Different neuropeptide Y receptor subtypes in rat and rabbit vas deferens

Prejunctional neuropeptide Y (NPY) receptors that inhibit the contractions evoked in rat and rabbit vas deferens by field stimulation were investigated by using NPY, [Leu31,Pro34]NPY and the fragments, NPY-(13-36) and NPY-(18-36). NPY, and especially [Leu31,Pro34]NPY, were more potent agonists on the twitch response of the rabbit vas deferens. In contrast the NPY C-terminal fragments, NPY-(13-36) and NPY-(18-36), inhibited the twitch response at lower concentrations in the rat vas deferens. These results indicate that distinct NPY receptor subtypes mediate the biological effect in these two tissues. We suggest that prejunctional receptors in the rat vas deferens are of the Y2-subtype and those in rabbit vas deferens of the Y1-subtype.

Differential regulation of cerebellar granule neurons by two types of quisqualate receptors

Stimulation of the quisqualate (QA) subtype of glutamate receptor increased the expression of phosphate activated glutaminase (needed for neurotransmitter glutamate synthesis) and the ability to release neurotransmitter glutamate in cultures of glutamatergic cerebellar granule neurons. In contrast, QA had no significant effects on the lactate dehydrogenase activity and amount of protein. The QA-mediated elevation in glutaminase activity was blocked by the ionotropic QA receptor antagonist CNQX and mimicked by the ionotropic QA receptor agonist AMPA, but not by the metatropic QA receptor agonist t-ACPD. The increase in Ca2+ influx essentially through activation of L-type channels, and not the mobilization of internal Ca2+ stores, was responsible for these QA receptor-mediated long-term changes in cerebellar granule neurons.

Antagonist binding reveals two heterogenous B2 bradykinin receptors in rat myometrial membranes

In rat myometrial membranes, two bradykinin binding sites with K1 values of 18 pM and 5.6 nM were identified. Three potent bradykinin antagonists were tested for their ability to compete for [3H]bradykinin binding. Two of them, D-Arg[Hyp3,Thi5.8,D-Phe7]bradykinin and [Hyp3,Thi5.8,D-Phe7]bradykinin, also bound to both the high- (KH) and the low-affinity (KL) site whereas [Thi5.8,D-Phe7]bradykinin identified only the low-affinity bradykinin receptor. There is a close correlation between the antagonistic potencies and the KL site affinities.

[Current topics on glutamate receptors]

Glutamate (Glu) receptors are classified into two major categories in the mammalian central nervous system: inotropic receptors linked to ion channels and metabotropic receptors linked to phosphatidylinositol (PI) metabolism. Classification of the inotropic Glu receptors is based on the differential sensitivity to excitement by N-methyl-D-aspartic acid (NMDA), DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and kainic acid (KA). The NMDA-sensitive subclass is supposed to be a receptor ionophore complex consisting of at least four different subcomponents, including an NMDA recognition site, a glycine (Gly) recognition site, a polyamine recognition site and a cation channel. The NMDA site is radiolabeled by both Glu and competitive antagonists, such as (+-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) and DL-(E)-2-amino-4-propyl-5-phosphono-3-pentenoic acid (CGP 39653). The Gly domain, which is labeled by both [3H]Gly and [3H]5,7-dichlorokynurenic acid, is sensitive to D-serine but insensitive to strychnine, and this domain seems to be absolutely required for an activation of the NMDA channel by agonists. The ionophore domain is identified by radiolabeled non-competitive NMDA antagonists that gain access to the binding sites within the channel only when it is gated by agonists. The opening of an NMDA channel is allosterically potentiated by Gly and several polyamines. In contrast, an activation of the NMDA channel is blocked by both H+ and divalent cations such as Mg2+ and Zn2+. [3H]AMPA binding displays pharmacological profiles of the AMPA-sensitive subclass with a rank order of agonistic potencies of quisqualic acid (QA) greater than or equal to AMPA greater than Glu greater than KA, which is apparently different from that found for the KA-sensitive subclass (domoic acid greater than or equal to KA greater than QA greater than Glu). In contrast, several quinoxaline derivatives competitively antagonize neuronal responses mediated not only by the AMPA receptor but also by the KA receptor. The metabotropic Glu receptors, which stimulate PI metabolism through an activation of the guanosine triphosphate-binding proteins, are activated by Glu, QA and trans-1-amino-cyclopentyl-1,3-di-carboxylic acid (ACPD). Responses mediated by the metabotropic receptors are competitively blocked by 2-amino-3-phosphonopropionic acid. Three or four cloned complementary deoxyribonucleic acids (cDNAs) encoding inotropic Glu receptors are isolated from a rat brain cDNA library. Pharmacological and electrophysiological properties of receptor-ion channels encoded by a transfection of these cDNAs are similar to those observed with the AMPA receptor as well as the KA receptor, but not with the NMDA receptor.

