Inhibition by dopamine of 3H-noradrenaline release elicited by nerve stimulation in the isolated cat's nictitating membrane

Argentinean Society of Experimental Pharmacology: Brief history and main scientific contributions to the discipline

Argentina Biomedical Science has been historically strong. The development of Human and Veterinary Pharmacology in our country as a pivotal discipline has been acknowledged worldwide because of the quality of its contributions. Argentinean Society of Experimental Pharmacology (SAFE) is a non- profit association whose research fields include Experimental and Clinical Pharmacology. SAFE main goals are described as follow (a) To meet active researchers for studying concerns regarding Experimental and Clinical Pharmacology (b) To launch an initiative for development of the discipline in mainly our country and other collaborative countries worldwide (c) To spread the pharmacological know-how obtained from different research teams (d) To strengthen relations between pharmacologists (e) To facilitate the presentation and discussion of scientific papers. This current article shows the SAFE's more important scientific contribution to pharmacology through its former research scientists to the present.

α2-Adrenoceptors in the treatment of major neuropsychiatric disorders

Presynaptic autoreceptors mediate a retrograde transfer of information by a negative feedback mechanism mediated by the transmitter of the neuron, and fulfill an autoregulatory function in neurotransmission in the peripheral and central nervous system (CNS). Starting with norepinephrine (NE), it was later reported that an autoreceptor-mediated negative feedback mechanism exists for other neurotransmitters, including dopamine (DA), serotonin, acetylcholine, histamine, GABA, and glutamate. This feedback mechanism regulates calcium-dependent transmitter release and synthesis through terminal presynaptic autoreceptors, while the firing rate of the neuron is regulated through somatodendritic autoreceptors.

Pharmacological evidence that dopamine inhibits the cardioaccelerator sympathetic outflow via D2-like receptors in pithed rats

It has been suggested that N,N-di-n-propyl-dopamine (dopamine analogue) decreased heart rate in rats through stimulation of dopamine receptors. Nevertheless, the role of prejunctional dopamine D1/2-like receptors or even α2-adrenoceptors to mediate cardiac sympatho-inhibition induced by dopamine remains unclear. Hence, this study identified the pharmacological profile of the cardiac sympatho-inhibition to dopamine in pithed rats. Male Wistar rats were pithed and prepared to stimulate the cardiac sympathetic outflow or to receive i.v. bolus of exogenous noradrenaline. I.v. continuous infusions of dopamine (endogenous ligand) or quinpirole (D2-like agonist) dose-dependently inhibited the tachycardic responses to sympathetic stimulation, but not those to exogenous noradrenaline. In contrast, SKF-38393 (100 μg/kg∙min, D1-like agonist) failed to modify both of these responses. The sympatho-inhibition to dopamine (1.8 μg/kg∙min) or quinpirole (100 μg/kg∙min): i) remained unaltered after saline or the antagonists SCH-23390 (D1-like, 300 μg/kg) and rauwolscine (α2-adrenoceptors, 300 μg/kg); and ii) was significantly antagonized by raclopride (D2-like, 300 μg/kg). These antagonists, at the above doses, failed to modify the sympathetically-induced tachycardic responses. The above results suggest that the inhibition of the cardiac sympathetic outflow to dopamine and quinpirole is primarily mediated by prejunctional D2-like receptors but not D1-like receptors or α2-adrenoceptors.

Therapeutic use of release-modifying drugs

Presynaptic inhibitory or facilitatory autoreceptors are targets for the endogenous neurotransmitter of the respective neuron, and also for exogenous agonists, partial agonists and antagonists which can produce pharmacological actions through changes in transmitter release. In addition, presynaptic inhibitory or facilitatory heteroreceptors can also be acted upon by exogenous agonists, partial agonists or antagonists to induce changes in transmitter release with useful therapeutic effects. This article summarizes drugs that are known or likely to produce their therapeutic effects through presynaptic modulation of neurotransmitter release. Included are drugs acting on alpha and beta adrenoceptors, dopamine receptors, angiotensin, opioid, cannabinoid, and nicotinic acetylcholine receptors. Also discussed are changes in presynaptic receptor mechanisms produced by drugs that inhibit transmitter re-uptake.

Presynaptic receptors for dopamine, histamine, and serotonin

Presynaptic receptors for dopamine, histamine and serotonin that are located on dopaminergic, histaminergic and sertonergic axon terminals, respectively, function as autoreceptors. Presynaptic receptors also occur as heteroreceptors on other axon terminals. Auto- and heteroreceptors mainly affect Ca(2+) -dependent exocytosis from the receptor-bearing nerve ending. Some additionally subserve other presynaptic functions.Presynaptic dopamine, histamine and serotonin receptors are involved in various (patho)physiological conditions. Examples are the following:Dopamine autoreceptors play a role in Parkinson's disease, schizophrenia and drug addiction. Dopamine heteroreceptors affecting the release of acetylcholine and of amino acid neurotransmitters in the basal ganglia are also relevant for Parkinson's disease. Peripheral dopamine heteroreceptors on postganglionic sympathetic terminals influence heart rate and vascular resistance through modulation of noradrenaline release. Blockade of histamine autoreceptors increases histamine synthesis and release and may support higher CNS functions such as arousal, cognition and learning. Peripheral histamine heteroreceptors on C fiber and on postganglionic sympathetic fiber terminals diminish neuropeptide and noradrenaline release, respectively. Both inhibititory effects are beneficial in myocardial ischemia. The inhibition of neuropeptide release also explains the antimigraine effects of some agonists of presynaptic histamine receptors. Upregulation of presynaptic serotonin autoreceptors is probably involved in the pathogenesis of major depression. Correspondingly, antidepressant treatments can be linked with a reduced density of 5-HT autoreceptors. 5-HT Heteroreceptor activation diminishes acetylcholine and GABA release and may therefore increase anxiety. In the periphery, presynaptic 5-HT heteroreceptor agonists shorten migraine attacks by inhibition of the release of neuropeptides from trigeminal afferents, apart from their constrictive action on meningeal vessels.

