Release of neuropeptide Y upon haemorrhagic hypovolaemia in relation to vasoconstrictor effects in the pig

Neuropeptide Y is co-stored with noradrenaline in peripheral sympathetic nerves, but is not present in the adrenal chromaffin cells in the pig. Plasma levels of neuropeptide Y-like immunoreactivity and catecholamines were studied upon haemorrhagic shock in the anaesthetized pig. The animals were bled in two successive steps (30 and 10 ml kg^–), resulting in a reduction of the mean arterial blood pressure by 44% and 53 %, respectively. Plasma levels of noradrenaline increased abruptly after the first bleeding from 1.21 ± 0.27 to 26.5 ± 6.3 nmol 1^-1. Plasma neuropeptide Y showed a progressive increase from 62 ± 8 pmol 1^–1 in the basal state to 365 ± 98 pmol 1^–1 at 60 min after the first bleeding. After the second bleeding plasma neuropeptide Y and noradrenaline showed a largely parallel increase and finally reached levels of 2524 ± 580 pmol 1^–1 and 316 ± 117 nmol 1^–1, respectively. A veno-arterial gradient of neuropeptide Y and noradrenaline indicating local release was present over the spleen after both bleeding steps. The overflow of neuropeptide Y was delayed about 15 min compared to noradrenaline after the initial bleeding. Depletion of the neuropeptide Y content after shock in the heart and skeletal muscle supported local release also from these organs. Infusions of neuropeptide Y to obtain similar plasma concentrations as during shock (nM range) caused reduction in blood flow as determined by the radionuclide-labelled microsphere technique in several organs including spleen and skeletal muscle (threshold response at 319 ± 22 pmol 1^–1) but not in heart and brain. In conclusion, both neuropeptide Y and noradrenaline were markedly elevated in plasma upon haemorrhagic shock, suggesting release from sympathetic nerve terminals. Neuropeptide Y could therefore have a role as a sympathetic neurotransmitter, and during severe stress, circulating plasma levels are in the range where vasoconstriction is evoked by exogenous NPY.     

Sympathetic vascular control of the pig nasal mucosa (2): Reserpine-resistant, non-adrenergic nervous responses in relation to neuropeptide Y and ATP

The possible occurrence of non-adrenergic mechanisms in the sympathetic vascular control of the nasal mucosa was studied in vivo using reserpine-treated pigs (1 mg kg-1, i.v., 24 h earlier) in combination with pharmacological blockade of alpha-adrenoceptors by local phenoxybenzamine (1 mg kg-1, i.a.) infusion. The nasal mucosal depletion (99%) of the content of noradrenaline (NA) in reserpinized animals was not influenced by preganglionic denervation while the depletion (44%) of neuropeptide Y (NPY) was prevented. Upon stimulation with single shocks, 25% of the arterial blood flow reduction and 47% of the nasal mucosal volume reduction (reflecting contraction of venous sinusoids) were still present after reserpine as compared with controls. In reserpinized animals, the vascular responses were slow developing and long-lasting, and about 60% remained at 0.59 Hz and more than 80% at 6.9 Hz. The vascular effects after reserpine were, however, subjected to fatigue, which may explain why phenoxybenzamine treatment still reduced the functional effects in the absence of NA. Local intra-arterial injections of NA, NPY and the metabolically stable adenosine-5'-triphosphate analogue alpha, beta-methylene ATP (mATP) caused reduction in both arterial blood flow and nasal mucosal volume. The C-terminal fragment of NPY (NPY 13-36) also induced nasal vasoconstriction although with a fivefold lower potency than NPY 1-36. Adenosine-5'-triphosphate caused a biphasic vascular effect with vasodilatatory actions at low doses and a short-lasting vasoconstriction followed by vasodilatation at very high doses (100-fold higher than the threshold response to mATP). In contrast to the response to NA, the long-lasting vascular effects of NPY and mATP were resistant to phenoxybenzamine treatment. In conclusion, although NA is likely to mediate most of the sympathetic vascular responses to low-frequency stimulation in the pig nasal mucosa, a large resistance and capacitance vessel component upon high-frequency stimulation seems to be non-adrenergic and mimicked by NPY rather than ATP.     

