Parachlorophenylalanine blocks the spontaneous pro-oestrous surge of prolactin as well as LH and affects the secretion of oestradiol-17 beta.

The effect of the 5-hydroxytryptamine synthesis inhibitor, parachlorophenylalanine (PCPA), on the spontaneous, pro-oestrous surges of prolactin and LH was investigated in conscious female rats implanted with an intra-atrial cannula. The LH surge was blocked in all animals treated with PCPA, but the prolactin surge was blocked in six out of ten animals and unaffected in four out of ten animals. There was a significant correlation between the plasma concentrations of prolactin and oestradiol-17 beta, surges of prolactin occurred in animals with plasma oestradiol-17 beta concentrations of 120 pmol/l and above, and the LH surge was blocked in animals in which the oestradiol-17 beta concentration was less than 188 pmol/l plasma. These results show that (i) PCPA can block the pro-oestrous prolactin as well as the LH surge, (ii) in addition to a central action PCPA may depend also on its peripheral effect on the secretion of oestradiol-17 beta and (iii) on average, lower plasma concentrations of oestradiol-17 beta are required for triggering the prolactin compared with the LH surge.