[Arg8]-Vasotocin

The effects of [Arg8]vasopressin and [Arg8]vasotocin on the firing rate of suprachiasmatic neurons in vitro

The excitatory effect of [Arg8]-vasopressin and its potential contribution to the circadian cycle of electrical activity in the suprachiasmatic nucleus of the rat was investigated using extracellular recordings from hypothalamic slices from virgin female rats. The majority of neurons tested for their responses to vasopressin and [Arg8]-vasotocin displayed coincident, dose-dependent excitation by both peptides, although the relative efficacy varied between neurons, with some showing a highly preferential excitation by vasotocin. Perifusion with the vasopressin receptor antagonist d(CH2)5[Tyr(OEt)2,Val4,Cit8]-vasopressin was able to block the majority of responses to vasopressin or vasotocin (20/25), and similar excitation could be induced by the selective agonist [Phe2,Orn8]-vasotocin, indicating a mainly V1 receptor-mediated effect. Few neurons (3/27; 11%) responded to the oxytocin-specific agonist, [Thr4,Gly7]-oxytocin, suggesting a low occurrence of oxytocin receptors. In addition to blocking the action of exogenous vasopressin, the V1 antagonist caused a reversible suppression of spontaneous basal activity in 7/25 cases, consistent with the presence of an endogenous excitatory vasopressin tone. In agreement with previous reports, the activity of suprachiasmatic nucleus neurons showed a significant correlation between spontaneous activity and the light-dark cycle, with activity decreasing during the subjective dark phase. When neurons were divided on the basis of their response to vasopressin and/or vasotocin, the peptide-sensitive neurons continued to show a strong correlation (r = 0.513, P < 0.01) while the insensitive neurons showed no correlation (r = 0.136, P > 0.05). These data confirm the presence of V1 type receptors in the suprachiasmatic nucleus and also indicate a small number of neurons possessing additional classes of receptor selective for either oxytocin or vasotocin. Contrary to previous reports, they also demonstrate that endogenous vasopressin tonically excites suprachiasmatic nucleus neurons. The fact that vasopressin-sensitive (but not vasopressin-insensitive) neurons show a level of basal activity correlated with time, suggests that this tone may contribute to the circadian cycle of electrical activity in the suprachiasmatic nucleus.

[Arg8]vasotocin excites neurones in the dorsal vagal complex in vitro: evidence for an action through novel class(es) of CNS receptors

Using extracellular recordings from brainstem slices in vitro, it was demonstrated that a high proportion (38/56) of neurones in the dorsal vagal complex of dioestrus, virgin female rats exhibit an excitatory response to [Arg8]-vasotocin (AVT). Pharmacological characterization suggests that these responses cannot be entirely explained by interaction with either of the currently known classes of central receptors for oxytocin (OT) and vasopressin (V1a). Comparison of the responses with those to the OT receptor-specific agonist [Thr4,Gly7]-OT (TGOT), showed that not all neurones that responded to TGOT also responded to AVT (3/27). Furthermore, while the effects of 10(-7) M TGOT could be blocked either by the broad-spectrum antagonist d(CH2)5[d-Tyr(OEt)2,Val4,Cit8]-vasopressin or by the selective OT receptor antagonist d(CH2)5[Tyr(Me)2,Thr4,Orn8,Tyr-NH2(9)]-vasotocin, these peptides did not completely block the responses to AVT, indicating that AVT is unlikely to act through the central OT receptor. The responses to AVT and [Arg8]-vasopressin (AVP) indicated the presence of at least 2 classes of receptor with which these agonists could act. Of 42 neurones tested with both AVP and AVT, none responded to AVP in the absence of a response to AVT, while 7/42 responded to AVT without a response to AVP. This might be explained by AVP acting through only the V1 receptor, while AVT acts through both the V1 and its own novel class of receptor. This was substantiated by the fact that two OT/V1 receptor antagonists, d(CH2)5[d-Tyr(OEt)2,Val4,Cit8]-VP and d(CH2)5[Tyr(Me)2,Tyr-NH2(9)]-AVP, were unable to block completely all the responses to AVT at a dose which suppressed responses to both AVP and TGOT.(ABSTRACT TRUNCATED AT 250 WORDS)

