Our study was the first experiment to demonstrate the level of AdipoR1 and AdipoR2 genes transcripts and proteins in both anterior and posterior pituitaries of cyclic gilts. We found that the stage of the oestrous cycle affects the abundance of mRNAs and proteins of both adiponectin receptors. The abundance of AdipoR1 and AdipoR2 mRNAs varied throughout the oestrous cycle, with a marked decrease in AdipoR1 levels on days 10–12 in AP and an increase on days 10–12 and 14–16 in NP. By contrast, AdipoR2 transcript content was higher on days 2–3 and 17–19 in AP and during the entire luteal phase in NP. Unlike the level of the AdipoR1 mRNA, the content of AdipoR1 protein was elevated during the entire luteal phase in AP, but it did not differ in NP during the oestrous cycle. AdipoR2 protein concentrations in AP and NP reached the highest level on days 10–12 of the oestrous cycle. Indicated in the study, lack of correlation between the protein concentration and gene transcripts may result from transcriptional regulation, post-transcriptional regulation (RNA processing and stability), translation regulation or protein stability, as well as functioning feedbacks, i.e. high protein concentration may suppress mRNA expression, and a high level of gene expression may diminish the post-transcriptional processes. The low concentration of protein with the simultaneous high gene expression level can also be caused by the action of interference RNA (RNAi). The above suggests that tissue mRNA and protein levels are determined by physiological state, and they are not always correlated.
There is a scarcity of data regarding adiponectin system (adiponectin and adiponectin receptors) expression in the pituitary. The presence of mRNA for all system components has been reported only in male rats [20, 21], humans  and chickens . Adiponectin receptor genes are also expressed by somatotroph cells isolated from transgenic GFP expressing mice, the GH3 cell line (rat pituitary tumor cell line) and LβT2 immortalized mouse gonadotrophs [20, 27]. The presence of adiponectin, AdipoR1 and AdipoR2 proteins in the human pituitary has been demonstrated by immunohistochemical methods. Interestingly, Psilopanagioti et al.  observed the colocalization of AdipoRs with gonadotrophs, somatotrophs and thyrotrophs, but not with corticotrophs or lactotrophs. Our study provides the first documented evidence of the expression of adiponectin receptors in the anterior and posterior lobes of the porcine pituitary during the oestrous cycle. The expression of the adiponectin in the studied endocrine gland was also determined (Kaminski et al., data not shown). The presence of both ligand and receptors in porcine pituitary may suggest adiponectin’s auto-/paracrine role in the regulation of the gland function.
The noted variations in the expression of AdipoRs during the cycle suggest a correlation with the animals’ hormonal milieu, primarily at the level of steroid hormones. Heightened levels of AdipoR1 and AdipoR2 mRNAs in the posterior lobe and lowered concentrations in the anterior lobe of the porcine pituitary during the luteal phase of the oestrous cycle could be attributed to ovarian hormones. The up-regulating effects of progesterone on AdipoRs transcripts in NP and its down-regulating effects in AP cannot be ruled out. This hypothesis seems to be confirmed by the results of Takemura et al.  who observed a similar pattern of AdipoRs mRNA expression in the human endometrium and attributed their findings to endometrial changes during the implantation period. In a study of rat placenta during gestation, Caminos et al.  noted that progesterone had a stimulating effect on AdipoR2 gene expression. In addition to progesterone, oestradiol could be yet another ovarian hormone to be involved in AdipoRs expression. Tabandeh et al.  postulated that enhanced expression of adiponectin receptors in theca, granulosa and cumulus cells of bovine ovarian follicles, especially in late stages of follicular growth, could result from increased oestradiol concentrations in follicular fluid. Lagaly et al.  reported a similar trend in AdipoR2 gene expression in theca cells from large follicles of beef cattle. Interestingly, Tan et al.  observed that oestradiol and testosterone increase AdipoRs mRNA and protein levels in cultured human adipocytes. An in vitro study of 3 T3-L1 cells (differentiated into adipocyte-like phenotype) revealed an increase in AdipoR1 and AdipoR2 gene transcript levels after oestradiol treatment . It has also been suggested that other factors, like insulin, prolactin, growth hormone or cytokines, may influence the abundance of AdipoRs [34–36]. Interestingly, several authors have suggested that adiponectin could be an important factor in modulating its own receptor levels. In a study by Rodriguez-Pacheco et al. , a significant decrease in AdipoR1 mRNA levels and an increase in AdipoR2 mRNA concentrations was noted in cultures of rat pituitary cells exposed to 10-8 M and 10-7 M of adipokine for 24 h, respectively. Data contrary to the above findings were reported by Caminos et al.  who observed a suppressing effect of adiponectin on AdipoR2 mRNA expression in cultured human placenta explants. The above results strongly suggest that steroids and other factors, including adiponectin itself, may affect the expression of adiponectin receptors in the pituitary.
Hormonal regulation of the adiponectin system is not limited to the receptors, and the production of adiponectin is also modified by steroid and protein hormones. Böttner et al.  demonstrated that serum adiponectin levels change between childhood and adulthood in a negative correlation with serum androgen levels. Elevated levels of adipokine were also reported in patients with hypogonadotropic hypogonadism and anorexia nervosa, whereas the administration of testosterone decreased adiponectin concentrations [38–40]. A similar relationship was observed in mice . The role of oestrogens remains unclear. Inhibitory effects of oestradiol on plasma adiponectin in women and female ovariectomized rats [42–44] as well as lack of any action [45, 46] were both noted. Some data suggest that other factors, like prolactin, growth hormone, insulin or hCG also influence adiponectin expression [35, 43, 47].
The expression of both adiponectin receptors in the pituitary suggests that adiponectin has a local modulatory effect on central endocrine axes and that it participates in central control of metabolic homeostasis. In vitro studies seem to confirm this hypothesis. In a pituitary cell culture, short-term exposure (4 h) to the discussed hormone inhibited LH and GH release. Adiponectin reduced the stimulatory effect of GnRH on LH release, whereas GH release evoked by GHRH was not affected . The above results were validated by Lu et al.  in whose study, adiponectin had an inhibitory effect on basal and GnRH-stimulated LH release, but not on FSH release by LβT2 cells. The discussed findings seem to confirm the hypothesis that adiponectin is involved in auto-/paracrine control of pituitary functions.
Adiponectin may also act at the hypothalamic level. The expression of all adiponectin system components was observed in porcine hypothalamic structures responsible for GnRH secretion: the medio-basal hypothalamus (MBH), preoptic area (POA) and median eminence (SME) (Kaminski et al., data not shown). Adiponectin receptors were also expressed in rat POA , as well as in the arcuate nucleus (ARH) and paraventricular hypothalamus (PVH) of mice . The presence of AdipoRs was observed in magnocellular (oxytocin or vasopressin secreting) neurons of rat PVH . Adiponectin’s influence on the excitability of oxytocin-secreting neurons could explain enhanced oxytocin secretion in an obese population . Globular adiponectin significantly decreased GnRH release in GT1-7 cells via AMPK activation. Specifically, low levels of adiponectin may contribute to chronically elevated LH levels . The discussed research findings indicate that adiponectin may be an important factor which regulates pituitary functions directly as well as by modulating hypothalamic activity.