Norepinephrine depletion as a possible mechanism of action of guanethidine (SU 5864), a new hypotensive agent

The Influence of an Adrenergic Antagonist Guanethidine on the Distribution Pattern and Chemical Coding of Caudal Mesenteric Ganglion Perikarya and Their Axons Supplying the Porcine Bladder

This study was aimed at disclosing the influence of intravesically instilled guanethidine (GUA) on the distribution, relative frequency and chemical coding of both the urinary bladder intramural sympathetic nerve fibers and their parent cell bodies in the caudal mesenteric ganglion (CaMG) in juvenile female pigs. GUA instillation led to a profound decrease in the number of perivascular nerve terminals. Furthermore, the chemical profile of the perivascular innervation within the treated bladder also distinctly changed, as most of axons became somatostatin-immunoreactive (SOM-IR), while in the control animals they were found to be neuropeptide Y (NPY)-positive. Intravesical treatment with GUA led not only to a significant decrease in the number of bladder-projecting tyrosine hydroxylase (TH) CaMG somata (94.3 ± 1.8% vs. 73.3 ± 1.4%; control vs. GUA-treated pigs), but simultaneously resulted in the rearrangement of their co-transmitters repertoire, causing a distinct decrease in the number of TH⁺/NPY⁺ (89.6 ± 0.7% vs. 27.8 ± 0.9%) cell bodies and an increase in the number of SOM-(3.6 ± 0.4% vs. 68.7 ± 1.9%), calbindin-(CB; 2.06 ± 0.2% vs. 9.1 ± 1.2%) or galanin-containing (GAL; 1.6 ± 0.3% vs. 28.2 ± 1.3%) somata. The present study provides evidence that GUA significantly modifies the sympathetic innervation of the porcine urinary bladder wall, and thus may be considered a potential tool for studying the plasticity of this subdivision of the bladder innervation.

No Evidence of Neurogenesis in Adult Rat Sympathetic Ganglia Following Guanethidine-Induced Neuronal Loss

The potential for neurogenesis in the cranial (superior) cervical ganglia (SCG) of the sympathetic nervous system was evaluated. Eleven consecutive daily doses of guanethidine (100 mg/kg/d) were administered intraperitoneally to rats in order to destroy postganglionic sympathetic neurons in SCG. Following the last dose, animals were allowed to recover 1, 3, or 6 months. Right and left SCG from guanethidine-treated and age-matched, vehicle-treated control rats were harvested for histopathologic, morphometric, and stereologic evaluations. Both morphometric and stereologic evaluations confirmed neuron loss following guanethidine treatment. Morphometric analysis revealed a 50% to 60% lower number of tyrosine hydroxylase (TH)-positive neurons per unit area of SCG at both 3 and 6 months of recovery, compared to ganglia of age-matched controls, with no evidence of restoration of neuron density between 3 and 6 months. Reductions in TH-positive neurons following guanethidine treatment were corroborated by unbiased stereology of total hematoxylin and eosin-stained neuron numbers in SCG. Stereologic analyses revealed that total neuron counts were lower by 37% at 3 months of recovery when compared to age-matched vehicle controls, again with no obvious restoration between 3 and 6 months. Thus, no evidence was found that postganglionic neurons of the sympathetic nervous system in the adult rat have a neurogenic capacity.

Antagonism of Guanethidine by Dexamphetamine and Other Related Sympathomimetic Amines

Dexamphetamine and certain other indirectly acting sympathomimetic amines prevent or reverse the sympathetic nerve blocking action of guanethidine in anaesthetised cats and dogs. Noradrenaline and dopamine do not antagonise the blocking action of guanethidine. These observations are discussed in relation to the mode of action of guanethidine and to a possible clinical significance of these findings.

Impaired gastric motor activity after abdominal surgery in rats

Postoperative ileus (POI) is a transient bowel dysmotility that occurs following abdominal surgery. Several mechanisms have been proposed such as neural reflex and inflammatory changes. We focused on gastric motility after abdominal surgery in rats. To investigate the time course of gastric motility after surgery, gastric motility was continuously recorded before, during and after surgery. After laparotomy, terminal ileum was manipulated for 10 min. Gastric motility was recorded by a strain gauge transducer implanted on the serosal surface of the stomach. To investigate whether peripheral sympathetic nerve is involved in the pathogenesis of POI, effects of guanethidine and celiac ganglionectomy were tested on the postoperative gastric motility. Although isoflurane anaesthesia reduced the gastric motility to 40%, the motility recovered immediately when isoflurane was withdrawn. Intestinal manipulation reduced the postoperative gastric motility for 3-24 h after surgery, compared with preoperative levels. Guanethidine administration and celiac ganglionectomy restored the impaired gastric motility. Feeding increased the gastric motility in each group. It is suggested that the pathogenesis of postoperative gastric ileus induced by intestinal manipulation involves viscero-sympathetic pathways. Intestinal manipulation causes impaired gastric motility via inhibitory sympathetic efferent pathway. Feeding may improve the postoperative gastric motility.

