A selective biotinylated probe for V1a vasopressin receptors

Lead (Pb) exposure induces physiological alterations in the serotoninergic and vasopressin systems causing anxiogenic-like behavior in Meriones shawi: Assessment of BDMC as a neuroprotective compound for Pb-neurotoxicity and kidney damages

BACKGROUND

Studies have shown that lead (Pb) is one of hazardous heavy metals with various adverse effects on human health including mental health; Pb can induce psychiatric disorders like anxiety. In the present work, we examined the potential of bisdemethoxycurcumin (BDMC) as a neuroprotective agent against lead induced anxiety inMeriones shawi (M. shawi).

METHODS

We asses, the potential of three consecutive day exposure to Pb (25 mg/kg body weight) in inducing anxiogenic effect, serotoninergic and vasopressinergic disruptions inM. shawi. This was done using neurobehavioral tests (open field, elevated plus maze), immunohistochemestry by anti-serotonin (5-HT), and anti-vasopressin (AVP) antibodies. We also measured the possible restorative potential of BDMC (30 mg/kg body weight), delivered by oral gavage. After that, a biochemical and histopathological studies were done.

RESULTS

Our results showed that lead exposure for three consecutive days increases significantly the 5-HT-immunoreactivity in dorsal raphe nucleus (DRN) accompanied with a significant enhancement of AVP-immunoreactivity in the cell bodies and fibers in the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus. In the collecting tube, AVP binds to the V2 receptor of the epithelial cells and increases the water permeability. Our results showed clearly the epithelial cells degeneration after lead exposure, then we suggest that the increased AVP could be a response to the hydric balance disrupted after degenerative effect of lead exposure on epithelial cells. BDMC produced an anxiolytic effect in meriones. Moreover, it restored 5-HT and AVP immunoreactivity within studying nuclei. The biochemical and histopathological studies showed that Pb induced renal damages. In addition, BDMC restored the renal alterations.

CONCLUSION

According to the obtained results, we suggest new pharmacological effects of BDMC; while it has an anxiolytic effect against Pb-induced anxiety by working on serotoninergic and vasopressinergic systems with an obvious restoration of the renal injuries induced by lead exposure.

Novel strategies for the design of receptor-selective vasopressin analogues: Aib-substitution and retro-inverso transformation

1. We determined the pharmacological profile of novel backbone-modified peptides designed as protease-resistant, selective analogues of AVP. Binding affinities of peptides were determined at both V1A and V2 subtypes of vasopressin receptor (VPR). Biological potencies of selected peptides were tested in pressor and antidiuretic bioassays. 2. Substitution of the achiral alpha-aminoisobutyric acid (Aib) at position 4 or 7 of AVP produced peptides that selectively bound the V2 VPR. Both [Aib4]AVP (140 IU mg-1) and [Aib7]AVP (36 IU mg-1) are selective antidiuretic agonists with little or no activity in uterotonic and pressor assays. 3. [Aib4] and [Aib7] derivatives of the linear V1A-selective antagonist [PhaaDTyr(Et)2Arg6Tyr(NH2)9]AVP bound selectively and with high affinity (Kd 0.51 and 4.1 nM respectively) to the V1A VPR. Bioassays confirmed that these peptides were potent antivasopressor agents (pA2 8.10 and 8.36 respectively). 4. A total retro-inverso strategy was used to prepare protease-resistant mimetics of both AVP and linear V1A-selective antagonists. Cyclic retro-inverso mimetics of AVP did not bind either V1A or V2 VPRs. In contrast, rationally designed retro-inverso mimetics of linear V1A-selective antagonists selectively bound the V1A VPR. 5. Our findings indicate novel methods to improve the pharmacodynamic and pharmacokinetic parameters of neurohypophysial hormone analogues which could be equally applicable to other peptide-receptor systems.

