Release of 3H-dopamine and analogous monoamines from rat striatal tissue

1. The release of previously accumulated 3H-dopamine (DA) from minces of striatal tissue prepared from the brains of pargyline-pretreated rats was evaluated by superfusion with a physiological buffer solution in a six-chamber apparatus with silver toroid electrodes to provide electrical field stimuli. The identity of released tritium as 3H-DA was demonstrated chromatographically and 3H-DA taken up was found in a synaptosomal subcellular fraction. 2. Release of 3H-DA previously accumulated at 0.3 microM was found to be linearly dependent on stimulus intensity between 1 and 10 V (for 60 sec); 5 V was selected as a standard stimulus. 3. Release of 3H-DA did not occur from minces of rat liver, nor was there release of previously accumulated labeled urea or leucine from striatal tissue by electrical stimulation, 50 mM KCL, or 0.1 mM (+)-amphetamine. When 3H-DA was taken up in the presence of cocaine (1 mM) or benztropine (100 microM), electrically induced release of 3H-DA was markedly reduced, while spontaneous efflux was much less altered. 4. Release of 3H-DA was also induced by depolarizing concentrations of K+, as well as by Rb+ or NH4+, and by veratridine. Electrical release and that induced by 50 mM K+ or 100 microM veratridine was blocked by the omission of Ca2+ (with EDTA added) and that induced by veratridine was blocked by tetrodotoxin (30 microM).

Characterization of the effects of serotonin on the release of [3H]dopamine from rat nucleus accumbens and striatal slices

The effect of serotonin agonists on the depolarization (K+)-induced, calcium-dependent, release of [3H]dopamine (DA) from rat nucleus accumbens and striatal slices was investigated. Serotonin enhanced basal 3H overflow and reduced K+-induced release of [3H]DA from nucleus accumbens slices. The effect of serotonin on basal 3H overflow was not altered by the serotonin antagonist, methysergide, or the serotonin re-uptake blocker, chlorimipramine, but was reversed by the DA re-uptake carrier inhibitors nomifensine and benztropine. With the effect on basal overflow blocked, serotonin did not modulate K+-induced release of [3H]DA in the nucleus accumbens or striatum. The serotonin agonists, quipazine (in the presence of nomifensine) and 5-methoxytryptamine, did not significantly affect K+-induced release of [3H]DA in the nucleus accumbens. This study does not support suggestions that serotonin receptors inhibit the depolarization-induced release of dopamine in the nucleus accumbens or striatum of the rat brain. The present results do not preclude the possibility that serotonin may affect the mesolimbic reward system at a site which is post-synaptic to dopaminergic terminals in the nucleus accumbens.

Dopamine-releasing action of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridine (MPP+) in the neostriatum of the rat as demonstrated in vivo by the push-pull perfusion technique: dependence on sodium but not calcium ions

This study examined the effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its metabolite, 1-methyl-4-phenylpyridine (MPP+) on the levels of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) in push-pull perfusates of the striatum in chloral hydrate-anaesthetized rats. In control animals the levels of DA and DOPAC remained stable for at least 6 h and responded rapidly to a depolarizing stimulus of 25 mM K+. This K+-induced DA release was Ca2+-dependent since no stimulation was observed when the striatal sites were perfused with high K+ in a Ca2+-free medium containing 2 mM EGTA thus verifying that the striatal sites were functionally active. MPTP (0.025 and 0.05 microgram/microliter) stimulated DA release and inhibited DOPAC output in a dose-related manner. MPP+ (0.01, 0.025 and 0.05 microgram/microliter) produced a more robust dose-dependent increase in DA levels in the perfusates; however, the level of suppression of DOPAC was similar to that in response to MPTP. The effect of MPP+ on DA release was attenuated by 10(-6) M benztropine, the DA re-uptake blocker and completely inhibited by 10 micrograms/kg i.p. benztropine and 10(-4) M ouabain, the Na+, K+-ATPase (Na pump) inhibitor. However, although these substances prevented the MPP+-induced release of DA, the levels of DOPAC in the perfusates did not recover and remained completely suppressed suggesting that MPP+ may inhibit extraneuronal rather than intraneuronal monoamine oxidase (MAO). Perfusion of the striatal sites with a Ca2+-free medium containing 2 mM EGTA did not prevent the MPP+-induced DA release indicating that MPP+ does not release DA from the striatal DA terminals by the Ca2+-dependent process of exocytosis. The responses of DA and DOPAC to 25 mM K+ were markedly suppressed in animals treated with MPTP and MPP+, these effects being most severe with the highest dose of MPP+. Moreover, this suppression of the K+-induced responses persisted in animals perfused with MPP+ in the presence of benztropine or ouabain, thus suggesting that MPP+ may have potent deleterious membrane effects. These studies have provided the first direct in vivo demonstration of the action of MPTP and MPP+ and the neuropharmacological basis of this action on DA metabolism in the rat striatum. The results show that the elevated levels of DA in the striatal perfusates are due to a direct action of MPTP and MPP+ on the nigrostriatal DA terminals and cannot be fully accounted for solely by their inhibition of MAO activity and/or inhibition of DA re-uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo studies on the dopamine re-uptake mechanism in the striatum of the rat: effects of benztropine, sodium and ouabain

