A comparison of the effects of 6-hydroxydopamine immunosympathectomy and reserpine on the cardiovascular reactivity in the rat

In the conscious rat, 6-hydroxydopamine, or reserpine (5 mg/kg) pretreatment produced a marked fall in the mean systolic blood pressure whilst immunosympathectomized rats had resting blood pressures just below that of control animals. In pithed preparations, 6-hydroxydopamine treatment or immunosympathectomy potentiated the pressor responses to injected noradrenaline; reserpine pretreatment did not potentiate the noradrenaline response to the same degree. Tyramine responses were abolished after 6-hydroxydopamine or reserpine pretreatment but were unaffected by immunosympathectomy. Stimulation of the sympathetic outflow by the Gillespie & Muir (1967) preparation was abolished after 6-hydroxydopamine and reserpine pretreatment, and reduced after immunosympathectomy. It is concluded that 6-hydroxydopamine produces a destruction of the sympathetic nerve endings, abolishing the physiological uptake process and, therefore, producing supersensitivity to injected noradrenaline. Immunosympathectomy, although showing a marked reduction in sympathetic nerve supply leaves a functional uptake process. Reserpine (5 mg/kg), given 6 h previously, depletes endogenous catecholamines without significantly altering the sensitivity to injected noradrenaline, the uptake process remaining functional.     

Presynaptic adrenergic supersensitivity induced by crude Latrodectus mactans venom

Pretreatment of isolated spleen strips of the cat with a supramaximal concentration of crude Latrodectus mactans venom (VLm) induced a supersensitivity to noradrenaline of similar magnitude to that produced by cocaine or surgical denervation. This effect persisted for 4 hr after washout of the venom. Reserpine pretreatment failed to modify this phenomenon. Cocaine significantly potentiated the response to noradrenaline in control reserpinized strips but failed to do so in spleen strips previously treated with the venom. Dose-response curves to methoxamine were not altered by VLm pretreatment. It is concluded that VLm-induced supersensitivity is the result of an irreversible impairment of the presynaptic neuronal uptake of amines. This effect appears to be due to an acute in vitro degeneration of adrenergic nerve terminals.     

Calcium influx and postjunctional supersensitivity in guinea pig aortic strips.

Both reserpine and 6-hydroxydopamine (6-OHDA) pretreatment potentiated the sensitivity of guinea pig aortic strips to norepinephrine (NE), barium, methoxamine and potassium indicating postjunctional supersensitivity. However, cocaine treatmetn only potentiated the NE response indicating prejunctional supersensitivity. 6-OHDA and reserpine-induced supersensitivity but not cocaine-induced supersensitivity was accompanied by an increase in 45Ca influx.     

On the cardiovascular action of dopamine in rats

Summary In rats anaesthetized with urethane the pressor effect of dopamine was not significantly altered by treatment with desipramine, which enhanced the action of noradrenaline on blood pressure and heart rate and abolished that of tyramine. Chemical sympathectomy with 6-hydroxydopamine in rats potentiated responses to noradrenaline and prevented the action of tyramine on blood pressure but did not significantly modify the pressor effects of dopamine. Furthermore, pretreatment with reserpine shifted the dose-response curves for dopamine and for tyramine on heart rate to the right. The dose-response curve for dopamine like that for noradrenaline on blood pressure was unaltered or shifted slightly to the left by reserpine. It is concluded that also in the rat an indirect, tyramine-like component contributes substantially to the cardiovascular action of dopamine.     

Effects of reserpine and 6-hydroxydopamine on the adrenergic and purinergic components of sympathetic nerve responses of the rabbit saphenous artery.

