Do presynaptic autoreceptors control dopamine release?

Cannabinoid receptor-mediated inhibition of acetylcholine release from hippocampal and cortical synaptosomes

In previous studies cannabinoid agonists have been found to inhibit and cannabinoid antagonists to enhance electrically-evoked [(3)H]-acetycholine (ACh) release in hippocampal slices. The present study was undertaken to determine if similar cannabinoid effects could be observed in synaptosomes. [(3)H]-ACh release was evoked by two methods, both sensitive to presynaptic receptor effects. The first involved the addition of 1.3 mM calcium following perfusion with calcium-free Krebs and the second the addition of 11 mM potassium following perfusion with normal Krebs. In hippocampal synaptosomes the 1.3 mM calcium-evoked release and the high potassium-evoked [(3)H]-ACh release were inhibited by the cannabinoid agonist, WIN 55212-2, by 59 and 39%, respectively, and with an EC(50) of approximately 1 nM. WIN 55212-2 produced a similar, although less potent, inhibition of [(3)H]-ACh release in cortical synaptosomes. No inhibitory effect of WIN 55212-2 on [(3)H]-ACh release in striatal synaptosomes was observed, supporting previous data collected in this area with brain slices. The cannabinoid antagonist, SR 141716A, produced a robust enhancement of 1.3 mM calcium-evoked [(3)H]-ACh release in hippocampal synaptosomes (EC(50)<0.3 nM) but had no effect on potassium-evoked release or on [(3)H]-ACh release in the cortex or striatum. In conclusion our data demonstrates the inhibitory effects of WIN 55212-2 observed on ACh release in brain slices can be observed in hippocampal and cortex synaptosomes, suggesting a direct action of these compounds on the synaptic terminals. The SR 141716A-induced enhancement of ACh release can similarly be observed in hippocampal synaptosomes and is probably due to an inverse agonist action at constitutively active receptors.

Lack of autoreceptor-mediated inhibitory control of dopamine release in striatal synaptosomes of D2 receptor-deficient mice

Mouse purified striatal synaptosomes were used to study the release of newly synthesised [3H]-dopamine ([3H]-DA) or of previously taken up [3H]-DA. Quinpirole (QP, 10 microM), a D2/D3 dopaminergic agonist, was found to reduce the release of newly synthesised [3H]-DA with a larger amplitude when 4-aminopyridine (100 microM) instead than veratridine (1 microM) or potassium (25 mM) was used to evoke DA release. Among the different D2/D3 dopaminergic agonists tested R(-)-propylnorapomorphine (NPA) and quinpirole were the most potent. These compounds reduced, in a concentration-dependent manner, the 4-aminopyridine-evoked release of [3H]-DA previously taken up by synaptosomes (50% maximal inhibition). In contrast, the D3 agonist PD-128,907 had little effect even when used at 100 nM. The QP (100 nM)-induced response was completely antagonised by sulpiride (1 microM). Strikingly, the NPA (100 nM) and PD-128,907 (100 nM)-evoked responses were completely suppressed in D2 receptor-deficient mice. This data strongly suggest that only D2 but not D3 receptors are involved in the autoreceptor-mediated inhibition of the evoked release of [3H]-DA. Interestingly, while amphetamine-induced release of [3H]-DA was not modified, a slight reduction of [3H]-DA efflux induced by the dopamine (DA) uptake inhibitor cocaine was observed in D2 receptor-deficient mice.

Effects of verapamil on the release of different neurotransmitters

The effect of verapamil on resting and depolarization-induced monoamine release was investigated in rat hippocampal synaptosomes prelabeled with [3H]-5-hydroxytryptamine (HT) or [3H]-norepinephrine (NE) and rat striatal synaptosomes prelabeled with [3H]-dopamine (DA). Verapamil (50 microM) completely abolishes high K(+)-induced [3H]-NE release, but paradoxically facilitates high K(+)-induced [3H]-5-HT and [3H]-DA release. All these high K(+)-evoked responses were Ca2+ dependent. Verapamil does not modify [3H]-NE baseline release, but increases dose dependently [3H]-5-HT and [3H]-DA baseline release. Verapamil (10 microM, for 5 min) increases endogenous DA release (70%) and endogenous 5-HT release (40%) independently on the presence of external Ca2+. The total amount of these monoamines (released plus retained by the preparation) and their metabolites (DOPAC and 5-HIAA) was similar in control and verapamil-treated synaptosomes. Verapamil displaces [3H]-spiroperidol specific binding (Ki of 2.4 x 10(-6) M) and [3H]-SCH-23390 specific binding (Ki of 9 x 10(-6) M) from striatal synaptosomal membranes, and [3H]-5-HT specific binding (Ki of 3 x 10(-5) M) from hippocampal synaptosomal membranes. It is concluded that in addition to the Ca2+ antagonistic properties of verapamil on the Ca(2+)-dependent, depolarization-induced release of some neurotransmitters [gamma aminobutyric acid (GABA and NE)], another mechanism probably mediated by presynaptic receptors underlies the effects of verapamil on DA and 5-HT release from discrete brain regions.

