Blood pressure changes modify the release rates of catecholamines in the intermediate nucleus of the solitary tract

Summary In anaesthetized cats, the intermediate aspect of the nucleus of the solitary tract (NTS) was bilaterally superfused with artificial CSF through push-pull cannulae. The release of the endogenous catecholamines dopamine, noradrenaline and adrenaline was determined in the superfusates radioenzymatically. Blood pressure changes were elicited by intravenous injections of drugs (noradrenaline or chlorisondamine), or electrical stimulation of the intermediate NTS with the tip of the push-pull cannula.Intravenous injections of noradrenaline (3 or 10 g/kg) elicited a rise in the arterial blood pressure which was associated with a decrease in the release rate of adrenaline in the intermediate NTS. The release rates of dopamine and noradrenaline were not influenced. The intravenous injection of chlorisondamine (3 mg/kg) lowered blood pressure and diminished the release rate of dopamine in the intermediate NTS. The release rate of noradrenaline was not modified by chlorisondamine. Electrical stimulation of the intermediate NTS contralateral to the superfused nucleus increased moderately the arterial blood pressure and decreased the release rate of noradrenaline and dopamine, while the release of adrenaline was not influenced. The findings suggest that experimentally induced changes in the arterial blood pressure by drugs injected intravenously modify the release rates of adrenaline and dopamine in the intermediate NTS so as to counteract the blood pressure change. In the intermediate NTS, release of adrenaline from adrenergic nerve terminals seems to act hypertensive. The results obtained with chlorisondamine point to a hypotensive function of endogenous dopamine in the intermediate NTS. Send offprint requests to N. Singewald at the above addressThis work was supported by the Fonds zur Förderung der wissenschaftlichen Forschung     

Release of endogenous catecholamines in the nucleus tractus solitarii during experimentally induced blood pressure changes

Summary The nucleus tractus solitarii (subnucleus medialis) of anaesthetized cats was bilaterally superfused through push-pull cannulae and the release of endogenous catecholamines (dopamine, noradrenaline and adrenaline) determined in the superfusates. A moderate increase in blood pressure elicited by intravenously injected noradrenaline (0.3 g per kg body weight) reduced the rate of release of endogenous adrenaline, while a pronounced rise in blood pressure (at least 47 mm Hg) evoked by noradrenaline (3 g per kg) or blood injection inhibited the release of adrenaline and noradrenaline in the nucleus tractus solitarii. Bilateral carotid occlusion also diminished the release rates of adrenaline and noradrenaline. Decreases in blood pressure induced by controlled bleeding, intravenous injection of chlorisondamine or nitroprusside did not alter the rates of release of adrenaline or noradrenaline, but the rate of release of endogenous dopamine seemed to be reduced. The decreased adrenaline and noradrenaline release elicited by increases in blood pressure and the reduced release of dopamine induced by decreases in blood pressure may reflect a counteracting mechanism in the nucleus tractus solitarii so as to normalize alterations in blood pressure. The results suggest a hypertensive function of noradrenaline, and possibly a hypotensive role of dopamine at the level of the nucleus tractus solitarii.This work was supported by the Fonds zur Förderung der wissenschaftlichen Forschung (P 5750) Send offprint requests to C. Kobilansky     

Central sites and mechanisms of the hypotensive and bradycardic effects of the narcotic analgesic agent fentanyl

Summary In dogs, anaesthetized with chloralose, fentanyl (5 g/kg i.v.) augmented the bradycardia produced by electrical stimulation of the carotid sinus nerves. In contrast, the same dose of the drug did not change the bradycardic response to stimulation of the nucleus of the solitary tract (NTS) indicating that a central facilitation of baroreceptor impulses occurs within the NTS, probably at the first synapse of baroreceptor reflex fibres.Bilateral destruction of the NTS caused a ful-minating hypertension and tachycardia similar to that after cutting the baroreceptor afferent fibres. After both procedures, fentanyl (20 g/kg i.v.) produced marked hypotension and bradycardia. The bradycardic effect was abolished by cutting both vagal nerves when the dogs were pretreated with a -adrenoceptor blocking agent (S 2395, 50 g/kg i.v.).The results provide evidence that the NTS is not the main site of action either for the hypotensive effect or for the vagally mediated bradycardia of fentanyl. Since the dorsal nucleus of the vagal nerve was destroyed together with the NTS, this nucleus does also not appear to be a major site of the action of fentanyl.Blockade of dopamine receptors by haloperidol or pimozide or of serotonin receptors by methysergide did not change the hypotensive, bradycardic and sympathoinhibitory effects of fentanyl.     