1 The effects of reserpine and of 6-hydroxydopamine on the contractions of the rabbit isolated saphenous artery produced by stimulation of the sympathetic nerves were studied. 2 In vessels exposed to reserpine, substantial contractions to nerve stimulation were recorded despite a 95.7% reduction in the noradrenaline content of the tissue. These responses of the vessel were not significantly affected by the alpha 1-antagonist, prazosin, whereas after desensitization of the P2-purinoceptor with alpha, beta-methylene ATP, no response to nerve stimulation remained. 3 In vessels exposed to 6-hydroxydopamine, no nerve-mediated responses were observed. 4 Noradrenaline-containing nerves were observed by fluorescence histochemistry in control tissues, but were not observed in tissues treated with reserpine or 6-hydroxydopamine. 5 The potencies of ATP and histamine were not significantly affected by reserpine or 6-hydroxydopamine treatment. However, there was a slight supersensitivity to noradrenaline in reserpine-treated and 6-hydroxydopamine-treated vessels compared with that of control vessels. Prazosin was selective for alpha-adrenoceptors, while alpha, beta-methylene ATP was selective for P2-purinoceptors. 6 These results substantiate the finding that ATP and noradrenaline are sympathetic cotransmitters in the rabbit isolated saphenous artery, and demonstrate that ATP can act as a transmitter independently of noradrenaline in this vessel.     

Effects of 6-hydroxydopamine treatment at birth on the submaxillary gland of the rat

Summary Six and 18 months after neonatal administration of 6-hydroxydopamine or surgical sympathetic denervation the submaxillary gland of the rat showed a marked depletion of noradrenaline stores. Six months after removal of the superior cervical ganglion the gland's endogenous noradrenaline was lowered to 0.032±0.004 g/g while after neonatal 6-hydroxydopamine the values were 0.228±0.023 g/g (controls 2.145±0.382 g/g). Eightheen months after either type of sympathetic denervation the neurotransmitter was still depleted. In rats treated with 6-hydroxydopamine the sialagogue effect of injected noradrenaline was potentiated 2.7-fold while the potentiation of the effect of noradrenaline was 3.6 times after surgical denervation. The magnitude of the supersensitivity developed to isoprenaline did not differ between both types of denervation. No supersensitivity to the cholinomimetic agent, methacholine, was observed. Cocaine administration or removal of the superior cervical ganglion slightly increased the supersensitivity to noradrenaline in rats treated with 6-hydroxydopamine. Eighteen months after surgical or chemical denervation, the activity of choline-acetyltransferase in the submaxillary gland was increased by about 50%. Of the respiratory enzymes studied, succinic dehydrogenase, fumarase and cytochrome oxidase, the activity of only the latter was markedly reduced by chronic sympathetic denervation. From the results obtained it is concluded that neonatal treatment with 6-hydroxydopamine causes a permanent and almost complete sympathectomy of the submaxillary gland of the rat.Supported partially by grant N⁰ 3211/73, Consejo Nacional de Investigaciones Cientificas y Técnicas.     

The consequences of loss followed by recovery of noradrenergic nerve function on muscarinic receptors in the chick expansor secundariorum muscle.

The effects of chemical sympathectomy with 6-hydroxydopamine on the response of the expansor secundariorum muscle to noradrenergic nerve stimulation, noradrenaline and acetylcholine have been investigated. Expansor muscles from 60 day old chicks were sensitive to noradrenergic nerve stimulation and exogenous noradrenaline but virtually unresponsive to acetylcholine. Chemical sympathectomy with 6-hydroxydopamine caused loss of function of noradrenergic nerves of the expansor muscle, induced supersensitivity to exogenous noradrenaline and gradually increased the response of the expansor muscle to acetylcholine. As the patency of noradrenergic nerves reappeared there was a decline in the extent of supersensitivity to noradrenaline and the response to acetylcholine gradually declined. The time courses of these changes differed, indicating that the mechanisms responsible for changes in response to noradrenaline and acetycholine are different.     

The effects of some antirheumatic drugs on an in vitro model of human polymorphonuclear leucocyte chemokinesis.

