Estradiol induction of proenkephalin messenger RNA in hypothalamus: dose-response and relation to reproductive behavior in the female rat

Hormone effects on proenkephalin (PE) mRNA allow an opportunity to compare a brain region-specific molecular change with a quantifiable behavior. Slot blots were used to measure PE mRNA levels in the ventromedial hypothalamus (VMN) and preoptic area (POA) as a function of the dose of estrogen administered to ovariectomized rats. Every rat used had been characterized for the ability to display lordosis behavior. Estradiol treatment led to a monotonic dose-dependent increase in PE mRNA level in VMN, while only a small effect was observed in POA at the higher estradiol doses. Lordosis behavior, assessed manually and in mating tests, also increased monotonically with estradiol dose. The data indicate that an apparent 'threshold' level of PE mRNA in VMN coincided with display of behavior, and suggest further that high levels of PE mRNA, alone, are not sufficient for lordosis. While the exact relationship of the eventual product, Met-enkephalin, to female reproductive behavior remains to be determined, the parallel changes in PE mRNA level and behavior encourage further analysis.