Calcium response to retinohypothalamic tract synaptic transmission in suprachiasmatic nucleus neurons

Robert P. Irwin, Charles N. Allen

Research output: Contribution to journalArticlepeer-review

58 Scopus citations


Glutamate released from retinohypothalamic tract (RHT) synapses with suprachiasmatic nucleus (SCN) neurons induces phase changes in the circadian clock presumably by using Ca2+ as a second messenger. We used electrophysiological and Ca2+ imaging techniques to simultaneously record changes in the membrane potential and intracellular calcium concentration ([Ca2+]i) in SCN neurons after stimulation of the RHT at physiologically relevant frequencies. Stimulation of the RHT sufficient to generate an EPSP did not produce detectable changes in [Ca2+] i, whereas EPSP-induced action potentials evoked an increase in [Ca2+]i , suggesting that the change in postsynaptic somatic [Ca2+]i produced by synaptically activated glutamate receptors was the result of membrane depolarization activating voltage-dependent Ca2+ channels. The magnitude of the Ca2+ response was dependent on the RHT stimulation frequency and duration, and on the SCN neuron action potential frequency. Membrane depolarization-induced changes in [Ca2+]i were larger and decayed more quickly in the dendrites than in the soma and were attenuated by nimodipine, suggesting a compartmentalization of Ca2+ signaling and a contribution of L-type Ca2+ channels. RHT stimulation at frequencies that mimicked the output of light-sensitive retinal ganglion cells (RGCs) evoked [Ca 2+]i transients in SCN neurons via membrane depolarization and activation of voltage-dependent Ca2+ channels. These data suggest that for Ca2+ to induce phase advances or delays, light-induced signaling from RGCs must augment the underlying oscillatory somatic [Ca2+]i by evoking postsynaptic action potentials in SCN neurons during a period of slow spontaneous firing such as occurs during nighttime.

Original languageEnglish (US)
Pages (from-to)11748-11757
Number of pages10
JournalJournal of Neuroscience
Issue number43
StatePublished - Oct 24 2007


  • Action potential
  • Calcium
  • Circadian rhythm
  • Retinal ganglion cells
  • Suprachiasmatic nucleus
  • Synaptic transmission

ASJC Scopus subject areas

  • Neuroscience(all)


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