Stimulation of presynaptic beta-adrenoceptors enhances [3H]-noradrenaline release druing nerve stimulation in the perfused cat spleen

Neural system modeling and simulation using Hybrid Functional Petri Net

The Petri net formalism has been proved to be powerful in biological modeling. It not only boasts of a most intuitive graphical presentation but also combines the methods of classical systems biology with the discrete modeling technique. Hybrid Functional Petri Net (HFPN) was proposed specially for biological system modeling. An array of well-constructed biological models using HFPN yielded very interesting results. In this paper, we propose a method to represent neural system behavior, where biochemistry and electrical chemistry are both included using the Petri net formalism. We built a model for the adrenergic system using HFPN and employed quantitative analysis. Our simulation results match the biological data well, showing that the model is very effective. Predictions made on our model further manifest the modeling power of HFPN and improve the understanding of the adrenergic system. The file of our model and more results with their analysis are available in our supplementary material.

Effect of papaverine on synaptic transmission in the guinea-pig ileum

1. The effect of papaverine, a well known smooth muscle relaxant, was investigated on neural transmission within the enteric nervous system. Segments of guinea-pig ileum were placed in a partitioned bath to enable drugs, including papaverine, to be applied to enteric nerve pathways without interfering with the recording of the smooth muscle contraction. Ascending excitatory enteric nerve pathways were activated by electrical field stimulation in the anal compartment (10 Hz for 2 s, 45 mA, 0.5 ms pulse duration) and the resulting contraction of the intestinal circular muscle in the oral compartment was recorded isotonically. 2. Tetrodotoxin (0.6 microM) and hexamethonium (100 microM) both abolished, or greatly reduced, the contractions when applied to either compartment indicating that nicotinic synapses are involved in this pathway. 3. Papaverine (0.3-30 microM) applied independently to each compartment depressed in a concentration-dependent manner, the nerve-mediated contractions. The IC50 of this inhibitory effect was 3.53 microM for the oral and 4.76 microM for the anal compartments, respectively. Two other phosphodiesterase (PDE) inhibitors, 3-isobutyl-1-methylxanthine (IBMX 10-300 microM) and theophylline (30-1000 microM) added to the anal compartment also inhibited the nerve mediated contractions. Papaverine applied to the anal bath, after IBMX 100 microM (or theophylline 300 microM) further inhibited the nerve-mediated contractions, but was less effective than when applied alone. 4. Phentolamine (1 microM), an alpha-adrenoceptor antagonist, reduced the inhibitory effect of papaverine, but not that of IBMX (100 microM) or theophylline (300 microM). A combination of phentolamine and IBMX (or theophylline) prevented the inhibitory effect of papaverine. 5. Tetrodotoxin, but not papaverine or hexamethonium, inhibited the contraction elicited by electrical stimulation just anal to the partition indicating that papaverine did not affect the generation or conduction of nerve action potentials. 6. Verapamil (1 microM) and nifedipine (1 microM), two smooth muscle relaxants which act by blocking L-type calcium channels, only inhibited the contractions when applied directly to the recording (oral) compartment. This indicates that L-type Ca2+ channels are probably not involved in synaptic transmission in these ascending pathways and thus that the PDE inhibitors do not inhibit synaptic transmission by acting on these channels. omega-Conotoxin GVIA (10 nM), a potent inhibitor of the N-type Ca2+ channels, blocked the nerve-mediated contractions applied to either compartment. Whether the PDE inhibitors exert their inhibitory actions via these channels remains to be established. 7. The results indicate that the PDE inhibitors, papaverine, IBMX and theophylline inhibit excitatory enteric neural pathways by depressing synaptic transmission. The inhibitory effect of papaverine (but not IMBX or theophylline) involves, at least in part, the release of noradrenaline from sympathetic nerves acting on alpha-adrenoceptors on enteric neurones.

Neurochemical, electrophysiological and immunocytochemical evidence for a noradrenergic link between the sympathetic nervous system and thymocytes

The object of these experiments was to investigate whether noradrenaline is the signal neurotransmitter between the sympathetic nervous system and rat thymocytes. Using immunocytochemistry, evidence was obtained that the rat thymus (thymic capsule, subcapsular region and connective tissue septa) is innervated by noradrenergic varicose axons terminals (tyrosine hydroxylase- and dopamine-beta-hydroxylase-immunostained nerve fibres). This innervation is mainly associated with the vasculature and separately from vessels along the thymic tissue septa it branches into the thymic parenchyma. Using electron microscopy, classical synapses between thymocytes and neuronal elements were not observed. The neurochemical study revealed that these nerve terminals are able to take up, store and release noradrenaline upon axonal stimulation in a [Ca2+]o-dependent manner. The release was tetrodotoxin (1 microM)-sensitive, and reserpine pretreatment prevented axonal stimulation to release noradrenaline, indicating vesicular origin of noradrenaline. In addition, it was found that the release of noradrenaline was subjected to negative feedback modulation via presynaptic alpha 2-adrenoreceptors. Using a patch-clamp technique, electrophysiological evidence was obtained showing that noradrenaline inhibits in a concentration-dependent manner outward voltage-dependent potassium (k+) current recorded from isolated thymocytes. Since noradrenergic varicose axon terminals enter the parenchyma thymocytes and the boutons are not in close apposition to their target cells, noradrenaline released from these terminals diffuses away from release site to reach its targets, thymocytes, and to exert its inhibitory effect on voltage-dependent K+ -current. Since K+ channels are believed to be involved in T cell proliferation and differentiation, the modulation of K+ channel gating by noradrenaline released in response to axonal activity suggests that signals from blood-born or locally released hormones and cytokines. In this respect, noradrenaline released from non-synaptic neuronal varicosities and exerting its effect within the radius of diffusion may serve as a chemical link between the sympathetic nervous system and thymocytes and may have physiological and pathological importance in the thymus during stress and inflammatory/immune responses.

