Amphetamine-induced effects on neuropeptide Y in the rat brain

Repeated (+)-amphetamine sulfate (AMPH) administration (5 mg/kg sc twice daily for 6 days and once on day 7) markedly and reversibly decreased (until 96 h after the final dose) neuropeptide Y-like immunoreactivity (NPY-LI) in the rat striatum (caudate-putamen) and nucleus accumbens, and had no effect on NPY-LI in the hippocampus. No significant alterations were detected in the hybridization signal of NPY mRNA4 and 24 h after the end of AMPH treatment. A single dose of AMPH (5 mg/kg sc) administered to rats 4 and 24 h prior to sacrifice had no effect on NPY-LI in the brain structures studied. Moreover, AMPH injected 8 days after the last dose of repeated AMPH administration did not change NPY-LI up to 72 h. The minimal dose of haloperidol, the strong mixed dopaminergic D2/D1 receptor antagonist, (0.75 mg/kg injected ip 30 min before each of the multiple AMPH administrations) that was sufficient to completely block stereotypy and hyperlocomotion elicited by multiple AMPH administrations enhanced the AMPH-induced decrease in the striatal and accumbens NPY-LI. Our results suggest that NPY neurons in the striatum, nucleus accumbens and hippocampus are not directly involved in the acute behavioral response to AMPH (stereotypy and hyperlocomotion) as well as in the initiation and expression of AMPH-induced behavioral sensitization.     

Endothelium-independent potentiating effects of neuropeptide Y in the rat tail artery.

The role of the endothelium in the potentiating action of neuropeptide Y (NPY) to contraction induced by KCl, alpha, beta-methylene ATP (mATP), and noradrenaline (NA) was tested on rat tail arteries. Endothelium-intact and denuded ring segments and freshly isolated single smooth muscle cells were used in the study. Contraction responses to KCl and mATP were potentiated by NPY (50 nM) in both intact and denuded arteries. Contraction to NA was potentiated by NPY at 500 nM but not at 50 nM. The potentiation effect of NPY was antagonized by nifedipine. Similarly, the shortening of single smooth muscle cells in response to KCl and mATP was potentiated by NPY (50 nM). The noradrenaline response was potentiated by NPY at 500 nM but not at 50 nM. Our results suggest that the potentiating effect of NPY is more specific to contraction mediated by nifedipine-sensitive calcium channels and is not dependent on the presence of an intact endothelium.     

Neuropeptide Y in rat detrusor and its effect on nerve-mediated and acetylcholine-evoked contractions.

1. Immunohistochemical and isolated organ bath techniques were used to detect the presence of neuropeptide Y (NPY) in the rat urinary bladder and to determine its effect on tone, spontaneous activity and contractile responses of the detrusor muscle to electrical field stimulation, acetylcholine and alpha,beta-methylene ATP (alpha,beta-MeATP). 2. A very rich presence of NPY-immunoreactive nerve fibres was found mainly within the bundles of detrusor muscle cells. Chronic treatment with 6-hydroxydopamine did not affect the density of NPY-positive nerve fibres. 3. NPY (> 1 nM) enhanced the force and frequency of spontaneous contractions and generated a rise in the resting tone of the detrusor. These effects of NPY on the tone and the spontaneous activity remained unaffected by atropine (3 microM), indomethacin (10 microM) and aspirin (100 microM) but were abolished by Ca(2+)-withdrawal from the bathing medium. 4. The enhancing effects of NPY on the spontaneous contractions and the resting tone were not prevented by the induction of purinoceptor desensitization. 5. NPY (1-250 nM) potentiated electrical field stimulation (EFS, 1-64 Hz, 0.1 ms pulses duration, 10s train duration)-evoked, tetrodotoxin (0.5 microM)-sensitive contractions. The atropine (3 microM)-resistant component of EFS-evoked contractions was also potentiated by NPY. By contrast, the nifedipine (1 microM)-resistant but atropine-sensitive component of EFS-evoked contraction was inhibited by NPY. 6. NPY (250 nM) did not affect acetylcholine-evoked contractions, but potentiated alpha,beta-MeATP-evoked contractions. 7. It is concluded that NPY-innervation of rat urinary bladder is largely confined to the detrusor muscle and is abundant and mainly non-adrenergic.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effects of neuropeptide Y and its fragments upon basal and electrically stimulated ion secretion in rat jejunum mucosa.

