Mapping loci for pentylenetetrazol-induced seizure susceptibility in mice.

DBA/2J (D2) and C57BL/6J (B6) mice exhibit differential sensitivity to seizures induced by various chemical and physical methods, with D2 mice being relatively sensitive and B6 mice relatively resistant. We conducted studies in mature D2, B6, F1, and F2 intercross mice to investigate behavioral seizure responses to pentylenetetrazol (PTZ) and to map the location of genes that influence this trait. Mice were injected with PTZ and observed for 45 min. Seizure parameters included latencies to focal clonus, generalized clonus, and maximal seizure. Latencies were used to calculate a seizure score that was used for quantitative mapping. F2 mice (n = 511) exhibited a wide range of latencies with two-thirds of the group expressing maximal seizure. Complementary statistical analyses identified loci on proximal (near D1Mit11) and distal chromosome 1 (near D1Mit17) as having the strongest and most significant effects in this model. Another locus of significant effect was detected on chromosome 5 (near D5Mit398). Suggestive evidence for additional PTZ seizure-related loci was detected on chromosomes 3, 4, and 6. Of the seizure-related loci identified in this study, those on chromosomes 1 (distal), 4, and 5 map close to loci previously identified in a similar F2 population tested with kainic acid. Results document that the complex genetic influences controlling seizure response in B6 and D2 mice are partially independent of the nature of the chemoconvulsant stimulus with a locus on distal chromosome 1 being of fundamental importance.

Extent and organization of opossum prefrontal cortex defined by anterograde and retrograde transport methods.

Prefrontal cortex is commonly defined as cortex which receives afferents from the thalamic mediodorsal nucleus (MD). The extent of opossum prefrontal cortex was mapped with anterograde and retrograde axonal transport methods. The prefrontal field was found to include not only cortex on the lateral convexity of the frontal lobe as reported in earlier studies, but, in addition, cortex within the rhinal fissure and cortex on the rostral medial wall of the hemisphere. The organization of the thalamic input to the medial wall was analyzed in some detail and compared with that of the rat. The reason for this emphasis stemmed from earlier observations which suggested that a lateral, nonolfactory segment of MD, prominent in the rat and other species, may not be present in opossum MD. In the rat, the lateral segment, which constitutes approximately one-third of MD, projects to a relatively large expanse of rostral medial cortex which is also projected upon by the anteromedial nucleus. The main projection field of the lateral one-third of opossum MD is to cortex on the lateral convexity of the frontal lobe which has no input from the anteromedial nucleus and has no counterpart in the rat. Only the most lateral edge of opossum MD projects to medial cortex, to a very small field, which is also projected upon by the anteromedial nucleus. In other respects, the organization of the rostral medial cortex is similar in rat and opossum. These results suggest that, rather than being absent, an equivalent of a nonolfactory segment may be present in opossum MD but is markedly reduced in size, compared to that in rat and other species.

Sources of olfactory inputs to opossum mediodorsal nucleus identified by horseradish peroxidase and autoradiographic methods.

Some sources of olfactory input to the opossum mediodorsal thalamic nucleus (MD) were identified by retrograde horseradish peroxidase and anterograde autoradiographic methods. One major source originated from the olfactory tubercle and a narrow strip of piriform cortex bordering the tubercle. The tubercle-MD projection exhibited a definite spatial organization and included all except the most medial part of MD. The fact that the projection reached the most lateral and ventral extent of MD abutting the intralaminar complex suggests that the entire opossum MD may correspond to only the medial, magnocellular division in the primate and that the equivalents of both the parvocellular and paralamellar divisions may be absent.

The prepiriform cortex in dementia of the Alzheimer type.

Histopathologic examination of the prepiriform cortex (PPC) in four documented cases of dementia of the Alzheimer type revealed increased numbers of neurofibrillary tangles and neuritic plaques. These findings indicate that some early- and late-onset cases of Alzheimer's disease may be associated with significant damage to the PPC, a relay center of the mammalian olfactory system that has extensive connections with structures concerned with cognitive and behavioral functions. Involvement of the PPC may be related to the complex neuropsychiatric manifestations observed in this disorder.