1 Pretreatment of dogs for 20 to 24 h before the start of experiments with reserpine (0.5 mg/kg) depleted noradrenaline from cerebral and mesenteric arteries, the diminution being greater in the latter arteries. 2 Contractile responses of helically-cut strips of cerebral and mesenteric arteries to noradrenaline were unaffected by pretreatment with reserpine. Tyramine-induced contractions of mesenteric arteries were markedly attenuated by reserpine-pretreatment, whereas the contraction of cerebral arteries was not influenced. The contractile response of mesenteric arteries to transmural nerve stimulation or nicotine was abolished by reserpine-pretreatment, but the relaxation induced by nicotine of cerebral arteries contracted with prostaglandin F2 alpha was not affected. Pretreatment with reserpine attenuated the contractions of mesenteric arteries induced by angiotensin II, but did not alter the response of cerebral arteries to 5-hydroxytryptamine. 3 In prostaglandin-contracted cerebral and mesenteric arterial strips, relaxant effects of acetylcholine, isoprenaline and K+ were not significantly influenced by reserpine-pretreatment. 4 It appears that tyramine and nicotine do not release noradrenaline from dog cerebral arteries in amounts sufficient to cause significant contractions. Attenuation of the response to angiotensin II by pretreatment with reserpine is not the result of depletion of noradrenaline from the mesenteric arterial wall but may be due to interference with the mechanism specific to actions of angiotensin II.     

The effects of nialamide on adrenergic functions

Nialamide potentiated the pressor effects of noradrenaline in the pithed cat. In cats treated with reserpine and then pithed, it prevented the restoration of the pressor effects of tyramine by slow intravenous infusions of noradrenaline. Nialamide produced a gradual decline in the pressor effects of repeated injections of tyramine whilst the effects of tyramine on the nictitating membrane were potentiated. The development of tachyphylaxis to tyramine in the isolated guinea-pig heart was associated with a 40% reduction in the myocardial concentration of noradrenaline. The onset of tachyphylaxis to tyramine was more rapid when nialamide was either included in the perfusion fluid or administered in vivo. Prior treatment with nialamide increased threefold the myocardial concentration of noradrenaline; however, the development of tachyphylaxis to tyramine was associated with a proportionate fall in the myocardial concentration of noradrenaline. In five out of nine experiments the acute administration of nialamide increased the output of noradrenaline per stimulus from the isolated cross-perfused spleen of the cat when the stimulus frequency was 30 shocks/sec, but not when the frequency was 10 shocks/sec. When nialamide (20 mg/kg) was given subcutaneously to cats 20 hr before their spleens were isolated and perfused, there was a rapid fall-off in the contractions of the spleen in response to periods of nerve stimulation. The outputs of noradrenaline per stimulus were decreased at both frequencies of stimulation. Nialamide decreased the concentrations of noradrenaline in the myocardium and spleen of the cat. The hypothesis is proposed that nialamide diminishes the availability of transmitter, as a consequence of a decreased re-entry of noradrenaline into a storage site present in nerve endings, and that such a decrease in the availability of noradrenaline in hyperactive nerve pathways may account for the antihypertensive effects of monoamine oxidase inhibitors in man.     

Restoration of tyramine responses by bretylium, BW392C60, bethanidine and monoamine oxidase inhibitors in reserpine-treated rats

1. Bretylium, BW392C60, bethanidine, nialamide and pheniprazine, but not guanethidine or ouabain, were all capable of restoring the cardiovascular response to tyramine in reserpine pretreated rats anaesthetized with sodium pentobarbitone.2. In parallel with their recorded in vitro activity as monoamine oxidase inhibitors, BW392C60 was found to be more potent at restoring the response to tyramine than bretylium or bethanidine.3. The restored responses to tyramine were completely blocked by desmethyl-imipramine or by a combination of phentolamine and propranolol.4. The effect of bretylium on the tyramine response was not influenced by bilateral adrenal demedullation, urethane anaesthesia, the dose or duration of the reserpine pretreatment and was not dependent upon the frequency of the tyramine injections.5. Bretylium, BW392C60 or bethanidine did not alter the pressor response to intravenous noradrenaline.6. Nialamide-induced restorations of the responses to tyramine were not further enhanced by the administration of bretylium, BW392C60 or bethanidine.7. In pithed reserpine-treated rats the ability of bretylium and BW392C60 to restore the response to tyramine was reduced.8. It is concluded that all the drugs which reversed the reserpine-induced subsensitivity to tyramine were acting as monoamine oxidase inhibitors, thus allowing the intra-neuronal accumulation of endogenously formed catecholamines. The presence of nerve impulses in the adrenergic fibres of reserpinized rats appears to be an important factor in mediating this effect.     