1. The effects of acute or chronic morphine treatment on the changes in blood pressure and pulse rate in response to ganglionic stimulation or blockade and to vagal stimulation, and of isolated atria to field stimulation or noradrenaline, were studied. 2. In pithed rats, intravenously injected hexamethonium significantly depressed the blood pressure responses to sympathetic nerve stimulation. The ganglionic blocking effects of hexamethonium were significantly greater in chronically morphine-treated rats, but were not significantly affected by acute morphine administration in naive animals. 3. Intravenous administration of nicotine dose-dependently increased blood pressure and pulse rate. The magnitudes of these changes were not significantly affected by acute or chronic morphine pretreatment. 4. Studies with rat isolated atrial preparations revealed that the changes in atrial contractile rate and force in response to noradrenaline or field stimulation were not influenced by either acute or chronic morphine treatment. 5. Cervical vagal stimulation produced voltage- or frequency-dependent decreases in pulse rate and blood pressure. The responses were not significantly affected by chronic morphine treatment. 6. These findings suggest that the site of the changes in sympathetic function following prolonged exposure to the opiate appears to be on the preganglionic nerve fibres.     

Vasoconstrictor effects of various neuropeptide Y analogues on the rat tail artery in the presence of phenylephrine.

1. The increase in perfusion pressure induced by neuropeptide Y (NPY), peptide YY (PYY) and related peptides were compared in the perfused rat tail artery precontracted by a submaximal concentration (1 microM) of the vasoconstrictor, phenylephrine. 2. NPY, PYY, [Leu31,Pro34]NPY, [Glu16,Ser18,Ala22,Leu28,31]NPY (ESALL-NPY) and the centrally truncated and stabilized analogues [D-Cys5,8-aminooctanoic acid7-20, Cys24]-NPY (D-Cys5-NPY) and [D-Cys7, 8-aminooctanoic acid8-17,Cys20]-NPY (D-Cys7-NPY) produced a concentration-dependent enhancement of the vasoconstrictor response induced by 1 microM phenylephrine. PYY was two times more potent than NPY and [Leu31,Pro34]NPY while ESALL-NPY, D-Cys7-NPY and D-Cys5-NPY were approximately 3, 5 and 16 times less potent than NPY respectively. NPY, D-Cys5-NPY and D-Cys7-NPY gave similar maximal responses whereas those observed for PYY, [Leu31,Pro34]NPY and ESALL-NPY were much greater than that of NPY. 3. NPY 13-36 and [des-Ser3,Lys4,Cys2,8-aminooctanoic acid3-24, D-Cys27]-NPY ([es-Ser3,Lys4]Cys2-NPY) were practically inactive at concentrations up to 3 microM, whereas [des-Ser3,Lys4,D-Cys2,8-aminooctanoic acid3-24,Cys27]-NPY ([des-Ser3,Lys4]D-Cys2-NPY), which differs from [des-Ser3,Lys4]Cys2-NPY in the disulphide bridge (a D-Cys in position 2 for [des-Ser3,Lys4]D-Cys2-NPY instead of an L-CYs for [des-Ser3,Lys4]Cys2-NPY) was a weak agonist the maximal effect of which could not be ascertained. 4. The contractile effects of [des-Ser3,Lys4]D-Cys2-NPY were additive with those of NPY and [Leu31,Pro34]NPY demonstrating that it is not a partial agonist but may simply not interact competitively with the receptor binding site for NPY.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vasoactive intestinal peptide receptor antagonists in rat seminal vesicle membranes.

