The influence of cortical lesions on penicillin induced convulsive activity in the awake rat

1. Experiments were performed to investigate the effects of cortical lesions on convulsive behaviour. Rats were lesioned in the left motor or sensory cortex by aspirating cortical tissue 2 to 3 months prior to the elicitation of convulsions. Convulsions were induced in the awake rats by the GABA antagonist Na-penicillin (Na-PCN) which was applied into the superficial layer of the foreleg field of their right motor cortex. Convulsive activity was recorded by means of the EEG. 2. The time courses of convulsive cortical activity were similar in the animals without or with a cortical lesion. Generalized seizures belonged to the tonic-clonic type in both intact and lesioned rats. 3. The early period of convulsive activity was described by the time to the onset (latency) of the first convulsive potential, jerk and seizure, and by the mean repetition rate of jerks during the first ten minutes, and the duration of the first generalized seizure. None of these parameters was significantly affected by a cortical lesion. 4. The median duration of the convulsive activity in intact animals was 162 min. In rats with a lesion in the sensory cortex it raised to more than 540 min while a lesion of the motor cortex increased the median duration to more than 273 min. The differences between intact and lesioned rats were significant (p less than 0.01 and p = 0.05, respectively). 5. The median time to the onset of the last generalized seizure in intact rats corresponded to 92 min with respect to the time of Na-PCN application. In rats with a lesion of the sensory cortex the last seizure was generated 433 min and in animals with a lesion of the motor cortex 167 min after Na-PCN treatment of the motor cortex of one side. This increase of latency of the last seizure was significant for the rats with a lesioned sensory area (p less than 0.02) or motor area (p = 0.05) compared to that of the intact rats. Additionally, the number of generalized seizures was significantly (p less than 0.01) increased by both groups of rats with a lesion of the motor or sensory cortex. 6. It is suggested that a substantial lesion of the cortex decreases predominantly the intrinsic cortical inhibition thus destabilizing brain function. This destabilizing effect becomes pronounced under the condition of superimposed suppression of the GABAergic cortical component. It is concluded that the intrinsic cortical inhibitory mechanism which in the intact brain acts against hyperexcitation and prevents the development of neuronal synchronization, i.e. the formation of seizures, becomes less effective in performing this task once an abnormal brain activation has developed.     

Effects of NMDA and non-NMDA ionotropic glutamate receptors in the medial preoptic area on body temperature in awake rats

Glutamate when microinjected at the medial preoptic area (mPOA) influences brain temperature (T_br) and body temperature (T_b) in rats. Glutamate and its various receptors are present at the mPOA. The aim of this study was to identify the contribution of each of the ionotropic glutamatergic receptors at the mPOA on changes in T_br and T_b in freely moving rats. Adult male Wistar rats (n=40) were implanted with bilateral guide cannula with indwelling styli above the mPOA. A telemetric transmitter was implanted at the peritoneum to record T_b and locomotor activity (LMA). A precalibrated thermocouple wire implanted near the hypothalamus was used to assess T_br. Specific agonist for each ionotropic glutamate receptor was microinjected into the mPOA and its effects on temperature and LMA were measured in the rats. The rats were also microinjected with the respective ionotropic receptor antagonists, 15 min prior to the microinjection of each agonist. Amongst amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), N-methyl-d-aspartate (NMDA) and kainic acid, AMPA increased T_b and LMA when injected at the mPOA. Specific antagonists for AMPA receptors was able to attenuate this increase (p<0.005). Pharmacological blockade of NMDA was able to lower T_br only. Microinjection of kainic acid and its antagonist had no effect on the variables. The finding of the study suggests that activation of the AMPA receptors at the mPOA, leads to the rise in body temperature.     