Role of cyclic AMP in the release of noradrenaline from isolated rat atria. Effect of pretreatment with clenbuterol

The possible role of cyclic AMP (cAMP) on tritium overflow evoked by stimulation of the cardio-accelerant nerves was studied in rat atria preincubated with [3H]-noradrenaline. Addition of the activator of adenylate cyclase forskolin (1 mumol/l), or of the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX, 100 mumol/l), did not affect both basal and evoked overflow. However, in the presence of the alpha 2-adrenoceptor antagonist yohimbine (0.03 mumol/l) both forskolin and IBMX increased the stimulation-induced transmitter overflow by 49% and 141%, respectively (compared to yohimbine 0.03 mumol/l). Thus, in rat atria the cAMP-dependent facilitation of noradrenaline release is only present when the autoinhibition exerted by activation of prejunctional alpha 2-adrenoceptors is blocked. Propranolol (0.1 mumol/l) that did not produce any effect on noradrenaline release markedly reduced the facilitatory response induced by forskolin in the presence of yohimbine. When rats were pretreated with the beta 2-adrenoceptor agonist clenbuterol (0.3 mg.kg-1, s.c., twice daily, 14 days), a treatment which desensitizes beta-adrenoceptor-mediated facilitation of noradrenaline release (Kazanietz and Enero 1989), the facilitatory effect of forskolin and IBMX in the presence of yohimbine was abolished. The results indicate that in rat atria the effect of forskolin and IBMX on noradrenaline release are only to be observed after blockade of presynaptic alpha 2-adrenoceptor autoinhibition. beta-adrenoceptor blockade or clenbuterol pretreatment decreases the facilitatory response to forskolin and hence prejunctional beta-adrenoceptor-mediated enhancement of noradrenaline release is linked to the stimulation of adenylate cyclase.

Accumulation and release of adrenaline, and the modulation by adrenaline of noradrenaline release from rabbit blood vessels in vitro

The accumulation of (-)-3H-adrenaline (3H-A) by rabbit isolated aorta was studied. In all experiments, monoamine oxidase and catechol-O-methyltransferase were inhibited by treatment with pargyline and 3',4'-dihydroxy-2-methyl-propiophenone, respectively. The relationship between the accumulation of 3H derived from 3H-A and the duration of incubation was linear. The 3H-accumulation after 3 h incubation was 22.5 ml/g. In reserpine-treated tissue, the 3H-accumulation levelled off after 30 min and was 8.5 ml/g after 3 h. The concentration of 3H-A or (-)-3H-noradrenaline (3H-NA) and the 3H-accumulation (ml/g) were inversely related. At 10(-8) M, the 1-hour accumulation of 3H derived from 3H-A and 3H-NA was 7.8 and 15.2 ml/g, respectively. With increasing concentrations the accumulation values approached each other. The accumulation of 3H derived from 3H-A by reserpine-treated tissue also showed an inverse relationship with concentration. The accumulation of 3H derived from 3H-A was dependent on the bath temperature. Storage of tissue (0-5 days in salt solution without equilibration with 95% O2/5% CO2; 4 degrees C) did not affect the accumulation of 3H derived from 3H-A. Thereafter (7-14 days), the accumulation decreased. The inhibitory potency (IC50; -log M) of desipramine, cocaine, propranolol, isoprenaline, and normetanephrine on accumulation of 3H derived from 3H-A was found to be 8.26; 6.50; 5.48; 4.88, and 4.02, respectively. The maximal degree of inhibition was almost the same for these drugs, while that of clonidine and corticosterone was 50 and 20%, respectively. In the presence of desipramine, either clonidine, corticosterone or isoprenaline reduces the accumulation of 3H derived from 3H-A. Ouabain and iodoacetic acid, but not sodium cyanide and 2,4-dinitrophenol, reduced the accumulation of 3H derived from 3H-A. Anoxia (95% N2/5% CO2; 37 degrees C; 1-24 h) did not alter the accumulation of 3H derived from 3H-A. Glucose deprivation alone or combined with anoxia markedly reduced the 3H-accumulation. The release of 3H-A from rabbit isolated aorta was studied. This release was compared with that of 3H-NA. The stimulation-evoked 3H-overflow from aorta preloaded with 3H-A decreased with repeated stimulation. In contrast, prestimulation enhanced subsequent stimulation-evoked 3H-overflows. For both 3H-amines, the 3H-overflow increased concomitantly to the same degree with the number of pulses. The time course of 3H-overflows with either 3H-A or 3H-NA was compared.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of age on presynaptic beta2 receptor mediated responses in the rat heart

Presynaptic alpha2 adrenergic receptor inhibition of norepinephrine (NE) release is diminished in older animals [1]. To determine whether presynaptic beta2 adrenergic receptor facilitation of NE release is also affected by age, the effect of propranolol on neurally-induced NE release was examined in perfused heart preparations isolated from 6- and 24-month-old rats. The heart was isolated with the right cardiac sympathetic nerve and then was stimulated electrically in the absence and presence of propranolol (10(-8)-10(-6) M). NE content in the effluent was measured by an HPLC/EC methodology. The data indicate that there is a greater suppression of NE release by propranolol in preparation isolated from younger animals. For example, in nerve-heart preparations of 6-month-old animals stimulated at 12 Hz, propranolol (10(-6) M) reduced the amount of NE released by 34% whereas in preparations from 24-month-old animals at 12 Hz there was no significant change in the NE overflow. Like the alpha2 receptor mechanism, autoregulation of NE release by presynaptic beta2 receptors is diminished in older animals. Lack of presynaptic autoregulation may explain in part, the reduced capacity of adrenergic influences to control and regulate cardiac function in older animals.