Vasoactive intestinal peptide (VIP) receptors coupled to activation of adenylate cyclase have been previously identified in seminal vesicle membranes of rat. In the present study we demonstrate that the synthetic peptides [4-Cl-D-Phe6,Leu17]VIP and the growth hormone releasing factor (GRF) analog [Ac-Tyr1,D-Phe2]GRF1-29-NH2 inhibit in a competitive manner the specific 125I-VIP binding to the same membrane preparation. The order of potency of the two peptides compared to VIP was: VIP (IC50 = 1.3 +/- 0.5 nM) greater than [4-Cl-D-Phe6,Leu17]VIP(IC50 = 1600 +/- 45.0 nM) greater than [Ac-Tyr1,D-Phe2]GRF1-29-NH2(IC50 = 290.0 +/- 59.4 nM). Whereas VIP showed a stimulatory activity upon adenylate cyclase with a potency (ED50 = 7.0 +/- 0.7 nM) compatible with the affinity of the VIP binding sites previously described, the other two peptides tested showed no effect at that level. The behavior as antagonists of both [4-Cl-D-Phe6,Leu17]VIP and [Ac-Tyr1,D-Phe2]GRF1-29-NH2 was confirmed by: (a) the parallel shifts of the VIP dose-response curves for stimulation of adenylate cyclase activity in the presence of the antagonists; (b) the close agreement between the binding affinity and the inhibition of adenylate cyclase activity for the two peptides; and (c) the lack of effect of the two antagonists upon the adenylate cyclase activity stimulated by the beta-adrenoceptor agonist isoproterenol which indicates the specificity of the interaction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparison of the effects of neuropeptide Y and noradrenaline on rat gastric mucosal blood flow and integrity.

1. The effects of neuropeptide Y (NPY) and noradrenaline on rat gastric mucosal blood flow, as estimated by laser Doppler flowmetry (LDF), have been examined. In addition, the ability of NPY and noradrenaline to induce acute mucosal haemorrhagic damage has also been assessed. 2. Close-arterial infusion of NPY (0.05-0.2 nmol kg-1 min-1) for 10 min in the anaesthetized rat induced a dose-dependent fall in LDF, but had minimal effects on systemic arterial blood pressure. Higher doses of NPY did not produce any further reduction in LDF. 3. Close-arterial infusion (0.1-0.4 nmol kg-1 min-1) of the structurally related peptide YY (PYY) or pancreatic polypeptide (PP), had inconsistent actions in decreasing LDF. 4. Close-arterial infusion of noradrenaline (30-90 nmol kg-1 min-1) dose-dependently reduced gastric LDF. 5. Local infusion of NPY (0.1 and 0.2 nmol kg-1 min-1) or noradrenaline (45 and 60 nmol kg-1 min-1) resulted in dose-related increases in the area of mucosal hemorrhagic damage. 6. Pretreatment with the alpha 1-adrenoceptor antagonist, prazosin (0.1 mg kg-1, i.v.) significantly reduced the effect of noradrenaline, but not NPY, on both LDF and mucosal damage. 7. These findings indicate that NPY and noradrenaline act directly on the gastric microvasculature to induce vasoconstriction and both can induce acute mucosal damage. Therefore endogenous NPY, like noradrenaline could play a modulatory role in regulating vascular tone and may influence mucosal integrity.     

Disparate effects of neuropeptide Y and clonidine on the excretion of sodium and water in the rat

Previous studies have demonstrated a similarity between the ability of neuropeptide Y (NPY) and clonidine to inhibit renin release and inhibit cAMP production. We therefore compared the effects of clonidine and NPY on the excretion of sodium and water in anesthetized rats which were unilaterally nephrectomized (right kidney) 10 days prior to the experiment. On the experimental day, rats were anesthetized (nembutal) and the left kidney exposed for the intrarenal infusion of the study drugs. The lowest dose of NPY (0.3 microgram/kg per min) investigated failed to alter renal function. Clonidine (0.3 microgram/kg per min) and NPY (1 microgram/kg per min) produced a similar increase in urine volume. Only NPY increased sodium excretion and osmolar clearance. Free water clearance was only increased by clonidine. Blood pressure and creatinine clearance were similar in all groups investigated. These effects were attenuated by pretreatment with pertussis toxin (5 days). The ability of pertussis toxin to block these effects suggests that the renal effects of NPY and clonidine are coupled to a G protein, conceivably the inhibitory Gi protein of the adenylate cyclase system. The disparate effects on sodium excretion and on free water and osmolar clearance indicate that the effects of these compounds may be mediated through the inhibition of different pools of hormonally stimulated cAMP.     

