Interneurones are probably not involved in the presynaptic dopaminergic control of dopamine release in rabbit caudate nucleus

Summary Slices of rabbit caudate nucleus were preincubated with ³H-dopamine and then superfused. The influence of apomorphine and haloperidol on the overflow of tritium evoked by 20 mmol/l potassium was investigated in the presence and in the absence of tetrodotoxin. The potassium-evoked overflow was largely calcium-dependent and consisted mainly of ³H-dopamine. The dopamine receptor agonist apomorphine 0.01–1.0 mol/l reduced, whereas the antagonist haloperidol 0.1 mol/l enhanced the potassium-evoked overflow of tritium. The effects of apomorphine and haloperidol were as pronounced in the presence as in the absence of tetrodotoxin 0.3 mol/l. It is concluded that the presynaptic dopaminergic modulation of dopamine release is not mediated by a tetrodotoxin-sensitive interneuronal pathway.     

Effect of extracellular dopamine on the release of dopamine in the rabbit caudate nucleus: Evidence for a dopaminergic feedback inhibition

Summary Slices of the head of the rabbit caudate nucleus were preincubated with 10^–7 M ³H-dopamine and then superfused, and the effect of unlabeled dopamine on the outflow of tritium was investigated. In most experiments, nomifensine was added throughout superfusion in order to block uptake of the unlabeled amine. Nomifensine was a potent inhibitor of the uptake of ³H-dopamine into rabbit caudate synaptosomes, with an IC₅₀ of 5·10^–8 M at a ³H-dopamine concentration of 4·10^–8 M.In the absence of nomifensine, unlabeled dopamine (10^–7 M and higher concentrations) accelerated the basal outflow of tritium from preincubated slices. 10^–5 M nomifensine strongly counteracted the acceleration. In the presence of nomifensine, unlabeled dopamine (10^–7 to 10^–6 M) caused a concentrationdependent decrease of the overflow of tritium evoked by electrical stimulation at 0.1 Hz. Chlorpromazine and haloperidol (in the presence of nomifensine) increased the stimulation evoked overflow and antagonized the inhibitory effect of dopamine.It is concluded that extracellular dopamine shares with other dopaminergic agonists the ability to inhibit action potential-evoked release of intraneuronal dopamine. The inhibition is mediated by specific receptors. The results support the hypothesis that previously released dopamine, by an action on these receptors, can inhibit further release of dopamine.     

Pathways of dopamine metabolism in the rabbit caudate nucleus in vitro

Summary Slices of rabbit caudate nucleus were preincubated with ³H-dopamine and then superfused. ³H-dopamine and its metabolites were separated by column chromatography.The basal outflow of tritium consisted of 68% DOPAC, 21% OMDA metabolites (most of which was HVA), 6% dopamine and 5% MTA. Except for an increase in ³H-dopamine, the basal outflow was little changed by nomifensine or cocaine. Amezinium reduced the outflow of ³H-DOPAC and ³H-OMDA metabolites and increased that of ³H-dopamine and ³H-MTA; its effect was antagonized by nomifensine. Haloperidol 10^–7 M selectively enhanced the outflow of ³H-DOPAC and ³H-OMDA metabolites. At haloperidol 10^–5M, a large increase in these two fractions was accompanied by a small increase in ³H-dopamine and ³H-MTA. Nomifensine diminished only slightly the outflow of ³H-DOPAC caused by haloperidol.The overflow of tritium elicited by electrical stimulation at 3 Hz consisted of 74% DOPAC, 15% OMDA metabolites, 10% dopamine and 0.4% MTA. Lowering the frequency reduced the overflow of total tritium and caused a decrease in the percentage of ³H-DOPAC and an increase in the percentage of ³H-dopamine. Nomifensine or cocaine greatly diminished the evoked overflow of ³H-DOPAC and ³H-OMDA compounds and increased the evoked overflow of ³H-dopamine and ³H-MTA. The effects of apomorphine and bromocriptine were similar to those caused by decreasing the stimulation frequency.The major pathways of the metabolism of previously taken up dopamine, under the conditions of our experiments, are as follows. When the neurones are at rest, dopamine metabolism is initiated by leakage of the amine into the axoplasm, where it is degraded through the aldehyde to DOPAC. Part of the DOPAC is methylated extraneuronally to HVA. Traces of MTA are formed by extraneuronal methylation of dopamine. When action potentials arrive, dopamine metabolism is initiated by exocytosis. The bulk of the extracellular dopamine is taken up back into the neurones. When the stimulation frequency is 3 Hz, most of the axoplasmic dopamine is subsequently transformed to DOPAC; little seems to be re-stored. HVA and MTA are generated essentially as during neuronal rest. When dopamine release is low (stimulation at low frequency; addition of apomorphine or bromocriptine), a larger portion seems to be re-stored, thus leading to a decrease in the percentage of DOPAC. Haloperidol, apart from its receptor blocking properties, acts on dopaminergic axons in a manner akin to the effect of reserpine.Abbreviations and Terminology AD Aldehyde dehydrogenase (E.C.1.2.1.3.) - COMT catechol-O-methyl transferase (E.C 2.1.1.6.) - DOPAC 3,4-dihydroxyphenylacetic acid - DOPEG 3,4-dihydroxyphenylglycol - DOPET 3,4-dihydroxyphenylethanol - HVA 3-methoxy-4-hydroxyphenylacetic acid - MAO monoamine oxidase (E.C. 1.4.3.4.) - MOPET 3-methoxy-4-hydroxyphenylethanol - MTA 3-methoxytyramine - OMDA metabolites O-methylated and deaminated metabolites. Intraneuronal refers to location in dopaminergic terminal axons, extraneuronal to location in other cells, neuronal as well as nonneuronal     

