Muscarinic inhibition of endogenous noradrenaline release from rabbit isolated trachea: receptor subtype and receptor reserve

The aim of the present study was to characterize putative muscarine receptors on sympathetic nerve terminals in the rabbit trachea. Release of endogenous noradrenaline from in vitro incubated rabbit tracheae was evoked by electrical field stimulation (3 Hz, 540 pulses) and quantified by high performance liquid chromatography with electrochemical detection.The muscarine receptor agonist oxotremorine inhibited the evoked release of noradrenaline completely at 1 ol/l (EC₅₀: 64 nmol/l). The concentration response curve was very steep (Hill coefficient of 2.3). Scopolamine shifted the concentration response curve of oxotremorine to the right (–log K_B 8.48) demonstrating specific, inhibitory muscarine receptors. Several subtype-preferring muscarine receptor antagonists also shifted the concentration response curve of oxotremorine to the right. The rank order of potency was (–log K_B or pA^* ₂): scopolamine (8.48) > AF DX 384 (7.88^*; slope of Schild plot 1.1) > (R)-trihexyphenidyl (7.87) > 4-DAMP (7.85) > AQ-RA 741 (7.77) methoctramine 6.18 > pirenzepine (6.0) >p-fluoro-hexahydrosiladifenidol (p-FHHSiD, 5.68). When these affinity constants were plotted against reported –log K_i values determined in binding studies on human cloned muscarine receptor subtypes (m₁-m₅), the best correlation was obtained for m₂. Indomethacin (3 mol/l), which on its own increased the evoked noradrenaline release by about 45%, affected neither the inhibitory effect of oxotremorine nor the antagonistic potency of methoctramine or p-FHHSiD. After preincubation for 48 min with 300 mol/l phenoxybenzamine, which has been shown to inactivate muscarine receptors irreversibly, the concentration response curve of oxotremorine was shifted 5.2 fold to the right and the maximal inhibition was reduced by 50%, whereas the slope remained steep (Hill coefficient 2.6). These experiments indicated that a fraction of about 22% of the muscarine receptors has to be occupied by oxotremorine to produce half-maximum inhibition of noradrenaline release; the dissociation constant of oxotremorine at the prejunctional muscarine receptors was 0.33 mol/l.In conclusion, the sympathetic nerve terminals in the rabbit trachea are endowed with inhibitory M₂-like muscarine receptors for which methoctramine displayed a low affinity. Since a large receptor reserve could be excluded, the steep concentration response curve of oxotremorine suggests that activation of muscarine receptors has to reach a threshold level before the onset of an inhibitory effect. Correspondence to: K. Racké at the above address     

Endogenous noradrenaline impairs the prostaglandin-induced inhibition of noradrenaline release

Summary The effects of prostaglandin E₂ (PGE₂) on electrically evoked noradrenaline release in rat brain cortex were studied under conditions under which autoinhibition of release was avoided. When stimulation was carried out with 36 pulses at 3 Hz, 1 mol/1 PGE2, produced about 50% inhibition of release. In the presence of the ₂-adrenoceptor antagonist yohimbine (1 gmol/1) the effect of PGE₂ was markedly increased. When release was elicited by 3 pulses/100 Hz the period of stimulation was too short to permit development of autoinhibition by released noradrenaline. Then the concentration-response-curve for PGE₂ was very similar to that obtained under the above conditions (36 pulses/3 Hz, in the presence of yohimbine). These data suggest that both the ₂-adrenoceptor and the PGE₂-receptor are linked to a common pathway. Since indometacin (10 mol/1) did not enhance evoked transmitter release, an influence of endogenous PG's on in vitro release of noradrenaline from rat brain cortex slices can be excluded.Abbreviation PG prostaglandin Send offprint requests to C. Allgaier at the above address     

(-)-Deprenyl inhibits tyramine-induced noradrenaline release, but not tyramine-induced dopamine release or potassium-induced noradrenaline release, from rat brain synaptosomes.