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.     

Differential effects of neuropeptide Y and opioids on neurogenic responses of the perfused rat mesentery

In perfused rat mesentery transmural nerve stimulation activates both adrenergic and capsaicin-sensitive sensory nerves. When adrenergic nerves were blocked with guanethidine and smooth muscle tone was increased, transmural nerve stimulation caused a dilator response which was attenuated by tetrodotoxin and abolished by capsaicin. Indomethacin increased the vasodilator response to transmural nerve stimulation, but did not affect the dilation to calcitonin gene-related peptide. Neuropeptide Y (NPY) potentiated vasoconstrictor responses to transmural nerve stimulation, but suppressed capsaicin-sensitive vasodilation, an effect which was unaltered by indomethacin. Opioid agonists selective for mu, delta or kappa receptors, DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin), DPDPE ([D-Pen2,D-Pen5]enkephalin) and ethylketocyclazocine, had no effect on the vasoconstrictor response to transmural nerve stimulation. DAMGO and DPDPE significantly inhibited vasodilator responses to transmural nerve stimulation, but ethylketocyclazocine was without effect. After treatment with indomethacin, DAMGO still inhibited the vasodilator response, but DPDPE was no longer effective. Prejunctional control of transmitter release by NPY or opioids is dependent on the specific nerve type as well as, in some cases, the participation of endogenous prostaglandins.     

Effects of butorphanol on feeding and neuropeptide Y in the rat

Butorphanol ([BT] an opioid receptor agonist/antagonist) is different from other opioid agonists in that a single dose of BT can elicit up to 12 g of chow intake in a satiated rat whereas most opioid agonists induce a mild feeding response (2-3 g). Here, we first examined whether the effectiveness of BT to elicit feeding was affected by dose, method of infusion and possible tachyphylaxis following administration. Secondly, we examined whether BT administration influenced hypothalamic NPY gene expression and peptide levels. A single dose administration of BT (4 mg/kg) significantly increased food intake at 2, 3 and 6 h after administration. However following repeated injections of BT at 4 mg/kg, the cumulative long-term intake of BT-treated rats did not differ from that of controls, indicating that the animals compensate for the increased feeding following BT injection by decreased feeding at a later time. An ascending dose schedule of repeated BT injections resulted in additional feeding. NPY gene expression in the ARC was influenced by how much food had been consumed, but not by BT. The amount of food consumed and the level of NPY mRNA were inversely correlated. This is consistent with NPY's role in normal feeding. BT treatment did not affect either NPY or leptin RIA levels. We conclude that the feeding produced by BT is sensitive to dose and dosing paradigm. Further, its mechanism of action does not appear to be mediated by NPY or leptin pathways.     

Effects of neuropeptide Y and agonists selective for neuropeptide Y receptor sub-types on arterioles of the guinea-pig small intestine and the rat brain.

1. The actions of neuropeptide Y (NPY) and agonists selective for NPY receptor subtypes were examined on arterioles from the guinea-pig small intestine and the rat pia in order to characterize the receptors mediating the vasoconstrictor and potentiating effects of NPY. 2. A method was developed for measuring the potentiating effects of NPY in situations where it was not possible to obtain a full concentration-response relationship for the vasoconstrictor. NPY, 50 nM, had a greater potentiating effect on the guinea-pig intestinal arterioles than those from the rat pia. 3. NPY and the Y1-selective agonist, NPY[Leu31,Pro34], potentiated the constrictor responses to U46619 in both arterioles and responses to noradrenaline in the guinea-pig arterioles. There was marked desensitization of the potentiating effect, and cross-desensitization between NPY and NPY[Leu31,Pro34]. Both NPY and NPY[Leu31,Pro34] caused constriction of the rat pial arterioles but not of those from the guinea-pig intestine. 4. The Y2-selective agonist PYY(13-36) caused no potentiation or vasoconstriction and did not affect the potentiation by NPY or NPY[Leu31,Pro34]. 5. The potentiating and vasoconstrictor effects of NPY on these arterioles were mediated by Y1 receptors.     

Clozapine decreases neuropeptide Y-like immunoreactivity and neuropeptide Y mRNA levels in rat nucleus accumbens.

