Nonnoradrenergic mechanism of reflex cutaneous vasoconstriction in men

We tested for a nonnoradrenergic mechanism of reflex cutaneous vasoconstriction with whole body progressive cooling in seven men. Forearm sites (<1 cm(2)) were pretreated with: 1) yohimbine (Yoh; 5 mM id) to antagonize alpha-adrenergic receptors, 2) Yoh plus propranolol (5 mM Yoh-1 mM PR id) to block alpha- and beta-adrenergic receptors, 3) iontophoretic application of bretylium tosylate (BT) to block all sympathetic vasoconstrictor nerve effects, or 4) intradermal saline. Skin blood flow was measured by laser Doppler flowmetry and arterial pressure by finger photoplethysmography; cutaneous vascular conductance (CVC) was indexed as the ratio of the two. Whole body skin temperature (T(SK)) was controlled at 34 degrees C (water-perfused suit) for 10 min and then lowered to 31 degrees C over 15 min. During cooling, vasoconstriction was blocked at BT sites (P > 0.05). CVC at saline sites fell significantly beginning at T(SK) of 33.4 +/- 0.01 degrees C (P <0.05). CVC at Yoh-PR sites was significantly reduced beginning at TSK of 33.0 +/- 0.01 degrees C (P < 0.05). After cooling, iontophoretic application of norepinephrine (NE) confirmed blockade of adrenergic receptors by Yoh-PR. Because the effects of NE were blocked at sites showing significant reflex vasoconstriction, a nonnoradrenergic mechanism in human skin is indicated, probably via a sympathetic cotransmitter.     

Neuropeptide Y Y1 receptor mediated mesenteric vasoconstriction in the pig in vivo

The object of the present study was to investigate the effects of the sympathetic cotransmitter neuropeptide Y (NPY), and the closely related gut hormone peptide YY (PYY), on splanchnic blood flow regulation in the anaesthetized pig in vivo. Systemic injections of NPY, PYY and the NPY Y(1) receptor agonist [Leu(31)Pro(34)]NPY (470 pmol kg(-1) each) evoked pressor and mesenteric vasoconstrictor responses that were largely abolished by the selective NPY Y(1) receptor antagonist H 409/22 (60 nmol kg(-1) min(-1)). In contrast, the NPY Y(2) receptor agonist N-acetyl[Leu(28)Leu(31)]NPY(24-36) (1.1 nmol kg(-1)), a dose of which potently evoked splenic NPY Y(2) receptor mediated (not affected by H 409/22) vasoconstriction, did not evoke any mesenteric vascular response. Mesenteric vascular responses to angiotensin II (10 pmol kg(-1)), alpha,beta-methylene ATP (10 nmol kg(-1)) and the alpha(1)-adrenoceptor agonist phenylephrine (15 nmol kg(-1)), were not inhibited by H 409/22. It is concluded that NPY and PYY evokes porcine mesenteric vasoconstriction mediated by the NPY Y(1) receptor subtype, as demonstrated by selective and specific inhibition exerted by the NPY Y(1) receptor antagonist H 409/22, in vivo.     

Relative roles of local and reflex components in cutaneous vasoconstriction during skin cooling in humans.

The reduction in skin blood flow (SkBF) with cold exposure is partly due to the reflex vasoconstrictor response from whole body cooling (WBC) and partly to the direct effects of local cooling (LC). Although these have been examined independently, little is known regarding their roles when acting together, as occurs in environmental cooling. We tested the hypothesis that the vasoconstrictor response to combined LC and WBC would be additive, i.e., would equal the sum of their independent effects. We further hypothesized that LC would attenuate the reflex vasoconstrictor response to WBC. We studied 16 (7 women, 9 men) young (30.5+/-2 yr) healthy volunteers. LC and WBC were accomplished with metal Peltier cooler-heater probe holders and water-perfused suits, respectively. Forearm SkBF was monitored by laser-Doppler flowmetry (LDF). Cutaneous vascular conductance (CVC) was calculated as LDF/blood pressure. Subjects underwent 15 min of LC alone or 15 min of WBC with and without simultaneous LC, either at equal levels (34-31 degrees C) or as equipotent stimuli (34-28 degrees C LC; 34-31 degrees C WBC). The fall in CVC with combined WBC and LC was greater (P<0.05) than for either alone (57.0+/-5% combined vs. 39.2+/-6% WBC; 34.4+/-4% LC) with equipotent cooling, but it was only significantly greater than for LC alone with equal levels of cooling (51.3+/-8% combined vs. 29.5+/-4% LC). The sum of the independent effects of WBC and LC was greater than their combined effects (74.9+/-4 vs. 51.3+/-8% equal and 73.6+/-7 vs. 57.0+/-5% equipotent; P<0.05). The fall in CVC with WBC at LC sites was reduced compared with control sites (17.6+/-2 vs. 42.4+/-8%; P<0.05). Hence, LC contributes importantly to the reduction in SkBF with body cooling, but also suppresses the reflex response, resulting in a nonadditive effect of these two components.     

