The rabbit saphenous vein: a tissue preparation specifically enriched in NPY-Y1 receptor subtype

Neuropeptide Y-induced enhancement of the evoked release of newly synthesized dopamine in rat striatum: Mediation by Y2 receptors

The purpose of the present study was to determine whether or not activation of neuropeptide Y (NPY) receptors resulted in an enhancement or attenuation of the KCl (50 mM) evoked release of [3H]dopamine newly synthesized from [3H]tyrosine in superfused striatal slices and, if so to identify the NPY receptor subtype mediating the effect. Rat striatal slices were prepared and placed in microsuperfusion chambers and continuously superfused with physiological buffer containing 50 microCi/ml of l-3-5-[3H]tyrosine. Superfusate effluents were collected and analyzed for [3H]dopamine by liquid scintillation spectrometry following amberlite CG50 and alumina chromatography. NPY agonists (NPY and PYY3-36) were added 6 min prior to the addition of KCl, while the Y1, Y2, and Y5 antagonist BIBO3304, BIIE0246 and CGP71683A, respectively were added 6 min prior to the agonists. Continuous superfusion with [3H]tyrosine resulted in the production of [3H]dopamine which reached a steady state at approximately 48 min. Depolarization with KCl resulted in a 2- to 3-fold increase in [3H]dopamine overflow. NPY and PYY3-36 produced a concentration dependent enhancement in the KCl induced increase in newly synthesized [3H]dopamine overflow. The Y2 antagonist BIIE0246 produced an attenuation of both the NPY and PYY3-36 induced enhancement while the Y1 antagonist BIBO3304 and theY5 antagonist CGP71683A failed to alter the NPY or PYY3-36 induced enhancement. These results are consistent with the NPY-Y2 receptor subtype mediating the facilitatory effect.

Marked neuropeptide Y-induced contractions via NPY-Y1 receptor and its desensitization in rat veins

The aim of this study was to investigate neuropeptide Y (NPY)-induced vasoconstrictions in rat blood vessels and which NPY receptor subtype is involved in vasoconstrictions. NPY produced marked contractions in rat common jugular, brachial, portal, femoral and tail veins, and vena cava inferior, whereas it produced little or no contractions in rat common carotid, brachial, femoral and tail arteries, and thoracic and abdominal aortae. The maximal NPY-induced contractions were larger than maximal phenylephrine (PE)-induced contractions in the veins. These NPY-induced contractions were blocked by the Y1 antagonists, SRL-21, and BIBP3226 but not by the Y5 antagonist, L-152804. A Y2 agonist, NPY (13-36), did not produce contractions. RT-PCR showed that NPY-Y1 was the only receptor subtype in the veins indicating that NPY-induced contractions are mediated through the Y1 receptor. Pretreatment with NPY showed a rapid and long-lasting desensitization of these contractions. The marked NPY-induced contractions and its desensitization in the veins suggest the physiological relevance of NPY in the venous circulation.

Neuropeptide Y is a cotransmitter with norepinephrine in guinea pig inferior mesenteric vein

Neuropeptide Y (NPY) is a cotransmitter with noradrenaline in guinea pig inferior mesenteric vein. Tyrosine hydroxylase-like immunoreactivity and NPY-like immunoreactivity were colocalized in a dense network of fibers within the adventitial layer of guinea-pig inferior mesenteric vein. Vasoconstrictor responses to electrical field stimulation (0.2-64 Hz, 0.1 ms, 12 V, for 10 s) appear to be mediated primarily by norepinephrine at 0.2 to 4 Hz and by NPY at 8 to 64 Hz. NPY Y1 receptors mediate the contractile responses to both endogenous and exogenous NPY. Norepinephrine and NPY are involved in neuromuscular transmission in guinea pig mesenteric vein suggesting that the sympathetic nervous system requires the coordinated action of norepinephrine and NPY to serve capacitance.

