Neuropeptide Y--a cotransmitter with noradrenaline and adenosine 5'-triphosphate in the sympathetic nerves of the mouse vas deferens? A biochemical, physiological and electropharmacological study

Structure-function analysis of stimulation of food intake by neuropeptide Y: effects of receptor agonists

Neuropeptide Y (NPY) is a potent natural orexigenic signal in the rat. In this study, we have compared the effects of several COOH-terminal fragments of NPY and NPY receptor agonists on cumulative food intake in male rats. Rats were implanted with permanent cannulae either into the third cerebroventricle or paraventricular nucleus (PVN). NPY1-36 and various COOH-terminal fragments of NPY, two agonist analogues [Leu31, Pro34]NPY and NPY 1-4-Aca (epsilon-amino-caproic acid)-25-36, were administered intracerebroventricularly (ICV) or directly into the PVN, and the cumulative 2-h food intake response was compared. We observed that peptides that were effective by ICV were also effective when administered into the PVN, but smaller amounts of the peptides were required after PVN injection to evoke an equivalent food intake response. Injection of NPY1-36 induced a dose-dependent increment in food intake. Surprisingly, deletion of NH2-terminal tyrosine residue did not adversely affect feeding behavior. In fact, NPY2-36 was consistently more effective than NPY1-36; the enhancement in feeding by NPY2-36 was dose-related and was higher than evoked by NPY1-36 at each dose tested. Further serial deletion of aminoacids at NH2-terminal resulted in complete loss of activity. In addition, NPY agonist analogue, NPY 1-4-Aca-25-36, failed to stimulate feeding. However, NPY Y1 receptor agonist, [Leu31, Pro34]NPY, but not Y2 receptor agonist, NPY13-36, stimulated feeding.(ABSTRACT TRUNCATED AT 250 WORDS)

Contraction-mediating alpha 2-adrenoceptors in the mouse vas deferens

The question of the existence of postjunctional, contraction-mediating alpha 2-adrenoceptors, in addition to the known alpha 1-adrenoceptors, was studied in the mouse isolated vas deferens. Both the alpha 1-selective agonist phenylephrine and the alpha 2-selective agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) caused contraction of the vas deferens. In the presence of the alpha 1-selective antagonist prazosin (added in order to prevent an alpha 1 component in the effect of high concentrations of UK 14,304), the alpha 2-selective antagonist yohimbine and idazoxan shifted the concentration-response curve of UK 14,304 to the right in a manner compatible with competitive antagonism and with dissociation constants KB indicating the involvement of alpha 2-adrenoceptors. The maximal contraction elicited by UK 14,304 (in the presence of prazosin) was much lower than the maximal contraction elicited by phenylephrine. The effect of UK 14,304 was not changed by the P2-purinoceptor agonist alpha,beta-methylene-ATP and was reduced by neuropeptide Y, but was markedly enhanced by relatively low concentrations of phenylephrine. When the sympathetic fibres of the vas deferens were stimulated by trains of ten widely spaced (0.5 Hz) electric pulses, the tissue responded with ten separate twitches in which purinergic and adrenergic components were isolated by prazosin and suramin, respectively. Prazosin reduced the first adrenergic twitch in these trains at concentrations close to its KB value at alpha 1-adrenoceptors, whereas yohimbine and idazoxan reduced the first adrenergic twitch at concentrations far lower than their KB values at alpha 1-adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Prejunctional modulatory action of neuropeptide Y on responses due to antidromic activation of peripheral terminals of capsaicin-sensitive sensory nerves in the isolated guinea-pig ileum

1. The effect of neuropeptide Y (NPY) on motor responses produced by activation of capsaicin-sensitive primary afferents in the guinea-pig isolated ileum was determined by use of capsaicin itself and electrical mesenteric nerve stimulation as stimuli. 2. NPY inhibited or suppressed the cholinergic contractile response produced by electrical mesenteric nerve stimulation while leaving the contractile response to a threshold concentration of capsaicin. 3. NPY had no effect on motor responses produced by a submaximal concentration of substance P, the putative endogenous mediator of the 'efferent' function of sensory fibres in this preparation. 4. It is concluded that NPY exerted a prejunctional inhibitory action on transmitter release from peripheral endings of capsaicin-sensitive nerves at interneuronal synapses.

The role of neuropeptides in cardiovascular regulation

During the past few years more than 30 novel, biologically active peptides have been discovered. Some are produced in endocrine glands and circulate as hormones in the blood; others are contained in the enterochromaffin cells of the gut and may be involved in the regulation of intestinal functions. The vast majority of new peptides, however, have been detected in the central and peripheral nervous systems, where they are synthesized in distinct neurons and stored in neurovesicles. Many of these neuropeptides may be involved in circulatory regulation. There is evidence supporting such a role, especially for centrally located angiotensin, opioid peptides, substance P, neuropeptide Y (NPY), vasopressin, atrial natriuretic peptide (ANP), kinins, corticotropin releasing factor, bombesin, and somatostatin. In this review we discuss the cardiovascular actions of angiotensin, neuropeptide Y, and calcitonin gene related peptide.

Sympathetic co-transmission to the cauda epididymis of the rat: characterization of postjunctional adrenoceptors and purinoceptors

1. Electrical field stimulation (10 Hz, 60 V, 1 ms, for 10 s) produced monophasic contractions of isolated preparations of rat cauda epididymis which could be abolished by guanethidine, and attenuated by prazosin and alpha, beta-methylene ATP. 2. The rank order of potency of adrenoceptor agonists in causing contraction of the preparation in the presence of the neuronal uptake blocker, nisoxetine (0.1 microM) was: adrenaline greater than or equal to phenylephrine greater than or equal to noradrenaline greater than clonidine greater than methoxamine greater than metaraminol greater than dopamine greater than or equal to isoprenaline greater than xylazine. 3. Responses to the agonists were blocked by prazosin but not by propranolol or idazoxan. 4. The rank order of potency of purinoceptor agonists in causing contraction of the cauda epididymis was: alpha,beta-methylene ATP greater than beta,gamma-methylene ATP greater than or equal to 2-methylthio ATP greater than ATP greater than ADP. AMP and adenosine did not cause contractions. 5. Contractile responses to the purine nucleotide analogues were blocked by repeated application of alpha,beta-methylene ATP. 6. It is concluded that both ATP and noradrenaline may act as co-transmitters in the sympathetic nerves supplying the smooth muscle of the rat cauda epididymis, and that alpha 1-adrenoceptors and P2x-purinoceptors are present postjunctionally.

