Pre- and postsynaptic effects of sulpiride in the rat isolated vas deferens

Effect of noni (Morinda citrifolia Linn.) fruit and its bioactive principles scopoletin and rutin on rat vas deferens contractility: an ex vivo study

This study examined the effect of methanolic extract of Morinda citrifolia Linn. (MMC) and its bioactive principles, scopoletin and rutin, on dopamine- and noradrenaline-evoked contractility in isolated rat vas deferens preparations. MMC (1-40 mg/mL), scopoletin (1-200 μg/mL), and rutin hydrate (0.6-312.6 μg/mL) dose-dependently inhibited the contractility evoked by submaximal concentrations of both dopamine and noradrenaline, respectively. Haloperidol and prazosin, reference dopamine D2, and α 1-adrenoceptors antagonists significantly reversed the dopamine- and noradrenaline-induced contractions, respectively, in a dose-dependent manner. Interestingly, MMC per se at higher doses (60-100 mg/mL) showed dose-dependent contractile response in rat vas deferens which was partially inhibited by high doses of haloperidol but not by prazosin. These results demonstrated the biphasic effects of MMC on dopaminergic system; that is, antidopaminergic effect at lower concentrations (<40 mg/mL) and dopaminergic agonistic effect at higher concentrations (>60 mg/mL). However, similar contractile response at high doses of scopoletin (0.5-5 mg/mL) and rutin hydrate (0.5-5 mg/mL) per se was not observed. Therefore, it can be concluded that the bioactive principles of MMC, scopoletin, and rutin might be responsible for the antidopaminergic and antiadrenergic activities of MMC.

Localization of peripheral dopamine D1 and D2 receptors in rat and human seminal vesicles

Dopamine, an established neurotransmitter in the central nervous system, is recognized for its role in penile erection and ejaculation in rats. However, its complete mechanism of action in the genitourinary tract is unknown. The objective of this study was to investigate the existence and expression of peripheral dopamine D1 and D2 receptor messenger RNAs (mRNAs) and corresponding proteins in rat and human seminal vesicles. The seminal vesicle tissues of male Sprague-Dawley rats and human radical prostatectomy specimens were used to extract total RNA and proteins, and to prepare slide sections. Rat hypothalamus tissue served as a control for dopamine D1 and D2 receptors. Testing for the presence and expression of peripheral dopamine D1 and D2 receptor mRNAs in rat and human seminal vesicle tissues was performed by reverse transcription-polymerase chain reaction. Western blotting was used to detect corresponding proteins of D1 and D2 receptors. Immunohistochemical staining using rabbit antipeptide polyclonal antibodies was employed to identify and anatomically localize dopamine D1 and D2 receptor proteins in rat and human seminal vesicles. Dopamine D1 and D2 receptor transcripts were detected in both human and rat seminal vesicle tissues. Western blot analysis demonstrated that peripheral dopamine D1 and D2 receptor proteins exist in both human and rat seminal vesicle tissues. Immunohistochemical analysis demonstrated the localization of peripheral dopamine D1 and D2 receptors to the smooth muscle layer of human and rat seminal vesicles. The results of this study demonstrate that peripheral dopamine D1 and D2 receptors are present in the seminal vesicle tissue in both rats and humans. Although these results suggest that seminal emission may be mediated in part by the stimulation of peripheral dopamine receptors located in the seminal vesicles, the functional significance of dopamine in male reproductive tract has yet to be fully defined.

Existence of postsynaptic dopamine D2 receptor as an enhancer of contractile response in vas deferens

Effects of dopamine and (+/-)-2-(N-phenylethyl-N-propyl)amino-5-hydroxy-tetralin hydrochloride (N-0434), a dopamine D2 receptor agonist, in the presence of prazosin on the ATP- and acetylcholine-induced contraction were investigated in the guinea-pig vas deferens in order to test for the existence of postsynaptic dopamine receptors. The contraction induced by ATP was potentiated by dopamine and N-0434. This potentiation was antagonized by spiperone, a dopamine D2 receptor antagonist, but not by a dopamine D1 receptor antagonist and an alpha2-adrenoceptor antagonist. Similar results were also observed by acetylcholine as well as ATP. The contraction induced by transmural nerve stimulation in the presence of alpha-adrenoceptor antagonists was also potentiated by N-0434, and this potentiation was antagonized by spiperone. The results suggest that dopamine D2 receptors are located on the postsynaptic site of guinea-pig vas deferens and that the contractile responses to ATP and acetylcholine are potentiated via activation of dopamine D2 receptor.

