The endogenous tripeptide Tyr-Gly-Gly as a possible metabolite of opioid peptides in rat brain: identification, regional distribution, effects of lesions and formation in depolarized slices

Using a sensitive radioimmunoassay, the tripeptide Tyr-Gly-Gly (YGG) which corresponds to the N-terminal sequence of opioid peptides was detected in rat brain and identified by HPLC. Its regional distribution paralleled that of (Met5)enkephalin (YGGFM), a marker of enkephalin neurons. Ablation of these neurons in the striato-pallidal pathway by intrastriatal kainate, induced a significant decrease in YGG levels in caudateputamen and globus pallidus (-49%), consistent with the hypothesis that YGG originates from enkephalin neurons. When pallidal slices were incubated under various conditions, YGG was mainly found in the incubation medium indicating a predominantly extracellular localization. Depolarization of these slices by a K+-stimulus elicited a release of YGGFM accompanied by a marked increase in YGG levels. Bestatin and amastatin further enhanced YGG levels, reflecting the participation of aminopeptidases in the metabolism of the tripeptide and its precursor. Captopril, an inhibitor of the angiotensin-converting enzyme (ACE) showed no effect on the recovery of YGGFM and YGG. In contrast, the formation of YGG was completely prevented by Thiorphan (IC50 value = 9 nM) and phosphoramidon, two inhibitors of "enkephalinase" (EC 3.4.24.11; membrane metallo-endopeptidase), thus identifying the latter as the YGG-forming enzyme. The K+-induced increase in YGG + YGGFM levels in medium containing bestatin exceeded by about 60% the amount of YGGFM released from tissues, suggesting that YGG was mainly formed by extracellular hydrolysis of the various opioid fragments of the proenkephalin molecule. In vivo, YGG levels of cerebral regions were also markedly reduced in rats treated with acetorphan, a parenterally active "enkephalinase" inhibitor. All data suggest that YGG levels constitute an index of opioid peptide release.     

Changes in levels of the tripeptide Tyr-Gly-Gly as an index of enkephalin release in the spinal cord: effects of noxious stimuli and parenterally-active peptidase inhibitors

The tripeptide Tyr-Gly-Gly (YGG), representing the product of enkephalin hydrolysis by enkephalinase (EC 3.4.24.11), was characterized and its levels measured in spinal cord perfusates of halothane-anaesthetized rats. During noxious pinching of the muzzle, which is known to trigger enkephalin release, YGG levels were enhanced more markedly and for longer than were those of [Met5]enkephalin (YGGFM), in the same samples. By contrast, neither YGG nor YGGFM levels were affected by pinching the tail. Treatment with carbaphethiol, a parenterally-active aminopeptidase inhibitor, markedly increased YGG levels and lengthened the duration of the increase produced by pinching the muzzle. Treatment with acetorphan, a parenterally-active enkephalinase inhibitor, given alone or in combination with carbaphethiol, completely prevented the rise in YGG triggered by noxious stimulation. By contrast, [Met5]enkephalin levels in the perfusates were increased by the combined administration of the two peptidase inhibitors but these levels were not further enhanced by noxious stimulation. Thus, spinal cord YGG appears to be formed under the influence of enkephalinase and to constitute a sensitive index of enkephalin release.     

Specific Inhibition of Dopamine D-1-Mediated Cyclic AMP Formation by Dopamine D-2, Muscarinic Cholinergic, and Opiate Receptor Stimulation in Rat Striatal Slices

The ability of different receptors to mediate inhibition of cyclic AMP accumulation due to a variety of agonists was examined in rat striatal slices. In the presence of 1 mM 3-isobutyl-1-methylxanthine, dopamine D-2, muscarinic cholinergic, and opiate receptor stimulation by RU 24926, carbachol, and morphine (all at 10(-8)-10(-5) M), respectively, inhibited the increase in cyclic AMP accumulation in slices of rat striatum due to dopamine D-1 receptor stimulation by 1 microM SKF 38393. In contrast, these inhibitory agents were unable to reduce the ability of a number of other agonists, including isoprenaline, prostaglandin E1, 2-chloroadenosine, vasoactive intestinal polypeptide, and cholera toxin, to increase cyclic AMP levels in striatal slices. These results suggest that in rat striatum either dopamine D-2, muscarinic cholinergic, and opiate receptors are only functionally linked to dopamine D-1 receptors or that the D-1 and D-2 receptors linked to adenylate cyclase lie on the cells, distinct from other receptors capable of elevating striatal cyclic AMP levels.     

