Pharmacological inhibition of forskolin-stimulated adenylate cyclase activity in rat brain by melatonin, its analogs, and diazepam

Preincubation of rat forebrain membranes for 30-60 min with micromolar concentrations of the pineal hormone, melatonin, significantly inhibited forskolin-stimulated adenylate cyclase (AC) activity. Melatonin had an EC25 (concentration which inhibited AC activity by 25%) of 600 microM and caused a maximal inhibitory effect of approximately 30% at a concentration of 1000 microM. A comparison of the effects of melatonin and its analogs, 6-chloromelatonin and 2-iodomelatonin, in the striatum revealed that these halogenated drugs were 2-3 times more potent than melatonin in inhibiting AC activity. The EC25 values were 611, 226 and 189 microM for melatonin, 6-chloromelatonin and 2-iodomelatonin respectively. The receptor antagonists phentolamine (alpha-adrenergic), propranolol (beta-adrenergic), and metergoline (serotonergic) did not block the effect of melatonin in forebrain membranes. The central-type benzodiazepine (BZ) antagonist, Ro 15-1788 (flumazenil), also failed to block the inhibitory effects of melatonin, and the benzodiazepines, diazepam and Ro 5-4864, on AC activity. Evidence that inhibition of adenylate cyclase activity may be involved in the prevention of seizures suggests that the reported anticonvulsant effect of large doses of melatonin may be due to this mechanism. The greater potency of the halogenated melatonin analogs in inhibiting AC suggests that further study of their potential usefulness as anticonvulsants would be worthwhile.     

Studies on the involvement of dopamine D-1 and D-2 receptors in the anticonvulsant effect of dopamine agonists in various rodent models of epilepsy

Dopamine agonists with different selectivity for dopamine D-1 and D-2 receptors in the brain were tested for their effects: on thresholds for maximal electroshock seizures in mice and rats and for pentylenetetrazol-induced clonic seizures in mice; on seizures induced by air blast stimulation in gerbils, and on seizures induced by amygdala-kindling in rats. The mixed D-1/D-2 agonist apomorphine exerted anticonvulsant effects in all models except kindling. In gerbils and mice, the anticonvulsant action of apomorphine could be antagonized by the D-2 selective dopamine antagonist sulpiride. When injected alone, sulpiride exerted no significant effect on seizure activity. The preferential D-2 receptor agonists lisuride and (+)-PHNO [+)-4-propyl-9-hydroxynaphthoxazine) differed in their profile of action. Both compounds displayed anticonvulsant efficacy in gerbils, while only lisuride proved capable of reducing kindled seizure severity. (+)-PHNO increased the threshold for electroconvulsions in mice while lisuride was ineffective in this respect or even decreased the threshold. The reverse was obtained in regard to electroshock seizures in rats. The threshold for seizures induced by pentylenetetrazol in mice was increased significantly by lisuride but not by (+)-PHNO. The selective dopamine D-1 receptor agonist SKF 38393-A exerted no anticonvulsant effect in any seizure test except a moderate increase of the electroconvulsive threshold in mice. In contrast, the dopamine precursor L-DOPA (injected after pretreatment with carbidopa) proved capable of reducing seizure activity in all models. In mice, the increase in the threshold for maximal electroshock seizures induced by L-DOPA was significantly reduced by sulpiride, which also attenuated the anticonvulsant effect of L-DOPA in gerbils. Collectively, the data indicate that dopamine D-2 receptors mediate the anticonvulsant effect of dopamine agonist and, at least in part, of L-DOPA whereas D-1 receptors seem not to be involved.     

下丘脑前核微量注射褪黑素及其受体拮抗剂对大鼠血压和心率的影响

目的：研究下丘脑前核微量注射褪黑素及其受体拮抗剂对正常血压和应激性高血压大鼠心血管活动的影响。方法：微量注射褪黑色及其受体拮抗剂至下丘脑前区前核,记录血压、平均动脉压和心率。结果：微量注射褪黑素可降低平均动脉压,其竞争性ML1受体拮抗剂luzindole可完全阻断褪黑素的降压反应,而其ML2受体拮抗剂prazosin不能阻断褪黑素的降压反应。结论：褪黑素为一种降压因子,其降压反应主要通过激活ML1受体,而不是ML2受体来介导的。下丘脑前核是褪黑素影响心血管活动的重要中枢闰。     

Anticonvulsant drug potentiation by glycine in maximal electroshock seizures is mimicked by D-serine and antagonized by 7-chlorokynurenic acid.