Time-dependent effects of ischaemia on neuropeptide Y mechanisms in pig renal vascular control in vivo

We have investigated the effects of ischaemia on neuropeptide Y (NPY) mechanisms involved in sympathetic vascular control of the pig kidney in vivo. Reperfusion after 2 h of renal ischaemia was associated with local overflow of noradrenaline (NA) but not of NPY-like immunoreactivity (-LI). Renal sympathetic nerve stimulation 10 min into reperfusion evoked markedly reduced vasoconstrictor effects and significantly less overflow of NA (reduced by 70% from the pre-ischaemic conditions), whereas NPY-LI overflow was unaltered. Renal vasoconstrictor responses to exogenous peptide YY (PYY), phenylephrine and angiotensin II were strongly attenuated after this ischaemic period, while vasoconstriction to α,β-methylene ATP was maintained to a larger extent. The renal vascular responses and NA overflow had become partially normalized within a 2 h recovery period. In contrast, the renal vasoconstrictor response and the overflow of NPY-LI upon sympathetic nerve stimulation were enhanced after 15 min of renal ischaemia. In parallel, the PYY-evoked renal vasoconstriction was selectively and markedly prolonged after the 15 min of ischaemia. In the presence of the NPY Y₁ receptor antagonist BIBP 3226, the augmented vascular response to nerve stimulation was significantly attenuated. We conclude that reperfusion after 2 h of renal ischaemia is associated with local overflow of NA, whereas the sympathetic nerve-evoked release of NA and the reactivity of the renal vasculature to vasoconstrictor stimuli are reversibly reduced. Furthermore, possibly due to an impaired local degradation, the role of neurogenically released NPY in renal sympathetic vasoconstriction is enhanced after short-term (15 min) ischaemia compared with control conditions.     

Release of endothelin-like immunoreactivity in relation to neuropeptide Y and catecholamines during endotoxin shock and asphyxia in the pig

The changes in endothelin-like immunoreactivity in plasma during various provocations in the pig were investigated and related to those of neuropeptide Y, noradrenaline and adrenaline. Release as revealed by overflow was determined in the spleen, kidney and femoral vascular bed (skeletal muscle) simultaneously by collecting local venous and arterial blood samples. Under basal conditions there was no net release of endothelin-like immunoreactivity from any region but a net removal (negative overflow) over the kidney. Endotoxin administration (20 micrograms kg-1 h-1 for 4 h) increased arterial endothelin-like immunoreactivity, neuropeptide Y-like immunoreactivity, noradrenaline and adrenaline seven-, 27-, 100- and 166-fold respectively, as well as splenic and renal vascular resistance. An increased overflow of endothelin-like immunoreactivity, neuropeptide Y-like immunoreactivity and noradrenaline, indicating local release, was observed in the spleen during endotoxin administration. The arterial plasma endothelin-like immunoreactivity during endotoxaemia correlated significantly with the splenic and renal vasoconstriction (r = 0.75 and 0.68 respectively). Chromatographic characterization revealed that the main portions of arterial plasma endothelin-like immunoreactivity collected during endotoxaemia corresponded to synthetic endothelin-1 and big endothelin. A similar uptake (50-90%) and plasma half-life (1-2 min) of exogenous endothelin-1-like immunoreactivity was observed both under control conditions and after endotoxin, suggesting that elevated plasma endothelin-like immunoreactivity after endotoxin was the result not of reduced clearance but rather of enhanced release. Asphyxia for 2 min did not increase arterial endothelin-like immunoreactivity but evoked an increased overflow of endothelin-like immunoreactivity, neuropeptide Y-like immunoreactivity and noradrenaline as well as vasoconstriction in the spleen. Capsaicin induced a release of neuropeptide Y-like immunoreactivity and noradrenaline from both the spleen and the kidney and of adrenaline from the adrenal, but no detectable overflow of endothelin-like immunoreactivity from any of the vascular regions. Renal nerve stimulation, renal artery occlusion for 30 min, haemorrhagic shock, hypotension induced by nitroprusside infusion or serotonin did not cause any detectable increase in arterial plasma levels or local overflow of endothelin-like immunoreactivity. It is concluded that plasma levels of endothelin-like immunoreactivity are increased, suggesting release in the pig in response to endotoxin administration and asphyxia. The possible involvement of endothelin as a mediator of the peripheral vasoconstrictor responses during these situations remains to be further established.     

