2-Amino-6-trifluoromethoxy benzothiazole, a possible antagonist of excitatory amino acid neurotransmission--II. Biochemical properties

Two models have been chosen to study the effect of 2-amino-6-trifluoromethoxy benzothiazole (PK 26124) on excitatory amino acid neurotransmission: the pool of cyclic guanosine monophosphate (cGMP) in the cerebellum and the release of acetylcholine in the striatum and olfactory tubercles. The release of acetylcholine induced by N-methyl-DL-aspartate in the striatum and olfactory tubercles was antagonized by PK 26124 which was less potent on the release of acetylcholine induced electrically. The increase in levels of cGMP in the cerebellum induced by excitatory amino acids such as glutamate and quisqualate was antagonized by PK 26124, but the drug was inactive against N-methyl-DL-aspartate, L-aspartate, kainate and cysteine sulphinate. In vivo it antagonized the increases of cGMP in the cerebellum elicited by all these excitatory compounds. All these results are compatible with a possible antagonism by PK 26124 of the excitatory amino acid neurotransmission and may explain its anticonvulsant properties.     

NMDA and AMPA/kainate receptors are involved in the anticonvulsant activity of riluzole in DBA/2 mice

The anticonvulsant activity of riluzole against sound-induced seizures was studied in the DBA/2 mouse model. Riluzole (0.1-4 mg kg(-1), intraperitoneal (i.p.)) produced dose-dependent effects with ED(50) values for the suppression of tonic, clonic and wild running phases of 0.72, 1.38 and 2.71 mg kg(-1), respectively. Riluzole also protected DBA/2 mice from seizures induced by an intracerebroventricular (i.c.v.) injection of N-methyl-D-aspartate (NMDA) with ED(50) values of 3.03 and 5.0 mg kg(-1) for tonus and clonus, respectively. Pretreatment with glycine, an agonist to the glycine/NMDA receptors, shifted the dose-response effect of riluzole to the right (ED(50)=6.53 against tonus and 9.34 mg kg(-1) vs. clonus). Similarly, D-serine, an agonist at the glycine site, shifted the ED(50) of riluzole against the tonic component of audiogenic seizures from 0.72 to 1.97, and that against clonus from 1.38 to 2.77 mg kg(-1). Riluzole was also potent to prevent seizures induced by administration of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), an AMPA/kainate receptor agonist (ED(50)=1.80 and 3.35 mg kg(-1), against tonus and clonus, respectively). Pretreatment with aniracetam, a positive allosteric modulator of AMPA/kainate receptors, shifted the dose-response curve of riluzole to the right (ED(50)=1.78 against tonus and 2.58 mg kg(-1) vs. clonus). The data indicate that riluzole is an effective anticonvulsant drug in the genetic model of seizure-prone DBA/2 mice. Our findings suggest that the anticonvulsant properties of riluzole depend upon its interaction with neurotransmission mediated by both the glycine/NMDA and the AMPA/kainate receptor complex.     

NNC-711, a novel potent and selective gamma-aminobutyric acid uptake inhibitor: pharmacological characterization.

NNC-711 (1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3- pyridinecarboxylic acid hydrochloride) is a novel, potent and selective gamma-aminobutyric acid (GABA) uptake inhibitor. NNC-711 inhibited synaptosomal (IC50 = 47 nM), neuronal (IC50 = 1238 nM) and glial (IC50 = 636 nM) GABA uptake in vitro NNC-711 lacked affinity for other neurotransmitter receptor binding sites, uptake sites and ion channels examined in vitro. In vivo, NNC-711 was a potent anticonvulsant compound against rodent seizures induced by methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) (ED50 (clonic) = 1.2 mg/kg i.p.), pentylenetetrazole (PTZ) (ED50 (tonic) = 0.72 mg/kg i.p., mouse; and ED50 (tonic) = 1.7 mg/kg, rat), or audiogenic (ED50 (clonic and tonic) = 0.23 mg/kg i.p.). At higher doses NNC-711 produced behavioral side effects characterized by inhibition of traction (ED50 = 23 mg/kg i.p.), rotarod (ED50 = 10 mg/kg i.p.) and exploratory locomotor activity (ED50 = 45 mg/kg i.p.) in the mouse. Following acute (3-h) in vivo pretreatment with NNC-711, behavioral tolerance developed to its motor impairing side effects (inhibition of traction, rotarod or exploratory locomotor activity) without corresponding tolerance to the anticonvulsant effects. These data suggest that NNC-711 will be useful for future in vitro and in vivo experiments to elucidate the role of the GABA uptake carrier in the central nervous system.     