Modulation of noradrenaline release by activation of presynaptic beta-adrenoceptors in the cardiovascular system

Peripheral sympathetic nerve terminals in many tissues, but not all, are endowed with beta-adrenoceptors. Activation of these result in an enhancement of noradrenaline release evoked by electrical nerve stimulation. These so-called presynaptic beta-adrenoceptors are possibly located on the outer surface of the varicosity of the noradrenergic nerves. A postsynaptic location, however, is also a possibility. The presynaptic beta-adrenoceptors appear to be of the beta 2-adrenoceptor subtype. However, specific classification is lacking. The stereospecificity of the beta-adrenoceptors is controversial. These receptors are not activated by noradrenaline released from sympathetic nerves. Adrenaline derived from the adrenal medulla may be the physiological activator. Either circulating adrenaline or adrenaline taken up by sympathetic nerve terminals and then released as a cotransmitter with noradrenaline activates the presynaptic beta-adrenoceptors. In the latter case, a "positive" feedback" loop may be formed.

Presynaptic beta-adrenoceptors in rat atria: evidence for the presence of stereoselective beta 1-adrenoceptors

1. Presynaptic beta-adrenoceptor activity was studied in rat isolated atria, previously loaded with [3H]-noradrenaline. The stimulation-induced release of 3H transmitter was measured in the presence of cocaine, and adrenaline was used as a facilitatory beta-adrenoceptor agonist. 2. Adrenaline (0.1 and 2 nM) increased, by about 50%, the evoked efflux of tritium. With phenoxybenzamine present, the same activity was shown with 10 nM adrenaline. 3. The beta 2-selective adrenoceptor blocking drugs: IPS 339 and ICI 118 551 caused a concentration-dependent decrease in the activity of adrenaline. Cardioselective beta-blocking drugs: acebutolol, beta-xolol, nebivolol and its isomers (R 67 138 and R 67 145) also reduced dose-dependently the agonistic action of adrenaline. The order of potency for nebivolol and its isomers was R 67 138 greater than nebivolol greater than R 67 145. The activity of pindolol was not concentration-dependent. The inhibitory effect of acebutolol was also observed in the presence of blockade of alpha-adrenoceptors. 4. The postsynaptic beta-adrenoceptor blocking activity of nebivolol and its isomers was studied in pithed rats. They reduced isoprenaline-induced tachycardia without altering hypotensive responses. The order of potency was: R 67 138 greater than nebivolol greater than R 67 145. 5. It is concluded that in rat isolated atria, presynaptic beta 2- and beta 1-adrenoceptors coexist and that facilitatory beta 1-adrenoceptors are stereospecific.

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.

Effect of isoprenaline on noradrenaline release from sympathetic neurones in rabbit isolated pulmonary artery

The aim of the present work was to demonstrate the presence of presynaptic beta-adrenoceptors in the rabbit isolated pulmonary artery by investigating the effect of isoprenaline on 3H-noradrenaline (3H-NA) release evoked by electrical field stimulation. (-)-Isoprenaline (10(-7)-10(-6) mol/l) had no effect on the 3H-overflow evoked by stimulation (3 Hz) of the pulmonary artery preloaded with 3H-NA. At 10(-5) mol/l, (-)-isoprenaline reduced the 3H-overflow by maximally 39%. (-)-Isoprenaline (10(-5) mol/l) caused an inhibition that remained almost constant with time. The same results were obtained with (-)-isoprenaline (10(-7)-3 X 10(-5) mol/l) in the presence of cocaine (3 X 10(-5) mol/l), corticosterone (4 X 10(-5) mol/l), and the catechol-O-methyltransferase inhibitor U-0521 (3',4'-dihydroxy-2-methylpropiophenone) (10(-4) mol/l). In the presence of cocaine plus corticosterone, (-)-isoprenaline (3 X 10(-10)-10(-7) mol/l) had no effect on the 3H-overflow evoked by stimulation at 1 Hz. At 10(-6) mol/l, (-)-isoprenaline slightly reduced the 3H-overflow. At 10 Hz, (-)-isoprenaline (10(-6)-3 X 10(-5) mol/l) decreased the 3H-overflow and had no effect at 10(-7) mol/l. In the presence of either rauwolscine (10(-6) mol/l), phentolamine (10(-6) mol/l) or the phosphodiesterase inhibitor ICI 63,197 (3 X 10(-5) mol/l), (-)-isoprenaline (10(-7)-10(-6) mol/l) did not enhance the stimulation-evoked 3H-overflow. (+/-)-Propranolol (10(-7)-10(-5) mol/l) did not alter the stimulation-evoked 3H-overflow.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of cyclic nucleotides in prejunctional modulation of noradrenaline release in mouse atria

1 In mouse isolated atria previously incubated with [3H]-noradrenaline, 8-bromo-cyclic AMP (3-270 microM) produced a concentration-dependent increase in the fractional stimulation-induced outflow of radioactivity. 8-Bromo-cyclic GMP induced a lesser increase in the stimulation-induced outflow. 2 The phosphodiesterase inhibitors: M&B 22948 (90 microM); ICI 63197 (30 and 90 microM) and 3-isobutyl-1-methylxanthine (90 microM) increased the fractional stimulation-induced outflow. Together these results indicate that cyclic AMP may have a modulatory effect on noradrenaline release. 3 The inhibition of the stimulation-induced outflow produced by clonidine (0.03 microM) and its facilitation produced by phentolamine (1 microM) were unaltered in the presence of 8-bromo-cyclic AMP (90 microM). However, in the presence of 8-bromo-cyclic AMP (270 microM), the facilitatory effect of phentolamine was enhanced, but the inhibitory effect of clonidine (0.03 microM) was unaltered. In the presence of ICI 63197 (30 microM) the inhibitory effect of clonidine (0.03 microM) was unaltered, but the facilitatory effect of phentolamine (1 microM) was slightly enhanced. 4 Isoprenaline (0.003-0.1 microM) enhanced the fractional stimulation-induced outflow, an effect blocked by propranolol (0.1 microM). In the presence of 8-bromo-cyclic AMP (90 microM), the facilitatory effect of isoprenaline (0.01 microM) was blocked. In the presence of ICI 63197 (30 microM) the facilitatory effect of isoprenaline (0.003 microM) was potentiated. 5 These results suggest that whereas beta-adrenoceptor-mediated enhancement of noradrenaline release is linked to the stimulation of adenylate cyclase and enhanced formation of cyclic AMP, alpha-adrenoceptor-mediated inhibition of noradrenaline release is not linked to inhibition of adenylate cyclase activity.