Differential effects of the stereoisomers of 3PPP in dopaminergic and cholinergic neurotransmission in superfused slices of the corpus striatum

Summary The two enantiomers of 3PPP were tested on the spontaneous and electrically-evoked release of ³H-dopamine from slices of the rabbit caudate nucleus and of ³H-acetylcholine (³H-ACh) from slices of the rat caudate nucleus.In caudate slices labelled with ³H-dopamine, exposure to (+)3PPP (0.1–1 M) facilitated the spontaneous outflow of radioactivity with a concomitant inhibition of the electrically-evoked release of ³H-dopamine. In the presence of cocaine 10 M, exposure to (+)3PPP (1 M) inhibited the electrically evoked release of ³H-dopamine without modifying the spontaneous outflow of radioactivity. This inhibitory effect was not significantly antagonized by S-sulpiride 0.01 M.Exposure to (+)3PPP 1 M inhibited the electrically-evoked release of ³H-ACh, and this effect was not modified by pretreatment with reserpine alone, or in combination with -methyl-p-tyrosine (-MT).In contrast to the (+) enantiomer, exposure to (-)3PPP (0.1–1 M) facilitated the electrically-evoked release of ³H-dopamine without affecting the spontaneous outflow of radioactivity. (-)3PPP antagonized the inhibitory effect of apomorphine on the electrically-evoked release of ³H-dopamine.Exposure to (-)3PPP 1 M did not modify the spontaneous or the electrically-evoked release of ³H-ACh. Yet, this concentration of (-)3PPP antagonized significantly the inhibitory effect of 0.03 M apomorphine, 1 M d-amphetamine, and 1 M (+)3PPP on the electrically-evoked release of ³H-ACh (-)3PPP (0.1–1 M) was about 100 times less potent than S-sulpiride at antagonizing the inhibitory effect of apomorphine on the electrically-evoked release of ³H-ACh.It is concluded that under in vitro conditions at the level of the dopamine receptor modulating the release of ³H-ACh from the cholinergic interneuron in the striatum, (+)3PPP behaves as a dopamine receptor agonist while (-)3PPP possesses dopamine receptor-antagonist properties. At the level of the dopaminergic nerve terminal, (-)3PPP facilitates the release of ³H-dopamine probably through the blockade of the dopamine autoreceptors. The dopamine autoreceptor agonists properties of (+)3PPP are difficult to establish in our model because of the dopamine releasing action of this enantiomer.Some of the results described in this publication have been presented at the British Pharmacological Society Meeting (Arbilla and Langer 1983).     

