Circadian rhythm and uptake of taurine by the rat pineal gland

Taurine is believed to be a modulator of membrane excitability in muscle and a neuroinhibitory transmitter in the central nervous system. The retina and pineal contain relatively large quantities of taurine. Taurine levels in the retina are reported to be responsive to variations in lighting conditions. We report here a carcadian rhythm for taurine in the mature male rat pineal gland. The maximum taurine concentration occurs at the midpoint of the light period, 24 ± 1.9 nmoles/gland, and the minimum at the beginning of the dark period, 13.9 ± 1.6 nmoles/gland. Sympathectomy by bilateral superior cervical ganglionectomy lowered pineal taurine levels. Constant light and blinding had no effect. Taurine was demonstrated to be taken up by the pineal gland $\text{in}$$\text{vitro}$ in organ culture. The uptake was saturable, K_m = 2.0 mM, and sodium dependent. The close structural analogs hypotaurine and β-alanine inhibited taurine uptake but α-alanine did not. We have demonstrated a circadian rhythm for taurine content in the rat pineal gland and the presence of a sodium-dependent transport system for taurine in the pineal $\text{in}$$\text{vitro}$ in organ culture.