Ethanol-Induced Increase in Endogenous Dopamine Release May Involve Endogenous Opiates

The effect of opiate peptides on basal and potassium-stimulated endogenous dopamine (DA) release from striatal slices was studied in vitro. Dual stimulation of the striatal slices gave a reproducible increase in DA release that was calcium dependent. Addition of the delta-opiate receptor agonists Met5-enkephalin, [D-Ala2,D-Leu5]enkephalin (DADLE), and [D-Ser2]Leu-enkephalin-Thr (DSLET), increased the basal DA release without affecting potassium-stimulated release in a dose-dependent manner. The effect of DADLE was antagonized by the addition of naloxone. In contrast, the mu-opioid receptor agonist [D-Ala2,N-MePhe4,Gly-ol5]enkephalin (DAGO) and the epsilon-opioid agonist beta-endorphin inhibited the stimulated DA release without changing the basal release. The inhibitory effect of DAGO on potassium-stimulated release was antagonized by naloxone. The addition of ethanol (75 mM) to the incubation media produced a delayed increase of both the basal and stimulated DA release. There was no change in stimulated DA release when the change in basal release was subtracted, suggesting that ethanol produced a dose-dependent, selective increase in basal DA release. Naloxone and the selective delta-opiate antagonist ICI 174864 inhibited the ethanol-induced increase in basal DA release. Naloxone and ICI 174864 added alone did not alter either basal or stimulated DA release. We therefore suggest that the ethanol-induced increase in basal DA release is an indirect effect involving an endogenous delta-opiate agonist.     

Influence of Cannabinoids on Electrically Evoked Dopamine Release and Cyclic AMP Generation in the Rat Striatum

Abstract: Using the endogenous cannabinoid receptor agonist anandamide, the synthetic agonist CP 55940 {[1α,2β( R )5α]-(−)-5-(1,1-dimethylheptyl)-2-[5-hydroxy-2-(3-hydroxypropyl)cyclohexyl]phenol}, and the specific antagonist SR 141716 [ N -(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1 H -pyrazole-3-carboxamide hydrochloride], second messenger activation of the central cannabinoid receptor (CB1) was examined in rat striatal and cortical slices. The effects of these cannabinoid ligands on electrically evoked dopamine (DA) release from [³H]dopamine-prelabelled striatal slices were also investigated. CP 55940 (1 µ M ) and anandamide (10 µ M ) caused significant reductions in forskolin-stimulated cyclic AMP accumulation in rat striatal slices, which were reversed in the presence of SR 141716 (1 µ M ). CP 55940 (1 µ M ) had no effect on either KCl- or neurotransmitter-stimulated ³H-inositol phosphate accumulation in rat cortical slices. CP 55940 and anandamide caused significant reductions in the release of dopamine after electrical stimulation of [³H]dopamine-prelabelled striatal slices, which were antagonised by SR 141716. SR 141716 alone had no effect on electrically evoked dopamine release from rat striatal slices. These data indicate that the CB1 receptors in rat striatum are negatively linked to adenylyl cyclase and dopamine release. That the CB1 receptor may influence dopamine release in the striatum suggests that cannabinoids play a modulatory role in dopaminergic neuronal pathways.     