Homotaurine (3 aminopropanesulfonic acid; 3APS) protects from the convulsant and cytotoxic effect of systemically administered kainic acid.

Pretreatment of rats with homotaurine (3 aminopropanesulfonic acid; 3APS), a synthetic gamma-aminobutyric acid (GABA) analog, protected from the convulsant and cytotoxic action of systemically injected kainic acid (KA). Wet dog shaking (WDS) behavior was significantly reduced. Taurine, an inhibitory non-GABA-mimetic amino acid, and muscimol (another direct GABA-agonist) reduced the number of seizures and lesions in the brain but were less effective than homotaurine. Progabide (a GABA-agonist) did not modify kainic acid effects. The neurotoxicity of kainic acid could have been due to repetitive convulsive activity. Activation of GABA-mediated inhibition is an effective, but not the determinant means of preventing KA-induced abnormalities.

Ketamine activation of experimental corticoreticular epilepsy.

Generalized corticoreticular epilepsy was established in adult cats by parenteral penicillin, and electroencephalographic monitoring was carried out. Ketamine HCl was injected intravenously in doses of 2.5 to 20 mg per kilogram. If doses of penicillin were inadequate to establish typical spike-wave activity, ketamine induced the spike-wave pattern typical of much higher doses of penicillin. At doses of penicillin that established the spike-wave pattern, ketamine potentiated the spike-wave activity and sometimes induced spike-and-wave status. These findings suggest caution in the clinical use of ketamine in patients with corticoreticular epilepsy. Because analogous effects have been observed upon administration of GABA-mimetic agents, GABA systems may play a role in ketamine anesthesia and corticoreticular epilepsy. Precollicular brain transections failed to modify ketamine effects, excluding a possible influence of mesencephalic centers on the observed potentiation.

Acute cocaine-induced seizures: differential sensitivity of six inbred mouse strains.

Mature male and female mice from six inbred stains were tested for susceptibility to behavioral seizures induced by a single injection of cocaine. Cocaine was injected ip over a range of doses (50-100 mg/kg) and behavior was monitored for 20 minutes. Seizure end points included latency to forelimb or hindlimb clonus, latency to clonic running seizure and latency to jumping bouncing seizure. A range of strain specific sensitivities was documented with A/J and SJL mice being most sensitive and C57BL/6J most resistant. DBA/2J, BALB/cByJ and NZW/LacJ strains exhibited intermediate sensitivity. EEG recordings were made in SJL, A/J and C57BL/6J mice revealing a close correspondence between electrical activity and behavior. Additionally, levels of cocaine determined in hippocampus and cortex were not different between sensitive and resistant strains. Additional studies of these murine strains may be useful for investigating genetic influences on cocaine-induced seizures.

The THIP-induced model of bilateral synchronous spike and wave in rodents.

Intraperitoneal administration of 5-10 mg/kg of THIP (4,5,6,7 tetrahydroxyisoxazolo (4,5,c) pyridine 3-ol) induced a transient, reliable model of bilaterally synchronous spikes and waves in rats. The paroxysmal bursts elicited by THIP had similar topographical distribution to the spontaneously-occurring spike and wave discharges often observed in naive rats. The responsiveness to electrical stimulation of subcortical nuclei was different in the two models. Systemic administration of THIP provided a simple, reliable model of bilaterally synchronous spike and wave discharges that may involve the same cerebral structures as those involved in the generation of the spontaneous paroxysms. The administration of the drug increased the yield of paroxysmal responses and provided a stable and predictable 4 hr test situation that may easily be quantified.

Penicillin spikes in rats. Limitations of a simple model for the study of anticonvulsants.

Direct GABA agonists that suppress spikes induced by penicillin in cats failed to do so in rats. Phenytoin and large doses of THIP increased the rate of spiking activity of the penicillin focus. Only progabide caused marked, initial, short-lasting suppression and a modest reduction of frequency of spikes for 1 hr. Homotaurine (3APS) reduced the amplitude and changed the morphology of the contralateral "mirror" spike. Antagonism of penicillin-induced spikes in rats is considered to be an unsuitable parameter for the screening of anticonvulsant agents.

Effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on levels of glutathione in the extrapyramidal system of the mouse.

Treatment of mice with the proximate neurotoxin MPTP depletes striatal dopamine levels. Depletion of striatal dopamine and metabolites in MPTP-treated mice is accompanied by depletion of glutathione (GSH) in the substantia nigra (SN). Striatal GSH and nigral amino acid levels were not significantly affected by MPTP. Results suggest that GSH depletion in SN may represent an index of regional vulnerability to metabolic oxidative stress and also of selective susceptibility to the toxic effects of MPTP.

Transient seizure activity induced by acetylcarnitine.

Intracerebral injection of L-acetylcarnitine in rats induced interictal and ictal epileptic phenomena with immediate onset, lasting up to 4 h. Pretreatment with systemic atropine prevents all epileptiform phenomena. Local injection of muscimol and THIP abolish ictal events, but not single spikes. L-carnitine induced only ictal discharges with a latency of 40-90 min. Acetylcarnitine epileptogenic properties are probably related to muscarinic agonism. The transition from interictal to ictal events may involve failure of GABAergic mechanisms.

Genotyping microsatellite polymorphisms by agarose gel electrophoresis with ethidium bromide staining: application to quantitative trait loci analysis of seizure susceptibility in mice.

Agarose gel electrophoresis with ethidium bromide staining (AGE/EBS) is an efficient and reliable method for analyzing microsatellite polymorphisms. We report the use of AGE/EBS for analyzing DNA microsatellite polymorphisms in a preliminary quantitative trait loci (QTL) study of seizure susceptibility in which a candidate gene strategy was used to direct initial mapping efforts. F2 intercross progeny, derived from seizure-sensitive DBA/2J (D2) and seizure-resistant C57BL/6J (B6) inbred strains of mice, were tested for their sensitivity to the seizure-inducing effect of pentylenetetrazol (PTZ), a gamma-aminobutyric acid (GABA) receptor antagonist. A semi-automated method is described, in which DNA microsatellites were amplified by polymerase chain reaction (PCR) to yield products of 100-200 base pair (bp) in length. Alleles were separated on 3-6% MetaPhor agarose gels, stained with ethidium bromide, and visualized by ultraviolet (UV) illumination. Univariate analysis of genotype and phenotype data provides evidence for a seizure-related QTL on chromosome 5, near genes coding for the GABAA receptor subunits alpha 5 and gamma 3. Interestingly, this suggestive QTL derives from the more resistant B6 strain, but it nonetheless provides impetus for the characterization of possible strain differences in these two candidate genes. Overall, these results demonstrate that AGE/EBS can be useful for rapid screening of genomic regions of special interest in QTL mapping studies.

Strain differences in convulsive response to the excitotoxin kainic acid.

We describe a strain of rats (Wistar-Furth) that is highly susceptible to the neurotoxic effects of kainic acid (KA) and presents a reliable and quantifiable (with low within-group variability) animal model of status epilepticus. Wistar-Furth rats are more sensitive and demonstrate a less variable convulsant response than Sprague-Dawley and Long-Evans rats when tested for total time in seizure activity, latency to onset of first seizure, latency to status epilepticus, seizure severity scores, and percentage exhibiting behavioral seizures and status epilepticus. Results suggest that significant heterogeneity exists in the rodent population with regard to neuronal sensitivity to an excitotoxic amino acid and indicate that strain differences are an important consideration in studies using KA.

The epileptogenic action of 6-aminomethyl-3-methyl,1-4H-1,2,6-benzothiadiazine-1,1-diazide hydrochloride (TAG): non-specific versus specific antitaurine pathogenesis.