Neuropeptide Y upregulates the adhesiveness of human endothelial cells for leukocytes

The nervous system and the autonomic system in particular have been associated with stress-induced changes in host resistance to infections and inflammatory reactions. Since a key step in initiation of inflammation is adhesion of leukocytes to the endothelium, we hypothesized that neuron-derived factors might be involved in this process. Neuropeptide Y (NPY), a 36-amino acid neuropeptide that is colocalized and released with norepinephrine from sympathetic nerves, has already been implicated in inflammatory reactions via modulation of histamine release from mast cells. This study was undertaken to examine the potential role of NPY in proinflammatory processes via modulation of endothelium-leukocyte interaction. NPY (0.01-10 microM) increased the adhesion of 51Cr-labeled human neutrophils or the human monocytic U937 cell line to human umbilical vein endothelial cells in a dose- and time-dependent manner. The stimulation of human umbilical vein endothelial cell adhesiveness occurred as early as 30 minutes and lasted over 48 hours. The increase of leukocyte adhesion to human umbilical vein endothelial cells by NPY was not inhibited by protein synthesis inhibitor cycloheximide, nor was it associated with expression of intercellular adhesion molecule-1 on human umbilical vein endothelial cells; in contrast, strong expression of intercellular adhesion molecule-1 was induced by tumor necrosis factor alpha and lipopolysaccharide endotoxin. These data suggest that neuron-derived factors such as NPY may serve as modulators of not only the neuromuscular unit but also the interaction of endothelial cells with leukocytes. In this capacity, the sympathetic nervous system might play an important role in the regulation of proaggregatory and hemostatic activity of microvessels.

Age-dependent modulation of neuropeptide Y on adrenergic transmission of guinea pig vas deferens

1. The effect of neuropeptide (NPY) on [3H]noradrenaline ([3H]NA) release- and on contractions evoked by field electrical stimulation (FES) was studied in vitro in vas deferens from mature and immature guinea pigs. 2. The evoked tritium overflow (which reflected [3H]NA release) was determined by liquid scintillation spectrometry. 3. Field electrical stimulation of 5 Hz (trains of 50 pulses in 20 sec intervals) evoked guanethidine-sensitive contractions. 4. NPY (0.01-1 microM) dose-dependently inhibited the evoked contractions in both groups of animals. NPY, 1 microM, almost completely inhibited the evoked contractions in mature animals, while those in immature guinea pigs were inhibited but only by 80.4 +/- 3.6%. 5. The amount of tritium overflow evoked by 5 Hz stimulation (300 pulses: 15 trains of 20 pulses in 20 sec intervals) was higher in immature guinea pigs (0.46 +/- 0.03%) compared with the amount of the evoked tritium overflow in mature guinea pigs (0.39 +/- 0.02%). 6. NPY, 1 microM, inhibited the evoked tritium overflow. The NPY inhibition was more pronounced in vas deferens of mature (45.3 +/- 2.0%) than in immature (25.1 +/- 3.5%) guinea pigs. 7. The results suggest that NPY modulation of adrenergic transmission at the prejunctional level increases with the maturity.

Influence of imipramine on neuropeptide Y immunoreactivity in the rat brain

The effects of treatment with the antidepressant drug imipramine on neuropeptide Y immunoreactivity were studied immunocytochemically in the rat brain cortex and hypothalamus. It was found that the level of neuropeptide Y immunoreactivity in the cortex was significantly lowered three and 24 h after the last dose of chronic (14 days) imipramine administration as well as 3 h after acute administration. A tendency to decrease neuropeptide Y immunoreactivity was also found in the hypothalamus. The results obtained suggest an important role of the cortical neuropeptide Y in the action of the drug.

Neuropeptide Y and sympathetic neurotransmission

The coexistence of neuropeptide Y (NPY) with noradrenaline (NA) in perivascular nerves as well as in sympathetic nerves to muscle in the heart, spleen and vas deferens suggests a role for NPY in autonomic transmission. Sympathetic nerve stimulation or reflexogenic activation in experimental animals or man are associated with NPY release as revealed by overflow mainly upon strong activation. This difference between NPY and NA secretion may be related to the partly separate subcellular storage whereby NPY seems to be exclusively present in the large dense-cored vesicles. The NPY secretion is likely to be regulated by the local biophase concentrations of NA acting on prejunctional alpha-2-adrenoceptors since alpha-2 agonists inhibit and antagonists enhance NPY overflow, respectively. Furthermore, after NA has been depleted by reserpine, the nerve stimulation-evoked release of NPY is enhanced leading to a progressive depletion of tissue content of NPY. Exogenous NPY binds to both pre- and postjunctional receptors, inhibits NA and NPY release, enhances NA-evoked vasoconstriction and induces vasoconstriction per se. The prejunctional action of NPY which is especially noticeable in the vas deferens may serve to reduce transmitter secretion upon excessive stimulation. The long-lasting vasoconstriction evoked by sympathetic stimulation in several tissues including skeletal muscle, nasal mucosa and spleen, which remains in animals pretreated with reserpine (to deplete NA) combined with preganglionic denervation (to prevent the concomitant excessive NPY release and depletion), is mimicked by NPY and highly correlated to NPY release. Under these circumstances the NPY content in the local venous effluent reaches levels at which exogenous NPY evokes vasoconstriction.

Pharmacology of noradrenaline and neuropeptide tyrosine (NPY)-mediated sympathetic cotransmission

Pharmacological and physiological aspects for neuropeptide Y (NPY) and noradrenaline (NA) cotransmission have been studied in the peripheral sympathetic nervous control of blood vessels, heart, spleen and vas deferens. NPY coexists with NA in large dense cored vesicles and is released compared to NA mainly upon high frequency stimulation or strong reflex sympathetic activation. NPY release is inhibited via prejunctional alpha-2 adrenoceptors and adenosine receptors but facilitated by angiotensin II or beta-receptor activation. NPY exerts prejunctional inhibitory actions on both NA and NPY release, enhances the vasoconstrictor effect of NA and evokes potent, long-lasting vasoconstriction. Specific receptor mechanisms for NPY exist at both the pre- and postjunctional levels; a large amidated C-terminal portion of NPY is necessary for receptor binding, inhibition of cyclic AMP formation and vasoconstrictor effects. Denervation results in supersensitivity for both NA and NPY-evoked vasoconstriction. Reserpine pretreatment is associated with depletion of NA as well as NPY; the effect on NPY is entirely dependent on an intact nerve activity. Reserpine treatment combined with preganglionic denervation depletes NA by 99% while NPY levels are maintained intact. The characteristic appearance of the nerve stimulation evoked vasoconstrictor response with a high correlation to NPY outflow after reserpine treatment, suggests that NPY may be involved as a transmitter in a variety of vascular beds. NPY-synthesis in ganglia seems to be regulated by nicotinic receptor activity; secondary stimulation by eg reserpine stimulates and nicotine antagonists decrease NPY-synthesis. Many classical pharmacological agents including guanethidine, clonidine, yohimbine, angiotensin II, nicotine and desipramine influence NPY release. A complex interplay therefore seems to occur at both the pre- and postjunctional levels of transmission for the classical transmitter NA and the coexisting peptide NPY, creating a great diversity of chemical signalling potential.

