Central monoamines and convulsine thresholds in mice and rats

Decrease in locus coeruleus [3H]idazoxan binding site density in genetically epilepsy-prone (GEPR) rats

Deficits in norepinephrine synthesis, transmitter level, turnover and reuptake have been reported in the brain of genetically epilepsy-prone (GEPR) rats. We investigated the hypothesis that these alterations may trigger a compensatory downregulation of locus coeruleus alpha 2-adrenergic receptors and an upregulation of postsynaptic alpha 2-adrenergic receptor density in forebrain regions of GEPR rat brain. alpha 2-adrenergic receptor density was measured in the locus coeruleus and 7 forebrain regions of control and GEPR rats by in vitro [3H]idazoxan autoradiography. Specific [3H]idazoxan binding site density was decreased significantly in the locus coeruleus of both GEPR-3 and GEPR-9 rats compared to controls. No significant differences in specific [3H]idazoxan binding were observed in the 7 forebrain regions of GEPR-9 rats compared to control. Reduced locus coeruleus alpha 2-adrenergic receptor density in GEPR rats may produce a net increase in locus coeruleus noradrenergic cell firing, an effect which could, in part, offset the impact of reduced noradrenergic influence in GEPR rat forebrain. Additionally, decreased norepinephrine levels in GEPR rat brain may be a long-term consequence of reduced alpha 2-adrenergic receptor-mediated inhibition of locus coeruleus firing activity.

Brain Na+/K(+)-ATPase regulation by serotonin and norepinephrine in normal and kindled rats

In this work we confirmed the activation of rat brain Na+/K(+)-ATPase by norepinephrine (NE) and observed a variable response of the enzyme according to the brain region considered. In isolated neuronal or glial fractions from normal cerebral cortices, we studied the response of the enzyme to increasing concentrations of serotonin (5-HT) (10(-9)-10(-3) M). A dose-dependent response over basal values was present in glial fractions, beginning at 10(-6) M. No such response was obtained in the neuronal fractions. In amygdaloid kindled brains, the pattern of activation by NE was different than in controls: less pronounced (cortex, brainstem, and diencephalon), inhibition-activation (cerebellum), or no change (striatum). The activation of Na+/K(+)-ATPase by 5-HT observed in the control glial fraction was not present in the kindled glial fraction. In conclusion, 5-HT seems to activate Na+/K(+)-ATPase preferentially in glial cells, and the kindling process markedly modifies this regulation. The normal response to NE in brain homogenates is less altered by kindling than is the response to 5-HT in the same regions.

Effects of pentylenetetrazol-induced seizures on dopamine and norepinephrine levels and on glucose utilization in various brain regions of the developing rat

Levels of dopamine and norepinephrine were measured in seven brain areas after 60 min of sustained seizure activity induced by intraperitoneal repetitive timed administrations of pentylenetetrazol in rats at 10, 14, 17 and 21 days of postnatal life. The tissue levels of norepinephrine were markedly reduced in the majority of brain structures, except for striatum at 10 and 14 days. Conversely, dopamine concentrations increased in many areas and at various ages, except in cerebral cortex at 10 and 14 days and in midbrain between 14 and 21 days. PTZ seizures induced marked increases over control levels in the rates of glucose utilization, measured by the quantitative autoradiographic [14C]2-deoxyglucose method, in all dopamine- and norepinephrine-innervated areas studied at 10 and 14 days, except in cerebellar cortex at both ages and in frontal cortex and anteroventral thalamus at 14 days. At 17 and 21 days, glucose utilization remained increased over control levels in some areas, mainly in catecholaminergic cell groupings such as substantia nigra, ventral tegmental area and locus coeruleus, but was significantly reduced in cortex, caudate nucleus and thalamus, and similar to control rates in other regions. The present results suggest that pentylenetetrazol-induced seizures lead to a simultaneous increase in functional activity of norepinephrine neurons and an inhibition of dopaminergic-mediated neurons. They also confirm the maturation of connections, of metabolic activity and of neurotransmitter interaction within the brain, occurring mainly during the third week of postnatal life, paralleled by an increased selective vulnerability of some regions to this kind of insult.

Role of serotonin receptor subtype in seizures kindled from the feline hippocampus

This study assessed the effects of a serotonin (5-HT)1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT2 agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and a 5-HT2 antagonist ketanserin on fully kindled seizures from the hippocampus. 8-OH-DPAT produced a marked suppression in hippocampal kindled seizures, while producing behavioral signs similar to those seen in the 5-HT syndrome. Although DOI and ketanserin did not affect kindled focal epileptic activity, DOI shortened and ketanserin prolonged the latency to onset of generalized convulsions. Our data suggest that 5-HT1A receptors play an inhibitory role in the generation of hippocampal seizures, whereas 5-HT2 receptors may participate in kindled seizure generalization from this region.

Dopaminergic regulation of epileptic activity

We evaluated the role of dopamine systems in the propagation of epileptic Focal, limbic seizures were produced by systemically administered pilocarpine (200 mg/kg, i.p.); as previously described this dose produces limbic stereotypes but neither convulsions nor seizure-related brain damage. The systemic pretreatment with D-1, but not D-2, agonists induced convulsions identical to those produced by a higher, convulsant dose of pilocarpine (400 mg/kg). Conversely, the pretreatment with D-1 receptor antagonists prevented the convulsions whereas the D-2 antagonists facilitated the pilocarpine-induced seizures. Furthermore, we studied the effects of intracerebral injections of dopamine agents on seizures induced by pilocarpine. Nigral microinjection of D-1 agonists strongly induced motor seizures in rats treated with the low dose of pilocarpine. On the other hand, microinjection of D-1 antagonists prevented the motor seizures induced by the high dose of pilocarpine. This study indicates that the two dopamine receptor subtypes, D-1 and D-2, exert opposing roles in the control of epilepsy propagation. Substantia nigra pars reticulata appears to be primarily involved in the dopamine-mediated modulation of seizures.