We used the push-pull perfusion technique to study the in vivo changes in dopamine (DA) levels in the rat striatum in response to treatments which could affect DA re-uptake into the nigrostriatal DA terminals. Benztropine (10(-6) M), a potent DA uptake inhibitor induced a 1.7-fold increase in DA levels in the perfusates compared to basal levels. Perfusion with a Na+-free medium in which Na+ was replaced with either Tris-Cl or choline-Cl in equimolar proportions induced respectively 6.5- and 8.5-fold increases in DA levels in the perfusates. Perfusion of media containing NaCl:Tris-Cl (50:50) or NaCl:choline-Cl (50:50) did not significantly alter the levels of DA in the perfusates. Ouabain (10(-6) M) did not significantly alter DA levels but at a concentration of 10(-4) M, there was a 5.3-fold increase in DA levels in the perfusates compared to basal levels. These results thus demonstrate that the raised DA levels in the extracellular space in response to benztropine is due to the action of the drug in blocking the uptake of DA. The dependence of the uptake mechanism on the presence of Na+ in the external medium and hence on metabolic energy (Na pump) is clearly demonstrated. However, the massive elevation of DA levels under these conditions cannot be due solely to an inhibition of DA uptake but to the carrier-mediated DA exit from cytoplasmic stores resulting from a running down of the ionic gradient.

Concurrent treatment with benztropine and haloperidol attenuates development of behavioral hypersensitivity but not dopamine receptor proliferation

Groups of male rats (n = 16 each) were treated with normal saline, haloperidol (0.75 mg/kg), benztropine (1.8 mg/kg) or haloperidol and benztropine once a day for 24 days. Following a 96 hour drug free interval, subsets of these animals were assessed for apomorphine-induced (0.75 mg/kg) stereotypic behavior, sacrificed and analyzed for striatal dopamine biochemistry or sacrificed and analyzed for spiroperidol binding sites. Benztropine cotreatment attenuated the development of behavioral hypersensitivity to haloperidol but did not alter either the dopamine receptor proliferation or the striatal dopamine biochemical changes induced by haloperidol. These results suggest that behavioral hypersensitivity is not an automatic manifestation of dopamine receptor proliferation but must depend, in part, on other factors.

Effects of cholinergic and anticholinergic drugs on ketamine-induced linguopharyngeal motor activity

Benztropine mesylate (Cogentin) and physostigmine salicylate (Antilirium), were tested for changes in tongue protrusions, retrusions, and swallowing acts in rats anesthetized with a 100 mg/kg IM injection of ketamine hydrochloride. These ketamine-induced linguopharyngeal events were monitored by means of a force displacement transducer fed onto a polygraph. Benztropine (0.05-1 mg/kg) caused mild to moderate reductions in the rate of these events for a short period of time, up to about 30 min. With physostigmine (5-25 micrograms/kg), linguopharyngeal activity was markedly increased, up to 50-fold by the highest dose within 5 min and returned almost to the baseline within 60 min. With lower doses, more moderate responses were obtained. If methscopolamine (1.4, 3, 6 mg/kg IM) preceded physostigmine, the physostigmine enhancement was preserved.

