Hyperpolarizing 'alpha 2'-adrenoceptors in rat sympathetic ganglia

Involvement of both dopaminergic and alpha-adrenergic receptors in the hypomotility induced by dibenzoyl-6,7-ADTN

The dopaminergic prodrug dibenzoyl-6,7-ADTN (DB-6,7-ADTN) can enter the brain following intraperitoneal injection and be hydrolysed to produce low concentrations of the dopamine agonist 6,7-ADTN. Intraperitoneal injections of DB-6,7-ADTN produce a decrease in motor activity and in the present study this response has been characterised, and the underlying mechanisms examined. Doses of 10-100 mumol/kg DB-6,7-ADTN elicit a strong hypomotive response, which is dose dependent. Treated animals are significantly less active than controls. DB-6,7-ADTN hypomotility was significantly attenuated by the non-sedative dopamine receptor antagonist sulpiride (62 mumol/kg, i.p.), but haloperidol (0.3 mumol/kg, i.p.) and cis-flupenthixol (0.45 mumol/kg, i.p.) were without effect. The hypomotility due to DB-6,7-ADTN was also antagonised by yohimbine (13 mumol/kg, i.p.) and piperoxane (21 mumol/kg i.p.), drugs which act mainly by blocking presynaptic (alpha 2) adrenergic receptors. Prazosin (1.5 mumol/kg, i.p.), drugs which act mainly by blocking presynaptic (alpha 2) adrenergic receptors. Prazosin (1.5 mumol/kg, i.p.), a postsynaptic (alpha 1) adrenergic blocker, did not affect the hypomotility, and nor did a range of other neurotransmitter antagonists. DB-6,7-ADTN (50 mumol/kg, i.p.) was also found to antagonise the alpha-methyltyrosine (alpha-MT, 1.02 mmol/kg, i.p.) induced fall in noradrenaline and dopamine levels in brain and spinal cord. Inhibition of the effects of DB-6,7-ADTN on noradrenaline and dopamine turnover by yohimbine and sulpiride, respectively, suggests that 6,7-ADTN (derived from the prodrug) has alpha adrenergic as well as dopaminergic activity. The results are discussed in connection with the hypomotive effects of other dopamine agonists.

Comparison of the antimuscarinic activity of mianserin and amitriptyline in the cat superior cervical ganglion

The antimuscarinic activity of mianserin and amitriptyline was investigated in the cat superior cervical ganglion using McN-A-343 as a muscarinic agonist. Mianserin was found to have a similar order of potency to amitriptyline in this preparation, in contrast to the marked difference in their antimuscarinic activities on non-neuronal muscarinic receptors. These findings suggest that the action of mianserin on neuronal muscarinic receptors is more important than that predicted from studies on non-neuronal receptors and may be related to its anti-depressant activity.

Site(s) and ionic basis of alpha-autoinhibition and facilitation of "3H'noradrenaline secretion in guinea-pig vas deferens