Anticonvulsant properties of flunarizine on reflex and generalized models of epilepsy

The anticonvulsant activity of 1-bis(4-fluorophenyl)methyl-4-(3-phenyl-2-propenyl)-piperazine, flunarizine, was studied after intraperitoneal administration in DBA/2 mice (seizures induced by sound), intravenous administration in Papio papio (myoclonus induced by photic stimulation) and oral administration in Wistar rats (seizures induced by cefazolin). Protection against sound-induced seizures was observed after intraperitoneal administration of flunarizine (5-40 mg/kg). The ED50 for suppression of tonic, clonic and wild running phases of sound-induced seizures was 3.3, 9.8 and 17.5 mg/kg, respectively. This protective action was significantly reduced by pretreatment with aminophylline (50 mg/kg, i.p.). In photosensitive baboons flunarizine (0.5-1.0 mg/kg, i.v.) provided partial protection against myoclonic responses to stroboscopic stimulation. After flunarizine (2 mg/kg, i.v.) this protection lasted for more than 5 hr (and was complete at 2-3 hr). Cefazolin-induced seizures in rats were prevented by administration of flunarizine (20-40 mg/kg, orally). The ED50 for the suppression of tonic and clonic seizures evoked by subsequent intravenous administration of cefazolin was 25 mg/kg. The protective effects of flunarizine (40 mg/kg, orally) were maximal after 3-6 hr and were maintained for 16-24 hr. Behavioural effects of flunarizine included signs of sedation in both mice and rats. Tolerance to the antiepileptic effects of flunarizine was not seen after chronic treatment in rats. The role of purinergic receptors and of calcium entry blockade in the anticonvulsant action of flunarizine requires further study.     

Anticonvulsant activity of two metabotropic glutamate group I antagonists selective for the mGlu5 receptor: 2-methyl-6-(phenylethynyl)-pyridine (MPEP), and (E)-6-methyl-2-styryl-pyridine (SIB 1893)

The selective mGlu5 antagonists, MPEP, 2-methyl-6-phenylethynyl-pyridine, and SIB1893, (E)-6-methyl-2-styryl-pyridine, have been evaluated as antiepileptic drugs in DBA/2 mice and lethargic mice. Clonic seizures induced by the selective mGlu5 agonist, (R,S)-2-chloro-5-hydroxyphenylglycine (CHPG), 3 micromol intracerebroventricularly (i.c.v.), are potently suppressed by both compounds (MPEP, ED(50)=0.42 [0.28-0.62] mg/kg intraperitoneally (i.p.); SIB 1893 ED(50)=0.19 [0.11-0.33] mg/kg i.p. ). Clonic seizures induced by the mGlu1,5 agonist, 3, 5-dihydroxyphenylglycine (DHPG), 1.5 micromol i.c.v., are less potently suppressed by both compounds (MPEP, ED(50)=22 [13-38] mg/kg i.p., 110 [67-180] nmol i.c.v.; SIB1893, ED(50)=31 [18-54] mg/kg i.p. , 95 [82-110] nmol i.c.v.). Sound-induced seizures in DBA/2 mice are suppressed at 15 min by MPEP and SIB 1893 (MPEP ED(50) clonic seizures=18 [10-32] mg/kg i.p., 93 [69-125] nmol i.c.v.; tonic seizures=6.1 [4.5-8.3] mg/kg i.p., 46 [26-80] nmol i.c.v.; SIB 1893 ED(50) clonic seizures=27 [17-44] mg/kg i.p., 825 [615-1108] nmol i. c.v., tonic seizures=5.4 [3.4-8.6] mg/kg i.p., 194 [113-332] nmol i. c.v.). The ED(50) for MPEP for impaired rotarod performance is 128 [83-193] mg/kg i.p., at 15 min, i.e. a therapeutic index for sound-induced seizures of 5-20. In lethargic mice (lh/lh), a genetic absence model, MPEP, 50 mg/kg i.p., caused a marked reduction in the incidence of spontaneous spike-and-wave discharges. These selective antagonists of mGlu5 block seizures due to activation of mGlu5 at very low systemic doses. At rather higher doses they block convulsive and non-convulsive primary generalised seizures.     

