Slow, regular discharge in suprachiasmatic neurones is calcium dependent, in slices of rat brain.

The Ca2+ dependence of suprachiasmatic firing patterns was studied in isolated hypothalamic slices. Single neurones were first recorded in a standard artificial cerebrospinal fluid and then followed through a change to a medium in which the Ca2+ was replaced with Mg2+, Co2+ or Mn2+. These test media caused disruption of the normal, slow, regular firing patterns of suprachiasmatic neurones. Some interspike intervals were shorter and some longer than any recorded under normal conditions, for a given firing frequency. In the absence of Ca2+, these cells could not be driven to fire more regularly as they fired more rapidly during glutamate applications. In contrast, in the presence of Ca2+ more slowly and irregularly firing suprachiasmatic cells can be driven to fire regularly if their firing rate is increased. The effects of these test media were reversible. When Ca2+ was replaced with Ba2+, a bursting pattern of discharge resulted. Periods of high frequency discharge, possibly superimposed on Ba2+ spikes, alternated with periods of slow, regular firing or silence. It is concluded that Ca2+ is necessary for the maintenance of regular firing in suprachiasmatic neurones. The possibility that Ca2+ channels similar to those present in other slow, regularly firing central neurones, play an important role in suprachiasmatic activity, is discussed.