Effects of guanosine 5'-O-(3-thiotriphosphate) on 2-[125I]iodomelatonin binding in the chicken lung, brain and kidney: hypothesis of different subtypes of high affinity melatonin receptors

Effects of 10 mumol/l guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), a non-hydrolyzable analog of guanosine 5'-triphosphate, on 2-[125I]iodomelatonin binding were investigated. In the chicken lung, 10 mumol/l GTP gamma S significantly increased (p < 0.05) the equilibrium dissociation constant (Kd) values, but did not affect the maximum number of binding sites (Bmax). Conversely, in the chicken brain, GTP gamma S significantly depressed (p < 0.05) the Bmax, but did not change the Kd of the 2-[125I]iodomelatonin binding sites in the brain tissue. A third variation was observed in the chicken kidney with GTP gamma S altering (p < 0.05) both the Kd and the Bmax of 2-[125I]iodomelatonin binding sites. The reason underlying the different effects of GTP gamma S on 2-[125I]iodomelatonin binding in the tissue preparations is not clear. However, we would like to hypothesize that they may represent distinct subtypes of the ML-1-type melatonin receptor with different receptor-G-proteins-effector complex. The group represented by the 2-[125I]iodomelatonin binding sites found in the chicken lung, which is downregulated by GTP gamma S with a consequent increase in the Kd value, has been designated ML-1 alpha. The second group, exemplified by the brain 2-[125I]iodomelatonin binding sites, which respond to GTP gamma S with a change in Bmax, has been labelled ML-1 beta. The third group, characterized by GTP gamma S-mediated alterations in both Bmax and Kd and found in the chicken kidney, has been called ML-1 gamma. Different subtypes of melatonin receptors may address the issue of the different physiological actions of melatonin reported in individual tissues within the same species or similar tissues but different species. Specialized responses could be generated depending on the predominant subtype of ML-1 receptors associated with the target tissue.

Substance P (NK-1 receptor) antagonists: in vivo and in vitro activities in rats and guinea pigs

NK-1 receptor subtypes have been identified by the use of CP-96,345 and RP-67,580, two non-peptide antagonists. These and other antagonists have been tested in vivo and in vitro in guinea pigs and rats to counteract the hypotensive and contractile (urinary isolated bladder) effects of a) SP, b) the NK-1 selective agonist [Sar9,Met(O2)11]SP and c) other neurokinins. CP-96,345 has been found to be more active on the guinea pig and RP-67,580 more active on the rat by at least 1 log unit both in vivo and in vitro. Both compounds are selective NK-1 antagonists and RP-67,580 appears to be weaker than CP-96,345. Two in vitro preparations, the guinea pig and rat urinary bladder are proposed as bioassays for the NK-1A (guinea pig) and NK-1B (rat) receptor subtypes, which have been shown to mediate smooth muscle contraction and hypotension, resulting from peripheral vasodilatation. CP-96,345 and RP-67,580 are more potent antagonists than spantide, its homologous octapeptide and the Fujisawa tri or dipeptides.

Effects on the isolated human bronchus of SR 48968, a potent and selective nonpeptide antagonist of the neurokinin A (NK2) receptors

Tachykinins produce concentration-dependent contraction of the human isolated bronchus by stimulation of receptors that belong to the NK2 type. The aim of this study was to investigate the inhibitory effects of a new, potent, and selective nonpeptide antagonist of the neurokinin A (NKA) (NK2) receptors, SR 48968 [(S)-N-methyl-N-[4-acetylamino-4-phenylpiperidino-2-(3,4-dichlorophenyl) butyl]benzamide] on human isolated airways. Our experiments were performed on human isolated bronchi obtained from patients with lung cancer. Phosphoramidon, 10(-5) M, was added to the bath to inhibit neurokinin metabolism. SR 48968 induced a parallel shift to the right of the concentration-response (C/R) curves to [Nle10]-NKA(4-10), a specific NK2 receptor agonist. The antagonism was of the competitive type, with a pA2 of 9.40 +/- 0.19 (slope = 0.95 +/- 0.08, n = 13). The (R)-enantiomer of SR 48968 was 100-fold less potent and a noncompetitive antagonist (slope = 0.56 +/- 0.11, n = 8); pA2 and slope of the racemate were 8.86 +/- 0.21 and 1.09 +/- 0.21 (n = 7), respectively. Under similar conditions, racemic CP-96,345, a nonpeptide NK1 antagonist, did not modify the C/R curves to [Nle10]-NKA(4-10) until 10(-7) M. SR 48968 did not modify C/R curves to acetylcholine, histamine, KCI, or PGF2 alpha on the human isolated bronchus. Finally, SR 48968 shifted to the right C/R curves to substance P on isolated human bronchi, whereas racemic CP-96,345 was without effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Distinct distributions of two bombesin receptor subtypes in the rat central nervous system

