Pre- and postsynaptic effects of yohimbine stereoisomers on noradrenergic transmission in the pulmonary artery of the rabbit

The Inhibitory Effect of Dopamine on Cat Gastric Smooth Muscle

The inhibitory effect of dopamine has been studied in longitudinal and circular muscle strips of the cat gastric fundus. When tone was raised by transmural electrical stimulation and by administration of methacholine, dopamine concentration-dependently relaxed the strips but the inhibitory effect of dopamine was clearly more pronounced on electrically-induced tone. The effect of dopamine was not influenced by the presence of cocaine or hydrocortisone. The relaxant effect of dopamine, when tone was raised by methacholine, was not influenced by α- and dopamine receptor antagonists but it was significantly reduced by propranolol and ICI 118551 (erythro-dl-1-(7-methylindan-4-yloxy)-3-isopropylaminobutan-2-ol). The inhibitory effect of dopamine on the electrically-induced tone was significantly reduced by phentolamine; domperidone tended to reduce the effect of the lower concentrations of dopamine. In the presence of propranolol, phentolamine and rauwolscine concentration-dependently antagonized the inhibitory effect of dopamine on electrically-induced tone, while prazosin was without influence. These results indicate that the inhibitory effect of dopamine in the cat gastric fundus is mainly due to interaction with postjunctional β-adrenoceptors on the smooth muscle cells and with prejunctional α-adrenoceptors on the intramural cholinergic neurons

Urethane, but not pentobarbitone, attenuates presynaptic receptor function in rats: a contribution to the choice of anaesthetic

BACKGROUND AND PURPOSE

We examined whether cannabinoid CB(1) and histamine H(3) receptors resemble alpha(2)-adrenoceptors in that their presynaptically mediated cardiovascular effects are less marked in urethane- than in pentobarbitone-anaesthetized pithed rats.

EXPERIMENTAL APPROACH

Effects of the cannabinoid agonist CP-55,940 and the H(3) receptor agonist imetit on electrically induced tachycardic and vasopressor responses, respectively, was compared in pithed rats anaesthetized with urethane or pentobarbitone. The affinity of urethane for the three receptors was measured by radioligand binding studies in rat brain cortex membranes and its potency assessed in superfused mouse tissues preincubated with (3)H-noradrenaline.

KEY RESULTS

The neurogenic tachycardic response was less markedly inhibited by CP-55,940 in urethane- than in pentobarbitone-anaesthetized pithed rats. Imetit inhibited the neurogenic vasopressor response after pentobarbitone but not after urethane. The catecholamine-induced tachycardic and vasopressor response did not differ between rats anaesthetized with either compound. Urethane 10 mM (plasma concentration reached under anaesthesia) did not affect binding to CB(1) or H(3) receptors and alpha(2) adrenoceptors, nor did it alter the inhibitory effect of agonists at the three receptors on electrically evoked (3)H-noradrenaline release.

CONCLUSIONS AND IMPLICATIONS

Urethane, but not pentobarbitone, abolished the H(3) receptor-mediated vascular response in pithed rats and attenuated the CB(1) receptor-mediated cardiac response much more than pentobarbitone. The weaker effects of CB(1), H(3) and alpha(2) receptor agonists cannot be explained by antagonism by urethane at the three receptors in vitro. Pentobarbitone, but not urethane, is suitable as an anaesthetic for investigations of inhibitory presynaptic receptor function in pithed and anaesthetized rats.

Dual effect of agmatine in the bisected rat vas deferens

The functional effects of the amine agmatine, the putative endogenous ligand for alpha(2)-adrenoceptors and imidazoline receptors, in rat vas deferens were investigated by using the epididymal and prostatic portions. Tissues were contracted by electrical stimulation or by exogenous drugs. In electrically stimulated portions, agmatine caused a dual effect on contractions. In the epididymal portion an inhibition on twitch contractions was observed, which was partially antagonised by idazoxan and yohimbine, indicating the involvement of at least a presynaptic alpha(2)-adrenoceptor-mediated mechanism, without the interference of imidazoline receptors. In the prostatic portion, agmatine enhanced the amplitude of twitches. In contractions induced by exogenous drugs, agmatine potentiated, only in the prostatic segment, the effects of noradrenaline (norepinephrine) or ATP; it also enhanced the effect of low concentrations of KCl and blocked the maximum effect of the higher concentrations. Effects induced by agmatine on the exogenous ATP in the prostatic portion were blocked by cromakalim, suggesting a blocking action on the postsynaptic K(+) channels, which explains, in part, the potentiation of the twitch amplitude. It was concluded that agmatine interferes with sympathetic neurotransmission, but the physiological relevance of this needs to be better understood because of the high doses employed to induce its effects.

Presynaptic modulation of cholinergic neurotransmission in the human proximal stomach

1. This study investigates whether the cholinergic neurones, innervating the human proximal stomach, can be modulated by nitric oxide (NO) or vasoactive intestinal polypeptide (VIP), or via presynaptic muscarinic, alpha(2)- or 5-hydroxytryptamine(4) (5-HT(4)-) receptors. 2. Circular muscle strips, without mucosa, were incubated with [(3)H]-choline to incorporate [(3)H]-acetylcholine into the cholinergic transmitter stores. The basal and electrically-induced release of tritium and [(3)H]-acetylcholine were analysed in a medium containing guanethidine (4 x 10(-6) M), hemicholinium-3 (10(-5) M), physostigmine (10(-5) M) and atropine (10(-6) M). Tissues were stimulated twice for 2 min (S(1) and S(2): 40 V, 1 ms, 4 Hz) and drugs were added before S(2). 3. The NO synthase inhibitor L-N(G)-nitroarginine methyl ester (3 x 10(-4) M) and the NO donor sodium nitroprusside (10(-5) M), as well as VIP (10(-7) M) did not influence the basal release nor the electrically-evoked release. 4. The alpha(2)-adrenoceptor agonist UK-14,304 (10(-5) M) significantly inhibited the electrically-evoked release of [(3)H]-acetylcholine, and this was prevented by the alpha(2)-adrenoceptor antagonist rauwolscine (2 x 10(-6) M). 5. The 5-HT(4)-receptor agonist prucalopride (3 x 10(-7) M) significantly enhanced the electrically-evoked release of [(3)H]-acetylcholine, and the 5-HT(4)-receptor antagonist SB204070 (10(-9) M) prevented this. 6. When atropine (10(-6) M) was omitted from the medium and added before the second stimulation, it significantly increased the release of [(3)H]-acetylcholine. 7. These results suggest that the release of acetylcholine from the cholinergic neurones, innervating the circular muscle in the human proximal stomach, can be inhibited via presynaptic muscarinic auto-receptors and alpha(2)-adrenoceptors, and stimulated via presynaptic 5-HT(4)-receptors. No evidence for modulation by NO or VIP was obtained.

Neural circuits regulating pulsatile luteinizing hormone release in the female guinea-pig: opioid, adrenergic and serotonergic interactions

We studied three neurotransmitters involved in the regulation of pulsatile luteinizing hormone (LH) release: opioid peptides, serotonin and norepinephrine, using the ovariectomized guinea-pig. This is an attractive animal model due to the regularity of its LH pulses, enabling any disruptions to be clearly ascertained. In all experiments, a specific agonist or antagonist was administered, either alone or serially to enable detection of interactions, and effects on mean LH concentrations, pulse amplitude and interpulse interval were determined by PULSAR analysis. In the ovariectomized guinea-pig, catecholamines are stimulatory (acting through the alpha1 and alpha2 but not beta receptors, unlike other species), opioids inhibitory and serotonin permissively stimulatory to pulsatile LH release. Stimulatory effects of the opiate antagonist were not blocked by pretreatment with an alpha1- or alpha2-adrenergic antagonist. Similarly, pretreatment with the opiate antagonist did not prevent the suppression of LH release by alpha1 and alpha2 antagonists. This suggests that, in the guinea-pig, effects of opiates and catecholamines on LH release are exerted by independent pathways to luteinizing hormone releasing hormone (LHRH) neurones. For the opiate-serotonin interactions, pretreatment with the serotonergic antagonist did not block the stimulatory effect of the opiate antagonist on LH release. However, pretreatment with the opiate agonist could not be overcome by the serotonergic agonist. This suggests that the effects of the serotonin system on LHRH release may be indirectly mediated by opioid neurones. Taken together, these studies demonstrate that the three neurotransmitter systems studied are critically involved in normal pulsatile LH release in the female guinea-pig, and demonstrate novel functional relationships between the opioid and the adrenergic and serotonergic systems.

Inhibition by midazolam of the adrenergic function in the isolated canine mesenteric vein

BACKGROUND

Midazolam has been reported to cause hypotension or to depress sympathetic activity following intravenous injection. However, little information is available concerning the mechanism of these effects. The aim of the present study was to determine the effects of midazolam on release of noradrenaline (NA) at nerve terminals and on receptors in the venous smooth muscle.