Tachykinin receptors in the guinea-pig isolated bronchi

The aim of the study was to assess which tachykinin receptors mediate the contractile response in the guinea-pig isolated bronchi. Experiments with natural tachykinins and receptor-selective tachykinin agonists were performed in the absence or presence of peptidase inhibitors and in bronchi pretreated with phenoxybenzamine. Both NK-1 (substance P, substance P methylester and septide) and NK-2 (neurokinin A, [beta-Ala8]neurokinin A-(4-10) and MDL 28,564) receptor agonists produced concentration-dependent contraction. NK-3 agonists (senktide and [MePhe7]neurokinin B) were active only at high concentrations. Phenoxybenzamine pretreatment reduced the maximal response to NK-1 agonists and produced a rightward shift of the curve to NK-2 agonists, without depression of the maximum. Five tachykinin antagonists selective for the NK-1 (L 668,169) or the NK-2 (MEN 10,207, MEN 10,376, L 659,877 and R 396) receptor were tested against substance P methylester and [beta-Ala8]neurokinin A-(4-10). The results indicated that these receptor-selective antagonists maintain their characteristic even when tested in a multireceptor assay such as the guinea-pig bronchus. The rank order of potency of NK-2 antagonists against [beta-Ala8]neurokinin A-(4-10) was MEN 10,207 = MEN 10,376 greater than L 659,877 much greater than R 396. This pattern, with the observation of the full agonist activity of MDL 28,564, indicates that in addition to NK-1 receptors, NK-2 receptors also are present in the guinea-pig bronchi and belong to the same subtype (NK-2A) as present in the rabbit pulmonary artery.

Reduction of desensitization of a glutamate ionotropic receptor by antagonists

The glutamate receptor channel subtype that responds to both quisqualate (QA) and alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) was expressed in Xenopus oocytes injected with rat cerebral cortex mRNA. Voltage-clamp current responses to QA, AMPA, and glutamate (GLU) exhibited a rapid increase followed by a decrease to a desensitized steady state (DS). Perfusion with high agonist concentrations produced smaller DS responses than perfusion with low concentrations. During the DS, the current was increased by lowering of the concentration of agonist or by application of low concentrations of a competitive antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX). This paradoxical increase of the agonist-induced currents during the DS was also observed in cultured Purkinje cells with another competitive antagonist, 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX). Dose-response curves obtained in oocytes were bell shaped, with a negative slope for high concentrations of QA. DNQX shifted these bell-shaped curves to the right. Together, these results indicate that the agonists are able to reversibly inhibit the AMPA receptor. The classical desensitization model of Katz and Thesleff [J. Physiol. (Lond.) 138:63-80 (1957)] cannot account for our observations.

Evidence that cultured airway smooth muscle cells contain bradykinin B2 and B3 receptors

We examined bradykinin-induced 45Ca2+ efflux and prostaglandin synthesis in guinea pig tracheal smooth muscle cells maintained in tissue culture. We also studied the effects of a B1 receptor agonist and antagonist, a B2 receptor antagonist, and the cyclooxygenase inhibitor indomethacin. In cultured smooth muscle cells, bradykinin (0.1 nM to 10 microM) stimulated efflux of 45Ca2+ and induced the synthesis of prostaglandin E2 and the prostacyclin metabolite 6-keto-prostaglandin F1 alpha. DesArg9-bradykinin, a B1 receptor agonist, had no effect on 45Ca2+ efflux or prostaglandin synthesis, and no responses to bradykinin were unaffected by the B1 receptor antagonist desArg9-[Leu8]-bradykinin. Indomethacin (1 microM) abolished bradykinin-induced prostaglandin synthesis but was without effect on 45Ca2+ efflux. NPC 567 (DArg[Hyp3,DPhe7]-bradykinin), a B2 receptor antagonist, had no effect on bradykinin-induced 45Ca2+ efflux, but abolished prostaglandin synthesis. Unlike in membranes prepared freshly from guinea pig tracheal smooth muscle, the B2 receptor antagonist inhibited completely (Ki, 12 nM) binding of [3H]-bradykinin to membranes prepared from cultured tracheal smooth cells. These data suggest that tracheal smooth muscle cells, maintained in culture, express B2 receptors that mediate bradykinin-induced prostaglandin synthesis. The observation that bradykinin-induced efflux of calcium ions was unaffected by B1 or B2 antagonists provides further evidence that airway smooth muscle may contain a novel B3 receptor.

