Implication of D2-like dopaminergic receptors in the median eminence during the establishment of long-day inhibition of LH secretion in the ewe

Abstract

In ewes, photoperiod modulates LH release and dopaminergic terminals in the median eminence (ME) have a critical role in the establishment of long-day inhibition of LH secretion. This study was undertaken to determine the type of dopaminergic receptors, D1-like or D2-like, that mediate the action of dopamine on LH secretion at the ME level in this situation. This was assessed, in ovariectomized and estradiol-treated ewes, with the use of reverse microdialysis in the ME in three experiments: first, when LH secretion was stimulated by short days, by determining the response to three doses (0·01, 0·1 or 1 mg/ml) of a D1-like (SKF38393) and a D2-like (quinpirole) agonist; secondly, during early long-day inhibition of LH secretion, by determining the ability of SKF38393 and quinpirole (1 mg/ml) to mimic the inhibitory effects of dopamine, after a blockade of its synthesis with -methyl-paratyrosine ( MPT; 2 mg/ml); and thirdly, during early long-day inhibition of LH secretion, by determining the response to three doses (0·009, 0·09 or 0·9 mg/ml) of a D1-like (SCH23390) and a D2-like (sulpiride) antagonist.In none of the conditions was effect of the D1-like analogs on LH secretion found, compared with the control treatments. In contrast, the D2-like analogs caused changes in LH secretion. First, with short days, quinpirole in the highest dose significantly reduced mean LH concentration (P<0·05) and LH pulse frequency (P<0·01). Secondly, with long days, addition of quinpirole to MPT caused a significant decrease in LH secretion relative to MPT alone (P<0·05). Thirdly, with long days, sulpiride at the highest dose significantly increased mean LH concentration (during the first 3 h of treatment, P<0·05) and LH pulse frequency (P<0·05). Prolactin secretion was also determined in these experiments, and D2-like agonist and antagonist caused an inhibition and a stimulation of prolactin secretion, respectively. These results demonstrate that, in the ME, inhibitory action of dopamine on LH secretion, critical for the initiation of long-dayinduced inhibition, is mediated by D2-like, not D1-like, dopaminergic receptors.

Plasma prolactin changes in anoestrous ewes after infusion of genistein into the third ventricle

“…Indeed, the suppression of prolactin concentrations following infusion of D2-like (quinpirole) agonist into MBH (Anderson et al, 1997) supports findings of Wolinska et al (1977) and Curlewis et al (1991). Dopamine might also act through dopamine D2 receptors located on lactotrophs in the anterior pituitary in the ewe (Anderson et al, 1997;Bertrand et al, 1999). On the other hand a stimulatory system controlling the secretion and production of prolactin in sheep was suggested to operate via dopamine Dl receptees in the ventromedial hypothalamus (VMH) (Curlewis et al, 1995).…”

Dopamine D1 and D2 receptor immunoreactivities in the arcuate-median eminence complex and their link to the tubero-infundibular dopamine neurons

“…There also exists evidence based on the use of reverse microdialysis of D2DR agonists into the median eminence of the ewe that D2DR but not D1DR in the median eminence establish long-day induced inhibition of LH secretion. 17 Thus, it is possible that both D2DR and D1DR mechanisms in the LPZ can be in operation in the inhibitory control of LHRH secretion. It should be noted that D2DR agonists in contrast to D1DR agonists increase DA synthesis and metabolism in the rat median eminence as determined from accumulation of DOPA after DOPA decarboxylase inhibition and measurement of DOPAC.…”

Dopamine Inhibits Luteinizing Hormone Synthesis and Release in the Juvenile European Eel: A Neuroendocrine Lock for the Onset of Puberty1

“…In mammals, DA has been shown in several species to be involved in the modulation of pulsatile LH release through direct or indirect actions. This has been extensively studied in sheep, in which DA participates in the inhibition of gonadotrophic activity before puberty as well as in the adult during seasonal anestrus by acting on GnRH neurons [59][60][61]. A direct inhibitor action of DA at the pituitary level was reported in the rabbit, in which DA inhibits GnRH-stimulated LH release in normal as well as in stalk-sectioned animals [62].…”