Baclofen, a gamma-aminobutyric acid B agonist, modifies hormonal secretion in pituitary cells from infantile female rats

Recent work from our laboratory has demonstrated that the activation of GABA B adenohypophyseal receptors by baclofen inhibits pituitary hormone secretion under basal (PRL) or stimulated conditions (PRL and LH) in adult female rats, suggesting a hypophyseal site of action in additon to the central site previously described. Since different patterns of hormone secretion are observed in infantile and adult rats, the purpose of the present study was to determine whether GABA B pituitary receptors were involved in endocrine responses at early stages of development. Pituitary cells of 12 day-old female rats were cultured in vitro and the effect of baclofen was determined in the presence or absence of stimulatory factors. Baclofen (1.10 −9, 1.10 −7 and 1.10 −5 M) did not alter basal LH or FSH secretion but significantly inhibited the LHRH induced gonadotropins release after 30 or 60 minutes of incubation (after 60 minutes of incubation LH (%): control: 100 ± 5.6, BACL(1.10 −7): 134.5 ± 25.8 ; LHRH(1.10 −7): 596.7 ± 85.9; LHRH(1.10 −7)-BACL(1.10 −7): 374.7±+ 48.0; p < 0.01. FSH (%): control: 100 ± 6.5; BACL(1.10 −7): 103.7 ±+ 6.5; LHRH(1.10 −7): 283.9 ± 29.3; LHRH(1.10 −7)-BACL(1.10 −7): 183.0 ± 20.0; p < 0.01). Baclofen did not significantly modify either basal or TRH-stimulated PRL or TSH secretion. These results show that baclofen has direct effects on the secretion of adenohypophyseal cells of immature rats and that such effects are different from those observed in adult rats, and depend on the stage of development of the neuroendocrine controls of each cellular type.