The role of presynaptic receptors in the release and synthesis of 3H-dopamine by slices of rat striatum

Enhanced dopamine D2 autoreceptor function in the adult prefrontal cortex contributes to dopamine hypoactivity following adolescent social stress

Adult psychiatric disorders characterized by cognitive deficits reliant on prefrontal cortex (PFC) dopamine are promoted by teenage bullying. Similarly, male Sprague-Dawley rats exposed to social defeat in mid-adolescence (P35-39) show impaired working memory in adulthood (P56-70), along with decreased medial PFC (mPFC) dopamine activity that results in part from increased dopamine transporter-mediated clearance. Here, we determined if dopamine synthesis and D2 autoreceptor-mediated inhibition of dopamine release in the adult mPFC are also enhanced by adolescent defeat to contribute to later dopamine hypofunction. Control and previously defeated rats did not differ in either DOPA accumulation following amino acid decarboxylase inhibition (NSD-1015 100 mg/kg ip.) or total/phosphorylated tyrosine hydroxylase protein expression, suggesting dopamine synthesis in the adult mPFC is not altered by adolescent defeat. However, exposure to adolescent defeat caused greater decreases in extracellular dopamine release (measured using in vivo chronoamperometry) in the adult mPFC upon local infusion of the D2 receptor agonist quinpirole (3 nM), implying greater D2 autoreceptor function. Equally enhanced D2 autoreceptor-mediated inhibition of dopamine release is seen in the adolescent (P40 or P49) mPFC, which declines in control rats by adulthood. However, this developmental decrease in autoreceptor function is absent following adolescent defeat, suggesting retention of an adolescent-like phenotype into adulthood. Current and previous findings indicate adolescent defeat decreases extracellular dopamine availability in the adult mPFC via both enhanced inhibition of dopamine release and increased dopamine clearance, which may be viable targets for improving treatment of cognitive deficits seen in neuropsychiatric disorders promoted by adolescent stress.

Alpha7 nicotinic receptors as therapeutic targets for Parkinson's disease

Accumulating evidence suggests that CNS α7 nicotinic acetylcholine receptors (nAChRs) are important targets for the development of therapeutic approaches for Parkinson's disease. This progressive neurodegenerative disorder is characterized by debilitating motor deficits, as well as autonomic problems, cognitive declines, changes in affect and sleep disturbances. Currently l-dopa is the gold standard treatment for Parkinson's disease motor problems, particularly in the early disease stages. However, it does not improve the other symptoms, nor does it reduce the inevitable disease progression. Novel therapeutic strategies for Parkinson's disease are therefore critical. Extensive pre-clinical work using a wide variety of experimental models shows that nicotine and nAChR agonists protect against damage to nigrostriatal and other neuronal cells. This observation suggests that nicotine and/or nAChR agonists may be useful as disease modifying agents. Additionally, studies in several parkinsonian animal models including nonhuman primates show that nicotine reduces l-dopa-induced dyskinesias, a side effect of l-dopa therapy that may be as incapacitating as Parkinson's disease itself. Work with subtype selective nAChR agonists indicate that α7 nAChRs are involved in mediating both the neuroprotective and antidyskinetic effects, thus offering a targeted strategy with optimal beneficial effects and minimal adverse responses. Here, we review studies demonstrating a role for α7 nAChRs in protection against neurodegenerative effects and for the reduction of l-dopa-induced dyskinesias. Altogether, this work suggests that α7 nAChRs may be useful targets for reducing Parkinson's disease progression and for the management of the dyskinesias that arise with l-dopa therapy.

Potassium markedly potentiates the effect of veratridine on dopamine release from rat superfused striatal ribbons

The effects of veratridine-induced depolarization on [3H]dopamine ([3H]DA) release in the presence of a physiological (5 Mm) or a depolarizing (25 Mm) concentration of K+ were studied in-vitro in rat superfused striatal ribbons. A combination of the two depolarizing agents induced a marked potentiation in the overflow of [3H]DA, giving an overall 3- to 5-fold increase in veratridine activity. This potentiation was completely antagonized by tetrodotoxin (100 Nm). These studies indicated that K+-induced depolarization can increase the potency of veratridine in releasing dopamine from terminals.

The pharmacokinetics and pharmacological effect of (S)-5-OH-DPAT following controlled delivery with transdermal iontophoresis

The pharmacokinetic (PK) and pharmacodynamic (PD) properties of the active (S)-enantiomer of the potent dopamine (DA) agonist 5-hydroxy-2-(N,N,-di-n-propylamino)tetralin (5-OH-DPAT) were investigated in a novel anesthetized animal model. First, the relationship between current density, in vivo transport, and plasma profile was characterized. Second, the effect of the anesthetic mixture, transdermal iontophoresis, and blood sampling on the striatal DA release (PD end point) was investigated. Third, the PK-PD relationship following transdermal iontophoresis was investigated during a controlled reversible pharmacological response. Given that striatal DA levels are unaltered during experimental procedures, this rat model can be used to investigate the PK-PD relationship. The in vivo flux was linearly correlated with the current density, indicating that drug delivery can be titrated by the current density. Following transdermal iontophoresis and intravenous infusion, a strong reversible effect was observed. Compartmental modeling showed that the relationship between plasma concentration and biomarker response is best characterized by an effect compartment, rather than an indirect response model. In addition, covariate analysis suggested that the delivery rate can affect the PD efficiency. Finally, PK-PD analysis revealed that steady delivery rates are translated into continuous dopaminergic stimulation. This can be of benefit for reducing side effects in the symptomatic treatment of Parkinson's disease with 5-OH-DPAT.