Neuropeptide Y and differential sympathetic control of splenic blood flow and capacitance function in the pig and dog

The roles of different mediators in the sympathetic regulation of the pig and dog spleens were investigated using a preparation with intact vascular perfusion in vivo. Sympathetic nerve stimulation caused overflow of neuropeptide Y-like immunoreactivity (NPY-LI) and noradrenaline (NA), arterial vasoconstriction, increase in venous blood flow and haematocrit. The dog spleen responded to single impulse stimulation, whereas more prolonged stimulation was required to elicit vascular responses in the pig spleen. Furthermore, the maximal splenic capacitance response was about 10 times larger in the dog than in the pig. After depletion of neuronal NA content by reserpine combined with preganglionic denervation, about 70% of the splenic arterial vasoconstrictor responses in the dog and pig still remained at 5 Hz stimulation. Fifty per cent of the capacitance response evoked by nerve stimulation still remained in the pig while in the dog spleen the capacitance response was virtually abolished after reserpine. The stimulation-evoked overflow of NPY-LI in pig spleen was increased several fold after reserpine treatment as compared to controls reaching levels in the venous effluent where exogenous NPY evokes vasoconstriction. In the dog spleen, overflow of NPY-LI was only observed after reserpine. Administration of NA caused arterial vasoconstriction with an initial increase in venous blood flow while NPY mainly reduced arterial blood flow. It is concluded that NA is involved in both the splenic arterial vasoconstriction and the capacitance responses while a non-adrenergic splenic vasoconstriction at least in the pig may be mediated by NPY.     

Mechanisms underlying pre- and postjunctional effects of neuropeptide Y in sympathetic vascular control

The effects of porcine neuropeptide Y (NPY) regarding sympathetic vascular control were studied in vitro on isolated rat blood vessels. The 10(-9)M NPY enhanced (about two-fold) the contractile responses to transmural nerve stimulation (TNS), noradrenaline (NA) and adrenaline (about two-fold) in the femoral artery. Higher concentrations of NPY (greater than 10(-8)M) caused an adrenoceptor-resistant contraction per se. The TNS-evoked [3H]NA efflux was significantly reduced by NPY in a concentration-dependent manner (threshold 10(-9)M). The calcium antagonist, nifedipine, abolished the contractile effects of NPY and the NPY-induced enhancement of NA contractions but did not influence the prejunctional inhibition of [3H]NA release. Receptor-binding studies showed that the ratio of alpha 1-to alpha 2-adrenoceptors in the femoral artery was 30:1. The NPY did not cause any detectable change in the number of alpha 1-or alpha 2-adrenoceptor binding sites or in the affinity of alpha 2-binding sites, as revealed by prazosin- and clonidine-binding, respectively. The NPY also inhibited the TNS-evoked [3H]NA release (by 42-86%) in the superior mesenteric and basilar arteries and in femoral and portal veins. The NPY still depressed TNS-evoked [3H]NA secretion from the portal vein in the presence of phentolamine. The NPY caused a clear-cut contraction in the basilar artery, increased the contractile force of spontaneous contractions in the portal vein, while only weak responses were observed in the superior mesenteric artery and femoral vein. The NA-induced contraction was markedly enhanced by NPY in the superior mesenteric artery, only slightly enhanced in the portal vein and uninfluenced in the femoral vein. In conclusion, in all blood vessels tested, NPY depresses the TNS-evoked [3H]NA secretion via a nifedipine-resistant action. Furthermore, NPY exerts a variable, Ca2+-dependent vasoconstrictor effect and enhancement of NA and TNS contractions.     