Renal dopaminergic mechanisms and hypertension: a chronology of advances

Dopamine (DA) has been shown to influence kidney function through endogenous synthesis and subsequent interaction with locally expressed dopamine receptor subtypes (D1, D5 as D1-like and D2, D3, and D4 as D2-like). DA, and DA-receptor specific agonists and antagonists can alter renal water and electrolyte excretion along with renin release when infused systemically or intrarenally. Such effects are brought about by a combination of renal hemodynamic and direct tubular effects evoked along the full length of the nephron. The cellular mechanisms that direct these dopamine-mediated renal electrolyte fluxes have recently been clarified and include alterations in adenylyl cyclase, phospholipase C, and phospholipase A1 activity. The dopaminergic system also interacts directly with the renal kallikrein-kinin, prostaglandin and other neurohumoral systems. Aberrant renal dopamine production and/or dopamine receptor function have been reported in salt-dependent and low-renin forms of human primary hypertension as well as in genetic models of animal hypertension, including the SHR and Dahl SS rat. DA D1 or D3 receptor knockout mice have been shown to develop hypertension.

Dopamine and sympathoadrenal activity in man

The sympathetic adrenal (SA) activity can be modulated by dopamine (DA) through D2 receptors. In man, using D2 antagonists, it has been demonstrated that endogenous DA plays an inhibitory modulation of the SA system during high degree of SA activation. D2 agonists are able to induce a decrease in norepinephrine (NE) release either in vitro or in vivo. This effect leads, in vivo, to a decrease in blood pressure (BP) and to an activation of arterial baroreceptors. Therefore, in vivo, the D2 mediated inhibition of epinephrine (E) release, which is clearly demonstrated in vitro, is overwhelmed by the baroreceptor-mediated activation of the splachnic nerve. As a consequence, the in vivo administration of D2 agonists can induce a different effect on the net peripheral sympathetic tone of an organ, depending on the balance between the degree of the baroreceptor-mediated sympathetic activation and the inhibitory D2-mediated inhibition of NE release at the tissue level. In the present paper we investigated the in vivo effect of placebo (PL) or acute oral bromocriptine (BC) administration on plasma CA and on the cardiac sympatho-vagal balance of 7 normal volunteers, as assessed by power spectral analysis of heart rate (HR) variability (autoregressive method), either in resting or sitting position. Low frequency (LF) and high frequency (HF) components, both expressed in normalized units (nU), and LF/HF ratio were calculated. BC caused a decrease in BP, plasma NE and no change in HR in resting and sitting position. Plasma E increased in sitting position. At the heart level, after BC, we observed, during rest, an increase in LF and LF/HF ratio and a decrease in HF while in sitting position LF did not increase further. These data show that BC, while reducing BP through a decrease of plasma NE, increases LF/HF ratio (sympathetic tone) without any change in heart rate. These data seem to confirm that BC causes an inhibitory modulation of the SA system acting predominantly at the periphery through D2 presynaptic receptors.

Measurement of endogenous dopamine and norepinephrine release from superfused slices of rat prefrontal cortex in vitro: modulation by D2 and alpha-2 presynaptic receptors

Endogenous dopamine (DA) and norepinephrine (NE) in the superfusate from slices of rat medial prefrontal cortex were measured by high-performance liquid chromatography coupled to electrochemical detection (HPLC-ECD). Stimulation with high K+ or methamphetamine (MAP) evoked a dose-dependent elevation in the release of DA and NE, although spontaneous release of DA or NE was barely detectable. The K(+)-evoked release was Ca++ dependent, whereas the MAP-evoked release was not. The K(+)-evoked DA release was inhibited by the DA agonist apomorphine (1 microM or 10 microM) and enhanced by the D2 antagonist (-)-sulpiride (1 microM). The K(+)-evoked NE release was inhibited by the alpha-2 agonist clonidine (0.1 microM or 1 microM) and enhanced by the alpha-2 antagonist idazoxan (1 microM). These results confirm the existence of release modulatory D2 and alpha-2 receptors in the medial prefrontal cortex. The present study is the first description of a method which allows evaluation of the release of endogenous DA or NE in the cortex slices and is competent to examine the properties of the two catecholamines release and their regulation by presynaptic receptors.

Ocular inhibitory effects of the dopamine DA2 agonist (Ha-118) in cats and rabbits

1. Topical administration of the dopamine (DA2) receptor agonist, Ha-118, produced unilateral ocular hypotension and miosis in normal cats. 2. The ocular hypotensive and mitotic effects of Ha-118 were not observed in surgically sympathectomized cats indicating that an intact sympathetic pathway is necessary to demonstrate activity. 3. Ha-118 caused dose-dependent suppression of contractions of the cat nictitating membrane (CNM) elicited by electrical stimulation of the pre- and postganglionic sympathetic nerve trunks but not by exogenously administered noradrenaline suggesting that Ha-118 affected prejunctional but not ganglionic or postjunctional receptors. 4. Sulpiride antagonized Ha-118-induced inhibition of neuronally mediated contractions in the CNM suggesting an interaction at DA2 receptors. 5. Topical administration of Ha-118 inhibited the rise in intraocular pressure induced by oral water loading in rabbits. 6. Topical pretreatment with metoclopramide, a DA2 antagonist, inhibited the ocular antihypertensive effect of Ha-118 in rabbits. 7. These studies demonstrate that Ha-118 decreased intraocular pressure (IOP) and pupil diameter (PD) in the cat and that this activity can be correlated with suppression of peripheral sympathetic tone in the CNM.

Cardiovascular dopamine receptors: role of renal dopamine and dopamine receptors in sodium excretion

Research efforts in the area of peripheral dopamine have now established the presence of two distinct subtypes--DA1 and DA2--of DA receptors, and have identified a potential role for dopamine produced within the kidney in the control of renal sodium excretion. Selective DA1 and DA2 receptor agonists are being developed because they exhibit therapeutic potential for treatment of cardiovascular and renal disorders. Furthermore, basic research efforts are aimed towards identifying the stimulus and/or stimuli for the production of dopamine within the kidney and characterizing the cellular signalling processes involved in mediating the renal effects of dopamine and selective DA receptor agonists.

Prejunctional adrenoceptor activity of N-0437: a relatively selective DA2 dopamine receptor agonist

Prejunctional adrenoceptor activity of N-0437, 2[N-n-propyl-N-2-(thienylethyl-amino)-5-hydroxytetralin], was investigated by means of the cat nictitating membrane (CNM) preparation. Intra-arterial (i.a.) administration of N-0437 produced a dose-related inhibition (ED50 = 14 micrograms) of the CNM contractions elicited by electrical stimulation of pre- and postganglionic sympathetic nerves of the superior cervical ganglion. Pretreatment with domperidone i.a., a relatively selective DA2 receptor antagonist, markedly attenuated the CNM response to racemic N-0437 (ED50 = 6.7 x 10(2) micrograms). However, pretreatment with rauwolscine i.a., a relatively selective alpha 2-receptor antagonist, did not alter the CNM responses to racemic N-0437. Evaluation of the (R)-(+) and (S)-(-) enantiomers showed that only the (S)-(-) enantiomer was active in suppressing the contractions in the CNM preparation. These results demonstrate that N-0437 is a potent agonist for prejunctional DA2 dopamine receptors on peripheral sympathetic nerves in the CNM and that these peripheral DA2 receptors appear to be enantioselective.

