Hippocampal-dependent learning requires a functional circadian system

Norman F. Ruby, Calvin E. Hwang, Colin Wessells, Fabian Fernandez, Pei Zhang, Robert Sapolsky, H. Craig Heller

Research output: Contribution to journalArticlepeer-review

189 Scopus citations

Abstract

Decades of studies have shown that eliminating circadian rhythms of mammals does not compromise their health or longevity in the laboratory in any obvious way. These observations have raised questions about the functional significance of the mammalian circadian system, but have been difficult to address for lack of an appropriate animal model. Surgical ablation of the suprachiasmatic nucleus (SCN) and clock gene knockouts eliminate rhythms, but also damage adjacent brain regions or cause developmental effects that may impair cognitive or other physiological functions. We developed a method that avoids these problems and eliminates rhythms by noninvasive means in Siberian hamsters (Phodopus sungorus). The present study evaluated cognitive function in arrhythmic animals by using a hippocampal-dependent learning task. Control hamsters exhibited normal circadian modulation of performance in a delayed novel-object recognition task. By contrast, arrhythmic animals could not discriminate a novel object from a familiar one only 20 or 60 min after training. Memory performance was not related to prior sleep history as sleep manipulations had no effect on performance. The GABA antagonist pentylenetetrazol restored learning without restoring circadian rhythms. We conclude that the circadian system is involved in memory function in a manner that is independent of sleep. Circadian influence on learning may be exerted via cyclic GABA output from the SCN to target sites involved in learning. Arrhythmic hamsters may have failed to perform this task because of chronic inhibitory signaling from the SCN that interfered with the plastic mechanisms that encode learning in the hippocampus.

Original languageEnglish (US)
Pages (from-to)15593-15598
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume105
Issue number40
DOIs
StatePublished - Oct 7 2008
Externally publishedYes

Keywords

  • GABA
  • Hippocampus
  • Memory
  • Sleep
  • Suprachiasmatic nucleus

ASJC Scopus subject areas

  • General

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