The interaction of cannabinoids and opioids on pentylenetetrazole-induced seizure threshold in mice

Marijuana, endocannabinoids, and epilepsy: potential and challenges for improved therapeutic intervention

Phytocannabinoids isolated from the cannabis plant have broad potential in medicine that has been well recognized for many centuries. It is presumed that these lipid soluble signaling molecules exert their effects in both the central and peripheral nervous system in large part through direct interaction with metabotropic cannabinoid receptors. These same receptors are also targeted by a variety of endogenous cannabinoids including 2-arachidonoyl glycerol and anandamide. Significant effort over the last decade has produced an enormous advance in our understanding of both the cellular and the synaptic physiology of endogenous lipid signaling systems. This increase in knowledge has left us better prepared to carefully evaluate the potential for both natural and synthetic cannabinoids in the treatment of a variety of neurological disorders. In the case of epilepsy, long standing interest in therapeutic approaches that target endogenous cannabinoid signaling systems are, for the most part, not well justified by available clinical data from human epileptics. Nevertheless, basic science experiments have clearly indicated a key role for endogenous cannabinoid signaling systems in moment to moment regulation of neuronal excitability. Further it has become clear that these systems can both alter and be altered by epileptiform activity in a wide range of in vitro and in vivo models of epilepsy. Collectively these observations suggest clear potential for effective therapeutic modulation of endogenous cannabinoid signaling systems in the treatment of human epilepsy, and in fact, further highlight key obstacles that would need to be addressed to reach that goal.

Effects of WIN 55,212-2 mesylate (a synthetic cannabinoid) on the protective action of clonazepam, ethosuximide, phenobarbital and valproate against pentylenetetrazole-induced clonic seizures in mice

The aim of this study was to determine the effect of WIN 55,212-2 mesylate (WIN - a non-selective cannabinoid CB1 and CB2 receptor agonist) on the protective action of four classical antiepileptic drugs (AEDs: clonazepam [CZP], ethosuximide [ETS], phenobarbital [PB], and valproate [VPA]) in the mouse pentylenetetrazole (PTZ)-induced clonic seizure model. WIN (15 mg/kg, i.p.) significantly enhanced the anticonvulsant action of ETS, PB and VPA, but not that of CZP against PTZ-induced clonic seizures. The ED(50) values of ETS, PB and VPA were reduced from 148.0, 13.9 and 137.1mg/kg to 104.0, 8.3 and 85.6 mg/kg, respectively (P<0.05). WIN (5 and 10mg/kg, i.p.) had no impact on the anticonvulsant action of all studied AEDs against PTZ-induced clonic seizures. WIN (15 mg/kg, i.p.) significantly elevated total brain concentrations of ETS and VPA, but not those of CZP and PB in mice. Moreover, WIN combined with CZP, ETS, PB and VPA significantly impaired motor performance, long-term memory and muscular strength in mice subjected to the chimney, passive avoidance and grip-strength tests, respectively. Pharmacodynamic enhancement of the anticonvulsant action of PB by WIN against PTZ-induced clonic seizures is favorable from a preclinical viewpoint. Advantageous effects of WIN in combination with ETS and VPA against PTZ-induced seizures were pharmacokinetic in nature. However, WIN combined with CZP, ETS, PB and VPA impaired motor coordination and long-term memory as well as reduced skeletal muscular strength in the experimental animals.

L-type calcium channel mediates anticonvulsant effect of cannabinoids in acute and chronic murine models of seizure

The anticonvulsant activities of cannabinoid compounds have been shown in various models of seizure and epilepsy. At least, part of antiseizure effects of cannabinoid compounds is mediated through calcium (Ca(2+)) channels. The L-type Ca(2+) channels have been shown to be important in various epilepsy models. However, there is no data regarding the role of L-type Ca(2+) channels in protective action of cannabinoids on acute and chronic models of seizure. In this study, the effects of cannabinoid compounds and L-type Ca(2+) channels blockers, either alone or in combination were investigated using acute model of pentylenetetrazole (PTZ)-induced seizure in mice and chronic model electrical kindling of amygdala in rats. Pretreatment of mice with both cannabinoid CB1 receptor agonist arachidonyl-2'-chloroethylamide (ACEA) and endocannabinoid degradating enzyme inhibitor cyclohexylcarbamic acid 3'-carbamoyl-biphenyl-3-yl ester (URB597) produced a protective effect against PTZ-induced seizure. Administration of various doses of the two L-type Ca(2+) channel blockers verapamil and diltiazem did not alter PTZ-induced seizure threshold. However, co-administration of verapamil and either ACEA or URB597 attenuated the protective effect of cannabinoid compounds against PTZ-induced seizure. Also, pretreatment of mice with diltiazem blocked the anticonvulsant activity of both ACEA and URB597. Moreover, (R)-(+)-[2,3-dihydro-5-methyl-3[(4-morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl) methanone mesylate (WIN55,212-2), the non-selective cannabinoid CB1 and CB2 receptor agonist showed anticonvulsant effect in amygdala-kindled rats. However, co-administration of WIN55,212-2 and verapamil attenuated the protective properties of WIN55,212-2. Our results showed that the anticonvulsant activity of cannabinoid compounds is mediated, at least in part, by L-type Ca(2+) channels in these two models of convulsion and epilepsy.