Review: alpha 2 and DA2 agonists as antiglaucoma agents: comparative pharmacology and clinical potential

Alpha-2 (alpha 2) and DA2 agonists lower intraocular pressure (IOP) in laboratory animals and man. Like beta-blockers, alpha 2 and DA2 agonists appear to lower IOP by reducing aqueous inflow. These agents share a common mode of action on sympathetic nerve terminals, where they modulate the release of neurotransmitters. However, one can demonstrate that peripheral prejunctional alpha 2 and DA2 receptors on sympathetic neurons are separate entities by utilizing selective agonists and antagonists. In addition to their prejunctional actions, alpha 2 agonists act postjunctionally in the iris root/ciliary body (IRCB). Moreover, utilizing selective postjunctional alpha 2 adrenoceptor antagonists, heterogeneity can be demonstrated between ocular pre- and postjunctional adrenoceptors. Stimulation of postjunctional alpha 2 adrenoceptors in the IRCB can inhibit the cellular responses to endogenous neurotransmitters and hormones that are coupled positively to adenylate cyclase. Based upon these observations, one can predict that alpha 2 agonists should have a broader spectrum of action in the eye than beta-receptor antagonists. Three bioassays were used in the activity analysis of alpha 2 and DA2 agonists. Prejunctional (neuronal) activity was determined in the cat nictitating membrane preparation in which frequency-related (2-8 Hz), neuronally induced contractions were inhibited by these compounds. Postjunctional activity was assayed on isolated rabbit IRCB tissue where cAMP levels were stimulated by either isoproterenol or VIP in the absence and presence of the test agonist (alpha 2 or DA2). In this system, it has been demonstrated that alpha 2 agonists have inhibitory properties, but DA2 agonists are inactive.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for the existence of prejunctional receptor sites for dopamine in the mouse vas deferens

The present work is focused on the effects of newly developed dopaminergic agonists and antagonists on the field-stimulated vas deferens. Both LY 171555 and SK&F 38393, relatively selective DA2 and DA1 receptor agonists, respectively, produced concentration-dependent inhibition of the field stimulation-evoked contractions in the mouse vas deferens; both compounds did not modify the baseline tone nor the contractile responses to exogenous noradrenaline. Control LY 171555 and SK&F 38393 concentration-response curves, were shifted rightward in a parallel manner in the presence of sulpiride (relatively specific DA2 antagonist) and SCH 23390 (relatively specific DA1 antagonist), respectively. Control concentration-response curves for dopaminergic agonists were not modified in the presence of specific blockers for H1 and H2 histamine receptors, serotonin receptors and alpha 2-adrenoceptors. These preliminary findings are suggestive of the existence of two dopaminergic receptor types both presumably located prejunctionally.

Release of 5-hydroxytryptamine, dopamine and noradrenaline in the rat vas deferens in the presence of compound 48/80, veratridine or K+

1. The release of 5-hydroxytryptamine (5-HT), dopamine and noradrenaline (NA) from the rat isolated vas deferens was estimated by high performance liquid chromatography with electrochemical detection. 2. Compound 48/80, which is known to release 5-HT and histamine from mast cells, induced a concentration-related, rapid overflow of 5-HT from the rat isolated vas deferens, releasing about 50% of the 5-HT present in the tissue at a concentration of 300 micrograms ml-1. The drug did not induce the release of either dopamine or NA. 3. The depolarizing agents veratridine (10 microM) or K+ (100 mM) induced the release of 5-10% of the 5-HT present in the vas deferens, but did not evoke the overflow of 5-HT from isolated mast cells. 4. Veratridine induced the release of a greater proportion of the dopamine than of the NA contained in the vas deferens (41.0 +/- 5.5 vs 17.9 +/- 2.0%, after 20 min of exposure to the drug, n = 6). In contrast, K+ evoked the release of both amines equally. 5. It is concluded that in the rat vas deferens 5-HT is stored in mast cells and also in cells responsive to depolarizing stimuli. It is suggested that 5-HT could be a neuromodulator or a neurotransmitter in this organ. Veratridine and K+ induce the release of dopamine in addition to NA.

Characterization of dopamine-induced potassium efflux in rat parotid acinar cells

The effects of dopamine and noradrenaline on potassium efflux from rat parotid gland were studied in a perifusion system. Tissue specimens were preincubated with 86RbCl and the efflux of 86Rb+ was used as a marker for potassium efflux. Noradrenaline induced 86Rb+ efflux more effectively than dopamine. The noradrenaline-induced efflux was inhibited by alpha-adrenoceptor blockers, especially the alpha 1-antagonist prazosin. The dopamine-induced 86Rb+ efflux was blocked by alpha-adrenoceptor antagonists, non-selective dopamine antagonists and a D-1 selective dopamine antagonist. The D-2 selective drug, sulpiride, did not affect the dopamine-induced 86Rb+ efflux. The dopamine effect was abolished when reserpinized animals were used, whereas the effect of noradrenaline was unaffected. The results suggest that dopamine has a presynaptic stimulatory effect in rat parotid gland, and that the presynaptic effect on potassium efflux seems to be mediated via the D-1 receptor subtype. Whether activation of the presynaptic D-1 receptors leads to noradrenaline release, or whether the D-1 receptor is coupled to the catecholamine transporter system remains to be studied further.