NMDA antagonists block restraint-induced increase in extracellular DOPAC in rat nucleus accumbens

The effects of the N-methyl-D-aspartate (NMDA) receptor antagonists CPP, TCP, PK 26124 and ifenprodil, and of the minor tranquillizer diazepam on stress-induced changes of dopamine metabolism in the nucleus accumbens were investigated in the rat. Dopamine metabolism was assessed by measuring the extracellular levels of 3,4-dihydroxyphenylacetic acid (DOPAC) by means of in vivo differential pulse voltammetry with electrochemically pretreated carbon fiber electrodes. Physical immobilization of the rats for 4 min caused a marked and long-lasting increase in extracellular DOPAC levels in the nucleus accumbens. A similar, though shorter-lasting, augmentation of extracellular DOPAC was observed in the nucleus accumbens after systemic administration of the anxiogenic agent methyl-beta-carboline-3-carboxylate (beta-CCM) (10 mg/kg s.c.). Pretreatment with CPP (1 mg/kg i.p.), TCP (3 mg/kg i.p.), PK 26124 (3 mg/kg i.p.), ifenprodil (3 mg/kg i.p.) or diazepam (2 mg/kg i.p.) totally antagonized the immobilization-induced increase in extracellular DOPAC in the nucleus accumbens. Diazepam and the benzodiazepine (omega 1-2) receptor antagonist flumazenil (30 mg/kg i.p.), but not ifenprodil, also antagonized the beta-CCM-induced activation of dopamine metabolism in the nucleus accumbens. Finally, systemic administration of haloperidol (25 micrograms/kg i.p.) increased the extracellular concentrations of DOPAC in the nucleus accumbens, but pretreatment with ifenprodil (3 mg/kg i.p.) did not modify this response. These data indicate that NMDA receptor antagonists prevent the activation of dopamine metabolism in the nucleus accumbens caused by immobilization stress but not by beta-CCM-induced anxiogenic stimulation. These results suggest that NMDA receptor antagonists may possess an anxiolytic-like action in the rodent, which is exerted via neuroanatomical circuits distinct from those acted upon by diazepam.     

Synthesis and anticonvulsant activity of novel 1-substituted-1,2-dihydro-pyridazine-3,6-diones

The synthesis and pharmacological evaluation of novel 1-substituted-1,2-dihydro-pyridazine-3,6-diones (4a--l, 5a--j) as potential anticonvulsant agents are described. The compounds were tested in vivo for the anticonvulsant activity. The compound which have maximum protection against MES induced seizures is 1-[3-(2-aminophenylamino)-2-hydroxypropyl)-1,2-dihydro-pyridazine-3,6-dione 4h (ED(50)=44.7 mg x kg(-1) i.p.) 1-[2-hydroxy-3-piperazin1-yl-propyl)-1,2-dihydro-pyridazine-3,6-dione 4c (ED(50)=72 mg x kg(-1) i.p.) and 1-[2-hydroxy-3-imidazol-1-yl-propyl)-1,2-dihydro-pyridazine-3,6-dione 4d (ED(50)=79 mg x kg(-1) i.p.) were also found to have maximum protection against MES induced seizures. Whereas all these compounds failed to protect the animals from subcutaneous pentylenetetrazole (Metrozol) seizure threshold test (sc-Met).     