Extending pharmacological spectrum of opioids beyond analgesia: multifunctional aspects in different pathophysiological states

Opioids are well known to exert potent central analgesic actions. In recent years, the numerous studies have unfolded the critical role of opioids in the pathophysiology of various diseases as well as in biological phenomenon of therapeutic interest. The endogenous ligands of opioid receptors are derived from three independent genes and their appropriate processing yields the major representative opioid peptides beta-endorphin, met-enkephalin, leu-enkephalin and dynorphin, respectively. These peptides and their derivatives exhibit different affinity and selectivity for the mu-, delta- and kappa-receptors located on the central and the peripheral neurons, neuroendocrine, immune, and mucosal cells and on many other organ systems. The present review article highlights the role of these peptides in central nervous system disorders such as depression, anxiety, epilepsy, and stress; gastrointestinal disorders such as diarrhea, postoperative ileus, ulceration, and irritable bowel syndrome; immune system and related inflammatory disorders such as osteoarthritis and rheumatoid arthritis; and others including respiratory, alcoholism and obesity/binge eating. Furthermore, the key role of opioids in different forms of pre- and post-conditioning including ischemic and pharmacological along with in remote preconditioning has also been described.

Synthetic cannabinoid WIN 55,212-2 mesylate enhances the protective action of four classical antiepileptic drugs against maximal electroshock-induced seizures in mice

The aim of this study was to determine the effect of WIN 55,212-2 mesylate (WIN--a non-selective cannabinoid CB1 and CB2 receptor agonist) on the protective action of four classical antiepileptic drugs (carbamazepine, phenytoin, phenobarbital, and valproate) in the mouse maximal electroshock seizure (MES) model. The results indicate that WIN (10 mg/kg, i.p.) significantly enhanced the anticonvulsant action of carbamazepine, phenytoin, phenobarbital and valproate in the MES test in mice. WIN (5 mg/kg) potentiated the anticonvulsant action of carbamazepine and valproate, but not that of phenytoin or phenobarbital in the MES test in mice. However, WIN administered alone and in combination with carbamazepine, phenytoin, phenobarbital and valproate significantly reduced muscular strength in mice in the grip-strength test. In the passive avoidance task, WIN in combination with phenobarbital, phenytoin and valproate significantly impaired long-term memory in mice. In the chimney test, only the combinations of WIN with phenobarbital and valproate significantly impaired motor coordination in mice. In conclusion, WIN enhanced the anticonvulsant action of carbamazepine, phenytoin, phenobarbital and valproate in the MES test. However, the utmost caution is advised when combining WIN with classical antiepileptic drugs due to impairment of motor coordination and long-term memory and/or reduction of skeletal muscular strength that might appear during combined treatment.

Dynamic changes of CB1-receptor expression in hippocampi of epileptic mice and humans

The endocannabinoid system plays a central role in retrograde synaptic communication, and controls both glutamatergic and gamma-aminobutyric acid (GABA)ergic transmission via type 1 cannabinoid (CB1) receptor. Both in sclerotic human hippocampi and in the chronic phase of pilocarpine-induced epilepsy in mice with sclerosis, CB1-receptor-positive interneuron somata were preserved both in the dentate gyrus and in the CA1 area, and the density of CB1-immunostained fibers increased considerably in the dentate molecular layer. This suggests that, although CB1 receptors are known to be reduced in density on glutamatergic axons, the CB1-receptor-expressing GABAergic axons sprout, or there is an increase of CB1-receptor levels on these fibers. The changes of CB1 immunostaining in association with the GABAergic inhibitory system appear to correlate with the severity of pyramidal cell loss in the CA1 subfield. These results confirm the involvement of the endocannabinoid system associated with GABAergic transmission in human temporal lobe epilepsy (TLE), as well as in the chronic phase of the pilocarpine model in mice. Pharmacotherapy aimed at the modulation of endocannabinoid-mediated retrograde synaptic signaling should take into account the opposite change in CB1-receptor expression observed on glutamatergic versus GABAergic axon terminals.

Δ⁹-Tetrahydrocannabivarin suppresses in vitro epileptiform and in vivo seizure activity in adult rats

PURPOSE

We assessed the anticonvulsant potential of the phytocannabinoid Δ⁹-tetrahydrocannabivarin (Δ⁹-THCV) by investigating its effects in an in vitro piriform cortex (PC) brain slice model of epileptiform activity, on cannabinoid CB1 receptor radioligand-binding assays and in a generalized seizure model in rats.

METHODS

Δ⁹-THCV was applied before (10 μm Δ⁹-THCV) or during (10-50 μm Δ⁹-THCV) epileptiform activity induced by Mg²(+) -free extracellular media in adult rat PC slices and measured using multielectrode array (MEA) extracellular electrophysiologic techniques. The actions of Δ⁹-THCV on CB1 receptors were examined using [³H]SR141716A competition binding and [³⁵S]GTPγS assays in rat cortical membranes. Effects of Δ⁹-HCV (0.025-2.5 mg/kg) on pentylenetetrazole (PTZ)-induced seizures in adult rats were also assessed.

RESULTS

After induction of stable spontaneous epileptiform activity, acute Δ⁹ -THCV application (≥ 20 μm) significantly reduced burst complex incidence and the amplitude and frequency of paroxysmal depolarizing shifts (PDSs). Furthermore, slices pretreated with 10 μm Δ⁹-THCV prior to induction of epileptiform activity exhibited significantly reduced burst complex incidence and PDS peak amplitude. In radioligand-binding experiments, Δ⁹-THCV acted as a CB1 receptor ligand, displacing 0.5 nm [³H]SR141716A with a Ki∼290 nm, but exerted no agonist stimulation of [³⁵S]GTPγS binding. In PTZ-induced seizures in vivo, 0.25 mg/kg Δ⁹-THCV significantly reduced seizure incidence.

DISCUSSION

These data demonstrate that Δ⁹-THCV exerts antiepileptiform and anticonvulsant properties, actions that are consistent with a CB1 receptor-mediated mechanism and suggest possible therapeutic application in the treatment of pathophysiologic hyperexcitability states.

