Characterization and presynaptic modulation of stimulation-evoked exocytotic co-release of noradrenaline and neuropeptide Y in guinea pig heart

Effect of Neuropeptide Y on Action Potential Generation in Working Cardiomyocytes of the Right Atrium in Rat Heart

We studied the effect of neuropeptide Y in concentrations of 10^-8-10^-6 M on electrical activity of adult rat right atrial cardiomyocytes with preserved spontaneous activity. Neuropeptide Y was found to modulate the amplitude-time parameters of action potential: in concentrations of 10^-7 and 10^-6 M it reduced the membrane potential, increased the amplitude of action potential and duration of the repolarization phase, and reduced the frequency of action potential generation. In concentration of 10^-6 M, neuropeptide Y produced stronger effect on the analyzed parameters, while in concentration of 10^-8 M it produced no significant changes.

The role of neuropeptides in adverse myocardial remodeling and heart failure

In addition to traditional neurotransmitters of the sympathetic and parasympathetic nervous systems, the heart also contains numerous neuropeptides. These neuropeptides not only modulate the effects of neurotransmitters, but also have independent effects on cardiac function. While in most cases the physiological actions of these neuropeptides are well defined, their contributions to cardiac pathology are less appreciated. Some neuropeptides are cardioprotective, some promote adverse cardiac remodeling and heart failure, and in the case of others their functions are unclear. Some have both cardioprotective and adverse effects depending on the specific cardiac pathology and progression of that pathology. In this review, we briefly describe the actions of several neuropeptides on normal cardiac physiology, before describing in more detail their role in adverse cardiac remodeling and heart failure. It is our goal to bring more focus toward understanding the contribution of neuropeptides to the pathogenesis of heart failure, and to consider them as potential therapeutic targets.

Dysregulation of Norepinephrine Release in the Absence of Functional Synaptotagmin 7

Synaptotagmin 7 (Syt7) is expressed in cardiac sympathetic nerve terminals where norepinephrine (NE) is released in both Ca(2+)-dependent exocytosis and Ca(2+)-independent norepinephrine transporter (NET)-mediated overflow. The role of Syt7 in the regulation of NE release from cardiac sympathetic nerve terminals is tested by employing a Syt7 knock-in mouse line that expresses a non-functional mutant form of Syt7. In cardiac sympathetic nerve terminals prepared from these Syt7 knock-in mice, the Ca(2+)-dependent component of NE release was diminished. However, these terminals displayed upregulated function of NET (∼130% of controls) and a significant increase in Ca(2+)-independent NE overflow (∼140% of controls), which is greater than the Ca(2+)-dependent component of NE exocytosis occurring in wild-type controls. Consistent with a significant increase in NE overflow, the Syt7 knock-in mice showed significantly higher blood pressures compared to those of littermate wild-type and heterozygous mice. Our results indicate that the lack of functional Syt7 dysregulates NE release from cardiac sympathetic nerve terminals.

Coreleased orexin and glutamate evoke nonredundant spike outputs and computations in histamine neurons

Stable wakefulness requires orexin/hypocretin neurons (OHNs) and OHR2 receptors. OHNs sense diverse environmental cues and control arousal accordingly. For unknown reasons, OHNs contain multiple excitatory transmitters, including OH peptides and glutamate. To analyze their cotransmission within computational frameworks for control, we optogenetically stimulated OHNs and examined resulting outputs (spike patterns) in a downstream arousal regulator, the histamine neurons (HANs). OHR2s were essential for sustained HAN outputs. OHR2-dependent HAN output increased linearly during constant OHN input, suggesting that the OHN→HAN^OHR2 module may function as an integral controller. OHN stimulation evoked OHR2-dependent slow postsynaptic currents, similar to midnanomolar OH concentrations. Conversely, glutamate-dependent output transiently communicated OHN input onset, peaking rapidly then decaying alongside OHN→HAN glutamate currents. Blocking glutamate-driven spiking did not affect OH-driven spiking and vice versa, suggesting isolation (low cross-modulation) of outputs. Therefore, in arousal regulators, cotransmitters may translate distinct features of OHN activity into parallel, nonredundant control signals for downstream effectors.