Central effect of mu-opioid agonists on antral motility in conscious rats

Centrally applied opioids delay gastric emptying and inhibit intestinal transit. However, the mechanism of inhibitory effects of central opioids on gastric motility still remains unclear. It also remains unclear which opioid receptor (mu, delta, and kappa) stimulation affects gastric motility. We studied the central effect of opioids on antral motility in conscious rats. A strain gauge transducer was implanted on the gastric antrum to record the circular muscle contractions. The area under the curve of the antral motility, calculated as a motility index, was evaluated before and after the intracerebroventricular (icv) injection of various opioid agonists in each rat. [D-Ala2, N-Me-Phe4, Gly5-ol] enkephalin (DAMGO, 0.1-10 nmol), a mu-opioid selective agonist, significantly inhibited antral motility in a dose-dependent manner (n=4). The motility index was significantly decreased to 47.3+/-10.8% (n=4) of controls at 20 min after icv injection of DAMGO (1.0 nmol). In contrast, [D-pen2, L-Pen5] enkephalin (DADLE, 1.0 nmol), a delta-opioid selective agonist, and U50,488 (1.0 nmol), a kappa-opioid selective agonist, had no significant effects on antral motility. Pretreatment with subcutaneous guanethidine (5 mg/kg) and propranolol (1 mg/kg), but not phentolamine (1 mg/kg), significantly antagonized the inhibitory effect of DAMGO (1.0 nmol). Reduced motility index induced by DAMGO (1.0 nmol) was restored from 48.7+/-3.5% to 88.6+/-10.9% (n=5) and 80.4+/-2.2% (n=5) by guanethidine and propranolol, respectively. Our findings suggest that central mu-opioid receptor has major inhibitory effects on antral motility in conscious rats. The inhibitory effects of mu-opioid receptors are mediated via sympathetic pathways and beta-adrenoceptors.

Restraint stress delays solid gastric emptying via a central CRF and peripheral sympathetic neuron in rats

Central corticotropin-releasing factor (CRF) delays gastric emptying through the autonomic nervous system. CRF plays an important role in mediating delayed gastric emptying induced by stress. However, it is not clear whether a sympathetic or parasympathetic pathway is involved in the mechanism of central CRF-induced inhibition of solid gastric emptying. The purpose of this study was to investigate whether 1) CRF inhibits solid gastric emptying via a peripheral sympathetic pathway and 2) stress-induced inhibition of solid gastric emptying is mediated via a central CRF and peripheral sympathetic pathways. Using male Sprague-Dawley rats, CRF was injected intracisternally with or without various adrenergic-blocking agents. To investigate whether central CRF-induced inhibition of solid gastric emptying is mediated via a peripheral sympathetic pathway, rats underwent celiac ganglionectomy 1 wk before the gastric emptying study. After solid meal ingestion (90 min), gastric emptying was calculated. To investigate the role of endogenous CRF in stress-induced delayed gastric emptying, a CRF type2 receptor antagonist, astressin2-B, was intracisternally administered. Rats were subjected to a restraint stress immediately after the feeding. Intracisternal injection of CRF (0.1-1.0 microg) dose-dependently inhibited solid gastric emptying. The inhibitory effect of CRF on solid gastric emptying was significantly blocked by guanethidine, propranolol, and celiac ganglionectomy but not by phentolamine. Restraint stress significantly delayed solid gastric emptying, which was improved by astressin2-B, guanethidine, and celiac ganglionectomy. Our research suggests that restraint stress inhibits solid gastric emptying via a central CRF type2 receptor and peripheral sympathetic neural pathway in rats.

Actions of bretylium and guanethidine on the uptake and release of [3H]-noradrenaline

The effect of bretylium and guanethidine has been studied on the uptake and the spontaneous and reserpine-induced release of [(3)H]-noradrenaline in the rat heart and in the splenic nerve endings of the cat. Bretylium and guanethidine inhibited the uptake by the heart of circulating [(3)H]-noradrenaline. Bretylium blocked spontaneous and reserpine-induced release of [(3)H]-noradrenaline, while guanethidine caused release and partially antagonized the reserpine-induced release. Both compounds produced a transient liberation of [(3)H]-noradrenaline from splenic nerves, but blocked the release of the [(3)H]-catechol amine following stimulation of the splenic nerve.

