Physiological Relevance of Pregnanolone Isomers and Their Polar Conjugates with Respect to the Gender , Menstrual Cycle and Pregnancy

5Ǐ/ǐ-Reduced progesterone metabolites (PM) including pregnanolone isomers (PI) and their polar conjugates (PIC), are efficient neuromodulators operating in a number of physiological and pathological processes including psychiatric diseases or problems connected with pregnancy, parturition and postpartum period. The substances mostly belong to the group of neuroactive steroids. The neuroactive steroids (including PI) originating directly in the central and peripheral nervous system are called neurosteroids. They have a wide variety of functions. Some neuroactive steroids are able to easily pass through the blood-brain barrier (Bixo, Andersson, Winblad, Purdy, & Backstrom, 1997; Kancheva, et al., 2010; M. D. Wang, Wahlstrom, & Backstrom, 1997). Disorders in their biosynthesis or malfunctions in interactions with the target sites can be the cause of many pathologies, including psychiatric illnesses (Backstrom, et al., 2003; Backstrom, Carstensen, & Sodergard, 1976). In contrast with the most common steroid hormones acting, neuroactive steroids largely affect non-genomic mechanisms and influence nerve excitability in both directions. Some neuroactive steroids, such as allopregnanolone and its derivates, also show neuroprotective effects (Ciriza, Azcoitia, & Garcia-Segura, 2004; Morfin & Starka, 2001; Shi, Schulze, & Lardy, 2000). The enzymes involved in the neurosteroidogenesis can be classified in two main groups: the cytochrome P450 and the non-P450 group. Experiments proved the direct biosynthesis of steroids in the brain independent of the periphery (Corpechot, Leclerc, Baulieu, & Brazeau, 1985; Corpechot, Robel, Axelson, Sjovall, & Baulieu, 1981). Three years later, Harrison and Simmonds published their work on the anesthetic effect of the synthetic pregnane steroid ganaxalone through modulation of the stimulation of the Ǒ-aminobutyric acid receptor (GABA-r) (Harrison & Simmonds, 1984). The following year Majewska and coworkers published the first study on the modulation effect of endogenous steroids on GABA-r, and thereby initiated an intense research effort focused on the mechanisms of action of neuroactive steroids (Majewska, Bisserbe, & Eskay, 1985).


Introduction
5Ǐ/ǐ-Reduced progesterone metabolites (PM) including pregnanolone isomers (PI) and their polar conjugates (PIC), are efficient neuromodulators operating in a number of physiological and pathological processes including psychiatric diseases or problems connected with pregnancy, parturition and postpartum period.The substances mostly belong to the group of neuroactive steroids.The neuroactive steroids (including PI) originating directly in the central and peripheral nervous system are called neurosteroids.They have a wide variety of functions.Some neuroactive steroids are able to easily pass through the blood-brain barrier (Bixo, Andersson, Winblad, Purdy, & Backstrom, 1997;Kancheva, et al., 2010; M. D. Wang, Wahlstrom, & Backstrom, 1997).Disorders in their biosynthesis or malfunctions in interactions with the target sites can be the cause of many pathologies, including psychiatric illnesses (Backstrom, et al., 2003;Backstrom, Carstensen, & Sodergard, 1976).In contrast with the most common steroid hormones acting, neuroactive steroids largely affect non-genomic mechanisms and influence nerve excitability in both directions.Some neuroactive steroids, such as allopregnanolone and its derivates, also show neuroprotective effects (Ciriza, Azcoitia, & Garcia-Segura, 2004;Morfin & Starka, 2001;Shi, Schulze, & Lardy, 2000).The enzymes involved in the neurosteroidogenesis can be classified in two main groups: the cytochrome P450 and the non-P450 group.Experiments proved the direct biosynthesis of steroids in the brain independent of the periphery (Corpechot, Leclerc, Baulieu, & Brazeau, 1985;Corpechot, Robel, Axelson, Sjovall, & Baulieu, 1981).Three years later, Harrison and Simmonds published their work on the anesthetic effect of the synthetic pregnane steroid ganaxalone through modulation of the stimulation of the Ǒ-aminobutyric acid receptor (GABA-r) (Harrison & Simmonds, 1984).The following year Majewska and coworkers published the first study on the modulation effect of endogenous steroids on GABA-r, and thereby initiated an intense research effort focused on the mechanisms of action of neuroactive steroids (Majewska, Bisserbe, & Eskay, 1985).