Aporphine derivatives affect ocular function in diverse ways

The sympathetic nervous system has been implicated in the ocular effects of several classes of dopamine receptor agonists. Two agonists of the aporphine class, (-) N-propylnorapomorphine [-)NPA) and norapomorphine (NA), were evaluated for effects on intraocular pressure (IOP) and pupil diameter (PD) in cats and normal (NL) and sympathectomized (SX) rabbits and on contractions of the cat nictitating membrane (CNM). Topically administered (-)NPA produced a monophasic IOP drop in the ipsilateral eye and a biphasic (increase, decrease) IOP response in the contralateral eye of normal rabbits. Pupil diameter increased bilaterally. In NL cats, the ipsilateral IOP and PD response to (-)NPA was biphasic (increase, decrease) and the contralateral IOP response was monophasic (increase). (-)NPA produced an increase in spontaneous motor activity (SMA) in rabbits and cats. NA lowered IOP unilaterally in NL rabbits. In SX rabbit eyes, (-)NPA lowered IOP as much as in NL eyes but with less mydriasis. NA did not lower IOP in SX rabbit eyes. Both (-)NPA and NA inhibited contractions of the CNM elicited by electrical stimulation of the pre- and post-superior cervical ganglionic nerves. (-)NPA, but not NA, inhibited contractions elicited by exogenous norepinephrine. These results suggest that aporphine derivatives produced two diametrically opposed effects on ocular function; 1) a centrally mediated effect that enhanced noradrenergic activity to elicit mydriasis, SMA and ocular hypertension, and 2) a peripherally mediated effect to produce miosis and ocular hypotension.

Evidence that dopamine regulates norepinephrine synthesis in the rat superior cervical ganglion during hypoxic stress

Electrical stimulation of preganglionic nerves is known to increase norepinephrine synthesis in the rat superior cervical ganglion in vitro, an effect which appears to be partially regulated by a non-cholinergic transmitter. In the present study, we sought to determine whether sympathetic stimulation also increases norepinephrine synthesis in the rat ganglion in vivo, and whether dopamine released from ganglionic interneurons might regulate this response. To tackle these questions, rats were pretreated with spiroperidol, a selective dopamine-receptor blocker, and then were sympathetically stimulated by exposure to severe hypoxic stress. Other rats were pretreated with vehicle alone before the hypoxic exposure. Norepinephrine synthesis in ganglia was assessed by measuring endogenous tyrosine hydroxylase activity and norepinephrine turnover. We found that hypoxic stress increased both of these indices of norepinephrine synthesis, but only in rats pretreated with spiroperidol. No such response was detected in rats pretreated with vehicle. These results indicate that sympathetic stimulation increases norepinephrine synthesis in the rat superior cervical ganglion in vivo, and that dopamine released from interneurons might regulate this response.

Presynaptic dopamine receptors in the cardiovascular system

Stimulation by agonists of presynaptic dopamine receptors on nerve terminals of peripheral sympathetic neurons results in inhibition of norepinephrine release and a concomitant reduction of end organ responses to sympathetic nerve stimulation. These presynaptic dopamine receptors are of the DA-2 subtype and can be blocked selectively by the antagonist S-sulpiride. Presynaptic DA-2 receptors are the target of action of agonists with potential antihypertensive and bradycardic effects. Under control conditions exposure to S-sulpiride on its own does not enhance norepinephrine release. Following chronic treatment of cats with pargyline, S-sulpiride produced a small but significant increase in the electrically-evoked release of 3H-norepinephrine from perfused atrial slices. The possible involvement of peripheral presynaptic DA-2 receptors in the antihypertensive effects of monoamine oxidase inhibitors is discussed.

Presynaptic dopamine receptors in the pithed rat: characterization with apomorphine and comparison with central dopamine autoreceptors

Apomorphine, a dopamine agonist with high affinity for presynaptic dopamine receptors, caused dose-dependent inhibition (10-300 micrograms/kg intravenously) of the stimulation-induced increase in diastolic blood pressure in the pithed rat. This effect of apomorphine could be antagonized with (-)-sulpiride or haloperidol but not with yohimbine or atropine, indicating the involvement of inhibitory dopamine receptors. The alpha-1-adrenoceptor mediated pressor response to phenylephrine (5 micrograms/kg intravenously) was not significantly attenuated by apomorphine. The sensitivity of peripheral presynaptic dopamine receptors was then studied in the cardiovascular system of rats treated subchronically with haloperidol (2 mg/kg, twice daily intraperitoneally for 10 days). The inhibition of sympathetic vasoconstrictor responses exerted by apomorphine was found to be enhanced after subchronic haloperidol treatment suggesting the development of presynaptic dopamine receptor supersensitivity in the periphery. In addition, the previously reported supersensitivity of central dopamine autoreceptors to low doses of apomorphine could be confirmed in behavioural experiments.

Characterization of a dopaminergic receptor that modulates adrenomedullary catecholamine release

Nicotine evokes the release of catecholamines from bovine adrenal glands perfused with oxygenated Krebs-bicarbonate solution. Two 2-min pulses of 5 microM nicotine, at 40-min intervals (S1 and S2), gave net catecholamine outputs of 45.2 +/- 3.6 and 29.1 +/- 3.5 micrograms/8 min, respectively. Apomorphine (1 or 10 microM) markedly inhibited catecholamine release during S2 to 9.1 +/- 2.2 and 0.5 micrograms/8 min, respectively. Haloperidol (0.5 microM) reversed the inhibitory effects of apomorphine. Haloperidol alone enhanced catecholamine release induced by nicotine to 67.9 +/- 7.9 micrograms/8 min. [3H]Spiperone binds to adrenomedullary membranes with a KD of 0.24 nM and a Bmax of 117 fmol/mg of protein. Whereas spiperone and haloperidol potently displaced such binding, 3,4-dihydroxyphenylethylamine (dopamine) and sulpiride were poorer displacers, and SCH23390, prazosin, phenoxybenzamine, propranolol, BAY-K-8644, and nitrendipine did not displace [3H]spiperone bound. These data strongly suggest that, as in the cat, the bovine adrenal medulla chromaffin cell contains a dopaminergic receptor that modulates the catecholamine secretory process triggered by stimulation of the nicotinic cholinoceptor. Such a receptor seems to be of the D2 type and might be involved in a sympatho-adrenal cooperative mechanism contributing to the maintenance of cardiovascular homeostasis during stressful situations as well as to the pathogenesis of hypertension. If so, selective dopaminergic agonists might prove clinically useful in the treatment of hypertension.

Is there feedback regulation of neurotransmitter release by autoreceptors?

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

Does dopamine suppress stress-induced intestinal and renal vasoconstriction?