In vivo release of catecholamines in the locus coeruleus

To investigate the release of endogenous dopamine, noradrenaline and adrenaline in the locus coeruleus, this brain area was superfused with artificial cerebrospinal fluid (CSF) through push-pull cannulae and the release of catecholamines was determined in the superfusate radioenzymatically.Collection of superfusates in time periods of 10 min revealed that release rates of the three catecholamines fluctuated, thus pointing to the existence of ultradian rhythms with following mean periods (minutes per cycle): noradrenaline 52±4, dopamine 37±2, adrenaline 36±2. The rhythm frequency of noradrenaline was significantly lower than the frequencies of dopamine and adrenaline.When the locus coeruleus was superfused with neuroactive drugs, superfusates were collected in time periods of 3 min. Superfusion with tetrodotoxin (1 mol 1^–1) for 12 min elicited a prompt and sustained decrease (–70%) in the release rates of dopamine and adrenaline. The release rate of noradrenaline was also reduced, although to a lesser extent (–40%). Superfusion with veratridine (50 mol 1^–1) led to an immediate and very pronounced enhancement in the release rates of dopamine, noradrenaline and adrenaline. The veratridine-induced increase in catecholamine outflow was decreased strongly by simultaneous superfusion with tetrodotoxin.The findings suggest that the release of endogenous catecholamines in the locus coeruleus fluctuates according to ultradian rhythms. Changes in the release on superfusion with veratridine and tetrodotoxin demonstrate the neuronal origin of the three catecholamines. The observed differences in the release characteristics between noradrenaline on the one hand and dopamine and adrenaline on the other might indicate that noradrenaline is partly released from somatodendritic sites of the noradrenergic cell bodies in the locus coeruleus.This work was supported by the Fonds zur Förderung der wissenschaftlichen Forschung Correspondence to: N. Singewald at the above address     

Dopaminergic modulation of hippocampal noradrenaline release

Summary ³H-Noradrenaline release in the rabbit hippocampus and its possible modulation via presynaptic dopamine receptors was studied. Hippocampal slices were preincubated with ³H-noradrenaline, continuously superfused in the presence of cocaine (30 mol/l) and subjected to electrical field stimulation. The electrically evoked tritium over-flow from the slices was reduced by 0.1 and 1 mol/l dopamine and apomorphine, but significantly enhanced by 10 mol/l apomorphine or by 0.1 and 1 mol/l bromocriptine. If the ₂-adrenoceptor antagonist yohimbine (0.1 mol/l) was present throughout superfusion, the inhibitory effects of dopamine and apomorphine were more pronounced and even 10 mol/l apomorphine and 1 mol/l bromocriptine inhibited noradrenaline release. Qualitatively similar observations were made in the presence of another ₂-antagonist, idazoxane (0.1 mol/l). In the presence of the D2-receptor antagonist domperidone (0.1 mol/l) the inhibitory effects of dopamine were almost abolished, whereas both apomorphine (>1 mol/l) and bromocriptine (>0.01 mol/l) greatly facilitated noradrenaline release. The D2-receptor agonist LY 171555 (0.1 and 1 mol/l) significantly reduced the evoked noradrenaline release whereas the D1-selective agonist SK & F 38393 was ineffective at similar concentrations. The effects of LY 171555 were abolished in the presence of domperidone (0.1 mol/l) but remained unchanged in the presence of yohimbine or idazoxane (0.1 mol/l, each).At 1 mol/l the D2-receptor antagonists domperidone and (-)sulpiride significantly increased the evoked noradrenaline release by about 10%. However, at this concentration, domperidone (but not (-)sulpiride) affected also basal tritium outflow. Bulbocapnine and the preferential D1-receptor antagonists SCH 23390 enhanced the evoked noradrenaline release already at 0.1 mol/l. Their marked facilitatory effects (50 to 60% increase at 1 mol/l) were reduced in the presence of idazoxane (0.1 mol/l) and almost abolished in the presence of 0.1 mol/l yohimbine, whereas the increase due to 1 mol/l (-)sulpiride persisted under these conditions.The evoked tritium efflux from rabbit hippocampal slices preincubated with ³H-serotonin was not affected by dopamine receptor agonists.From our results we conclude that hippocampal noradrenaline, but not serotonin release, is modulated via D2-dopamine receptors. In addition, our results provide evidence for more or less pronounced ₂-adrenoceptor agonistic properties of dopamine and ₂-adrenoceptor antagonistic properties of apomorphine, bromocriptine, SCH 23390 and bulbocapnine in this noradrenaline release model from CNS tissue.     