Neuropeptide Y1 receptor vasoconstriction in exercising canine skeletal muscles.

Existing evidence suggests that neuropeptide Y (NPY) acts as a neurotransmitter in vascular smooth muscle and is coreleased with norepinephrine from sympathetic nerves. We hypothesized that release of NPY stimulates NPY Y(1) receptors in the skeletal muscle vasculature to produce vasoconstriction during dynamic exercise. Eleven mongrel dogs were instrumented chronically with flow probes on the external iliac arteries of both hindlimbs and a catheter in one femoral artery. In resting dogs (n = 4), a 2.5-mg bolus of BIBP-3226 (NPY Y(1) antagonist) infused into the femoral artery increased external iliac conductance by 150 +/- 82% (1.80 +/- 0.44 to 3.50 +/- 0.14 ml.min(-1).mmHg(-1); P < 0.05). A 10-mg bolus of BIBP-3226 infused into the femoral artery in dogs (n = 7) exercising on a treadmill at a moderate intensity (6 miles/h) increased external iliac conductance by 28 +/- 6% (6.00 +/- 0.49 to 7.64 +/- 0.61 ml.min(-1).mmHg(-1); P < 0.05), whereas the solvent vehicle did not (5.74 +/- 0.51 to 5.98 +/- 0.43 ml.min(-1).mmHg(-1); P > 0.05). During exercise, BIBP-3226 abolished the reduction in conductance produced by infusions of the NPY Y(1) agonist [Leu(31),Pro(34)]NPY (-19 +/- 3 vs. 0.5 +/- 1%). Infusions of BIBP-3226 (n = 7) after alpha-adrenergic receptor antagonism with prazosin and rauwolscine also increased external iliac conductance (6.82 +/- 0.43 to 8.22 +/- 0.48 ml.min(-1).mmHg(-1); P < 0.05). These data support the hypothesis that NPY Y(1) receptors produce vasoconstriction in exercising skeletal muscle. Furthermore, the NPY Y(1) receptor-mediated tone appears to be independent of alpha-adrenergic receptor-mediated vasoconstriction.     

Neuropeptide Y reduces ovarian blood flow in the rabbit

Neuropeptide Y-containing nerve fibers have previously been demonstrated to innervate the mammalian ovary. These nerve fibers innervate primarily the vasculature. In this study we have developed a method for in vivo measurement of the ovary blood flow rate by means of the 133Xe method. Using this technique we measured the ovary blood flow rate and investigated the dose-response relationship between close intraarterial-injected NPY and the ovary blood flow rate. A monoexponential washout curve for 133Xe was found for the whole washout process, ensuring that the blood flow rate at any time could be calculated from the curve. We found a mean blood flow rate in the nonpregnant rabbit ovary at 43.6 +/- 4.4 ml.(100 g)-1.min-1 (mean +/- SEM). Injection of NPY (20, 200, 2000 pM) in the aorta close to a. ovarica resulted in a dose-dependent decrease in the ovarian blood flow rate with a maximum reduction to 40.7 +/- 6.3% (mean +/- SEM) of the control blood flow rate. These findings make it likely that receptors able to interact with NPY are present in the vasculature of the rabbit ovary.     

