L. Turski

Excitatory neurotransmission within substantia nigra pars reticulata regulates threshold for seizures produced by pilocarpine in rats: Effects of intranigral 2-amino-7-phosphonoheptanoate and n-methyl-d-aspartate

Seizures produced by pilocarpine given i.p. to rats provide an animal model for studying the initiation, spread and generalisation of convulsive activity within the forebrain. Pilocarpine, 380 mg/kg, produces a sequence of behavioural and electroencephalographic alterations indicative of motor limbic seizures and status epilepticus, which is followed by widespread damage to the limbic forebrain resembling that occurring subsequent to prolonged intractable seizures. Microinjections of a selective antagonist at the N-methyl-D-aspartate receptor, (+/-)-2-amino-7-phosphonoheptanoate, into the substantia nigra pars reticulata, bilaterally, protects against the behavioural, electrographic and morphological features of seizures produced by pilocarpine, 380 mg/kg, with an ED50 of 0.0007 mumol (0.0004-0.0011). Microinjections of (+/-)-2-amino-7-phosphonoheptanoate, 0.005 or 0.01 mumol, into the substantia nigra pars compacta or into the dorsal part of mid-anterior striatum do not modify the electrographic and morphological sequelae of pilocarpine, 380 mg/kg. In rats pretreated with microinjections of N-methyl-D-aspartate into the substantia nigra pars reticulata, a non-convulsive dose of pilocarpine, 100 mg/kg, results in recurrent motor limbic seizures and status epilepticus. The ED50 of N-methyl-D-aspartate for the generation of seizures after pilocarpine, 100 mg/kg, is 0.0014 mumol (0.001-0.0019). Electrographic monitoring shows a pattern and sequence of evolution of convulsant activity within the hippocampus and cortex similar to that produced with pilocarpine, 380 mg/kg, alone. Morphological examination of brains from rats treated with N-methyl-D-aspartate in the substantia nigra pars reticulata and subsequently given pilocarpine, 100 mg/kg, which underwent status epilepticus, reveals widespread damage to the amygdala, thalamus, olfactory cortex, substantia nigra, neocortex, and hippocampus. Microinjections of N-methyl-D-aspartate, 0.002 mumol, into either the substantia nigra pars compacta or dorsal striatum, bilaterally, do not augment seizures produced by pilocarpine, 100 mg/kg. The results indicate that the threshold for pilocarpine-induced seizures in rats is modulated by excitatory amino acid neurotransmission within the substantia nigra pars reticulata.

Injections of picrotoxin and bicuculline into the amygdaloid complex of the rat: An electroencephalographic, behavioural and morphological analysis

Bicuculline methiodide (0.5-3 nmol) and picrotoxin (0.5-4 nmol) were injected uni- or bilaterally into the rat amygdala and the resulting behavioural, electroencephalographic and morphological alterations were studied. In rats treated unilaterally with lowest doses of either bicuculline or picrotoxin (0.5 and 1 nmol) increase in the locomotor activity, occasional myoclonus of the hindlimbs and wet dog shakes were observed. At doses of 2-3 nmol, both gamma-aminobutyrate antagonists produced a sequence of repetitively occurring behavioural alterations including limbic gustatory automatisms, tremor and myoclonus of the forelimbs, head nodding and rearing, that developed over 15-30 min and built up progressively into the recurrent motor limbic seizures lasting for 1-6 h. In animals injected bilaterally with either bicuculline (0.5-3 nmol) or picrotoxin (0.5-3 nmol) motor limbic seizures rapidly developed into the status epilepticus lasting for several hours. Bicuculline and picrotoxin produced both ictal and interictal epileptiform activity in the electroencephalogram. A spectrum of electroencephalographic changes consisted of high voltage fast activity, slow and fast voltage spiking, paraoxysmal bursts and periods of postictal depression. The earliest electrographic alterations appeared in the amygdala and then rapidly spread to cortical areas. Electrographic seizures started 1-10 min after unilateral injections of large doses of bicuculline and pictrotoxin (2-4 nmol). Ictal periods lasted for 1-2 min, recurred every 5-10 min and were followed by periods of depression of the electrographic activity. Bilateral injections of large doses of both gamma-aminobutyrate antagonists (2-3 nmol) resulted in the status epilepticus. Morphological examination of frontal forebrain sections with light microscopy revealed a widespread damage to the amygdala, olfactory cortex, substantia nigra, thalamus, hippocampus and neocortex. Pretreatment of animals with diazepam prevented the build-up of convulsive activity and brain damage produced by bicuculline or picrotoxin. Muscimol retarded the appearance and shortened the duration of convulsive activity, but did not alter the sequence and intensity of seizures. The results indicate that gamma-aminobutyrate antagonists, bicuculline and picrotoxin when directly applied to the amygdala can elicit in rats motor limbic seizures, epileptic changes in the electroencephalogram indicative of repetitive limbic seizures, and status epilepticus accompanied by seizure-related brain damage.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of excitatory amino-acid antagonists on the anticonvulsant action of phenobarbital or diphenylhydanton in mice