Ammonia-induced release of neurotransmitters from rat brain synaptosomes: differences between the effects on amines and amino acids

The effect of NH4Cl on release of amine and amino acid transmitters from rat brain synaptosomes was investigated. Ammonia (0.1-10 mM) stimulated the secretion of dopamine and 5-hydroxytryptamine in a dose-dependent manner, in a process which was additive with the effect of 40 mM K+, almost unaffected by withdrawal of Ca2+, and markedly decreased by increasing [H+] in the medium. The NH4Cl-induced dopamine efflux, in contrast to that caused by high [K+]e, was inhibited by benztropine. The release of gamma-aminobutyric acid, aspartate, and glutamate was unaltered by [NH4Cl] less than 5 mM, but somewhat stimulated at higher levels. Transmembrane pH gradient, acid inside, was dissipated by NH4Cl in a concentration-dependent manner and the internal alkalinization correlated with the stimulation of the rate of dopamine efflux. Transmembrane electrical potential was unaffected by [ammonia] less than 5 mM, but a small depolarization was observed at higher levels. It is postulated that ammonia-induced alkalinization of the intrasynaptic storage granules causes extrusion of amines into the cytoplasm and their subsequent leakage into the medium through a reversal of the plasma membrane transporters. A lack of correlation between the release of amino acid neurotransmitters and the dissipation of the delta pH suggests that in rat brain intrasynaptic vesicles, acidic inside, are unlikely to store substantial amounts of gamma-aminobutyric acid, aspartate, or glutamate.

Mechanisms contributing to the recovery of striatal releasable dopamine following MFB stimulation

In vivo voltammetry was used to monitor the concentration of dopamine released into striatal extracellular fluid during electrical stimulation of the medial forebrain bundle of anesthetized rats. During stimulation the dopamine concentration increases rapidly to micromolar levels. After cessation of the stimulus, uptake mechanisms rapidly clear the released dopamine. That dopamine is the compound detected during stimulation is confirmed by voltammetry and pharmacology. The interval between stimulations influences the amount of dopamine released. When 10 s stimulations are spaced by 30 s, only 50% of the amount of dopamine released by the first stimulation is released by the second stimulation. Twenty minutes are required for full recovery of the stimulation response. Full recovery requires dopamine synthesis but during the initial 2 min following stimulation the recovery exceeds synthesis rates. Possible mechanisms that contribute to the rapid initial recovery are the mobilization of intraneuronally stored dopamine and the recycling of the released dopamine via presynaptic uptake mechanisms. Dopaminergic receptors are able to modulate the rapid recovery.

A double-blind crossover comparison of antiparkinson drug therapy: amantadine versus anticholinergics in 90 normal volunteers, with an emphasis on differential effects on memory function

Anticholinergic antiparkinson drugs administered orally at standard clinically prescribed doses impaired new memory acquisition and mood in normal volunteer subjects, based on tests of free recall, recognition memory, and time production, self-rating of memory function, and an evaluation of mood states. Elderly subjects were more severely impaired than were young adults. Amantadine did not impair new memory acquisition, and on self-report measures, it was significantly better tolerated than were anticholinergics. Among patients being treated for psychotic illness, there are two groups in which an effort to avoid anticholinergic therapy is especially worthwhile because of the severe consequences of memory dysfunction. These individuals are young neuroleptic-responsive patients who are in an early stage of their disease and elderly patients. For these two groups, amantadine should be considered as the initial mode of treatment, with low-dose anticholinergics being used for those patients who do not achieve adequate relief from extrapyramidal side effects with amantadine.

Lack of selectivity between the uptake of [3H]adrenaline and [3H]noradrenaline into rat hypothalamic slices

The uptake of [3H]adrenaline and [3H]noradrenaline into rat hypothalamic slices was compared for determination of whether adrenaline uptake was independent of uptake into noradrenergic neurones. Kinetic analysis revealed a similar high-affinity uptake process for both adrenaline and noradrenaline, with Km and Vmax values within similar ranges. These uptakes were inhibited by desipramine and maprotiline in a dose-dependent manner, but the selective dopamine and 5-hydroxytryptamine uptake inhibitors benztropine and fluoxetine, respectively, were without effect. Competition for uptake sites by unlabelled adrenaline with [3H]adrenaline and [3H]-noradrenaline and by unlabelled noradrenaline with [3H]-adrenaline and [3H]noradrenaline was very similar. Lesioning of the major adrenaline-containing cell group (C1 cell group) decreased the hypothalamic adrenaline concentration but had no effect on hypothalamic [3H]adrenaline or [3H]noradrenaline uptake. The results suggest that exogenous adrenaline is largely taken up by high-affinity sites on noradrenergic nerve terminals.