Anticonvulsant activity of (+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine (MK-801), a substance with potent anticonvulsant,central sympathomimetic,and apparent anxiolytic properties

MK-801 prevented tonic extensor seizures in the rat induced by bicuclline with the ED₅₀ being 23 μg/kg p.o. Clonazepam, phenobarbital, diazepam, phenytoin, γ-acetylenic GABA, sodium valproate, and trimethadione were all less potent. In mice, MK-801 was likewise the most potent (ED₅₀ = 0.35 mg/kg p.o.) compound in protecting against tonic seizures induced by electroshock. Clonazepam (ED₅₀ = 0.41 mg/kg p.o.) and MK-801 (ED₅₀ = 0.67 mg/kg p.o.) were by far more potent than any of the other anticonvulsants tested versus bicuculline-elicited seizures in mice. Besides being a potent anticonvulsant, MK-801 demonstrated selectivity, since much higher doses were required in mice to block clonic convulsions produced by pentylenetetrazol (ED₅₀ = 11 mg/kg p.o.) and tonic seizures caused by strychnine (ED₅₀ > 15 mg/kg p.o.) than were needed against electroshock or bicuculline. The anticonvulsant (electroshock) effect of MK-801 in mice was unaffected by pretreating the animals with various receptor antagonists (atropine, mecamylamine, chlorpheniramine, tripelennamine, cyproheptadine, cinanserin, methysergide, cimetidine, and propranolol). MK-801 was slightly, but significantly, antagonized by methergoline, naloxone, and theophylline, whereas haloperidol and especially α-adrenoceptor blockers (prazosin, HEAT, phenoxybenzamine) markedly reduced the anticonvulsant effect of MK-801. Haloperidol was selective for MK-801, not affecting the anticonvulsant actions of phenytoin or phenobarbital. Prazosin antagonized phenytoin and phenobarbital, but to a much lesser extent than it antagonized MK-801. MK-801 is an extremely potent and selective anticonvulsant acting at least partly via a catecholaminergic mechanism.     

Anti-epileptic activity of group II metabotropic glutamate receptor agonists (--)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate (LY379268) and (--)-2-thia-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate (LY389795).

The selective group II metabotropic glutamate receptor (mGlu(2/3)) agonists (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate (LY379268) and (-)-2-thia-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate (LY389795) have been evaluated as anti-epileptic drugs in dilute brown agouti (DBA/2) mice, lethargic (lh/lh) mice, genetically epilepsy-prone-9 (GEP) rats and amygdala-kindled rats. Sound-induced clonic seizures in DBA/2 mice were transiently inhibited by both agonists intracerebroventricularly (i.c.v.), LY379268 ED(50)=0.08 [0.02-0.33]nmol and LY389795 ED(50)=0.82 [0.27-3.24]nmol or intraperitoneally (i.p.), LY379268 ED(50)=2.9 [0.9-9.6]mg/kg and LY389795 ED(50)=3.4 [1.0-11.7]mg/kg. Both mGlu(2/3) agonists inhibited seizures induced by the group I mGlu receptor agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG), where LY379268, i.c.v. ED(50)=0.3 [0.02-5.0]pmol and LY389795, i.c.v. ED(50)=0.03 [0.05-0.19]nmol. The spike and wave discharge (SWD) duration of absence seizures in lh/lh mice was significantly reduced by both agonists at 1 and 10nmol (i.c.v.) up to 90min following infusion. The electrically induced seizure score and afterdischarge duration of amygdala-kindled rats was partially inhibited by the agonists 30min after i.p. injection of 10mg/kg. The agonists did not inhibit sound-induced seizures in GEP rats (0.1-1mg/kg, 30min 1h, i.p.), but were proconvulsant following sound stimulus (> or =0.1mg/kg). These findings identify a potential role for mGlu(2/3) agonists in the amelioration of generalised and partial epileptic seizures.     

Antiparkinsonian drugs memantine and trihexyphenidyl potentiate the anticonvulsant activity of valproate against maximal electroshock-induced seizures.

Memantine increased the threshold for electroconvulsions, when administered at 1.0-6.0 mg/kg (i.p.) and given in subthreshold doses of 0.0156, 0.0625, 0.125 and 0.5 mg/kg (i.p.) potentiated the protective efficacy of valproate, against maximal electroshock (50 mA)-induced seizures in mice, lowering the ED50 from 235 to 197, 172, 164 and 130 mg/kg, respectively. Trihexyphenidyl, applied in doses of 30 and 50 mg/kg (i.p.), did not influence the electroconvulsive threshold per se but when combined with valproate, strongly enhanced its anticonvulsant activity against maximal electroshock-induced seizures lowering the ED50 from 206 to 103 and 46 mg/kg, respectively. The chimney test and retention testing in mice revealed that administration of memantine at 0.5 mg/kg (i.p.) or trihexyphenidyl at 30 mg/kg (i.p.) together with valproate in doses of 130 or 103 mg/kg (i.p.), respectively, resulted in motor impairment and caused impairment of long-term memory, similar to the effects of valproate alone, when applied at its ED50 against maximal electroshock. Neither memantine nor trihexyphenidyl altered the total level of valproate in plasma. It may be concluded that the potentiation of the anticonvulsant activity of valproate, by memantine and trihexyphenidyl, is not associated with a pharmacokinetic interaction.     

