Release of newly synthesized 3H-dopamine in the striatum: an adaptation of the push-pull cannula method to awake restrained and anesthetized rats

The release of newly synthesized 3H-dopamine (3H-DA) was measured in the rat striatum superfused, through a push-pull cannula, with a physiological medium enriched in 3H-tyrosine. The level of spontaneous 3H-DA release was dependent on the topographical localisation of the cannula in the striatum (anterior parts displayed higher levels than posterior ones) and on the anesthetic state (halothane anesthetized rats demonstrated higher levels than awake ones). Inhibition of DA inactivation processes by local application of benztropine (a DA reuptake inhibitor, 10(-6) M) or by IV administration of pargyline (a MAO inhibitor, 100 mg/kg) enhanced the detectable outflow of 3H-DA from the striatum in both halothane anesthetized and awake rats. Local application of D-amphetamine (10(-5) M) or acetylcholine (5 X 10(-5) M) in the presence of eserine (5 X 10(-5) M) evoked respectively a fivefold and a 30% increase in spontaneous 3H-DA release in halothane anesthetized rats. Inhibition of the firing of dopaminergic neurons by IV injection of gamma-hydroxybutyrate (400 mg/kg) produced a 30% decrease in striatal 3H-DA release. The present results demonstrate that the push-pull cannula method is suitable for the study of DA release in both the anesthetized and the awake rat.     

Apomorphine and haloperidol effects on striatal H-dopamine release in anesthetized, awake restrained and freely moving rats

The ability of apomorphine (APO) and haloperidol (HAL) to affect the spontaneous release of newly synthesized ³H-DA in the striatum was studied in halothane anesthetized, gallamine paralyzed, awake restrained and freely moving rats. The striatum was continuously superfused through a push-pull cannula with a physiological medium enriched in ³H-tyrosine. Basal levels of ³H-DA release were different in the four experimental models: highest in halothane anesthetized rats, intermediate in awake restrained and gallamine treated rats and lowest in freely moving rats. In all experimental models IV or SC injection of APO (1 mg/kg) inhibited the release of ³H-DA (30–50%) from 15 to 90 min following its administration. In awake restrained and freely moving rats, stereotyped behaviour was observed for one hour following the APO injection. In halothane anesthetized rats the inhibitory effect of APO on ³H-DA release was prevented by pretreatment with HAL (2 mg/kg IV). Injection of HAL (2 mg/kg IV or SC) failed to enhance the release of ³H-DA in anesthetized and awake restrained rats, whilst a long-lasting increase in ³H-DA release was observed in gallamine treated and freely moving animals (55% and 120% respectively). However, catalepsy was observed in both restrained and freely moving rats. It is concluded that the modifications of ³H-DA release produced by HAL but not those produced by APO are dependent on the experimental model used, a fact possibly related to the different sites of action of these two drugs.     

Apomorphine and haloperidol effects on striatal 3H-dopamine release in anesthetized, awake restrained and freely moving rats

The ability of apomorphine (APO) and haloperidol (HAL) to affect the spontaneous release of newly synthesized 3H-DA in the striatum was studied in halothane anesthetized, gallamine paralyzed, awake restrained and freely moving rats. The striatum was continuously superfused through a push-pull cannula with a physiological medium enriched in 3H-tyrosine. Basal levels of 3H-DA release were different in the four experimental models: highest in halothane anesthetized rats, intermediate in awake restrained and gallamine treated rats and lowest in freely moving rats. In all experimental models IV or SC injection of APO (1 mg/kg) inhibited the release of 3H-DA (30-50%) from 15 to 90 min following its administration. In awake restrained and freely moving rats, stereotyped behaviour was observed for one hour following the APO injection. In halothane anesthetized rats the inhibitory effect of APO on 3H-DA release was prevented by pretreatment with HAL (2 mg/kg IV). Injection of HAL (2 mg/kg IV or SC) failed to enhance the release of 3H-DA in anesthetized and awake restrained rats, whilst a long-lasting increase in 3H-DA release was observed in gallamine treated and freely moving animals (55% and 120% respectively). However, catalepsy was observed in both restrained and freely moving rats. It is concluded that the modifications of 3H-DA release produced by HAL but not those produced by APO are dependent on the experimental model used, a fact possibly related to the different sites of action of these two drugs.     