Neuropeptide Y neuromodulation of sympathetic co-transmission in the guinea-pig vas deferens

1. We examined the neuromodulatory effects of neuropeptide Y (NPY) on purinergic and adrenergic co-transmission in the guinea-pig vas deferens. 2. In superfused vas deferens preparations, NPY (0.3 microM) inhibited the stimulus-evoked overflow of both ATP and [3H]-noradrenaline ([3H]-NA) at 2 Hz, but only the stimulus-evoked release of [3H]-NA at 20 Hz. 3. Postjunctionally, NPY greatly enhanced responses to alpha,beta-methylene ATP and to a lesser extent to exogenous NA. 4. Preparations stimulated in organ baths showed frequency-dependent contractions to field stimulation. NPY abolished responses to field stimulation at low frequency and a small number of pulses. At high frequency (20 Hz), NPY abolished responses elicited by 10 pulses, inhibited responses by 50% at 20 pulses and had little effect on preparations stimulated for 240 pulses. 5. Our study suggests that NPY neuromodulates co-transmission in the vas deferens by inhibiting the release of ATP and NA and that these effects predominate over the postjunctional enhancement by NPY. These results also show that the physiological effect of NPY will be determined both by the frequency at which the nerves are discharging and the duration of their firing.

Neuronal and adrenal enkephalins and catecholamines in response to acute CNS ischemia and reserpine in pig

Co-storage of enkephalins and catecholamines in coronary artery, mesenteric artery and vein, middle cerebral artery, vas deferens and adrenal medulla was studied in domestic pig (Sus scrofa). Responses to acute CNS ischemia were correlated with time to peak plasma levels of central venous and adrenal vein outflow samples in controls, during reserpine treatment and after drug withdrawal. Endogenous enkephalins are co-stored in chromaffin granules of adrenal epinephrine-type cells and large dense cored vesicles of noradrenergic terminals. After a lag period, reserpine at near 'therapeutic' doses caused an apparent induction of opioid peptide precursor synthesis accompanied by processing to enkephalins in adrenal medulla up to 8-fold by 30 days and in mesenteric vein up to 4.5-fold by 14 days. Upon 14 days recovery from reserpine, elevated adrenal enkephalins were maintained and depleted catecholamines were largely replenished. Acute CNS ischemia produced rises in MAP (approx. 80 mmHg), marked net depletions of noradrenergic enkephalin stores, and net increases in adrenal vein outflow and central venous levels of enkephalins and catecholamines. Noradrenergic terminals contributed significantly to circulating enkephalins as well as norepinephrine. Reserpine for 7 days nearly abolished all tested responses to acute CNS ischemia, but immediate net 200-400% elevations of endogenous enkephalin stores occurred in coronary artery and mesenteric artery and vein (apparent processing of reserpine-induced neuronal precursor stores). Thus, induction of new synthesis of precursor opioid peptides by reserpine, with or without parallel processing to enkephalins, occurs in noradrenergic terminals in many tissues. All effects of reserpine on endogenous enkephalins implicate a central mechanism to inhibit sympathoadrenal outflow to the periphery. At 14 days recovery from reserpine, when near normal cardiovascular responses to acute CNS ischemia were regained, there was increased net release of the elevated adrenal enkephalins, exaggerated peak plasma enkephalin concentrations, but only minimal depletions of enkephalins from noradrenergic terminals.

Evidence for noradrenergic-purinergic cotransmission in the hepatic artery of the rabbit

1. Transmural electrical field stimulation produced a transient contraction of the isolated hepatic artery of the rabbit that was frequency-dependent up to 64 Hz. A contraction was rarely evoked at a stimulation frequency of less than 8 Hz and never at 4 Hz or less. All contractions were abolished in the presence of tetrodotoxin. 2. Neurogenic contractions were partially inhibited by prazosin. After desensitization of the P2X-purinoceptor with alpha, beta-methylene ATP, contractions in response to electrical stimulation were significantly reduced at all frequencies tested (4-64 Hz). In most cases, contractions were abolished by a combination of both drugs. 3. In vessels treated with 6-hydroxydopamine, no nerve-mediated contractile responses were observed. 4. In arteries from reserpine-treated rabbits, nerve stimulation evoked contractions that were resistant to prazosin. After desentization of the P2X-purinoceptor with alpha,beta-methylene ATP, no neurogenic contractile response remained. 5. The tissue contracted to exogenously applied noradrenaline and alpha,beta-methylene ATP. There was an increase in sensitivity to noradrenaline in 6-hydroxydopamine-treated vessels compared with control vessels, but no difference in potency to alpha,beta-methylene ATP was detected. The potency of noradrenaline and alpha,beta-methylene ATP was not significantly affected by reserpine treatment. 6. In control tissues, fluorescence histochemistry demonstrated the presence of noradrenergic nerve fibres. Noradrenaline-containing nerves were not observed in 6-hydroxydopamine-treated or reserpine-treated vessels. 7. It is concluded that noradrenaline and ATP are cotransmitters in the sympathetic nerves supplying the hepatic artery of the rabbit. In contrast to other vessels, the hepatic artery requires a relatively high frequency of stimulation to evoke contractions and the purinergic component is not frequency-dependent. The significance of this finding in terms of the physiological demands of blood flow to the liver are discussed.

Neuropeptide Y potentiates purinergic as well as adrenergic responses of the rabbit ear artery

The localisation of neuropeptide Y in the rabbit central ear artery and its pharmacological action on this preparation were investigated. Immunohistochemical studies demonstrated the presence of neuropeptide Y-like immunoreactivity in the perivascular nerves supplying the rabbit ear artery. Forty-eight hours after treatment with reserpine (5 mg/kg i.p. 48 h and 3 mg/kg i.p. 24 h prior to the experiment) catecholamine fluorescence in the rabbit central ear artery was abolished and the neuropeptide Y-like immunoreactivity was substantially reduced, suggesting that noradrenaline and neuropeptide Y were colocalised in sympathetic nerves. Contractile responses to exogenous noradrenaline (1 microM) and alpha,beta-methylene ATP (1 microM) were both significantly potentiated following incubation with neuropeptide Y (0.3 microM); the degree of potentiation being similar for both agonists. Electrical field stimulation of the rabbit central ear artery (16 and 64 Hz) produced frequency-dependent contractile responses which were abolished by tetrodotoxin (1 microM) and which were significantly potentiated in the presence of neuropeptide Y (0.3 microM). The responses to stimulation at 16 Hz were enhanced to a greater extent than the responses at 64 Hz. After blocking the noradrenergic component of the neurogenic response with prazosin (1 microM), the residual purinergic component, at both 16 and 64 Hz, was significantly enhanced in the presence of neuropeptide Y. However, following desensitisation of the P2-purinoceptor with alpha,beta-methylene ATP, neuropeptide Y had no significant effect on the residual noradrenergic component.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of peptide cotransmitters from a cholinergic motor neuron under physiological conditions

In previous studies, we demonstrated that B15, one of the two cholinergic motor neurons of the accessory radula closer muscle of Aplysia, synthesizes two peptides, small cardioactive peptides A and B (SCPA and SCPB), that, when exogenously applied, increase the size and relaxation rate of muscle contractions elicited by motor neuron stimulation. In the present experiments, we obtained evidence that the SCPs are released under physiological conditions. Specifically, we characterized firing patterns of motor neuron B15 during normal behavior, simulated them in vitro, and demonstrated that this type of neuronal activity produces decreases in SCP levels in neuronal processes and terminals. We also obtained evidence that suggests that enough SCP is released under physiological conditions to modulate neuromuscular activity in the accessory radula closer. We demonstrated that physiological activity of neuron B15 produces significant increases in muscle cAMP levels. Furthermore, increases in the size and relaxation rate of muscle contractions can be produced by changes in stimulation parameters that are also likely to maximize effects of released endogenous SCPA and SCPB.