The effects of L-glutamic acid diethyl ester (GDEE), D,L-alpha-aminoadipic acid (alpha-AA) and D,L-2-aminophosphonovaleric acid (APV) on the anticonvulsant action of phenobarbital and of diphenylhydantoin were studied in mice against electroconvulsions. Anticonvulsants were administered intraperitoneally 60 min and amino-acid antagonists 30 min before the test, by the same route. Neither GDEE (up to 400 mg/kg) nor alpha-AA (up to 100 mg/kg) were found to affect the seizure threshold whilst APV (100 and 200 mg/kg) raised the threshold moderately from 6.2 to 8.4 and 9.0 mA. APV and alpha-AA (up to 100 mg/kg) and GDEE (up to 400 mg/kg) did not affect the anticonvulsant potency of diphenylhydantoin. Only APV in the dose of 200 mg/kg potentiated the protective efficacy of this antiepileptic against maximal electroshock to a relatively low degree. The anticonvulsant action of phenobarbital was enhanced by APV (25-200 mg/kg) and alpha-AA in the dose of 50 but not in the dose of 100 mg/kg, GDEE being completely ineffective. These results suggest that the blockade of N-methyl-D-aspartic acid receptors by alpha-AA and APV is mainly responsible for the potentiation of the anticonvulsant activity of phenobarbital. The anticonvulsant effects of both antiepileptics do not seem to be related to the suppression by GDEE of events mediated by receptors for quisqualic acid.

Catalepsy after microinjection of haloperidol into the rat medial prefrontal cortex

SummaryTo investigate the behavioural role of mesocortical dopamine innervation we performed bilateral microinjections of haloperidol into various parts of the rat frontal cortex and into adjacent subcortical forebrain structures. Haloperidol (2.5 μg/ 0.5 μl) locally injected into the medial prefrontal cortex or into the rostral part of the neostriatum resulted in the development of catalepsy as measured in the bar test. In contrast, injections of haloperidol into the nucleus accumbens, more caudal parts of the neostriatum, anterior cingulate cortex, rostral and lateral parts of the prefrontal cortex and into the lateral ventricles failed to induce catalepsy. It is concluded that blockade of dopamine receptors located in the rostral neostriatum and in the medial prefrontal cortex contributes to the development of haloperidol induced catalepsy.

δ-Aminovaleric acid antagonizes the pharmacological actions of baclofen in the central nervous system

SummaryThe action of δ-aminovaleric acid (AVA) on the muscle relaxant properties of baclofen, a GABAB receptor agonist, was investigated in two experimental models: (1) the pathologically increased muscle tone of the gastrocnemius muscle in spastic mutant Han-Wistar rats and (2) the Hoffmann (H)-reflex recorded from plantar foot muscles after electrical stimulation of the tibial nerve in barbiturate (60 mg/kg) anaesthetized rats. In both paradigms coadministration of AVA (500 nmol/5 μl) antagonized the muscle relaxant action of intrathecally applied baclofen (0.2–2 nmol), but failed to affect the muscle relaxant effects of intrathecally injected muscimol (2–20 nmol). In contrast, coadministration of bicuculline (1 nmol) did block the muscle relaxant action of muscimol, but failed to alter the effects of baclofen. When administered alone, bicuculline (1 nmol), or AVA (500 nmol–2 μmol) were without intrinsic action in both paradigms. In an additional series of experiments we investigated the action of AVA on a supraspinal effect of baclofen. Coadministration of AVA (12.5 nmol/0.5 μl) in the ventromedial thalamic nucleus antagonized the catalepsy induced by baclofen (ED50 10 pmol/0.5 μl), as indicated by an increase in ED50 of baclofen by a factor of 4.835 and a parallel shift of the probit-log dosage regression line to the right. The parallel shift seems to be consistent with a competitive mechanism of action of AVA. This study presents evidence that AVA antagonizes central pharmacological actions of baclofen at both spinal and supraspinal sites without affecting the actions of a GABAA agonist, muscimol.