Activation of β2-adrenoceptors by isoprenaline and adrenaline enhances noradrenaline release in cortical kidney slices of young spontaneously hypertensive rats

Summary The aim of the present study was to investigate -adrenoceptor modulation of noradrenaline release from sympathetic nerves in superfused cortical kidney slices of 4-week-old spontaneously hypertensive rats (SHR) and age-matched controls (WKY). After preincubation with ³H-noradrenaline the kidney slices were electrically stimulated in superfusion chambers. The stimulation induced (S-I) outflow of radioactivity was mainly composed of unmetabolized 3H-noradrenaline in both strains and thus taken as an index of noradrenaline release. There was a frequency-dependent (1.25–20 Hz) increase in the S-1 outflow of radioactivity. At all stimulation frequencies tested S-I outflow of radioactivity was similar or even slightly lower in SHR than in WKY kidney slices in either the absence or presence of cocaine (10 mol/l). The non-selective -adrenoceptor agonists isoprenaline (0.l gmol/1) and adrenaline (0.01 and 0.1 mol/l) enhanced S-I outflow of radioactivity. The facilitatory effects of isoprenaline (0.1 mol/l) and adrenaline (0.1 mol/l) were blocked by the selective ₂-adrenoceptor antagonist ICI 118551 (0.1 mol/l) but not by the selective ₁-adrenoceptor antagonist atenolol (0.3 mol/l). The cell-permeable CAMP analogue 8-bromo-cAMP (300 mol/l) enhanced S-1 outflow of radioactivity to a similar extent in both SHR and WKY kidney slices. A combination of 8-bromo-cAMP (300 mol/l) and adrenaline (0.1 mol/l) did not enhance S-1 outflow of radioactivity to a greater extent than 8-bromo cAMP (300 mol/l) alone in both strains. However, the facilitatory effects of isoprenaline (0.1 mol/l) and adrenaline (0.1 mol/l) but not that of adrenaline (0.01 mol/l) were significantly greater in SHR than in WKY. The results suggest that stimulation of prejunctional ₂-adrenoceptors by adrenaline even in the absence of a-adrenoceptor blockade enhances noradrenaline release in kidney cortex of young SHR and WKY. This ₂-adrenoceptor mediated effect may possibly be dependent on cAMP formation. The greater facilitatory effects of isoprenaline (0.1 mol/l) and adrenaline (0.1 mol/l) in SHR as compared to WKY are in accord with receptor binding studies which show a higher density of ₂-adrenoceptors in SHR than in WKY kidney cortex.Abbreviations SHR Spontaneously hypertensive rats - WKY WistarKyoto rats - cAMP 3-5-cyclic adenosine monophosphate - S-I stimulation induced Send offprint requests to: L. C. Rump     

In vivo modulation of histamine release by autoreceptors and muscarinic acetylcholine receptors in the rat anterior hypothalamus

The modulation of histamine release by histamine and muscarinic acetylcholine receptors was investigated by using the push-pull technique. The anterior hypothalamic area of the conscious, freely moving rat was superfused through the push-pull cannula with CSF or with CSF containing drugs and the release of endogenous histamine was determined in the superfusate.Hypothalamic superfusion with tetrodotoxin (10 mol/1) led to a pronounced and sustained decrease in the histamine release rate. Superfusion with compound 48/80 (100 mg/1) was ineffective. Hypothalamic superfusion with the H₃ agonist (R)--methylhistamine inhibited, while superfusion with the H₃ antagonist thioperamide enhanced the release of histamine. The release of histamine was inhibited on hypothalamic superfusion with the muscarinic receptor agonists carbachol or oxotremorine. Histamine release was enhanced by atropine, and this release-enhancing effect was abolished by oxotremorine. The selective M₁ antagonist pirenzepine (100 mol/I) and 4-diphenylacetoxy-N-methylpiperidine (4-DAMP, 10 ol/1), which blocks M₁ and M₃ receptors, also enhanced the release rate of histamine. On the other hand, 50 and 100 moI/I methoctramine (M₂ receptor antagonist) 10 and 100 moI/l p-fluoro-hexahydro-siladifenidol (p-F-HHSiD, a M₃ receptor antagonist) were ineffective.It is concluded that histamine released in the hypothalamus originates predominantly from neurons. The release of histamine is modulated by H₃ autoreceptors. The histamine release is also modulated by cholinergic neurons which modify histamine release from histaminergic neurons by stimulating M₁ muscarinic acetylcholine heteroreceptors probably located on histaminergic neurons.Supported by the Fonds zur Förderung der wissenschaftlichen Forschung Correspondence to: H. Prast at the above address     

Nonexocytotic noradrenaline release and ventricular fibrillation in ischemic rat hearts