A functional response to D1 dopamine receptor stimulation in the central nervous system: inhibition of the release of [3H]-serotonin from the rat substantia nigra

The influence of dopamine receptor agonists on the calcium dependent electrically-evoked release of [3H]-serotonin was studied in the superfused rat substantia nigra. The electrically-evoked overflow of [3H]-serotonin was significantly inhibited by micromolar concentrations of exogenous dopamine and of the D1-selective dopamine receptor agonists, fenoldopam and SKF 38393. The inhibitory effects of dopamine and fenoldopam on [3H]-serotonin release were antagonized by the D1-selective dopamine receptor antagonist SCH 23390. S-sulpiride, an antagonist of D2 receptors failed to antagonize the inhibitory effects of dopamine on [3H]-serotonin release. Moreover, quinperol, the selective D2 receptor agonist did not modify the electrically evoked release of [3H]-serotonin. In the presence of nomifensine, SCH 23390 but not S-sulpiride enhanced in a concentration dependent manner the electrically-evoked release of [3H]-serotonin. Under similar experimental conditions, exposure to fenoldopam did not inhibit the electrically-evoked release of [3H]-serotonin from rat hippocampal or hypothalamic slices. These results indicate that, in rat substantia nigra, exogenous as well as endogenous dopamine causes inhibition of serotonin release by activation of dopamine receptors of the D1-subtype.

Evidence for prejunctionally located beta 2-adrenoceptors in the cat spleen

The number of beta-adrenoceptors in myocardium and spleen from 6-hydroxydopamine (6-OH-DA) treated cats was determined by radioligand binding with [125I]-iodohydroxybenzylpindolol (IHYP). The effectiveness of 6-OH-DA pretreatment was assessed by analyses of the tissue content of catecholamines and the contractile response of isolated splenic strips to electrical stimulation. Since no effect on the splenic strip was produced by the beta-agonist isoprenaline, whereas noradrenaline caused contraction, it is concluded that the smooth muscle of the splenic capsule is controlled by postjunctional alpha-adrenoceptors. The number of specific IHYP binding sites were reduced by 70% in whole spleen tissue and totally abolished in the splenic capsule by pretreatment with 6-OH-DA. Subclass analysis revealed that the reduction in total splenic beta-adrenoceptor number was due to a loss of beta 2-adrenoceptors. However, the 6-OH-DA induced chemical sympathectomy did not produce any alteration either in beta-adrenoceptor density or the relative distribution of the beta-adrenoceptor subtype in the myocardium. It is suggested that a loss of prejunctional beta-adrenoceptors, due to chemical sympathectomy, might be compensated for by an increased number of postjunctional beta-adrenoceptors in the myocardium due to the development of denervation supersensitivity in this tissue. In conclusion, the findings provide direct biochemical evidence for existence of prejunctional beta 2-adrenoceptors on the sympathetic nerve terminals of the cat spleen.

Presynaptic beta-adrenoceptors

The existence of facilitatory presynaptic beta-adrenoceptors has been shown in approximately 30 tissues of 6 different species including human. A positive feed back loop for further release of the transmitter appears to be activated by an endogenous agonist, epinephrine, taken up and released as a cotransmitter with norepinephrine rather than norepinephrine itself released from peripheral noradrenergic nerve terminals. Presynaptic beta-adrenoceptors are mainly of a beta 2-subtype. Some beta 1-subtype receptors are also suggested. There coexist presynaptic beta 1- and beta 2-adrenoceptors in cat and rat hypothalamus. Higher sensitivity of peripheral presynaptic beta-adrenoceptors to isoproterenol may be implicated in the early development of hypertension in SHR. Epinephrine taken up and released initiates the development of hypertension in rats via activation of these receptors. Increased activation of these receptors by epinephrine may play a role in the development of essential hypertension. The antihypertensive action of beta-antagonists may be in part due to blockade of these facilitatory presynaptic beta-adrenoceptors.

Prejunctional beta-adrenoceptors in rabbit pulmonary artery and mouse atria: effect of alpha-adrenoceptor blockade and phosphodiesterase inhibition