BIIE0246, a potent and highly selective non-peptide neuropeptide Y Y(2) receptor antagonist

1. BIIE0246, a newly synthesized non-peptide neuropeptide Y (NPY) Y(2) receptor antagonist, was able to compete with high affinity (8 to 15 nM) for specific [(125)I]PYY(3 - 36) binding sites in HEK293 cells transfected with the rat Y(2) receptor cDNA, and in rat brain and human frontal cortex membrane homogenates. 2. Interestingly, in rat brain homogenates while NPY, C2-NPY and PYY(3 - 36) inhibited all specific [(125)I]PYY(3 - 36) labelling, BIIE0246 failed to compete for all specific binding suggesting that [(125)I]PYY(3 - 36) recognized, in addition to the Y(2) subtype, another population of specific NPY binding sites, most likely the Y(5) receptor. 3. Quantitative receptor autoradiographic data confirmed the presence of [(125)I]PYY(3 - 36)/BIIE0246-sensitive (Y(2)) and-insensitive (Y(5)) binding sites in the rat brain as well as in the marmoset monkey and human hippocampal formation. 4. In the rat vas deferens and dog saphenous vein (two prototypical Y(2) bioassays), BIIE0246 induced parallel shifts to the right of NPY concentration-response curves with pA(2) values of 8.1 and 8.6, respectively. In the rat colon (a Y(2)/Y(4) bioassay), BIIE0246 (1 microM) completely blocked the contraction induced by PYY(3 - 36), but not that of [Leu(31), Pro(34)]NPY (a Y(1), Y(4) and Y(5) agonist) and hPP (a Y(4) and Y(5) agonist). Additionally, BIIE0246 failed to alter the contractile effects of NPY in prototypical Y(1) in vitro bioassays. 5. Taken together, these results demonstrate that BIIE0246 is a highly potent, high affinity antagonist selective for the Y(2) receptor subtype. It should prove most useful to establish further the functional role of the Y(2) receptor in the organism.

Food intake regulation in rodents: Y5 or Y1 NPY receptors or both?

Neuropeptide Y (NPY), one of the most abundant peptides in rat and human brains, appears to act in the hypothalamus to stimulate feeding. It was first suggested that the NPY Y1 receptor (Y1R) was involved in feeding stimulated by NPY. More recently a novel NPY receptor subtype (Y5R) was identified in rat and human as the NPY feeding receptor subtype. There is, however, no absolute consensus since selective Y1R antagonists also antagonize NPY-induced hyperphagia. Nevertheless, new anti-obesity drugs may emerge from further pharmacological characterization of the NPY receptors and their antagonists. A large panel of Y1R and Y5R antagonists (such as CGP71683A, BIBO3304, BIBP3226, 1229U91, and SYNAPTIC and BANYU derivatives but also patentable in-house-synthesized compounds) have been evaluated through in vitro and in vivo tests in an attempt to establish a predictive relationship between the binding selectivity for human receptors, the potency in isolated organs assays, and the inhibitory effect on food intake in both normal and obese hyperphagic rodents. Although these results do not allow one to conclude on the implication of a single receptor subtype at the molecular level, this approach is crucial for the design of novel NPY receptor antagonists with potential use as anti-obesity drugs and for evaluation of their possible adverse peripheral side effects, such as hypotension.Key words: obesity, weight reduction, food intake, neuropeptide Y, rodents.

Potent and selective tools to investigate neuropeptide Y receptors in the central and peripheral nervous systems: BIBO3304 (Y1) and CGP71683A (Y5)

We have evaluated 3 newly developed neuropeptide Y receptor antagonists in various in vitro binding and bioassays: BIBO3304 (Y1), T4[NPY33-36]4 (Y2), and CGP71683A (Y5). In rat brain homogenates, BIBO3304 competes for the same population of [125I][Leu31,Pro34] peptide YY (PYY) binding sites (75%) as BIBP3226, but with a 10 fold greater affinity (IC50 of 0.2 ± 0.04 nM for BIBO3304 vs. 2.4 ± 0.07 nM for BIBP3226),while CGP71683A has high affinity for 25% of specific [125I][Leu31,Pro34]PYY binding sites. Both BIBO3304 and CGP71683A (at 1.0 µM) were unable to compete for a significant proportion of specific [125I]PYY3-36/Y2 sites. The purported Y2 antagonist T4[NPY33-36]4 competed against [125I]PYY3-36 binding sites with an affinity of 750 nM. These results were confirmed in HEK 293 cells transfected with either the rat Y1, Y2, Y4, or Y5 receptor cDNA. BIBO3304, but not CGP71683A, competed with high affinity for [125I][Leu31,Pro34]PYY binding sites in HEK 293 cells transfected with the rat Y1 receptor cDNA, whereas the reverse profile was observed upon transfection with the rat Y5 receptor cDNA. Additionally, both molecules were inactive at Y2 and Y4 receptor subtypes expressed in HEK 293 cells. Receptor autoradiographic studies revealed the presence of [125I][Leu31,Pro34]PYY/BIBO3304-insensitive sites in the rat brain as reported previously for BIBP3226. Finally, the selective antagonistic properties of BIBO3304 were demonstrated in a Y1 bioassay (rabbit saphenous vein; pA2 value of 9.04) while being inactive in Y2 (rat vas deferens) and Y4 (rat colon) bioassays. These results confirm the high affinity and selectivity of BIBO3304 and CGP71683A for the Y1 and Y5 receptor subtypes, respectively, while the purported Y2 antagonist, T4[NPY33-36]4 possesses rather low affinity for this receptor.Key words: NPY receptor antagonist, receptor subtypes, bioassays, receptor binding assays, autoradiographic studies, receptor distribution.

