In vivo release rates of substance P in the preoptic/anterior hypothalamic area of ovariectomized and ovariectomized estrogen-primed rats: correlation with luteinizing hormone and prolactin levels.

Push-pull cannulae were implanted into the preoptic/anterior hypothalamic area (POA) of ovariectomized (OVX) rats. After recovery animals were treated with estradiol (E2) or corn oil and they were perfused 3 days later. Substance P (SP) concentrations were measured in 15 min perfusate fractions, blood samples were taken at similar intervals. SP concentration in POA perfusates were readily measurable. Following estrogen priming SP release increased significantly each afternoon prior to the estrogen induced prolactin and luteinizing hormone (LH) surges. No such increase of SP release was observed in OVX rats with constant LH and prolactin levels throughout the day. Mean SP rates in OVX rats were significantly higher in comparison to OVX estrogen-primed rats. These results indicate that SP may be involved in the feedback mechanisms of estrogen on prolactin and LH release. Authenticity of SP in POA perfusates was made probable by high-performance liquid chromatography (HPLC) where synthetic SP eluted at the same retention time as the signal measured in push-pull perfusates.