1. Mechanisms controlling the secretion of [(3)H]noradrenaline from the noradrenergic nerves of guinea-pig isolated vas deferens, prelabelled by incubation with [(3)H]noradrenaline, were studied using (a) different modes of (extramural or transmural) electrical nerve stimulation (a total of 300 shocks of varying strength, and a duration of 2 msec) at 1-30 Hz, or (b) depolarizing concentrations of K(+) (60-110 mm).2. The fractional rise in efflux of (3)H-labelled material (Deltat) was used to measure the secretion of [(3)H]noradrenaline.3. The dependence of [(3)H]noradrenaline secretion on the external Ca(2+) concentration (1-8 mm) was essentially hyperbolic. Double reciprocal plot analysis (1/Deltat vs. 1/Ca(2+)) of the data yields that blockade of alpha-autoinhibition (phentolamine 1 mum) does not increase the maximal secretory velocity, but does enhance the apparent affinity of the secretory mechanism for external Ca(2+). Exogenous noradrenaline has (qualitatively) opposite effects. The interaction between alpha-autoinhibition and external Ca(2+) thus shows a ;competitive' pattern, indicating that restriction of the utilization of external Ca(2+) is a major mechanism in alpha-autoinhibition of noradrenaline secretion, in this system.4. Phenoxybenzamine (10 mum) and phentolamine (1 mum) increased the secretion of [(3)H]noradrenaline evoked by depolarization with K(+) much less than that caused by electrical nerve stimulation (frequencies up to 10 Hz). Exogenous noradrenaline (1-5 mum) depressed the secretion evoked by both modes of stimulation. The results indicate that alpha-autoinhibition of [(3)H]noradrenaline secretion is mainly operative when the secretory stimulus requires conduction of nerve impulses between varicosities.5. The frequency dependence of [(3)H]noradrenaline secretion was hyperbolic, both in the presence and in the absence of alpha-autoinhibition; at each frequency the secretion (Deltat per shock) increased with the Ca(2+) concentration in the medium (0.6-8 mm). Double reciprocal plot analysis (1/Deltat vs. 1/frequency) of the data yields that the pattern of interaction between external Ca(2+) and facilitation depends on the presence or absence of alpha-autoinhibition (phentolamine 1 mum); in the former case it is ;non-competitive', in the latter ;competitive'. Similar analysis of the effect of facilitation by increasing the length of stimulus trains (from 5 to 300 pulses) at a constant frequency (5 Hz), on the Ca(2+) dependence of Deltat (1/Deltat vs. 1/Ca(2+)) in the absence of alpha-autoinhibition also yields that facilitation promotes utilization of external Ca(2+). These results apparently imply that a rise in external Ca(2+), in the presence of alpha-autoinhibition, augments the secretory response to electrical nerve stimulation mainly by promoting recruitment of active units (varicosities?), without markedly altering their ;affinity' for facilitation. In the absence of autoinhibition (when all units are already recruited?), the results seem to imply that facilitation promotes depolarization-secretion coupling in each, by more efficient utilization of external Ca(2+).6. The pattern of interaction between alpha-autoinhibition and facilitation depends on the Ca(2+)concentration in the medium. At or below the physiological level of Ca(2+) in extracellular fluid (1.2 mm) it is ;non-competitive', indicating that alpha-autoinhibition and facilitation act, at least in part, at separate targets under these conditions. At high (5.4 mm) external Ca(2+) the pattern becomes almost purely ;competitive', indicating that facilitation can, under suitable conditions, overcome all manifestations of alpha-autoinhibition.7. The secretion evoked by electrical nerve stimulation (Deltat per shock, at 1 or 10 Hz) increased with the strength of applied shocks, both when applied extra- or transmurally, in the presence or absence of alpha-autoinhibition. In the former case the rise in (Deltat per shock) vs. (current strength) was hyperbolic, in the latter it followed a biphasic pattern. Double reciprocal plot analysis (1/Deltat vs. 1/current) of the data yields a ;non-competitive' pattern of interaction between facilitation or alpha-autoinhibition, and exogenous current, when stimulation was extramural. When it was transmural the pattern is ;competitive'. The results seem to imply that hyperpolarization, or depolarization, of nerve terminals are major mechanisms whereby alpha-autoinhibition and facilitation, respectively, exert their effects on the secretory response to electrical nerve stimulation.8. Neither activation of Na(+), K(+)-ATPase, nor promotion of G(Cl) appear to be critically involved in alpha-autoinhibition. Experiments with known blockers of G(K) (tetraethylammonium, 4-aminopyridine and Rb(+)) did not give support to the notion that promotion of K(+) efflux is a mechanism whereby prejunctional alpha-adrenoceptors cause (hyperpolarization of nerve terminals and) autoinhibition of secretion. If alpha-autoinhibition does involve K(+) channels in the nerve terminal membrane, then these must be different from the (voltage-sensitive) K(+) channels blocked by the above mentioned inhibitors of K(+) efflux.9. The results are discussed in the context of a model that assumes that local control of noradrenaline secretion from noradrenergic nerves may be exerted both by control of invasion of terminals, and by control of depolarization-secretion coupling in each invaded varicosity. Under suitable conditions facilitation and alpha-autoinhibition may interact at both levels. It proposed that utilization of external Ca(2+) plays a pivotal role for both, and that restriction of invasion of nerve terminal varicosities is the main effect of alpha-autoinhibition, while promotion of depolarization-secretion coupling is the main effect of facilitation, at physiological concentrations of Ca(2+) in the medium. For the nerve the role of this dual control system is proposed to be to ensure ;rotational' activation of varicosities, and for the effector cell of noradrenergic junctions, to increase the signal/noise ratio.