The effect of (-)-deprenyl (selegiline), a therapeutic agent for Parkinson's disease, on the tyramine-induced release of catecholamine from rat brain synaptosomes was studied using a superfusion system. Tyramine (10(-7) to 10(-5)M) enhanced the release of [3H]noradrenaline (NA) and [3H]dopamine (DA) from forebrain and striatal synaptosomes in a dose-dependent manner. (-)-Deprenyl (5x10(-5)M) had no effect on spontaneous catecholamine release, suggesting that it has no tyramine-like catecholamine releasing effect. Pretreatment with (-)- or (+)-deprenyl (5x10(-5)M) significantly prevented the tyramine (10(-6)M)-induced NA release, but not DA release. The inhibitory action of (-)-deprenyl was not observed on potassium (15mM)-induced NA release. (-)-Desmethyldeprenyl (5x10(-5)M), a metabolite of (-)-deprenyl, and a monoamine oxidase-A (MAO-A) inhibitor, clorgyline (5x10(-5)M), failed to block the tyramine-induced NA and DA release. Although (+)-deprenyl, a potent DA uptake inhibitor, did not inhibit tyramine-induced DA release, a catecholamine uptake inhibitor nomifensine (5x10(-5)M) did. In summary, (-)-deprenyl at a dose inhibiting tyramine-induced NA release did not have any effect on tyramine-induced DA release or potassium-induced NA release.     

Muscarinic modulation of endogenous noradrenaline release from adrenergic terminals in the guinea-pig colon

1 The present study examined the role of muscarinic receptors in the modulation of noradrenaline (NA) release in the guinea-pig isolated distal colon. The spontaneous endogenous NA overflow assayed by HPLC-ED was taken as an index of NA release from enteric noradrenergic nerve terminals. 2 Physostigmine (10 μm ) significantly enhanced spontaneous endogenous NA overflow. Hyoscine (muscarinic antagonist), (R)-(-)-trihexyphenidyl and telenzepine (M₁-selective antagonists), and 11[[2-[(diethylamino)methyl]-1-piperydil]acetyl]-5,11-dihydro-6H-pyrido[2,3-b][1,4]benzodiazepine-6-one (AF-DX 116, M₂-selective antagonist) inhibited NA overflow in a concentration dependent manner, with the following EC₅₀ values: 131.74 (18.19–953.96), 101.62 (58.83–175.60), 150 (60–330), 30 (5–170) nm , respectively. 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, M₁- and M₃- selective antagonist) had no significant effect up to 100 μm . 3 The muscarinic agonist oxotremorine inhibited NA overflow in a concentration dependent manner, with an EC₅₀ value of 0.67 (0.30–1.51) μm . The response to oxotremorine was inhibited by muscarinic antagonists with the following order of potency: hyoscine = (R)-(-)-trihexyphenidyl = telenzepine > 4-DAMP >> AF-DX 116. 4 In the presence of 3 μm tetrodotoxin (TTX), the effect of oxotremorine and 4-DAMP was unchanged, while hyoscine, (R)-(-)-trihexyphenidyl, telenzepine and AF-DX 116, instead of inhibiting, significantly enhanced NA overflow. 5 The present results indicate that, in the guinea-pig colon, endogenous acetylcholine sustains spontaneous NA release by activating muscarinic receptors possibly located on interneurones. In addition, inhibitory muscarinic receptors may exist on adrenergic terminals.     

Stimulation by vanadate of [3H]noradrenaline release from rabbit pulmonary artery and its inhibition by noradrenaline

Vanadate, the +5 oxidation state of vanadium, present in mammalian tissues, even in nerve tissue, and a competitive inhibitor of NaK-ATPase, significantly enhanced the release of [3H]noradrenaline evoked from rabbit isolated perfused pulmonary artery by electrical stimulation. Its effect proved to be concentration-dependent. Noradrenaline (10(-6) M) reduced the vanadate-potentiated release of [3H]noradrenaline. The effect of noradrenaline is mediated via alpha 2-adrenoceptors as evidenced by the finding that yohimbine 3 x 10(-7) M prevented its action. The effect of vanadate was dependent on external K ions. When the effect of vanadate on [3H]noradrenaline release was studied under conditions when the NaK-ATPase enzyme activity was inhibited by removal of external K for 45 min, vanadate was ineffective. This finding indicates that the effect is related to the inhibition of NaK-ATPase activity, a condition known to result in transmitter release.     

