Humans can taste many compounds but are able to distinguish between five basic tastes, bitter, sweet, umami, sour and salt. Bitter taste, which is mediated by 25 bitter taste receptors (T2Rs) in humans, acts as a central warning signal against the ingestion of toxic compounds. In addition to their expression in the oral cavity, T2Rs are expressed in various extra-oral tissues, suggesting that they have additional functions apart from sensing taste. The recent finding that T2Rs, upon activation with bitter tastants, cause muscle relaxation and bronchodilation of pre-contracted airway smooth muscle, has been a topic of consideration. The bronchodilatory role of T2Rs, which was three fold greater than that elicited by currently used beta-adrenergic receptor agonists, has implicated them as potential therapeutic targets for the treatment of asthma. In view of the importance of T2R function in extra-oral tissues, it is of fundamental importance to determine their physiological role in extra-oral regions like the vascular tissues, and understand how T2R signal is regulated. In this study, two representative bitter taste receptors, T2R1 and T2R4, were selected to elucidate the function and signal regulation of T2Rs. The expression of T2Rs was characterized in pulmonary artery smooth muscle cells, and studies were pursued to explore the effects of dextromethorphan (DXM) on the pulmonary artery. DXM caused vasoconstriction in pulmonary arterial cells by activating endogenous T2R1. The structure-function role of the C-terminus of T2R4 receptor was characterized by site-directed mutagenesis. A conserved KLK motif was identified in the C-terminus and many residues involved in cell surface targeting and function of T2R4 were revealed. A constitutively active mutant (CAM) was also discovered within the T2R4 C-terminus. To identify the regulatory proteins involved in T2R4 desensitization, a molecular and pharmacological approach was used. The kinases involved in this process were identified by performing either a knockdown or by using activators and inhibitors. The potential residues of T2R4 involved in desensitization were assessed for function. In addition, the internalization status of few T2Rs was determined by using different bitter compounds. This study provides novel insights into the function and desensitization of T2Rs and reports a pharmacochaperone activity for quinine.

The third intracellular loop (IL3) and cytoplasmic tail (CT), which are the most divergent regions between human D1-class dopaminergic receptors (hD1R and hD5R), have been implicated in modulating their subtype-specific functional phenotypes. The importance of the IL3 for Guanine nucleotide-binding protein (G-protein) coupling and specificity has long been acknowledged in the G-protein-coupled receptor (GPCR) field. However, the exact role the central region of the IL3, notably the N- and C-terminal moieties, plays in GPCR receptor functionality remains unclear. Studies in our laboratory indicated that the IL3/N-terminal moiety of hD1-class receptors appears to be critical for facilitating agonist-independent and dependent activation of hD1R and hD5R. Furthermore, the IL3/C-terminal portion of hD1-class receptors constrains the receptor in the inactive state and reduces receptor affinity for agonists and G-protein coupling. I put forward the following hypothesis: 1. The functional properties of hD1-class receptors are regulated via a molecular micro-switch present within the IL3 central region modulating the functional properties of the receptor distinctly, 2. The functional differences between D1R and D5R require structural elements from both N- and C-terminal halves of the IL3 central region, and 3. The molecular interplay between the N- and C-terminal halves of the IL3 central region is dependent on the amino acid chain length and content. Herein, I have employed site-directed mutagenesis, and alanine replacement approaches to analyze comprehensively the structural determinants within the N- and C-terminal moieties of the IL3 central region that regulate ligand binding and G-protein coupling properties of hD1-class receptors. Moreover, my Ph.D. research aimed to pinpoint whether the IL3 length and/or structural motif(s) regulate ligand binding and activation properties of hD1R and hD5R. The results of my study highlight the importance of structural elements from both the proximal and distal segments of the IL3/central region of hD1-class receptors for the ligand binding and receptor activation status. Additionally, my results underline the significance of preserving the length of the IL3 regardless of the amino acid content. This study also shows the pivotal role played by a phenylalanine residue, F2646.27, in the signaling properties of hD1R. Notably, mutating F2646.27 leads to a mutant hD1R with characteristics resembling those of constitutively active mutant GPCRs. Unraveling the amino acid/amino acids constraining the receptor in the inactive state will perhaps provide an attractive target for drug design. Future work aims at developing drugs that particularly bind to the intracellular face of hD1R and improving selectivity towards hD1R may prove useful in limiting the side effects associated with the conventional therapy of brain disorders such as in the case of L-DOPA induced dyskinesia (LID) seen in individuals suffering from Parkinson’s disease.