Inhibition of dopamine uptake by D2 antagonists: an in vivo study

D(2)-like antagonists potentiate dopamine release. They also inhibit dopamine uptake by a mechanism yet to be clarified. Here, we monitored dopamine uptake in the striatum of anesthetized mice. The dopamine overflow was evoked by brief electrical stimulation of the medial forebrain bundle (four pulses at 100 Hz) and was monitored with carbon fiber electrodes combined with continuous amperometry. The decay phase of evoked overflows reflects dopamine half-life, which entirely depends on uptake. The D(2)-like antagonists haloperidol and eticlopride enhanced the half-life by 45% and 48%, respectively, a moderate effect as compared to the uptake blocker nomifensine (528%). Both D(2)-like antagonists did not affect dopamine uptake in mice lacking D(2) receptors. Inhibition of tonic dopamine release by gamma-butyrolactone did not mimic the enhancing effect of D(2) antagonists on dopamine half-life. However, prolonged stimulation boosted dopamine uptake and this effect was not observed after haloperidol treatment or in mice lacking D(2) receptors. Therefore, dopamine uptake is accelerated in conditions of excessive D(2) stimulation but not finely tuned in resting conditions. Inhibition of dopamine uptake by D(2) antagonists synergizes with the potentiation of dopamine release to strongly alter the phasic dopamine signaling.     

Modulation of In Vivo Dopamine Release by D2 but Not D1 Receptor Agonists and Antagonists

The capacity of D1 and D2 agonists and antagonists to regulate the in vivo release and metabolism of dopamine (DA) in mesolimbic and nigrostriatal DA neurons of the mouse was determined using gas chromatographic and mass fragmentographic (GC-MF) analysis. DA release was inferred from levels of 3-methoxytyramine (3-MT) and DA metabolism was inferred from levels of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). DA release was increased by the D2 antagonists haloperidol and metoclopramide but not by the D1 antagonists SCH 23390 and SKF 83566. DA metabolism was increased by each of the four antagonists but to a greater extent with the D2 antagonists. The D2 agonists CGS 15855A and LY 171555 decreased DA release whereas the D1 agonist SKF 38393, at relatively high doses, only slightly affected DA release. Each of the three agonists decreased DA metabolism but again metabolism was more affected by the D2-selective drugs. The in vivo release of DA from mesolimbic and neostriatal DA neurons appears to be modulated by D2 but not by D1 receptors, whereas both receptor types can modulate DA metabolism.     

Dopamine Release in Rat Striatum: Physiological Coupling to Tyrosine Supply

Intracerebral microdialysis was used to monitor dopamine release in rat striatal extracellular fluid following the intraperitoneal administration of dopamine's precursor amino acid, L-tyrosine. Dopamine concentrations in dialysates increased transiently after tyrosine (50-100 mg/kg) administration. Pretreatment with haloperidol or the partial lesioning of nigrostriatal neurons enhanced the effect of tyrosine on dopamine release, and haloperidol also prolonged this effect. These data suggest that nigrostriatal dopaminergic neurons are responsive to changes in precursor availability under basal conditions, but that receptor-mediated feedback mechanisms limit the magnitude and duration of this effect.     

Biotransformation of l-DOPA to Dopamine in the Substantia Nigra of Freely Moving Rats: Effect of Dopamine Receptor Agonists and Antagonists

Abstract: We investigated the effects of continuous intranigral perfusion of dopamine D1 and D2 receptor agonists and antagonists on the biotransformation of locally applied l -DOPA to dopamine in the substantia nigra of freely moving rats by means of in vivo microdialysis. The "dual-probe" mode was used to monitor simultaneously changes in extracellular dopamine levels in the substantia nigra and the ipsilateral striatum. Intranigral perfusion of 10 µ M l -DOPA for 20 min induced a significant 180-fold increase in extracellular nigral dopamine level. No effect of the intranigral l -DOPA administration was observed on dopamine levels in the ipsilateral striatum, suggesting a tight control of extracellular dopamine in the striatum after enhanced nigral dopamine levels. Continuous nigral infusion with the D1 receptor agonist CY 208243 (10 µ M ) and with the D2 receptor agonist quinpirole at 10 µ M (a nonselective concentration) attenuated the l -DOPA-induced increase in dopamine in the substantia nigra by 85 and 75%, respectively. However, perfusion of the substantia nigra with a lower concentration of quinpirole (1 µ M ) and the D1 antagonist SCH 23390 (10 µ M ) did not affect the nigral l -DOPA biotransformation. The D2 antagonist (−)-sulpiride (10 µ M ) also attenuated the l -DOPA-induced dopamine release in the substantia nigra to ∼10% of that of the control experiments. We confirm that there is an important biotransformation of l -DOPA to dopamine in the substantia nigra. The high concentrations of dopamine formed after l -DOPA administration may be the cause of dyskinesias or further oxidative stress in Parkinson's disease. Simultaneous administration of D1 receptor agonists with l -DOPA attenuates the biotransformation of l -DOPA to dopamine in the substantia nigra. The observed effects could occur via changes in nigral GABA release that in turn influence the firing rate of the nigral dopaminergic neurons.     