In myocardial ischemia, nonexocytotic noradrenaline release has been identified as underlying mechanism of ischemia-evoked noradrenaline release. Nonexocytotic noradrenaline release can be suppressed by inhibitors of the neuronal noradrenaline carrier (uptake), such as desipramine. Utilizing this pharmacological intervention the role of local noradrenaline release in the genesis of ischemia-induced ventricular arrhythmias was studied. Regional ischemia was induced in rat isolated perfused hearts by ligature of the left anterior descending coronary artery, and the venous effluent obtained during the first 2 min of reperfusion was used to measure the release of endogenous noradrenaline by high-performance liquid chromatography methods. Coronary occlusion caused ventricular fibrilation in a well reproducible manner with an incidence of 70 to 80% during a 30 min observation period. Blockade of uptake, by desipramine decreased the occurrence of ischemia-induced ventricular fibrillation to 60% (0.01 mol/1) or 20% (0.1 mol/l), and ventricular fibrillation was completely suppressed by 1 mol/l desipramine. Likewise, desipramine (0.01–1 mol/l) concentration-dependently reduced endogenous noradrenaline release during 30 min of regional myocardial ischemia. Nisoxetine, a structurally unrelated inhibitor of uptake,, also suppressed ischemia-evoked ventricular fibrillation.In contrast to its antifibrillatory effect during regional myocardial ischemia, desipramine precipitated arrhythmias when ventricular fibrillation was induced by perfusing normoxic hearts with exogenous noradrenaline. Combination of desipramine (0.1 mol/1) with exogenous noradrenaline (0.01 to 1 mol/l) increased the incidence of ventricular fibrillation compared to noradrenaline perfusion alone. Under these conditions, uptake₁-blockade is known to increase the extracellular concentration of the perfused noradrenaline. Finally, in the isolated, spontaneously beating papillary muscle of the left rat heart, desipramine (0.1 and 1.0 mol/l) had no effect on the upstroke velocity of action potentials, the action potential duration and the effective refractory period.In conclusion, the findings demonstrate that nonexocytotic noradrenaline release is an important mediator of ischemia-induced ventricular fibrillation in isolated hearts of the rat. It is also documented that uptake, inhibitors such as desipramine reveal their effects on ventricular fibrillation secondary to their action on transmembrane noradrenaline transport.     

Release of endogenous acetylcholine in the hypothalamus of conscious rats

Summary The release of endogenous acetylcholine was investigated by the push-pull technique. The posterior hypothalamus of conscious rats was superfused through a push-pull cannula with artificial cerebrospinal fluid (ACSF) which contained 1 mol/l neostigmine. Acetylcholine was determined in the superfusate by high pressure liquid chromatography and electrochemical detection. Hypothalamic superfusion with potassium-rich (100 mmol/l) ACSF led to a pronounced increase in the release rate of acetylcholine. Tetrodotoxin (1 mol/l) almost abolished the basal release of the neurotransmitter. Superfusion of the hypothalamus with atropine (10 or 50 mol/l) led to a concentration-dependent increase, whereas superfusion with oxotremorine (50 mol/l) inhibited the release rate of acetylcholine. It is concluded that acetylcholine released into the superfusate of the hypothalamus originates from cholinergic neurons. Furthermore, the release of acetylcholine seems to be modulated by muscarinic acetylcholine receptors, probably located on cholinergic neurons of the hypothalamus.This work was supported by the Fonds zur Förderung der wissenschaftlichen Forschung Send offprint requests to H. Prast at the above address     

Cardiovascular effects in rats of alpha1 and alpha2 adrenergic agents injected into the nucleus tractus solitarii

Summary The cardiovascular effects of selective alpha₁ and alpha₂ agonists and antagonists injected into the nucleus tractus solitarii (NTS) were studied in urethane-anesthetized rats. Methoxamine (0.3–3 g) injected bilaterally into the NTS caused a dose-dependent increase in blood pressure and heart rate. Phenylephrine (6 g) and an imidazolidine derivative St 587 (3 g) similarly injected also produced an increase in blood pressure, whereas a-methylnoradrenaline and an azepine derivative B-HT 920 (1 and 3 g) caused a decrease in blood pressure and heart rate. The pressor response to methoxamine (1 g) was markedly inhibited by prazosin (0.3 pg) injected into the same sites or hexamethionum (25 mg/kg, i. v.). Prazosin (0.3 g) alone injected bilaterally into the NTS did not affect the blood pressure, while yohimbine (0.1 g) similarly injected increased the pressure. These results suggest that in the rat NTS there exist alpha₁ adrenoceptors responsible for an increase in arterial pressure. The NTS alpha2 adrenoceptors seem to be involved in the tonic regulation of arterial pressure. Send offprint requests to T. Kubo at the above address     

Nitric oxide influences the release of histamine and glutamate in the rat hypothalamus

