The stimulatory effect of the octadecaneuropeptide (ODN) on cytosolic Ca2+ in rat astrocytes is not mediated through classical benzodiazepine receptors

Cytoprotective and Neurotrophic Effects of Octadecaneuropeptide (ODN) in in vitro and in vivo Models of Neurodegenerative Diseases

Octadecaneuropeptide (ODN) and its precursor diazepam-binding inhibitor (DBI) are peptides belonging to the family of endozepines. Endozepines are exclusively produced by astroglial cells in the central nervous system of mammals, and their release is regulated by stress signals and neuroactive compounds. There is now compelling evidence that the gliopeptide ODN protects cultured neurons and astrocytes from apoptotic cell death induced by various neurotoxic agents. In vivo, ODN causes a very strong neuroprotective action against neuronal degeneration in a mouse model of Parkinson's disease. The neuroprotective activity of ODN is based on its capacity to reduce inflammation, apoptosis, and oxidative stress. The protective effects of ODN are mediated through its metabotropic receptor. This receptor activates a transduction cascade of second messengers to stimulate protein kinase A (PKA), protein kinase C (PKC), and mitogen-activated protein kinase (MAPK)-extracellular signal-regulated kinase (ERK) signaling pathways, which in turn inhibits the expression of proapoptotic factor Bax and the mitochondrial apoptotic pathway. In N2a cells, ODN also promotes survival and stimulates neurite outgrowth. During the ODN-induced neuronal differentiation process, numerous mitochondria and peroxisomes are identified in the neurites and an increase in the amount of cholesterol and fatty acids is observed. The antiapoptotic and neurotrophic properties of ODN, including its antioxidant, antiapoptotic, and pro-differentiating effects, suggest that this gliopeptide and some of its selective and stable derivatives may have therapeutic value for the treatment of some neurodegenerative diseases.

Glial endozepines and energy balance: Old peptides with new tricks

The contribution of neuroglial interactions to the regulation of energy balance has gained increasing acceptance in recent years. In this context, endozepines, endogenous analogs of benzodiazepine derived from diazepam-binding inhibitor, are now emerging as major players. Produced by glial cells (astrocytes and tanycytes), endozepines have been known for two decades to exert potent anorexigenic effects by acting at the hypothalamic level. However, it is only recently that their modes of action, including the mechanisms by which they modulate energy metabolism, have begun to be elucidated. The data available today are abundant, significant, and sometimes contradictory, revealing a much more complex regulation than initially expected. Several mechanisms of action of endozepines seem to coexist at the central level, particularly in the hypothalamus. The brainstem has also recently emerged as a potential site of action for endozepines. In addition to their central anorexigenic effects, endozepines may also display peripheral effects promoting orexigenic actions, adding to their complexity and raising yet more questions. In this review, we attempt to provide an overview of our current knowledge in this rapidly evolving field and to pinpoint questions that remain unanswered.

The gliotransmitter ACBP controls feeding and energy homeostasis via the melanocortin system

Glial cells have emerged as key players in the central control of energy balance and etiology of obesity. Astrocytes play a central role in neural communication via the release of gliotransmitters. Acyl-CoA binding protein (ACBP)-derived endozepines are secreted peptides that modulate the GABAA receptor. In the hypothalamus, ACBP is enriched in arcuate nucleus (ARC) astrocytes, ependymocytes and tanycytes. Central administration of the endozepine octadecaneuropeptide (ODN) reduces feeding and improves glucose tolerance, yet the contribution of endogenous ACBP in energy homeostasis is unknown. We demonstrated that ACBP deletion in GFAP+ astrocytes, but not in Nkx2.1-lineage neural cells, promoted diet-induced hyperphagia and obesity in both male and female mice, an effect prevented by viral rescue of ACBP in ARC astrocytes. ACBP-astrocytes were observed in apposition with proopiomelanocortin (POMC) neurons and ODN selectively activated POMC neurons through the ODN-GPCR but not GABAA, and supressed feeding while increasing carbohydrate utilization via the melanocortin system. Similarly, ACBP overexpression in ARC astrocytes reduced feeding and weight gain. Finally, the ODN-GPCR agonist decreased feeding and promoted weight loss in ob/ob mice. These findings uncover ACBP as an ARC gliopeptide playing a key role in energy balance control and exerting strong anorectic effects via the central melanocortin system.

Glial Endozepines Inhibit Feeding-Related Autonomic Functions by Acting at the Brainstem Level

Endozepines are endogenous ligands for the benzodiazepine receptors and also target a still unidentified GPCR. The endozepine octadecaneuropeptide (ODN), an endoproteolytic processing product of the diazepam-binding inhibitor (DBI) was recently shown to be involved in food intake control as an anorexigenic factor through ODN-GPCR signaling and mobilization of the melanocortinergic signaling pathway. Within the hypothalamus, the DBI gene is mainly expressed by non-neuronal cells such as ependymocytes, tanycytes, and protoplasmic astrocytes, at levels depending on the nutritional status. Administration of ODN C-terminal octapeptide (OP) in the arcuate nucleus strongly reduces food intake. Up to now, the relevance of extrahypothalamic targets for endozepine signaling-mediated anorexia has been largely ignored. We focused our study on the dorsal vagal complex located in the caudal brainstem. This structure is strongly involved in the homeostatic control of food intake and comprises structural similarities with the hypothalamus. In particular, a circumventricular organ, the area postrema (AP) and a tanycyte-like cells forming barrier between the AP and the adjacent nucleus tractus solitarius (NTS) are present. We show here that DBI is highly expressed by ependymocytes lining the fourth ventricle, tanycytes-like cells, as well as by proteoplasmic astrocytes located in the vicinity of AP/NTS interface. ODN staining observed at the electron microscopic level reveals that ODN-expressing tanycyte-like cells and protoplasmic astrocytes are sometimes found in close apposition to neuronal elements such as dendritic profiles or axon terminals. Intracerebroventricular injection of ODN or OP in the fourth ventricle triggers c-Fos activation in the dorsal vagal complex and strongly reduces food intake. We also show that, similarly to leptin, ODN inhibits the swallowing reflex when microinjected into the swallowing pattern generator located in the NTS. In conclusion, we hypothesized that ODN expressing cells located at the AP/NTS interface could release ODN and modify excitability of NTS neurocircuitries involved in food intake control.