Enhancement of [H]dopamine release and its [H]metabolites in rat striatum by nicotinic drugs

Recent evidence has identified directly muscarinic acetylcholine receptor (m-ACh R) and nicotinic acetylcholine receptor (n-ACh R) in the brain utilizing receptor binding assay. Several studies suggest that release of dopamine (DA) in the striatum is regulated by presynaptic receptors present on dopaminergic terminals. In the present study, the effects of cholinergic drugs on [³H]DA release were examined using micropunched tissue and synaptosomes obtained from rat striatum. ACh (5 × 10⁻⁴M) significantly increased spontaneous [³H]DA release, and the overflow was partially inhibited by d-tubocurarine (1 mM) but not atropine. Nicotine, lobeline, coniine and spartein, nicotinic agonists, significantly increased spontaneous and 25 mM K⁺ evoked [³H]DA release almost in a dose-dependent manner. In contrast, oxotremorine (2 × 10⁻⁴M), muscarinic agonist, did not any change in [³H]DA efflux. Furthermore, the metabolites of [³H]DA were separated by column chromatography. The main metabolite of [³H]DA in the spontaneous release from rat striatal synaptosomes was [³H]DOPAC (3,4-dihydroxyphenylacetic acid). Lobeline (5 × 10⁻⁵M) accelerated the outflow of [³H]DOPAC and [³H]OMDA metabolites (O-methylated and deaminated metabolites).These results could give rise to the suggestion that there was n-ACh R on the dopaminergic nerve terminals in the striatum and n-ACh R might have related to a directly excitatory effect on the DA release.     

Local and distal effects induced by unilateral striatal application of opiates in the absence or in the presence of naloxone on the release of dopamine in both caudate nuclei and substantiae nigrae of the cat

Halothane-anesthetized cats implanted with push-pull cannulae in both caudate nuclei (CN) and substantiae nigrae (SN) were used to study the effects of naloxone and various opiates when applied into the left CN on the release of newly synthetized tritiated dopamine (DA) from nerve terminals and dendrites of the two nigro-striatal dopaminergic pathways. In all cases, the drugs (naloxone, opiates alone or in the presence of naloxone) were applied for 30 min into the left CN. When applied alone, naloxone (10⁻⁶ M) induced a delayed reduction in tritiated DA release both in the ipsilateral and contralateral CN. These effects were seen after removal of the drug from the superfusion fluid. Complementary experiments made with tritiated naloxone (10⁻⁶ M) revealed that the contralateral effect on DA release was not due to a diffusion of the opiate antagonist from its application site. Locally,d-Ala₂, Met-enkephalinamide (d-Ala₂, Met-Enk, 10⁻⁶ M) and the potent δ agonist Tyr-d-Ser-Gly-Phe-Leu-Thr (DSThr, 5 × 10⁻⁸ M) induced a biphasic increase in tritiated DA release. The local changes in tritiated DA release evoked by morphine (10⁻⁶ M) and μ agonists such as Tyr-d-Ala-Gly-NH-C₆H₁₃ (10⁻⁸ M) and fentanyl (10⁻⁸ M) differed from those of δ agonists and furthermore differed from each other. For instance, morphine induced a delayed increase in tritiated DA release whereas a biphasic increase followed by a delayed inhibition occurred with fentanyl. Among all the opiates testedd-Ala₂-Met-Enk was the only one which elicited a distal effect, that is a reduction of tritiated DA release in the ipsilateral SN. Marked differences in these opiates' effects on tritiated DA release occurred both locally and in distal structures when opiates were applied simultaneously with naloxone (10⁻⁶ M). Locally, the changes induced by μ agonists were particularly altered since during morphine's application with naloxone a reduction of tritiated DA release occurred. In addition, the opiate antagonist prevented the second increase and the delayed inhibition of tritiated DA release evoked by fentanyl (10⁻⁸ M). Interestingly, the combined application of naloxone with eitherd-Ala₂,Met-Enk (10⁻⁶ M) DSThr (5 × 10⁻⁸ M) or morphine (10⁻⁶ M) resulted in the appearance of changes in tritiated DA release in contralateral structures. The most striking effect was seen withd-Ala₂,Met-Enk which enhanced tritiated DA release in the contralateral CN and SN. These results are discussed in the light of the involvement of several types of opiate receptors and of the polysynaptic pathways responsible for the distal changes in dopaminergic transmission.     