To investigate the influence of nitric oxide (NO) on the release of histamine and glutamate, the anterior hypothalamus of anaesthetized rats was superfused through a push-pull cannula either with artificial cerebrospinal fluid (CSF) or with various drugs dissolved in CSF.Hypothalamic superfusion with the NO-donating compounds linsidomine (200 mol/l) or diethylamine-NO (DEANO, 100 mol/l) led to a pronounced and sustained decrease in the histamine release rate, whereas the release rate of glutamate was enhanced. Superfusion with the inhibitor of NO synthase L-N^G-nitro-L-arginine methyl ester (L-NAME, 200 mol/l) increased the histamine release rate. The inhibitory effect of 200 mol/l linsidomine was abolished by atropine (10 mol/l). Superfusion with the glutamate receptor agonists glutamate (100 mol/l) or N-methyl-D-aspartate (NMDA, 50 mol/l) enhanced the histamine release rate. In the presence of linsidomine, the releasing effect of NMDA was not changed.These findings demonstrate that the release of histamine in the hypothalamus is diminished by endogenous NO. This effect of NO on histamine release seems to be due to enhanced release of acetylcholine from vicinal cholinergic neurons via stimulation of muscarinic acetylcholine receptors located presynaptically on histaminergic neurons. The NO-induced glutamate release seems to exert a subordinate stimulatory effect on histamine release. Finally, the inhibition of histamine release by NO is not due to blockade of NMDA receptors.This work was supported by the Jubiläumsfonds der österreichischer Nationalbank     

Methoxamine enhances the release of endogenous noradrenaline from rabbit ear artery: possible involvement of ATP

Summary The effect of methoxamine, an ₁-adrenoceptor agonist, on the electrically-evoked release of endogenous noradrenaline was examined in the isolated rabbit ear artery. Noradrenaline was quantified by high performance liquid chromatography-electrochemical detection. The release of adenine nucleotides and nucleosides by methoxamine was examined using high performance liquid chromatography-fluorescence detection.The release of noradrenaline evoked by electrical field stimulation (EFS) at 4 Hz was reduced by tetrodotoxin 0.3 mol/l and clonidine 1 mol/l by approximately 80% and 50%, respectively. On the other hand, methoxamine at 10 but not 1 mol/l enhanced the release of noradrenaline to approximately twice the control, and the enhancement was prevented by prazosin 1 mol/l. The facilitatory action of methoxamine was also abolished after desensitization of P₂-purinoceptors by ,-methylene ATP 30 mol/l as well as by the presumed P₂-purinoceptor antagonist suramin given at 10 mol/l. Exogenous ATP 10 mol/l significantly enhanced the EFS-evoked release of noradrenaline, and the enhancement was abolished by ,-methylene ATP and suramin. None of the drugs changed the spontaneous outflow of noradrenaline. These results indicate that endogenous ATP, acting at prejunctional purinoceptors, may participate in the facilitatory effect of methoxamine. Indeed, methoxamine 10 mol/l significantly enhanced the spontaneous outflow of ATP and, less so, ADP. The methoxamine evoked release of ATP and ADP was antagonized by prazosin 1 mol/l.It is concluded that methoxamine releases endogenous ATP from postjunctional sites which then, via prejunctional purinoceptors, facilitates action potential-evoked release of noradrenaline in rabbit ear artery.Supported by grants from the Mita Research Foundation, Matsue, Japan and Kanae Research Foundation, Osaka, JapanCorrespondence to K. Takeuchi at the above address     

Inhibition by the putative potassium channel opener pinacidil of the electrically-evoked release of endogenous dopamine and noradrenaline in the rat vas deferens

Summary The effect of pinacidil on the release of endogenous noradrenaline and dopamine from the sympathetic innervation of the rat vas deferens was examined. Amine release was evoked by electrical stimulation (1, 2, 5 and 10 Hz) or by depolarization with high potassium (75 mmol/l) in the medium. Dopamine and noradrenaline were measured by means of high pressure liquid chromatography with electrochemical detection.Pinacidil (1, 5, 10 and 50 mol/l) produced a concentration-dependent inhibition of the electrically stimulated (2 Hz) overflow of noradrenaline and dopamine. Only pinacidil 50 mol/l increased the spontaneous loss of dopamine and noradrenaline. The inhibitory effects of pinacidil (5 mol/l) on amine overflow were also observed at other frequencies of stimulation (1, 5 and 10 Hz). The magnitude of the inhibitory effect on noradrenaline release was approximately the same at all frequencies (63% to 56% reduction); for dopamine, the higher the frequency of stimulation, the greater the inhibitory effect of pinacidil (up to 73% reduction). When the preparations were continuously stimulated for 70 min at 2 Hz, pinacidil (5 mol/l) reduced the overflow of dopamine and noradrenaline during the first 40 or 30 min of stimulation only. The addition of phentolamine (1 mol/l) to the perifusion medium slightly reduced the inhibitory effect of pinacidil on amine overflow, but the inhibition by pinacidil remained statistically significant. Tetraethylammonium (10 mmol/l) completely abolished the inhibitory effect of pinacidil (10 mol/l). Pinacidil (5 mol/l) did not reduce the potassium-evoked release of the amines.The results demonstrate that pinacidil impairs transmitter release from the sympathetic innervation of the rat vas deferens, probably as a consequence of the opening of potassium channels. Send offprint request to P. Soares-da-Silva at the above adress     

