Long-Term Survival of Intrastriatal Dopaminergic Grafts: Modulation of Acetylcholine Release by Graft-Derived Dopamine

The nigrostriatal dopaminergic system of rats was unilaterally lesioned with 6-hydroxydopamine. Part of the animals was grafted 2 weeks later with fetal dopaminergic cells on the lesioned side; untreated rats of the same strain served as controls. Both 3 and 12-14 months after surgery the striatal dopamine (DA) content and the in vivo rotational response following injection of D-amphetamine showed significant changes in grafted as compared to lesioned animals. At 12-14 months after transplantation, the electrically evoked release of tritiated DA and acetylcholine (ACh) in slices (preincubated with [3H]DA or [3H]choline, respectively) of striata of intact, lesioned, or grafted animals was also investigated. Electrical field stimulation of striatal slices of the lesioned side did not evoke any significant [3H]DA overflow, whereas a marked [3H]DA release was observed in slices of grafted and control striata. Moreover, both DL-amphetamine (3 microM) and nomifensine (10 microM) strongly enhanced basal 3H outflow in these slices. Electrically evoked [3H]ACh release was significantly reduced in slices from all striatal tissues by 0.01 microM apomorphine. In slices from denervated striata a clearcut hypersensitivity for this action of apomorphine was present, indicating supersensitivity of DA receptors on cholinergic terminals; this hypersensitivity was significantly reduced in graft-bearing striata. Furthermore, because this hypersensitivity was unchanged in slices of lesioned striata under stimulation conditions (four pulses/100 Hz) avoiding inhibition by endogenously released DA, it is concluded that lesion-induced DA receptor supersensitivity is caused by an increase in receptor density or efficacy rather than by a decreased competition between endogenous and exogenous agonists. Both reuptake blockade of DA with nomifensine (10 microM) and release of endogenous DA by DL-amphetamine (3 microM) potently reduced [3H]ACh release only in control and grafted but not in lesioned tissue. In experiments using potassium-evoked [3H]ACh release, tetrodotoxin had no effect on the inhibitory activity of amphetamine and nomifensine, indicating that the DA receptors involved in their indirect inhibitory action are located directly on the cholinergic terminals.     

The Relationship Between the Stimulation of Dopamine Synthesis and Release Produced by Amphetamine and High Potassium in Striatal Slices

The effects of amphetamine (amph) and high K+ on the synthesis and release of dopamine (DA) were compared in striatal slices. Both agents stimulated DA synthesis as well as release. For both agents, Ca2+ was required for the initiation of synthesis stimulation as well as for the maintenance of this stimulation. The addition of EGTA to medium containing slices that were already stimulated by 1.0 microM-amph or 55 mM-K+ markedly reduced the stimulation of DA synthesis. Although it has been reported that high K+ activates soluble tyrosine hydroxylase (TH), neither high K+ nor amph appeared to increase the affinity of the synthetic cofactor, 6-MPH4, or decrease the affinity of the catechol, DA, for TH. This finding was supported by the observation that the inhibitory effect of L-DOPA on DA synthesis in slices, in which synthesis was stimulated by either agent, was not decreased. Although both 1.0 microM-amph and 55 mM-K+ stimulated the release of [3H]DA from striatal slices, the release produced by K+ was Ca2+-dependent, whereas release produced by amph did not occur at any concentration tested. Studies on pH requirements for both synthesis and release also confirmed a similarity between amph and K+ in stimulating synthesis but not in stimulating release. These results suggest that depolarizing agents, such as high K+, couple synthesis and release of DA by a Ca2+-dependent mechanism. In contrast, the simultaneous stimulation of synthesis and release by amph is not regulated by Ca2+.     

The in vivo modulation of dopamine synthesis by calcium ions: influences on the calcium independent release

To investigate the contribution of the dopamine (DA) synthesis to both the calcium-dependent and the carrier-mediated, mechanisms of DA release in the striatum, anaesthetized rats were locally superfused in the striatum with a push–pull cannula supplied with an artificial CSF containing tritiated tyrosine. DA, dihydroxyphenylacetic acid (DOPAC) and their respective specific activity were measured in effluent and used to evaluate changes in the DA synthesizing rate. Excluding calcium ions from the CSF only partially reduced spontaneous DA release (70%) still leaving a possible carrier-mediated DA release. This effect was not additive with a local superfusion with 0.1 mM a-methyl-p-tyrosine, a blocker of DA synthesis, suggesting that synthesis could already be reduced by calcium-free superfusion. Local superfusion with 100 μM cadmium in the presence or not of calcium ions, increased the DA release (220 and 350%, respectively), simultaneously reducing DA synthesis. Local application of 1 μM calcium ionophore (A23187) was without effect on the basal release of DA but enhanced DA synthesis and increased the amphetamine-evoked and carrier-mediated amine release. We conclude that DA synthesis can be a modulatory process of the firing-independent and carrier-mediated amine release while it weakly affects the classical calcium-dependent release.     