We have previously demonstrated the presence of two distinct bombesin receptor subtypes in the rat CNS and distinguished them as bombesin/gastrin-releasing peptide (BBS/GRP) and neuromedin B (NMB)-preferring binding sites. In the present study, we conducted a complete evaluation of the distribution of these binding sites throughout the rat brain using in vitro receptor autoradiography. The BBS/GRP-preferring binding sites were characterized as those that bound 125I-(Tyr4)BBS but not 125I-(D-Tyr0)NMB. At these sites 125I-(Tyr4)BBS binding was inhibited in the presence of 100 nM BBS but not by the same concentration of NMB. In contrast, NMB-preferring sites bound both radioligands and binding at these sites was inhibited in the presence of 100 nM NMB. Our results indicate that the distributions of BBS/GRP and NMB-preferring binding sites are widespread and distinct at all levels of the rat brain suggesting these peptides mediate separate functions in the rat central nervous system.

Receptor classes and the transmitter-gated ion channels

Transmitter-gated channels, which can be selective for cations or for anions, form an important class among the membrane receptors responsible for signal transduction. Thirteen principal types of these channels can now be recognized and most of these are available for analysis in recombinant form. It is instructive to contrast their characteristic structural features with those of the two other primary classes of the signal-transducing receptors of membranes.

Gamma-D-glutamylaminomethyl sulfonic acid (GAMS) distinguishes subtypes of glutamate receptor in the chick cochlear nucleus (nuc. magnocellularis)

Because kainic acid (KA) is more potent than other excitatory amino acids (EAAs) in affecting synaptic transmission in the cochlear nucleus, previous reports have concluded that primary afferent neurotransmission to the cochlear nucleus in birds and mammals is mediated by KA-preferring non-N-methyl-D-aspartate (non-NMDA) EAA receptors. Since this conclusion is at odds with a number of studies suggesting that rapid excitatory neurotransmission in the CNS is mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-preferring non-NMDA receptors, we re-examined the pharmacology of synaptic transmission between the cochlear nerve and nucleus magnocellularis (NM) in chickens, using bath application of drugs and recording of field potentials evoked in NM by electrical stimulation of the cochlear nerve in vitro. A series of EAA agonists produced complete, concentration-dependent and reversible suppression of postsynaptic responses: the order of potency was domoic acid (DO) greater than KA greater than AMPA much greater than quisqualic acid much greater than L-glutamic acid (Glu). Three quinoxalinedione antagonists of non-6-nitro-7-sulphamobenzo[f]quinoxaline-2,3-dione NMDA receptors also produced complete, concentration-dependent and reversible suppression of postsynaptic responses in NM without affecting the presynaptic action potential; the half-maximal inhibitory concentrations (IC50's) were 2.7 +/- 0.4 microM for 6-nitro-7-sulphamobenzo[f]quinoxaline-2,3-dione (NBQX), 5.3 +/- 0.1 microM for 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and 10.6 +/- 1.2 microM for 6,7-dinitroquinoxaline-2,3-dione (DNQX).(ABSTRACT TRUNCATED AT 250 WORDS)

Possible location and function of neuropeptide Y receptor subtypes in the rat mesenteric arterial bed

Earlier investigation of the vascular actions of Neuropeptide Y (NPY) led us to propose that distinct receptors mediated the prejunctional inhibition of periarterial nerve-stimulated norepinephrine (NE) release and the postjunctional potentiation of the increase in perfusion pressure elicited by vasoconstrictors. These receptors were designated Y2 and Y1, respectively, based on the ability of C-terminal fragments to mimic the former action. The present study investigates further the involvement of these putative receptor subtypes in the isolated and perfused mesenteric arterial bed. [Leu31Pro34]NPY, a novel analog with specificity at the Y1 receptors, potentiated the increase in perfusion pressure elicited by exogenously administered NE and arginine vasopressin, confirming the existence of this NPY subtype postjunctionally. This immediate and prolonged potentiation was abolished by phentolamine, attenuated by benextramine and the reputed NPY antagonist, PYX1. [11-36]NPY also produced a concentration-dependent potentiation of NE-stimulated increase in perfusion pressure suggesting that the Y2 receptor subtype may also be present postjunctionally in this model of the vascular neuroeffector junction. The finding that the profile of this potentiation differed from that elicited by [Leu31Pro34]NPY and, in contrast to the latter, was not attenuated by PYX1, intimates the existence of both distinct subtypes postjunctionally. [Leu31Pro34]NPY also reduced periarterial nerve-stimulated release of NE with a concomitant reduction in perfusion pressure indicating, in addition to the Y2 subtype, the presence of the Y1 receptor prejunctionally in the rat mesenteric arterial bed.