Involvement of neuropeptide Y in neuroendocrine stress responses. Central and peripheral studies

Neuropeptide Y (NPY) is closely associated to stress-reactive structures in the central and peripheral nervous system. In the periphery, the peptide is colocalized with catecholamines in postganglionic sympathetic fibres and the adrenal medulla. In the brain, the paraventricular nucleus of the hypothalamus receives a dense innervation of NPYergic neurons, some of which also contain monoamines. With the use of a specific immunoradiometric assay, we have demonstrated that NPY is released into the peripheral circulation during psychological stress together with catecholamines. The postganglionic origin of the peptide was demonstrated by the activity of the nicotinic antagonist hexamethonium to attenuate the response. Adrenalectomy or insulin-induced hypoglycemia did not alter basal or stimulated NPY plasma levels, showing that the adrenal is not a major source of circulating NPY in the rat. Although NPY and noradrenaline are frequently released in parallel in various experimental conditions, a clear dissociation can be found in several cases, such as cold stress or the response to phentolamine, where no change can be seen in plasma NPY despite a large activation of noradrenergic terminals. Furthermore, the neuropeptide may play a role in stress-induced pathological states such as hypertension, since its release is greater in animals previously submitted to chronic stress and high-sodium diet. On the other hand, its role in the central nervous system control mechanisms of the stress response is far from being clear, but to understand the interaction of NPY we need a better knowledge of the role of noradrenergic neurons in the central control of the adrenocortical axis or sympathetic nervous system activity.

125I-neuropeptide Y binding activity of pig spleen cell membranes: effect of solubilisation

Crude preparations of pig spleen cell membranes were obtained by differential centrifugation. 125I-NPY bound specifically to these membranes with a KD of 56 +/- 13 pM and Bmax of 44 +/- 4.0 fmols/mg protein. After treatment with 1% CHAPS* and 10 mM 2-mercaptoethanol in the presence of 2 microM leupeptin, 2 microM pepstatin A, 10 microM phosphoramidon, 200 microM PMSF and 0.1% bacitracin, followed by centrifugation at 100,000 g a soluble preparation was obtained that contained a single population of specific 125I-NPY binding sites. The KD of the soluble receptor was significantly higher at 1.38 +/- 0.2 nM (P less than 0.01) but the Bmax of 59.6 +/- 6.6 fmols/mg protein was similar (N.S.). This is the first description of a method for obtaining NPY receptors in soluble form and should enable their purification and characterisation, though the low affinity of the soluble receptor may reflect disruption of the ligand binding site upon removal of the receptor from the membrane.

Peptides and vasomotor mechanisms

The multiple and diverse roles played by neuropeptide Y, vasoactive intestinal polypeptide, substance P, calcitonin gene-related peptide and other biologically active peptides in the cardiovascular system are considered. A model of the vascular neuroeffector junction is described, which illustrates the interactions of peptidergic and nonpeptidergic transmitters that are possible at pre- and postjunctional sites. The effects of peptides on specific endothelial receptors are also described, which highlights the ability of these agents to act as dual regulators of vascular tone at both adventitial and intimal surfaces, following local release from nerves, or from endothelial cells themselves. Changes in expression of vascular neuropeptides that occur during development and aging in some disease situations and following nerve lesion are discussed.

Influence of reserpine administration on neuropeptide Y immunoreactivity in the locus coeruleus and caudate-putamen nucleus of the rat brain

The effects of treatment with reserpine (10 mg/kg, i.p.) a monoamine depleting agent, on neuropeptide Y immunoreactivity were studied immunohistochemically in neurons of two rat brain structures: locus coeruleus and caudate-putamen nucleus. It was found that reserpine after 24 h increased neuropeptide Y immunoreactivity level but no significant changes were observed 4 and 72 h or 5 days after the injection. The results indicate that despite the known co-existence of neuropeptide Y and noradrenaline in some neurons of the locus coeruleus no concomitant decrease in neuropeptide Y immunoreactivity level was found after reserpine when noradrenaline was depleted from nerve cell bodies and terminals. The increase in neuropeptide Y immunoreactivity observed 24 h after reserpine injection may suggest that the neuropeptide Y-containing neuronal systems of the locus coeruleus and caudate-putamen nucleus are controlled by monoaminergic afferents.

Role of adenylate cyclase in modulatory effect of neuropeptide Y on [3H]noradrenaline release in guinea-pig vas deferens

1. The effect of neuropeptide Y (NPY) on [3H]noradrenaline ([3H]NA) release-evoked by 5-Hz electrical stimulation or 5 microns calcium ionophore A23187 was studied in vitro in guinea-pig vas deferens. 2. The evoked tritium overflow (which reflected [3H]NA release) was determined by liquid scintillation spectrometry. 3. NPY, 1 microM, reduced electrically-evoked tritium overflow. NPY reduction was more pronounced upon 20-sec, 3 msec continuous stimulation (73.2 +/- 4.4%) and upon 5-min, 1 msec intermittent stimulation (47.8 +/- 2.4%) as compared to the reduction upon 5-min, 1 msec continuous stimulation (24.3 +/- 3.8%). Forskolin (0.1-1 microM) and theophylline (0.65-1.25 mM) dose-dependently diminished this NPY reducing effect. 4. NPY, 1 microM, reduced A23187-evoked tritium overflow by 52.3 +/- 7.1%. Forskolin (5 microM) and theophylline (1.25 mM) significantly decreased the effect of NPY. 5. It is concluded that in guinea-pig vas deferens NPY reduces [3H]NA release through affecting adenylate cyclase and the processes responsible for calcium mobilization.

Prejunctional modulatory action of neuropeptide Y on peripheral terminals of capsaicin-sensitive sensory nerves

1. We have determined the effect of neuropeptide Y (NPY) on motor responses produced by activation of capsaicin-sensitive primary afferents in the guinea-pig isolated left atria (reserpine-pretreatment, atropine in the bath) and bronchi (atropine and indomethacin in the bath) using capsaicin itself and electrical field stimulation as stimuli. 2. In both preparations, NPY inhibited or suppressed the response produced by electrical field stimulation while leaving the response to a submaximal concentration of capsaicin unaffected. 3. NPY had no effect on motor responses produced by a submaximal concentration of calcitonin gene-related peptide (atria) or neurokinin A (bronchi), the putative endogenous mediators of the responses produced by activation of the 'efferent' function of sensory fibres in these preparations. 4. We conclude that NPY exerts a prejunctional inhibitory action on transmitter release from peripheral endings of capsaicin-sensitive nerves. Failure of NPY to modulate responses activated by capsaicin provides further evidence for the existence of two independent modes of activation of the 'efferent' function of capsaicin-sensitive sensory nerves.