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Natural orexin release generates unique signatures of brain activity

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Unlike classical transmitter glutamate, orexin release produces enduring communication

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Orexin transmission requires a distinct neural firing code

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Orexin transmission is necessary for brain histamine neurons to integrate inputs

Neuronal and non-neuronal modulation of sympathetic neurovascular transmission

Noradrenaline, neuropeptide Y and adenosine triphosphate are co-stored in, and co-released from, sympathetic nerves. Each transmitter modulates its own release as well as the release of one another; thus, anything affecting the release of one of these transmitters has consequences for all. Neurotransmission at the sympathetic neurovascular junction is also modulated by non-sympathetic mediators such as angiotensin II, serotonin, histamine, endothelin and prostaglandins through the activation of specific pre-junctional receptors. In addition, nitric oxide (NO) has been identified as a modulator of sympathetic neuronal activity, both as a physiological antagonist against the vasoconstrictor actions of the sympathetic neurotransmitters, and also by directly affecting transmitter release. Here, we review the modulation of sympathetic neurovascular transmission by neuronal and non-neuronal mediators with an emphasis on the actions of NO. The consequences for co-transmission are also discussed, particularly in light of hypertensive states where NO availability is diminished.

Preserved norepinephrine reuptake but reduced sympathetic nerve endings in hypertrophic volume-overloaded rat hearts

BACKGROUND

In congestive heart failure (CHF), an activation of the cardiac sympathetic nervous system results in depleted cardiac norepinephrine (NE) stores. The underlying regulatory mechanisms are discussed controversially and were investigated in the present study in CHF resulting from volume overload.

METHODS AND RESULTS

Aorto-caval shunt (AVS) was performed in rats. Plasma NE levels were determined by radioenzymatic assay, left ventricular NE by high-performance liquid chromatography, endothelin-1 by enzyme-linked immunosorbent assay. Tyrosine-hydroxylase (TH)- and nerve growth factor (NGF)-mRNA was determined by Northern blot analysis and ribonuclease-assay. Cardiac [3H]-NE uptake was measured in isolated perfused hearts. Glyoxylic acid-induced histofluorescence was used to quantify cardiac sympathetic nerves. Compared with sham-operated animals (SH), AVS rats were characterized by depleted cardiac NE stores and enhanced NE plasma levels. Neither TH-mRNA levels in stellate ganglia, nor cardiac [3H]-NE-uptake were reduced in AVS. The left ventricular density of sympathetic nerves was markedly decreased. Gene expression of myocardial NGF (a positive regulator of NE reuptake and cardiac sympathetic nerve density) and left ventricular endothelin-1 (a negative regulator of NE reuptake and positive regulator of cardiac NGF expression) were unchanged.

CONCLUSION

In volume-overloaded hypertrophic hearts, depletion of cardiac NE stores is caused by a reduction of the sympathetic nerve density, whereas cardiac NE reuptake is preserved.

The role of adenosine A2A and A3 receptors on the differential modulation of norepinephrine and neuropeptide Y release from peripheral sympathetic nerve terminals

The pre-synaptic sympathetic modulator role of adenosine was assessed by studying transmitter release following electrical depolarization of nerve endings from the rat mesenteric artery. Mesentery perfusion with exogenous adenosine exclusively inhibited the release of norepinephrine (NA) but did not affect the overflow of neuropeptide Y (NPY), establishing the basis for a differential pre-synaptic modulator mechanism. Several adenosine structural analogs mimicked adenosine's effect on NA release and their relative order of potency was: 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride = 1-[2-chloro-6-[[(3-iodophenyl)methyl]amino]-9H-purin-9-yl]-1-deoxy-N-methyl-beta-d-ribofuranuronamide = 5'-(N-ethylcarboxamido)adenosine >> adenosine > N(6)-cyclopentyladenosine. The use of selective receptor subtype antagonists confirmed the involvement of A(2A) and A(3) adenosine receptors. The modulator role of adenosine is probably due to the activation of both receptors; co-application of 1 nM 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride plus 1 nM 1-[2-chloro-6-[[(3-iodophenyl)methyl]amino]-9H-purin-9-yl]-1-deoxy-N-methyl-beta-D-ribofuranuronamide caused additive reductions in NA released. Furthermore, while 1 nM of an A(2A) or A(3) receptor antagonist only partially reduced the inhibitory action of adenosine, the combined co-application of the two antagonists fully blocked the adenosine-induced inhibition. Only the simultaneous blockade of the adenosine A(2A) plus A(3) receptors with selective antagonists elicited a significant increase in NA overflow. H 89 reduced the release of both NA and NPY. We conclude that pre-synaptic A(2A) and A(3) adenosine receptor activation modulates sympathetic co-transmission by exclusively inhibiting the release of NA without affecting immunoreactive (ir)-NPY and we suggest separate mechanisms for vesicular release modulation.