In rabbit isolated pulmonary artery previously incubated with [3H]-noradrenaline, isoprenaline (0.3 microM) had no effect on the stimulation-induced outflow of radioactivity. However, if the phosphodiesterase inhibitor ICI 63,197 (30 microM) or the alpha-adrenoceptor blocker phentolamine (1 microM) was present, then isoprenaline significantly enhanced the stimulation-induced outflow, an effect blocked by propranolol (0.1 microM). ICI 63,197 (30 microM) but not phentolamine significantly enhanced the stimulation-induced outflow of radioactivity. In mouse isolated atria previously incubated with [3H]-noradrenaline and stimulated at a frequency of 10 Hz, isoprenaline had no effect on the stimulation-induced outflow of radioactivity; this is in contrast to its release-enhancing effects at stimulation frequencies of 4 Hz and 2 Hz. The facilitation of stimulation-induced outflow by isoprenaline at 4 Hz was blocked by propranolol (0.08 microM) which, by itself, had no effect on the stimulation-induced outflow. At a stimulation frequency of 2 Hz in mouse atria the facilitatory effect of isoprenaline (0.01 microM) was significantly greater in the presence of ICI 63,197 (30 microM) which, by itself, had no effect on the stimulation-induced outflow. Similarly, the facilitatory effect of isoprenaline was significantly greater in the presence of phentolamine (1 microM) but, in this case, phentolamine significantly enhanced the stimulation-induced outflow. These results suggest that facilitatory prejunctional beta-adrenoceptors are present in both rabbit pulmonary artery and mouse atria. The effects of the phosphodiesterase inhibitor ICI 63,197 suggest that they are linked to adenylate cyclase in both tissues and we propose that the ability of phentolamine to facilitate the release and enhance the effect of isoprenaline may be due to the blockade of alpha-adrenoceptor inhibition of adenylate cyclase. This latter proposition needs further investigation.

Is there feedback regulation of neurotransmitter release by autoreceptors?

Neurotransmitter release does not seem to be regulated by neuronal receptors mediating feedback and the mechanism of action of presynaptically active agents is still uncertain. In a recent set of papers [27, 82], experiments were described in which major modifications were made to the amount of neurotransmitter released per impulse, with all other parameters of field stimulation, such as pulse number, voltage and frequency, fully controlled. These studies done with a number of sympathetically innervated tissues give some insight into an antagonist action presynaptically which is independent of the ambient concentration of extracellular transmitter. It appears to involve, instead, the gating mechanisms which control neuronal membrane depolarization and repolarization. It was found that the effects of yohimbine and also of phenoxybenzamine on stimulation-induced efflux appeared to be essentially "all or none". That is, the absolute total release of tritiated transmitter with 100 pulses was elevated to roughly the same dpm value by the presynaptic antagonist at each of the pulse durations between 50 and 1000 microsec, in a variety of test tissues. The declining percentage effect of the antagonist on tritium efflux, as the pulse duration was enlarged between 50 and 1000 microsec, referred to earlier (Fig. 3), was due to rising values for transmitter release in the controls not matched by proportionally similar increases in the antagonist-treated tissues. Values for the amount of transmitter released during stimulation in the presence of yohimbine, at pulse lengths between 50 and 1000 microsec, were all in the range of values achieved in the absence of yohimbine with long pulse lengths (1000-2000 microsec). In other words, prolongation of the pulse duration from 50 to 1000 microsec and the exposure of tissues to a presynaptic antagonist, such as yohimbine or phenoxybenzamine, may involve a common mechanism, and the effects of these two procedures are not additive. In fact, with much prolonged pulse durations (2000-5000 microsec), the presynaptic antagonists are virtually ineffective. It is known that the release of transmitter from sympathetic nerves is directly related to the duration of the action potential. If it is prolonged, the calcium channels stay open longer leading to greater entry of calcium and to an increased release of transmitter [45, 46]. Yohimbine and phenoxybenzamine may prolong the duration of depolarization by indirect modification of the calcium gating mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

Involvement of cAMP in modulation of noradrenaline release in the human pulmonary artery

After incubation with 3H-noradrenaline, strips of human pulmonary arteries from patients undergoing surgery for lung tumour were superfused with physiological salt solution containing cocaine and corticosterone. Forskolin, AH 21-132 (a cAMP phosphodiesterase inhibitor), 8-Br-cAMP and isoprenaline did not affect the basal tritium efflux from the strips, but produced a concentration-dependent facilitation of the tritium overflow evoked by transmural electrical stimulation (2 Hz). The facilitatory effect of isoprenaline was potentiated by forskolin which produced a shift to the left of the concentration-response curve of isoprenaline. It is concluded that cAMP plays a role in the modulation of noradrenaline release in the human pulmonary artery and that presynaptic beta-adrenoceptors appear to be coupled to an adenylate cyclase in the sympathetic nerve terminals.

Role of cAMP for regulation of impulse-evoked noradrenaline release from the rabbit pulmonary artery and its possible relationship to presynaptic ACTH receptors

Strips of the rabbit pulmonary artery preincubated with 3H-noradrenaline were superfused with physiological salt solution containing cocaine, corticosterone and propranolol. Basal tritium efflux and electrically evoked tritium overflow were determined. The basal efflux of tritium was not affected by forskolin 0.01-10 mumol/l, 8-Br-cAMP and dibutyryl-cAMP 10-330 mumol/l, or the phosphodiesterase inhibitors rolipram 1-10 mumol/l and AH 21-132 l mumol/l; it was increased by AH 21-132 10-100 mumol/l. Forskolin concentration-dependently increased the evoked 3H overflow, and this effect was not attenuated by omission of cocaine. The facilitatory effect of forskolin was more pronounced at 0.66 Hz than at 2 Hz. Rolipram, AH 21-132, 8-Br-cAMP or dibutyryl-cAMP also produced a concentration-dependent increase in evoked 3H overflow (8-Br-cAMP was more effective than dibutyryl-cAMP in this respect). Except for the highest concentration investigated, AH 21-132 was more effective in facilitating evoked overflow than in increasing basal efflux. Forskolin, AH 21-132 or 8-Br-cAMP did not alter the percentages of 3H-noradrenaline and 3H-metabolites contained in basal tritium efflux or in stimulation-evoked tritium overflow. When a combination of AH 21-132 plus 8-Br-cAMP or AH 21-132 plus forskolin was administered, the facilitatory effect on evoked tritium overflow was more pronounced than with the single compounds alone. ACTH1-24 also facilitated the evoked tritium overflow. Combined exposure to ACTH1-24 plus forskolin, ACTH1-24 plus AH 21-132 or ACTH1-24 plus forskolin plus AH 21-132 resulted in a clearly more pronounced increase in evoked tritium overflow than exposure to the single compounds alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenaline mediates a positive feedback loop in noradrenergic transmission: its possible role in development of hypertension

