Baclofen as an anticonvulsant in experimental models of convulsions

The anticonvulsant activity of baclofen, a putative γ-aminobutyric acid agonist, was determined in four different models of experimentally induced convulsions. These convulsions were induced by (i) transcorneally administered electroconvulsive shock, (ii) pentylenetetrazol administration, (iii) ringing sound presented to animals during barbital abstinence syndrome, and (iv) administration of 3-mercaptopropionic acid. The results showed the drug to be active as an anticonvulsant whether convulsions were induced by electroshock, 3-mercaptopropionic acid, or auditory stimulation. No effect was seen on convulsions induced by pentylenetetrazol administration. The participation of mechanisms involving the neurotransmitter γ-aminobutyric acid is suggested, and possible mechanisms of baclofen action are discussed.     

Locomotor and convulsive responses to picrotoxin in amygdala-kindled rats

Amygdala-kindled rats and handled controls were tested for differences in locomotion and convulsive activity produced by picrotoxin, a specific synaptic antagonist of the putative neurotransmitter GABA. Compared with controls, the gross activity of kindled rats decreased more rapidly with dose (0.5 to 2.0 mg/kg, i.p.) and kindled rats were more likely to convulse with the higher doses (1.0 to 2.0 mg/kg). These results suggest a functional deficit in the GABA system of kindled rats as measured by locomotion and convulsive activity.     

Acupuncture reduces the trigeminal evoked response in decerebrate cats

Electroacupuncture was administered to 52 decerebrate, unanesthetized cats in which the trigeminal electrically evoked response was recorded from the surface of the brain stem. In 15 of these, the first negative wave of the evoked potential was reliably depressed by 20 to 80% after electroacupuncture, and spontaneous recovery of the responses occurred in 3 to 15 min. This is a previously unreported finding and is discussed in the light of current theories of acupuncture and analgesia.     

Ultrastructural identification of physiologically recorded neurons in the cat cerebellum

Neurons from the cerebellar cortex of cats were examined in electron microscopic preparations after intracellular recording and pressure injections of horseradish peroxidase (HRP). Neurons that contained HRP reaction product within dendrites were identified as either Purkinje or Golgi II cells. This identification was based on specific ultrastructural criteria that included the presence of synapses on the surfaces of dendritic shafts or spines, the identification of the presynaptic component of these synapses, and the presence of certain visible intracellular organelles. In addition, we examined specimens that contained two types of labeled dendrites after a single HRP injection. These dendrites were identified as arising from Purkinje and Golgi II cells and were shown to interdigitate with each other in a dendritic glomerulus. Dendritic appendages or sheet-like spines emanated from the Purkinje cell dendrite and sent small finger-like protrusions that surrounded and invaginated the Golgi II cell dendrite. In this glomerulus, the dendrites were shown to approach each other, and preliminary results suggest the presence of a gap junction at this site of direct apposition. This finding supports physiologic data which suggest electrical coupling between Purkinje and Golgi II cells. In addition, the results of this study demonstrate the usefulness of combining intracellular electrophysiology with HRP staining for the ultrastructural identification of recorded neurons.     

Action of inhibitory amino acids on acute epileptic foci: an electrographic study.

Epileptiform activity was induced in adult cats anesthetized with Ketamine or Na-pentobarbital. Acute models of focal epilepsy were created by application of various epileptogenic agents to neocortical or limbic structures. Inhibitory amino acids were injected intravenously and their effects on epileptiform discharges monitored for 2 h after administration. Amino acid solutions were adjusted to pH between 5.5 and 8. Glycine (to 250 mg/kg) did not induce any change. Short-lasting inhibitory effects (5 s to 9 min) were noted with β-alanine, γ-aminobutyric acid (GABA) (>80 mg/kg), taurine (>50 mg/kg), and 3-aminopropane sulfonic acid (3-APS, >5 mg/kg). The action of 3-APS was particularly powerful in abolishing cortical spiking with only moderate depression of background EEG activity. GABA, taurine, and 3-APS also induced depression of respiration in animals under barbiturate anesthesia. In addition, 3-APS caused a 20% decrease in systolic blood pressure. Similar and even greater pressure decreases were observed after injection of control drugs which did not, however, affect the epileptic firing rate. 3-APS seems to deserve further investigation as a possible antiepileptic and GABA-mimetic agent.     

