Effect of the TRH analogue, CG 3703, on prolactin secretion in the pseudopregnant rat.

The effect of the TRH analogue, CG 3703 (AG), on secretion of prolactin (Prl) was studied in pseudopregnant rats. AG stimulated Prl secretion of incubated pituitary glands in the presence of 5.10(-8) M dopamine (DA) but not in the presence of 0 or 10(-6) M DA. AG did not inhibit the in vitro secretion of Prl. AG postponed the peaks of Prl which were generated between 0100 and 1200 h (delay after 500 micrograms AG: 3-4 h); the height of these nocturnal Prl peaks was not affected by AG. AG did not induce Prl secretion during the phase of low Prl secretion (the interphase) between 2000 and 0100 h. The DA-release blocking drug, HA 966, induced peaks of Prl during the interphase. These peaks were as high as spontaneous nocturnal Prl peaks and were blocked by AG. The results show that AG has two opposite effects on Prl secretion: a direct, stimulating effect, which is modulated by DA, and an indirect inhibitory effect, which is probably executed by DA, released by AG from the tuberoinfundibular DA neurons.