Direct inhibition of levodopa-induced beginning-of-dose motor deterioration by subthalamic nucleus stimulation in a patient with Parkinson disease

✓Beginning-of-dose motor deterioration (BDMD) is a complication of levodopa medications in Parkinson disease (PD) that is presumably caused by inhibitory effects of levodopa. Only limited experience of BDMD has been described in the literature. The authors report the case of a patient with PD who demonstrated a marked BDMD while being treated with standard levodopa medications. This 55-year-old woman had a 12-year history of PD and a 10-year history of levodopa treatment. Marked exacerbation of symptoms occurred 15 to 20 minutes after every dose of levodopa at 100 mg and lasted approximately 15 minutes. The PD symptoms, particularly tremor and rigidity, were exacerbated more markedly during this period than during the wearing-off deterioration. The BDMD could be controlled very well by subthalamic nucleus (STN) stimulation without any change in the regimen of levodopa medications. These observations suggest that the BDMD was inhibited by STN stimulation through a direct effect.

Diphenyl diselenide decreases the prevalence of vacuous chewing movements induced by fluphenazine in rats

RATIONALE

Chronic treatment with neuroleptics causes, as a side effect, tardive dyskinesia in humans; however, the mechanisms involved in its pathophysiology remain unclear.

OBJECTIVES

The purpose of this study was to examine the effects of diphenyl diselenide, an organoselenium compound with antioxidant properties, in an animal model of vacuous chewing movements (VCMs) induced by long-term treatment with fluphenazine.

RESULTS

Adult male rats were treated during 24 weeks with fluphenazine (25 mg/kg, intramuscularly [i.m.], once every 21 days) and diphenyl diselenide (1 mg/kg, subcutaneously, three times a week). VCMs and body weight gain were quantified every 3 weeks. The fluphenazine treatment produced VCMs in the majority of the treated rats (87% after 24 weeks). Concomitant treatment with diphenyl diselenide decreased the prevalence of VCMs to 50%. Additionally, we separated the rats that developed or did not develop VCMs. We did not find any statistical differences among the groups when oxidative stress parameters were evaluated. Chronic fluphenazine treatment significantly decreased [(3)H]-dopamine uptake. Concomitant treatment with diphenyl diselenide was not able to prevent this decrease in those rats that developed VCMs.

CONCLUSIONS

Our data suggest that the reduction in dopamine transport can be a possible mechanism related to the maintenance of VCMs in rats. Moreover, diphenyl diselenide seems to be a promising pharmacological agent in the reduction in the prevalence of VCMs in rats.

Continuous delivery of rotigotine decreases extracellular dopamine suggesting continuous receptor stimulation

Rotigotine, a non-ergolinic dopamine receptor agonist for treatment of Parkinson's disease was continuously administered over 48 h (0.5 mg/kg s.c., slow release formulation) to conscious rats striatally implanted with a microdialysis probe. Subsequently, the levels of rotigotine increased to a maximum of 3.42 + 2.1 nmol/l and remained at a level of 2.81 +/- 0.82 nmol/l for 48 h. Concomitantly, the dopamine levels consistently decreased to 20% of the control level. This suggests that the sustained administration of rotigotine provides stable extracellular drug levels in the striatum resulting in continuous stimulation of dopamine receptors.

Differential effects of methamphetamine and cocaine on behavior and extracellular levels of dopamine and 3,4-dihydroxyphenylalanine in the nucleus accumbens of conscious rats

The central dopamine system plays a prominent role in the effect of psychostimulants such as methamphetamine, cocaine and nicotine. l-3,4-Dihydroxyphenylalanine (DOPA), a precursor of dopamine, has been proposed as a neurotransmitter in the central nervous system. We have studied the effects of these psychostimulants on the release of DOPA and dopamine from the nucleus accumbens shell in conscious rats using in vivo microdialysis. Methamphetamine and cocaine increase the extracellular levels of dopamine. The effect of methamphetamine (1 mg/kg s.c.) on the release of dopamine was almost comparable to that of cocaine (10 mg/kg i.p.). However, methamphetamine increases, but cocaine decreases the extracellular levels of DOPA. In a behavioral study, methamphetamine (1 mg/kg s.c.) induced chewing, walking and sniffing behavior. Cocaine (10 mg/kg i.p.) produces weak effects on these behavioral parameters, when compared to the effects of methamphetamine (1 mg/kg s.c.). The behavioral changes produced by methamphetamine are suppressed by DOPA cyclohexyl ester (30 mg/kg i.p.), a competitive DOPA antagonist. Endogenous DOPA in the nucleus accumbens thus appears to be in involved in the behavioral responses to these psychomotor stimulants.