Drosophila neuropeptide F and its receptor, NPFR1, define a signaling pathway that acutely modulates alcohol sensitivity

Alcohol is likely to affect neurons nonselectively, and the understanding of its action in the CNS requires elucidation of underlying neuronal circuits and associated cellular processes. We have identified a Drosophila signaling system, comprising neurons expressing neuropeptide F (NPF, a homolog of mammalian neuropeptide Y) and its receptor, NPFR1, that acutely mediates sensitivity to ethanol sedation. Flies deficient in NPF/NPFR1 signaling showed decreased alcohol sensitivity, whereas those overexpressing NPF exhibited the opposite phenotype. Furthermore, controlled functional disruption of NPF or NPFR1 neurons in adults rapidly confers resistance to ethanol sedation. Finally, the NPF/NPFR1 system selectively mediates sedation by ethanol vapor but not diethyl ether, indicating that the observed NPF/NPFR1 activity reflects a specialized response to alcohol sedation rather than a general response to intoxication by sedative agents. Together, our results provide the molecular and neural basis for the strikingly similar alcohol-responsive behaviors between flies and mammals.

Endothelin (ET)-1-induced inhibition of ATP release from PC-12 cells is mediated by the ETB receptor: differential response to ET-1 on ATP, neuropeptide Y, and dopamine levels

During sympathetic neurotransmitter release, there is evidence for differential modulation of cotransmitter release by endothelin (ET)-1. Using nerve growth factor (NGF)-differentiated PC12 cells, the effects of ET-1 on K(+)-stimulated release of ATP, dopamine (DA), and neuropeptide Y (NPY) were quantified using high-pressure liquid chromatography or radioimmunoassay. ET-1, in a concentration-dependent manner, inhibited the release of ATP, but not DA and NPY. Preincubation with the ET(A/B) antagonist, PD 142893 (N-acetyl-beta-phenyl-D-Phe-Leu-Asp-Ile-Ile-Trp), reversed the inhibitory effect of ET-1 on ATP release, which remained unaffected in the presence of the ET(A)-specific antagonist BQ123 [cyclo(D-Asp-Pro-D-Val-Leu-D-Trp)]. The ET(B) agonists, sarafotoxin 6c (Cys-Thr-Cys-Asn-Asp-Met-Thr-Asp-Glu-Glu-Cys-Leu-Asn-Phe-Cys-His-Gln-Asp-Val-Ile-Trp), BQ 3020 (N-acetyl-[Ala(11,15)]-endothelin 1 fragment 6-21Ac-Leu-Met-Asp-Lys-Glu-Ala-Val-Tyr-Phe-Ala-His-Leu-Asp-IIe-IIe-Trp), and IRL 1620 (N-succinyl-[Glu(9), Ala(11,15)]-endothelin 1 fragment 8-21Suc-Asp-Glu-Glu-Ala-Val-Tyr-Phe-Ala-His-Leu-Asp-Ile-Ile-Trp), decreased K(+)-stimulated release of ATP in a dose-dependent manner, and this effect was reversed by the ET(B) antagonists RES 701-1 [cyclic (Gly1-Asp9) (Gly-Asn-Trp-His-Gly-Thr-Ala-Pro-Asp-Trp-Phe-Phe-Asn-Tyr-Tyr-Trp)] and BQ 788 (N-[N-[N-[(2,6-dimethyl-1-piperidinyl)carbonyl]-4-methyl-l-leucyl]-1-(methoxycarbonyl)-D-tryptophyl]-D-norleucine sodium salt). Preincubation of PC12 cells with pertussis toxin reversed the ET-1-induced inhibition of the K(+)-evoked ATP release. Real-time intracellular calcium level recordings were performed on PC-12 cell suspensions, and ET-1 induced a dose-dependent decrease in the K(+)-evoked calcium levels. Nifedipine, the L-type voltage-dependent Ca(2+) channel antagonist, caused inhibition of the K(+)-stimulated ATP release, but the N-type Ca(2+) channel antagonist, omega-conotoxin GVIA, did not reverse the effect on ATP release. These data suggest that ET-1 modulates the release of ATP via the ET(B) receptor and its associated G(i/o) G-protein through attenuation of the influx of extracellular Ca(2+) through L-type channels.

Modulation of neurotransmitter release by NO is altered in mesenteric arterial bed of spontaneously hypertensive rats

Nitric oxide (NO) reacts with catecholamines resulting in their deactivation. In the present study with the use of the perfused mesenteric arterial bed as a model of the sympathetic neuroeffector junction, the NO synthase (NOS) inhibitor Nω-nitro-l-arginine methyl ester (l-NAME) resulted in the enhancement of the periarterial nerve stimulation-induced increase in perfusion pressure and norepinephrine overflow while decreasing neuropeptide Y (NPY) overflow. These changes were prevented by l-arginine, demonstrating that the effects of l-NAME were specific to the inhibition of NOS. From the fact that norepinephrine acts on prejunctional α2-adrenoceptors to inhibit the evoked release of sympathetic cotransmitters, we carried out experiments in the presence of the α2-adrenergic receptor antagonist yohimbine to investigate the possibility that the decrease in NPY observed in the presence of l-NAME was due to the increase in bioactive norepinephrine acting on its autoreceptor. Periarterial nerve stimulation in the presence of both l-NAME and yohimbine prevented the previously observed decrease in NPY, indicating that the cause of this decrease was, as predicted, due to α2-adrenoceptor activation. The periarterial nerve stimulation-induced increase of norepinephrine overflow was greater in the spontaneously hypertensive rat compared with normotensive rats. In contrast to what was observed in the isolated perfused mesenteric arterial bed obtained from normotensive animals, inhibition of NOS did not result in a further increase in the overflow of norepinephine or in a subsequent decrease in NPY. These results demonstrate that, in addition to being a direct vasodilator, NO, by deactivating norepinephrine, can modulate sympathetic neurotransmission and that this modulation is altered in the spontaneously hypertensive rat.