In vitro acetylcholine release from rat caudate nucleus as a new model for testing drugs with dopamine-receptor activity

Summary The effects of a number of dopamine-receptor agonists on depolarization-induced (26 mM K⁺) release of ³H-acetylcholine from slices of rat caudate nucleus were examined with a superfusion method. Apomorphine (10^–6 M) and N,N-dipropyliso-ADTN (10^–7 M) inhibited acetylcholine-release in vitro by about 50% and these inhibitory effects were antagonized by haloperidol. For N,N-dipropyl-iso-ADTN an EC₅₀ of approximately 3×10^–9 M was estimated from its dose-response curve. However, dopamine (10^–6 M) itself and bromocriptine (10^–6 M) inhibited acetylcholine-release less. Presumably: the weak effect of exogenous dopamine is due to its (partial) uptake in dopaminergic nerve terminals and to the fact that released endogenous dopamine already partially activates the receptors involved in the inhibition of acetylcholine-release.Pretreatment of young rats with 6-hydroxydopamine (+ desipramine) increased the inhibitory effects of dopamine-receptor agonists, including dopamine itself, on acetylcholine-release from caudate slices, indicating dopamine-receptor supersensitivity. This was corroborated by the finding that apomorphine-induced stereotyped behavior was significantly higher in rats lesioned with 6-hydroxydopamine than in controls.It is suggested that K⁺-induced release of radiolabelled acetylcholine from caudate nucleus slices provides a functional model to study the characteristics of post-synaptic dopamine-receptors in vitro. The concentrations of dopamine-receptor agonists needed to inhibit acetylcholine-release appear to be in the nanomolar range, in agreement with their affinities as determined in dopamine-receptor binding studies. In contrast, these concentrations are much lower than those required for stimulation of dopamine-sensitive adenylate cyclase activity.     

Effects of neuroleptics on release of 3H-dopamine from slices of rat corpus striatum

Summary The characteristics of ³H-DA release from striatal slices by electrical stimulation were analyzed and the effects of a number of neuroleptics thereon were examined under different experimental conditions. The butyrophenones, haloperidol and spiroperidol, already at low concentrations (0.1–1 M) increased basal tritium efflux in a dose-dependent manner. The phenothiazines, chlorpromazine and fluphenazine, were much less effective in this respect.The butyrophenones strongly inhibited the electrically stimulated overflow of both ³H-DA and ¹⁴C-GABA, while the phenothiazines again had little effect. The action of 1 M haloperidol on ³H-DA release could be blocked by 10 M cocaine, but not with 1 M apomorphine. Apomorphine itself had no significant effect on ³H-DA release.Our data do not support the suggestion that presynaptic DA receptors on dopaminergic nerve terminals may modulate the release of newly taken-up ³H-DA. Some neuroleptics, particularly the butyrophenones may have presynaptic effects not related to interaction with DA receptors. It is suggested that different mechanisms may be involved in the local presynaptic receptor-mediated feedback regulation of transmitter release in noradrenergic and dopaminergic systems in the CNS.     

Increase in plasma ACTH after dopaminergic stimulation in rats

The effects of a dopaminergic agonist, apomorphine, and a dopaminergic antagonist, haloperidol, on plasma ACTH, and corticosterone levels were evaluated in adult male rats. Subcutaneous administration of apomorphine in the dose range of 50–500 g · kg^–1 significantly increased plasma corticosterone levels. Acute treatment with apomorphine (250 g · kg^–1) resulted in an elevation of plasma ACTH concentration, peak values being reached 15 min after the injection. The apomorphine-induced rise in plasma ACTH levels was completely inhibited by pretreatment with haloperidol (1 mg · kg^–1). A stimulatory role for dopamine receptors in the control of pituitary ACTH release in the rat is suggested.     