Les récepteurs aux hormones glycoprotéiques (rGpHs) rTSH, rFSH et rLH/CG appartiennent à la classe A des GPCRs. Les récepteurs da la classe A des GPCRs sont caractérisés par la similitude de séquence de leur domaine transmembranaire avec la rhodopsine. Outre ce domaine dit « serpentin », qu’ils partagent avec tous les GPCRs, les rGpHs offrent la particularité de présenter un grand domaine extracellulaire (ECD) responsable de la liaison et de l’affinité de leurs ligands respectifs. Les récepteurs couplés aux protéines G (GPCRs) sont les cibles de 50% des médicaments actuellement sur le marché pharmaceutique. La compréhension de leur mode de fonctionnement est donc essentielle au développement de nouvelles molécules capables de cibler spécifiquement ces récepteurs. Ces dernières années, il est apparu clairement que les GPCRs étaient présent à la surface cellulaire sous forme d’oligomères. Le but de ce travail était d’explorer de manière approfondie, le mécanisme d’activation et de dimérisation du rTSH en identifiant les résidus du domaine transmembranaire des récepteurs aux hormones glycoprotéiques impliqués dans le processus d’activation et de dimérisation. Au cours de ce travail de thèse, nous avons dans premier temps généré une banque de mutants aléatoires du rTSH. Ces mutants ont ensuite été criblés par une approche HTRF pour mettre en évidence des mutants dont l’activité constitutive est augmentée ou ayant perdu la capacité de dimériser. Nous avons ainsi déterminé de nouveaux résidus importants pour le mécanisme d’activation du rTSH. Les résultats que nous avons obtenus permettent d’apporter des éléments de réponse et une base de travail sur le mécanisme d’activation. Cependant une description détaillée de l’activation reste toutefois indéterminée.

El receptor Neuroquinin-1 (NK1R) és un GPCR que es troba en el sistema nerviós central i perifèric dels vertebrats i és responsable dels processos fisiològics com la transmissió de dolor, secreció endocrina i exocrina, vasodilatació, modulació de la proliferació cel·lular i molts altres. Els antagonistes del NK1R poden ser potencials analgèsics i antidepressius i també poden ser utilitzats per al tractament del trastorn bipolar, l'alcoholisme, càncer, malalties del sistema immune i algunes infeccions. Les tècniques espectroscòpiques i les estructurals d'alta resolució com NMR i la cristal·lografia requereixen quantitats de l'ordre de mil·ligrams del receptor actiu purificat. Una de les estratègies que permet produir els GPCRs recombinants per estudis estructurals és el sistema d'expressió en E.coli. No obstant, molts GPCRs degut al seu efecte tòxic per a la cèl·lula bacteriana s'expressen en forma dels cossos d'inclusió i han de ser sotmesos al procés de renaturalització. La renaturalització dels GPCRs és una tasca complexa que implica complexos ajustaments dels tampons. La primera part d'aquest treball s'ha centrat en la renaturalització de las formes truncades hNK1R-366 i hNK1R-311 del receptor expressades en cossos d'inclusió d'E.coli. Per a l'obtenció del receptor ben plegat hem establert un original protocol de la renaturalització en columna. Hem utilitzat diferents tècniques espectroscòpiques per a estudiar el receptor renaturalitzat. Els resultats de CD han demostrat que el hNK1R-366 i el hNK1R-311 renaturalitzats en DDM presenten un percentatge d'hèlix-α similar al de la rodopsina extreta de retines bovines i solubilitzada en DDM. En els estudis de fluorescència intrínseca de triptòfans a baixes concentracions del GuHCl hem pogut observar el desplaçament cap el blau en l'espectre d'emissió, típic de triptòfans que es troben en un entorn hidrofòbic. A més a més, els espectres d'emissió del hNK1R-366 expressat en cèl·lules COS-1 i solubilitzat en DDM presenten el màxim d'emissió a 335 nm, molt similar al del receptor renaturalitzat a partir dels cossos d'inclusió, indicant la seva correcte renaturalització. El hNK1R-366, renaturalitzat en tampó fisiològic presenta agregació en 24 hores. No obstant, la presència de 0.05% DDM és capaç d'estabilitzar el receptor. Els assajos de radioligand binding de saturació del hNK1R-366 renaturalitzat indiquen que el receptor actiu constitueix l’1% de la proteïna total de la mostra. No obstant, la unió de la SP al receptor en el rang de nanomols és significatiu i és un resultat important, donat que, per primera vegada s’ha obtingut NK1 funcional a partir de E.coli. Per altre costat, no hem pogut obtenir una corba de saturació del hNK1R-311, degut possiblement al plegament defectuós del receptor per falta dels darrers 96 residus. La segona part d'aquest estudi està centrada en l'expressió, purificació i caracterització estructural del C-terminal del receptor hNK1. El domini C-terminal és important per l'acoblament de la proteïna G i la β-arrestina, i també és essencial per a la dessensibilització, internalització i reciclatge del receptor. No