Adrenaline activates prejunctional beta-adrenoceptors of the beta 2-subtype on sympathetic nerve terminals and enhances noradrenergic transmission. Adrenaline can be incorporated in transmitter stores of noradrenergic nerves and, when released as a cotransmitter, activates the prejunctional beta 2-adrenoceptors, thereby mediating autofacilitation of noradrenergic transmission. Adrenaline released from the adrenal medulla in stress may be incorporated in noradrenergic transmitter stores and reach a sufficient concentration as a cotransmitter to activate the autofacilitatory feedback loop involving prejunctional beta 2-adrenoceptors, resulting in prolongation of the increases in vasomotor tone and cardiac activity that occur acutely: with frequent repetition of stress, there may be progression into a hypertensive state. In accord with this hypothesis, adrenaline administration produces persistent increases in blood pressure in rats, and plasma levels of adrenaline are elevated in a proportion of hypertensive patients; furthermore, repeated stress produces prolonged increases in blood pressure in animals and man.

Minireview. Presynaptic receptors and alterations in norepinephrine release in spontaneously hypertensive rats

The ability of blood vessels to constrict to a given stimulus is significantly increased in spontaneously hypertensive rats (SHR). Such an increase in the vasoconstrictor responsiveness contributes to the elevated peripheral vascular resistance noted in SHR. The present review discusses evidence in support of the concept that an increased release of norepinephrine during sympathetic nerve stimulation may contribute to the increase in vasoconstrictor responsiveness and, subsequently, to an increase in vascular resistance in the SHR. Several studies suggest that the exocytotic release of norepinephrine from sympathetic nerves may be altered by endogenously occurring neurohumoral substances which produce their effects by interacting with presynaptic receptors located on postganglionic sympathetic nerves. Therefore, it is postulated that alterations in presynaptic regulation of norepinephrine release, resulting from changes in the functioning of one or more of these presynaptic receptors, may lead to a greater release of norepinephrine in the SHR. This review summarizes the results of studies evaluating presynaptic receptor mechanisms and norepinephrine release in the SHR. These studies suggest that norepinephrine release during sympathetic nerve stimulation is greater in the SHR and that alterations in some of the presynaptic receptor mechanisms may be responsible for this phenomenon.

Effect of epinephrine on norepinephrine release from rat kidney during sympathetic nerve stimulation

Experiments were performed to study presynaptic beta-adrenoceptor facilitation of sympathetic neurotransmitter release in the isolated perfused rat kidney and evaluate the effect of epinephrine on norepinephrine release during sympathetic nerve stimulation. The right kidney was isolated and perfused with Krebs-Ringer solution. Norepinephrine storage sites were labelled with [3H]norepinephrine. Increasing concentrations of isoproterenol and salbutamol when perfused through the kidney, caused an enhancement of the stimulus-induced release of [3H]norepinephrine at 0.5 and 2 Hz, with the maximum facilitatory effect being observed at 0.5 Hz. The effect of salbutamol on [3H]norepinephrine release was concentration-dependent and more pronounced than that of isoproterenol. While propranolol (10(-9)-10(-5)M) by itself did not cause any significant changes in the stimulus-induced release of [3H]norepinephrine, it antagonized the facilitatory action of salbutamol on [3H]norepinephrine release during periarterial nerve stimulation. When epinephrine (10(-10)-10(-7)M), was perfused through the kidney in the presence of cocaine, it caused a concentration-dependent inhibition of the stimulus-induced release of [3H]norepinephrine release elicited during periarterial nerve stimulation. However, when epinephrine was perfused in the presence of cocaine, phentolamine and corticosterone it caused a slight but significant increase in the stimulus-induced release of [3H]norepinephrine; the highest concentration (10(-7)M) still caused a decrease in the [3H]norepinephrine release. These results, while providing evidence for the existence of presynaptic facilitatory beta-adrenoceptors on renal sympathetic nerves, fail to support the hypothesis that these receptors have a physiological role in the regulation of sympathetic neurotransmitter release.

Prejunctional beta-adrenoceptors in human and canine saphenous veins

Experiments were designed to compare the functional importance of prejunctional beta-adrenoceptors in human and canine blood vessels. Rings of saphenous veins were mounted for isometric tension recording in organ chambers filled with physiological salt solution. Isoproterenol caused concentration-dependent relaxations of comparable magnitude during contractions of canine veins caused by electrical stimulation or norepinephrine. In the human veins, isoproterenol caused smaller relaxations during norepinephrine-induced contractions, and augmented the contractile response to electrical stimulation. Helical strips of veins were incubated with [3H]norepinephrine and mounted for superfusion and isometric tension recording. Isoproterenol augmented the release of intact labeled transmitter during electrical stimulation more in the human than in the canine vein. In veins from both species the effects of isoproterenol were inhibited by propranolol. These experiments demonstrate that the adrenergic nerves of the human saphenous vein have a greater responsiveness to prejunctional beta-adrenergic activation than those of the corresponding canine vessel. As a consequence, in the human vein, beta-adrenergic agonists augment, rather than depress the contractile response to activation of the adrenergic nerve endings.