Mechanism of the initial adrenergic effects of bretylium and guanethidine

Treatment with phenoxybenzamine and dichloroisoprenaline prevented the rise of blood pressure, contraction of the nictitating membrane and increase in cardiac contractile force produced by intravenous injections of bretylium and guanethidine in anaesthetized or spinal cats. Treatment with cocaine, imipramine or reserpine reduced the sensitivity to bretylium and guanethidine of the spinal cat. In the spinal cat treated with reserpine, sensitivity to the drugs could be restored by an infusion of noradrenaline. Chlorpromazine also blocked the pressor and nictitating membrane responses to bretylium and guanethidine. The drug effects were unaltered 1 hr after bilateral adrenalectomy. During the intravenous infusion of noradrenaline into spinal cats the pressor responses to bretylium and guanethidine were increased, whilst those to adrenaline and to noradrenaline were decreased. Guanethidine (3 to 5 mg/kg) injected intravenously into the cat caused a sudden relaxation of the rat isolated stomach-strip bathed in blood. In seven similar experiments bretylium (3 to 5 mg/kg) relaxed the strip only once; in the other six experiments there was either no effect (four experiments) or an increase in the tone of the strip (two experiments). It is concluded that the initial adrenergic effects of bretylium and guanethidine are mediated, at least in part, through a release of catechol amines from stores in the effector organ.

Effect of guanethidine in revealing cholinergic sympathetic fibres

Guanethidine abolished the inhibitory response of segments of rabbit intestine to stimulation of the sympathetic nerves which accompany the mesenteric arteries. In the majority of experiments a motor response of the intestinal segment was then revealed; it was more readily observed in intestinal segments from young than from adult rabbits. The motor response of the intestine to sympathetic stimulation after guanethidine was blocked by atropine. It was not blocked by hexamethonium and was present in rabbits in which the vagal innervation to the small intestine had been sectioned. In the cat isolated atria, guanethidine blocked the accelerator response to sympathetic nerve stimulation and revealed a response resembling that to stimulation of the vagus. It was concluded that guanethidine blocked the release of noradrenaline and thus revealed the response to the direct action of acetylcholine released from cholinergic sympathetic nerves.

Adrenergic neurone blockade and other acute effects caused by N-benzyl-N'N"-dimethylguanidine and its ortho-chloro derivative

N-Benzyl-N'N"-dimethylguanidine sulphate (BW 467C60) and its ortho-chloro derivative (BW 392C60) had adrenergic neurone blocking and sympathomimetic effects resembling those of bretylium and guanethidine in cats, dogs and monkeys, but they were more potent in blocking adrenergic mechanisms in the cat. BW 467C60 was more active than its chloro derivative. Each compound inhibited release of noradrenaline during stimulation of the splenic nerve of cats, and increased smooth muscle responses to adrenaline and noradrenaline. Pressor responses to standard doses of tyramine were also increased except when large doses of BW 467C60 or BW 392C60 were given. The adrenergic neurone block by BW 467C60 was inhibited by dopamine, cocaine and amphetamine in situations in which these amines inhibit the effects of bretylium and guanethidine. In contrast to guanethidine, BW 467C60 and BW 392C60 did not lower the pressor amine content of the iris of cats 24 hr after administration of single doses of the compounds. BW 467C60 depressed the slope of curves relating the frequency of stimuli applied to the cervical sympathetic nerves and the resulting contraction of the nictitating membrane, but the effects of the lower rates of stimulation were preferentially inhibited. Large intravenous doses of BW 467C60 and BW 392C60 blocked autonomic cholinergic mechanisms and caused neuromuscular paralysis of voluntary muscle. These effects were brief, in contrast to the adrenergic neurone blockade. Both BW 467C60 and BW 392C60 were well absorbed from the alimentary tract. In contrast to guanethidine, BW 467C60 did not cause diarrhoea in guinea-pigs.