Involvement of Dopamine D1 and D2 Receptors in the Regulation of Proenkephalin mRNA Abundance in the Striatum and Accumbens of the Rat Brain

The effects of short-term treatment (6 h) with selective D1 or D2 agonists and antagonists on the mRNA for proenkephalin in the medial and anterior aspects of the caudate-putamen and the nucleus accumbens were assessed by in situ hybridization histochemistry. Proenkephalin mRNA abundance was significantly changed in the striatum and accumbens in response to D2 receptor manipulation. D2 blockade with haloperidol or raclopride increased, whereas D2 stimulation with LY-171555 (D2 agonist) decreased, striatal and accumbens proenkephalin mRNA abundance. Antagonism of D1 receptor activity with SCH-23390 significantly decreased proenkephalin mRNA abundance in all brain regions. Concurrent administration of the D1 agonist SKF-38393 prevented the SCH-23390 effect in all brain areas. The data demonstrate that acute treatment with dopaminergic D2 agonists and antagonists affects proenkephalin mRNA abundance in the striatum and accumbens via a D2 receptor mechanism, consistent with the concept that D2 receptor function inhibits the synthesis of the mRNA encoding the enkephalin peptides. Moreover, D1 receptor activity, directly or indirectly, exerts modulatory effects on proenkephalin mRNA abundance in the striatum and nucleus accumbens.     

Effects of Lithium on Inositol Phospholipid Hydrolysis and Inhibition of Dopamine D1 Receptor-Mediated Cyclic AMP Formation by Carbachol in Rat Brain Slices

The in vitro and ex vivo effects of lithium on muscarinic cholinergic inositol phospholipid hydrolysis and muscarinic cholinergic inhibition of dopamine D1-receptor-stimulated cyclic AMP formation were examined in rat brain slices. Following chronic lithium feeding, carbachol-stimulated inositol phosphate accumulation was reduced ex vivo in slices of cerebral cortex but not in striatal slices. Lithium (1 mM) in vitro had no direct effect on dopamine D1-receptor-stimulated cyclic AMP formation, but enhanced the inhibitory effect of carbachol on the D1 response, in striatal slices, and this was not significantly altered by prior lithium feeding. Lithium therefore has effects on two discrete muscarinic responses in rat brain which are apparently maintained after chronic exposure to the ion and might be relevant to its antimanic actions.     

Blockade of Striatal 5-Hydroxytryptmine2 Receptors Reduces the Increase in Extracellullar Concentrations of Dopamine Produced by the Amphhetamine analogue 3,4-Methylenedioxymethamphetamine

Abstract: 5-Hydroxytryptamine₂ (5-HT₂) receptor antagonists have been shown to interfere with the stimulation of striatal dopamine synthesis and release produced by the amphetamine analogue 3,4-methylenedioxymethamphetamine (MDMA). To localize the receptors responsible for the attenuation of MDMA-induced release, 5-HT₂ receptor antagonists were infused via the microdialysis probe directly into the brains of awake, freely moving rats before the systemic administration of MDMA. Intrastriatal infusions of the selective 5-HT₂ antagonist MDL 100, 907 produced a concentration-dependent inhibition of MDMA-induced dopamine release. Similar results were observed with intrastriatal infusions of the 5-HT₂ antagonist amperozide. In contrast, infusion of MDL 100, 907 into the midbrain region near the dopaminergic cell bodies was with out effect on the MDMA-induced elevation of extracellular dopamine in the ipsilateral striatum. Neither antagonist attenuated basal transmitter efflux nor the MDMA-stimulated release of [³H]dopamine from striatal slices in vitro indicating that the in vivo effect of the antagonists was not due to inhibition of the dopamine uptake carrier. Intrastriatal infusion of tetrodotoxin reduced both basal and MDMA-stimulated dopamine efflux and eliminated the effect of intrastriatal MDL 100, 907. The results indicate that 5-HT₂ receptors located in the striatum augment the release of dopamine produced by high doses of MDMA. Furthermore, these 5-HT₂ receptors appear to be located on nondopaminergic elements of the striatum.     