Detection, characterization and biological activities of [bisphospho-thr3,9]ODN, an endogenous molecular form of ODN released by astrocytes

Astrocytes synthesize and release endozepines, a family of regulatory neuropeptides, including diazepam-binding inhibitor (DBI) and its processing fragments such as the octadecaneuropeptide (ODN). At the molecular level, ODN interacts with two types of receptors, i.e. it acts as an inverse agonist of the central-type benzodiazepine receptor (CBR), and as an agonist of a G protein-coupled receptor (GPCR). ODN exerts a wide range of biological effects mediated through these two receptors and, in particular, it regulates astrocyte activity through an autocrine/paracrine mechanism involving the metabotropic receptor. More recently, it has been shown that Müller glial cells secrete phosphorylated DBI and that bisphosphorylated ODN ([bisphospho-Thr(3,9)]ODN, bpODN) has a stronger affinity for CBR than ODN. The aim of the present study was thus to investigate whether bpODN is released by mouse cortical astrocytes and to compare its potency to ODN. Using a radioimmunoassay and mass spectrometry analysis we have shown that bpODN as well as ODN were released in cultured astrocyte supernatants. Both bpODN and ODN increased astrocyte calcium event frequency but in a very different range of concentration. Indeed, ODN stimulatory effect decreased at concentrations over 10(-10)M whereas bpODN increased the calcium event frequency at similar doses. In vivo effects of bpODN and ODN were analyzed in two behavioral paradigms involving either the metabotropic receptor (anorexia) or the CBR (anxiety). As previously described, ODN (100ng, icv) induced a significant reduction of food intake. Similar effect was achieved with bpODN but at a 10 times higher dose (1000 ng, icv). Similarly, and contrasting with our hypothesis, bpODN was also 10 times less potent than ODN to induce anxiety-related behavior in the elevated zero maze test. Thus, the present data do not support that phosphorylation of ODN is involved in receptor selectivity but indicate that it rather weakens ODN activity.

The octadecaneuropeptide stimulates somatolactin release from cultured goldfish pituitary cells

The present study aimed to investigate the distribution of the octadecaneuropeptide (ODN) in the goldfish brain and to look for a possible effect of ODN on somatolactin (SL) release from pituitary cells. A discrete population of ODN-immunoreactive neurones was localised in the lateral part of the nucleus lateralis tuberis. These neurones sent projections through the neurohypophyseal tract towards the neurohypophysis, and nerve fibres were seen in the close vicinity of SL-producing cells in the pars intermedia. Incubation of cultured goldfish pituitary cells with graded concentrations of ODN (10(-9) -10(-5 ) m) induced a dose-dependent stimulation of SL-β, but not SL-α, release. ODN-evoked SL release was blocked by the metabotrophic endozepine receptor antagonist cyclo(1-8) [DLeu(5) ]OP but was not affected by the central-type benzodiazepine receptor antagonist flumazenil. ODN-induced SL release was suppressed by treatment with the phospholipase C (PLC) inhibitor U-73122 but not with the protein kinase A (PKA) inhibitor H-89. These results indicate that, in fish, ODN produced by hypothalamic neurones acts as a hypophysiotrophic neuropeptide stimulating SL release. The effect of ODN is mediated through a metabotrophic endozepine receptor positively coupled to the PLC/inositol 1,4,5-trisphosphate/protein kinase C-signalling pathway.

The octadecaneuropeptide ODN prevents 6-hydroxydopamine-induced apoptosis of cerebellar granule neurons through a PKC-MAPK-dependent pathway

Oxidative stress, induced by various neurodegenerative diseases, initiates a cascade of events leading to apoptosis, and thus plays a critical role in neuronal injury. In this study, we have investigated the potential neuroprotective effect of the octadecaneuropeptide (ODN) on 6-hydroxydopamine (6-OHDA)-induced oxidative stress and apoptosis in cerebellar granule neurons (CGN). ODN, which is produced by astrocytes, is an endogenous ligand for both central-type benzodiazepine receptors (CBR) and a metabotropic receptor. Incubation of neurons with subnanomolar concentrations of ODN (10⁻¹⁸ to 10⁻¹² M) inhibited 6-OHDA-evoked cell death in a concentration-dependent manner. The effect of ODN on neuronal survival was abrogated by the metabotropic receptor antagonist, cyclo₁₋₈ [DLeu⁵]OP, but not by a CBR antagonist. ODN stimulated polyphosphoinositide turnover and ERK phosphorylation in CGN. The protective effect of ODN against 6-OHDA toxicity involved the phospholipase C/ERK MAPK transduction cascade. 6-OHDA treatment induced an accumulation of reactive oxygen species, an increase of the expression of the pro-apoptotic gene Bax, a drop of the mitochondrial membrane potential and a stimulation of caspase-3 activity. Exposure of 6-OHDA-treated cells to ODN blocked all the deleterious effects of the toxin. Taken together, these data demonstrate for the first time that ODN is a neuroprotective agent that prevents 6-OHDA-induced oxidative stress and apoptotic cell death.

Down-regulation of GABA(A) receptor via promiscuity with the vasoactive peptide urotensin II receptor. Potential involvement in astrocyte plasticity

GABA_A receptor (GABA_AR) expression level is inversely correlated with the proliferation rate of astrocytes after stroke or during malignancy of astrocytoma, leading to the hypothesis that GABA_AR expression/activation may work as a cell proliferation repressor. A number of vasoactive peptides exhibit the potential to modulate astrocyte proliferation, and the question whether these mechanisms may imply alteration in GABA_AR-mediated functions and/or plasma membrane densities is open. The peptide urotensin II (UII) activates a G protein-coupled receptor named UT, and mediates potent vasoconstriction or vasodilation in mammalian vasculature. We have previously demonstrated that UII activates a PLC/PIPs/Ca²⁺ transduction pathway, via both G_q and G_i/o proteins and stimulates astrocyte proliferation in culture. It was also shown that UT/G_q/IP₃ coupling is regulated by the GABA_AR in rat cultured astrocytes. Here we report that UT and GABA_AR are co-expressed in cerebellar glial cells from rat brain slices, in human native astrocytes and in glioma cell line, and that UII inhibited the GABAergic activity in rat cultured astrocytes. In CHO cell line co-expressing human UT and combinations of GABA_AR subunits, UII markedly depressed the GABA current (β₃γ₂>α₂β₃γ₂>α₂β₁γ₂). This effect, characterized by a fast short-term inhibition followed by drastic and irreversible run-down, is not relayed by G proteins. The run-down partially involves Ca²⁺ and phosphorylation processes, requires dynamin, and results from GABA_AR internalization. Thus, activation of the vasoactive G protein-coupled receptor UT triggers functional inhibition and endocytosis of GABA_AR in CHO and human astrocytes, via its receptor C-terminus. This UII-induced disappearance of the repressor activity of GABA_AR, may play a key role in the initiation of astrocyte proliferation.

Effect of GABA A receptor activation on UT-coupled signaling pathways in rat cortical astrocytes

Cultured rat cortical astrocytes express two types of urotensin II (UII) binding sites: a high affinity site corresponding to the UT (GPR14) receptor and a low affinity site that has not been fully characterized. Activation of the high affinity site in astroglial cells stimulates polyphosphoinositide (PIP) turnover and provokes an increase in intracellular calcium concentration. We have hypothesized that the existence of distinct affinity sites for UII in rat cortical astrocytes could be accounted for by a possible cross-talk between UT and the ligand-gated ion channel GABA(A) receptor (GABA A R). Exposure of cultured astrocytes to UII provoked a bell-shaped increase in cAMP production, with an EC50 stimulating value of 0.83+/-0.04 pM, that was totally blocked in the presence of the adenylyl cyclase inhibitor SQ 22,536. In contrast, UII was found to inhibit forskolin-induced cAMP formation. In the presence of the specific PKA inhibitor H89, UII provoked a sustained stimulation of cAMP formation. Inhibition of PKA by H89 strongly reduced the stimulatory effect of UII on PIP metabolism. GABA and the GABA A R agonist isoguvacine provoked a marked inhibition of UII-induced cAMP synthesis and a significant reduction of UII-evoked PIP turnover. These data suggest that functional interaction between UT and GABA(A)R negatively regulates coupling of UT to the classical PLC/IP(3) signaling cascade as well as to the adenylyl cyclase/PKA pathway.