Transmitter release patterns of noradrenergic,dopaminergic and cholinergic axons in rabbit brain slices during short pulse trains,and the operation of presynaptic autoreceptors

Summary Slices of rabbit brain were field-stimulated either by single electrical pulses or by trains of 4 or 8 pulses at 1 or 100 Hz in order to study transmitter release patterns and the autoinhibition of transmitter release. The slices were preincubated with ³H-noradrenaline (cortex), ³H-dopamine (caudate nucleus) or ³H-choline (caudate nucleus).Slices preincubated with ³ H-noradrenaline were superfused with medium containing desipramine 1 gmol/l. The overflow of tritium elicited by single pulses amounted to 0 .19% of the tritium content of the tissue. The overflow elicited by 4 pulses/1 Hz was similar, whereas that elicited by 4 pulses/100 Hz was 5.1-fold higher. Yohimbine 101000 nmol/l increased up to 2.5-fold the overflow evoked by 4 pulses/1 Hz but did not change the overflow evoked by single pulses or 4 pulses/100 Hz. - Slices preincubated with ³ H-dopamine were superfused with medium containing nomifensine 1 mol/l. The overflow of tritium elicited by single pulses was 0.39% of the tritium content of the tissue. The overflow elicited by 4 pulses/1 Hz was 1.3-fold and the overflow elicited by 4 pulses/100 Hz 1.4-fold higher. Domperidone 1–100 nmol/l and sulpiride 10–1000 nmol/1 increased up to 2.4-fold the overflow evoked by 4 pulses/ 1 Hz but increased only slightly the overflow evoked by single pulses or 4 pulses/100 Hz. - Slices preincubated with ³ H-choline were superfused either with physostigmine-free medium or with medium containing physostigmine 1 mol/l. In physostigmine-free medium, atropine did not increase the evoked overflow of tritium at any stimulation condition. In physostigmine-containing medium, the overflow elicited by single pulses was 0.18% of the tritium content of the tissue. The overflow elicited by 8 pulses/1 Hz was 2.0-fold and the overflow elicited by 8 pulses/100 Hz 2.2-fold higher. Atropine 2–200 nmol/1 increased up to 2.4-fold the overflow evoked by 8 pulses/1 Hz but increased only slightly the overflow evoked bysingle pulses or 8 pulses/100 Hz. In physostigmine-free medium, sulpiride 10–1000 nmol/1 did not change the single-pulse-evoked overflow of tritium in the absence but increased it in the presence of nomifensine 1 mol/l.Single pulses elicit a large release of ³H-noradrenaline, ³H-dopamine and ³H-acetylcholine under the conditions of these experiments. Release elicited by single pulses is not subject to autoinhibition except for a small inhibition by spontaneously released transmitter in the case of dopaminergic and cholinergic axons. When 3 or 7 further pulses follow the first one at intervals of 1 s, they elicit much smaller release. At least a great part of the fall is due to autoreceptor mediated inhibition (for 3H-acetylcholine release in the presence of physostigmine only). When 3 or 7 further pulses follow at intervals of 10 ms, they elicit release that is either similar to that evoked by the first pulse (³H-noradrenaline) or much smaller (³H-dopamine, ³H-acetylcholine). However, the fall is not due to stimulation-dependent, auto-receptor-mediated inhibition; autoinhibition does not develop in these short high-frequency trains. Overall, the results are in accord with the autoreceptor theory. They demonstrate the role of autoinhibition in determining the transmitter release patterns of central noradrenergic, dopaminergic and cholinergic neurones. Send offprint requests to N. Limberger at the above address     

Inhibitory effects of apomorphine and pergolide on neurogenic vasoconstriction in the hindquarters of the rat

Summary The effects of locally administered apomorphine and pergolide were studied in the isolated autoperfused hindquarters of the rat, in an attempt to assess the possible role of presynaptic dopamine receptors at the level in the hypotensive effect of these dopamine agonists.Local infusion of apomorphine (1g·kg^–1·min^–1 for 5 min) or pergolide (1g·kg^–1·min^–1 for 5 min) [into the hindquarters] did not alter perfusion pressure per se, but reduced the pressor response to electrical stimulation of the lumbar sympathetic chains for the whole frequency range used during a cumulative frequency-response curve (0.25–16 Hz, 1 ms, supramaximal voltage). Apomorphine and pergolide reduced the pressor response elicited by 4 Hz electrical stimulation (applied until maximum response was reached) to 54.8±7.1% and 53.9±1.7% respectively, but they did not modify similar increases of perfusion pressure produced by locally administered noradrenaline.The inhibition by apomorphine and pergolide of the 4 Hz stimulation-evoked pressor response was completely antagonized by local administration of the dopamine antagonist haloperidol (1g·kg^–1), but was not influenced by the ₂-antagonist rauwolscine (100g·kg^–1). This dose of rauwolscine antagonized the inhibitory effect of the ₂-agonist UK-14,304, which was not influenced by haloperidol.Local administration of rauwolscine increased the pressor response to stimulation at 4 Hz by 37.4–46.2%. In contrast, local administration of haloperidol did not influence the 4 Hz stimulation-evoked pressor response.These results indicate that dopamine receptors are pressent on the sympathetic innervation of the vascular bed in the rat hindquarters but do not provide evidence for a physiological role of these receptors in modulating peripheral sympathetic neurotransmission. Stimulation of these receptors, leading to a decrease of noradrenaline release and thus of vasomotor tone, might—at least in part—explain the blood pressure lowering effects of intravenous apomorphine and pergolide in the rat.     