Presynaptic control of dopamine synthesis and release by excitatory amino acids in rat striatal synaptosomes

Purified striatal synaptosomes were continuously superfused with L,3,5[3H]tyrosine in order to estimate the synthesis ([3H]water) and release of newly formed [3H]dopamine. In the presence of magnesium, L-glutamate, D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate (AMPA) and kainate, but not N-methyl-D-aspartate (NMDA) and 1-aminocyclopentane-1S,3R-dicarboxylate (t-ACPD), stimulated the release of [3H]dopamine, in a dose-dependent manner. When magnesium was omitted or in the presence of AMPA, NMDA also increased the release of [3H]dopamine. The effects of AMPA and kainate were competitively inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or 6,7-dinitro-quinoxaline-2,3-dione (DNQX), whereas those of NMDA were reduced by 2-amino-5-phosphonovalerate (APV) or (+)-5-methyl-10,11-dihydro-5-H-dibenzo(a,d)cyclo-hepten-5,10-imine maleate (MK801). The stimulation of [3H]dopamine release by a high concentration of glutamate resulted from the concomitant activation of AMPA and NMDA receptors since this effect was potentiated by glycine and reduced by 2-amino-5-phosphonovalerate or MK801. This reduction was almost complete in the combined presence of DNQX and MK801. Surprisingly, glutamate and NMDA (in the absence of magnesium) reduced the efflux of [3H]water. The reduction of [3H]dopamine synthesis was blocked by 2-amino-5-phosphonovalerate indicating the involvement of NMDA receptors. Neither AMPA nor kainate affected dopamine synthesis. The inhibition of [3H]dopamine synthesis resulting from the stimulation of NMDA receptors was prevented when synaptosomes were continuously superfused with adenosine deaminase and quinpirole, a combined treatment known to markedly reduce the phosphorylation of tyrosine hydroxylase by cAMP-dependent protein kinase. The opposite effects of a high concentration of glutamate on [3H]dopamine synthesis and release were mimicked by ionomycin. As a working hypothesis, it is proposed that the NMDA-triggered calcium influx could lead to a reduction of tyrosine hydroxylase phosphorylation, possibly through an activation of calcineurin.

Opposite presynaptic regulations by glutamate through NMDA receptors of dopamine synthesis and release in rat striatal synaptosomes

Purified striatal synaptosomes were superfused continuously with L-[3,5-3H]tyrosine to measure simultaneously the synthesis ([3H]water formed during the conversion of [3H]tyrosine into [3H]DOPA) and the release of [3H]dopamine ([3H]DA). Glutamate (10(-3) M) and NMDA (10(-3) M, in the absence of Mg2+) stimulated the release of [3H]DA, but they reduced the efflux of [3H]water. This reduction of [3H]DA synthesis was blocked by 2-amino-5-phosphonovalerate indicating the involvement of NMDA receptors. Although D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate (AMPA) and kainate stimulated the release of [3H]DA, they did not affect its synthesis. The glutamate-evoked inhibition of [3H]DA synthesis was prevented when synaptosomes were superfused continuously with adenosine deaminase plus quinpirole, a treatment which markedly reduces the phosphorylation of tyrosine hydroxylase by cAMP dependent protein kinase. The opposite effects of glutamate on [3H]DA synthesis and release were mimicked by ionomycin (10(-6) M). It is proposed that both an activation of a cyclic nucleotide phosphodiesterase and a dephosphorylation of tyrosine hydroxylase linked to the influx of calcium through NMDA receptors is responsible for the inhibition of dopamine synthesis by glutamate and that calcineurin could play a critical role in these processes.