Presynaptic Dopamine Autoreceptors Control Tyrosine Hydroxylase Activation in Depolarized Striatal Dopaminergic Terminals

The possible control of tyrosine hydroxylase (TH) activity by dopaminergic receptor-dependent mechanisms was investigated using rat striatal slices or synaptosomes incubated in the presence of various 3,4-dihydroxyphenylethylamine (dopamine or DA) agonists and antagonists. Under "normal" conditions (4.8 mM K+ in the incubating medium), the DA agonists apomorphine, 6,7-dihydroxy-N,N-dimethyl-2-aminotetralin (TL-99), 7-hydroxy-N,N-dipropyl-2-aminotetralin (7-OH-DPAT), Trans-(-)-4,4a,5,6,7,8,8a,9-octahydro-5-propyl-2H-pyrazolo-3,4- quinoline, and 3-(3-hydroxyphenyl)-N-n-propylpiperidine decreased TH activity in soluble extracts of incubated tissues. In the case of the catechol-containing drugs apomorphine and TL-99, this effect was partly due to a direct inhibition of the enzyme, but in all other cases it appeared to depend on the stimulation of presynaptic DA autoreceptors. No effect of DA antagonists was detected on TH activity under "normal" conditions. In contrast, when tissues were incubated in a K+ -enriched (60 mM) medium, (-)-sulpiride and other DA antagonists enhanced TH activation due to depolarization whereas DA agonists were ineffective. Because (-)-sulpiride also increased the enzyme activity in striatal slices exposed to drugs inducing release of DA, such as veratridine and d-amphetamine, it is concluded that the stimulating effect of the DA antagonist resulted in fact from the blockade of the negative control of TH normally triggered by endogenous DA acting on presynaptic autoreceptors. In contrast to TH activation due to K+ -induced depolarization, the activation evoked by tissue incubation with dibutyryl cyclic AMP was unaffected by the typical agonist 7-OH-DPAT or the antagonist (-)-sulpiride. This would suggest that TH control via presynaptic DA autoreceptors normally concerns possible modulations of the cyclic AMP-dependent phosphorylation of the enzyme.     

Inhibition of the Electrically Evoked Release of [3H]Acetylcholine in Rat Striatal Slices: An Experimental Model for Drugs that Enhance Dopaminergic Neurotransmission

The activation by endogenous dopamine of the inhibitory 3,4-dihydroxyphenylethylamine (dopamine) receptors modulating the electrically evoked release of [3H]acetylcholine [( 3H]ACh) and [3H]dopamine in rat striatal slices is a function of the concentration of dopamine accumulated in the synaptic cleft during electrical stimulation. When the release of 3H-neurotransmitters was elicited with a 2-min period of stimulation at a frequency of 1 Hz, neither dopamine autoreceptors nor dopamine receptors modulating [3H]ACh were activated by endogenously released dopamine. On the other hand, exposure to (S)-sulpiride facilitated the release of [3H]dopamine and [3H]ACh elicited when the 2-min stimulation was carried out at a frequency of 3 Hz but this effect was not observed at a lower frequency of stimulation (1 Hz). In the presence of amphetamine the dopamine receptors modulating the electrically evoked release of [3H]ACh can be activated by endogenous dopamine even at the lower frequency of stimulation (1 Hz). Similar effects can be obtained if the neuronal uptake of dopamine is inhibited by cocaine or nomifensine. The inhibition by amphetamine of the release of [3H]ACh elicited by electrical stimulation at 1 Hz involves dopamine receptors and can be fully antagonized by clozapine, haloperidol, chlorpromazine, or pimozide. The stereoselectivity of this antagonism can be demonstrated with the optical enantiomers of sulpiride and butaclamol. This inhibitory effect of amphetamine on cholinergic neurotransmission appears to be the result of the stimulation of dopamine receptors of the D2 subtype, as they were resistant to blockade by the preferential D1 receptor antagonist SCH 23390.(ABSTRACT TRUNCATED AT 250 WORDS)     