Mini-review. The postsynaptic alpha 2-adrenoreceptor

The discovery of postsynaptic alpha 2-adrenoreceptors with the drug specificities of presynaptic alpha 2-adrenoreceptors has contributed to a refinement of the classification of alpha-adrenoreceptors. postsynaptic alpha 2-adrenoreceptors have been identified by pharmacological means and with the aid of direct radioligand-receptor binding studies. The evidence for the existence of this class of alpha 2-adrenoreceptors in the brain and in vascular smooth muscle is particularly strong. Central postsynaptic alpha 2-adrenoreceptors play a major part in the hypotensive action of centrally acting antihypertensive drugs such as clonidine and alpha-methyl-DOPA. Vascular smooth muscle cells contain postsynaptic alpha 2-adrenoreceptors which mediate vasoconstriction, like the more classical alpha 1-adrenoreceptors. The simultaneous occurrence of contractile alpha 1- and alpha 2-adrenoreceptors in vascular smooth muscle offers a simple model for the characterization of alpha-adrenoreceptor agonists and antagonists. At present, highly selective agonists of alpha 2-adrenoreceptors have been found. These new compounds may be useful for the classification of other alpha-adrenoreceptor populations. It has been suggested that the vascular postsynaptic alpha 2-adrenoreceptor might be located at extrasynaptic sites. Accordingly, adrenaline released by the adrenal medulla would be the endogenous stimulant. Finally, observations have been made in vivo indicating that a transmembrane influx of calcium ions is necessary for linking the drug-induced activation of these alpha 2-adrenoreceptors to vasoconstriction.

Muscarinic receptors in rat sympathetic ganglia

1 Potential changes in isolated superior cervical ganglia of the rat produced by muscarinic-receptor agonists were recorded by an extracellular ;air-gap' method.2 Muscarinic agonists produced a delayed low-amplitude ganglion depolarization, frequently preceded by a hyperpolarization. Potentials were enhanced by reducing [K(+)](o) or [Ca(2+)](o).3 Mean ED(50) values (muM) for depolarization at 25 degrees C were: oxotremorine 0.004, methylfurmethide 0.11, (+/-)-muscarine 0.24, furmethide 1.56, pilocarpine 4.81 and AHR-602 (N-benzylpyrrolidylacetate methobromide) 10.8. Responses produced by oxotremorine, pilocarpine and AHR-602 showed some characteristics of ;partial agonism'. ED(50) values (muM) for choline esters (measured in the presence of 2.5 mM hexamethonium) were: acetylcholine 3.2, methacholine 59 and bethanechol 78.4 Responses to muscarine were antagonized by hyoscine (K(I) 0.49 nM) atropine (K(I) 0.24 nM) methylscopolamine (K(I) 0.09 nM) lachesine (K(I) 0.15 nM) and (weakly) by hexamethonium (K(I) 0.2 mM). Propylbenzilylcholine mustard produced irreversible antagonism with an apparent onset rate constant of 2 x 10(5) M(-1)S(-1).5 Depolarization was accompanied by facilitation of submaximal ganglionic transmission.6 Muscarine (1 to 100 muM) initially reduced, then increased, the rate of (86)Rb(+)-efflux from isolated ganglia at both 6 and 120 mM [K(+)](o). These effects were reduced by 1 muM hyoscine.7 No consistent change in the amounts of cyclic 3',5'-guanosine monophosphate in isolated ganglia accompanying muscarinic depolarization could be detected.8 Mean against ED(50) values (muM) for contracting the rat isolated ileum were: oxotremorine 0.012, methylfurmethide 0.29, (+/-)-muscarine 0.48, pilocarpine 7.8 and AHR-602 9.9. Mean antagonist K(I) values (nM) were: hyoscine 0.17, atropine 0.34 and lachesine 0.27.9 It is concluded that ganglionic muscarinic receptors are quite similar to ileal receptors in terms of agonist ED(50) and antagonist K(I) values, and that the major difference between them lies in the greater ;efficacy' of certain agonists (pilocarpine, AHR-602 and McN-A-343) on the ganglion.

The role of sodium influx mediated by nicotinic receptors as an initial event in trans-synaptic induction of tyrosine hydroxylase in adrenergic neurons