Dopamine interference with intestinal and renal sympathetic reflex vasoconstrictor responses was studied in cats anaesthetized with diazepam, fentanyl and nitrous oxide. Vasoconstriction was induced by electric stimulation of the hypothalamic defence-alarm area and by stimulation of somatic and visceral afferents. In addition, intestinal vasoconstriction was elicited by direct stimulation of postganglionic sympathetic efferent nerves. In the intestine, dopamine administration (7.5 microgram X kg-1 X min-1) was not associated with an attenuation of the investigated sympathetic vasoconstrictor responses, although dopamine per se decreased intestinal vascular resistance by 36 +/- 4%. Due to this dopamine-induced background vasodilation, the intestinal blood flow level during stimulation procedures and concomitant dopamine infusion was higher than during similar stimulations prior to dopamine (for defence-alarm area stimulation 45 +/- 16%, for afferent nerve stimulation 79 +/- 22% and for efferent postganglionic nerve stimulation 66 +/- 16%). In the kidney, dopamine per se had only minor effects on vascular resistance and on changes in vascular tone elicited by the stimulation procedures. The renal blood flow level in response to the stimulation procedures was not significantly affected by dopamine. In conclusion, dopamine may contribute to a sustained intestinal blood flow level when administered during supervening stress-related sympathetic activation.

Interaction between presynaptic facilitatory angiotensin II receptors and inhibitory muscarinic cholinoceptors on 3H-noradrenaline release in the rabbit heart

The existence of a functional interaction between presynaptic receptors modulating the release of noradrenaline was studied in the rabbit heart. Isolated right atria were prelabelled with 3H-noradrenaline and the overflow of tritium was induced by field stimulation (2 Hz, 0.1 ms duration, supramaximal voltage for a total of 180 pulses). In atria superfused with Krebs' solution containing 10 mumol/l cocaine and 30 mumol/l corticosterone, angiotensin II (10 nmol/l) increased the stimulation-evoked overflow of 3H-transmitter by 2.8-fold. The addition of atropine (0.3 mumol/l) to the perfusion medium, either in the presence or in the absence of uptake inhibitors, further enhanced the facilitatory effect of angiotensin II (3H-transmitter release increased by 3.5-fold). Exposure to 1 mumol/l carbachol decreased by 65% the stimulation-evoked release of 3H-transmitter while the facilitatory effect of angiotensin II determined in the presence of the muscarinic cholinoceptor agonist was enhanced (3H-transmitter release increased by 6.6-fold). Conversely, during sustained activation of presynaptic angiotensin receptors producing a 2.5-fold increase in the release of 3H-transmitter, the inhibitory effect of carbachol remained unchanged. These results suggest a functional interaction between presynaptic inhibitory muscarinic cholinoceptors and the presynaptic facilitatory angiotensin receptor which modulate the release of noradrenaline from cardiac noradrenergic nerves.

Dopamine increases potassium efflux in the rat parotid gland by stimulating noradrenaline release from sympathetic nerve endings

By studying 86Rb+ efflux (tracer for potassium) and labelled NA ([3H]NA) efflux from preloaded parotid glands in vitro it is suggested that dopamine stimulates potassium efflux through an indirect sympathomimetic activity--i.e. releasing noradrenaline from sympathetic nerve endings.

Neuronal dopamine receptors of the rabbit ear artery: pharmacological characterization of the receptor

Dopamine and apomorphine were examined in the rabbit isolated perfused ear artery for both direct effects on vascular smooth muscle and effects on the response to field stimulation of sympathetic nerve terminals. The neuroinhibitory effect of both dopamine (EC50 = 37 nM) and apomorphine (EC50 = 44 nM) occurred at concentrations which did not produce vasoconstriction. The neuroinhibitory effect of dopamine was shown to be due to inhibition of noradrenaline release by measurement of 3H-overflow from prelabelled tissues. At relatively high concentrations dopamine produced vasoconstriction. In a superfused segment of ear artery, dopamine was found to be a full agonist at the alpha 1-adrenoreceptor, with an EC50 (15 microM) about 75 fold higher than the EC50 for noradrenaline. At concentrations up to 3 microM, apomorphine had no vasoconstrictor activity in the perfused ear artery. Representative examples of several classes of dopamine antagonists, including the phenothiazines, butyrophenones, diphenylbutylpiperidines and benzamides produced competitive antagonism of dopamine or apomorphine-induced inhibition, with nearly identical Kb values against these two agonists. The pharmacological characteristics of the neuronal dopamine receptor on the rabbit ear artery would indicate this receptor to be typical of the D2 subclass, and this tissue to be a useful model for quantitative studies on dopamine receptor agonists and antagonists.

Does endogenous dopamine modulate human sympathetic activity through DA2 receptors?

The effects of placebo and DA2 antidopaminergic agents on basal and stimulated catecholamine (CA) plasma levels in humans have been evaluated by administration either of domperidone or metoclopramide to separate groups of subjects. The latter group was subjected domperidone to sympathetic stimulation by handgrip coldpressor tests, and standing. Pharmacological DA2 receptor blockade, either peripheral by or central and peripheral by metoclopramide, did not modify plasma CA whilst resting or during acute sympathetic stimulation. It is concluded that endogenous dopamine does not seem to play a significant role in modulating NE release in man, at least not under the physiological conditions examined.

In vivo and in vitro dopaminergic effects of three ergoline fragments

The pharmacological effects of three ergoline fragments (BD-179, BD-271 and BD-214) were studied in vivo using the cat cardioaccelerator nerve preparation and in vitro using field stimulated isolated cat right atria. BD-179 and BD-271 produced dose dependent inhibition of tachycardia due to electrical stimulation of the right postganglionic cardioaccelerator nerve in anesthetized cats, BD-214 was inactive. BD-179 produced primarily hypotension, BD-271 produced a transient pressor response followed by hypotension and BD-214 produced only pressor effects. The tachycardia inhibitory effects and hypotension produced by BD-179 and BD-271 were antagonized by the dopamine receptor antagonist sulpiride. BD-179, BD-271 and BD-214 antagonized the presynaptic inhibitory effects of the dopamine receptor agonist apomorphine in vitro on field stimulated isolated cat right atria. All three ergoline fragments facilitated stimulation-induced increases in systolic tension development and BD-214 facilitated stimulation-induced tachycardia in isolated cat right atria.

