Formation and effects of neuroactive steroids in the central and peripheral nervous system

This chapter summarizes several observations that emphasize the importance of neuroactive steroids in the physiology of the central and peripheral nervous systems. A new, and probably important, concept is emerging: Neuroactive steroids not only modify neuronal physiology but also intervene in the control of glial cell functions. The data presented here underscore that (1) the mechanism of action of the various steroidal molecules may involve both classical (progesterone and androgens) and nonclassical steroid receptors [gamma-aminobutyric acid type A (GABAA) receptor], (2) in many instances, the actions of hormonal steroids are not due to their native molecular forms but to their 5 alpha- and 3 alpha,5 alpha-reduced metabolites, (3) several neuroactive steroids exert dramatic actions on the proteins proper of the peripheral myelin (e.g., glycoprotein Po and peripheral myelin protein 22), and (4) the effects of steroids and of their metabolites might have clinical significance in cases in which the rebuilding of the peripheral myelin is needed (e.g., aging, peripheral injury).

Interactions between growth factors and steroids in the control of LHRH-secreting neurons

How the gene expression and the release of luteinizing hormone releasing hormone (LHRH) are controlled in LHRH-secreting neurons is a very crucial and still debated topic of the neuroendocrinology. Several observations present in literature have recently indicated that glial cells may influence the activity of hypothalamic LHRH-secreting neurons, via the release of growth factors. The present review will summarize data obtained in our laboratory indicating that: (a) type 1 astrocytes, a kind of glial cells, are able to release in vitro growth factors belonging to the transforming growth factors beta (TGFbeta) family (i.e. TGFbeta1 and TGFbeta2) which influence the gene expression and the release of the decapeptide in immortalized LHRH-secreting neurons; (b) glial cells are also able to influence the steroid metabolism occurring in these neurons and in some cases this effect is exerted by TGFbeta1; (c) the mRNA levels of TGFbeta1 and of basic fibroblast growth factor (bFGF), another growth factor involved in the control of LHRH-secreting neurons, are modified in the rat hypothalamus during the different phases of the estrous cycle; (d) steroid hormones are able to modulate the gene expression of TGFbeta1 and bFGF both in vivo (i.e. in the whole hypothalamus of ovariectomized rats) and in vitro (cultures of type 1 astrocytes). On the basis of these results a possible functional correlation in the control of LHRH-secreting neurons between growth factors and gonadal steroids will be discussed and proposed.

Neuroactive steroids and peripheral myelin proteins

The present review summarizes observations obtained in our laboratories which underline the importance of neuroactive steroids (i.e., progesterone (PROG), dihydroprogesterone (5alpha-DH PROG), tetrahydroprogesterone (3alpha, 5alpha-TH PROG), testosterone (T), dihydrotestosterone (DHT) and 5alpha-androstan-3alpha,17beta-diol (3alpha-diol)) in the control of the gene expression of myelin proteins (i.e. glycoprotein Po (Po) and the peripheral myelin protein 22 (PMP22)) in the peripheral nervous system. Utilizing different in vivo (aged and adult male rats) and in vitro (Schwann cell cultures) experimental models, we have observed that neuroactive steroids are able to stimulate the mRNA levels of Po and PMP22. The effects of these neuroactive steroids, which are able to interact with classical (progesterone receptor, PR, and androgen receptor, AR) and non-classical (GABA(A) receptor) steroid receptors is further supported by our demonstration in sciatic nerve and/or Schwann cells of the presence of these receptors. On the basis of the observations obtained in the Schwann cells cultures, we suggest that the stimulatory effect of neuroactive steroids on Po is acting through PR, while that on PMP22 needs the GABA(A) receptor. The present findings might be of importance for the utilization of specific receptor ligands as new therapeutical approaches for the rebuilding of the peripheral myelin, particularly in those situations in which the synthesis of Po and PMP22 is altered (i.e. demyelinating diseases like Charcot-Marie-Tooth type 1A and type 1B, hereditary neuropathy with liability to pressure palsies and the Déjérine-Sottas syndrome, aging, and after peripheral injury).