This study evaluated a possible mechanism by which glycine potentiates the activity of anticonvulsant drugs against maximal electroshock seizures in rats. Administered concurrently, glycine (40 mmol/kg p.o.) significantly enhanced the anticonvulsant effect of phenobarbital, carbamazepine and phenytoin as determined by the occurrence of tonic hindlimb extension. Likewise, concurrent administration of the strychnine-insensitive glycine receptor agonist, D-serine (20 mmol/kg p.o.) significantly enhanced the anticonvulsant effect of phenobarbital, carbamazepine and phenytoin. L-Serine was ineffective. Administration of the strychnine-insensitive glycine receptor antagonist, 7-chlorokynurenic acid (100 nmol i.c.v.), significantly antagonized the potentiation of anticonvulsant activity induced by glycine co-administered with either phenobarbital or phenytoin. 7-Chlorokynurenic acid did not block tonic hindlimb extension when administered alone and did not affect the activity of the anticonvulsants in the absence of glycine. These results provide evidence for the potentiation of certain anticonvulsant drugs by glycine as a specific effect that may be mediated by the strychnine-insensitive glycine receptor.     

Time-dependent anticonvulsant activity of melatonin in hamsters

The objective of the present study was to assess whether the anticonvulsant activity of melatonin displays diurnal variability in hamsters. Convulsions were induced by administering 3-mercaptopropionic acid (3-MP). There was a significant diurnal variation in 3-MP-induced convulsions, hamsters being more prone to exhibit seizures during the night than during the day. Melatonin (50 mg/kg i.p.) had a maximal anticonvulsive effect in the early evening (20:00 h). The administration at 20:00 h of the central-type benzodiazepine antagonist, Ro 15-1788, although unable by itself to modify seizure threshold, blunted the anticonvulsant response to melatonin. The results indicate that the time-dependent anticonvulsant activity of melatonin is sensitive to central-type benzodiazepine antagonism.     

Comparison of anticonvulsant effect of pentobarbital and phenobarbital against seizures induced by maximal electroshock and picrotoxin in rats

Pentobarbital and phenobarbital exhibited anticonvulsant effect against maximal electroshock (MES) and picrotoxin-induced seizures in rats. Bicuculline, a GABAA receptor antagonist, reversed the anticonvulsant effect of pentobarbital, but not of phenobarbital, at a dose having no effect per se. Although picrotoxin (2 mg/kg, IP) potentiated MES seizures, it did not reverse the anticonvulsant effect due to either pentobarbital or phenobarbital. GABAB receptor antagonists such as delta-amino-n-valeric acid and homotaurine failed to modify the anticonvulsant effect due to pentobarbital or phenobarbital. Furthermore, GABAA agonist muscimol but not baclofen, a GABAB receptor agonist, exhibited the anticonvulsant effect against MES-induced seizures. However, baclofen when combined with sub-effective dose of pentobarbital or phenobarbital offered protection against MES seizures. Pentobarbital and phenobarbital were effective in almost equivalent doses against MES, as well as against picrotoxin-induced seizures. These observations indicated that pentobarbital exhibits anticonvulsant effect against MES seizures through the involvement of GABAA receptors, and activation of GABAB receptors alone does not seem to play any significant role in MES seizures and in the anticonvulsant effect of pentobarbital. However, activation of GABAB receptor does potentiate the facilitatory effect of barbiturates on GABAAergic transmission and in their anti-MES effect. Moreover, these results also suggest that the anticonvulsant effect of barbiturates against MES-seizures may involve other mechanisms in addition to GABAAergic transmission.     