Cyclic AMP facilitates the electrically evoked release of radiolabelled noradrenaline,dopamine and 5-hydroxytryptamine from rat brain slices

Summary The adenylate cyclase activator forskolin as well as 8-bromo-cyclic AMP enhanced the electrically evoked release of³H-noradrenaline and³H-5-hydroxytryptamine from superfused rat neocortical slices and that of³H-dopamine from neostriatal slices with comparable EC₅₀'s of about 0.5 and 50 M, respectively, without affecting spontaneous tritium efflux. The phosphodiesterase inhibitor ZK 62771 (3–100 M) also enhanced³H-noradrenaline and³H-dopamine release but slightly reduced³H-5-hydroxytryptamine release. However, this drug profoundly enhanced spontaneous tritium release in the latter case. The facilitatory effect of forskolin (0.3 M) on the release of the amine neurotransmitters was potentiated in the presence of ZK 62771 (30 M). Therefore, cyclic AMP appears to exert a general facilitatory effect on the release of these biogenic amines from central nerve terminals.     

Influence of morphine and naloxone on the release of noradrenaline from rat brain cortex slices

Summary In slices of rat brain cortex preincubated with (–)-³H-noradrenaline, the influence of morphine and naloxone on the efflux of tritium was investigated. The spontaneous outflow of tritium was not changed by 10^–7–10^–5 M morphine and by 10^–6–10^–4 M naloxone, but was accelerated by 10^–4 M morphine. Electrical field stimulation augmented tritium outflow. The overflow evoked per ppulse decreased as the frequency of stimulation was increased from 0.3 to 3 Hz, but remained approximately constant when it was further increased to 10 Hz. At frequencies of 0.3, 1, and 3 Hz, but not at 10 Hz, morphine in concentrations of 10^–7–10^–5 M depressed the stimulation-induced overflow of tritium. 10^–4 M morphine did not influence the overflow induced by stimulation at 0.3 and 1 Hz and increased that evoked by stimulation at 10 Hz. Naloxone (10^–6–10^–4 M) did not change the response to stimulation. In the presence of 10^–4 M naloxone, 10^–6 M morphine did not diminish, and 10^–5 M morphine even enhanced the stimulation-induced overflow of tritium. The inhibitory effect of 10^–6 M morphine was not reduced, after tyrosine hydroxylase had been blocked by -methyltyrosine-methylester. It is concluded that morphine through an action on specific opiate receptors inhibits the release of transmitter from cerebrocortical noradrenergic neurones evoked by nerve impulses. By an action unrelated to opiate receptors, morphine at high concentrations increases the stimulation-induced overflow of noradrenaline, presumably by inhibiting its re-uptake into nerve endings.     

Mepiprazole,a new psychotropic drug: Effects on uptake and retention of monoamines in rat brain synaptosomes

The influence of mepiprazole (EMD 16,923), a new pyrazol-ylalkyl-piperazine derivative, on the uptake of ³H-norepinephrine (NE), ³H-dopamine (DA), and ³H-serotonin (5-HT) into rat brain synaptosomes from cerebral cortex, corpus striatum, and hypothalamus was investigated in comparison with several psychotropic drugs, including oxypertine, d-amphetamine, imipramine, desipramine, chlorimipramine, amitriptyline, and chlorpromazine in vitro. Mepiprazole was a relatively weak inhibitor of monoamine uptake and exhibited its strongest action on the hypothalamic 5-HT uptake, being almost equipotent with desipramine (IC₅₀=0.9 M). Furthermore, the influence of the drugs on the retention of ³H-amines previously taken up by whole rat brain synaptosomes was studied. Unlike the tricyclic antidepressants, mepiprazole as well as oxypertine and d-amphetamine markedly increased the efflux of radioactivity during a 20-min incubation at 37°C at low concentrations (10^–6 to 10^–5 M), whereas at 10^–4M all drugs greatly enhanced the efflux. The ability of mepiprazole to increase 5-HT concentration at the receptor level by a combination of neuronal uptake inhibition and release is discussed in relationship to the central actions of the drug.     

