IN-VIVO INTERACTION OF BACLOFEN, TRH AND SEROTONIN ON PRL AND TSH SECRETION IN THE DEVELOPING AND ADULT MALE AND FEMALE RATS

Gamma-aminobutyric acid (GABA) is involved in the neural control of hy pophyseal hormones, including PRL and TSH. In the present work we inve stigated the ontogeny of the effect of baclofen, a GABA B agonist, on basal PRL and TSH release and in the presence of releasing stimulus wh ich act at two different levels: TRH, at the hypophyseal level, and se rotonin, at the central nervous system. Ages studied were 4, 12, 20, 2 8-29, 37-38 day-old and adult male and female animals. Rats of each ag e and sex were separated in groups and each group received two intrape ritoneally injections, one 45 minutes after the other: saline-saline, saline-TRH, baclofen-saline, baclofen-TRH, saline-serotonin or baclofe n-serotonin. Rats were decapitated 15 minutes after the last injection and serum hormones were measured by RIA. Baclofen (7 mg/kg) significa ntly elevated basal prolactin levels at 4, 12 and 20 days of age and t he stimulating effect increased with age. At 28 days of age baclofen s ignificantly inhibited PRL whereas from 38 days of age onwards it had no effect on basal PRL levels. No sex differences were evident. Intera ction of TRH (4 mu g/kg) and baclofen on PRL secretion resulted in an additive effect on days 4 and 12, this effect was not observed when ba clofen was administered with serotonin (10 mg/kg). In 28 day-old and o lder animals baclofen completely blunted the PRL releasing effect of T RH or serotonin. Again, no sex differences were observed. With regard to TSH, baclofen did not alter either basal or TRH stimulated TSH secr etion regardless of sex and age. The present experiments indicate that GABA B receptors are involved in the regulation of basal and stimulat ed PRL secretion from the first days of life to adulthood. Receptor ac tivation results in stimulation or inhibition of PRL release depending on the age of the animals and the site of action. This GABA B regulat ion of PRL secretion is sex independent. In contrast, pituitary GABA B receptors do not seem to be involved in the regulation of TSH secreti on.