Biochemical Indices of Catecholaminergic Neuronal Activity in the Median Eminence during the Estrous Cycle of the Rat

Abstract

An attempt was made to correlate the changes in blood concentrations of prolactin and luteinizing hormone (LH) which occur during the estrous cycle of rats with changes in biochemical estimates of the activities of dopamine (DA)- and norepinephrine (NE)-containing neurons in the median eminence. The serum concentrations of prolactin and LH markedly increased on the afternoon and evening (16.00–24.00 h) of proestrus. The steady state concentrations of NE and DA in the median eminence did not change at any time during the estrous cycle, but the α-methyltyrosine-induced decline of NE was increased during the morning of proestrus (8.00–12.00 h) while the decline of DA was decreased (12.00 h). The rate of total catecholamine synthesis (dopa accumulation after a decarboxylase inhibitor) in the median eminence was not markedly altered at any time during the estrous cycle; the lack of change of dopa accumulation on proestrus may be due, in part, to opposing changes in the rates of amine synthesis in NE and DA neurons.