Biosynthesis of neuroactive steroids in the cells of neuronal system
The 3Ǐ,5Ǐ-THP is present in human post-mortem brain tissue at considerably higher concentrations than typically observed in blood (Marx, et al., 2006).These neurosteroids are synthesized in brain, peripheral glial cells and neurons (Schumacher, et al., 2000).As demonstrated on rats, the enzymes that are necessary for synthesis of neuroactive 5Ǐ/ǐ-PM as type 2 3ǐ-hydroxysteroid dehydrogenase (HSD3B2) and 5Ǐ-reductase of types 1 (SRD5A1) and 2 (SRD5A2) are present in the CNS.While the SRD5A1 was identified for the most part in glial cells of white matter, SRD5A2 was found in oligodendrocytes, neurons and astrocytes of the grey matter.The enzyme isoforms, which are effective as the 3Ǐhydroxysteroid dehydrogenase are present in oligodendrocytes, neurons and astrocytes of white and grey matter (Patte-Mensah, Penning, & Mensah-Nyagan, 2004;Schumacher, et al., 2004;Stoffel-Wagner, et al., 2000;Tsuruo, 2005).The important system mediating changes or even reversion of neuromodulating activity involves a steroid sulfatase (STS) and sulfotransferases controlling the balance between neuroinhibitory 3Ǐ-PI and PIC that exert an opposite effect.However, relatively high STS activity but very low sulfotransferase activity were detected in the brain (Compagnone, Salido, Shapiro, & Mellon, 1997;Kriz, Bicikova, Hill, & Hampl, 2005).The GABAergic steroids can be inactivated by their 3Ǐ-oxidation to yield 5Ǐdihydroprogesterone (5Ǐ-DHP).It was found that 5Ǐ-DHP levels in HEK293 cells expressing type 10 17ǐ-hydroxysteroid dehydrogenase (HSD17B10) increased as 3Ǐ,5Ǐ-THP was added to culture media.Brain astrocytes contain a moderate level of HSD17B10, which is elevated in activated astrocytes of brains with Alzheimer type pathology.Cerebral cortex has the lowest level of HSD17B10; whereas the hippocampus, hypothalamus, and amygdala possess relatively higher levels of this enzyme.The catalysis of HSD17B10 appears to be essential for maintaining normal functions of GABAergic neurons (He, Wegiel, & Yang, 2005).

Gonadal function and neuroactive steroids
The most part of neuroactive steroids in women in the luteal phase of menstrual cycle (LP) consists of metabolites of progesterone, which is formed in corpus luteum (Ottander, et al., 2005).The levels of PI and PIC strongly depend on the menstrual cycle, reflecting changes in progesterone formation.The mRNA of 5Ǐ-reductase (SRD5A), 5ǐ-reductase (AKR1D1) and 3Ǐ-hydroxysteroid oxidoreductase (3Ǐ-HSOR) mRNA are all expressed in human corpus www.intechopen.comPhysiological Relevance of Pregnanolone Isomers and Their Polar Conjugates with Respect to the Gender, Menstrual Cycle and Pregnancy 357 luteum and the release of 3Ǐ,5Ǐ-THP and 3Ǐ,5ǐ-THP herein is stimulated by a trophic hormone.The dominant PI is the 3Ǐ,5Ǐ-THP (Havlikova, et al., 2006;M. Hill, et al., 2005).In women, progesterone and its reduced metabolites exhibit a decrease with increasing age and a qualitative change after menopause (Genazzani, et al., 1998).It is likely that the gonadal 5Ǐ/ǐ-PM easily overcome the blood-brain-barrier (Bixo, et al., 1997;Kancheva, et al., 2010).