obstant, el paper d'aquest domini ha estat infravalorat pels investigadors durant molt temps i existeix poca informació sobre la seva estructura. Els estudis espectroscòpics de UV i de fluorescència posen de manifest anomalies en l'absorbància a 292 nm i en l'emissió intrínseca de les tirosines a 345 nm, atribuïdes a formes ionitzades de l’aminoàcid, degut a la seva proximitat a grups carboxil de residus glutàmics o aspàrtics. A partir de la predicció de l'estructura secundària i terciària i dels resultats dels estudis espectroscòpics hem proposat un model tridimensional pel C-terminal del hNK1 que conté: 25% d’hèlix-α, 27% d’estructura desordenada i 48% de fulles-β i girs-β. En conjunt, els resultat obtinguts, indiquen que el C-terminal del hNK1R no és un domini desordenat, sinó que té una estructura secundària i terciària clarament definides que poden relacionar amb les seves funcions. / Neurokinin-1 receptor (NK1R) is a GPCR found in the central and peripheral nervous system of vertebrates, responsible for such physiological processes as pain transmission, exocrine and endocrine secretion, vasodilatation, modulation of cell proliferation and many others. NK1R antagonists could be potential analgesics and anti-depressants and may also be used for treatment of bipolar disorder, alcoholism, cancer, immune system diseases and selected infections. Spectroscopic studies and high resolution structural techniques, as NMR and crystallography, require milligram amounts of active purified receptor. One of the strategies to produce recombinant GPCRs for structural studies is an E.coli expression system. However, many GPCRs due to their toxic effect for bacterial host cell are expressed in form of inclusion bodies and require refolding. The refolding of GPCRs is a complicated task that requires screening and adjustment of buffer conditions. The first part of this work was centered on the refolding of hNK1R-366 and hNK1R-311 truncated forms expressed in E.coli inclusion bodies. To obtain properly folded receptor, we established an original on-column refolding protocol. Different spectroscopic techniques were applied to study the refolded receptor. The results obtained from CD measurements showed that hNK1R-366 refolded in DDM presents similar α-helical content as rhodopsin extracted from bovine retinas and solubilized in non-denaturing DDM micelles. In the intrinsic tryptophan fluorescence studies, at low concentrations of GuHCl we observed a blue-shift in the emission spectrum peak, typical for tryptophan in hydrophobic environment. Furthermore, the emission spectra of hNK1R-366 expressed in COS-1 cells and solubilized in DDM micelles show very similar emission maximum around 335 nm to that of the receptor refolded from the inclusion bodies, which may be indicative of proper protein refolding. The refolded in physiological buffer hNK1R-366 was prone to aggregate in about 24 hours, however, the presence of 0.05% DDM was found to stabilize the receptor. Saturation radioligand-binding assays for the refolded hNK1R-366 showed that the amount of the active receptor is about 1% of the total protein the sample. However, the binding of SP to the refolded hNK1R-366 in nanomolar range is significant and can be considered as a promising result, since until now the intents to produce any detectable amounts of functional NK1 receptor in E. coli were unsuccessful. On the other hand, we were unable to get any saturation binding curve for hNK1R-311 truncated form of the receptor, which could be explained by incorrect folding caused by the lack of 96 residues of the C-terminus of the receptor. The second part of the present study is centered on hNK1R C-terminus expression, purification and characterization to elucidate its structural properties. The C-terminus domain seems to be essential for the coupling to corresponding G protein and β-arrestin, and is essential for receptor desensitization, internalization and recycling. However, the role of this domain was underestimated by researchers for a long time and as a result very little information is known about its structure. UV and fluorescence spectroscopic studies revealed abnormal tyrosine red-shifted absorbance band at 292 nm and intrinsic tyrosine emission at 345 nm which could be attributed to ionized form of tyrosine and possibly arises from the proximity of one or more tyrosines to carboxyl groups of glutamic o aspartic residues. Based on secondary and tertiary structure prediction as well as on the results of spectroscopic studies we propose a 3D-model for hNK1R C-terminus. Th following assignment of the secondary structure was made: 25% α-helix, 27% unordered structure, 48% β-sheets and β-turns. The obtained ressults give evidence that hNK1R C-terminus is not an unordered region but has clearly defined secondary and tertiary structures which certainly are tightly related to its multiple functions.