Anticonvulsant effects of ipsilateral but not contralateral microinjections of the dopamine D2 agonist LY 171555 into the nucleus accumbens of amygdala-kindled rats

Recent radioligand binding studies demonstrated an increase in the density of dopamine D2 receptors in the nucleus accumbens ipsilateral to the stimulating electrode in amygdala- or hippocampal-kindled rats. In the present study we examined the anticonvulsant effect of dopamine agonists by unilateral microinjections into the nucleus accumbens in rats kindled from the right basolateral amygdaloid nucleus. Microinjections of the D2 agonist LY 171555 into the ipsilateral nucleus accumbens 15 min prior to the kindling stimulation in fully kindled rats decreased significantly kindling parameters such as seizure severity, seizure duration and afterdischarge duration, whereas the D1 agonist SKF 38393 had no anticonvulsant effects. After ipsilateral microinjection of 40 pmol LY 171555 focal and generalized kindled seizures were totally blocked in almost 50% of the rats. The anticonvulsant effect of LY 171555 could be completely antagonised by systemic administration of the D2 antagonist sulpiride. Microinjection of the D1 or D2 agonist into the nucleus accumbens contralateral to the stimulating electrode had no anticonvulsant effects. In accordance with other reports our data indicate a possible topographic limitation of D2 receptor mediated anticonvulsant effects to specific regions of the basal ganglia.

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

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

Suppressive effects of L-5-hydroxytryptophan in a feline model of photosensitive epilepsy

We recently demonstrated that long-lasting photosensitivity is acquired as a result of kindling of the lateral geniculate nucleus (LGN), and that the LGN-kindled cat pretreated with D, L-allylglycine represents a useful model of epilepsy for drug studies. The present experiments studied anticonvulsant effects of a serotonin precursor, L-5-hydroxytryptophan (5-HTP), on photosensitivity in the LGN-kindled cat under D,L-allylglycine and on LGN-kindled seizures. 5-HTP suppressed both myoclonic responses and paroxysmal EEG discharges induced by photic stimulation in a dose-related manner. Photically-induced seizures were completely blocked 1.5-2 h after injection of 20 mg/kg 5-HTP. 5-HTP was also effective in reducing the afterdischarge duration and behavioral seizure stage in LGN-kindled seizures; following 40 mg/kg administration, no electroclinical seizures were elicited in the LGN-kindled cats. Serotonergic mechanisms may play an important role in epileptic photosensitivity; the 5-HTP suppressive effect on photosensitivity is at least partly due to reduced neuronal activity at the level of the LGN via serotonergic inhibition.

Prenatal nicotine exposure increased susceptibility to electroconvulsive shock (ECS) in adult rats

Female rats were treated during pregnancy (days 1-20) with nicotine (1 mg/kg SC, b.i.d.). When some aspects of maternal behavior and developmental parameters were recorded in mothers and pups, respectively, no changes were detected in the experimental group as compared with controls. However, adult offspring of nicotine-treated rats showed an increased susceptibility to ECS. These results demonstrate long-lasting deleterious effects induced by nicotine exposure during fetal life.

The role of technical, biological and pharmacological factors in the laboratory evaluation of anticonvulsant drugs. III. Pentylenetetrazole seizure models

Although seizure models using systemic administration of the chemoconvulsant pentylenetetrazol (PTZ) for induction of generalized clonic seizures in rodents are widely employed to identify potential anticonvulsants, the important role of diverse technical, biological and pharmacological factors in interpretation of results obtained with these models is often not recognized. The aim of this study was to delineate factors other than sex, age, diet, climate, and circadian rhythms, which are generally known. For this purpose, experiments with 8 clinically established antiepileptic drugs were undertaken in the following PTZ models: (1) the threshold for different types of PTZ seizures, i.e., initial myoclonic twitch, generalized clonus with loss of righting reflexes, and tonic backward extension of forelimbs (forelimb tonus), in mice; (2) the traditional PTZ seizure test with s.c. injection of the CD97 for generalized clonic seizures in mice; and (3) the s.c. PTZ seizure test in rats. In rats, in addition to evaluating drug effects on generalized clonic seizures, a ranking system was used to determine drug effects on other seizure types. When drugs were dissolved in vehicles which themselves did not exert effects on seizure susceptibility, the most important factors which influenced drug potencies were: (1) bishaped dose-response curves, i.e., a decline in anticonvulsant dose-response at high doses of some drugs, leading to misinterpretations of drug efficacy if only a single high drug dosage is tested; (2) effects of route of PTZ administration (i.v. infusion vs. s.c. injection) on estimation of anticonvulsant potency; (3) species differences in drug metabolism; (4) differences in drug potencies calculated on the basis of administered doses compared to potency calculations based on 'active' drug concentrations in plasma; (5) qualitative and quantitative species differences in drug actions; (6) endpoints used for PTZ tests; (7) misleading predictions from PTZ seizure models. Analysis of anticonvulsant drug actions indicated that myoclonic or clonic seizures induced by i.v. or s.c. PTZ might be suitable for predicting efficacy against myoclonic petit mal seizures in humans, but certainly not to predict efficacy against absence seizures. Tonic seizures induced by PTZ were blocked by drugs, such as ethosuximide, which exert no effect on tonic seizures in humans. In order to reduce the variability among estimates of anticonvulsant activity in PTZ seizure models, the various factors delineated in this study should be rigidly controlled in experimental situations involving assay of anticonvulsant agents.

Monoamine turnover in the brain of mice during development of tolerance to the anticonvulsant effect of clonazepam

Mice were treated for 14 days with clonazepam, 0.5 mg/kg i.p. twice daily, during which time partial tolerance to the anticonvulsant effect against pentetrazole developed. The development of tolerance was paralleled by a reduced turnover of noradrenaline in the whole brain, and of dopamine in the midbrain. The turnover of 5-HT was increased during the first week of treatment, but decreased thereafter. These changes in monoamine turnover, which are thought to be GABA-mediated, are consistent with an increased seizure susceptibility, and may contribute to the development of tolerance to the anticonvulsant effect of benzodiazepines.