The role of adrenals in the biosynthesis of neuroactive steroids
Zona glomerulosa controlled by the renin-angiotensin axis produces deoxycorticosterone (DOC), the metabolites of which are neuroactive like the 3Ǐ,5Ǐ-tetrahydro-DOC (3Ǐ,5Ǐ-THDOC) and its isomers.However, DOC is produced in substantially greater quantities in zona fasciculata, which is controlled by the CRH-ACTH system.In contrast to the 3Ǐ,5Ǐ-THP reaching about 10% of progesterone concentration, the basal levels of DOC and 3Ǐ,5Ǐ-THDOC are almost comparable (<0.5 nmol/L) (Reddy, 2006).Zona fasciculata primarily produces cortisol in relatively high amounts.3Ǐ-5Ǐ/ǐ-Reduced metabolites of cortisol are GABAergic such as the 3Ǐ-PI.The 3Ǐ,5Ǐ-tetrahydrocortisol and 3Ǐ,5Ǐ-THP posses a comparable activity on GABA A -r (Stromberg, Backstrom, & Lundgren, 2005).In addition, adrenal zona fasciculata produces relatively abundantly pregnenolone sulfate (PregS) (20-400 nmol/l), which, like the cortisol, readily reacts to adrenocorticotropin (ACTH) stimulation (de Peretti, et al., 1986).PregS appears to be the most important precursor of progesterone and DOC of adrenal origin.Growing formation of PregS in adrenals that is further metabolized up to neuroactive PI may explain the increased levels of brain 5Ǐ/ǐ-PM in patients with diagnoses associated with stress (Higashi, Takido, & Shimada, 2005).Increasing peripheral production of pregnane steroids and their precursors and their subsequent transport across the blood-brain-barrier could contribute to the physiological compensation of stress.Most probably, the peripheral levels of pregnane steroids primarily depend on adrenal activity in women in FP (Havlikova, et al., 2006;M. Hill, et al., 2005), postmenopausal women, children and men (Fig. 2A,B).However, the proportion of 5Ǐ/ǐ-PM derived from the adrenal activity is pronouncedly lower compared to the quantity originating in the corpus luteum (Meczekalski, et al., 2000) (Fig. 2C).
3.2 Human CNS-related pathologies that are linked to the 5α/ -reduced pregnanes 3.2.1 Premenstrual syndrome Changes in progesterone levels and respective changes in its neuroactive metabolites are apparently the cause of premenstrual dysphoric disorder in women (PMDD).Withdrawal effect in case of abrupt drop of steroid positive modulators rapidly supervenes like the addiction effect while increasing the steroid levels.Changing 5Ǐ/ǐ-PI concentrations induce a decreased affinity of GABA A -r for these steroids due to the changed expression of the receptor subunits and/or as a result of the changed phosphorylation status of the specific sites on the GABA A -r (Brussaard, Wossink, Lodder, & Kits, 2000;Koksma, et al., 2003;Leng & Russell, 1999;Maguire & Mody, 2009).The aforementioned mechanism requires synchronization, the disturbances of which could have significant neuropsychiatric consequences in physiological and pathological situations like pregnancy, parturition, onset of menopause, traumas, endocrine diseases, and stress.Several GABA A -r modulators, including 3Ǐ,5Ǐ-THP, exert biphasic effect.The low concentrations induce an adverse, anxiogenic effect whereas the higher concentrations decrease this effect and show calming properties (Andreen, et al., 2009).The severity of these mood symptoms is related to the

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3Ǐ,5Ǐ-THP serum concentrations in a manner similar to a bell-shaped curve.Negative mood symptoms occur when the serum concentration of 3Ǐ,5Ǐ-THP is similar to the endogenous LP levels, while low and high concentrations have no such effect.Progesterone/3Ǐ,5Ǐ-THP treatment in women increases the activity in the amygdala in a similar manner as the changes seen during anxiety reactions.Women with PMDD in LP show changes in GABA Ar sensitivity and GABA concentrations that are related to the severity of the condition (Andreen, et al., 2009).

Chronic fatigue syndrome
Increased inhibition through GABA A -r due to the accumulation of neuroinhibitory steroids may represent an important pathophysiological mechanism of fatigue in chronic liver diseases.The levels of 3Ǐ,5Ǐ-THP and 3ǐ,5Ǐ-THP are increased in plasma of patients with chronic fatigue syndrome (Ahboucha, et al., 2008).