GPCRs are known to form dimeric structures, and this affects their pharmacological properties. The ?2AR and AT1aR are GPCRs that are involved the regulation of the adrenergic and renin-angiotensin systems. The ?2AR is polymorphic at position 164, affecting its responsiveness to adrenergic ligands. Both receptors have been shown to form dimers, but little is known on how dimerization affects their trafficking and signalling following ligand treatments. Plasma membrane localization, arrestin-2 recruitment, and G-protein interactions were determined between receptor dimers using molecular biological techniques. This study demonstrates that the formation of heterodimers can change the expected response to ligand treatments, along with associated trafficking events. It was determined that ligands bind to dimers, resulting in conformational changes to the dimeric complexes. Both the ?2AR and AT1aR are targeted in cardiovascular disease and this research demonstrates the importance of dimerization when prescribing drug therapies to avoid potential unwanted drug side effects.

Este trabalho teve por objetivo dissecar o papel potencial de fosforilação/desfosforilação como efeitos da ativação de PfSR25, um candidato a receptor serpentina de P. falciparum. Como a sinalização é um evento celular complexo, não excluímos a possibilidade de que outros mecanismos moleculares ocorram além dos aqui descritos. Nossas conclusões são que potássio modula PfSR25, ativando quinases/fosfatases, levando à ativação de moléculas efetoras. Encontramos MSP1, proteína já caracterizada e Pf4-4-13 e o fator básico de transcrição 3B (PfBTF3B), que ainda não foram caracterizados em P. falciparum, como efetores. Estes dados sugerem que pelo menos em parte, o mecanismo pelo qual PfSR25 exerce seu papel no desenvolvimento de P. falciparum seja através da ativação de quinases/fosfatases. Isto não é surpreendente, pois a sinalização de PfSR25 ocorre através de K+/cálcio e o segundo mensageiro é um modulador destas classes de proteínas. No entanto, deve ser investigado se cálcio tem algum efeito direto sobre o processamento/ativação dos efetores aqui identificados. / This work aimed at dissecting the potential role of phosphorylation/dephosphorylation as downstream effect of PfSR25 activation. PfSR25 is a serpentine receptor candidate from P. falciparum. As signaling is a quite complex cellular event, we do not exclude the possibility that other molecular mechanisms, take place additionally to those here described. Our conclusions are that potassium modulates PfSR25 by activation of kinases/phosphatases, leading to activation of effector molecules. We found MSP1 already characterized protein and Pf4.4.13 and the basic transcription factor 3B (PfBTF3B), which have not yet been characterized in P. falciparum as effectors. These data suggest that, at least in part, the mechanism by which PfSR25 exerts its role in the P. falciparum development is through the activation of kinase/phosphatase. This is not surprising, since PfSR25 signaling occurs through K+/Calcium and the second messenger is a modulator of these classes of proteins. However, remains to be investigated rather calcium has a direct effect on processing/activation the effectors here identified.

