Dopaminergic modulation of hippocampal noradrenaline release

Summary ³H-Noradrenaline release in the rabbit hippocampus and its possible modulation via presynaptic dopamine receptors was studied. Hippocampal slices were preincubated with ³H-noradrenaline, continuously superfused in the presence of cocaine (30 mol/l) and subjected to electrical field stimulation. The electrically evoked tritium over-flow from the slices was reduced by 0.1 and 1 mol/l dopamine and apomorphine, but significantly enhanced by 10 mol/l apomorphine or by 0.1 and 1 mol/l bromocriptine. If the ₂-adrenoceptor antagonist yohimbine (0.1 mol/l) was present throughout superfusion, the inhibitory effects of dopamine and apomorphine were more pronounced and even 10 mol/l apomorphine and 1 mol/l bromocriptine inhibited noradrenaline release. Qualitatively similar observations were made in the presence of another ₂-antagonist, idazoxane (0.1 mol/l). In the presence of the D2-receptor antagonist domperidone (0.1 mol/l) the inhibitory effects of dopamine were almost abolished, whereas both apomorphine (>1 mol/l) and bromocriptine (>0.01 mol/l) greatly facilitated noradrenaline release. The D2-receptor agonist LY 171555 (0.1 and 1 mol/l) significantly reduced the evoked noradrenaline release whereas the D1-selective agonist SK & F 38393 was ineffective at similar concentrations. The effects of LY 171555 were abolished in the presence of domperidone (0.1 mol/l) but remained unchanged in the presence of yohimbine or idazoxane (0.1 mol/l, each).At 1 mol/l the D2-receptor antagonists domperidone and (-)sulpiride significantly increased the evoked noradrenaline release by about 10%. However, at this concentration, domperidone (but not (-)sulpiride) affected also basal tritium outflow. Bulbocapnine and the preferential D1-receptor antagonists SCH 23390 enhanced the evoked noradrenaline release already at 0.1 mol/l. Their marked facilitatory effects (50 to 60% increase at 1 mol/l) were reduced in the presence of idazoxane (0.1 mol/l) and almost abolished in the presence of 0.1 mol/l yohimbine, whereas the increase due to 1 mol/l (-)sulpiride persisted under these conditions.The evoked tritium efflux from rabbit hippocampal slices preincubated with ³H-serotonin was not affected by dopamine receptor agonists.From our results we conclude that hippocampal noradrenaline, but not serotonin release, is modulated via D2-dopamine receptors. In addition, our results provide evidence for more or less pronounced ₂-adrenoceptor agonistic properties of dopamine and ₂-adrenoceptor antagonistic properties of apomorphine, bromocriptine, SCH 23390 and bulbocapnine in this noradrenaline release model from CNS tissue.     

An analysis of theα-adrenoceptor modulation of vasomotor tone at the level of lateral medullary pressor area (LMPA)

Summary Microinjection of noradrenaline and clonidine into lateral medullary pressor area (LMPA) of chloralose anaesthetized cats produced dose dependent decrease in blood pressure without affecting heart rate, while phenylephrine did not elicit any cardiovascular response. Selective ₂-adrenoceptor, antagonists idazoxan and piperoxan, microinjected locally, blocked the effects of the agonists but prazosin and phenoxybenzamine, which are relatively selective for ₁-adrenoceptors, failed to do so. Clonidine did not elicit any response in guanethidine pretreated cats but noradrenaline microinjected into LMPA of these animals induced a pressor response which was blocked by prazosin pretreatment. It is concluded that catecholaminergic fibres impinging upon this are inhibit the activity of the inhibitory second order baroreceptor neurone by activating ₁-adrenoceptors while ₂-adrenoceptors situated presynaptically on these inhibitory catecholaminergic nerve terminals are responsible for the manifestation of the hypotensive effect of clonidine and exogenously administered noradrenaline.     

Pharmacology in vivo of the phenylindan derivative,lu 19-005, a new potent inhibitor of dopamine,noradrenaline and 5-hydroxytryptamine uptake in rat brain

