Biomedical subjects

C Bouvier

Publications and source records attributed to C Bouvier.

At least 19 recordsLinked to original sources

PubMed1998

Catecholaminergic CATH.a cells express predominantly delta-opioid receptors.

CATH.a cells are a catecholaminergic cell line of neuronal origin. The opioid receptor complement expressed by CATH.a cells was defined pharmacologically and by reverse transcription-polymerase chain reaction (RT-PCR). CATH.a cells were found to express mRNA encoding all three of the major subtypes of opioid receptors. The relative abundance of CATH.a cell opioid receptor transcripts was delta > kappa> mu. Pharmacological and functional data were in agreement with the results of RT-PCR inasmuch as delta-opioid receptor was identified as the most abundant opioid receptor subtype expressed by CATH.a cells. In addition, at least one of the opioid signalling pathways, inhibition of adenylyl cyclase activity, was found to be operant in this cell line. CATH.a cells should be of general utility for the study of opioid receptor signalling mechanisms in the context of catecholaminergic neurons.

Adenylyl Cyclases↗

PubMed1997

Automated and quantitative immunocytochemical assays of Nm23/NDPK protein in breast carcinomas.

A series for Nm23-protein immunodetection was investigated in human breast carcinomas. Frozen sections were processed by automated immunoperoxidase procedure, and immunoprecipitates in positive tumors were quantified by processing digitized microscopic images. Nm23 immunohistochemical expression in tumors was correlated with clinicopathological data and with intra-tumoral proteins also detected by automated and quantitative immunohistochemistry. A positive Nm23 immunoreaction was observed in 58% of tumors, within cell cytoplasm. Nm23 expression was independent of the patient's age, and of tumor size, type and grade, but an inverse relationship was observed between Nm23 expression and axillary-lymph-node metastasis. An inverse relationship was also observed between Nm23 and P-53, CD-31, cathepsin D, tenascin and P-gp immunohistochemical expressions. But Nm23 expression was independent of c-erb-B product, growth fraction (MIB1/Ki67), and immunohistochemical expression of hormone receptors/P-S2. The results suggest that the anti-metastatic nm23 gene may partly act upon the regulation of tumor-cell proliferation (correlation with P-53) and may have some effects on epithelial-cell/stroma interactions (regulation of extracellular-matrix protease and of angiogenesis) independently of hormone sensitivity.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1997

E-cadherin quantitative immunocytochemical assays in breast carcinomas.

The reduction of E-cadherin expression, which is involved in the initial step of invasion and metastasis of cancer, was investigated in 218 human breast carcinomas. Quantitative immunohistochemical assays (ICAs) were performed on frozen sections. Quantitation was assessed by processing digitized microscopic images of immunoreactions using a computerized system of image analysis (SAMBA). The results were correlated with clinicopathological data and quantitative immunodetection of other molecules. E-cadherin expression was significantly (P < 0.001) stronger in ductal carcinomas than in lobular carcinomas and stronger (P < 0.01) in low grades than in high grades, but E-cadherin was independent of lymph node status and tumour size. Also an inverse significant (P < 0.01) relationship was observed between E-cadherin expression on tissue sections and positive immunoreactions with anti-P53, MIB1 (growth fraction), and anti-c-erb-B2 product. Conversely, strong positive and anti-E-cadherin immunoreactions correlated with strong positive anti-ER and anti-PR immunoreactions (P < 0.01). No relationship was observed between E-cadherin and the results of quantitative ICAs of cathepsin D, CD31, and P-glycoprotein, assessed on consecutive sections from the same frozen tissue samples. The results show that preserved E-cadherin expression correlates with high degree of tumour differentiation, low proliferative activity, and low expression of prognostic markers. The deregulation of E-cadherin is independent of other steps of tumour invasion, such as protease digestion of extracellular matrix and angiogenesis.

Adult↗

PubMed1997

Automated and quantitative immunocytochemical assays of CD44v6 in breast carcinomas.