Effect of arachidonyl-2'-chloroethylamide, a selective cannabinoid CB1 receptor agonist, on the protective action of the various antiepileptic drugs in the mouse maximal electroshock-induced seizure model

The aim of this study was to determine the influence of arachidonyl-2'-chloroethylamide (ACEA - a highly selective cannabinoid type 1 [CB1] receptor agonist) on the protective action and acute adverse effects of carbamazepine, lamotrigine, oxcarbazepine, phenobarbital, phenytoin, and topiramate in the maximal electroshock seizure model and chimney test in mice. Tonic hind limb extension (seizure activity) was evoked in adult male albino Swiss mice by a current (sine-wave, 25 mA, 500 V, 50 Hz, 0.2s stimulus duration) delivered via auricular electrodes. Acute adverse-effect profiles of the studied antiepileptic drugs with respect to motor coordination was assessed in the chimney test. Additionally, long-term memory and skeletal muscular strength were measured along with free plasma (non-protein bound) and total brain antiepileptic drug concentrations. To inhibit the rapid metabolic degradation of ACEA by the fatty-acid amide hydrolase, phenylmethylsulfonyl fluoride (PMSF) was used at a constant ineffective dose of 30 mg/kg. Results indicate that ACEA (2.5 mg/kg, i.p.) co-administered with PMSF (30 mg/kg, i.p.), significantly enhanced the anticonvulsant activity of phenobarbital, but not that of carbamazepine, lamotrigine, oxcarbazepine, phenytoin, or topiramate in the maximal electroshock seizure test in mice. Moreover, ACEA (2.5 mg/kg) with PMSF (30 mg/kg) had no significant impact on the acute adverse effects of all examined antiepileptic drugs in the chimney test in mice. The protective index values (as quotients of the respective TD(50) and ED(50) values denoted from the chimney and maximal electroshock seizure tests, respectively) for the combinations of ACEA (2.5 mg/kg) and PMSF (30 mg/kg) with carbamazepine, oxcarbazepine, phenobarbital, and topiramate were greater than those denoted for the antiepileptic drugs administered alone. Only, the protective index values for the combination of ACEA (2.5 mg/kg) and PMSF (30 mg/kg) with lamotrigine and phenytoin were lower than those determined for the antiepileptic drugs administered alone. Pharmacokinetic experiments revealed that ACEA (2.5 mg/kg) and PMSF (30 mg/kg) affected neither free plasma (non-protein bound) nor total brain concentrations of phenobarbital in mice. Moreover, ACEA and PMSF in combination with carbamazepine, lamotrigine, oxcarbazepine, phenobarbital, phenytoin, and topiramate did not alter long-term memory or skeletal muscular strength in experimental animals. In conclusion, the enhanced anticonvulsant action of phenobarbital by ACEA and PMSF, lack of pharmacokinetic interaction and no acute adverse effects between the examined compounds, make the combination of ACEA and PMSF with phenobarbital of pivotal importance for further experimental and clinical studies. The combinations of ACEA and PMSF with carbamazepine, lamotrigine, oxcarbazepine, phenytoin, and topiramate are neutral from a preclinical viewpoint.

Involvement of nitrergic system in the anticonvulsant effect of the cannabinoid CB(1) agonist ACEA in the pentylenetetrazole-induced seizure in mice

Cannabinoid system plays a pivotal role in the seizure threshold modulation which is mainly mediated through activation of the cannabinoid CB(1) receptor. There is also several evidence of interaction between cannabinoid system and other neurotransmitters including nitric oxide (NO) system. Using model of clonic seizure induced by pentylenetetrazole (PTZ) in male NMRI mice, we investigated whether NO is involved in the effects of cannabinoids on the seizure threshold. Injection of the selective cannabinoid CB(1) agonist ACEA (2mg/kg, i.p.) significantly (P<0.01) increased the seizure threshold which was prevented (P<0.001) by pretreatment with the selective CB(1) antagonist AM251 (1mg/kg, i.p.). The NO precursor l-arginine (50 and 100mg/kg, i.p.) potentiated the anticonvulsant effects of the sub-effective dose of ACEA (1mg/kg, i.p.). Pretreatment with non-effective doses of the non-specific NOS inhibitor l-NAME (15 and 30mg/kg, i.p.) and the specific neuronal NOS inhibitor 7-NI (40 and 80mg/kg, i.p.) but not the inducible NOS inhibitor aminoguanidine (10, 50 and 100mg/kg, i.p.) prevented the anticonvulsant effect of ACEA (2mg/kg, i.p.). Co-administration of non-effective dose of AM251 (0.5mg/kg) with both low and per se non-effective doses of l-NAME (1mg/kg, i.p.) and 7-NI (10mg/kg, i.p.) had significant (P<0.01) effect in preventing the anticonvulsant effect of ACEA (2mg/kg, i.p.). Our findings demonstrated that central NO system could be involved in the anticonvulsant properties of the specific cannabinoid CB(1) agonist ACEA, emphasizing on the interaction between two systems in the seizure modulation.

Pharmacological enhancement of endocannabinoid signaling reduces the cholinergic toxicity of diisopropylfluorophosphate

