Letters (1977), ¹⁷⁹⁴ and Aickin, Deisz and Lux, Journal of Physiology (1978),¹⁷¹⁶ found that PHT (as low as 10 ^-9 M) increased inhibitory synaptic conductance and prolonged inhibitory postsynaptic potentials of crayfish stretch receptor neurons. The authors conclude that PHT decreases the rate of closing of the GABA-activated chloride channel, thereby enhancing inhibition.

1794. Deisz, R. A. and Lux, H. D., Diphenylhydantoin prolongs post-synaptic inhibition and iontophoretic GABA action in the crayfish stretch receptor, Neuroscience Letters, 5: 199-203, 1977.
1716. Aickin, C. C., Deisz, R. A. and Lux, H. D., The effect of diphenylhydantoin and picrotoxin on post-synaptic inhibition, J. Physiol., 284: 125-6, 1978.

Schwarz and Vogel, European Journal of Pharmacology (1977), ²⁰⁷² studied the excitability-reducing action of PHT in single myelinated frog nerve fibers.PHT hyperpolarized the resting membrane potential, raised the threshold potential. It also reduced the peak sodium current by 45%. The onset of PHT's effect on the sodium current occurred within 3.8 seconds. The authors suggest that the excitability-reducing action of PHT is mainly due to a potential-dependent blockage of the sodium channel.

2072. Schwarz, J. R. and Vogel, W., Diphenylhydantoin: excitability reducing action in single myelinated nerve fibres, European J. Pharmacol, 44: 241-9, 1977.

Pollen, Brain Behavior and Evolution (1977), ²⁰²⁴ working on a prosthesis for the blind, found PHT useful in minimizing the production of after discharges following electrical stimulation of the visual cortex in cats.

2024. Pollen, D. A., Responses of single neurons to electrical stimulation of the surface of the visual cortex, Brain Behav. Evol., 14: 67-86,  1977.

Selzer, Annals of Neurology (1978), ²⁰⁷³ in studies on a composite electrical-chemical synapse in the lamprey spinal cord, found that PHT (20 µg/ml) reversibly reduced the chemical component of the excitatory postsynaptic potential by up to 70%. PHT also greatly reduced or eliminated post-tetanic potentiation of the chemical component, but produced little or no decrease in the size of the electrical component, which reflects the presynaptic spike.

2073. Selzer, M. E., The action of phenytoin on a composite electrical-chemical synapse in the lamprey spinal cord, Ann. Neurol., 3(3): 202-6, 1978.

Tuttle and Richelson, Journal of Pharmacology and Experimental Therapeutics (1979), ²¹⁰⁵ studied the effect of PHT (40 µM) on passive and active membrane properties of mouse neuroblastoma cells. They found PHT's effect was greatest on the calcium-dependent components of the action potential. PHT reduced the hyperpolarization following an action potential and its potentiation under a repetitive stimulus.

2105. Tuttle, J. B. and Richelson, E., Phenytoin action on the excitable membrane of mouse neuroblastoma, J. Pharmacol. Exp. Ther., 2(3): 632-7, 1979.

Grafova and Danilova, Bulletin of Experimental Biology and Medicine (1980), ²²⁰² demonstrated, in a rat model of generalized myoclonus, that PHT abolished the spontaneous clonic discharges from tetanus toxin-induced hyperexcitable generators in the ventral horns of the spinal cord. Evoked tonic activity was more resistant.

2202. Grafova, V. N. and Danilova, E. I., Action of antiepileptic drugs on myoclonia of spinal origin, Bull. Exp. Biol. Med., 90(11): 1503-6, 1980.

Fromm, Chattha, Terrence and Glass, European Journal of Pharmacology (1982),²⁵¹⁵ studied the effects of PHT and carbamazepine on segmental and periventricular inhibition elicited in cat spinal trigeminal nucleus by stimulation of the maxillary nerve. Therapeutic levels of PHT and carbamazepine facilitated the segmental inhibition, but depressed the periventricular inhibition, while also depressing the response of trigeminal nucleus neurons to maxillary nerve stimulation. The authors suggest that PHT's ability to produce a relative facilitation of inhibitory feedback mechanisms is important to its regulation of abnormal electrical activity in the CNS.

2515. Fromm, G. H., Chattha, A. S., Terrence, C. F., Glass, J. D., Do phenytoin and carbamazepine depress excitation and/or facilitate inhibition?, Eur. J. Pharmacol., 78: 403-9, 1982.

McLean and MacDonald, The Journal of Pharmacology and Experimental Therapeutics (1983), ²⁷⁸³ studied the concentration dependence of several actions of PHT in mouse spinal cord neurons in culture using intracellular microelectrode techniques. At 1-2 µg/ml (equivalent to therapeutic cerebrospinal fluid concentrations), PHT reduced high-frequency repetitive firing during long depolarizing current pulses. There was a progressive reduction of the maximal rate of rise of the action potential (V_max) during the train until firing failed. While PHT did not affect V_max of single action potentials, recovery of V_max after repetitive firing was prolonged. PHT concentrations above 3 µg/ml reduced V_max, spontaneous neuronal firing, and calcium-dependent action potential amplitude. PHT abolished convulsant-induced paroxysmal bursting, and augmented postsynaptic responses to iontophoretically-applied GABA. The authors conclude that, at therapeutic levels, PHT prolongs sodium channel recovery from inactivation, an effect which is critical to its ability to limit sustained high-frequency repetitive firing. (See also Refs. 2745, 2746, 2747, 2748, 2782.)