NPY receptor subtype in the rabbit isolated ileum

1. The purpose of this work was to verify the hypothesis that the rabbit ileum is a selective preparation for the NPY Y5 receptor by using new selective antagonists recently synthesized. Spontaneous contractions of the rabbit isolated ileum were recorded and binding experiments were performed in cells expressing the human NPY Y1, Y2, Y4 or Y5 receptor subtype. 2. NPY analogues produced a concentration-dependent transient inhibition of the spontaneous contractions of the rabbit ileum with the following order of potency hPP > rPP > PYY > or = [Leu31,-Pro34]-NPY > NPY >> NPY13-36. Pre-exposure to rPP, PYY, [Leu31,Pro34]-NPY or NPY (but not NPY13-36) inhibited the effect of subsequent administration of hPP suggesting cross-desensitization of the preparation. The apparent affinity of the various agonists studied was correlated to the affinity reported for the human Y4 receptor subtype (and to a lesser extent for the rat Y4 subtype) but not to the affinity for the Y5 receptor subtype. 3. BIBO 3304, a selective NPY Y1 receptor antagonist, and CGP 71683A, a selective NPY Y5 receptor antagonist, did not affect the response to hPP. JCF 109, another NPY Y5 receptor antagonist, produced an inhibition of the response to hPP but only at the highest dose tested (10 microM) which also, by itself, produced intrinsic inhibitory effects. 4. 1229U91, a non-selective ligand for Y1, Y2, Y4 and Y5 receptors with high affinity toward the Y1 and Y4 receptor subtypes, produced a concentration-dependent transient inhibition of the spontaneous contractions of the rabbit ileum and a dose-dependent inhibition of the response to hPP (apparent pKB: 7.2). 5. These results suggest that in the rabbit ileum, the NPY receptor involved in the inhibition of the spontaneous contractile activity is a NPY Y4 receptor subtype.

NPY receptor subtypes involved in the contraction of the proximal colon of the rat

Experiments were designed to determine the receptor subtype(s) involved in the contraction of the rat proximal colon to NPY. In this tissue, mRNA of Y2 and Y4 NPY receptor subtypes were highly expressed, whereas Y5 mRNA levels were very low and Y1 mRNA levels were intermediate. NPY analogues induced contractions with the following order of potency: rPP > hPP = PYY = NPY = [Leu31,Pro34]NPY > NPY(2-36) = [D-Trp32]NPY > NPY(33-36). Responses to NPY, PYY and NPY(13-36) were not or partially affected by tetrodotoxin, in contrast to the responses to [Leu31,Pro34]NPY, rPP, hPP and [D-Trp32]NPY which were fully blocked. Atropine did not inhibit the contractions to NPY, PYY and [Leu31,Pro34]NPY but significantly affected those to NPY(13-36), [D-Trp32]NPY, rPP and hPP. The specific Y1 receptor antagonist BIBP 3226 was ineffective but JCF 104 and JCF 105 (two compounds with preferential affinity toward the hY5 receptor versus the hY1 or hY2 receptor) abolished the contractions provoked by the NPY analogues. These results suggest that NPY activates three receptor subtypes, a Y2 subtype possibly by a direct action on the smooth muscle cells, as well as a Y4 and a Y5 (or 'Y5-like') subtype which, respectively, release acetylcholine and an unknown neurotransmitter.