Diisopropylfluorophosphate (DFP) elicits cholinergic toxicity by inhibiting acetylcholinesterase, leading to accumulation of the neurotransmitter acetylcholine and excessive stimulation of cholinergic receptors throughout the body. Endocannabinoids inhibit the release of neurotransmitters including acetylcholine via a widely distributed retrograde signaling pathway. Endocannabinoid signaling is therefore a potential therapeutic target for the management of OP poisoning. We first evaluated the relative in vitro and in vivo (2.5mg/kg, sc) effects of DFP on cholinesterase, fatty acid amide hydrolase (FAAH, an endocannabinoid degrading enzyme), monoacylglycerol lipase (MAGL, another endocannabinoid degrading enzyme) and cannabinoid receptor (CB1) binding in rat hippocampus. The effects of WIN 55212-2 (cannabinoid receptor agonist, 1.5mg/kg), URB597 (FAAH inhibitor, 3mg/kg), URB602 (MAGL inhibitor, 10mg/kg) or AM404 (endocannabinoid uptake inhibitor, 10mg/kg) on DFP toxicity were then examined. Adult male rats were given either peanut oil or DFP followed immediately by vehicle or one of the four cannabinomimetic drugs. Functional signs of toxicity were evaluated for 24h and then rats were sacrificed for neurochemical measurements. DFP inhibited cholinesterase, FAAH, MAGL and CB1 receptor binding in vitro in a concentration-dependent manner, with highest and lowest potency against cholinesterase and FAAH, respectively. In vivo, DFP inhibited hippocampal cholinesterase (89%) and FAAH (42%), but had no significant effect on MAGL or CB1 binding. Rats treated with DFP alone showed typical signs of cholinergic toxicity including involuntary movements and excessive secretions (SLUD signs). WIN 55212-2, URB597, URB602 and AM404 all significantly reduced involuntary movements following DFP exposure in a time-dependent manner, and most (URB597, URB602 and AM404) also significantly reduced DFP-induced SLUD signs. These results suggest that enhancing endocannabinoid signaling can attenuate the acute toxicity of DFP and provide rationale for further investigations on the role of endocannabinoids in cholinergic toxicity.

Effect of metal chelating agents on pentylenetetrazole-induced seizure threshold in cholestatic mice

Zinc has been proven to be anticonvulsant in several studies which indicate that diphenylthiocarbazone (dithizone) and diethyldithiocarbamate (DEDTC), zinc chelating agents, enhance seizure activities. There is also evidence that nitric oxide (NO) generators increase zinc concentration in the brain. On the other hand, the increased level of NO in the nervous system and the consequently increased seizure threshold in cholestatic mice have been well studied. Thus, it could be hypothesized that one of the reasons for the increased seizure threshold in cholestasis is partly the enhanced endogenous zinc concentration, at least in part, due to the overproduction of NO. In this study, we examined the hypothesis that zinc chelating agents might decrease seizure activity to its pre-cholestatic level in bile duct-ligated (BDL) mice. Mice were intra-peritoneally injected with dithizone and diethyldithiocarbamate (DEDTC) before the induction of seizure by pentylenetetrazole (PTZ) and then the seizure activity was recorded. Dose response (dithizone: 5, 30, 100 and 200mg/kg; DEDTC: 25, 50 and 100mg/kg) and time course (only for dithizone: 15, 30, 60 and 120 min) studies were performed first. Then, the effects of cholestasis, with and without dithizone injection, on seizure activity were assessed. Proconvulsant effect of dithizone and DEDTC was proved to be dose dependent although time interval between dithizone and PTZ injections did not play any significant role in the seizure activity. Cholestasis decreased seizure activity and increased lag phase before seizure and both effects were decreased by dithizone injection. It is elicited that zinc may mediate the cholestasis-induced decrement in seizure activity.

Elevation of pentylenetetrazole-induced seizure threshold in cholestatic mice: interaction between opioid and cannabinoid systems

BACKGROUND AND AIM

Several studies have reported that endogenous opioid and cannabinoid systems may be involved in some pathophysiological changes occurring in cholestatic liver disease. It is well known that endogenous opioids and cannabinoids alter the susceptibility of experimental animals to different models of seizure.

METHODS

The alterations in pentylenetetrazole-induced clonic seizure thresholds were evaluated from 1 to 6 days after bile duct ligation in mice. Whether the pretreatment of cholestatic mice with different doses of opioid receptor antagonist naltrexone or cannabinoid CB(1) receptor antagonist AM251 (AM251) would have changed the clonic seizure threshold was also examined.

RESULTS

Although the clonic seizure threshold was similar between sham-operated and unoperated mice, there was a time-dependent increase in the threshold in cholestatic mice, reaching a peak on day 3 after bile duct ligation and declining partially after day 4. Chronic pretreatment with naltrexone (2, 5, and 10 mg/kg) reversed the increased threshold in cholestatic mice on day 3 after operation in a dose-dependent manner with the highest doses used restoring the threshold to that of the control animals. A similar reversal of the increased threshold was observed after acute (0.5, 0.75, and 1 mg/kg) or chronic (0.5 mg/kg for 4 days) pretreatment with AM251. Moreover, concurrent administration of doses of AM251 and naltrexone that each separately induced a partial reversal of increased seizure threshold in cholestasis caused a complete restoring of the threshold to the control level.

CONCLUSIONS

Both opioid and cannabinoid CB(1) receptors may be involved in the dramatic increase in pentylenetetrazole-induced seizure threshold in cholestasis.

The cannabinoid anticonvulsant effect on pentylenetetrazole-induced seizure is potentiated by ultra-low dose naltrexone in mice

Cannabinoid compounds are anticonvulsant since they have inhibitory effects at micromolar doses, which are mediated by activated receptors coupling to G(i/o) proteins. Surprisingly, both the analgesic and anticonvulsant effects of opioids are enhanced by ultra-low doses (nanomolar to picomolar) of the opioid antagonist naltrexone and as opioid and cannabinoid systems interact, it has been shown that ultra-low dose naltrexone also enhances cannabinoid-induced antinociception. Thus, concerning the seizure modulating properties of both classes of receptors this study investigated whether the ultra-low dose opioid antagonist naltrexone influences cannabinoid anticonvulsant effects. The clonic seizure threshold was tested in separate groups of male NMRI mice following injection of vehicle, the cannabinoid selective agonist arachidonyl-2-chloroethylamide (ACEA) and ultra-low doses of the opioid receptor antagonist naltrexone and a combination of ACEA and naltrexone doses in a model of clonic seizure induced by pentylenetetrazole (PTZ). Systemic injection of ultra-low doses of naltrexone (1pg/kg to 1ng/kg, i.p.) significantly potentiated the anticonvulsant effect of ACEA (1mg/kg, i.p.). Moreover, the very low dose of naltrexone (500pg/kg) unmasked a strong anticonvulsant effect for very low doses of ACEA (10 and 100microg/kg). A similar potentiation by naltrexone (500pg/kg) of anticonvulsant effects of non-effective dose of ACEA (1mg/kg) was also observed in the generalized tonic-clonic model of seizure. The present data indicate that the interaction between opioid and cannabinoid systems extends to ultra-low dose levels and ultra-low doses of opioid receptor antagonist in conjunction with very low doses of cannabinoids may provide a potent strategy to modulate seizure susceptibility.

