Rapid desensitization of dopamine release induced by neurotensin and neurotensin fragments

Neurotensin (NT), a tridecapeptide, induced a concentration-dependent release of dopamine (DA) from the striatum. In addition, NT8-13 and Nacetyl-NT8-13, the carboxy-terminal-containing hexapeptides, were much more effective as DA releasers than the amino-terminal, NT1-6 peptide. The potency and efficacy of NT in inducing DA release was markedly enhanced by increases in the extracellular concentration of potassium (K+). Similar to electrical stimulation and to elevated extracellular K+, NT-induced DA release was inhibited by 70% in the presence of 0.13 mM calcium. Desensitization to NT was observed after a first exposure to NT for 2.5 to 10 min, despite a 20- to 85-min washout period between exposures, with NT-free medium. The loss of response was not due to degradation or inactivation of the peptide, nor it was due to activation of DA autoreceptors or the DA transporter. NT-induced desensitization was not associated to a loss of responsiveness to DA release elicited by electrical stimulation or by high K+. In addition, desensitization occurred even if NT-induced DA release was markedly enhanced by high extracellular K+ (10 and 15 mM). Inhibition of NT-induced DA release by low calcium (on the first exposure) did not prevent the development of desensitization. Similar to the parent peptide, desensitization was observed with the active carboxy-terminal NT fragments. However, a first exposure to NT1-6 did not induce desensitization to NT8-13. These results are compatible with the view that NT-induced DA release and the development of desensitization are mediated through an action of NT on NT receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

A further evaluation of the effects of K+ depolarization on glutamate-evoked [3H]dopamine release from striatal slices.

Exogenous glutamate will evoke dopamine (DA) release from striatal slices in vitro. To further characterize glutamate-evoked DA release from striatal slices, experiments were designed to: 1) determine if sufficient endogenous glutamate can be released in vitro to presynaptically mediate [3H]DA release in the absence of Mg++ and 2) reevaluate how K+ depolarization affects glutamate-evoked [3H]DA release. Removal of Mg++ to potentiate N-methyl-D-aspartate (NMDA) receptor-mediated DA release increased 15 mM K(+)-evoked [3H]DA release to about 200% of control. The potentiation of this release was probably not mediated by NMDA receptors because it was not blocked by the glutamate receptor antagonists MK-801, 6,7-dinitroquinoxalinedione (DNQX) or kynurenate. Furthermore, the removal of Mg++ increased DA release substantially (200%) in the presence of 5 microM sulpiride and 10 microM nomifensine, indicating that DA reuptake and DA D2 autoreceptors are not primarily responsible for increased DA release. In the absence of Mg++, depolarization produced by 20 mM or greater [K+] inhibited DA released by exogenous glutamate, whereas a much higher [K+] was necessary to evoke endogenous glutamate release. In the presence of 1.5 mM Mg++, a reduction of the "Mg++ blockade" of NMDA receptors by 15 mM K+ depolarization during glutamate-evoked DA release was evaluated with and without the DA reuptake inhibitor nomifensine and the DA D2 antagonist sulpiride. DA released by K+ depolarization (Mg++ present) was markedly increased by 1 mM glutamate, but this effect was only partially reversed by kynurenate or high concentrations of either MK-801 (25 microM) or DNQX (100 microM).(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in sensitivity of release modulating dopamine autoreceptors after chronic treatment with haloperidol