Induction of c-fos mRNA Expression in Rat Striatum by Neuroleptic Drugs

The effect of selective dopamine D2 receptor-acting drugs on striatal c-fos mRNA expression in the rat has been investigated by Northern hybridization and autoradiography to determine a possible role for c-fos in the initiation of adaptive changes in D2 receptor number by neuroleptic drugs. The neuroleptic drug haloperidol, a D2 receptor antagonist, was found to produce a rapid and transient induction of c-fos mRNA expression as compared with the expression in animals treated with saline. This induction by haloperidol was found to be dose dependent and D2 receptor mediated, inasmuch as a D2 agonist completely reversed the induction and the inactive isomer of the neuroleptic butaclamol, which does not produce an increase in D2 receptors, had no effect on c-fos mRNA expression. From these data, it can be concluded that c-fos expression in striatum is under dopamine D2 receptor-mediated inhibitory control. It is suggested that c-fos may play a role in the initiation of the increase in D2 receptor number produced by chronic neuroleptic drug treatment.     

Opposing Roles of Dopamine D1 and D2 Receptors in Nigral γ-[3H]Aminobutyric Acid Release?

This study examined the effects of dopamine D1 and D2 receptor agonists and antagonists on the spontaneous and calcium-dependent, K+-induced release of gamma-[3H]aminobutyric acid [( 3H]GABA) accumulated by slices of rat substantia nigra. SKF 38393 (D1 agonist) and dopamine (dual D1/D2 agonist) were without effect on [3H]GABA efflux by themselves (1-40 microM), or in the presence of the phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) (0.5 mM), but potentiated evoked release in the presence of forskolin (0.5 microM), an adenylate cyclase activator. These increases in release were prevented by the D1 antagonist SCH 23390 (0.5 microM), but not by the D2 antagonist metoclopramide (0.5 microM). Higher concentrations of forskolin (10-40 microM) augmented stimulus-evoked [3H]GABA release directly, whereas dibutyryl cyclic AMP (100-200 microM) depressed it. Apomorphine, noradrenaline, and 5-hydroxytryptamine (1-40 microM) had no effect. The D2 stimulants lisuride, RU 24213, LY 171555, and bromocriptine dose-dependently inhibited depolarisation-induced but not basal [3H]GABA outflow. These inhibitory responses were not modified by the additional presence of SKF 38393 (10 microM) or SCH 23390 (1 microM), or by injection of 6-hydroxydopamine into the medial forebrain bundle 42 days earlier, but were attenuated by metoclopramide (0.5 microM). Higher amounts (10 microM) of SCH 23390, metoclopramide, or other D2 antagonists (loxapine, haloperidol) reduced evoked GABA release by themselves, probably by nonspecific mechanisms. These results suggest D1 and D2 receptors may have opposing effects on nigral GABA output and could explain the variable effects of mixed D1/D2 dopaminomimetics in earlier release and electrophysiological experiments.     

