Calcium excitability and oscillations in suprachiasmatic nucleus neurons and glia in vitro

Abstract

Converging lines of evidence suggest that the hypothalamic suprachiasmatic nucleus (SCN) is the site of the endogenous biological clock controlling mammalian circadian rhythms. To study the calcium responses of the cellular components that make up the clock, computer-controlled digital video and confocal scanning laser microscopy were used with the Ca2+ indicator dye fluo-3 to examine dispersed SCN cells and SCN explants with repeated sampling over time. Ca2+ plays an important second messenger role in a wide variety of cellular mechanisms from gene regulation to electrical activity and neurotransmitter release, and may play a role in clock function and entrainment. SCN neurons and astrocytes showed an intracellular Ca2+ increase in response to glutamate and 5-HT, two major neurotransmitters in afferents to the SCN. Astrocytes showed a marked heterogeneity in their response to the serial perfusion of different transmitters; some responded to both 5-HT and glutamate, some to neither, and others to only one or the other. Under constant conditions, most neurons showed irregular temporal patterns of Ca2+ transients. Expression of regular neuronal oscillations could be blocked by the inhibitory transmitter GABA. Astrocytes, on the other hand, showed very regular rhythms of cytoplasmic Ca2+ concentrations with periods ranging from 7 to 20 sec. This periodic oscillation could be initiated by in vitro application of glutamate, the putative neurotransmitter conveying visual input to the SCN critical for clock entrainment. Long-distance communication between glial cells, seen as waves of fluorescence moving from cell to cell, probably through gap junctions, was induced by glutamate, 5-HT, and ATP. These waves increased the period length of cellular Ca2+ rises to 45-70 sec. Spontaneously oscillating cells were common in culture medium, serum, or rat cerebrospinal fluid, but rare in HEPES buffer. One source for cytoplasmic Ca2+ increases was an influx of extracellular Ca2+, as seen under depolarizing conditions in about 75% of the astroglia studied. All neurotransmitter-induced Ca2+ fluxes were not dependent on voltage changes, as Ca2+ oscillations could be initiated under both normal and depolarizing conditions. Since neurotransmitters could induce a Ca2+ rise in the absence of extracellular Ca2+, the mechanisms of ultradian oscillations appear to depend on cycles of intracellular Ca2+ fluxes from Ca(2+)-sequestering organelles into the cytoplasm, followed by a subsequent Ca2+ reduction. In the adult SCN, fewer astrocytes are found than neurons, yet astrocytes frequently surround glutamate-immunoreactive axons in synaptic contact with SCN dendrites, isolating neurons from each other while maintaining contact with other astrocytes by gap junctions.(ABSTRACT TRUNCATED AT 400 WORDS)