Modulation of fibroblast growth factor-2 by stress and corticosteroids: from developmental events to adult brain plasticity

Neurotrophic factors are a heterogeneous group of peptides that play important roles on brain function at different development stages. Basic fibroblast growth factor (FGF-2), one of these molecules, is highly expressed in developing and adult brain. Its expression can be regulated under different experimental situations and this may be relevant for cellular vulnerability and brain plasticity. Stress and glucocorticoid hormones produce short- and long-term effects on brain function, which can involve the regulation of specific neurotrophic factors within selected brain structures. Treatments with corticosterone or dexamethasone up-regulate FGF-2 expression in different rat brain regions as well as in cultured astroglial cells. A similar elevation of FGF-2 biosynthesis is also observed in several brain regions following an acute restraint stress. This response is rapid and transient and, as FGF-2 is neuroprotective, may represent a defense mechanism through which the brain may limit the deleterious effect of stress over time. Moreover exposure to corticosterone during late stage of embryonic life (E18-E20) produces a significant reduction of FGF-2 mRNA levels in the adult hippocampus of male rats as well as changes in its acute modulation in response to stress or corticosterone. These data suggest that stress-related events taking place during brain maturation can modulate the expression of FGF-2 within selected brain regions thus contributing to permanent structural and functional alterations leading to an increased vulnerability to challenging life events.

Steroid effects on the gene expression of peripheral myelin proteins

The present article summarizes recent observations obtained in our laboratory which clearly indicate that sex steroids exert relevant effects on the peripheral nervous system. In particular, the following important points have emerged: (1) Steroids exert stimulatory actions on the synthesis of the proteins proper of the peripheral myelin (e.g., glycoprotein Po and peripheral myelin protein 22) in vivo and on the Schwann cells in culture; (2) in many cases the actions of hormonal steroids are not due to their native molecular forms but rather to their metabolites (e.g., dihydroprogesterone and tetrahydroprogesterone in the case of progesterone; dihydrotestosterone and 5 alpha-androstane-3 alpha,17 beta-diol in the case of testosterone); (3) the mechanism of action of the various steroidal molecules may involve both classical (progesterone and androgen receptors) and nonclassical steroid receptors (GABA(A) receptor); and finally, (4) the stimulatory action of steroid hormones on the proteins of the peripheral myelin might have clinical significance in cases in which the rebuilding of myelin is needed (e.g., aging, peripheral injury, demyelinating diseases, and diabetic neuropathy).

Hypothalamic transforming growth factor beta1 and basic fibroblast growth factor mRNA expression is modified during the rat oestrous cycle

The present observations show that the mRNA levels of two growth factors, previously described to be involved in the control of neurones synthesizing the luteinizing hormone releasing hormone (LHRH) [i.e. transforming growth factor beta1 (TGFbeta1) and basic fibroblast growth factor (bFGF)], fluctuate in the hypothalamus of adult female rats during the oestrous cycle. In particular, the expression of TGFbeta1-mRNA shows a peak on the morning of the day of proestrus, which precedes the increased secretion of the two gonadotrophins that occurs on that day. In the case of bFGF, the peak is evident in the evening of the same day and is concomitant with that of the gonadotrophins. We evaluated the effects of ovariectomy and of exogenous oestrogens on the mRNA levels of these two growth factors in the hypothalamus. The data indicate that 3 weeks of ovariectomy are not able to change the hypothalamic messenger levels of the two growth factors considered, which remain at the levels found in diestrus 1, and that 17beta-oestradiol is able to induce a significant increase of both TGFbeta1- and of bFGF-mRNA levels in the hypothalamus of the ovariectomized rat. The present in vivo observations support the concept, previously proposed on the basis of in vitro data, that growth factors, such as TGFbeta1 and bFGF, play a role in the hypothalamic control of reproduction, and suggest that the control of LHRH dynamics involves a strict cooperation between gonadal steroids and growth factors.

Corticosteroids protect oligodendrocytes from cytokine-induced cell death

The present data show that the simultaneous exposure to tumor necrosis factor-alpha (TNFalpha) and interferon-gamma (IFNgamma) induces cell death with characteristics of apoptosis in cultured rat oligodendrocytes; TNFalpha alone was ineffective. We have also demonstrated that different corticosteroids (aldosterone, deoxycorticosterone, dexamethasone and corticosterone) protect rat oligodendrocytes in culture from apoptosis induced by TNFalpha plus IFNgamma. This effect seems to be exerted via the interaction with both type I and type II corticosteroid receptors since all steroids considered are effective. Since oligodendrocyte apoptosis represents an important event in multiple sclerosis and in several demyelinating diseases, the present observations might be considered an interesting background for further researches directed to the possibility of controlling in vivo the death of these cells.

