U. Heinemann

Extracellular free calcium and potassium during paroxysmal activity in the cerebral cortex of the cat

SummaryExtracellular calcium and potassium activities (aCa and aK) as well as neuronal activity were simultaneously recorded with ion-sensitive electrodes in the somatosensory cortex of cats. Baseline aCa was 1.2–1.5 mM/1, baseline ak 2.7–3.2 mM/1. Transient decreases in aCa and simultaneous increases in aK were evoked by repetitive stimulation of the contralateral forepaw, the nucleus ventroposterolateralis thalami and the cortical surface. Considerable decreases in aCa (by up to 0.7 mM/1) were found during seizure activity. A fall in aCa preceded the onset of paroxysmal discharges and the rise in aK after injection of pentylene tetrazol. The decrease in aCa led also the rise in aK during cyclical spike driving in a penicillin focus. It is concluded that alterations of Ca++ dependent mechanisms participate in the generation of epileptic activity.

Epileptiform activity in combined slices of the hippocampus, subiculum and entorhinal cortex during perfusion with low magnesium medium ☆

Reduction of [Mg2+]o induced spontaneous epileptiform activity consisting of 40-100-ms bursts of population spikes in hippocampal slices. This activity disappeared from area CA1 when the connections to area CA3 were cut, but persisted in isolated minislices of area CA3. Spontaneous activity was also observed in the dentate gyrus, provided that the connections to the subiculum and entorhinal cortex (EC) were intact. In the parasubiculum and EC longer lasting epileptiform events were observed which resembled seizure-like behaviour. The epileptiform activity was completely suppressed by 2-aminophosphonovalerate (30 microM) suggesting that N-methyl-D-aspartate receptors for excitatory amino acid transmitters participate in the generation of this activity. These findings show that the EC possesses properties which permit the generation of seizure-like activity in contrast to the hippocampus where the activity resembled recurrent interictal events.

Transient Changes in the Size of the Extracellular Space in the Sensorimotor Cortex of Cats in Relation to Stimulus-induced Changes in Potassium Concentration*

SummaryThe time course of local changes of the extracellular space (ES) was investigated by measuring concentration changes of repeatedly injected tetramethylammonium (TMA+) and choline (Ch+) ions for which cell membranes are largely impermeable. After stimulus-induced extracellular [K+] elevations the δ[TMA+] and δ[Ch+] signals recorded with nominally K+-selective liquid ion-exchanger microelectrodes increased by up to 100%, thus indicating a reduction of the ES down to one half of its initial size. The shrinkage was maximal at sites where the K+ release into the ES was also largest. At very superficial and deep layers, however, considerable increases in extracellular K+ concentration were not accompanied by significant reductions in the ES. These findings can be explained as a consequence of K+ movement through spatially extended cell structures. Calculations based on a model combining the spatial buffer mechanism of Kuffler and Nicholls (1966) to osmolarity changes caused by selective K+ transport through primarily K+ permeable membranes support this concept.Following stimulation additional iontophoretically induced [K+]o rises were reduced in amplitude by up to 35%, even at sites where maximal decreases of the ES were observed. This emphasizes the importance of active uptake for K+ clearance out of the ES.

Stimulus-induced changes in extracellular Na + and Cl − concentration in relation to changes in the size of the extracellular space

SummaryExtracellular Na+- and Cl−-concentrations ([Na+]o, [Cl−]o) were recorded with ion-selective microelectrodes during repetitive stimulation and stimulus-induced self-sustained neuronal afterdischarges (SAD) in the sensorimotor cortex of cats. In all cortical layers [Na+]o initially decreased by 4–7 mM. In depths of more than 600 μm below the cortical surface such decreases usually turned into increases of 2–6 mM during the course of the SADs, whereas in superficial layers [Na+]o never rose above its resting level. [Cl−]o always showed an increase in the course of the SADs often preceded by an initial small decrease. The average increase at a depth of 1,000 μm was about 7 mM. [Cl−]o reached peak values at about the end of the ictal period, whereas [Na+]o reached its maximum shortly after the end of the SAD, at times when [K+]o was still elevated above the baseline concentration.These data indicate that the extracellular osmolarity can increase during SAD by up to 30 mM. Such an increase in osmolarity can be explained by an increase in the number of intracellular particles, caused by cleavage of larger molecules during enhanced metabolism. This could lead to cell-swelling due to passive water influx from the extracellular space (ES). However, the resulting reduction of the size of the ES is calculated to be less than 10% for an increase in intracellular osmolarity by 30 mOsm. This value is too small as compared to previously measured ES-reductions under similar conditions (i.e., 30% reduction at 1,000 μm; Dietzel et al. 1980). Reductions of the size of the ES that accompany the observed changes in the ionic environment, are quantitatively explained on the basis of the extended glial buffering mechanism described in the preceding paper.

Undershoots following stimulus-induced rises of extracellular potassium concentration in cerebral cortex of cat.

Extracellular potassium activity (ak) was recorded with potassium-sensitive electrodes in the sensorimotor cortex of cats. Resting activity was 2.8--3.4 mEquiv/l. Electric stimulation of the cortical surface and the nucleus ventroposterolateralis of the thalamus brought about an increase in aK followed by an undershoot and return to normal value. The lowest observed value of aK was 2.1 mEquiv./l. Size and duration (range 0.5--4 min) of the undershoots of aK increased with increasing peak amplitudes of the preceding rise in aK. Following the rise in aK, a period of reduced neuronal activity was observed which usually shorter lasting than the decrease in extracellular aK. An undershoot of aK and a concomitant reduction of neuronal discharge frequency can also occur in immediate response to antidromic stimulation of the pyramidal tract. To compare the K+ redistribution at normal and reduced levels of aK electrophoretic K+ signals were produced with constant current pulses from a proximate KCl-filled capillary. Both amplitudes and half times of decay of these K+ signals were found to decrease during the phase of poststimulatory undershoot in aK (19 and 23% respectively). It is suggested that an activated reuptake of potassium contributes to the decrease in extracellular aK in addition to inhibitory processes.