Summary Behavioural effects on dopaminergic transmission of a phenylindane derivative, Lu 19-005 [(±)-trans-3-(3,4-dichlorophenyl)-N-methyl-l-indanamine, HCl], with potent inhibitory effect on dopamine (DA), noradrenaline (NA) and serotonin (5-HT) uptake in rats and the effect on DA, NA and 5-HT activity in mice have been studied and compared with those of other known DA, NA and 5-HT uptake inhibitors with different selectivity ratios.Lu 19-005 induced stereotyped behaviour after parenteral and oral administration with a duration of action of more than 24 h. The stereotyped licking and biting induced by Lu 19-005 was antagonized by reserpine and cis(Z)-flupentixol, but not affected by prazosin, p-chlorophenylalanine and -methyl-p-tyrosine pretreatments. Metergoline slightly facilitated the onset of stereotypy. Lower doses of Lu 19-005 induced ipsilateral circling in unilaterally 6-hydroxy-DA-lesioned rats. Finally, Lu 19-005 antagonized the catalepsy induced by perphenazine. In mice, Lu 19-005 potentiated the apomorphine-induced gnawing, reversed tetrabenazine-induced ptosis and potentiated the behavioural effects of 5-HTP within a similar dose range.The effects of Lu 19-005 were compared with those of other reference compounds. Nomifensine had qualitatively similar effects in rats although of much shorter duration. In mice, nomifensine selectively reversed tetrabenazine-induced ptosis. Weaker effects in all test models were found with bupropion, LR 5182 and GBR 13.069, compounds with inhibitory effect on DA and NA uptake. The DA-, NA-and 5-HT-uptake inhibitor diclofensine, however, had no effect in rats except in the 6-hydroxy-DA-circling test and had low potency in mice. The specific 5-HT-and NA-uptake inhibitors citalopram and talsupram, respectively, were ineffective in all rat models. They selectively potentiated 5-HTP or reversed tetrabenazine0induced ptosis in mice, respectively, as expected according to their in vitro profile. These results indicate that effect on DA mechanisms are responsible for the behavioural activity of the test compounds in the rat models and that the circling model is the most sensitive. Since DA may be involved in some depressive states Lu 19-005 could be an attaactive new antidepressant as it combines the pharmacological profile of established antidepressants (effect on NA and/or 5-HT uptake) with equipotent activity on DA uptake.     

The membrane potential of vascular smooth muscle appears to modulate uptake2 of 3H-isoprenaline

Summary Segments of the rabbit main pulmonary artery and of its two branches were exposed for 10 min to 50 nmol/l ³H-(±)-isoprenaline, and the accumulation of tritium in the tissue was determined; COMT was inhibited in all experiments. 1. The accumulation of the tritium label was sensitive to 3-O-methyl-isoprenaline (OMI), showing that this vascular smooth muscle possesses uptake₂. 2. In the presence of 0.01 to 1 mol/l (–)-noradrenaline, the accumulation of tritium was depressed (in a concentration-dependent manner). This decline involved the OMI-sensitive accumulation of ³H-isoprenaline. 3. The presence of any one of three selective alpha₁-adrenoceptor antagonists (1 gmol/l prazosin, 1 mol/l WB4101, 10 gmol/l corynanthine) prevented the effect of 1 mol/l (–)-noradrenaline on the accumulation of tritium. However, in the absence of (–)-noradrenaline, the three antagonists failed to affect the accumulation of tritium. 4. 10 mol/l nicorandil caused the accumulation of tritium to increase. 5. As stimulation of alpha₁-adrenoceptors is known to result in depolarization, and as nicorandil is known to hyperpolarize this smooth muscle, it is concluded that the resting membrane potential modulates uptake₂. Send offprint requests to U. Trendelenburg at the above address     

Differential effects of α-β-methylene ATP on responses to nerve stimulation in SHR and WKY tail arteries

Summary The effects of ,-methylene-adenosine triphosphate, (,-methylene ATP, a P₂-receptor desensitising agent) have been evaluated on vasoconstrictor responses elicited by exogenous agonists or electrical field stimulation in isolated perfused SHR or WKY tail arteries and on tritium release elicited by electrical field stimulation in SHR-tail arteries pre-labeled with ³H-noradrenaline.Exposure to ,-methylene ATP (0.1 mol/l) significantly inhibited vasoconstrictor responses to electrical field stimulation in SHR tail arteries. These inhibitory effects were not further increased at a higher concentration of ,-methylene ATP (1 mol/l). In WKY tail arteries, ,-methylene ATP (1 mol/l) failed to significantly inhibit vasoconstrictor responses to electrical stimulation.In SHR tail arteries prelabelled with ³H-noradrenaline, ,-methyleneATP (1 mol/l) did not inhibit the stimulation evoked release of tritium. However, at this concentration, ,-methylene ATP significantly antagonized the vasoconstrictor responses of SHR tail arteries induced by exogenous ATP (1 mol/l), ,-methylene ATP (30 mol/l), a stable agonist at P₂-receptors, or 60 mmol/l KCl. These effects of ,-methylene ATP on contractile responses to KCl were not observed in WKY-tail arteries.In tail arteries obtained from reserpine pretreated SHR, despite a 85–95% decrease in endogenous noradrenaline tissue content, the vasoconstrictor responses induced by periarterial field stimulation were greatly diminished, but not abolished. These residual responses to periarterial field stimulation were not antagonized by prazosin (0.1 mol/l), but were practically abolished by the addition of ,-methylene ATP (1 mol/l).In tail arteries from WKY rats pretreated with reserpine, exposure to prazosin (0.1 mol/l) further reduced the residual responses elicited by electrical field stimulation. In these WKY-tail arteries, addition of ,-methylene ATP (1 mol/l) did not further inhibit the remaining vasoconstrictor response obtained in the presence of prazosin.While our results suggest a significantly greater cotransmitter role for ATP with noradrenaline in tail arteries of SHR compared with control normotensive WKY rats, additional effects of ,-methylene ATP not involving P₂ receptors cannot be entirely excluded.     