Downregulation of the CB1 cannabinoid receptor and related molecular elements of the endocannabinoid system in epileptic human hippocampus

Endocannabinoid signaling is a key regulator of synaptic neurotransmission throughout the brain. Compelling evidence shows that its perturbation leads to development of epileptic seizures, thus indicating that endocannabinoids play an intrinsic protective role in suppressing pathologic neuronal excitability. To elucidate whether long-term reorganization of endocannabinoid signaling occurs in epileptic patients, we performed comparative expression profiling along with quantitative electron microscopic analysis in control (postmortem samples from subjects with no signs of neurological disorders) and epileptic (surgically removed from patients with intractable temporal lobe epilepsy) hippocampal tissue. Quantitative PCR measurements revealed that CB(1) cannabinoid receptor mRNA was downregulated to one-third of its control value in epileptic hippocampus. Likewise, the cannabinoid receptor-interacting protein-1a mRNA was decreased, whereas 1b isoform levels were unaltered. Expression of diacylglycerol lipase-alpha, an enzyme responsible for 2-arachidonoylglycerol synthesis, was also reduced by approximately 60%, whereas its related beta isoform levels were unchanged. Expression level of N-acyl-phosphatidylethanolamine-hydrolyzing phospholipase D and fatty acid amide hydrolase, metabolic enzymes of anandamide, and 2-arachidonoylglycerol's degrading enzyme monoacylglycerol lipase did not change. The density of CB(1) immunolabeling was also decreased in epileptic hippocampus, predominantly in the dentate gyrus, where quantitative electron microscopic analysis did not reveal changes in the ratio of CB(1)-positive GABAergic boutons, but uncovered robust reduction in the fraction of CB(1)-positive glutamatergic axon terminals. These findings show that a neuroprotective machinery involving endocannabinoids is impaired in epileptic human hippocampus and imply that downregulation of CB(1) receptors and related molecular components of the endocannabinoid system may facilitate the deleterious effects of increased network excitability.

Ultra-low dose cannabinoid antagonist AM251 enhances cannabinoid anticonvulsant effects in the pentylenetetrazole-induced seizure in mice

Several lines of evidence suggest that cannabinoid compounds are anticonvulsant since they have inhibitory effects at micromolar doses, which are mediated by activated receptors coupling to Gi/o proteins. Surprisingly, both the analgesic and anticonvulsant effects of opioids are enhanced by ultra-low doses (nanomolar to picomolar) of the opioid antagonist naltrexone and as opioid and cannabinoid systems interact, it has been shown that ultra-low dose naltrexone also enhances cannabinoid-induced antinociception. However, regarding the seizure modulating properties of both classes of receptors this study investigated whether ultra-low dose cannabinoid antagonist AM251 influences cannabinoid anticonvulsant effects. The clonic seizure threshold (CST) was tested in separate groups of male NMRI mice following injection of vehicle, the cannabinoid selective agonist arachidonyl-2-chloroethylamide (ACEA) and ultra-low doses of the cannabinoid CB1 antagonist AM251 and a combination of ACEA and AM251 doses in a model of clonic seizure induced by pentylenetetrazole (PTZ). Systemic administration of ultra-low doses of AM251 (10 fg/kg-100 ng/kg) significantly potentiated the anticonvulsant effect of ACEA at 0.5 and 1 mg/kg. Moreover, inhibition of cannabinoid induced excitatory signaling by AM251 (100 pg/kg) unmasked a strong anticonvulsant effect for very low doses of ACEA (100 ng/kg-100 microg/kg), suggesting that a presumed inhibitory component of cannabinoid receptor signaling can exert strong seizure-protective effects even at very low levels of cannabinoid receptor activation. A similar potentiation by AM251 (100 pg/kg and 1 ng/kg) of anticonvulsant effects of non-effective dose of ACEA (0.5 and 1 mg/kg) was also observed in the generalized tonic-clonic model of seizure. The present data suggest that ultra-low doses of cannabinoid receptor antagonists may provide a potent strategy to modulate seizure susceptibility, especially in conjunction with very low doses of cannabinoids.

Role of ATP-sensitive potassium channels in the biphasic effects of morphine on pentylenetetrazole-induced seizure threshold in mice

Although several studies have indicated that the opioid receptor agonist morphine exerts biphasic effects on clonic seizure threshold, as yet little is known of the underlying mechanisms in this effect. In the present study, using the specific ATP-sensitive K(+) (K(ATP)) channel blocker glibenclamide and the specific K(ATP) channel opener cromakalim, the possible involvement of K(ATP) channels in the effects of morphine on pentylenetetrazole (PTZ)-induced seizure threshold in mice was investigated. Acute administration of lower doses of morphine (1, 3 and 7.5 mg/kg, i.p.) increased and higher doses of morphine (30 and 60 mg/kg, i.p.) decreased the PTZ-induced seizure threshold. Glibenclamide (2.5-5 mg/kg) increased the PTZ-induced seizure threshold. Non-effective dose of cromakalim (0.1 microg/kg) inhibited anticonvulsant effect of glibenclamide (5 mg/kg). Acute administration of non-effective dose of glibenclamide (1 mg/kg) interestingly inhibited both anticonvulsant and pro-convulsant effects of morphine and this effect was significantly reversed by cromakalim (0.1 microg/kg). These results support the involvement of K(ATP) channels in the modulation of seizure threshold by morphine.

