Central stimulatory influence of oxytocin on preovulatory gonadotropin-releasing hormone requires more than the median eminence

The study was designed to determine whether the ability of central oxytocin (OT) to stimulate gonadotropin-releasing hormone (GnRH) on the afternoon o f proestrus (PE) in the cycling female rat is mediated at the level of GnRH terminals within the median eminence (ME), or at higher hypothalamic level s where GnRH cell bodies and axons are located. Determining the location of this OT effect in vivo has proven difficult. Therefore, an in vitro system utilizing ME or basal hypothalamic (BH) explants containing GnRH terminals , or GnRH neurons including the cell bodies, axons and terminals, respectiv ely, were harvested from regular cycling female rats at 15:00 h on PE or di estrus (DI). The explants were allowed to preincubate in Krebs Ringer Bicar bonate Buffer containing glucose, ascorbic acid, calcium, and a metalloprot ease inhibitor (KRBG) and enriched with 95% O-2/5% CO2 at 37 degreesC until a stable baseline release of GnRH was achieved (30 min). The 0.05 level of probability was used as the minimum criterion of significance in all exper iments. The ability of OT (10(-15)-10(-9) M) to stimulate the release of Gn RH was determined in both ME and BH explants on PE and DI. The results demo nstrated a sensitive, dose-dependent ability of OT to stimulate GnRH releas e from PE BH explants which was observed only in PE. Furthermore, OT failed to significantly stimulate GnRH release from ME explants on either PE or D l. The data indicate that the PE BH explant paradigm can be used to examine the manner and mechanisms by which OT influences GnRH release on the after noon of PE. Furthermore, the results indicate for the first time that the s timulatory action of OT by itself on preovulatory GnRH release in cycling f emale rats is not mediated at the level of the GnRH terminals within the ME , but requires neuronal interactions and mechanisms within the BH explants. Copyright (C) 2001 S. Karger AG, Basel.