Bilateral asymmetrical changes in the nigral release of [H]GABA induced by unilateral application of acetylcholine in the cat caudate nucleus

In halothane anaesthetized cats, a push-pull cannula was implanted into the right caudate nucleus (CN) and in each substantia nigra (SN). The release of [³H]GABA continuously formed from [³H]glutamine was estimated in each structure. Acetylcholine (ACh, 5 × 10⁻⁵M) added in presence of eserine (5 × 10⁻⁵M) for 50 min in the right caudate nucleus 2 h after the onset of superfusion with [³H]glutamine, stimulated the [³H]GABA release locally. The effect was biphasic when ACh application was made in the median two-thirds of the structure and it was monophasic and transient when the ACh application was restricted to the lateral part. ACH application in the right caudate nucleus also induced changes in [³H]GABA release in the anterior (pars reticulata) and posterior (pars compacta) parts of both SN. While [³H]GABA release was enhanced in the ipsilateral anterior SN, it was reduced in the contralateral anterior SN. Respective opposite effects were observed in the posterior parts of the ipsi- and contralateral SN. These bilateral asymmetrical changes in [³H]GABA release were not dependent on the site of ACh application in the right caudate nucleus. These results indicate that the facilitation of cholinergic transmission in one caudate nucleus influences in an opposite way the striato-nigral GABA neurones on both sides of the brain.     

A role for striatal beta-adrenergic receptors in the regulation of dopamine release

The effect of (−) isoproterenol on the spontaneous release of [³H]dopamine ([³H]DA) continuously formed from [³H]tyrosine has been studied both in vitro using rat striatal slices and in vivo in halothane-anaesthetized cats implanted with push-pull cannulae in each caudate nucleus and each substantia nigra.In vitro, (−) isoproterenol (10⁻⁶M) stimulated [³H]DA release. This effect was stereospecific since (+) isoproterenol (10⁻⁶M) was without effect. (±) Propranolol (10⁻⁶M), which did not block the acetylcholine (10⁻⁵M)-induced release of [³H]DA, completely prevented the stimulatory effect of (−) isoproterenol on [³H]DA release. (±) Practolol (10⁻⁵M) significantly reduced the (−) isoproterenol-induced release of [³H]DA whereas phentolamine (10⁻⁵M) was uneffective. These results suggest that (−) isoproterenol acts on [³H]DA release through β₁-receptors. Since the adrenergic agonist still stimulated [³H]DA release in the presence of tetrodotoxin, the β-receptors involved may be located on DA terminals.Confirming in vitro results, (−) isoproterenol (10⁻⁶M) enhanced the local release of [³H]DA when applied into one caudate nucleus. This effect was biphasic and prevented by (±) propranolol (10⁻⁶M). The β-antagonist by itself slightly reduced [³H]DA spontaneous release suggesting that noradrenergic neurons which may innervate the caudate nucleus could exert a tonic facilitatory influence on DA release. No change in [³H]DA release occured in other structures studied during the application of (−) isoproterenol into one caudate nucleus.These various results indicate that striatal β-adrenergic receptors play a role in the regulation of DA release from DA nerve terminals and suggest an interaction between noradrenergic and dopaminergic neurones in the caudate nucleus.     