Dopamine D1 and D2 receptors mediate opposite functions in seizures induced by lithium-pilocarpine

The effect of selective dopamine receptor blockade on epileptic activity was tested in rats, using the lithium-pilocarpine seizure model. One day after lithium pretreatment, systemic administration of the dopamine D1 antagonist, SCH 23390, prevented the convulsive activity induced by either 10 or 15 mg/kg of pilocarpine in a dose-dependent manner as revealed by behavioral and electroencephalographic alterations. No anticonvulsant effect was observed when SCH 23390 was injected at the same time as lithium and 24 h prior to pilocarpine. Furthermore, the D2 antagonists, raclopride and haloperidol, potently reduced the threshold for convulsions induced by 10 mg/kg of pilocarpine, following lithium pretreatment. Neither dopamine D1 nor D2 antagonists altered the limbic stereotypies induced by pilocarpine, supporting the view that the dopamine system is primarily involved in the mechanisms of convulsion generation and seizure spreading. These results indicate that dopamine receptor subtypes exert opposite functions on the regulation of convulsive activity.

Influence of antidepressant drugs on seizure susceptibility and the anticonvulsant activity of valproate in mice

The tricyclic antidepressants, amitriptyline (20-30 mg/kg, i.p.) and imipramine (30-40 mg/kg), provided a significant protection against electro-convulsions (12 mA, 0.2 s stimulus duration) but desipramine (up to 40 mg/kg) remained ineffective. On the other hand, all drugs, amitriptyline (10 mg/kg), desipramine (20 mg/kg), and imipramine (20 mg/kg) distinctly potentiated the protective efficacy of valproate against maximal electroshock, reducing its ED 50 values from 255 mg/kg to 150, 135, and 128 mg/kg, respectively. In one case the plasma valproate level was measured and it was evident that desipramine (20 mg/kg) did not affect the plasma level of this antiepileptic.

Nigral influence on focal epilepsy

The substantia nigra (SN) has been proposed as a structure involved in epileptiform phenomena. Previous investigations demonstrated that SN is able to elicit hippocampal rhythmic slow activity (RSA) as well as to inhibit hippocampal interictal spikes induced by parenteral administration of penicillin. The present series of experiments was carried out in order to characterize the action of SN on a focal model of hippocampal epilepsy. Experiments were performed on encéphale isolé cats in which steady epileptiform activity was induced by locally applied penicillin. Electrical stimulation of SN pars reticulata (pr) caused a statistically significant decrease of hippocampal spike frequency and amplitude in 30% of the total number of stimulation sessions. Stimulation of SN pars compacta (pc) was even more effective. It induced inhibitory effects on hippocampal spikes in 91% of the cases. In 30% of the cats, RSA was noted on hippocampal recordings in correspondence to nigral activation. Experimental data support the hypothesis that the SNpc influences hippocampal excitability: a differential role may be played by SNpc and SNpr in the control of seizure processes.

Opposite effects of stimulation of D1 and D2 dopamine receptors on the expression of motor seizures in mouse and rat

The ability of drugs, selective for dopamine D1 and D2 receptors, to influence the production of motor seizures was studied in mice and rats. Mice, which had been injected with reserpine (5 mg/kg) to deplete stores of monoamines in brain, could be made to convulse 24 hr later by injecting the D1 agonists, SKF 38393 (15-30 mg/kg) and CY 208-243 (0.3-3 mg/kg). The D2 agonists, lisuride (0.5-5 mg/kg) and RU 24213 (0.5-15 mg/kg) and the mixed D1/D2 agonist, apomorphine (0.05-0.5 mg/kg), had no effect on the seizure thresholds by themselves. However, the proconvulsant action of SKF 38393, 15 mg/kg, was prevented by the simultaneous injection of lisuride (5 mg/kg), RU 24213 (5 mg/kg) or apomorphine (0.5 mg/kg) and also by the selective D1 blocking drug, SCH 23390 (0.1 mg/kg). Rats were made to convulse by injecting the cholinergic agonist, pilocarpine (200-600 mg/kg) coupled with methyl scopolamine (1 mg/kg), to prevent peripheral autonomic effects. The smallest dose of pilocarpine (200 mg/kg) was subconvulsant, whereas the larger ones (400 and 600 mg/kg) dose-dependently induced tonic convulsions. The drug SKF 38393 (30 mg/kg) was found to be proconvulsant and caused seizures to develop in 100% of animals, at all dose levels of pilocarpine. This effect was blocked by SCH 23390 (0.25 mg/kg) which, by itself, reduced the severity and increased the latency of pilocarpine-induced convulsions, but not their frequency. The D2 agonist LY 171555 (0.5 mg/kg) was also anticonvulsant in this model and was antagonised by the D2 blocking drug metoclopramide (1.25 mg/kg), which was ineffective alone.

5-Hydroxyindoleacetic acid and homovanillic acid in cerebrospinal fluid of children with febrile convulsions

5-Hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA) were measured by high-performance liquid chromatography (HPLC) in lumbar cerebrospinal fluid (CSF) obtained from febrile children subdivided according to the presence or absence of convulsions. Lumbar puncture was made either early (mean time 2 h) or late (3-6 days) after the febrile convulsion. The level of 5-HIAA was significantly decreased in children early and late after the febrile convulsion as compared with the convulsion-free group, but the HVA level was reduced only early after the febrile convulsion. These results support the hypothesis that a decrease in CSF 5-HIAA may be a biologic marker of susceptibility to convulsions and indicate that the transient decrease in HVA is a secondary phenomenon related to occurrence of convulsions.