Extracellular dopamine in rat striatum following uptake inhibition by cocaine, nomifensine and benztropine

A microdialysis/smallbore chromatographic system was used to monitor changes in extracellular dopamine concentration in the striatum of the rat following administration of drugs that block catecholamine uptake. Analysis of 0.5 microliter of dialysate every 5 min showed dose-dependent elevations in extracellular dopamine following systemic administration of nomifensine (1 and 10 mg/kg), benztropine (5 and 25 mg/kg) and cocaine (3, 10 and 30 mg/kg). The order of potency in vivo was nomifensine greater than cocaine greater than benztropine. The short sampling interval allows accurate temporal profiles following pharmacological manipulations to be acquired.

Chronic treatment with antidepressants decreases the number of [3H]SCH 23390 binding sites in the rat striatum and limbic system

Binding of [3H]SCH 23390 to dopamine D-1 receptors and of [3H]spiperone to D-2 sites was measured in identical membrane preparations of the striatum and limbic system of rats treated chronically (twice daily, for two weeks) with antidepressants. Chronic administration of imipramine, amitriptyline, mianserin, citalopram, bupropion, iprindole and electroconvulsive shocks, but not benztropine or cyproheptadine (non-antidepressants) decreased the number of [3H]SCH 23390 binding sites, while no change in the parameters of [3H]spiperone binding was observed. The serotonin2 receptor antagonist ketanserin when added to the incubation medium had no effect on [3H]SCH 23390 binding to D-1 sites. The results suggest that D-1 receptor subsensitivity is a component of the therapeutic effect of antidepressants.

Drug levels and antiparkinsonian drugs in neuroleptic-treated schizophrenic patients

The limitations of antiparkinsonian treatment strategy when using anticholinergic drugs are determined by their side effects induced through excessive inhibition of parasympathetic functions. In the present study we have investigated the peripheral effects of antiparkinsonian agents on blood levels of concomitantly administered neuroleptic drugs. We have compared the anticholinergic and a dopamine mimetic antiparkinsonian agent in their effects on serum neuroleptic activity (SNA) and serum anticholinergic activity (SAA). Sixteen schizophrenic patients on chronic neuroleptic therapy with steady state neuroleptic levels were receiving either amantadine, 200 mg/day, or anticholinergic drugs (trihexyphenidyl, 10 mg/day, or benztropine, 6 mg/day) for the first 2 weeks, after which the amantadine group was crossed over to anticholinergic and the anticholinergic group to amantadine for the following 2 weeks. Blood samples were obtained once a week along with clinical testing. The results indicate that SAA was fivefold higher with benztropine than with trihexyphenidyl and that amantadine had no effect on SAA. Moreover, SNA was not altered either by anticholinergics or amantadine coadministration, indicating that the therapeutic blood neuroleptic levels are not compromised by antiparkinsonian administration.

Nonmuscarinic neurotoxicity of oxotremorine

The ability of various treatments to prevent peripheral parasympathetic actions, central effects and lethality of the muscarinic agonist oxotremorine was studied in rats. The percentage of animals exhibiting effects of oxotremorine was dose and time dependent. The ED50 for producing lacrimation, salivation, tremor, convulsions and death was 2.5, 1.3, 1.6, 3.2 and 8.3 mg/kg i.p., respectively. Pretreatment with 5 mg/kg of atropine completely prevented all observable effects of oxotremorine at doses of 5 mg/kg and below. Doses of oxotremorine in excess of 5 mg/kg produced tremor, generalized clonic convulsions and death that could not be prevented by atropine when given at up to 160 mg/kg; lacrimation and salivation were not present in atropine-treated rats. In the presence of 40 mg/kg of atropine, ED50 values for oxotremorine were shifted more than 12-fold for lacrimation, salivation and tremor, whereas convulsions and death were maximally altered by a factor of 2. Scopolamine, benactyzine and benztropine were also incapable of completely preventing tremor, convulsions and death induced by 10 or 15 mg/kg of oxotremorine. Atropine methyl nitrate had effects comparable to atropine sulfate on lacrimation, salivation and lethality induced by oxotremorine (10 or 15 mg/kg) but had no effect on tremor or convulsions. A similar profile of atropine-insensitive effects was produced by pilocarpine and arecoline. Doses of diazepam 4 times higher (4 mg/kg) than necessary to prevent tonic-clonic convulsions induced by pentylenetetrazol were ineffective against tremor, convulsions or death produced by oxotremorine (10 or 15 mg/kg) unless given in conjunction with atropine.(ABSTRACT TRUNCATED AT 250 WORDS)

Does brain 3,4-dihydroxyphenylacetic acid reflect dopamine release?