Glutamate and GABA receptors in vertebrate glial cells

Glial cells of the central nervous system express receptors for the main inhibitory and excitatory neurotransmitters, GABA and glutamate. The glial GABA and glutamate receptors share many properties with the neuronal GABAA and kainate/quisqualate receptors, but are molecularly and, in some aspects, pharmacologically distinct from their neuronal counterparts. The functional role of these receptors is as yet speculative: They have been proposed to control proliferation of astrocytes, serve to balance ion changes at GABAergic synapses, or they could enable the glial cell to detect neuronal synaptic activity.

Novel selective agonists and antagonists confirm neurokinin NK1 receptors in guinea-pig vas deferens

1. This study investigated the recognition characteristics of neurokinin receptors mediating potentiation of the contractile response to field stimulation in the guinea-pig vas deferens. 2. A predominant NK1 receptor population is strongly suggested by the relative activities of the common naturally-occurring tachykinin agonists, which fall within less than one order of magnitude. This conclusion is supported by the relative activities of the synthetic NK1 selective agonists substance P methyl ester, [Glp6,L-Pro9]-SP(6-11) and delta-aminovaleryl-[L-Pro9,N-MeLeu10]- SP(7-11) (GR73632) which were 0.78, 9.3 and 120 as active as substance P, respectively. Furthermore, the NK2 selective agonist [Lys3, Gly8,-R-gamma-lactam-Leu9]-NKA(3-10) (GR64349) was active only at the highest concentrations tested (greater than 10 microM), and the NK3 selective agonist, succ-[Asp6,N-MePhe8]-SP(6-11) (senktide) was essentially inactive (10 nM-32 microM). 3. [D-Arg1,D-Pro2,D-Trp7,9,Leu11]-SP(1-11) antagonized responses to neurokinin A, neurokinin B, physalaemin, eledoisin, [Glp6,D-Pro9]-SP(6-11), GR73632 and GR64349 (apparent pKB s 5.6-6.2), but was less potent in antagonizing responses to substance P, substance P methyl ester and [Glp6,L-Pro9]-SP(6-11) (apparent pKB s less than or equal to 5.0-5.0). 4. In contrast, the recently developed NK1-selective receptor antagonist [D-Pro9[Spiro-gamma-lactam]Leu10,Trp11]-SP(1-11) (GR71251) did not produce agonist-dependent pKB estimates. Schild plot analysis indicated a competitive interaction with a single receptor population where the antagonist had an estimated overall pKB of 7.58 +/- 0.13 for the four agonists of differing subtype selectivity tested (GR73632, GR64349, substance P methyl ester and neurokinin B). This estimate is similar to that we obtained for NK1-mediated (substance P methyl ester) contraction in the guinea-pig ileum preparation (pKB= 7.86+ 0.05). 5. Tachykinin action appears not to depend on release of a number of intermediary mediators including acetylcholine, histamine or cyclo-oxygenase products, nor to involve interaction with neuronal mechanisms including alpha 2-adrenoceptor feedback, noradrenergic Uptake-I or opioid-release, since antagonism or inhibition of these mechanisms did not modify responses to tachykinins. 6. We conclude that tachykinin action in the field-stimulated guinea-pig vas deferens preparation is mediated through interaction with a predominant neurokinin NK, receptor population and this preparation can therefore be used to study NK, modulation of sympathetic neurotransmission.

Autoradiographic characterization of [3H]L-glutamate binding sites in developing mouse cerebellar cortex

Postnatal changes of [3H]L-glutamate binding sites in mouse cerebellum were studied by in vitro autoradiography. These sites were already present at birth, their density globally increased until postnatal day 25, and at all ages it was higher when Cl- and Ca2+ were present in the incubation buffer. At birth, these binding sites were diffused through the whole cerebellar mass, but became distinctly concentrated in the molecular and the internal granular layers by postnatal day 10. From this age on, binding site sensitivity to ions and glutamate analogues takes a different course in each layer. The external granular layer and the white matter never displayed significant amounts of binding. In the molecular layer the Cl-/Ca2+ effect increased during ontogeny until, in adults, the ion-dependent binding was threefold higher than the ion-independent binding. Quisqualate-sensitive sites accounted for 80% of the total binding sites already at postnatal day 15, while displacement by alpha-amino-3-hydroxy-methyl-4-isoxazolepropionic and ibotenic acids attained the maximum (68%) at postnatal day 60. N-Methyl-D-aspartate displaced glutamate binding (50%) only in the presence of Cl- and Ca2+. Starting from postnatal day 15, binding site density in the molecular layer of lobules VIb and VII of the vermis was lower than in other lobules. In the internal granular layer, the Cl-/Ca2+ effect observed in young animals decreased during development. These transient binding sites were sensitive to quisqualic and ibotenic acid. In adults, the majority of glutamate binding sites were ion-independent and mainly sensitive to D,L-amino-5-phospho-valeric acid and N-methyl-D-aspartate. Throughout development and in both layers, sites displaced by kainate were present at low density and sites displaced by D,L-2-amino-4-phosphonobutyric acid were not detected. The localized postnatal changes of the [3H]L-glutamate binding sites were correlated with the events occurring during growth and maturation of cerebellar structures. The increase of the Cl-/Ca(2+)-dependent binding in the molecular layer is simultaneous with the growth of Purkinje cell dendrites and of parallel fibres and with the formation of the synapses between them. This suggests that these binding sites are localized in these synapses. The changing pattern of sensitivity to different agonists during development might correspond to the maturation of these synapses. The low density of [3H]L-glutamate binding in the molecular layer of lobules VIb and VII probably indicates the presence of specific nerve projections to these areas.(ABSTRACT TRUNCATED AT 400 WORDS)