The release of recently taken up [3H]dopamine ([3H]DA) elicited by electrical stimulation (3 Hz, 2 min, 16 mA) from slices of the rabbit caudate nucleus is inhibited by apomorphine (0.01-0.1 microM) in a concentration-dependent manner. This action is mediated through the activation of presynaptic inhibitory DA autoreceptors. The inhibition of [3H]DA release by apomorphine (0.1 microM) was antagonized 2 hr, but not 24 hr after the single administration of haloperidol (1 mg/kg s.c.). After 2 days of withdrawal after 28 days of chronic treatment with haloperidol (1 mg/kg s.c.) once daily, apomorphine (0.01-0.1 microM) was more effective in inhibiting [3H]DA release elicited by electrical stimulation when compared with rabbits injected chronically with either the vehicle for haloperidol or with saline. In superfused slices of the rabbit caudate nucleus, exposure to S-sulpiride (0.1 and 1 microM) increased in a concentration-dependent manner the release of [3H] DA elicited by electrical stimulation. After 28 days of chronic treatment with haloperidol, the facilitation of [3H]DA release by S-sulpiride was significantly reduced when compared with the controls. The inhibition of central noradrenergic transmission by DA receptor agonists was studied in hypothalamic slices prelabeled with [3H]norepinephrine ([3H-NE]). Apomorphine (0.01-1 microM) inhibited the electrically evoked (5 Hz, 2 min, 26 mA) release of [3H]NE from hypothalamic slices of untreated rabbits. The sensitivity to the inhibitory effect of apomorphine on [3H]NE overflow remained unaffected after 2 days of withdrawal following 28 days of chronic treatment with haloperidol. In summary, our results indicate that chronic haloperidol administration induces changes in sensitivity of the DA autoreceptors regulating dopaminergic neurotransmission but does not affect the sensitivity of DA receptors modulating NE release in the central nervous system. These results suggest that the DA autoreceptors that regulate dopaminergic neurotransmission may play a physiological role in the modulation of transmitter release and consequently are susceptible to the development of changes in sensitivity after chronic receptor blockade. The possible implication of changes in sensitivity of the DA autoreceptor during the treatment of schizophrenia with neuroleptics is discussed.     

Effects of acute and subchronic administration of typical and atypical antipsychotic drugs on the neurotensin system of the rat brain

The acute and subchronic effects of a variety of doses of a prototype typical (haloperidol) or one of several atypical antipsychotic drugs (clozapine, olanzapine, risperidone, quetiapine, or sertindole) on regional brain neurotensin (NT) tissue concentrations, and NT receptor binding were examined. Acute administration of haloperidol, clozapine, olanzapine, and risperidone dose-dependently increased NT tissue concentrations in the nucleus accumbens. Haloperidol, olanzapine, risperidone, and sertindole also increased NT tissue concentrations in the caudate nucleus. NT tissue concentrations in the nucleus accumbens and caudate remained elevated after 14-day administration of haloperidol, olanzapine, sertindole, and risperidone. In contrast, at the doses studied, quetiapine decreased NT tissue concentrations in the nucleus accumbens; clozapine had no effect. Haloperidol significantly increased NT receptor binding in the substantia nigra after 14-day administration. All of the atypical antipsychotic drugs decreased NT receptor binding in the nucleus accumbens and in the substantia nigra. Although these studies do not conclusively support the hypothesis that increased NT neurotransmission is involved in the clinically relevant effects of all antipsychotic drugs, the extant evidence clearly suggests that further study is warranted. Inconsistencies in the data suggest that differential effects of antipsychotic drug administration on subpopulations of NT neurons must be scrutinized further.     

Neurotensin modulates differently potassium, veratridine and 4-aminopyridine-evoked release of dopamine in rat striatal slices

Summary— We have studied the effects of neurotensin (NT) on the release of [³H]dopamine ([³H]DA) evoked by terminal depolarization with either K+, veratridine or 4-aminopyridine (4-AP). NT (1–1000 nM) induced a net potentiation (up to 170%) of the K+ (25 mM) -evoked release of [³H]DA. The capacity of NT to potentiate the effect of K+ ions decreased as the K+ concentration rose from 25 to 50 mM and totally disappeared at this high K+ concentration. NT (100 nM; 1000 nM) had no significant effect on the veratridine (1.5; 5μM) or 4-AP (20 μM) -evoked release of [³H]DA. The relevance of these experimental models of DA release to physiological transmitter release remains to be established. Those data highlight the complexity of the modulation of evoked neurotransmitter release by pharmacological agents.     

Kappa receptor regulation of dopamine release from striatum and cortex of rats and guinea pigs

