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Subthreshold oscillation of the membrane potential in magnocellular neurones of the rat supraoptic nucleus

Gerd Boehmer, Wolfgang Greffrath, Erich Martin and Sven Hermann

1. Electrophysiological properties and ionic basis of subthreshold oscillation of the membrane potential were examined in 104 magnocellular neurones of the rat supraoptic nucleus using intracellular recording techniques in a brain slice preparation.

2. Subthreshold oscillation of the membrane potential occurring in all neurones examined was voltage dependent. Oscillation was initiated 7-12 mV negative to the threshold of fast action potentials. Oscillation was the result of neither excitatory nor inhibitory synaptic activity nor of electric coupling.

3. Frequency analyses revealed a broad band frequency distribution of subthreshold oscillation waves (range 10-70 Hz). The frequency band of 15-33 Hz was observed in neurones depolarized close to the threshold of discharge.

4. Subthreshold oscillation was blocked by TTX (1·25-2·5 µM) as well as by TEA (15 mM). Subthreshold oscillation was not blocked by low Ca²⁺-high Mg²⁺ superfusate, CdCl₂, TEA (1-4·5 mM), 4-aminopyridine, apamin, charybdotoxin, iberiotoxin, BaCl₂, carbachol and CsCl.

5. During application of TTX, stronger depolarization induced high-threshold oscillation of the membrane potential at a threshold of about -32 mV. These oscillation waves occurred at a mean frequency of about 35 Hz and were blocked by CdCl₂.

6. Effects of ion channel antagonists suggest that subthreshold oscillation is generated by the interaction of a subthreshold sodium current and a subthreshold potassium current. The generation of high-threshold oscillation during TTX involves a high-threshold calcium current.

7. Subthreshold oscillation of the membrane potential may be important for the inter-neuronal synchronization of discharge and for the amplification of synaptic events.

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