Testosterone metabolites in patients reduce the levels of very long chain fatty acids accumulated in X-adrenoleukodystrophic fibroblasts

Testosterone metabolites (dihydrotestosterone, DHT) and 5 alpha-androstan-3 alpha,17 beta-diol (3 alpha-diol), but not testosterone itself, were shown to reduce the levels of very long chain fatty acids which accumulate in cultured skin fibroblasts from X-adrenoleukodystrophic patients (X-ALD). In addition, in X-ALD fibroblasts, testosterone is less actively converted into DHT vs. controls (skin fibroblasts retrieved from normal subjects) whereas the additional conversion of DHT to the final product 3 alpha-diol is enhanced. This is the first report of altered testosterone metabolism in X-ALD fibroblasts and of the effects of androgens in lowering the abnormal accumulation of very long chain fatty acids in this type of cells.

Corticosteroids regulate the gene expression of FGF-1 and FGF-2 in cultured rat astrocytes

The present data show that the gene expression of FGF-1 and FGF-2 is regulated by corticosteroids in rat type 1 astrocytes. In particular, the gene expression of FGF-1 is modulated by corticosteroids acting both on type I (minerocorticoid) and type II (glucocorticoid) receptors. In fact, at short times of exposure (2 h) a slight decrease in FGF-1 mRNA levels is induced by deoxycorticosterone, a steroid able to interact with the type I receptors; a similar effect is observed at 6 h following exposure to corticosterone or its 5alpha-reduced metabolite, dihydrocorticosterone. Conversely, at longer times of exposure (24 h) corticosterone is able to strongly increase FGF-1 mRNA levels. Both effects of corticosterone (inhibition and stimulation) were duplicated by dexamethasone, indicating that both effects occur via the type II receptors. Interestingly, the 5alpha-3alpha-reduced metabolite of deoxycorticosterone, tetrahydrodeoxycorticosterone, which does not interact with either corticosteroid receptors, is able to stimulate (at 6 and 24 h of exposure) the gene expression of FGF-1. It is possible that this effect might be induced via the GABA(A) receptor, since muscimol, an agonist of this receptor, exerts a similar effect. The situation is different in the case of FGF-2. The mRNA levels of this growth factor are only stimulated by steroids interacting with type II receptors. Altogether, these observations indicate that corticosteroids modulate the levels of FGF-1 and FGF-2 gene expression in astroglial cells by interaction with classical (type I and II) or nonclassical (GABA(A) receptor) steroid receptors.

The action of steroid hormones on peripheral myelin proteins: a possible new tool for the rebuilding of myelin?

The present paper summarizes recent results we have obtained while studying the effect of sex steroids on the gene expression of two peripheral myelin proteins, the glycoprotein Po (Po) and the peripheral myelin protein 22 (PMP22). In particular, we have analyzed the effect of progesterone (P), testosterone (T) and their 5alpha- and 3alpha-5alpha-reduced derivatives [respectively, dihydrotestosterone (DHTT) and 5alpha-androstan-3alpha, 17beta-diol (3alpha-diol) for T, and dihydroprogesterone (DHP) and tetrahydroprogesterone (THP) for P]. The data obtained, utilizing different in vivo and in vitro experimental models, have indicated that: a) DHP is able to enhance the low messenger levels of Po present in the sciatic nerve of aged male rats; b) P, DHP and THP treatments stimulate the gene expression of Po in the sciatic nerve of adult male rats or in cultures of rat Schwann cells, while only THP is effective on PMP22; c) P and DHP are also able to increase the low messenger levels of Po present in transected sciatic nerve; d) the removal of circulating androgens by castration is able to decrease the mRNA levels of Po in the sciatic nerve, a phenomenon which is counteracted by the consequent treatment with DHT; e) the stimulatory effect of DHT on the gene expression of Po is also evident in cultures of rat Schwann cells, but in this case the effect seems to be due to the interaction of this steroid with the progesterone receptor; f) in cultures of Schwann cells PMP22 mRNA levels are stimulated only by 3alpha-diol treatment. Taken together, these observations showing the positive effects of sex steroid hormones on the gene expressions of Po and PMP22, suggest that a treatment with these molecules or their synthetic agonists may be useful in cases in which the rebuilding of myelin is necessary.