Immunological detection of glutamate receptor subtypes in human central nervous system

Glutamate receptors are the principal excitatory neurotransmitter receptors in the central nervous system and are involved in a number of normal and pathological neuronal processes. Using subunit-specific antipeptide antibodies developed against the predicted amino acid sequences of several rat glutamate receptor cDNAs, we have identified these proteins in post-mortem human central nervous system tissue. Immunoblotting of dissected brain regions demonstrates that these receptor proteins are differentially distributed. The ability to identify these proteins in post-mortem human tissues should allow examination of the changes in levels of receptor subtypes that occur in a variety of neurological and psychiatric diseases.

Molecular cloning of a cDNA encoding a novel member of the mouse glutamate receptor channel family

The primary structure of a novel putative subunit of the mouse glutamate receptor channel, designated as delta 1, has been deduced by cloning and sequencing the cDNA. The delta 1 subunit shows 21-25% amino acid sequence identity with previously characterized rodent glutamate receptor channel subunits and thus may represent a new subfamily of the glutamate receptor channel.

Differences in neurokinin receptor pharmacology between rat and guinea-pig superior cervical ganglia

1. The depolarizations elicited by seven neurokinin receptor agonists were examined in both rat and guinea-pig superior cervical ganglia by use of grease-gap methodology in the presence of tetrodotoxin (0.1 microM). Responses were normalised with respect to 1 microM eledoisin. 2. The rank order of agonist potency in the rat ganglia was senktide greater than substance P greater than substance P methyl ester = eleidosin = Sar-Met-substance P greater than neurokinin B greater than neurokinin A, whereas in guinea-pig superior cervical ganglion (SCG) the rank order was senktide greater than Sar-Met-substance P greater than neurokinin B = eledoisin = substance P methyl ester. The concentration-effect curves for substance P and neurokinin A in guinea-pig ganglia were biphasic which precluded the determination of meaningful potency values. 3. The maximal depolarization achieved by subtype selective ligands was different between these two species. On rat and guinea-pig SCG, the NK3-selective ligand, senktide, produced a maximal depolarization of 27% and 274% respectively, whereas the NK1-selective ligand, substance P methyl ester, produced depolarizations of 77% and 64% respectively. 4. The depolarizations induced by substance P methyl ester and senktide in either species were unaffected by atropine (1 microM), suggesting a lack of involvement of presynaptic neurokinin receptors in the generation of the response. 5. The potency of substance P methyl ester, senktide, and neurokinin A were unaffected by pretreating ganglia with the peptidase inhibitors bacitracin (40 micrograms ml-1), leupeptin (4 micrograms ml-1), and chymostatin (2 micrograms ml-1). Similarly, these peptidase inhibitors had no effect on the maximal depolarizations achieved by any of these agonists.6. It is evident that rat and guinea-pig superior cervical ganglia possess both NK, and NK3 receptors, but that their net contribution to depolarizations are different between the two species. The depolarizations in guinea-pig SCG are mediated predominantly by an NK3 subtype and in rat SCG by an NK, receptor subtype.

Antagonists that demonstrate species differences in neurokinin-1 receptors

125I-Bolton-Hunter-substance P (125I-BH-SP) binding properties of three novel classes of neurokinin-1 (NK-1) receptor antagonists were investigated in tissues derived from humans, guinea pigs, and rats. 125I-BH-SP was shown to bind to a single class of binding sites, with similar dissociation constants, Kd, in human astrocytoma cells (U-373 MG), human urinary bladder, guinea pig forebrain, guinea pig ileum longitudinal smooth muscle, rat forebrain, and rat duodenum. In each tissue preparation, known peptide agonists and peptide antagonists yielded potencies typical for a NK-1 receptor profile, with little difference in binding properties between the various tissues. However, when the three classes of compounds, heterosteroids, cyanines, and modified peptides, were tested for their ability to displace 125I-BH-SP binding from the NK-1 receptor, very different binding profiles were observed. The heterosteroids were shown to be as much as 3 orders of magnitude more potent in tissues derived from rats than from humans or guinea pigs. A distinct species-dependent structure-activity relationship (SAR) was also observed for this class of compounds. Like the heterosteroids, the cyanines displaced 125I-BH-SP with 10-30-fold higher affinity in rat tissues than in human and guinea pig tissues. However, the SAR generated by the cyanines was comparable in all tissues studied. The modified peptides, on the other hand, were up to 10-100-fold more potent in human and guinea pig than rat tissues, producing a SAR that differed between the various species. No differences in binding properties between central nervous system and peripheral tissues from the same species were seen with these compounds. These results provide evidence for species differences in NK-1 receptors in humans, guinea pigs, and rats. Because it is known that there exists great sequence identity between rat and human NK-1 receptors, it is hypothesized that key amino acid changes or different lipid environments within the transmembrane binding region of the receptor may account for the observed species difference. Furthermore, this study emphasizes that caution is necessary in the choice of species to be used in development programs targeted towards therapeutic entities in the NK-1 receptor antagonist area.