Cooperative synthesis of dopamine in rat mediobasal hypothalamus as a compensatory mechanism in hyperprolactinemia

Dopamine (DA), synthesized in the mediobasal hypothalamus by dopaminergic neurons containing two enzymes of DA synthesis–tyrosine hydroxylase and decarboxylase of aromatic L-amino acids, or by monoenzymatic non-dopaminergic neurons containing one DA synthesis enzyme in cooperation, is known to have an inhibitory effect on prolactin secretion. Deterioration of this inhibitory control leads to an increase in prolactin concentration in the blood and to the development of hyperprolactinemia syndrome. In a rat model of hyperprolactinemia induced by administration of a neurotoxin causing degeneration of dopaminergic and noradrenergic neurons, the level of DA first decreases, leading to an increase in prolactin level (decompensation stage), while later both levels are restored to normal (compensation stage). However, the mechanism of such compensation is still not clear. The aim of the present study was to analyze whether the increase in cooperative synthesis of DA by monoenzymatic neurons during hyperprolactinemia is a manifestation of a compensatory mechanism representing a particular case of neuroplasticity. The level of cooperative synthesis in the hyperprolactinemia model and in the control was estimated as the level of synthesis of DA and L-dihydroxyphenylalanine (L-DOPA)–an intermediate product of DA synthesis, when L-DOPA transfer from neurons containing tyrosine hydroxylase into neurons containing aromatic L-amino acid decarboxylase is inhibited. The level of DA synthesis during the decompensation stage was not changed, while during the compensation stage it was lower than the control. Along with a reduction in DA level, during the compensation stage an increase in the extracellular L-DOPA level in the medium was detected. Thus, the compensation of DA deficiency after degeneration of dopaminergic neurons in the mediobasal hypothalamus is due to the increase in cooperative synthesis of DA by monoenzymatic neurons containing one of the complementary enzymes of the DA synthesis pathway.     

Reduction of Dopamine Synthesis Inhibition by Dopamine Autoreceptor Activation in Striatal Synaptosomes with In Vivo Reserpine Administration

Abstract: In an attempt to clarify the mechanisms by which dopamine (DA) autoreceptor activation inhibits DA synthesis, the efficacy and potency of the D₂ DA agonists bromocriptine, lisuride, and pergolide, and the D₁,-D₂ DA agonist apomorphine were studied in rat striatal synapto- somes, in which the rate of DA synthesis (formation of ¹⁴CO₂ from l -[1–¹⁴C]tyrosine) was increased 103% by treating the animals from which the synaptosomes were obtained with reserpine (5 mg/kg i.p. twice, 24 and 2 h before they were killed), using the striatal total homogenate as the standard synaptosomal preparation. The increase in DA synthesis evoked by reserpine was additive with that produced by treatment of the synaptosomes with dibutyryl cyclic AMP, suggesting that, not a cyclic AMP-dependent, but possibly a Ca²⁺-dependent mechanism was involved. The DA agonists showed a concentration-dependent inhibition of DA synthesis in the control synaptosomes, which was antagonized by the selective D₂ DA antagonist (-)-sulpiride. In the synaptosomes with increased rate of DA synthesis obtained from the rats treated with reserpine, the concentration-response curves of DA synthesis inhibition for the other DA agonists were shifted to the right, and the effect of bromocriptine was completely eliminated, whereas bromocriptine antagonized the effect of apomorphine. The increased rate of DA synthesis was not preserved in the striatal P₁+ P₂ fraction obtained from the reserpine-treated rats, but the effects of the DA agonists were still reduced to the same degree as those in the total homogenate. (-)-Sulpiride did not enhance DA synthesis in synaptosomes from the reserpine- treated rats. The results presented indicate that the reduced effect of the DA agonists in synaptosomes from the reserpine-treated rats was not due to endogenous DA occupying the DA autoreceptors. Because it is known from the literature that reserpine in vivo increases impulse activity in DA neurons and, as a result, increases the Ca²⁺ concentration, these results suggest that the effect of DA agonists was reduced because DA autoreceptors may normally control DA synthesis by decreasing the free intraneuronal Ca²⁺ concentration, and consequently, the Ca²⁺-dependent phosphorylation of tyrosine hydroxylase.     