The aim of this study was to evaluate the effect of acute, subchronic (14 days) and chronic (28 days) intraperitoneal (i.p.) administration of clozapine (10 or 25 mg/kg) on neuropeptide Y (NPY) system activity in the nucleus accumbens of the rat. NPY-like immunoreactivity (NPY-LI) decreased 24 h after subchronic clozapine while NPY mRNA after both acute and subchronic clozapine treatment. NPY-LI levels were also reduced 8 days after cessation of chronic lower-dose treatment. Subchronic (14 days) administration of the 5-HT2A antagonist ketanserin (1 mg/kg i.p.) or the dopamine D2/D3 antagonist (+/-) sulpiride (100 mg/kg i.p.) reduced NPY-LI levels, whereas the dopamine D1-like antagonist SCH 23390 (0.5 mg/kg i.p.), dopamine D4 antagonist L-745,870 (1 mg/kg per os), and alpha1-adrenergic antagonist prazosin (0.2 mg/kg i.p.) had no effect. There were no significant differences between the ketanserin-induced decrease in NPY-LI levels and the effects of the following two-drug combinations: ketanserin and SCH 23390, ketanserin and L-745,870, and ketanserin and prazosin. The study has shown that clozapine reduces NPY system activity in the rat nucleus accumbens. It seems that the action of clozapine is partly mediated by blockade of 5-HT2A and D2/D3 dopaminergic receptors.     

Characterization of the contractile activity of dopamine on the rat isolated seminal vesicle

The mechanism of the contractile effect of dopamine (DA) on the rat isolated seminal vesicle was studied. Cocaine (10 microM/1 in the organ bath, 30 min before DA) and 6-OHDA (50 mg/kg i.v. 24 hr before removal of the seminal vesicle) almost completely prevented the contractile effect of DA. Drugs known to have an affinity for DA receptors or for alpha-adrenoceptors antagonized the contractile effect of DA, the rank order of potency being: prazosin greater than phentolamine greater than yohimbine greater than clonidine greater than sulpiride greater than apomorphine greater than haloperidol. The antagonism was in each case greater against DA than against noradrenaline (NA), used for comparison; selectivity for DA being highest in the case of prazosin and sulpiride. Taken together, these findings indicate that DA makes the rat seminal vesicle contract mostly by means of an indirect mechanism, binding presynaptic DA-receptors and, in part, presynaptic alpha-adrenoceptors as well; or, alternatively, binding presynaptic DA-receptors which have some links with alpha 2-adrenoceptors; the consequence being in either case the release of NA from sympathetic nerve endings.     

Ionic currents and inhibitory effects of glibenclamide in seminal vesicle smooth muscle cells.

1. Whole-cell voltage-clamp recordings were made from smooth muscle cells isolated from guinea-pig seminal vesicle. 2. When the recording pipette solution contained 130 mM KCl and a low concentration of EGTA (0.2 mM), a dominant outward current was elicited by depolarization to positive of -30 mV from a holding potential of -50 mV. The current was non-inactivating, stimulated by intracellular Ca2+ and blocked by bath-applied 1 mM tetraethylammonium but not 1 mM 3,4 diaminopyridine. 3. If 10 mM EGTA was added to the KCl pipette solution and the holding potential was -50 mV, or more negative, the major current elicited by depolarization to positive of -30 mV was an A-type K(+)-current. This current inactivated rapidly (within 100 ms) and was blocked by bath-applied 1 mM 3,4-diaminopyridine but not 10 mM tetraethylammonium. 4. An inward voltage-gated Ca channel current was observed on depolarization to positive of -30 mV with 1.5 mM Ca2+ or 10 mM Ba2+ in the bath solution and when Ca+ replaced K+ in the pipette. The Ba(2+)-current was shown to be abolished by bath-applied 100 microM Cd2+ and inhibited by 90% by 1 microM nifedipine, and thus appeared to be carried by L-type Ca channels. 5. High concentrations of glibenclamide (10-500 microM) inhibited A-type K(+)-current, Ba(2+)-current and contraction of the whole tissue induced by noradrenaline or electrical field stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Discrimination between neuropeptide Y and peptide YY in the rat tail artery by the neuropeptide Y1-selective antagonist, BIBP 3226.

