Supraoptic neurons sustain high frequency firing when extracellular Ca 2+ is replaced with other divalent cations in rat brain slices

Abstract

In slices of hypothalamus, maintained in vitro, the discharge of supraoptic neurons in standard artificial cerebrospinal fluid was compared with that produced when extracellular Ca 2+ was replaced with Mg 2+, Co 2+ or Mn 2+. Interspike interval histograms were constructed for periods before, during and after replacement of extracellular Ca 2+. Of the 31 cells recorded in normal medium, 16 fired slowly and irregularly, 9 were phasic, 3 fired continuously at more than 3 spikes/s and 3 produced short, high frequency bursts of activity that were separated by slow irregular discharge. Interspike interval distributions were broad showing little preference for any one interval and intervals shorter than 30–50 ms were rare. The cell firing rate could be increased by the electrophoretic application of glutamate and under these conditions, the interval distributions became narrower as shorter intervals predominated. However, when cells discharged above 20 spikes/s, the spike amplitude declined rapidly and became indistinguishable from the noise. Replacement of extracellular Ca 2+ with Mg 2+, Co 2+ or Mn 2+ produced reversible changes in interspike interval distribution, although no consistent change in mean firing frequency was observed. Supraoptic neurons were now able to maintain relatively high frequency discharge (15–25 spikes/s) for longer periods; firing either continuously or periodically and interspike intervals became grouped more closely at the shorter end of the normal distribution. However, no very short interspike intervals were recorded. Less than 2% of all recorded intervals were shorter than 30 ms, even in cells exposed to test medium for 1–3 h and excited by application of glutamate. In cells that were transynaptically activated by electrical stimulation of the slice, exposure to test medium blocked stimulus-evoked spike activity. Calcium may be a necessary factor in the mechanism that prevents sustained high frequency discharge in these neurons under normal conditions, but the mechanism that regulates the minimum interspike interval, which is a striking feature of the normal firing of these cells, appears to be less dependent on extracellular Ca 2+.