Effects of some AMPA receptor antagonists on the development of tolerance in epilepsy-prone rats and in pentylenetetrazole kindled rats

The non-selective alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) receptor antagonists, 2,3-benzodiazepine derivatives CFM-1 (3,5-dihydro-7,8-dimethoxy-1-phenyl-4H-2,3-benzodiazepin-4-one) and CFM-2 (1-(4'-aminophenyl)-3,5-dihydro-7,8-dimethoxy-4H-2,3-benzodiazepin -4-one), following intraperitoneal (i.p.) administration, were studied against audiogenic seizures in genetically epilepsy-prone rats (GEPRs) or pentylenetetrazole induced kindling in rats. After acute i.p. administration the ED50 values of CFM-1 against the clonic and tonic phases of the audiogenic seizures 30 min after pretreatment were 40 (16-100) and 13 (8-25) micromol kg(-1), respectively. The animals used for chronic study were treated i.p. daily (at 10 h) for 4 weeks with CFM-1 (20 or 50 micromol kg(-1)). Chronic treatment for 2 weeks with CFM-1 gave ED50 values against clonic and tonic seizures of 39 (22-69) and 16 (8-25) micromol kg(-1), respectively, whereas chronic treatment for 4 weeks gave ED50 values against clonic and tonic seizures of 42 (18-98) and 17 (7-41.3) micromol kg(-1), respectively. The duration of anticonvulsant activity observed between 0.5 and 4 h following administration of CFM-1 was similar for acute and chronic treatment. Two groups of Sprague-Dawley rats received CMF (20 or 50 micromol kg(-1)) 30 min before a subconvulsant dose of pentylentetrazole (25 mg kg(-1) i.p.) which is able to increase seizure severity in control animals (i.e., chemical kindling). Pretreatment with CFM-2 delayed the progression of seizure rank during repeated administration of pentylentetrazole. At the end of the period of repeated pentylentetrazole treatment (6 weeks) the mean seizure score was 0 in vehicle treated controls, 4.3 in animals treated with vehicle + pentylentetrazole, 2.2 in rats treated chronically with CFM-2 (20 micromol kg(-1) i.p.) + pentylentetrazole and 1.0 in rats treated repeatedly with CFM-2 (50 micromol kg(-1) i.p.) + pentylenetetrazole. CFM-2 was also able to antagonize the long-term increase in sensitivity of the convulsant effects of GABA function inhibitors in pentylentetrazole-kindled animals. Thus, the administration of a challenge dose of pentylentetrazole (15 mg kg(-1) i.p.) or picrotoxin (1.5 mg kg(-1) i.p.) 15 or 30 days after the end of the repeated treatment showed that animals treated with CFM-2 were significantly protected against seizures induced by pentylentetrazole or picrotoxin. The data suggest that, following repeated treatment, tolerance to the novel AMPA receptor antagonists does not develop (CFM-1 in genetically epilepsy-prone rats and CFM-2 in the pentylentetrazole kindling model of epilepsy). Thirteen minutes after drug injection on days 1, 14 and 28 of chronic treatment the motor impairment induced by these compounds was studied with a rotarod apparatus. The TD50 values for CFM-1 or CFM-2-induced impairment of locomotor performance were similar following acute and repeated treatment. The data also suggest that some novel 2,3-benzodiazepines may have clinical potential for some types of epilepsy.     