Characterisation of the mechanisms for hyperactivity induction from the nucleus accumbens by phenylethylamine derivatives

A wide variety of phenylethylamine derivatives were injected bilaterally into the nucleus accumbens of rat following a nialamide pretreatment and hyperactivity was recorded. 2-Phenylethylamine was shown to induced a low intensity hyperactivity but the inrroduction of hydroxyl functions on to the phenyl ring at the 3- and/or 4-positions enhanced activity and m- and p-tyramine and dopamine each caused marked hyperactivity in the 0.4–25 g dosage range. Methylation of one hydroxyl function reduced activity (3-methoxy-4-hydroxy- and 3-hydroxy-4-methoxy-phenylethylamine); 2(3,4-methylenedioxyphenyl) ethylamine was inactive. Agents with substitution of the side chain, such as noradrenaline, d-amphetamine and -methyldopamine, were all shown to induced marked hyperactivity at doses of 1.6–25 g. Alterations in the chain length markedly reduced activity (4-(3,4-dihydroxyphenyl) butylamine, 3,4-dihydroxybenzylamine). A variety of N-substituted compounds were shown to be potent inducers of hyperactivity from the nucleus accumbens (adrenaline, epinine, N-ethyldopamine, N-isopropyldopamine, isoprenaline) (0.2–25 g). However, N-methyl-N-isopropyldopamine showed only weak activity and N,N-dimethyldopamine was inactive. All hyperactivity effects were shown to be dose-dependent. The hyperactivities induced by dopamine, noradrenaline and isoprenaline were each inhibited in a dose-dependent manner by subsequent injections of fluphenazine (1.25–25 g) into the nucleus accumbens, although no reductions were recorded following similar injections of saline, solvent, 2% procaine, 50 g propranolol or 50 g piperoxan.     

The role of α1- and α2-adrenoceptors in the coronary vasoconstrictor responses to neuronally released and exogenous noradrenaline in the dog

Summary 1. Coronary vasoconstriction was examined in response to the neuronal release of noradrenaline produced by bilateral carotid occlusion and the infusion of tyramine (5 – 50 Erg/kg/min i. v.) in anaesthetized dogs which had been vagotomized and treated with the -adrenoceptor antagonist propranolol (1.0 mg/kg i. v.). These responses were compared to those produced by the infusion of noradrenaline (0.1 – 0.5 g/kg/min i. v.). 2. Similar increases in late diastolic coronary resistance were produced by bilateral carotid occlusion (0.70 ± 0.25 mmHg min/ml), and intravenous infusions of tyramine, 20 g/kg/min (0.70 ± 0.12 mm Hg min/ml) and noradrenaline, 0.5 gg/kg/min (0.59 ± 0.11 mm Hg min/ml). 3. Selective antagonism at ₁-adrenoceptors with prazosin (0.5 mg/kg i. v.) attenuated the coronary constrictor response to bilateral carotid occlusion (0.36 ± 0.09 mm Hg min/ml), tyramine (0.12 ± 0.06 mm Hg min/ml) and noradrenaline (0.18 ± 0.07 mm Hg min/ml). Antagonism at ₂-adrenoceptors with idazoxan (1 mg/kg i. v.) attenuated the coronary vasoconstriction produced by bilateral carotid occlusion (0.30 ± 0.06 mmHg min/ml), tyramine (0.17 ± 0.08 mmHg min/ml) and noradrenaline (0.12 ± 0.03 mm Hg min/ml). Combined antagonism at both ₁- and ₂-adrenoceptors with prazosin and idazoxan abolished the responses to bilateral carotid occlusion, tyramine and noradrenaline. 4. These results show that coronary vasoconstriction produced by either neuronally released or exogenous noradrenaline is mediated by both ₁ and ₁-adrenoceptors. It appears that in the coronary resistance vessels of the dog postjunctional ₁- and ₂-adrenoceptors are both innervated by sympathetic nerves.     