The relationship between calcium and increased sensitivity of rabbit aortae four hours after reserpine.

1 Four hours after reserpine, rabbit aortic strips were supersensitive to acetylcholine, isoprenaline and noradrenaline. The threshold concentration of the drugs necessary to induce a response was less and the maximum tension developed by the tissues was greater than in control strips. 2 Reserpine-treatment potentiated the contractile responses to CaCl2. 3 Reserpine-treatment resulted in an increase in calcium uptake and an increase in the slow component of 45Ca2+ efflux. 4 After resperine-treatment, the rate of relaxation from a potassium-induced contraction was decreased. 5 It is concluded that reserpine-induced supersensitivity is related to an enhanced ability of the tissue to retain and utilize calcium.     

Bretylium potentiation of the contractor responses of isolated rabbit aortic strips to potassium and tyramine

1. Pretreatment of rabbit aortic strips with bretylium potentiated the contractor response to potassium and tyramine but not to noradrenaline. On the other hand, such pretreatment inhibited the response to nicotine.2. Even in reserpinized or cold stored aortic strips, pretreatment with bretylium enhanced the contractor response to potassium and tyramine.3. Pretreatment of fresh, reserpinized, or cold stored aortic strips with pheniprazine potentiated the contractor response to potassium and tyramine.4. Pretreatment of aortic strips with bretylium or pheniprazine did not potentiate the response to 5-hydroxytryptamine (5-HT).5. The results indicate that both bretylium and pheniprazine potentiate the action of tyramine and potassium, not by presynaptic mechanisms, but by postsynaptic action, causing an increase in the sensitivity of the effector cells to the stimulants.     

Effect of neonatal 6-hydroxydopamine treatment on the blood pressure response to noradrenaline and tyramine in rats

Neonatal treatment of rats with 6-hydroxydopamine (6-OHDA) caused a permanent depletion of noradrenaline (NA) in the heart and the spleen, but not in the adrenals. In 10–12 week old animals, anesthetized with pentobarbital, sensitivity of the pressor response to various doses of i.v. administered NA increased 5-fold whereas the pressor response to i.v. administered tyramine was greatly diminished; the response to tyramine was further reduced after adrenalectomy, but not in controls. These results suggest that the pressor responses evoked by tyramine and NA can be used as a test for functional sympathetic denervation after 6-OHDA treatment.     

An action of 5-hydroxytryptamine on adrenaline receptors

Contractions of isolated strips of cat spleen due to 5-hydroxytryptamine, adrenaline, histamine and acetylcholine were antagonized by phenoxybenzamine. Responses to both 5-hydroxytryptamine and adrenaline were not blocked in strips which were protected by a high concentration of either 5-hydroxytryptamine or adrenaline throughout exposure to phenoxybenzamine. The contraction due to a large dose of 5-hydroxytryptamine lasted less than 1 hr even when the drug was still present. Strips thus desensitized to 5-hydroxytryptamine responded normally to acetylcholine and histamine but did not respond to adrenaline. The actions of 5-hydroxytryptamine and adrenaline were blocked by 2-bromolysergic acid diethylamide or by dihydroergotamine. These results indicated that 5-hydroxytryptamine and adrenaline act on the same receptors. Cocaine potentiated the action of adrenaline but inhibited the action of 5-hydroxytryptamine. The sensitivity to 5-hydroxytryptamine of spleen strips from cats treated 24 hr earlier with reserpine was only one-fiftieth of that of normal strips. Cocaine potentiated the action of 5-hydroxytryptamine on strips from reserpine-treated cats. A high concentration of 5-hydroxytryptamine in spleen strips from reserpine-treated cats and in cocaine-treated strips prevented phenoxybenzamine from blocking the actions of adrenaline. The effects of tyramine on spleen strips almost exactly paralleled the effects of 5-hydroxytryptamine. Strips showing tachyphylaxis to tyramine did not respond to 5-hydroxytryptamine. It is concluded that 5-hydroxytryptamine has a dual action, viz., a major action due to release of stored noradrenaline and a minor direct action of adrenaline receptors.     