Prejunctional dopamine receptors modulate twitch responses to parasympathetic nerve stimulation in the rabbit isolated rectococcygeus muscle

Field stimulation of the parasympathetic nerves supplying the rabbit isolated rectococcygeus muscle produced individual twitch responses that were inhibited by dopamine (1 x 10(-8)-1 x 10(-5)M). The twitch-inhibitory effect of dopamine was reversed by haloperidol or sulpiride (1 x 10(-8)-1 x 10(-5)M for either) but not by phentolamine or yohimbine (up to 1 x 10(-4)M). Haloperidol and sulpiride were potent, specific, competitive antagonists of the twitch-inhibitory effect of dopamine; their pA2 values were 8.39 and 7.75, respectively. In contrast, cis alpha-flupenthixol, fluphenazine, bulbocapnine and thioridazine were weak or inactive against dopamine. Concentrations of dopamine that inhibited the twitch response to parasympathetic nerve stimulation had little or no effect on contractions elicited by carbachol or by direct muscle stimulation after abolition of neuronal conduction by tetrodotoxin. Thus, the effects of dopamine on responses elicited by parasympathetic nerve stimulation seem to be exerted at a prejunctional level rather than directly on the smooth muscle. The twitch-inhibitory effect of dopamine was mimicked by epinine, N,N-diethyldopamine, N,N-di-n-propyldopamine, 5,6-ADTN, N,N-di-n-propyl 5,6-ADTN, 6,7-ADTN, apomorphine and Sandoz 27-403. Sulpiride reversed the effects of all these agonists. N,N-di-n-propyl-6,7-ADTN and SK & F 82526 also inhibited the twitch response but their effects were not reversed by sulpiride. SK & F 38393 and DPI had little effect on the twitch response. The pharmacological characteristics of the presynaptic dopamine receptors in the rabbit rectococcygeus muscle show that they resemble those in the cat heart and rabbit ear artery in some respects but there are differences that suggest that the presynaptic dopamine receptors in the rabbit rectococcygeus muscle constitute a specific subgroup of receptors.

Alteration in ocular function induced by phenylethylamine analogs of dopamine

Dopamine (DA) and three methylated analogs (N-methyldopamine, NMDA; N, N- dimethyldopamine , DMDA ;N,N-di-n- propyldopamine , DPDA) were examined for effects on intraocular pressure (IOP) and pupil diameter (PD) in normal rabbits, sympathectomized (SX) rabbits and rabbits with transected extraocular muscles ( EOMX ) following topical administration. In normal rabbits, the predominant effect of DA, NMDA and DMDA was transient, unilateral ocular hypertension with minimal effects on PD. In contrast, DPDA produced bilateral ocular hypotension in normal rabbits. DA and NMDA did not produced ocular hypertension in EOMX rabbits indicative of an involvement of extraocular muscles in normal rabbits. In SX rabbits, NMDA produced mydriasis and exaggerated ocular hypertension followed by significant ocular hypotension; the ocular hypotensive phase was antagonized by timolol pretreatment. The ocular hypotensive activity of DPDA seen in normal rabbits was absent in SX rabbits suggestive of a neuronal site of action for DPDA. DPDA inhibited contraction of the cat nictitating membrane elicited by stimulation of pre- and postganglionic sympathetic nerves. This effect was antagonized by a dopamine (DA2) antagonist metoclopramide, indicative of a prejunctional site of action. NMDA and DA suppressed ocular hypertension induced by water loading. Only NMDA depressed the IOP recovery rate in response to infusion of hypertonic saline indicating suppression of aqueous humor formation. These results suggest that DA, NMDA and DMDA produce an initial ocular hypertension by contracting extraocular muscles. Timolol antagonized the ocular hypotensive effect of NMDA in SX rabbits indicating that this response is, in part, a function of beta-adrenoceptor stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory dopamine receptors on sympathetic neurons innervating the cardiovascular system of the pithed rat. Characterization and role in relation to presynaptic alpha 2-adrenoceptors

Additional experimental evidence was obtained for an inhibitory function of prejunctional alpha 2-adrenoceptors and/or dopamine receptors located on noradrenergic neurons innervating the heart and resistance vessels of the pithed normotensive rat. Mixed alpha 2-adrenoceptor/dopamine receptor agonists, differing in selectivity towards either receptor type, i.e. N,N-di-n-propyldopamine (DPDA), 2-N, N-di-n-propylamino-6, 7-dihydroxy-1,2,3,4-tetrahydro-naphthalene (DP-6,7-ADTN), B-HT 920 and B-HT 933 (azepexole) were used. In pithed normotensive rats, DPDA (30 and 100 micrograms/kg/min) dose-dependently inhibited the electrical stimulation-induced increase in diastolic pressure, but did not significantly affect the stimulation-evoked increase in heart rate. The inhibition exerted by DPDA was blocked by haloperidol and sulpiride (0.3 mg/kg of each), but not by yohimbine (1 mg/kg), indicating the involvement of dopamine receptors. In this respect, sulpiride and haloperidol were found approximately equipotent. DP-6,7-ADTN (10 and 30 micrograms/kg/min) impaired both tachycardic and vasoconstrictor responses in a dose-dependent manner. Sulpiride (0.3 mg/kg) only partially restored the DP-6,7-ADTN-depressed stimulation-evoked increase in diastolic pressure, whereas yohimbine (1 mg/kg) alone was without effect. The combination of both antagonists completely prevented the inhibition caused by DP-6,7-ADTN. On the other hand, yohimbine (1 mg/kg), but not sulpiride (0.3 mg/kg), selectively antagonized the DP-6,7-ADTN-induced inhibition of stimulation-evoked tachycardia. B-HT 920 (1, 3 and 10 micrograms/kg/min) very effectively reduced the increase in diastolic pressure and heart rate caused by electrical stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Ocular hypotensive action of ergoline derivatives in rabbits: effects of sympathectomy and domperidone pretreatment

Ergot derivatives, such as bromocriptine, lergotrile and pergolide, are potent dopaminergic agonists in various biological systems. In topical doses ranging from 0.001 to 1% applied unilaterally, each agent produced dose-related ocular hypotension in normal rabbits. Utilizing an intraocular pressure (IOP) recovery rate method (aqueous formation index), each agent was observed to suppress the recovery rate of IOP in normal rabbits. Pretreatment with a prejunctional dopamine receptor antagonist (domperidone) inhibited the ocular hypotensive effect of bromocriptine and pergolide more than that of lergotrile. In rabbits with unilateral superior cervical ganglionectomies, IOP was lowered appreciably less by these compounds in the denervated eyes. These studies indicate that: a) lowering of IOP by these ergot derivatives is dependent, in part, on suppression of sympathetic neuronal activity; b) the most probable sites of action are DA2 receptors on sympathetic nerve endings or in sympathetic ganglia; c) ocular hypotension is produced, in part, by suppressing aqueous humor formation.