The influence of central monoaminergic systems on the anticonvulsive action of propranolol

The influence of substances modulating the activity of central monaminergic systems on the anticonvulsant effect of propranolol was investigated with the maximal electroshock test in mice. The results demonstrated that especial the noradrenergic system plays an important role in modulating the efficacy of propranolol. Pharmacological stimulation of the noradrenergic system (e. g. with desipramine, pargyline, yohimbine) resulted in an enhanced anticonvulsant effect of propranolol. Compounds that suppress the noradrenergic transmission reduced this action. On the other hand manipulations of serotonergic (with 5,7-DHT, 5-HTP, PCPA) or dopaminergic (with 6-OHDA + desipramine, apomorphine) mechanisms seems to be without marked influence on the protective effect of propranolol in maximal electroshock.     

Interaction of felbamate and diazepam against maximal electroshock seizures and chemoconvulsants in mice.

The anticonvulsant effects of felbamate alone or in combination with diazepam were investigated against maximal electroshock-, pentylenetetrazol-, isoniazid- and bicuculline-induced seizures in mice. A single subprotective dose of felbamate, a dose which offers no protection to animals when combined with diazepam, enhanced the protective effects of diazepam against seizures induced by electroshock, pentylenetetrazol and isoniazid, as measured by significant reduction of ED50 values. However, felbamate failed to significantly affect the protective action of diazepam against bicuculline. Felbamate does not interact directly with the GABA-benzodiazepine-ionophore complex. Thus the enhancement of anticonvulsant activity of diazepam by felbamate against maximal electroshock and pentylenetetrazol may involve an indirect effect at benzodiazepine receptors. The anticonvulsant action of felbamate against isoniazid does not seem to involve benzodiazepine receptors and may be due to reversing the inhibitory effect of isoniazid on glutamate decarboxylase (GAD) activity. The interaction between felbamate and diazepam may also involve other mechanisms.     

Modulation of antiepileptic effect of phenytoin and carbamazepine by melatonin in mice

The pharmacokinetic and pharmacodynamic interaction of phenytoin and carbamazepine with melatonin was studied in a maximal electroshock seizure (MES) model in mice. The anticonvulsant ED(20), ED(33), ED(50) and ED(100) of phenytoin and carbamazepine, and ED(50) of melatonin were determined. Thereafter, the subanticonvulsant doses of phenytoin and carbamazepine were combined with ED(50) dose of melatonin. In combination with melatonin, 100% protection against seizures was achieved with phenytoin and carbamazepine in doses as low as ED(50) and ED(33), respectively. Serum levels of phenytoin and carbamazepine in animals that received ED(50) dose of phenytoin and carbamazepine per se, were not significantly different to those of the groups that received melatonin also. The study suggests that the synergistic antiepileptic effect is most likely a pharmacodynamic interaction, and not due to pharmacokinetic changes. Melatonin, thus, can be a potential adjunct to antiepileptic drugs, achieving a therapeutic effect at lower concentrations, hence limiting their dose-related toxicities.     

Melatonin in experimental seizures and epilepsy

Although melatonin is approved only for the treatment of jet-lag syndrome and some types of insomnia, clinical data suggest that it is effective in the adjunctive therapy of osteoporosis, cataract, sepsis, neurodegenerative diseases, hypertension, and even cancer. Melatonin also modulates the electrical activity of neurons by reducing glutamatergic and enhancing GABA-ergic neurotransmission. The indoleamine may also be metabolized to kynurenic acid, an endogenous anticonvulsant. Finally, the hormone and its metabolites act as free radical scavengers and antioxidants. The vast majority of experimental data indicates anticonvulsant properties of the hormone. Melatonin inhibited audiogenic and electrical seizures, as well as reduced convulsions induced by pentetrazole, pilocarpine, L-cysteine and kainate. Only a few studies have shown direct or indirect proconvulsant effects of melatonin. For instance, melatonin enhanced low Mg2+-induced epileptiform activity in the hippocampus, whereas melatonin antagonists delayed the onset of pilocarpine-induced seizures. However, the relatively high doses of melatonin required to inhibit experimental seizures can induce some undesired effects (e.g., cognitive and motor impairment and decreased body temperature). In humans, melatonin may attenuate seizures, and it is most effective in the treatment of juvenile intractable epilepsy. Its additional benefits include improved physical, emotional, cognitive, and social functions. On the other hand, melatonin has been shown to induce electroencephalographic abnormalities in patients with temporal lobe epilepsy and increase seizure activity in neurologically disabled children. The hormone showed very low toxicity in clinical practice. The reported adverse effects (nightmares, hypotension, and sleep disorders) were rare and mild. However, more placebo-controlled, double-blind randomized clinical trials are needed to establish the usefulness of melatonin in the adjunctive treatment of epilepsy.     