Involvement of septal and striatal dopamine D-2 receptors in yawning behavior in rats

A behavioral study was performed in an attempt to understand the neuronal mechanisms involved in yawning behavior in rats. Subcutaneous injections of low doses of apomorphine (0.05–0.25 mg/kg) or piribedil (0.2–1.0 mg/kg), which preferentially activate presynaptic dopamine autoreceptors at those doses, evoked yawning. Marked yawning responses were also elicited by both 3-PPP (5–20 mg/kg, SC) and TL-99 (1–2 mg/kg, SC). SK & F 38393, a dopamine D-1 receptor agonist, at doses ranging from 0.1 to 8.0 mg/kg (SC) induced neither yawning nor stereotypy. However, bromocriptine (0.5–32.0 mg/kg, SC), a dopamine D-2 receptor agonist, induced yawning for which the dose-response curves showed a bell-shaped form. After a higher dose of 32 mg/kg (SC) bromocriptine, some rats occasionally showed sniffing and sawdust chewing. Yawning responses induced by systemic injection of apomorphine, piribedil, 3-PPP or bromocriptine were wholly suppressed after treatment with sulpiride (10 mg/kg SC), a dopamine D-2 receptor antagonist. Bilateral injections of apomorphine (20 g/side×2), piribedil (100 g/side×2) or 3-PPP (50, 100 g/side×2) into the striatum or septum also elicited marked yawning. The results indicate that low doses of apomorphine, piribedil, 3-PPP, TL-99 or bromocriptine elicit yawning by stimulating dopamine D-2 receptors and striatal and septal dopaminergic systems may be related to the occurrence of yawning behavior.     

Noradrenaline release from slices of the thalamus of normal and morphine-dependent rats

Summary The effect of morphine on potassium-induced stimulation of (³H)-noradrenaline release from slices of the rat thalamus was investigated. The in vitro addition of morphine (10^–6 M) significantly depressed potassium-induced tritium overflow by 42% and this was prevented by the prior addition of naloxone (3×10^–6 M) to the medium. The stimulation-evoked overflow of tritium from slices of the thalamus of morphine-dependent rats was not significantly different from normal controls. Addition of naloxone (10^–5 M) 10 min before exposure of the tissues to 20 mM K⁺ significantly enhanced noradrenaline release from dependent slices. The results suggest that the basic release mechanism may have adapted to the continuous presynaptic inhibition of release by morphine.     

Effects of apomorphine and haloperidol on “spontaneous” stereotyped licking behaviour in the Cebus monkey

Three recently arrived drug naive Cebus apella monkeys with spontaneous stereotyped oral movements were treated with apomorphine and haloperidol using a wide dose range. Low doses of apomorphine (0.05–0.1 mg/kg) suppressed the oral stereotypies without affecting normal behaviour such as grooming and scratching. Higher doses of apomorphine (0.25–1.0 mg/kg) and haloperidol (0.01–0.1 mg/kg) also decreased or abolished the oral stereotypies, but induced generalized stereotypies (apomorphine) or dystonia/parkinsonism (haloperidol), suppressing normal behaviour. The findings indicate that dopamine is involved in these presumably stress-induced (not drug-induced) stereotypies.     