Evidence for a dopaminergic innervation of the cat lateral habenula: its role in controlling serotonin transmission in the Basal ganglia

The presence of a dopaminergic innervation of the cat lateral habenula and its possible role in modulating serotonin transmission within the basal ganglia were investigated using both in vitro and in vivo approaches. A high density of [³H]spiroperidol binding sites with similar affinities for domperidone and apomorphine as those present in the cat striatum were found in the habenula. By means of the push-pull cannula technique, a substantial release of [³H]dopamine continuously formed from [³H]tyrosine was detected in the lateral habenula of halothane-anesthetized cats since the amount of [³H]catecholamines was enhanced in the presence of benztropine, an inhibitor of dopamine uptake into dopaminergic nerve terminals. Furthermore, in anesthetized animals with a push-pull cannula implanted in each caudate nucleus and substantia nigra habenular applications of dopamine (10⁻⁷ M) reduced nigral but not striatal release of [³H]serotonin continuously formed from [³H]tryptophan. This change was prevented either by the delivery of domperidone to the lateral habenula or by the blockade of GABAergic transmission (picrotoxin 10⁻⁵ M) in the dorsal raphe. These data support the involvement of habenula-raphe pathways in the regulation of serotonin transmission in the cat basal ganglia and indicate that dopaminergic inputs to the lateral habenula participate in such a control.     

Dopamine-induced changes in protein phosphorylation and polyphosphoinositide metabolism in rat hippocampus

Effects of dopamine (DA) on endogenous phosphorylation of hippocampal proteins and polyphosphoinositides were studied in subcellular fractions from a crude mitocondrial/synaptosomal preparation. DA induced a concentration-dependent decrease in the in vitro phosphorylation of the protein B-50 (−22.1% at 10⁻⁵ M DA), whereas no changes were found in phosphoproteins in other subcellular fractions. Treatment of hippocampal slices with 5 × 10⁻⁴ M DA resulted in a 45.8% increase in post hoc phosphorylation of B-50 in SPM and it affected post hoc phosphorylation of several proteins in a cytosolic fraction. In vitro phosphorylation of SPM with DA (5 × 10⁻⁴ M) increased endogenous TPI phosphorylation (+ 51.6%), whereas treatment of slices with DA (5 × 10⁻⁴ M) resulted in a 39.4% decrease in post hoc TPI phosphorylation. This decrease could be blocked by haloperidol. Significant changes induced by DA (5 × 10⁻⁴ M) were also found in³²P-incorporation into PA (in vitro:−32.4% and post hoc:+ 39.3%), but were not found in DPI labeling. The data provide evidence for DA-induced changes in phosphorylation of proteins and polyphosphoinositides in rat hippocampal SPM.     

Amphetamine-induced circling behavior in rats: Effects of unilateral microinjections of GABA and GABA-related drugs into substantia nigra

In these experiments, the GABA control upon dopaminergic nigrostriatal neurons has been investigated using circling behavior in the rat. Chronically cannulated rats were givend-amphetamine (2 mg/kg i.p.) 45 min before unilateral microinjection (0.2 μl/2 min) into the substantia nigra (SN) of GABA, muscimol, chlordiazepoxide (CDP) or bicuculline. Circling behavior was continuously recorded for 165 min using an automated rotometer. (1) Non-microinjected control rats exhibit a ‘spontaneous’ circling behavior after amphetamine. (2) When applied to the SN contralateral to the preferential side of the ‘spontaneous’ rotations, saline enhances contraversive circling; GABA (5 × 10⁻⁵M), CDP (5 × 10⁻⁵M) and muscimol (5 × 10⁻⁷M, 5 × 10⁻⁸M) counteract this effect and induce (except CDP) light ipsiversive rotations; GABA (10⁻²M) and muscimol (5 × 10⁻⁵M) further enhance contraversive turning. (3) When applied to the SN ipsilateral to the preferential side of the ‘spontaneous’ rotations, saline has no marked effect on the ipsiversive circling behavior but induces weak contraversive turning; GABA (5 × 10⁻⁵M), CDP (5 × 10⁻⁵M) and muscimol (5 × 10⁻⁸M) enhance the ipsiversive rotations; GABA (10⁻²M) and muscimol (5×10⁻⁵M) transiently decrease the ipsiversive circling; bicuculline (5 × 10⁻⁵M) induces a vigorous contraversive turning associated with a transient inhibition of the ipsiversive rotations. These results suggest that the activity of the nigral neurons is presumably stimulated by the microinjection itself, by bicuculline and, to a lesser extent, by high concentrations of GABA and muscimol, and inhibited by low concentrations of GABA, muscimol and CDP. These findings could further support the hypotheses of a GABAergic inhibitory control upon DA nigrostriatal pathways and of a GABA-like activity of CDP.     

