Phenytoin: effects on calcium flux and cyclic nucleotides.

Previous studies have demonstrated that phenytoin alters calcium conductance in isolated presynaptic nerve endings (synaptosomes) from rat or rabbit brain. Drug concentrations of 0.08 mM (20 microgram/ml) or higher inhibit stimulated calcium influx into synaptosomes depolarized by high concentrations of potassium (69 mM) by 7-58%. Calcium transport into undepolarized synaptosomes is only inhibited by 0.4 mM or greater concentrations of phenytoin. Recent investigations show that in mouse brain slices, phenytoin inhibited elevations of cyclic GMP and cyclic AMP produced by ouabain or veratridine. In contrast, elevations of the two cyclic nucleotides produced by high concentrations of potassium were not inhibited by phenytoin, suggesting that the anticonvulsant suppresses depolarization-induced elevation of cyclic nucleotide levels in brain slices by inhibiting influx of sodium into cells. These data indicate that phenytoin inhibits both sodium and calcium influx into cells during cellular depolarization and alters regulation of brain cyclic nucleotide levels. Both of these actions may be important for the antiepileptic effect of phenytoin.     

AHR-12245: a potential anti-absence drug

AHR-12245, 2-(4-chlorophenyl)-3H-imadazo[4,5-b]pyridine-3-acetamid, ethosuximide, Na valproate, phenytoin, and clonazepam were evaluated in mice and rats with a battery of well-standardized anticonvulsant test procedures. The results obtained indicate that the anticonvulsant profile of AHR-12245 is similar to that for ethosuximide and clonazepam. AHR-12245 is effective in nontoxic intraperitoneal doses in mice by the maximal electroshock seizure (MES), pentylenetatrazol (s.c. PTZ), bicuculline, and picrotoxin tests but ineffective against strychnine-induced seizures; it is effective after nontoxic oral doses in both mice and rats by the s.c. PTZ test and ineffective by the MES test. The candidate antiepileptic substance was also ineffective against seizures induced in amygdala and corneally kindled rats. The PIs for AHR-12245 by the s.c. PTZ test were 4.5 to 12 times higher than those for the prototype agents, except that for clonazepam when administered orally in mice. The in vitro studies indicate that AHR-12245 is a weak inhibitor of benzodiazepine (BDZ) receptor binding but does inhibit adenosine uptake. These results indicate that AHR-12245 is a relatively nontoxic agent with a profile of anticonvulsant action which suggests it should be useful in generalized absence seizures.     

Comparative anticonvulsant activity and neurotoxicity of clobazam, diazepam, phenobarbital, and valproate in mice and rats

The 1.5-benzodiazepine (clobazam), the 1,4-benzodiazepine (diazepam), and two nonbenzodiazepine antiepileptic drugs (phenobarbital and valproate) were evaluated in mice and rats with a battery of well-standardized anticonvulsant test procedures. The results obtained indicate that clobazam and valproate exhibit a wider range of experimental anticonvulsant activity than either diazepam or phenobarbital. Except for clobazam by the maximal electroshock seizure (MES) test in rats, clobazam and valproate are effective in nontoxic doses against MES and all four chemically induced seizures (Metrazol, bicuculline, picrotoxin, and strychnine). Clobazam is effective by the MES test in rats only in doses that exceed the median minimal toxic dose. Phenobarbital is effective against all of the above tests, but minimal toxic doses must be employed to prevent strychnine seizures. Diazepam, on the other hand, is effective in nontoxic doses against seizures induced by Metrazol, bicuculline, and picrotoxin, but protects animals from maximal electroshock and strychnine seizures only when given in toxic doses. When compared on the basis of protective indices (PI = TD50/ED50) calculated from intraperitoneal data, the PIs for clobazam were 1.6 to 13 times higher than those for diazepam. Overall, except for the MES test in rats, the PIs for clobazam were from 1.5 to 44 times higher than those for any of the other three substances. With respect to the MES test in rats, the PI for clobazam was 10.8 times higher than that for diazepam; however, the PIs for phenobarbital and valproate were 3.5 and 4.4 times higher, respectively, than that for clobazam. These data suggest that the spectrum of anticonvulsant activity for the 1,5-benzodiazepine (clobazam) is superior to that for the 1,4-benzodiazepine (diazepam). Also, the broad experimental profile of anticonvulsant activity of clobazam agrees well with its reported broad clinical efficacy.     