Neuropeptide Y Y1 receptors in vascular pharmacology

The existence of neurogenic mediator candidates apart from noradrenaline and acetylcholine involved in the control of vascular tone has attracted enormous attention during the past few decades. One such mediator is neuropeptide Y (NPY), which is co-localized with noradrenaline in sympathetic perivascular nerves. Stimulation of sympathetic nerves in vitro and in vivo causes non-adrenergic vasoconstriction which can be blocked by experimental manipulations that inhibit NPY mechanisms. Thus, the vasopressor response to stimulation of sympathetic nerves can be attenuated by chemical or surgical sympathectomy, treatment with reserpine or other pharmacological agents, and tachyphylaxis to NPY or by NPY antagonists. The NPY field was long plagued by a lack of specific antagonists, but with the recently developed, selective, non-peptide and stable NPY antagonists it has now become possible to study subtypes of this receptor family. For instance, it has become clear that the NPY Y1 receptor mediates most of the direct peripheral effects of NPY on vascular tone. These antagonists promise to stimulate NPY research and will likely unravel the true significance of NPY in cardiovascular control under physiological conditions as well as in pathophysiological states.

The dog saphenous vein: a sensitive and selective preparation for the Y2 receptor of neuropeptide Y

The dog saphenous vein responds to neuropeptide Y with a dose-dependent contraction and this vasopressor effect is not altered by the removal of the endothelium nor by the neuropeptide Y Y1 receptor antagonist, BIBP 3226 ((R)-N2-(diphenylacetyl)-N-[(n-hydroxyphenyl)methyl]-argininami de). The dose-response curves obtained with neuropeptide Y, peptide YY and with C-terminal fragments such as neuropeptide Y-(2-26), neuropeptide Y-(13-36) and peptide YY-(3-36) have similar slopes and maxima. EC50 values of these compounds vary between 30 +/- 10 and 89 +/- 47 nM. The neuropeptide Y Y1 receptor-selective agonist [Leu31,Pro34]neuropeptide Y and human pancreatic polypeptide are inactive up to 1 microM. This pharmacological profile suggests that the contraction of the dog saphenous vein induced by neuropeptide Y and its homologues or fragments is mediated by a neuropeptide Y Y2 receptor type. Moreover, this neuropeptide Y Y2 receptor appears to be localized in the venous smooth muscle, where it exerts a direct myotropic effect that may be useful for the pharmacological characterization of new compounds acting as agonists or antagonists of the neuropeptide Y Y2 receptor.

Neuropeptide Y release and contractile properties: differences between canine veins and arteries

During intense sympathetic activation, as occurs during hemorrhage, veins constrict to a greater degree than do arteries. This study determined if differences in the amounts or actions of the sympathetic cotransmitter neuropeptide Y released from perivascular nerves could contribute to these differences. Strips of canine mesenteric and popliteal arteries and of saphenous and portal veins were superfused, and the releases of noradrenaline and neuropeptide Y evoked by transmural stimulation were assessed. Both compounds were released in greater amounts in the veins than in the arteries. In other experiments rings of each vessel were mounted in organ chambers for isometric-tension recording. Neuropeptide Y (up to 10(-4) M) did not contract any vessel; however, at 3 x 10(-7) M it shifted the frequency-response and concentration-response curves to noradrenaline in the arteries only. In the veins neuropeptide Y had no postsynaptic effect on strong contractions. These results suggest that neuropeptide Y functions locally to affect vasoconstriction of the arteries studied, and may have a different role in the veins. Further, processes involving neuropeptide Y do not appear to account for the differences in responsiveness of these arteries as compared to the veins during intense sympathetic stimulation.

Characterization of neuropeptide Y (NPY) receptors in human cerebral arteries with selective agonists and the new Y1 antagonist BIBP 3226