Effects of the non-NMDA antagonists NBQX and the 2,3-benzodiazepine GYKI 52466 on different seizure types in mice: comparison with diazepam and interactions with flumazenil.

1. GYKI 52466 is a benzodiazepine derivative that has muscle relaxant and anticonvulsant properties thought to be mediated by highly selective, noncompetitive antagonism of non-NMDA receptors. However, recent electrophysiological data showed that, in addition to non-NMDA receptors, the GABAA-receptor associated benzodiazepine site is involved in the depressant effect of GYKI 52466 on spinal reflex transmission. In view of the structural similarities between the 2,3 benzodiazepine derivative GYKI 52466 and 1,4-benzodiazepines such as diazepam, the benzodiazepine site of GABAA receptor complex could also be involved in the anticonvulsant activity of GYKI 52466, which has not yet been proven. This prompted us to study the effect of the benzodiazepine receptor antagonist, flumazenil, on anticonvulsant and adverse effects of GYKI 52466 in different seizure models in mice. The non-NMDA antagonist, NBQX and diazepam were used for comparison. 2. Seizure threshold models for different types of generalized seizures were used. The threshold for maximal (tonic) electroshock seizures (MES) was significantly increased by GYKI 52466 (10-20 mg kg-1), NBQX (80-120 mg kg-1) and diazepam (5 mg kg-1) shortly after i.p. drug administration. The same dose-range of the non-NMDA antagonists also significantly increased the threshold for myoclonic and clonic seizures induced by i.v. infusion of pentylenetetrazol (PTZ), although the magnitude of threshold increases obtained with the respective drugs, differed, at least in part, from that seen in the MES experiments. GYKI 52466 was clearly less potent in increasing PTZ thresholds for myoclonic and clonic seizures than on the MES threshold, while NBQX exerted about the same potency in both models.(ABSTRACT TRUNCATED AT 250 WORDS)     

Genistein modulation of seizure: involvement of estrogen and serotonin receptors

Genistein, a major source of phytoestrogen exposure for humans and animals, has been shown to mediate neuroprotection in Alzheimer’s disease and status epilepticus. In the present study, we investigated the effect of genistein on pentylenetetrazole-induced seizures in ovariectomized mice and the possible involvement of estrogenic and serotonergic pathways in the probable effects of genistein. Intraperitoneal (i.p.) administration of genistein (10 mg/kg) significantly increased the seizure threshold 30 min prior to induction of seizures 14 days after ovariectomy surgery. Administration of fulvestrant (1 mg/kg, i.p.), an estrogen receptor antagonist, completely reversed the anticonvulsant effect of genistein (10 mg/kg) in ovariectomized mice. Administration of the antagonist of serotonin receptor (5-HT3), tropisetron (10 mg/kg, i.p.), eliminated the anticonvulsant effect of genistein, whereas co-administration of m-chlorophenylbiguanide (5-HT3 receptor agonist; 1 mg/kg) and a non-effective dose of genistein (5 mg/kg) increased the seizure threshold. To conclude, it seems that estrogenic/serotonergic systems might be involved in the anticonvulsant properties of genistein.     

Are neuronal nicotinic receptors a target for antiepileptic drug development? Studies in different seizure models in mice and rats