Characterization of the tachykinin NK2 receptor subtype in the rabbit pulmonary artery

Contractile responses to neurokinin A (NKA), neuropeptide gamma(NP gamma), and the NK2 receptor-selective analogs [Lys5,MeLeu9,Nle10]NKA(4-10) and MDL 28,564 were determined in the endothelium-denuded rabbit pulmonary artery. Responses to NKA, NP gamma, and [Lys5,MeLeu9,Nle10]NKA(4-10) were antagonized by the NK2 receptor antagonist MDL 29,913, with pA2 values of 6.67, 6.46, and 7.32, respectively. Autoradiographic studies failed to demonstrate any specific binding sites for [125I]-iodohistidyl NKA (INKA) over the pulmonary artery. These data suggest the presence in rabbit pulmonary artery of an unusual "nonclassical" NK2 receptor subtype, which appears to lack affinity for INKA.

Putative novel bradykinin B3 receptors in the smooth muscle of the guinea-pig taenia caeci and trachea

The bradykinin receptors mediating contraction in smooth muscle of the guinea-pig taenia caeci were compared with the proposed novel B3 receptors of the guinea-pig trachea. The activities of several antagonists in functional and binding studies were found to be very similar between these two guinea-pig preparations, but pKBs were markedly lower than in a number of typical B2 preparations from other species, suggesting that the characteristics of the proposed B3 receptor may be in part species-related.

Non-selectivity of new bradykinin antagonists for B1 receptors

Two new B1 receptor antagonists, [Hyp3,Thi5,DTic7,Oic8]desArg9-BK and DArg[Hyp3,Thi5,DTic7,Oic8]desArg9-BK were tested in vitro on the rabbit jugular vein and the guinea pig ileum (preparations containing B2 receptors) and on the rabbit aorta (preparation containing B1 receptors) for pharmacological characterization. The results indicate that both compounds are antagonists on both B1 and B2 receptors, are competitive and discriminate between B2A and B2B receptor subtypes.

Evidence for tachykinin NK-2B-like receptors in guinea-pig alveolar macrophages

Mammalian tachykinins dose-dependently activate guinea-pig alveolar macrophages, by interacting with tachykinin NK-2 receptors, mainly. By evaluating the effects of different NK-2 tachykinin receptor antagonists, we now provide evidence that tachykinin NK-2 receptors in guinea-pig alveolar macrophages meet the pharmacological criteria used to define the NK-2B subtype.

Anti-idiotypic antibodies against the kinin receptor

Three sets of monoclonal antibodies against bradykinin (MBK1, MBK2, MBK3) were generated by somatic cell fusion, characterized by their peptide specificity and compared to the known ligand specificity of the kinin receptor subtypes. By these criteria the paratope of MBK3 resembled the B2 receptor binding site whereas MBK1 shared principal binding characteristics with the B1 recrptor. Anti-idiotypic antibodies against MBK1, MBK2 and MBK3 were raised in rabbit and sheep. Specificity of the network components was verified by inhibition experiments on the level of peptide, idiotype and anti-idiotype. Anti-idiotypic antibodies against MBK3 recognized a conformation-dependent epitope which was binding site-related. Binding studies on human foreskin fibroblasts and guinea pig ileum showed mutual displacement of the anti-idiotypic antibody and bradykinin at the binding site pointing to a specific interaction of the antibody with the receptor from various species. An agonist activity of the antibodies, demonstrated in human (inositolphosphate pathway) and mouse (prostaglandin pathway) fibroblasts indicated that the anti-idiotypes bear an internal image of the ligand epitope. This molecular mimicry which was further substantiated by the detection of bradykinin specific anti-idiotypic antibodies, provides the structural basis for the observed cross-reactivity over species borders.