METHODS

The effect of midazolam at nerve terminals was examined by measuring the amount of NA release from superfused canine mesenteric vein helical strips during electrical stimulation (ES; 5 Hz, 2 ms, 9 V). The NA was quantified by high-performance liquid chromatography with electrochemical detection; tension development evoked by ES was also recorded simultaneously. In a separate series of experiments, ring preparations from the isolated vein were mounted in Krebs-Ringer solution for isometric tension recording to assess the effect of midazolam on alpha-adrenoceptors.

RESULTS

Application of tetrodotoxin (10(-6) M) or replacement of superfusate with Ca(2+)-free solution decreased both the release of NA and the tension development evoked by ES. Yohimbine (5 x 10(-8) M) increased the ES-evoked release of NA, whereas it decreased tension development in the vein strips. Midazolam (10(-4) M) did not affect either the basal release of NA or the basal tension, but inhibited both the NA release (P < 0.01) and the tension development (P < 0.01) during ES; midazolam at 10(-5) M inhibited the tension development (P < 0.05) but had no effect on NA release. In the ring preparations, midazolam (10(-5) and 10(-4) M) attenuated responses to NA (a mixed alpha 1- and alpha 2-adrenoceptor agonist, 10(-8) to 10(-3) M), phenylephrine (the alpha 1-adrenoceptor agonist, 10(-8) to 10(-3) M) and 5-bromo-6-[2-imidazolin-2yl-amino]-quinoxaline (UK14304; the alpha 2-adrenoceptor agonist, 10(-7) to 10(-3) M) in a dose-dependent manner.

CONCLUSION

The data obtained in the present study suggest that midazolam at 10(-4) M may reduce venous tone by inhibiting the release of NA from sympathetic nerve endings and both alpha 1- and alpha 2-adrenoceptor mediated smooth muscle contractions. It is also postulated that a stage of the post-receptor transduction mechanism linked to the venous smooth muscle contraction may be more sensitive to midazolam than the NA release mechanism at nerve terminals since midazolam at the low concentration tested inhibited ES-evoked tension development with no effect on the release of NA.

Neuronal-associated tumor necrosis factor (TNF alpha): its role in noradrenergic functioning and modification of its expression following antidepressant drug administration

Tumor necrosis factor-alpha (TNF alpha) and the alpha 2-adrenergic agonist clonidine regulate norepinephrine (NE) release from noradrenergic nerve terminals in the central nervous system (CNS). In the present study, superfusion and electrical field stimulation were applied to a series of rat hippocampal brain slices in order to investigate the regulation of [3H]-NE release. NE release had been previously determined to be decreased by TNF alpha in a concentration-dependent manner, an effect which was potentiated by the alpha 2-adrenergic antagonist idazoxan. Presently, we demonstrate that similar to alpha 2-adrenergic activation, TNF alpha regulation of NE release in a region of the brain rich in noradrenergic nerve terminals, is dependent upon the frequency of electrical stimulation applied to the hippocampal slice. Furthermore, immunoperoxidase staining has verified our previous findings of constitutive TNF alpha protein in the rat brain. Staining for TNF alpha appears to be largely localized to neurons and neuronal processes, further substantiating the proposal that TNF alpha is either synthesized de novo or is accumulated in and released by neurons. After administration of the tricyclic antidepressant desipramine, tissue sections obtained from the rat hippocampus and locus coeruleus are devoid of neuronal-associated TNF alpha immunoreactivity. TNF alpha localization in neurons and its modification of NE release comparable to alpha 2-adrenergic receptor activation, explains a functional role for the cytokine as a neuromodulator in the CNS.

Effects of iodoproxyfan, a potent and selective histamine H3 receptor antagonist, on alpha 2 and 5-HT3 receptors

We determined the affinity and/or potency of the novel H3 receptor antagonist iodoproxyfan at alpha 2 and 5-HT3 receptors. Iodoproxyfan and rauwolscine (a reference alpha 2 ligand) (i) monophasically displaced 3H-rauwolscine binding to rat brain cortex membranes (pKi 6.79 and 8.59); (ii) facilitated the electrically evoked tritium overflow from superfused mouse brain cortex slices preincubated with 3H-noradrenaline (pEC50 6.46 and 7.91) and (iii) produced rightward shifts of the concentration-response curve (CRC) of (unlabelled) noradrenaline for its inhibitory effect on the evoked overflow (pA2 6.65 and 7.88). In the guinea-pig ileum, iodoproxyfan 6.3 mumol/l failed to evoke a contraction by itself but depressed the maximum of the CRC of 5-hydroxytryptamine (pD'2 5.24). Tropisetron (a reference 5-HT3 antagonist) produced rightward shifts of the CRC of 5-hydroxytryptamine (pA2 7.84). In conclusion, the affinity/potency of iodoproxyfan at H3 receptors (range 8.3-9.7 [1]) exceeds that at alpha 2 receptors by at least 1.5 log units and that at 5-HT3 receptors by at least 3 log units.

Comparison of the interaction of agmatine and crude methanolic extracts of bovine lung and brain with alpha 2-adrenoceptor binding sites

1. In the present study we have evaluated whether alpha 2-adrenoceptor binding sites on bovine cerebral cortex membranes labelled by [3H]-clonidine, [3H]-idazoxan and [3H]-RX-821002 can distinguish between known agonists and antagonists. This model has then been used to compare the binding profiles of the putative non-catecholamine, clonidine-displacing substance (CDS), agmatine and crude methanolic extracts of bovine lung and brain. 2. Saturation studies carried out in the presence and absence of noradrenaline, 10 mumol 1(-1), revealed that the maximum number of binding sites on bovine cerebral cortex membranes for [3H]-idazoxan and [3H]-RX-821002 were approximately 60-80% greater than those for [3H]-clonidine (62.6 fmol mg-1 protein). Rauwolscine, the selective alpha 2-adrenoceptor antagonist, was approximately 100 fold more potent against each of the ligands than the selective alpha 1-adrenoceptor diastereoisomer, corynanthine. Also, the pKi value for the selective alpha 1-adrenoceptor prazosin against each ligand was less than 6. 3. Adrenaline, UK-14034, rauwolscine, corynanthine, RX-811059 and prazosin produced concentration-dependent inhibition of binding of all three 3H-ligands. The agonists, adrenaline and UK-14304, were approximately 5 and 10 fold less potent against [3H]-idazoxan and [3H]-RX-821002, respectively, than against [3H]-clonidine. In marked contrast, the antagonists, rauwolscine, corynanthine, RX-811059 and prazosin exhibited a different profile, being approximately 2-3 fold more potent against sites labelled by [3H]-RX-821002 and [3H]-idazoxan compared to sites labelled by [3H]-clonidine. 4. Agmatine and histamine produced a concentration-dependent displacement of [3H]-clonidine, [3H]-idazoxan and [3H]-RX-821002 binding to bovine cerebral cortex membranes. The pKi values for agmatine and histamine were independent of the 3H-ligand employed, approximately 4.8 and 4.5,respectively.5. Crude methanolic extracts of bovine brain and lung produced a concentration-dependent inhibition of [3H]-clonidine binding to bovine cerebral cortex membranes (>90%). Based on the volume of the extract that caused 50% inhibition of [3H]-clonidine binding, bovine lung contains 3 fold more CDS than bovine brain. Both extracts were at least 5 fold more potent against a2-adrenoceptor sites labelled by[3H]-clonidine than those labelled by [3H]-idazoxan and [3H]-RX-821002.6. All three 3H-ligands label the same population of alpha2-adrenoceptor binding sites on bovine cerebral cortex membranes, but [3H]-clonidine appears to label selectively the 'agonist' state of the sites: for which known agonists, adrenaline and UK-14304, exhibit a higher affinity. Our results indicate that neither agmatine nor histamine can account for the CDS activity present in crude extracts of bovine brain and lung. Moreover, these extracts appear to possess a binding profile similar to that of adrenaline and UK-14304, suggesting that they may possess agonist activity.

Studies on the protective effect of azepexole on ouabain-induced cardiac arrhythmias and lethality in guinea-pig

Azepexole, an alpha 2-adrenoceptor agonist (125, 250 and 500 micrograms/kg i.v.), was examined for its effect on ouabain-induced ventricular premature beats, ventricular tachyarrhythmias and lethality in guinea-pigs. The doses of ouabain required to cause ventricular arrhythmias and lethality were significantly higher in azepexole-treated animals. However, it did not offer any protection in reserpinised guinea-pigs. Idazoxan, the alpha 2-adrenoceptor antagonist (100 micrograms/kg i.v.) inhibited the protective action of azepexole while corynanthine, the alpha 1-adrenoceptor antagonist (1 mg/kg i.v.), potentiated the effect. Azepexole inhibited the rate of the ouabain-induced rise in mean arterial blood pressure and the peak pressor response. In isolated paced left atria of guinea-pig, azepexole (2.76 x 10(-3) M) did not offer any protection against extrasystolic contractions induced by ouabain. Therefore the protective effect of azepexole may be mediated through the stimulation of alpha 2-adrenoceptors and the resultant suppression of the indirect neural components of ouabain toxicity.