Renal bradykinin and vasopressin receptors: ligand selectivity and classification

We studied the specific binding of radiolabeled bradykinin ([3H]BK) and vasopressin ([3H]AVP) to membrane preparations of bovine and porcine kidney medulla. [3H]BK reversibly labeled a single site (Kd = 1.06 nM) in bovine kidney medulla independently of [Mg2+]. The number of BK receptors in bovine kidney medulla, Bmax = 122 fmol/mg protein, is markedly (2- to 3-fold) higher than that reported in other tissues. Further characterization by ligand binding indicated that the bovine bradykinin receptor was the B2a subtype, pharmacologically related to B2a receptors expressed by human and rabbit tissues. In contrast, the specific binding of [3H]BK, but not [3H]AVP, to porcine kidney medulla (Kd = 0.32 nM, Bmax = 45 fmol/mg) was dependent upon the presence of enzyme inhibitors to prevent the rapid and selective degradation of bradykinin. Interspecies differences were revealed for renal medulla V2 vasopressin receptors with respect to their abundance and their affinity for several V2-selective ligands. In summary, (i) bovine kidney medulla is a convenient source of tissue for studying the B2a bradykinin receptor subtype; (ii) there are significant species-dependent differences in both the abundance of renal medulla B2a and V2 receptors and the ligand selectivity of V2 receptors; and (iii) these findings are significant in relation to the physiological and pathological roles of renal kinins and their interaction with the neurohypophysial peptide hormone system.

Probing the V1a vasopressin receptor binding site with pyroglutamate-substituted linear antagonists

We have synthesized eight analogues of the linear vasopressin antagonist DTyr(Et)2-Phe3-Gln4-Asn5-Arg6-Pro7-Arg8-Tyr(NH2)9 substituted with L-, or D-, pyroglutamate at position-1, Asn or Val at position-4 and Arg or Met at position 6. All of these peptides bound to the V1a vasopressin receptor with affinities ranging 33.6-5, 470 nM. Of this series, only two peptides, [LpGlu1Val4Arg6Tyr(NH2)9]AVP Kd = 48.4 nM and [DpGlu1Val4Arg6Tyr(NH2)9]AVP Kd = 691 nM, bound to the V2 vasopressin receptor. All of the neurohypophysial hormone receptors studied (V1a VPR, V2 VPR and OTR) were found to be stereoselective with respect to the N-terminal pGlu residue. The effect on binding characteristics of L-pGlu1 and D-pGlu1 analogues was dependent on both the sequence of the peptide and on the receptor subtype in question. From these data we found that peptide 5, which has the structure DpGlu-DTyr(Et)-Phe-Val-Asn-Arg-Pro-ARg-Tyr(NH2), exhibited the highest V1a/OTR selectivity reported to date (V1aVPR Kd = 82 nM; OTR no binding at 10 microM). As such, peptide 5 will provide useful leads to the development of ligands with enhanced V1a/OTR selectivity. The binding affinity and hydrophobicity of pyroglutamate-substituted peptides was compared with previously characterized V1a receptor antagonists which contained a range of position-1 substitutions. The hydrophobicity of both cyclic and linear antagonists was markedly increased relative to the agonists AVP and [Phe2Orn8]VT but increased hydrophobicity alone did not exclusively lead to high affinity antagonists. Data presented support the contention that in addition to a general increase in hydrophobicity/lipophilicity, position-1 influences the pharmacophore of vasopressin antagonists by providing molecular determinants for ligand/receptor interaction.

Molecular pharmacology of V1a vasopressin receptors

1. Vasopressin, a mammalian neurohypophysial peptide hormone, has diverse physiological actions. 2. Pharmacological studies, using a range of mammalian tissues, have identified three subtypes of vasopressin receptor. 3. The V1a subtype of vasopressin receptor is widely distributed and mediates many central and peripheral actions of vasopressin. 4. The development of subtype-selective vasopressin analogues has provided valuable tools for pharmacological and physical studies of the V1a receptor protein. 5. Pharmacological differences indicate species heterogeneity in the characteristics of V1a receptors and in the expression of hepatic V1a receptors. 6. The cloning of neurohypophysial hormone receptor proteins allows structural and functional comparison of the V1a vasopressin receptors with other G-protein-coupled receptors.