Cardiovascular changes induced by central hypertonic saline are accompanied by glutamate release in awake rats

To elucidate neurochemical mechanisms responsible for cardiovascular responses induced by central salt loading, we directly perfused the paraventricular nucleus (PVN) of the hypothalamus region with hypertonic saline (0.3 or 0.45 M) by using an in vivo brain microdialysis technique. We then measured the extracellular concentrations of glutamate in the PVN region in conscious rats along with the blood pressure and heart rate. Blood pressure, heart rate, and glutamate levels were increased by perfusion of 0.45 M saline; however, they did not change by perfusion of 0.3 M saline. Next, we examined the possible involvement of glutamate in the cardiovascular responses induced by hypertonic saline. Dizocilpine, a noncompetitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, attenuated the increases of blood pressure and heart rate, although 6-cyano-7-nitroquinoxaline-2,3-dione, an antagonist of the non-NMDA receptor, did not affect the blood pressure and heart rate. Our results show that local perfusion of the hypothalamic PVN region with hypertonic saline elicits a local release of glutamate, which may act via NMDA-type glutamate receptors to produce cardiovascular responses.     

Protective effect of aspartate and glutamate on cardiac mitochondrial function during myocardial infarction in experimental rats

The present study investigates the effect of aspartate and glutamate on mitochondrial function during myocardial infarction (MI) in wistar rats. Male albino wistar rats were pretreated with aspartate [100 mg(kgbody weight)(-1) day(-1)] or glutamate [100 mg(kg body weight)(-1) day(-1)] intraperitoneally for a period of 7 days. Following amino acid treatment, MI was induced in rats by subcutaneous injection of isoproterenol [200 mg(kg body weight)(-1) day(-1)] for 2 days at an interval of 24 h. Isoproterenol (ISO) induction resulting in significant (P<0.05) increase in the levels of cardiac mitochondrial lipid peroxidation with a decrease in reduced glutathione level. The activities of glutathione peroxidase and glutathione reductase were significantly (P<0.05) decreased by ISO. ISO-induction also caused significant (P<0.05) decrease in the activities of mitochondrial tricarboxylic acid cycle enzymes (malate dehydrogenase, isocitrate dehydrogenase, succinate dehydrogenase, alpha-ketoglutarate dehydrogenase) and respiratory chain enzymes (NADH dehydrogenase and cytochrome-c-oxidase). ISO significantly (P<0.05) reduced the cytochrome contents, ATP production, ADP/O ratio and oxidation of succinate in state 3/state 4 whereas significantly (P<0.05) increased NADH oxidation. Pretreatment with aspartate or glutamate significantly (P<0.05) reduced the alterations induced by ISO and maintained normal mitochondrial function. The present findings reveal the protective effect of aspartate and glutamate on cardiac mitochondrial function in myocardial infarction-induced rats.     

Effect of aspartate and glutamate on experimental myocardial infarction in rats

Cardiac necrosis was produced in rats by administering isoproterenol sulphate (85 mg/kg, sc for 4 days). The myocardial damage was proved by observing the elevated levels of serum aspartate amino-transferase, lactate dehydrogenase and creatine phosphokinase and the changes were confirmed by histopathology of the tissue. Both aspartate and glutamate (100 mg/kg, ip) significantly reduced the elevated levels of these enzymes. The average degree of cardiac necrosis produced in these rats when observed macroscopically and histologically was also found to be significantly reduced on pretreatment with aspartate and glutamate.     

An examination of aspartate decarboxylase and glutamate decarboxylase activity in mosquitoes

A major pathway of beta-alanine synthesis in insects is through the alpha-decarboxylation of aspartate, but the enzyme involved in the decarboxylation of aspartate has not been clearly defined in mosquitoes and characterized in any insect species. In this study, we expressed two putative mosquito glutamate decarboxylase-like enzymes of mosquitoes and critically analyzed their substrate specificity and biochemical properties. Our results provide clear biochemical evidence establishing that one of them is an aspartate decarboxylase and the other is a glutamate decarboxylase. The mosquito aspartate decarboxylase functions exclusively on the production of beta-alanine with no activity with glutamate. Likewise the mosquito glutamate decarboxylase is highly specific to glutamate with essentially no activity with aspartate. Although insect aspartate decarboxylase shares high sequence identity with glutamate decarboxylase, we are able to closely predict aspartate decarboxylase from glutamate decarboxylase based on the difference of their active site residues.     

