Clonidine and morphine increase [3H]-noradrenaline overflow in mouse vas deferens

Autoinhibition, sympathetic cotransmission and biphasic contractile responses to trains of nerve stimulation in the rodent vas deferens

1. The present review critically discusses the evidence for and against the various hypotheses that have been proposed to explain the biphasic contractile response of the rodent vas deferens to trains of electrical field stimulation (EFS). 2. It is widely accepted that the initial component of the biphasic response of the rodent isolated vas deferens to trains of EFS is mediated by ATP and the second slower tonic contractions is mediated by noradrenaline (NA). This theory is based on the ability of antagonists of the post-junctional receptors for these neurotransmitters to inhibit the respective components of the biphasic response and on the ability of exogenous application of either ATP or NA to mimic the responses of each phase. 3. Prejunctional autoinhibition has also been proposed as the cause of the biphasic response. This is based primarily on the ability of alpha 2-adrenoceptor antagonists to transform responses from biphasic to monophasic and on the ability of neuronal NA uptake inhibitors to accentuate the separation of the two phases. 4. Atypical or extrajunctional NA receptors have also been proposed to be the mediators of the component of the response to nerve stimulation that is resistant to the traditional alpha-adrenoceptor antagonists. 5. Different contractile mechanisms and/or sources of calcium have also been postulated to cause the biphasic response. Blockers of intracellular Ca2+ mobilization are able to block the initial component, while blockers of extracellular Ca2+ entry inhibit the second tonic phase. 6. It is concluded that because alpha 1-adrenoceptor antagonists and blockers of P2 purinoceptors have also been shown to block both phases of the response to trains of EFS, prejunctional auto-inhibitory mechanisms perhaps provide the most sound explanation for the phenomenon of the biphasic contractile response to trains of EFS.

Evidence for the differential release of the cotransmitters ATP and noradrenaline from sympathetic nerves of the guinea-pig vas deferens

1. Experiments were carried out to quantify the stimulation-evoked overflow of catecholamines and purines (ATP, ADP, AMP and adenosine) from an in vitro sympathetic nerve-smooth muscle preparation of the guinea-pig vas deferens and from isolated bovine adrenal chromaffin cells. The superfused preparations were stimulated for 60 s with electrical field stimulation (EFS; vas deferens), dimethylphenylpiperazinium (chromaffin cells) or KCl (both preparations). 2. Samples of superfusate were taken at 10 s intervals during the 60 s stimulation period for analysis of purines by HPLC-fluorescence detection and catecholamines by HPLC-electrochemical detection. 3. The evoked overflow of catecholamines and purines from chromaffin cells occurred with the same time course and in a constant ratio of approximately 4:1 (catecholamine to purine). These findings are compatible with the release of catecholamines and purines from a homogeneous population of exocytotic vesicles in the chromaffin cells. 4. The evoked overflow of purines and noradrenaline (NA) from the vas deferens preparation differed from the pattern of overflow from chromaffin cells and there was also some temporal disparity in the overflow of the two cotransmitters. The evoked overflow of ATP exceeded that of NA. In addition, the overflow of NA was tonic while the overflow of ATP and the other purines was phasic. 5. The EFS-evoked overflow of NA and the purines from the guniea-pig vas deferens preparation was examined after treatment with the neuronal amine-uptake inhibitors desipramine and cocaine, the alpha 1-adrenoceptor agonist methoxamine, the alpha 1-adrenoceptor antagonist prazosin, the alpha 2-adrenoceptor antagonists idazoxan and yohimbine, the noradrenaline-depleting drug reserpine and the adrenergic neuron-blocking agent guanethidine. The results of these studies, together with an analysis of the metabolic degradation of extracellular ATP, indicated that the temporal disparity in the overflow of NA and ATP is unlikely to be due to differences in the clearance of the cotransmitters or to the release of purines from non-neuronal sites. These results indicate that evoked overflow of the cotransmitters accurately reflects release from nerves. This pattern of release from nerves suggests that the two cotransmitters are released from two separate populations of exocytotic vesicles. 6. Superfusion of the vas deferens with exogenous epsilon-ATP, a fluorescent derivative of ATP, revealed that there was essentially no metabolism of the nucleotide over 60 s unless the tissue was subjected to EFS. Upon EFS, there was a rapid and nearly complete degradation of ATP with a corresponding increase in ADP, AMP and adenosine. This indicates the presence of a nerve stimulation-dependent metabolism of ATP.