Depression
Antidepressants elevate 3Ǐ-PI levels in rodent brain (Uzunova, et al., 1998).However, recent studies suggest that changes in plasma neuroactive steroid levels may not be a general mandatory component of clinically effective antidepressant treatment per se, but may reflect distinct properties of pharmacotherapy only (Uzunova, Sampson, & Uzunov, 2006).

Epilepsy
Women with epilepsy show reduced progesterone levels in the LP.The progesterone deficit results in the debit of neuroinhibitory 3Ǐ-hydroxy-5Ǐ/ǐ-reduced progesterone metabolites (Stoffel-Wagner, 2001).Rat models of catamenial epilepsy exhibit an abstinence effect at lowered 3Ǐ,5Ǐ-THP concentrations, which however results in higher sensitivity after restitution of its original levels.Various authors demonstrate that catamenial epilepsy is linked to the disturbed biosynthesis of progesterone and its reduced metabolites (Backstrom, et al., 2003), particularly in the LP (Bonuccelli, et al., 1989).

Schizophrenia
GABAergic steroids may be candidate modulators for the pathophysiology of schizophrenia and bipolar disorder, and relevant to the treatment of these disorders.3Ǐ,5Ǐ-THP levels tend to be decreased in parietal cortex in subjects with schizophrenia compared to control subjects (Marx, et al., 2006).

Neurodegenerative disorders
3Ǐ,5Ǐ-THP is reduced in prefrontal cortex in male patients with Alzheimer disease (AD) compared to male cognitively intact control subjects, and inversely correlated with neuropathological disease stage.3Ǐ,5Ǐ-THP levels are reduced in temporal cortex in patients with AD compared to control subjects and inversely correlated with neuropathological disease stage.Patients carrying an APOE4 allele demonstrate reduced 3Ǐ,5Ǐ-THP levels in temporal cortex (Naylor, et al., 2010).

Eating disorders
Compared with healthy women, the patients with eating disorders exhibit increased plasma levels of 3Ǐ,5Ǐ-THP.However, the relevance of such hormonal alteration to the pathophysiology of eating disorders remains to be elucidated (Monteleone, et al., 2001) (Monteleone, et al., 2003).

5α/ -Reduced pregnanes in human pregnancy
4.1 Fetal adrenal is the primary source of pregnancy steroids 4.1.1Placental CRH controls the steroid biosynthesis in the fetal adrenal The machinery regulating production of pregnancy steroids (including pregnanolone isomers and their polar conjugates) is based on the excessive placental production of corticoliberin (CRH) (Goland, Wardlaw, Stark, Brown, & Frantz, 1986;Rainey, Rehman, & Carr, 2004;R. Smith, et al., 2009).CRH in non-pregnant subjects is a hypothalamic hormone controlling the pituitary secretion of adrenocorticotropic hormone (ACTH) and, in turn, the corticosteroid production in adult adrenal.The hypothalamic-pituitary-adrenal axis in these subjects is based on a negative feedback loop between the final active hormone, ACTH and CRH.Alternatively, the pregnant women after luteo-placental shift produce CRH primarily in placenta and instead of the negative feedback loop cortisol-ACTH-CRH; there is a positive one between cortisol and CRH, while the ACTH production stagnates.The rising CRH levels in the last four weeks of pregnancy stimulate the synthesis of conjugated Δ 5 steroids (Sirianni, Mayhew, Carr, Parker, & Rainey, 2005;R. Smith, Mesiano, Chan, Brown, & Jaffe, 1998) in the fetal zone of the fetal adrenal (FZ), which is a specific transient tissue gradually converting to zona reticularis after labor.The excessive production of placental CRH is unique for primates and the boosting CRH production near term is exclusive for humans and great apes (Power & Schulkin, 2006).The sulfated Δ 5 steroids, originating in the FZ represent the largest fraction of steroids in pregnancy (Ingelman-Sundberg, Rane, & Gustafasson, 1975;Lacroix, Sonnier, Moncion, Cheron, & Cresteil, 1997;Leeder, et al., 2005;Moghrabi, Head, & Andersson, 1997) (Fig. 2D).Sulfotransferase 2A1 (SULT2A1) transcript shows even 13-fold higher levels in the fetal adrenal.Alternatively, HSD3B2 mRNA expression in midgestation is 127-fold lower than the one in the adult adrenal due to preferential synthesis of the Δ 5 C-21 steroids over corticoids.The FZ is similar to the adult zona reticularis but unlike the adult zona reticularis, the FZ produces excessive amounts of sulfated C-21 Δ 5 steroids, including pregnenolone sulfate (PregS) (M.Hill, Parizek, Cibula, et al., 2010;M. Hill, Parizek, Jirasek, et al., 2010;Rainey, et al., 2004).The Δ 5 steroid sulfates (originating in the FZ) serve as precursors for the placental production of estradiol (Sirianni, et al., 2005;R. Smith, et al., 1998) and progesterone (M.Hill, Parizek, Jirasek, et al., 2010;Jaffe & Ledger, 1966;Komatsuzaki, et al., 1987;Walsh, 1988).