Corynanthine inhibits, while idazoxan potentiates, cardiotoxic effects of ouabain

1. Ouabain, infused intravenously to anaesthetized guinea-pigs induced ventricular premature beats, ventricular tachyarrhythmias and lethality. 2. Corynanthine (1, 2 and 4 mg kg-1), an alpha 1-adrenoceptor antagonist and idazoxan (100, 200 and 400 micrograms kg-1), an alpha 2-adrenoceptor antagonist were administered 10 min prior to ouabain. Corynanthine (2 and 4 micrograms kg-1) showed significant increase in the amount of ouabain required to cause arrhythmia and lethality, whereas idazoxan (200 and 400 micrograms kg-1) decreased it. 3. Corynanthine inhibited the ouabain-induced pressor response while idazoxan potentiated it. 4. Effects of these agents on the sympathetic nervous system appear to have played a significant role in its anti- and proarrhythmic actions.

Pharmacological characterization of noradrenaline-induced contractions of the porcine isolated palmar lateral vein and palmar common digital artery

1. The aim of this study was to examine the pharmacological characteristics of alpha-adrenoceptor-mediated contractions in two porcine isolated blood vessels, the palmar lateral vein (PLV) and the palmar common digital artery (PCDA). This was carried out with noradrenaline used as the agonist throughout, and either phentolamine (non-selective alpha-adrenoceptor antagonist), prazosin and YM-12617 (selective alpha 1-adrenoceptor antagonists) or rauwolscine and CH-38083 (selective alpha 2-adrenoceptor antagonists). 2. Noradrenaline (0.003-10 microM) produced concentration-dependent contractions in both vessels, with the PCDA (pD2 = 6.33 +/- 0.07, n = 10) being approximately 10 fold less sensitive to noradrenaline compared to the PLV (pD2 = 7.39 +/- 0.09, n = 8). Also, the maximal response to noradrenaline was greater in the PCDA compared to the PLV. Phentolamine (0.03-30 microM) produced parallel rightward shifts in the CRC to noradrenaline in both tissue preparations. The pA2 values were similar and slopes of the Schild plots were not significantly different from unity, indicating an interaction between phentolamine and a single receptor in each preparation. 3. In the PCDA the alpha 1-adrenoceptor antagonists, prazosin (0.01-1 microM) and YM-12617 (0.01-1 microM) produced non-parallel rightwards shifts in the CRC to noradrenaline, with the lower 10-15% of the CRC exhibiting greater resistance to the effects of these antagonists compared to the upper part. In contrast, rauwolscine (1-10 microM) and CH-38083 (10 microM) produced parallel displacement of the CRC to noradrenaline. In the PLV, low concentrations of either alpha l- (0.01 microM) or alpha2-adrenoceptor antagonists(0.1-1 microM) produced a large shift in the CRC, but subsequent higher concentrations had only small additional effects. Based upon pKB values estimated from the effects of the lower concentrations of antagonists, the results are consistent with a large population of alpha1-adrenoceptors in the PCDA and a mixture of alpha l- and alpha2-adrenoceptors in the PLV.4. In both tissues, when an ac,- and an a2-adrenoceptor antagonist were used in combination the effect produced was greater than that with either agent alone. In contrast, the combination of the alpha1-adrenoceptor antagonists (prazosin and YM-12617 together) or the alpha2-adrenoceptor antagonists (CH-38083 and rauwolscine together) were no more effective than that produced by the individual antagonists. These findings suggest the presence of functional alpha l- and alpha2-adrenoceptors in the PLV andPCDA.5. Phenoxybenzamine (0.3-3 microM, 60min exposure) produced a concentration-dependent reduction in the maximal response to noradrenaline which was more pronounced in the PCDA than the PLV. After a 60 min exposure to a combination of phenoxybenzamine (1 microM) and rauwolscine (1 microM), the remaining NA-induced contraction after washout was resistant to prazosin (0.1 microM) and sensitive to rauwolscine(1 microM) in both tissue preparations, indicating the existence of functional alpha2-adrenoceptors in both vessels.6. Evidence suggests that post-junctional alpha l- and alpha2-adrenoceptors contribute to noradrenaline-induced contractions in the PCDA and PLV, with the latter possessing a larger population of functional alpha2-adrenoceptors.

Effects of hydralazine on contractile responses to alpha 1 and alpha 2-adrenoceptor agonists in isolated rubbed rat aorta

1. Effects of hydralazine on contractile responses to noradrenaline (an alpha 1- and alpha 2-adrenoceptor agonist) to phenylephrine and methoxamine (both selective alpha 1-adrenoceptor agonists) and to clonidine and BHT-920 (both relatively selective alpha 2-adrenoceptor agonists) were examined in isolated rat aorta deprived of endothelium. Hydralazine (1 mM) produced a rightward shift with depression of the maximal tension of the concentration-response curves for all the agonists tested. The effects on curves for clonidine and BHT-920 (partial agonists) were greater than on curves for noradrenaline, phenylephrine and methoxamine (full agonists). 2. The inhibitory effect of prazosin (pA2, about 10) was much greater than that of yohimbine (pA2, about 7) for all the agonists. 3. In tissues pretreated with phenoxybenzamine, hydralazine (1 mM) inhibited the residual response to all the agonists. The inhibitory effect on residual response to full agonists was similar to that observed on response to partial agonists in tissues not treated with phenoxybenzamine. 4. The relationship between maximal response and percentage receptor occupancy was nonlinear for full agonists, but near-linear for partial agonists. 5. These results indicate that the responses induced by noradrenaline, phenylephrine, methoxamine, clonidine and BHT-920 in the rat aorta are due to the activation of alpha 1-adrenoceptors and confirm the vasorelaxant action of hydralazine. 6. These results also suggest that the differential effects of hydralazine on the responses to alpha-adrenoceptor agonists may be due to differences in the amount of receptor reserve available available in this blood vessel for full agonists (noradrenaline, phenylephrine or methoxamine) and partial agonists (clonidine or BHT-920).

Effects of the central analgesic tramadol and its main metabolite, O-desmethyltramadol, on rat locus coeruleus neurones

1. Tramadol is a centrally acting analgesic with low opioid receptor affinity and, therefore, presumably additional mechanisms of analgesic action. Tramadol and its main metabolite O-desmethyltramadol were tested on rat central noradrenergic neurones of the nucleus locus coeruleus (LC), which are involved in the modulation of nociceptive afferent stimuli. 2. In pontine slices of the rat brain the spontaneous discharge of action potentials of LC cells was recorded extracellularly. (-)-Tramadol (0.1-100 microM), (+)-tramadol (0.1-100 microM), (-)-O-desmethyl-tramadol (0.1-100 microM) and (+)-O-desmethyltramadol (0.01-1 microM) inhibited the firing rate in a concentration-dependent manner. (+)-O-desmethyltramadol had the highest potency, while all other agonists were active at a similar range of concentrations. 3. (-)-Tramadol (10, 100 microM) was less inhibitory in brain slices of rats pretreated with reserpine (5 mg kg-1, 5 h before decapitation) than in controls. 4. The effect of (-)-tramadol (10 microM) was abolished in the presence of the alpha 2-adrenoceptor antagonist, rauwolscine (1 microM), whilst that of (+)-O-desmethyltramadol (0.3 microM) virtually disappeared in the presence of the opioid antagonist, naloxone (0.1 microM). (+)-Tramadol (30 microM) and (-)-O-desmethyl-tramadol (10 microM) became inactive only in the combined presence of naloxone (0.1 microM) and rauwolscine (1 microM). 5. In another series of experiments, the membrane potential of LC neurones was determined with intracellular microelectrodes. (-)-Tramadol (100 microM) inhibited the spontaneous firing and hyper-polarized the cells; this effect was abolished by rauwolscine (1 microM). (+)-O-desmethyltramadol (10 microM)had a similar but somewhat larger effect on the membrane potential than (-)-tramadol. The (+)-O-desmethyltramadol-(10 microM) induced hyperpolarization was abolished by naloxone (0.1 microM).6. The hyperpolarizing effect of noradrenaline (30 microM) was potentiated in the presence of (-)-tramadol(100 microM), but not in the presence of (+)-O-desmethyltramadol (10 microM). There was no potentiation of the noradrenaline (30 microM) effect, when the cells were hyperpolarized by current injection to an extent similar to that produced by (-)-tramadol (100 microM).7. Both noradrenaline (100 microM) and (- )-tramadol (100 microM) decreased the input resistance.8. The results confirm that the analgesic action of tramadol involves both opioid and non-opioid components. It appears that (-)-tramadol inhibits the uptake of noradrenaline and via a subsequent increase in the concentration of endogenous noradrenaline indirectly stimulates alpha2-adrenoceptors. (+)-0-desmethyltramadol seems to stimulate directly opioid micro-receptors. The effects of (+)-tramadol and(-)-O-desmethyltramadol consist of combined micro-opioid and alpha2-adrenergic components.

