Two channels reduced to one

In vitro and in vivo characterisation of glial cells immortalised with a temperature sensitive SV40 T antigen-containing retrovirus

An oncogene-carrying replication-defective retrovirus was used to establish immortalised lines of murine glial cells. Primary cultures of early postnatal cerebellar cells were infected with a retrovirus based on the Murine Moloney Leukemia Virus containing a temperature-sensitive mutant of the Simian Virus 40 large T antigen (SV40 T) oncogene and a gene coding for resistance to the antibiotic G418. Infected cells were selected in G418 and after several in vitro passages cells expressing the O4 antigen were established as a cell line. At a later time point O4-positive single-cell clones were established. Two different types of clones were obtained: 1) "plastic" clones consisting of cells which initially had a morphological and antigenic phenotype of young glial precursor cells but which gradually lost these features, and 2) "stable" cell clones including a clone with the immunological and electrophysiological characteristics of Schwann cells. Culture of the latter cells in the presence of 1 mM dibutyryl cyclic adenosine monophosphate for a period of at least 10 days induced a change in shape and a shift in antigen expression towards a more "differentiated" maturation stage. When the SV40 T O4-positive immortalised cell line isolated on the cell sorter was transplanted into demyelinated lesions in adult rats, cells were observed ensheathing axons and forming limited amounts of PNS-type myelin. Glial cells immortalised with a temperature-sensitive mutant of the SV40 T oncogene thus retain many physiological properties of their primary culture counterparts and can be induced to undergo limited differentiation in vitro and in vivo. These cell lines, which represent immature CNS glia or Schwann cells, are providing useful tools for investigating the role of cell surface antigens involved in neuron-glial interactions.

Glutamate and glycine induce a negative wave on hippocampal field response through NMDA receptors

In rats under urethane anesthesia, iontophoresis of large amounts (30-300 nA) of glutamate in the hippocampus induced a negative wave on the field potential evoked by stimulation of fimbria/commissura or perforant pathway. The amplitudes of the negative waves ranged between 0.2 and 9.8 mV and their mean duration was 341 +/- 12 ms. This activity was antagonized by iontophoresis of N-methyl-D-aspartate (NMDA) antagonists: Mg2+ (80-100 nA), ketamine (50-150 nA), MK-801 (50-150 nA) and by systemic ketamine (5 mg/kg, i.v.) administration. Iontophoresis of N-methyl-DL-aspartate (NMDLA) (20-40 nA) and glycine (25-100 nA) also elicited a negative wave which was blocked by NMDA antagonists. The negative waves were induced in all hippocampal layers except the dentate hilus by glutamate, NMDLA and glycine. Pyramidal regions were found to be as sensitive as dendritic layers; the mean amplitudes of glutamate-induced negative waves on the field response were 4.1 +/- 0.6 and 4.2 +/- 0.5 mV for CA1 stratum pyramidale and radiatum, respectively. These data suggest that large amounts of glutamate activate NMDA receptor/ion channels causing appearance of a long-lasting negative wave on the hippocampal field response. The data also demonstrate that glycine leads to a significant participation of NMDA receptors during glutamatergic transmission which is largely mediated through non-NMDA receptors.

Accumulation of calcium in substantia nigra lesions induced by status epilepticus. A microprobe analysis

Status epilepticus of sufficient duration (greater than 30 min) causes a unique lesion of substantia nigra pars reticulata (SNPR), and of globus pallidus (GP). This lesion, which encompasses a pan-necrotic destruction of neurons and glial cells seems to develop during ongoing seizures. We decided to investigate if the lesion is accompanied by net calcium accumulation. Seizures of 20 and 60 min duration were induced by the administration of flurothyl, and the tissue was frozen in situ either at the end of the seizure periods, or after recovery periods of 15 or 120 min. The total calcium and potassium contents of caudoputamen, neocortex, GP and SNPR were measured using particle induced X-ray emission (PIXE) in the microprobe mode. Seizures of 20 min duration did not cause net accumulation of calcium. When the duration of seizures was extended to 60 min the results varied depending on the location. In caudoputamen, which does not incur neuronal damage, no calcium accumulation was observed. In GP and SNPR, such a rise was unequivocally demonstrated, with calcium content increasing to about 150% of controls. The increase in calcium correlated to a decrease in potassium content. It is concluded that epileptic cell death occurs pari passu with accumulation of calcium although it cannot be stated that this accumulation is the cause of the cell death. It is speculated that seizures increase the permeability of the blood-brain barrier to calcium, and that enhanced blood to tissue transfer increases the calcium load of metabolically strained cells.