Neuropeptide Y and peptide YY mediate nonadrenergic vasoconstriction and modulate sympathetic responses in rats

The modulation of cardiovascular sympathetic responses by neuropeptide Y (NPY) and peptide YY (PYY) was assessed in vivo, in pithed rats. Both peptides (0.02-2 nmol/kg) caused similar dose-dependent pressor responses, resistant to adrenergic blockade but antagonized by the calcium channel blocker, nifedipine. Only NPY, at the lowest dose, slightly accelerated heart rate (by 10 +/- 4 beats/min). At the pressor dose (0.6 nmol/kg) but not subpressor dose (0.2 nmol/kg), the increase in blood pressure induced by stimulation of the sympathetic outflow (ST: 0.3 Hz, 50 V, 1 min) was attenuated by PYY (by 40%), whereas ST-evoked tachycardia was reduced by NPY (by 35%). Neither NPY- nor PYY-pretreatment affected ST-induced increments in plasma norepinephrine (NE) and epinephrine concentrations. In addition, regional hemodynamic effects of NPY were studied in conscious rats instrumented with Doppler flow probes. The hypertension caused by NPY was attended by reflex bradycardia and marked rise in peripheral vascular resistance in renal (+ 233 +/- 59%), superior mesenteric (+ 183 +/- 65%) and hindquarter (+ 65 +/- 10%) circulation. The pattern of hemodynamic responses of NPY was similar to that of NE but, unlike the latter, persisted after adrenergic blockade.     

Intravenous buprenorphine reduces pupil size and the light reflex in humans

The pupillary effects of intravenous buprenorphine were studied in eight nondependent male subjects who reported previous opiate use. Buprenorphine (0.3, 0.6, and 1.2 mg) decreased pupil size, the amplitude of the light reflex, and the velocities of constriction and dilation. Significant pupillary effects occurred within 15 min of the injection and persisted for 24 hr. At 48 hr most measures returned to baseline levels. Generally the magnitude of the effect was not dose related although recovery occurred sooner after the lower dose. The time course of the pupillary effects of buprenorphine exceeds duration of its analgesic and subjective effects. Previous studies have reported that pupillary measures are especially sensitive to the acute effects of full opiate agonists. The results of the present study indicate that buprenorphine, a partial opiate agonist, causes profound and persistent effects on pupillary size and dynamic measures.     

In vivo mechanisms of cutaneous vasodilation and vasoconstriction in humans during thermoregulatory challenges.

This review focuses on the neural and local mechanisms that have been demonstrated to effect cutaneous vasodilation and vasoconstriction in response to heat and cold stress in vivo in humans. First, our present understanding of the mechanisms by which sympathetic cholinergic nerves mediate cutaneous active vasodilation during reflex responses to whole body heating is discussed. These mechanisms include roles for cotransmission as well as nitric oxide (NO). Next, the mechanisms by which sympathetic noradrenergic nerves mediate cutaneous active vasoconstriction during whole body cooling are reviewed, including cotransmission by neuropeptide Y (NPY) acting through NPY Y1 receptors. Subsequently, current concepts for the mechanisms that effect local cutaneous vascular responses to direct skin warming are examined. These mechanisms include the roles of temperature-sensitive afferent neurons as well as NO in causing vasodilation during local heating of skin. This section is followed by a review of the mechanisms that cause local cutaneous vasoconstriction in response to direct cooling of the skin, including the dependence of these responses on intact sensory and sympathetic, noradrenergic innervation as well as roles for nonneural mechanisms. Finally, unresolved issues that warrant further research on mechanisms that control cutaneous vascular responses to heating and cooling are discussed.     

Neuropeptide Y5 receptor antagonism does not induce clinically meaningful weight loss in overweight and obese adults

Neuropeptide Y (NPY) is a potent orexigenic neuropeptide, and antagonism of NPY Y1 and NPY Y5 receptors (NPYxR) is considered a potentially important anti-obesity drug target. We tested the hypothesis that blockade of the NPY5R will lead to weight loss in humans using MK-0557, a potent, highly selective, orally active NPY5R antagonist. The initial series of experiments reported herein, including a multiple-dose positron-emission tomography study and a 12 week proof-of concept/dose-ranging study, suggested an optimal MK-0557 dose of 1 mg/day. The hypothesis was then tested in a 52 week, multicenter, randomized, double-blind, placebo-controlled trial involving 1661 overweight and obese patients. Although statistically significant at 52 weeks, the magnitude of induced weight loss was not clinically meaningful. These observations provide the first clinical insight into the human NPY-energy homeostatic pathway and suggest that solely targeting the NPY5R in future drug development programs is unlikely to produce therapeutic efficacy.     

