Cholinergic kindling of the amygdala requires the activation of muscarinic receptors

Role of NMDA receptors in the pathophysiology and treatment of status epilepticus

This review considers the role of N-methyl-d-aspartate receptors in the pathophysiology and treatment of status epilepticus (SE). NMDA receptors play a critical role in sustaining SE by mediating the plasticity of γ-aminobutyric acid (GABA)-_A and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, neuronal loss, and epileptogenesis. In parallel, there is growing interest in using the NMDA receptor antagonist ketamine in the treatment of refractory SE. Ketamine has proved to be safe for use in refractory and super-refractory SE in patients. The pilot studies also suggest that ketamine may be efficacious for termination of refractory SE.

Competitive NMDA receptor antagonists do not block cholinergic kindling with carbachol

The role of NMDA receptor activity in kindling was examined in rats pretreated with the competitive NMDA receptor antagonists aminophosphonovaleric acid (APV) or NPC17742 (2R,4R,2S-(2-amino-4,5(cyclohexyl)-7-phosphonoheptanoic acid). After pretreatment, the rats received an infusion of carbachol, a muscarinic agonist, into the amygdala or hippocampus. Kindling sessions with carbachol occurred once every 48 h until a stage 5 convulsion was displayed. Electrical kindling of the amygdala after pretreatment with NPC17742 was also examined. Both APV and NPC17742 retarded the rate of carbachol kindling in its early stages, but all rats displayed kindled stage 5 convulsions under APV or NPC17742 in fewer than 10 sessions. Convulsion development was accompanied by growth in the duration and strength of the accompanying epileptiform activity. All rats exhibited a stage 5 convulsion on the first or second session after cross-over to vehicle pretreatment, confirming the development of kindled convulsions under pretreatment with NMDA antagonists. NPC17742 retarded electrical kindling, but after cross-over to vehicle there was savings in the rate of kindling to stage 5 convulsions. These findings indicate that carbachol kindling of the amygdala or hippocampus readily occurs under NMDA antagonism. They are consistent with the view that NMDA receptor activity may contribute to, but is not required for, the kindling of seizures.

Change in neurotrophins and their receptor mRNAs in the rat forebrain after status epilepticus induced by pilocarpine

We studied the effects of status epilepticus (SE) induced by lithium chloride/pilocarpine treatment on gene expression of neurotrophins of the nerve growth factor (NGF) family and of their high-affinity receptors of the tyrosine protein kinase (trk) family in the forebrain. Using in situ hybridization (ISH), we demonstrated an early (3 h after treatment) increase in brain-derived neurotrophic factor (BDNF) and trkB mRNA expression in the dentate gyrus, amygdala, and piriform cortex, as well as widespread increases in the cerebral cortex. NGF mRNA, but not the mRNA of its receptor trkA, was increased in the dentate gyrus. In contrast, 12 h after treatment, neurotrophin-3 (NT-3) decreased, and its receptor trkC mRNA increased. There was no change in NT-4 mRNA levels. All changes were blocked by pretreatment with scopolamine, a muscarinic antagonist. The noncompetitive N-methyl-D-aspartate (NMDA) antagonist ketamine blocked NGF, BDNF, and trkB mRNA increases in the hippocampus and cerebral cortex, but not in the amygdala and piriform cortex. In contrast, ketamine did not affect NT-3 and trkC changes. These results provide a complete description of changes in mRNA levels of neurotrophins and their receptors in the forebrain after SE and supply additional data supporting the view that neurotrophin gene expression is related to abnormal neuronal activity.