The effects of opioid agonists with selectivity for kappa, mu and delta types of opioid receptors on the K+-stimulated release of [3H]dopamine (DA) from striatum and cortex of rat and guinea pig loaded previously with the monoamine have been studied. The kappa agonist U50488H did not affect base-line release of [3H]DA measured in 5 mM K+, but produced a dose-dependent inhibition of the release of [3H]DA stimulated by 20 mM K+ from slices of striatum in rat and guinea pig, with an IC50 of about 0.5 nM in each case. In contrast, the mu-selective agonist, Tyr-D-Ala-Gly-(Me)Phe-Gly-ol, and the delta-selective agonist, [D-Pen2-D-Pen5]enkephalin, did not inhibit stimulated release from the slice preparations at concentrations up to 1 microM. The inhibitory effects of U50488H were antagonized by naloxone, and the potent and selective kappa antagonist, nor-binaltorphimine (nor-BNI). Similar results were obtained when release of [3H]DA from rat and guinea pig cortex slices was examined. In guinea pig cortex, the maximum inhibition of DA release induced by U50488H was 80% of control-stimulated fractional release. In rat cortex and in striatum of both species the maximum release was about 40% of control fractional release. Thus, in the guinea pig, the mesocortical dopaminergic pathway appears more sensitive to the inhibitory effects of U50488H than the nigrostriatal system. The effects of the opioids on the K+ (12.5 mM)-stimulated release of [3H]DA from guinea pig striatal synaptosomes also were determined.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibitory effects of amphetamine on potassium-stimulated release of [3H]dopamine from striatal slices and synaptosomes

Amphetamine, 10(-7) M or greater, evoked the release of [3H]dopamine ([3H]DA) and inhibited subsequent K+-evoked [3H]DA release from striatal synaptosomes superfused at a flow rate (1 ml/min) that prevented reuptake. Amphetamine inhibited the K+-evoked release of [3H]DA to a lesser extent in striatal slices or in synaptosomes superfused at a flow rate (0.35 ml/min) that allowed reuptake. The observed decrease in amphetamine inhibition of K+-evoked release was primarily due to amphetamine blocking [3H]DA reuptake. Interneuronal interactions may account for some of the inhibitory effects of amphetamine on K+-evoked release in the slice. Inhibition of K+-evoked release from either slices or synaptosomes was still evident when 10(-6) M amphetamine was removed from the superfusion buffer and the spontaneous release had returned to control levels. The presence of Ca++ during amphetamine exposure was required for subsequent inhibition of K+-evoked release in synaptosomes. Amphetamine in the presence of Ca++ did not affect the subsequent release of [3H]DA evoked by the Ca++ ionophore, A23187. Therefore, amphetamine inhibition of the K+-evoked release of [3H]DA cannot be explained by prior depletion of Ca++-releasable pools. Nifedipine, 1 microM, failed to block either the Ca++-dependent release of [3H]DA or the inhibition of K+-evoked release by amphetamine. However, 1 mM cobalt inhibited the Ca++-dependent release of [3H]DA by amphetamine and antagonized the inhibition of K+-evoked release after amphetamine exposure. This suggests that amphetamine may open voltage-dependent Ca++ channels sensitive to cobalt but not nifedipine. Amphetamine may desensitize these voltage-dependent Ca++ channels and inhibit their activation by K+ depolarization.     

Dopamine modulates the function of group II and group III metabotropic glutamate receptors in the substantia nigra pars reticulata

Recent findings have shown that dendritically released dopamine (DA) plays an important modulatory role in the substantia nigra pars reticulata (SNr). It is therefore possible that the loss of DA observed in Parkinson's disease (PD) could hold important consequences for nigral function. Previously, we have shown that activation of presynaptically localized group II metabotropic glutamate receptors (mGluRs) inhibits excitatory transmission at the subthalamic nucleus (STN)-SNr synapse and that activation of presynaptically localized group III mGluRs decreases excitatory and inhibitory transmission in the SNr. To test the hypothesis that nigral DA can modulate mGluR function in the SNr, we performed whole-cell patch-clamp recordings from gamma-aminobutyric acidergic SNr neurons in slices obtained from rats that were acutely reserpinized. In slices obtained from reserpinized animals, the effect of group II mGluR activation by the selective agonist (+)-2-aminobicyclo[3.1.0]-hexane-2,6-dicarboxylate monohydrate (LY354740) (100 nM), but not group III mGluR activation [L-(+)-2-amino-4-phosphonobutyric acid, L-AP4, 500 microM], at STN-SNr synapses is significantly decreased. This effect could be mimicked in control slices by prior bath application of haloperidol (20 microM) and R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH23390) (20 microM) but not sulpiride (50 microM). Furthermore, application of dopamine (100 microM) and (+/-)-6-chloro-7,8-dyhydroxy-3allyl-1-phenyl-2,3,4,5-tetra-hydro-1H-benzazepine (SKF82958) (1 microM) but not quinpirole (10 microM) could rescue the group II mGluR effect in reserpinized slices. The effect of group III mGluR activation (L-AP4, 100 microM) on inhibitory synaptic transmission was also significantly reduced in slices from reserpine-treated animals. This effect was mimicked by haloperidol (20 microM), SCH23390 (20 microM), and sulpiride (50 microM) in control slices. Thus, in a Parkinsonian state, the loss of nigral DA may add to the overall pathophysiological changes in basal ganglia output.     