Effects of dopamine on neuromuscular transmission in the guinea-pig mesenteric artery

The effects of dopamine on neuromuscular transmission in the guinea-pig mesenteric artery were investigated using a microelectrode method. Dopamine did not modify the membrane potential or the membrane resistance of the smooth muscle, but did reduce the amplitude of excitatory junction potentials (e.j.p.) and enhance the facilitation of e.j.p. produced by repetitive stimulation. Phentolamine (10(-7) M) enhanced the amplitude and the facilitation of the e.j.p., and with the addition of dopamine (10(-6) M) there was a reduction in the amplitude of e.j.p. but not in the facilitation. Haloperidol and sulpiride (greater than 10(-6) M) increased the amplitude of e.j.p. without altering the postjunctional membrane properties. Haloperidol and sulpiride did not increase the facilitation of e.j.p. produced by repetitive stimulation. In the presence of haloperidol or sulpiride (10(-5) M), dopamine (10(-6) M) did not suppress the amplitude of the e.j.p. These results indicate that in the guinea-pig mesenteric artery, dopamine inhibits the release of transmitter at the presynaptic membrane.

Effects of dopamine and apomorphine on the response of rabbit isolated atria to sympathetic nerve stimulation

In isolated rabbit atria, dopamine (10(-6) M to 3 X 10(-6) M), in the presence of cocaine and atropine, inhibits the chronotropic responses to electrical stimulation of the sympathetic nerves without influencing the responses to isoprenaline. The inhibitory effect of dopamine is antagonized by cis-flupenthixol in a concentration (2.5 X 10(-6) M) that does not antagonize the inhibitory effect of clonidine. Phentolamine, in a concentration (10(-5) M) that antagonizes clonidine, does not influence the inhibitory effect of dopamine. Apomorphine (10(-6) M) also produces an inhibition of the nerve stimulation-induced chronotropic responses that can be prevented by cis-flupenthixol. These data suggest that prejunctional dopamine receptors are present in rabbit atria.

Antagonist properties of phentolamine at both presynaptic alpha 2-adrenoceptors and presynaptic dopamine receptors using field stimulated right cat atria

Phentolamine, which is considered a specific alpha-adrenoceptor antagonist, was tested for antagonist properties at presynaptic dopamine receptors and presynaptic alpha 2-adrenoceptors present on sympathetic nerve terminals in isolated right cat atria. Field stimulation induced a transient tachycardia which was inhibited by stimulation of presynaptic dopamine receptors using apomorphine or by stimulation of presynaptic alpha 2-adrenoceptors using clonidine. The presynaptic inhibitory effects of apomorphine were competitively antagonized by sulpiride, which is considered a specific dopamine receptor antagonist, and by phentolamine and yohimbine which are considered alpha-adrenoceptor antagonists. The presynaptic inhibitory effects of clonidine were competitively antagonized by phentolamine and yohimbine but not by sulpiride. pA2 values for phentolamine against apomorphine and phentolamine against clonidine suggest that phentolamine may be an antagonist at both presynaptic dopamine receptors and presynaptic alpha 2-adrenoceptors.

Mechanisms involved in the blocking effects of pimozide on smooth muscle contraction

Pimozide, a dopamine-receptor antagonist, shifted to the right the concentration-response curves (CRC) induced by (-)-noradrenaline in the isolated posterior mesenteric artery of the cat. Since this antagonism could be related to a blockade of vascular alpha-adrenoceptors, we compared the effects of pimozide (0.20 microM) with those of phentolamine (0.31 microM). The onset of the blockade by pimozide on the (-)-noradrenaline-induced contractions was slower when compared with that of phentolamine. In addition, the effects of pimozide did not reverse by washing while those of phentolamine did. Pimozide but not phentolamine inhibited the contractions elicited by potassium and it decreased the maximum responses of the curves. This effect was partially reversed by CaCl2 10.2 mM. Pimozide displaced the CRC induced by CaCl2 to a greater extent than phentolamine. Even if a blocking effect on alpha-adrenoceptors cannot be discarded, our results suggest that pimozide may interfere with calcium in the arterial smooth muscle. Furthermore, vascular smooth muscle seems to be more sensitive than non-vascular tissue to the action of pimozide.

Presynaptic dopamine receptors mediate the inhibitory action of dopamine agonists on stimulation-evoked pressor responses in the rat

The effects of the dopamine agonists TL-99, M-7 (N, N-dimethyl analogues of aminotetralins) and N, N-dinpropyldopamine (NNPD) on stimulation-evoked pressor responses and tachycardia in pithed Sprague-Dawley rats were investigated when pressor responses to the compounds per se had subsided. Various antagonists were used to characterise the effects of the dopamine agonists. 2 M-7 (3 micrograms/kg i.v.), and NNPD (1 mg/kg i.v.), but not TL-99 (1-30 micrograms/kg i.v.), inhibited pressor responses evoked by low frequency electrical stimulation of the spinal cord in the pithed rat. 3 M-7 (3 micrograms/kg i.v.), but neither NNPD (1 mg/kg i.v.) nor TL-99 (1-30 micrograms/kg), inhibited tachycardia evoked by low frequency electrical stimulation of the spinal cord in the pithed rat. 4 The inhibition of stimulation-evoked pressor responses by M-7 and NNPD was prevented by pimozide, metoclopramide and sulpiride but not by yohimibine, atropine, cimetidine or propranolol. 5 The inhibition of stimulation-evoked tachycardia by M-7 was prevented by yohimbine (and to a certain extent by sulpiride) but not pimozide, metoclopramide, atropine or cimetidine. 6 Pressor responses elicited by TL-99, M-7 and NNPD were selective antagonised by yohimbine, but not by prazosin, indicating that these responses were mediated by stimulation of vascular postsynaptic alpha 2-adrenoreceptors. 7 This study demonstrates that, in the rat, presynaptic dopamine receptors exist on sympathetic pre- or postganglionic nerve endings to blood vessels, but not on sympathetic pre- or postganglionic nerve endings to the heart, where inhibition by M-7 of stimulation-evoked tachycardia is mediated by stimulation of presynaptic alpha 2-adrenoreceptors.