Release of secretoneurin and noradrenaline from hypothalamic slices and its differential inhibition by calcium channel blockers

Secretoneurin is a newly discovered peptide found in high concentrations in brain. We have studied the release of secretoneurin and noradrenaline from superfused hypothalamic slices from rat brain. Both electrical stimulation and potassium induced depolarisation released secretoneurin and noradrenaline from these slices in a calcium-dependent manner. Electrical stimulation caused a preferential release of noradrenaline when compared to the secretion elicited by high potassium. The time course of secretoneurin release was more protracted than that of noradrenaline. The calcium channel blocker -conotoxin inhibited only the electrically induced release of noradrenaline, whereas nifedipine inhibited only that of secretoneurin. These results establish that secretoneurin is secreted from neurons. Inhibition of this release by nifedipine is consistent with the concept that secretion from large dense core vesicles occurs at sites different from that of small vesicles and depends on calcium influx via L-type calcium channels. Correspondence to: H. Winkler at the above address     

P2-Receptor-mediated inhibition of noradrenaline release in the rat pancreas

The aim of the study was to find out whether, and if so through which receptors, nucleotides modulate the release of noradrenaline in the rat pancreas. Segments of the pancreas were preincubated with [³H]-noradrenaline, superfused with medium containing desipramine (1μM) and yohimbine (1μM), and stimulated electrically, in most experiments by 60 pulses/1Hz. The adenosine A₁-receptor agonist N⁶-cyclopentyl-adenosine (CPA; EC₅₀ 32nM), the non-subtype-selective adenosine receptor agonists adenosine (EC₅₀ 15μM) and 5’-N-ethylcarboxamidoadenosine (NECA; EC₅₀ 135nM), and the nucleotides ATP (EC₅₀ 13μM), adenosine-5’-O-(3-thiotriphosphate) (ATPγS; EC₅₀ 19μM) and adenosine-5’-O-(2-thiodiphosphate) (ADPβS; EC₅₀ 16μM) decreased the evoked overflow of tritium. The adenosine A_2A-agonist 2-p-(2-carboxyethyl)-phenethylamino-5’-N-ethylcarboxamido-adenosine (CGS 21680) caused no change. The concentration-response curve of CPA was shifted to the right by the A₁-antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX 10nM; pK_d 9.1) but, like the concentration-response curve of adenosine, hardly affected by the P2-receptor antagonist cibacron blue 3GA (30μM). Combined administration of a high concentration of DPCPX (1μM) and 8-phenyltheophylline (10μM) abolished the effects of CPA and NECA. The concentration-response curves of ATP and ADPβS were shifted to the right by both DPCPX (10nM; pK_d 8.7 and 8.9, respectively) and cibacron blue 3GA (30μM; pK_d 5.0 and 5.2, respectively). The antagonist effects of DPCPX (10nM) and cibacron blue 3GA (30μM) against ATP were additive in a manner compatible with the blockade of two separate receptors for ATP. In the presence of the high concentration of DPCPX (1μM) and 8-phenyltheophylline (10μM), ATP and ADPβS still decreased evoked tritium overflow, and this decrease was attenuated by additional administration of cibacron blue 3GA (30μM). The P2-antagonists cibacron blue 3GA, reactive blue 2, reactive red 2, and to a limited extent also suramin and 8-(3,5-dinitro-phenylenecarbonylimino)-1,3,5-naphthalenetrisulphonate (XAMR0721), increased the evoked overflow of tritium by up to 114%. Pyridoxalphosphate-6-azophenyl-2’,4’-disulphonate (PPADS) caused no change. The results indicate that the postganglionic sympathetic axons of the rat pancreas possess A₁-adenosine and P2-receptors. Both receptors mediate an inhibition of noradrenaline release. The presynaptic P2-receptors are activated by an endogenous ligand, presumably ATP, during appropriate trains of action potentials. This is the first demonstration of presynaptic P2-receptors at postganglionic sympathetic neurons that are located in prevertebral ganglia. Received: 6 November 1997 / Accepted: 6 January 1998     