CD44 variants carrying sequences encoded by exon v6 are preferentially expressed in metastatic animal cancer cell lines. CD44v6 overexpression correlates tumor dedifferentiation and progression in some human carcinomas, but the relationship of CD44v6 overexpression with metastatic behavior of tumor observed in animal models is controversial, particularly in breast carcinomas. The discrepancies probably result from analytical bias. We investigated CD44v6 and CD44s expression in 218 frozen samples of primary breast carcinomas. Immunocytochemical procedure was performed under optimal technical conditions using commercially available 2F-10 monoclonal antibody (MAb), a microprocessor-controlled automated device (Ventana Medical Systems, Tucson, AZ), and quantitative evaluation of results by processing digitized-colored microscopic images (SAMBA, Grenoble, France). CD44v6 expression in tissue sections was shown to be independent of the patient age, tumor size, histological types and grades, and the lymph node status. CD44v6 expression was also independent of the expression of molecules endowed with poor prognostic significance detected by MAbs (anti-p53, anti-c-erb B-2 protein, MIB1) on consecutive sections. No significant relationship could be evidenced either between CD44v6 expression, and CD31 involved stromal angiogenesis and cathepsin D. Finally, CD44v6 was independent of markers of hormone dependence (estrogen and progesterone receptors, pS2) and of multidrug resistance (P-glycoprotein). Similar results were observed with anti-CD44s. We conclude that the true prognostic significance of CD44v6 overexpression still remains to be shown under rigorous technical conditions (frozen samples, well-documented MAbs, and optimal standardization of procedure using automation and quantitative analysis) providing data appropriate for further correlation with long-term patient follow-up.

Adult↗

PubMed1997

c-erbB-2 oncoprotein detected by automated quantitative immunocytochemistry in breast carcinomas correlates with patients' overall and disease-free survival.

The prognostic significance of c-erbB-2 oncoprotein overexpression detected in tumours by immunocytochemical assays (ICAs) was investigated in 148 breast carcinomas. ICAs were performed under optimal technical conditions with frozen tissue sections and included automated immunoperoxidase technique and computer-assisted analysis (densitometry) of digitized coloured microscopic images. Results of quantitative ICAs (expressed in percentages of c-erbB-2-positive surfaces and mean optical densities) were correlated with the patients' follow-up in axillary lymph node-positive (N+) and node-negative (N-) subgroups of patients. Patients' follow-up ranged from 9 months (for the first death) to 101 months (for the 121 alive patients) with a 62.5 months mean overall follow-up. It was shown that marked c-erbB-2 immunocytochemical expression in tumours (cut-off point 35%) significantly correlated with the patients' poor overall survival in N+ and in N- patients (Kaplan-Meier, log-rank test, P = 0.045 and P = 0.015). Also, marked c-erbB-2 immunohistochemical expression correlates with short disease-free (P = 0.005), recurrence-free (P = 0.048) and metastasis-free survival (P = 0.05) (Kaplan-Meier, log-rank test) in N+, but not in N- subgroups. It is concluded that in optimal conditions (automated and quantitative ICAs on frozen sections) c-erbB immunohistochemical expression is a significant prognostic indicator in terms of overall and disease-free survival. The c-erbB-2 protein prognostic significance is independent of node status in terms of overall survival, but not of disease-free survival.

Adult↗

PubMed1997

Automated and quantitative immunocytochemical assays of Bcl-2 protein in breast carcinomas.

Expression of the bcl-2 gene was investigated in 218 human breast carcinomas by immunohistochemical analysis. Immunodetections were assessed using (1) frozen sections, (2) documented commercially available monoclonal antibody (bcl-2/124, Dako), (3) automation of immunoperoxidase technique (Ventana) and (4) quantitative evaluation of results by image analysis (SAMBA) and statistical analysis of quantitative data (BMDP software). Bcl-2 protein expression was correlated with current prognostic indicators and with molecular markers detected by the same procedure as for Bcl-2. It was shown that Bcl-2 expression is not related to patients' age, tumour size and type or lymph node status, but an inverse relationship was observed between Bcl-2 and tumour grade (P < 0.0001). An inverse relationship was also observed between Bcl-2 expression and p53 (P < 0.0001), Ki67/MIB1 antigen- (P = 0.0012), and P-gp- (P = 0.002) positive immunoreactions. In contrast, anti-Bcl-2 positive reaction was significantly associated with ER-positive (P < 0.001) and with ER/PR-positive or ER/PR/pS2-positive immunoreactions (P < or = 0.005). Bcl-2 expression was independent of CD31 and cathepsin D expression. Thus, Bcl-2 protein, thought to be antiapoptotic, exhibits parodoxical expression in human breast carcinomas. It is strongly detected in low-grade tumours (well-differentiated) with low (MIB1) growth fraction, but is independent of the tumour progression (size, node status, CD31, and cathepsin D). Bcl-2 acting on apoptosis is related to p53 gene abnormalities in breast carcinomas. Bcl-2 protein expression may also be involved in response to endocrine therapy (associated to ER/PR/pS2 positive immunoreactions) and probably with chemoresistance mechanisms (inverse relationship with P-gp).