Convulsant action of morphine, [d-ala2, d-leu5]-enkephalin and naloxone in the rat amygdala: Electroencephalographic, morphological and behavioural sequelae

Morphine hydrochloride (25-200 nmol), [D-Ala2, D-Leu5]enkephalin (10-200 nmol) and naloxone hydrochloride (100-1000 nmol) were injected unilaterally into the rat amygdala and the following electrographic, behavioural and neuropathological responses were studied. Microinjections of low doses of morphine (25-50 nmol) resulted in behavioural alterations characterized by staring, gustatory automatisms and wet shakes, whereas higher doses additionally produced motor limbic seizures and status epilepticus. The first changes in the electroencephalogram appeared in the amygdala immediately after the administration of morphine and rapidly spread to hippocampal and cortical areas. Electrographic alterations consisted of high voltage fast activity, spiking, bursts of polyspiking, electrographic seizures and periods of postictal depression. Neuropathological analysis of frontal forebrain sections by means of light microscopy revealed widespread, seizure-related damage confined to amygdala, olfactory cortex, thalamus, hippocampal formation, neocortex and substantia nigra. Pretreatment of animals with naloxone, 2-20 mg/kg s.c., as well as simultaneous microinjection of the non-convulsant dose of naloxone, 100 nmol, with morphine, 100 nmol, into the amygdala failed to block the development of convulsant activity and seizure-related brain damage produced by the opiate. In contrast, diazepam, 10 mg/kg i.p., when administered prior to the microinjection of morphine into the amygdala, abolished the epileptogenic effects of the drug. [D-Ala2, D-Leu5]Enkephalin, 10-200 nmol, elicited electrographic and behavioural responses similar to those seen after low doses of morphine, when administered into the amygdala. High voltage fast activity, single spikes, bursts of polyspiking, electrographic seizures and periods of postictal depression were seen in the electroencephalogram, but no behavioural signs of motor limbic seizures could be detected. The only behavioural correlates of epileptiform electrographic activity were wet shakes, myoclonic head twiches and gustatory automatisms. The examination of frontal forebrain sections from rats receiving [D-Ala2, D-Leu5]enkephalin revealed no morphological changes. Pretreatment of rats with either naloxone, 2 mg/kg, or diazepam, 10 mg/kg, blocked the development of behavioural and electrographic sequelae of the peptide. Naloxone, 100-1000 nmol, when microinjected into the amygdala, produced electrographic, behavioural and morphological alterations resembling those seen after high doses of morphine.(ABSTRACT TRUNCATED AT 400 WORDS)

Distinct sites of functional interaction between dopamine, acetylcholine and γ-aminobutyrate within the neostriatum: An electromyographic study in rats

In order to study the functional interaction between dopamine, acetylcholine and gamma-aminobutyrate within the rat neostriatum, we investigated the effect of intrastriatal injection of different drugs acting on these transmitter systems on muscle tone measured as tonic activity in the electromyogram of the gastrocnemius muscle. Bilateral injection of haloperidol (500 ng) into the rostral neostriatum (rostral injection: A8920-9650(46] induced tonic activity in the electromyogram, whereas injection into the intermediate part (intermediate injection; A7020-7890(46] was ineffective. Muscimol (25 ng) induced tonic activity in the electromyogram, when injected into the intermediate part and not into the rostral part, while bethanechol (1 microgram) was effective when injected into either site. Haloperidol-induced tonic activity in the electromyogram was prevented by coadministration of apomorphine (500 ng) or scopolamine (1 microgram), but not of bicuculline (300 ng). Haloperidol-induced tonic activity in the electromyogram was also reduced by subsequent intermediate injection of scopolamine or bicuculline, while apomorphine was ineffective. Tonic activity in the electromyogram induced by rostral injection of bethanechol was prevented by coadministration of scopolamine, but not of apomorphine. Intermediate injection of scopolamine or bicuculline reduced the tonic activity in the electromyogram after rostral or intermediate injection of bethanechol. Tonic activity in the electromyogram induced by intermediate injection of muscimol was prevented by coadministration of bicuculline, but not of scopolamine. Rostral injection of apomorphine or scopolamine failed to alter the tonic activity in the electromyogram induced by intermediate injection of bethanechol or muscimol. These results point to the existence of: a functional interaction between dopamine and acetylcholine in the rostral neostriatum; a functional interaction between acetylcholine and gamma-aminobutyrate in the intermediate neostriatum, and a functional flow of information from the rostral to the intermediate neostriatum.