5α/ -Reductases
The liver has also high activity of SRD5A and AKR1D1 (Charbonneau & The, 2001;Meikle, Stringham, Wilson, & Dolman, 1979).From the two isoforms of SRD5A, SRD5A1 is widely distributed in the body, with the highest levels in the liver and converts testosterone into 5Ǐdihydrotestosterone and progesterone, and corticosterone into their corresponding 3-oxo-5Ǐ-reduced steroids.In the androgen-dependent structures, 5Ǐ-DHT is almost exclusively formed by SRD5A2 (Poletti, et al., 1998).In the peripheral tissues, including the liver, SRD5A1 and 3Ǐ-HSD reductive AKR1Cs and HSD17Bs work consecutively eliminating the androgens, protecting against the hormone excess (Jin & Penning, 2001) and producing GABAergic steroids, which are, however, extensively sulfated in the liver.Liver AKR1D1 efficiently catalyzes the reduction of both C-19 and C-21 3-oxo-Δ 4 steroids to the corresponding 5ǐ-PM (Kochakian, 1983;Okuda & Okuda, 1984).The higher levels of 5ǐ-PM in the fetus than in maternal compartment as well as the arteriovenous differences in the fetus indicate that steroid 5ǐ-reduction in the fetal liver (but not in the placenta) is important for production of 5ǐ-PM in both maternal and fetal compartment (M.Hill, Parizek, Cibula, et al., 2010).

Balance between polar conjugates and unconjugated steroids
The sulfotransferase SULT2A1 is highly expressed in human liver (Comer & Falany, 1992;Geese & Raftogianis, 2001;Meloche & Falany, 2001;Zhang, Varlamova, Vargas, Falany, & Leyh, 1998).However, the liver also strongly expresses the STS (Selcer, Difrancesca, Chandra, & Li, 2007) like the placenta.The formation of sulfated steroids with a 3Ǐhydroxy-5Ǐ configuration may account for 50% of the metabolism of progesterone in late pregnancy (Anderson, et al., 1990).The sulfation of 3ǐ,5Ǐ-THP is an important metabolic step contributing to progesterone catabolism and significantly affecting the balance between neuroinhibitory steroids and their antagonists in pregnant women.The further major pathway of progesterone catabolism in the maternal compartment proceeds in the sequence progesterone → 5ǐ-DHP → 3Ǐ,5ǐ-THP→ conjugated 3Ǐ,5ǐ-THP (Kancheva, et al., 2007), which is analogous to the situation out of pregnancy (Havlikova, et al., 2006).In this pathway, the 5ǐ-reduction and the reduction of the 3-oxo-group in 5ǐ-DHP, resulting in the synthesis of 3Ǐ,5ǐ-THP, appear to be critical metabolic steps (Kancheva, et al., 2007).Humans with low progesterone production exhibit very low concentrations of unconjugated 3Ǐ,5ǐ-THP (men, women in the FP).In these subjects, the 3Ǐ,5ǐ-THP is rapidly conjugated.Alternatively, in woman in the LP and so much the more in pregnant women, the conjugation capacity for 3Ǐ,5ǐ-THP may be limited.The increased conjugation of 5Ǐ-PI probably further diminishes the difference between the 3Ǐ,5Ǐ-THP and 3Ǐ,5ǐ-THP levels in pregnant women and may also regulate the proportions between neuroinhibitory 3Ǐ,5Ǐ-THP and antagonistic conjugated 5Ǐ-PI (Kancheva, et al., 2007).