Raising the ambient potassium ion concentration enhances carbachol stimulated phosphoinositide hydrolysis in rat brain hippocampal and cerebral cortical miniprisms

Summary The influence of the ambient potassium ion concentration ([K⁺]_ upon agonist stimulated hydrolysis of phosphoinositides (PI) has been studied in isolated miniprisms of rat hippocampus and cerebral cortex. When the external [K⁺] was raised from 6 to 18 mmol/l, there was little or no increase in the hydrolysis of PI in the absence of agonist, however, carbachol (100 mol/l) stimulated hydrolysis was greatly enhanced in both brain regions studied. Thus, carbachol stimulated the hydrolysis of PI to 146% and 386% of control levels at potassium concentrations of 5.8 and 18.2 mmol/l, respectively, in the rat hippocampus. A similar enhancement of muscarine (100 mol/l) stimulation was observed in cortical miniprisms with 18 mmol/l [K⁺]. A further enhancement was seen at higher ambient [K⁺], although basal hydrolysis of PI was then also increased. The carbachol-stimulated hydrolysis of PI found at both 6 and raised [K⁺] was prevented by atropine (1 and 10 mol/l) and tetraethylammonium (20 mmol/l), but not by 10 mmol/l Mg²⁺. Pirenzepine (50 nmol/l) also reduced this response. The ions Cs⁺ and Rb⁺ (but not Li⁺ or Tris⁺) produced a similar enhancement of the carbachol stimulation to that found with K⁺. At a buffer [K⁺] of 6 mmol/l, noradrenaline (100 mol/l) produced a 2-fold increase in the hydrolysis of PI whereas 5-hydroxytryptamine (100 mol/l) and histamine (500 mol/l) had little or no effect. However, histamine and 5-hydroxytryptamine did stimulate the hydrolysis of PI when [K⁺] was increased. Miniprism ATP content was not changed by a rise in [K⁺] to 18 mmol/l. The significance of these results is discussed in terms of the postsynaptic cellular events following cholinergic stimulation.     

Different effects of noradrenaline,angiotensin II and BAY K 8644 on the abolition of autoregulation of renal blood flow by verapamil

Summary To examine the role of Ca channels in autoregulation of renal blood flow in response to changes of perfusion pressure, experiments were performed with perfused kidney in anesthetized dogs using a Ca channel activator, BAY K 8644, and vasoconstrictors such as noradrenaline and angiotensin II. Control observations usually showed excellent autoregulation of renal blood flow at pressures between 120 and 200 mm Hg, the autoregulatory index being less than 0.2. Verapamil (50 g/min, i.a. infusion) obviously inhibited the renal autoregulation. Simultaneous infusion of 5 g/min of BAY K 8644 with verapamil prevented both the increase of renal blood flow and the impairment of the autoregulation caused by verapamil. Whereas simultaneous infusion of noradrenaline (1 and 3 g/min) or angiotensin 11 (0.1 and 0.3 g/min) with verapamil dose-dependently reduced renal blood flow, these drugs could not antagonize the inhibitory effect of verapamil on autoregulation. The present experiments show that Ca channels play an important role in establishing renal autoregulation, and that a mere vasoconstriction by noradrenaline and angiotensin II is distinguished from autoregulatory performance.     