Involvement of peripheral neuropeptide y receptors in sympathetic modulation of acute cutaneous flare induced by intradermal capsaicin

In a recent study, we have demonstrated that the dorsal root reflex (DRR)-mediated acute cutaneous neurogenic inflammation following intradermal injection of capsaicin (CAP) is sympathetically dependent and subject to modulation by peripheral alpha(1)-adrenoceptors. Postganglionic sympathetic neurons contain not only adrenergic neurotransmitters, but also non-adrenergic substances, including neuropeptide Y (NPY). In this study, we examined if peripheral NPY receptors participate in the flare following CAP injection. Different NPY receptor subtypes were studied by using relatively specific agonists and antagonists for the Y(1) and Y(2) subtypes. Changes in cutaneous blood flow on the plantar surface of the foot were measured using a laser Doppler flowmeter. Following CAP injection, cutaneous flare spread more than 20 mm away from the site of CAP injection. Removal of the postganglionic sympathetic nerves by surgical sympathectomy reduced dramatically the CAP-evoked flare. If the foot of sympathectomized rats was pretreated with either NPY or Y(2) receptor agonists by intra-arterial injection, the spread of flare induced by CAP injection could be restored and prolonged. However, if the spinal cord was pretreated with a GABA(A) receptor antagonist, bicuculline, to prevent DRRs, NPY or an Y(2) receptor agonist no longer restored the CAP-evoked flare. A Y(1) receptor agonist did not affect the CAP-evoked flare in sympathectomized rats. In sympathetically intact rats, blockade of either peripheral NPY or Y(2) receptors with [D-Trp(32)]-NPY or BIIE0246 markedly reduced the flare induced by CAP injection, whereas blockade of peripheral Y(1) receptors by BIBP3226 did not obviously affect the flare. It is suggested that NPY is co-released with NE from the postganglionic sympathetic terminals to activate NPY Y(2) and alpha(1) receptors following CAP injection. Both substances are involved, at least in part, in modulation of the responses of CAP sensitive afferents thereby affecting their ability to evoke the release of inflammatory agents from primary afferents.

Increased cardiac norepinephrine release in spontaneously hypertensive rats: role of presynaptic alpha-2A adrenoceptors

OBJECTIVES AND DESIGN

An increased sympathoadrenergic activation is thought to contribute to the maintenance of elevated blood pressure levels in hypertension. Therefore, the regulation of cardiac presynaptic sympathetic neurotransmission was investigated in spontaneously hypertensive (SHR) and Wistar-Kyoto rats (WKY).

METHODS AND RESULTS

Electrical field stimulation (1 min, 4 Hz) evoked a higher norepinephrine (NE) overflow from isolated perfused SHR than from WKY hearts (171 +/- 78 versus 111 +/- 27 nmol/g; means +/- SD, n = 7, P < 0.05). The difference in stimulation-evoked NE overflow was neither due to increased NE stores nor to a higher density of sympathetic nerve endings in SHR hearts. Furthermore, impairment of cardiac NE re-uptake was ruled out, as pharmacological inhibition of NE re-uptake by desipramine (300 nmol/l) similarly increased NE overflow from SHR (+ 54 +/- 17%) and WKY hearts (+ 59 +/- 18%). However, inhibition of presynaptic alpha-2 adrenoceptors (alpha-2R) with yohimbine (1 micromol/l) resulted in a significantly larger increase in NE overflow from WKY (+ 244 +/- 42%) than from SHR hearts (+ 162 +/- 47%, P < 0.05 versus WKY), indicating impairment of presynaptic inhibitory effect of alpha-2R in SHR. Supporting this notion, mRNA concentrations of alpha-2(A), the predominant presynaptic alpha-2R subtype, were reduced in SHR compared with WKY (738 +/- 251 versus 1468 +/- 518 mRNA molecules/10 ng, n = 7, P < 0.01), as quantified by competitive reverse transcription-polymerase chain reaction derived from left stellate ganglia.