[Cys(O2NH2)2]enkephalin analogues and dalargin: selectivity for delta-opioid receptors

To investigate the structure-activity relationships for potent and selective action of enkephalins at the delta-opioid receptors, two newly synthesized analogues, [Cys(O2NH2)2,Leu5]enkephalin and [Cys(O2NH2)2, Met5] enkephalin and the hexapeptide [D-Ala2,Leu5]enkephalyl-Arg (dalargin) were tested and compared with [Leu5]enkephalin and [Met5]enkephalin, for their effectiveness to inhibit electrically evoked contractions of the mouse vas deferens (predominantly enkephalin-selective delta-opioid receptors) and the guinea pig ileum (mu- and kappa-opioid receptors). The mouse vas deferens assays included evaluation of the effects of opioid agonists on the first, purinergic, and the second, adrenergic, components of electrically evoked biphasic responses (10 Hz and 20 Hz) and on ATP- or noradrenaline-evoked, tetrodotoxin-resistant responses. The opioids tested inhibited in a similar manner: (i) the purinergic and the adrenergic components of the electrically evoked contractions; and (ii) the ATP- and noradrenaline-induced postjunctional responses of the mouse vas deferens. Extremely low IC50 values (of 2-5 orders) were found for [Cys(O2NH2)2,Leu5] enkephalin, whose relative potency was between 239 and 1316 times higher than that of [Leu5]enkephalin. The order of potency for the other peptides in this tissue was: [Cys(O2NH2)2,Met5]enkephalin > [Leu5]enkephalin > dalargin > [Met5]enkephalin. The highest IC50 values in the guinea pig ileum assays, indicating the lowest affinity for mu-/kappa-opioid receptors, were obtained for the cysteine sulfonamide analogues, while dalargin showed a potency four times higher than that of [Met5]enkephalin. The order of potency in this tissue was: dalargin > [Met5]enkephalin > [Leu5]enkephalin > [Cys(O2NH2)2,Met5]enkephalin > [Cys(O2NH2)2,Leu5]enkephalin. The ratio, IC50 in guinea pig ileum: IC50 in mouse vas deferens, indicating selectivity of the respective peptide for delta-opioid receptors, was extremely high for [Cys(O2NH2)2,Leu5]enkephalin and especially for the adrenergic component of the responses. This ratio for [Cys(O2NH2)2,Met5]enkephalin was higher than the ratios for dalargin, [Leu5]enkephalin and [Met5]enkephalin, which were about 3 orders of magnitude lower. The results suggest that incorporation of hydrophilic Cys(O2NH2) in the enkephalin molecule greatly increases the potency and selectivity of the analogues at delta-opioid receptors, while both D-Ala2 substitution and lengthening of the peptide chain by Arg6 in the molecule of [Leu5]enkephalin decrease them.

Functional responses of the rat isolated seminal vesicle to electrical field stimulation: a pharmacological analysis