Effect of catecholaminergic drugs on quinolinate- and kynurenine-induced seizures in mice

Administration of reserpine, trifluperidol, chlorpromazine, haloperidol, spiroperidol, and thioproperazine to adult mice shortened the latency and increased the number of animals with clonic seizures induced by 1-kynurenine sulfate or its metabolite quinolinic acid. Haloperidol dose-dependently intensified kynurenine-induced seizures and did not alter pentylenetetrazole seizures. Dopamine abolished the effect of haloperidol while serotonin was ineffective. Pretreatment with 6-hydroxydopamine potentiated kynurenine-induced seizures, but not quinolinic acid-induced seizures. The seizure thresholds of kynurenine and quinolinic acid were not affected by pretreatments with yohimbine, clonidine, piperoxan, phentolamine and tricyclic antidepressants. Apomorphine and amphetamine (i.p.), noradrenaline and adrenaline (i.c.v.) possess anticonvulsant action against kynurenine and not against quinolinic acid. The data obtained suggest a similarity of kynurenine and known convulsants in the involvement of the catecholaminergic processes in their convulsant action. Quinolinic acid markedly differs from kynurenine in its mechanism of action as indicated by their interactions with numerous endogenous substances.

Dopamine D1 receptor modulation of pilocarpine-induced convulsions

The contribution of dopaminergic mechanisms to the generalization of epileptic activity was studied in rats given pilocarpine after pretreatment with selective dopamine agonists. At the dose of 200 mg/kg, pilocarpine produced limbic stereotypes but not convulsions or seizure-related brain damage. Pilocarpine, 200 mg/kg, following pretreatment with the D1 agonist (RS)-2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3 benzazepine, but not its (S)-enantiomer, induced convulsive activity as revealed by behavioral, electroencephalographic alterations and widespread brain damage. These features were identical to those produced by a higher, convulsant dose of pilocarpine (400 mg/kg). On the other hand, pretreatment with the D2 agonist 4,4a,5,6,7,8,8a,9-octahydro-5-n-propyl-2H-pyrazolo-3,4-g-quinoline failed to induce convulsions. Furthermore, the D1 receptor antagonist (R)-(+)-8-chloro-2,3,4,5-n-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine -7-ol prevented the convulsive activity induced by both 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3 benzazepine plus pilocarpine (200 mg/kg) and pilocarpine (400 mg/kg), given alone. However, neither dopamine agonists nor antagonists altered the limbic stereotypes induced by pilocarpine, suggesting a dopamine system involvement primarily in the mechanisms of epilepsy generalization. The results suggest that pharmacological manipulation of dopaminergic transmission may provide an alternative approach to therapy of secondarily generalized epilepsy.

The effects of perinatal hypoxia on pentylenetetrazol-induced seizures in developing rats

The effects of perinatal hypoxia on susceptibility to seizures due to a single dose (55 mg/kg, i.p.) of pentylenetetrazol (PTZ) were examined, in 15-, 20-, 30-, 60- and 90 to 120-day-old rats. The rats exposed to hypoxia at 10 days of age and the unexposed controls showed similar developmental changes in the types of seizures, ictal electro-encephalograms and severity scores, the last being lowest at 30 days of age. However, the susceptibility to seizures induced by PTZ, which was measured by the mean number of generalized convulsions (GCs) as well as a mean duration of the 1st GC and severity score, was more enhanced in the rats exposed to perinatal hypoxia than in the control group at every age. The present study suggests that hypoxia in rats results in greater seizure susceptibility throughout the developmental period.

A simple intravenous infusion method in rodents for determining the potency of anticonvulsants acting through GABAergic mechanisms

A simple method of intravenous infusion of convulsant drugs (pentetrazol and bicuculline) into the tail vein of rats has been used to determine seizure threshold and construct log-dose seizure threshold response curves for several anticonvulsant drugs (diazepam, phenobarbitone, pentobarbitone, chlormethiazole and valproate). It has been shown that an index of the dose required to increase seizure threshold by 50% (TI50) can be obtained using small numbers of animals. The advantages of this method over that of the subcutaneous pentetrazol method are several: small numbers of animals required, speed and reproducibility. The data also demonstrate the concordance between TI50 values obtained using pentetrazol and bicuculline as the convulsant agent. It is suggested that this method can be used routinely in screens of anticonvulsant drugs thought to work through GABAergic mechanisms.

Effects of long-term aldrin administration on seizure susceptibility of rats

The effects of long-term administration of sub-convulsive doses of aldrin, an organochlorinated pesticide, on seizure susceptibility of rats were studied. Results show that treatment with the pesticide made the rats more susceptible to sound-induced seizures; an increment in post-seizure depression time after maximal electroshock administration was also observed in aldrin-treated animals. Aldrin administration displaced the control dose-response curves constructed for amphetamine and pentylenetetrazol to the left and right, respectively; no differences were detected between control and aldrin-treated rats for strychnine, picrotoxin or 3-mercaptopropionic acid. The results, discussed in the light of the effects of aldrin on biogenic amines or on the electrophysiology of neurones suggested an involvement of both noradrenergic and alteration excitability of the CNS with the observed increments on seizure susceptibility.