The NMDA antagonist procyclidine, but not ifenprodil, enhances the protective efficacy of common antiepileptics against maximal electroshock-induced seizures in mice

Procyclidine (up to 20 mg/kg i.p.) did not influence the electroconvulsive threshold per se, but when given in a dose of 10 mg/kg, it potentiated the protective activity of carbamazepine, diphenylhydantoin, phenobarbital and valproate, and in a dose of 20 mg/kg, that of diazepam against maximal electroshock-induced convulsions in mice. Ifenprodil increased the threshold for electroconvulsions when applied at 20 and 40 mg/kg (i.p.), but surprisingly, when combined with all antiepileptics tested, it did not influence their anticonvulsant actions. The chimney test in mice revealed, that application of procyclidine at 10 mg/kg together with phenobarbital and valproate, and procyclidine at 20 mg/kg with diazepam resulted in motor impairment. However, when procyclidine was applied at 10 mg/kg together with carbamazepine or diphenylhydantoin, no motor impairment was noted. The combined treatment of procyclidine (10 mg/kg) with carbamazepine, diphenylhydantoin, phenobarbital or valproate, as well as procyclidine (20 mg/kg) with diazepam caused significant worsening of long-term memory. Finally, procyclidine did not alter the total plasma levels of carbamazepine, diazepam, diphenylhydantoin, phenobarbital and valproate. It may be concluded that not all agents interfering with NMDA receptor complex-mediated events lead to the potentiation of the anticonvulsant activity of antiepileptic drugs.

Antinociception and behavioral changes induced by carbachol microinjected into identified sites of the rat brain

The sites of the rat brain in which intracerebral administration of carbachol (0.4 microgram/0.5 microliter) elevates the nociceptive threshold to thermic (tail-flick test) and mechanical (calibrated-pinch test) noxious stimuli were examined. An extensive mapping (510 sites) ranging from AP + 10.5 to AP-0.1 mm revealed that antinociception was obtained from 119 sites (23%) widely scattered in the brain, and reached structures distant from, or within the immediate vicinity of the ventricular system. The effects from most placement were demonstrated using the tail-flick test, whereas a smaller proportion (approximately 13%) of sites was effective in reducing the response to mechanical stimuli only. Structures containing sensitive sites include the dorsal raphe nucleus, lateral border of the superior cerebellar peduncle, caudal portion of the superior colliculus, medial geniculate body, habenular complex, amygdala, temporal pole of the ventral hippocampus, rostral aspect of the dorsal hippocampus, lateral septal area, and triangular nucleus of the septum. Analysis of the distribution of responsive sites indicated that they are poorly superposed to the known distribution of opiate-sensitive areas. Most of the structures found to be responsive to carbachol are also known to possess cholinergic receptors and to evoke antinociception following focal electrical stimulation. In various placements, particularly in limbic structures, microinjection of carbachol evoked jumping to mechanical noxious stimulation, hyperexcitability to non-noxious stimuli, convulsive reactions, and other less frequent reactions. On few occasions, however, these changes were accompanied by antinociception.

N-methyl-D-aspartate receptors mediate activation of the c-fos proto-oncogene in a model of brain injury

The proto-oncogene c-fos is rapidly and transiently induced in the CNS by a variety of stimuli. Brain injury, disruption of pia-arachnoid in a limited area, is one of the situations that leads to a dramatic increase in c-fos immunoreactivity. This increase is limited to the lesioned hemisphere. Injections of atropine (25 mg/kg, i.p.), naltrexone (5 mg/kg, i.p.), nifedipine (5 mg/kg, i.p.), and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (20 mg/kg, i.p.), prior to the injury, did not affect the activation of c-fos as assessed by immunohistochemistry in adult Sprague-Dawley rats perfused 2 h after the lesion. The non-competitive N-methyl-D-aspartate antagonists ketamine (100 mg/kg, i.p.) and MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate] (1 and 3 mg/kg, i.p.) markedly reduced c-fos activation. Phencyclidine (10 mg/kg, i.p.) produced a slight reduction in damage-induced fos activation. This study suggests that c-fos activation in this particular model is N-methyl-D-aspartate receptor-mediated and supports the idea that the fos proto-oncogene might play a role in plasticity and/or neurotoxic changes following brain damage.