Modulation of noradrenaline release through activation of presynaptic beta-adrenoreceptors

On peripheral noradrenergic nerve endings there exist beta-adrenoreceptors activation of which results in an enhanced release of noradrenaline in response to nerve stimulation. These presynapatic beta-adrenoreceptors do not appear to be activated by neuronally-released noradrenaline. However, adrenaline may be a physiological activator during enhanced adrenomedullary secretion. Adrenaline can also be incorporated into the noradrenergic transmitter stores and be released as a co-transmitter. Under these conditions presynaptic beta-adrenoreceptors may be activated by neuronally-released adrenaline, thus forming a 'positive feedback loop'. The release of adrenaline from the adrenal medullae may also be modulated through facilitatory beta-adrenoreceptors, but the release of noradrenaline from noradrenergic nerves in the central nervous system is not. The facilitatory presynaptic beta-adrenoreceptors appear to be in the main of the beta 2-subtype although precise receptor characterization has not been carried out. Increased activation of presynaptic beta-adrenoreceptors by adrenaline may be implicated in the development of essential hypertension. Part of the antihypertensive action of beta-adrenoreceptor blocking drugs may be due to blockade of these facilitatory presynaptic beta-adrenoreceptors.

Noradrenergic modulation of glutamate release in the cerebellum

The effect of alpha- and beta-adrenergic agonists has been studied on the release of newly synthesized [3H]glutamate and [3H]GABA from slices of rat cerebellum. The beta 2-adrenergic agonist salbutamol, and also noradrenaline in the presence of the alpha-adrenergic antagonist phentolamine, both potentiated the K+-evoked release of [3H]glutamate. This potentiation appears to be mediated by adenylate cyclase activation. No effects of beta-adrenergic stimulation were observed on [3H]GABA release. The alpha-adrenergic agonist clonidine inhibited both the [3H]glutamate and the [3H]GABA release evoked by K+. The results suggest that noradrenergic modulation of cerebellar activity may have a presynaptic as well as postsynaptic origin.

Stereoselectivity of presynaptic autoreceptors modulating dopamine release

The effects of the (R)- and (S)-enantiomers of sulpiride and butaclamol were studied on the spontaneous and field stimulation-evoked release of total radioactivity from slices of rabbit caudate nucleus prelabelled with [3H]dopamine. (S)-Sulpiride in concentrations ranging from 0.01--1 microM enhanced the electrically evoked release of [3H]dopamine while (R)-sulpiride was 10 times less potent than (S)-sulpiride. Exposure to (S)-butaclamol (0.01--1 microM) but not to (R)-butaclamol (0.1--10 microM) enhanced the field-stimulated release of [3H]dopamine. The facilitatory effects of (S)- and (R)-sulpiride and (S)-butaclamol on the stimulated release of the labelled neurotransmitter were observed under conditions in which these drugs did not modify the spontaneous outflow of radioactivity. Only the active enantiomers of sulpiride and butaclamol antagonized the inhibition by apomorphine (1 microM) of the stimulated release of [3H]dopamine. Our results indicate that the presynaptic inhibitory dopamine autoreceptors modulating the stimulation-evoked release of [3H]dopamine in the caudate nucleus are, like the classical postsynaptic dopamine receptors, chemically stereoselective.

Effect of beta-adrenergic receptor blockade with propranolol on the response of plasma catecholamines and renin activity to upright tilting in normal subjects

1 Relationship between plasma catecholamines (measured as noradrenaline and adrenaline) and plasma renin activity (PRA) were examined at rest and during passive head-up tilting for 30 min in nine normal subjects, before and after treatment with propranolol 160 mg daily for 7 days. 2 Noradrenaline (NA) and adrenaline (A) increased substantially after tilting for 15 min. There were no changes in PRA. After 30 min tilting, NA remained elevated, whereas A had returned to resting levels. A significant increase in PRA was apparent at 30 min. Pulse rate and diastolic blood pressure increased progressively during tilting. Systolic pressure did not change. 3 Treatment with propranolol reduced pulse rate and systolic blood pressure at rest and during tilting. Resting catecholamine concentrations and the response of NA to tilting were unaffected. In contrast, treatment prolonged the A response leading to significantly higher levels after 30 min tilting. Propranolol reduced PRA in six of the nine subjects and prevented the increase with tilting observed before treatment.

The effects of (+)- and (-)-propranolol on 3H-transmitter efflux in guinea-pig atria and the presynaptic beta-adrenoceptor hypothesis

The effects of isoprenaline and of the (+)- and (-)-isomers of propranolol on the stimulation-induced overflow of 3H-transmitter was assessed in guinea-pig atria to evaluate the hypothesis of presynaptic beta-adrenoceptors. 2 Isoprenaline (1.2 x 10(-8) M) enhanced the efflux of tritium at 2 and 5 Hz with 100 pulses and did so to a similar extent at both frequencies. 3 The (-)-isomer of propranolol (1.0 x 10(-7) M) blocked the enhancing effect of isoprenaline but did not by itself modify transmitter efflux. 4 The (+)-isomer of propranolol, almost devoid of beta-adrenoceptor blocking properties, was also effective at 1.0 x 10(-7) M in blocking the enhancement of tritium efflux by isoprenaline. 5 The (-)-isomer of propranolol (1.0 x 10(-7) M) blocked almost entirely the inotropic response to isoprenaline (3 x 10(-7) M) but even 3.0 x 10(-6) M (+)-propranolol was ineffective in antagonizing the beta-adrenoceptor-mediated contractile responses to the catecholamine. 6 It is concluded that the presynaptic site of isoprenaline action does not show the requisite stereo-specificity of beta-adrenoceptors and that a 'non-specific' action of the antagonist probably accounts for its reduction of the effect of isoprenaline.