Cardiovascular dopamine receptors: physiological, pharmacological and therapeutic implications

Dopamine receptor activation can lead to pronounced changes in cardiovascular function. The myriad of effects produced by dopamine receptor agonists results from the activation of dopamine receptors located at different anatomical sites in the cardiovascular system. Further basic research is required to better characterize these dopamine receptors so as to allow the development of more specific dopamine receptor agonists. Endogenous dopamine may be involved in the physiological control of fluid and electrolyte balance and continuing research efforts in this area should provide for a better understanding of the role of cardiovascular dopamine receptors in the maintenance of overall circulatory homeostasis. Cardiovascular dopamine receptor stimulation represents an important and promising approach for the development of novel therapeutic agents for the treatment of hypertension, angina pectoris, congestive heart failure and acute renal failure.

Dopamine decreases mesenteric blood flow in the anaesthetised dog through the stimulation of postsynaptic alpha 2-adrenoceptors

The decrease in mesenteric blood flow produced by dopamine administered intra-arterially in the anaesthetised dog was investigated by means of drugs selective for alpha 1- and alpha 2-adrenoceptors. The selective alpha 1-adrenoceptor agonist phenylephrine (0.3-100 microgram) given by intra-arterial injection (i.a.) into the superior mesenteric artery of the anaesthetized dog produced a decrease in mesenteric blood flow which was preferentially blocked by the alpha 1-adrenoceptor antagonist prazosin (30-300 microgram/kg i.v.). On the other hand, i.a. injections of the selective alpha 2-adrenoceptor agonist M7 (1-100 microgram) or of dopamine (1-300 microgram) produced a decrease in mesenteric blood flow which was blocked by the alpha 2-adrenoceptor antagonist yohimbine (100-300 microgram/kg i.v.) but was not significantly reduced by prazosin (300 microgram/kg i.v.). These results demonstrate that the mesenteric vascular bed of the dog contains both alpha 1- and alpha 2-adrenoceptors located postsynaptically and mediating vasoconstriction. The decrease in mesenteric blood flow produced by i.a. injections of dopamine is mediated predominantly via postsynaptic alpha 2-adrenoceptors.

Effects of dopaminergic agonists on somato-autonomic reflexes

In cats lightly anesthetized with urethane (600 mg/kg, i.p.) reflexes of the blood pressure (BP) and of the nictitating membrane (NM) were elicited by stimulation of the sciatic nerve (16 V, 0.3 ms, 1-128 Hz, 2 s or 2 min) prior to and after the administration of apomorphine (0.05-0.2 mg/kg, i.v.) or piribedil (0.4-1.0 mg/kg, i.v.). In case of short-train (2 s) stimulation, both dopaminergic agonists shifted the frequency-response curves of NM contractions to the right, i.e. depressed NM reflexes in the entire range of the stimulation frequencies applied. At the same time, BP reflexes were depressed only in the range of lower frequencies (1-4 Hz). At higher rates (32-128 Hz) BP reflexes were potentiated. The reactions of BP to sustained (2 min) stimulations displayed a flat pressor plateau in response to lower-frequency stimulation, and a two-component pattern (an initial pressor peak followed by a plateau) to the higher-frequency one. Compatibility with the effects seen to short-train stimulations, the dopaminergic agonists prolonged the rise-time and augmented the amplitude of the initial pressor peak to sustained stimulations with lower and higher frequencies, respectively. The plateau of the pressor response to higher frequencies was depressed by higher doses (greater than 0.4 mg/kg) of piribedil. Administration of haloperidol (0.05-0.2 mg/kg, i.v.) resulted only in a partial restoration of the reflexes of BP and NM. The manifold effects of dopaminergic agonists on the somato-autonomic reflexes studied support the thought than NM and BP reflexes are organized, at least partially, in different ways.

Interactions between dopamine and prostaglandins on vascular reactivity to noradrenaline: dopamine inhibits the action of PGE1

Interactions between dopamine and responses to noradrenaline as modulated by prostaglandins (PGs) were studied in the perfused rat mesenteric vascular bed. When perfused alone dopamine up to a concentration of 10(-7)M neither changed baseline pressure nor modified the pressor response to noradrenaline. Dopamine at 10(-9) to 10(-7)M significantly inhibited responses to noradrenaline which had been enhanced by the presence of 10(-10) to 10(-8) M PGE1. In preparations in which vascular responses to noradrenaline had been abolished by indomethacin and restored by adding PGE1, 10(-9) to 10(-7)M dopamine significantly inhibited the restored responses. Dopamine also attenuated the inhibitory effects of prostacyclin on pressor responses to noradrenaline but it did not change the actions of either PGE2 or PGF2 alpha. Pimozide, a mainly centrally acting dopamine antagonist, did not interfere with these peripheral actions of dopamine. The dopamine effects were blocked by another dopamine antagonist, metoclopramide. Dopamine can inhibit the effects of PGE1 and prostacyclin in the rat mesenteric vascular bed, possibly by interacting with specific dopamine receptors.

Presynaptic dopamine receptors and alpha-adrenoceptors as mediators of the bradycardic action of N-n-propyl-N-n-butyl dopamine

The role of presynaptic receptors in the bradycardic action of N-n-butyl dopamine (PBDA) was investigated. A biphasic effect on blood pressure and a decrease in heart rate were seen upon intravenous administration of PBDA to pentobarbital-anesthetized dogs. The bradycardia produced by PBDA was unaffected by bilateral vagotomy; however, it was abolished by cardiac sympathetic denervation. When PBDA was readministered following restoration of the cardiac sympathetic nerve activity by electrical stimulation, at frequency of 1 Hz, decrease in heart rate was again observed. The cardioinhibitory action of PBDA was completely abolished by sulpiride, whereas phentolamine and yohimbine caused only partial attenuation, suggesting the involvement of both presynaptic alpha-adrenoceptors as well as dopamine receptors in the bradycardiac action of PBDA. Additional experiments were performed to study the influence of stimulus frequency on the cardioinhibition produced by PBDA. Administration of PBDA to animals with different levels of cardiac sympathetic nerve activity (0.25-2 Hz) resulted in decreases in heart rate. However, yohimbine antagonized this action of PBDA only at the two higher frequencies of cardiac nerve stimulation. Sulpiride completely abolished the bradycardia observed at all the different frequencies of cardiac nerve stimulation. These results demonstrate that activation of presynaptic receptors on cardiac sympathetic nerves can result in a decrease in heart rate. PBDA causes bradycardia via an action on presynaptic dopamine receptors when the cardiac sympathetic nerve activity is low, while both presynaptic dopamine receptors as well as alpha-adrenoceptors are involved in the decrease in heart rate produced by this compound at higher levels of sympathetic nerve activity.