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1997

CD31/PECAM automated and quantitative immunocytochemical assays in breast carcinomas: correlation with patient follow-up.

The purpose of this study was to determine the prognostic significance of quantitative CD31 immunohistochemical assays. CD31 assays were performed on a series of 167 breast carcinoma specimens under optimal technical conditions that involved frozen sections, an automated immunoperoxidase technique, and computer-assisted analysis of digitized colored microscopic images. Results of automated quantitative immunohistochemical assays were correlated with patient follow-up (9.6 years). Patients were divided into two subgroups: those who had axillary lymph node-positive (N+) disease and those who had lymph node-negative (N-) disease. The marked immunocytochemical expression of CD31 in tumors (cutoff point, 20%) was significantly (P = .033) associated with a poor overall survival rate (Kaplan-Meier, log rank test); however, a significant association was not observed in the N+ and N- subgroups. CD31-immunostained tumor cell surfaces larger than 20% correlated with the metastasis-free survival rate (P = .004) in all patients and in the N+ subgroup (P = .005) but not in the N- subgroup. In addition, marked immunocytochemical expression of CD31 correlated with the short-term disease-free survival rate (P = .04) in the N+ subgroup but not in the N- subgroup. In multivariate analysis (proportional hazards regression, Cox model) the prognostic significance of CD31 was independent of tumor size and histologic type but not of grade. The results suggest that, under optimal technical conditions (automated and quantitative immunohistochemical assays on frozen sections), immunohistochemical expression of CD31 is a significant prognostic indicator of overall and metastasis-free survival rates. CD31 has limited prognostic value, however, and is not a completely independent prognostic indicator because it is related to nodal status.

Adult↗

PubMed1997

Cathepsin D detected by automated and quantitative immunohistochemistry in breast carcinomas: correlation with overall and disease free survival.

AIM: To determine the prognostic significance and clinical relevance of cathepsin D detected by immunocytochemical assays (ICAs) in breast carcinomas. METHODS: 151 patients presenting with palpable or impalpable breast carcinomas and who had not received any kind of adjuvant chemotherapy or endocrine therapy who were operated from January 1986 to May 1987 were studied. ICAs of tumour specimens were performed in optimal technical conditions (frozen sections, automated immunoperoxidase technique (Ventana), and computer assisted analysis of digitised coloured microscopic images (SAMBA)) to determine cathepsin D immunocytochemical expression. Results of quantitative ICAs were correlated with overall and disease free survival over 8.4 years of follow up in axillary lymph node positive and negative patients. RESULTS: Cathepsin D immunocytochemical expression in tumours of 15% or more was significantly associated with poor overall survival in the whole group and in node positive patients (Kaplan Meier, log rank test p = 0.003 and p = 0.007); however, it was not correlated with survival in node negative patients. Cathepsin D immunocytochemical expression (> 15%) correlated with short disease free (p = 0.015) and short recurrence free survival (p = 0.021) in the group as a whole but not when node positive and negative patients were evaluated separately. CONCLUSIONS: In optimal conditions (automated and quantitative ICAs on frozen sections) cathepsin D immunohistochemical expression is a significant prognostic indicator in terms of overall, disease free, and recurrence free survival; however, there is no correlation when node negative patients are considered separately.

Adult↗

PubMed1996

Effect of intracellular iron loading on lipid peroxidation of brain slices.