The ontogeny of apomorphine-induced alterations of neostriatal dopamine release: effects on potassium-evoked release

The effects of apomorphine (0.05, 0.1, and 1.0 mg/kg, s.c.) on K(+)-evoked dopamine release were studied through the use of in vivo microdialysis in the neostriatum of developing and adult rats. Fifteen-minute samples were collected from urethane-anesthetized rats 5, 10-11, 21-22, 35-36 days of age, and adults, and quantified by high performance liquid chromatography with electrochemical detection. Apomorphine attenuated K(+)-evoked dopamine release in all age groups, suggesting that the dopamine autoreceptor modulating release in the neostriatum is functional by 5 days of age. A dose-response effect of apomorphine was observed in all age groups except at 5 and 10 days of age. Absolute levels of extracellular dopamine were significantly lower at 5 and 10 days of age compared with the other ages, and the effectiveness of a high-K+ artificial cerebrospinal fluid to evoke dopamine release increased with age.

Effect of cocaine and 5-HT3 receptor antagonists on 5-HT-induced [3H]dopamine release from rat striatal synaptosomes

The effect of serotonin (5-HT) on the release of tritium from striatal synaptosomes previously loaded with [3H]dopamine ([3H]DA) was studied. 5-HT stimulated both the spontaneous and Ca(2+)-evoked efflux of tritium in a concentration-dependent manner. This effect was not mimicked by the non-selective 5-HT agonist, d-lysergic acid diethylamide. Further, the stimulatory effects of 5 muM 5-HT were unaffected by the selective 5-HT3 receptor antagonists, MDL-72222 and GR-38032F. On the other hand, cocaine and the selective DA uptake inhibitor, nomifensine completely antagonized the effect of 5 muM 5-HT on spontaneous tritium efflux with IC50 values of 0.2 and 0.09 muM, respectively. The effect of 5-HT on Ca(2+)-evoked tritium efflux was also blocked by these DA uptake inhibitors, albeit at somewhat higher concentrations. These data support the hypothesis that 5-HT induces the release of DA from striatal nerve terminals via a mechanism involving the transport of 5-HT into the dopaminergic terminal, rather than by activating 5-HT3 receptors as has been proposed to account for the effect of 5-HT observed in striatal slices.

The effect of verapamil on GABA and dopamine release does not involve voltage-sensitive calcium channels

The effect of verapamil, which belongs to the group of drugs collectively referred to as 'organic Ca2+ channel blockers', was investigated on the basal and stimulated release of the neurotransmitters dopamine and GABA in rat striatum synaptosomes. Verapamil inhibits the Na(+)-dependent release of GABA in response to depolarization with an IC50 of 25 microM, whereas it is unable to modify the Na2(+)-independent, Ca2(+)-dependent fraction of GABA release induced by high K+ depolarization. Verapamil does not modify the basal release of GABA but stimulates the basal release of dopamine in a dose-dependent manner (ED50 5 microM). This verapamil-induced outflow of dopamine is independent of Ca2+ and occurs in the presence of tetrodotoxin, indicating that it is not mediated by voltage-sensitive Ca2+ or Na+ channels of the presynaptic membrane. Dopamine release induced by verapamil is cumulative with that induced by depolarizing agents (high K+ or veratridine). As verapamil, pimozide, a neuroleptic of the diphenylbutylpiperidine type, increases the basal and stimulated release of dopamine. We conclude that the opposite effects of verapamil of GABA and dopamine release are due to differences in the releasable fractions of these 2 types of neurotransmitters. Besides, none of these effects are directly linked with the blockade of voltage-operated Ca2+ channels of the presynaptic membrane.

Chronic cocaine treatment impairs the regulation of synaptosomal 3H-DA release by D2 autoreceptors

The effect of repeated administration of cocaine on presynaptic D2 autoreceptor sensitivity in synaptosomes was studied. In rats treated chronically with saline, the dopamine D2 agonist 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin (N-0437) caused a significant inhibition of the Ca2(+)-evoked 3H-DA release from synaptosomes prepared from the nucleus accumbens and from the striatum; this effect was blocked by the D2 antagonist sulpiride. However, chronic cocaine pretreatment abolished the effect of N-0437 in both areas, suggesting a subsensitivity of release-modulating terminal DA autoreceptors. Subsensitive DA autoreceptors would enhance stimulated DA release from mesolimbic and nigrostriatal terminals and may play a role in the behavioral sensitization observed in this paradigm.