Altered function of neuronal nicotinic acetylcholine receptors in the brain has recently been associated with an idiopathic form of partial epilepsy, suggesting that functional alterations of these receptors can be involved in the processes leading to epileptic seizures. Thus, nicotinic acetylcholine receptors may form a novel target for antiepileptic drug development. In the present study, various nicotinic acetylcholine receptor antagonists, including novel amino-alkyl-cyclohexane derivatives, were evaluated in two animal models, namely the maximal electroshock seizure test in mice and amygdala-kindling in rats. For comparison with these standard models of generalized and partial seizures, the effects against nicotine-induced seizures were examined. Because some of the agents tested showed an overlap between channel blocking at nicotinic acetylcholine receptors and NMDA receptors, the potency at these receptors was assessed by using patch clamp in a hippocampal cell preparation. Preferential nicotinic acetylcholine receptor antagonists were potent anticonvulsants in the maximal electroshock seizure test and against nicotine-induced seizures. The anticonvulsant potency in the maximal electroshock seizure test was decreased by administration of a subconvulsant dose of nicotine. Such a potency shift was also seen with selective NMDA receptor antagonists, which were also efficacious anticonvulsants against both maximal electroshock seizures and nicotine-induced seizures. Experiments with agents combining nicotinic acetylcholine receptor and NMDA receptor antagonistic effects suggested that both mechanisms contributed to the anticonvulsant effect of the respective agents in the maximal electroshock seizure test. This was not found in kindled rats, in which nicotinic acetylcholine receptor antagonists exerted less robust effects. In conclusion, it may be suggested that nicotinic acetylcholine receptor antagonism might be a valuable therapeutic approach to treat generalized epileptic seizures but rather not complex partial seizures.     

Involvement of a GABAergic mechanism in the anticonvulsant effect of pentobarbital against maximal electroshock-induced seizures in rats

The interaction between pentobarbital and other modulators of GABAergic transmission (diazepam, ethanol and progabide) was investigated on maximal electroshock seizures and on the loss of righting reflexes in rats. Pentobarbital, diazepam and ethanol produced a dose-dependent protection against electroshock seizures, with pentobarbital being more potent (3- and 50-times) than diazepam and ethanol. Progabide neither provided protection nor caused loss of righting reflex. Subprotective doses of pentobarbital and diazepam, together or when combined with a single ineffective dose of ethanol or progabide, caused protection against seizures and loss of righting reflex for variable durations, while ethanol and progabide combination did not provide protection. The protective effect of diazepam was antagonized by RO15-1788, picrotoxin and bicuculline pretreatments. The antagonism of pentobarbital protection by a specific GABA receptor antagonist, bicuculline suggests involvement of the GABAergic system in the anticonvulsant effect of pentobarbital. These results indicate that, like diazepam, the anticonvulsant effect of pentobarbital appears to be mediated through a GABAergic mechanism.     

Convulsant and anticonvulsant cyclopentanones and cyclohexanones

The convulsant and/or anticonvulsant activity of unsubstituted and mono-alkyl-substituted cyclopentanones and cyclohexanones were examined by testing the ability of these compounds to produce seizures or to inhibit seizures induced by pentylenetetrazol and maximal electroshock in CF-1 mice. In addition, these compounds were tested for their ability to bind to the picrotoxin receptor. The unsubstituted compounds, cyclopentanone and cyclohexanone, prevented both pentylnetetrazol- and maximal electroshock-induced seizures. Cyclopentanones and cyclohexanones with small (less than 3 carbon atoms) alkyl substituents in the 2-position were also anticonvulsant; all of these compounds, except 2-ethylcyclohexanone, blocked both pentylenetrazol- and maximal electroshock-induced seizures. 2-Ethylcyclohexanone was very effective against pentylenetetrazol seizures but did not prevent maximal electroshock seizures. Cyclohexanones with larger alkyl substituents in the 2-position, 2-propylcyclohexanone and 2-t-butylcyclohexanone, caused clonic seizures following injection into mice. Of the cyclopentanones and cyclohexanones with alkyl substitutions in the 3-position that were studied, one was an anticonvulsant (3-methylcyclopentanone), one was a mixed convulsant/anticonvulsant (3-ethylcyclohexanone), and the other two (3-ethylcyclopentanone and 3-t-butylcyclohexanone) were convulsants. Finally, two cyclohexanones with alkyl substituents in the 4-position were studied. Both 4-ethylcyclohexanone and 4-t-butylcyclohexanone produced convulsions when injected into mice. All the neuroactive cyclopentanones and cyclohexanones competitively displaced [35S]t-butylbicyclophosphorothionate, a ligand specific for the picrotoxin receptor, from rat brain membranes. The convulsant compounds were generally more potent than the anticonvulsants. The cyclohexanones were more potent than their corresponding cyclopentanones and the binding potency of both increased as the size of the alkyl substituent increased. These results suggest that cyclopentanone, cyclohexanone, and their alkyl-substituted derivatives act at the picrotoxin receptor to increase or decrease neuronal activity. Thus, they appear to have sites and mechanisms of action similar to those of the neuroactive gamma-butyrolactones and gamma-thiobutyrolactones.     