Environmental stress and domoic acid production by Pseudo-nitzschia: a physiological perspective

Production of domoic acid (DA) by the pennate diatom Pseudo-nitzschia multiseries is associated with physiological stress caused by silicate (Si) and/or phosphate (P) limitation. Such limitation may promote DA synthesis by (1) reducing primary metabolic activity, thus making available necessary precursors, high energy compounds, and cofactors, and (2) favoring the expression of genes involved in the biosynthesis of this toxin. In the case of Si and P-limitation, DNA synthesis and the progression through the cell division cycle are slowed, perhaps prolonging or arresting the cells in the stage of the division cycle which is most conducive to DA production. However, N-limitation results in an insufficient pool of cellular free N, which restricts synthesis of this nitrogenous toxin. A continuous supply of photophosphorylated high-energy intermediates (e.g., ATP and NADPH) is necessary for DA synthesis. In order to better understand the mechanism(s) of DA production, more studies are needed to elucidate: (1) the details of the biosynthetic pathway, (2) the regulation of enzymes involved in the pathway, (3) the relation between DA synthesis and the cell division cycle, (4) the cellular compartmentalization of DA biosynthesis, and (5) other environmental factors that may trigger DA production. Finally, these studies should be extended to include toxigenic Pseudo-nitzschia species other than P. multiseries, to confirm the commonality of these mechanisms.     

Synthesis and Release of Dopamine in Rat Brain: Comparison Between Substantia Nigra Pars Compacta, Pars Reticulata, and Striatum

Dopamine (DA) is synthesized and released not only from the terminals of the nigrostriatal dopaminergic neuronal pathway, but also from the dendrites in the substantia nigra. We have investigated the regulation of the DA turnover, the DA synthesis rate, and the DA release in the substantia nigra pars compacts (SNpc) and pars reticulata (SNpr) in vivo. As a measure of DA turnover, we have assessed the concentrations of 3,4-dihydroxyphenylacetic acid and homovanillic acid. As a measure of the DA synthesis rate, we have determined the 3,4-dihydroxyphenylalanine accumulation after inhibition of aromatic L-amino acid decarboxylase by 3-hydroxybenzylhydrazine. As a measure of DA release, we have investigated the disappearance rate of DA after inhibition of its synthesis by alpha-methyl-p-tyrosine and the 3-methoxytyramine accumulation following monoamine oxidase inhibition by pargyline. Both the DA turnover and the DA synthesis rate increased following treatment with the DA receptor antagonist haloperidol and decreased following treatment with the DA receptor agonist apomorphine in the SNpc and in the SNpr, but the effects of the drugs were less pronounced than in the striatum. gamma-Butyrolactone treatment, which suppresses the firing of the dopaminergic neurons, increased the DA synthesis rate in the striatum (165%), but had no such effect in the SNpc or SNpr. Haloperidol, apomorphine, and gamma-butyrolactone increased, decreased, and abolished, respectively, the DA release in the striatum, but the drugs had no or only slight effects on the alpha-methyl-p-tyrosine-induced DA disappearance and on the pargyline-induced 3-methoxytyramine accumulation in the SNpc or SNpr. Taken together, these results indicate that the DA synthesis rate, but not the DA release, are influenced by DA receptor activity and neuronal firing in the SNpc and SNpr. This is in contrast to the situation in the striatum, where both the DA synthesis rate and the DA release are under such control.     

Effects of Phenylalanine on the Release of Endogenous Dopamine from Rat Striatal Slices

We examined the effect of phenylalanine (50-400 microM) on the electrically stimulated release of endogenous 3,4-dihydroxyphenylethylamine (dopamine or DA) from superfused rat striatal slices. In the absence of tyrosine, phenylalanine (25 microM) partially sustained DA release, but less well than an equimolar concentration of tyrosine. In the presence of tyrosine (50 microM), phenylalanine (in concentrations of greater than or equal to 200 microM) inhibited DA release into the superfusate. This inhibition was not associated with changes in tissue levels of tyrosine or DA, nor was it mimicked by addition of high concentrations of tyrosine or leucine to the medium. We conclude that phenylalanine is a less effective precursor of DA in rat striatum than tyrosine and that it can also act to inhibit DA synthesis, depending on its concentration.     