Splanchnic and renal vasoconstrictor and metabolic responses to neuropeptide Y in resting and exercising man

The local clearance of neuropeptide Y (NPY) and whether NPY influences splanchnic and renal metabolism in man have not been investigated previously. The influence of NPY on splanchnic and renal blood flows at physiologically elevated levels has also not been investigated. The effects of a 40-min constant NPY infusion (3 pmol kg^-1 min^-1) at rest and during 130 min of exercise (50% of Vo_2max) were studied in six healthy subjects and compared with resting and exercising subjects receiving no NPY. Blood samples were drawn from arterial, hepatic and renal vein catheters for the determination of blood flows (indicators: cardiogreen and paraaminohippuric acid [PAH]), NPY, catecholamines, glucose, lactate and glycerol. NPY infusion was accompanied by: (1) significant fractional extraction of NPY-like immunoreactivity (NPY-Li) by splanchnic tissues at rest (58±5%) and during exercise (53±6%), while no arterial–venous differences could be detected across the kidney; (2) a reduction in splanchnic and renal blood flows of up to 18 and 13% respectively (P < 0.01–0.001) at rest without any additional changes during exercise; and (3) metabolic changes as reflected in: (a) a more marked fall in arterial glucose during exercise compared to the reference group (P < 0.05); (b) a 35% lower splanchnic glucose release (P < 0.01) during exercise due to diminished glycogenolysis (P < 0.01); and (c) a lower arterial lactate level (18%P < 0.05) together with unchanged splanchnic lactate uptake during exercise, suggesting reduced lactate production by extrahepatic tissues. The disappearance of plasma NPY-Li after the infusions was biphasic with two similar half-lives at rest (4 and 39 min) and during exercise (3 and 43 min).     

Splanchnic and renal vasoconstrictor and metabolic responses to neuropeptide Y in resting and exercising man.

The local clearance of neuropeptide Y (NPY) and whether NPY influences splanchnic and renal metabolism in man have not been investigated previously. The influence of NPY on splanchnic and renal blood flows at physiologically elevated levels has also not been investigated. The effects of a 40-min constant NPY infusion (3 pmol kg-1 min-1) at rest and during 130 min of exercise (50% of VO2max) were studied in six healthy subjects and compared with resting and exercising subjects receiving no NPY. Blood samples were drawn from arterial, hepatic and renal vein catheters for the determination of blood flows (indicators: cardiogreen and para-aminohippuric acid [PAH]), NPY, catecholamines, glucose, lactate and glycerol. NPY infusion was accompanied by: (1) significant fractional extraction of NPY-like immunoreactivity (NPY-Li) by splanchnic tissues at rest (58 +/- 5%) and during exercise (53 +/- 6%), while no arterial-venous differences could be detected across the kidney; (2) a reduction in splanchnic and renal blood flows of up to 18 and 13% respectively (P less than 0.01-0.001) at rest without any additional changes during exercise; and (3) metabolic changes as reflected in: (a) a more marked fall in arterial glucose during exercise compared to the reference group (P less than 0.05); (b) a 35% lower splanchnic glucose release (P less than 0.01) during exercise due to diminished glycogenolysis (P less than 0.01); and (c) a lower arterial lactate level (18% P less than 0.05) together with unchanged splanchnic lactate uptake during exercise, suggesting reduced lactate production by extrahepatic tissues. The disappearance of plasma NPY-Li after the infusions was biphasic with two similar half-lives at rest (4 and 39 min) and during exercise (3 and 43 min).     

Prolonged non-adrenergic inhibition of cardiac vagal action following sympathetic stimulation: neuromodulation by neuropeptide Y?

After short periods of intense sympathetic activation, the action of the vagus nerve on heart rate is inhibited for periods of many minutes to several hours. This effect is not abolished by effective blockade of alpha- or beta-adrenoceptors and is not duplicated by administration of noradrenaline or isoprenaline. A similar, long lasting vagal inhibition is caused by administration of neuropeptide Y, a probable cotransmitter in sympathetic nerves.     