Regulation of Dopamine D1 and D2 Receptors on Striatal Acetylcholine Release in Rats

The effects of dopamine (DA) D₁ and D₂ receptors on striatal acetylcholine (ACh) releases were investigated by in vivo microdialysis. All drugs were applied via dialysis membrane directly to the striatum. The levels of ACh release were increased by 10⁻⁴ M SKF38393, a D₁ receptor agonist. Although 10⁻⁴ M SCH23390, a D₁ receptor antagonist, exhibited an increase in the levels of ACh release, the agonist (10⁻⁴ M) induced-increase in the levels of ACh release was suppressed by coperfusion of the antagonist (10⁻⁴ M). In contrast, the levels of ACh release were decreased by the D₂ receptor agonist, N-434, in a dose-dependent manner (10⁻⁵ M to 10⁻⁷ M) and increased by the D₂ receptor antagonist, sulpiride, in a dose-dependent manner (10⁻⁵ M to 10⁻⁷ M). The agonist (10⁻⁵ M) induced-decrease in the levels of ACh release was suppressed by coperfusion of the antagonist (10⁻⁶ M). Coperfusion of D₁ (10⁻⁴ M) and D₂ (10⁻⁵ M) agonists blocked both effects of respective drug alone. In order to clarify the effect of endogenous DA, two drugs with different mechanisms for enhancing DA concentration in the synaptic cleft, the DA release-inducer methamphetamine, and the DA uptake inhibitor nomifensine were perfused separately. Both (10⁻⁴ M to 10⁻⁶ M) produced a dose- and a time-dependent decrease in the levels of ACh release. Significant higher levels of ACh release were observed in the striatum of the 6-hydroxydopamine (8

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)-treated rats with significant depletion of striatal DA content. These results suggest that in striatal DA-ACh interaction ACh release, as cholinergic interneuron's activity, is tonically inhibited via the D₂ receptor, mainly by dopaminergic input, and the D₁ receptor probably modifies the effect of the D₂ receptor indirectly.     

The effects of intranigral GABA and dynorphin A injections on striatal dopamine and GABA release: Evidence that dopamine provides inhibitory regulation of striatal GABA neurons via D2 receptors

The effects of injections of γ-aminobutyric acid (GABA) and dynorphin A into the substantia nigra, pars reticulata on the levels of extracellular dopamine (DA) and GABA in the ipsilateral striatum of halothane-anaesthetized rats were studied using microdialysis. The effects of intranigral injections of substance P and neurokinin A were also studied. Intranigral GABA (300 nmol) or dynorphin A (0.5 nmol) injections produced a simultaneous decrease in DA and increase in GABA levels, while intranigral substance P (0.07 nmol) or neurokinin A (0.09 nmol) injections produced an increase in DA but had no effect on GABA levels. DA agnonists, apomorphine (D₁/D₂), SKF 38393 (D₁) and pergolide (D₂) were applied locally by perfusing them through the microdialysis probe, each at a concentration of 10⁻⁵ M. All 3 agonists decreased the levels of DA in the striatum. However, while apomorphine and SKF 38393 increased, pergolide decreased the levels of GABA in the striatum. The increase in striatal GABA produced by intranigral injections of GABA (300 nmol) was reversed by local perfusion with pergolide (10⁻⁵ M), but was not reversed by local perfusion with SKF 38393 (10⁻⁵ M). These findings suggest that D₁ and D₂ receptors differentially regulate striatal GABA release, and are stimulatory and inhibitory, respectively. Furthermore, it is suggested that nigrostriatal DA functions as an inhibitory modulator of striatal GABA neurons, acting via D₂ receptors.     