P2-Receptor-mediated inhibition of noradrenaline release in the rat hippocampus

Experiments on hippocampal slices were carried out in order to find out whether the release of noradrenaline in the hippocampus can be modulated through P2-receptors. The slices were preincubated with [³H]-nor-adrenaline, superfused with medium containing desipramine (1 μM), and stimulated electrically, in most experiments by 4 pulses/100 Hz. The adenosine A₁-receptor agonist N⁶-cyclopentyl-adenosine (CPA) and the nucleotides ATP, adenosine-5’-O-(3-thiotriphosphate) (ATPγS) and adenosine-5’-O-(2-thiodiphosphate) (ADPβS) decreased the evoked overflow of tritium by up to 55 %. The adenosine A_2a-agonist 2-p-(2-carboxyethyl)-phenethylamino-5’-N-ethylcarboxamido-adenosine (CGS 21680; 0.003-0.3 μM) caused no change. The concentration-response curve of CPA was shifted to the right by the A₁-antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 3 nM) but not by the P2-receptor antagonists cibacron blue 3GA (30 μM) and reactive blue 2 (30 μM); the apparent pK_B value of DPCPX against CPA was 9.0. In contrast, the concentration-response curve of ATP was shifted to the right by DPCPX (3 nM), apparent pK_B 8.7, as well as by ciba-cron blue 3GA (30 μM), apparent pK_B 5.2, and reactive blue 2 (30 μM), apparent pK_B 5.6; the antagonist effects of DPCPX and cibacron blue 3GA were additive in a manner compatible with the blockade of two separate receptors for ATP. The same pattern was obtained with ATPγS: its concentration-response curve was shifted to the right by DPCPX as well as by cibacron blue 3GA and reactive blue 2. Suramin (300 μM) antagonized neither the effect of ATP nor that of ATPγS. The 5’-nucleotidase inhibitor α,β-methylene-ADP (100 μM) did not change the effect of ATP. Only cibacron blue 3GA (30 μM) but not reactive blue 2 (30 μM), given alone, consistently caused a small increase of the evoked overflow of tritium. Hippocampal slices degraded exogenous ATP, and this degradation was reduced by cibacron blue 3GA (30 μM), reactive blue 2 (30 μM) and suramin (300 μM). The results indicate that the noradrenergic terminal axons of the rat hippocampus possess P2-receptors in addition to the known A₁-adenosine receptors. The presynaptic P2-receptors mediate an inhibition of noradrenaline release, are activated by nucleotides but not nucleosides, and are blocked by cibacron blue 3GA and reactive blue 2. ATP and ATPγS act at both the A₁- and the P2-receptors. An autoreceptor function of cerebral presynaptic P2-receptors remains doubtful. Received: 20 November 1996 / Accepted: 10 February 1997     

Sympathomimetic inhibition of noradrenaline release: Mediated by prostaglandins?

Summary In isolated perfused rabbit hearts, the effec of oxymetazoline and phentolamine on the stimulation-induced overflow of noradrenaline was tested after the synthesis of prostaglandins had been blocked by indomethacin. Indomethacin changed neither the decrease of overflow caused by oxymetazoline nor the increase caused by phentolamine. It is concluded that prostaglandins are not involved to any major degree in the modulation of noradrenaline release by drugs with affinity to -receptors, or in the feed-back inhibition of transmitter release mediated by the effect of liberated noradrenaline on -receptive sites.     

强啡肽对培养脊髓神经元高钾刺激性胞内钙增高的双向作用

为探讨强啡肽(Dyn)镇痛与致瘫的细胞机理，采用Fura-2显微荧光光度技术观测了不同浓度的Dyn A(1-17)对原代培养脊髓神经元单个细胞内游离钙离子浓度(［Ca²⁺］_i)的影响. Dyn A 0.1-100 μmol·L^-1对基础［Ca²⁺］_i均无影响. Dyn A 0.1和1 μmol·L^-1使高钾(50 mmol·L^-１)刺激的Ca²⁺内流峰值反应分别下降94%(n=6)和83%(n=4, P<0.05); Dyn A 10和100 μmol·L^-1对高钾刺激反应峰值无明显影响，但所有测试细胞均呈现持续性［Ca²⁺］_i升高；预先给予低浓度的Dyn A (0.1和1 μmol·L^-1), 则高浓度Dyn A (10 和100 μmol·L^-1)的促进作用明显 减弱甚至消失. 结果表明低浓度和高浓度Dyn A(1-17)对培养脊髓神经元的基础［Ca²⁺］_i无影响，但可分别抑制和促进去极化性钙离子内流，低浓度Dyn A 可对抗高浓度Dyn A 的促进作用.     