CONCLUSIONS

The impairment of the alpha-2R mediated presynaptic negative feedback mechanism by a reduced expression of the alpha-2R subtype A may increase cardiac net secretion of NE in SHR and could therefore contribute to their hypertensive phenotype.

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.

Evidence of systemic neuropeptide Y release after carbachol administration into the posterior hypothalamic nucleus

The unilateral microinjection of the cholinergic agonist carbachol (CCh) directly into the posterior hypothalamic nucleus (PHN) of conscious rats evokes a dose-dependent increase in mean arterial pressure (MAP). Blockade of peripheral alpha-adrenoceptors and V1-vasopressin receptors completely inhibits this response, suggesting that the increase in MAP is mediated by increases in sympathoadrenal excitation and circulating vasopressin. Combining beta-adrenoceptor blockade with alpha-adrenoceptor and V1-vasopressin receptor blockade results in the return of a pressor response. To determine if neuropeptide Y (NPY) might be responsible for this increase, the putative NPY and irreversible alpha 1-adrenoceptor antagonist benextramine was added to alpha 2- and beta-adrenoceptor and V1-vasopressin receptor blockade provided by yohimbine, propranolol, and [D(CH2)5-Tyr(Me)]AVP (AVPX), respectively. Benextramine noncompetitively inhibited the pressor response to intravenous injection of NPY and the increase in MAP evoked by CCh microinjection into adrenergic and V1-vasopressin receptor-blocked rats, whereas benextramine competitively inhibited the pressor response to angiotensin II (AII). Furthermore, the combination of losartan, the selective AT1-AII receptor antagonist that completely blocked the increase in MAP evoked by intravenous AII, and adrenergic and V1-vasopressin receptor antagonists did not attenuate the pressor response evoked by CCh microinjection into the PHN or the increase in MAP evoked by intravenous injection of NPY. These results indicate that AII was not responsible for the CCh-evoked increase in MAP in the presence of adrenergic and V1-vasopressin receptor blockade. The similarity in the antagonism of the increase in MAP evoked by intravenous NPY injection and by CCh microinjection into the PHN of adrenergic- and V1-vasopressin receptor-blocked rats suggests that NPY might be released from sympathetic neurons after activation of the sympathetic nervous system by central administration of CCh into the PHN.

Calcium and sodium currents evoked by action potential waveforms in rat sympathetic neurones

1. Calcium channel currents evoked by action potential waveforms were recorded from rat sympathetic neurones using the whole-cell variant of the patch-clamp technique. A voltage template was created in which the unmodified action potential (AP) was played back followed by a rectangular pulse. 2. The inhibitory effects of noradrenaline (NA) and neuropeptide Y (NPY) on the AP and rectangular pulse-evoked currents, were compared. The percentage inhibition of the Ca2+ current produced by NA and NPY was significantly greater in the case of currents evoked using AP waveforms. 3. A train of APs was applied in the voltage command (40 or 75 Hz) and the inhibition produced by NPY on the AP-evoked Ca2+ currents was examined. The inhibitory effect of NPY on the Ca2+ currents evoked by the high frequency APs did not change significantly during the train. 4. The AP falling phase was artificially prolonged and the resulting Ca2+ currents were compared. AP prolongation increased the amount of Ca2+ entering into the cell. However, the peak value of the Ca2+ current was primarily determined by the AP height. The AP plateau phase was also prolonged in defined voltage ranges. Prolongation in the positive voltage region was most effective in increasing the Ca2+ current. 5. Ion substitution studies were used to isolate Na+ and Ca2+ currents evoked by AP waveforms. The inhibitory effects of NA and oxotremorine (OXO-M) on Ca2+ currents evoked by AP waveforms were examined in the presence and absence of the Na+ current.(ABSTRACT TRUNCATED AT 250 WORDS)

Time-course and frequency dependence of sympathetic stimulation-evoked inhibition of vagal effects at the sinus node