Kainic acid-induced seizures: potentiation by alpha-methyl-p-tyrosine

We have investigated the influence of central noradrenergic and dopaminergic systems on the susceptibility of rats to seizures in the kainic acid (KA)-model of epilepsy. In the dose range of 0.75 to 10 mg/kg s.c., KA dose-dependently induced characteristic behavioural changes. Partial depletion of noradrenaline (NA) and dopamine (DA) in the brain by pretreatment with the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine (AMPT; 250 mg/kg, i.p.) markedly potentiated KA-induced epileptic symptoms. A low dose of KA (1.5 mg/kg s.c.), which was ineffective in normal rats, triggered in AMPT-pretreated rats a high incidence of wet dog shakes (WDS) and a seizure activity (seizure rating: 3.17 +/- 0.31) which was comparable in degree to that resulting from 10 mg/kg KA in rats with normal catecholamine synthesis (seizure rating: 3.33 +/- 0.28). In AMPT-pretreated rats a higher dose of KA (10 mg/kg) further enhanced seizure activity and was associated with a mortality rate of up to 80%. Within 6.5 h after AMPT-pretreatment the levels of NA and DA in amygdala/pyriform cortex declined from 0.56 +/- 0.02 (control) to 0.23 +/- 0.01 ng/mg tissue and from 0.21 +/- 0.03 to 0.05 +/- 0.01 ng/mg tissue, respectively. At a dose of 1.5 mg/kg KA was ineffective on the levels of NA and DA in normal rats, but further reduced these levels in AMPT-pretreated rats to 0.08 +/- 0.02 and 0.020 +/- 0.004 ng/mg tissue, respectively. Induction of seizure activity and decline in NA and DA levels in amygdala/pyriform cortex after AMPT/KA (1.5 mg/kg) treatment was antagonized by the alpha-adrenoceptor agonist clonidine (0.1 mg/kg, i.p.).(ABSTRACT TRUNCATED AT 250 WORDS)

Role of ion channels and intraterminal calcium homeostasis in the action of deltamethrin at presynaptic nerve terminals

Using a continuous perfusion system, synaptosomes prepared from rat brain released [3H]norepinephrine in a Ca2+-dependent manner when pulse depolarized by briefly elevating external potassium concentrations. Tetrodotoxin (10(-7) M), a sodium channel blocker, inhibited 48% of this pulsed release, and D595 (10(-5) M), a phenethylamine-type calcium channel blocker, inhibited 21%. In combination, these two specific ion channel antagonists appear to function independently of each other in an additive fashion. Addition of deltamethrin to this preparation resulted in an enhanced release of [3H]norepinephrine which occurred in a biphasic fashion. At 10(-7) M, deltamethrin produced a 42% enhancement in the first or initial peak of [3H]norepinephrine release and a 100% enhancement in the second or tailing peak. Addition of deltamethrin to tetrodotoxin-pretreated synaptosomes resulted in a net 37% enhancement of the initial peak release and a net increase of 277% in the tailing peak. Addition of deltamethrin to D595-pretreated synaptosomes produced no significant effect on enhanced [3H]norepinephrine release from either peak. Since tetrodotoxin is a specific sodium channel blocker, deltamethrin may be enhancing [3H]norepinephrine release by increasing the uptake of Ca2 via other voltage-gated channels (e.g. calcium) or exchange mechanisms in addition to its action at voltage-gated sodium channels. To determine whether deltamethrin may also have an effect on intraterminal Ca2+ homeostasis, external Ca2+ was replaced with Ba2+ and synaptosomes were depolarized with pentylenetetrazole (PTZ). At 10(-5) M, deltamethrin produced a 66% increase in neurotransmitter release over that produced by PTZ alone. An estimated EC50 value of deltamethrin for PTZ-induced release was calculated to be 2.4 x 10(-10) M.

Neuromodulatory role of serotonin in the anticonvulsant activity of 2-phenylbenzoate of 3-diethylamino-1-propanol.HCl (JAW-669)

The role of serotonin in the mediation of the anticonvulsant activity of JAW-669 was investigated against maximal electric shock (MES)-induced seizures in mice. A dose-dependent protection against seizures was provided by JAW-669 (4, 6 and 8 mg/kg, IP) and the calculated ED50 value was 6.01 mg/kg, IP. Pretreatment of mice with 5-hydroxytryptophan (50 mg/kg, IP) 2 hr before the administration of JAW-669 (6.01 mg/kg, IP) was found to cause a 40% increase in the ability of JAW-669 to provide protection against MES-induced seizures. Similar pretreatment with tryptophan (100 mg/kg, IP, 1 hr) caused a 30% decrease in the anticonvulsant activity of JAW-669. Prior administration of p-chlorophenylalanine (300 mg/kg, IP, 48 hr) and methysergide (10 mg/kg, IP; 0.5 hr) before administration of JAW-699 caused a 66% and 74% decrease, respectively, in the ability of JAW-669 to provide protection against MES-induced seizures. These results suggest a facilitatory role of serotonin in the anticonvulsant activity of JAW-669.

Newborn humans and rats malnourished in utero: free plasma L-tryptophan, neutral amino acids and brain serotonin synthesis

In the present study we report results concerning 5-hydroxytryptamine (5-HT) metabolism in two groups of small for date (SFD) human babies (gestational age 36 and 3 weeks), who suffered intrauterine nutritional restriction. A complementary study in the brain of rat fetuses with two types of intrauterine deprivation, in which brain L-tryptophan (L-Trp), tryptophan-5-hydroxylase (T5-H) activity and 5-HT content were determined on days 17, 19 and 21 of gestation. The same parameters studied prenatally were followed in both species during the immediate postnatal period. In the SFD babies the results were: (a) the free fraction of plasma L-Trp was significantly elevated; (b) plasma neutral amino acids were not substantially modified; (c) the bound fraction of L-Trp and plasma proteins were significantly low, as compared to controls. In the fetal brain of intrauterine malnourished rats, L-Trp, activity of T5-H and 5-HT content, were significantly elevated, since day 17, as related to normal littermates. These alterations in 5-HT metabolism persisted during the early postnatal period in both species. Elevation of the free fraction of plasma L-Trp in early malnourished SFD human babies suggest an increased transport of this amino acid to the brain with a possible enhancement of serotonin synthesis, during a critical period of brain differentiation.

The effect of amygdala kindled seizures on locus coeruleus activity

Locus coeruleus (LC) neuronal activity was recorded in anesthetized and in awake behaving rats during the production of amygdala afterdischarges (AD's) using kindling protocol. Both LC multiple and single unit discharges were temporally correlated with the appearance of AD's in the amygdala. Seizures were manifested in single unit activity as significant increases in firing rate. In awake animals, a bursting pattern of discharges was observed. We postulate that the observed changes in the LC activity pattern may have a modulatory role in the development of kindled seizures.

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.