[Orn5]URP acts as a pure antagonist of urotensinergic receptors in rat cortical astrocytes

Cultured rat astrocytes, which express functional urotensin II (UII)/UII-related peptide (URP) receptors (UT), represent a very suitable model to investigate the pharmacological profile of UII and URP analogs towards native UT. We have recently designed three URP analogs [D-Trp4]URP, [Orn5]URP and [D-Tyr6]URP, that act as UT antagonists in the rat aortic ring bioassay. However, it has been previously reported that UII/URP analogs capable of inhibiting the contractile activity of UII possess agonistic activity on UT-transfected cells. In the present study, we have compared the ability of URP analogs to compete for [125 I]URP binding and to modulate cytosolic calcium concentration ([Ca2+]c) in cultured rat astrocytes. All three analogs displaced radioligand binding: [D-Trp4]URP and [D-Tyr6]URP interacted with high- and low-affinity sites whereas [Orn5]URP only bound high-affinity sites. [D-Trp4]URP and [D-Tyr6]URP both induced a robust increase in [Ca2+]c in astrocytes while [Orn5]URP was totally devoid of activity. [Orn5]URP provoked a concentration-dependent inhibition of URP- and UII-evoked [Ca2+]c increase and a rightward shift of the URP and UII dose-response curves. The present data indicate that [D-Trp4]URP and [D-Tyr6]URP, which act as UII antagonists in the rat aortic ring assay, behave as agonists in the [Ca2+]c mobilization assay in cultured astrocytes, whereas [Orn5]URP is a pure selective antagonist in both rat aortic ring contraction and astrocyte [Ca2+]c mobilization assays.

Translocator protein (18 kDa) ligand PK 11195 induces transient mitochondrial Ca2+ release leading to transepithelial Cl- secretion in HT-29 human colon cancer cells

BACKGROUND INFORMATION

TSPO (translocator protein), known previously as PBR (peripheral-type benzodiazepine receptor), is a 18 kDa protein expressed in the mitochondrial membrane of a variety of tissues. TSPO has been reported to be over-expressed in human colorectal tumours and cancer cell lines, but its function is not well characterized.

RESULTS

We investigated the expression and function of TSPO in the human colon cancer cells HT-29. Immunohistochemical studies revealed that TSPO is localized in mitochondria, and its endogenous ligand, the polypeptide diazepam-binding inhibitor, in the cytosol. Radioligand binding studies using the specific high-affinity drug ligand [(3)H]PK 11195 and membrane fraction demonstrated saturable binding, with K(d) and B(max) values of 13.5+/-1.5 nM and 10.1+/-1.0 pmol/mg respectively. PK 11195 induced a rapid and transient dose-dependent rise in intracellular [Ca(2+)], which was unaffected by extracellular Ca(2+), but was blocked by the PTP (permeability transition pore) inhibitor, cyclosporin A, and by the TSPO partial agonist, flunitrazepam. Using HT-29 clone 19A cell line, which forms cell monolayers, we demonstrated that TSPO ligand stimulated a Ca(2+)-dependent transepithelial Cl(-) secretion. This secretion was inhibited: (i) after removal of extracellular Cl(-); (ii) by apical addition of the Cl(-) channel blocker NPPB [5-nitro-2-(3-phenylpropylamino)-benzoate]; and (iii) by basolateral addition of the Na(+)-K(+)-2Cl(-) co-transporter inhibitor bumetanide. Furthermore, the intracellular Ca(2+) chelator BAPTA/AM [bis-(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid tetrakis(acetoxymethyl ester)] and cyclosporin A abolished the rise in PK 11195-induced Cl(-) secretion.

CONCLUSIONS

These findings indicate that TSPO is located in mitochondrial membranes of HT-29 and reveal that its activation induces a rise in cytosolic Ca(2+), leading to the stimulation of Cl(-) secretion.

Role of PACAP and VIP in astroglial functions

Astrocytes represent at least 50% of the volume of the human brain. Besides their roles in various supportive functions, astrocytes are involved in the regulation of stem cell proliferation, synaptic plasticity and neuroprotection. Astrocytes also influence neuronal physiology by responding to neurotransmitters and neuropeptides and by releasing regulatory factors termed gliotransmitters. In particular, astrocytes express the PACAP-specific receptor PAC1-R and the PACAP/VIP mutual receptors VPAC1-R and VPAC2-R during development and/or in the adult. There is now clear evidence that PACAP and VIP modulate a number of astrocyte activities such as proliferation, plasticity, glycogen production, and biosynthesis of neurotrophic factors and gliotransmitters.

Pharmacological characterization of the receptor mediating the anorexigenic action of the octadecaneuropeptide: evidence for an endozepinergic tone regulating food intake

Peptides of the endozepine family, including diazepam-binding inhibitor, the triakontatetraneuropeptide, and the octadecaneuropeptide (ODN), act through three types of receptors, that is, central-type benzodiazepine receptors (CBR), peripheral-type (mitochondrial) benzodiazepine receptors (PBR) and a metabotropic receptor positively coupled to phospholipase C via a pertussis toxin-sensitive G protein. We have previously reported that ODN exerts a potent anorexigenic effect in rat and we have found that the action of ODN is not affected by the mixed CBR/PBR agonist diazepam. In the present report, we have tested the possible involvement of the metabotropic receptor in the anorexigenic activity of ODN. Intracerebroventricular administration of the C-terminal octapeptide (OP) and its head-to-tail cyclic analog cyclo(1-8)OP (cOP) at a dose of 100 ng mimicked the inhibitory effect of ODN on food intake in food-deprived mice. The specific CBR antagonist flumazenil and the PBR antagonist PK11195 did not prevent the effect of ODN, OP, and cOP on food consumption. In contrast, the selective metabotropic endozepine receptor antagonist cyclo(1-8)[DLeu(5)]OP (100-1000 ng; cDLOP) suppressed the anorexigenic effect of ODN, OP, and cOP. At the highest concentration tested (1000 ng), cDLOP provoked by itself a significant increase in food intake. Taken together, the present results indicate that the anorexigenic effect of ODN and OP is mediated through activation of the metabotropic receptor recently characterized in astrocytes. The data also suggest that endogenous ODN, acting via this receptor, exerts an inhibitory tone on feeding behavior.

Structure−Activity Relationships of a Series of Analogs of the Endozepine Octadecaneuropeptide (ODN11-18) on Neurosteroid Biosynthesis by Hypothalamic Explants

We have previously shown that the endozepine octadecaneuropeptide (ODN) stimulates the biosynthesis of neurosteroids from frog hypothalamic explants. In the present study, we have investigated the structure-activity relationships of a series of analogs of the C-terminal octapeptide of ODN (OP) on neurosteroid formation. We found that OP and its cyclic analog cyclo1-8OP stimulate in a concentration-dependent manner the synthesis of various steroids including 17-hydroxypregnenolone, progesterone, 17-hydroxyprogesterone and dehydroepiandrosterone. Deletion or Ala-substitution of the Arg1 or Pro2 residues of OP did not affect the activity of the peptide. In contrast, deletion or replacement of any of the amino acids of the C-terminal hexapeptide fragment totally abolished the effect of OP on neurosteroid biosynthesis. The present study indicates that the C-terminal hexapeptide of ODN/OP is the minimal sequence retaining full biological activity on steroid-producing neurons.