Cannabinoid CB1 receptor-mediated inhibition of noradrenaline release in the human and guinea-pig hippocampus

We examined the question of whether cannabinoid receptors modulating noradrenaline release are detectable in the brain of humans and experimental animals. For this purpose, hippocampal slices from humans, guinea-pigs, rats and mice and cerebellar, cerebrocortical and hypothalamic slices from guinea-pigs were incubated with [³H]noradrenaline and then superfused. Tritium overflow was evoked either electrically (0.3 or 1Hz) or by introduction of Ca²⁺ ions (1.3μM) into Ca²⁺-free, K⁺-rich medium (25μM) containing tetrodotoxin 1μM. Furthermore, the cAMP accumulation stimulated by forskolin 10μM was determined in guinea-pig hippocampal membranes. We used the following drugs: the cannabinoid receptor agonists (–)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol (CP-55,940) and R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-yl]-(1-naphthalenyl)methanone (WIN 55,212-2), the inactive S(–)-enantiomer of the latter (WIN 55,212-3) and the CB₁ receptor antagonist N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazole-carboxamide (SR 141716). The electrically evoked tritium overflow from guinea-pig hippocampal slices was reduced by WIN 55,212-2 (pIC_30% 6.5) but not affected by WIN 55,212-3 up to 10μM. The concentration-response curve of WIN 55,212-2 was shifted to the right by SR 141716 (0.032μM) (apparent pA₂ 8.2), which by itself did not affect the evoked overflow. WIN 55,212-2 1μM also inhibited the Ca²⁺-evoked tritium overflow in guinea-pig hippocampal slices and the electrically evoked overflow in guinea-pig cerebellar, cerebrocortical and hypothalamic slices as well as in human hippocampal slices but not in rat and mouse hippocampal slices. SR 141716 (0.32μM) markedly attenuated the WIN 55,212-2-induced inhibition in guinea-pig and human brain slices. SR 141716 0.32μM by itself increased the electrically evoked tritium overflow in guinea-pig hippocampal slices but failed to do so in slices from the other brain regions of the guinea-pig and in human hippocampal slices. The cAMP accumulation stimulated by forskolin was reduced by CP-55,940 and WIN 55,212-2. The concentration-response curve of CP 55,940 was shifted to the right by SR 141716 (0.1μM; apparent pA₂ 8.3), which by itself did not affect cAMP accumulation. In conclusion, cannabinoid receptors of the CB₁ subtype occur in the human hippocampus, where they may contribute to the psychotropic effects of cannabis, and in the guinea-pig hippocampus, cerebellum, cerebral cortex and hypothalamus. The CB₁ receptor in the guinea-pig hippocampus is located presynaptically, is activated by endogenous cannabinoids and may be negatively coupled to adenylyl cyclase. Received: 5 June 1997 / Accepted: 6 August 1997     

P2-purinoceptor-mediated inhibition of noradrenaline release in rat atria.

1. We looked for P2-purinoceptors modulating noradrenaline release in rat heart atria. Segments of the atria were preincubated with [3H]-noradrenaline and then superfused with medium containing desipramine (1 microM) and yohimbine (1 microM) and stimulated electrically, by 30 pulses/1 Hz unless stated otherwise. 2. The adenosine A1-receptor agonist, N6-cyclopentyl-adenosine (CPA; EC50 9.7 nM) and the nucleotides, ATP (EC50 6.6 microM) and adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S; EC50 4.8 microM), decreased the evoked overflow of tritium. The adenosine A2a-agonist, 2-p-(2-carbonylethyl)-phenethylamino-5'-N-ethylcarboxamido-a denosine (CGS-21680; 0.03-0.3 microM) and the P2x-purinoceptor agonist beta, gamma-methylene-L-ATP (30 microM) caused no change. 3. The concentration-response curve of CPA was shifted to the right by the adenosine A1-receptor antagonist, 8-cyclopentyl-1,3-dipropyl-xanthine (DPCPX; 3 nM; apparent pKB value 9.7) but hardly affected by the P2-purinoceptor antagonist, cibacron blue 3GA (30 microM). In contrast, the concentration-response curves of ATP and ATP gamma S were shifted to the right by DPCPX (3 nM; apparent pKB values 9.3 and 9.4, respectively) as well as by cibacron blue 3GA (30 microM; apparent pKB values 5.0 and 5.1, respectively). Combined administration of DPCPX and cibacron blue 3GA caused a much greater shift of the concentration-response curve of ATP than either antagonist alone. The concentration-response curve of ATP was not changed by indomethacin, atropine or the 5'-nucleotidase blocker alpha, beta-methylene-ADP. 4. Cibacron blue 3GA (30 microM) increased the evoked overflow of tritium by about 70%.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential effects of acidosis, high potassium concentrations, and metabolic inhibition on noradrenaline release and its presynaptic muscarinic regulation.