1. We have characterized pharmacologically the receptor subtype(s) responsible for the neuropeptide Y (NPY)-induced vasoconstriction in human cerebral arteries. NPY, PYY and several of their derivatives with well defined affinities at the known Y1 and Y2 receptor subtypes were used. Moreover, we tested the ability of the new Y1 receptor antagonist, BIBP 3226, to antagonize the NPY-induced cerebral vasoconstriction. 2. NPY, PYY and their agonists with high affinities at the Y1 receptor subtype ([Leu31-Pro34]-NPY and [Leu31-Pro34]-PYY) elicited strong, long lasting and concentration-dependent contractions of human cerebral arteries. Compounds with Y2 affinity such as PYY3-36 or NPY13-36 either elicited a submaximal contraction at high concentrations or failed to induce any significant vasomotor response. Also, the application of NPY or the specific Y1 agonist, [Leu31-Pro34]-NPY, to human cerebral vessels pretreated with the Y1 agonist, NPY13-36, resulted in contractile responses identical to those obtained when these compounds were tested without prior application of NPY13-36. 3. The order of agonist potency at the human cerebrovascular receptor was: [Leu31-Pro34]-NPY = [Leu31-Pro34]-PYY > or = NPY > PYY > PYY3-36 > > > NPY13-36, which corresponded to that reported previously at the neuronal and vascular Y1 receptors. 4. Increasing concentrations (10(-9)-10(-6) M) of the Y1 receptor antagonist, BIBP 3226, to human cerebral vessels caused a parallel and rightward shift in the NPY dose-response curves without any significant change in the maximal contractile response. The calculated pA2 was 8.52 +/- 0.13, a value compatible with the reported affinity at the rodent and human Y1 receptor. 5. We conclude that Y1 receptors exclusively, mediate the NPY-induced contraction in human cerebral arteries and we show that BIBP 3226 is a potent and competitive antagonist of this YI-mediated vasoconstriction.

Discrimination by benextramine between the NPY-Y1 receptor subtypes present in rabbit isolated vas deferens and saphenous vein

1. In order to characterize the neuropeptide Y (NPY) Y1 receptors known to be present in rabbit isolated vas deferens and saphenous vein, the pharmacological activity of the selective NPY Y1 receptor agonists, [Leu31,Pro34] NPY and various other peptide agonists, together with the putative NPY antagonist, benextramine, were compared in the two tissues. 2. In rabbit isolated saphenous vein, cumulative dose-response curves to various NPY agonists were obtained. All the peptides tested caused contractions which developed quite slowly. The rank order of potency obtained was: PYY > NPY > [Leu31,Pro34] NPY = NPY2-36 > hPP >> NPY13-36 = NPY18-36. Incubation with benextramine (BXT) at 100 microM for 30 min irreversibly abolished the contractile response to [Leu31,Pro34] NPY but was ineffective against NPY18-36-induced contractions. 3. Cumulative dose-response curves to [Leu31,Pro34] NPY were performed in the same preparation before and after incubation with 100 microM BXT for 20 min in order to inactivate NPY Y1 receptors. The pKA (-logKA) estimation for [Leu31,Pro34] NPY was 7.60 +/- 0.30 using the operational model and 7.20 +/- 0.33 using the null method; the difference between the two methods was not statistically significant (P = 0.36). 4. Prostatic segments of rabbit vas deferens were electrically stimulated with single pulses. Immediately after stabilization of the contractile response, a cumulative dose-response curve to various NPY agonists was obtained in each tissue. The rank order of potency for twitch inhibition was: PYY> [Leu31,Pro34]NPY > NPY > hPP>NPY2- 36 >>NPY13-36>> NPY 18-36 which indicates the presence of a prejunctional NPY Y1 receptor. BXT at 100 microM incubated for 10 or 60 min did not antagonize the response to[Leu31,Pro34] NPY.5. We conclude that rabbit isolated saphenous vein contains a population of post-junctional NPY Y1 receptors irreversibly blocked by BXT, as well as a population of post-junctional NPY Y2 receptors,which are insensitive to BXT. In contrast, the rabbit isolated vas deferens express a pre-junctional NPYY1 receptor subtype which is not blocked by BXT. Tetramine disulphides such as BXT could be useful tools in classifying NPY receptors.

Neuropeptide Y receptor subtypes

Neuropeptide Y (NPY) is an amidated 36-amino acid peptide with a wide distribution in the central and peripheral nervous system. It can evoke numerous physiological responses by activating specific receptors. Studies using NPY analogs in various model systems and cell types demonstrate different orders of ligand potency and receptor binding affinity. These studies suggest the existence of multiple subtypes of NPY receptors. NPY has been described to bind to at least three different receptors, Y1, Y2 and Y3. NPY has also been shown to interact with sigma receptor in vivo and in vitro. There are indications that more subtypes might exist. Ligand binding studies reveal that Y1, Y2 and Y3 receptors are all G-protein coupled. It is not yet confirmed whether the sigma receptor that interacts with NPY is G-protein coupled. Some studies show that NPY receptors may interact with other classical receptors, including alpha- and beta-adrenoceptors and cholinergic receptors. In the case of alpha- and beta-adrenoceptors, the receptor-receptor interaction is possibly via a pertussis toxin-sensitive G-protein. NPY receptors are coupled to various signal transduction mechanisms including inhibition of adenylate cyclase, and stimulation or inhibition of increases in intracellular Ca2+. Specific links between individual NPY receptor subtype and a particular signal transduction pathway are not established.

