Effect of suckling on the in vivo release of thyrotropin-releasing hormone, dopamine and adrenaline in the lactating rat.

The present study was concerned with the effect of suckling on the hypothalamic release of thyrotropin-releasing hormone (TRH), dopamine and adrenaline in lactating rats as estimated by push-pull perfusion of the median eminence-arcuate nucleus area. The push-pull cannula was implanted on day 15 of pregnancy. This surgery did not interfere with pregnancy, time of delivery or lactation. Push-pull perfusion was performed on day 8 or 14 of lactation and 30 out of 42 perfusions were successful. On the day of perfusion mothers and young were separated. Six hours later push-pull perfusion was begun and 6 samples at 15-min intervals were collected. In control animals, not allowed to nurse pups during perfusion, the release of TRH, dopamine and adrenaline did not change during the 90-min period. In experimental animals, reunited with their litter after 30 min of perfusion, the hypothalamic release of adrenaline did not change. However, both on day 8 and 14 suckling induced a 50% decrease in the release of dopamine (p less than or equal to 0.025) which lasted for 15-30 min. Suckling on day 14 did not affect the concentration of TRH in the perfusate, but on day 8 the TRH output gradually decreased for 45 min after the onset of suckling.