Experimental hyperphenylalaninemia: dendritic alterations in cerebellum of rat

A quantitative analysis was made of the morphology of the cerebellar vermis in rats rendered hyperphenylalaninemic by daily injections on postnatal days 3 through 82 of the phenylalanine hydroxylase inhibitor, p-chlorophenylalanine (60 mg/kg), in combination with phenylalanine (300 mg/kg). This treatment resulted in a significant hyperphenylalaninemia (HPA) compared with saline-injected controls. In HPA animals there was a reduction in the cerebellar cross-sectional areas and in molecular layer thickness of midsagittal sections. Utilization of a classification strategy showed no specific qualitative changes in Purkinje cell structure between groups, but did indicate a hetereogeneity in Purkinje cell morphology between specific lobular sites of both HPA and control groups. Areas encompassed by Golgi-Cox-stained Purkinje cell dendritic trees were significantly smaller in HPA animals than in controls. The changes produced by HPA were of lesser degree in the nodulus than in the remaining lobes of the vermis. It is proposed that the decreased effect of HPA in the nodulus is related to normal differences in monoaminergic innervation between the nodulus and the other regions of the cerebellum.     

Acute effects of pyridoxine hydrochloride on monomethylhydrazine seizure latency and amygdaloid kindled seizure thresholds in cats

A number of similarities between the acute systemic convulsant, monomethylhydrazine (MMH), and the chronic focal epilepsy model, kindling, were recently identified. Those findings suggested a common neurophysiologic basis for the two experimental epilepsy models, which the present experiment further explored. Pyridoxine, which can facilitate neurohumoral transmitters thought to be deficient in both models, has been shown to prevent MMH seizures, but its effect on kindling is unknown. This report documented the effects of this substance on generalized tonic-clonic convulsions with both models. Eight, fully kindled (basolateral amygdala) cats underwent and A₁B₁A₂ design in which kindled seizure thresholds and MMH seizure latencies were examined sequentially during an initial, untreated baseline condition (A₁), after 20 mg/kg injections of pyridoxine (B₁), and during a final, untreated baseline condition (A₂). The MMH latencies were calculated in minutes postinjection and kindling thresholds represented the minimum level of stimulation (μA) necessary to elicit stage 6 convulsions. Pyridoxine completely blocked convulsions in seven of eight animals exposed to MMH. In addition, this substance yielded modest but statistically significant elevations in kindled seizure thresholds compared with initial and final baselines. These findings extend previous evidence of pyridoxine's anticonvulsant effect on MMH to kindling. Differences in the potency of effects suggested that the precise mechanism involved in MMH-induced and kindled convulsions may not be the same. Nevertheless, the fact that pyridoxine afforded some protection against seizures with both procedures provides some support for the initial hypothesis that similar neurochemical abnormalities may underlie the two experimental models.     

Relation of monomethylhydrazine (MMH) seizure thresholds to afterdischarge thresholds with amygdaloid stimulation in cats

Studies conducted in our laboratory for the last 15 years focused on an acute seizure model involving exposure to the convulsant compound, monomethylhydrazine (MMH). An important feature of this model is the latency in minutes postinjection to the onset of generalized tonic clonic convulsions. This index varies between naive animals but is systematically altered by procedures which facilitate or protect against seizures in other experimental animal models and human seizure disorders. MMH latencies may therefore provide a sensitive index of seizure susceptibility in general, despite its systemic and acute nature. To further test this hypothesis, the present study compared postoperative MMH latencies in eight naive cats with subsequent afterdischarge thresholds (in μA) obtained from basolateral amygdala kindling electrodes. The correlation between those two indices was statistically significant. In addition, animals divided on the basis of the median population value into high or low MMH seizure latencies differed significantly from one another on that index. Afterdischarge thresholds evaluated in the same animals also differed significantly and in the same direction. Thus, animals with prolonged seizure latencies also showed significantly higher afterdischarge thresholds, whereas shorter latencies accompanied lower afterdischarge thresholds. Finally, the gender, weight, electrode impedance, or electrode sites of individuals did not account for this relationship. Given the dissimilarity of the two models used, the results suggested endogenous differences in seizure susceptibility that are definable regardless of procedure. Alternatively, common features of the two models may have contributed to the present findings.     