Circulating adrenaline and blood pressure: the metabolic effects and kinetics of infused adrenaline in man

Six normotensive volunteers were infused with L-adrenaline at 0.01, 0.03, 0.05, 0.075 and 0.10 microgram/kg-1 min-1, each increment lasted 10 min. Plasma adrenaline rose from 0.27 to 4.61 nmol/l, and there were dose-related increases in plasma renin activity, blood glucose, plasma cyclic AMP and plasma free fatty acids, but not in plasma noradrenaline and cyclic GMP. Levels of circulating adrenaline previously noted in essential hypertensives had minimal cardiovascular effects. The secretion rate of adrenaline and its rate of clearance from the circulation were calculated from plasma samples taken during an hour-long infusion (0.083 +/- 0.006 microgram kg-1 min-1) of L-adrenaline in the same individuals. The secretion rate ranged from 1.40 to 6.01 nmol/min with a mean (+/- SEM, 6) of 2.82 +/- 0.76 nmol/min. Mean clearance (+/- SEM, 6) was 9.41 +/- 1.37 l/min and ranged from 4.86 to 14.61 l/min. The decline of plasma adrenaline following the infusion was biexponential. Plasma adrenaline is unlikely to be of primary importance in the elevation of blood pressure, either directly, via renin release or by noradrenaline release via presynaptic beta receptors. However, variation in clearance between subjects limits the use of plasma levels as an interindividual index of adrenal release of adrenaline. The relationship between sympathoadrenal activity and plasma adrenaline may be further perturbed by equilibration between the circulation and sites of tissue uptake. The lower levels of plasma adrenaline than of noradrenaline appear to result from both a slower rate of secretion and a higher rate of clearance from the circulation.

Recent developments in noradrenergic neurotransmission and its relevance to the mechanism of action of certain antihypertensive agents

This report reviews a number of significant developments in the fields of noradrenergic transmission and adrenergic receptors which suggest that, in addition to the classical postsynaptic adrenoceptors, there are also presynaptic adrenoceptors that help modulate the release of norepinephrine (NE) from peripheral as well as central noradrenergic nerve endings during nerve stimulation. In particular, stimulation of presynaptic alpha-adrenoceptors reduces this release of transmitter and the reverse is observed after blockade of these receptors. Clearcut pharmacological differences exist between the postsynaptic alpha 1-adrenoceptors that mediate the responses of certain organs and the presynaptic alpha 2-adrenoceptors that modulate the NE release during nerve stimulation. Therefore, subclassification of alpha-adrenoceptors into alpha 1 and alpha 2 subtypes is warranted but must be considered to be independent of the anatomical location of these receptors. Some noradrenergic nerve endings have also been shown to possess beta-adrenergic receptors, the stimulation of which increases the quantity of transmitter released by nerve impulses. Physiologically, these receptors could be activated by circulating epinephrine (E) and be involved in essential hypertension. A third type of catecholamine receptor found at the noradrenergic nerve ending is the inhibitory dopamine (DA) receptor, which might be of significance in the development of new antihypertensive agents. Application of these new concepts of noradrenergic neurotransmission and the subclassification of alpha-adrenoceptors to the treatment of hypertension is presented. Clonidine, for example, appears to be a potent alpha 2-adrenoceptor agonist; the central receptor involved in its antihypertensive action is pharmacologically an alpha 2-type but located postsynaptically. Clonidine also induces activation of peripheral presynaptic alpha 2-adrenoceptors, which might contribute to its cardiovascular action. The antihypertensive effects of alpha-methyldopa are related to the formation of alpha-methylnorepinephrine, a preferential alpha 2-adrenoceptor agonist, which can stimulate peripheral presynaptic alpha 2-adrenoceptors leading to a decrease of NE release and a reduction in sympathetic tone. Prazosin is a new antihypertensive agent the mechanism of action of which involves a selective blockade of postsynaptic alpha 1-adrenoceptors. This drug does not antagonize several effects of clonidine that are mediated via alpha 2-adrenoceptors. The mechanisms presently considered to account for the antihypertensive activity of beta-adrenoceptor blocking agents are numerous. It is proposed that blockade of peripheral presynaptic facilitatory beta-adrenoceptors could be of significance in the antihypertensive action of these drugs.

Propranolol decreases sympathetic nervous activity reflected by plasma catecholamines during evolution of myocardial infarction in man

Plasma 1-norepinephrine and epinephrine contents were strikingly elevated in 70 patients during evolution of myocardial infarction. Propranolol or placebo, 0.1 mg/kg i.v., was administered randomly an average of 10 h after infarction and continued orally for 3 d. Propranolol, but not placebo, acutely decreased 1-norepinephrine contents from 2.24 +/- 1.33 (mean +/- SD) to 1.31 +/- 0.74 microgram/liter, P less than 0.001, and epinephrine contents from 0.97 +/- 0.42 to 0.74 +/- 0.42 microgram/liter, P less than 0.02. Decreases in 1-norepinephrine contents were related to the initial plasma concentrations, r = 0.85, P less than 0.001. A similar, but less strong relationship was observed between the initial epinephrine contents and propranolol-induced changes, r = -0.51, P less than 0.01. Propranolol reduced plasma-free fatty acid contents from 1,121 +/- 315 to 943 +/- 274 mumol/liter, P less than 0.001. Decreases in plasma contents of free fatty acids were related to decreases in epinephrine, r = 0.66, P less than 0.001. Propranolol did not cause significant additional changes in plasma catecholamine contents during the subsequent 3 d. In the placebo group 1-norepinephrine contents had decreased 24 h after infarction from 1.92 +/- 0.99 to 1.37 +/- 0.93 microgram/liter, P less than 0.02. Plasma epinephrine contents did not change. Heart rate remained below the control values during the entire study period in the propranolol, but increased in the placebo group. The data indicate that sympathetic hyperactivity, indirectly reflected by plasma catecholamine contents, is acutely reduced by propranolol during evolution of myocardial infarction.