Inhibition of noradrenergic neurotransmission by apomorphine and pergolide in the in situ autoperfused rat renal and superior mesenteric vascular beds

Summary In vitro studies have provided evidence that presynaptic dopamine receptors are present in the rat renal and superior mesenteric vascular beds. To confirm this in vivo, the effects of locally administered apomorphine and pergolide were studied in the in situ autoperfused renal and superior mesenteric vascular beds.Local infusion of apomorphine (1 g · kg^–1 · min^–1 for 5 min) or pergolide (1 g · kg^–1 · min^–1 for 5 min) into either the renal or the superior mesenteric artery had no effect on perfusion pressure per se. In the renal vascular bed, the pressure response to electrical stimulation (4 Hz, 1 ms, supramaximal voltage) was reduced to 49.8±4.8% by apomorphine and to 54.8±2.7% by pergolide; in the mesenteric vascular bed, apomorphine reduced the pressure response to electrical stimulation (4 Hz, 1 ms, supramaximal voltage) to 53.8±2.9, pergolide to 52.0±1.8%. Increases of perfusion pressure in the renal and in the mesenteric vascular bed induced by locally administered noradrenaline were not modified by apomorphine or pergolide.In both vascular beds, the inhibition of the stimulation-evoked pressure responses by apomorphine or pergolide was completely antagonized by local administration of the dopamine receptor antagonist haloperidol in a dose (1 g · kg^–1) which did not influence the inhibitory effect of the ₂-adrenoceptor agonist UK-14,304; the ₂-adrenoceptor antagonist rauwolscine, in a dose (100 g · kg^–1) which completely antagonized the inhibitory effect of UK-14,304, did not antagonize the inhibitory effects of apomorphine and pergolide.Local administration of rauwolscine per se increased the pressure response to stimulation at 4 Hz in both vascular beds. In contrast, local administration of haloperidol did not influence the stimulation-evoked pressure response.These results provide evidence for the presence of presynaptic, inhibitory dopamine receptors on sympathetic nerves in the rat renal and mesenteric vascular beds; these receptors could be involved in the blood pressure lowering effects of dopamine receptor agonists, such as apomorphine and pergolide.Research Associate of the National Fund for Scientific Research (Belgium)     

Involvement of different dopamine receptors in rat diphasic motility response to apomorphine

A critical dose of apomorphine (300 g/kg SC) given immediately before placing rats into a novel environment produced a diphasic motility response (initial sedation followed by enhanced locomotion). Various neuroleptics having different clinical and/or pharmacological profiles were studied by using such a model. (–)-Sulpiride and sultopride preferentially antagonized apomorphine inhibition; haloperidol and tiapride antagonized both phases of apomorphine response at similar doses; chlorpromazine, fluphenazine, thioridazine, metoclopramide and SCH 23390 preferentially antagonized apomorphine stimulation. The results are discussed in terms of the dopamine receptor subtypes involved in the two phases of apomorphine effect. Apomorphine stimulation can be antagonized by D-1 as well as D-2 receptor blockade. A higher affinity for D-2 receptors seems a necessary requisite for the antagonism of apomorphine inhibition; moreover, the ability of neuroleptics to antagonize apomorphine inhibition seems to depend on the ratio of their presynaptic versus postsynaptic D-2 activity. Offprint requests to: N. Montanaro     

Dopamine-induced neurogenic vasodilatation in isolated perfused muscle preparation of the dog

Summary Dopamine, injected into the lumbar aorta of the dog in doses which produce a reversible inhibition of synaptic transmission in the lumbar paravertebral ganglia (0.5–64×10^–8 moles), produces a neurogenic vasodilatation in the isolated perfused hindleg or gracilis muscle. This was abolished by acute preganglionic decentralization and by administration into the perfused preparation of -adrenoceptor blocking agents, but not of atropine or diphenhydramine. After decentralization, preganglionic electrical stimulation restored the dopamine-induced indirect vasodilatation. The neurogenic vasodilatation was also seen with intra-aortic injections of epinine (2–32×10^–8 moles) and apomorphine (1.2–19.2×10^–8 moles) and was preferentially blocked by haloperidol (0.26×10^–6 moles). (-)-Noradrenaline, injected into the lumbar aorta in baroreceptor-denervated dogs, was found to be equipotent with dopamine in eliciting the neurogenic vasodilatation; this (-)-noradrenaline-induced effect was preferentially blocked by phentolamine (8×10^–6 moles).The possibility that the neurogenic vasodilatation, which occurs upon intra-aortic injection of dopamine in the dog, is due to its ganglionic-inhibitory effect is discussed.This work was aided by grants from the Coordination Committee for Scientific Research (C.C.A.S.) and from the Fund for Interdisciplinary Research, Belgium.     