Neurotensin Regulation of Endogenous Acetylcholine Release from Rat Striatal Slices Is Independent of Dopaminergic Tone

The effects of neurotensin (NT) alone or in combination with the dopamine antagonist sulpiride were tested on the release of endogenous acetylcholine (ACh) from striatal slices. NT enhanced potassium (25 mM)-evoked ACh release from striatal slices in a dose-dependent manner. This effect was tetrodotoxin-insensitive, suggesting an action directly on cholinergic elements. The dopamine antagonist sulpiride (5 x 10(-5) M) significantly increased (63%) potassium-evoked ACh release from striatal slices; potassium-evoked ACh release was further increased (90%) in the presence of NT (10(-5) M) and sulpiride (5 x 10(-5) M). The second set of experiments tested the effects of 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra on NT-induced increases of potassium-evoked ACh release. These lesions did not alter the NT regulation of potassium-evoked ACh release from striatal slices, but did significantly increase spontaneous (33%) and potassium-evoked (40%) ACh release from striatal slices. Striatal choline acetyltransferase activity was not affected by 6-OHDA lesions. In addition, following 6-OHDA lesions, sulpiride was ineffective in altering ACh release from striatal slices. Furthermore, evoked ACh release in the presence of the combination of NT and sulpiride was not different from that in the presence of NT alone. These results suggest that in the rat striatum, NT regulates cholinergic interneuron activity by interacting with NT receptors associated with cholinergic elements. Moreover, the NT modulation of cholinergic activity is independent of either an interaction of NT with D2 dopamine receptors or the sustained release of dopamine.     

In vivo Effects of the Anatoxin-a on Striatal Dopamine Release

Anatoxin-a is an important neurotoxin that acts a potent nicotinic acetylcholine receptor agonist. This characteristic makes anatoxin-a an important tool for the study of nicotinic receptors. Anatoxin-a has been used extensively in vitro experiments, however anatoxin-a has never been studied by in vivo microdialysis studies. This study test the effect of anatoxin-a on striatal in vivo dopamine release by microdialysis.The results of this work show that anatoxin-a evoked dopamine release in a concentration-dependent way. Atropine had not any effect on dopamine release evoked by 3.5 mM anatoxin-a. However, perfusion of nicotinic antagonists mecamylamine and α-bungarotoxin induced a total inhibition of the striatal dopamine release. Perfusion of α7*-receptors antagonists, metillycaconitine or α-bungarotoxin, partially inhibits the release of dopamine stimulated by anatoxin-a. These results show that anatoxin-a can be used as an important nicotinic agonist in the study of nicotinic receptor by in vivo microdialysis technique and also support further in vivo evidences that α7*nicotinic AChRs are implicated in the regulation of striatal dopamine release.     

Neurochemical Interactions of Competitive N-Methyl-D-Aspartate Antagonists with Dopaminergic Neurotransmission and the Cerebellar Cyclic GMP System: Functional Evidence for a Phasic Glutamatergic Control of the Nigrostriatal Dopaminergic Pathway

Direct intrastriatal injection of N-methyl-D-aspartate (NMDA; 100 micrograms/rat) increased striatal dopamine (DA) release in vivo. However, parenteral administration of (+/-)-3-(2-carboxypiperizin-4-yl)propyl-1-phosphonic acid (CPP) and cis-4-phosphonomethyl-2-piperidine carboxylic acid (CGS-19755) did not alter DA metabolism and release in several brain regions in the rat and mouse. Intracerebroventricular administration of the competitive NMDA antagonists CPP, CGS-19755, 2-amino-5-phosphonopentanoate, and 2-amino-7-phosphonoheptanoate did not alter rat striatal DA metabolism and release but profoundly reduced cerebellar cyclic GMP (cGMP) levels in the same animals. CPP and CGS-19755 decreased basal cerebellar cGMP levels in the mouse with ED50 values of 6 and 1 mg/kg, i.p., respectively. CPP antagonized the harmaline-induced increases in cGMP levels with an ED50 value of 5.0 mg/kg, i.p. CPP (25 mg/kg, i.p.) also decreased basal cGMP levels in mouse cerebellum for up to 3 h, a result suggesting brain bioavailability and a long duration of NMDA receptor antagonism in vivo. These contrasting patterns suggest that NMDA receptors exert a tonic excitatory tone on the guanine nucleotide signal transduction pathway in the cerebellum while exerting a phasic control over nigrostriatal dopaminergic neurotransmission. These results also indicate that competitive NMDA antagonists, unlike phencyclidine receptor agonists, may not mediate biochemical and behavioral effects via dopaminergic mechanisms.