Factors released by rat type 1 astrocytes exert different effects on the proliferation of human neuroblastoma cells (SH-SY5Y) in vitro

Brain metastases derived from abdominal neuroblastoma are an uncommon complication of this tumour; however, an increase in their occurrence has recently been reported. In the present study, we have investigated the influence of factors derived from central nervous system glial cells on the proliferation of human neuroblastoma cells (SH-SY5Y) in vitro. Co-culture experiments show that a 24-h exposure to factors released by type 1 astrocytes (A1) may induce a significant decrease in [(3)H]thymidine ([(3)H]TdR) incorporation by SH-SY5Y cells. This effect was not duplicated by fresh A1-conditioned medium (A1-CM); A1-CM became active only when it was heated or frozen. In contrast to this short-lived inhibitory effect, long-term treatment (3, 6 and 9 days) with A1-CM produced a significant and dose-dependent increase in SH-SY5Y cell number. Immunoneutralisation of A1-CM with an anti-transforming growth factor-beta antibody eliminated the inhibitory effect on [(3)H]TdR uptake in SH-SY5Y cells, but did not affect the increased number of viable cells observed after long-term treatments. In conclusion, these results showed that factor(s) released by A1 may affect the proliferation/survival of a human neuroblastoma cell line in vitro inducing: (a) a short transient negative effect on DNA synthesis and (b) an overall sustained trophic action. These results are suggestive of a possible role of glial cells in the establishment of brain metastases of neuroblastomas.

Aging in peripheral nerves: regulation of myelin protein genes by steroid hormones

The process of aging deeply influences morphological and functional parameters of the peripheral nerves. Interestingly, recent observations performed in our laboratory on the rat sciatic nerves have indicated that the deterioration of myelin occurring in the peripheral nerves during aging may be explained by the fall of the messenger levels of the major peripheral myelin proteins (glycoprotein Po, myelin basic protein and peripheral myelin protein 22). At least in the case of the Po, the low levels of its messengers and of the protein itself found in aged animals are increased by the treatment with a physiological progesterone derivative like dihydroprogesterone. It has also been found that in normal adult male rats the levels of the messengers for Po in the sciatic nerve are increased by progesterone, dihydroprogesterone and tetrahydroprogesterone; surprisingly, the gene expression of peripheral myelin protein 22 is stimulated only by tetrahydroprogesterone. These observations have been confirmed in parallel studies performed on Schwann cell cultures. Since tetrahydroprogesterone does not bind to the progesterone receptor but is a ligand for the GABAA receptor, the hypothesis has been put forward that part of the steroidal effects reported might occur not through the classical progesterone receptor, but rather via an interaction with the GABAA receptor. In other experiments it has been found that the gene expression of Po may be decreased by orchidectomy and restored by treatment with the androgen dihydrotestosterone. Altogether, these observations suggest the future use of physiological and/ or synthetic steroid hormones as a possible therapeutic approach for some pathological situations occurring in peripheral nerves during aging and demyelinating diseases.

Interactions between type 1 astrocytes and LHRH-secreting neurons (GT1-1 cells): modification of steroid metabolism and possible role of TGFbeta1

The hypothesis that type 1 astrocytes (A1) might modify the activities of the enzymes 5alpha-reductase (5alpha-R) and 3alpha-hydroxysteroid dehydrogenase (3alpha-HSD) present in the GT1-1 cells has been tested. The data obtained indicate that, utilizing a co-culture technique, A1 are able to: (1) decrease the formation of dihydrotestosterone (DHT) from testosterone (T); (2) increase the formation of dihydroprogesterone (DHP) from progesterone (P); (3) decrease the conversion of DHP into tetrahydroprogesterone (THP) in GT1-1 cells. Moreover, GT1-1 cells are able to increase the formation of DHP in A1; that of DHT was unchanged. The present data might suggest the possible existence of a third isoform of the enzyme 5alpha-R; details on this hypothesis are provided in the text. Interestingly, the inhibitory effect exerted by A1 on the formation of DHT in GT1-1 cells can be mimicked by transforming growth factor beta1 (TGFbeta1). Since TGFbeta1 had been previously shown to be directly involved in the stimulatory control of LHRH secretion by GT1-1 cells, acting both on LHRH release [R.C. Melcangi, M. Galbiati, E. Messi, F. Piva, L. Martini, M. Motta, Type 1 astrocytes influence luteinizing hormone-releasing hormone release from the hypothalamic cell line GT1-1: is transforming growth factor-beta the principle involved? Endocrinology 136 (1995) 679-686.] and gene expression [M. Galbiati, M. Zanisi, E. Messi, I. Cavarretta, L. Martini, R.C. Melcangi, Transforming growth factor-beta and astrocytic conditioned medium influence LHRH gene expression in the hypothalamic cell line GT1, Endocrinology 137 (1996) 5605-5609], the present data also show that TGFbeta1 might intervene in modulating feedback signals reaching hypothalamic LHRH producing neurons. The present findings underline once more the importance of the physiological cross-talk between A1 and neurons.