Mechanism of action of guanethidine

The early hypotensive action of intravenous guanethidine in rabbits, rats and cats anaesthetized with urethane is reversed after pretreatment with iproniazid. The fall in blood pressure following injection of guanethidine in rabbits is reduced after previous administration of reserpine. Reserpine, like adrenalectomy and splenectomy, suppresses the early pressor effect of guanethidine in cats anaesthetized with chloralose. Guanethidine inhibits the action of tyramine and nicotine, but potentiates the effect of noradrenaline on isolated rabbit atria. Guanethidine is also a weak inhibitor of monoamine oxidase activity. The results are discussed and compared with those shown by reserpine. It is concluded that the early effects of guanethidine are mainly due to the release of endogenous catechol amines.

Effect of drugs on the noradrenaline content of brain and peripheral tissues and its significance

Large single doses of methoserpidine (12 mg/kg) given to rabbits lowered the noradrenaline content of sympathetic ganglia but not that of brain; no sedation was observed. Cats responded to doses ranging from 12 to 0.5 mg/kg with loss of noradrenaline from ganglia as well as from brain, and were sedated by the drug. The effect in man resembles that in the rabbit. Only within the group of reserpine-like drugs do sedation and loss in hypothalamic noradrenaline run parallel. These effects are therefore not causally related. Guanethidine lowers the noradrenaline content of sympathetic ganglia (cats and rabbits), but this effect does not explain the blocking action of the drug on the adrenergic nerve. Effects on the noradrenaline of the brain are variable and may be caused reflexly rather than by direct central action of guanethidine. Repeated intravenous injections of dimethylphenylpiperazinium iodide for a period of 4 hr did not produce any significant change in the noradrenaline content of ganglia or brain of rabbits. In contrast, dexamphetamine (20 mg/kg) produced a small but significant mean fall in noradrenaline content of the superior cervical ganglia and in that of the brain, but the effects were not seen in every rabbit. Prolonged administration of the mono-amine oxidase inhibitors pheniprazine and phenylhydrazinobutane raised the noradrenaline content of the brain of rabbits but not that of cats, whereas it raised the noradrenaline of the ganglia of cats but not (or rarely) that of rabbits. The question of correlation between a rise in the noradrenaline content of the brain and certain clinical signs is discussed. Finally, a comparison is made in rabbits between the changes produced by drugs in the noradrenaline content of the heart and of the superior cervical ganglion. The changes run parallel and are only occasionally more pronounced in the heart.

Effect of sympathomimetic amines on the blocking action of guanethidine, bretylium and xylocholine

Experiments were carried out in which the adrenergic neurone blocking activity of xylocholine, bretylium and guanethidine was studied by the use of the inhibitory responses of the isolated rabbit ileum to lumbar sympathetic nerve stimulation, and the contractions of the nictitating membrane of the anaesthetized cat in response to stimulation of the cervical sympathetic nerves. In both these preparations, after blockade of the effects of sympathetic nerve stimulation had been produced with xylocholine, bretylium or guanethicdine, the sympathomimetic amines, dexamphetamine, mephentermine, hydroxyamphetamine, ephedrine and phenethylamine, reversed the blockade; if these amines were given first, then the adrenergic neurone blocking agents were ineffective. Tyramine and dopamine were effective on the isolated rabbit ileum but not on the cat's nictitating membrane. Effective antagonism of the adrenergic neurone blocking drugs was also shown by some substances which inhibit mono-amine oxidase but only those which in addition possess sympathomimetic effects. Thus phenelzine, pheniprazine and tranylcypromine were effective whereas iproniazid and nialamide were not. Since xylocholine, bretylium and guanethidine were all antagonized by the same agents, it seems likely that they all produce sympathetic blockade by a similar mechanism. The possibility is discussed that the sympathomimetic amines which antagonize the adrenergic neurone blocking drugs are competing with these substances for the same receptor sites.

Comparison of bretylium and guanethidine: tolerance, and effects on adrenergic nerve function and responses to sympathomimetic amines