Status epilepticus causes a long-lasting redistribution of hippocampal cannabinoid type 1 receptor expression and function in the rat pilocarpine model of acquired epilepsy

Activation of the cannabinoid type 1 (CB1) receptor, a major G-protein-coupled receptor in brain, acts to regulate neuronal excitability and has been shown to mediate the anticonvulsant effects of cannabinoids in several animal models of seizure, including the rat pilocarpine model of acquired epilepsy. However, the long-term effects of status epilepticus on the expression and function of the CB1 receptor have not been described. Therefore, this study was initiated to evaluate the effect of status epilepticus on CB1 receptor expression, binding, and G-protein activation in the rat pilocarpine model of acquired epilepsy. Using immunohistochemistry, we demonstrated that status epilepticus causes a unique "redistribution" of hippocampal CB1 receptors, consisting of specific decreases in CB1 immunoreactivity in the dense pyramidal cell layer neuropil and dentate gyrus inner molecular layer, and increases in staining in the CA1-3 strata oriens and radiatum. In addition, this study demonstrates that the redistribution of CB1 receptor expression results in corresponding functional changes in CB1 receptor binding and G-protein activation using [3H] R+-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-yl](1-napthalen-yl)methanone mesylate (WIN55,212-2) and agonist-stimulated [35S]GTPgammaS autoradiography, respectively. The redistribution of CB1 receptor-mediated [35S]GTPgammaS binding was 1) attributed to an altered maximal effect (Emax) of WIN55,212-2 to stimulate [35S]GTPgammaS binding, 2) reversed by the CB1 receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A), 3) confirmed by the use of other CB1 receptor agonists, and 4) not reproduced in other G-protein-coupled receptor systems examined. These results demonstrate that status epilepticus causes a unique and selective reorganization of the CB1 receptor system that persists as a permanent hippocampal neuronal plasticity change associated with the development of acquired epilepsy.

Opposing Modifications in Intrinsic Currents and Synaptic Inputs in Post-Traumatic Mossy Cells: Evidence for Single-Cell Homeostasis in a Hyperexcitable Network

Recent experimental and modeling results demonstrated that surviving mossy cells in the dentate gyrus play key roles in the generation of network hyperexcitability. Here we examined if mossy cells exhibit long-term plasticity in the posttraumatic, hyperexcitable dentate gyrus. Mossy cells 1 wk after fluid percussion head injury did not show alterations in their current-firing frequency ( I-F) and current-membrane voltage ( I-V) relationships. In spite of the unchanged I-F and I-V curves, mossy cells showed extensive modifications in Na+, K+ and h-currents, indicating the coordinated nature of these opposing modifications. Computational experiments in a realistic large-scale model of the dentate gyrus demonstrated that individually, these perturbations could significantly affect network activity. Synaptic inputs also displayed systematic, opposing modifications. Miniature excitatory postsynaptic current (EPSC) amplitudes were decreased, whereas miniature inhibitory postsynaptic current (IPSC) amplitudes were increased as expected from a homeostatic response to network hyperexcitability. In addition, opposing alterations in miniature and spontaneous synaptic event frequencies and amplitudes were observed for both EPSCs and IPSCs. Despite extensive changes in synaptic inputs, cannabinoid-mediated depolarization-induced suppression of inhibition was not altered in posttraumatic mossy cells. These data demonstrate that many intrinsic and synaptic properties of mossy cells undergo highly specific, long-term alterations after traumatic brain injury. The systematic nature of such extensive and opposing alterations suggests that single-cell properties are significantly influenced by homeostatic mechanisms in hyperexcitable circuits.

Prevention of plasticity of endocannabinoid signaling inhibits persistent limbic hyperexcitability caused by developmental seizures

Depolarization-induced suppression of inhibition (DSI) is an endocannabinoid-mediated short-term plasticity mechanism that couples postsynaptic Ca2+ rises to decreased presynaptic GABA release. Whether the gain of this retrograde synaptic mechanism is subject to long-term modulation by glutamatergic excitatory inputs is not known. Here, we demonstrate that activity-dependent long-term DSI potentiation takes place in hippocampal slices after tetanic stimulation of Schaffer collateral synapses. This activity-dependent, long-term plasticity of endocannabinoid signaling was specific to GABAergic synapses, as it occurred without increases in the depolarization-induced suppression of excitation. Induction of tetanus-induced DSI potentiation in vitro required a complex pathway involving AMPA/kainate and metabotropic glutamate receptor as well as CB1 receptor activation. Because DSI potentiation has been suggested to play a role in persistent limbic hyperexcitability after prolonged seizures in the developing brain, we used these mechanistic insights into activity-dependent DSI potentiation to test whether interference with the induction of DSI potentiation prevents seizure-induced long-term hyperexcitability. The results showed that the in vitro, tetanus-induced DSI potentiation was occluded by previous in vivo fever-induced (febrile) seizures, indicating a common pathway. Accordingly, application of CB1 receptor antagonists during febrile seizures in vivo blocked the seizure-induced persistent DSI potentiation, abolished the seizure-induced upregulation of CB1 receptors, and prevented the emergence of long-term limbic hyperexcitability. These results reveal a new form of activity-dependent, long-term plasticity of endocannabinoid signaling at perisomatic GABAergic synapses, and demonstrate that blocking the induction of this plasticity abolishes the long-term effects of prolonged febrile seizures in the developing brain.