Expression of the enzymes of dopamine synthesis in non-dopaminergic neurons: functional significance and regulation

Dopamine(DA), the most widely distributed in the nervous system and functionally important chemical signal, is synthesized in DA-ergic neurons from L-tyrosine by means of two enzymes, tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AADC). Apart from the enzymes, specific DA transporter is an attribute of DA-ergic neurons. In the mid eighties of the last century, in addition to DA-ergic neurons, those expressing only one enzyme, TH or AADC, have been discovered. These "monoenzymatic" neurons occurred to be more numerous and more widely distributed in the brain compared to DA-ergic neurons that manifests their wide involvement to the brain functioning. It has been demonstrated that the monoenzymatic neurons expressing complementary enzymes of DA synthesis produce this neurotransmitter in cooperation. In this case, L-tyrosine is transformed to L-DOPA in TH containing neurons that is followed by L-DOPA release and uptake from the intercellular space to AADC containing neurons for DA synthesis. Moreover, the L-DOPA uptake to DA-ergic or serotoninergic neurons results either in the increase or the onset of DA synthesis in addition to serotonin, respectively. The expression of the enzymes of DA synthesis in non-dopaminergic neurons is one of the adaptive reactions serving to compensate the functional insufficiency of DA-ergic neurons. For instance, hyperprolactinemia and the deficiency of DA, prolactin-inhibiting hormone, which is developed under degeneration of DA-ergic neurons of the arcuate nucleus, are compensated with time due to the increase of the number of monoenzymatic neurons and cooperative synthesis of DA in the nucleus. It is supposed that the same compensatory cooperative synthesis of DA is turned on under the degeneration of DA-ergic neurons of the nigrostriatal system that is manifested by the appearance of non-dopaminergic neurons expressing enzymes of DA synthesis in the deafferentated striatum. The expression of the enzymes of DA synthesis in non-dopaminergic neurons is under the control by intercellular signals, catecholamines, neurotrophic (growth) factors and, perhaps, hormones. Thus, non-dopaminergic monoenzymatic neurons expressing enzymes of DA synthesis produce this neurotransmitter in cooperation that is a compensatory reaction under functional insufficiency of DA-ergic neurons, in neurodegenerative diseases, hyperprolactinemia and Parkinson's disease, in particular.     

Influence of noradrenaline on progesterone synthesis and post-translational processing of oxytocin synthesis in the bovine corpus luteum

Noradrenaline (NA) influences secretory function of the bovine corpus luteum (CL), stimulating secretion of progesterone and ovarian oxytocin (OT). To study whether NA is able to stimulate progesterone synthesis and to affect post-translational OT processing, different doses of NA alone or in combination with different doses of OT were added to bovine CL slices from 8 to 13 d of the estrous cycle. To determine which receptors NA affects, and if dopamine (DA) also affects CL function, we used NA or DA combined with a beta-antagonist (propranolol). The results indicated that NA stimulates both luteal progesterone and OT content; furthermore, it increased the activity of 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) and peptidyl glycine-alpha-amidating mono-oxygenase (PGA), terminal enzymes in synthesis of these 2 hormones. The stimulating effect of NA was inhibited by propranolol and by pre-treatment of CL slices with high OT doses. Post-translational processing of OT synthesis by PGA activation was also stimulated by DA, but this effect was inhibited by beta-receptor blocker. Thus DA acts in CL as a NA precursor. In conclusion, it can be assumed that the noradrenergic system affects CL secretory function on different levels of regulation. Furthermore, a high concentration of OT in CL prevents NA from activating PGA and thus decreases post-translational OT synthesis.     

Reduction of food intake by piribedil in the rat: relation to dopamine receptor stimulation

Piribedil, given either intraperitoneally or intracerebroventricularly to rats trained to eat 4 h a day, induced a dose- and time-related anorexia. In this context it was less potent than either amphetamine or fenfluramine.The anorectic effect of piribedil was selectively antagonized by blockade of dopamine (DA) receptors in the central nervous system but not either inhibition of catecholamine synthesis, blockade of α- or β-adrenoceptors or serotoninergic receptors. Also a blocker of “peripheral” DA receptors failed to antagonize piribedil-induced anorexia.Piribedil, as opposed to amphetamine, failed to increase locomotor activity or to induce stereotyped behaviour at doses lower than that required to cause an approximate 80% reduction of food intake.These findings indicate that stimulation of central DA receptors involved in feeding regulation is responsible for the anorexigenic effect of piribedil. This effect in most instances occurs at dose levels of the compound which fail to induce other central stimulant effects.     