1. Isolated superior cervical ganglia of the rat were incubated for 2--30 min (37 degrees C) in Krebs' solution or tissue culture medium (BGJb) containing 22Na and then washed for 30 min in ice-cold 22Na-free Krebs' solution (to clear extracellular space). The radioactivity remaining in the ganglia was taken as a measure of 22Na influx into the intracellular compartment of the ganglion. 2. Addition of cholinomimetics (100 microM nicotine or 100 microM carbachol) to the incubation led to an increase in 22Na influx. This increase reached maximal values after 10 min of incubation; it was more pronounced after incubation in Krebs' solution than in BGJb medium. 3. While chlorisondamine (3 microM) or dopamine (100 microM) greatly reduced the carbachol-induced 22Na influx, tetrodotoxin (2 microM) did not have any effect. 4. In ganglia obtained from animals treated with 6-hydroxydopamine in the early postnatal phase (resulting in an extensive destruction of peripheral sympathetic neurons) neither carbachol (100 microM) nor nicotine (100 microM) produced an increase in 22Na influx demonstrating that the intraneuronal compartment is responsible for this enhanced influx. 5. The effects of dopamine, chlorisondamine and tetrodotoxin on the carbachol-induced 22Na uptake into superior cervical ganglia are similar to their effects on carbachol-mediated induction of tyrosine hydroxylase in superior cervical ganglia kept in tissue culture (Thoenen and Otten 1977b). It is concluded that the induction of tyrosine hydroxylase via nicotinic receptors is closely linked to the enhanced sodium influx into the adrenergic neurons mediated by the same receptors.

Alpha-drenergic inhibition of calcium-dependent potentials in rat sympathetic neurones

1. Post-ganglionic neurones of the rat superior cervical ganglion were studied in vitro (21-26 degrees C) using single intracellular micro-electrode methods. 2. Three Ca2+-dependent potentials were studied: the shoulder on the normal action potential, the hyperpolarizing afterpotential (h.a.p.), and th Ca2+ spike. 3. Bath-applied noradrenaline reversibly inhibited these Ca2+-dependent potentials. The EC50 for inhibition of peak h.a.p. amplitude was about 1 microM. The order of catetholamine potency was: L-adrenaline > L-noradrenaline > D-noradrenaline congruent to dopamine > DL-isoprenaline. Phentolamine (10 microM), an alpha-blocker, but not MJ-1999 (10 microM), a beta-blocker, antagonized the action of noradrenaline. 4. Noradrenaline (10 microM) hyperpolarized most neurones (1-6 mV) studied, with no detectable change in resting membrane conductance. 5. Superfusion with low external Ca2+ and high Mg2+ mimicked the effect of noradrenaline. Either procedure alone antagonized the h.a.p. conductance increase but did not alter the h.a.p. reversal potential. However, in the presence of low Ca2+, high Mg2+, the remaining action potential and h.a.p. were not further reduced by noradrenaline. 6. The Ca2+-dependent shoulder of the action potential did not appear dependent upon GK. Noradrenaline and low Ca2+ antagonized the shoulder when enhanced by TEA+ or Ba2+. 7. Both the rate of rise and amplitude of the Ca2+ spike were antagonized by noradrenaline. 8. We propose that activation of an alpha-adrenoceptor inhibits a voltage-sensitive Ca2+ conductance (GCa(V)), thereby reducing the inward Ca2+ current which may generate the noraml action potential shoulder and the rising phase of the Ca2+ spike. Reduction of Ca2+ current would also reduce the Ca2+-dependent portion of outward K+ current underlying the h.a.p.

Dopamine depolarisation of mammalian primary afferent neurones

Dopamine (DA) is an important neurotransmitter or neuromodulator in the mammalian nervous system. As such, it is implicated in the aetiology and therapy of various disease conditions--for example, Parkinson's disease, schizophrenia, Huntington's disease and tardive dyskinesia. However, only limited electrophysiological information is presently available concerning dopamine receptors in the mammalian nervous system, and there are only three reports in which intracellular techniques have successfully recorded the action of DA on individual central neurones. In all cases, DA depolarised the respective neurones. In the periphery, DA is reported to hyperpolarise superior cervical ganglia. However, this hyperpolarisation has been shown to be due to activation of alpha-adrenoreceptors and not to a response of DA on a DA receptor. Peripheral DA actions have also been described presynaptically, but are difficult to study electrophysiologically for technical reasons. As a result, little is known at the membrane level about the effects of drugs thought to modulate or interact with DA receptors. In the present report, we describe a depolarising action for DA on the cat dorsal root ganglion.

