Four novel tachykinins in frog (Rana catesbeiana) brain and intestine

De novo sequencing of peptides secreted by the skin glands of the Caucasian Green Frog Rana ridibunda

Amphibian skin glands are known to secrete various types of bioactive peptides. The array of these peptides is specific for every frog species. The present research deals with the identification of peptides isolated from the skin secretion of the Marsh frog R. ridibunda inhabiting the Kolkhida Canyon of the Caucasian region. The research is based on comprehensive high-performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS) analysis of intact and chemically modified peptides. In particular, an oxidation procedure was applied directly to the crude skin secretion to open S--S loops whereas N-terminal acetylation was additionally carried out for one individual peptide. Sequences were determined by manual interpretation of electron capture dissociation (ECD) and collisionally induced dissociation (CID) tandem mass spectra. A total of 29 peptides were identified in the skin secretion of the Caucasian Marsh frog. The peptide profile is represented with disulfide-containing peptides belonging to the brevinin, esculentin and ranatuerin families, neuropeptides of the bradykinin and bombesin families. Two identified peptides belonging to the ranatuerins are the first peptides of this family discovered in the skin secretions of European frogs. Ten of the identified peptides coincide with those reported earlier for the European Edible frog. Another ten are identical to those found in R. ridubunda from the Moscow region. This fact verifies the described method as being an efficient analytical tool to compare intra- and interspecific variabilities.

Role of glutamate and substance P in the amphibian respiratory network during development

This study tested the hypothesis that glutamatergic ionotropic (AMPA/kainate) receptors and neurokinin receptors (NKR) are important in the regulation of respiratory motor output during development in the bullfrog. The roles of these receptors were studied with in vitro brainstem preparations from pre-metamorphic tadpoles and post-metamorphic frogs. Brainstems were superfused with an artificial cerebrospinal fluid at 20-22 degrees C containing CNQX, a selective non-NMDA antagonist, or with substance P (SP), an agonist of NKR. Blockade of glutamate receptors with CNQX in both groups caused a reduction of lung burst frequency that was reversibly abolished at 5 microM (P<0.01). CNQX, but not SP, application produced a significant increase (P<0.05) in gill and buccal frequency in tadpoles and frogs, respectively. SP caused a significant increase (P<0.05) in lung burst frequency at 5 microM in both groups. These results suggest that glutamatergic activation of AMPA/kainate receptors is necessary for generation of lung burst activity and that SP is an excitatory neurotransmitter for lung burst frequency generation. Both glutamate and SP provide excitatory input for lung burst generation throughout the aquatic to terrestrial developmental transition in bullfrogs.

Functional selectivity of NK1 receptor signaling: peptide agonists can preferentially produce receptor activation or desensitization

A cascade of events follows binding of an agonist ligand to the tachykinin NK(1) receptor. These events include activation of multiple signal transduction pathways as well as cellular modulation of receptor function by the process of desensitization. This study examines the differences in the abilities of naturally occurring peptide agonist ligands of the tachykinin NK(1) receptor to preferentially direct signaling through the receptor to produce signal activation versus receptor desensitization. The differential effects of tachykinin peptides with respect to ligand competition binding, receptor-G protein coupling, intracellular Ca(2+) elevations, and receptor desensitization have been measured. In relation to its potency in competition binding studies, substance P produces desensitization at lower concentrations, whereas higher concentrations are required to elicit a Ca(2+) response. In contrast to this, a related peptide, ranatachykinin C, is more effective at activating a Ca(2+) response relative to its ability to produce desensitization. This difference in functional selectivity is conserved for an amphibian and two mammalian ortholog tachykinin receptors. The present study demonstrates that peptide agonist ligands of NK(1) receptors can preferentially produce signal activation or desensitization.

