Activity-dependent ionic changes and neuronal plasticity in rat hippocampus.

We describe here the ionic changes which occur during repetitive stimulation of a type which will induce long-term potentiation and kindling plasticity. The causes of these ionic changes, particularly of changes in [Ca2+]o, are discussed. Evidence will be presented which shows that only a fraction of the decreases in [Ca2+]o is due to movement through N-methyl-D-aspartate (NMDA)-operated channels. Since NMDA-receptor activation is critical in many synapses for induction of long-term potentiation (LTP) and since the initial response to a stimulus in hippocampus is a long-lasting slow inhibitory postsynaptic potential (IPSP), mechanisms must be defined which ultimately permit activation of NMDA receptors. We conclude that increases in [K+]o and reductions in [Ca2+]o and [Mg2+]o, together with a K(+)-dependent reduction of slow IPSP promote the activation of NMDA receptors during a stimulus train and help to overcome the blocking effect which the long-lasting hyperpolarizations exert on NMDA receptors. Preliminary evidence derived from analysis of quisqualate and NMDA-induced changes in [Ca2+]o suggests that NMDA-receptor activation slows the extrusion of Ca2+ from cells. This mechanism may be important for induction of long-term changes. Finally, we document that a number of long-term changes in neuronal excitability are associated with alterations of stimulus and excitatory amino acid (EAA)-induced changes in the ionic microenvironment, which give some insight into the mechanisms underlying stimulus-induced plasticity and, perhaps, progression of temporal lobe epilepsy.

Reflections of low calcium epileptiform activity from area CA1 into dentate gyrus in the rat hippocampal slice.

Lowering of [Ca2+]o induces epileptiform activity in hippocampal area CA1, characterized by slow negative field potentials with superimposed trains of population spikes and by rises in [K+]o. In dentate gyrus slow positive field potentials occur simultaneously with the activity in area CA1. The accompanying small rises in [K+]o may stem from spatial K+ redistribution through glial cells from area CA1.

Lowering extracellular calcium reverses paired pulse habituation into facilitation in dentate granule cells and removes a late IPSP.

Simultaneous measurements of cellular responses and extracellular field potentials as well as extracellular Ca2+ concentration [( Ca2+]0) were performed in the dentate gyrus granule cell layer during paired pulse stimulation of the lateral perforant path at resting [Ca2+]o and during washout of calcium. At resting [Ca2+]o the second response to a paired stimulus was smaller than the first response. This frequency habituation reversed into frequency potentiation (second response larger than the first one) during lowering of [Ca2+]o at about the same time when a late presumed inhibitory postsynaptic potential (IPSP) was abolished. This suggests that a slow inhibition can account for part of frequency habituation in the dentate gyrus.

Effects of pentetrazol on neuronal activity and on extracellular calcium concentration in rat hippocampal slices.

Effects of pentetrazol (PTZ) were studied on neuronal responses in dentate granule cells and area CA1 hippocampal pyramidal cells with intra- and extracellular recording techniques. PTZ induced spontaneous epileptiform field potential transients in areas CA3 and CA1, but not in the dentate gyrus. The concentration optimum for induction of spontaneous epileptiform activity was 2 mM. The epileptiform activity compared in many respects to that induced by GABA antagonists such as picrotoxin, bicuculline and penicillin. Paired pulse stimulus induced responses were affected by concentrations of 0.5 mM. In the concentration range 0.5-2 mM mostly disinhibitory effects were noted. Stimulus induced Ca2+ concentration changes were found to be maximally augmented at concentrations of 2-5 mM. In this range, intracellular studies revealed a block of frequency habituation and an increase in input resistance. The convulsant action of PTZ decreased at concentrations above 5 mM, probably due to a decrease of inward currents. We suggest that the action of PTZ in screening studies for anticonvulsants is mostly due to a decrease of GABAA-receptor mediated IPSPs.

Effects of changes in extracellular potassium, magnesium and calcium concentration on synaptic transmission in area CA1 and the dentate gyrus of rat hippocampal slices.

The dependence of stimulus-induced synaptic potentials on changes of extracellular ionic concentrations of potassium ([K+]o 3, 5, 8 mM), magnesium ([Mg2+]o 2, 4, 8 mM) and calcium [Ca2+]o (2 mM and continuous lowering by washing with Ca2(+)-free solutions) was investigated in area CA1 and dentate gyrus of rat hippocampal slices. Field potentials (fps), [K+]o and [Ca2+]o were measured with double-barreled ion selective/reference microelectrodes. Paired pulse stimulation (interval 50-ms) was applied either to the lateral perforant path or to the Schaffer collaterals. Elevation of [K+]o from 5 to 8 mM and of [Mg2+]o from 2 to 8 mM depressed the rise of excitatory postsynaptic potentials, as well as the amplitude of population spikes. With elevation of [K+]o, the effect was stronger in the dentate gyrus, while with elevation of [Mg2+]o, the reduction was more pronounced in area CA1. During washout of Ca2+, synaptic potentials became reduced and finally depressed. The [Ca2+]o at which synaptic transmission was blocked increased with higher [Mg2+]o and decreased with a change of [K+]o from 3 to 5 mM, whereas with an elevation of [K+]o from 5 to 8 mM, it rose in area CA1 but was reduced in dentate gyrus. All ionic changes also affected frequency habituation and potentiation in paired pulse experiments. In dentate gyrus, frequency habituation was reversed to frequency potentiation with moderate lowering of [Ca2+]o and with elevation of [Mg2+]o and [K+]o. In contrast, in area CA1 frequency potentiation was reduced upon elevation of [K+]o.

Slow synaptic inhibition in relation to frequency habituation in dentate granule cells of rat hippocampal slices.

In paired pulse stimulation experiments the mechanism underlying frequency habituation of postsynaptic potentials in dentate granule cells of rat hippocampal slices was studied by measuring extra- and intracellular potentials as well as changes in extracellular calcium [( ([Ca2+]0) and potassium concentrations ([K+]0). Orthodromic stimulation of the perforant path induced in most granule cells a late, slow hyperpolarization (SH), lasting for up to 1.2 s. During the SH the membrane conductance was increased by up to 40%. The reversal potential of the SH was around -90 mV and varied with the [K+]0. Frequency habituation was seen in all cells with the SH, whereas cells which display frequency potentiation had no SH. Lowering of [Ca2+]0 reversed paired pulse induced frequency habituation into frequency potentiation at [Ca2+]0 levels where the SH disappeared. Phaclofen blocked the SH and reversed frequency habituation into frequency potentiation. Elevating [Mg2+]0 also reversed frequency habituation into frequency potentiation and reduced the SH. We conclude that the SH represents a late, slow IPSP which is responsible for frequency habituation in dentate granule cells. We noted that during repetitive stimulation the SH soon started to fade. This effect can in part be attributed to extracellular K+-accumulation as suggested by the K+-dependence of the slow IPSP and the observations of changes in [K+]0 during repetitive stimulation. This could explain why frequency habituation reverses into frequency potentiation during repetitive stimulation.