Seizures decrease regional enzymatic hydrolysis of N-acetyl-aspartylglutamate in rat brain

Previous results have shown that kindled seizures increase N-acetyl-aspartylglutamate (NAAG) levels in the entorhinal cortex, while non-kindled convulsions have no effect. To further explore possible relationships between epilepsy and the physiology of NAAG, the effect of amygdaloid kindling on the activity of a NAAG-hydrolyzing enzyme was examined in specific brain regions associated with limbic seizures. NAAG is hydrolyzed into glutamate (Glu) and N-acetyl-aspartate (NAA) by N-acetylated-alpha-linked acidic dipeptidase (NAALADase), a membrane-bound peptidase. We found that convulsions decreased NAALADase activity and these effects were generalized to several brain regions. While small decreases in the hippocampus were specific to kindling, the decreases in other limbic regions were larger, non-specific, and appear to be aftereffects of convulsions; i.e. not specific to kindling. Although there is evidence that NAAG may be an excitatory neurotransmitter, it could also function as a storage form of Glu. Thus, a reduction in NAALADase activity could reduce the availability of Glu at certain synapses, which might be a homeostatic mechanism for lessening susceptibility to further seizures.

Neurochemical consequences of status epilepticus induced in rats by coadministration of lithium and pilocarpine

Status epilepticus was produced in rats by administering pilocarpine (30 mg/kg, s.c.) 16 h after treatment with LiCl (3 meq/kg, i.p.). After 35 min of status epilepticus, several parameters of cholinergic activity were measured. Seizures had no effect on the in vivo concentration of acetylcholine or choline in cerebellum, cortex, hippocampus, or striatum. Synaptosomal high-affinity choline transport was also not changed by seizures in hippocampus, cortex, or striatum. Cortical slices from seizing rats had elevated concentrations of acetylcholine and released acetylcholine at a greater rate than did controls, but these effects seemed to be due to a reduction in the postmortem hydrolysis of acetylcholine. Synaptosomal 45calcium uptake during 2 to 60 s of incubation was no different from control rates in tissue prepared from seizing rats. These results indicate that presynaptic cholinergic activity is not markedly altered by 35 min of continuous seizure activity induced by lithium and pilocarpine. In contrast, the in vivo concentration of cyclic guanosine 5'-monophosphate was elevated above control values in seizing rats by 57 to 170% in cerebellum, cortex, hippocampus, and striatum.

Characterization of lithium potentiation of pilocarpine-induced status epilepticus in rats

Subcutaneous administration of pilocarpine to rats that were pretreated with a small dose of lithium chloride results in the evolution of generalized convulsive status epilepticus. The production of status epilepticus is absolutely reproducible, has a very consistent time to onset (22 min), has a duration of several hours, and is extremely severe with a high mortality rate. Experimental results show that this animal model of status epilepticus: (i) requires activation of muscarinic receptors because the initiation of seizures is blocked by atropine; (ii) requires presynaptic cholinergic activity because it is attenuated by hemicholinium-3; (iii) recruits noncholinergic cells because when status epilepticus is established it is not altered by atropine administration; and (iv) is blocked by pretreatment with diazepam and ongoing seizures are terminated by administration of diazepam, similar to certain forms of status epilepticus in humans. The reproducibility, prolonged nature, and involvement of a clearly defined neurochemical system as the triggering mechanism, i.e., cholinergic activation, makes this a potentially valuable animal model of generalized convulsive status epilepticus.

Effect of cocaine and pentylenetetrazol on cortical kindling

The effect of drug-induced convulsions on subsequent cortical kindling was studied in male Long-Evans rats. Animals experienced three intravenous infusions of physiological saline at 3 day intervals, or three convulsions induced by the infusion of cocaine or pentylenetetrazol (PTZ). Beginning eight days after the last infusion, all animals were kindled by stimulation of the anterior neocortex (area 6). PTZ-induced convulsions facilitated the development of both the behavioral convulsion and the electrographic seizure during cortical kindling, while cocaine-induced convulsions facilitated only the development of the electrographic seizure. Comparison of these results with previous research indicates that convulsions induced by these two drugs have long-lasting effects on brain function which differ both in their anatomical distribution and in the nature of the effects produced. These drugs also differed in their acute effects at subconvulsant doses on the expression of cortically kindled seizures. Cocaine (and lidocaine, another local anesthetic) substantially elevated afterdischarge (AD) threshold and inhibited the focal component of the cortically kindled seizure. PTZ had no significant effect on either of these variables but significantly increased AD duration. In addition to these drug effects, a substantial inhibitory effect on seizure expression was observed, both during kindling and afterwards, when ADs were elicited daily but not when they were separated by 3 days or more. This finding suggests that the large number of ADs typically required for cortical kindling may be due in part to daily stimulation.

