The vasotocin receptor family is homologous to the mammalian vasopressin G-protein coupled receptor (GPCR) family. The vasotocin receptor 2 (VT2R) and 4 (VT4R) have recently been shown to play important role(s) in the neuroendocrine regulation of stress in birds. A homology-based structural model of VT4R of the domestic chicken, Gallus gallus, was built using the sophisticated SYBYL-X suite. The structure of VT4R built with and without extra- and intracellular unstructured loops showed a seven-helix transmembrane domain, which is a characteristic feature of GPCRs. Several agonists and antagonists were screened by molecular docking to map their potential binding sites on the structure of VT4R. Interestingly, the presence of the N-terminal, intracellular and extracellular loops and C-terminal amino acid sequences emerging from the transmembrane domains during molecular docking appeared to influence the binding interface of the peptide agonists and peptide/non-peptide antagonists on the VT4R. The presence of unstructured loops, however, did not affect the relative binding affinity ranking of the peptide antagonists to VT4R. In general, the natural ligand, arginine vasotocin and the peptide/non-peptide antagonists were observed to be more deeply buried in the receptor. Results of in vitro inhibition experiments, using cultured anterior pituitary cells, showed excellent agreement with the binding affinity of the antagonists predicted by molecular docking. The results of this study provide valuable clues for the rational design of novel pharmaceutical compounds capable of blocking or attenuating the stress response.

This study evaluated the utility of (S)-N-[(1-ethyl-2-pyrrolidinyl)methyl]-5-(3-[18F]fluoropropyl)-2,3 - dimethoxybenzamide ([18F]fluorpropylepidepride), [18F]5-FPrEpid, as a ligand for PET studies of cerebral dopamine D2 receptors. The in vitro affinity for the rat striatal dopamine D2 receptor, KD 138 pM, was determined by Scatchard analysis of in vitro binding to rat striatal homogenate. The apparent lipophilicity, log kw 1.6, was measured with reverse phase HPLC at pH 7.5. The receptor specificity was determined by competitive displacement of [18F]5-FPrEpid by a variety of neurotransmitter ligands. Only dopamine D2 ligands displaced [18F]5-FPrEpid with high affinity. Positron tomographic imaging studies in primates of [18F]5-FPrEpid demonstrated a stable striatal uptake of 0.02% injected dose/ml for up to 5 h after injection. The striatal: cerebellar ratio increased from 2 at 15 min, to 7 at 200 min, and to 10 at 300 min. Striatal uptake was displaceable by haloperidol (1 mg/kg) or raclopride (2.5 mg/kg) to cerebellar levels with a t1/2 of washout of 9 or 15 min. Striatal uptake was mildly susceptible to displacement by d-amphetamine (1-2 mg/kg) released endogenous dopamine; d-amphetamine administration produced a 10% h increase in the rate of striatal washout. Although uptake in the striatum is reversible, an equilibrium between receptor bound [18F]5-FPrEpid in striatum and [18F]5-FPrEpid in plasma is not reached within 5 h postinjection.