Functional damage of dopamine nerve terminals following intrastriatal kainic acid injection

The release of [3H]dopamine ([3H]DA) previously taken up into rat striatal slices was studied one week after a monolateral intrastriatal injection of kainic acid (KA). Different releasing stimuli (electrical pulses, veratrine, high-K+) were applied. The electrically evoked release in the KA-lesioned striata was drastically reduced with respect to the unlesioned contralateral striata. In contrast, KA had no effect on the release of [3H]DA evoked by veratrine or high-K+. In unlesioned striatal slices, depolarized with 15 mM KCl, apomorphine reduced and (-)sulpiride increased the release of [3H]DA. The effect of apomorphine was antagonized by (-)sulpiride indicating the presence of an autoreceptor system similar to that seen in unlesioned striata stimulated electrically. However, the effects of apomorphine and of (-)sulpiride were dramatically reduced in K+-depolarized slices prepared from KA-lesioned striata. The results suggest that the axon terminals in KA-treated areas remain intact in several of their properties but may be damaged in some critical processes.

Time resolved dopamine overflow from synaptosomes and chopped striatal tissue with rapid superfusion

Overflow of dopamine has been measured with a rapid superfusion apparatus in an attempt to obtain a system in which overflow is a measure of the primary release process. The tissue samples employed, chopped tissue and synaptosomes, were prepared from rat striatum. The superfusion system employed an on-line amperometric detector to provide temporal information. In addition, liquid chromatography with electrochemical detection was used for identification and quantification of dopamine. Dopamine release could be induced from both samples by exposure to K+ in the presence of Ca2+. The presence of pargyline (0.1 mM) did not significantly affect overflow from either sample. In addition, dopamine stores could be replenished in both samples by exposure to 0.5 microM DA, an effect blocked by amphetamine, nomifensine, and amfonelic acid. However, overflow from synaptosomes showed considerably less distortion from interactions of released substances with the tissue than from chopped tissue. The temporal profile of overflow was more rapid and uptake inhibitors did not affect overflow during depolarization. Since overflow from synaptosomes appears to be more closely related to release, the temporal response of this preparation to K+ stimulations was examined in more detail. A linear relation between dopamine overflow and log (K+) was obtained with 3-s exposures to K+. In contrast, a sigmoidal relationship was obtained with 30-s exposures. Thus, the data support the concept that depolarization of nerve terminals by K+ is a biphasic process that can be temporally resolved.

The effect of the dopamine agonist pergolide on tyrosine hydroxylase activity in rat striatal and limbic miniprisms in vitro: a model for the dopamine autoreceptor?

A method for the assay of tyrosine hydroxylase activity in rat striatal and limbic (nucleus accumbens + olfactory tubercle) brain miniprisms is described. The dopamine agonists apomorphine (1 mumol/l) and pergolide (0.01-100 mumol/l) inhibited the tyrosine hydroxylase activity in both regions. The inhibition produced by 1 mumol/l pergolide was antagonised partially in striatal miniprisms and completely in limbic miniprisms by 1 mumol/l haloperidol. The dopamine D2-selective antagonist raclopride, at concentrations up to 300 nmol/l, did not antagonise the inhibition produced by pergolide in striatal miniprisms, but appeared partially to antagonise the inhibition in limbic miniprisms. It is concluded that whilst pergolide potently inhibits tyrosine hydroxylase activity in striatal and limbic miniprisms, the inhibition is of doubtful value as a predictive model of dopamine autoreceptor function in striatal miniprisms, but may be useful when limbic miniprisms are used.

Dopamine autoreceptors and the effects of drugs on locomotion and dopamine synthesis

Criteria for distinguishing dopamine autoreceptor agonism from other mechanisms of inhibiting locomotion were examined, together with the relationship between inhibition of locomotion and dopamine synthesis. ED50 potencies to inhibit locomotion of mice were established for drugs from a number of categories. Spiperone 0.02 mg kg-1 significantly (P less than 0.05) reversed inhibition of locomotion by known dopamine agonists but not that by the other types of drug. Idazoxan antagonized inhibition of locomotion due to alpha 2-agonists but not dopamine agonists. RU 24926 (N-propyl-N,N-di[2-(3-hydroxyphenyl)ethyl]amine) was antagonized by both spiperone and idazoxan. Only for dopamine agonists was there good correlation (r = 0.97) between potencies to inhibit locomotion in mice and L-dihydroxyphenylalanine (L-DOPA) accumulation in the nucleus accumbens of rats treated with gamma-butyrolactone and 3-hydroxybenzylhydrazine. The specific dopamine D1-agonist, SK&F 38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine), was inactive in both tests at doses up to 10 mg kg-1. The mixed dopamine agonist/antagonist, (-)-3-(3-hydroxyphenyl)-N-propylpiperidine, commonly known as (-)-3-PPP, acted as a dopamine agonist in both tests but inhibited locomotion more potently than L-DOPA accumulation. The inhibitory effects of dopamine agonists on locomotion were not prevented by alpha-methyl-p-tyrosine pretreatment. The data suggest that spiperone-reversible inhibition of locomotion in mice is a good criterion for dopamine autoreceptor agonists. The receptors involved are affected by low doses of both dopamine agonists and antagonists and seem similar to those involved in the autoreceptor mediated inhibition of dopamine synthesis. However, inhibition of locomotion is not due simply to suppression of dopamine release brought about as a secondary consequence of effects on synthesis; a separate mechanism for inhibiting dopamine release is probably involved.