Effects of noradrenaline and somatostatin on basal and stimulated mucosal ion transport in the guinea-pig small intestine

Summary Noradrenaline (NA) and somatostatin (SOM) stimulate intestinal water and ion absorption and are found in mucosal nerve fibres and nerve terminals in submucous ganglia of the guinea-pig small intestine. As the main projection of submucous neurons is to the mucosa, NA and SOM might alter mucosal transport either by a direct effect on the epithelium or indirectly, by affecting submucous neurons. In this study these two possible sites of action of NA and SOM have been investigated in mucosa-submucosa preparations of guinea-pig ileum. In addition, the actions of NA and SOM on the secretory responses caused by stimulation of different populations of submucous neurons have been studied. The stimulants of secretion used were a nicotinic agonist, 1,1-dimethyl-4-phenylpiperazinium (DMPP, 10^–5 M), 5-hydroxytryptamine (5-HT, 10^–7 M) and electrical field stimulation (EFS), which activate cholinergic, noncholinergic and mixed populations of submucous secretomotor neurons, respectively.Segments of intestine were dissected free of external muscle and myenteric plexus and mounted in Ussing chambers. Short-circuit current (I _sc) was measured as an indication of net active ion transport across the tissue. NA (10^–8 M) and SOM (>10^–10 M) each caused a decrease in I _sc, indicating a net increase in ion absorption. The NA response was abolished and the magnitude of the SOM response was reduced to 20% by tetrodotoxin (10^–7 M). DMPP, 5-HT and EFS each stimulated nerves that increased I _sc and each of these responses was significantly diminished by NA and SOM; for both NA and SOM the decrease in the DMPP response was significantly greater than the decrease observed in the response to carbachol (10^–6 M). Phentolamine (10^–6 M) abolished all of the effects of NA but caused no change in the SOM effects. These studies have shown that NA and SOM cause similar changes in net ion transport, that their actions are primarily on submucous secretomotor neurons and that NA and SOM can diminish the responses to stimulation of both cholinergic and noncholinergic submucous neurons.In this tissue it is also known that SOM coexists with NA in noradrenergic nerve terminals in the submucosa. However, when applied together, NA and SOM caused no greater decrement in the carbachol and 5-HT responses than would be predicted by adding the separate effects of NA and SOM. Hence there was no obvious interaction between NA and SOM effects on mucosal transport.     

Polymyxin B,a selective inhibitor of protein kinase C,diminishes the release of noradrenaline and the enhancement of release caused by phorbol 12,13-dibutyrate

Summary Slices of the rabbit hippocampus were labelled with ³H-noradrenaline, superfused continuously with a modified Krebs-Henseleit medium containing the uptake inhibitor cocaine and stimulated electrically (2 ms, 3 Hz, 24 mA, 5 V/cm). Phorbol 12,13-dibutyrate (PDB), a potent activator of protein kinase C (PKC), strongly enhanced the electrically-evoked overflow of tritium. In contrast, polymyxin B, a relatively selective inhibitor of PKC, diminished the evoked tritium overflow in a time-and concentration-dependent manner. The enhancement of the evoked overflow of tritium caused by PDB was strongly reduced in the presence of polymyxin B (100 mol/l). These results suggest 1. that PKC may be involved in the physiological mechanism of action-potential-induced noradrenaline release from noradrenergic nerve terminals and 2. that the PDB-induced enhancement of noradrenaline release may be due to a direct activation of PKC.Abbreviations PKC protein kinase C - PDB phorbol 12,13-dibutyrate - TPA 12-O-tetradecanoyl 13-acetate     

Differential effects of nifedipine,verapamil, and diltiazem on noradrenaline-induced contractions,adrenergic transmitter release,and alpha-adrenoceptor binding in the female rabbit urethra

Summary In order to study differences in action between Ca²⁺-entry blockers on smooth muscle and peripheral nerves, the effects of nifedipine, verapamil, and diltiazem on noradrenaline (NA)-induced contractions and electrically evoked release of ³H-NA were investigated in the female rabbit urethra. In addition, possible influences of Ca²⁺-entry blockers on alpha-adrenoceptors were studied with radioligand binding technique. Exposure to Ca²⁺-free medium completely abolished the contractile response to 1 M NA in the rabbit urethra, indicating that the contraction was entirely dependent on influx of extracellular Ca²⁺. The Ca²⁺-entry blockers inhibited the NA-induced contractions in the following order of potency: nifedipine>verapamildiltiazem. In contrast to nifedipine and diltiazem, which produced a maximum inhibition of between 50 and 60%, verapamil was able to abolish the contractile responses to NA. The electrically evoked efflux of ³H-NA was decreased by diltiazem and increased by verapamil, whereas nifedipine failed to alter the ³H-NA efflux. Only verapamil was effective in inhibiting specific ³H-DHE binding to a crude membrane preparation of the rabbit bladder base and urethra, and the inhibition appeared to be of the competitive type. It is suggested that the effects of verapamil on electrically evoked efflux of ³H-NA and on NA-induced contractions can be partly explained by blockade of pre- and post-junctional alpha-adrenoceptors. The failure of nifedipine and diltiazem to abolish the NA-induced contraction might indicate the existence of different Ca²⁺-entry pathways in urethral smooth muscle.