Local ascorbate administration augments NO- and non-NO-dependent reflex cutaneous vasodilation in hypertensive humans

Full expression of reflex cutaneous vasodilation (VD) is dependent on nitric oxide (NO) and is attenuated with essential hypertension. Decreased NO-dependent VD may be due to 1) increased oxidant stress and/or 2) decreased L-arginine availability through upregulated arginase activity, potentially leading to increased superoxide production through uncoupled NO synthase (NOS). The purpose of this study was to determine the effect of antioxidant supplementation (alone and combined with arginase inhibition) on attenuated NO-dependent reflex cutaneous VD in hypertensive subjects. Nine unmedicated hypertensive [HT; mean arterial pressure (MAP) = 112 +/- 1 mmHg] and nine age-matched normotensive (NT; MAP = 81 +/- 10 mmHg) men and women were instrumented with four intradermal microdialysis (MD) fibers: control (Ringer), NOS inhibited (NOS-I; 10 mM N(G)-nitro-L-arginine), L-ascorbate supplemented (Asc; 10 mM L-ascorbate), and Asc + arginase inhibited [Asc+A-I; 10 mM L-ascorbate + 5 mM (S)-(2-boronoethyl)-L-cysteine-HCl + 5 mM N(omega)-hydroxy-nor-L-arginine]. Oral temperature was increased by 0.8 degrees C via a water-perfused suit. N(G)-nitro-L-arginine was then ultimately perfused through all MD sites to quantify the change in VD due to NO. Red blood cell flux was measured by laser-Doppler flowmetry over each skin MD site, and cutaneous vascular conductance (CVC) was calculated (CVC = flux/MAP) and normalized to maximal CVC (%CVC(max); 28 mM sodium nitroprusside + local heating to 43 degrees C). During the plateau in skin blood flow (Delta T(or) = 0.8 degrees C), cutaneous VD was attenuated in HT skin (NT: 42 +/- 4, HT: 35 +/- 3 %CVC(max); P < 0.05). Asc and Asc+A-I augmented cutaneous VD in HT (Asc: 57 +/- 5, Asc+A-I: 53 +/- 6 %CVC(max); P < 0.05 vs. control) but not in NT. %CVC(max) after NOS-I in the Asc- and Asc+A-I-treated sites was increased in HT (Asc: 41 +/- 4, Asc+A-I: 40 +/- 4, control: 29 +/- 4; P < 0.05). Compared with the control site, the change in %CVC(max) within each site after NOS-I was greater in HT (Asc: -19 +/- 4, Asc+A-I: -17 +/- 4, control: -9 +/- 2; P < 0.05) than in NT. Antioxidant supplementation alone or combined with arginase inhibition augments attenuated reflex cutaneous VD in hypertensive skin through NO- and non-NO-dependent mechanisms.     

Transient receptor potential vanilloid type 1 channels contribute to reflex cutaneous vasodilation in humans

Mechanisms underlying the cutaneous vasodilation in response to an increase in core temperature remain unresolved. The purpose of this study was to determine a potential contribution of transient receptor potential vanilloid type 1 (TRPV-1) channels to reflex cutaneous vasodilation. Twelve subjects were equipped with four microdialysis fibers on the ventral forearm, and each site randomly received 1) 90% propylene glycol + 10% lactated Ringer (vehicle control); 2) 10 mM l-NAME; 3) 20 mM capsazepine to inhibit TRPV-1 channels; 4) combined 10 mM l-NAME + 20 mM capsazepine. Whole body heating was achieved via water-perfused suits sufficient to raise oral temperature at least 0.8°C above baseline. Maximal skin blood flow was achieved by local heating to 43°C and infusion of 28 mM nitroprusside. Systemic arterial pressure (SAP) was measured, and skin blood flow was monitored via laser-Doppler flowmetry (LDF). Cutaneous vascular conductance (CVC) was calculated as LDF/SAP and normalized to maximal vasodilation (%CVC(max)). Capsazepine sites were significantly reduced compared with control (50 ± 4%CVC(max) vs. 67 ± 5%CVC(max), respectively; P < 0.05). l-NAME (33 ± 3%CVC(max)) and l-NAME + capsazepine (30 ± 4%CVC(max)) sites were attenuated compared with control (P < 0.01) and capsazepine (P < 0.05); however, there was no difference between l-NAME and combined l-NAME + capsazepine. These data suggest TRPV-1 channels participate in reflex cutaneous vasodilation and TRPV-1 channels may account for a portion of the NO component. TRPV-1 channels may have a direct neural contribution or have an indirect effect via increased arterial blood temperature. Whether the TRPV-1 channels directly or indirectly contribute to reflex cutaneous vasodilation remains uncertain.     