Multiple receptors for brain dopamine in behavior regulation: concept of dopamine-E and dopamine-I receptors

From current knowledge, it is possible to substantiate the original concept of DAe and DAi receptors including the predicted correlation with the anatomical, histochemical, biochemical and functional features of the distinct neuronal structures, in which they occur; labelling them as neostriatal DAe receptors and mesolimbic DAi receptors appears to be justified. Available data warrant a revision of currently employed behavior and pharmacological tests. When revised in terms of the DAe-DAi concept, assessment of such tests reveals that agents such as (-)NPA, 6, 7ADTN and certain ergot alkaloids like lergotrile, lisuride and, to a less degree, CB-154 are weak DAe agonists and strong DAi antagonists (Table I). The discovery that mesolimbic α-like NE receptors which regulate the DA activity at the level of the mesolimbic DAi, but not neostriatal DAe, receptors show adaptational changes following priming or subacute treatments with apomorphine or haloperidol, opens new perspectives for understanding phenomena such as the development of hypersensitivity to apomorphine etc. Presynaptic DA receptors located within the DA synaps and DA receptors located at DA cell-bodies resemble closely the DAe receptors, although absence of linkage to the enzyme adenylyl cyclase hints at some distinction. It is possible that the distinct classes of DA receptors identified by behavior and pharmacological studies in mice correspond with the DAe and DAi receptors in snails, rats and cats. There is no evidence to suggest that DAe and DAi receptors are directly related to a) so-called DA1 and DA2 receptors which are coupled and uncoupled respectively to the enzyme adenylyl cyclase, or to b) any of the DA-specific binding sites identified with radiolabelled DA agonists or antagonists. Nonetheless, it cannot be excluded that DAe receptors may correspond with DA-specific binding sites identified with tritiated DA and/or haloperidol, and DAi receptors with a particular subclass of DA-specific binding sites identified within certain mesolimbic structures with radiolabelled spiperone. Thus, future work is still required to relate DAe and DAi receptors to particular, molecular entities within the brain.     

Role of excitatory amino acids in the regulation of dopamine synthesis and release in the neostriatum

Summary We have explored the role of excitatory amino acids in the increased dopamine (DA) release that occurs in the neostriatum during stress-induced behavioral activation. Studies were performed in awake, freely moving rats, usingin vivo microdialysis. Extracellular DA was used as a measure of DA release; extracellular 3,4-dihydroxyphenylalanine (DOPA) after inhibition of DOPA decarboxylase provided a measure of apparent DA synthesis. Mild stress increased the synthesis and release of DA in striatum. DA synthesis and release also were enhanced by the intra-striatal infusion of N-methyl-D-aspartate (NMDA), an agonist at NMDA receptors, and kainic acid, an agonist at the DL-a-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate (AMPA)/kainate site. Stress-induced increase in DAsynthesis was attenuated by co-infusion of 2-amino-5-phosphonovalerate (APV) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), antagonists of NMDA and AMPA/kainate receptors, respectively. In contrast, intrastriatal APV, CNQX, or kynurenic acid (a non-selective ionotropic glutamate receptor antagonist) did not block the stress-induced increase in DArelease. Stress-induced increase in DA release was, however, blocked by administration of tetrodotoxin along the nigrostriatal DA projection. It also was attenuated when APV was infused into substantia nigra. Thus, glutamate may act via ionotropic receptors within striatum to regulate DA synthesis, whereas glutamate may influence DA release via an action on receptors in substantia nigra. However, our method for monitoring DA synthesis lowers extracellular DA and this may permit the appearance of an intra-striatal glutamatergic influence by reducing a local inhibitory influence of DA. If so, under conditions of low extracellular DA glutamate may influence DA release, as well as DA synthesis, by an intrastriatal action. Such conditions might occur during prolonged severe stress and/or DA neuron degeneration. These results may have implications for the impact of glutamate antagonists on the ability of patients with Parkinson's disease to tolerate stress.     