The proconvulsant effect of sildenafil in mice: role of nitric oxide-cGMP pathway

Recent evidence indicates that sildenafil may exert some central effects through enhancement of nitric oxide (NO)-mediated effects. NO is known to have modulatory effects on seizure threshold, raising the possibility that sildenafil may alter seizure susceptibility through NO-mediated mechanisms. This study was performed to examine whether sildenafil influences the threshold of clonic and/or generalized tonic seizures through modulation of nitric oxide (NO)-cGMP pathway. The effect of sildenafil (1-40 mg kg(-1)) was investigated on clonic seizures induced by intravenous administration of GABA antagonists pentylenetetrazole (PTZ) and bicuculine and on generalized tonic seizures induced by intraperitoneal administration of high dose PTZ in male Swiss mice. The interaction of sildenafil-induced effects with NO-cGMP pathway was examined using nitric oxide synthase (NOS) inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), NOS substrate L-arginine, NO donor, sodium nitroprusside (SNP) and guanylyl cyclase inhibitor methylene blue (MB). Sildenafil induced a dose-dependent proconvulsant effect in both models of clonic, but not generalized tonic type of seizures. Pretreatment with either MB or L-NAME inhibited the proconvulsant effect of sildenafil, indicating the mediation of this effect by NO-cGMP pathway. In addition, a subeffective dose of sildenafil induced an additive proconvulsant effect when combined with either L-arginine or SNP. Sildenafil induces a proconvulsant effect on clonic seizure threshold that interacts with both exogenously and endogenously released NO and may be linked to activation of NO-cGMP pathway.

Pharmacological studies of the opioids, mood stabilizer and dopaminergic drugs on pilocarpine-induced seizures and status epilepticus

This work was designed to study the influence of drugs during seizures and status epilepticus (SE) induced by pilocarpine and mortality in adult rats. Morphine (0.1 and 0.2 mg/kg), SCH 23390 (0.1 and 0.2 mg/kg), haloperidol (5 and 10mg/kg) and lithium (30 and 60 mg/kg) were administered intraperitoneally (i.p.), 30 min before to pilocarpine (400 mg/kg, s.c.). The animals were observed (24 h) to determine: number of peripheral cholinergic signs, tremors, stereotyped movements, seizures, SE, latency to first seizure and number of deaths after pilocarpine treatment. Morphine and haloperidol had proconvulsant effects in both doses tested. Smaller and higher doses of these drugs no protected and increased pilocarpine-induced seizures, SE and/or mortality. SCH 23390 protected against seizures, increased the latency to first seizure and reduced the mortality of the animals treated with pilocarpine Theses results suggest that dopamine receptor system receptor subtypes exert opposite functions on the regulation of convulsive activity. The morphine is proconvulsant in lower doses. The opioids in high doses tested exert an action proconvulsant during the establishment of epileptic activity induce by pilocarpine. The lithium no protected the animals against seizures induced by pilocarpine and is used which a model of epilepsy associated with lower doses of pilocarpine in several studies, suggesting absence of the effect anticonvulsants in rodents.

Modulation of paraoxon toxicity by the cannabinoid receptor agonist WIN 55,212-2

Cannabinoids can reduce the pre-synaptic release of acetylcholine and other neurotransmitters in the mammalian brain through a retrograde signaling pathway. Organophosphorus insecticides elicit toxicity by inhibiting acetylcholinesterase and thereby increasing synaptic acetylcholine levels. Several studies suggest that some organophosphorus toxicants can potentially modify cannabinergic signaling by direct binding to cannabinoid receptors and inhibition of enzymes responsible for cannabinoid degradation (i.e., fatty acid amide hydrolase and monoacylglycerol lipase). We hypothesized that exposure to the cannabinoid receptor agonist WIN 55,212-2 (WIN) could alter the acute toxicity of the prototype anticholinesterase, paraoxon. In vitro, paraoxon inhibited hippocampal cholinesterase and fatty acid amide hydrolase activities, and displaced specific binding to the cannabinoid receptor ligand ([(3)H]CP 55,940) in a concentration-dependent manner. WIN (0.5, 1.5 or 5mg/kg/day) had a complex dose-related effect on locomotor activity when evaluated for 2h after either the first or last of seven daily exposures, and significantly decreased hippocampal CB1 binding following repeated dosing. Four hours after dosing, paraoxon (0.4 mg/kg, sc) elicited classical signs of cholinergic toxicity and significantly reduced hippocampal cholinesterase and fatty acid amide hydrolase activities as well as [(3)H]CP 55,940 binding. A single exposure to WIN (1.5 mg/kg) significantly reduced involuntary movements and SLUD signs following acute paraoxon exposure (0.4 and 0.6 mg/kg, sc). In contrast, when rats were challenged with paraoxon (0.4 mg/kg) after the seventh daily exposure to WIN (1.5mg/kg/day), involuntary movements were significantly increased at later timepoints, while SLUD signs were unaffected. These results suggest that acute and repeated exposure to cannabinoid agonists may differentially modify acute cholinergic toxicity, possibly through modulation of acetylcholine release and adaptation in cannabinergic signaling associated with repeated cannabinoid exposures.

Arachidonyl-2'-chloroethylamide, a highly selective cannabinoid CB1 receptor agonist, enhances the anticonvulsant action of valproate in the mouse maximal electroshock-induced seizure model