Neuropeptide Y inhibits Ca2+ oscillations, cyclic AMP, and secretion in melanotrope cells of Xenopus laevis via a Y1 receptor

The melanotrope cells in the pituitary gland of Xenopus laevis are innervated by neurons containing neuropeptide Y (NPY). In the present study, the mechanism of action of NPY on the melanotropes has been investigated. NPY inhibited in vitro secretion from melanotropes in intact neurointermediate lobes as well as from isolated, single melanotropes. Inhibition of secretion from neurointermediate lobes was mimicked by the NPY analogues PYY and [Leu31,Pro34]NPY, whereas NPY(13-36) was inactive. Secretion from isolated melanotropes was inhibited by [Leu31,Pro34]NPY and NPY(13-36), but NPY(13-36) was 10-fold less potent than [Leu31,Pro34]NPY. Studies on isolated cells revealed that NPY and its analogues inhibited the occurrence of intracellular Ca2+ oscillations with the same potency as they inhibited secretion from isolated cells. In addition to inhibiting basal secretion and spontaneous Ca2+ oscillations, NPY inhibited the basal production of cyclic AMP. On the basis of these results it is proposed that NPY inhibits secretion from Xenopus melanotropes by inhibiting cyclic AMP-dependent spontaneous Ca2+ oscillations through a Y1-like receptor.

Peptide YY derivatives as selective neuropeptide Y/peptide YY Y1 and Y2 agonists devoided of activity for the Y3 receptor sub-type

Peptide YY derivatives were evaluated for their respective ability to bind and activate the NPY/PYY receptor sub-types (Y1, Y2 and Y3) present in various preparations. The analogue [Leu31,Pro34]PYY demonstrated high (nM) affinity in rat frontoparietal cortical membrane preparations (Y1-enriched tissue) and the rabbit saphenous vein (Y1 in vitro bioassay) but only low affinity in a Y2-enriched preparation (rat hippocampus). In contrast, PYY C-terminal fragments such as PYY3-36 and PYY13-36 were more potent in Y2 than Y1 assays. Interestingly, and in contrast to [Leu31,Pro34]NPY and NPY13-36, the PYY derivatives [Leu31,Pro34]PYY and PYY3-36 were inactive in a purported Y3 bioassay (rat colon). These results suggest that [Leu31,Pro34]PYY and PYY3-36 respectively represent the first selective and potent Y1 and Y2 agonists, devoided of significant affinity/activity for the Y3 receptor class.

Evaluation of truncated neuropeptide Y analogues with modifications of the tyrosine residue in position 1 on Y1, Y2 and Y3 receptor sub-types

Substitutions of the tyrosine residue in position 1 of truncated neuropeptide Y (N-terminal fragment 1-4 linked to C-terminal fragment 18-36 by the epsilon-aminocaproic acid) produced analogues that compete for specific [125I]polypeptide YY (PYY) binding in the frontoparietal cortex (Y1-enriched) with a profile best fitted to a two site-model with KD values in the low and high nM range, respectively. In the hippocampal membrane preparations (Y2-enriched), halogen substitutions on the aromatic ring generated analogues with competition profiles best fitted to a one-site model, revealing differences between the two binding assays and the interaction of these analogues with the Y1 and Y2 receptor sub-types. In the rat vas deferens (Y2-enriched), all truncated analogues inhibited the twitch response with similar or slightly weaker potency than the native molecule. In contrast, these molecules were markedly less potent than neuropeptide Y (NPY) in the rabbit saphenous vein (Y1-enriched) and the rat distal colon (Y3-enriched). Some of the truncated analogues were inactive at up to microM concentrations in the rat distal colon, demonstrating the distinct structural requirement of the receptor sub-type present in this bioassay. These results revealed that amino acid residues between positions 5 and 17 are critical for the maintenance of optimal affinity for the NPY receptors present in the rabbit saphenous vein and the rat distal colon.(ABSTRACT TRUNCATED AT 250 WORDS)