The effect of artificially elevated cell iron content on oxygen-derived free radical production was assessed in brain slices by use of an iron ligand, 8-hydroxyquinoline (HQ). The iron complex Fe(3+)-HQ exhibited a high lipid solubility evidenced by n-octanol/water partition coefficient and was avidely taken up by brain slices. The catalytically active form of Fe3+ within the complex was evidenced by measuring the rate of ascorbate oxidation. Lipid peroxidation was assessed by measuring the thiobarbituric acid-reactive substances (TBARS) in brain homogenates or slices exposed to two doses of Fe(3+)-HQ (10 microM/20 microM, 100 microM/200 microM) or Fe(3+)-citrate (10 microM, 100 microM). Addition of the iron complexes to homogenates or slices resulted in a dose-dependent increase in lipid peroxidation. In homogenates, the effects were grossly similar with both complexes, whereas in slices the effects of Fe-HQ were significantly higher than those of Fe-citrate. Lipid peroxidation persisted in washed slices preexposed to Fe-HQ, but not in slices preexposed to the hydrophilic iron complex Fe-citrate. Fe-HQ-induced lipid peroxidation in slices was enhanced in the presence of H2O2, an effect that was not seen using Fe-citrate. Addition of Fe-HQ to brain homogenates in the presence of salicylic acid resulted in the production of 2,3-dihydroxybenzoic acid and the effect was potentiated in the presence of H2O2. This model of iron cell loading may be useful for evaluating the efficacy of antioxidant drugs.

Animals↗

PubMed1996

Asymmetrical distribution of G proteins in syncytiotrophoblastic brush-border and basal-plasma membranes of human term placenta.

In human placental syncytiotrophoblast brush-border (BBM, facing the mother) and basal-plasma membranes (BPM, facing to fetus) we have recently demonstrated the presence of calcaemic hormone-specific receptors for parathyroid hormone and calcitonin, which could be implicated in calcium transport from the mother to the fetus. It is well recognized that signal transducing G proteins (guanosinc nucleotide-binding proteins) can associate with various transmembrane receptors and effector proteins, and regulate a variety of second-messenger systems and ion channels. In this present paper, we investigated the presence of a variety of alpha and beta subunits of G proteins in both syncytiotrophoblast, BBM and BPM by Western blot technique. For the first time, we were able to demonstrate the presence of G proteins in the bipolar syncytiotrophoblast membranes, which were evaluated by immunoblotting using affinity purified antiserum raised against the alpha subunits of Gi1, Gi1/i2, Gi3, G0, Gq, Gs, G7 and against the beta subunits. In BBM, we identified the alpha subunits of Gi1, Gi3, G0, Gq, Gs (42, 46 kDa), Gz and beta subunits. The same alpha subunits of G proteins were found in BPM, although alpha subunits of Gi1, Gq, Gs (46 kDa) were located predominantly in the BBM, and the alpha subunit of G0 was found preferentially in BPM. Moreover, in BBM and BPM, a purified antisera raised against the alpha subunits of Gi1 and Gs, detected a 105 kDa protein and a 67 kDa protein, respectively. Interestingly, the 67 kDa protein was preferentially located in BBM, and none of these proteins were detectable in membranes prepared from brain (control). The asymmetrical distribution of the alpha subunits of G proteins among the two different placental bipolar membranes might reflect the very specialized function of these syncytiotrophoblast membranes in ions and nutrients transport from the mother to the fetus.

Blotting, Western↗

PubMed1996

Quantitative immunocytochemical assays on frozen sections of p53: correlation to the follow-up of patients with breast carcinomas.

A series of 222 tumor samples stored at -80 degrees C in the authors' tumor library were investigated with anti-p53 (PA 1801) and streptavidin-biotin-peroxidase complex. The p53 immunoprecipitates were quantified by densitometry assessed by image analysis of digitized microscopic images. Two parameters, percentage of positive surface and mean optical densities, were compared with the patient's outcome (follow-up = 96.8 months) (life table method, Mantel Cox test, BMDP statistical software). The p53 expression significantly correlated with a poor overall survival (P = .0063), metastasis-free survival (P = .024), and recurrence-free survival (P = .022) at a 20% cutoff point of positive immunoreactive tumor surface. A strong prognostic significance was observed in the node-positive subset of patients but not in the node-negative subset, except for recurrence-free survival (P = .047). The results indicate the clinical relevance p53 evaluated by quantitative immunocytochemistry.

Adult↗

PubMed1995

Functional characterization of the human dopamine D4.2 receptor using vaccinia virus as an expression system.

