Spindle-like thalamocortical synchronization in a rat brain slice preparation.

We obtained rat brain slices (550-650 microm) that contained part of the frontoparietal cortex along with a portion of the thalamic ventrobasal complex (VB) and of the reticular nucleus (RTN). Maintained reciprocal thalamocortical connectivity was demonstrated by VB stimulation, which elicited orthodromic and antidromic responses in the cortex, along with re-entry of thalamocortical firing originating in VB neurons excited by cortical output activity. In addition, orthodromic responses were recorded in VB and RTN following stimuli delivered in the cortex. Spontaneous and stimulus-induced coherent rhythmic oscillations (duration = 0.4-3.5 s; frequency = 9-16 Hz) occurred in cortex, VB, and RTN during application of medium containing low concentrations of the K(+) channel blocker 4-aminopyridine (0.5-1 microM). This activity, which resembled electroencephalograph (EEG) spindles recorded in vivo, disappeared in both cortex and thalamus during application of the excitatory amino acid receptor antagonist kynurenic acid in VB (n = 6). By contrast, cortical application of kynurenic acid (n = 4) abolished spindle-like oscillations at this site, but not those recorded in VB, where their frequency was higher than under control conditions. Our findings demonstrate the preservation of reciprocally interconnected cortical and thalamic neuron networks that generate thalamocortical spindle-like oscillations in an in vitro rat brain slice. As shown in intact animals, these oscillations originate in the thalamus where they are presumably caused by interactions between RTN and VB neurons. We propose that this preparation may help to analyze thalamocortical synchronization and to understand the physiopathogenesis of absence attacks.

Responses to N-methyl-D-aspartate are enhanced in rats with petit mal-like seizures.

The responses to the glutamate agonist N-methyl-D-aspartate (NMDA) were studied in the sensori-motor cortex of rats with petit mal-like seizures. In a first study, the changes in extracellular concentration of calcium elicited through ionophoretic application of NMDA at various depths in the cortex were measured in vivo. The results show that in the cortex of epileptic rats the NMDA responses are much more widely distributed than in the cortex of control rats. In a second study, a current-source density analysis of the responses elicited through electrical stimulation of the white matter was performed in slices of neocortex in vitro. These findings show that the NMDA-dependent component of the synaptic responses are more widely distributed and of longer duration in the cortex of epileptic rats than in that of control rats. Taken together, these results suggest that in this model of absence epilepsy NMDA-dependent mechanisms are important in the triggering and maintenance of epileptic activity.

Ionic changes induced by excitatory amino acids in the rat cerebral cortex.

The ionic mechanisms underlying the action of excitatory amino acids were investigated in the rat motor cortex. Ion-selective microelectrodes were attached to micropipettes such that their tips were very close and local changes in extracellular concentration of sodium, calcium, and potassium ions elicited through ionophoretic applications of glutamate (Glu) and of its agonists N-methyl-D-aspartate (NMDA), quisqualate (Quis), and kainate (Ka) were measured. These agents produced moderate increases in [K+]o (up to 13 mM) but, in contrast, substantial tetrodotoxin-insensitive decreases in [Na+]o (maximally of 60 mM). NMDA-induced sodium responses could be blocked by manganese, while the Quis- and Ka-induced responses were not. Quis and Ka produced increases in [Ca2+]o or biphasic responses while NMDA, even with small doses, induced each time drastic decreases in [Ca2+]o (maximally of 1.15 mM), which could be attenuated or blocked by manganese but not by organic calcium channel blockers. NMDA responses could be abolished by reduced doses of 2-amino-phosphonovalerate. The largest Glu- and NMDA-induced calcium responses were observed in the superficial cortical layers, but such maxima disappeared after selective degeneration of pyramidal tract neurons. All amino acids produced sizeable reductions in the extracellular space volume. The following can be concluded. (i) All the excitatory amino acids tested induce an increased permeability to sodium and potassium ions. (ii) In addition, the NMDA-operated channels have specifically a large permeability for calcium, although calcium ions contribute only by less than 10% to the NMDA-induced inward currents.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term alterations in amino acid-induced ionic conductances in chronic epilepsy.