Comparison of the effects of biogenic amines on cyclic GMP and cycle AMP levels in mouse cerebellum in vitro.

Norepinephrine (NE) elevates levels of both 3',5'-guanosine monophosphate (cyclic GMP) and 3',5'-adenosine monophosphate (cyclic AMP) in incubated slices of mouse cerebellum. As little as 1 muM NE is capable of increasing the level of either cyclic nucleotide. Maximal elevations of cyclic AMP and cyclic GMP levels produced by NE are 15- to 40-fold and 2- to 4-fold, respectively. Dopamine, serotonin and histamine, other biogenic amines considered to be neurotransmitters in CNS, have no effect on mouse cerebellum cyclic nucleotide levels except at relatively high (1 mM) concentrations. NE-induced accumulation of cyclic GMP, but not cyclic AMP, is blocked by omission of Ca2+ from the incubation media. Theophylline does not alter the effect of this catecholamine on either cyclic nucleotide. In tissue slices incubated in buffered sucrose or in choline-Krebs buffer, NE is still capable of increasing both cyclic GMP and cyclic AMP levels; however, these elevations are less than those observed in brain slices incubated in Krebs-Ringer buffer. NE, in combination with glutamate, produces supra-additive elevations of both cyclic GMP and cyclic AMP levels. However, NE in combination with adenosine or high levels of K+ only has a synergistic effect on cyclic AMP and not on cyclic AMP and not on cyclic GMP combination. The elevation of cyclic AMP levels produced by NE appears to be mediated via both alpha- and beta-adrenergic receptor sites. In contrast, the receptor site(s) mediating cyclic AMP accumulation do not appear to be either typical alpha- or beta-adrenergic receptors.     

Changes in cerebrospinal fluid cyclic nucleotides in alcohol-dependent patients suffering from delirium tremens

In eight patients with a history of alcoholism (on average 9 years), cyclic 3',5'-monophosphate (AMP) and cyclic 3',5'-monophosphate (GMP) were determined in CSF in the acute untreated phase of delirium tremens and at 2 weeks later when clinical symptoms had vanished. Before the second lumbar puncture a drug-free period of 1 week had existed. The results were compared with an age- and sex-matched neurological control group. CSF cyclic AMP concentrations were markedly reduced by 62% (p less than 0.005) in the acute state of delirium tremens and remained at the same level 2 weeks later; cyclic GMP concentrations were increased by 37% (p = 0.05) and showed a small further increase (p less than 0.05) at the second lumbar puncture. A negative correlation existed between age and cyclic AMP content of CSF (r = -0.756; p less than 0.05) in the patients group. The data indicate that the earlier observed increase in norepinephrine turnover in the acute state of delirium tremens (Athen et al., 1977) seems not to induce an increase of cyclic AMP content in CSF.     

Effects of reserpine, propranolol, and aminophylline on seizure activity and CNS cyclic nucleotides.

Convulsant doses of pentylenetetrazol (100 mg/kg) increase levels of both cyclic adenosine monophosphate (AMP) and cyclic guanosine monophosphate (GMP in mouse cerebral cortex and hippocampus. In animals pretreated with reserpine, propranolol, or aminophylline, pentylenetetrazol seizures were more severe, cyclic AMP elevations were attenuated or blocked, and cyclic GMP increases were unaffected or augmented. These data indicate that norepinephrine, adenosine, and perhaps other biogenic amines have a regulatory effect on cyclic AMP, but not cyclic GMP, levels in epileptic brain. An increased level of cyclic AMP in brain tissue may have an antiepileptic effect leading to seizure attenuation or termination. By contrast, elevated levels of cyclic GMP may have an epileptogenic effect in initiating or maintaining seizure activity.     