Galanin receptors inhibit the spontaneous firing of locus coeruleus neurones and interact with mu-opioid receptors

Electrophysiological experiments were performed in a pontine slice preparation of rat brain containing the locus coeruleus (LC). The extracellular part of this study showed that galanin (0.003-0.1 mumol/l), [Met5]enkephalin (0.01-10 mumol/l) and noradrenaline (0.1-100 mumol/l) concentration dependently inhibited the firing rate. Noradrenaline (1 and 3 mumol/l) had the same effect both before and during the application of galanin (0.001 or 0.01 mumol/l). Similarly, [Met5]enkephalin (0.03 and 0.1 mumol/l) produced identical inhibition, regardless of the presence or absence of 0.01 mumol/l galanin. Whereas rauwolscine (1 mumol/l) potentiated the effect of galanin (0.03 mumol/l), idazoxan (1 mumol/l) was inactive. In contrast, both naloxone (0.1 mumol/l) and beta-funaltrexamine (0.1 mumol/l) facilitated the galanin-induced inhibition. In the intracellular experiments, galanin (0.3 mumol/l) abolished the spontaneous discharge of action potentials, hyperpolarized the cells and decreased their input resistance. In conclusion, galanin may depress the firing rate by increasing a potassium permeability. Moreover, galanin receptors appear to interact with mu-opioid receptors but not with alpha 2-adrenoceptors.

Norepinephrine release in rat vas deferens. Effect of rauwolscine and BHT 920

Norepinephrine (NE) overflow from field-stimulated rat was deferens preparations was quantified directly by electrochemical detection using high performance liquid chromatography. The effect of agonist (BHT 920) and antagonist (rauwolscine) of prejunctional alpha 2-adrenoceptors on NE overflow was assessed and compared with their effect on the smooth muscle mechanical response to field stimulation. Increasing the stimulation frequency from 2 to 30 Hz resulted in an increase in muscle tension together with an increase in NE overflow. Addition of 1 microM rauwolscine to the medium resulted in a significant increase in muscle contraction to field stimulation which reached a maximum at 5 Hz. On the other hand, NE overflow increased linearly with the frequency of stimulation within the range studied. Addition of 0.1 microM BHT 920 to the medium significantly decreased the amplitude of contractions at lower stimulation frequencies (2 to 10 Hz) but elicited no significant changes at high frequencies. BHT 920 did not significantly affect NE overflow for all range of stimulation frequency. The simultaneous recording of field-stimulation induced contractions and NE overflow indicates that in the rat vas deferens, rauwolscine acts like a pure alpha 2 adrenoceptor antagonist at a prejunctional level. BHT 920 did not appear to affect selectively prejunctional alpha 2 adrenoceptors but also may activate postjunctional alpha 1 adrenoceptors.

Modulation of non-adrenergic non-cholinergic inhibitory neurotransmission in rat gastric fundus by the alpha 2-adrenoceptor agonist, UK-14,304

1. The influence of the alpha 2-adrenoceptor agonist, UK-14,304, on non-adrenergic non-cholinergic (NANC) relaxation induced by electrical field stimulation was investigated in longitudinal muscle strips of the gastric fundus of reserpinized rats. 2. In tissues where tone was raised by 3 x 10(-7) M prostaglandin F2 alpha (PGF2 alpha), the inhibitory effect of 10(-6) M UK-14,304, on the NANC relaxations induced by short train stimulation (40 V, 1 ms, 20 s) was inversely related to the stimulus frequency (1-4-16 Hz). UK-14,304 (10(-6) M) did not influence relaxations induced by administration of exogenous nitric oxide (NO, 2 x 10(-6) M-10(-4) M). The inhibitory effect of UK-14,304 on the electrically induced relaxations was antagonized by 10(-6) M rauwolscine but not by 10(-6) M prazosin. 3. UK-14,304 (10(-6) M) also reduced the amplitude of the sustained NANC relaxation, induced by electrical field stimulation (40 V, 1 ms, 4 Hz) for 5 min. The effect of UK-14,304 was also antagonized by 10(-6) M rauwolscine but not by 10(-6) M prazosin. UK-14,304 (10(-6) M) did not reduce the relaxation induced by 3 x 10(-9) M vasoactive intestinal polypeptide (VIP). 4. These results suggest that the release of the inhibitory NANC neurotransmitter during short train stimulation, thought to be NO, and during sustained stimulation, thought to be VIP, is inhibited by stimulation of presynaptic alpha 2-adrenoceptors in the rat gastric fundus.

Pharmacological characterization of presynaptic alpha 2-autoreceptors in rat submaxillary gland and heart atrium

1. The pharmacological properties of presynaptic alpha 2-autoreceptors were studied in rat isolated submaxillary glands and atria. Tissue pieces were preincubated with [3H]-noradrenaline, then superfused with medium containing desipramine, and stimulated electrically. In one series of experiments, pEC30 values of 12 alpha-adrenoceptor antagonists were determined, i.e., negative logarithms of concentrations that increased the electrically evoked overflow of tritium by 30%. In another series, pKD values of 9 alpha-adrenoceptor antagonists against the release-inhibiting effect of 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304), and of 3 antagonists against the release-inhibiting effect of methoxamine, were determined. 2. In submaxillary glands, the pEC30 values of the antagonists correlated well with their pKD values against UK 14304 (r = 0.93). The same was true for atria (r = 0.92). 3. In submaxillary glands, the pKD values of 3 antagonists against UK14304 were very similar to their pKD values against methoxamine, with a maximal difference of 0.4. The same was true for atria where the maximal difference was 0.3. 4. The pEC30 values obtained in submaxillary glands correlated significantly with those obtained in atria (r = 0.81). The same was true for the pKD values (r = 0.79). However, the pEC30 and pKD values also indicated consistent differences between the two tissues. 5. It is concluded that the sites of action of the imidazoline UK 14304 (alpha 2-selective), the phenylethylamine noradrenaline, and the phenylethylamine methoxamine (alpha 1-selective) are exclusively alpha 2-adrenoceptors. There is no indication for presynaptic alpha 1-adrenoceptors or for an effect of UK 14304 mediated by presynaptic imidazoline receptors.The 02-autoreceptor population in the submaxillary gland differs from that in the atrium.6. Comparison with studies from the literature indicates that the submaxillary autoreceptors are closely similar to the a2D radioligand binding site found in the bovine pineal gland and probably the rat submaxillary gland. The atrial autoreceptors also conform best to this site, but the agreement is more limited; the atrial autoreceptors may represent a type related to, but distinct from, the a2D site, or a mixture of different types.

Endotoxin impairs the response of rabbit mesenteric artery to electrical stimulation via a prejunctional mechanism

1. The effect of E. coli lipopolysaccharide (LPS) on sympathetic neuro-effector transmission was studied in the rabbit mesenteric artery. The experiments were performed on artery rings isolated 5 or 20 h after intravenous treatment with LPS or saline as well as on artery rings isolated from non-treated rabbits (for assessment of the effect of in vitro preincubation with LPS). In most experiments, neural elements in the arteries were stimulated electrically (10 V, 2 ms, 1-32 Hz). 2. Preincubation with LPS (10 micrograms ml-1) for 5 or 20 h had no effect on the contraction responses of endothelium-intact artery rings to electrical stimulation. In contrast, in vivo intravenous pretreatment with LPS (10 micrograms) led to an inhibition of the contraction; LPS elicited this effect when injected 20 h, but not 5 h, before the experiment. The effect of LPS was eliminated in artery rings isolated from animals receiving an inhibitor of protein synthesis (actinomycin D or cycloheximide) before treatment with LPS. LPS (injected 20 h before the experiment) had no effect on the concentration-response curves for exogenous noradrenaline and tyramine in endothelium-intact artery rings. 3. The inhibition of electrically induced contractions produced by LPS treatment in endothelium-intact artery rings was attenuated by atropine and yohimbine, but not by phentolamine. Yohimbine plus atropine restored the depressed contraction to the normal level. Clonidine and acetylcholine mimicked the effect of LPS in endothelium-intact artery rings isolated from saline-treated animals. 4. When steady-state contractions were induced by 5 min of stimulation at 16 Hz, acetylcholine or clonidine reduced the contraction in endothelium-denuded artery rings from both saline-treated rabbits and animals receiving LPS 20 h before the experiment. The reduction produced by acetylcholine or clonidine of the contraction in artery rings from LPS-treated rabbits was significantly greater than in artery rings from saline-treated animals.5. These results suggest that treatment of rabbits with LPS inhibits noradrenaline release from sympathetic nerve endings via increased sensitivity of both prejunctional inhibitory muscarinic receptors and x2-adrenoceptors in mesenteric arteries. They also suggest that the effect of LPS is independent of endothelial cells but linked to protein synthesis.