Dopaminergic agents including 3-PPP and its enantiomers on medial septal self-stimulation

The effects of several dopamine agonists were determined on medial septal self-stimulation in rats and compared with selected dopamine antagonists and with the psychostimulants, d-amphetamine and nomifensine. Apomorphine, 3-PPP, TL-99, N,N-dipropyl-5,6-ADTN and N,N-dipropyl-6,7-ADTN inhibited self-stimulation at dose ranges selective for the dopamine autoreceptor as indicated by biochemical studies. Haloperidol and molindone produced dose-related inhibition but sulpiride increased self-stimulation. D-amphetamine and nomifensine also increased responding. The agonist-induced inhibition differed from neuroleptic-induced inhibition of self-stimulation. Both (+) and (-) 3-PPP inhibited responding by a similar amount over the dose range 0.25-1.0 mg/kg. At higher doses, (-) 3-PPP further decreased responding whereas the effects of (+) 3-PPP plateaued at approximately 55% of controls. These studies show that dopamine agonists, like neuroleptics, inhibit medial septal self-stimulation. This effect appears to be mediated via autoreceptor activation. Differences between neuroleptic- and agonist-induced inhibition and the 3-PPP stereoisomer data accord with the hypothesis that behavioural inhibitory effects caused by autoreceptor activation are less severe than those caused by dopamine postsynaptic blockade.

Avoidance and ICSS behavioral models dissociate TL-99 and 3-PPP from dopamine receptor antagonists

The behavioral effects of the putative dopamine autoreceptor agonists, TL-99 and 3-PPP, were explored in animal procedures that reveal highly characteristic effects of neuroleptics currently in clinical use. Sidman avoidance responding in rats was not altered appreciably by doses up to 10 mg/kg TL-99 or 30 mg/kg 3-PPP. Higher doses of TL-99 attenuated Sidman avoidance performance in squirrel monkeys, although 3-PPP had no effect. Lever pressing for intracranial self-stimulation (ICSS) was attenuated in a dose-related fashion by TL-99 and 3-PPP, with relatively shallow dose-response relationships. A low dose of haloperidol (0.03 mg/kg) partly reversed the effects of 3-PPP (3 mg/kg) on lever pressing ICSS, but not those of TL-99 (3 mg/kg). Yohimbine (3 mg/kg) failed to alter the effects of TL-99 at a dose that abolished the suppressant effect of clonidine on ICSS. Analysis of within-session ICSS response decrement patterns indicated that TL-99 reduced ICSS to a greater extent towards the end of the session than during the first 5 min. No such within-session trend was produced by 3-PPP, suggesting that 3-PPP attenuates ICSS by virtue of a performance deficit. Similar conclusions were reached using a shuttlebox task that involved self-regulation of ICSS duration by rats. Therefore, the clinical profile of neuroleptics is unlikely to be mimicked precisely by 3-PPP or TL-99. Clinical trials of DA autoreceptor agonists for antipsychotic efficacy will indicate whether or not avoidance and ICSS behaviors are relevant to the detection of the intrinsic antipsychotic activity of drugs.

Measurement of release of endogenous GABA and catabolites of [3H]GABA from synaptosomal preparations using ion-exchange chromatography

Picomole quantities of endogenous GABA in acidified superfusates of synaptosomal preparations have been measured using micro-bore ion-exchange chromatography and post-column formation of the fluorescent iso-indole derivative. Using this technique superfusates have been analyzed directly, without further manipulations, to investigate the release of endogenous GABA. Spontaneous release of GABA was 2-5 pmol/200 microliters superfusate increasing to 20 pmol/200 microliters with potassium stimulation. When gamma-vinyl GABA (RMI 71754), an inhibitor of GABA-T was injected into rats (750 mg/kg) and synaptosomes prepared the potassium-evoked release of GABA was increased 3-fold compared to controls. Chromatographic separations and measurement of release of endogenous and radiolabeled GABA allowed the real specific activity of released GABA to be calculated. Only when 500 microM amino-oxyacetic acid was added during isolation of synaptosomes was the specific activity of released GABA the same as the initial specific activity.