Neuropeptide Y Y5 receptors inhibit kindling acquisition in rats

Neuropeptide Y inhibits neuronal excitability and seizures in various experimental models. This peptide delays kindling epileptogenesis but the receptors involved in this action are unknown. We have studied the role of Y5 receptors in kindling using the selective antagonist GW438014A (IC50=210 nM), a small heterocycle molecule that crosses the blood-brain barrier, and the selective peptide agonist Ala31Aib34 NPY (IC50=6.0 nM). Intraperitoneal injection of GW438014A (10 mg/kg), 30 min before the beginning of a rapid-kindling protocol, significantly accelerated the rate of kindling acquisition as compared to vehicle-injected rats. Thus, the number of electrical stimuli required to reach stages 3 and 4-5 of kindling were reduced by 50% and 25%, respectively. The average afterdischarge duration in the stimulated hippocampus was prolonged by 2-fold. Conversely, kindling rate was delayed by intracerebroventricular administration of 24 nmol Ala31Aib32 NPY. Thus, the number of stimuli necessary to reach stages 2 and 3 of kindling was increased by 3- and 4-fold, respectively. During the stimulation protocol (40 stimuli) none of the rats treated with the Y5 agonist showed stages 4-5 seizures. Twenty-four hours after the last kindling stimulation, thus during the re-test session, Y5 agonist- or antagonist-treated rats had stages 4-5 seizures as their controls. In rats treated with both the antagonist and the agonist, kindling rate was similar to vehicle-injected rats. These data indicate that Y5 receptors mediate inhibitory effects of NPY in kindling and display anticonvulsant rather then antiepileptogenic effects upon agonist stimulation.     

Regulation of Neuropeptide Y Release by Neuropeptide Y Receptor Ligands and Calcium Channel Antagonists in Hypothalamic Slices

Neuropeptide Y (NPY) is an important regulator of energy balance in mammals through its orexigenic, antithermogenic, and insulin secretagogue actions. We investigated the regulation of endogenous NPY release from rat hypothalamic slices by NPY receptor ligands and calcium channel antagonists. High-potassium stimulation (60 mM) of the slices produced a calcium-dependent threefold increase in NPY release above basal release. The Y2 receptor agonists NPY(13-36) and N-acetyl[Leu28,Leu31]NPY(24-36), the Y4 agonist rat pancreatic polypeptide (rPP), and the Y4/Y5 agonist human pancreatic polypeptide (hPP) significantly reduced both basal and stimulated NPY release. NPY(13-36)-induced reduction of NPY release could be partially prevented in the presence of the weak Y2 antagonist T4-[NPY(33-36)]4, whereas the hPP- and rPP-induced inhibition of release was not affected by the Y5 antagonist CGP71683A or the Y1 antagonist BIBP3226. The selective Y1, Y2, and Y5 antagonists had no effect on either basal or potassium-stimulated release when administered alone. The calcium channel inhibitors omega-conotoxin GVIA (N-type), omega-agatoxin TK (P/Q-type), and omega-conotoxin MVIIC (Q-type) all significantly inhibited potassium-stimulated NPY release, without any effect on basal release, whereas nifedipine had no effect on either basal or stimulated release. Addition of both omega-conotoxin GVIA and omega-agatoxin TK together completely inhibited the potassium-stimulated release. In conclusion, we have demonstrated that NPY release from hypothalamic slices is calcium-dependent, involving N-, P-, and Q-type calcium channels. NPY release is also inhibited by Y2 agonists and rPP/hPP, suggesting that Y2 and Y4 receptors may act as autoreceptors on NPY-containing nerve terminals.     