Tachykinin peptide-induced activation and desensitization of neurokinin 1 receptors

The actions of four tachykinins on inhibition and desensitization of the M-current of bullfrog sympathetic neurons have been characterized. Radioligand binding parameters of the tachykinins were determined at a neurokinin receptor in a heterologous expression system. The correlation between binding, signaling and receptor regulation was investigated. A correlation between receptor binding and signaling was found between the peptides; however, their ability to produce desensitization was not correlated with binding and signaling. These results show that the ability of a tachykinin peptide to induce signal activation is not indicative of its ability to induce receptor regulation.

Solution structures in SDS micelles and functional activity at the bullfrog substance P receptor of ranatachykinin peptides

A set of novel tachykinin-like peptides has been isolated from bullfrog brain and gut. These compounds, ranatachykinin A (RTKA), ranatachykinin B (RTKB), and ranatachykinin C (RTKC), were named for their source, Rana catesbeiana, and their homology to the tachykinin peptide family. We present the first report of the micelle-bound structures and pharmacological actions of the RTKs. Generation of three-dimensional structures of the RTKs in a membrane-model environment using (1)H NMR chemical shift assignments, two-dimensional NMR techniques, and molecular dynamics and simulated annealing procedures allowed for the determination of possible prebinding ligand conformations. RTKA, RTKB, and RTKC were determined to be helical from the midregion to the C-terminus (residues 4-10), with a large degree of flexibility in the N-terminus and minor dynamic fraying at the end of the C-terminus. The pharmacological effects of the RTKs were studied by measuring the elevation of intracellular Ca(2+) in Chinese hamster ovarian cells stably transfected with the bullfrog substance P receptor (bfSPR). All of the RTKs tested elicited Ca(2+) elevations with a rank order of maximal effect of RTKA >/= SP > RTKC >/= RTKB. A high concentration (1 microM) of the neuropeptides produced varying degrees of desensitization to a subsequent challenge with the same or different peptide, while a low concentration (1 pM) produced sensitization at the bfSPR. Our data suggest differences in amino acid side chains and their charged states at the C-terminal sequence or differences in secondary structure at the N-terminus, which do not overlap according to the findings in this paper, may explain the differing degree and type of receptor activation seen at the bfSPR.

Molecular characterization and functional expression of a substance P receptor from the sympathetic ganglion of Rana catesbeiana

Substance P is an important neuropeptide neurotransmitter in the central, autonomic and enteric nervous systems. In sympathetic ganglia, substance P is thought to play a role in modulating synaptic transmission. Release of substance P by neuronal stimulation or direct application of substance P to ganglionic neurons increases neuronal excitability. An amphibian substance P receptor complementary DNA has been cloned and characterized from bullfrog, Rana catesbeiana, sympathetic ganglion complementary DNA libraries. The deduced primary structure contains features indicative of a seven transmembrane domain G-protein-coupled receptor. The deduced protein sequence shows 69% identity to previously cloned mammalian substance P receptors. In situ hybridization analysis performed on bullfrog sympathetic ganglia using digoxigenin-labelled complementary RNA probe demonstrated that approximately 75% of the principal neurons displayed reaction product above background levels. Radioligand binding studies were performed on stably transfected cells with [(125)I]Tyr-1-substance P as the ligand. Substance P had an IC50 of 16 nM and the agonist potency profile was substance P>neurokinin A >> neurokinin B. The order of potency for three tachykinins to increase intracellular calcium when applied to a stably transfected clonal cell line was substance P>neurokinin A >> neurokinin B. This order of agonist potency also held for inhibition of the M-type potassium current in intact bullfrog sympathetic neurons. The non-peptide substance P antagonists CP-96345 and RP-67580 at concentrations that block mammalian substance P receptors had little or no effect on the responses to substance P at the bullfrog receptor. Overall, these results demonstrate that the cloned sequence has the features consistent with and characteristic of a substance P receptor. The results are discussed with reference to the established pharmacology of the bullfrog substance P receptor and known structure activity relationships of mammalian tachykinin receptors.