Neurally evoked responses of human isolated resistance arteries are mediated by both alpha 1- and alpha 2-adrenoceptors

1. Human subcutaneous resistance arteries (internal diameter 113-626 microns) were mounted in an isometric myograph. Electrical field stimulation was applied either continuously in the form of a frequency-response curve or intermittently at 16 Hz. The magnitude of the maximum contraction induced by continuous stimulation expressed as a percentage of the response to a supramaximal concentration of noradrenaline (10 microM) was highly variable but unrelated to vessel calibre. Contractile responses to both continuous and intermittent stimulation were abolished by 1 microM tetrodotoxin. 2. Prazosin (100 nM and 1 microM, alpha 1-adrenoceptor antagonist) inhibited responses to continuous stimulation over a range of frequencies (2-8 Hz). The response to continuous stimulation at 8 Hz was inhibited by 78 +/- 6% by 1 microM prazosin. Rauwolscine (100 nM, alpha 2-adrenoceptor antagonist) had a smaller effect on contractions induced by continuous stimulation. Rauwolscine inhibited the response at 8 Hz by 36 +/- 6%. Rauwolscine at a higher concentration (1 microM) caused further inhibition of the response to continuous stimulation. Prazosin and rauwolscine in combination almost completely inhibited the response to continuous stimulation at concentrations of 1 microM. 3. Prazosin and rauwolscine inhibited responses to intermittent stimulation in a concentration-dependent manner. The IC50 for this action of prazosin was 3.7 +/- 1.6 nM and the maximum inhibition induced by 100 nM prazosin was 78 +/- 6%. The IC50 of rauwolscine was 12.0 +/- 1.3 nM and 100 nM rauwolscine caused a 86 +/- 7% reduction in the response to intermittent stimulation.Prazosin and rauwolscine in combination (each at 100 nM) caused marked inhibition of the response to intermittent stimulation leaving only 7.0 +/- 2.6% of the response.4. These data suggest that neurally released noradrenaline evokes contractions of human resistance arteries by activation of both alpha 1,- and alpha 2-adrenoceptors postjunctionally.

Interactions between the effects of endothelin-1, clonidine and yohimbine on electrically-induced contractions in rat vas deferens

1. The relationship between endothelin-1(ET-1)-induced effects on the contractile responses of epididymal portion of rat vas deferens elicited by field electrical stimulation (FES: 80 V, 1 msec, 0.1 Hz) and the effects of the alpha 2-adrenoceptor agonist clonidine and the alpha 2-adrenoceptor antagonist yohimbine were studied. 2. ET-1 (0.01 nM-0.1 microM) concentration-dependently increased the FES-induced contractions. 3. ET-1 (0.1 nM-0.1 microM) reversed the inhibitory effect of clonidine on the FES-evoked contractions whereas ET-1 applied before clonidine exerted a dual effect on the clonidine-induced inhibition of the FES-evoked contractions. 4. The ET-1-induced enhancement of FES-induced contractions was potentiated in the presence of 1 microM yohimbine and was not observed at all in the presence of 10 microM yohimbine. Yohimbine, applied at concentrations of 1 and 10 microM exerted similar blocking effects on the alpha 1-adrenoceptor agonistic effects of phenylephrine. However, yohimbine at a concentration of 10 microM markedly potentiated the contractile effect of exogenous adenosine 5'-triphosphate (ATP), 30 microM. Tetrodotoxin abolished this effect of yohimbine. 5. The results presented here suggest the existence of modulating interactions between the ET-1-evoked increase of FES-induced contractions of rat vas deferens and the alpha 2-adrenoceptor drugs clonidine and yohimbine.

Contributions of alpha 1-adrenoceptors, alpha 2-adrenoceptors and P2x-purinoceptors to neurotransmission in several rabbit isolated blood vessels: role of neuronal uptake and autofeedback

1. The roles of autofeedback and neuronal uptake in neurotransmission produced by electrical field stimulation in several rabbit isolated blood vessels were examined. 2. Blocking drugs were used to separate the possible purinergic and noradrenergic contributions to the end organ response: prazosin, antagonist at postjunctional alpha 1-adrenoceptors; rauwolscine and yohimbine, antagonists at pre- and postjunctional alpha 2-adrenoceptors; alpha,beta-methylene ATP, desensitizing agent at postjunctional P2x-purinoceptors. In addition to desensitizing postjunctional P2x-purinoceptors, alpha,beta-methylene ATP potentiated the noradrenergic component of the nerve-induced responses. 3. In the presence of an intact neuronal uptake mechanism, the vessels showed different contributions of purinergic (via P2x-purinoceptors) and noradrenergic (via alpha 1-adrenoceptors and alpha 2-adrenoceptors) components to the end organ response to nerve stimulation: saphenous artery (approximately equal contributions from P2x-purinoceptors and alpha 1-adrenoceptors), ileocolic artery (mainly P2x-purinoceptors with a smaller contribution from alpha 1-adrenoceptors), plantaris vein (mainly alpha 1-adrenoceptors with a small contribution from alpha 2-adrenoceptors and P2x-purinoceptors) and saphenous vein (alpha 1-adrenoceptors). 4. The presence of alpha 2-adrenoceptor-mediated autofeedback could be demonstrated for both purinergic and noradrenergic components of the nerve-induced responses in the artery preparations. In the veins, potentiation of nerve-induced responses by alpha 2-adrenoceptor antagonists could not be studied due to blockade of postjunctional alpha 2-adrenoceptor-mediated vasoconstriction. 5. Blockade of neuronal uptake with cocaine potentiated the noradrenergic component of the nerve-induced responses. Both alpha 1-adrenoceptor- and alpha 2-adrenoceptor-mediated components were potentiated, with a relatively greater potentiation of the alpha 2-adrenoceptor-mediated component. In the case of saphenous vein an alpha 2-adrenoceptor-mediated component which was previously absent was uncovered.6. Blockade of neuronal uptake with cocaine had no effect or reduced the purinergic component of responses, the latter effect presumably due to enhanced alpha 2-adrenoceptor-mediated autofeedback.7. In the presence of cocaine, nerve-induced responses in the saphenous vein were biphasic. Rauwolscine potentiated the first phase and inhibited the second phase thus demonstrating effects of pre- and postjunctional alpha 2-adrenoceptor-mediated activation in the same preparation.8. In conclusion, neuronal uptake and autofeedback processes play important and complex interacting parts in determining the relative contributions of alpha 1,- and alpha 2-adrenoceptors and P2.-purinoceptors in the end organ response to neurotransmission in blood vessels.

Protective action of clonidine against the arrhythmogenic and lethal effects of ouabain in guinea-pigs

1. Clonidine (1.25, 2.5 and 5.0 micrograms kg-1) was studied for its effect on the cardiac arrhythmias and lethality induced by slow intravenous infusion of ouabain in guinea-pigs. 2. Clonidine produced significant delays in the onset of the arrhythmic stages and lethality. However, clonidine did not offer any such protection in reserpinised guinea-pigs, whereas its effects were unaltered in atropinized guinea-pigs. 3. Idazoxan (100 micrograms kg-1, i.v.) abolished the antiarrhythmic effect of clonidine whereas corynanthine (1 mg kg-1, i.v.) had no such effect. 4. Clonidine inhibited the rate of the ouabain-induced rise in blood pressure and the peak pressor response. 5. In isolated paced left atria of the guinea-pig, clonidine (3.75 x 10(-4) M) did not offer any protection against rapid and/or irregular extrasystolic contractions induced by ouabain. 6. It is concluded that the antiarrhythmic effect of clonidine is due to its effects on the indirect neural components of digitalis toxicity mediated by the stimulation of alpha 2-adrenoceptors, without any direct antiarrhythmic effect on the myocardium.