Evidence for a glycinergic cortico-lateral hypothalamic inhibitory pathway in the rat

To determine the neurotransmitter involved in the frontal cortex (FC)-lateral hypothalamic (LHA) inhibitory pathway, the effects of transmitter candidates and their antagonists on FC-evoked inhibitions in LHA neurons were analyzed using multibarrel iontophoretic and extracellular recording methods. The result suggests that glycine is involved in this inhibitory pathway.     

Effects of adenosine and related compounds on an inhibitory process in rat cerebral cortex

Inhibition of neurons in the rat cerebral cortex was evoked by local cortical stimulation. Adenosine, AMP, and ATP applied by microiontophoresis produced no change of this inhibition. Theophylline and aminophylline, administered intravenously or iontophoretically, blocked the depression of neuronal firing by adenosine, but did not themselves affect the duration of inhibition. Dipyridamole and hexobendine, at iontophoretic doses which potentiated responses to adenosine, produced some reduction of inhibitory duration. We conclude that endogenous purines do not normally contribute to local cortical inhibition, but that their accumulation can inhibit this phenomenon, possibly presynaptically.     

Interaction of norepinephrine with cerebrocortical activity evoked by stimulation of somatosensory afferent pathways in the rat

This study investigated the action of norepinephrine (NE) on transmission of information through somatosensory cortical neuronal circuits. In the forelimb region of rat somatosensory cortex (SI), single-unit responses to natural stimulation (foot tap) of afferent pathways were recorded before, during, and after microiontophoretic application of NE. Actions of NE were quantitatively assessed by computer-based analysis of poststimulus time histograms. Overall, NE was found to enhance both excitatory and inhibitory responses generated by the afferent synaptic input. In 78% of the cells tested, low doses of NE differentially suppressed background discharge more than stimulus-bound excitation such that the signal to noise ratio was enhanced approximately twofold. Evoked spiking in 12 cells was quantitatively increased above control values during NE administration. In 82% of the units examined, NE augmented stimulus-bound inhibition and postexcitatory suppression of activity. Potentiation of inhibition was observed in 5 cells at doses of NE which caused little or no depression of spontaneous activity. These observed effects on neuronal responsiveness to afferent synaptic input often persisted for several minutes after termination of NE iontophoresis. Such modulatory actions of NE were demonstrated for cells situated throughout the vertical extent of the cortex. These results suggest that low amounts of NE may facilitate transfer of afferent information within the cerebral cortical circuitry and are consistent with a modulatory role rather than a specific information transfer function for NE.     

Thalamus as a relay station for catalepsy and rigidity

The aim of the study was to determine to what extent catalepsy and tonic rigidity of muscles induced by muscimol administration into the ventral thalamic nuclei disturb the motor activity of rats. This study also aimed to test whether the ventromedial thalamic nucleus (Vm) was involved in transmitting effects evoked by the systemic injection of neuroleptics or opioids. For this purpose muscimol and/or picrotoxin was injected into the ventral thalamic nuclei and the behaviour of the animals was assessed in a series of test situations. It was found that muscimol administration to the Vm disturbs not only the initiation and performance of voluntary movements but also the occurrence of avoidance when the animal's life is endangered. Postural reflexes remained, however, undisturbed. Those effects seemed to be GABA- and site-specific to Vm. The haloperidol catalepsy was strongly inhibited by administration of picrotoxin to the Vm while the morphine catalepsy remained unchanged after picrotoxin. The Vm plays a crucial role in the motor behaviour and transmission of cataleptogenic effects of haloperidol, whereas similar effects produced by morphine appear to by-pass the investigated thalamic region.     