Effects of cervical sympathetic nerve stimulation and neuropeptide Y (NPY) on cranial blood flow in the cat

Effects of cervical sympathetic nerve stimulation (SNS) at 10 Hz and intravenous infusion of neuropeptide Y (NPY), 10 and 100 pmol x kg body wt-1 x min-1 for 5 min, on regional blood flow in the cat were investigated with radioactive microspheres. Sympathetic nerve stimulation caused significant reductions in blood flows in the facial tissues including the eye. Alpha-adrenoceptor blockade with phenoxybenzamine and combined beta- and alpha-adrenoceptor blockade with propranolol and phenoxybenzamine abolished the effects of sympathetic nerve stimulation in most facial tissues except in the tongue, upper eyelid and masseter muscle. In most cranial tissues, neuropeptide Y reduced regional blood flow and increased vascular resistance. No effect of neuropeptide Y on vascular resistance was observed in the choroid. In the present study, evidence for a non-adrenergic component in sympathetic vasoconstriction was found in the tongue, upper eyelid and masseter muscle but not in the majority of feline facial tissues. Neuropeptide Y was a potent vasoconstrictor in many cranial tissues, while in parts of the uvea, the effects of neuropeptide Y were less pronounced.     

Renal sympathetic nerve activation in relation to reserpine-induced depletion of neuropeptide Y in the kidney of the rat

The effect of reserpine treatment on renal sympathetic nerve activity and tissue levels of neuropeptide Y (NPY)-like immunoreactivity (LI) and noradrenaline (NA) were studied in rats. Injection of reserpine (1 mg kg-1 i.v.) caused a clear-cut (about 50%) increase in rectified activity of the post-ganglionic sympathetic nerves to the kidney within 15 min in chloralose-anaesthetized rats compared to a saline-treated control group. This increase in nerve activity was still maintained 120 min after the reserpine injection. The renal nerve activation was accompanied by a progressive fall in mean arterial blood pressure and an initial tachycardia. In a separate group of conscious rats, the levels of NPY-LI (1.3 +/- 0.06 pmol g-1) and NA (1.6 +/- 0.07 nmol g-1) in the kidney were significantly reduced (by 74 and 83%, respectively) 24 h after reserpine treatment (1 mg kg-1 i.v.). The reserpine-induced depletion of NPY-LI, but not that of NA, was inhibited by pretreatment with the ganglionic blocking agent chlorisondamine or the alpha 2-adrenoceptor agonist clonidine, both of which are known to decrease renal sympathetic nerve activity. The tissue content of NPY-LI in the right atrium (16.3 +/- 0.7 pmol g-1) was not reduced by reserpine. Arterial plasma NPY-LI in the rat was high (222 +/- 5 pmol l-1), and this value did not change after pretreatment with reserpine, chlorisondamine or clonidine, indicating that, in the rat, circulating NPY-LI is not a good indicator of sympatho-adrenal activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

C-peptide potentiates the vasoconstrictor effect of neuropeptide Y in insulin-dependent diabetic patients

Recent findings suggest that proinsulin C-peptide improves renal and nerve function as well as microcirculation in patients with insulin-dependent diabetes possibly by stimulating Na-K+-ATPase activity. Furthermore, in vitro studies on proximal rat renal tubule cells show that the effect of C-peptide on Na+, K+-ATPase activity is potentiated in the presence of the vasoconstrictor peptide neuropeptide Y. The aim of the present study was to examine whether the effects of neuropeptide Y on resting forearm blood flow in insulin-dependent patients is altered in the presence of C-peptide. Forearm blood flow was measured by a plethysmographic method in eight insulin-dependent patients and six healthy control subjects. Neuropeptide Y (20, 200 and 2000 pmol min(-1)) was infused into the brachial artery before and during an i.v. infusion of C-peptide (5 pmol kg(-1) min(-1)). Basal blood flow was 36.7 +/- 2.2 mL min(-1) L(-1) tissue. It decreased in a dose dependent manner by 11 +/- 2, 18 +/- 3 and 25 +/- 3%, respectively, during infusion of neuropeptide Y. Administration of C-peptide increased basal blood flow by 25 +/- 6%, to 46.3 +/- 3.5 mL min(-1) L(-1) tissue (P < 0.01) and forearm glucose uptake by 76 +/- 34% (P < 0.05). Infusion of the three doses of neuropeptide Y during administration of C-peptide decreased forearm blood flow by 14 +/- 4, 22 +/- 3 and 42 +/- 4%. There was a significant difference (43%, P < 0.001) between the reduction in blood flow evoked by the high dose (2000 pmol min(-1)) of neuropeptide Y before and during C-peptide infusion. Similar differences were also obtained when data were calculated as changes in vascular resistance. C-peptide did not affect resting forearm blood flow or the response to neuropeptide Y in healthy controls. In conclusion, the present data demonstrate that C-peptide increases resting forearm blood flow and augments the vasoconstrictor effects of neuropeptide Y in insulin-dependent patients.     