Cobalt blocks postsynaptic responses induced by neurotransmitters in the hippocampus in vitro

Divalent metals such as cobalt are frequently used by neurophysiologists to prevent synaptic transmission, because they are thought to selectively block presynaptic calcium conductance. Recording intracellularly from hippocampal CA3 pyramidal cells we show that Co2+ (2 mM) is not specific in this action but also diminishes postsynaptic responses mediated by agonists acting at ionotropic and metabotropic glutamatergic receptors, as well as GABAA, GABAB, adenosine, and cholinergic receptors. These findings indicate that a more selective substance should be employed for experiments where neurotransmitter release must be blocked.

Two distinct receptor subtypes for mammalian bombesin-like peptides

The mammalian bombesin-like peptides, gastrin-releasing peptide (GRP) and neuromedin B (NMB), are structurally related neuropeptides that elicit a wide spectrum of biological activities including regulation of smooth muscle contraction, stimulation of secretion, modulation of neural activity, and growth regulation. Earlier studies have shown that GRP and NMB are expressed in different regions of both the CNS and peripheral organs. Recent ligand-binding and molecular-cloning studies have revealed two pharmacologically distinct G-protein-coupled receptor subtypes for mammalian bombesin-like peptides that have different relative affinities for GRP, NMB and bombesin receptor antagonists. Similar to the peptide ligands, the two receptor subtypes are expressed in a distinct but overlapping set of CNS regions, some of which have been identified in functional studies as sites where bombesin peptides elicit defined biological responses. Delineation of these peptide ligands and receptor subtypes will be important in future studies that explore the molecular basis for the heterogeneous nature of the responses to bombesin observed in mammalian systems.

Inhibitory role on gastric secretion of a central NK-3 tachykinin receptor agonist, senktide

When administered intracerebroventricularly, the highly selective NK-3 tachykinin receptor agonist senktide possesses a potent and dose-related inhibitory effect on gastric acid secretion. The central mechanism governing the antisecretory effect of senktide was examined in perfused-stomach rats by studying its influence on gastric acid secretion elicited by the secretagogues histamine, pentagastrin and bethanechol. Given intracerebroventricularly, senktide reduced the acid response to histamine, but not that to pentagastrin or bethanechol. Stimulation of NK-3 receptors in rat brain thus appears to inhibit gastric acid secretion through histaminergic pathways.

Analysis of neuropeptide Y-induced feeding: dissociation of Y1 and Y2 receptor effects on natural meal patterns

To differentiate NPY receptor subtypes, Y1 and Y2, in terms of their impact on feeding behavior, the intact molecule NPY(1-36) and the 3 fragments, NPY(2-36), the Y1 agonist [Leu31,Pro34]NPY, and the Y2 agonist NPY(13-36), were injected (100 pmol/0.3 microliters) into the hypothalamic paraventricular nucleus (PVN) of freely feeding rats. A computer-automated data acquisition system was employed in these experiments to permit a detailed analysis of feeding over the 12-h nocturnal cycle, in animals maintained on pure macronutrient diets. The results demonstrate that: 1) NPY(1-36) potentiates feeding behavior, primarily carbohydrate ingestion, by increasing the size and duration of the first meal after injection, rather than by affecting meal number of feeding rate, suggesting that NPY acts through mechanisms of satiety. The potentiation of carbohydrate intake occurs in association with a suppression of protein intake, which is strongest during the second meal after injection and which further increases the proportion of carbohydrate in the diet. No changes in fat ingestion are seen. 2) NPY(2-36), with the N-terminal tyrosine residue deleted, is equally potent to NPY(1-36) in potentiating carbohydrate intake and increasing meal size; however, it is less selective than NPY(1-36), producing an additional, smaller increase in consumption of protein. 3) The stimulatory effect of these peptides on carbohydrate intake and meal size is similarly observed, with somewhat reduced potency, after PVN injection of the selective Y1 agonist [Leu31,Pro34]NPY which, like NPY(1-36), also reduces protein intake. 4) The Y2 receptor agonist, NPY(13-36), causes a decrease in the ingestion of carbohydrate, a smaller decline in protein intake, and a reduction in meal size. It is proposed that hypothalamic Y1 receptors mediate the stimulatory effect of NPY on carbohydrate intake and meal size, while Y2 receptors have the opposite effect of suppressing carbohydrate intake, possibly by altering presynaptic release of monoamines known to influence nutrient ingestion.