Analysis of presynaptic inhibitory actions of various dopamine analogs on sympathetic neurotransmission in mongrel dogs

Inhibitory effects of apomorphine and pergolide on neurogenic vasoconstriction in the hindquarters of the rat

The effects of locally administered apomorphine and pergolide were studied in the isolated autoperfused hindquarters of the rat, in an attempt to assess the possible role of presynaptic dopamine receptors at that level in the hypotensive effect of these dopamine agonists. Local infusion of apomorphine (1 micrograms . kg-1 . min-1 for 5 min) or pergolide (1 micrograms . kg-1 . min-1 for 5 min) [into the hindquarters] did not alter perfusion pressure per se, but reduced the pressor response to electrical stimulation of the lumbar sympathetic chains for the whole frequency range used during a cumulative frequency-response curve (0.25-16 Hz, 1 ms, supramaximal voltage). Apomorphine and pergolide reduced the pressor response elicited by 4 Hz electrical stimulation (applied until maximum response was reached) to 54.8 +/- 7.1% and 53.9 +/- 1.7% respectively, but they did not modify similar increases of perfusion pressure produced by locally administered noradrenaline. The inhibition by apomorphine and pergolide of the 4 Hz stimulation-evoked pressor response was completely antagonized by local administration of the dopamine antagonist haloperidol (1 microgram . kg-1), but was not influenced by the alpha 2-antagonist rauwolscine (100 micrograms . kg-1). This dose of rauwolscine antagonized the inhibitory effect of the alpha 2-agonist UK-14,304, which was not influenced by haloperidol. Local administration of rauwolscine increased the pressor response to stimulation at 4 Hz by 37.4-46.2%. In contrast, local administration of haloperidol did not influence the 4 Hz stimulation-evoked pressor response.(ABSTRACT TRUNCATED AT 250 WORDS)

Central and peripheral dopaminergic mechanisms in the cardiovascular actions of pergolide in neurogenic hypertensive dogs

The cardiovascular actions of the dopaminergic ergoline, pergolide were examined in pentobarbital-anesthetized dogs. When administered intravenously (i.v.) to dogs made hypertensive by sino-aortic deafferentation, pergolide produced a sustained reduction in blood pressure, dilatation of the hindlimb vasculature and bradycardia. The antihypertensive action of pergolide in neurogenic hypertensive dogs was converted into a pressor action when it was given to dogs pretreated with i.v. sulpiride or hexamethonium plus atropine. Pergolide caused significant impairment of renal vasoconstriction elicited during stimulation of renal sympathetic nerves but not that caused by exogenous norepinephrine. The inhibitory action of pergolide on renal sympathetic nerve function was antagonized by sulpiride suggesting that pergolide activated presynaptic dopamine receptors. Direct administration of pergolide within the central nervous system via the cisterna magna (i.c.) to normotensive dogs also resulted in hypotension, bradycardia and iliac vasodilatation. The central actions of pergolide were prevented by sulpiride (i.c.) but not by yohimbine (i.c.), which indicates that specific activation of central dopamine receptors was responsible for these actions of pergolide. The antihypertensive, bradycardic and iliac vasodilatory actions of i.v. pergolide in neurogenic hypertensive dogs were significantly attenuated by i.c. sulpiride. It was determined that while this dose of sulpiride antagonized central dopamine receptors, it did not antagonize peripheral presynaptic dopamine receptors. These results suggest that pergolide exerts antihypertensive and bradycardic actions via simultaneous stimulation of dopamine receptors in the brain and on postganglionic sympathetic neurons. While presynaptic dopamine receptor activation contributes to the blood pressure lowering action of pergolide, stimulation of central dopamine receptors appears to be the dominant mechanism by which pergolide elicits the observed antihypertensive and bradycardic effects. Pergolide appears to lower blood pressure by a novel mechanism of action and may offer an additional therapeutic approach for the treatment of hypertension.