1. The ability of the novel, nonpeptide, neuropeptide Y (NPY) Y1-selective antagonist, BIBP 3226 ¿(R)-N2-(diphenylacetyl)-N-[(4-hydroxyphenyl)methyl]-D-arginine amide¿, to antagonize the increase in perfusion pressure induced by NPY and peptide Y (PYY) was tested in the perfused rat tail artery, a postjunctional Y1-receptor bioassay, precontracted by 1 microM phenylephrine. 2. NPY and PYY produced a concentration-dependent enhancement of the vasoconstrictor response evoked by 1 microM phenylephrine. Although NPY and PYY are roughly equipotent, the maximal contractile response elicited by PYY was about twice that elicited by NPY. 3. Increasing concentrations of BIBP 3226 caused a parallel and rightward shift in the NPY concentration-response curve without depressing the maximal response. The contractile effect of NPY was potently inhibited in a competitive manner. The pA2 value for BIBP 3226 was 7.01 +/- 0.08, a value equivalent to that observed in the rabbit saphenous vein. Although increasing concentrations of BIBP 3226 shifted the concentration-response curve of PYY to the right without any significant decrease in the maximal vasoconstrictor response, the antagonism appeared non-competitive as the slope of the Schild plot was significantly different from unity (0.58 +/- 0.04). 4. In conclusion, these data confirm that BIBP 3226 is a potent and selective nonpeptide Y1 receptor antagonist. Moreover, they show that complex interactions occur between BIBP 3226 and postjunctional receptors activated by PYY. We postulate that BIBP 3226 might discriminate between the effects of NPY and PYY at the postjunctional level in the rat tail artery. It may be that distinct receptors for NPY and PYY exist; these may or may not allosterically interact with each other. Another working hypothesis would be that there is a single receptor complex with allosterically interacting binding sites for the two peptides.     

Potentiation by neuropeptide Y of vasoconstriction in rat resistance arteries.

1. The effects of neuropeptide Y (NPY) on resistance arteries were investigated on 3rd generation mesenteric arterioles of the rat. 2. Contractions were elicited by noradrenaline (NA), 5-hydroxytryptamine (5-HT), prostaglandin F2 alpha (PGF2 alpha), depolarization (KCl substituted for NaCl) and by the calcium agonist Bay K 8644, in the absence and in the presence of NPY (100 nM), a concentration which by itself did not induce vasoconstriction. 3. NPY produced a leftward shift of the concentration-response curves to the agonists and to KCl, without any alteration of maximal contractions. 4. NPY also potentiated contractions elicited by addition of CaCl2 to KCl-depolarized vessels, but its effect on calcium-induced contractions decreased with increasing KCl concentrations (from 20 to 100 mM). 5. Calcium-induced contractions were inhibited by the calcium channel blocker nitrendipine, both in the presence and absence of NPY (100 nM). NPY increased slightly (but significantly) the sensitivity to nitrendipine (the apparent KB increased from 2.9 x 10(-10) M to 1.6 x 10(-10) M). 6. The KCl concentration necessary for the maximal effect of Bay K 8644 was decreased in the presence of NPY, and the sensitivity to the calcium channel agonist was increased. 7. Elevating the KCl concentration in the bath from 5 to 20 mM (which gives the same displacement to the left of the KCl concentration-effect curve seen in the presence of NPY) induced a parallel leftward shift of NA and 5-HT concentration-response curves. This shift was identical to the one induced by NPY on 5-HT-evoked contractions, but it was significantly smaller (P less than 0.001) than the shift of the NA concentration-response curve observed in the presence of NPY. In the latter case, NPY enhanced more markedly the contractions induced by low NA concentrations (between 10(-9) and 3 x 10(-8 M) than those induced by high concentrations (up to 3 x 10(-7) M), thus giving a shallow concentration-response curve. 8. The results strongly suggest that NPY partially depolarizes the arterioles and induces an increase in calcium entry through voltage-dependent channels, thus enhancing contractions elicited by agonists or by KCl-depolarization. In addition, they support the view that another mechanism also plays a part in the potentiation by NPY of the effects of low concentrations of NA.