Steroid metabolism in placenta 4.4.1 Steroid sulfatases and placental production of sex hormones
The principal metabolic step that is indispensable for placental metabolism of sulfated Δ 5 steroids originating in FZ is their desulfation, which is catalyzed by the placental STS.Placental STS activity is independent of substrate concentration (Watanabe, et al., 1990) and of gestational age (GA) (Fukuda, Okuyama, & Furuya, 1986;Ishida, et al., 1985;Leslie, et al., 1994).The placental STS expression in pregnancy explicitly outweighs the production in other tissues (Miki, et al., 2002) and allows access of Δ 5 steroids to the HSD3B1 and CYP19A1 within the syncytiotrophoblast layer and their conversion to estrogens (Siiteri, 2005) and progestogens (M.Hill, Parizek, Cibula, et al., 2010;M. Hill, Parizek, Jirasek, et al., 2010).The latter substances are subsequently converted to 5Ǐ/ǐ-PM by placental and liver enzymes.

5α/ -reduced progesterone metabolites around parturition
Pearson Murphy et al. demonstrate that during the period 2-7 day postpartum, the level of progesterone fall precipitously, whereas those of pregnenolone and the metabolites decrease more slowly and their levels are still elevated compared with FP levels 2 weeks after delivery.By the 7 th week postpartum only 3Ǐ,5Ǐ-THP and 3ǐ,5ǐ-THP remains slightly elevated (Pearson Murphy, et al., 2001).Our recent report (M.Hill, Parizek, Cibula, et al., 2010;M. Hill, Parizek, Kancheva, et al., 2010) as well as our previous data for PI around parturition display significantly lower PIC/PI ratios in the umbilical venous plasma than in the maternal plasma (M.Hill, et al., 2001;Klak, et al., 2003).Changes in concentrations of PI in the maternal serum exhibit a similar pattern, falling mostly within the first hour after the  Hill, et al., 2001;Klak, et al., 2003).The PIC/PI ratios significantly decrease within the first hour and the first day after delivery in all PI (M.Hill, et al., 2001;Klak, et al., 2003).These results indicate an intensive sulfation of GABAergic substances in the maternal compartment during pregnancy but attenuating sulfation activity shortly after labor.The sulfation of GABAergic steroids (transforming them to antagonistic substances) might represent a mechanism counterbalancing their placental overproduction.The ratios of 3Ǐ/3ǐ-PI decrease around parturition (M.Hill, et al., 2001;Klak, et al., 2003), which may indicate that the placental and possibly also the liver reductive conversion of the 3-oxo-and 3ǐ-hydroxy-5Ǐ/ǐ-PI to the 3Ǐ-isomers may be of importance for pregnancy sustaining.
On the other hand, rising steroid sulfation that catabolizes both 3Ǐ,5Ǐ-THP and 5ǐ-reduced steroids, produces high amounts of PIC.PIC induce neuroexcitatory effect via GABA A -r and may shift the biological activity towards induction of labor (Park-Chung, et al., 1999).Majewska and Vaupel (Majewska & Vaupel, 1991) reported that 3Ǐ,5Ǐ-THP interact with GABA A -r to modulate uterine contractility: 3Ǐ,5Ǐ-THP inhibits while PregS increases contractions.Further, 3Ǐ,5Ǐ-THP rapidly antagonizes the stimulatory effect of PregS, but progesterone inhibits the contractions after a delay, suggesting that the known pregnancy sustaining effect of progesterone on the uterus is at least partly mediated via the metabolite 3Ǐ,5Ǐ-THP, which potentiates the neuroinhibitory function of GABA A -r (Majewska & Vaupel, 1991).On the other hand, Lofgren et al. (Lofgren, Holst, & Backstrom, 1992) reported contradictory data.Concerning the 3Ǐ-hydroxysteroid oxidoreductase-mediated turnover of 5Ǐ-DHP and 5ǐ-DHP to their metabolites 3Ǐ,5Ǐ-THP and 3Ǐ,5ǐ-THP, respectively, which reflects the ratios between these GABAergic 3Ǐ-PI and their inactive precursors, Gilbert Evans et al. (Gilbert Evans, et al., 2005) reported that the turnover of 5Ǐ-DHP to 3Ǐ,5Ǐ-THP rise during pregnancy and drops at the late prenatal visit.At 6 weeks postpartum, all steroids are significantly reduced compared with late prenatal values.Although, we have found no significant change of the ratio 3Ǐ/3ǐ-PI during pregnancy (Parizek, et al., 2005), our more recent study shows contradictory results to the data of Gilbert Evans et al. and demonstrates a moderate but significant shift from the 3Ǐ-PI to the-3-oxo-isomers (M.Hill, Parizek, Cibula, et al., 2010).
With the exception of two 5Ǐ/ǐ-PM (5Ǐ-DHP and 3ǐ,5Ǐ-THP), the remaining ones used in the present study are more potent than progesterone.It is important that when the tissues are washed, the contractile activity is recovered.This rapid and reversible relaxing effect is not blocked by antiprogestin RU 486, which suggests its independence of receptor-mediated genomic action (Perusquia & Jasso-Kamel, 2001).
Whereas the turnovers of 5Ǐ-DHP/progesterone and 5ǐ-DHP/progesterone in the 3 rd trimester show that the metabolism of progesterone to 5Ǐ-DHP inconspicuously culminates in the 35 th week, the conversion of progesterone to 5ǐ-DHP significantly declines from the 31 st week of gestation (M.Hill, et al., 2007).This is in accordance with results of other authors as well as with our current data (Gilbert Evans, et al., 2005;M. Hill, Parizek, Cibula, et al., 2010;M. Hill, Parizek, Kancheva, et al., 2010;Sheehan, 2006;Sheehan, et al., 2005).
Besides the modulation of ionotropic receptors, the 5ǐ-reduced metabolites of progesterone may act chronically in pregnancy as uterine relaxants through a mechanism mediated by pregnane X-type receptors.Moreover, acute in vitro treatment with 5ǐ-DHP causes rapid uterine relaxation that is independent of pregnane X-type receptors (Mitchell, et al., 2005;Putnam, Brann, Kolbeck, & Mahesh, 1991).The aforementioned data demonstrate that the progesterone metabolite 5ǐ-DHP is a potent tocolytic (Mitchell, et al., 2005).In the placenta and myometrium, the relative expression of AKR1D1 decreases in association with labor by about two-fold and 10-fold, respectively (Sheehan, et al., 2005).Therefore, it is likely that the decrease in AKR1D1 activity during the third trimester is associated with a reduced ability to sustain the pregnancy (Gilbert Evans, et al., 2005;M. Hill, et al., 2007;M. Hill, Parizek, Cibula, et al., 2010;M. Hill, Parizek, Kancheva, et al., 2010;Sheehan, et al., 2005).AKR1D1 activity participates in the formation of almost 40% of pregnancy-sustaining PI.

4.7
Effects of 5α/ -reduced pregnanes on pain perception, induction of tolerance, receptor plasticity 4.7.1 The effects of 5α/ -reduced pregnanes in the fetal CNS 3Ǐ,5Ǐ-THP may interact with GABA A -r to inhibit fetal CNS activity from mid-gestation.This inhibition may contribute to maintaining the sleep-like behavior and low incidence of arousal-type activity typical of fetal life (Crossley, et al., 2003).