Enkephalinase is involved in the degradation of endogenous substance P released from slices of rat substantia nigra

The effects of various peptidase inhibitors were examined upon the K+-evoked overflow of substance-like immunoreactive material (SPLI) from slices of rat substantia nigra in order to assess the possible involvement of "enkephalinase," angiotensin-converting enzyme (ACE) and calpain in the enzymatic inactivation of endogenous substance P in brain tissues. The calpain inhibitor leupeptin and the enkephalinase inhibitors thiorphan and phosphoramidon increased markedly SPLI overflow, whereas the two ACE inhibitors, captopril and enalaprilat (up to 10 microM in the superfusing medium), were inactive. Surprisingly kelatorphan, which inhibits not only enkephalinase but also aminopeptidase and dipeptidylaminopeptidase activities, was less potent than thiorphan or phosphoramidon to enhance SPLI overflow. However, in the presence of ICI-154129 or naloxone to block opiate receptors, kelatorphan was as potent as thiorphan, therefore suggesting some negative influence of endogenous opioids on SPLI release with kelatorphan but not thiorphan. In agreement with this interpretation, the direct stimulation of delta opiate receptors by deltakephalin was found to significantly reduce SPLI overflow. Furthermore, an increased outflow of [Met]enkephalin-like material was observed from substantia nigra slices superfused with kelatorphan but not thiorphan. These results indicate that endogenous substance P released within the substantia nigra is very probably inactivated by enkephalinase and calpain, but not ACE. They also demonstrate that endogenous opioids can exert a negative control upon substance P release in this brain region.     

Ca2+ channel blockers enhance neurotensin (NT) binding and inhibit NT-induced inositol phosphate formation in prostate cancer PC3 cells

Neurotensin (NT) stimulates Ca2+ release and Ca2+ influx in many cells. Its contractile effects in smooth muscle are inhibited by removal of Ca2+ and by Ca2+ channel blockers (CCBs). To better understand NT signaling in prostate cancer PC3 cells, blockers of voltage-gated and store-operated Ca2+ channels (VGCC and SOCC) were tested for effects on NT-binding and signaling. Eight chemical types of agents, including VGCC-blocker nifedipine and SOCC-blocker SKF-96365 (1-[beta-[3-(4-methoxyphenyl)-propoxy]-4-methoxyphenyl]-1H-imidazole), enhanced cellular NT binding up to 3-fold, while inhibiting (by congruent with 70%) NT-induced inositol phosphate (IP) formation. The ability to enhance NT binding correlated with the ability to inhibit NT-induced IP formation, and both effects were relatively specific for NT. Although cellular binding for beta2-adrenergic, V1a-vasopressin, and epidermal growth factor receptors was not enhanced by these drugs, bombesin receptor binding was increased approximately equal to 19% and bombesin-induced IP formation was inhibited approximately equal to 15%. One difference was that the effect on NT binding was Ca2+-independent, whereas the effect on IP formation was Ca2+-dependent (in part). The Ca2+-dependent part of the IP response seemed to involve SOCC-mediated Ca2+ influx to activate phospholipase C (PLC)delta, while the Ca2+-independent part probably involved PLCbeta. Photoaffinity labeling of the NT receptor NTR1 was enhanced in CCB-treated cells. NTR1 affinity was increased but NTR1 number and internalization were unchanged. Since CCBs did not alter NT binding to isolated cell membranes, the effects in live cells were indirect. These results suggest that CCBs exert two effects: 1) they inhibit NT-induced IP formation, perhaps by preventing Ca2+ influx-dependent activation of PLCdelta; and 2) they enhance NTR1 affinity by an unexplained Ca2+-independent mechanism.     