Stimulation of dopamine synthesis and activation of tyrosine hydroxylase by phorbol diesters in rat striatum

In rat striatal synaptosomes, 4 beta-phorbol 12-myristate 13-acetate (PMA) and 4 beta-phorbol 12,13-dibutyrate (PDBu), two activators of Ca2+-phospholipid-dependent protein kinase (protein kinase C) increased dopamine (DA) synthesis measured by following the release of 14CO2 from L-[1-14C] tyrosine. Maximal stimulation (21-28% increase of basal rate) was produced by 0.5 microM PMA and 1 microM PDBu. 4 beta-Phorbol and 4 beta-phorbol 13-acetate, which are not activators of protein kinase C, were ineffective at 1 microM. PMA did not change the release of 14CO2 from L-[1-14C]DOPA. Addition of 1 mM EGTA to a Ca2+-free incubation medium failed to affect PMA stimulation. KC1 (60 mM) enhanced DA synthesis by 25%. Exposure of synaptosomes to either PMA or PDBu prior to KC1 addition resulted in a more than additive increase (80-100%) of DA synthesis. A similar synergistic effect was observed when the phorbol diesters were combined with either veratridine or d-amphetamine but not with forskolin and dibutyryl cyclic AMP. Pretreatment of striatal synaptosomes with phorbol diesters produced an activation on of tyrosine hydroxylase (TH) associated with a 60% increase of the Vmax and a decrease of the Km for the pterine cofactor 6-methyl-5,6,7,8-tetrahydropterin. These results indicate that protein kinase C participates in the regulation of striatal TH in situ and that its activation may act synergistically with DA releasing agents in stimulating DA synthesis.     

多巴胺类似物抑制二氢蝶啶还原酶的研究

多巴胺类似物对二氢蝶啶还原酶有明显的非竞争性抑制作用(Ki或I_(50)值为10~(-5)—10~(-6)mol/L)。其中阿朴吗啡是最强的抑制剂之一(Ki或I~(50)=1-2×10~(-6)mol/L)。由于酪氨酸羟化酶和二氢蝶啶还原酶包含于同一酶促反应过程中,且限制了多巴胺合成的决定速度的那一步。这些结果可能提示出被多巴胺抑制的酪氨酸羟化作用包含着对这二种酶的抑制作用。     

Dopamine Sulfoconjugation in the Rat Brain: Regulation by Monoamine Oxidase

An increase of free 3,4-dihydroxyphenylethylamine (DA, dopamine) in the rat brain such as is found following 3,4-dihydroxyphenylalanine (L-DOPA) administration or an intraventricular injection of free dopamine did not result in DA sulfate formation, despite the presence of phenolsulfotransferase activity in various regions of the brain and the high affinity of DA for this enzyme. However, when rats were pretreated with pargyline, a monoamine oxidase inhibitor, the same treatment with L-DOPA or free DA led to active synthesis of DA sulfate. The increase in DA sulfate was significantly correlated with the degree of monoamine oxidase inhibition and directly proportional to free DA concentrations in the hypothalamus (r = 0.86), striatum (r = 0.54), and brainstem (r = 0.89). The highest ratio of DA sulfate to free DA was found in the hypothalamus, suggesting that sulfoconjugation is most active in this region. Prior treatment of rats with 6-hydroxydopamine did not decrease DA sulfate concentrations, indicating that sulfoconjugation occurs most likely in extraneuronal tissues not destroyed by the neurotoxin. The results are compatible with the notion that phenolsulfotransferase may be highly compartmentalized and that inhibition of monoamine oxidase allows the newly generated free DA to become accessible to the sulfoconjugating enzyme, resulting in increase in DA sulfation.     

Effect of cold exposure on synthesis of cerebral dopamine (author's transl)]

The synthesis of dopamine (DA) has been studied in the striatum and cortex of rats exposed to a temperature of 40C for 2.5 or 24 h. The synthesis rate has been estimated 30 mn after an i.v. injection of 3H tyrosine (TY), by the evaluation of the ratio: 3H-DA specific activity 3H-TY specific activity. Cold exposure modified DA synthesis differently in the two brain areas. In the striatum, DA synthesis was multiplied by a factor of 1.5 after 2.5 h of cold exposure and returned to normal value after 24 h. In the cortex, cold exposure did not significantly change DA synthesis (at any of the two times studied).     