Serotonin-dependent (Na+,K+)ATPase in kindled rats: a study in various brain regions

Serotonin (5-HT) modulation of brain (Na+,K+)ATPase, has recently been proposed. Activation curves of the enzyme activity dependent on 5-HT concentration have previously been observed in various brain regions of normal rats. In the present study, we report the absence of 'normal' (Na+,K+)ATPase response to 5-HT in cerebral cortex, striatum and diencephalon of kindled rats.

Noradrenaline and 5-hydroxytryptamine release in the hippocampus during seizures induced by hippocampal kindling stimulation: an in vivo microdialysis study

The intracerebral microdialysis technique has been used to monitor extracellular levels of noradrenaline, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid in the rat hippocampus in vivo in response to focal and generalized seizures induced by hippocampal kindling stimulation. In fully kindled animals a stimulus-induced generalized seizure gave rise to a three-fold increase of noradrenaline levels in the stimulated hippocampus as compared to baseline levels (15-min samples). The maximal increase of noradrenaline levels occurred within the first minutes after onset of seizure activity, as assessed in 2-min sample fractions with the noradrenaline uptake blocker desipramine added to the perfusion medium. After the peak increase, the noradrenaline levels tapered off, reaching baseline after 8-10 min. In 6-hydroxydopamine-treated animals, baseline noradrenaline levels were markedly reduced and there was no significant increase in noradrenaline release in response to a generalized seizure. These data support the hypothesis that the high extracellular levels of noradrenaline measured in seizures are of neuronal origin. There were no significant changes in extracellular 5-hydroxytryptamine or 5-hydroxyindoleacetic acid levels after a generalized seizure. In non-kindled animals the steady state noradrenaline levels during uptake blockade were two-three times higher than in the kindled rats. However, the peak noradrenaline levels measured in both hippocampi after the first two electrical kindling stimulations giving rise to focal epileptiform activity (afterdischarge) were similar to those observed in the kindled animals in response to generalized seizures. The increase of noradrenaline release in the non-kindled animals was significantly correlated to the duration of afterdischarge. In conclusion, the present study demonstrates the usefulness of the intracerebral dialysis technique for monitoring noradrenaline, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid release during seizures. The results indicate that both focal and generalized hippocampal seizures evoked by electrical kindling stimulation lead to a marked increase of transmitter release from noradrenergic but not from serotonergic neurons in the hippocampus. The ability of the noradrenergic system to respond by increased transmitter release to epileptic seizures is thus retained also in the kindled state.

A function for REM sleep: regulation of noradrenergic receptor sensitivity

We hypothesize that REM sleep serves to upregulate and/or prevent downregulation of brain norepinephrine (NE) receptors. This hypothesis is based on the following observations: (1) NE neurons of the locus coeruleus (LC) are tonically active in waking and non-REM sleep, but the entire population of LC NE neurons is inactive during REM sleep. (2) Continuous presence of NE or adrenoceptor agonists downregulates NE receptors, while a reduction in NE availability upregulates these receptors. (3) The effects of REM sleep deprivation are similar to those of NE receptor downregulation. Recent biochemical studies of NE receptor sensitivity provide strong experimental support for this hypothesis. The functional consequence of enhanced NE receptor 'tone' brought about by REM sleep would be improved signal processing in diverse brain systems, thus endowing the organism with a selective advantage. This hypothesis makes a number of specific predictions which can be tested with currently available techniques, and suggests new ways of understanding the evolution and postnatal development of REM sleep.

Chronic treatment with diazepam or the inverse benzodiazepine receptor agonist FG 7142 causes differential changes in the effects of GABA receptor stimulation

Daily treatment of mice with diazepam leads to the development of tolerance to the anxiolytic and anticonvulsant effect of the benzodiazepine, while daily treatment with the proconvulsant benzodiazepine receptor inverse agonist FG 7142 produces sensitization to its effects in that seizures develop (chemical kindling). In the present study, the effects of GABA receptor stimulation were studied 2 days after termination of 13 days treatment with diazepam, 20 mg/kg i.p./day, and FG 7142, 40 mg/kg i.p./day. For GABA receptor stimulation, the GABA agonist progabide was chosen because among several GABA receptor stimulants tested it was the only compound that induced increases in seizure threshold in non-toxic doses. Using the threshold for maximal (tonic extension) electroconvulsions as a measure for anticonvulsant efficacy, the anticonvulsant effect of progabide (100 mg/kg i.p.) was unchanged after chronic treatment with diazepam but was lost in FG 7142 kindled animals. Conversely, the hypothermic effect of progabide was reduced after treatment with diazepam but not with FG 7142. Baseline seizure threshold was unchanged 2 days after chronic administration of diazepam but increased in the FG 7142 pretreated mice. The data indicate that tolerance to benzodiazepines and kindling by FG 7142 are associated with different changes in GABA receptor function.

A proconvulsant action of selective alpha 2-adrenoceptor antagonists

The effects of alpha 2-adrenoceptor agonists and antagonists were examined on seizure thresholds determined by intravenous infusions of convulsants in rodents. alpha 2-Adrenoceptor antagonists were proconvulsant; dose dependently reducing the threshold for pentylenetetrazol- or bicuculline-induced tonic seizures. Strychnine-induced tonic seizures were unaffected. The alpha 2-adrenoceptor agonists clonidine, BHT-933 and UK 14,304 did not modify pentylenetetazol-induced seizures at low or moderate doses but at high doses clonidine and BHT-933 were proconvulsant. The facilitatory effect of alpha 2-adrenoceptor antagonists on pentylenetetrazol-induced tonic seizures was blocked by clonidine or UK 14,304. The proconvulsant action of alpha 2-adrenoceptor antagonists was contrasted with that of ethyl-beta-carboline-3-carboxylate, a benzodiazepine receptor contragonist, which markedly reduced the threshold for seizure initiation rather than the tonic seizure threshold. The selective facilitatory action of alpha 2-adrenoceptor blockade on tonic seizures suggests that a noradrenergic mechanism is involved in the control of seizure propagation rather than seizure initiation.

