[Postnatal lactotropic hypoactivity in rats: possible implication of VIP, endogenous opioids, dopamine and...

In this study, the possible intervention of VIP (Vasoactive Intestinal Peptide), beta-endorphine, glycocorticoids and dopamine, in postnatal lactotrophs hypoactivity at the rat, was investigated. Our results show that the injection of VIP (40 micrograms/kg), beta-endorphine (125 micrograms/kg) or its antagonist, naloxone (2 mg/kg), does not provide a change in serum PRL before or after ether stress at 6-day-old rats. However, after adrenalectomy, 6 day-old neonates became sensitive to ether stress as do adults, while acute treatment with dexamethasone (2 mg/kg) or dopamine (1 mg/kg), abolished this response completely. Sulpiride injection (1 mg/kg), on the contrary potentialized response. This lactotroph insensitiveness to ether stress, TRH, VIP, and beta-endorphine, during the beginning of postnatal life in the rat, might be explained, partially, by the failure of stimulatory factors "PRF" (Prolactin Releasing Factors), together with strength of inhibitory factors "PIF" (Prolactin Inhibitory Factors), such as dopamine and glycocorticoids.