Angiotensin neuromodulation of adrenergic and purinergic co-transmission in the guinea-pig vas deferens

1. The effects of angiotensin II (AII) and angiotensin III (AIII) on the isolated vas deferens of the guinea-pig were studied via three parameters: the overflow of adenosine 5'-triphosphate (ATP) and tritiated noradrenaline (NA), the mechanical response to field stimulation and the mechanical response to exogenous NA and alpha, beta-methylene ATP (alpha, beta-mATP). 2. At 2 Hz, AII enhanced the overflow of ATP and NA, whereas at 20 Hz, AII enhanced the overflow of NA but was without significant effect on ATP overflow. AIII, at 2 Hz, inhibited the overflow of ATP, but enhanced NA overflow, whereas at 20 Hz ATP overflow was unaffected, but NA overflow was still enhanced. 3. At 2 Hz, AII enhanced both phases of the response to field stimulation and at 20 Hz the overall response. AIII at 2 Hz enhanced the adrenergic response, but was without effect on the purinergic response to field stimulation. At 20 Hz, AIII was without effect on the overall response. 4. AII enhanced responses to exogenous NA and alpha, beta-mATP, whereas AIII was without effect. 5. These results provide evidence that both ATP and NA release are capable of being modulated by angiotensins. Furthermore, modulation of ATP release is frequency-dependent, whereas [3H]-NA release is not. These results raise questions about the mechanisms of storage and release of the sympathetic co-transmitters NA and ATP; they also show that angiotensin receptors in the guinea-pig vas deferens are not a homogeneous population.

Indirect evidence for separate vesicular neuronal origins of norepinephrine and ATP in the rabbit vas deferens

Various modulators of neurotransmission were examined for selective effects on the non-adrenergic or adrenergic components of neurotransmission in the vas deferens to test the hypothesis that ATP and norepinephrine are secreted from the same vesicles. The ATP receptor antagonist, arylazido aminopropionyl ATP (ANAPP), selectively depressed the non-adrenergic contraction and prazosin selectively depressed the adrenergic contraction in response to electrical stimulation. These results are consistent with the presence of two neurotransmitters, ATP and norepinephrine, which mediate neurogenic contractions. Prostaglandin E2 inhibited non-adrenergic, but enhanced adrenergic, electrically induced (10 Hz) contractions, presumably via a prejunctional mechanism. The adrenergic component of the neurogenic response was significantly more sensitive to treatment with guanethidine, guanabenz, and 6-hydroxy-dopamine. These results with a variety of agents are inconsistent with the hypothesis that the neurotransmitters. ATP and norepinephrine, are released in tandem from the same neuronal granules.

Pharmacological characterization of neuropeptide Y and noradrenaline mechanisms in sympathetic control of pig spleen

The mechanisms underlying the functional effects of neuropeptide Y (NPY)-like immunoreactivity (LI) and noradrenaline (NA) and their release evoked by nerve stimulation were studied with the blood-perfused pig spleen in vivo. Infusion of selective agonists and antagonists suggested the presence of alpha 1- and beta 2-adrenoceptors mediating vasoconstriction and vasodilatation, respectively. NPY caused a slight inhibition of stimulation-evoked [3H]NA release and a clearcut non-adrenergic vasoconstriction. Local pretreatment with phentolamine and prazosin as well as with clonidine and UK 14304 reduced the perfusion pressure response to nerve stimulation. Phentolamine, yohimbine and idazoxan enhanced while clonidine and UK 14304 decreased the output of [3H]NA or NA and NPY-LI. The subsequent addition of propranolol to the alpha-adrenoceptor antagonists was followed by reappearance at a considerable portion of the perfusion pressure response while the output of [3H]NA or NA and NPY-LI was slightly reduced. It is concluded that NPY exerts pre- and post-junctional actions in pig spleen that regulate both NA release and vascular tone. alpha 1-Adrenoceptors are mainly involved in vasoconstriction, and prejunctional alpha 2 mechanisms inhibit both NA and NPY release at a low frequency of stimulation. beta 2-Adrenoceptors mediate vasodilatation when NA release is enhanced with a minor effect on mediator secretion.

Some pharmacological applications of an extracellular recording method to study secretion of a sympathetic co-transmitter, presumably ATP

Extracellular recording in guinea-pig or mouse vas deferens or rat tail artery was used to study the effects of some pharmacological agents on the nerve terminal spike (NTS) and the secretion of a sympathetic co-transmitter (presumably ATP), as reflected in the excitatory junction current (EJC). A negative-going EJCi (i for inside) was assumed to reflect release from sites inside, and a positive-going EJCo (o for outside) release from sites outside the recording electrode. Passage into or out of the electrode seemed to be slow. Tetrodotoxin (TTX) in the outer medium blocked the NTS and ECJo as well as EJCi; TTX in the pipette blocked stimulus-evoked but not spontaneous EJCi. The dihydropyridine Ca2+ channel blocking agent, nifedipine, was without effect, but Cd2+ in the external medium blocked EJCo and also, by an effect apparently 'upstream' of varicosities, inhibited EJCi (i.e. release within the patch) but not the NTS. When present in the outer medium the alpha 2-adrenoceptor agonists, clonidine and xylazine, blocked both EJCo and EJCi, but not the NTS. The effects of clonidine were blocked by yohimbine, which in itself increased the EJCo by about 50%. Neuropeptide Y and met-enkephalin in the outer medium blocked EJCo; the effect of met-enkephalin was blocked by naloxone. The K+ channel blocking agents, tetraethylammonium and 4-aminopyridine, inside or outside the electrode, increased dramatically the size of EJCi or EJCo, respectively.

Noradrenergic, purinergic, and cholinergic transmission in the mouse vas deferens: influence of field-stimulation parameters, reserpinization, 6-hydroxydopamine and 4-aminopyridine

In the field-stimulated mouse vas deferens the twitch inhibiting potency of prazosin (1 microM) and alpha,beta-methylene ATP (MeATP, 10 microM) was studied, using two types of stimulation-response curves, (a) variation of frequency from 3 to 100 Hz at a constant pulse width of 0.1 ms and (b) variation of pulse width from 0.04 to 0.8 ms at a constant frequency of 15 Hz. Prazosin and MeATP reduced the twitch response by eliminating the noradrenergic and purinergic component, respectively. After the combined application of both compounds a small third twitch component remained that was most prominent at high frequencies. Reserpinization reduced the effect of prazosin but enhanced that of MeATP and increased the cholinergic component. 6-Hydroxydopamine enhanced the effects of prazosin and MeATP to the same extent, but left the cholinergic component intact. In vasa pre-loaded with [3H]-noradrenaline, field stimulation induced a larger release of tritium at high frequency and short pulse duration (100 Hz, 0.1 ms) than at lower frequency and long pulse duration (15 Hz, 0.3 ms). Prazosin (1 microM) augmented both the spontaneous and the stimulation-induced overflow of tritium, whereas MeATP (10 microM) had only a negligible negative effect on the outflow of label. In conclusion, the twitch contraction of the mouse vas deferens has a small cholinergic component in addition to the noradrenergic and purinergic components. Adrenergic and purinergic transmission seem not to run strictly in parallel: the purinergic component dominates during stimulation at low frequency and long pulse duration, and after reserpinization; 4-aminopyridine enhances the adrenergic mechanism more than the purinergic one.

