Monoamine Content of the Stalk‐Median Eminence and Hypothalamus in Adult Female Sheep as Affected by Daylength

Abstract

In the ewe, plasma luteinizing hormone and prolactin concentrations exhibit seasonal variations. During long days, inhibition of pulsatile luteinizing hormone secretion is mediated by monoamines. In a model of ovariectomized ewes bearing a subcutaneous oestradiol implant, we previously showed that the steroid-dependent inhibition of luteinizing hormone involves the A15 dopaminergic nucleus of the retrochiasmatic area. In the present work, we compared the aminergic activities of tele-diencephalic structures in groups of ovariectomized ewes under artificial illumination for short versus long days (8 versus 16 h/day of light, respectively). Half the animals in each group were bearing a subcutaneous oestradiol implant. Using high-performance liquid chromatography and electrochemical detection, we measured the levels of amines and amine metabolites in 'punches' of tissues from regions containing luteinizing hormone-releasing hormone axon terminals or cell bodies and catecholaminergic structures. Concurrently, we checked the pulsatile luteinizing hormone release and plasma prolactin concentration to assess the ability of our model to mimic seasonal changes in the hormonal status.As expected, ovariectomized ewes with a subcutaneous oestradiol implant showed an inhibition of the pulsatile luteinizing hormone release under long days. A higher concentration of plasma prolactin was also observed under long days, without any effect of the steroid treatment. Under this light regimen, statistically significant higher contents of dopamine than under short days were found in the stalk-median eminence. Larger contents of homovanillic acid, a dopamine metabolite, and 4-hydroxy-3-methoxyphenylethyleneglycol (MHPG), a noradrenaline metabolite were observed in the infundibular nucleus, while the catecholamines themselves remained unchanged. Furthermore, oestradiol also significantly increased the content of MHPG in the latter structure. During long days, animals without oestradiol treatment exhibited a significant lower content of noradrenaline in the A15 nucleus, without any alteration of the dopamine content. Daylength or oestradiol treatment had no significant effects on the levels of amines or amine metabolites in the preoptic or septa1 areas. Thus, our results in the ewe underline the role played by the medial basal hypothalamus in the catecholaminergic regulation of seasonal changes in hormone release and suggest modifications in the turnover of the neurotransmitters in some structures.In the ewe, which is a short day breeder, annual variations in the secretion of luteinizing hormone (LH) and prolactin (PRL) accompany seasonal changes in reproductive activity (1 -3). The seasonal effect on LH (4, 5 ) and PRL secretion (6) has also been demonstrated in the ovariectomized ewe. During the period corresponding to anoestrus in the intact animals, the frequency of LH pulses in the ovariectomized females was slightly lower than during the breeding season, (steroid-independent inhibition), but the effect was fully expressed wh...

Identification of salsolinol in the mediobasal hypothalamus of lactating ewes and its relation to suckling-induced prolactin and GH release

“…In the current study, we have shown that salsolinol is present in the IN/ME of lactating ewes. A higher concentration of salsolinol and its variability found in the anterior than the posterior part of the IN/ME suggest that the main source of this compound may be the dopaminergic cells from the IN A12 and/or these from the retrochiasmatic area (Thiery 1991). Our data did not show, however, a direct proof for salsolinol synthesis in these dopaminergic nuclei.…”

“…Because of the different localization of dopaminergic nuclei in the ovine hypothalamus (Thiery 1991), the statistical analysis for salsolinol was done separately for the anterior (nZ4) and posterior (nZ6) parts of the IN/ME, while the changes in the concentrations of prolactin and GH were analyzed together (nZ10) due to the same protocol of the experiment used for all ewes. The effect of suckling on salsolinol concentration was first examined by ANOVA for repeated measurements and then for consecutive perfusates by one-factor ANOVA followed by the Tukey's post hoc test (Statistica, StatSoft, Inc., Tulsa, OK, USA).…”

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

“…Several arguments suggest that these terminals are implicated in one of the final steps of the long-day-induced inhibition of LH secretion. Indeed, ME dopamine content and tyrosine hydroxylase (rate-limiting enzyme of the catecholamine biosynthesis) activity are lower under short days (high LH secretion) than under long days (low LH secretion; Thiéry 1991, Viguié et al 1996, 1997. Importantly, blockade of dopamine synthesis in the ME, by local infusion of -methyl-para-tyrosine ( MPT, a competitive tyrosine hydroxylase inhibitor) induces a stimulation of LH secretion in long day-inhibited OVX+E ewes (Viguié et al 1998).…”

Melatonin and the seasonal control of reproduction

“…Exposure to stimulatory short days resulted in decreased dopaminergic activity in the median eminence as assessed both by a reduction in dopamine content and in TH activity [62,72]. No effect of short-day exposure on noradrenaline content in this structure or on TH activity in the other hypothalamic areas investigated was found [72].…”