Role of androgens and glucocorticoids in the regulation of diazepam-binding inhibitor mRNA levels in male mouse hypothalamus

In peripheral organs, gonadal and adrenal steroids regulate diazepam-binding inhibitor (DBI) mRNA expression. In order to further investigate the involvement of peripheral steroid hormones in the modulation of brain DBI mRNA expression, we studied by semiquantitative in situ hybridization the effect of adrenalectomy (ADX) and castration (CX) and short-term replacement therapy on DBI mRNA levels in the male mouse hypothalamus. Cells expressing DBI mRNA were mostly observed in the arcuate nucleus, the median eminence and the ependyma bordering the third ventricle. In the median eminence and the ependyma bordering the third ventricule, the DBI gene expression was decreased in ADX rats and a single injection of corticosterone to ADX rats induced a significant increase in DBI gene expression at 3 and 12 h time intervals without completely restoring the basal DBI mRNA expression observed in intact mice. In the arcuate nucleus, ADX and corticosterone administration did not modify DBI mRNA expression. CX down-regulated DBI gene expression in the ependyma bordering the third ventricle. The administration of dihydrotestosterone (3-24 h) completely reversed the inhibitory effect of CX. In the median eminence and arcuate nucleus, neither CX or dihydrotestosterone administration modified DBI mRNA levels. These results suggest that the effects of glucocorticoids on the hypothalamo-pituitary-adrenocortical axis and androgens on the hypothalamo-pituitary-gonadal axis are mediated by DBI.

Biochemical and functional characterization of high-affinity urotensin II receptors in rat cortical astrocytes

The urotensin II (UII) gene is primarily expressed in the central nervous system, but the functions of UII in the brain remain elusive. Here, we show that cultured rat astrocytes constitutively express the UII receptor (UT). Saturation and competition experiments performed with iodinated rat UII ([(125)I]rUII) revealed the presence of high- and low-affinity binding sites on astrocytes. Human UII (hUII) and the two highly active agonists hUII(4-11) and [3-iodo-Tyr9]hUII(4-11) were also very potent in displacing [(125)I]rUII from its binding sites, whereas the non-cyclic analogue [Ser5,10]hUII(4-11) and somatostatin-14 could only displace [(125)I]rUII binding at micromolar concentrations. Reciprocally, rUII failed to compete with [(125)I-Tyr0,D-Trp8]somatostatin-14 binding on astrocytes. Exposure of cultured astrocytes to rUII stimulated [(3)H]inositol incorporation and increased intracellular Ca(2+) concentration in a dose-dependent manner. The stimulatory effect of rUII on polyphosphoinositide turnover was abolished by the phospholipase C inhibitor U73122, but only reduced by 56% by pertussis toxin. The GTP analogue Gpp(NH)p caused its own biphasic displacement of [(125)I]rUII binding and provoked an affinity shift of the competition curve of rUII. Pertussis toxin shifted the competition curve towards a single lower affinity state. Taken together, these data demonstrate that rat astrocytes express high- and low-affinity UII binding sites coupled to G proteins, the high-affinity receptor exhibiting the same pharmacological and functional characteristics as UT.

Catabolism of the octadecaneuropeptide ODN by prolyl endopeptidase: identification of an unusual cleavage site

The octadecaneuropeptide ODN (QATVGDVNTDRPGLLDLK), a biologically active fragment of diazepam-binding inhibitor, exerts a number of behavioral and neurophysiological activities. The presence of a proline residue in the sequence of ODN led us to investigate the role of proline endopeptidase (PEP) in the catabolism of this neuropeptide. The effect of PEP on the breakdown of ODN and related analogs was studied by combining RP-HPLC analysis and MALDI-TOF MS characterization. Incubation of ODN with PEP generated two products, i.e. ODN3-18 and ODN5-18 which resulted from cleavage of the Ala-Thr and Val-Gly peptide bonds. S 17092, a specific PEP inhibitor, significantly reduced the PEP-induced cleavages of ODN. Similarly, [Ala2]OP showed S 17092-sensitive post-alanine cleavage, while [pGlu1]ODN and OP (ODN11-18) were not catabolized by the enzyme. For all these peptides, cleavage of the Pro-Gly peptide bond by PEP was never observed, even after prolonged incubation times. In contrast, PEP hydrolyzed human urotensin II at the canonical post-proline site. Collectively, these data suggest that the Ala2 residue is the preferential cleavage site of ODN and that the Pro-Gly bond of ODN is not hydrolyzed by PEP. In addition, this study reveals for the first time that the endoproteolytic activity of PEP can specifically take place after a valine moiety.

Solid phase synthesis of a redox delivery system with the aim of targeting peptides into the brain

A solid phase approach for the preparation of peptides attached to a redox chemical delivery system derived from stable annulated NADH models is reported. The synthesis starts with the grafting on a Merrifield resin of quinoline 4b, precursor of the redox carrier. From the resulting quinoline supported resin 4d, the stepwise SPPS of both octapeptide OP (RPGLLDLK) and octadecaneuropeptide ODN (QATVGDVNTDRPGLLDLK), two neuropeptides exhibiting anorexigenic effects, was successfully achieved by conventional methods. Quaternization of the quinoline moiety prior to cleavage of the modified OP and ODN peptides from the resin, led to the expected quinolinium salt 8a and 8b respectively linked to OP or ODN peptides. Finally, the reduction with NaBH4 monitored by UV-vis, provided the desired annulated NADH models as peptides carriers with either the OP (11a,b) or ODN (12a,b) moiety.

In vivo action of a new octadecaneuropeptide antagonist on neuropeptide Y and corticotropin-releasing hormone mRNA levels in rat

It has been reported that several of the effects induced by an octadecaneuropeptide (ODN), derived from an 86-amino-acid polypeptide termed diazepam-binding inhibitor, could be mediated by activation of a metabotropic receptor. In order to investigate the role and mechanism of action of ODN in the regulation of corticotropin-releasing factor (CRH) and neuropeptide Y (NPY) expression in the paraventricular nucleus and arcuate nucleus, respectively, we studied the effects of the acute intracerebroventricular administration of ODN (2 microg/rat) and the ODN antagonist to metabotropic receptor, cyclo(1-8)[Dleu5]OP (20 microg/rat), on the gene expression of the two neuropeptides in castrated male rat. ODN administration resulted in a 45% increase in CRH mRNA expression, an effect which was reversed by cyclo(1-8)[Dleu5]OP. When cyclo(1-8)[Dleu5]OP was administered alone, it induced a 19% decrease in CRH mRNA levels. ODN administration induced a 17% decrease in NPY mRNA expression while cyclo(1-8)[Dleu5]OP increased by 21% the hybridization signal. The administration of both ODN and ODN antagonist completely abolished the depressing effect of ODN on NPY mRNA. These data suggest that the effects of ODN on CRH and NPY mRNA might be mediated by interaction with metabotropic receptors. Moreover, since cyclo(1-8)[Dleu5]OP can by itself influence the expression of two peptide mRNAs, it might be suggested that ODN is exerting a tonic influence on NPY and CRH neurons.