Interactions of calcium antagonists and the calcium channel agonist Bay K 8644 on neurotransmission of the mouse isolated vas deferens

1. The present study compares the effects of verapamil and Bay K 8644 on twitches of the mouse vas deferens induced by field stimulation at 0.1 Hz. The influence of interactions between these drugs and nifedipine on neurotransmission was also investigated. 2. Bay K 8644 (0.1 nM-3 microM) and verapamil (1-100 microM) potentiated twitches maximally by about 1000% (EC50 17.3 nM) and 300% (EC50 17.5 microM), respectively. Nifedipine (0.1 nM-1 microM) only reduced twitch magnitude (IC50 7.7 nM). All effects were reversed following washout. 3. Yohimbine (1-100 microM) reversed twitch potentiation caused by verapamil but not by Bay K 8644. Prazosin (1 microM) did not reduce basal twitch tension nor antagonize twitch potentiation by verapamil. 4. Twitch inhibition by nifedipine was unaltered by previous incubation with verapamil (30 microM), but Bay K 8644 (1 microM) shifted the curve to nifedipine 120 fold to the right. Previous incubation with nifedipine (1 microM) blocked potentiation induced by verapamil but did not modify responsiveness to Bay K 8644. 5. Previous addition of verapamil (30 microM) markedly enhanced twitch potentiation caused by Bay K 8644 in a supra-additive fashion. In experiments conducted in the reversed condition, Bay K 8644 (1 nM but not 10 nM) potentiated the effect of verapamil in a similar manner but to a lesser extent. 6. It is concluded that verapamil, in contrast to nifedipine, markedly enhances neurally-evoked twitches of the mouse vas deferens. Bay K 8644 produces essentially the same effect as verapamil, but its potency is 1000 fold and its maximal effect about 3 fold greater than that observed for verapamil. It is suggested that the mechanism of twitch potentiation by verapamil is different from that of Bay K 8644 and may involve an increased release of non-adrenergic co-transmitter(s).

Novel autonomic neurotransmitters and intestinal function

A wide variety of substances, including amines and peptides, have been detected within the complex neuronal pathways of the enteric nervous system using immunohistochemical techniques. In this article we have discussed some of the more recent data on the effects of these substances on intestinal activity. We have also commented on the many difficulties associated with ascribing neurotransmitter status to individual compounds. The technique of immunoblockade of neurogenic functional responses has been used in an attempt to identify some of the putative neurotransmitter substances. The search for selective antagonists continues.

Central administration of neuropeptide Y induces hypothermia in mice. Possible interaction with central noradrenergic systems

Neuropeptide Y (0.24 and 1.17 nmol icv) and clonidine (0.025, 0.05 and 0.1 mg/Kg ip) induced a slight decrease of short duration of the rectal temperature in mice in a dose-dependent manner. While pretreatment with yohimbine (0.5 mg/Kg sc), was without effect on neuropeptide Y-induced hypothermia, it attenuated the hypothermic effect of clonidine. The association of neuropeptide Y (0.05 and 0.24 nmol icv) with clonidine (0.0125, 0.025, 0.05 and 0.1 mg/Kg ip) induced a synergistic effect, but it only was significant when neuropeptide Y 0.05 and 0.24 nmol icv was associated with clonidine 0.1 mg/Kg ip and when neuropeptide Y 0.05 nmol icv was associated with clonidine 0.05 mg/Kg ip. These results suggest that the effect of neuropeptide Y is not mediated by an interaction on alpha 2-adrenoceptor, but in accordance with these results, the existence of a collaborative mechanism between both neuropeptide Yergic and noradrenergic systems cannot be ruled out.

An increase in the expression of neuropeptidergic vasodilator, but not vasoconstrictor, cerebrovascular nerves in aging rats

Perivascular nerve fibres containing noradrenaline (NA), serotonin (5-HT), substance P (SP), vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY) and calcitonin gene-related peptide (CGRP) were localized in whole-mount stretch preparations of the arteries of the rat circle of Willis using fluorescence and immunohistochemical techniques. Changes in the pattern and density of these perivascular nerves were studied from birth to 27 months of age. All perivascular nerve types reached a peak density of innervation at 1 month of age. This was followed by a general fall in the density of fluorescent nerve fibres. However, with aging, there was a decrease in the expression of vasoconstrictor neurotransmitters (NA and 5-HT) in cerebrovascular nerves, whereas the expression of vasodilator neurotransmitter (VIP and CGRP) in perivascular nerve fibres supplying the rat cerebral arteries was strikingly increased in old age. The density of NPY- and SP-containing nerve fibres was not significantly altered in old age. These changes are discussed in relation to the increased incidence of cerebrovascular disorders in the elderly.

Concurrent separation of catecholamines, dihydroxyphenylglycol, vasoactive intestinal peptide, and neuropeptide Y in superfusate and tissue extract

A method is described for separation and quantification of 3,4-dihydroxyphenylglycol (DO-PEG), norepinephrine (NE), dopamine (DA), vasoactive intestinal peptide (VIP), and neuropeptide Y (NPY) from single samples of tissue homogenate and from superfusate from in vitro dog blood vessel preparations using cartridges containing 0.4 g of octadecylsilane (Sep-Pak C-18). Samples were passed through the cartridge at pH 7.4. A step-gradient system was used to first selectively desorb the catechols (DOPEG, NE, DA) with a moderately polar eluent; subsequently VIP and NPY were eluted with 2.5 ml of a mixture of 1% trifluoroacetic acid, 80% acetonitrile. Five Sep-Pak catechol eluents were tested. Catechols were quantified by HPLC with electrochemical detection and peptides by radioimmunoassay. An HPLC solvent system is described which is particularly useful for chromatography of the more hydrophilic catechols DOPEG, 3,4-dihydroxymandelic acid, and 3,4-dihydroxyphenylalanine concurrently with catecholamines. For superfusion studies, sample cleanup time was reduced to about 4 min per sample by attachment of the cartridges directly to the bottom of the superfusion chamber. Superfusate was subsequently pulled through the cartridges immediately after they were passed over the tissue. Batches of 12 high-speed tissue supernates were processed through the method in about 30 min. The method was used to analyze DOPEG, NE, DA, VIP, and NPY in various rat and dog tissues. The values obtained were similar to values obtained previously by other methods. Because the catechols and peptides are separated from a single sample, the method has several advantages over those described previously; e.g., it is rapid, simple, and more sensitive.

Structure and action of buccalin: a modulatory neuropeptide localized to an identified small cardioactive peptide-containing cholinergic motor neuron of Aplysia californica

A model system that consists of a muscle utilized in biting, the accessory radula closer (ARC), and the two cholinergic motor neurons innervating this muscle, neurons B15 and B16, has been used to study the expression of food-induced arousal in the marine mollusk Aplysia. The ARC muscle receives modulatory input from an extrinsic source, the serotonergic metacerebral cells, which partially accounts for the progressive increase in the strength of biting seen in aroused animals. Another source of modulation may arise from the ARC motor neurons themselves, which synthesize neuropeptides that can potentiate ARC contractions. Neuron B15 synthesizes the two homologous peptides, small cardioactive peptides A and B, whereas neuron B16 synthesizes the structurally unrelated peptide myomodulin. Here we report the purification and sequencing of a neuropeptide termed buccalin and show that it is colocalized with the small cardioactive peptides to neuron B15. Buccalin is also bioactive at the ARC neuromuscular junction but, in contrast to the small cardioactive peptides, when exogenously applied, it decreases rather than increases the size of muscle contractions elicited by firing of the motor neurons. Also unlike the small cardioactive peptides, which exert postsynaptic actions, buccalin seems to act only presynaptically. It has no effect on muscle relaxation rate and decreases motor neuron-elicited excitatory junction potentials in the ARC without affecting contractions produced by direct application of acetylcholine to the muscle. Neuron B15, therefore, appears to contain three modulatory neurotransmitters, two of which may act postsynaptically on the muscle to potentiate the action of the primary neurotransmitter acetylcholine and one of which may act presynaptically on nerve terminals to inhibit acetylcholine release.

