Indication of diurnal variations in the rat choroid plexus for cerebrospinal fluid secretion

The epithelial tissue of the choroid plexus (CP) is responsible for producing cerebrospinal fluid (CSF). The CP possesses a robust circadian clock, suggesting that CSF secretion from the CP is regulated by the circadian clock; however, the underlying mechanism is not yet fully understood. In the present study, we investigated diurnal variations in the expression of mRNA and proteins that contribute to CSF secretion in the rats. Of these gene products, only the expression of Glut1 mRNA in the CP of the lateral and fourth ventricles (CP-LV and CP-4V, respectively) and Cldn2 mRNA in the CP-4V exhibited significant diurnal variation using quantitative PCR analysis. On the other hand, the expression of Aqp1, Nkcc1, and Cldn2 mRNA showed fluctuation that alternate peaks and troughs every 3 h, and the timing of the peak in expression of the three genes was consistent. Western blot analysis did not reveal any significant diurnal variation in GLUT1 expression, whereas AQP1 in the CP-4V and CLDN2 in the CP-LV and CP-4V exhibited clear diurnal variation. The above results indicate that the diurnal variation observed in AQP1 and CLDN2 occur as the result of diurnal variations in translation or degradation activity. AQP1 and CLDN2 expression levels are likely to determine the changes in CSF secretion volume during a day. Fluorescence immunohistochemistry clearly detected AQP1 immunoreactivity in the apical side of the CP during the night, whereas almost no immunoreactivity was detected during the daytime. This result is suggestive of loss of antigenicity due to modified AQP1.