The role of dopaminergic receptors in the behavioral effects induced by lisuride in male rats

Lisuride increased the incidence of stretching and yawning (SY) as well as of penile erection (PE) and elicited stereotyped behavior (SB), aggressive behavior and mounting in male rats, depending on the dose used. SY was prevented by two dopaminergic antagonists, haloperidol and sulpiride, but not by methysergide, a serotoninergic antagonist, while PE was antagonized by all three drugs. With regard to SB, aggressive behavior and mounting, all three were suppressed by haloperidol; sulpiride, while partially antagonizing aggressiveness, failed to affect SB and mounting; methysergide did not significantly influence any of the three. This suggests that lisuride principally affects the dopaminergic system. Although further detailed studies are required to elucidate which type of the complex population of DA-receptors is involved in each kind of behavior, we suggest that SY at least is due to the activation by lisuride of presynaptic DA-receptors.

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.

Evaluation of peripheral dopamine receptor and alpha-adrenoceptor blocking activity of sulpiride

The effect of sulpiride, a dopamine receptor antagonist, on peripheral alpha-adrenoceptors and dopamine receptors was determined. Positive chronotropic responses to cardioaccelerator nerve stimulation in pentobarbital-anesthetized dogs were potentiated by 1.0 and 2.0 mg/kg but not by 0.5 mg/kg intravenous sulpiride. This effect persisted after neuronal uptake blockade with desipramine, but was prevented by alpha 2-adrenoceptor blockade with yohimbine. Positive chronotropic effects of intravenous norepinephrine were unchanged by sulpiride, suggesting that sympathetic nerve function was facilitated via a presynaptic mechanism. Since yohimbine prevented the facilitatory action of sulpiride, an analysis of the ability of sulpiride to antagonize alpha-adrenoceptors was made. The reduction in stimulus-induced tachycardia caused by the alpha 2-adrenoceptor agonist tramazoline was significantly antagonized by sulpiride. Furthermore, while sulpiride did not antagonize the pressor effect of the alpha-adrenoceptor agonist phenylephrine, it significantly attenuated the increases in blood pressure produced by tramazoline. The alpha 2-adrenoceptor blocking action of sulpiride lasted for approximately 15 min at 1.0 mg/kg adn for 90 min at 2.0 mg/kg. In additional experiments, it was determined that the dopamine receptor-mediated bradycardic and hypotensive effects of N,N-di-n-propyldopamine were antagonized for at least 2 h by 0.1, 0.5, and 1.0 mg/kg sulpiride. These studies establish that peripheral neuronal and vascular dopamine receptors may be antagonized by sulpiride without affecting alpha-adrenergic mechanisms. However, at doses of 1.0 mg/kg and higher, sulpiride facilitates sympathetic nerve function via a preferential antagonism of alpha 2-adrenoceptors.

Dopaminergic inhibition of cardiac sympathetic nerve function by pergolide

The effect of pergolide, a dopaminergic ergoline, on cardiac sympathetic neurotransmission was examined in pentobarbital-anesthetized dogs. Positive chronotropic responses to right postganglionic cardioaccelerator nerve stimulation were significantly reduced following infusion of pergolide, 1.25 microgram/kg per min for 10 min, whereas positive chronotropic responses to exogenous norepinephrine were unchanged. Pretreatment with sulpiride but not with yohimbine completely antagonized the inhibitory effect of pergolide. These results demonstrate that pergolide inhibits sympathetic neurotransmission to the heart by a specific activation of presynaptic 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.