Effect of chronic hyperprolactinemia on daily changes of glutamate and aspartate concentrations in the median eminence and different hypothalamic areas of male rats

The 24h changes of glutamate (GLU) and aspartate (ASP) were studied in the median eminence (ME) and hypothalamic areas. It was analyzed whether prolactin may change their daily patterns. The hypothalamic concentration of these amino acids was measured by high-performance liquid chromatography (HPLC) with fluorometric detection. Plasma prolactin levels increased over the 24h light-dark cycle after pituitary grafting compared to controls, and its circadian rhythm was disrupted. In controls, aspartate and glutamate in the hypothalamic areas studied followed a specific daily variation or showed no rhythmicity. In the median eminence, hyperprolactinemia seem to phase advance the aspartate or glutamate peaks from 16:00 to 12:00. In the mediobasal hypothalamus, hyperprolactinemia altered daily changes of aspartate and significantly decreased its concentration. Also, it seems to delay the nocturnal glutamate peak compared to controls. In the posterior hypothalamus, hyperprolactinemia did not change aspartate and glutamate concentrations and their daily changes, although it increased the glutamine concentration. These data show the existence of 24h changes of amino acid concentration in three of the hypothalamic regions studied. Increased plasma prolactin levels differentially affected these patterns depending on the hypothalamic area analyzed. (Chronobiology International, 17(5), 631-643, 2000)     

Effect of ionotropic glutamate receptors antagonists on the modifications in extracellular glutamate and aspartate levels during picrotoxin seizures: a microdialysis study in freely moving rats

Our previous studies have shown a local decrease in glutamate and aspartate levels during seizures, induced by picrotoxin microdialysis in the hippocampus of chronic freely moving rats. In this paper, we study the effect of continuous hippocampal microperfusion of the NMDA, AMPA and kainate glutamate receptor inhibitors 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine (MK-801); 6,7-dinitroquinoxaline-2,3-dione (DNQX), and 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2,3-benzodiazepine hydrochloride (GYKI 52466). We also examine the action of L(-)-threo-3-hydroxyaspartic acid (THA), a glutamate and aspartate reuptake blocker, on the modification of extracellular glutamate and aspartate levels induced by picrotoxin, using the microdialysis method in freely moving rats. We found that changes in extracellular hippocampal concentrations in both amino acids are prevented by NMDA, AMPA and kainate receptor inhibitors. Seizures elicited under DNQX also induce a transient increase in aspartate extracellular levels coincident with seizure time. L(-)-threo-3-hydroxyaspartic acid increased the basal extracellular concentrations of both amino acids, but did not prevent the seizure-related decrease. Our results suggest that glutamate, the major neurotransmitter at the synaptic level, may also play an important role in non-synaptic transmission during seizures.     

Regional brain glutamate transport in rats at normal and raised concentrations of circulating glutamate

the permeability of the blood-brain barrier to glutamate was measured by quantitative autoradiography in brains of control rats (average plasma glutamate concentration of 95 ) and rats infused with glutamate (average plasma glutamate concentration of 837 m). Measurements of glutamate permeability were initiated by the injection of [¹⁴C]glutamate and stopped at 1 min to avoid the accumulation of [¹⁴C]glutamate metabolites. Glutamate entered the brain at a slow rate, with an average permeability-surface area product of 7 l.min^...g^-1, except in those areas known to have fenestrated capillaries. Glutamate accumulated in the choroid plexus of ventricles, but did not seem to enter the cerebrospinal fluid in detectable amounts regardless of the circulating concentration. Glutamate accumulated in circumventricular organs, such as the median eminence, where the radioactivity was localized without detectable spread. Infusion of glutamate to create high plasma concentrations did not result in greater spread of [¹⁴C]glutamate beyond the immediate vicinity of the circum ventricular organs.     