1. Electrical field stimulation (EFS) of the rat isolated seminal vesicle elicited frequency-dependent and tetrodotoxin sensitive contractions which were unaltered by hexamethonium or mecamylamine. 2. Prazosin alone was not sufficient to abolish these responses, but a combination of atropine and prazosin was fully effective, indicating involvement of both noradrenergic and cholinergic mechanisms. 3. Responses were predominantly cholinergic (blocked by atropine, potentiated by ecothiopate but not significantly altered by prazosin or guanethidine) at 1-8 Hz but became increasingly noradrenergic (blocked by prazosin or guanethidine but relatively unaltered by atropine or ecothiopate) with increasing frequencies of stimulation. 4. Electrical field stimulation of seminal vesicles removed from reserpine or 6-hydroxydopamine (6-OHDA)-pretreated rats produced contractions that were clearly cholinergic in nature. 5. After exposing the seminal vesicles to guanethidine, or after pretreatment of rats with 6-OHDA, responses to EFS remained, indicating that activation of discrete cholinergic and noradrenergic innervations seem to underlie the contractile responses observed. 6. Yohimbine and prazosin potentiated the predominantly cholinergic responses at 1, 2 and 4 Hz in tissues from untreated rats, but not in those from animals pretreated with reserpine or 6-OHDA, indicating the possibility of an interaction between the two innervations. 7. No inhibitory responses to EFS could be demonstrated in tissues precontracted with KCl in the presence of a combination of atropine and prazosin suggesting the absence of a nonadrenergic, noncholinergic inhibitory innervation.

Effects of mu-, delta- and kappa-agonists on isolated right atria of the rat

The present study examined the effects of morphine, DAMGO, DPDPE and U-50, 488H on auricular rate on isolated right atria of the rat. All the opioid agonists tested induced a decrease of auricular rate. The maximal effect obtained with U-50,488H (75 +/- 8.3%) was significantly (p < 0.001) higher than that obtained with morphine (12 +/- 2.7%), DAMGO (8 +/- 0.6%) or DPDPE (11 +/- 1.8%). The inhibitory effects of U-50,488H were not antagonized by the presence of naloxone (10(-7) or 5 x 10(-7) M) or MR-2266 (10(-7) or 5 x 10 (-7) M). Moreover, U-50,488H did not change the auricular chronotropism in the presence of atropine (5 x 10(-7) M). In this case the maximal inhibitory effect was 79 +/- 6.7%, similar to that obtained with the kappa-agonist alone (75 +/- 8.3%). Propranolol (10(-8) or 5 x 10(-8) M) modified the inhibitory effect of U-50,488H. The maximal effect obtained by the kappa-agonist in presence of propranolol was 100 +/- 0 significantly (p < 0.01) higher than that obtained with U-50,488H alone. These results demonstrated that the depressant action of U-50,488H was not blocked in the presence of opioid receptor antagonists and probably does not involve opioid receptors. Furthermore, propranolol caused a dose-dependent potentiation of the effects of the kappa-agonist supporting the conclusion that it is not mediated by opioid receptors.

Effect of opioid receptor subtype-selective agonists on purinergic and adrenergic components of neurogenic contractions of mouse vas deferens

1. Effects of opioid agonists on the purinergic and adrenergic components of neurogenic contractions and in some experiments on transmitter overflow were studied in the mouse isolated vas deferens. 2. When the vas deferens was stimulated every 2 min by pairs of pulses 2 s apart in the presence of prazosin 0.3 microM (to isolate the purinergic component) or alpha,beta-methylene-ATP 3 microM (to isolate the adrenergic component), each pulse elicited a separate twitch. The opioid agonists [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO, mu-receptor-selective), [D-Pen2,D-Pen5]enkephalin (DPDPE, delta-selective) and trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]- benzeneacetamide (U-50488, kappa-selective) concentration-dependently reduced both purinergic and adrenergic contractions. For each agonist, maximal effects and concentrations causing half-maximal effects were very similar for inhibition of the purinergic component on the one hand and for inhibition of the adrenergic component on the other hand, although the adrenergic component was inhibited with a slight preference. Moreover, effects on contractions elicited by the first and the second pulse of the pairs were very similar. 3. When vasa deferentia preincubated with [3H]-noradrenaline were stimulated with trains of 100 pulses delivered at 20 Hz, morphine 10 microM reduced significantly both evoked tritium overflow and evoked contractions. Its effect was antagonized by naloxone. 4. It is concluded that, in contrast to drugs acting at some other presynaptic receptors, opioid mu-, delta- and kappa-agonists inhibit purinergic and adrenergic neurogenic contractions of the mouse vas deferens in a similar manner. In contrast to a previous report, no enhancement by morphine of the release of noradrenaline elicited by high frequency pulse trains was observed.