Cortical superfusion with 6-aminomethyl-3-methyl, 1-4H-1,2,6-benzothiadiazine-1,1-diazide hydrocholoride (TAG) at a concentration which selectively blocks taurine (Tau) action fails to modify electroencephalographic (EEG) activity, cortical neuronal firing and caudate-induced inhibition of cortical neuronal activity. Higher concentrations of TAG increase neuronal firing rate and eventually induce EEG interictal spikes that can be suppressed by topical gamma-aminobutyric acid (GABA), but not by glycine or beta-alanine. Topical Tau consistently enhances the epileptiform activity. It is concluded that specific blockade of Tau does not affect any of the physiological function under observation and that the epileptogenic effect of TAG is due to its GABA antagonistic action.

The distribution of olfactory input in the opossum mediodorsal nucleus.

Discharges of single cells in the thalamic mediodorsal nucleus (MD) of the opossum were recorded during electrical stimulation of the lateral olfactory tract. Responsive sites were histologically localized throughout the entire mediolateral extent of MD. In both rabbit and squirrel monkey responses are confined to the medial half of MD. Thus the lateral non-olfactory nuclear subdivision, common to both rabbit and squirrel monkey, was not found in the opossum. Firing patterns of cells were similar to those observed in rabbit and squirrel monkey. They commonly consisted of an early spike or burst of spikes, followed by a period of inactivity and then, in many cells, by a later period of response or of resumed spontaneous activity. The results indicate that olfactory input is characteristic of MD in a diverse sample of mammals but that topographic organization of the input is distinctly different in the opossum.

Cortical projections of the thalamic mediodorsal nucleus in the rabbit.

The cortical projection of the thalamic mediodorsal nuclear complex (MD) in the rabbit was mapped retrograde horseradish peroxidase and anterograde tritiated proline techniques. The projection field occupied the entire medial wall rostral to a mid corpus callosal level, wrapped around the frontal pole onto the lateral convexity and tailed off caudally on the dorsal bank of the rhinal sulcus. The projection of the lateral approximately one-half of MD, the half which does not receive olfactory input, was confined to medial cortex supply all but the most rostral region. This projection field of lateral MD was precisely organized in two dimensions with the most lateral part projecting most caudally and the most dorsal part projecting most ventrally. A representation for the third, anterior-posterior (A-P), dimension was not evident since any cortical point within the field was supplied by a cylinder of cells extending the entire A-P extent of lateral MD. The medial half of MD, which does receive olfactory input, projected to the remaining rostral medical cortex, the lateral convexity and rhinal sulcal region. The inverse dorsoventral relationship was partially preserved and on overlapping A-P gradient was present with sulcal projections originating more caudally in medial MD and the rostral medial projection originating more rostrally.

Partial characterization of kainic acid-induced striatal dopamine release using in vivo microdialysis.

The aim of this study was to characterize interactions between striatal kainate (KA) receptors and dopamine (DA) release using in vivo microdialysis. After insertion of a microdialysis probe and establishment of baseline DA release, each preparation was standardized with a pulse of an iso-osmotic solution of 100 mM KCl in Ringer's solution. DA release following pharmacological manipulation was compared to potassium-induced release and expressed as a percent value. In one group of animals, KA (12.5 mM in Ringer's solution) was administered via the microdialysis probe in 2, 3, 5 or 10 min pulses 30 min following standardization with potassium resulting in release of DA which was 15.7 +/- 3.9, 30.3 +/- 11.3, 67.5 +/- 15.0 and 92.9 +/- 19.8% of potassium-induced DA release, respectively. Perfusion of CdCl2 (0.6 mM in Ringer's solution) 30-45 min prior to a 10 min KA pulse significantly reduced KA-induced DA release compared to control values. Intrastriatal administration of kynurenate (Kyn) attenuated KA-induced DA release in a dose-dependent manner. Levels of DA metabolites in striatal perfusates were significantly reduced following KA administration. This effect was partially reversed by cadmium pretreatment but not affected by Kyn pretreatment. Findings of this study indicate that KA induces striatal DA release in a dose-dependent manner, and this effect is at least partially dependent upon activation of calcium channels. Results also indicate dose-dependent inhibition of KA-induced striatal DA release by the excitatory amino acid receptor antagonist, Kyn, suggesting that this compound interacts with striatal KA receptors and that these receptors are involved with modulating striatal DA release in vivo.