Mediation of nitric oxide in inhibitory effect of morphine against electroshock-induced convulsions in mice

Nitric oxide (NO) and morphine have been coupled in many physiological as well as pathological processes. The present study examined the involvement of the L-arginine/NO pathway in the anticonvulsant properties of systemic morphine (2-30 mg/kg) against electroshock seizures (ECS) in mice. Morphine decreased the intensity of maximal electroshock seizures (MES) and increased the threshold for ECS. Neither the NOS substrate L-arginine (30, 60, and 100 mg/kg), the reversible nonspecific NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME; 3, 10, and 30 mg/kg), the irreversible specific inducible NOS inhibitor aminoguanidine (20, 50, and 100 mg/kg), nor the opioid receptor antagonist naloxone (0.1, 0.3, and 1 mg/kg) did alter per se the ECS threshold or the intensity of MES at doses used. However, both naloxone and L-NAME, but not aminoguanidine, inhibited the anticonvulsant effects of morphine (30 mg/kg) against ECS, while L-arginine potentiated the anticonvulsant effects of lower doses of morphine (2 or 10 mg/kg). Low doses of naloxone (0.1 or 0.3 mg/kg) or L-NAME (3 mg/kg), which did not alter morphine effect per se, showed additive anticonvulsant effects against MES. Thus, the L-arginine/NO pathway seems to play a role in the anticonvulsant properties of morphine against ECS and this mediation involves the constitutive, but not the inducible, form of nitric oxide synthase.     

On the mechanism of anticonvulsant effect of tramadol in mice

The present study was conducted to examine the effects of tramadol, an atypical opioid on convulsive behaviour in maximal electroshock (MES) seizure test on mice. Moreover, an attempt was also made to investigate the role of possible receptor mechanisms involved. MES seizures were induced via transauricular electrodes (60 mA, 0.2 sec). Seizure severity was determined by (1) the duration of tonic hindlimb extensor (THE) phase and by (2) mortality due to electroconvulsions. Intraperitoneal (i.p.) administration of tramadol dose-dependently (10-50 mg/kg) decreased the duration of THE phase of MES. The anticonvulsant effect of tramadol was antagonized by the opioid antagonists, naloxone in high dose, and MR2266, a selective kappa antagonist but not by naltrindole, a delta opioid antagonist. Coadministration of either gamma-aminobutyric acid (GABA)-ergic drugs (diazepam, GABA, muscimol and baclofen) or N-methyl-D-aspartate (NMDA) receptor antagonist, MK801 with tramadol augmented the anticonvulsant effect of the latter drug. By contrast, flumazenil, a central benzodiazepine (BZD) receptor antagonist, counteracted the diazepam-induced facilitation of anti-MES effect of tramadol. Similarly, delta-aminovaleric acid (DAVA), a GABAB receptor antagonist, abolished the facilitatory effect of baclofen, a GABAB agonist on anti-MES action of tramadol. These BZD-GABAergic antagonists, flumazenil or DAVA, on their own also antagonized the anti-MES effect of tramadol administered alone. No significant effect on mortality was observed in any of the studied groups. Taken together, the current results have demonstrated a possible role for multitude of important neurotransmitter systems, i.e., opioid (kappa), GABAA-BZD receptors system, GABAB receptors and NMDA channel involvement in the antielectroshock effect of tramadol in mice.     