Recombinant vaccinia viruses harboring the human dopamine D4 receptor cDNA containing two 48 base pair-repeats (D4.2) or the rat dopamine D2 short (D2s) receptor cDNA were used to infect rat-1 fibroblasts. Heterologous expression of both dopamine receptors was demonstrated in binding assays. The affinity constants of these receptors were consistent with values previously reported, including D4.2's higher affinity for the antipsychotic clozapine and raclopride's selectivity for D2 receptors. In the presence of 200 microM 5'-guanylyl-imidodiphosphate (Gpp[NH]p) both receptors exhibited reduced affinities for dopamine. Furthermore, when rat-1 cells were infected with the D2s or the D4.2 recombinant vaccinia viruses and exposed to dopamine agonists, the inhibition of adenylyl cyclase activity was prevented in pertussis toxin-treated cells. This study demonstrates the utility of recombinant receptor-vaccinia viruses in studies of expression, pharmacology and functional coupling of inhibitory G protein-coupled receptors.

Adenylyl Cyclase Inhibitors↗

PubMed1995

Characterization and distribution of a cloned rat mu-opioid receptor.

We have cloned and expressed a rat brain cDNA, TS11, that encodes a mu-opioid receptor based on pharmacological, physiological, and anatomical criteria. Membranes were prepared from COS-7 cells transiently expressing TS11 bound [3H]diprenorphine with high affinity (KD = 0.23 +/- 0.04 nM). The rank order potency of drugs competing with [3H]diprenorphine was as follows: levorphanol (Ki = 0.6 +/- 0.2 nM) approximately beta-endorphin (Ki = 0.7 +/- 0.05 nM) approximately morphine (Ki = 0.8 +/- 0.5 nM) approximately [D-Ala2, N-Me-Phe4,Gly-ol5]-enkephalin (DAMGO; Ki = 1.6 +/- 0.5 nM) uch much greater than U50,488 (Ki = 910 +/- 0.78 nM) > [D-Pen2,5]- enkephalin (Ki = 3,170 +/- 98 nM) > dextrorphan (Ki = 4,100 +/- 68 nM). The rank order potencies of these ligands, the stereospecificity of levorphanol, and morphine's subnanomolar Ki are consistent with a mu-opioid binding site. Two additional experiments provided evidence that this opioid-binding site is functionally coupled to G proteins: (a) in COS-7 cells 50 microM 5'-guanylylimidodiphosphate shifted a fraction of receptors with high affinity for DAMGO (IC50 = 3.4 +/- 0.5 nM) to a lower-affinity state (IC50 = 89.0 +/- 19.0 nM), and (b) exposure of Chinese hamster ovary cells stably expressing the cloned mu-opioid receptor to DAMGO resulted in a dose-dependent, naloxone-sensitive inhibition of forskolin-stimulated cyclic AMP production. The distribution of mRNA corresponding to the mu-opioid receptor encoded by TS11 was determined by in situ hybridization to brain sections prepared from adult female rats. The highest levels of mu-receptor mRNA were detected in the thalamus, medial habenula, and the caudate putamen; however, significant hybridization was also observed in many other brain regions, including the hypothalamus.

Amino Acid Sequence↗

PubMed1995

Differential coupling of D1 and D5 dopamine receptors to guanine nucleotide binding proteins in transfected GH4C1 rat somatomammotrophic cells.

D1 and D5 dopamine receptor genes, stably expressed in GH4C1 rat somatomammotrophic cells, display identical binding values and stimulate adenylate cyclase. Approximately 60% of D1 receptors were in the agonist high-affinity state and were converted to the low-affinity state by 100 microM guanyl-5'-ylimidodiphosphate [Gpp(NH)p]. Of the 48% of D5 receptors in the high-affinity state, only half were modulated by 100 microM Gpp(NH)p; in the presence of the G protein activator, AIF4-, the high-affinity sites of D5 receptors were abolished by Gpp(NH)p, suggesting tight coupling between D5 receptors and G proteins. The high-affinity sites of D1, but not D5, receptors were reduced after pertussis toxin treatment of cells. Thus, whereas D1 receptors in GH4C1 cells couple to both Gs, the G stimulatory protein, and a pertussis toxin-sensitive G protein, D5 receptors couple to Gs and a pertussis toxin-insensitive G protein. Neither D1 nor D5 receptors were able to stimulate phosphoinositide metabolism in these cells. The ability of D5, but not D1, receptors to couple to novel G proteins may be significant in assigning a functional role for these receptors.