Characterization of the subtype of presynaptic alpha 2-adrenoceptors modulating noradrenaline release in cat and bovine cerebral arteries

The possible existence of a heterogeneous population of alpha 2-adrenoceptors (alpha 2A and alpha 2B, demonstrated by binding studies) in adrenergic nerve endings of cat and bovine cerebral arteries modulating noradrenaline release was investigated. Electrical field stimulation elicited an increase of tritium secretion from these vessels preincubated with (+/-)-[3H]noradrenaline, which was reduced by the alpha 2-agonists, clonidine (1 microM) and B-HT 920 (0.01 and 0.1 microM), in cat cerebral arteries but only by B-HT 920 in bovine cerebral arteries. This reduction was inhibited by the antagonist of the alpha 2B-subtype, prazosin, and the antagonists of alpha 2A- and alpha 2B-subtypes yohimbine and particularly rauwolscine. The effect of B-HT 920 was partially inhibited by clonidine in bovine, but not in cat cerebral arteries. In both types of arteries, prazosin, yohimbine and the alpha 1-agonist methoxamine (all at 1 microM) failed to modify the stimulated radioactivity liberation, whereas it was increased by 1 microM rauwolscine, and by yohimbine plus prazosin in cat cerebral arteries. The basal tritium release was enhanced by rauwolscine and prazosin in cat cerebral arteries but only by the latter in bovine cerebral arteries. These results suggest: (1) the existence of presynaptic alpha 2-adrenoceptors, mainly of the alpha 2B-subtype, in these vessels negatively modulating noradrenaline release, their activity being greater in cat than in bovine cerebral arteries, and (2) clonidine has no agonistic but a weak antagonistic action in the latter vessels.

Accumulation and release of adrenaline, and the modulation by adrenaline of noradrenaline release from rabbit blood vessels in vitro

The accumulation of (-)-3H-adrenaline (3H-A) by rabbit isolated aorta was studied. In all experiments, monoamine oxidase and catechol-O-methyltransferase were inhibited by treatment with pargyline and 3',4'-dihydroxy-2-methyl-propiophenone, respectively. The relationship between the accumulation of 3H derived from 3H-A and the duration of incubation was linear. The 3H-accumulation after 3 h incubation was 22.5 ml/g. In reserpine-treated tissue, the 3H-accumulation levelled off after 30 min and was 8.5 ml/g after 3 h. The concentration of 3H-A or (-)-3H-noradrenaline (3H-NA) and the 3H-accumulation (ml/g) were inversely related. At 10(-8) M, the 1-hour accumulation of 3H derived from 3H-A and 3H-NA was 7.8 and 15.2 ml/g, respectively. With increasing concentrations the accumulation values approached each other. The accumulation of 3H derived from 3H-A by reserpine-treated tissue also showed an inverse relationship with concentration. The accumulation of 3H derived from 3H-A was dependent on the bath temperature. Storage of tissue (0-5 days in salt solution without equilibration with 95% O2/5% CO2; 4 degrees C) did not affect the accumulation of 3H derived from 3H-A. Thereafter (7-14 days), the accumulation decreased. The inhibitory potency (IC50; -log M) of desipramine, cocaine, propranolol, isoprenaline, and normetanephrine on accumulation of 3H derived from 3H-A was found to be 8.26; 6.50; 5.48; 4.88, and 4.02, respectively. The maximal degree of inhibition was almost the same for these drugs, while that of clonidine and corticosterone was 50 and 20%, respectively. In the presence of desipramine, either clonidine, corticosterone or isoprenaline reduces the accumulation of 3H derived from 3H-A. Ouabain and iodoacetic acid, but not sodium cyanide and 2,4-dinitrophenol, reduced the accumulation of 3H derived from 3H-A. Anoxia (95% N2/5% CO2; 37 degrees C; 1-24 h) did not alter the accumulation of 3H derived from 3H-A. Glucose deprivation alone or combined with anoxia markedly reduced the 3H-accumulation. The release of 3H-A from rabbit isolated aorta was studied. This release was compared with that of 3H-NA. The stimulation-evoked 3H-overflow from aorta preloaded with 3H-A decreased with repeated stimulation. In contrast, prestimulation enhanced subsequent stimulation-evoked 3H-overflows. For both 3H-amines, the 3H-overflow increased concomitantly to the same degree with the number of pulses. The time course of 3H-overflows with either 3H-A or 3H-NA was compared.(ABSTRACT TRUNCATED AT 400 WORDS)

Different sensitivities of rabbit isolated blood vessels exhibiting co-transmission to the slow calcium channel blocker, nifedipine

1. Antagonist drugs were used to separate the purinergic and adrenergic contributions as well as the adrenoceptor sub-types involved in the sympathetic vasoconstrictor responses produced by electrical field stimulation in rabbit isolated ileocolic and proximal saphenous arteries. Blocking drugs were applied either alone or in various combinations and sequences. 2. Nifedipine attenuated vasoconstrictor responses to sympathetic nerve stimulation both in the presence and in the absence of alpha-adrenoceptor blocking agents. However in the presence of alpha,beta-methylene ATP, nifedipine produces at best only a small attenuation of the vasoconstrictor response. 3. These results suggest that the purinergic component of the response to sympathetic nerve stimulation, at least in these tissues, can be antagonized by nifedipine, whereas the alpha 1-adrenoceptor-mediated response is relatively resistant.

Subclassification of the presynaptic alpha 2-autoreceptors in rabbit brain cortex

1. alpha 2-Adrenoceptor binding sites have been subclassified into alpha 2A sites of which a main characteristic is very low affinity for prazosin, and alpha 2B sites with relatively high affinity for prazosin. The presynaptic alpha 2-autoreceptors in rabbit brain cortex were studied in order to classify them in terms of alpha 2A and alpha 2B. Release of [3H]-noradrenaline in cortical slices was elicited by trains of 4 pulses delivered at 100 Hz. 2. Clonidine caused concentration-dependent inhibition of the stimulation-evoked overflow of tritium, with an EC50 of 7.5 nM and a maximal inhibition by 96%. 3. The following alpha-adrenoceptor antagonists shifted the concentration-response curve of clonidine to the right (antagonist-receptor dissociation constants KD in brackets): yohimbine (14 nM), 2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazo le (BRL 44408; 15 nM) and 1,2-dimethyl-2,3,9,13betetrahydro-1H-dibenzo[c,f]imidazo[1,5-a]aze pine (BRL 41992; 630 nM). Prazosin 1 microM and 2-[2-[4-(o-methoxyphenyl)piperazine-1-yl]-ethyl]-4,4-dimethyl-1,3 (2H,4H)-isoquinolinedione (AR-C 239) 1 microM failed to antagonize the effect of clonidine. Higher concentrations of prazosin and AR-C 239 greatly accelerated the basal efflux of tritium. 4. The method used permits the functional determination of antagonist affinities undistorted by endogenous alpha 2-autoinhibition. A comparison with affinities derived from radioligand binding experiments indicates that the presynaptic alpha 2-autoreceptors in rabbit brain cortex are markedly different from the alpha 2B-subtype and probably belong to the prazosin-insensitive alpha 2A-subtype.

Alpha 2-adrenoceptor activation inhibits noradrenaline release in human and rabbit isolated renal arteries

The aim of the present study was to investigate alpha 2-adrenoceptor modulation of noradrenaline release in superfused strips of human and rabbit renal arteries. The arteries were field-stimulated after incubation with [3H]noradrenaline. The stimulation-induced outflow of radioactivity was taken as an index of noradrenaline release. At a high stimulation frequency (4 Hz), the alpha 2-adrenoceptor agonist clonidine (0.1 mumol/l) failed to inhibit stimulation-induced outflow of radioactivity in human and rabbit renal arteries whereas the alpha 2-adrenoceptor agonist UK 14304 (0.1 mumol/l) did inhibit stimulation-induced outflow. The inhibitory effect of UK 14304 in human renal arteries was blocked by the alpha 2-adrenoceptor blocking drug rauwolscine (1 mumol/l). At a lower stimulation frequency (2 Hz), both clonidine and UK 14304 inhibited stimulation-induced outflow of radioactivity from rabbit renal arteries; both effects were blocked by rauwolscine. Rauwolscine by itself enhanced stimulation-induced outflow of radioactivity in both preparations. The results suggest that activation of prejunctional alpha 2-adrenoceptors in human and rabbit renal arteries inhibits noradrenaline release. Neuronally released noradrenaline exerts inhibitory feed-back modulation of its own release through activation of prejunctional alpha 2-adrenoceptors. At a higher stimulation frequency most of the prejunctional alpha 2-adrenoceptors are already occupied by endogenous noradrenaline and clonidine fails to inhibit noradrenaline release since it seems to act as a partial agonist at these prejunctional alpha 2-adrenoceptors.

