Arthur Raines

Differential selectivity of several barbiturates on experimental seizures and neurotoxicity in the mouse.

Summary: Six barbiturates with diverse time‐action characteristics– thiopental, pentobarbital, butabarbital, phenobarbital, diphenylbarbiturate, and barbital–were evaluated for “anticonvulsant” and “neurotoxic” effects. For the former, the MES test, clonic seizures induced by pentylenetetrazol, 90 mg/kg, s.c, and maximal seizures produced by pentylenetetrazol, 200 mg/kg, s.c, were employed. For the latter, we used a rotorod technique. Time to peak activity in the MES test was employed as the time for other tests. Pentobarbital required at least neurotoxic doses to produce substantial “anticonvulsant” activity, its protective index ranging from 0.79 to 0.98 in the three tests. Among the drugs tested, phenobarbital and diphenylbarbiturate exhibited the most favorable protective indices, ranging from 2.71 to 3.41 for phenobarbital and from 3.85 to 5.0 for diphenylbarbiturate. Barbital, another drug with a prolonged duration of action, exhibited a range from 0.84 to 2.81. Although a prolonged duration of action is an important characteristic for antiepileptic activity, this property does not confer per se a favorable protective index.

Protection by phenytoin and calcium channel blocking agents against the toxicity of diisopropylfluorophosphate

Pretreatment of male Swiss-Webster mice with phenytoin, 25 mg/kg, verapamil, 25 to 3.0 mg/kg, nifedipine, 0.05 to 0.1 mg/kg, nitrendipine, 0.1 mg/kg, and nimodipine, 1 to 2.5 mg/kg, elevated the LD50 of diisopropylfluorophosphate (DFP) to a significant degree. In addition, these agents enhanced the protection that can be obtained from atropine and 2-pralidoxime. The protective effects of phenytoin cannot be attributed to an anticonvulsant action, per se, since carbamazepine, phenobarbital, and diphenylbarbituric acid in anticonvulsant doses did not influence DFP lethality. The mechanism of action of phenytoin and the other effective calcium channel blockers in providing protection over and above that achieved with atropine and 2-pralidoxime appears to be due to a protective action of the former agents on central respiratory centers and peripheral nicotinic sites and may involve the movement of calcium into excitable membranes.

Effects of Acute Cerebellectomy on Maximal Electroshock Seizures and Anticonvulsant Efficacy of Diazepam in the Rat

Rats were cerebellectomized 72–96 hr prior to evaluation (1) during maximum electroshock seizures and (2) for their capacity to respond to pentylenetetrazol‐induced clonic seizures. Cerebellectomized rats failed to exhibit tonic hindlimb extension, an endpoint characteristic of maximal electroshock seizures. The dose of pentylenetetrazol required to produce clonic seizures or death was not different in cerebellectomized and sham‐operated controls. The anticonvulsant efficacy of diazepam, when assessed as a pentylenetetrazol antagonist, was not influenced by removal of the cerebellum. These data indicate that whereas cerebellar influences may suppress seizure activity which is largely focal, seizures of more diffuse origin are not markedly influenced by cerebellar activity. It is, therefore, essential that the role of the cerebellum in suppressing seizures be characterized for each kind of experimentally induced seizure process.

Short communicationProtection by phenytoin and calcium channel blocking agents against the toxicity of diisopropylfluorophosphate

Pretreatment of male Swiss-Webster mice with phenytoin, 25 mg/kg, verapamil, 25 to 3.0 mg/kg, nifedipine, 0.05 to 0.1 mg/kg, nitrendipine, 0.1 mg/kg, and nimodipine, 1 to 2.5 mg/kg, elevated the LD50 of diisopropylfluorophosphate (DFP) to a significant degree. In addition, these agents enhanced the protection that can be obtained from atropine and 2-pralidoxime. The protective effects of phenytoin cannot be attributed to an anticonvulsant action, per se, since carbamazepine, phenobarbital, and diphenylbarbituric acid in anticonvulsant doses did not influence DFP lethality. The mechanism of action of phenytoin and the other effective calcium channel blockers in providing protection over and above that achieved with atropine and 2-pralidoxime appears to be due to a protective action of the former agents on central respiratory centers and peripheral nicotinic sites and may involve the movement of calcium into excitable membranes.

The Effects of 5,5‐Diphenylbarbituric Acid on Experimental Seizures in Rats: Correlation Between Plasma and Brain Concentrations and Anticonvulsant Activity

Diphenylbarbituric acid was administered by gavage to male albino Sprague‐Dawley rats for evaluation of anticonvulsant activity and analysis of blood plasma and brain drug concentrations. A method for gas‐liquid chromatographic analysis of blood plasma and brain for diphenylbarbituric acid is described. The drug was effective in the maximum electro‐shock test and as an antagonist to clonic seizures and death produced by pentylenetetrazol. Brain concentrations associated with 50 to 70% protection in the maximum electroshock seizure test were of the order of 2 to 6 Mg/g of brain and are thus equal to values for phenobarbital in this test. Large doses of diphenylbarbituric acid (up to 2,800 mg/kg) failed to produce signs of neurotoxicity. The results of the present investigation continue to indicate that diphenylbarbituric acid exhibits promise as a potential antiepileptic agent.