Beta-amyloid peptides stimulate endozepine biosynthesis in cultured rat astrocytes

Accumulation of beta-amyloid peptide (Abeta), which is a landmark of Alzheimer's disease, may alter astrocyte functions before any visible symptoms of the disease occur. Here, we examined the effects of Abeta on biosynthesis and release of diazepam-binding inhibitor (DBI), a polypeptide primarily expressed by astroglial cells in the CNS. Quantitative RT-PCR and specific radioimmunoassay demonstrated that aggregated Abeta(25-35), at concentrations up to 10(-4) m, induced a dose-dependent increase in DBI mRNA expression and DBI-related peptide release from cultured rat astrocytes. These effects were totally suppressed when aggregation of Abeta(25-35) was prevented by Congo red. Measurement of the number of living cells revealed that Abeta(25-35) induced a trophic rather than a toxic effect on astrocytes. Administration of cycloheximide blocked Abeta(25-35)-induced increase of DBI gene expression and endozepine accumulation in astrocytes, indicating that protein synthesis is required for DBI gene expression. Altogether, the present data suggest that Abeta-induced activation of endozepine biosynthesis and release may contribute to astrocyte proliferation associated with Alzheimer's disease.

Somatostatin down-regulates the expression and release of endozepines from cultured rat astrocytes via distinct receptor subtypes

Endozepines, a family of regulatory peptides related to diazepam-binding inhibitor (DBI), are synthesized and released by astroglial cells. Because rat astrocytes express various subtypes of somatostatin receptors (sst), we have investigated the effect of somatostatin on DBI mRNA level and endozepine secretion in rat astrocytes in secondary culture. Somatostatin reduced in a concentration-dependent manner the level of DBI mRNA in cultured astrocytes. This inhibitory effect was mimicked by the selective sst4 receptor agonist L803-087 but not by the selective sst1, sst2 and sst3 receptor agonists L779-591, L779-976 and L797-778, respectively. Somatostatin was unable to further reduce DBI mRNA level in the presence of the MEK inhibitor U0126. Somatostatin and the sst1, sst2 and sst4 receptor agonists induced a concentration-dependent inhibition of endozepine release. Somatostatin and the sst1, sst2 and sst4 receptor agonists also inhibited cAMP formation dose-dependently. In addition, somatostatin reduced forskolin-induced endozepine release. H89 mimicked the inhibitory effect of somatostatin on endozepine secretion. In contrast the PLC inhibitor U73122, the PKC activator PMA and the PKC inhibitor calphostin C had no effect on somatostatin-induced inhibition of endozepine release. The present data demonstrate that somatostatin reduces DBI mRNA level mainly through activation of sst4 receptors negatively coupled to the MAPK pathway, and inhibits endozepine release through activation of sst1, sst2 and sst4 receptors negatively coupled to the adenylyl cyclase/PKA pathway.

Pathogenesis of peroxisomal deficiency disorders (Zellweger syndrome) may be mediated by misregulation of the GABAergic system via the diazepam binding inhibitor

Background

Zellweger syndrome (ZS) is a fatal inherited disease caused by peroxisome biogenesis deficiency. Patients are characterized by multiple disturbances of lipid metabolism, profound hypotonia and neonatal seizures, and distinct craniofacial malformations. Median live expectancy of ZS patients is less than one year. While the molecular basis of peroxisome biogenesis and metabolism is known in considerable detail, it is unclear how peroxisome deficiency leads to the most severe neurological symptoms. Recent analysis of ZS mouse models has all but invalidated previous hypotheses.

Hypothesis

We suggest that a regulatory rather than a metabolic defect is responsible for the drastic impairment of brain function in ZS patients.

Testing the hypothesis

Using microarray analysis we identify diazepam binding inhibitor/acyl-CoA binding protein (DBI) as a candidate protein that might be involved in the pathogenic mechanism of ZS. DBI has a dual role as a neuropeptide antagonist of GABA(A) receptor signaling in the brain and as a regulator of lipid metabolism. Repression of DBI in ZS patients could result in an overactivation of GABAergic signaling, thus eventually leading to the characteristic hypotonia and seizures. The most important argument for a misregulation of GABA(A) in ZS is, however, provided by the striking similarity between ZS and "benzodiazepine embryofetopathy", a malformation syndrome observed after the abuse of GABA(A) agonists during pregnancy.

Implications of the hypothesis

We present a tentative mechanistic model of the effect of DBI misregulation on neuronal function that could explain some of the aspects of the pathology of Zellweger syndrome.

In vivo action of a new octadecaneuropeptide (ODN) antagonist on gonadotropin-releasing hormone gene expression in the male rat brain

It has been reported that several of the effects induced by octadecaneuropeptide (ODN) could be mediated by an activation of a metabotropic receptor. In order to investigate the role and mechanism of action of ODN in gonadotropin-releasing hormone (GnRH) neuron regulation, we studied the effects of the acute i.c.v. administration of ODN and of a new ODN antagonist to metabotropic receptor, cyclo(1-8)[Dleu(5)]OP, on GnRH mRNA expression as evaluated by in situ hybridization in castrated male rats. The administration of ODN produced a decrease in the hybridization signal while the administration of cyclo(1-8)[Dleu(5)]OP alone produced an 18% increase. When administrated concomitantly with ODN, the antagonist both inhibited the depressing effect of ODN and induced a 22% increase over the values detected in ODN-treated rats. The data suggest that the effect of ODN on GnRH mRNA expression might be mediated by interaction with metabotropic receptors.

Effect of intracerebroventricular administration of the octadecaneuropeptide on the expression of pro-opiomelanocortin, neuropeptide Y and corticotropin-releasing hormone mRNAs in rat hypothalamus

Intracerebroventricular (i.c.v.) administration of the octadecaneuropeptide (diazepam-binding inhibitor [33-50]; ODN) exerts a potent anorexigenic effect in the rat. We studied the effect of ODN on three neuropeptides involved in feeding behaviour: the orexigenic peptide neuropeptide Y (NPY) and two anorexigenic peptides, corticotropin-releasing hormone (CRH) and the pro-opiomelanocortin (POMC)-derived peptide alpha-melanocyte-stimulating hormone. The effect of i.c.v. administration of ODN (0.1 microg/kg and 1 microg/kg) on mRNA expression of the peptides in male rat hypothalamus was evaluated by semiquantitative in situ hybridization. In the arcuate nucleus, NPY-expressing neurones were mostly found in the inner zone in close proximity of the third ventricle. ODN at the dose of 0.1 microg/kg induced a significant decrease of 17.4% in NPY mRNA expression, while the depressing effect was more marked (31.4%) with the highest dose of ODN (1 microg/kg). POMC-expressing neurones were more laterally located in the arcuate nucleus. Administration of ODN at 0.1 microg/kg and 1 microg/kg doses induced increases of 33.5% and 27.4% in POMC mRNA expression, respectively. Labelling obtained with the CRH cRNA probe was essentially distributed throughout the medial parvocellular area of the hypothalamic paraventricular nucleus. ODN, at doses of 0.1 and 1 microg/kg, resulted in 17.8% and 32.8% decreases in CRH mRNA expression, respectively. The present data suggest that ODN might exert its anorexigenic effect by increasing mRNA expression of POMC and decreasing mRNA expression of NPY in the arcuate nucleus.