Existence of dopamine D1 receptor on the sympathetic nerve endings in the guinea-pig vas deferens

The effects of selective dopamine receptor agonists and antagonists on sympathetic neuromuscular transmission were investigated in the guinea-pig vas deferens in order to test for the presence of presynaptic dopamine receptors. A single-pulse field stimulus induced a rapid monophasic contraction which was strongly inhibited by alpha,beta-methylene ATP, a P2X purinoceptor desensitizing agent. The contraction was also inhibited by 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK 14,304), a selective alpha2-adrenoceptor agonist. This inhibition was antagonized by idazoxan, an alpha2-adrenoceptor antagonist, but not by R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzaz epine hydrochloride (SCH-23390), a dopamine D1 receptor antagonist. Furthermore, the contractions were inhibited in a dose-dependent manner by R(+)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrochloride (SKF-38393) and (+/-)-6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-be nzazepine hydrobromide (SKF-82958), dopamine D1 receptor agonists, and the inhibition was antagonized by both SCH-23390 and idazoxan, but not by spiperone, a dopamine D2 receptor antagonist. The results suggest that dopamine D1 receptors are located on the sympathetic nerve endings of guinea-pig vas deferens.

Effects of some antipsychotic drugs on cardiovascular catecholamine receptors in the rat

1. Experiments were performed to determine the activity of four antipsychotic drugs on several catecholamine receptors that control the sympathetic cardiovascular responses in rats. 2. Chlorpromazine, thioridazine (0.03 and 0.1 mg kg-1) and haloperidol (0.3 and 1 mg kg-1) inhibited methoxamine-induced diastolic blood pressure increases in the pithed rat, whereas sulpiride (1 and 3 mg kg-1) was without effect. 3. Only sulpiride (3 mg kg-1) antagonized the pressor responses induced by xylazine. 4. Xylazine inhibited the heart rate increase induced by electrical stimulation of the spinal cord (C7-Th1) in the pithed rat. This effect was partially prevented by sulpiride (1 and 3 mg kg-1) and chlorpromazine (0.3 mg kg-1). A higher dose of chlorpromazine (1 mg kg-1) abolished the inhibitory effect of xylazine. 5. Apomorphine infusion inhibited the pressor responses induced by electrical stimulation (Th5-L4) in pithed rats. This effect was reversed by sulpiride (0.01, 0.03 and 0.1 mg kg-1) and partially antagonized by haloperidol (0.1 mg kg-1). 6. The depressor response to fenoldopam in anaesthetized rats was only inhibited by the higher dose of chlorpromazine and thioridazine (3 mg kg-1). 7. Our results suggest that, in the peripheral nervous system of the rat, haloperidol and sulpiride act as antagonists of DA2 receptors while chlorpromazine and thioridazine antagonized DA1 receptors. Furthermore, thioridazine and haloperidol show alpha 1-adrenoreceptor antagonist properties, whereas sulpiride antagonizes alpha 2-adrenoreceptors. Chlorpromazine shows mixed alpha 1/alpha 2-adrenoreceptor antagonism.

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.

Effects of ergotamine on cardiovascular catecholamine receptors in the pithed rat

1. Ergotamine (3-10 micrograms/kg) inhibited the electrical stimulation-induced pressor and cardiac responses without modifying pressor responses of noradrenaline and tyramine in the pithed rat. 2. Yohimbine (0.3 mg/kg) partially prevented the ergotamine cardiac and vascular inhibitory effects but sulpiride (0.3 mg/kg) only prevented it at vascular level. Both antagonists together abolished the ergotamine inhibition of electrical stimulation-induced pressor responses. 3. The cumulative dose-response curve of ergotamine (1-100 micrograms/kg) vasoconstrictor effects was partially inhibited to the same extent by prazosin (1 mg/kg) and yohimbine (0.3 mg/kg). A greater inhibition was observed with both antagonists administered together. 4. Ergotamine (30 micrograms/kg), in presence of yohimbine, inhibited the pressor responses of methoxamine, without any effect on xylazine pressor responses. 5. These data indicate that ergotamine acts as an agonist of both the presynaptic dopamine receptors and alpha 2-adrenoceptors, of alpha 1 and alpha 2-postsynaptic adrenoceptors, and also as an antagonist of the postsynaptic alpha 1-adrenoceptors.

Actions of dopamine and related amines on reserpinized and chronically denervated vasa deferentia of the rat

1. Reserpine, chronic guanethidine denervation and alpha-adrenoceptor antagonists were used to distinguish between presynaptic and postsynaptic actions of exogenous dopamine (DA), octopamine (OA), tyramine (TA) and noradrenaline (NA) on the rat vas deferens. 2. TA has only a presynaptic action while DA and OA have mixed presynaptic actions (releasing endogenous NA) and postsynaptic actions. 3. The postsynaptic actions of DA and OA are likely to be mediated by alpha-adrenoceptors rather than by specific receptors.

