OSMOTIC EFFECTS ON THE CA1 NEURONAL POPULATION IN HIPPOCAMPAL SLICES WITH SPECIAL REFERENCE TO GLUCOSE

1. Lowered osmolality promotes epileptiform activity both clinically and in the hippocampal slice preparation, but it is unclear how neurons are excited. We studied the effects of altered osmolality on the electrophysiological properties of CA 1 pyramidal cells in hippocampal slices by the use of field and intracellular recordings. The excitability of these neurons under various osmotic conditions was gauged by population spike (PS) amplitude, single cell properties, and evoked synaptic input. 2. The orthodromic PS recorded in stratum pyramidale and the field excitatory postsynaptic potential (EPSP) in stratum radiatum were inversely proportional in amplitude to the artificial cerebro-spinal fluid (ACSF) osmolality over a range of +/- 80 milliosmoles/kgH2O (mosM). The effect was osmotic because changes occurred within the time frame expected for cellular expansion or shrinkage and because permeable substances such as dimethyl sulfoxide or glycerol were without effect. Dilutional changes in ACSF constituents were experimentally ruled out as promoting excitability. 3. To test whether the field data resulted from a change in single-cell excitability, CA 1 cells were intracellularly recorded during exposure to +/- 40 mosM ACSF over 15 min. There was no consistent effect upon CA 1 resting potential, cell input resistance, or action potential threshold. 4. Osmotic alteration of orthodromic and antidromic field potentials might involve a change in axonal excitability. However, the evoked afferent volley recorded in CA 1 stratum pyramidale or radiatum, which represents the compound action potential (CAP) generated in presynaptic axons, remained osmotically unresponsive with regard to amplitude, duration, or latency. This was also characteristic of CAPs evoked in isolated sciatic and vagus nerve preparations exposed to +/- 80 mosM. Therefore axonal excitability and associated extracellular current flow generated periaxonally are not significantly affected by osmotic shifts. 5. The osmotic effect on field potential amplitudes appeared to be independent of synaptic transmission because the inverse relationship with osmolality held for the antidromically evoked PS. Moreover, as recorded with respect to ground, the intracellular EPSP-inhibitory postsynaptic potential (IPSP) sequence (evoked from CA3 stratum radiatum) was not altered by osmolality. 6. The PS could occasionally be recorded intracellularly as a brief negativity interrupting the evoked EPSP. In hyposmotic ACSF, the amplitude increased and action potentials arose from the trough of the negativity as expected for a field effect. This is presumably the result of enhanced intracellular channeling of current caused by the increased extracellular resistance that accompanies cellular swelling. 7. Under hyposmotic conditions, the constant evoked EPSP (recorded intracellularly with respect to ground) and the associated increased field EPSP will result in an altered transmembrane potential, depending on the recording site. In distal CA 1 dendrites (negative field EPSP) the evoked transmembrane EPSP will be enhanced, whereas proximally (positive field EPSP) it will be reduced. 8. We conclude that osmotic effects on the excitability of the hippocampal CA 1 population are mediated primarily by altered field effects during synchronous discharge but not by changes in single cell properties or axonal excitability. 9. Unlike the impermeant substance mannitol, PS amplitude was not reduced by D-glucose below a threshold concentration approximately 26 mM, above which it acted osmotically effective (i.e., cell impermeant). Thus a 40-mosM increase using D-glucose reversibly blocked electrographic seizures in six of seven hippocampal slices in zero Mg2+ ACSF. Considering that plasma glucose levels can fluctuate dramatically in poorly controlled diabetes mellitus, our findings suggest that associated changes in CNS excitability have, at least in part, an osmotic basis.