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates endozepine release from cultured rat astrocytes via a PKA-dependent mechanism

Astroglial cells synthesize and release endozepines, neuropeptides that are related to the octadecaneuropeptide ODN. Glial cells also express PACAP/VIP receptors. We have investigated the possible effect of PACAP on the release of ODN-like immunoreactivity (ODN-LI) by cultured rat astrocytes. Administration of PACAP27 and PACAP38 induced a concentration-dependent increase in secretion of ODN-LI whereas VIP was approximately 1000-fold less potent. The maximum effect of PACAP38 occurred after 5 min, then gradually declined during the next 10 min. The stimulatory effects of PACAP and VIP were abrogated by the PACAP antagonist PACAP6-38. PACAP38 stimulated cAMP formation, activated polyphosphoinositide turnover, and provoked calcium mobilization from IP3-sensitive pools. The PKA inhibitor H89 suppressed PACAP-induced secretion of ODN-LI, whereas PLC inhibitor U73122 and the PKC inhibitor chelerythrine had no effect. In contrast, U73122 restored the stimulatory action of PACAP on ODN-LI release and cAMP formation during prolonged (15 min) incubation with the peptide, and this effect was prevented by PMA. The present results demonstrate that PACAP stimulates endozepine release through activation of PAC1 receptors coupled to the AC/PKA pathway. Our data also show that activation of the PLC/PKC pathway down-regulates the effect of PACAP on endozepine release.

Synthesis, conformational analysis and biological activity of cyclic analogs of the octadecaneuropeptide ODN. Design of a potent endozepine antagonist

The octadecaneuropeptide (ODN; QATVGDVNTDRPGLLDLK) and its C-terminal octapeptide (OP; RPGLLDLK), which exert anxiogenic activity, have been previously shown to increase intracellular calcium concentration ([Ca2+]i) in cultured rat astrocytes through activation of a metabotropic receptor positively coupled to phospholipase C. It has also been found that the [d-Leu5]OP analog possesses a weak antagonistic activity. The aim of the present study was to synthesize and characterize cyclic analogs of OP and [d-Leu5]OP. On-resin homodetic backbone cyclization of OP yielded an analog, cyclo1-8 OP, which was three times more potent and 1.4-times more efficacious than OP to increase [Ca2+]i in cultured rat astrocytes. Cyclo1-8 OP also mimicked the effect of both OP and ODN on polyphosphoinositide turnover. Conversely, the cyclo1-8 [d-Leu5]OP analog was totally devoid of agonistic activity but suppressed the effect of OP and ODN on [Ca2+]i and phosphoinositide metabolism in astrocytes. The structure of these cyclic analogs has been determined by two-dimensional 1H-NMR and molecular dynamics. Cyclo1-8 OP exhibited a single conformation characterized by a gamma turn comprising residues Pro2-Leu4 and a type III beta turn encompassing residues Leu5-Lys8. Cyclo1-8 [d-Leu5]OP was present as two equimolar conformers resulting from cis/trans isomerization of the Arg-Pro peptide bond. These pharmacological and structural data should prove useful for the rational design of non peptidic ODN analogs.

The triakontatetraneuropeptide TTN increases [CA2+]i in rat astrocytes through activation of peripheral-type benzodiazepine receptors

Astrocytes synthesize a series of regulatory peptides called endozepines, which act as endogenous ligands of benzodiazepine receptors. We have recently shown that one of these endozepines, the triakontatetraneuropeptide TTN, stimulates DNA synthesis in astroglial cells. The purpose of the present study was to determine the mechanism of action of TTN on cultured rat astrocytes. Binding of the peripheral-type benzodiazepine receptor ligand [3H]Ro5-4864 to intact astrocytes was displaced by TTN, whereas its C-terminal fragment (TTN[17-34], the octadecaneuropeptide ODN) did not compete for [3H]Ro5-4864 binding. Microfluorimetric measurement of cytosolic calcium concentrations ([Ca2+]i) with the fluorescent probe indo-1 showed that TTN (10(-10) to 10(-6) M) provokes a concentration-dependent increase in [Ca2+]i in cultured astrocytes. Simultaneous administration of TTN (10(-8) M) and Ro5-4864 (10(-5) M) induced an increase in [Ca2+]i similar to that obtained with Ro5-4864 alone. In contrast, the effects of TTN (10(-8) M) and ODN (10(-8) M) on [Ca2+]i were strictly additive. Chelation of extracellular Ca2+ by EGTA (6 mM) or blockage of Ca2+ channels with Ni2+ (2 mM) abrogated the stimulatory effect of TTN. The calcium influx evoked by TTN (10(-7) M) or by Ro5-4864 (10(-5) M) was not affected by the N- and T-type calcium channel blockers omega-conotoxin (10(-6) M) and mibefradil (10(-6) M), but was significantly reduced by the L-type calcium channel blocker nifedipine (10(-7) M). Patch-clamp studies showed that, at negative potentials, TTN (10(-7) M) induced a sustained depolarization. Reduction of the chloride concentration in the extracellular solution shifted the reversal potential from 0 mV to a positive potential. These data show that TTN, acting through peripheral-type benzodiazepine receptors, provokes chloride efflux, which in turn induces calcium influx via L-type calcium channels in rat astrocytes.

The octadecaneuropeptide [diazepam-binding inhibitor (33-50)] exerts potent anorexigenic effects in rodents

The effects of intracerebroventricular administration of the octadecaneuropeptide ODN on food intake have been investigated in rat and mouse. In rats deprived of food from 9:00 a.m. to 7:00 p.m., i.c.v. injection of ODN (30 to 100 ng) provoked a dose-dependent reduction of food consumption during the following 12-h nocturnal period. At a dose of 100 ng, ODN almost completely suppressed food intake. Treatment of rats with diazepam (2 mg/kg s.c.; 15 min before ODN administration) did not affect the anorexigenic response evoked by 100 ng ODN. Continuous i.c.v. infusion of ODN (10 ng/h during 15 days) using osmotic minipumps, significantly reduced food intake during the 2nd, 3rd and 4th days of treatment. The decrease in food consumption was associated with a significant reduction in body weight, which persisted during the 15-day duration of the experiment. In mice deprived of food for 18 h, i.c.v. administration of a low dose of ODN (5 ng) significantly reduced food intake. Treatment of mice with diazepam (1 mg/kg s.c.; 10 min before ODN administration) did not prevent the inhibitory effect of ODN (100 ng) on food intake. The C-terminal octapeptide fragment of ODN mimicked the anorexigenic effect of the intact peptide. Taken together, the present data demonstrate that i.c.v. injection of ODN causes, in both rat and mouse, a long-lasting anorexigenic effect that is not mediated through central-type benzodiazepine receptors. The biologically active region of ODN appears to be located in the C-terminal domain of the peptide.