Evidence for involvement of catecholamines in the effect of morphine on ventricular automaticity in the rat

1. The present study examined the effects of morphine on ectopic automaticity induced by local injury in the isolated right ventricle of the rat. 2. Morphine (10(-7)-5 x 10(-5) M) induced a significant increase of ventricular rate similar to that produced by noradrenaline. The excitatory effect of morphine was not modified by the presence of naloxone (5 x 10(-5) M). The maximal effect obtained with morphine in the presence of naloxone was 60 +/- 7%, similar to that obtained with morphine alone (67 +/- 15%). The EC50 values for morphine in the absence (0.89 x 10(-7) M) and presence of naloxone (0.87 x 10(-7) M) were also similar. Apparently this effect is not mediated by postsynaptic opioid receptors. 3. The ventricular automaticity induced in isolated right ventricle of the rat was significantly decreased by the highest concentrations of naloxone (5 x 10(-5) and 10(-7) M). 4. Morphine (10(-9)-5 x 10(-5) M) did not significantly change ventricular automaticity in the presence of propranolol (5 x 10(-8) M) or in reserpinized rats (5 mg kg-1 i.p. 24 h before the experiments). The maximal increases induced by morphine in the presence of propranolol or in reserpinized rats were 5 +/- 0.8% and 16 +/- 14.7% respectively. These results were significantly different from the maximal increase obtained without propranolol or with non-reserpinized animals. It is possible that the effects of morphine on ventricular automaticity could be mediated by an indirect effect located presynaptically at the adrenergic nerve terminals through the release of catecholamines.

Interaction between morphine and noradrenaline on isolated heart muscle

1. We evaluated the interaction between morphine and noradrenaline on the right atria. 2. Noradrenaline significantly increased atrial contraction rate, with an effective concentration 16 (EC16) of 85 +/- 0.8 x 10(-9) M. 3. The EC16 of noradrenaline significantly decreased in the presence of morphine. 4. These results demonstrate that morphine increases the potency of noradrenaline on the isolated right atria.

Interactions of calcium antagonists and the calcium channel agonist Bay K 8644 on neurotransmission of the mouse isolated vas deferens

1. The present study compares the effects of verapamil and Bay K 8644 on twitches of the mouse vas deferens induced by field stimulation at 0.1 Hz. The influence of interactions between these drugs and nifedipine on neurotransmission was also investigated. 2. Bay K 8644 (0.1 nM-3 microM) and verapamil (1-100 microM) potentiated twitches maximally by about 1000% (EC50 17.3 nM) and 300% (EC50 17.5 microM), respectively. Nifedipine (0.1 nM-1 microM) only reduced twitch magnitude (IC50 7.7 nM). All effects were reversed following washout. 3. Yohimbine (1-100 microM) reversed twitch potentiation caused by verapamil but not by Bay K 8644. Prazosin (1 microM) did not reduce basal twitch tension nor antagonize twitch potentiation by verapamil. 4. Twitch inhibition by nifedipine was unaltered by previous incubation with verapamil (30 microM), but Bay K 8644 (1 microM) shifted the curve to nifedipine 120 fold to the right. Previous incubation with nifedipine (1 microM) blocked potentiation induced by verapamil but did not modify responsiveness to Bay K 8644. 5. Previous addition of verapamil (30 microM) markedly enhanced twitch potentiation caused by Bay K 8644 in a supra-additive fashion. In experiments conducted in the reversed condition, Bay K 8644 (1 nM but not 10 nM) potentiated the effect of verapamil in a similar manner but to a lesser extent. 6. It is concluded that verapamil, in contrast to nifedipine, markedly enhances neurally-evoked twitches of the mouse vas deferens. Bay K 8644 produces essentially the same effect as verapamil, but its potency is 1000 fold and its maximal effect about 3 fold greater than that observed for verapamil. It is suggested that the mechanism of twitch potentiation by verapamil is different from that of Bay K 8644 and may involve an increased release of non-adrenergic co-transmitter(s).