The role of presynaptic receptors in the cardiovascular actions of N,N-di-n-propyldopamine in the cat and dog

Intravenous administration of N,N-di-n-propyldopamine (DPDA: 50 and 200 micrograms/kg/min, i.v.) produces hypotensive and bradycardic effects in anaesthetized cats and dogs. These effects were abolished by ganglionic blockade and antagonized by haloperidol or (SR)-sulpiride suggesting a neurogenic mechanism of action, mediated by specific dopamine receptors. The renal blood flow increases to DPDA in dogs were resistant to ganglionic blockade indicating some activity at postsynaptic vascular dopamine receptors. Studies with DPDA in vivo administered via the intravertebral and intravenous routes suggested a peripheral site of action for the hypotensive effects of this compound. In vitro, in isolated perfused cat spleens prelabeled with 3H-noradrenaline, DPDA (0.01--1 microM) produced a concentration-dependent inhibition of tritium release elicited by nerve stimulation at 1 Hz which was selectively antagonized by 1 microM (SR)-sulpiride implicating presynaptic inhibitory dopamine receptors in the mediation of this effect. In isolated rabbit splenic arteries, contracted by prostaglandin-F2 alpha, dopamine,ADTN (2-amino-6,7-dihydroxytetrahydronaphthalene) and apomorphine produced concentration-dependent relaxations while DPDA exhibited only weak postsynaptic dopamine-like effects. In vivo DPDA reduced, in a frequency dependent manner, the end organ responses to sympathetic nerve stimulation in the cat nictitating membrane and in the dog renal vascular bed. Both effects were mediated through activation of presynaptic inhibitory dopamine receptors by DPDA. In conclusion these results suggest a predominantly presynaptic agonist effect for DPDA in vitro and a similarly important action in vivo, mediated mainly via dopamine receptors. Furthermore evidence is presented which suggests that pre- and postsynaptic dopamine receptors may differ in their pharmacological properties and that presynaptic dopamine receptors could be important target receptors in the development of novel antihypertensive drugs.

Presynaptic inhibition by ergotamine of the responses to cardioaccelerator nerve stimulations in the cat

The effects of ergotamine on the tachycardia elicited by postganglionic stimulation of the cardioaccelerator nerve have been investigated in both intact and spinal (desmethylimipramine-pretreated) cats. Ergotamine (2.5--20 microgram kg-1) selectively antagonized the response to low-frequency (0.3--3 Hz) stimulation. The responses elicited by stimulation of the nerve with higher frequencies (greater than 10 Hz) either remained unaltered or were facilitatated by the drug. Pretreatment of the cats with pimozide (0.2 mg kg-1) completely antagonized the inhibitory effects of ergotamine on the sympathetic neuron. Although phentolamine (0.5 mg kg-1) did appear to attenuate the effects of ergotamine, the changes were, however, not statistically significant. Since the drug did not reduce the cardiac effects of isoprenaline or tyramine, it is concluded that ergotamine interferes with the release of noradrenaline from the cardioaccelerator nerve, mainly by stimulating the presynaptic dopaminergic receptors. This effect may be an important mechanism by which the drug causes bradycardia in almost all species regardless of the blood pressure response.

Analysis of the effects of bromocriptine on blood pressure and sympathetic nerve function

Bromocriptine, a dopamine receptor agonist, lowered arterial blood pressure and caused significant impairment of the cardioacceleration observed during the stimulation of the right postganglionic cardiac sympathetic nerve fibers in pentobarbital-anethetized dogs. Prior administration of pimozide antagonized the hypotensive and the inhibitory action of bromocriptine on cardiac sympathetic function. These results suggest that while presynaptic dopamine receptors seem to be involved in mediating the inhibitory action of bromocriptine on cardiac sympathetic function, the role of presynaptic as well as dopamine receptors located at other sites in mediating the hypotensive action of bromocriptine needs to be further investigated.