Anticonvulsant effect of denzimol in DBA/2 mice

The anticonvulsant activity of N-[beta-[4-beta-phenylethyl)phenyl]-beta-hydroxyethyl]-imidazole hydrochloride, denzimol, was studied following intraperitoneal administration in DBA/2 mice (seizures induced by sound). Protection against sound-induced seizures was observed after intraperitoneal administration of denzimol (3-15 mg/kg). The ED50 values for the suppression of tonic, clonic and wild running phases of sound-induced seizures were 1.24, 2.61 and 6.03 mg/kg, respectively. This protective action was significantly reduced by pretreatment with aminophylline (25 mg/kg i.p.), CGS 8216 (1 or 5 mg/kg i.p.) and Ro 15-1788 (2.5 mg/kg i.p). The present experiments suggest an involvement of purinergic and benzodiazepine mechanisms in the anticonvulsant action of denzimol.     

Effects of compounds acting on GABA(B) receptors in the pentylenetetrazole kindling model of epilepsy in mice

The involvement of GABA(B) receptors in the behavioural and epileptic electrocortical discharges occurring in chemical kindling induced by repeated treatments with a subconvulsant dose of pentylenetetrazole (25 mg/kg i.p.) has been investigated in CD1 mice. Behavioural and electrocorticographic epileptic seizures following kindling induced by pentylenetetrazole (25 mg/kg i.p.) were attenuated or completely antagonized in a dose-dependent manner by the GABA(B) receptor agonist R-baclofen (2 and 6 mg/kg) whilst the GABA(B) receptor antagonist 3-amino-propyl-diethoxy-methyl-phosphinic acid (CGP 35348, 25, 50 or 100 mg/kg) and 3-[1-(S)-(3, 4-dichloro-phenyl-ethyl]amino-2-(S)-hydroxy-propyl-benzyl-phosphinic acid (CGP 55845A, 10 or 20 mg/kg) produced a more rapid development of kindling and an increase in behavioural and electrocorticographic epileptic changes. In addition, all GABA(B) receptor antagonists were able to induce an increase in Fos and Jun protein expression in pentylenetetrazole (25 mg/kg i.p.) treated mice whilst the GABA(B) receptor agonist R-baclofen (2 or 6 mg/kg) attenuated the expression of Fos and Jun protein, at cortical and limbic structures. In order to study the persistence of changes induced by pentylenetetrazole kindling, different groups of mice were rechallenged with a kindling stimulus 15 or 30 days after withdrawal from the last injection of vehicle+pentylenetetrazole, R-baclofen+pentylenetetrazole or GABA(B) receptor antagonists+pentylenetetrazole. The groups receiving GABA(B) receptor antagonists+pentylenetetrazole showed a higher incidence of seizures following the kindling stimulus than mice receiving vehicle+pentylenetetrazole whilst animals treated with R-baclofen were protected from the kindling stimulus. The different effects observed following repeated treatment with the GABA(B) receptor agonist and antagonist used revealed that GABA(B) receptors are able to affect the development of the epileptic kindling state induced by pentylenetetrazole.     

Lack of protection by pyridoxine or hydrazine pretreatment against monosodium L-glutamate-induced seizures.

Pretreatment of rats with hydrazine (100 mg/kg), a compound which raises brain gamma-aminobutyric acid (GAGA) 175 percent in 12 hr was not able to prevent the occurrence of seizures induced by monosodium L-glutamate (MSG). Pyridoxine (50 mg/kg) the cofactor essential in the conversion of glutamate to GABA, also failed to prevent convulsions induced by parenteral MSG administration. It is concluded that the mechanism of action of MSG-induced seizures is neither by decreasing brain GABA levels or interfering with the pyridoxine cofactor.     

Substitution for PCP, disruption of prepulse inhibition and hyperactivity induced by N-methyl-D-aspartate receptor antagonists: preferential involvement of the NR2B rather than NR2A subunit