NMDA regulation of dopamine release from proximal and distal dendrites in the cat substantia nigra

The NMDA regulation of the dendritic release of [³H]dopamine ([³H]DA) synthesized from [³H]tyrosine was investigated in vitro using a microsuperfusion procedure in the pars compacta (SNC) and the pars reticulata (SNR) of the cat substantia nigra. The spontaneous release of [³H]DA was threefold higher in the SNC than in the SNR and amphetamine (1 μM) enhanced similarly [³H]DA release in both nigral areas. In the absence of magnesium, NMDA (50 μM) stimulated markedly the release of [³H]DA in the SNC and SNR, these effects being completely prevented by MK 801 (1 μM), the NMDA receptor antagonist. The DA uptake inhibitor, nomifensine (5 μM), increased the amount of [³H]DA recovered in SNC (×2) and SNR (×3) superfusates but did not significantly modify the NMDA-evoked responses. The effects of NMDA seen in the absence or presence of nomifensine persisted when the two nigral areas were continuously superfused with tetrodotoxin (1 μM). These results are in favor of the presence of NMDA receptors on dopaminergic dendritic arborizations and indicate that the stimulation of these receptors facilitates in a similar way the release of DA from proximal and distal dendrites.     

Modulation of glycine-induced currents by zinc and other metal cations in neurons acutely dissociated from the dorsal motor nucleus of the vagus of the rat

Effects of Zn²⁺ and other polyvalent cations on glycine-induced currents in the freshly dissociated rat dorsal motor nucleus of the vagus neurons were investigated under voltage-clamp conditions by the use of the nystatin-perforated patch recording configuration. Glycine evoked a Cl⁻ current in a concentration-dependent manner with a half-maximum effective concentration (EC₅₀) of 3.3×10⁻⁵ M. Strychnine inhibited the 3×10⁻⁵ M glycine-induced current in a concentration-dependent manner with a half-maximal inhibitory concentration (IC₅₀) of 6.8×10⁻⁷ M. At low concentrations (3×10⁻⁸ M–3×10⁻⁶ M), Zn²⁺ potentiated the current elicited by 3×10⁻⁵ M glycine. On the other hand, at concentrations higher than 10⁻⁵ M, Zn²⁺ inhibited the glycine response. The biphasic action of Zn²⁺ was mimicked by Ni²⁺, but La³⁺ and Co²⁺ had only potentiating effect. Zn²⁺ shifted the concentration–response curve for the glycine-induced current without changing the maximum response, and the EC₅₀ values for the glycine response in the absence and presence of 10⁻⁶ M and 10⁻⁴ M Zn²⁺ were 3.3×10⁻⁵ M, 1.3×10⁻⁵ M and 1.3×10⁻⁴ M, respectively. These results suggest that the biphasic modulation of glycine response by Zn²⁺ results from changes in apparent glycine affinity.     

Striatal dopamine release is altered by glucose and insulin during push-pull perfusion of the rat's caudate nucleus

