Evidence for Dopamine Receptor-Mediated Inhibition of Prolactin Cell Function in the Female Syrian Hamster

Abstract

Since the exact nature of the hypothalamic regulation of prolactin (PRL) cell activity in the photosensitive female Syrian hamster is unknown, the present investigation was designed to determine whether dopamine (DA), a physiological PRL-inhibitory hormone in the rat, inhibits the synthesis and release of female hamster PRL in vitro via a DA receptor-mediated mechanism. Anterior pituitary glands from long photoperiod-exposed adult female Syrian hamsters were incubated in the presence of increasing concentrations of DA (5 nM, 500nM and 50 microM) in Kreb's Ringer Bicarbonate medium for 2 hours following a preincubation period of 1 hour in medium not containing DA. While PRL synthesis, as measured by the incorporation of 3H-leucine into PRL, was unaffected by DA, the release of immunoreactive (RIA)-PRL into the medium was inhibited by 55% and 53% by 500 nM and 50 microM DA, respectively; however, these same concentrations of DA inhibited the release of 3H-PRL into the medium by only 25% and 23%, respectively. The DA receptor blocker, pimozide (PIM) was effective in blocking the PRL-inhibitory effects of DA (500 nM) on both RIA- and 3H-PRL release in vitro. These are the first data suggesting that DA directly inhibits PRL release from female hamster anterior pituitary glands via a DA receptor-mediated mechanism.

Influence of neonatally-administered monosodium glutamate on the neuroendocrine regulation of prolactin cell activity in adult Syrian hamsters

“…This contention is further supported by the fact that unlike in the rat, hypothalamic extracts from hamsters fail to inhibit the release of RIA-PRL from hamster pituitaries in vitro (Orstead and Blask, 1985). Moreover, whereas DA inhibits both the synthesis and release of PRL in the rat, it only inhibits the release of PRL in hamsters Orstead and Blask, 1987). Additionally, when PRL levels are at their nadir in short dayexposed hamsters, hypothalamic DA turnover is also at its lowest rate (Benson, 1987) without a concomitant increase in pituitary sensitivity to DA (Burns et al, 1987).…”

“…On the other hand, the ability of DA to inhibit PRL release in vitro Orstead and Blask, 1987;Burns etal., 1987), the identification of high affinity anterior pituitary DA receptors (Burns et al, 1986) and the hyperprolactinemia which results from pituitary transplants in hamsters (Reiter and Ferguson, 1979) suggest that DA and/or other PIFs may play an important PRL-regulatory role in this species. However, neither DA nor other PIFs may be the only or even the primary influences on PRL regulation in the Syrian hamster.…”