Endogenous interaction of glutamate and dopamine in the basal ganglia of the awake rat during aging

The interaction of glutamate and dopamine in the basal ganglia of fully conscious rat during the normal process of aging is reviewed. Using a novel approach, that of blocking the reuptake of glutamate, the effects of increasing concentrations of endogenous glutamate on the extracellular concentrations of dopamine in striatum and nucleus accumbens in the young rat were investigated. It was found that increasing concentrations of glutamate correlated significantly with increasing concentrations of dopamine in striatum and nucleus accumbens. Moreover the increase of dopamine in both structures was significantly reduced after blockade of NMDA and AMPA/kainate glutamate receptors, suggesting that the increase of dopamine was mediated by glutamate. The interaction glutamate/dopamine expressed by its ratio showed a significant age-related decrease in nucleus accumbens but not in striatum, so that to a given amount of glutamate less increase of dopamine is produced. It is suggested that the interaction glutamate-dopamine represents a balanced input to the GABA neuron in the basal ganglia and that during aging this balance is disrupted. In addition, we also speculate on the significance of this glutamate-dopamine disruption in relation to the changes in motor behavior found with age.     

Electrophysiological properties of mesencephalic ventral tegmental area neurons in awake rats

A statistical analysis was made of spike activity and the shape and duration of individual action potentials in cells from the mesencephalic ventral tegmental area and adjacent regions during experiments on awake rats. Four groups of cells were identified in this test population whith electrophysiological features which may relate to their distinctive neurochemical profile and to their specific afferent connections.Institute of Pharmacology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 18, No. 6, pp. 729–737, November–December, 1986.     

Stimulation induced recurrent epileptiform discharges block cortical and subcortical spreading depression in rats

Blockade of the spread depression [SD] in chemically or electrically stimulated areas of the cerebral cortex was analyzed in a series of experiments performed in 40 curarized, locally anaesthetised rats. Longlasting stimulation of the cerebral cortex (0.5 to 1.7 mA, 6 Hz, 0.1 ms) elicited recurrent episodes of enhanced evoked potentials propagating to remote cortical areas [Co], the caudate nucleus [Cd], the hippocampus [Hi] and the thalamus [Th] and was accompanied by marked slow potential shifts (4-6 mV amplitude, 2-3 min duration, at 2-5 min intervals]. The projected discharge interfered with SD initiation and propagation in all the examined structures. The SD blockade was most pronounced during the episodes and almost absent in the intervals between them. The block was manifested by reduced amplitude and duration of the slow potential of SD. Pentobarbital [20 mg/kg] suppressed the recurrent discharges and eliminated the corresponding SD blockade. Recurrent excitability changes induced by Cd and Th stimulation elicited similar effects but the threshold was higher in Cd and Th than in Co. SD was less effectively blocked by the projected discharge than by stimulation of the same structure, particularly in the vicinity of the recording electrodes, where the blockade could be observed even under pentobarbital. The SD blockade outlasted stimulation by a considerably longer period of time in Th (about 10 min) than in the Cd (about 3 min). The onset of stimulation and the projected episodes sometimes elicited SD waves but SD blockade prevailed with continued stimulation. The present findings support the hypothesis that excessive neural activity increases the potassium clearance and thus prevents the autoregenerative accumulation of potassium ions, mediating SD propagation.     

Synthesis of glutamate and aspartate in rat brain synaptosomes

ATP and glutamine are the sources of endogenous ammonia in rat brain synaptosomes. The amount of endogenous ammonia formed from exogenous ATP is not sufficient to assure the maximum rate of aspartate and glutamate accumulation in the synaptosomes utilizing pyruvate + malate. Addition of exogenous NH4+ or depolarization of synaptosome plasma membranes with high K+ concentration led to a twofold increase in the rate of accumulation of these amino acids. This indicates that both exogenous and endogenous NH4+ is involved in the synthesis of aspartate and glutamate in nerve terminals. Accumulation of glutamate was stimulated by aminooxyacetate and inhibited by haloperidol which indicates that NH4+ is bound in the reaction catalysed by glutamate dehydrogenase. Endogenous oxaloacetate derived from pyruvate metabolism was the substrate for synthesis of aspartate. Additive inhibition of aspartate accumulation by fluorocitrate and (-) hydroxyacetate shows that, in addition to the tricarboxylic acid cycle, the reaction catalysed by ATP-citrate lyase serves in the synaptosomes as another source of oxaloacetate.