Bretylium depresses the slope of regression lines relating frequency of sympathetic nerve stimulation to magnitude of contractions of the cat nictitating membrane. In contrast, guanethidine and reserpine preferentially abolish responses to low rates of nerve stimulation and cause a roughly parallel shift of the regression lines. The hypersensitivity of the nictitating membranes of cats to intravenous adrenaline or noradrenaline is far greater after a series of small daily doses of bretylium or guanethidine than after single large doses. The maximal sensitivity produced was similar to that after postganglionic sympathetic nerve section and exceeded that produced by ganglion blockade. The development of hypersensitivity to catechol amines is accompanied by some return of responses of the nictitating membranes to sympathetic nerve stimulation despite continued daily administration of bretylium or guanethidine. In cats given bretylium daily, responses to low rates of nerve stimulation become greater than in controls unless the dose of bretylium given subcutaneously is 50 mg/kg or more. When marked hypersensitivity to catechol amines has been produced by giving bretylium or guanethidine daily for 7 or 14 days, the sympathomimetic effects of these compounds are greater. Responses to intravenous dimethylphenylpiperazinium are also increased and the results suggest that even large daily doses of adrenergic neurone blocking agents do not appreciably impair the functioning of the adrenal medulla. The pressor effects of intravenous adrenaline, noradrenaline and dimethylphenylpiperazinium iodide increase less than the corresponding nictitating membrane responses. These results are discussed in relation to tolerance to adrenergic neurone blockade, and differences between the effects of bretylium and guanethidine found in man. Bretylium and guanethidine depress the slopes of the dose-response curves for the pressor and nictitating membrane contracting effects of tyramine. When single doses or a short series of daily doses were given, guanethidine caused more depression of the slopes than did bretylium, but nevertheless large depressions of slope were found after giving bretylium daily for several weeks. The magnitude of the responses can be greater or less than in controls depending on the dose of the sympathomimetic amine, the dose of the adrenergic neurone blocking agent and the duration of its administration. The results suggest that injection of tyramine produces a progressively smaller release of adrenaline or noradrenaline during the daily administration of bretylium (or guanethidine) but that in some test situations this is more than compensated for by the development of hypersensitivity to the catechol amine released. Some corresponding changes in responses to amphetamine and ephedrine are also described.

Evidence for a competitive antagonism of guanethidine by dexamphetamine

After guanethidine had blocked the response of the cat nictitating membrane to sympathetic nerve stimulation, dexamphetamine restored the responses to all frequencies of stimulation. Dexamphetamine antagonized the sympathetic nerve block by guanethidine in the isolated sympathetically innervated rabbit ileum; the evidence suggests that the antagonism was competitive. Dexamphetamine antagonized the sympathetic nerve block by guanethidine in the isolated hypogastric nerve-vas deferens preparation of the guinea-pig. Doses of dexamphetamine, larger than those required to antagonize the blocking action of guanethidine, abolished the responses of the nictitating membrane, ileum and vas deferens to nerve stimulation. Dexamphetamine did not influence the depletion of noradrenaline by guanethidine in the heart and spleen of rabbits. The hypothesis is advanced that both dexamphetamine and guanethidine act on the store of noradrenaline at sympathetic nerve endings.

Studies on sympathetic mechanisms in isolated intestinal and vas deferens preparations

The effects of various drugs which block sympathetic nerves have been studied in Finkleman preparations of rabbit and cat ileum. Contractile responses could be produced in many cases after the normal inhibitory responses to periarterial nerve stimulation had been blocked. In almost all cases this contractile response was abolished by ganglion-blocking drugs. The normal rabbit ileum Finkleman preparation was found to behave differently towards bretylium than do preparations taken from animals treated with reserpine. The block of the guinea-pig isolated hypogastric vas deferens preparation caused by hemicholinium HC-3 was found to be reversed in the presence of noradrenaline, histamine, or 5-hydroxytryptamine, as was the block caused by guanethidine and bretylium. The results are discussed in relation to the Burn-Rand theory of sympathetic nerve mechanism.

Tissue amine levels and sympathetic blockade after guanethidine and bretylium

A single dose of guanethidine produces a substantial, long-lasting depletion of tissue catecholamines in the rat, whereas a similar dose of bretylium has no effect. Both drugs produce block of the eserine-induced sympathetic pressor effect. Block by guanethidine is induced more rapidly than is amine depletion. When amine depletion is maximal, a noradrenaline infusion is capable of restoring the response to eserine, but no restoration of the response to eserine occurs after noradrenaline infusion into bretylium-treated rats. Catecholamine levels in isolated tissues are not reduced when complete block of sympathetic nerve stimulation has been produced by guanethidine. It is suggested that guanethidine possesses a primary bretylium-like, and a secondary reserpine-like, blocking action. Guanethidine produces a transient lowering of intestinal 5-hydroxytryptamine, and this coincides with increased intestinal motility.

Influence of syrosingopine on the cardiovascular response to acute hypoxemia and exercise

Large parenteral doses of a rauwolfia alkaloid, syrosiiigopine, administered to normal volunteers, although inducing a consistent reduction in pulse rate, did not interfere with the increase in cardiac output resulting from acute hypoxemia and exercise. The physiological and pharmacological implications of these observations are discussed.