The non-competitive NMDA receptor antagonist phencyclidine (PCP) is known to produce a discriminative stimulus in rats. The first aim of the present study was to investigate which NMDA receptor subtype(s) is involved in this effect of PCP. Rats were trained to discriminate PCP (2 mg/kg; i.p.) from saline in a two lever operant task. The NMDA channel blocker, (+)MK-801 (0.1 mg/kg; i.p.) and the competitive NMDA receptor antagonist SDZ 220-581 (3 mg/kg; i.p.) produced 76% of PCP-lever selection (ED50=0.045 and 2 mg/kg, respectively), whereas their respective inactive enantiomers (-)MK-801 (0.025-0.1 mg/kg) and SDZ 221-653 (2-5 mg/kg) induced less than 30% of PCP-appropriate responding. Another competitive NMDA antagonist, SDZ EAB-515 (30 mg/kg; i.p.), induced 63% of PCP-lever responding (ED50=23.48 mg/kg). The selective antagonist of NMDA receptors containing the NR1A/NR2B-subunits Ro 25-6981 (20 mg/kg; i.p.) resulted in a complete substitution (more than 80% of PCP-lever selection) for PCP (ED50=8.59 mg/kg). In contrast, the NR1A/NR2A NMDA receptor-preferring antagonist NVP-AAM077 (2-10 mg/kg; i.p.) failed to produce PCP-like discriminative stimuli. At high doses SDZ 220-581 (ED50=2.44), NVP-AAM077 (ED50=8.33) and SDZ EAB-515 (ED50=25.81) decreased the performance of the rats in this operant task. The ability of these NMDA receptor antagonists to disrupt the prepulse inhibition (PPI) of the startle response and to alter locomotor activity was also studied. PCP (0.5-2 mg/kg; s.c.), SDZ 220-581 (0.5-5 mg/kg; s.c.), SDZ EAB-515 (1-30 mg/kg; i.p.) and Ro 25-6981 (5-20 mg/kg; i.p.) disrupted PPI and at high doses produced hyperlocomotion. In contrast, NVP-AAM077 (5-20 mg/kg; i.p.) did not disrupt PPI and reduced locomotor activity. In conclusion, it appears that the NMDA receptor containing the NR2B, rather than the NR2A subunit, may play a major role in the PCP-like discriminative stimulus. In addition, sensory motor gating disturbances associated with NMDA antagonists do not seem to result from a blockade of NR1/NR2A-containing NMDA receptors.     

Non-competitive N-methyl-D-aspartate antagonists protect against sound-induced seizures in DBA/2 mice

Non-competitive antagonists of the N-methyl-D-aspartate (NMDA) receptor have been evaluated as anticonvulsants against sound-induced seizures in DBA/2 mice. The ED50 values for protection against sound-induced clonic seizures 15 min following the intracerebroventricular (i.c.v.) and intraperitoneal (i.p.) administration are: MK-801, ED50 = 0.5 nmol (i.c.v.); 0.14 mumol/kg (i.p.); phencyclidine, ED50 = 14 nmol (i.c.v.); 1.9 mumol/kg (i.p.); dextrorphan, ED50 = 35 nmol (i.c.v.); 18.5 mumol/kg (i.p.); tiletamine, ED50 = 40 nmol (i.c.v.); 5.6 mumol/kg (i.p.); SKF-10047, ED50 = 50 nmol (i.c.v.); 23.5 mumol/kg (i.p.); dextromethorphan, ED50 = 70 nmol (i.c.v.); 28.0 mumol/kg (i.p.); ketamine, ED50 = 110 nmol (i.c.v.); 15.5 mumol/kg (i.p.). The anticonvulsant effects of ketamine and tiletamine are of short duration (10-30 min), whereas the anticonvulsant effects of MK-801 and dextromethorphan last for 45 min or longer. The effects of phencyclidine, SKF-10047 and dextrorphan are of intermediate duration. Mild to moderate behavioural excitation is associated with the anticonvulsant activity of all the non-competitive NMDA antagonists. For MK-801, phencyclidine, dextrorphan, SKF-10047 and ketamine there is a close correlation between their relative anticonvulsant potencies and their potencies for displacing [3H]MK-801. The anticonvulsant effect is likely to be primarily mediated via NMDA antagonism at the PCP/MK-801 site.     