Pharmacological characterization of linear analogues of vasopressin generated by the systematic substitution of positions 1 and 6 by L-amino acids

Eighteen linear analogues of [Arg8]vasopressin (AVP) were synthesized by systematically substituting the cysteine residues at positions 1 and 6 with a range of L-amino acids. Screening by competition ligand binding revealed that the combinations of amino acid residues tolerated at these positions was very restricted with respect to retention of vasopressin receptor (VPR) binding. Consequently, only three of the eighteen analogues investigated, [Pro1,Met6]AVP, [Gly1,Met6]AVP and [Phe1,Lys6]AVP, bound to the V1a receptor. Furthermore, these three peptides were all selective for the V1a receptor rather than the V1b, V2 and vasotocin receptors. In addition, although very homologous to the natural agonist, these analogues were in fact antagonists at V1a receptors. These data provide insights into the biophysical requirements at positions 1 and 6 of linear ligands for binding to V1a receptors and furthermore, supply clues to the nature of the receptor:ligand interaction.

Hepatic vasopressin receptors (VPRs) exhibit species heterogeneity--absence of VPRs in sheep liver

1. We have studied the binding characteristics of the hepatic VPRs expressed by rat, cow, pig, sheep and human and have demonstrated species heterogeneity. 2. These species differences are manifested as variation in the VPR capacity (rat > cow > pig > human > sheep), and in the affinity of these receptors for their natural ligand AVP (rat = human > pig = cow). 3. A single class of VPRs, with high affinity for AVP, is present in rat, cow, human and pig liver. In contrast, ovine hepatocytes do not express a VPR. 4. The affinity of the V1a-selective antagonist d(CH2)5Tyr(Me)2AVP is highly dependent on the species utilised, such that rat > human > cow.

Fluorescent and biotinylated linear peptides as selective bifunctional ligands for the V1a vasopressin receptor

We have designed and synthesized a linear peptide analogue of arginine vasopressin. This peptide, [1-phenylacetyl, 2-O-methyl-D-tyrosine, 6-arginine, 8-arginine, 9-lysinamide]vasopressin (PhAcALVP), has a lysinamide residue substituted for the more usual glycinamide at position 9. Derivatization of PhAcALVP at the N epsilon-lysyl amino group with N-hydroxysuccinimide esters of aminomethylcoumarin (Mec) and biotin (Btn) produced the bifunctional ligands PhAcAL(Mec)VP and PhAcAL(Btn)VP, respectively. Pharmacological characterization of these peptides revealed that all were high-affinity V1a-selective antagonists. PhAcAL(Btn)VP can simultaneously bind to both the rat liver V1a receptor and avidin conjugates. Using this strategy, we were able to study the distribution of V1a receptors on the surface of the rat mammary tumour cell line, WRK-1. Routine epifluorescent microscopy and confocal image analysis were used to observe the distribution of avidin-Texas-Red associated with receptor-bound PhAcAL(Btn)VP. We conclude that PhAcALVP is a useful precursor for the production of hetero-bifunctional V1a-selective ligands. Both PhAcAL(Mec)VP and PheAcAL(Btn)VP can be used selectively to probe the V1a receptor and will be versatile tools for a variety of histocytochemical applications, including receptor localization and purification.

Signal transduction of V1-vascular vasopressin receptors

This review covers the recent developments gained in the exploration of V1-vascular vasopressin (AVP) receptors. We examine the different radioligands available for the pharmacological characterization of these receptors. The immediate transmembrane signaling of V1-vascular AVP receptors involves ligand-receptor complex formation, receptor lateral mobility and internalization, coupling to a Gq protein, activation of phospholipases A2, C and D, translocation and activation of protein kinase C, production of inositol 1,4,5-triphosphate and 1,2-diacylglycerol, mobilization of intracellular calcium, alteration of intracellular pH with activation of the Na+/H+ exchanger, calmodulin activation and myosin light chain phosphorylation. The secondary nuclear signal mechanisms triggered by activation of V1-vascular AVP receptors includes tyrosine phosphorylation, induction of gene expression and protein synthesis.