Effect of aminophylline on muscle relaxant action of diazepam and phenobarbitone in genetically spastic rats: Further evidence for a purinergic mechanism in the action of diazepam

The effect of aminophylline on the muscle relaxant action of both diazepam and phenobarbitone was studied in genetically spastic rats of the Han-Wistar strain which exhibit spontaneous tonic activity in the electromyogram of the gastrocnemius-soleus muscle. Both diazepam (0.8 and 4.0 mg/kg i.p.) and phenobarbitone (20 and 30 mg/kg i.p.) reduced the spontaneous activity measured in the electromyogram in a dose-related manner. Aminophylline (50 mg/kg i.p.), a methylxanthine with potent antagonistic activity of adenosine-mediated inhibition, partially reversed the muscle relaxant action of diazepam (4 mg/kg) but not that produced by phenobarbitone. The muscle relaxant effect of phenobarbitone (30 mg/kg) was antagonised by beta-carboline-3-carboxylic acid methylester (beta-CCM), 2 mg/kg i.p. The reversal of the muscle relaxant effect of phenobarbitone produced by beta-CCM was abolished by CGS 8216 (2-phenylpyrazolo-(4,3c)quinolin-3(5H)-one), 5 mg/kg i.p. Aminophylline altered neither the muscle relaxant effect of a low dose of diazepam (0.8 mg/kg) nor the reversal of the muscle relaxant effect of phenobarbitone produced by beta-CCM. These findings indicate that the interaction between diazepam and aminophylline does not involve competition for the benzodiazepine receptor and add further support to the suggestion that purinergic mechanisms may be engaged in the muscle relaxant action of diazepam.

Unusual interactions of excitatory amino acid receptor agonists: α- and β-kainate antagonize motor responses to N-methyl-d-aspartate in rodents

Abstract The α- and β-stereoisomers of kainate correspond sterically to the l - and d -isomers of glutamate. α-Kainate is a potent excitant at a specific membrane receptor site (kainate receptor). β-Kainate has been proposed as a functional N-methyl- d -aspartate antagonist in vivo . Because of the structural similarities between the α- and β-stereoisomers of kainate we have investigated the interactions of both compounds with N-methyl- d -aspartate-mediated excitation in two well established animal models for assessing the action of excitatory amino acids and their antagonists in vivo : (a) determination of Cd 50 (convulsant dose) for myoclonic seizures in mice and (b) electromyographic measurement of muscle tone in genetically spastic rats. We find that α-kainate and β-kainate produce myoclonic seizures in mice when given intracerebroventricularly and increase the muscle tone in genetically spastic rats when given intrathecally. α-Kainate is about 5000 times more potent than β-kainate as a convulsant and about 1000 times more active than β-kainate in increasing the muscle tone. The excitatory actions of α-kainate and of β -kainate are blocked by γ- d -glutamylaminomethylsulphonate, a preferential kainate/quisqualate antagonist, but not by ( ± )-2-amino-7-phosphonoheptanoate, a specific N-methyl- d -aspartate antagonist. Surprisingly, α-kainate and β-kainate antagonize the myoclonic seizures and the increase in muscle tone produced by N-methyl- d -aspartate and potentiate both the anticonvulsant and myorelaxant actions of ( ± )2-amino-7-phosphonoheptanoate. Quisqualate induces myoclonic seizures in mice after intracerebroventricular application and increases muscle tone in genetically spastic rats following intrathecal injection. Quisqualate potentiates the convulsant action of N-methyl- d -aspartate and does not enhance the anticonvulsant action of ( ± )2-amino-7-phosphonoheptanoateagainst N-methyl- d -aspartate. In genetically spastic rats, quisqualate neither antagonizes the increase in the muscle tone induced by N-methyl- d -aspartate nor enhances the myorelaxant action of ( ± )2-amino-7-phosphonoheptanoate. In conclusion, α-kainate and β-kainate behave in vivo as kainate agonists and as N-methyl- d -aspartate antagonists. Quisqualate does not display N-methyl- d -aspartate antagonist activity in vivo .

Neurotransmitters in the Basal Ganglia and Motor Thalamus: Their Role for the Regulation of Muscle Tone

Stimulated by Ehringer and Hornykiewicz’s (1960) fundamental finding of a highly decreased dopamine content in the neostriata of Parkinsonian patients there has been increasing interest in neurotransmitter mechanisms in the basal ganglia and their role in physiological and pathological processes. Aided by the advent of new neuroanatomical and neurochemical technigues and the use of modern electrophysiological and pharmacological methods, new concepts of basal ganglia function have emerged which, although still inadaeguate, allow a better integration of the experimental and clinical data of different disciplines involved in basal ganglia research.