Vagally-induced chronotropic responses have been shown to be inhibited after the termination of sympathetic stimulation. We sought to characterize the onset and time-course of the sympathetically evoked inhibition of vagal effects by measuring vagally induced chronotropic responses during concomitant sympathetic stimulation. In anesthetized dogs we recorded lead II of the electrocardiogram, arterial pressure and cardiac cycle length. In 7 dogs, the vagi were stimulated for 15 s every minute before, during and after 10-min trains of sympathetic stimulation. The sympathetic stimulation was applied at frequencies of 0.5, 1, 2, 5 and 10 Hz. During the 1, 2 and 5-Hz trains of sympathetic stimulation, the vagally induced changes in cycle length diminished progressively and thus, were less (P < 0.001) at 3, 5 and 10 min compared with 1 min into the sympathetic stimulation. The magnitude of the attenuation of vagal effects on cycle length depended (P < 0.001) on the frequency of sympathetic stimulation. To determine the role of alpha- and beta-adrenergic receptors, we measured vagally-induced changes in cycle length during 5-min trains of sympathetic stimulation (1, 2, 5, 10 Hz) in the presence and absence of phentolamine and propranolol (n = 6). Both before and after combined alpha- and beta-adrenergic receptor blockade, the vagally-induced changes in cycle length decreased (P < 0.03) progressively during the 1, 2, 5 and 10-Hz trains of sympathetic stimulation and the magnitude of the inhibition depended (P < 0.002) on the frequency of sympathetic stimulation. These data show that the effects of short trains of vagal stimulation on cardiac cycle length are inhibited progressively during continuous trains of sympathetic stimulation before and after combined alpha- and beta-adrenergic receptor blockade. Thus, substances other than norepinephrine may contribute to the inhibition of cardiac vagal effects that occurs during a continuous train of sympathetic stimulation.

Neuronal control of coronary blood flow

Controversies on acetylcholine-induced increases or decreases in coronary blood flow arise from obvious species differences, the role of endothelium in mediating vascular smooth muscle responses, and the marked negative chronotropic and inotropic effects of acetylcholine. In man, there appears to be a predominant dilation of intact epicardial coronary arteries and a constriction of artherosclerotic segments. However, at present there is no evidence for a vagal initiation of myocardial ischemia. Coronary vascular beta-adrenergic receptors mediate dilation, but appear to be functionally insignificant during sympathetic activation. The beta-adrenergic mechanism contributing to myocardial ischemia are indirect, mediated by a tachycardia-related redistribution of blood flow away from the ischemic myocardium. alpha-Adrenergic receptors mediating epicardial coronary artery constriction in experimental studies appear not to be responsible for the initiation of ischemia in patients with angina at rest. However, alpha-adrenergic constriction of coronary resistance vessels resulting in the precipitation of post-stenotic myocardial ischemia was demonstrated in experimental studies and recently confirmed in patients with effort angina. Non-adrenergic, non-cholinergic neurotransmitters exist; however, their role in regulating coronary blood flow remains entirely unclear.

Chronotropic cardiac effects of NPY in conscious dogs: interactions with the autonomic nervous system and putative NPY receptors

The chronotropic cardiac effects of neuropeptide Y (NPY) were studied in the conscious dog with chronic atrioventricular block. NPY (0.2-5 micrograms/kg i.v.) produced no effect on atrial cycle length (ACL), and increased ventricular cycle length (VCL) and mean arterial blood pressure (MBP). After atropine, NPY produced no effect on ACL and increased MBP. At 0.2 microgram/kg, it shortened VCL, whereas at 1 and 5 micrograms/kg, it lengthened this parameter. After pindolol, NPY produced no effect on ACL, shortened VCL and increased MBP. These results indicate that in the conscious dog, NPY (0.2-5 microgram/kg i.v.) does not exert any chronotropic effect on the sinoatrial node, most likely because of competition between opposite chronotropic effects and/or absence of specific NPY receptors in the sinoatrial node. They also suggest that the ventricular bradycardic effects produced by NPY result mainly from a reflex withdrawal of beta-adrenergic tone and that its ventricular tachycardic effects result from a direct action of NPY on specific receptors located in the His bundle.

Sympatho-adrenal secretion in humans: factors governing catecholamine and storage vesicle peptide co-release