Melatonin signaling regulates locomotion behavior and homeostatic states through distinct receptor pathways in Caenorhabditis elegans

Melatonin is a hormone that controls circadian rhythms and seasonal behavioral changes in vertebrates. Recent studies indicate that melatonin participates in diverse physiological functions including the modulation of neural activities. Melatonin is also detected in many other organisms that do not exhibit obvious circadian rhythms, but their precise functions are not known. To understand the role of melatonin and its genetic pathway in vivo, we examined the effects of melatonin and its receptor antagonists on various behaviors in Caenorhabditis elegans. Exogenously applied melatonin specifically decreased locomotion rates in 15-min treatments, suggesting that melatonin directly regulates neural activities for locomotion. This melatonin signaling functions through MT1-like melatonin receptors, because the MT1/2 receptor antagonist luzindole effectively blocked the effect of melatonin on locomotion, while MT2-specific antagonist 4-phenyl-2-propionamidotetralin (4-P-PDOT) and MT3-selective antagonist prazosin had no effect. Alternatively, long-term treatment with prazosin specifically altered homeostatic states of the worm, suggesting another melatonin-signaling pathway through MT3-like receptors. We also found that two G-protein alpha subunit mutants and newly isolated five mutants exhibited defects in response to melatonin. Our findings imply that melatonin acts as a neuromodulator by regulating locomotion behavior and as a ligand for homeostatic control through distinct receptor pathways in C. elegans.     

Evaluation of CPP, a selective NMDA antagonist, in various rodent models of epilepsy. Comparison with other NMDA antagonists, and with diazepam and phenobarbital

3-(2-Carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), a novel antagonist at the N-methyl-D-aspartate (NMDA)-preferring subtype of excitatory amino acid receptor, was evaluated in four rodent models of epilepsy, i.e. maximal electroshock seizures and pentylenetetrazol (PTZ)-induced seizures in mice, epileptic gerbils and amygdala-kindled rats. The effect of CPP after systemic (i.p.) injection was compared with that of the clinical antiepileptics, phenobarbital and diazepam, and in gerbils, in addition, with the effect of the NMDA antagonist 2-amino-5-phosphonopentanoate (AP5) and 2-amino-7-phosphonoheptanoate (AP7). CPP, 5 mg/kg i.p., increased the threshold for tonic electroshock seizures but this effect was associated with motor impairment in the chimney test whereas phenobarbital had comparable anticonvulsant potency without motor impairment. The threshold for clonic PTZ seizures was increased by CPP only at high doses (20 mg/kg) which induced ataxia and marked motor impairment in the chimney test, whereas both diazepam and phenobarbital were active in this test at doses which exerted no side-effects. CPP, 2-20 mg/kg i.p., could not reduce the severity or duration of focal and generalized clonic seizures or the duration of amygdalar afterdischarges in the amygdala-kindling model in rats but instead caused ataxia and reduced muscle tone at the higher doses examined. Diazepam and phenobarbital both had anticonvulsant efficacy in this model. CPP at doses of 5-10 mg/kg did not reduce seizure severity in gerbils in which generalized tonic-clonic seizures were induced by air-blast stimulation, but, as in mice and rats, it caused motor impairment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Assessment of the role of CB1 receptors in cannabinoid anticonvulsant effects.

The cannabinoid CB1 receptor has been shown to be the primary site of action for cannabinoid-induced effects on the central nervous system. Activation of this receptor has proven to dampen neurotransmission and produce an overall reduction in neuronal excitability. Cannabinoid compounds like delta9-tetrahydrocannabinol and cannabidiol have been shown to be anticonvulsant in maximal electroshock, a model of partial seizure with secondary generalization. However, until now, it was unknown if these anticonvulsant effects are mediated by the cannabinoid CB1 receptor. Likewise, (R)-(+)-[2,3-Dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone (WIN 55,212-2), a cannabimimetic compound that has been shown to decrease hyperexcitability in cell culture models via the cannabinoid CB1 receptor, has never been evaluated for anticonvulsant activity in an animal seizure model. We first show that the cannabinoid compounds delta9-tetrahydrocannabinol (ED50 = 42 mg/kg), cannabidiol (ED50 = 80 mg/kg), and WIN 55,212-2 (ED50 = 47 mg/kg) are anticonvulsant in maximal electroshock. We further establish, using the cannabinoid CB1 receptor specific antagonist N-(piperidin-1-yl-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamidehydrochloride (SR141716A) (AD50 = 2.5 mg/kg), that the anticonvulsant effects of delta9-tetrahydrocannabinol and WIN 55,212-2 are cannabinoid CB1 receptor-mediated while the anticonvulsant activity of cannabidiol is not. This study establishes a role for the cannabinoid CB1 receptor in modulating seizure activity in a whole animal model.