A comparison of domperidone and haloperidol effects on different dopaminergic neurons in the rat brain

Domperidone, a dopamine (DA) receptor antagonist with reportedly preferential actions outside of the blood-brain barrier, and haloperidol, a centrally active DA antagonist, were compared with respect to their abilities to increase the activity of dopaminergic neurons in the rat brain. The activity of nigrostriatal, mesolimbic, tuberohypophyseal and tuberoinfundibular dopamine nerves was estimated by measuring the $\text{in vivo}$ rate of DA synthesis (dihydroxyphenylalanine accumulation following administration of an inhibitor of aromatic L-amino acid decarboxylase) in the striatum, olfactory tubercle, posterior pituitary and median eminence, respectively. In an initial study, the rates of DA synthesis in striatum, olfactory tubercle, and posterior pituitary were determined at 2, 8, and 16 h after subcutaneous administration of 0.25, 2.5, or 25 mg/kg domperidone. At the lowest dose of domperidone, DA synthesis was increased only in the posterior pituitary at 8 and 16 h; at the intermediate dose, DA synthesis increased in the posterior pituitary at 8 and 16 h and in the olfactory tubercle at 8 h. Only at 8 h after the highest dose of domperidone was DA synthesis increased in the striatum. When 2.5 mg/kg of doperidone or haloperidol were administered, DA synthesis in posterior pituitary and median eminence was increased in a similar fashion (in the latter region only at 16 h). In contrast, domperidone promoted only modest and delayed increases in DA synthesis in the olfactory tubercle and had no effect in the striatum. These results indicate that systemically administered domperidone preferentially increases DA synthesis in neurons terminating outside the blood-brain barrier, but after a pronounced delay, high doses of the drug can also activate DA neurons which project to the forebrain.     

Dopamine Synthesis in Synaptosomes: Relation of Autoreceptor Functioning to pH, Membrane Depolarization, and Intrasynaptosomal Dopamine Content

Abstract: Factors affecting dopamine (DA) synthesis in rat striatal synaptosomes were examined by measuring the conversion of [³H]tyrosine (Tyr) to [³H]DA. Any [³H]DA that was synthesized was extracted into a toluene-based scintillation cocktail and quantitated by liquid scintillation spectrometry. The extraction was facilitated using di-(2-ethylhexyl) phosphoric acid (DEHP), a liquid cation exchanger. DA, apomorphine, and other DA agonists were much less potent inhibitors of DA synthesis in striatal synaptosomes at pH 6.2 than at pH 7.2. 3-(3-Hydroxyphenyl)- N - n -propylpiperidine (3-PPP), a putative DA autoreceptor agonist, was inactive at pH 6.2. However, at pH 7.2, 3-PPP did inhibit DA synthesis. This inhibition was reversed by sulpiride, a DA receptor antagonist, but not by benztropine, a DA uptake blocker, suggesting that 3-PPP inhibits DA synthesis by stimulating the DA autoreceptor. DA release from synaptosomes was much greater at pH 6.2 than at pH 7.2, most probably because the synaptosomal membrane appears to be depolarized at pH 6.2, as measured by the accumulation of [³H]tetraphenylphosphonium ions. Since tyrosine hydroxylase is inhibited by DA, this finding suggested that low assay buffer pH (i.e., pH 6.2) might interfere with the ability of 3-PPP and other DA agonists to inhibit DA synthesis, by promoting DA release. Likewise, reserpine and tetrabenazine, compounds which disrupt vesicular DA storage, were much less effective inhibitors of DA synthesis at pH 6.2 (high basal DA release). Moreover, d -amphetamine and high buffer potassium concentrations, treatments which promote DA release, also interfered with the ability of 3-PPP to inhibit DA synthesis. Thus, modulation of the release of DA in equilibrium with tyrosine hydroxylase may be a mechanism by which the DA autoreceptor regulates DA synthesis.     

Rat striatal dopamine and tetrahydrobiopterin content following an intrastriatal injection of manganese chloride

Injection of manganese into the rat corpus striatum causes a rapid fall in the biopterin and dopamine (DA) content ipsilateral to the lesion. Two weeks after the lesion both biopterin and DA are partially recovered. Controls, injected with saline or magnesium, do not show alterations in their DA or cofactor levels. It is proposed that the fall in DA levels results from a rapid displacement of the amine from its storage sites by manganese followed by a decrease in the rate of DA synthesis causes by the drop in cofactor levels.