1. In postganglionic sympathetic neurones and adrenal chromaffin cells, catecholamines are co-stored in vesicles with soluble peptides, including chromogranin A (CgA) and neuropeptide Y (NPY), which are subject to exocytotic co-release with catecholamines. 2. Plasma catecholamine, CgA and NPY responses to stimulators and inhibitors of sympatho-adrenal catecholamine storage and release were measured in humans. Short-term, high-intensity dynamic exercise, prolonged low-intensity dynamic exercise, and assumption of the upright posture, in decreasing order of potency, predominantly stimulated noradrenaline (NA) release from sympathetic nerve endings. Only high-intensity exercise elevated CgA and NPY, which did not peak until 2 min after exercise cessation. Stimulated NA correlated with plasma CgA 2 min after exercise, and with NPY 5 min after exercise. 3. Insulin-evoked hypoglycaemia and caffeine ingestion, in decreasing order of potency, predominantly stimulated adrenaline (AD) release from the adrenal medulla. During insulin hypoglycaemia AD and CgA rose, but NPY was unchanged. Neither NPY nor CgA were altered by caffeine. The rise in CgA after intense adrenal medullary stimulation was greater than its rise after intense sympathetic neuronal stimulation (1.4-versus 1.2-fold, respectively). 4. Infusion of tyramine, which disrupts sympathetic neuronal vesicular NA storage, elevated systolic blood pressure and NA, while NPY and CgA were unchanged. After reserpine, another disruptor of neuronal NA storage, NA transiently rose and then fell; NPY and CgA were unaltered. After the non-exocytotic adrenal medullary secretory stimulus glucagon. AD rose while NA, CgA and NPY did not change. After amantadine, an inhibitor of protein endocytosis, both CgA and fibrinogen rose, while NA and NPY remained unaltered. Neither CgA, NPY, nor catecholamines were altered by the catecholamine uptake and catabolism inhibitors desipramine, cortisol, and pargyline. 5. Human sympathetic nerve contained a far higher ratio of NPY to catecholamines than human adrenal medulla, while adrenal medulla contained far more CgA than sympathetic nerve. 6. It is concluded that peptides are differentially co-stored with catecholamines, with greater abundance of CgA in the adrenal medulla and NPY in sympathetic nerve. Activation of catecholamine release from either the adrenal medulla or sympathetic nerves, therefore, results in quite different changes in plasma concentrations of the catecholamine storage vesicle peptides CgA and NPY. Only profound, intense stimulation of chromaffin cells or sympathetic axons measurably perturbs plasma CgA or NPY concentration; lesser degrees of stimulation perturb plasma catecholamines only. Neither CgA nor NPY are released during non-exocytotic catecholamine secretion.

Neuropeptide Y effector systems: perspectives for drug development

Neuropeptide Y was isolated in 1982 and has since attracted considerable interest. It is widely distributed in central and peripheral neurones and can produce a multitude of biological effects in the brain and the periphery. For example, the peptide has been associated with stimulation of food and water intake, control of mood, and regulation of central autonomic functions. In the periphery, sympathetic neuropeptide Y plays a role as a vasopressor and vasoconstrictor. Neuropeptide Y acts on at least three distinct receptor types, referred to a Y1, Y2 and Y3. This review by Lars Grundemar and Rolf Håkanson focuses on some neuropeptide Y-dependent mechanisms that may be implicated in certain disorders and may be promising targets for drugs active at neuropeptide Y receptors.

Development of neuropeptide Y and tyrosine hydroxylase immunoreactive innervation in postnatal rat heart

Neuropeptide Y (NPY), immunoreactive (IR), and tyrosine hydroxylase (TH)-IR nerve fibers were scarce at birth in rat heart, but increased rapidly during the first 2 postnatal weeks, reaching approximately adult levels by the third week. The sequence of development was: interatrial septum and atrial wall, free ventricular wall starting from the epicardium, and finally the atrial appendages and interventricular septum. In ventricles and atrial appendages both fiber types developed similarly. In interatrial septum and atrial walls more NPY-IR than TH-IR fibers were evident, and NPY-IR, but not TH-IR, neurons were detected in intrinsic ganglia. Double-label immunohistochemistry provided further evidence that NPY is located in ventricular and atrial noradrenergic nerves, but is also located in nonnoradrenergic nerves in atria.

Multiple neuropeptide Y receptors are involved in cardiovascular regulation. Peripheral and central mechanisms

1. Neuropeptide Y (NPY) occurs in both the central and peripheral nervous system. In the periphery, NPY coexists with noradrenaline (NA) in perivascular sympathetic fibers. 2. NPY has a vasopressor effect, reflecting direct vasoconstriction of blood vessels and potentiation of the NA-evoked response. NPY also suppresses the release of NA from sympathetic fibers. 3. The post- and pre-junctional NPY receptors are referred to as Y1 and Y2, respectively. They recognize not only NPY but also the homologous gut hormone peptide YY (PYY). 4. The Y1 and Y2 receptors have been characterized in numerous test systems using analogs of NPY/PYY. Already the deletion of the first N-terminal amino acid (NPY 2-36) results in a marked loss of potency at the Y1 receptor. The Y2 receptor is much less dependent upon an intact N-terminus, and a wide range of C-terminal NPY fragments retain quite high potency. 5. Recently, yet another NPY receptor, Y3, that is distinct from Y1 and Y2 in that it recognizes PYY poorly, has been demonstrated in the brainstem and in the periphery. 6. Further attempts to characterize the various receptor types have relied on truncated and substituted analogs of NPY/PYY. Although such studies suggest the existence of at least three types of NPY receptors, the lack of antagonists has represented a problem. 7. Since NPY may regulate cardiovascular functions via peripheral and central receptors its physiological and possibly pathophysiological significance has attracted much attention. 8. The responsiveness to NPY seems to be altered in animal models of hypertension and elevated plasma levels of NPY have been found in patients under various conditions of stress and in primary hypertension. A number of studies have suggested that NPY may be a pathogenetic factor behind primary hypertension. 9. Antagonists for the various NPY receptors would be useful for an analysis of which effects of these peptides are physiologically relevant. It is tempting to predict that both agonists and antagonists of the NPY receptors could be useful as drugs, for instance, in the treatment of primary hypertension.