Presynaptic actions of piribedil on the cardiovascular system of the pithed rat

The influence of piribedil on cardiovascular sympathetic responses has been studied in the pithed rat. Piribedil (0.3-1 mg kg-1) inhibited the increases of diastolic blood pressure induced by spinal cord electrical stimulation at the level Th5-L4. This effect was reversed by sulpiride (0.3 mg kg-1) but not by yohimbine (0.3 mg kg-1). The cardiovascular responses induced by noradrenaline were unaffected by piribedil (0.3-1 mg kg-1). However piribedil (0.3-1 mg kg-1) did not modify the heart rate increase induced by spinal cord electrical stimulation at the C7-Th1 level. These results suggest that piribedil inhibits the vascular sympathetic transmission in the pithed rat via stimulation of presynaptic dopamine receptors.

Effects of bromocriptine on catecholamine receptors mediating cardiovascular responses in the pithed rat

The interaction of bromocriptine with several catecholamine receptors that control the sympathetic responses at cardiac and vascular level has been studied in pithed adrenalectomized and vagotomized normotensive rats. Bromocriptine (30 and 100 micrograms/kg) inhibited the stimulation-induced pressor responses in the pithed rat without modifying the pressor responses induced by noradrenaline. Sulpiride (0.3 mg/kg) abolished the effects of bromocriptine (30 micrograms/kg) but only partially prevented the effects of bromocriptine (100 micrograms/kg) on the stimulation-induced pressor responses. Yohimbine (0.3 mg/kg) partially antagonised the inhibitory effect of bromocriptine on stimulation-induced pressor responses. Combination of yohimbine and sulpiride abolished attenuation of the stimulation-induced pressor responses by bromocriptine (100 micrograms/kg). Bromocriptine (0.3 and 1 mg/kg) shifted to the right the frequency-response curve of increases in heart rate. This effect was prevented by yohimbine (0.3 mg/kg) but not by sulpiride (0.3 mg/kg). The same doses of bromocriptine were ineffective on heart rate increases induced by noradrenaline. Bromocriptine (0.3 and 1 mg/kg) shifted to the right the increases in diastolic blood pressure induced by methoxamine without modifying those induced by xylazine and noradrenaline. These results suggest that bromocriptine acts on the peripheral sympathetic nervous system of the pithed rat as an agonist of presynaptic dopamine receptors and alpha 2-adrenoreceptors and as an antagonist of postsynaptic alpha 1-adrenoreceptors.

Characterization of the contractile activity of dopamine on the rat isolated seminal vesicle

The mechanism of the contractile effect of dopamine (DA) on the rat isolated seminal vesicle was studied. Cocaine (10 microM/1 in the organ bath, 30 min before DA) and 6-OHDA (50 mg/kg i.v. 24 hr before removal of the seminal vesicle) almost completely prevented the contractile effect of DA. Drugs known to have an affinity for DA receptors or for alpha-adrenoceptors antagonized the contractile effect of DA, the rank order of potency being: prazosin greater than phentolamine greater than yohimbine greater than clonidine greater than sulpiride greater than apomorphine greater than haloperidol. The antagonism was in each case greater against DA than against noradrenaline (NA), used for comparison; selectivity for DA being highest in the case of prazosin and sulpiride. Taken together, these findings indicate that DA makes the rat seminal vesicle contract mostly by means of an indirect mechanism, binding presynaptic DA-receptors and, in part, presynaptic alpha-adrenoceptors as well; or, alternatively, binding presynaptic DA-receptors which have some links with alpha 2-adrenoceptors; the consequence being in either case the release of NA from sympathetic nerve endings.

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)

Dopamine acts at the same receptors as noradrenaline in the rat isolated vas deferens

1 The proposal that dopamine activates a different population of receptors from those activated by noradrenaline and phenylephrine to cause contraction of the rat vas deferens has been investigated using a preparation of the epididymal half of this tissue. 2 In preparations preincubated in cocaine, oestradiol and propranolol, to block sites of amine loss and beta-adrenoceptors, noradrenaline was the most, and dopamine the least, potent of the three agonists. Phentolamine competitively inhibited each of the agonists to a similar extent. Prazosin also inhibited the actions of the three agonists to a similar extent. These results indicate that the three agonists activate a single population of alpha 1-adrenoceptors to cause contraction in this preparation. 3 In experiments using the prostatic half of the rat vas deferens, in the presence of cocaine, oestradiol, propranolol and prazosin, noradrenaline was approximately 40 times more potent than dopamine in causing inhibition of twitches induced by electrical field stimulation. Yohimbine competitively antagonized the effects of the two agonists to a similar extent indicating that both act at the same population of alpha 2-adrenoceptors. 4 Taken together, these findings do not lend support to proposals that there are populations of specific dopamine receptors located pre- and postjunctionally in the rat vas deferens.