Effects of serotonin and metergoline on 125[I]-iodocyanopindolol binding parameters to beta-adrenergic receptors in rat brain.

Most ligands which have been employed to investigate the regulation of beta-adrenergic receptors (betaAR) under pathophysiological conditions and in response to pharmacological manipulations have also been shown to have affinity for 5-HT1B receptors. We examined the effects of serotonin and metergoline (10 microM) on 125I-iodocyanopindolol (ICYP, 5-100 pM) binding to betaAR in rat frontal cortex and hippocampus membranes. In both brain regions, the presence of either serotonin or metergoline significantly lowered iodocyanopindolol dissociation constant (Kd) and maximum binding capacity (Bmax). Isoproterenol displacement curves showed that the decrease in receptor density was primarily due to a significant decrease in the receptors in the low-conformational state. Thus, a significant fraction of the apparent ICYP binding to betaAR in the low-conformational state was due to binding to 5-HT1B receptors. Neither serotonin nor metergoline had an effect on the agonist isoproterenol dissociation constant from betaAR in either conformational state.