Possible involvement of neuropeptide Y in sympathetic vascular control of canine skeletal muscle

Sympathetic nerve stimulation (2 min, 2 and 10 Hz) increased perfusion pressure in the blood perfused canine gracilis muscle in situ after pretreatment with atropine, desipramine and beta-adrenoceptor antagonists. This vasoconstriction was accompanied by clear-cut increases in the overflow of endogenous noradrenaline (NA) at both frequencies and, at 10 Hz but not at 2 Hz, also of neuropeptide Y-like immunoreactivity (NPY-LI). The irreversible alpha-adrenoceptor antagonist phenoxybenzamine enhanced the nerve stimulation induced overflows of NA and NPY-LI five- to eightfold and threefold, respectively. The fractional overflows of NA and NPY-LI per nerve impulse were similar in response to the high-frequency stimulation, indicating equimolar release in relation to the tissue contents of the respective neurotransmitter. The maximal vasoconstrictor response elicited by 10 Hz was reduced by about 50% following a dose of phenoxybenzamine which abolished the effect of exogenous NA and the remaining response was more long-lasting. Local i.a. infusion of NPY evoked long-lasting vasoconstriction in the presence of phenoxybenzamine, while the stable adenosine 5(1)-triphosphate (ATP) analogue alpha-beta-methylene ATP was without vascular effects. Locally infused NPY reduced the nerve stimulation evoked NA overflow by 31% (P less than 0.01) at 1 microM in arterial plasma, suggesting prejunctional inhibition of NA release. In conclusion, NPY-LI is released from the canine gracilis muscle upon sympathetic nerve stimulation at high frequencies. There is nerve stimulation evoked vasoconstriction, which is resistant to alpha-adrenoceptor blockade. This may in part be mediated by NPY released together with NA from the sympathetic vascular nerves.     

The relationship of terminals containing neuropeptide Y-like immunoreactivity to lumbar sympathetic vasoconstrictor neurones in the rabbit

The distributions of preganglionic neurones retrogradely labelled with horseradish peroxidase and of immunohistochemically identified varicose axons reactive for neuropeptide Y (NPY-IR) in the intermediate zone of rabbit lumbar spinal segments have been compared. Neurones projecting in the caudal lumbar sympathetic chain (LSC), which are mainly vasoconstrictor, were located laterally, associated with dense accumulations of NPY-IR terminals in mid L3 to caudal L4. In all other regions of the lumbar intermediate zone, there was relatively little correlation between the location of either LSC neurones, or those projecting to the pelvic viscera, and aggregations of NPY-IR axons. Most of these axons probably arise in the C1 nucleus, but it seems possible that this nucleus innervates only some groups of lumbar vasoconstrictor neurones.     

Neuropeptide Y may mediate effects of sympathetic nerve stimulations on colonic motility and blood flow in the cat