Effect of mu- and kappa-opioid agonists on the electroconvulsive seizure threshold in mice and antagonism by naloxone and MR 2266

The effects of mu-agonists (morphine, fentanyl) and kappa-agonists (U-50,488, U-69,593, bremazocine, nalbuphine, tifluadom) on the electroconvulsive threshold were studied in mice. The threshold could be significantly elevated by all drugs tested in a dose range that was in the order of magnitude of the antinociceptive ED50. Mice tolerant to the antielectroshock effect of morphine still reacted to U-69,593. The antagonism of the anticonvulsant effect by the mu-antagonist naloxone and the kappa-antagonist MR 2266 was receptor-specific only with fentanyl and U-50,488. The other opioid agonists were either antagonized by both drugs (morphine, U-69,593, bremazocine, nalbuphine) or even by the opposite antagonist (tifluadom). A synergistic effect of mu- and kappa-stimulation is assumed for the mediation of the antielectroshock effect of opioid drugs, but drugs with high affinity and intrinsic activity at one receptor type (fentanyl, U-50,488) are obviously able to bring about their antielectroshock effect through the one respective opioid binding site.

A possible role for spinal noradrenaline in the mechanisms of 6-hydroxydopamine against pentylenetetrazol induced convulsions in rats

The threshold of the generalized clonic convulsions induced by intravenous infusion of pentylenetetrazol (PTZ) was significantly increased by the intraperitoneal administration of noradrenaline (NA) neurotoxin, 6-hydroxydopamine, which produced no changes in the levels of catecholamines in discrete areas of rat brain, but the effect was accompanied by spinal depletion of NA. Moreover, the anticonvulsant effects of phenobarbitone (PB) and diphenylhydantoin (DPH) against PTZ convulsions were also significantly increased in the animals pretreated with 6-OHDA. These results suggest that the observed elevation of PTZ convulsive threshold and the potentiation of anticonvulsant activity of PB and DPH in 6-OHDA treated rats were possibly mediated through spinal cord depletion of NA.

A grand mal seizure associated with desipramine and haloperidol

Authors describe a case of grand mal seizure associated with administration of desipramine and haloperidol. Neuroleptic induced inhibition of desipramine metabolism is discussed as a possible explanation of unusually high desipramine plasma level of 610 ng/ml at the day of seizure.

Increased antinociception by alpha-adrenoceptor drugs after spinal cord noradrenaline depletion

Animals depleted of the bulbospinal NA fiber tracts have been reported to be supersensitive to antinociceptive effects of intrathecally administered noradrenaline (NA) in vivo. In the present investigation, the antinociceptive effects were determined after systemic or intrathecal injections of noradrenergic agents. NA and the selective alpha 2-adrenoceptor agonists guanfacine and clonidine were used. NA depletion was performed by treatment neonatally with 6-hydroxydopamine (6-OHDA), or in adult animals by intrathecal 6-OHDA administration or systemic N-2-chloroethyl-N-ethyl-2-bromobenzylamine hydrochloride (DSP4). The neurotoxins were found to cause a severe depletion of spinal NA without affecting dopamine (DA) or 5-hydroxytryptamine (5-HT) levels. The antinociceptive effects of intrathecal injection of NA, clonidine and guanfacine were more strongly enhanced in the depleted than in the control rats. It was also found that clonidine and guanfacine given systemically had a stronger effect in depleted than in control animals. In conclusion, depletion of descending NA pathways induces functional supersensitivity both to intrathecally administered NA and to the selective alpha 2-adrenoceptor agonists clonidine and guanfacine. It was also found that systemically administered clonidine and guanfacine had a stronger effect in NA-depleted than in control animals.

Comparison of drugs with different selectivity for central alpha 1-and alpha 2-adrenoceptors in animal models of epilepsy

The effects of two drugs which differ in selectivity for central alpha 1- and alpha 2-adrenoceptors were compared in different animal models of epilepsy. Clonidine, a selective alpha2-adrenoceptor agonist, up to 0.5 mg/kg i.p. was inactive against electroconvulsions in mice, but decreased the threshold for electroconvulsions in rats, whereas it exerted anticonvulsant effects against seizures induced by pentylenetetrazol in mice, amygdala kindling in rats and air blast stimulation in seizure-sensitive gerbils. In gerbils, the anticonvulsant effect of clonidine was counteracted by pretreatment with the alpha2-antagonist yohimbine (2.5 mg/kg i.p.), but not by the alpha 1-selective antagonist corynanthine (10 mg/kg i.p.). St 587 [2-(2-chloro-5-trifluoromethylphenylimino)imidazolidine], a highly alpha 1-selective agonist which easily penetrates into the brain, up to 20 mg/kg i.p. exerted no effects on the thresholds for electroshock and pentylenetetrazol-induced seizures in mice and rats, but displayed significant anticonvulsant efficacy in kindled rats and epileptic gerbils. In gerbils, corynanthine but not yohimbine antagonized the anticonvulsant effect of St 587. The data indicate that, at least in certain seizure models, anticonvulsant effects can be reached via stimulation of both alpha 1- and alpha 2-adrenoceptors.

Interactions between morphine-like analgesics and anticonvulsant drugs

The effect of pretreatment with morphine-like analgesics on the anticonvulsant effect of phenytoin and phenobarbital against electroconvulsions and of phenobarbital and ethosuximide against pentetrazole-induced convulsions was studied in mice. High doses of morphine (4 mg/kg subcutaneously), fentanyl (100 micrograms/kg subcutaneously), pethidine and pentazocine (4 and 8 mg/kg subcutaneously) increased the anti-electroshock effect of phenytoin; that of phenobarbital was likewise increased by morphine (2 and 4 mg/kg), fentanyl (25-100 micrograms/kg) and pentazocine (2-8 mg/kg), whereas pethidine had no corresponding effect. There was no consistent effect of pretreatment with the analgesics studied on the anticonvulsant effects of phenobarbital and ethosuximide in the pentetrazole seizure threshold test.