GABAergic modulation of D-1 dopamine receptor-mediated 3H-acetylcholine release from rabbit retina

Summary Dopamine evokes calcium-dependent release of ³H-acetylcholine from superfused rabbit retina labeled in vitro with ³H-choline, through activation of a D-1 dopamine receptor. This study investigates the activation of this receptor by endogenous dopamine and the modulation of the spontaneous and dopamine-evoked release of ³H-acetylcholine from rabbit retina labeled with ³H-choline by GABAergic agonists and antagonists. Endogenous dopamine, released from dopaminergic amacrine neurons by the indirect amines tyramine or D-amphetamine evoked the calcium-dependent release of ³H-acetylcholine from rabbit retina. The release of ³H-acetylcholine elicited by tyramine (10 M) or D-amphetamine (10 M) was attenuated by the selective D-1 antagonist SCH 23390 (0.1 M) and by the dopamine uptake inhibitor nomifensine (3 M). At concentrations of 1 mM and 1 M respectively, GABA and muscimol inhibited the spontaneous release of tritium from rabbit retina labeled in vitro with ³H-choline. Picrotoxin and bicuculline (10 M) increased the spontaneous release of tritium. GABA and the GABA agonist muscimol (0.01–100 M) inhibited in a concentration-dependent manner the release of ³H-acetylcholine elicited by 100 M dopamine with IC₅₀ values of 4.5 M and 0.02 M respectively. The inhibition of dopamine-evoked ³H-acetylcholine release by GABA (10 M) and muscimol (0.1 M) was antagonized by the GABA antagonists bicuculline and picrotoxin. Picrotoxin and bicuculline (10 M) increased the spontaneous release of tritium, and potentiated the release of ³H-acetylcholine evoked by 100 M dopamine consistant with a tonic, inhibitory GABAergic input to the cholinergic amacrine neurons in rabbit retina. Dopamine-evoked acetylcholine release in rabbit retina may be of physiological importance as D-1 dopamine receptor-mediated increases in ³H-acetylcholine release from rabbit retina can be elicited by endogenous dopamine. In addition, activation of GABA receptor sites modulates the spontaneous and dopamine-evoked acetylcholine release from rabbit retina. Send offprint requests to M. L. Dubocovich at the above address     

Effects of verapamil,diltiazem and ryosidine on the release of dopamine and acetylcholine in rabbit caudate nucleus slices

Summary Slices of the rabbit caudate nucleus were preincubated with ³H-dopamine or ³H-choline and then superfused with label-free medium. Release of ³H-dopamine and ³H-acetylcholine was elicited by either electrical stimulation at 8 (in one series 2) Hz, or an increase in the K⁺ concentration by 50 mmol/l, or addition of L-glutamate 1 mmol/l. Verapamil 1 mol/l, diltiazem 1 and 10 mol/l, and ryosidine 1 mol/l failed to the reduce the electrically-, K⁺- and glutamate-evoked overflow of tritium. Verapamil 1 mol/l and diltiazem 10 mol/l also failed to reduce the electricallyevoked overflow (2 Hz) when dopamine receptors, neuronal dopamine uptake, and neuronal choline uptake were blocked by domperidone, nomifensine and hemicholinium, respectively. Inhibition of the evoked overflow of tritium was only obtained when concentrations were increased to verapamil 10 mol/l, diltiazem 100 mol/l and ryosidine 10 mol/l. The inhibition was generally small. It was more evident for slices preincubated with ³H-choline than for those preincubated with ³H-dopamine, because in the latter verapamil, diltiazem and (much less) ryosidine accelerated the basal efflux of tritium. The inhibition of the K⁺-evoked overflow of tritium was probably due to blockade of Ca²⁺ channels because this overflow was Ca²⁺-dependent but tetrodotoxin-resistant. In contrast, the inhibition of the electrically- and glutamateevoked overflow possibly involved blockade of Na⁺ channels as well. The results indicate that three calcium antagonists from different chemical classes are very weak inhibitors of Ca²⁺ entry into, and hence transmitter release from, the terminal axons of central dopaminergic and cholinergic neurones. The function of the high affinity calcium antagonist binding sites that have been identified in brain remains unknown.