Blockade of the Tonic Hindlimb Extensor Component of Maximal Electroshock and Pentylenetetrazol‐Induced Seizures by Drugs Acting on Muscle and Muscle Spindle Systems: A Perspective on Method

Blockade of tonic hindlimb extension (THE) in the maximal electroshock seizure test was demonstrated for C‐28 ‘882‐Ba, a muscle spindle suppressant, chlorpromazine, a depressant of fusimotor drive, and gallamine, a neuromuscular blocking drug. These agents also blocked THE produced by very large doses of pentylenetetrazol. Dantrolene, a muscle contraction antagonist, and MI‐65‐S, a muscle spindle suppressant, significantly delayed THE. The data indicate that blockade of THE may be effected at a multiplicity of loci and may not be an expression of an “anticonvulsant” effect.

Effects of Phenytoin on the Cyclic Nucleotide System in the Motor Nerve Terminal

The effects of phenytoin on the motor nerve terminal were evaluated on the in vivo cat soleus nerve muscle preparation. Phenytoin, 10 mg/kg, reduced the repetitive aftercharges in motor nerve endings due to tetanic conditioning. It also reduced the repetitive activity due to adenylate cyclase activation with NaF, or to exogeneous dibutyryl cyclic AMP. These effects of phenytoin could be reversed by administering theophylline, a phosphodiesterase inhibitor, or by increasing the extracellular concentration of calcium. The effects of phenytoin could also be reversed by 3‐aminopyridine, but not by tetraethylammonium chloride. Verapamil, a calcium current antagonist, produced effects that were identical to phenytoin. It is concluded that phenytoin blocks a cyclic nucleotide‐mediated calcium influx that is associated with transmitter release. This calcium flux also appears to control a slow potassium current that is responsible for post‐tetanic hyperpolarization.

The influence of heart rate on ouabain cardiotoxicity in cats with spinal cord transection.

The dose, serum level and ventricular content of ouabain needed to produce cardiotoxicity were examined in control cats, cats with transected spinal cords and cats with transected spinal cords whose heart rates were restored to control values by artificial pacing. The lethal dose of ouabain was higher in cats with transected spinal cords and not paced than it was in the control group. However, the lethal dose of ouabain in spinal-sectioned cats with ventricular pacing was no different from that in controls. However, in both groups of spinal-sectioned cats, death was associated with higher ventricular and serum levels of ouabain than in controls. The ventricular ouabain content of paced animals with transected spinal cords was higher than that of controls and lower than that of unpaced spinal cats. Thus, restoration of heart rate to control levels in spinal animals appeared to accelerate myocardial ouabain uptake. The lower myocardial ouabain content in the spinal-sectioned animals which were paced suggests that pacing sensitizes the heart to cardiotoxicity. Spinal section itself appears to decrease the sensitivity to ouabain partly through a decrease in cardiac rate and partly through a loss of neurogenic influence.

Neuroexcitatory and depressant effects of penicillin at the cat soleus neuromuscular junction.

Penicillin administered intraarterially in a dose of 200 units per kg produced high frequency (ca 300 Hz) repetitive discharges originating in motor nerve endings in response to single stimuli applied to the cat soleus nerve. The high frequency repetitive discharges were antidromically conducted and recorded on ventral root filaments. Simultaneously each repetitive burst was transmitted to the muscle, producing an increase in contractile strength by converting a twitch into a brief tetanic contraction. Posttetanic potentiation in this system, which is mediated by repetitive discharges originating in nerve terminals after high frequency stimulation, is augmented by penicillin; larger doses depressed posttetanic potentiation, and still larger doses produced varying degrees of neuromuscular block. Events similar to those observed after intraarterial injections were recorded when penicillin was administered intravenously in larger doses. These data suggest that in the presence of penicillin the motor nerve terminals remain depolarized for a prolonged period after excitation by the stimulus, thus providing a current sink for the parent axon. Under the influence of this constant cathodal focus the axon fires repetitively at a high frequency.

Selective diphenylhydantoin suppression of auditory evoked potentials in the cat cerebellar cortex

Abstract Evoked potentials were recorded in Larsells lobules VI, VIIB and the left paramedian lobule of the cerebellum in adult cats anaesthetized with ketamine hydrochloride (Vetelar ® ) and α-chloralose. Diphenylhydantoin (Dilantin ® ) reduced auditory responses produced by a 2800 Hz 60 dB tone in all three cerebellar areas, 5 mg/kg being below threshold and doses of 20 mg/kg or more completely abolishing the response. In addition, cerebellar responses produced by dropping a 10-g weight on the forepaw or stimulation of the ipsilateral superficial radial nerve were resistant to diphenylhydantoin doses as great as 40 mg/kg. The onset time was 15–30 min and the duration of the depression was in excess of 6 hr in most experiments. Diphenylhydantoin seemed to have no effect on spontaneous cerebellar activity. Ketamine produced a short-lived depression of cerebellar evoked potentials, lasting less than 1 hr. α-Chloralosc produced substantial depression of evoked potentials only at exceedingly large doses, with little effect observed with the doses required for anaesthesia. Therefore, the selective depression of auditory-evoked cerebellar activity is due to diphenylhydantoin.