Circadian variation of β-adrenoceptor binding sites in the pineal gland of the Syrian hamster and prevention of the nocturnal reduction by light exposure or propranolol treatment

Radioreceptor assays with (-)[¹²⁵I]-iodopindolol (IPIN) were used to describe circadian variations of β-adrenoceptors in the pineal gland of male Syrian hamsters (Mesocricetus auratus). Single-point experiments (200 pM IPIN, 8-9 pineals pooled per time point) showed a significant circadian variation (p < 0.01) with maximal values (28.9 ± 1.8 fmol/mg protein, x̄ ± SEM, n = 7) between 07:00 and 02:30 h, and minimal values at 04:00 (7.2 ± 2.9 fmol/mg protein, n = 3), 8 h after darkness onset. Either exposure of animals to light at night or treating dark exposed hamsters with a β-adrenergic receptor blocker, propranolol, prevented the nocturnal drop in the number of β-adrenoceptors. Scatchard analysis of saturation isotherms at 02:30, 04:00 and 08:00 h (30-600 pM IPIN, one saturation experiment with 25 pineals pooled per time point) confirmed the circadian variation.