After sulpiride (75 mg kg-1) administration, a significant increase in dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) occurred in the rat striatum. No attenuation in the sulpiride-induced dopamine and DOPAC increase was seen in the striatum from rats previously treated with benztropine (25 mg kg-1). On the other hand, in the benztropine + sulpiride-treated group, HVA levels were significantly lower compared with those of rats to which only sulpiride had been given. The results presented suggest that under appropriate conditions, as when sulpiride is used, dopamine can be deaminated before its release has occurred.

Modulation of dopamine receptors in the Tapes clam by dextroamphetamine and phenylethanolamine

The mechanism underlying the modulation, by dextroamphetamine and compounds related to phenylethanolamine, of responses to dopamine and serotonin has been studied in the isolated ventricle and aortic bulb of the clam Tapes watlingi. Dextroamphetamine and phenylethanolamine but not cocaine and benztropine have the ability to unmask inhibitory responses to both dopamine and serotonin in the ventricle. Chlordimeform but not clozapine attenuates the inhibitory response to both dextroamphetamine and phenylethanolamine in concentrations which have little or no effect on the inhibitory response to dopamine in the ventricle. Phenylethanolamine, dextroamphetamine, phenylpropylolamine and p-chloro-phenylethanolamine but not octopamine or noradrenaline attenuate the contractile responses to both dopamine and serotonin in preparations of the quiescent aortic bulb. These data show that there are specific receptors for phenylethanolamine in the Tapes heart capable of modulating responses to dopamine and serotonin, and suggests that this biogenic phenethylamine can act as an environmental and physiological factor which may determine how the mollusc heart responds to dopamine.

Quantification of the dopamine innervation in adult rat neostriatum

Conditions leading to specific and integral visualization of dopamine axon terminals (varicosities) were tested in adult rat cerebral hemisphere slices incubated with [3H]dopamine and processed for high resolution radioautography. Specific visualization of the dopamine endings was achieved after incubation with 10(-6) M [3H]dopamine in the presence of a monoamine oxidase inhibitor (pargyline 10(-4) M), and of desipramine (5 X 10(-6) M), an inhibitor of catecholamine uptake by noradrenaline and serotonin neurons. [3H]Dopamine varicosity labeling was eliminated by the addition of 5 X 10(-5) M benztropine (an inhibitor of catecholamine uptake by catecholamine neurons), and was almost absent when dopamine nerve cell bodies of the midbrain had been previously destroyed with 6-hydroxydopamine. In dopamine-denervated neostriatum incubated without desipramine, a second set of labeled terminals was also visible. These were identified as serotoninergic, since their labeling was suppressed by citalopram, an inhibitor of monoamine uptake highly specific for serotonin neurons. There was no desipramine-sensitive but citalopram-resistant varicosity labeling suggestive of neostriatal noradrenaline innervation. In normal striatum, incubation at 35 degrees C always resulted in a labeling of dopamine varicosities restricted to a narrow band which followed the contours and cut surface of this anatomical region. This unusual distribution was the result of an uptake barrier generated by the tightly packed dopamine varicosities. Indeed, the striatal dopamine varicosity labeling was more widespread after partial 6-hydroxydopamine denervation or in normal tissue incubated either with a higher [3H]dopamine concentration (5 X 10(-6) M), in the presence of relatively low benztropine concentrations (10(-5) M), or at lower temperature (15 degrees C). Material incubated at 15 degrees C for 90 min was suitable for purposes of quantification: labeled varicosities were then visualized throughout the striatum and across the full thickness of the slices; moreover, the number of labeled varicosities plotted against radioautographic exposure time increased in parallel and reached a plateau at the same time in neostriatal sectors with widely different innervation densities. At a rostral transverse level across neostriatum, the dorsolateral quadrant showed hyperdense "patches" of labeled terminals distinguishable from an already dense surrounding "matrix", whereas, ventromedially, the dopamine innervation appeared more uniform and somewhat less dense.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of xylamine binding to proteins of PC12 pheochromocytoma cells