Prolonged head-down tilt exposure reduces maximal cutaneous vasodilator and sweating capacity in humans.

Cutaneous vasodilation and sweat rate are reduced during a thermal challenge after simulated and actual microgravity exposure. The effects of microgravity exposure on cutaneous vasodilator capacity and on sweat gland function are unknown. The purpose of this study was to test the hypothesis that simulated microgravity exposure, using the 6 degrees head-down tilt (HDT) bed rest model, reduces maximal forearm cutaneous vascular conductance (FVC) and sweat gland function and that exercise during HDT preserves these responses. To test these hypotheses, 20 subjects were exposed to 14 days of strict HDT bed rest. Twelve of those subjects exercised (supine cycle ergometry) at 75% of pre-bed rest heart rate maximum for 90 min/day throughout HDT bed rest. Before and after HDT bed rest, maximal FVC was measured, via plethysmography, by heating the entire forearm to 42 degrees C for 45 min. Sweat gland function was assessed by administering 1 x 10(-6) to 2 M acetylcholine (9 doses) via intradermal microdialysis while simultaneously monitoring sweat rate over the microdialysis membranes. In the nonexercise group, maximal FVC and maximal stimulated sweat rate were significantly reduced after HDT bed rest. In contrast, these responses were unchanged in the exercise group. These data suggest that 14 days of simulated microgravity exposure, using the HDT bed rest model, reduces cutaneous vasodilator and sweating capacity, whereas aerobic exercise training during HDT bed rest preserves these responses.     

Neuropeptide Y receptor in the rat brain

The specific binding of the chloramine-T iodinated neuropeptide Y (125I-NPY) to membranes from rat cerebral cortex was investigated using equilibrium binding and kinetic methods. The equilibrium binding of 125I-NPY at 37 degrees C was characterized by a Kd value of 0.38 nM. The receptor densities in the cerebral cortex, hypothalamus and cerebellum were 0.45 pmol/mg, 0.47 pmol/mg and 0.04 pmol/mg protein respectively. The binding site for 125I-NPY was sensitive to treatment with proteolytic enzymes and thiol reagents. The binding showed a sharp optimum at pH 7-7.7 and was inhibited by increasing concentrations of Mg2+.     

Reduced Neuropeptide Y Concentrations in Suicide Brain

Neuropeptide Y (NPY) was measured in postmortem brain tissue from victims of suicide and from individuals dying a sudden natural or accidental death (controls). Concentrations of NPY-immunoreactivity were measured by radioimmunoassay in frontal cortex (BA 10), temporal cortex (BA 22), caudate nucleus, and cerebellum. Concentrations of NPY-immunoreactivity were significantly lower in postmortem frontal cortex (-14%) and caudate nucleus (-27%) from suicide victims compared with age-matched controls. A subgroup of suicides with evidence of a history of depression revealed more robust reductions in concentrations of NPY-immunoreactivity in frontal cortex and caudate nucleus, as did four individuals who died from natural causes and also were described as having a possible history of depression. Concentrations of NPY-immunoreactivity in temporal cortex and cerebellum from victims of suicide or from the subgroup of subjects with a possible history of depression were not significantly different from those of age-matched controls. We suggest there is a deficit in the brain NPY system leading to region-specific reductions in peptide concentrations in subjects who have a history of depression.     

Neuropeptide Y, ubiquitous and elusive

This paper reviews aspects of NPY research that were emerging in 1985, shortly after the isolation and characterization of the peptide. NPY had become known for its widespread distribution especially in the central and peripheral nervous systems, but also in the gastro-intestinal and respiratory tracts and in fibers innervating smooth muscle around blood vessels. Consistent with its distribution, it was determined that NPY is a potent vasoconstrictor, affects neuroendocrine systems and is involved in appetite regulation--areas of research still relevant today. Through advances in technology knowledge about NPY's role in these and newly discovered physiological functions has deepened considerably. Successful cloning of a series of NPY receptors has opened up new and complex research vistas. Lately, the creation of mice genetically modified for NPY as well as for several receptor subtypes has brought many puzzling observations--followed by questions yet to be answered.