The present study investigated sympathetic mechanisms involved in the regulation of colonic motility and blood flow in the cat. Infusion of neuropeptide Y (NPY) close i.a. produced an inhibition of colonic motility and a vasoconstriction of long duration but no post-infusion vasodilatation. In contrast to NPY, porcine pancreatic polypeptide did not evoke any vascular or motility response. On a molar basis, NPY was 25 times more potent than noradrenaline in producing 50% reduction of the colonic blood flow. These vascular and motility effects of NPY were resistant to guanethidine, phentolamine, phenoxybenzamine and propranolol. Thus, the action of NPY on vascular and colonic smooth muscle did not seem to be mediated via adrenergic receptors. Noradrenaline administered close i.a. produced inhibition of colonic motility, and vasoconstriction followed by a rapid vasodilatation. These effects were completely blocked by combined alpha- and beta-adrenoceptor blockade. Electrical stimulation of the splanchnic and lumbar colonic nerves produced an overall inhibition of colonic motility, and vasoconstriction of the proximal and distal colon, respectively, with a rapid post-stimulatory vasodilatation. After combined alpha- and beta-adrenoceptor blockade the inhibitory effect of the nerve stimulations on colonic motility partly remained together with a marked vasoconstriction, which was most pronounced upon lumbar colonic nerve stimulation. All vascular effects of sympathetic nerve stimulation were eradicated by guanethidine, which also abolished the inhibitory motility response to splanchnic nerve stimulation. However, lumbar colonic nerve stimulation elicited a colonic contraction, possibly due to stimulation of afferent C-fibres. The present findings indicate the existence of a sympathetic nonadrenergic neuronal mechanism mediating vasoconstriction and inhibition of colonic motility in the cat. Thus, NPY may be released from noradrenergic neurons to act on colonic smooth muscle and vessels.     

Neuropeptide Y (NPY) and sympathetic control of blood flow in oral mucosa and dental pulp in the cat

Neuropeptide Y (NPY) immunoreactivity (-IR) was found to be present in perivascular nerves in the cat dental pulp and oral mucosa. Many ganglion cells in the superior cervical ganglion also contained NPY-IR. Ligation of the inferior alveolar or lingual nerves produced an accumulation of NPY-IR in axons proximal to the site of ligation, suggesting an anterograde axonal transport of the peptide. After unilateral sympathectomy the NPY-IR disappeared in the dental pulp and oral mucosa on the ipsilateral side. Reversed phase high performance liquid chromatography showed that the main peak of NPY-like immunoreactivity found in the superior cervical ganglion co-chromatographed with synthetic porcine NPY. Changes in blood flow in dental pulp or oral mucosa were measured indirectly by recording local clearance of ¹²⁵I during electrical stimulation of the sympathetic nerve or during close intra-arterial infusion of noradrenaline or NPY. All three procedures resulted in a pronounced decrease in local blood flow of a similar magnitude in both tissues. After a-adrenoceptor blockade with phentolamine, the vasoconstrictor effect of noradrenaline was abolished. However, the effect of sympathetic stimulation after phentolamine was only partially reduced (23–54%) and the response to NPY was almost unaffected by the a-receptor blockade. The remaining effect of sympathetic stimulation after phentolamine was abolished by guanethidine. However, the response to NPY was not changed by the latter drug. In conclusion, the vasoconstrictor response in the dental pulp and oral mucosa caused by activation of sympathetic nerves is more resistant to phentolamine than the response induced by infusion of exogenous noradrenaline. Since NPY is probably co-localized with noradrenaline in the sympathetic perivascular nerves and NPY reduces local blood flow,it is proposed that this peptide is involved in sympathetic vascular control in oral tissues.     

Decoding rule from vasoconstrictor skin sympathetic nerve activity to nonglabrous skin blood flow in humans at normothermic rest

Although an importance of vasoconstrictor skin sympathetic nerve activity (SNA) in control of cutaneous circulation is widely recognized, the decoding rule that translate dynamic fluctuations of vasoconstrictor skin SNA into skin blood flow is not fully understood. In 10 male subjects who rested in supine position under normothermic condition, we measured skin blood flow index (by laser-Doppler flowmetry) at the dorsum pedis, and vasoconstrictor skin SNA (by microneurography) that was confirmed to innervate the same region as the flow index. We determined the transfer and coherence functions from the neural activity input to the flow and quantified the contribution and predictability from the input to output by system engineering technique. The results showed that in frequency-domain analysis, the transfer function from vasoconstrictor skin SNA to skin blood flow had low-pass filter characteristics with 3.6 ± 0.1 s of pure time delay. The coherence function was approximately 0.5 between 0.01 and 0.1 Hz and less above 0.1 Hz. In time-domain analysis, the predictability from the SNA to the skin blood flow was approximately 50%. These findings indicate that at normothermic rest, the decoding rule from vasoconstrictor skin SNA to skin blood flow of skin is characterized by low-pass filter with 3–4 s of pure time delay, and that the vasoconstrictor skin SNA contributes to a half of fluctuation of skin blood flow in the condition. The incomplete dependence of skin blood flow on vasoconstrictor skin SNA may confirm nonneural mechanisms to control cutaneous circulation even at normothermic rest.     