2783. McLean, M. J., Macdonald, R. L., Multiple actions of phenytoin on mouse spinal cord neurons in cell culture, Pharmacol. Exp. Ther., 227(3): 779-89, 1983.
2745. Macdonald, R. L., Barbiturate and hydantoin anticonvulsant mechanisms of action, Basic Mechanisms of Neuronal Hyperexcitability, Jasper, H. H. and Van Gelder, N. M., Eds., Alan R. Liss, Inc., New York, 361-87, 1983.
2746. Macdonald, B. L., Anticonvulsant and convulsant drug actions on vertebrate neurones in primary dissociated cell culture, Electrophysiology of Epilepsy, Schwartzkroin, P.A. and Wheal, H. V., Eds., Academic Press, London, 353-87, 1984.
2747. Macdonald, R. L., McLean, M. J., Skerritt, J. H., Anticonvulsant drug mechanisms of action, Fed. Proc., 44: 2634-9, 1985.
2748. Macdonald, R. L., Skerritt, J. H., McLean, M. J., Anticonvulsant drug cations on GABA responses and sustained repetitive firing neurons in cell culture, Neuropharmacology, 23(7): 843-4, 1984.
2782. McLean, M., Carbamazepine and phenytoin limit rapid firing of action potentials of dorsal root ganglion neurons in cell culture, Soc. Neurosci. Abstr., 12 (Pt 2): 1015, 1986.

Selzer, David and Yaari, The Journal of Neuroscience (1985), ²⁹⁴⁵ reported that PHT (200-300 µM) dramatically reduced the early component of the post-tetanic potentiation (PTP) produced by a 30-second, 30-Hz tetanus at the frog neuromuscular junction. The late component of PTP was also reduced, although to a lesser extent. PHT reduced PTP even when there was no failure of the endplate potential during tetanus. The PHT effect did not require a complete block of the presynaptic action potential. PHT had no effect on PTP in the presence of tetrodotoxin. The authors suggest that their results indicate that PHT reduces PTP by producing a graded reduction of sodium influx into nerve terminals during high rates of axonal stimulation.

2945. Selzer, M. E., David, C., Yaari, Y., On the mechanism by which phenytoin blocks post-tetanic potentiation at the frog neuromuscular junction, J. Neurosci., 4(11): 2894-9, 1985.

Yaari, David and Selzer, Personal Communication (1985), ³⁰⁸⁹ utilizing a frog sciatic nerve preparation, reported that PHT has no effect at low-frequency stimulation but, at higher rates of stimulation, it produces a progressive decline in compound action potential amplitude and integral. They state that PHT's use- and frequency-dependent depression of axonal excitability is likely to account for its ability to suppress abnormal brain discharge, without significantly altering normal activity. (See also Refs. 3090, 3092, 3093.)

Adler, Yaari, David and Selzer, Brain Research (1986),²²⁷⁶ reported on the use- and frequency-dependent effects of PHT on the sodium-dependent action potential of lamprey spinal axons. Control lamprey action potential amplitude and conduction velocity showed little reduction during a 500-stimulus train at frequencies of 2 to 100 Hz. With 20 µg/ml PHT, there was a progressive attenuation of the action potential amplitude and velocity with the same stimulus conditions. The higher the frequency of stimulation, the greater PHT's effect. PHT's effects at a given concentration were much greater when the extracellular potassium concentration was raised from 2.1 to 5 mM. The authors conclude that increased potassium may, in part, account for PHT's ability to distinguish between normal and abnormal neuronal excitability.

2276. Adler, E. M., Yaari, Y., David, G., Selzer, M. E., Frequency-dependent action of phenytoin on lamprey spinal axons, Brain Research, 362(2): 271-80, 1986.

Hartman, Fiamengo and Riker, Anesthesiology (1986), ²⁵⁷⁶ found that pretreatmet with intravenous PHT (30 mg/kg) suppressed succinylcholine-induced motor terminal repetitive firing and post-tetanic potentiation, as well as muscle fasciculations in an in situ cat soleus  neuromuscular preparation. PHT was more effective than d-tubocurarine in suppressing the fasciculations. In addition, PHT enhanced succinylcholine's desired blocking effect, while d-tubocurarine reduced it. The authors comment that succinylcholine, a depolarizing neuromuscular blocker used in anesthesia, has a number of undesirable side effects which can be reduced by suppressing fasciculations. They suggest that PHT may be clinically useful in preventing these side effects.

2576. Hartman, G. S., Flamengo, S. A., Riker, W. F., Succinylcholine: mechanism of fasciculations and their prevention by d-tubocurarine or diphenylhydantoin, Anesthesiology, 65(4): 405-13, 1986.