Twenty-four hour variation of alpha 1-adrenergic receptors in the pineal gland of the male Syrian hamster.

Specific binding of [125I]iodo-[beta-(4-hydroxyphenyl)-ethylaminomethyl]tetralone ([ 125I]HEAT) was used to assess alpha 1-adrenergic receptors on pineal gland membranes of male Syrian hamsters (Mesocricetus auratus) housed under a 14:10 h light-dark cycle (lights on at 06.00 h). Saturation experiments with pooled pineal membrane preparations showed the presence of alpha 1-adrenergic receptor sites (dissociation constant Kd approx. 0.1 nM). Analysis of 4 time points indicated no significant change in Kd, but significant (P less than 0.01) changes of receptor density (Bmax) with a minimum recorded at night. Binding of a constant amount of [125I]HEAT (200 pM) to pineal membranes at 8 time points exhibited a circadian variation (P less than 0.001) of receptor density with lowest values around midnight and highest levels during daytime.