Extracellular free sodium (Na+)o and calcium (Ca2+)o concentration changes were measured in the rat motor cortex, using ion-selective microelectrodes. During ionophoretic applications of excitatory amino acids, decreases in (Na2+)o and in (Ca2+)o were observed. Ca2+ signals were not or very little modified by applications of tetrodotoxin while Na+ signals were slightly depressed, up to 20%. Laminar profile analysis revealed that, while the magnitude of Na+ signals was rather constant throughout the cortex, Ca2+ signals were largest in upper cortical layers. Lesioning and pharmacological experiments indicated that the corresponding permeabilities were most probably located on apical dendrites of pyramidal tract neurons. The relative amplitude of Na+ and Ca2+ signals induced by the release of the glutamate agonists N-methyl-D-aspartate, quisqualate and kainate and the shape of the laminar profile of such responses indicated that different ionic permeabilities located on different neurons underlie such responses. Similar experiments performed on chronic epileptogenic motor foci in rats indicated that the amino acid-induced ionic responses were altered. The significance of such alterations for epileptogenesis is discussed.

Extracellular calcium and potassium concentration changes in chronic epileptic brain tissue.

Repetitive electrical stimulation and application of excitatory amino acids lead to decreases in extracellular Ca2+ concentration and to rises in extracellular K+ concentration [( Ca2+]o, [K+]o) with a typical laminar distribution in a given neo- or allocortical structure. These ionic changes result from transmembrane ion fluxes along their respective electrochemical gradients. Epileptogenic drugs that impair repolarizing K+ conductances or inhibitory synaptic transmission enhance such extracellular ionic changes, but they do not alter the laminar distribution of [K+]o and [Ca2+]o changes. Enhanced [Ca2+]o concentration changes are also observed in chronic epilepsies such as the chronic alumina cream and cobalt focus, the kindling epilepsy, and during photically induced seizures in the baboon Papio papio. In chronic epilepsies, the sites of maximal [Ca2+]o changes shift to other layers, suggesting changes in the distribution of ion channels over the surface of nerve cells that may be involved in epileptogenesis in chronic epilepsies. The K+ and Ca2+ concentration changes associated with seizure contribute to the generation and spread of epileptic activity. This is demonstrated by the fact that lowering of extracellular free calcium concentration can induce spreading epileptiform activity in the absence of chemical synaptic transmission, with [K+]o rises preceding epileptiform activity.

Ectopic action potential generation: its occurrence in a chronic epileptogenic focus.

Intracortical axon terminals of thalamocortical relay neurons projecting into chronic cortical epileptogenic foci may generate high frequency bursts of action potentials during spontaneous cortical epileptiform paroxysms. Such bursts occur regularly before onset of the corresponding surface paroxysms in a large proportion of the tested neurons. Therefore, such axon terminals bursting may participate in the triggering of the synchronous, prolonged intracellular depolarization shifts which are observed in cortical neurons during epileptic events.

Effects of Ba2+ and tetraethylammonium on cortical neurones.

1. Ba(2+), applied by micro-iontophoresis, excites most cortical neurones that are excitable by ACh; other neurones tend to be depressed.2. The discharges evoked by Ba(2+) resemble those evoked by ACh, but they have an even slower time course and are characterized by firing in high frequency bursts.3. The excitatory action of Ba(2+), unlike that of ACh, is not abolished by muscarine antagonists; but it can be prevented with dinitrophenol.4. The depolarizing effect of Ba(2+) is associated with a rise in membrane resistance and it has a reversal level 24 mV more negative than the resting potential.5. These observations suggest that, as in other tissues, Ba(2+) reduced the K(+) conductance by a direct action on the cell membrane. Some diminution in Na(+) inactivation is indicated by the repetitive firing at high frequency.6. TEA has a predominantly depressant effect on all neurones tested. Like Ba(2+), it often increases greatly the duration of spikes, but there is no regular change in resting membrane resistance and no tendency to repetitive firing. TEA probably reduces only the delayed K(+) current.7. Even in large doses neither Ba(2+) nor TEA interferes with the conductance increase that generates the typical prolonged IPSPs recorded in cortical neurones.