阿尔茨海默病脑内深部核团磁敏感相位值与MMSE的相关性研究

目的利用磁敏感加权成像(susceptibility-weighted imaging,SWI)评价阿尔茨海默病(Alzheimer's disease,AD)患者深部脑核团铁沉积,并分析其与简易智力状况评分(mini-mental state examination,MMSE)的相关性。材料与方法自2015年11月至2016年12月期间,募集AD患者17例,年龄47～82岁,同期就诊的21名认知正常者作为对照,年龄48～78岁。扫描设备选用Philip Ingenia3.0 T全数字磁共振扫描仪,头部8通道相控阵线圈,受试者均行头磁共振常规扫描及磁敏感加权成像序列扫描,测量双侧尾状核头、丘脑、壳核、苍白球、黑质及红核的相位值。采用SPSS 19.0和Med Cal方法比较病例组与对照组相位值的差异,并对相位值与简易智力状况评分进行Spearman相关分析,对相关系数r值进行t检验(P0.05)。结果病例组与对照组双侧尾状核头、右侧黑质相位值差异有统计学意义(P0.01);双侧苍白球、双侧壳核、双侧丘脑、双侧红核及左侧黑质相位值差异无统计学意义(P0.05)。病例组中双侧黑质、双侧尾状核相位值与其MMSE评分呈正相关。结论磁敏感相位值能半定量评估脑内铁沉积,对AD诊断有一定价值。     

Ethanol excitation of dopaminergic ventral tegmental area neurons is blocked by quinidine

The dopaminergic (DA) neurons in the ventral tegmental area (VTA) are important for the reinforcing effects of ethanol. We have shown that ethanol directly excites DA VTA neurons and reduces the afterhyperpolarization (AHP) that follows spontaneous action potentials in these neurons. These data suggested that ethanol may be increasing the firing rate of DA VTA neurons by modulating currents that contribute to the AHP, either by reducing a K+ current or by increasing the inward current Ih. In the present study, different blockers of K+ channels and Ih were tested to determine whether any could prevent the ethanol excitation of DA VTA neurons. Extracellular single-unit recordings and whole-cell patch-clamp recordings were made from DA VTA neurons in brain slices from Fischer-344 rats and ethanol (40-120 mM) and channel blockers were applied in the bath. Ethanol excitation was not reduced by blockade of Ih with cesium (5 mM) or ZD7288 (30 microM), or by block of G-protein-coupled inwardly rectifying K+ channels with barium (500 microM). Tetraethylammonium (TEA) ion (2-10 mM), which blocks the large conductance calcium-dependent potassium K+ current and some types of delayed rectifier currents, had no effect on the ethanol-induced excitation. Interestingly, ethanol excitation of DA VTA neurons was blocked by quinidine (20-80 microM), a drug that blocks many types of delayed rectifier K+ channels, including some insensitive to TEA. This effect of quinidine was concentration-dependent and reversible. These results suggest that ethanol excites DA VTA neurons by reducing a quinidine-sensitive K+ current.     

Membrane events and ionic processes involved in dopamine release from tuberoinfundibular neurons. I. Effect of the inhibition of the Na+,K+-adenosine triphosphatase pump by ouabain

In the present study we investigated the membrane events and the ionic processes which mediate the stimulatory effect of ouabain on the release of endogenous dopamine (DA) and "previously taken-up" [3H]DA release from rat hypothalamic tuberoinfundibular dopaminergic (TIDA) neurons. Ouabain (0.1-1 mM) dose-dependently stimulated endogenous DA and "newly taken-up" [3H]DA release. This effect was counteracted partially by nomifensine (10 microM). Removal of Ca++ ions from the extracellular space in the presence of the Ca++-chelator ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid prevented completely ouabain-elicited [3H]DA release. Lanthanum (1 mM) and cobalt (2 mM), two inorganic Ca++-entry blockers, were able to inhibit this stimulatory effect, whereas verapamil (10 microM) and nitrendipine (50 microM), two organic antagonists of the voltage-operated channel for Ca++ ions, failed to affect ouabain-induced [3H]DA release. By contrast, adriamycin (100-300 microM), a putative inhibitor of cardiac Na+-Ca++ antiporter, dose-dependently prevented ouabain-induced [3H]DA release from TIDA neurons. Finally, tetrodotoxin reduced digitalis-stimulated [3H]DA release. In conclusion, these results seem to be compatible with the idea that the inhibition of Na+,K+-adenosine triphosphatase by ouabain stimulates the release of [3H]DA from a central neuronal system like the TIDA tract and that this effect is critically dependent on the entrance of Ca++ ions into the nerve terminals of these neurons. In addition the Na+-Ca++ exchange antiporter appears to be the membrane system which transports Ca++ ions into the neuronal cytoplasm during Na+,K+-adenosine triphosphatase inhibition. The enhanced intracellular Ca++ availability triggers DA release which could occur partially through a carrier-dependent process.     