Primidone, phenobarbital, and PEMA: I. Seizure protection, neurotoxicity, and therapeutic index of individual compounds in mice

Neurotoxicity and protection against maximal electroshock and Metrazol seizures from primidone (PRM), phenobarbital (PB), and phenylethylmalonamide (PEMA) were determined in mice for each drug separately and expressed in terms of brain concentrations. Compared with PB, PEMA was 16 times less potent against electroshock and Metrazol seizures but only 8 times less toxic. Primidone was markedly less neurotoxic than PB and equally potent against electroshock, but PRM had no effect against Metrazol or bicuculline. PRM is a relatively nontoxic anticonvulsant with a different action than PB, and PEMA is both a weak and a relatively toxic anticonvulsant.     

An evaluation of the anticonvulsant effects of vitamin E

The anticonvulsant effects of D-alpha-tocopherol (vitamin E) were studied in 4 animal seizure models: the Metrazol threshold model (MET), the maximal electroshock model (MES), the kindling model (well-established seizures), and the ferrous chloride model. Vitamin E failed to antagonize seizures in the MES, MET, or the kindling models. It was, however, able to significantly delay the onset of electrographic seizures in the intracerebral ferrous chloride model. Thus, vitamin E shows activity in the ferrous chloride model, but not in the animal models commonly used to screen for anticonvulsant drug actions.     

Guanylate cyclase—Cyclic GMP in mouse cerebral cortex and cerebellum: Modification by anticonvulsants

Incubated tissue slices from mouse cerebral cortex and cerebellum readily accumulate cyclic GMP in response to a challenge by ouabain, NaN₃, NH₂OH, or KCl. Under similar conditions, l-glutamate, l-aspartate, glycine, γ-aminobutyric acid, kainic acid, and the calcium ionophore, A-23187, were ineffective. Inhibition of the ouabain-induced accumulation of cyclic GMP was evident with valproate, carbamazepine, clonazepam, phenytoin, and phenobarbital. Only phenytoin blocked the action of KCl and cyclic GMP responses to NaN₃ were inhibited by high concentrations of valproate, carbamazepine, phenytoin, or phenobarbital. The effects of NH₂OH were attenuated by high concentrations of carbamazepine, phenobarbital, and clonazepam (cortex only). Guanylate cyclase activity in homogenates of cortex and cerebellum was enhanced in the presence of NaN₃, NH₂OH, or Ca²⁺ (in low concentrations of Mn²⁺). The enzyme activation induced by Ca⁺ was blocked only by large (1 mm) amounts of carbamazepine. In like manner, large concentrations of carbamazepine, phenytoin (cortex), or clonazepam (cortex) were effective in reducing guanylate cyclase stimulation by NaN₃. No agent affected the NH₂OH responses. The results suggest that anticonvulsant drug actions with regard to central cyclic GMP systems are related to the Na⁺-induced depolarization of nerve tissue and not to any direct actions on the guanylate cyclase enzyme.     

Melatonin inhibits cyclic AMP and cyclic GMP accumulation in the rat pituitary

Subnanomolar concentrations of melatonin inhibit cyclic AMP and cyclic GMP accumulation in neonatal rat anterior pituitary stimulated in vitro with luteinizing-hormone releasing-hormone. Melatonin also inhibited forskolin-stimulated cyclic AMP accumulation in pars tuberalis. Inhibition of cyclic AMP accumulation is specific for melatonin, since its analogs N-acetylserotonin and 5-methoxytryptamine are 1000 times less potent. Cyclic nucleotides may thus serve as second messengers transducing the effect of melatonin on cellular level.     