Interaction of selective alpha-adrenoceptor agonists and antagonists with human and rabbit blood platelets

1 The selectivity of alpha-adrenoceptors mediating the pro-aggregatory response of human and rabbit platelets to adrenaline and the conditions required to permit expression of an aggregatory response to partial agonists at these alpha-adrenoceptors have been studied.2 Yohimbine causes effective blockade of the pro-aggregatory responses whereas indoramin and prazosin are ineffective.3 The clonidine analogue, UK-14304, is nearly as effective as adrenaline in inducing an aggregatory response in human platelets and a pro-aggregatory response in rabbit platelets. Cross-tachyphylaxis between adrenaline and UK-14304 has been demonstrated.4 Clonidine is a weak agonist for the pro-aggregatory response of rabbit platelets and in some donors for the aggregatory response of human platelets.5 Methoxamine induces a pro-aggregatory response in human platelets which is blocked by indoramin or prazosin but not by yohimbine. No such response to methoxamine is observed in rabbit platelets.6 The divalent cation ionophore, A-23187, induces an aggregatory response to clonidine (in platelets from a non-responsive donor), phenylephrine and methoxamine in human platelets and to adrenaline, UK-14304 and clonidine in rabbit platelets. A secretory response to clonidine is also induced by A-23187 in human platelets.7 The adenylate cyclase inhibitor, SQ-22536, is ineffective in either inducing a response to the alpha-agonists or potentiating the effect of A-23187.8 The aggregatory responses to adrenaline and UK-14304 in rabbit platelets and to clonidine in human and rabbit platelets, which can be induced by A-23187, are blocked by yohimbine but not by prazosin or indoramin.9 From these studies we conclude that the pro-aggregatory responses of human and rabbit platelets to adrenaline are mediated primarily by alpha(2)-adrenoceptors. The presence of alpha(1)-adrenoceptors on human platelets is confirmed but these receptors do not appear to be present on rabbit platelets. The conditions required for expression of an aggregatory response to partial agonists at the human and rabbit platelet alpha-adrenoceptors implicate an increase in cytosolic Ca(2+) concentration as a key event in stimulus-response coupling but do not indicate such a role for depression of cyclic adenosine-3',5'-monophosphate concentration.

Effects of metoclopramide and isoprenaline in the rat vas deferens; interactions with alpha-adrenoceptors

1 Metoclopramide (2.8 to 280 microM) augmented contractions of rat vas deferens preparations induced by field stimulation (6 Hz for 1 s). This effect was antagonized by phentolamine (0.1 microM). Metoclopramide (2.8 to 280 microM) did not affect phenylephrine-induced contractions. 2 Metoclopramide (2.8 to 280 microM) antagonized the inhibitory effects of clonidine on the contractions induced by field stimulation, but not the inhibitory effects of purine nucleosides. 3 From these results it is concluded that metoclopramide (2.8 to 280 microM) is a presynaptic alpha-adrenoceptor antagonist in the rat vas deferens. 4 Following beta-adrenoceptor blockade with (+/-)-propranolol (3.3 microM), (-)-isoprenaline (0.47 to 14 microM) inhibited responses to field stimulation but not to phenylephrine. These propranolol-resistant effects of isoprenaline were antagonized by metoclopramide (2.8 to 280 microM) and by phentolamine (0.1 to 10 microM), indicating that isoprenaline may stimulate presynaptic alpha-adrenoceptors in this preparation.

Relation between catecholamine-induced cyclic AMP changes and hyperpolarization in isolated rat sympathetic ganglia

1. The effect of catecholamines on cyclic adenosine 3'5'-monophosphate (cyclic AMP) production in isolated rat superior cervical ganglia has been measured under experimental conditions in which they also produce ganglion hyperpolarization.2. (+/-)Isoprenaline (1 muM) increased cyclic AMP levels by 8-100 times after 15 min incubation at 25 degrees C. Half-maximal stimulation occurred at about 0.03 muM. This was due to stimulation of beta-receptors, since it was prevented by 1 muM-propranolol but not by 1 muM-phentolamine.3. The alpha-agonists phenylephrine (100 muM), dopamine (100 muM) and clonidine (1 muM) did not produce a detectable increase in ganglionic cyclic AMP. Dopamine (100 muM) was also ineffective at 37 degrees C in the presence of 10 mM-theophylline.4. Exogenous cyclic AMP (0.01-1 mM) hyperpolarized the ganglion. This effect was replicated by other adenosine compounds, most effectively by adenosine and by adenosine 5'-monophosphate, and was antagonized by theophylline. Dibutyryl cyclic AMP was weaker than cyclic AMP.5. Neither theophylline nor the non-xanthine phosphodiesterase inhibitor, Ro 20-1724, enhanced the hyperpolarizing actions of noradrenaline or dopamine.6. Since catecholamine-induced hyperpolarization of the isolated rat ganglion is induced via alpha-receptors, whereas cyclic AMP-production is induced via beta-receptors, it is concluded that cyclic AMP is unlikely to mediate the hyperpolarization. The effect of exogenous cyclic AMP may be due to an action on external adenosine-receptors.