The action of guanethidine with particular reference to the sympathetic nervous system

It has been suggested that guanethidine can release and then deplete postganglionic sympathetic nerve endings of noradrenaline. However, no release of noradrenaline from postganglionic nerve endings or from the adrenal medulla by guanethidine was found by direct experiment. Although release of noradrenaline from postganglionic sympathetic nerve endings in response to nerve stimulation was rapidly reduced and finally abolished by guanethidine, the drug did not appear to affect the release of catechol amines from the adrenal medulla in response to splanchnic nerve stimulation. The nature of the action of guanethidine is discussed, and it is concluded that it blocks the effect of postganglionic sympathetic nerve stimulation by interfering with the synthesis of transmitter and that it also has a direct sympathomimetic effect.

Study of the relationship between the neurotransmitter store and adrenergic nerve block induced by reserpine and guanethidine

The heart rate response to cardioaccelerator nerve stimulation and the corresponding levels of myocardial norepinephrine content were determined and correlated in dogs at various time intervals following the intravenous injection of either 3 mg/kg reserpine, or 10 mg/kg guanethidine. Guanethidine produced complete blockade of the cardiac accelerator response before producing measurable myocardial depletion of norepinephrine. In contrast, reserpine reduced the positive chronotropic response to cardioaccelerator nerve stimulation only after myocardial norepinephrine levels have been reduced to approximately 0.3 µg/g. An infusion of norepinephrine did not restore the heart rate response to cardioaccelerator nerve stimulation in either the reserpine or guanethidine treated dogs. These data suggest that the interference with adrenergic transmission produced by guanethidine is independent of changes in the level of stored adrenergic transmitter. The reserpine-induced blockade of adrenergic transmission may ultimately be dependent upon the mission may ultimately be dependent upon the depletion of adrenergic transmitter, but almost complete depletion of stored adrenergic transmitter must occur before reserpine-induced adrenergic blockade occurs.

Effect of guanethidine, hemicholinium and mebutamate on the hypertensive response to eserine and catechol amines

Guanethidine, hemicholinium and mebutamate were used to study the site and mechanism of the hypertensive response to eserine in the rat. Guanethidine was found to block very effectively the hypertensive effect of eserine and to produce at the same time a very strong potentiation of the response to catechol amines. Hemicholinium, after a certain latent period, also blocked the effect of eserine, at the same time leaving the response to adrenaline and noradrenaline intact. Mebutamate was also found to block the effect of eserine. The results of the present experiments suggest that eserine produces a central adrenergic activation in the rat.

THE EFFECTS OF GUANETHIDINE ON THE NORADRENALINE CONTENT OF THE HYPOTHALAMUS IN THE CAT AND RAT

Single injections of guanethidine 15 mg./kg. i.p. did not lower the hypothalamic noradrenaline in cats, but, daily injections of 15 mg./kg. s.c. over a 7 day period consistently produced a decrease in the hypothalamic noradrenaline. Attempts to lower the hypothalamic noradrenaline in rats after single injections or daily injections of guanethidine were unsuccessful.

5-HT agonist induced analgesia modulated by central but not peripheral noradrenaline depletion in rats

The antinociceptive effect elicited by the 5-hydroxytryptamine (5-HT) agonist 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) was reversed or blocked in animals which had previously sustained severe spinal noradrenaline (NA) depletion via either systemic N-2-chlorethyl-N-ethyl-2-bromobenzylamine hydrochloride (DSP 4), neonatal 6-hydroxydopamine (neon. 6-OHDA), or intrathecal 6-OHDA treatment. Biochemical analysis of the lumbar spinal cord samples confirmed severe central NA depletions. Animals were tested with nondamaging heat pain (tail-flick test, hot-plate test) and electric footshock titration to determine the amount of antinociception or nociception. Peripheral NA depletion following intravenous (i.v.) 6-OHDA injection to adult rats had no effect on the antinociception induced by 5-MeODMT, but did cause severe NA depletions in the left heart atrium. These results suggest a modulatory effect of central and not peripheral noradrenergic system upon 5-HT agonist induced analgesia, and also give evidence that this effect is spinally mediated.