Opposite modulation of cotransmitter release in guinea-pig vas deferens: increase of noradrenaline and decrease of ATP release by activation of prejunctional β-adrenoceptors

Effects of isoprenaline on contraction, release of noradrenaline and release of ATP elicited by electrical field stimulation (210 pulses, 7 Hz) as well as on contractions elicited by exogenous noradrenaline and ATP were studied in the isolated vas deferens of the guinea pig. Release of noradrenaline was assessed as overflow of total tritium after preincubation with [³H]-noradrenaline. ATP was measured by means of the luciferin-luciferase technique.In [³H]-noradrenaline-pretreated tissues, electrical stimulation elicited an overflow of tritium and ATP and a biphasic contraction. Isoprenaline (1–100 nM) reduced the contraction, mainly phase I, and enhanced the evoked overflow of tritium; evoked overflow of ATP was not changed significantly. No, or almost no, contraction remained in [³H]-noradrenaline-pre-treated tissues exposed to both prazosin (0.3 M) and suramin (300 M), and the evoked overflow of ATP was reduced by about 82%. Under these conditions, isoprenaline (1–100 nM) again enhanced the evoked overflow of tritium, but it now decreased the evoked overflow of ATP. Propranolol (1 M), when added on top of prazosin and suramin, prevented the effects of isoprenaline (1–100 nM). In some tissues not pretreated with [³H]-noradrenaline, purinergic and adrenergic components of the neurogenic contraction (again to 210 pulses, 7 Hz) were isolated by exposure to prazosin (0.3 M) and suramin (300 M), respectively. Isoprenaline (1–100 nM) decreased the isolated purinergic component but did not change significantly the isolated adrenergic component. Contractions elicited by ATP (1000 M) were not changed and contractions elicited by noradrenaline (100 M) were slightly increased by isoprenaline (1–100 nM). Isoprenaline (100 nM) did not change the degradation of ATP (100 M) by pieces of the vas deferens.It is concluded that, in the guinea-pig vas deferens, activation of prejunctional -adrenoceptors modulates the neural release of noradrenaline and ATP in opposite directions: release of noradrenaline is enhanced, whereas release of ATP is decreased.     

Effects of blood pressure changes on the catecholamine release in the locus coeruleus of cats anaesthetized with pentobarbital or chloralose

Summary Effects of carotid occlusion and drugs applied intravenously on the release of endogenous catecholamines in the locus coeruleus of cats anaesthetized with pentobarbital or chloralose were investigated. The locus coeruleus was superfused bilaterally with artificial cerebrospinal fluid through push-pull cannulae inserted stereotaxically. Dopamine, noradrenaline and in some experiments also adrenaline were determined radioenzymatically in the superfusate.Under pentobarbital anaesthesia, a bilateral carotid occlusion increased the release rate of noradrenaline in the locus coeruleus, while the release of dopamine was decreased. These changes were due to the fall of blood pressure in the carotid sinus caused by the occlusion. Loading of baroreceptors by elevating blood pressure with phenylephrine (10 g·kg^–1·min^–1, i.v. infusion) was accompanied by a decreased release of noradrenaline in the locus coeruleus. This decrease in noradrenaline release was not detected in the caudal aspect of the locus coeruleus. Under chloralose anaesthesia, phenylephrine diminished the release rate of noradrenaline to about the same extent as under pentobarbital anaesthesia. The release rate of adrenaline was also decreased. A prolonged infusion of phenylephrine led to a prolonged pressor response associated with a sustained decrease in the noradrenaline release rate. Intravenous injection of chlorisondamine (3 mg·kg^–1) did not change the release of noradrenaline, while dopamine release was reduced.It is concluded that the release of catecholamines in the locus coeruleus is influenced by signals originating from peripheral baroreceptors. The influences are similar under pentobarbital and chloralose anaesthesia. Noradrenergic neurons responding to haemodynamic signals are not uniformly distributed within the locus coeruleus. It is suggested that noradrenergic and possibly dopaminergic and adrenergic neurons of the locus coeruleus are involved in the baroreceptor reflex, thus contributing to central homeostasis of blood pressure.     

Differential distribution in,and release from,sympathetic nerve endings of endogenous noradrenaline and recently incorporated catecholamines

Summary Guinea-pig vasa deferentia or hypogastric nerve-vas deferens preparations, preincubated with pargyline (to irreversibly inhibit monoamine oxidase), were exposed to 2.3 mol/l of unlabelled adrenaline or of ³H-7-(–)-noradrenaline in the presence of hydrocortisone (to inhibit extraneuronal uptake). The vasa deferentia were then mounted in perifusion chambers and subjected to transmural electrical stimulation, electrical stimulation of the nerve, depolarization by potassium (50 mmol/l), or addition of tyramine (40 mol/l). The evoked overflow of tritium and of unlabelled catecholamines was expressed as a fraction of their tissue content. For all stimuli, the fractional release of the exogenous amines was higher than that of endogenous noradrenaline. Thus, recently incorporated amines are preferentially mobilized irrespective of the particular type of releasing mechanism or the chemical nature of the amine.In vasa deferentia which had been loaded with increasing amounts of adrenaline (by incubating the tissues with adrenaline at concentrations ranging from 0.6 to 160 mol/1), the fractional release of adrenaline decreased and became closer to that of endogenous noradrenaline. Hence, the access of exogenous catecholamines to the deepest storage sites requires higher concentrations of amines than those needed to reach the more easily releasable pools.Light microscope autoradiographs obtained from slices of vasa deferentia previously loaded with 2.3 mol/l ³H-(–)-noradrenaline showed that the outer layers were strongly labelled with silver grains whereas the inner layers were poorly marked. It is concluded that recently incorporated amines are preferentially stored in varicosities close to the surface of the tissue and, in comparison with endogenous noradrenaline, are preferentially released from sympathetically innervated organs.Send offprint requests to D. Moura at the above address     