Facilitation of cardiac sympathetic function by angiotensin II: role of presynaptic angiotensin receptors

Intravenous infusion of two separate doses of angiotensin II in pentobarbital-anesthetized, desipramine-treated animals produced dose-related increases in arterial blood pressure and caused significant potentiation of the cardioacceleration observed during the stimulation of the right postganglionic cardiac sympathetic nerve fibers. Positive chronotropic effects of intravenous norepinephrine were not altered during angiotensin II infusion. Prior administration of Saralasin, an angiotensin receptor antagonist, caused significant attenuation of the pressor action of angiotensin II, and also significantly antagonized the facilitatory effect of angiotensin II on sympathetic transmission to the myocardium. These results suggest that angiotensin II can cause facilitation of sympathetic nerve function to the myocardium via an action on angiotensin receptors which may be located on sympathetic nerve terminals.

Presynaptic inhibitory effects of catecholamines on cholinergic transmission in the smooth muscle of the chick stomach

In the vagus nerve--smooth muscle preparation isolated from the chick proventriculus, adrenaline, clonidine (10(-8) - 2.5 x 10(-7) M), noradrenaline (10(-7) - 2.5 x 10(-6) M) and dopamine (10(-5) - 10(-4) M) inhibited the contraction induced by low frequency (0.5 Hz) stimulation of the vagus nerve, but they did not inhibit the contraction elicited by acetylcholine (5 x 10(-8) - 5 x 10(-7) M). The concentration producing 50% inhibition was 10(-7) M for adrenaline and clonidine, 10(-6) M for noradrenaline, and 5 x 10(-5) M for dopamine. Isoproterenol (5 x 10(-8) - 5 x 10(-7) M) inhibited the responses induced by both stimulation of the vagus nerve and acetylcholine. The inhibitory effects of the catecholamine and clonidine were blocked by phentolamine (2.7 x 10(-6) M) but not by 5-(3-tert-Butylamino-2-hydroxy)-propoxy-3, 4-dihydrocarbostyril hydrochloride (OPC 1085) which blocked the effect of isoproterenol. It is suggested that presynaptic alpha-receptors are present in the myenteric plexus of the chick proventriculus, and that the catecholamines and clonidine exert their inhibitory effects on cholinergic transmission via these receptors.

Presynaptic dopamine receptors as mediators of dopamine-induced inhibition of neurogenic vasoconstriction

I.v. infusion of dopamine (10 microgram/kg/min) caused significant attenuation of renal vasoconstrictor responses to sympathetic nerve stimulation in pentobarbital-anesthetized dogs, whereas renal vasoconstrictor responses to exogenously administered norepinephrine were unaffected. Desipramine treatment significantly potentiated the inhibitory effect of dopamine on neurogenic vasoconstriction. Pretreatment with pimozide, a dopamine receptor blocking agent, prevented the inhibitory influence of dopamine on renal sympathetic nerve function. These results demonstrate that dopamine can inhibit neurogenic vasoconstriction by activating presynaptic dopamine receptors present on renal sympathetic nerves.

Effects of several catecholamines on sympathetic transmission to the myocardium: role of presynaptic alpha-adrenoceptors

Effects of several alpha-adrenoceptor agonists on presynaptic alpha-adrenoceptors were evaluated by studying chronotropic responses to cardioaccelerator nerve stimulation in anesthetized dogs. I.v. infusions of norepinephrine (NE), methylnorepinephrine (MNE), epinephrine (E) or phenylephrine (PHE) in desipramine-treated dogs caused significant attenuation of nerve stimulation responses. The inhibitory influence of all these agents could be prevented by prior treatment with phentolamine, but not with haloperidol. Comparisons of relative pressor and presynaptic inhibitory actions of these compounds revealed that in equipressor doses, MNE caused a significantly greater attenuation of nerve stimmulation responses than NE, while PHE had similar pressor and presynaptic inhibitory activity to that of NE. The inhibition of chronotropic responses to nerve stimulation observed following E was significantly greater compared to its relative pressor effect. These results demonstrate the existence of a presynaptic alpha-adrenoceptor mechanism modulating cardiac rate in intact dogs and indicate that the false transmitter MNE may be more potent than NE in imparing neuronal transmission by an action on presynaptic alpha-adrenoceptors.