The anticonvulsive action of adenosine: a postsynaptic, dendritic action by a possible endogenous anticonvulsant.

Neural afterdischarges generated in the presence of penicillin or low extracellular calcium concentrations were found to be inhibited by adenosine in the rat hippocampus in vitro. This anticonvulsant effect of adenosine is observed in the absence, as well as in the presence, of chemical synaptic transmission and apparently occurs at a postsynaptic site which is most sensitive in the apical dendritic region of the CA1 pyramidal cells. The methylxanthine theophylline antagonizes the effect of adenosine; and, the anticonvulsant action of the L-isomer of the adenosine analogue phenylisopropyladenosine (PIA) is substantially more potent than the D-isomer, findings which are characteristic of an A1 type adenosine receptor. The endogenous release of adenosine may therefore serve to tonically reduce the tendency for repetitive discharge in CA1 pyramidal cells via an interaction with a high affinity A1 receptor which appears to be preferentially localized in the apical dendrites.

Spontaneous epileptiform activity of ca1 hippocampal neurons in low extracellular calcium solutions

SummaryLowering extracellular [Ca2+] in rat hippocampal slices induces spontaneous epileptiform activity in area CA1, which is characterized by rhythmic burst firing of CA1 neurons and by prolonged negative potential shifts at the pyramidal cell body layer. This activity is accompanied by transient decreases of [Na+] and increases of [K+] in the extracellular space. In spite of the complete blockade of synaptic transmission, the wave of epileptiform activity propagates across area CA1. These findings suggest, that non-synaptic mechanisms may play a role in the generation and spread of epileptiform activity in the mammalian CNS.

Epileptiform activity induced by lowering extracellular [Mg2+] in combined hippocampal-entorhinal cortex slices: Modulation by receptors for norepinephrine and N-methyl-d-aspartate

Reduction of extracellular Mg2+ concentration induced spontaneous and evoked epileptiform activity in the entorhinal cortex (EC) and dentate gyrus (DG) of combined hippocampus (HC)-EC slices. Extracellular field potentials, as well as changes in extracellular Ca2+ and K+ concentrations, were measured in EC and DG with ion-selective/reference electrodes during both repetitive and single stimuli. In the EC, lowering extracellular [Mg2+] induces both spontaneous and single stimulus evoked ictal events consisting of extracellular negative potential shifts (up to 5 mV, 30 sec), decreases in [Ca2+]0 and increases in [K+]0. In the DG, spontaneous events were much shorter, but similar changes in [Ca2+]0, [K+]0 and field potentials (FPs) could be evoked by brief high-frequency stimulation. In both areas, the N-methyl-D-aspartate (NMDA) receptor antagonist 2-aminophosphonovalerate (2-APV) completely blocked spontaneous as well as stimulus evoked epileptiform events. The neurotransmitter norepinephrine (NE), which has previously been shown to modulate long-term potentiation in the DG, was found to exhibit differential modulation of epileptiform activity in the EC and DG. In the EC, NE, acting via alpha 1-receptors, completely blocked low Mg2+-induced epileptiform activity. In contrast, in the DG, NE exhibited a beta-receptor mediated prolongation of the low Mg2+-induced ictal events, and enhanced the stimulus-induced ionic and field potential changes. From these results, we conclude that lowering extracellular [Mg2+], acting in large part through the removal of the Mg2+ voltage-dependent blockade of NMDA receptors, leads to induction of epileptiform activity in both the EC and DG.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium and epileptogenesis

ConclusionsIn conclusion, both acutely induced and chronic epilepsies are associated with an enhanced Ca uptake capability into nerve cells. This finding may in future help to identify areas in the brain with chronic epileptogenic potential and thereby facilitate the study of mechanisms involved in the generation of chronic epilepsies. The enhanced Ca uptake observed in many experimental and aquired epilepsies itself may depend on Ca fluxes through voltage or NMDA operated channels. Intrinsic currents may be involved in amplifying EPSPs into PDS by relieving the block which Mg exerts normally on NMDA operated ionophores. The many consequences of decreases in [Ca]o as those of the regularly associated rises in [K]o provide positive feedback which supports the initiation and spread as well as the maintenance of ictal activity. The resulting intracellular load with Ca may be one factor involved in the degeneration of nerve cells as a result of epileptic activity.

Developmental changes in neuronal sensitivity to excitatory amino acids in area CA1 of the rat hippocampus

The laminar distribution of decreases in extracellular free calcium and concomitant field potentials induced by repetitive orthodromic stimulation, ionophoretic application of N-methyl-D-aspartate and quisqualate, was studied in the CA1 field of rat hippocampal slices, at two different stages during postnatal development. While stimulation-elicited and quisqualate-induced signals remain maximal in stratum pyramidale during the first postnatal month, the laminar profiles of responses to N-methyl-D-aspartate (NMDA) depend on age: the responses to this agonist are maximal in stratum pyramidale in 5-9-day-old rats and in stratum radiatum in 12-30-day-old rats. Our findings suggest that, during the second postnatal week, the apical dendrites of pyramidal neurons in area CAl become more sensitive to NMDA, which is expressed by big influxes of calcium at this level.