Modulation of the Ca++-evoked release of [3H]dopamine from striatal synaptosomes by dopamine (D2) agonists and antagonists

Release of [3H]dopamine ([3H]DA) from striatal synaptosomes is evoked most commonly by elevating potassium levels in the presence of calcium. However, it has been difficult to show that DA agonists or antagonists can modify K+-evoked release of [3H]DA. DA. In this study [3H]DA release evoked by exposure of synaptosomes (isolated and superfused previously with 0.0 mM Ca++ and 0.1 mM ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid) to 1.25 mM Ca++ can be modulated by the DA (D2) agonists apomorphine, pergolide and quinpirole and antagonists l-sulpiride and domperidone. The release was evoked under low potassium (6 mM or less) concentrations and the potassium concentration in the superfusion medium was not elevated before or during Ca++ exposure. Analysis of the superfusates obtained during Ca++ exposure revealed that approximately 80% of the tritium released was [3H]DA. The ability of DA (D2) agonists to inhibit the Ca++-evoked release from synaptosomes superfused with 9 mM K+ was greatly reduced. Therefore, prolonged depolarization may block DA (D2) regulation of [3H]DA release from synaptosomes. The Ca++-evoked release of [3H]DA was reduced greatly when 1 microM tetrodotoxin was present indicating sodium channels play a role in triggering the processes involved in Ca++-evoked [3H]DA release.     

Dopamine efflux from striatal slices after intracerebral 6-hydroxydopamine: evidence for compensatory hyperactivity of residual terminals

In this study, we have examined the spontaneous efflux and stimulation-induced overflow of endogenous dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) from striatal slices prepared from adult rats. Partial destruction of striatal DA terminals with 6-hydroxydopamine reduced the electrically evoked overflow of DA and DOPAC, but by less than the DA content of the tissue slices. This resulted in an increase in the fractional overflow of DA and DOPAC, a measure of overflow from residual dopaminergic terminals. The increase in fractional DA overflow was related to the lesion size, reaching 7-fold with DA depletions of greater than 90%. Inhibition of DA re-uptake with nomifensine (10 microM) increased DA overflow and reduced DOPAC overflow by an equivalent amount, indicating that a portion of the DA released by electrical stimulation is recaptured and metabolized before appearing as DOPAC in the superfusate. However, 6-hydroxydopamine lesions further elevated fractional DA overflow above control even in the presence of nomifensine, suggesting that slices prepared from lesioned animals exhibit an increase in DA release from residual dopaminergic terminals. DA overflow was enhanced by the DA receptor antagonist sulpiride (1 microM) in control tissue but not in slices prepared from lesioned animals, suggesting that increased efflux per terminal was accompanied by reduced autoinhibition of release. Over a range of firing frequencies typical of nigrostriatal neurons in vivo (2-8 Hz), fractional DA overflow per pulse from lesioned slices was 3-fold higher than control overflow; however, fractional overflow per pulse was reduced from lesioned but not control slices when slices were exposed to a higher frequency (12 Hz). Thus, the lesion appeared to have increased DA release at moderate frequencies, but had reduced the effective range of frequencies over which the DA terminals could operate. Finally, 3-iodotyrosine (2 mM), an inhibitor of tyrosine hydroxylase, reduced DA overflow from intact slices, but completely abolished overflow from lesioned slices, suggesting that 6-hydroxydopamine had increased the dependence of DA efflux on a sustained rate of DA synthesis. Taken together, these data suggest that after lesioning with 6-hydroxydopamine, DA released per pulse from residual terminals is increased relative to control, so long as the stimulation frequency is within the physiological range. This increase in release may serve a compensatory function, maintaining dopaminergic control over striatal function despite extensive loss of DA neurons.     