Intramural ganglia in the human urethra

The urethras from five patients with a thoracic or cervical spinal cord lesion and one patient with carcinoma of the bladder were studied immunohistochemically for neuropeptide Y and vasoactive intestinal polypeptide. Autonomic ganglia, containing two to 21 nerve cell bodies, were found in the smooth and striated muscle regions of the intrinsic external urethral sphincter; they were present rarely in the distal urethra and were absent from the prostatic urethra. Neuropeptide Y immunoreactivity was observed in some of the nerve cell bodies (diameter 25 to 50 microns). Vasoactive intestinal polypeptide immunoreactivity was observed in small cells (diameter 15 to 25 microns.) in the urethral smooth muscle and in the walls of blood vessels that resembled small intensely fluorescent cells but may be nerve cell bodies. Both neuropeptide Y- and vasoactive intestinal polypeptide-immunoreactive nerve fibres were found in the smooth muscle and around blood vessels in the urethra of all patients. Both types of peptide-containing nerves were found associated with striated muscle of the intrinsic external urethral sphincter in patients with spinal cord injury, but only vasoactive intestinal polypeptide-immunoreactive nerves were found in the patient with carcinoma of the bladder in this region. The functions of the autonomic ganglia and vasoactive intestinal polypeptide- and neuropeptide Y-immunoreactive nerves in the human urethra remain to be elucidated.

Neuropeptide Y (NPY), an endogenous presynaptic modulator of adrenergic neurotransmission in the rat vas deferens: structural and functional studies

The role of neuropeptide tyrosine (NPY) on adrenergic neurotransmission was assessed in the rat vas deferens transmurally stimulated with square pulses of 0.15 or 15 Hz. Nanomoles of NPY inhibited the electrically-induced contractions on the prostatic half but not on the epididymal end of the ductus. NPY was at least 200-fold more potent than norepinephrine or adenosine to produce an equivalent inhibition. Complete amino acid sequence of NPY is required for full agonist activity; deletion of tyrosine at the amino terminus, i.e., NPY fragment 2-36 was 3-fold less potent than the native peptide. NPY fragment 5-36, 11-36 or 25-36 were proportionally less potent than NPY. Avian pancreatic polypeptide was inactive. The presynaptic nature of the NPY activity was established measuring the outflow of 3H-norepinephrine from the adrenergic varicosities of the vas deferens electrically stimulated. In this assay, NPY was more potent than NPY 2-36 or NPY fragment 5-36. No inhibitory action of NPY was detected in K+ depolarized tissues. The inhibitory effect of NPY on the rat vas deferens neurotransmission was not significantly modified by yohimbine, theophylline or naloxone, indicating that the effect of NPY is not due to the activation of alpha 2-adrenoceptors, adenosine receptors or opiate receptors respectively. Picrotoxin or apamin did not modify the inhibitory potency of NPY; verapamil or methoxyverapamil significantly reduced its potency. The inhibitory action of NPY is best explained through the activation of presynaptic NPY receptors that regulate norepinephrine release via a negative feedback mechanism. Structure activity studies give support to the notion of NPY receptors.

Pre- and postjunctional effects of neuropeptide Y on the rabbit isolated ear artery

1. In the isolated perfused and superfused rabbit ear artery, neuropeptide Y (NPY, 0.3-100 nmol/l) had no direct vasoconstrictor action, but produced a concentration-dependent and reversible enhancement of vasoconstrictor responses to both sympathetic nerve stimulation and exogenous noradrenaline. 2. In arteries in which the noradrenergic transmitter stores had been radiolabelled with [3H]-noradrenaline, 100 nmol/l NPY inhibited the stimulation-induced (1 Hz for 30 s) release of radioactivity, but the lower concentrations tested (10 and 30 nmol/l) had no effect. NPY (10, 30 and 100 nmol/l) had no effect on the resting release of radioactivity. 3. Thus, NPY in low concentrations enhances vasoconstrictor responses in the rabbit ear artery by a postjunctional action; prejunctionally, NPY inhibits stimulation-induced transmitter release when it is present in high concentrations.

A study of the actions of P1-purinoceptor agonists and antagonists in the mouse vas deferens in vitro

1. We have examined the effects of purinoceptor agonists and antagonists on the mechanical 'twitch' response, excitatory junction potential (e.j.p.) amplitude and [3H]-noradrenaline overflow in the mouse vas deferens. 2. The agonist profile for inhibition of the mechanical response was N6-([R]-2-phenylisopropyl)adenosine (L-PIA) congruent to N6-cyclohexyladenosine (CHA) greater than 5' N-ethylcarboxamido-adenosine (NECA) greater than 2-chloroadenosine (2ClA) congruent N6-([S]-2-phenylisopropyl)adenosine (D-PIA). 3. The P1-purinoceptor agonists inhibited the e.j.p. with an agonist profile of CHA greater than L-PIA congruent to NECA greater than 2ClA. 4. 2ClA inhibited [3H]-noradrenaline overflow with an EC50 of 1.2 microM which was not significantly different from the values for inhibition of the e.j.p. and the mechanical response. 5. The inhibitory action of 2ClA on the mechanical response was antagonized by 5 microM 8-phenyltheophylline (8-PT). However, neither blockade of P1-purinoceptors by 8-PT nor increasing the rate of degradation of endogenous adenosine by addition of adenosine deaminase had any effect on the mechanical response per se. 8-PT (5 microM) also failed to alter the e.j.p. amplitude or [3H]-noradrenaline overflow. 6. These results indicate that there are P1-purinoceptors present on sympathetic nerve terminals of the mouse vas deferens which are more like A1- than A2-receptors, but may be better classified as being of the A3-subtype (Ribeiro & Sebastiao, 1986). These receptors are not normally involved in the feedback regulation of transmitter release in this tissue.

Neuropeptide Y immunoreactivity in the locus coeruleus of the rat brain

The detailed distribution of neuropeptide Y immunoreactive neurons and fibres is given for the rat locus coeruleus. The studies were carried out using indirect immunofluorescence and avidin-biotin-peroxidase techniques. It was shown that in colchicine pretreated rats, about 15-20% of locus coeruleus neurons contain neuropeptide Y immunoreactivity. Neuropeptide Y immunoreactive neurons form two populations: (1) medium-sized or large neurons, poorly immunostained, situated mainly in the dorsal and central locus coeruleus nucleus, and (2) small, strongly immunostained neurons in ventromedial parts of the nucleus. Neuropeptide Y immunoreactive fibres and terminals are scattered throughout the locus coeruleus, but are more numerous in its ventromedial and ventrorostral parts.