[Arg8]vasopressin-induced contractions of rabbit urinary bladder smooth muscle

The effects of a number of neurohypophyseal hormones and analogues on the contractility of rabbit urinary bladder smooth muscle in vitro were examined. The order of potency was [Arg8]vasopressin = [Lys8]vasotocin greater than [Lys8]vasopressin greater than [Arg8]vasotocin much greater than [deamino1,D-Arg8]vasopressin greater than oxytocin much greater than pressinoic acid. The maximum tension induced by [Arg8]vasopressin was 4.4 N . cm-2 and the pD2 was 8.7. The effects of [Arg8]vasopressin were competitively antagonized by [(beta-mercapto-beta,beta-cyclopentamethylenepropionic acid)1,(O-methyl)Tyr2,Arg8]vasopressin and by [deaminopenicillamine1,Val4,D-Arg8]vasopressin with pA2s of 8.4 and 6.6, respectively. It was concluded that rabbit urinary bladder smooth muscle contains receptors for the neurohypophyseal hormones which have recognition properties similar to V1-vasopressin receptors.

Structure, function, and phylogeny of [Arg8]vasotocin receptors from teleost fish and toad

[Arg8]Vasotocin (AVT) is considered to be the most primitive known vertebrate neurohypophyseal peptide of the vasopressin/oxytocin hormone family and may thus be ancestral to all the other vertebrate peptide hormones. The molecular evolution of the corresponding receptor family has now been studied by cloning an AVT receptor, consisting of 435 amino acid residues, from the teleost fish, the white sucker Catostomus commersoni. Frog oocytes injected with the AVT receptor-encoding cRNA respond to the application of AVT, but not to its structural and functional counterpart isotocin, by an induction of membrane chloride currents indicating the coupling of the AVT receptor to the inositol phosphate/calcium pathway. The pharmacological properties of the expressed AVT receptor show that it represents, or is closely related to, an ancestral nonapeptide receptor: oxytocin, aspargtocin, mesotocin, and vasopressin activated the receptor, but other members of the vasopressin/oxytocin family tested showed little or no potency; antagonists of the mammalian vasopressin V1 and oxytocin receptors blocked the AVT response. Comparison of AVT receptor sequences spanning transmembrane domains two to five, deduced by cloning cDNAs from the Pacific salmon Oncorhynchus kisutch, the cave-dwelling fish Astyanax fasciatus, and the anuran Xenopus laevis, with those of their mammalian counterparts emphasizes amino acid residues that are involved in hormone binding. The presence of a 5.0-kb transcript in various teleost tissues (pituitary, liver, gills, swim bladder, and lateral line) points to a physiological role for the fish AVT receptor in metabolic, osmoregulatory, and sensory processes.

Mutational analysis and molecular modeling of the nonapeptide hormone binding domains of the [Arg8]vasotocin receptor

To identify determinants that form nonapeptide hormone binding domains of the white sucker Catostomus commersoni [Arg8]vasotocin receptor, chimeric constructs encoding parts of the vasotocin receptor and parts of the isotocin receptor have been analyzed by [(3,5-3H)Tyr2, Arg8]vasotocin binding to membranes of human embryonic kidney cells previously transfected with the different cDNA constructs and by functional expression studies in Xenopus laevis oocytes injected with mutant cRNAs. The results indicate that the N terminus and a region spanning the second extracellular loop and its flanking transmembrane segments, which contains a number of amino acid residues that are conserved throughout the nonapeptide receptor family, contribute to the affinity of the receptor for its ligand. Nonapeptide selectivity, however, is mainly defined by transmembrane region VI and the third extracellular loop. These results are complemented by a molecular model of the vasotocin receptor obtained by aligning its sequence with those of other G-protein coupled receptors as well as that of bacteriorhodopsin. The model indicates that amino acid residues of transmembrane regions II-VII that are located close to the extracellular surface also contribute to the binding of vasotocin.