Alpha-adrenoceptor subtypes in dog saphenous vein that mediate contraction and inositol phosphate production

1. Studies have been made of the contractile responses to the alpha-adrenoceptor agonists phenylephrine (Phen), cirazoline (Cir) or BHT-920 (BHT) in dog isolated saphenous vein (DSV) rings, using the antagonists yohimbine (Yoh), idazoxan (Idaz), prazosin (Praz), WB-4101 (WB) and nitrendipine or zero Ca2+ medium. 2. Contractile concentration-response curves to Phen or BHT were displaced to the right of controls by Yoh (0.01-3 microM) with mean apparent antagonist dissociation constants (pKBs) of 7.9 and 8.6 respectively. Yoh did not show simple competitive antagonism against either agonist, since the Schild plot slopes were significantly less than unity. Neither the antagonist affinity of Yoh against Phen, nor the slope of the Schild plot was modified in the presence of catecholamine uptake inhibitors, nor in the presence of alpha,beta-methylene ATP, which desensitizes P2-purinoceptors, suggesting that Phen does not release ATP, or noradrenaline to cause contraction in DSV. In the presence of Praz (0.3 microM) the antagonist potency of Yoh (mean pKB 7.4) against Phen was slightly decreased. Yoh had low potency against responses induced by Cir (pKB 6.3). 3. WB (0.001-1.0 microM) was a very potent antagonist of Phen-induced contractions, however, the biphasic Schild plot against Phen could be separated into two affinity sites, a high pKB of 9.3 (equivalent to that obtained using Cir as the agonist; pKB 9.6) and a lower affinity (pKB 8.6). WB showed an even lower antagonist affinity (pKB 7.4) against BHT-induced contractions, suggesting that these effects might be mediated by alpha 2A-adrenoceptors. Praz also appeared to identify two sites using Phen-induced contractions, a high pKB of 8.4 was equivalent to that obtained with Cir (pKB 8.2) and a lower affinity site (pKB 7.7; pA2 7.6; slope 1.1) at which Praz showed competitive antagonism. Higher concentrations of Praz were required to antagonize contractions to BHT (pKB 5.9). 4. Idaz was a weak partial agonist in this tissue with threshold contractile effects at concentrations in excess of 3 microM. Idaz (0.1-1 microM) competitively antagonized the contractile effects of BHT, but showed low antagonist affinity against Phen at these concentrations. 5. Contractions to Phen were slightly antagonized by nitrendipine (1 microM), with a 36% decrease in Emax. Contractions to Phen and Cir were also markedly attenuated in zero calcium medium (with EGTA), but maximum responses of 4.2 +/- 0.1 and 3.6 +/- 0.1 g, could be obtained with these agonists respectively. Only part of the contractile effects to Phen or Cir are therefore due to calcium influx (but L-type channels are not totally implicated), while the contractile effects of BHT were abolished in zero Ca2 + medium. Yoh (0.1 microm) retained its antagonist effects on Phen-induced responses in zero Ca2 + medium. 6. The formation of inositol phosphates (InsPs) in the presence of lithium (10mM) was measured after incubation of intact DSV strips with myo-2-[3H]-inositol. Phen (1-1OO0 microM) and Cir (O.O1-1O microm) induced concentration-dependent increases in total labelled InsP1_3, but BHT showed minimal InsP stimulation. InsPs were recovered after Phen (100,M) stimulation (10min) as labelled InsP1 (71%), InsP2 (25%) and InsP3 (4%). Phen (100 microM)-stimulated InsP1-3 formation was significantly antagonized by Praz (10nM), but was not fully inhibited even after Praz 1 microM. Yoh and Praz (0.1 and 1.0 microM) were equipotent inhibitors of this response, while Idaz (0.3 microM) showed no effects. 7. The receptors in DSV which are stimulated by Phen to cause contraction show characteristics of the alpha lA-adrenoceptor (high pM antagonist affinity for WB-4101 and extracellular calcium sensitivity) and the alpha lB-adrenoceptor (contraction in calcium-free medium, increase in InsP and low nm antagonist affinity of WB). The paradoxical results obtained with Yoh (potent antagonist effects on Phen-stimulated PI and pKB 7.9 on contraction) and Praz (low affinity competitive antagonist of Phen-induced contraction, pKB 7.7 and failure to inhibit completely the PI response at 1 microM), cannot fully exclude an alpha 2B-subtype characterization of these responses. These pharmacological differences suggest that the adrenoceptor involved in the contractile and in particular the second messenger effects of Phen in DSV is not typically an alpha lB-adrenoceptor.

Role of cyclic AMP in the prejunctional alpha 2-adrenoceptor modulation of noradrenaline release from the rat tail artery

Experiments were designed to evaluate the effect of cyclic AMP on the electrically-induced release of noradrenaline from vascular sympathetic nerve terminals. The possible implication of the inhibition of adenylate cyclase in the negative feed-back control by prejunctional alpha 2-adrenoceptors of neurotransmitter release was also investigated. Rat isolated tail arteries were preincubated with [3H]-noradrenaline; the preparations were subsequently perfused/superfused with [3H )-noradrenaline-free medium and their perivascular nerves were field stimulated with 24 pulses at 0.4 Hz (0.3 ms, 200 mA). 2 compounds known to enhance the intracellular concentration of cyclic AMP, namely the membrane permeant analogue 8-Br-cAMP (10-300 mumol/l) and forskolin (0.3-10 mumol/l), an activator of adenylate cyclase, concentration-dependently enhanced the stimulation-evoked tritium overflow. The 1,9-dideoxy derivative of forskolin, which does not stimulate adenylate cyclase, was ineffective. Exposure to the cyclic AMP phosphodiesterase inhibitor rolipram 30 mumol/l produced a moderate increase (about 20%) in tritium overflow. However, in the presence of rolipram the facilitatory effect of forskolin was significantly more pronounced than in its absence. Whereas 8-Br-cAMP produced a slight concentration-dependent enhancement of the stimulation-induced vasoconstriction, forskolin and rolipram depressed it. The alpha 2-adrenoceptor agonist B-HT 933 (3-30 mumol/l) concentration-dependently inhibited the tritium overflow. The effect of B-HT 933 30 mumol/l was slightly, but significantly reduced in the presence of 8-Br-cAMP 100 and 300 mumol/l, but was not changed in the presence of forskolin 3 mumol/l. The facilitatory effect of rauwolscine 1 mumol/l was enhanced in the presence of 8-Br-cAMP 100 mumol/l. During perfusion with 8-Br-cAMP 100 mumol/l, the current strength and frequency were decreased to 150 mA and 0.2 Hz, respectively in order to obtain similar amounts of tritium overflow to those observed in the absence of the cyclic AMP analogue with the initial stimulation parameters. Under these conditions, the inhibition of the overflow by B-HT 933 30 mumol/l and the facilitation by the alpha 2-adrenoceptor antagonist rauwolscine 1 mumol/l were unaltered as compared to controls under initial stimulation conditions. It is concluded that, in the rat tail artery, the terminals of perivascular sympathetic nerves are endowed with an adenylate cyclase system. Cyclic AMP is able to modulate noradrenaline release, but does not appear to play a role in the initiation of the release process itself. In addition, the results do not support the hypothesis that prejunctional alpha 2-adrenoceptors depress noradrenaline release through the inhibition of adenylate cyclase.

Blockade of alpha 2-adrenoceptors increases opioid mu-receptor-mediated inhibition of the firing rate of rat locus coeruleus neurones

In pontine slices of the rat brain, the frequency of spontaneous action potentials of locus coeruleus (LC) neurones was recorded extracellularly. Noradrenaline 0.1-100 mumol/l, UK 14,304 0.01-100 nmol/l, [Met5]-enkephalin 1-10,000 nmol/l and [D-Ala2,D-Leu5]enkephalin 0.1-1,000 nmol/l, all depressed the firing rate. Rauwolscine 1 mumol/l antagonized the effects of both noradrenaline and UK 14,304, but potentiated the effects of [Met5]enkephalin and [D-Ala2, D-Leu5]enkephalin. Idazoxan 1 mumol/l acted in a similar manner. Prazosin 1 mumol/l did not change the effects of either noradrenaline or [Met5]enkephalin. Naloxone 0.1 mumol/l antagonized both [Met5]enkephalin and [D-Ala2, D-Leu5]enkephalin, but failed to alter the effects of either noradrenaline or UK 14,304. Rauwolscine, idazoxan and prazosin, all 1 mumol/l, as well as naloxone 0.1 mumol/l, did not influence the firing rate when given alone. Desipramine 1 mumol/l inhibited the discharge of action potentials in a rauwolscine-antagonizable manner. Noradrenaline 10 mumol/l produced the same depression of firing, both in the presence of noradrenaline 1 mumol/l and [Met5]enkephalin 0.03 mumol/l. Likewise, the effect of [Met5]enkephalin 0.3 mumol/l was the same, irrespective of whether it was added to a medium containing [Met5]enkephalin 0.03 mumol/l or noradrenaline 1 mumol/l. The spontaneous activity of LC neurones is inhibited by somatic alpha 2-adrenoceptors and opioid mu-receptors. We suggest that the two receptors interact with each other at a site located between themselves and not in the subsequent common signal transduction system.

