The role of catecholamines in the regulation of gonadotropin secretion

Earlier neuropharmacological evidence suggested a role for the central catecholamine (CA) system in the regulation of gonadotropin secretion. To resolve whether dopamine (DA) or norepinephrine (NE) was the important CA involved in evoking the preovulatory release of LHRH and to evaluate the temporal sequence of changes in the CAs which occur prior to and during preovulatory-like gonadotropin surges, we evaluated turnover rates (T/R) of DA and NE in micro-dissected regions of the hypothalamus of proestrous rats. We also examined whether similar changes in T/R occur in ovariectomized (OVX), estrogen-(E2)-treated rats and how progesterone (P) amplifies and advances the time of the LH surge in this rat model system. In these studies we used a radioenzymatic assay to measure CA content and alpha-MPT to block CA synthesis and provide information on the rate of CA efflux. This information was used to calculate T/R. In proestrous rats, a dramatic increase in NE T/R occurred prior to and during the LH surge whereas DA T/R initially increased and then markedly declined during the afternoon of proestrus. The NE T/R changes occurred in the medial preoptic nucleus (MPN), suprachiasmatic nucleus (SCN), arcuate nucleus (AN) and median eminence (ME) whereas DA T/R changes were observed only in AN and ME. When phenobarbital was administered to block spontaneous proestrous LH surges it also completely blocked the increase in NE T/R which occur in ME and SCN but did not affect DA T/R. In OVX E2-treated rats similar increases in MPN, SCN and ME NE T/R occurred during the time of the LH surge while DA T/R remained unchanged. The P advancement of the time of the LH surge (by 1 h) was accompanied by an advancement of the time of increased NE T/R in the hypothalamus. In androgen-sterilized rats, neither spontaneous nor E2-induced LH surges occurred. All microdissected brain regions in ASR contained less NE than controls but DA values in ME were comparable in both groups. No increase in NE T/R occurred between morning and afternoon in MPN or ME while DA T/R declined in both control and ASR between morning and afternoon. These observations suggest that NE rather than DA is the important CA responsible for triggering the preovulatory-like release of LHRH from ME axon terminals. They further suggest that E2 affects function of NE perikarya in the mid and hind-brain to increase the frequency of depolarization which accounts for the increase in NE secretion into the preoptico-suprachiasmatic, tuberoinfundibular system.