Involvement of norepinephrine activity in the regulation of α1 adrenergic receptors in the medial preoptic nucleus of estradiol-treated rats

To establish whether the diurnal decrease in the density of α1 receptors observed in the medial preoptic nucleus (MPN) of estrogen (E₂)-treated rats is related to the concomitant diurnal increase in norepinephrine (NE) turnover rates, we quantitated the density of [³H]-Prazosin binding to α1 receptors after blockade of NE turnover with alpha-methyl-paratyrosine (αMPT). A series of preliminary studies was performed to rule out an interference of this drug with [³H]-Prazosin binding to α1 adrenergic receptors in vitro and in vivo. Incubation of brain slices with αMPT produced a dosedependent inhibition of [³H]-Prazosin binding to α1 adrenergic receptors with an IC₅₀ of approximately 6 mM. Scatchard analysis demonstrated that αMPT exhibited a simple competitive interaction with [³H]-Prazosin binding sites as shown by an increase in the apparent dissociation constant (Kd) of the ligand and no change in the number of α1 receptors (B_max). In contrast, preincubation of brain slices with αMPT and prior in vivo administration of αMPT did not affect [³H]-Prazosin binding to α1 adrenergic receptors. Once we established that αMPT could be used to suppress NE turnover without interfering with the measurement of α1 receptor densities, we repeatedly injected this drug to ovariectomized (OVX) and E₂-implanted rats. The density of α1 adrenergic receptors in MPN was quantitated autoradiographically. Blockade of NE turnover with αMPT only partially prevented the reduction in α1 receptor density observed in the E₂-treated rats, suggesting that the decrease in the level of [³H]-Prazosin binding sites cannot be completely ascribed to increased NE turnover rates.