Tachykinin receptor subtypes involved in the central effects of tachykinins on water and salt intake

The present study was aimed at investigating which tachykinin receptor subtypes mediate the inhibitory effects of tachykinins a) on salt intake induced by sodium depletion, b) on water intake induced by subcutaneous hypertonic NaCl administration and c) on water intake induced by central angiotensin II injection. The study was carried out by evaluating the potency of action, following intracerebroventricular injection, of several peptides, including both naturally occurring tachykinins and synthetic peptides selective for a given receptor subtype. The results obtained show different rank orders of potency of the agonists in the different behavioral tests, thus suggesting that different receptor subtypes are involved in the effects of tachykinins on water and salt intake. NK-3 receptors appear to be involved in the inhibitory effect of tachykinins on depletion-induced salt appetite. NK-2 receptors apparently mediate the inhibitory effect of tachykinins on drinking induced by hyperosmotic NaCl administration, while NK-1 receptors are probably involved in the inhibition of angiotensin II-induced drinking.

Progress in the development of potent bombesin receptor antagonists

Bombesin and the mammalian-related peptides gastrin-releasing peptide (GRP), GRP and neuromedin B have been shown to have numerous actions in the CNS, gastrointestinal tract and on growth. However, the role of the peptides in various physiological processes has remained unclear because of the lack of potent antagonists. Recent in vitro studies have described four different classes of bombesin receptor antagonist, some of which are active in the nanomolar range and in vivo. Robert Jensen and David Coy describe recent insights into peptide structural determinants of biological activity. Evidence from structure-function studies have resulted in identification of some analogues that function as potent antagonists in all systems examined. Furthermore, various subtypes of bombesin receptors can now be differentiated by these various classes of antagonist.

Cerebellar excitatory amino acid binding sites in normal, granuloprival, and Purkinje cell-deficient mice

Using quantitative autoradiography, the cellular localization and characterization of cerebellar excitatory amino acid binding sites in normal, Purkinje cell-deficient and granuloprival (granule cell-deficient) mouse cerebella were investigated. In the molecular layer of normal mouse cerebellum, the quisqualate subtype of excitatory amino acid receptor (assayed by [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, quisqualate-sensitive L-[3H]glutamate, and [3H]6-cyano-7-nitroquinoxaline-2,3-dione binding) predominated. In the granule cell layer of the cerebellum, N-methyl-D-aspartate-sensitive L-[3H]glutamate and [3H]glycine binding sites were predominant. In the molecular layer of Purkinje cell-deficient mutant mice, [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding sites and [3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding were reduced to 24% (P less than 0.01) and 36% (P less than 0.001) of control, respectively, while quisqualate-sensitive [3H]glutamate binding sites were reduced to 54% of control (P less than 0.01). N-Methyl-D-aspartate-sensitive [3H]glutamate and [3H]glycine binding were unchanged. In the granule cell layer of these mouse cerebella, there was no change in excitatory amino acid receptor binding. In the molecular layer of granuloprival mouse cerebella, [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding was increased to 205% of control (P less than 0.01), [3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding was increased to 136% of control (P less than 0.02), and quisqualate-sensitive [3H]glutamate binding was increased to 152% of control (P less than 0.01). N-Methyl-D-aspartate-sensitive [3H]glutamate and [3H]glycine binding were unchanged. In areas of granule cell depletion N-methyl-D-aspartate-sensitive [3H]glutamate and [3H]glycine binding were reduced to 68% (P less than 0.01) and 59% (P less than 0.01) of control, respectively. In the granule cell layer, binding to quisqualate receptors was not significantly different from binding in controls with any of the ligands tested. These results suggest that three different receptor assays: [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, quisqualate-sensitive L-[3H]glutamate, and [3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding can be used to demonstrate that quisqualate receptor specific binding sites are located on Purkinje cell dendrites in the molecular layer of cerebellum, and that these binding sites apparently up-regulate in response to granule cell ablation and Purkinje cell deafferentation.