Aging and photoperiod regulate glutamic acid decarboxylase₆₇ messenger RNA expression

Aging and short photoperiod exposure both induce similar long-term changes in circadian rhythms, including alterations in the timing and the amplitude of rhythms. Furthermore, these chronic conditions affect the function of the circadian pacemaker in the suprachiasmatic nuclei (SCN) by altering rhythmic expression of neuropeptide messenger RNAs (mRNAs). Because GABA modulates SCN neuronal activity, and GABAergic neurons innervate peptidergic neurons in the SCN, the present study investigated whether photoperiod or aging affect the expression of mRNA for GAD₆₇, the enzyme responsible for regulating the tonic levels of GABA. As a control for regional specificity, the reticular thalamic nucleus (RTN) was also examined. In situ hybridization for GAD₆₇ mRNA was performed on brain sections derived from Siberian hamsters exposed to a long day or a short day photoperiod for 15 days, and on brain sections from young (3-4 months old) and old (12-17 months old) Syrian hamsters exposed to a long photoperiod. The results showed that photoperiod and aging have different effects on GAD₆₇ mRNA expression. Exposure to short day photoperiod significantly increased GAD₆₇ mRNA expression in both the SCN and RTN of Siberian hamsters, while aging significantly decreased GAD₆₇ mRNA expression in the RTN of Syrian hamsters but had no effect on GAD₆₇ mRNA expression in the SCN. These findings suggest that modulation of GAD₆₇ mRNA expression in the SCN is associated with photoperiodic regulation of neuropeptide mRNA expression, but is not a common mechanism for chronic regulation of circadian rhythms. Also, GAD₆₇ mRNA expression in the RTN is differentially affected by photoperiod and aging.