Abstract
Glutamate stimulates LH secretion in adult female rats after activation of N-methyl-D-aspartic acid (NMDA), kainate, and 2-amino-3-hydroxy-5-methyl-4-isoxazol propionic acid receptors. In contrast to the positive role of kainate receptors in the control of LH secretion in adult females, neither activation nor antagonization of kainate receptors in immature rats modified the onset of puberty. The present experiments were carried out to establish why, if kainate stimulates LH release in adult rats, it fails to advance puberty in immature rats, and to determine whether the role of kainate receptors is sexually dimorphic around puberty. In Expt 1, 4-, 8-, 12-, 16-, 20- and 30-day-old females were investigated 15 min after administration of vehicle or kainic acid, a kainate receptor agonist (2.5 mg kg-1). In Expt 2, 30-day-old female rats were studied 2, 5 and 10 min after administration of vehicle, 2.5 mg kainic acid kg-1 or NMDA, an NMDA receptor agonist (15 mg kg-1). In Expt 3, female and male rats were gonadectomized or sham-gonadectomized on day 23 and investigated on day 30 after injection of vehicle, kainic acid (2.5 mg kg-1 at -15 min) or 6,7 dinitroquinoxaline-2,3 dione (DNQX), a kainate antagonist (2 mg kg-1 in two injections at -120 and -60 min). Finally, 30-day-old female and male rats were investigated 15 min after injection of vehicle or NMDA (15 mg kg-1) or 60 min after administration of different doses of 5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801), an NMDA antagonist (0.1, 0.25 or 0.50 mg kg-1). The results indicate that the role of kainate receptors in the control of gonadotrophin secretion is sexually dimorphic around puberty, since: (a) LH secretion was stimulated by kainic acid in male rats but inhibited in females; (b) FSH secretion was inhibited by kainic acid in ovariectomized females, but not in orchidectomized males; and (c) DNQX inhibited LH secretion in males but not in females. These differences were specific for kainate receptors since, in both sexes, NMDA stimulated and MK-801 inhibited LH secretion. It may be concluded that the secretion of gonadotrophins is modulated differently by kainate receptors in prepubertal male and female rats.
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