An individual site in the caudate-putamen complex of the unrestrained, fasted rat was tritium-labeled with 1.0 μCi of either³H-DA or ³H-NE. The amine was microinjected in a volume of 0.5 μl through a permanently implanted 20 ga guide cannula. Then 30 min later, a push-pull cannula assembly was lowered through the guide and the labeled site was perfused with an artificial CSF solution at a rate of 25 μl/min. Each perfusion lasted for 5 min with a 10 min interval between successive perfusions. At the mid-point of a sequence of seven perfusions, one of two experimental procedures was introduced: (1) the rat was offered food pellets which were readily consumed; or (2) one of three compounds was added to the CSF perfusate: (a) 55 μg/gml of D-glucose; (b) 4 mU/μl of insulin; or (c) 10 μg/μl of 2-DG. The results showed that the efflux of³H-DA increases as the rat feeds and during the subsequent perfusions. An immediate rise occurs in the output of the amine also when insulin is added to the perfusate; conversely, a slight suppression in ³H output is caused by glucose during its perfusion but a consistent elevation of radioactivity follows thereafter. 2-DG has no effect on the kinetics of catecholamine activity in the caudate-putamen. The patterns of release of ³H-NE are similar to those of ³H-DA under the same experimental conditions; however, the magnitude of the change is substantially attenuated. Although catecholamine activity within the caudate-putamen is usually implicated in motor control, our results support the possibility that dopamine in this structure may play a functional role in the regulation of food intake.     

Assessment of the role of TRH in the release of [H]-dopamine from rat nucleus accumbens-lateral septum slices

We have studied [³H]-dopamine ([³H]-DA) release from rat nucleus accumbens lateral septum slices in response to various paradigms aimed at increasing endogenous or exogenous thyrotropin releasing hormone (TRH) concentrations in the extracellular space. High KCl concentrations significantly enhanced [³H]-DA release by fourfold. TRH (10⁻⁴ or 5 × 10⁻⁴ M) did not affect [³H]-DA release. The release of [³H]-DA was not stimulated by TRH either in the presence of N-l-carboxy-2-phenylethyl (N^imbenzyl)-histidylβnaphthylamide, a specific pyroglutamyl peptidase II inhibitor, or that of specific inhibitors of prolyl endopeptidase and pyroglutamyl peptidase I. None of the peptidase inhibitors modified the [³H]-DA release by themselves. These results suggest that the TRH stimulation of [³H]-DA release in vitro observed in previous studies is not due to peptide inactivation but may be due to a nonspecific effect. TRH enhancement of DA release in nucleus accumbens in vivo may not be the result of a direct effect of TRH on DA terminals.     

Uric acid is reduced in the substantia nigra in parkinson's disease: Effect on dopamine oxidation

Postmortem caudate and substantia nigra tissue samples from human parkinsonian patients (PD) and age-matched controls (NC) were analyzed for uric acid (UA), dopamine (DA), and ascorbic acid (AA) by HPLC/UV/ED. Uric acid and DA levels were significantly lower in the substantia nigra of PD by 54% and 85%, respectively. In the caudate, DA levels were significantly lower while UA levels were nonsignificantly reduced (0.10 < p < 0.05). Ascorbic acid levels were not significantly different from the controls in either brain region. Conditions favorable for oxidative stress were evaluated by measuring the oxidation of DA in individual brain homogenates. The rate constant for DA oxidation in control caudate was 0.34 × 10⁻²min⁻¹ and in parkinsonian caudate was 4.20 × 10⁻²min^t-1. In control and parkinsonian substantianigra DA oxidation rate constants were 2.82 × 10⁻²min^t−1 and 4.57 × 10⁻²min⁻¹, respectively. Addition of UA or catalase to parkinsonian homogenate decreased the rate of DA oxidation, while addition of uncase to control homogenate increased the rate of DA oxidation. The data support the hypothesis that UA is decreased in nigrostriatal dopamine neurons in parkinsonian patients which contributes to an environment susceptible to oxidative stress, particularly through dopamine oxidation reactions.     