大鼠局灶性脑缺血再灌注后GABA_A受体亚基α1和γ2在梗死对侧皮层表达的动态变化

目的研究大鼠大脑中动脉缺血再灌注后梗死对侧皮层GABAA受体α1和γ2亚基的表达变化。方法采用大鼠大脑中动脉缺血再灌注模型(MCAO),分为MCAO组和假手术组,分别于术后7、30d和6月断头取脑,采用免疫组织化学技术检测各组中梗死对侧皮层GABAA受体α1和γ2亚基的表达水平。结果 MCAO组大鼠与假手术组大鼠比较,在缺血再灌注7d时GABAA受体α1和γ2亚基在梗死对侧皮层各个脑区的表达无明显差异(P0.05);30d和6个月时α1、γ2亚基在梗死对侧皮层各个脑区的表达均明显增加(P0.05)。结论 MCAO再灌注后梗死对侧皮层GABAA受体α1和γ2亚基发生了泛发而长效的变化,这种变化提示脑缺血再灌注后的远期GABAA受体可能出现了结构和功能上的改变,从而为进一步阐释脑缺血和GABAA受体间的关系,理解缺血性脑卒中后患者出现的相关病理生理学改变提供了理论依据。     

Baclofen and the brain stem auditory evoked potential

Baclofen, a commonly used antispastic drug, is believed to block the release of excitatory amino-acid neurotransmitters. Auditory distortions are one side effect of this drug. Peak 1 of the brain stem auditory evoked potential of the cat was not affected by intravenously applied baclofen (2 to 3 mg/kg). Peaks 2, 4, and 5, however, were strongly suppressed or blocked. The results indicate a basis for the occasional auditory side effects of baclofen and suggest that the transmitter of the auditory nerve is an excitatory amino acid.     

Location of commissural neurons in the vestibular nuclei of the cat

Commissural neurons were studied in 14 cats by examining the contralateral location of neurons labeled 24 h after the injection of horseradish peroxidase into the vestibular nuclei. After establishing the distribution of labeled cells in a group of seven animals, the changes in this pattern were examined when the retrograde flow of tracer was impeded by midline transection in a second group of seven cats. These results indicated that commissural units were located mainly in the superior and medial vestibular nuclei. The Group Y nucleus also represents a connecting link to the contralateral brain stem, and a moderate number of commissural neurons were found in the descending vestibular nucleus.     

Suppressing action of cholinergic agents on synaptic transmissions in the corpus striatum of rats.

The action of acetylcholine (Ach) and of carbamylcholine (Cch) on activities of the corpus striatum were studied in vitro in thin striatal sections of rats and in vivo in rats anesthetized with α-chloralose. Cch and Ach suppressed field potentials (N₂-wave) and single-cell discharges elicited in slices by electric stimulation. They caused weak and inconsistent changes in firing evoked by glutamate (Glu). When the cholinergic agents were administered at concentrations more than 10 times that needed to suppress the N₂-wave, background firing was initiated or was modified in pattern by Cch. Atropine or tubocurarine was without effect on the N₂-wave. The suppressing action of Ach and Cch on the N₂-wave was blocked by atropine. Cch suppressed similarly negative field potentials elicited in vivo by local stimulation in the striatum of anesthetized rats. Whereas Cch suppressed single-cell discharges evoked by local electric stimulation, it was without effect on firing induced by Glu. Likewise, Cch suppressed firing elicited by motor cortex stimulation, but did not block activation of striatal neurons by nigral stimulation. These observations suggest that the cholinergic agents block synaptic transmission in the striatum without causing excitation or inhibition in postsynaptic neurons, and that the action of these agents is input-specific.     