Protection of presynaptic α-adrenoceptors against irreversible blockade by phenoxybenzamine: preservation of the modulatory effects of exogenous noradrenaline and yohimbine

Summary 1. Receptor protection experiments were carried out in order to study the site of action of -adrenoceptor agonists and antagonists on the release of noradrenaline. Cerebrocortical slices from rabbits were preincubated with ³H-noradrenaline. They were then superfused with medium containing cocaine 30 mol/l and stimulated electrically (3 Hz) three times, after 60, 250 and 295 min of superfusion (S₁, S₂, S₃). Phenoxybenzamine 10 mol/1 when used, was added between S₁ and S₂ for 30 min; putative protecting drugs (clonidine 100 mol/1 or yohimbine 10 mol/1) were present 5 min before and during the exposure to phenoxybenzamine and then washed out together with the latter. Either the voltage drop between the electrodes at S₂ and S₃ or the Ca²⁺-concentration of the superfusion medium at S₂ and S₃ was diminished, if necessary, in order to bring the overflow evoked by S₂ close to the overflow at S₁. Blockade by phenoxybenzamine, or protection against the blockade, was examined by addition of the test compounds noradrenaline 0.1 mol/1 or yohimbine 1 mol/1 before S₃. 2. In slices not exposed previously to -adrenoceptor ligands, noradrenaline 0.1mol/1 greatly reduced, whereas yohimbine 1 mol/1 greatly increased the evoked overflow of tritium. Both effects were abolished in slices treated with phenoxybenzamine 10 pmol/1 alone between S₁ and S₂. 3. In contrast to phenoxybenzamine alone, exposure to phenoxybenzamine 10 mol/1 in the presence of either clonidine 100 pmol/1 or yohimbine 10 mol/1 failed to abolish the effects of the test compounds noradrenaline 0.1 mol/1 and yohimbine 1 mol/1, although the effects were reduced. 4. It is concluded that the irreversible antagonist phenoxybenzamine, the protecting agents clonidine and yohimbine, the test compounds noradrenaline and yohimbine, and by inference endogenous noradrenaline as well, all act at the same site, namely the presynaptic -autoreceptor. Send offprint requests to K. Starke at the above address     

The kinetic characteristics of the extraneuronal O-methylating system of the dog saphenous vein and the supersensitivity to catecholamines caused by its inhibition

Summary The half-saturating outside concentration and the V _max of the extraneuronal O-methylating system of the dog saphenous vein were determined in vitro for 3 catecholamines: isoprenaline, adrenaline and noradrenaline.Strips pretreated with 1 mmol/l pargyline were exposed to 30 mol/l cocaine for 30 min before and during the 30 min incubation with the amines.Two methods were used to reach our aims: a) the classical one in which the ³H-O-methylated metabolites formed from a mixture of unlabelled and labelled amine were determined by using final concentrations of the substrate ranging from 0.2 and 12.8 mol/l and b) an indirect one in which 0.2 mol/l ³H-(±)-isoprenaline was used to assess the extraneuronal O-methylation of the tracer amine, and then those concentrations of unlabelled amines were determined which reduce the O-methylation of ³H-(±)-isoprenaline by 50% (IC₅₀).The half-saturating outside concentrations and the V _max obtained by the first method were: 1.3, 2.5 and 3.4 mol/l and 241, 317 and 294 pmol/g/min for ³H-(±)-isoprenaline, ³H-(±)-adrenaline and ³H-(-)-noradrenaline, respectively. The IC₅₀s obtained by the second method used were: 1.1, 0.6, 0.7 and 1.4 mol/l for (±)-isoprenaline, (-)-isoprenaline, (-)-adrenaline and (-)-noradrenaline, respectively.It was observed that the contraction of the strips caused by adrenaline and noradrenaline distorted IC₅₀ values. In the presence of 1 mol/l phentolamine the IC₅₀ for adrenaline and noradrenaline was about 2.5 times higher than in its absence.The relationship between the ratio of half-saturating outside concentration/ED₅₀ for -adrenoceptor-mediated responses for the three amines and the supersensitivity to them caused by inhibition of COMT was also assessed. It was observed that the ratio was 37, 8 and 1.3 for isoprenaline, adrenaline and noradrenaline, respectively and the supersensitivity caused by inhibition of COMT was 11.8-, 8.9- and 1.8-fold, respectively.