G-protein–coupled receptors are the largest superfamily in the human genome, and involved in critical cellular signaling processes in living cells. Protein structural fluctuations are the key for GPCR function that is driven and modulated by a variety of factors that are not well understood. This dissertation focusses on understanding the activation of GPCRs using the visual receptor, rhodopsin as the prototype. Rhodopsin is an ideal candidate for this study, as it represents the largest class of GPCRs, and is known to demonstrate more noticeable structural changes upon activation compared to the other GPCRs. What structural fluctuations occur, the role of water, and how the retinal cofactor regulates the protein dynamics during rhodopsin activation are specific research problems addressed in this work. Hypothesizing an ensemble activation mechanism, experiments were conducted using a variety of techniques to probe structural and dynamical fluctuations of rhodopsin in native membranes, as well as in membrane mimetics such as detergent micelles. Time-resolved wide-angle X-ray scattering (TR-WAXS), small-angle neutron scattering (SANS), quasielastic neutron scattering (QENS), and electronic spectroscopy are among the prominent techniques used to gain insights into the photo-intermediates that are key to understanding the rhodopsin activation process. The small-angle neutron scattering (SANS) experiments revealed a volumetric expansion of the protein molecule upon photoactivation of rhodopsin. Electronic spectroscopy together with the differential hydration study revealed the crucial role of water in rhodopsin signaling process and signal amplification by water. The quasielastic neutron scattering study conducted on powdered rhodopsin probed the changes in the local dynamics that are regulated by the retinal cofactor of the rhodopsin molecule. The increased local steric crowding in the ligand-free opsin is consistent with collapsing of the apoprotein structure in the absence of the retinal chromophore leading to inactive opsin conformation. Finally, a time-resolved wide-angle X-ray scattering study was conducted using the X-ray free electron laser at the SLAC national laboratory to probe the early structural fluctuations in rhodopsin photoactivation. The preliminary pump-probe experiments conducted on rhodopsin in CHAPS detergent micelles revealed a light-triggered protein quake that occurs during the early activation stages of rhodopsin photoactivation. Thus the protein fluctuations underlying the GPCR function are revealed by neutrons, X-rays, and other photons in a combined implementation of both spectroscopic and scattering techniques as applied to the investigation of rhodopsin activation.

Doctor of Philosophy / Entomology / Yoonseong Park / Tick salivary secretion is crucial for survival and for successful feeding. Tick saliva includes excretory water/ions and bioactive components for compromising the hosts' immune responses, and provides a direct route for pathogen transmission. Control of the tick salivation involves autocrine/paracrine dopamine, the most potent stimulator of tick salivation. Our research group reported the presence of two dopamine receptors in the salivary glands of the blacklegged tick (Ixodes scapularis): dopamine receptor (D1) and invertebrate specific D1-like dopamine receptor (InvD1L). Dopamine-induced salivary secretion was orchestrated by two distinct physiological roles via activation of the two dopamine receptors (Chapter 2). Low concentration of dopamine activated D1 receptor on epithelial cells of salivary gland acini leading inward fluid transport. High concentration of dopamine activated InvD1L receptors on axonal projections innervating myoepithelial cells modulating pumping/gating actions for emptying luminal saliva into the main duct. Thus, ticks coordinated salivary secretion with duo dopamine receptors. Dopamine-mediated saliva production involves an important downstream component, Na/K-ATPase (Chapter 3). Na/K-ATPase was found in the epithelial cells of all types of acini. However, Na/K-ATPase had two different functions in salivary secretion in different acini: 1) dopamine-mediated production of primary saliva in distally located salivary gland acini type-2/- 3, and 2) dopamine-independent resorption in proximally located salivary gland acini type-1. Type-1 acini were also found to function in direct water absorption of off-host ticks, which could be a potential route for delivery of acaricides. Chapter 4 investigated the comparative transcriptomics of the lone star tick underlying the processes of pathogen acquisition. Differential expression analyses in pathogen-exposed ticks revealed a number of transcripts that are important in the tick-pathogen interaction. These included genes for tick immunity against pathogen and for modulation of tick physiology facilitating a pathogen’s invasion and proliferation. My study expanded the understanding of physiological mechanisms controlling tick salivation. In addition, transcriptomics of ticks in interaction with pathogen identified several genes that are relevant in vector/pathogen interactions. The knowledge obtained in my study will facilitate to the development of novel methods for the disruption of tick feeding and pathogen transmission.

Regulators of G protein signaling (RGS) are a diverse group of proteins, which play a fundamental role in modulation of G protein coupled receptor signal transduction. RGS proteins are primarily known as GTPase activating proteins (GAPs) for Gá subunits. In addition to the RGS domain, which is responsible for GAP activity, most RGS proteins also contain other structural motifs. The R7 family of RGS proteins for example, which consists of RGS-6, 7, 9 and 11 gene products, also contains DEP, DHEX and GGL domains. All R7 RGS proteins are obligatory binding partners with G protein beta subunit, G beta sub five, which binds to the GGL domain. In my dissertation work, I provide insights into significance of the multi-domain architecture of G beta sub five-RGS7. I have identified a novel intramolecular interaction within the G beta sub five-RGS7 complex; between the DEP domain of RGS7 and G beta sub five subunit. My experimental evidence supports the idea that G beta sub five-RGS7 can exist in at least two hypothetical conformations: "closed" where the DEP domain and G beta sub five subunit are bound to each other, and "open" where DEP and G beta sub five are not interacting, and as a result both these proteins can associate with other binding partners. My results indicate that in its "open" conformation, G beta sub five-RGS7 can selectively inhibit calcium mobilization elicited by stimulated muscarinic acetylcholine receptor type 3 (M3R). This inhibition is mediated by direct interaction between the third intracellular loop of M3R and the DEP domain of RGS7. In addition to the effect on M3R signaling, I observed that the G beta sub five-RGS7 complex redistributes from the cytosol to endocytic vesicles in an M3R-specific manner. These results identify a novel molecular mechanism that can impart receptor-subtype selectivity on signal transduction via G protein-coupled receptors. Lastly, I have identified a small group of compounds that inhibits the DEP-G beta sub five interaction. These compounds may serve as starting points for design of G beta sub five-RGS7 modulators in the future.

