MICRODIALYSIS MEASUREMENT OF NEUROCHEMICAL CHANGES IN THE MEDIOBASAL HYPOTHALAMUS OF OVARIECTOMIZED EWES DURING ESTRUS

Oestrus behaviour and the luteinizing hormone (LH) surge are induced i n ovariectomized ewes by oestradiol (E2) after a period of progesteron e priming with a low level of E2 (Pge2) and we have previously shown t hat these effects are primarily mediated through their action on the m ediobasal hypothalamus (MBH). The aim of the present study was to asse ss what neurochemical changes in the MBH are induced by these steroids that might mediate their action on oestrus behaviour and LPI release. Eight ovariectomized ewes were implanted with microdialysis probes in the MBH and submitted to three artificial cycles, so that they exhibi ted either both oestrus behaviour and an LH surge (Pge2 + E2), an LH s urge alone (E2 alone) or neither oestrus behaviour nor an LH surge (Pg e2 alone). Microdialysis and blood samples were collected every 30 min from 4 h before the end of Pge2 treatment until the end of oestrus. B ehavioural tests with a ram were made to assess receptivity. Dopamine (DA) levels were found to increase significantly at the termination of Pge2 treatment after both Pge2 + E2 and Pge2 treatments. When the ewe s received E2 after a Pge2 + low estradiol priming (Pge2 + E2), DA lev els decreased 16 h later (4 h after E2) whereas they did not change af ter E2 or Pge2 alone. By contrast, serotonin (5HT) levels did not chan ge significantly during the first 24 h but then increased when ewes re ceived E2 alone and decreased when they were treated with Pge2 + E2. g amma-Aminobutyric acid (GABA) concentrations decreased significantly a t the beginning of the sampling period after all treatments but this d ecrease lasted longer after Pge2 + E2 and was most pronounced at the b eginning of receptivity. No significant long term effects of these ste roid treatments were found on noradrenaline (NA), aspartate, glutamate , glycine and taurine levels. However, E2 administration was followed during the next few hours by a significant increase in glycine and to a smaller extent in glutamate and GABA. More importantly, when ewes we re treated with Pge2 + E2, NA levels increased significantly following the behavioural interactions with a ram when the ewes were sexually r eceptive. In contrast to this, DA levels only increased during interac tions with the ram when the ewes were not receptive. 5HT levels increa sed after tests where the ewe was either receptive or unreceptive to t he male. GABA, aspartate and glycine levels increased in the sample ju st preceding the test and then decreased during it. These results show that a number of neurotransmitter changes occur in the MBH during hor monal induction of oestrus. Changes mainly occurred in DA, NA, 5HT and GABA concentrations during oestrus when animals were treated with Pge 2 + E2 and suggest that they may be potentially involved in the contro l of female sexual behaviour. The changes occurring when animals recei ved E2 alone and showed only prolactin and LH surges, but no behaviour al oestrus, are much more limited. Furthermore, they do not correspond to changes after the E2 injection in the Pge2 + E2 treatment, so it i s unlikely that the neurochemical changes we observed are directly rel ated to these endocrine events.