The octadecaneuropeptide ODN stimulates neurosteroid biosynthesis through activation of central-type benzodiazepine receptors

Neurosteroids may play a major role in the regulation of various neurophysiological and behavioural processes. However, while the biochemical pathways involved in the synthesis of neuroactive steroids in the central nervous system are now elucidated, the mechanisms controlling the activity of neurosteroid-producing cells remain almost completely unknown. In the present study, we have investigated the effect of the octadecaneuropeptide (ODN), an endogenous ligand of benzodiazepine receptors, in the control of steroid biosynthesis in the frog hypothalamus. Glial cells containing ODN-like immunoreactivity were found to send their thick processes in the close vicinity of neurones expressing the steroidogenic enzyme 3 beta-hydroxysteroid dehydrogenase. Exposure of frog hypothalamic explants to graded concentrations of ODN (10(-10)-10(-5) M) produced a dose-dependent increase in the conversion of tritiated pregnenolone into various radioactive steroids, including 17-hydroxypregnenolone, progesterone, 17-hydroxyprogesterone, dehydroepiandrosterone and dihydrotestosterone. The ODN-induced stimulation of neurosteroid biosynthesis was mimicked by the central-type benzodiazepine receptor (CBR) inverse agonists methyl beta-carboline-3-carboxylate (beta-CCM) and methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM). The stimulatory effects of ODN, beta-CCM and DMCM on steroid formation was markedly reduced by the CBR antagonist flumazenil. The ODN-evoked stimulation of neurosteroid production was also significantly attenuated by GABA. Collectively, these data indicate that the endozepine ODN, released by glial cell processes in the vicinity of 3 beta-hydroxysteroid dehydrogenase-containing neurones, stimulates the biosynthesis of neurosteroids through activation of central-type benzodiazepines receptors.

The triakontatetraneuropeptide (TTN) stimulates thymidine incorporation in rat astrocytes through peripheral-type benzodiazepine receptors

Astrocytes and astrocytoma cells actively express the diazepam-binding inhibitor (DBI) gene, suggesting that DBI-processing products may regulate glial cell activity. In the present study, we have investigated the possible effect of one of the DBI-derived peptides, the triakontatetraneuropeptide (TTN), on [(3)H]thymidine incorporation in cultured rat astrocytes. Reversed-phase HPLC analysis of incubation media indicated that TTN is the major form of DBI-derived peptides released by cultured astrocytes. At very low concentrations (10(-14)-10(-11) M), TTN induced a dose-dependent increase in [(3)H]thymidine incorporation, whereas at higher concentrations (10(-10)-10(-5) M) the effect of TTN gradually declined. In the same range of concentrations, the specific peripheral-type benzodiazepine receptor (PBR) agonist Ro 5-4864 mimicked the bell-shaped stimulatory effect of TTN on [(3)H]thymidine incorporation. The PBR antagonist PK11195 (10(-6) M) suppressed the stimulatory action of both TTN and Ro 5-4864 on [(3)H]thymidine incorporation, whereas the central-type benzodiazepine receptor antagonist flumazenil (10(-6) M) had no effect. The present study demonstrates that the endozepine TTN stimulates DNA synthesis in rat glial cells through activation of PBRs. These data strongly suggest that TTN exerts an autocrine/paracrine stimulatory effect on glial cell proliferation.

Specific binding of benzodiazepines to human breast cancer cell lines

Binding of [3H]Ro5-4864, a peripheral benzodiazepine receptor (PBR) agonist, to BT-20 human, estrogen- (ER) and progesterone- (PR) receptor negative breast cancer cells was characterized. It was found to be specific, dose-dependent and saturable with a single population of binding sites. Dissociation constant (K(D)) was 8.5 nM, maximal binding capacity (Bmax) 339 fM/10(6) cells. Ro5-4864 (IC50 17.3 nM) and PK 11195 (IC50 12.3 nM) were able to compete with [3H]Ro5-4864 for binding, indicating specificity of interaction with PBR. Diazepam was able to displace [3H]Ro5-4864 from binding only at high concentrations (>1 microM), while ODN did not compete for PBR binding. Thymidine-uptake assay showed a biphasic response of cell proliferation. While low concentrations (100 nM) of Ro5-4864, PK 11195 and diazepam increased cell growth by 10 to 20%, higher concentrations (10-100 microM) significantly inhibited cell proliferation. PK 11195, a potent PBR ligand, was able to attenuate growth of BT-20 cells stimulated by 100 nM Ro5-4864 and to reverse growth reduction caused by 1 and 10 microM Ro5-4864, but not by 50 microM and 100 microM. This indicates that the antimitotic activity of higher concentrations of Ro5-4864 is independent of PBR binding. It is suggested, that PBR are involved in growth regulation of certain human breast cancer cell lines, possibly by supplying proliferating cells with energy, as their endogenous ligand is a polypeptide transporting Acyl-CoA.

The endozepine ODN stimulates [3H]thymidine incorporation in cultured rat astrocytes

High concentrations of diazepam-binding inhibitor (DBI) mRNA have been detected in astrocytoma, suggesting that DBI-derived peptides may play a role in glial cell proliferation. In the present study, we have investigated the effect of a processing product of DBI, the octadecaneuropeptide ODN, on DNA synthesis in cultured rat astrocytes. At very low concentrations (10(-14) to 10(-11) M), ODN caused a dose-dependent increase of [3H]thymidine incorporation. At higher doses (10(-10) to 10(-5) M), the effect of ODN gradually declined. The central-type benzodiazepine receptor antagonist flumazenil (10(-6) M) completely suppressed the stimulatory action of ODN whereas the peripheral-type benzodiazepine receptor ligand, PK11195 (10(-6) M) had no effect. The ODN-induced stimulation of [3H]thymidine incorporation was mimicked by methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM). The GABAA receptor antagonist bicuculline (10(-4) M) suppressed the effect of both ODN and DMCM on DNA synthesis. Exposure of cultured astrocytes to the specific GABAA agonist 3APS (10(-10) to 10(-4) M) also induced a dose-related increase of [3H]thymidine incorporation. The present study indicates that ODN, acting through central-type benzodiazepine receptors associated with the GABAA receptor complex, stimulates DNA synthesis in rat glial cells. These data provide evidence for an autocrine role of endozepines in the control of glial cell proliferation.

Ontogeny of diazepam binding inhibitor/acyl-CoA binding protein mRNA and peripheral benzodiazepine receptor mRNA expression in the rat

The Diazepam Binding Inhibitor/Acyl-CoA Binding Protein (DBI/ACBP) has been implicated in different functions, as acyl-CoA transporter and as an endogenous ligand at the GABA(A) receptor and the peripheral benzodiazepine receptor (PBR). The latter is thought to be involved in control of steroidogenesis. We studied the ontogeny of DBI/ACBP and PBR mRNA expression in embryos and offspring of time-pregnant Long Evans rats by in-situ hybridization with 33P-endlabelled oligonucleotides. Both mRNAs were present in embryo and placenta at gestational day (G)11, the earliest stage studied. DBI/ACBP mRNA was strongly expressed from embryonic through mid-foetal stages in central nervous system (maximum in neuroepithelium), cranial and sympathetic ganglia, anterior pituitary, adrenal cortex, thyroid, thymus, liver and (late foetal) brown adipose tissue, moderately in testis, heart, lung and kidney. In brain, a late foetal decrease of DBI/ACBP mRNA was followed by an increase at postnatal day 6. Peripheral benzodiazepine receptor mRNA expression started very low and increased to moderate levels in adrenal cortex and medulla, testis, thyroid, brown adipose tissue, liver, heart, lung, salivary gland at mid- to late-foetal stages. Data suggest a significant role of DBI/ACBP at early developmental stages. Both proteins may be involved in the control of foetal steroidogenesis. However, differences in developmental patterns indicate that additional functions may be equally important during ontogeny, such as the involvement in lipid metabolism in the case of DBI/ACBP.