PC12 pheochromocytoma cells take up 3,4-dihydroxyphenylethylamine (dopamine) and norepinephrine by a Na+-dependent, cocaine-sensitive system. The kinetics suggest that the same transporter functions for both substrates. Xylamine, a nitrogen mustard that blocks catecholamine uptake into neurons, irreversibly inhibited norepinephrine uptake into PC12 (IC50 = 15 microM). Pretreatment with 10 microM xylamine did not inhibit norepinephrine transport if 10 microM cocaine or 100 microM norepinephrine was also present during the pretreatment period or if Na+ was absent. These results indicate that xylamine must interact with the norepinephrine transporter to inhibit norepinephrine uptake. PC12 accumulated [3H]xylamine; this uptake had Na+-dependent and Na+-independent components. The Na+-dependent uptake was saturable (Km = 13 microM), and it was inhibited by cocaine (IC50 = 0.6 microM), desipramine (IC50 less than 1 nM), and norepinephrine (IC50 = 1 microM). Several proteins became prominently labeled when intact PC12 cells were incubated with [3H]xylamine; these proteins were enriched in a plasma membrane fraction and have molecular weights of 17,000, 24,000, 31,000, 33,000, 41,000, 42,000, 52,000, and 80,000. Other proteins were labeled less prominently. The labeling of all proteins was markedly decreased when the incubation with [3H]xylamine occurred in the presence of cocaine, desipramine, gramicidin D, or in a Na+-free buffer. These results indicate that xylamine must be transported into the cells for covalent binding to proteins to occur. [3H]Xylamine labeled essentially the same proteins when incubated with cell homogenates, but competition experiments with bretylium, desipramine, and cocaine failed to reveal which of the [3H]xylamine-labeled proteins is associated with the norepinephrine transporter.

Evidence that amphetamine and Na+ gradient reversal increase striatal synaptosomal dopamine synthesis through carrier-mediated efflux of dopamine

Amphetamine (AMPH) releases dopamine (DA) from striatal synaptosomes and concomitantly increases DA synthesis. Since AMPH may release DA through carrier-mediated diffusion via reversal of the DA uptake system, the increase in DA synthesis might depend on a functioning uptake carrier. Consistent with such a mechanism, the uptake inhibitors nomifensine (NMF) and benztropine (BZT) completely prevented the AMPH-induced increase in DA synthesis at concentrations known to inhibit DA uptake. Changes in the Na+ gradient across the synaptosomal membrane also promote DA release, since DA and Na+ are cotransported by the neuronal uptake carrier. Incubation of synaptosomes in medium containing decreasing Na+ increased DA synthesis inversely proportional to Na+ over the range 128 to 20 mM. Similarly, incubations in the presence of 10(-4) M ouabain to inhibit Na+, K+-ATPase and allow intracellular accumulation of Na+ also increased DA synthesis. These changes in DA synthesis could also be prevented by BZT and were non-additive with the AMPH-induced increase in DA synthesis. However, a concentration of ouabain (10(-6) M) which by itself did not increase DA synthesis, and does not promote DA release, potentiated the AMPH-induced increase in DA synthesis. Further, the increased DA synthesis promoted by all three manipulations was only marginally dependent on the presence of Ca2+ in the incubation medium. However, at 5 and 10 mM Na+, a second component of increased DA synthesis was observed which was insensitive to BZT, but was prevented by Ca2+ removal. These results suggest that the increase in DA synthesis, and presumably DA release promoted by AMPH, lowered Na+, and ouabain, depend on the availability of the DA carrier at the internal face of the neuronal membrane and the intracellular content of Na+. The second component of increased DA synthesis which is evident at 5 and 10 mM Na+ is discussed in terms of a possible Ca2+-mediated change in DA synthesis which is independent of the DA carrier.

Sub-second striatal dopamine release measured by in vivo voltammetry

High speed cyclic voltammetry was used to measure electrically stimulated striatal dopamine release in vivo with 25 ms time resolution. Dopamine release after 1 s stimulations was readily detectable and could be manipulated by drugs known to influence dopaminergic neurones. Using data-averaging dopamine release could be detected following stimulus trains as short as 100 ms. The use of carbon fibre microelectrodes gave a spatial resolution better than 5 micron.