Activation of neuropeptide Y1 and neuropeptide Y2 receptors by substituted and truncated neuropeptide Y analogs: identification of signal epitopes

Neuropeptide Y (NPY-(1-36)) acts on Y1 and Y2 receptors at the sympathetic neuroeffector junction. Various truncated NPY analogs were tested in the isolated guinea-pig caval vein where NPY is a vasoconstrictor (Y1 receptors) and in isolated rat vas deferens, by monitoring the suppression of electrically evoked contractions (Y2 receptors). The aim of this study was to define which parts of the NPY-(1-36) molecule were required to activate these receptors. NPY-(1-36), [Pro34]NPY and [Glu16,Ser18,Ala22,Leu28,31]NPY (ESALL-NPY), the latter being an analog with increased alpha-helicity in the 14-31 region, evoked vasoconstriction with similar potency and efficacy. Cyclic as well as linear NPY analogs having the 4 to 7 N-terminal amino acid residues linked to the 9 to 19 C-terminal residues by an 8-aminooctanoic acid (Aoc) residue were 25-50 times less potent than NPY-(1-36) itself. In the cyclic analogs, a disulfide bond was introduced to bring the N- and C-termini close together. Linear Aoc-2-27-NPY was virtually inactive. The Y1 receptor needs an intact N-terminal end of NPY in order to become fully activated. The requirements for the C-terminus are less stringent, since substitutions in this part of the molecule resulted in fully active analogs. The central portion of the molecule may impose steric constraints on the N- and C-terminal ends, thereby facilitating Y1 receptor activation, but it does not seem to be essential for receptor recognition. NPY-(2-36) and NPY-(5-36) were only slightly less potent than the parent molecule in suppressing electrically evoked twitches in the vas deferens.(ABSTRACT TRUNCATED AT 250 WORDS)

Norepinephrine release in rat vas deferens. Effect of rauwolscine and BHT 920

Norepinephrine (NE) overflow from field-stimulated rat was deferens preparations was quantified directly by electrochemical detection using high performance liquid chromatography. The effect of agonist (BHT 920) and antagonist (rauwolscine) of prejunctional alpha 2-adrenoceptors on NE overflow was assessed and compared with their effect on the smooth muscle mechanical response to field stimulation. Increasing the stimulation frequency from 2 to 30 Hz resulted in an increase in muscle tension together with an increase in NE overflow. Addition of 1 microM rauwolscine to the medium resulted in a significant increase in muscle contraction to field stimulation which reached a maximum at 5 Hz. On the other hand, NE overflow increased linearly with the frequency of stimulation within the range studied. Addition of 0.1 microM BHT 920 to the medium significantly decreased the amplitude of contractions at lower stimulation frequencies (2 to 10 Hz) but elicited no significant changes at high frequencies. BHT 920 did not significantly affect NE overflow for all range of stimulation frequency. The simultaneous recording of field-stimulation induced contractions and NE overflow indicates that in the rat vas deferens, rauwolscine acts like a pure alpha 2 adrenoceptor antagonist at a prejunctional level. BHT 920 did not appear to affect selectively prejunctional alpha 2 adrenoceptors but also may activate postjunctional alpha 1 adrenoceptors.

Modulation by adrenergic transmitters of the efferent function of capsaicin-sensitive nerves in cardiac tissue

In atrial preparations obtained from reserpine-pre-treated guinea-pigs, incubated in the presence of 1 microM atropine plus 1 microM CGP 20712A (a beta 1 blocking drug), a positive inotropic effect due to CGRP release from capsaicin-sensitive sensory neurons was induced by electrical field stimulation (EFS). This response was concentration-dependently reduced by noradrenaline (0.01-3 microM), neuropeptide Y (NPY, 3-300 nM) and adenosine triphosphate (ATP, 1-30 microM). On the other hand, the overflow of [3H]-noradrenaline from sympathetic nerve terminals induced by EFS in isolated atria obtained from normal untreated animals was not modified in 10 nM calcitonin gene-related peptide (CGRP). Substance P (SP) and neurokinin A (NKA), at concentrations ranging from 0.01 to 1 microM did not affect the cardiac response to field stimulation of adrenergic terminals of atrial tissue. These findings demonstrate that all the co-transmitters stored in adrenergic nerve terminals have a modulatory role on the efferent function of cardiac capsaicin-sensitive sensory neurons, while cardiac adrenergic neurotransmission is not influenced by the peptidergic transmitters released from sensory neurons.