Effects of morphine and naloxone on the K-stimulated release of methionine-enkephalin from slices of rat corpus striatum

Methionine-enkephalin (ME) released from superfused slices of rat corpus striatum was estimated by radioimmunoassay (RIA). The basal release of2.5 ± 0.2pmol/g/min (0.15% of content per min) was increased approximately 3-fold upon exposure of tissue to 30 mM K⁺ for 5 min. This increase in release was not observed in the absence of Ca²⁺. Both morphine (10⁻⁵ M) and (−)-naloxone (10−⁵ and 10−⁶ M) significantly depressed the release of ME evoked by 30 mM K⁺ but did not alter basal release. The +-isomer of naloxone, which lacks opiate antagonist activity, did not affect basal or evoked release. A consistent depression of release was not observed when 47 mM K⁺ was used to evoke the release of ME. The issue of whether a feedback mechanism controls the release of ME from the striatum cannot be resolved until it is known whether the effect of morphine and naloxone on ME release are mediated by opiate or non-opiate mechanisms.     

ACTH1–24 and lysine vasopressin selectively activate dopamine synthesis in frontal cortex

The accumulation of [³H]catecholamines from [³H]tyrosine in frontal cortical, septal, striatal and hippocampal slices was examined following intracerebroventrcular (i.c.v.) injections of ACTH₁₋₂₄, lysine vasopressin (LVP) and saline. Both ACTH₁₋₂₄ and LVP (1 μg) selectively increased the accumulation of [³H]dopamine (DA) in frontal cortical slices, but did not affect that of [³H]norepinephrine (NE). LVP but not ACTH₁₋₂₄ also inhibited the accumulation of [³H]DA in striatal slices. ACTH₁₋₂₄ did not alter the accumulation of [³H]NE in hippocampal slices, nor did LVP alter the accumulation of either catecholamine (CA) in septal slices. In vitro incubations with ACTH analogs or LVP failed to alter the rate of accumulation of [³H]CAs in striatal, substantia nigral and frontal cortical slices, except for an inhibitory effect at high doses. This effect is believed to be an artifact of precursor dilution caused by release of tyrosine following degradation of the peptides. Neither peptide modified the increased [³H]CA accumulation stimulated by 26 mM K⁺, nor did ACTH_1–24 modify the inhibition of [³H]CA accumulation caused by 3 × 10⁻⁶ M haloperidol or 3 × 10⁻⁷ M apomorphine. Selective activation of the mesocortical DA system has also been reported to occur in response to footshock, suggesting the possibility that endogenous ACTH and/or LVP might mediate the stress-induced activation of mesocortical DA synthesis. Alternatively, i.c.v. injections of these peptides may themselves be stressful and thus indirectly elicit the response.     

Intrastriatal injection of [H]dopamine through a chronic cannula to produce rotation: Distribution and concentration of the tracer in specific brain regions

As others have shown, rats rotated contralaterally following unilateral intrastriatal injections of dopamine. However, the concentration of the injection solution necessary to elicit rotation in our and others' studies was high, 50 μg/0.5 μl (10,000 times endogenous concentration) and the latency to turn was 20–60 min. Thus, we injected [³H]dopamine ([³H]DA) to determine the concentration of the injected DA in the striatum and to determine if the long latency to turn was related to spread of DA over time to distal nuclei such as the globus pallidus, subthalamic nucleus and substantia nigra. Microinjections (0.5 μl) of [³H]DA were made through a chronic striatal cannula in pargyline-treated freely moving rats. Determinations of [³H]DA concentrations in individual brain areas were made at 20 min and 60 min following intrastriatal injection. The major accumulation of [³H]DA and its metabolites was in the dorsal striatum, near the cannula tip, and in the sensorimotor cortex where the cannula passed through to the striatum. The concentration of [³H]DA in the dorsal striatum was only 10–100 times the endogenous concentration, depending upon the cannula injection system used. The 60 min latency to rotate could not be attributed to spread of DA to the subthalamic n., entopeduncular n. and substantia nigra. Rotation following an intrastriatal injection of DA was dependent upon the dorsal striatal tissue concentration of DA. The ventral striatum and globus pallidus were also possibly involved. The highest striatal [³H]DA concentrations were, at most, 10 times greater than the concentrations of systemically injectedl-[³H]DOPA, and, at the lower concentrations which produced twisting and weak rotation, the DA concentration was similar to that seen with systemicl-DOPA.