Muscarinic actions in hippocampus are probably not mediated by cyclic GMP

It has been proposed that, in a variety of tissues, guanosine 3':5'-monophosphate (cyclic GMP) is the intracellular mediator of muscarinic effects. This hypothesis was tested in the CA1 region of the hippocampus, in urethane-anaesthetized rats, by studying extracellularly muscarinic disinhibition of disfacilitation and the effect of dibutyryl cyclic GMP, muscarinic agents and an inhibitor of cyclic nucleotide-dependent kinase (H-8), all applied by microiontophoresis. The main findings were: (a) cyclic GMP analogues do not mimic disfacilitation or disinhibition produced by muscarinic agents; (b) N-(2-(methylamino)ethyl)-5-isoquinoline sulfonamide (H-8) does not prevent the excitatory actions of muscarinic agents; and (c) H-8 alone does not change the field responses. In conclusion, cyclic nucleotide-dependent kinases do not seem to play a major role in the on-going modulation of excitability in the hippocampus and cyclic GMP is unlikely to be a major intracellular messenger mediating directly or indirectly the excitatory actions of acetylcholine.     

Somatostatin pretreatment desensitizes somatostatin receptors linked to adenylate cyclase and facilitates the stimulation of cyclic adenosine 3':5'-monophosphate accumulation in anterior pituitary tumor cells

Somatostatin-14 (SRIF) inhibits both hormone- and forskolin-stimulated cyclic adenosine 3':5'-monophosphate (cyclic AMP) formation in tumor cells of the mouse anterior pituitary (AtT-20/D16-16). However, long-term pretreatment of cells with SRIF modifies the responsiveness of this system in two ways: The response of adenylate cyclase to stimulatory agents is enhanced, whereas the ability of SRIF to inhibit stimulated cyclic AMP formation is reduced. The supersensitive adenylate cyclase response and the SRIF desensitization were dependent on the concentration and duration of SRIF pretreatment. Enhancement of forskolin-stimulated cyclic AMP formation occurred within 4 hr, whereas that of corticotropin-releasing-factor-, (-)-isoproterenol-, and vasoactive intestinal peptide-induced cyclic AMP accumulation required 16 hr of pretreatment. The elevated responses to each of these stimulants were due to increases in their maximal ability to stimulate cyclic AMP formation. Cycloheximide treatment blocked the enhanced cyclic AMP response induced by SRIF pretreatment, suggesting a requirement for protein synthesis. In membrane preparations, SRIF pretreatment facilitated activation of adenylate cyclase by forskolin, sodium fluoride, and guanosine 5'-(beta,tau-imido)-triphosphate without affecting basal activity. These results suggest that desensitization of an inhibitory input to adenylate cyclase is accompanied by a supersensitivity of adenylate cyclase to stimulatory agents through a process requiring protein synthesis.     

Comparative trial of valproate sodium and clonazepam in chronic epilepsy.

A crossover comparative study of valproate sodium and clonazepam in the treatment of 32 adult epileptic patients receiving multiple drug therapy is described. Serum concentrations of other anticonvulsant drugs were unchanged by the addition of clonazepam. However, patients receiving high doses of other anticonvulsant drugs had lower serum concentrations of clonazepam (p less than .01). With valproate sodium, phenobarbital concentrations increased (P less than .05) in patients receiving phenobarbital but not significantly in patients receiving primidone. Phenytoin concentrations were reduced (P less than .05) during treatment with valproate sodium. Both drugs significantly reduced the frequency of minor seizures, with valproate sodium having the greater effect. However, it is important to monitor serum concentrations of other anticonvulsant drugs during treatment with valproate sodium since changes in these may influence seizure control or cause side effects.     