Co-transmission in the rat vas deferens: postjunctional synergism of noradrenaline and adenosine 5'-triphosphate

In the isolated prostatic half of the rat vas deferens, joint application of noradrenaline (NA) and adenosine 5'-triphosphate (ATP) produced a contractile response whose magnitude was greatly larger than the addition of the tension generated by the application of each agent alone. The effect of ATP was mimicked by two non-hydrolyzable ATP analogs, but not by GTP, AMP or adenosine. In sympathectomized rats, ATP potentiated NA effects, increasing both the peak tension and the duration of the vas deferens contractile response. The synergism was concentration related. Prazosin antagonized the NA synergism but not the ATP response. Likewise, desensitization of the P2-purinoceptor blocked the ATP synergism without modifying the NA-induced contraction.

Modulation of cholinergic neurotransmission in guinea-pig trachea by neuropeptide Y

1. Neuropeptide Y (NPY) is localized to adrenergic nerves in guinea-pig airways but its function is not known. 2. NPY (1 X 10(-10)-3 X 10(-7) M) had no direct effect on guinea-pig tracheal smooth muscle in vitro. 3. NPY produced a concentration- and frequency-dependent inhibition of the cholinergic component of responses elicited by electrical field stimulation (EFS) whilst having no effect on the contractile response to exogenously applied acetylcholine (ACh). 4. Yohimbine was able to reverse significantly the inhibitory effect of noradrenaline on the cholinergic component to EFS without having any significant effect on the inhibition produced by NPY. 5. Neither blockade of beta-adrenoceptors by propranolol, nor depletion of adrenergic nerves by incubation with 6-hydroxydopamine caused any significant alteration in the response to EFS in the presence of 3 X 10(-7) M NPY. Bretylium tosylate incubation to prevent noradrenaline release produced a small but significant enhancement of the inhibitory effect of NPY on EFS at high frequencies. 6. NPY appears to reduce the cholinergic component to EFS via a prejunctional mechanism, acting directly on receptors on cholinergic nerve terminals, rather than affecting adrenergic mechanisms. NPY released by adrenergic nerves may modulate cholinergic neurotransmission in guinea-pig airways.

Direct evidence for concomitant release of noradrenaline, adenosine 5'-triphosphate and neuropeptide Y from sympathetic nerve supplying the guinea-pig vas deferens

Concomitant release of noradrenaline (NA) (using tritium labelling), adenosine 5'-triphosphate (ATP) (using the luciferin-luciferase firefly technique) and neuropeptide Y (using the enzyme-linked immunosorbant assay) during electrical stimulation of the guinea-pig vas deferens has been demonstrated. In addition it has been shown: (1) that release of NA and ATP is unaffected following selective desensitization of the P2 (ATP)-purinoceptor by alpha, beta-methylene ATP and (2) that total block of contractile responses of the vas deferens to nerve stimulation by a combination of prazosin and alpha, beta-methylene ATP does not reduce the release of NA and ATP. These results are consistent with the hypothesis that NA, ATP and neuropeptide Y are released as co-transmitters or modulators in sympathetic nerves supplying the vas deferens.

Depletion of brainstem epinephrine stores by alpha-methyldopa: possible relation to attenuated sympathetic outflow

The antihypertensive effect of alpha-methyldopa (MD) is believed to be critically dependent on its ability to deplete endogenous catecholamines or cause the synthesis of false neurotransmitters. We used liquid chromatography with electrochemical detection (LCEC) and negative chemical ionization gas chromatography-mass spectrometry (GC-MS) for quantitation of catecholamines and MD metabolites in rat. MD intraperitoneally (100 mg/kg q12 hr X 12 days), significantly increased alpha-methylnorepinephrine (MNE) in brain (1.02 +/- 0.33 micrograms/g), heart (1.67 +/- 0.57 micrograms/g) and adrenal glands (114.93 +/- 50.47 micrograms/g) Endogenous norepinephrine (NE), epinephrine (E) and dopamine (DA) were reduced. ME levels were 2.19 +/- 0.44 micrograms/g (n = 6) in the adrenal gland but only 99 +/- 26 pg/g (n = 3) in the brainstem. MD-induced endogenous brainstem NE depletion was more than compensated by MNE production, but brainstem E depletion was not compensated for by a stoichiometric production of brainstem ME. We conclude (1) although ME is a metabolite of MD, it is present in extremely low concentrations in brainstem and (2) central epinephrine-containing neurons are depleted of neurotransmitter by MD therapy. If this selective epinephrine depletion occurs in the bulbospinal tract neurons responsible for maintaining sympathetic tone, then this effect could contribute to the antihypertensive effect of MD.

Leucine-enkephalin- and neuropeptide Y-modulation of [3H]noradrenaline release in the oviduct of mature and juvenile rabbits

1. The effects of leucine-enkephalin and neuropeptide Y (NPY) on [3H]noradrenaline release induced by electrical field stimulation were studied in the isthmic part of the oviduct of juvenile and mature rabbits. 2. [3H]noradrenaline and total tritium overflow in the presence of cocaine, corticosterone and hyoscine were determined by liquid scintillation spectrometry. 3. Tritium overflow evoked by electrical stimulation (1 or 4 Hz, 1 msec) was calcium dependent. [3H]noradrenaline content (measured by ion exchange chromatography) accounted for 85% of the total tritium overflow. 4. Leucine-enkephalin (1 microM) in the presence of the peptidase inhibitor bacitracin reduced the stimulation-evoked tritium overflow in mature rabbits by 26.1 +/- 1.6% and in juvenile rabbits by 11.9 +/- 1.9%. Naloxone (1 microM) antagonized the effect of leucine-enkephalin. 5. NPY (0.2 microM) reduced the evoked tritium overflow in mature rabbits by 23.4 +/- 2.4% and in juvenile rabbits by 17.2 +/- 4.3%. 6. It is concluded that leucine-enkephalin and NPY inhibited [3H]noradrenaline release in rabbit oviduct and the modulatory effect of leucine-enkephalin depends on maturity while NPY modulation is a more independent system.

Presynaptic inhibition of cardiac vagal postganglionic nerves by neuropeptide Y

Stimulation of cardiac sympathetic nerves evokes prolonged non-adrenergic, non-cholinergic attenuation of the action of the vagus nerve on heart rate-an effect mimicked by, and proposed to be due to neuropeptide Y (NPY), a peptide released from sympathetic nerve terminals. In anaesthetised dogs, the effects on heart rate of the cholinomimetic bethanechol were unaltered by sympathetic stimulation or administration of NPY sufficient to cause prolonged inhibition of cardiac vagal action. In isolated guinea pig atria, during effective ganglion blockade, the effects on heart rate of the cholinomimetic methacholine were unaltered by exogenous NPY which inhibited cardiac slowing induced by stimulation of vagal nerve terminals. It is suggested that NPY released from sympathetic nerves inhibits cardiac vagal effectiveness by an action on postganglionic nerve terminals.