Protection against induction of supersensitivity to catecholamines by cocaine

1. Adrenaline, noradrenaline, isoprenaline, tyramine, phentolamine, pronethalol, histamine and acetylcholine were each tested for their ability to prevent cocaine from causing supersensitivity to catecholamines in cat spleen strips in vitro. A high concentration of one of these drugs was added to the bath 5 min before cocaine hydrochloride (10 mug/ml). The effect on subsequent responses to catecholamines was compared with the effect of cocaine in control strips in the absence of an interfering drug.2. Phentolamine completely abolished the potentiating effect of cocaine. Large doses of adrenaline or noradrenaline reduced, but did not completely prevent, potentiation. Tyramine, isoprenaline, pronethalol, histamine and acetylcholine did not prevent potentiation.3. The ability of these drugs to interfere with potentiation does not correlate well with their ability to interfere with uptake of noradrenaline. Interference with uptake by cocaine is therefore unlikely to account fully for potentiation.     

Potentiation by cocaine and 3,3-di(p-aminophenyl)-propylamine (TK 174) of the effect of isoprenaline and noradrenaline on isolated strips of cat spleen

Contractions of cat isolated spleen strips by noradrenaline and isoprenaline are brought about by interaction with α-adrenergic receptors. The effect of isoprenaline cannot be diminished by in vivo reserpine pretreatment. The effects of both noradrenaline and isoprenaline are potentiated by cocaine and by a new diphenyl-propylamine derivative TK 174 [3,3-di(p-aminophenyl)propylamine]. Noradrenaline is potentiated more by TK 174 than by cocaine while the reverse is seen with isoprenaline. These results suggest that beside the inhibition of noradrenaline uptake another factor may be involved in the mechanism of action of cocaine, and to a lesser extent of TK 174.     

Steroid potentiation of responses to sympathomimetic amines in aortic strips

1. Responses to catecholamines (adrenaline, noradrenaline, nordefrine) were enhanced by 17beta-oestradiol, progesterone and desoxycorticosterone in untreated and reserpine pretreated aortic strips. Responses to tyramine, believed mediated via endogenous catecholamines, were enhanced only in untreated strips.2. Responses to sympathomimetic amines lacking the catechol nucleus (phenylephrine, synephrine, methoxamine) were potentiated inconsistently by the steroids and reserpine pretreatment reduced markedly the frequency of potentiated responses.3. Known inhibitors of catechol-O-methyl transferase (tropolone, U-0521, pyrogallol) potentiated responses to catecholamines and abolished the enhancing effects of the steroids-when the steroids were given first, there was no further increase in response to catecholamines on adding inhibitors of catechol-O-methyl transferase.4. Experiments with the oil-immersion technique, to eliminate diffusion of drug from the tissue, indicated that 17beta-oestradiol, progesterone and desoxycorticosterone decreased the rate at which aortic strips inactivated adrenaline by O-methylation.5. It is concluded that 17beta-oestradiol, progesterone and desoxycorticosterone potentiate responses to catecholamines in aortic strips by inhibiting a major mechanism for their inactivation.     

Effect of tyramine on isolated guinea-pig atria in relation to their noradrenaline stores