Primidone, phenobarbital, and PEMA: II. Seizure protection, neurotoxicity, and therapeutic index of varying combinations in mice

Neurotoxicity and protection against maximal electroshock (MES) and pentylenetrazol (Metrazol) seizures were determined in mice for various combinations of primidone (PRM), phenobarbital (PB), and phenylethylmalonamide (PEMA). The results suggest that PRM and PB together are superior to either one alone in terms of spectrum of activity and relative toxicity. The protection against Metrazol and the toxicity of PB are both potentiated by PEMA at low concentrations. PEMA also potentiates the toxicity of combined PRM plus PB, without altering their protection against MES, thus lowering their therapeutic index. We conclude that PRM and PB together have an advantage over PB alone, especially when their brain concentration ratio is at or above 1 and PEMA concentrations are low. These conditions are usually not present at steady state in patients treated with PRM.     

Anticonvulsant effect of GMP depends on its conversion to guanosine

Studies on the purinergic system normally deal with adenine-based purines, namely, adenine nucleotides and adenosine. However, a guanine-based purinergic system may also have important neuromodulatory roles. Guanine-based purines exert trophic effects on neural cells, protect brain slices in a model of hypoxia and stimulate glutamate uptake. In vivo, both guanosine 5'-monophosphate (GMP) and guanosine (GUO) protected against seizures. In this study, we investigated if the anticonvulsant effect of GMP is mediated by guanosine and if guanosine or GMP treatments were able to increase adenosine levels. Intraperitoneal (i.p.) treatments with 7.5 mg/kg GMP or guanosine prevented 50% of seizures by quinolinic acid (QA) and increased guanosine cerebrospinal fluid (CSF) levels around twofold and threefold, respectively; GMP and adenosine levels remained unchanged. Intracerebroventricular treatment with 960 nmol GMP prevented 80% of seizures and the 5'-nucleotidase inhibitor alpha-beta-methyleneadenosine 5'-diphosphate (AOPCP), when injected 3 min before, reduced this anticonvulsant effect to 30% protection as well as significantly decreased the conversion of GMP into guanosine measured in the CSF. This study shows that the previously reported effect of GMP as an anticonvulsant seems to be related to its ability to generate guanosine through the action of ecto-5'-nucleotidase.     

Inositol trisphosphate, cyclic AMP, and cyclic GMP in rat brain regions after lithium and seizures.

The mechanism of action of lithium, the primary treatment for bipolar affective disorder, is unknown but may involve inhibition of second messenger production in the brain. Therefore, the concentrations of three second messengers, inositol 1,4,5 trisphosphate (Ins 1,4,5P3), cyclic adenosine monophosphate (AMP), and cyclic guanosine monophosphate (GMP), were measured in rat cerebral cortex and hippocampus after acute or chronic lithium administration, as well as after treatment with the cholinergic agonist pilocarpine alone or in combination with lithium at a dose that induces seizures only in lithium pretreated rats. Neither acute nor chronic lithium treatment altered the hippocampal or cortical concentration of Ins 1,4,5P3, cyclic AMP, or cyclic GMP. Pilocarpine administered alone increased Ins 1,4,5P3 in both regions, did not alter cyclic AMP, and slightly increased cyclic GMP in the cortex. Coadministration of lithium plus pilocarpine caused large increases in the concentrations of all three second messengers and the production of each of them was uniquely attenuated: lithium reduced pilocarpine-induced increases of Ins 1,4,5P3 in the cortex at 60 min; chronic lithium administration reduced stimulated cyclic AMP production in the hippocampus; and chronic lithium treatment impaired stimulated cyclic GMP production in both regions. In summary, chronic lithium treatment appeared only to reduce Ins 1,4,5P3 and cyclic AMP concentrations after a long period of stimulation whereas cyclic GMP production was reduced by chronic lithium administration after both short and long periods of stimulation. Thus cyclic GMP was most sensitive to lithium and lithium attenuation of second messenger formation may be most important in excessively activated pathways.     