Differences in pharmacological properties of dopamine release between the substantia nigra and striatum: an in vivo electrochemical study

The properties of dopamine (DA) release in the rat substantia nigra (SN) and striatum were investigated using high-speed chronoamperometric recordings in brain slices. In both brain regions, a 2-min bath superfusion with 30 mM KCl produced robust DA-like electrochemical signals, with the mean amplitude of the signal being >10-fold greater in the striatum than the SN. The reproducibility of the response was confirmed by a second stimulus (S2)/first-stimulus (S1) ratio of >0.8 in both regions. The bath application of tetrodotoxin significantly reduced the S2/S1 ratio in both the striatum and SN, implicating the requirement for voltage-sensitive sodium channels in the DA-release process. However, the application of cadmium chloride, a nonselective blocker of voltage-sensitive calcium channels, reduced the S2/S1 ratio only in the striatum and not within the SN. Moreover, removal of Ca2+ from the buffer did not significantly affect release within the SN, despite a >85% reduction in release within the striatum. In addition, although the D2 receptor antagonist sulpiride enhanced the S2/S1 ratio in the striatum, no effect of this agent was seen in the SN. Finally, the application of d-amphetamine produced DA-like electrochemical signals in both the striatum and SN. However, the amplitude of the d-amphetamine-evoked response, relative to the KCl-evoked release, was much smaller in the striatum than in the SN. Taken together, these data support the hypothesis that differences in the mechanism or mechanisms of release exist between somatodendritic and axonal elements within the nigrostriatal pathway.     

D-1 and D-2 dopamine receptor blockade: interactive effects in vitro and in vivo

Haloperidol, at low concentrations that block D-2 dopamine (DA) receptors but not D-1 DA receptors (less than 10 microM), potentiated the enhancement of adenylate cyclase activity produced by the D-1 agonist SKF 38393. Low concentrations of haloperidol (less than or equal to 5 microM) also potentiated the K+-evoked release of [3H]acetylcholine from superfused striatal tissue slices. Both of these effects of haloperidol were blocked by nanomolar concentrations of SCH 23390, a D-1 receptor antagonist. In addition, SCH 23390 reduced the ability of haloperidol to antagonize the inhibition of [3H]acetylcholine release produced by the DA agonist apomorphine. By itself, SCH 23390 did not alter either basal adenylate cyclase activity or the K+-evoked release of [3H]acetylcholine. These findings suggest that SCH 23390 can attenuate in vitro responses to D-2 receptor blockade. Likewise, in vivo, very low doses (less than 1 microgram/kg) of SCH 23390 reduced the ability of haloperidol to elevate striatal DA metabolite concentrations and plasma prolactin concentrations. Thus, D-1 receptor blockade may attenuate the effects of D-2 DA receptor blockade both in vitro and in vivo.     

Met-enkephalin release from slices of the rat striatum and globus pallidus: stimulation by excitatory amino acids.

The present study evaluated the effects of high K+ and four excitatory amino acids (EAAs) on the release of met-enkephalin-like immunoreactivity (ME-i.r.) from slices of the rat striatum and globus pallidus. High K+ (15-50 mM) increased the release of ME-i.r. in a concentration-dependent manner in both regions, the release response in the globus pallidus being consistently greater than in the striatum. This release was highly Ca(++)-dependent and was significantly enhanced in the absence of external Mg++. D-2-Amino-7-phosphonoheptanoic acid (0.5 mM), a competitive N-methyl-D-aspartate (NMDA) receptor antagonist, did not alter this enhanced action of K+, suggesting that the activation of NMDA receptors by an endogenous agonist did not contribute to the enhancement. Exposure of pallidal or striatal slices to four EAA receptor agonists, NMDA, L-glutamate, kainate (KA) and quisqualate, increased the release of ME-i.r. above the base line, an effect that was Ca(++)-dependent. Both L-glutamate and NMDA, at concentrations of 1 and 5 mM, produced a graded increase in the ME-i.r. release, but a higher concentration (10 mM) produced a lower release. In both regions the NMDA (5 mM)-evoked release was effectively inhibited by Mg++ (1.2 mM), 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) (5 microM), a competitive NMDA receptor antagonist and thienylcyclohexylpiperidine (10 microM), a noncompetitive NMDA receptor antagonist. Tetrodotoxin (0.3 microM), a Na+ channel blocker, did not affect the NMDA-evoked release of ME-i.r. in the striatum, but decreased it by 52% in the globus pallidus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Modulation of neurotransmitter release in the basal ganglia of the rat brain by dynorphin peptides.