Combined treatment with muscarinic and nicotinic cholinergic antagonists slows development of kindled seizures

Rats were given daily injections of atropine, mecamylamine or both drugs in combination, 1 h prior to daily electrical amygdaloid kindling stimulation. Neither drug was effective alone, but the two drugs in combination significantly increased the latency to develop stage 5 kindled seizures.

Pharmacological and neurochemical aspects of kindling

Rats implanted with amygdaloid stimulating and cortical recording electrodes were kindled by daily low-intensity electrical stimulation. In one experiment amino acid concentrations were measured in amygdala, cortex and hippocampus at behavioural stages 1, 2 and 4 (Racine). Control groups consisted of unstimulated rats. Only alanine showed a significant enhancement of concentration in the kindled rats (stage 4 of Racine). In a second experiment, a group of rats was treated daily with 10mg/kg p.o. of diazepam. Diazepam significantly inhibited kindling and no changes in amino acid concentrations were observed in this group. Increased alanine levels are seen after various seizure types; since pentetrazole, isoniazid and beta-vinyllactic acid seizures were associated with alanine level increases only after and never before seizure occurrence, it is suggested that the alanine increases are a consequence rather than a cause of convulsions. In 3H-flunitrazepam binding studies, no change in affinity or receptor number could be demonstrated during kindling.

Chemical kindling by muscarinic amygdaloid stimulation in the rat

507 Holtzman rats received injections, through chemitrodes chronically implanted into the basolateral amygdala, of 0.2-1 microliter of sterile isotonic solution containing nanomolar quantities of cholinergic muscarinic agonists and/or antagonists. The bulk of the injected solution diffused only a short distance as judged by autoradiography. Once daily injections of 2.7 nmoles of carbamylcholine, an initially subconvulsive dose, kindled the progressive development of epileptic seizures similar to those seen in electrical amygdaloid kindling. This response was dependent on dose and on interstimulus interval, and once established persisted at least 8 weeks without further stimulation. Spontaneous seizures were observed in some fully kindled animals. No kindling-specific changes were seen by light microscopy. Muscarine (3 nmol) and the active (+), but not the inactive (-), isomer of acetyl-beta-methylcholine also kindled seizures. The action of (+)-acetyl-beta-methylcholine was potentiated by the cholinesterase inhibitor physostigmine. The muscarinic antagonists atropine and quinuclidinyl benzylate (QNB) blocked kindling by carbamylcholine or muscarine. Atropine, QBN and scopolamine greatly reduced agonist-induced seizures in previously kindled rats. Highly significant transfer effects were observed between muscarinic agonists, i.e. muscarine-kindled rats had widespread seizures on their first carbamylcholine exposure and vice versa. Kindled animals had a lowered seizure threshold for muscarinic agonists. Dibutyryl cyclic GMP produced seizures but no kindling. Those results demonstrate that in this model the stimulation of a group of muscarinic cholinergic synapses is both necessary and sufficient to induce a kindled state characterized by both evoked and spontaneous seizures, and support the view that epilepsy can be acquired and expressed transsynaptically.

A study of the role of the cholinergic system in amygdaloid kindling in rats

The effect of atropine on kindling the amygdala of rats was tested by administering the drug in a dose of 25 mg/kg 1 h before each stimulus was applied. Rats tested with atropine kindled at the same rate as saline-treated controls. Cholinergic activity in the amygdala of rats was assessed, 4 weeks after the completion of kindling, by measuring both muscarinic receptor numbers and sodium-dependent high affinity choline uptake in tissue homogenates. There was no change in either of these parameters attributable to kindling. These results suggest that changes in the cholinergic system are not fundamental either to the development or the maintenance of kindling in the rat amygdala.