Steroid metabolism and effects in central and peripheral glial cells

Hormonal steroids participate in the control of a large number of functions of the central nervous system (CNS); recent data show that they may also intervene at the level of the peripheral nervous system (PNS). Both the CNS and the PNS metabolize endogenous as well as exogenous steroids; one of the major enzymatic system is represented by the 5alpha-reductase-3alpha-hydroxysteroid complex. This is a versatile system, since every steroid possessing the delta 4-3keto configuration (e.g., testosterone, progesterone, deoxycorticosterone) may be a substrate. High levels of 5alpha-reductase are found in the white matter of the CNS and in purified myelin. The observation that, in addition to neurons, glia may be a target for steroid action is an important recent finding. The effects of progesterone, testosterone, corticoids, and their respective 5alpha and 3alpha-5alpha derivatives on the expression of glial genes are presented and discussed. It has also been found that progesterone and/or its 5alpha-reduced metabolites increase the mRNA for the two major proteins of peripheral myelin, the glycoprotein Po and the peripheral myelin protein 22, in the sciatic nerve of normal and aged animals and in Schwann cells. The hypothesis has been put forward that glycoprotein Po might be under the control of progestagens acting mainly via the progesterone receptor, and that peripheral myelin protein 22 might be controlled via an interaction of steroids with the gamma-aminobutyric acid (GABA)ergic system. It is known that tetrahydroprogesterone, the 3alpha-5alpha-reduced metabolite of progesterone, interacts with the GABA(A) receptor. Our recent data show that several subunits of this receptor are present in sciatic nerve as well as in Schwann cells that reside in this nerve. These data open multiple possibilities for new therapeutic approaches to demyelinating diseases.

Transforming growth factor beta2 is able to modify mRNA levels and release of luteinizing hormone-releasing hormone in a immortalized hypothalamic cell line (GT1-1)

On the basis of our previous observations which indicated that transforming growth factor beta1 (TGFbeta1) affects the gene expression and the release of luteinizing hormone-releasing hormone (LHRH) in GT1-1 cells, we have presently evaluated whether also TGFbeta2 might be effective on these parameters. The data here reported show that also TGFbeta2 is able to affect LHRH dynamics, and that this action presents a different kinetics than that reported by TGFbeta1. In particular TGFbeta2 is able to facilitate LHRH release and to decrease the mRNA levels of this decapeptide. The present data have also shown that, GT1-1 cells express the messengers for the two most important receptors of the TGFbeta family, namely TGFbetaRI and TGFbetaRII and consequently represent a target for the action of the different isoforms of TGFbeta. Since the two isoforms of TGFbeta are produced and released from astrocytes, the present data add new support to the hypothesis that astrocytes participate in the control of LHRH secretion in a paracrine fashion.

Po gene expression is modulated by androgens in the sciatic nerve of adult male rats

The present results show that androgens are able to modulate the Po gene expression in different models. In particular, we have shown that: (1) the messenger for the androgen receptor (AR) is present in the rat sciatic nerve but not in cultured Schwann cells; (2) castration induces a decrease of Po mRNA levels in the sciatic nerve of male rats, which is counteract by the subsequent treatment with dihydrotestosterone (DHT), the 5alpha-reduced metabolite of testosterone; (3) castration is also able to significantly decrease in the sciatic nerve the activity of the enzyme 5alpha-reductase (which converts testosterone into DHT); and (4) DHT is able to stimulate Po gene expression in cultured Schwann cells. These observations seem to indicate that androgens may exert their effect on Po gene expression via indirect mechanisms; modulation of neuronal influences reaching the Schwann cells through the binding of the androgen to the AR present in neurons may be postulated. However, alternative mechanisms may also be taken in consideration. The data presented suggest indeed that androgens might act on Schwann cells via the progesterone receptor (PR) rather than the AR. It has been observed that: (1) the messenger for PR is present in Schwann cells; (2) DHT may activate the transcriptional activity of a PR-responsive gene by binding to the PR; and (3) putative steroid responsive elements have been described in this paper to be present in the Po promoter region.