Effect of temperature on alpha 2- and alpha 1-adrenoceptor-mediated responses in the pithed rat and in the rat vena cava

1. The influence of body temperature on the alpha 1- and alpha 2-adrenoceptor-mediated pressor responses has been investigated in the pithed rat. 2. The pressor responses to noradrenaline, to the full alpha 1-adrenoceptor agonists phenylephrine and cirazoline and to tyramine were not influenced by lowering the temperature from 36-37 degrees C to 27-29 degrees C. In contrast, the dose-response curves for the pressor effects of the partial alpha 1-adrenoceptor agonist ST 587 and of the alpha 2-adrenoceptor agonist B-HT 920 (2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]-azepine), B-HT 933 (2-amino-6-ethyl-5,6,7,8-tetrahydro-4H-[4,5-d]azepine), clonidine, moxonidine and M-7 (2-dimethylamino-5,6-dihydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide) were markedly depressed (without change in the ED50 values), when the body temperature was lowered from 36-37 degrees C to 27-29 degrees C. 3. After i.v. administration of yohimbine, there was a rightward shift of the dose-response curve for B-HT 920, and the degree of this shift was the same at all temperatures investigated. 4. In the rat vena cava preincubated with [3H]-noradrenaline, the B-HT 920-induced alpha 2-adrenoceptor-mediated inhibition of electrically evoked tritium overflow was also reduced at lower temperature. 5. These results are compatible with the suggestion that cooling decreases the alpha 2-adrenoceptor-mediated pre- and postsynaptic responses in the rat vena cava and pithed rat respectively, leaving the pressor effect induced by full, but not partial alpha 1-adrenoceptor agonists in the pithed rat unaffected. 6. These differences may partly be related to differences in receptor reserve for alpha 1- and alpha 2-adrenoceptors in the pithed rat preparation.

Inhibitory effects of catecholamines in the paravertebral sympathetic ganglia of the anesthetized dog

alpha-Adrenoceptor agonists decreased mean arterial pressure when injected into the arterial blood supply of the paraspinal sympathetic ganglia of pentobarbital-anesthetized open-chest dogs. The hypotensive response occurred concomitantly with selective decreases of vascular resistance in the vessels innervated by neurons arising from these ganglia, and both of these responses were blocked by the ganglionic blocking agent, hexamethonium. The hypotensive response to phenylephrine was selectively blocked by terazosin; alpha 1 selective agonist, and antagonist, respectively, while the hypotension produced by intra-arterial clonidine was blocked by rauwolscine; alpha 2 selective agonist and antagonist, respectively. Either terazosin or rauwolscine reduced the hypotension produced by noradrenaline or dopamine. These results demonstrated the presence of both alpha 1- and alpha 2-adrenoceptors in the paraspinal sympathetic ganglia. Activation of either alpha-adrenoceptor subtype inhibited ganglionic transmission.

Cardiovascular effects of bromocriptine in rats: role of peripheral adrenergic and dopaminergic receptors

1. Experiments were designed to study the involvement of alpha-adrenoceptors and dopamine receptors in the hypotensive and bradycardic actions of bromocriptine in rats. 2. Intravenous administration of bromocriptine reduced blood pressure and heart rate which was inhibited by ganglionic blocking agents or by pithing. 3. The fall in blood pressure produced by bromocriptine was not modified by atropine, atenolol, prazosin, yohimbine, bilateral vagotomy or carotid ligation, but was blocked by sulpiride, domperidone and haloperidol. 4. The bradycardia produced by bromocriptine in intact rats was assumed to be mediated by the autonomic nervous system since it was partly reduced by bilateral vagotomy or atenolol, and entirely prevented by pithing. Furthermore, sulpiride but not yohimbine antagonized this effect. 5. In pithed rats, bromocriptine decreased both the pressor response (above 10 micrograms kg-1) and the tachycardia (above 50 micrograms kg-1) elicited by electrical stimulation of spinal cord outflow. Both effects were inhibited by sulpiride or yohimbine. 6. In pithed rats, bromocriptine did not affect the hypertension due to exogenous noradrenaline, phenylephrine, B-HT 920, nor the bradycardia evoked by stimulation of the cardiac muscarinic receptors by carbachol. 7. These results suggest that, in rats, bromocriptine produces hypotension via an action on presynaptic and/or ganglionic dopamine receptors, and causes bradycardia by activation of central dopamine receptors.

Antiarrhythmic and antinecrotic effects of yohimbine stereoisomers in rats during coronary occlusion and reperfusion

In pentobarbital-anesthetized rats the left coronary artery was ligated for 5 or 30 min and then opened for reperfusion of the ischemic myocardial area. Twelve min prior to the coronary occlusion yohimbine stereoisomers, namely corynanthine and rauwolscine, or saline solution were given intravenously. In the saline controls both ischemia and reperfusion provoked severe tachyarrhythmias with ventricular fibrillation in 37.5% or 54.6%, respectively. Using corynanthine and rauwolscine a highly significant antiarrhythmic effect was observed. Corynanthine completely prevented ventricular fibrillation and delayed significantly the development of myocardial necrosis. The factors likely underlying the antiarrhythmic and antinecrotic effects are discussed.

An examination of the sources of calcium for contractions mediated by postjunctional alpha 1- and alpha 2-adrenoceptors in several blood vessels isolated from the rabbit

1. The roles of intracellular and extracellular-derived Ca2+ in alpha-adrenoceptor-mediated contractions to noradrenaline (NA) have been investigated in several isolated blood vessels from the rabbit by examining responses in the presence of a modified Krebs-Henseleit saline with 2.5 mM Ca2+ and a Ca2(+)-buffered saline with 0.1 microM free Ca2+. 2. NA was tested in preparations of the abdominal aorta, distal saphenous artery, renal vein, lateral saphenous vein, plantaris vein and ear vein exposed to a Ca2(+)-buffered saline with 0.1 microM [Ca2+]. A concentration of NA which was maximally effective in modified Krebs-Henseleit saline, produced an initial transient contraction (ITC) followed by a relaxation towards baseline. This is evidence that alpha-adrenoceptor-mediated responses in all these blood vessels depend upon calcium from both sources. 3. The ITC was particularly pronounced in the arteries and was associated more closely with the alpha 1-receptor subtype. In the abdominal aorta, distal saphenous artery and renal vein the ITC can almost exclusively be attributed to an alpha 1-adrenoceptor (prazosin-sensitive, rauwolscine-resistant). In the ear vein, and to a lesser extent the plantaris vein, the ITC was mediated in part by an alpha 2-adrenoceptor (prazosin-resistant, rauwolscine-sensitive). 4. alpha 2-Adrenoceptors in the lateral saphenous vein largely account for the response to NA in modified Krebs-Henseleit saline, but alpha 1-adrenoceptors mediate the ITC in Ca2(+)-buffered saline. After selective inactivation of alpha 1-adrenoceptors with a combination of phenoxybenzamine and rauwolscine, responses to NA in modified Krebs-Henseleit saline are slow in onset and there is no ITC in Ca2(+)-buffered saline. 5. The possible significance of the coupling of postjunctional alpha 2-adrenoceptors to dual sources of Ca2 + is discussed in relation to the interaction between alpha-adrenoceptor subtypes and the ease of demonstrating functional alpha 2-adrenoceptors in isolated blood vessels.

Prejunctional alpha 1-adrenoceptors modify release of [3H]noradrenaline in the guinea-pig vas deferens

1. Several alpha 1- and alpha 2-adrenoceptor agonists and antagonists were examined for effects on spontaneous and stimulus-evoked release of [3H]noradrenaline from sympathetic nerves in guinea-pig vas deferens. 2. Prazosin (0.1 and 1 microM), phentolamine (30 microM) and yohimbine (10 microM) each enhanced the stimulus-evoked release of [3H]noradrenaline. 3. Prazosin and phentolamine increased the spontaneous outflow of [3H]noradrenaline, whereas yohimbine was without effect. 4. Methoxamine (10 microM) and clonidine (0.1 microM) inhibited the stimulus-evoked release of [3H]noradrenaline, whereas only methoxamine (1 microM) decreased the spontaneous outflow of [3H]noradrenaline. 5. The identity of prejunctional alpha-adrenoceptors in the guinea-pig vas deferens is discussed.

Central effect of yohimbine on sexual behavior in the rat

A large range of doses of yohimbine (Y) was administered intracerebroventricularly (ICV) (5-100 micrograms/rat) or intraperitoneally (IP) (0.35-10 mg/kg) to male rats and the effects on sexual, locomotor and general behavior were evaluated. For both routes there was a clear-cut inverted-U effect (stimulating/depressing), calculable as parabolic regressions on the log of administered doses. The maximal stimulating doses (15 micrograms/rat ICV and 1 mg/kg IP) significantly shortened mount, intromission and ejaculation latencies and the mean interintromission interval. These data indicate the importance of CNS mechanisms in the sexual effect of Y.