It was the aim of the present study to characterize the effect of single components of ischaemia, such as inhibition of aerobic and anaerobic energy production by combined anoxic and glucose-free perfusion (metabolic inhibition), high extracellular potassium concentrations (hyperkalaemia), and acidosis, on (1). the stimulated release of noradrenaline from the in situ perfused guinea-pig heart and (2). its presynaptic modulation by the muscarinic agonist carbachol. The release of endogenous noradrenaline from efferent cardiac sympathetic nerve endings was induced by electrical stimulation of the left stellate ganglion (1 min, 5 V, 12 Hz) and quantified in the coronary venous effluent by high-performance liquid chromatography. Under control conditions, two consecutive electrical stimulations (S1, S2) elicited a similar noradrenaline overflow (S2/S1: 0.98 plus minus 0.05). After 10 min of global myocardial ischaemia overflow of endogenous noradrenaline was significantly reduced (S2/S1: 0.18 plus minus 0.03; P< 0.05). When studied separately, metabolic inhibition, hyperkalaemia (16 mM), and acidosis (pH 6.0) each markedly attenuated stimulated noradrenaline overflow (S2/S1: 0.65 plus minus 0.05, 0.43 plus minus 0.14, and 0.37 plus minus 0.09, respectively; P< 0.05). The muscarinic agonist carbachol (10 microM) inhibited stimulated noradrenaline release under normoxic conditions (S2/S1: 0.41 plus minus 0.07; P< 0.05). However, after 10 min of global myocardial ischaemia the inhibitory effect of carbachol on noradrenaline overflow was completely lost. Single components of ischaemia had a differential effect on presynaptic muscarinic modulation. Whereas hyperkalaemia (8-16 mM) did not affect muscarinic inhibition of noradrenaline release, carbachol lost its inhibitory effect during acidosis and metabolic inhibition. In conclusion, hyperkalaemia, metabolic inhibition, and severe acidosis each contribute to reduced overflow of noradrenaline after 10 min of myocardial ischaemia. However, presynaptic muscarinic inhibition of noradrenaline release was not affected by hyperkalaemia, but was sensitive to metabolic inhibition and low degrees of acidosis.     

Muscarinic inhibition of acetylcholine release from a novel in vitro preparation of the guinea-pig trachea

Summary An isolated preparation of the guinea-pig trachea is described which allows the simultaneous measurement of acetylcholine release and smooth muscle contraction. Incubation of the epithelium-free preparation with [³H]choline resulted in the formation of [³H]acetylcholine. Electrical stimulation caused the release of [³H]acetylcholine and a contractile response. Tetrodotoxin and omission of calcium from the medium abolished both the evoked release and contractions.The muscarinic agonists oxotremorine, carbachol and pilocarpine concentration-dependently inhibited the electrically evoked acetylcholine release and contracted the tracheal smooth muscle. Pre- and postsynaptic EC₅₀ values for a given agonist were not different. Atropine (100 nmol/l) significantly faciliated the evoked acetylcholine release. A concentration of 10 nmol/l atropine did not change the evoked release but antagonized the inhibitory effect of oxotremorine. It is concluded that presynaptic muscarine autoreceptors inhibit the release of acetylcholine from parasympathetic nerves of the guinea-pig trachea.Send offprint requests to G. D'Agostino at the above address     

Changes in plasma noradrenaline concentration as a measure of release rate.

1 A method is described for repeated sampling of plasma noradrenaline (NA) in freely moving rats. Na concentration does not change during the day or after adrenalectomy. 2 Exogenous NA has a half-life of 1.5 min; drugs which block neuronal and extra-neuronal uptake lengthen this to 6.3 min. 3 Swim-stress leads to a steep rise followed by a rapid decline in plasma NA concentration. 4 This method of plasma NA sampling can serve as a measure of both steady and rapid changes in release rate over long periods of time.