Pharmacological profiles of the putative dopamine autoreceptor agonists 3-PPP and TL-99

The putative dopamine (DA) autoreceptor agonists, N-n-propyl-3-(3-hydroxyphenyl)-piperidine (3-PPP) and 6, 7-dihydroxy-2-dimethylaminotetralin (TL-99) were compared with apomorphine in a series of tests indicative of DA receptor activation. All three agents displaced [3H] apomorphine and [3H] spiroperidol from DA recognition sites in rat brain and caused contralateral turning in the 6-hydroxydopamine lesioned rat. Apomorphine and TL-99 were generally more potent than 3-PPP. All three agents were also active at the DA autoreceptor that controls the synthesis of dopamine as indicated in vivo using the gamma-butyrolactone (GBL) procedure and in vitro using a synaptosomal preparation. In addition, all agents produced emesis in beagles. clear differences in the drugs' actions were observed in other test procedures. In the rat, apomorphine was the only compound which caused stereotypy or rotation following a reversible KCI-induced lesion of the striatum. Conversely, TL-99 and 3-PPP lacked activity in these procedures. Presumably, activity in these two tests indicates postsynaptic DA receptor activation. Each of the putative autoreceptor agonists produced a monotonic dose-related decrease in the mouse locomotor activity as opposed to the biphasic effect exerted by apomorphine. This action on the mouse locomotor activity, coupled with the results for the GBL test, provides an index of autoreceptor activation. In contrast to 3-PPP, both apomorphine and TL-99 increased locomotor activity in animals pretreated with reserpine and alpha-methyl-p-tyrosine and caused rotation in unilaterally caudectomized mice. In these test procedures thought to reflect activity at the postsynaptic DA receptor, TL-99 differed in its action from 3-PPP in a manner which suggests 3-PPP may be a more selective DA autoreceptor agonist.

Autoregulation of acetylcholine release in isolated hippocampal nerve endings

The existence of presynaptic autoreceptors modulating acetylcholine release from central cholinergic nerve endings was investigated by using rat hippocampal synaptosomes in a superfusion system. The presence of exogenous acetylcholine, carbachol or oxotremorine in the superfusion fluid produced a dose-dependent inhibition of the release of [3H]acetylcholine elicited by 15 mM KCl in synaptosomes prelabeled with tritiated choline. The inhibition was counteracted by atropine. Another well known muscarinic agonist, bethanechol, had no effect on [3H]acetylcholine release. Our results indicate that central cholinergic nerve terminals possess autoreceptors of the muscarinic type for the control of acetylcholine release. Moreover, differences seem to exist between pre-and postsynaptic muscarinic receptors in the central nervous system.

Dopamine biosynthesis is regulated by the amine newly recaptured by dopaminergic nerve endings

The synthesis of dopamine from labeled tyrosine (but not from labeled DOPA) in rat striatal synaptosomes was effectively inhibited by exogenous dopamine only when the amine was allowed to enter the nerve endings. In the presence of the uptake blocker nomifensine, extracellular dopamine was almost inactive. The evolution of 14CO2 from [14C]tyrosine was consistently higher when synaptosomes were 'incubated' in the presence of nomifensine than in its absence. This effect disappeared when synaptosomes were 'superfused' with labeled tyrosine (with or without nomifensine) in conditions in which dopamine reuptake cannot occur. The monoaminoxidase inhibitor pargyline inhibited 14CO2 evolution from [14C]tyrosine. However, the effect was almost abolished if dopamine reuptake was prevented (by nomifensine or in superfusion). Our results suggest that dopaminergic nerve endings do not possess autoreceptors controlling dopamine synthesis. In the present paper it is proposed that the regulation of dopamine synthesis occurs through inhibition of tyrosine hydroxylase, according to the classical and end-product concept; however, the function of 'end-product' would be primarily exerted by the amine newly taken up by the nerve terminals.

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

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 mumol/l reduced, whereas the antagonist haloperidol 0.1 mumol/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 mumol/l. It is concluded that the presynaptic dopaminergic modulation of dopamine release is not mediated by a tetrodotoxin-sensitive interneuronal pathway.