Endogenous opioids and post-ictal increase in seizure threshold in Mongolian gerbils

In gerbils, the convulsive thresholds both for electroconvulsions and for pentetrazole-induced convulsions were increased when determined 15 min after a convulsion elicited by an air blast to the back of the animals. Contrary to a recently offered hypothesis, this sign of post-ictal depression could not be reversed by pretreatment with the morphine antagonists naloxone and naltrexone. This result speaks against a mediation of post-ictal depression by endogenous opioids.

Alterations in brain catecholamines during pregnancy

During pregnancy mice are more susceptible to flurothyl-induced seizures than are non-pregnant controls. The potential role of brain catecholamines in mediating this behavior was examined in the present study. The concentration and turnover of norepinephrine (NE) and dopamine (DA) were measured in hippocampus, striatum, midbrain and cortex in control, pregnant and delivery-day mice. There were no significant changes from control in DA levels during pregnancy and parturition. The turnover of DA was not altered during pregnancy, except for a small increase in turnover rate in the hippocampus. The concentration of NE decreased during pregnancy, and rose at parturition. This effect was most striking in the hippocampus. The turnover of NE was markedly depressed during pregnancy, with the hippocampus again being most affected. These data imply a role for NE, but not DA in the mediation of increased seizure susceptibility during pregnancy.

A single dose of FG 7142 causes long-term increases in mouse cortical beta-adrenoceptors

There is evidence that central monoamines are involved in the actions of benzodiazepines. We have investigated the effects of a single dose of the benzodiazepine partial inverse agonist, FG 7142, on radioligand binding to alpha 2- and beta-adrenoceptors in mouse cerebral cortex. We found that seven days after a single injection of FG 7142 there was a large increase in the density of beta-adrenoceptors. This rise was not detectable either 15-30 min, or 24 h after the injection and no statistically significant changes in alpha 2-adrenoceptor binding were found at any of these times. Administration of the benzodiazepine antagonist Ro 15-1788 at the same time as FG 7142 prevented the rise in beta-adrenoceptor density. We discuss the possibility that the beta-adrenoceptor upregulation is related to the behavioural effects of FG 7142.

Noradrenergic sub-sensitivity in the cerebral cortex of the tottering mouse, a spontaneously epileptic mutant

The glycogenolytic action of norepinephrine (NE) was examined in the tottering mouse, a spontaneously epileptic mutant which presents a noradrenergic hyperinnervation of various CNS areas, including the cerebral cortex. The potency and efficacy of NE in promoting glycogenolysis were markedly decreased in cerebral cortical slices prepared from homozygous tottering (tg/tg) when compared to control C57BL/6j (+/+) mice, indicating a sub-sensitive response to a cellular action of NE. The metabolic nature of this adaptive change suggests that an impaired capacity of NE in mobilizing energy substrates may be related to the expression of the epileptic symptomatology in this mutant.

Differential effects of alpha-methyl-p-tyrosine and 6-hydroxydopamine on pentylenetetrazol seizures in mice

An intraperitoneal injection of the tyrosine hydroxylase inhibitor, alpha-methyl-p-tyrosine, did not alter the incidence of seizures induced by pentylenetetrazol, but increased the severity and duration of the tonic and clonic phases which resulted in death of some animals. By contrast, pentylenetetrazol seizures' characteristics were significantly changed in response to the intraperitoneal administration of the norepinephrine antagonist, 6-hydroxydopamine, by abolishing the tonic and clonic phases of the seizure. Moreover, alpha-methyl-p-tyrosine slightly attenuated the protective effect of 6-hydroxydopamine against pentylenetetrazol-induced seizures. Neurochemically, alpha-methyl-p-tyrosine significantly lowered the brain contents of both norepinephrine and dopamine while 6-hydroxydopamine caused no changes in the brain contents of these amines.

Determination of neurotransmitter release into the caudate nucleus during convulsions induced by pentylenetetrazole using in vivo differential pulse voltammetry

In vivo differential pulse voltammetry using an electrochemically treated carbon fiber electrode was applied to the investigation of the in vivo release of indoleamine and catecholamine within the caudate nucleus of freely moving and immobilized rats during convulsions induced by pentylenetetrazole (PTZ). Two distinct oxidation peaks, on at 130 mV (3,4-dihydroxyphenylacetic acid (DOPAC] and the other at 300 mV (5-hydroxyindoleacetic acid), were observed in voltammograms obtained from the caudate nucleus. In the caudate nucleus of freely moving rats, the in vivo oxidation current that peaked at 300 mV showed almost no change during and after tonic convulsions induced by 60 mg/kg of PTZ i.p. During tonic convulsions, the amplitude of the DOPAC oxidation peak significantly decreased to 75% of the peak height recorded prior to the injection of PTZ, and the minimum lasted for about 30 min; then the mean curve slowly recovered to the control level within 60 min. These results suggest that the release of dopamine (DA) in the caudate nucleus of freely moving rats decreased during tonic convulsions induced by PTZ. In another experiment, the EEGs of immobilized rats were recorded simultaneously, and the changes in the EEG pattern were used as an index of convulsion. In voltammograms from the caudate nucleus of immobilized rat, the peak height of the 130-mV oxidation potential significantly increased during ictal seizures. The increase lasted for 3-6 min after the ictal seizures. The severe electrical activity of the brain during ictal seizures interfered with the recording of some voltammograms.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological assessment of monoamine synaptic function in neuronal circuits of seizure susceptible brains

The following report reviews evidence suggesting a role for the monoamines, norepinephrine (NE) and serotonin (5-HT), in the pathophysiology of seizure disorders and outlines a strategy for electrophysiologically evaluating monoaminergic function at the synaptic level in central neuronal circuits of two animal models of epilepsy.

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

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