Galanin (GAL) participates in the regulation of the hypothalamic-pituitary-gonadal axis. On the other hand, oestrogen dramatically stimulates GAL gene expression in the anterior pituitary of mice and rats (Shen et al. 1999). In these studies, we have evaluated whether oestrogen effects on anterior pituitary GAL gene expression require the physical connection between the pituitary and the hypothalamus.
Fifty-day-old Sprague-Dawley female rats were housed under a 14 h-10 h light-dark cycle and treated according to the European Community Guidelines for Care and Use of Laboratory Animals; all the results are expressed as means ± S.E.M.; n = 5. They were ovariectomized under I.P. ketamine-xylazine anaesthesia ((100 mg-10 mg) kg-1 b.w.), and allowed to recover for 2 weeks. Rats were treated orally for either 24 h (h) or 4 days (d) with oestrone sulphate (E1S) or vehicle (0.5 % carboxymethyl-cellulose), and killed humanely. Changes in anterior pituitary GAL gene expression were measured by ribonuclease protection assay.
E1S increased GAL mRNA levels in a time- and dose-dependent fashion. Maximal effects were reached 12 h after a single E1S dose at doses greater than 3 mg kg-1, with 14.72-fold increase (ED50 = 2.17 ± 1.24 mg kg-1). After 4 days of treatment (killing conducted 24 h after the last dose), maximal increases in GAL gene expression were reached at the 10 mg kg-1 E1S dose, with a 99.77-fold increase (ED50 = 2.93 ± 1.25 mg kg-1). The effects of E1S on GAL gene expression were also studied in ovariectomized rats bearing a pituitary transplant under the kidney capsule. GAL mRNA levels increased in both the native and the transplanted pituitaries after E1S treatment. However, while in the native pituitaries GAL gene expression increased 7.68- and 23.23-fold after 24 h and 4 days treatment, respectively, in the transplanted pituitaries elevations of only 2.11- and 2.40-fold were observed for the 24 h and 4 day regimens. Serum PRL levels indicated that the transplanted pituitaries were fully functional.
We conclude that oestrogen action requires a physical connection between the pituitary and the hypothalamus to induce maximal pituitary GAL gene expression. Therefore, although the nature of a hypothetical hypothalamic mechanism(s) and its precise action are unknown, these data suggest that oestrogen-induced GAL gene expression results from a combined action of the steroid on both the hypothalamus and the anterior pituitary.
This work was supported by grants SAF2001-3614-C03-01 and BFI2002-00485.