Frequency- and reserpine-dependent chemical coding of sympathetic transmission: differential release of noradrenaline and neuropeptide Y from pig spleen

The importance of impulse pattern and stimulation frequency for the release of noradrenaline (NA) and the coexisting peptide neuropeptide Y (NPY) in relation to vasoconstriction (perfusion-pressure increase) was studied in the blood-perfused pig spleen in vivo. Splenic nerve stimulation with intermittent bursts at high frequency (20 Hz) caused a several-fold larger release of NPY-like immunoreactivity (-LI) in relation to NA than a continuous stimulation at a low frequency (2 Hz), giving the same total number of impulses. alpha-Adrenoceptor blockade by phentolamine enhanced markedly both NA and NPY release, especially at low stimulation frequency, suggesting prejunctional adrenergic inhibition of release. Addition of propranolol unmasked a large remaining perfusion-pressure response to nerve stimulation. Reserpine treatment reduced the NA content of the spleen as well as the stimulation-evoked NA release by greater than 90%. However, the perfusion-pressure increase in response to nerve stimulation was well maintained. A marked increase in the stimulation-evoked release of NPY-LI occurred after reserpine. Adrenoceptor blockade after reserpine treatment reduced only slightly the perfusion-pressure response in parallel with a decline in NPY output. NPY caused an adrenoceptor-resistant perfusion-pressure increase at plasma concentrations that were in the same range as the maximal increase during nerve stimulations. In conclusion, the present data suggest a frequency-dependent, chemical coding of sympathetic transmission with preferential release of the classical transmitter NA at low, continuous frequencies and release of NPY, mainly at high frequencies. Reserpine treatment enhances markedly NPY release, which may explain why the functional response is largely intact in spite of adrenoceptor blockade and marked NA depletion.     

Modulation of sympathetic neurotransmission by neuropeptide Y Y2 receptors in rats and guinea pigs

We have investigated the effect of the Y₂ receptor agonist (Y₂ agonist; N-acetyl [Leu^28,31] NPY 24-36), on contractions evoked by transmural electrical stimulation of sympathetic nerves of isolated arteries from a range of vascular beds in rats and guinea pigs. Contractions evoked by transmural stimulation of the rat renal, mesenteric and femoral arteries were significantly attenuated in the presence of the Y₂ agonist. In these arteries, contractions were significantly inhibited in the presence of an α-adrenoceptor antagonist (76–97%). So we conclude that these responses were primarily mediated by noradrenaline and that the Y₂ agonist attenuates the release of noradrenaline via presynaptic Y₂ receptors. Contractions of the rat carotid artery were not attenuated by the Y₂ agonist but were completely abolished in the presence of an α-adrenoceptor antagonist suggesting that in this artery the Y₂ agonist has no effect on release of noradrenaline. In the guinea pig, carotid arteries contractions evoked by transmural nerve stimulation were attenuated in the presence of the Y₂ agonist and inhibited by an α-adrenoceptor antagonist 75–87% suggesting that the Y₂ agonist attenuates the release of noradrenaline via presynaptic Y₂ receptors in this vessel. In the guinea pig femoral artery contractions evoked by transmural stimulation were not modified in the presence of the Y₂ agonist but were completely abolished in the presence of an α-adrenoceptor antagonist. This suggests that the Y₂ agonist does not modify noradrenaline release in this vessel. Contractions of the guinea pig mesenteric artery were significantly potentiated by the Y₂ agonist, possibly by potentiation of neuropeptide Y (NPY) at the Y₁ receptor. The Y₁ antagonist inhibited more than 70 % of the response, indicating that the majority of the contraction was mediated by NPY. The current study demonstrates heterogeneity of neurotransmitter substances in sympathetic nerves supplying vascular beds within and across species and in subsequent functional response.Supported by the National Health and medical Research Council of Australia.