Characteristics of the vagally driven non-adrenergic, non-cholinergic inhibitory innervation of ferret gastric corpus

This paper reports a quantitative in vivo study on the vagal activation of the intramural non-adrenergic, non-cholinergic inhibitory nerves in the ferret gastric corpus. The nature of the inhibitory neurotransmitter was also investigated. In the atropinized, guanethidine-treated, urethane-anaesthetized ferret, electrical stimulation (10 s at 20 V, 1-20 Hz, 0.5 ms pulses) of the cervical vagi produced a prompt fall in intracorpus pressure that was related to the stimulus frequency. The maximal response was achieved at 10 Hz. The time taken for the intracorpus pressure to return to pre-stimulus levels after a 10 s period of stimulation was related to the stimulus frequency; at 10 Hz the pressure took approximately 11 min to recover. In contrast to studies in the cat (Martinson & Muren, 1963), there was no detectable difference in the electrical threshold for activation of the vagal excitatory and vagal inhibitory fibres. The nature of the vagal non-adrenergic, non-cholinergic inhibitory neurotransmitter was investigated using a variety of antagonists and agonists. Adenosine triphosphate (ATP), adenosine, alpha beta-methylene ATP and beta gamma-methylene ATP all contracted the corpus in the presence of vagotomy, atropine, guanethidine and indomethacin. The vagally induced fall in corpus pressure was not blocked by high doses of alpha beta-methylene ATP. A variety of peptides were investigated for their effects on corpus pressure in the presence of atropine, guanethidine and vagotomy. Bombesin, pentagastrin, substance P, cholecystokinin octapeptide (CCK-8) and bradykinin all produced an increase in intracorpus pressure. Neurotensin and vasoactive intestinal polypeptide (VIP) both decreased intracorpus pressure, and of the two VIP most closely mimicked the response to vagal activation of the non-cholinergic, non-adrenergic inhibitory neurones. The results provide support for the involvement of a peptide (possibly VIP) rather than a purine in the vagally driven decrease in intracorpus pressure in the ferret.

Participation of adrenal medulla in 6-amino-nicotinamide-induced hyperglycaemia in the rat

In fasted rats 6-aminonicotinamide (6-AN) produced delayed hyperglycaemia, the peak effect being seen by 7 h. Fasting plasma insulin concentrations were not significantly altered but liver glycogen concentrations were decreased following treatment with 6-AN. Adrenalectomy, demedullation and pretreatment with reserpine, phentolamine, nicotinamide and nicotinic acid completely blocked the hyperglycaemic response whereas guanethidine and propranolol or oxprenolol were ineffective. Catecholamine concentrations in the adrenal venous plasma were markedly increased by treatment with 6-AN, the peak effect being seen by 5 h. It is concluded that adrenal medullary release, which is slow in onset, is mainly responsible for the development of sustained hyperglycaemia in the rat.

The relation between the adrenergic neurone-blocking and noradrenaline-depleting actions of some guanidine derivatives

1 The effects of some guanidine derivatives, (-)-N-(1-phenylethyl) guanidine (PEG), guanethidine and debrisoquine have been investigated on the content and subcellular distribution of noradrenaline in cat spleen and on the overflow of noradrenaline from this organ during stimulation of the splenic nerve.2 PEG and guanethidine, at a dose of 5 mg/kg, produced adrenergic neurone blockade within 15 min and the same dose of debrisoquine produced blockade within 30 minutes.3 All three compounds produced a decrease of similar magnitude in the noradrenaline content of the high-speed particulate (P(2)) and supernatant (S) fractions of splenic homogenates; these actions were temporally correlated with the adrenergic neurone-blocking action of the compounds.4 PEG did not produce any further decrease in the noradrenaline content of the subcellular fractions at times up to 18 h after its administration; adrenergic neurone blockade was maintained throughout this period but had disappeared after 7 days when the noradrenaline content of the subcellular fractions was restored to control levels.5 Guanethidine, in contrast, caused a marked progressive loss of the transmitter from all subcellular fractions-an effect which was maximal 18 h after its administration but continued, as did the adrenergic neurone-blocking action, for at least 72 hours. This additional loss of noradrenaline, over and above that seen after 15 min, is unlikely to be connected with the adrenergic neurone-blocking action of the drug.6 Dexamphetamine both prevented and antagonized the neurone blockade and the subcellular noradrenaline-depleting action of PEG and guanethidine. The restoration of nerve function after administration of dexamphetamine to animals pretreated with 5 mg/kg of either of these compounds was temporally correlated with a selective repletion of the noradrenaline content of the supernatant fraction of the spleen.7 Larger doses (15 mg/kg) of PEG or guanethidine produced a selective depletion of noradrenaline in only the supernatant fraction of the spleen. This depletion was temporally correlated with the adrenergic neurone-blocking action of these compounds. The lack of effect of the compounds at this dose level on the noradrenaline contained in the P(2) fraction may be due to ;stabilization' of the store of noradrenaline in vivo which gives rise to this fraction on homogenization.8 It is suggested that the guanidine-type adrenergic neurone-blocking agents displace the noradrenaline which is readily available for release by nerve impulses, and that it is this action that may account for their sympathomimetic properties.9 The ability of these guanidines to impair the release of noradrenaline by nerve impulses could occur because whilst they are present within the neurone the ;nerve-releasable store', which may in these experiments appear in the supernatant fraction after homogenization, may be unable to refill with transmitter.