Anesthetic-dependent excitability changes in the hypothalamic ventromedial nucleus of the rat

In acute experiments in rats anesthetized with urethane, the field potentials, population spike, and unit activity evoked in the hypothalamic ventromedial nucleus (HVM) by amygdaloid stimulation are significantly increased with respect to control when preceded by a conditioning volley at 20- to 100-ms intervals. Under pentobarbital anesthesia, in contrast, the evoked responses were inhibited by the conditioning stimulus for similar interstimulus intervals. In unanesthetized animals chronically implanted with stimulating and recording electrodes, a facilitation of responses by a conditioning stimulus was observed when they were awake or anesthetized with urethane. When the same animals were anesthetized with pentobarbital the HVM evoked response was inhibited by a conditioning pulse. Frequency facilitation and post-tetanic potentiation of HVM responses were markedly enhanced under urethane, whereas in pentobarbital-anesthetized animals inhibition predominated. Picrotoxin reversed the inhibition under pentobarbital to facilitation. These results suggest that the HVM neuron population is under both excitatory and inhibitory influences from the amygdala, the former being predominant in awake and urethane-anesthetized animals and the latter being expressed under pentobarbital anesthesia and is probably mediated by γ-aminobutyric acid.     

Uptake and transport of H-GABA (γ-aminobutyric acid) injected into the cat dentate nucleus

The uptake and transport of γ-aminobutyric acid (GABA) by neurons in the cat dentate nucleus were studied using light microscopy autoradiography after injections of ³H-GABA into the nucleus. Injections of ³H-GABA were also made into the overlying cerebellar cortex to provide positive evidence for uptake of the label in each experiment. In autoradiographs where labeled neurons were observed in the cortex, ³H-GABA was also observed to be incorporated by some dentate neurons. Along the periphery of the injection sites in the dentate nucleus, some neuronal soma and their processes were very heavily labeled as indicated by the dense accumulation of autoradiographic silver grains overlying those neuronal elements. Immediately adjacent there were nonlabeled neurons which were quite conspicuous due to the paucity of silver grains overlying their cell bodies and processes. Labeled axons were also observed leaving the injected regions of the dentate nucleus. Some labeled fibers coursed from the lateral and ventrolateral edge of the nucleus into the corpus medullare and the white matter of individual cerebellar folia. These axons could not, however, be traced into the cortex. Other labeled axons projected from the ventromedial hilus of the dentate into the brachium conjunctivum. These results demonstrate an uptake and intraaxonal transport system for ³H-GABA within the cat dentate nucleus.     

Electrophysiological and neurochemical study of the rat geniculo‐cortical pathway. Evidence for glutamatergic neurotransmission

The projection from the dorsal lateral geniculate nucleus to the primary visual cortex of the rat was studied electrophysiologically. Electrical stimulation of the dorsal lateral geniculate nucleus and the optic tract produced three types of responses on neurons of area 17: excitation followed by inhibition, excitation and inhibition. These results extend and confirm, in adult rats, previous studies done in rat geniculate-visual cortex cocultures preparations in vitro. The role of glutamate in the neurotransmission of the rat geniculo-cortical pathway was also investigated. In a first set of experiments, the effects of kynurenate, an antagonist of glutamate receptors, on visual cortex neurons with a monosynaptic excitatory response to dorsal lateral geniculate nucleus stimulation were studied. Microiontophoresis of kynurenate in area 17 neurons selectively suppressed the excitatory response to dorsal lateral geniculate nucleus and optic tract stimulation. In a second set of experiments, the effects of electrical stimulation of the dorsal lateral geniculate nucleus and the optic tract on the release of amino acids in the rat visual cortex in vivo were studied. Using the push–pull method, we perfused a discrete region of the visual cortex with artificial cerebrospinal fluid (CSF), and the amino acid content of the perfusates was analysed by high performance liquid chromatography (HPLC). Stimulation of either the dorsal lateral geniculate nucleus or the optic tract significantly increased glutamate release in area 17. The rest of the amino acids studied did not show significant changes. The results provide evidence for the participation of glutamate in the neurotransmission of the geniculo-cortical pathway in the rat.