Microinjection studies have found that although dynorphin peptides decrease dopamine release in the rat basal ganglia, the nonselective opiate antagonist naloxone produces the opposite effect. To investigate the contribution of the dynorphin pathways to a tonic modulation of dopamine release, a microdialysis study was undertaken, with probes implanted in the substantia nigra and the ipsilateral neostriatum. Perfusion of the substantia nigra with the nonselective antagonist naltrexone (NTX; 1-10 microM), the selective kappa-opoid receptor antagonist, nor-binaltorphimine (nor-BNI; 1-10 microM), and the selective mu-opioid receptor antagonist, D-Pen-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP; 1-10 microM) produced an increase in dopamine release, both in substantia nigra and neostriatum. nor-BNI also produced an increase in dynorphin B release, and a similar effect was observed with the higher concentration of NTX (10 microM). At the higher concentration of NTX and CTOP, an increase in glutamate release was also observed. Perfusion of the neostriatum with NTX, nor-BNI, or CTOP increased striatal dopamine, and dynorphin B release and increased dynorphin B in the ipsilateral substantia nigra. NTX and CTOP, but not nor-BNI, increased striatal glutamate and aspartate release. The kappa-opioid agonist U-50,488H (10 microM) induced a decrease in dopamine levels, both in the substantia nigra and neostriatum, and a paradoxical increase in striatal aspartate levels. Finally, systemic administration of NTX (4 mg/kg s.c.) in awake animals significantly increased striatal dopamine levels. The results suggest that opioid peptides, either dynorphins acting on kappa-opioid receptors or enkephalins acting on mu-opioid receptors, exert tonic inhibition on dopamine and dynorphin B release in both substantia nigra and neostriatum.     

Effects of d-amphetamine and dopamine synthesis inhibitors on dopamine and acetylcholine neurotransmission in the striatum. II. Release in the presence of vesicular transmitter stores

The release of endogenous dopamine (DA) elicited by electrical stimulation and by d-amphetamine (AMPH) from superfused striatal slices of untreated rabbits was examined. AMPH (0.3-10 microM) produced a concentration-dependent increase in basal DA efflux (30-fold increase at 10 microM) and stimulation-evoked (SE) DA overflow (11-fold increase at 10 microM). Although AMPH had little effect on the basal efflux of dihydroxyphenylacetic acid (DOPAC), the drug was an effective inhibitor of the SE overflow of the DA metabolite (66% inhibition at 0.3 microM). AMPH increased significantly the total basal efflux of endogenous compounds (DA + DOPAC) only at high concentrations (3-10 microM) whereas the total SE overflow of total endogenous compounds was decreased at all concentrations of AMPH tested. AMPH inhibited SE [3H]acetylcholine (ACh) release in a concentration-dependent manner (71% inhibition at 10 microM). Inhibition of DA synthesis with alpha-methyl-p-tyrosine (100 microM) or 3-iodotyrosine (100 microM) reduced both the basal efflux and SE overflow of endogenous DA and DOPAC; synthesis inhibition had greater effects on the SE overflow. Neither synthesis inhibitor altered SE [3H]ACh release. alpha-Methyl-p-tyrosine and 3-iodotyrosine reduced the absolute values of the basal efflux and SE overflow of DA elicited by AMPH by approximately 60%; however, the inhibition of SE [3H]ACh release produced by AMPH was attenuated only slightly (approximately 20%). Synthesis inhibitors also reduced tissue DA levels (approximately 30%). These results suggest that: basal efflux of endogenous DA from superfused rabbit striatal slices may derived both from DA newly synthesized in the cytoplasm and from spontaneous leakage of DA from storage vesicles. In addition, synthesis may provide a continuous supply of DA to vesicles that are used for exocytotic DA release during electrical stimulation. However, the depletion of tissue DA produced by synthesis inhibitors as well as other extraneous pharmacological actions of these drugs makes firm conclusions difficult. AMPH increases the synaptic concentration of DA by accelerating the basal efflux as well as the SE overflow of unchanged DA. At concentrations less than 1 microM AMPH has no effect on basal efflux of DA or DOPAC but reduces SE overflow of DOPAC via an unknown mechanism. At higher concentrations (greater than or equal to 1 microM) acceleration of carrier-mediated DA efflux coupled with displacement of DA from vesicular stores, as well as interference with the uptake of exocytotically released DA produces a marked increase in synaptic DA which in turn inhibits SE ACh release.(ABSTRACT TRUNCATED AT 400 WORDS)