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the infant.The authors suggest that the uterus plays a key role in keeping the fetus continuously asleep.Despite the presence of intact nociceptive pathways from around midgestation, the critical aspect of cortical awareness in the process of pain perception is missing.The mechanism providing the permanent sleeping status in the fetus combines neuroinhibitory actions of a powerful EEG suppressor and sleep inducing agent (adenosine), two GABAergic steroids anesthetics (3Ǐ,5Ǐ-THP, 3Ǐ,5ǐ-THP) and a potent sleep-inducing hormone (prostaglandin D2), acting together with a putative peptide inhibitor and other factors produced by the placenta (Mellor, Diesch, Gunn, & Bennet, 2005).
Concerning the role of GABAergic steroids in suppressing the nociceptive pathways in the fetus, our current data shows 2-3 times lower 3Ǐ,5Ǐ-THP levels in the fetal circulation than in the maternal one, while 3Ǐ,5ǐ-THP levels in UV exceed those in MV 1-2.5 times (M.Hill, et al., 2011).The total amount of GABAergic PI is only slightly higher in the fetal compartment than in the maternal, mainly due to the contribution of unconjugated 3Ǐ,5ǐ-THP.These results indicate that the peripheral GABAergic steroids exert a comparable effect on the maternal and fetal CNS.Even when considering the 1.5-3 fold excess of progesterone in the fetal circulation when compared to the maternal blood, a possibility of progesterone transport into the brain, and its conversion to the GABAergic steroids herein, the resulting contribution of GABAergic steroids originating from peripheral sources do not pronouncedly differ between mother and fetus.Therefore the importance of GABAergic steroids for maintenance of permanent fetal sleeping is open to discussion.

The effects of 5α/ -reduced pregnanes in the maternal CNS
Increases in the brain levels of 5Ǐ/ǐ-PM during pregnancy are causally related to changes in the expression of specific GABA A -r subunits and the function of extrasynaptic GABA A -r in the cerebral cortex and hippocampus (Concas, Follesa, Barbaccia, Purdy, & Biggio, 1999;Mostallino, Sanna, Concas, Biggio, & Follesa, 2009).Turkmen et al. demonstrated that 3Ǐ,5Ǐ-THP treatment induce a partial tolerance against acute 3Ǐ,5Ǐ-THP effects in the Morris water maze (Turkmen, Lofgren, Birzniece, Backstrom, & Johansson, 2006).Alterations in ǒGABA A -r expression during pregnancy result in region-specific increases in neuronal excitability in brain that are res t o r e d b y t h e h i g h l e v e l s o f 3 Ǐ,5Ǐ-THP under normal conditions.On the contrary, under pathological conditions may result in neurological and psychiatric disorders associated with pregnancy and postpartum period (Maguire, Ferando, Simonsen, & Mody, 2009).Besides the GABAergic effects in the CNS and periphery, 5ǐ-PM also exert peripheral analgesic effects via blockade of testosterone-type calcium channels controlling pain perception (Todorovic, et al., 2004).These data as well as those mentioned previously, allow a speculation, whether these steroids might operate as endogenous analgesics around parturition.

Summary
Although the effects of bioactive reduced progesterone metabolites in human and laboratory animals were extensively studied, their physiological importance remains commonly uncertain due to the lack of metabolomic data.Therefore, we focused on the intersection between steroid metabolomics and neurophysiology so as to give a comprehensive insight into the physiological and pathophysiological relevance of the aforementioned compounds.

SHIFT Fetal adrenals Fetal livers 5α-DHP, 5β-DHP, 3α/β,5α-THP, 3α/β,5β-THP Progesterone, 5α-DHP Pregnenolone sulfate
Physiological Relevance of Pregnanolone Isomers and Their Polar Conjugates with Respect to the Gender, Menstrual Cycle and Pregnancy 365 delivery.The decrease in PIC is shifted to the interval within the first hour and first day after delivery (M. www.intechopen.com Mellor et al. reviewedthe role of endogenous neuro-inhibitors that contribute to fetal sleep states, and thus mediate the suppression of fetal awareness.The authors show that there are several suppressors in utero, which inhibits neural activity in the fetus to a far greater degree than is seen postnatally in www.intechopen.comPhysiological Relevance of Pregnanolone Isomers and Their Polar Conjugates with Respect to the Gender, Menstrual Cycle and Pregnancy