Habituation of the local cyclic GMP response during amygdaloid carbachol kindling in the rat

Seizures kindled with amygdaloid carbachol injections are transynaptic, dependent on activation of a specific population of muscarinic receptors, and some components of their expression could be mediated by intracellular second messengers. We measured cyclic GMP and cyclic AMP concentrations in micropunch biopsies of multiple brain regions after microwave fixation during the development and the expression of carbachol-kindled seizures in the rat. In the naive carbachol-injected amygdala, cyclic GMP concentrations rose from 1.03 +/- 0.15 pmol/mg protein to 2.21 +/- 0.46 after 2 min, and significant rises occurred in caudate, hypothalamus and contralateral amygdala. This response did not occur in implanted controls, after injection of mock cerebrospinal fluid, or when carbachol actions were blocked with atropine. The rise in cyclic GMP progressively disappeared upon repeated stimulation (injected amygdala on tenth stimulation: 0.72 +/- 0.23 pmol/mg protein). However, a late rise in both cyclic GMP and cyclic AMP concentrations occurred in many brain regions during convulsive seizures. These data suggest that during the development of kindling, changes in neuronal and synaptic excitability are associated with changes in intracellular second messengers.     

Amygdaloid kindling and cerebrospinal cyclic nucleotides

The kindling model of experimental epilepsy is characterized by a persistent seizure pattern and long-lasting seizure susceptibility without associated tissue damage. In order to examine the relationship between CSF cyclic nucleotides and epilepsy. CSF cAMP and cGMP were measured before and after kindling, or after electrically induced seizures. Cyclic AMP and cGMP levels in cisternal CSF decreased significantly 1 week after the amygdaloid kindling. This finding suggests decreased levels of brain cAMP and cGMP in this type of epileptogenesis. A slight increase in CSF cyclic nucleotides concentrations was found after triggering both partial and generalized seizures. There was, however, no difference in increase of cAMP and cGMP levels between partial seizure and generalized convulsion, indicating that differences in intensity ictal or postictal events cannot be reflected in the CSF cyclic nucleotide concentrations.     

Cyclic nucleotides in platelet function.

Inhibition of adenylate cyclase in intact platelets by addition of compounds such as 2', 5' - dideoxyadenosine prevented the inhibition of platelet aggregation by PGE1 but did not affect the responses of platelets to aggregating agents in the absence of PGE1. This confirms that cyclic AMP mediates the effects of PGE1 but indicates that the level of cyclic AMP in unstimulated platelets is too low to affect the actions of aggregating agents. Studies on the phosphorylation of proteins in intact 32P-labelled platelets showed that PGE1 increased the phosphorylation of a membrane-bound polypeptide (P24) and prevented the increased phosphorylation of other polypeptides (P47 and P20) that occurred on addition of inducers of the release reaction. It is suggested that the cyclic AMP-dependent phosphorylation of P24 stimulates the active transport of Ca(2+) out of the platelet cytosol, so preventing phosphorylation of P47 and P20, reactions which may be involved in the release mechanism. As increases in platelet cyclic GMP could be dissociated from both platelet aggregation and the release reaction, it is proposed that the bidirectional regulation of platelet function is achieved primarily by the opposing actions of increases in the concentrations of Ca(2+) and cyclic AMP.     

Cyclic AMP accumulation in cerebral cortical slices: effect of carbamazepine, phenobarbital, and phenytoin.

Recent investigations have suggested that abnormal increases in brain cyclic 3',5'-adenosine monophosphate (cAMP) may play a role in epileptogenesis. Therefore, the effect of three commonly used antiepileptic drugs on cAMP accumulation in rat cortex slices was investigated. Ouabain, a depolarizing agent which produces seizures when applied to rat cortex, produced a five- to sevenfold increase in cAMP accumulation, and both carbamazepine and and phenytoin inhibited this increase. Ouabain stimulation may be mediated by the release of endogenous adenosine, and carbamazepine antagonized adenosine stimulation of cAMP accumulation whereas phenytoin did not. Carbamazepine had no effect on adenosine efflux. The augmentation of cAMP accumulation by norepinephrine was inhibited by carbamazepine and phenobarbital but slightly increased by phenytoin. If increases in brain cAMP are involved in epileptogenesis, the antagonism of cAMP accumulation by antiepileptic drugs may play a role in their anticonvulsant action.