Adenylate Cyclase Toxin↗

PubMed1994

Molecular cloning and tissue distribution of a putative member of the rat opioid receptor gene family that is not a mu, delta or kappa opioid receptor type.

A novel G protein-coupled receptor was cloned by PCR and homology screening. Its deduced amino acid sequence is 47% identical overall to the mu, delta and kappa opioid receptors and 64% identical in the putative transmembrane domains. When transiently expressed in COS-7 cells this receptor did not bind any of the typical mu, delta or kappa opioid receptor ligands with high affinity. In situ hybridization analysis revealed that LC132 mRNA is highly expressed in several rat brain areas, including the cerebral cortex, thalamus, subfornical organ, habenula, hypothalamus, central gray, dorsal raphe, locus coeruleus and the dorsal horn of the spinal cord. Based on this distribution and its high homology with the mu, delta and kappa opioid receptors, it is proposed that LC132 is a new member of the opioid receptor family that is involved in analgesia and the perception of pain.

Amino Acid Sequence↗

PubMed1994

Platelet activating factor impairs pressor responses to noradrenaline in the anaesthetized rat but does not mediate endotoxin-induced hyporeactivity.

A nonhypotensive dose of endotoxin (Escherichia coli lipopolysaccharide, 250 micrograms kg-1 h-1) impaired both the pressor responsiveness to noradrenaline and its effects in reducing renal and hindquarter blood flow, measured using ultrasound Doppler flow probes. Platelet activating factor (PAF, 50 ng kg-1 h-1) similarly impaired pressor responsiveness to noradrenaline, although this effect was accompanied by marked hypotension. These actions of PAF were prevented by pretreatment with the PAF antagonists WEB 2086 (20 mg kg-1) or BN 50739 (10 mg kg-1) 15 min before commencing the infusion. However, neither antagonist modified the effect of endotoxin in impairing vascular responsiveness to noradrenaline. Thus, these results do not support a role for PAF in mediating endotoxin-induced vascular hyporeactivity, at least in the early stages of endotoxaemia.

Anesthesia↗

PubMed1993

Dopaminergic activity measured in D1- and D2-transfected fibroblasts by silicon-microphysiometry.

The dopaminergic system implicated in human disorders such as Parkinson's disease, schizophrenia and prolactinomas, exerts its effects through several dopamine receptors. The diversity of the dopaminergic system has been revealed by the application of molecular biology techniques to this system, which allowed the identification of five different types of dopamine receptors to date. Even though the structure of these receptors has now been identified, their physiological roles are still under investigation. The coupling of the D1 and D2 dopamine receptor to second messengers has been investigated using cell lines transfected with the cDNAs of these receptors. However, until recently, there was no technique allowing non-invasive real-time measurement of the metabolic activity of cells after agonist stimulation. We present here real-time measurement of events induced by dopaminergic agents on either the D1 or the D2 dopamine receptors using a novel technique employing a silicon-based microphysiometer.

Animals↗

PubMed1992

Effect of acidosis and anoxia on iron delocalization from brain homogenates.

Cortical homogenates were prepared from rat brain in Krebs-Ringer phosphate media adjusted to pH 7, 6 or 5 and incubated for 1 hr under aerotic or anaerobic conditions in the presence of dipyridyl, an iron chelator. Low molecular weight species (LMWS) iron was measured spectrophotometrically after passing of the homogenates through a 10,000-Mr ultrafiltration membrane. Following aerobic incubation, LMWS iron reached 1.24 micrograms/g tissue at pH 7, and increased 1.7-fold at pH 6 and 3.1-fold at pH 5. Anoxia enhanced significantly the amount of ultrafiltrable iron at the three pH values, the LMWS iron level being increased by 190% at pH 7, by 113% at pH 6, and by 77% at pH 5. Addition of the ultrafiltrates to brain membranes caused significant rises in the production of lipid peroxides assessed by the thiobarbituric acid test, indicating that LMWS iron was in a form capable for catalysing oxygen-derived free radical-mediated lipid peroxidation. It was concluded that decompartmentalization of intracellular iron may be an important factor in the initiation of peroxidative damage to ischemic cells.

2,2'-Dipyridyl↗