Influence of light irradiance on hydroxyindole-O-methyltransferase activity, serotonin-N-acetyltranferase activity, and radioimmunoassayable melatonin levels in the pineal gland of the diurnally active Richardson's ground squirrel

Russel J. Reiter, Edward C. Hurlbut, George C. Brainard, Stephan Steinlechner, Bruce A. Richardson

Research output: Contribution to journalArticle

45 Scopus citations


When Richardson's ground squirrels were kept under light:dark cycles of 14:10 h there was no nocturnal rise in pineal hydroxyindole-O-methyltransferase (HIOMT) activity. Conversely, the 10 h dark period was associated with large nocturnal rises in both pineal serotonin-N-acetyltransferase (NAT) activity and radioimmunoassayable melatonin levels. The nighttime rises in pineal NAT and melatonin were not suppressed by the exposure of the animals to a light irradiance of 925 μW/cm2 during the normal dark period. On the other hand, when the light irradiance was increased to 1850 μW/cm2 the rise in pineal NAT activity was eliminated while the melatonin rise was greatly reduced. When ground squirrels were acutely exposed to a light irradiance of 1850 μW/cm2 for 30 min beginning at 5.5 h after lights out, pineal NAT activity and melatonin levels were reduced to daytime values within 30 min. The half-time (t 1 2) for each constituent was less than 10 min. Exposure to a light irradiance of either 5 s or 5 min (beginning at 5.5 h intoim dark period) was equally as effective as 30 min light exposure in inhibiting pineal NAT activity and melatonin levels. When animals were returned to darkness after a 30 min exposure to a light irradiance of 1850 μW/cm2 at night, both pineal NAT activity and melatonin levels were restored to high nighttime levels within 2 h of their return to darkness. The results indicate that the pineal gland of the wild-captured, diurnal Richardson's ground squirrel is 9000× less sensitive to light at night than is the pineal gland of the laboratory raised, nocturnal Syrian hamster.

Original languageEnglish (US)
Pages (from-to)151-157
Number of pages7
JournalBrain Research
Issue number1-2
StatePublished - Dec 12 1983



  • N-acetyltransferase
  • hydroxyindole-O-methyltransferase
  • irradiance
  • light
  • melatonin
  • pineal

ASJC Scopus subject areas

  • Neuroscience(all)
  • Molecular Biology
  • Clinical Neurology
  • Developmental Biology

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