Endogenous cannabinoid ligands and cannabinoid CB(1) receptor agonists have been shown to exert potent anticonvulsant effects in various experimental models of epilepsy. The purpose of this study was to determine the effects of arachidonyl-2'-chloroethylamide (ACEA; N-(2-chloroethyl)-5Z,8Z,11Z,14Z-eicosatetraenamide, a highly selective cannabinoid CB(1) receptor agonist) on the threshold for electroconvulsions and the anticonvulsant activity of valproate in the maximal electroshock-induced seizures in mice. To inhibit the rapid metabolic degradation of ACEA by the fatty-acid amide hydrolase, phenylmethylsulfonyl fluoride (PMSF) was used at a constant ineffective dose of 30 mg/kg (i.p.). Moreover, the effects of ACEA and PMSF on the acute adverse-effect profile of valproate were determined in the chimney test. Additionally, the adverse-effect potentials of combination of ACEA, PMSF with valproate were examined in the step-through passive avoidance task (long-term memory) and grip-strength test (neuromuscular strength). To ascertain any pharmacokinetic contribution of ACEA and PMSF to the observed interaction between tested drugs, both free (non-protein bound) plasma and total brain concentrations of valproate were estimated. Results indicated that ACEA (5 and 7.5 mg/kg; i.p.) combined with PMSF increased significantly (P<0.001) the electroconvulsive threshold in mice. ACEA at low doses of 1.25 and 2.5 mg/kg, i.p., with PMSF had no impact on threshold for electroconvulsions. Similarly, neither PMSF (30 mg/kg) nor ACEA (15 mg/kg) administered alone affected the electroconvulsive threshold in mice. Moreover, ACEA (at a subthreshold dose of 2.5 mg/kg; i.p.) co-administered with PMSF potentiated significantly the antielectroshock activity of valproate by reducing its ED(50) from 258.3 to 195.1 mg/kg (P<0.01). Isobolographic transformation of data revealed that the interactions between valproate and ACEA (at 1.25 and 2.5 mg/kg) combined with PMSF were additive. In the chimney test, the combination of ACEA (2.5 mg/kg) and PMSF (30 mg/kg) had no effect on acute adverse effect of valproate and its TD(50) (356.4 mg/kg) did not differ significantly from that for valproate administered alone (TD(50)=404.4 mg/kg). Moreover, none of the examined drugs administered either alone or in combinations produced long-term memory deficits in the step-through passive avoidance task and impaired neuromuscular strength in the grip-strength test in mice. In contrast, ACEA (2.5 mg/kg; i.p.) combined with PMSF (30 mg/kg; i.p.) considerably increased both, the free plasma (by 42%; P<0.01) and total brain (by 49%; P<0.001) concentrations of valproate (administered at 195 mg/kg; i.p.) in mice. Hence, the observed interaction between valproate and ACEA with PMSF in the maximal electroshock test was pharmacokinetic in nature. Finally, based on this preclinical study, one can conclude that ACEA--a cannabinoid CB(1) receptor agonist co-administered with PMSF pharmacokinetically interacted with valproate and thus, providing the enhancement of the antielectroshock activity of valproate in mice, although, the isobolographically determined interaction between drugs was additive. To elucidate the protective role of cannabinoids in the brain during seizures, more advanced neurochemical studies are required.

Activation of the cannabinoid type-1 receptor mediates the anticonvulsant properties of cannabinoids in the hippocampal neuronal culture models of acquired epilepsy and status epilepticus

Cannabinoids have been shown to have anticonvulsant properties, but no studies have evaluated the effects of cannabinoids in the hippocampal neuronal culture models of acquired epilepsy (AE) and status epilepticus (SE). This study investigated the anticonvulsant properties of the cannabinoid receptor agonist R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolol[1,2,3 de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone (WIN 55,212-2) in primary hippocampal neuronal culture models of both AE and SE. WIN 55,212-2 produced dose-dependent anticonvulsant effects against both spontaneous recurrent epileptiform discharges (SRED) (EC50 = 0.85 microM) and SE (EC50 = 1.51 microM), with total suppression of seizure activity at 3 microM and of SE activity at 5 microM. The anticonvulsant properties of WIN 55,212-2 in these preparations were both stereospecific and blocked by the cannabinoid type-1 (CB1) receptor antagonist N-(piperidin-1-yl-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamidehydrochloride (SR141716A; 1 microM), showing a CB1 receptor-dependent pathway. The inhibitory effect of WIN 55,212-2 against low Mg2+-induced SE is the first observation in this model of total suppression of SE by a selective pharmacological agent. The clinically used anticonvulsants phenytoin and phenobarbital were not able to abolish low Mg2+-induced SE at concentrations up to 150 microM. The results from this study show CB1 receptor-mediated anticonvulsant effects of the cannabimimetic WIN 55,212-2 against both SRED and low Mg2+-induced SE in primary hippocampal neuronal cultures and show that these in vitro models of AE and SE may represent powerful tools to investigate the molecular mechanisms mediating the effects of cannabinoids on neuronal excitability.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

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.

Ultra-low dose naltrexone potentiates the anticonvulsant effect of low dose morphine on clonic seizures

Significant potentiation of analgesic effects of opioids can be achieved through selective blockade of their stimulatory effects on intracellular signaling pathways by ultra-low doses of opioid receptor antagonists. However, the generality and specificity of this interaction is not well understood. The bimodal modulation of pentylenetetrazole-induced seizure threshold by opioids provide a model to assess the potential usefulness of this approach in seizure disorders and to examine the differential mechanisms involved in opioid anti- (morphine at 0.5-3 mg/kg) versus pro-convulsant (20-100 mg/kg) effects. Systemic administration of ultra-low doses of naltrexone (100 fg/kg-10 ng/kg) significantly potentiated the anticonvulsant effect of morphine at 0.5 mg/kg while higher degrees of opioid receptor antagonism blocked this effect. Moreover, inhibition of opioid-induced excitatory signaling by naltrexone (1 ng/kg) unmasked a strong anticonvulsant effect for very low doses of morphine (1 ng/kg-100 microg/kg), suggesting that a presumed inhibitory component of opioid receptor signaling can exert strong seizure-protective effects even at very low levels of opioid receptor activation. However, ultra-low dose naltrexone could not increase the maximal anticonvulsant effect of morphine (1-3 mg/kg), possibly due to a ceiling effect. The proconvulsant effects of morphine on seizure threshold were minimally altered by ultra-low doses of naltrexone while being completely blocked by a higher dose (1 mg/kg) of the antagonist. The present data suggest that ultra-low doses of opioid receptor antagonists may provide a potent strategy to modulate seizure susceptibility, especially in conjunction with very low doses of opioids.