Structure-activity relationships of a series of analogues of the octadecaneuropeptide ODN on calcium mobilization in rat astrocytes

The octadecaneuropeptide ODN (QATVGDVNTDRPGLLDLK), originally characterized as an endogenous ligand for central-type benzodiazepine receptors, increases intracellular calcium concentration ([Ca2+]i) in rat astroglial cells. A series of ODN analogues was synthesized, and each compound was studied for its ability to induce Ca2+ mobilization in cultured rat astrocytes. Replacement of each amino acid by an L-alanine residue (AlaScan) showed that the N-terminal region of the molecule was relatively tolerant to alanine substitution (2-8, 10), except for the Ala9-substituted analogue (9) which was totally devoid of activity. Pyroglutamization (21) and acetylation (22) of the Gln1 residue reduced the Ca2+ response suggesting that a free N-terminal amine function is required for full activity of ODN. Alanine substitution of the residues in the C-terminal region of the molecule (11-14, 16-18) significantly reduced the biological activity of ODN. In particular, modifications of the Leu15 residue (15, 20) abolished the Ca2+-mobilizing activity. The analogues [Ala9]ODN (9), [Ala15]ODN (15), [D-Thr9]ODN (19), and [D-Leu15]ODN (20) partially antagonized the Ca2+ response evoked by ODN. Most importantly, the octapeptide ODN11-18 (OP, 24) produced a dose-response curve that was superimposable to that obtained with ODN, indicating that the C-terminal region of the molecule possesses full biological activity. Finally, the AlaScan of OP revealed that replacement of the Leu5 residue by Ala (29) or D-Leu (33) totally suppressed the calcium response, confirming the crucial contribution of the Leu15 residue of ODN to the biological activity of the neuropeptide.

The octadecaneuropeptide ODN inhibits apomorphine-induced yawning in rats

High concentrations of diazepam-binding inhibitor (DBI) have been detected in brain areas containing dopaminergic cell bodies and nerve terminals. In the present study, we have investigated the effect of a proteolytic fragment of DBI, the octadecaneuropeptide ODN, on apomorphine-induced yawning in Sprague-Dawley rats. Injection of graded doses of ODN (12.5 to 100 ng i.c.v.) caused a dose-dependent inhibition of apomorphine-induced yawning and penile erections. At a dose of 100 ng, intracerebroventricularly administered ODN was able to inhibit, during more than 3 h, the apomorphine-evoked yawning. ODN also inhibited pilocarpine-induced yawning. Apomorphine induces a bell-shaped dose-dependent effect on yawning with a maximum response at the dose of 100 microg/kg and a much lower effect at a dose of 200 microg/kg. Injection (i.c.v.) of 100 ng ODN markedly attenuated the number of yawns induced by 100 microg/kg apomorphine but partially restored the yawning behavior in rats treated with a 200 microg/kg dose of apomorphine. At doses of 0.5 or 5 mg/kg s.c., diazepam did not modify the inhibitory effect of ODN on the apomorphine-induced yawning. Taken together, the present data suggest that ODN inhibits yawning downstream dopaminergic as well as cholinergic synapses involved in yawning. In addition, the effect of ODN cannot be ascribed to an inverse agonistic activity on central-type benzodiazepine receptors.

The octadecaneuropeptide-induced response of corticotropin-releasing hormone messenger RNA levels is mediated by GABA(A) receptors and modulated by endogenous steroids

The involvement of endogenous benzodiazepine octadecaneuropeptide in the regulation of corticotropin-releasing hormone messenger RNA expression has been studied using in situ hybridization technique. Intracerebroventricular injection of octadecaneuropeptide (4 microg/kg) induced a 26% decrease in the corticotropin-releasing hormone messenger RNA expression in the hypothalamic paraventricular nucleus. Concomitant injection of octadecaneuropeptide and i.p. injection of the GABA(A) receptor agonist muscimol (4 mg/kg) potentiated the corticotropin-releasing hormone messenger RNA decrease ( - 34%). The depressing effect of octadecaneuropeptide on corticotropin-releasing hormone gene expression was totally reversed by pretreatment of the animals with the GABA(A) receptor antagonist picrotoxin (5 mg/kg; i.p.) or by pretreatment with the benzodiazepine receptor antagonist flumazenil (4 mg/kg; i.p.). To determine the reciprocal involvement of adrenal and sexual steroids in this regulation, animals are adrenalectomized and/or castrated. Adrenalectomy reversed the effect induced by octadecaneuropeptide, which increased corticotropin-releasing hormone messenger RNA expression (+21%), while castration did not modify the negative influence of octadecaneuropeptide. When rats were adrenalectomized and castrated, the adrenalectomy influence was predominant, since octadecaneuropeptide increased significantly the hybridization signal (+18%). The involvement of neurosteroids, especially reduced metabolites of progesterone was also investigated. The concomitant injection of octadecaneuropeptide and subcutaneous injection of the 5alpha-reductase inhibitor MK-906 (14 mg/kg) to adrenalectomized and castrated rats, reduced significantly by 60% the increase of corticotropin-releasing hormone messenger RNA expression induced by octadecaneuropeptide. These results indicate that in vivo the endogenous benzodiazepine octadecaneuropeptide, via an activation of the benzodiazepine sites of the GABA(A) receptor, negatively modulates corticotropin-releasing hormone neuronal activity and that this modulation can be negatively or positively influenced by central and peripheral steroids.

The octadecaneuropeptide ODN induces anxiety in rodents: possible involvement of a shorter biologically active fragment

The octadecaneuropeptide ODN has been originally characterized as an endogenous ligand of central-type benzodiazepine receptors, on its ability to displace the anxiogenic compound beta-[3H]carboline-3-carboxylate methyl ester from its binding sites. The aim of the present study was to investigate the anxiogenic effects of intracerebroventricular administration of ODN in mice and rats. At doses ranging from 10 to 100 ng, ODN increased in mice the latency to explore a white compartment when the animals were placed in a black one. ODN also reduced the first stay duration in the white compartment. These effects were antagonized by diazepam (0.075 mg/kg, s.c.) as well as flumazenil (1 mg/kg, s.c.), indicating that ODN acts as an inverse agonist on central-type benzodiazepine receptors. In rats, ODN reduced the latency to enter a black compartment when the animals were placed in the white one. In the plus-maze elevated test, ODN reduced, in both mice and rats, the number of entries and the time spent in the open arm. In mice, ODN (100 ng) increased the thigmotaxis index, i.e. the distance traveled in the peripheral zone of the open field. Time-course studies revealed that a significant effect of ODN (100 ng) in the black/white compartment test was only observed 40 min after the injection and lasted between 3 and 6 h. The effect of a 1000-ng dose of ODN appeared more tardily than that of a 10-ng dose. In addition, a 1000-ng dose of ODN occluded the early effect of a 100-ng dose on the white compartment first stay duration. The COOH-terminal octapeptide of ODN was more rapidly effective than ODN in the black/white compartment test, suggesting that the anxiogenic effect of the peptide requires the formation of biologically active proteolytic fragment.