The relation between the noradrenaline content of isolated guinea-pig atria and the rate-increasing action of tyramine was studied by the use of pretreatment with reserpine as a pharmacological tool for graded depletion of the noradrenaline stores. Reserpine was more potent in depleting the stores than in reducing the biological response to tyramine; 50% depletion had little effect on the response to tyramine; 50% reduction of the response to tyramine occurred when the noradrenaline content fell to about 10% of normal. Depletion of the stores of guinea-pig atria did not result in supersensitivity to noradrenaline. Exposure of heavily pretreated atria to 3 × 10^-6 noradrenaline for 10 min (followed by repeated washing for 45 min) restored the response to tyramine to 70% of normal; it failed, however, to restore the noradrenaline content to the level expected from the experiments with reserpine alone. Restoration of the response to tyramine was accompanied by a small but significant increase in the noradrenaline content of the atria; a change in sensitivity to added noradrenaline did not occur. The results are consistent with the view that (a) the noradrenaline stores consist of two compartments the smaller of which is important for the action of tyramine, that (b) this smaller compartment can be at least partially refilled by exposure of the atria to noradrenaline, and that (c) there is no direct relationship between the noradrenaline content and the sensitivity to noradrenaline in guinea-pig atria.     

Effects of reserpine on the disposition of sympathomimetic amines in vascular tissue

1. The effects of reserpine pretreatment on the intrinsic inactivation of low concentrations of phenylephrine and noradrenaline in strips of rabbit thoracic aorta were assessed by measuring the rates of relaxation, after oil immersion to prevent loss of active amine by diffusion into the surrounding medium.2. Reserpine pretreatment considerably augmented the amplitude of responses to low concentrations of phenylephrine, noradrenaline and nordefrine (Cobefrine).3. Reserpine pretreatment did not reduce the overall rate of inactivation of either phenylephrine or noradrenaline, but it did appear to decrease the contribution of uptake and storage, measured as an increased effect of enzyme inhibition and a decreased effect of cocaine on the rate of inactivation.4. The role of catechol-O-methyl transferase (COMT), but not that of monoamine oxidase (MAO), in terminating the action of noradrenaline was increased in strips from animals pretreated with reserpine. Thus it appears that interference with intraneuronal storage diverts active amine to inactivation by COMT in vascular tissue, rather than by MAO as has been previously suggested.5. As in preparations not treated with reserpine, inhibition of MAO alone had little effect on the rate of inactivation of noradrenaline, and this enzyme appears to function predominantly as an alternate pathway of little importance as long as COMT activity is unimpaired. Enzymatic processes accounted for about 85 and 70% of the inactivation of a low concentration of noradrenaline in reserpine pretreated and untreated preparations, respectively.6. Cocaine potentiated responses to noradrenaline and phenylephrine as effectively in reserpine pretreated as in untreated preparations, and inhibition of the pathways of enzymatic inactivation did not appreciably decrease the potentiation produced by this agent.7. The present results cannot be explained by the hypothesis that interference with amine inactivation by nerve uptake and storage is responsible for the potentiation of responses to noradrenaline or phenylephrine by either reserpine or cocaine, and emphasize the unrealiability of potentiation as an index of interference with mechanisms involved in terminating the action of sympathomimetic amines.     

Secretory effects of 6-hydroxydopamine in normal and denervated submaxillary glands of the rat

Summary Injection of 6-hydroxydopamine (6-OH-DA) elicited a marked and sustained secretory response of control and surgically sympathectomized submaxillary glands of rats. These responses were diminished by previous treatment with reserpine 0.1 mg/kg 48 and 24h before the experiment and almost abolished by 5 mg/kg reserpine 6 h before the administration of 6-OH-DA. The responses to 6-OH-DA were potentiated in control glands by previous preganglionic denervation of either the parasympathetic or sympathetic nerves of the gland. Development of postjunctional supersensitivity in denervated glands also increased the responses to 6-OH-DA, while atropine had a feeble blocking action. For all these responses, the adrenal catecholamines played no role. After two consecutive doses of 6-OH-DA a third dose of the drug still elicited a secretory response that was 50% of that of the first dose.It is concluded that for the responses to 6-OH-DA the leakage of noradrenaline from the degenerating adrenergic nerve endings of the submaxillary gland plays a partial role. Noradrenaline released by the drug from other tissues and reaching the gland via the circulation also contributes to the responses observed. A muscarinic component may also participate in the secretory effects of 6-OH-DA.Members of Carrera del Investigador Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos AiresPartially supported by Grant no. 3211/74, Consejo Nacional de Investigaciones Científicas y Técnicas, Buenos Aires