Thyrotropin releasing hormone (TRH): Apparent receptor binding in rat brain membranes

Thyrotropin releasing hormone (TRH) binds to membranes of rat brain tissue via high- and low-affinity binding components. The high-affinity binding of TRH to brain membranes resembles binding to pituitary membranes in terms of its affinity and specificity for most peptides. In equilibrium studies, the dissociation constant for high-affinity binding to brain membranes is about 50 nM, which is about the same as for rat pituitary membranes, while low-affinity binding to brain membranes has a dissociation constant of about 5 μM. Liver membranes display low-affinity binding for TRH with a dissociation constant similar to the low-affinity binding component of brain membranes. No high-affinity binding can be detected with liver membranes. Negligible saturable binding of TRH can be detected with membranes of any tissues examined other than liver, pituitary and brain. The dissociation constants for TRH binding to liver and pituitary determined by kinetic experiments measuring the rate constants for association and dissociation are close to dissociation constants determined by equilibrium experiments. In the cerebral cortex, the dissociation constant from kinetic experiments for the low-affinity binding site resembles the value obtained in equilibrium experiments, while a lower dissociation constant for the high-affinity binding site is obtained in the kinetic experiments. Saturable TRH binding occurs to a similar extent in most regions of the brain, though no high-affinity binding can be detected in the cerebellum. The dissociation constant of the binding site in the brain for TRH corresponds well to endogenous levels of TRH in the brain. The properties of high-affinity TRH binding in brain membranes correspond to those which might be anticipated for the physiological receptor for TRH in the brain.