Transmission to the longitudinal muscle of the guinea-pig vas deferens: The effect of pretreatment with guanethidine

1 Tissue was taken from guinea-pigs that had been injected with guanethidine (100 mg/kg, i.p.) 24 h before they were killed, and from paired control animals.2 Pretreatment with guanethidine caused a significant, substantial, and sometimes complete reduction of the nerve-mediated contractions of the vas deferens. There were no significant changes in the responses of the ileum to stimulation of cholinergic nerves or of the distal colon to stimulation of intrinsic (non-adrenergic) inhibitory nerves. Responses of the vas deferens and ileum to acetylcholine were unchanged, but contractions of the vas deferens elicited by exogenous noradrenaline were enhanced.3 The nerve-mediated contractions of the vas deferens were restored by exposing it to (+)-amphetamine followed by noradrenaline in vitro.4 It is concluded that noradrenaline is the transmitter released from motor nerves to the longitudinal muscle of the guinea-pig vas deferens. Possible explanations for the ineffectiveness of receptor blocking agents in antagonizing transmission are discussed.

Autonomic and non-autonomic circulatory components in essential hypertension in man

The circulatory effects of blocking cardiac and peripheral autonomic effectors were studied in 32 subjects with established essential hypertension, and in 15 normotensives. The mean resting arterial pressure, heart rate and total peripheral resistance index (TPRI) were significantly higher in the hypertensives, but cardiac index was the same in both groups. In subjects with blocked cardiac effectors (atropine + beta-blocking drugs, i.v.) the sympathetic constrictor effects on TPRI were estimated from the changes after giving i.v. guanethidine + phentolamine. The autonomic component of TPRI was higher in hypertensives than in normotensives. The residual resistance after `total' autonomic block (non-autonomic TPRI) was higher in hypertensives, accounting for 60 to 80% of the initial difference in resting TPRI between the two groups. With an increase in non-autonomic TPRI, the increased autonomic TPRI effect in hypertension is not necessarily due to increased sympathetic nerve activity. Vagal and cardiac sympathetic effects on heart rate were compared in the two groups. Each estimate was based on the average of the responses to the appropriate blocking drug (1) in subjects not previously given a blocking drug, and (2) in subjects with the other cardiac effector pathway already blocked. The higher heart rate in established hypertension was predominantly due to change in vagal rather than cardiac sympathetic effects.

Interaction of bretylium and guanethidine on the relaxations of the rat isolated fundal strip preparation, evoked by indirect stimulation

1. Isolated rat stomach fundal strip bathed in Krebs solution containing atropine (1 mug/ml), responded to indirect stimulation by a relaxation which was frequency dependent. These responses were blocked by phenoxybenzamine (6 mug/ml) or phentolamine (8 mug/ml).2. Strips obtained from rats previously treated with reserpine did not show relaxation to indirect stimulation. These responses were therefore adrenergic in nature.3. Bretylium (0.1-100 mug/ml) failed to block the relaxations produced by indirect stimulation, in fact relaxations were potentiated by the drug.4. Guanethidine (10 mug/ml) blocked the relaxations induced by indirect stimulation.5. Guanethidine may be taken up by adrenergic nerves actively since its action is not seen at 12 degrees C.6. Bretylium (10 mug/ml) prevented the actions of guanethidine at 37 degrees C.

The effects of sympathetic nerve stimulation and guanethidine on parasympathetic neuroeffector transmission; the inhibition of acetylcholine release

The effect of noradrenaline released either by sympathetic nerve stimulation or guanethidine added to the organ bath has been studied on acetylcholine release from parasympathetic nerve terminals and compared with the effect of exogenous noradrenaline. Sympathetic nerve stimulation, guanethidine and noradrenaline reduced the release of acetylcholine from resting rabbit intestine by up to 70%. Sympathetic stimulation and guanethidine failed to reduce acetylcholine release in preparations previously depleted of noradrenaline. Noradrenaline added to the bath still remained effective. The fact that noradrenaline released is capable of inhibiting acetylcholine release supports the concept that noradrenaline physiologically controls the release of acetylcholine.