Measurement of stimulated dopamine release in the rat by in vivo voltammetry: the influence of stimulus duration on drug responses

High-speed cyclic voltammetry at carbon fibre microelectrodes was used to monitor stimulated striatal dopamine (DA) release in the anaesthetised rat. The effects of metoclopramide, chlorpromazine, benztropine and dexamphetamine on DA release after 1 s and 10 s stimulations were investigated. The antagonists enhanced DA release most clearly with 1 s stimuli. Benztropine had no effect on DA release with the 10 s stimulations, but elevated release after 1 s trains. Dexamphetamine reduced release with 10 s stimuli but enhanced release after 1 s trains. The results are discussed in terms of neuronal DA uptake and releasable DA pool size.

Evidence for a non-precursor dopamine pool in noradrenergic neurones of the dog mesenteric artery

The effects of dopamine beta-hydroxylase inhibition by disulfiram on dopamine and noradrenaline concentrations were investigated at various times in the main trunk and proximal branches of the dog mesenteric artery. The effects of desmethylimipramine and benztropine on the uptake of exogenous dopamine in the proximal branches of the mesenteric artery were also analyzed. The rate constant of dopamine accumulation after dopamine beta-hydroxylase inhibition in the main trunk (0.1137 h-1) was significantly higher than that observed in the proximal branches (0.075 h-1). The rate constants of noradrenaline decline were similar in both segments of the mesenteric artery (about 0.052 h-1). The uptake of exogenous dopamine in the proximal branches of the mesenteric artery was insensitive to benztropine and only blocked by desmethylimipramine. These results suggest the existence of a non-precursor dopamine pool in noradrenergic neurones supplying the proximal branches of the dog mesenteric artery, and also provide evidence in favour of the absence of independent dopaminergic neurones in this vascular area.

In vivo voltammetric characterization of low affinity striatal dopamine uptake: drug inhibition profile and relation to dopaminergic innervation density

Electrical stimulation of the median forebrain bundle evoked dopamine release in the ipsilateral striatum which was monitored with high-speed cyclic voltammetry. After stimulation, the extracellular concentration of dopamine fell due to uptake in a biphasic manner, showing zero-order and first-order components. The zero-order phase corresponded to a Vmax of 42.8 +/- 1.8 nmol/min/g tissue. The uptake system could be blocked by D-amphetamine, methylphenidate and nomifensine, but not by benztropine, amfonelic acid or mazindol. The density of uptake sites showed a correlation with the density of striatal dopamine innervation.

Dopamine receptor-mediated inhibition of serotonin N-acetyltransferase activity in retina

The possible involvement of catecholamines in the regulation of serotonin N-acetyltransferase (NAT) activity in retina of the African clawed frog was investigated using an in vitro eye cup preparation. Dopamine (10 microM) and norepinephrine (50 microM) had no significant effect on NAT activity of eye cups incubated in the light. However, dopamine inhibited the increase of retinal NAT activity that occurs in eye cups incubated in darkness; the ED50 for dopamine was 0.3 microM. The effect of dopamine on NAT activity was mimicked by the dopamine receptor agonists apomorphine and bromocriptine, but not by agonists of alpha 1-, alpha 2- or beta-adrenergic receptors. Dopamine-mediated inhibition of NAT activity was antagonized by spiroperidol and by alpha-flupenthixol, but not by beta-flupenthixol, phentolamine or timolol. Benztropine, an inhibitor of dopamine reuptake, also decreased NAT activity in eye cups incubated in the dark. The inhibitory effect of benztropine was antagonized by spiroperidol, suggesting that it was mediated by an increase in the extracellular concentration of endogenous dopamine. These studies indicate that the regulation of NAT activity in the retina is subject to modulation by a dopamine receptor-mediated mechanism and suggest that dopamine may play a role in the inhibition of NAT activity by light.

The effect of antimuscarinic agents on haloperidol induced behavioral hypersensitivity

Varying doses of scopolamine, trihexyphenidyl and benztropine were administered to rats or guinea-pigs by themselves or in combination with 0.5 mg/kg haloperidol for 24 days. All animals were then challenged with 0.75 mg/kg apomorphine and assessed for stereotypic behavior following a 96 h drug free interval. Animals treated with haloperidol alone exhibited behavioral hypersensitivity to apomorphine challenge. Animals treated with both an antimuscarinic agent and haloperidol exhibited a significant reduction in behavioral responsiveness relative to animals treated with only haloperidol. This reduction was directly proportional to the antimuscarinic dose administered. A non-significant trend toward hyposensitivity was observed in animals who had been treated with antimuscarinic agents alone. These results suggest that the development of behavioral hypersensitivity may reflect CNS alterations in cholinergic as well as dopaminergic activity.