MPTP and convulsive responses in rodents.

Administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (30 mg/kg s.c. for 5 days) to mice resulted in complete abolishment of strychnine seizure and of the tonic phase of the maximal electroshock response. Bicuculline and picrotoxin convulsions were not significantly affected by MPTP treatment. The severity of the pentylenetetrazole seizures was mildly, but significantly affected in the protective way. MPTP depleted neostriatal dopamine and its metabolites, together with hippocampal norepinephrine. No nigral neuronal loss was detected histologically. Strychnine seizures and the tonic phase of the maximal electroshock response are thought to depend mostly on hindbrain (bulbo-spinal) structures. Thus, these experiments suggest that a caudally projecting system originates from the substantia nigra, pars compacta, and/or locus coeruleus, controlling seizures that involve bulbo-spinal centers. While neostriatal dopamine depletion offers a good index of seizure resistance, its role in the protection from seizures remains to be established.

In vivo modulation of excitatory amino acid receptors: microdialysis studies on N-methyl-D-aspartate-evoked striatal dopamine release and effects of antagonists.

Striatal dopamine (DA) release was measured following intrastriatal (i.s.) administration of N-methyl-D-aspartate (NMDA) to unanesthetized, freely-moving rats. One hour after insertion of a removable microdialysis probe and perfusion with normal Ringer's solution, a modified Ringer's solution containing 100 mM potassium (high-K+ Ringer's) was used to standardize the preparation. DA release following i.s. administration of NMDA (12.5 mM in normal Ringer's) was dose-dependent. When NMDA (12.5 mM) was administered in high-K+ Ringer's, DA release was greatly potentiated. Administration of the competitive NMDA receptor antagonist aminophosphonovalerate (APV) in normal Ringer's prior to treatment with NMDA in high-K+ Ringer's resulted in a significant reduction of DA release compared to control animals. In contrast, administration of APV priot to treatment with NMDA in normal Ringer's resulted in a significantly increased release of DA compared to controls. Administration of the non-competitive NMDA antagonist, dextromethorphan (DXT) prior to treatment with NMDA in normal Ringer's or NMDA in high-K+ Ringer's caused significant reductions of DA release compared to controls. Intrastriatal DXT also caused dose-dependent inhibition of high-K+ Ringer's-induced DA release. Similarly, administration of the non-specific calcium channel blocker, cadmium, prior to treatment with NMDA resulted in a significant decrease when compared to control values. Results of this study indicate that dose-dependent NMDA-induced striatal DA release is greatly potentiated by potassium suggesting that under physiological conditions in vivo, striatal NMDA receptors are mostly inactivated.(ABSTRACT TRUNCATED AT 250 WORDS)

Kainate and AMPA receptor binding in seizure-prone and seizure-resistant inbred mouse strains.

Glutamate and its receptors represent the major excitatory neurotransmission system in the mammalian brain and are considered important in the pathogenesis of many neurological diseases. The present study describes saturation binding experiments performed to measure the affinity (Kd) and density (Bmax) of kainate and AMPA receptors in striatum, cortex and hippocampus from mature DBA/2J (DBA) and C57BL/6J (C57) mice. Previous studies have documented that these two strains differ significantly in seizure susceptibility, with DBA mice exhibiting greater sensitivity in various convulsant tests compared to C57 mice. Non-linear regression analysis of binding data together with Student's t-test and ANOVA revealed significantly higher densities of kainate receptors in striatum and of AMPA receptors in cortex of DBA mice. C57 mice exhibited higher striatal [3H]AMPA binding. There were no significant differences between the mouse strains in binding sites prepared from hippocampus and no differences in affinity for either receptor in any brain region studied. The results support a role for kainate and AMPA receptors in seizure sensitivity, possibly by influencing glutamate transmission in specific pathways. It is unlikely, however, that these receptors account for the generation of seizures alone but rather cooperate with other glutamatergic and non-glutamatergic neurotransmitter systems.