H2-receptor antagonist and gastric acid antisecretory properties of Wy-45,662

Investigations were carried out to delineate the biological activity of Wy-45,662, a new H2-receptor antagonist. In the pylorus-ligated rat after intraduodenal administration, total acid output (TAO) over 4 hours was inhibited by Wy-45,662 with an ED50 of 0.3 mg/kg as compared to ranitidine (ED50 = 7 mg/kg) and cimetidine (ED50 = 12 mg/kg); i.v. or i.m. administration increased Wy-45,662's potency 10-fold. In dogs with innervated gastric pouches Wy-45,662 inhibited food-stimulated TAO with ED50's of 0.35 mg/kg (p.o.), 0.045 mg/kg (i.v.) and 0.065 mg/kg (i.m.); cimetidine (ED50 = 6 mg/kg p.o.) and ranitidine (ED50 = 1 mg/kg p.o.) were less potent. Wy-45,662 also inhibited pentagastrin- or histamine-stimulated acid secretion in the conscious fistula rat. In vitro, Wy-45,662 antagonized the histamine-stimulated a) positive chronotropism in guinea pig atria and b) [14C]aminopyrine uptake by rat gastric mucosal cells, confirming its H2-receptor antagonist properties.     

An antinociceptive profile of kojic amine: an analogue of gamma-aminobutyric acid (GABA)

Kojic amine [2-(aminomethyl)-5-hydroxy-4H-pyran-4-one], an analogue of gamma-aminobutyric acid (GABA), produced dose-related, but short-lived, antinociceptive activity in the 48 degrees C [ED50 = 9.2 (8.2-10.3) mg/kg i.p.] and 55 degrees C [ED50 = 13.8 (12.2-15.7) mg/kg i.p.] hot-plate tests in the mouse. The antinociceptive activity of kojic amine at 48 degrees C was found to be insensitive to bicuculline (1.0 mg/kg i.p.) and picrotoxin (0.5 mg/kg i.p.). At this temperature, antinociception was distinctly separate from the impairment of motor function (measured by a rotorod assay) and was not significantly affected by prior treatment with the cholinergic antagonist, atropine sulfate (10.0 mg/kg i.p.). However, at 55 degrees C, the antinociceptive effect of a large dose (20 mg/kg i.p.) of kojic amine was significantly attenuated by similar pretreatment with atropine sulfate, but not by the peripheral cholinergic antagonist, atropine methylnitrate (10.0 mg/kg i.p.). Kojic amine exhibited no significant interaction with haloperidol (0.5 mg/kg i.p.) at this temperature. In animals made tolerant to morphine, THIP or baclofen, there was analgesic cross-tolerance between kojic amine, morphine and baclofen but not between kojic amine and THIP. It is suggested that kojic amine-induced antinociception is similar to that produced by both THIP and baclofen. Thus, kojic amine represents a unique tool with which to study GABA-ergic antinociceptive processes.     

GABA-related enzymes in the hypothalamus of rats treated with estradiol

In a naloxone-reversible, dose-dependent manner, morphine (10-50 mg/kg i.p.) protected against seizures induced by maximal electroshock and increased the incidence and severity of seizures induced by bicuculline, in rats. Morphine also potentiated seizures induced by isoniazid and by picrotoxin. Thus, opiate activity influences the expression of seizures in contrasting ways depending upon the mode of seizure induction. Since morphine consistently potentiated seizures induced by interference with GABA transmission, it appears that GABAergic systems may be of particular significance for the elucidation of the varied effects of morphine on seizure susceptibility.     

Antagonists of adenosine and alpha-2-adrenoceptors reverse the anticonvulsant effects of tizanidine in DBA/2 mice

The anticonvulsant activity of 5-chloro-4-(2-imidazolin-2yl-amino)-2,1,3-benzothiazole, tizanidine, was studied following intraperitoneal (i.p.) administration in DBA/2 mice (which show sound-induced seizures). Protection against sound-induced seizures was observed after tizanidine, (0.5-4 mg/kg i.p.). The ED50 values for suppression of the tonic, clonic and wild running phases of sound-induced seizures were 0.54, 0.76 and 1.43 mg/kg, respectively. This protective action was significantly reduced by pretreatment with aminophylline (25 mg/kg i.p.), yohimbine (1 mg/kg i.p.) or piperoxan (20 mg/kg i.p.). Methysergide, a serotonin antagonists, did not significantly reduce the anticonvulsant effects of tizanidine. The present experiments suggest an involvement of purinergic and adrenergic mechanisms in the anticonvulsant action of tizanidine.