Stress impacts upon all organisms and a robust stress response is required for adaptive interactions of the organism with the environment. In most higher organisms, an individual’s response to stress is mediated by the hypothalamic pituitary adrenal (HPA) axis. Inappropriate regulation of this axis can cause debilitating mental health disorders including depression and anxiety. These disorders can affect an individual’s ability to interact and respond appropriately as different situations arise. An important component of this axis is the vasopressin V1b receptor (V1bR), which mediates adrenocorticotropin (ACTH) secretion from the anterior pituitary in response to stimulation by arginine vasopressin (AVP). AVP also potentiates the ACTH secretion mediated by corticotropin-releasing hormone type 1 receptor (CRH-R1) in response to corticotropin- releasing hormone (CRH) stimulation. Both the V1bR and CRH-R1 are G protein coupled receptors (GPCRs). A common feature of GPCR signalling is desensitisation of the response following prolonged or repeated exposure to an agonist. Phosphorylation of the receptor is one of the mechanisms of desensitisation. This directly, or indirectly, results in rapid and reversible uncoupling of the receptor from its heterotrimeric guanine nucleotide binding protein (G-protein). Previous research has shown that G protein coupled receptor kinases (GRKs) are key phosphorylators involved in the molecular mechanism of GPCR desensitisation. One of the mains goals of the research carried out in the Mason laboratory is to examine the molecular mechanisms of V1bR desensitisation. The current short term aim is to examine the potential role for GRKs in this mechanism. It is difficult to study a single receptor type and the molecular mechanisms involved in its regulation in a system larger than a cell based assay. As the proposed method of assessing the involvement of GRKs in desensitisation of the V1bR is to use RNA interference (RNAi) to knock down the expression of the GRKs, primary cell cultures of pituitary corticotrophs are an inappropriate choice. This is due to a number of factors, including the difficulty involved in transfecting primary cells, and the difficulty involved in interpreting the results from primary cell culture experiments as these cultures are composed heterogenous population of cells. Therefore, the main aim of this research was to develop a model cell system from an immortalised cell line, stably-transfected with the V1bR, in which the involvement of GRKs in the molecular mechanism of V1bR desensitisation could be studied. Development of stably-transfected cell lines requires substantial preliminary work and planning in order to produce a successful outcome. Once developed, characterisation of the clonal cell lines is required. The preliminary work involved determining the cell proliferation rate of the parental cell line, plasmid sub-cloning and production of a large quantity of plasmid DNA, optimisation of the antibiotic selection conditions, and optimisation of the transfection protocol, as well as modification of the inositol phosphate (IP) assay protocol. The V1bR activates the phospholipase Cβ (PLCβ) second messenger signalling pathway in response to stimulation with AVP. This results in the production of IPs and therefore, measurement of IPs in response to AVP stimulation of cells labelled with myo-[³H]inositol can be used as an indicator of functional V1bR expression. In this research a total of nine clonal cell lines resistant to the antibiotic G418 were generated. Initial testing of these lines indicated that four probably expressed the V1bR and these were selected for characterisation in greater detail. All of these four lines showed significantly increased IP production in response to AVP stimulation (P<0.05; t-test). A significant decrease in IP production in response to AVP stimulation following an AVP pre-treatment was also seen with all four lines (P<0.05; t-test). Current evidence therefore suggests that the V1bR in these clonal cell lines signals and desensitises in the normal way. Although further characterisation of the clonal cell line is desirable, the data to date indicate that these lines should be considered to provide an appropriate model system for examining the molecular mechanisms involved in the regulation of the V1bR. It appears that there are some minor differences in signalling between the clonal cell lines and therefore this should be a consideration when deciding which line is most appropriate to use for investigating a particular question.