Progesterone derivatives are able to influence peripheral myelin protein 22 and P0 gene expression: possible mechanisms of action

The present study has analyzed the effect of progesterone and its derivatives (dihydroprogesterone and tetrahydroprogesterone) on the gene expression of the peripheral myelin protein 22 utilizing in vivo and in vitro models. The data obtained indicate that tetrahydroprogesterone is able to stimulate the gene expression of peripheral myelin protein 22 both in vivo (in adult but not in old animals) and in Schwann cell cultures. An effect of this steroid, which is known to interact with the GABA(A) receptor, would not be surprising, since in the present study we show the presence in Schwann cells and in the sciatic nerve of the messengers for several subunits (alpha2, alpha3, beta1, beta2, and beta3) of the GABA(A) receptor. An effect of tetrahydroprogesterone is also evident on the gene expression of another myelin protein, the peripheral myelin protein zero. However, in this case also dihydroprogesterone, which is able to bind the progesterone receptor, is involved, both in old and adult animals, in the stimulation of messengers levels of this myelin protein. In conclusion, the present data show that the gene expression of two important peripheral myelin proteins can be influenced by progesterone derivatives. The hypothesis has been put forward that part of their effects might occur not through the classical progesterone receptor, but rather via an interaction with the GABA(A) receptor.

Effects of steroid hormones on gene expression of glial markers in the central and peripheral nervous system: variations induced by aging

The present article summarizes our data regarding: (a) the effect of sex steroids on the expression of a specific astrocytic marker in glial cell cultures (GFAP); (b) the effects of aging on two markers of the peripheral myelin (glycoprotein Po and the myelin basic protein, MBP); (c) the possible modification of the damaging effects of aging on these two markers by the in vivo administration of progesterone and its derivatives; and, finally, (d) the effect of progesterone derivatives on the gene expression of Po in cultures of rat Schwann cells. The data obtained have indicated that progesterone and its 5 alpha-reduced metabolites may play an important role in the control of gene expression of GFAP and Po, respectively, in type 1 astrocytes and Schwann cells. It has also been found that the gene expression of Po and MBP is dramatically decreased in the myelin of the sciatic nerve of aged male rats and that the aged-linked decrease of the gene expression of Po is partially reversible with steroid treatment.

Age-induced decrease of glycoprotein Po and myelin basic protein gene expression in the rat sciatic nerve. Repair by steroid derivatives

The data here reported show that the gene expression of the glycoprotein Po and of the myelin basic protein, the major components of myelin in the peripheral nervous system, dramatically decreases with ageing in the sciatic nerve of normal male rats. A one-month treatment with dihydroprogesterone, the 5alpha-reduced derivative of progesterone, is able to partially restore the fall in Po gene expression occurring in the sciatic nerve of aged male rats, without significantly modifying the gene expression of the myelin basic protein. In cultures of neonatal Schwann cells (the peripheral nervous system elements involved in the synthesis of myelin), the addition of progesterone and of dihydroprogesterone significantly increases Po gene expression; the 3alpha-reduced metabolite of dihydroprogesterone, tetrahydroprogesterone proved to be even more effective. These data suggest that the effect of progesterone is linked to its conversion into dihydroprogesterone and especially into tetrahydroprogesterone, since Schwann cells possess the 5alpha-reductase-3alpha-hydroxysteroid dehydrogenase system. The data provide the first demonstration that ageing decreases the gene expression of two major components of the peripheral myelin in the sciatic nerve; they also show that this phenomenon may be partially reversed by progesterone derivatives, which might act by stimulating Po gene expression in the Schwann cells.

The 5alpha-reductase in the central nervous system: expression and modes of control

The present paper will summarize two important aspects of the interactions between steroids and the brain, which have recently been studied in the authors' laboratory. In particular the paper will consider data on: (1) the significance of the two isoforms of the 5alpha-R during brain ontogenesis and development, and (2) the cross-talk between glial and neuronal elements, particularly in relation to the metabolism of sex hormones. (1) The data obtained have shown that the 5alpha-R type 1 enzyme is constitutively expressed in the rat CNS at all stages of brain development. Moreover, the expression of the 5alpha-R type 1 is similar in males and in females, and does not appear to be controlled by androgens. The gene expression of the 5alpha-R type 2 is totally different. This isoform appears to be expressed in the rat brain almost exclusively in the late fetal/early post-natal life and is controlled by testosterone. (2) The present data show that two cell lines derived respectively from a rat glioma (C6 cell line) and from a human astrocytoma (1321N1 cell line) are able to convert testosterone and progesterone into their corresponding 5alpha-reduced metabolites dihydrotestosterone and dihydroprogesterone. The possibility that secretory products of normal and tumoral brain cells might be able to influence steroid metabolism occurring in the two glial cell lines previously mentioned has been considered.