N-methyl-D-aspartate activates different channels than do kainate and quisqualate

In the mammalian central nervous system, the excitatory amino acid transmitter L-glutamate activates three pharmacologically distinguishable receptors, the N-methyl-D-aspartate (NMDA), kainate, and quisqualate receptors. The present paper addresses the issue of whether these three receptors operate independent channels or whether they share channels that may have several conductance substates. The Xenopus oocyte provides a system for expression of exogenous mRNAs that permits detailed study of receptor structure and function. In oocytes injected with rat brain mRNA, NMDA has a stoichiometry of channel activation different from that for kainate and quisqualate. NMDA activates its own channels as indicated by simple summation or near-summation of currents evoked by NMDA with those evoked by quisqualate or kainate. Deviations from summation are ascribable to lack of selectivity in which an agonist at one receptor acts as a weak antagonist at another receptor. A further indication of separate channels is that block of NMDA channels by Mg2+ or phencyclidine has no effect on kainate or quisqualate responses evoked during the block. Interactions of kainate and quisqualate are more complex, but they can be explained by lack of complete specificity of these agonists for their own receptors.

Synaptic plasticity and learning. II: Do different kinds of plasticity underlie different kinds of learning?

This paper discusses certain issues connected with the question of whether synaptic plasticity is involved in information storage by the brain. We begin by contrasting two well documented types of synaptic plasticity--activity-dependent modulation of presynaptic facilitation ("Kandel synapses") and NMDA receptor triggered alterations in excitatory amino acid transmission ("Hebb synapses"). We then propose that, embedded-into appropriate circuitry, these different forms of plasticity might underlie different kinds of learning. In partial support of this idea, we show that, in freely moving rats, intrahippocampal microinfusion of the NMDA-receptor antagonist D,L-AP5 causes a dose-dependent impairment of a type of spatial learning known to be sensitive to disruption by hippocampal lesions (water-maze place-navigation). In a second experiment, the same drug infusion protocol is shown to cause a blockade of hippocampal long-term potentiation in vivo across a comparable dose range. Finally, third, autoradiographic experiments indicate that diffusion of the drug was largely restricted to the hippocampus. Together, these results (1) suggest that blockade of hippocampal NMDA receptors, under conditions which leave baseline synaptic transmission relatively unaffected, blocks a type of learning with which the vertebrate hippocampus has been implicated on the basis of neuropsychological work; and (2) supports Abrams and Kandel's Trends Neurosci. 11 (1988) recent proposal that there may be several logically distinct forms of synaptic plasticity.

Accumulation of calcium and loss of potassium in the hippocampus following transient cerebral ischemia: a proton microprobe study

This study explored (a) whether postischemic accumulation of calcium in hippocampal neurons precedes or occurs pari passu with light microscopical signs of delayed neuronal necrosis, and (b) whether calcium initially accumulates in dendritic domains, presumed to have a high density of agonist-operated calcium channels. Transient ischemia of 10-min duration was induced in rats, and the animals were studied after 1, 2, 3, and 4 days of recovery. We measured total calcium and potassium contents in the stratum oriens, pyramidale, radiatum, and moleculare of the CA1 and CA3 sectors, using particle induced x-ray emission (PIXE) in the proton microprobe mode. The results showed significant accumulation of calcium and loss of potassium after 3 and 4 days of recovery in the CA1 sector, which developed neuronal necrosis, but not in the CA3 sector, which showed only occasional damage. In a few animals, calcium accumulation (and loss of potassium) was observed with no or only mild visible damage, but in the majority of animals the accumulation of calcium correlated to signs of neuronal necrosis. Since calcium accumulation was similar in all strata examined, the results failed to reveal preferential accumulation in dendritic or somal regions. Based on our results and those of Dux et al., we emphasize the possibility that delayed neuronal death is, at least in part, caused by increased calcium cycling of plasma membranes and gradual calcium overload of mitochondria.