Subthalamic high frequency stimulation induced rotations are differentially mediated by D1 and D2 receptors

High frequency stimulation (HFS) of the subthalamic nucleus (STN) has clinically emerged as a promising approach in the treatment of Parkinson's disease, epilepsy, dystonia as well as compulsive and possibly other mood disorders. The underlying mechanisms are incompletely understood, but are definitely related to high frequency and likely to involve the dopamine (DA)-system. To further test this hypothesis the present study investigated the modulation of STN-HFS-induced circling by systemic and intracerebral injection of drugs acting on DA receptors in naive freely moving rats. Within this experimental setup, unilateral STN-HFS alone induced intensity-dependent circling. Systemic injections of selective D1- (SCH-23390) and D2-((-)-sulpiride) antagonists as well as the mixed D1 and D2 agonist apomorphine dose-dependently reduced STN-HFS-induced rotational behavior. Intracerebral microinjections of (-)-sulpiride but not SCH-23390 decreased circling when injected intrastriatally and increased the number of rotations when injected intranigrally (pars reticulata (SNr)). These data reveal that STN-HFS-induced contralateral circling is differentially modulated by D1 and D2 receptors. While D2 receptor-mediated effects involve the dorso-/ventrolateral striatum and the SNr, D1 receptors probably exert their actions via brain areas outside the striatum and SNr. These findings suggest the nigrostriatal DA-system to be specifically involved in the mediation of STN-HFS-induced motor effects.

Nicotine and nicotinic receptors; relevance to Parkinson's disease

The development of nicotinic agonists for therapy in neurodegenerative disorders such as Parkinson's disease is an area currently receiving considerable attention. The rationale for such work stems from findings that reveal a loss of nicotinic receptors in Parkinson's disease brains. These results, coupled with reports that nicotine treatment relieves some of the symptoms of this disorder, provides support for the contention that nicotine and/or nicotinic agonists may be beneficial for acute symptomatic treatment. Moreover, the observation that there is a decreased incidence of Parkinson's disease with tobacco use, possibly due to the nicotine in tobacco products, may imply that such drugs are useful for long-term neuroprotection. However, there are multiple nicotinic receptor populations in the brain with different functional properties. Identification of the subtypes involved in nigrostriatal dopaminergic activity is therefore critical for the rational use of selective therapeutic agents for symptomatic treatment and/or neuroprotection. Accumulating evidence, both in rodents and nonhuman primates now indicate that alpha6* nicotinic receptors are present on nigrostriatal dopaminergic neurons, and furthermore, that receptors containing this subunit may be most vulnerable to nigrostriatal damage, at least in nonhuman primates. These data suggest that nicotinic receptor ligands directed to alpha6* nicotinic receptors might be particularly relevant for Parkinson's disease therapeutics.

Effects of intrastriatal infusion of D2 receptor antisense oligonucleotide on apomorphine-induced behaviors in the rat

An antisense oligonucleotide strategy was employed to specifically deplete postsynaptic striatal D2 receptors in order to determine the possible role of presynaptic D2 autoreceptors in mediating behavioral responses induced by low doses of apomorphine. A phosphorothioate-modified antisense oligonucleotide complementary to the first 19 bases of the coding region of D2 receptor mRNA, a scrambled sequence comprising the same bases, or saline was infused bilaterally into the striatum of adult rats, twice daily for 2 days via indwelling cannulae. After an interval of 8-12 h, rats were habituated and challenged with high (300 micrograms/kg; subcutaneous) or low (50 micrograms/kg; s.c.) doses of apomorphine or its vehicle (0.1% ascorbic acid). Yawning, vacuous chewing mouth movements, hypoexploration, and penile grooming induced by low-dose apomorphine were unaffected by antisense infusion into the striatum, whereas stereotypic sniffing following high-dose apomorphine was markedly suppressed. Intrastriatal infusion of antisense resulted in significantly diminished [3H]-raclopride binding, while binding of [3H]-SCH 23390 (D1 receptors) and [3H]-WIN 35428 (dopamine transporter) was unchanged. D2 mRNA levels determined by quantitative in situ hybridization were normal in the striatum and the substantia nigra. Our results confirm that stereotypic sniffing is mediated via postsynaptic D2 receptors in the striatum, and favor the notion that behavioral responses induced by low doses of apomorphine are mediated by presynaptic D2 autoreceptors.

Locomotor inhibition, yawning and vacuous chewing induced by a novel dopamine D2 post-synaptic receptor agonist

The N-n-propyl analog of dihydrexidine ((+/-)-trans-10, 11-dihydroxy-5,6,6a,7,8,12b-hexahydrobenzo[a]phenanthridine) is a dopamine receptor agonist with high affinity for dopamine D2 and D3 receptors (K0.5 = 26 and 5 nM, respectively). Members of the hexahydrobenzo[a]phenanthridine structural class are atypical because they display high intrinsic activity at post-synaptic dopamine D2 receptors, but low intrinsic activity at dopamine D2 autoreceptors. The present study examined the effects of (+/-)-N-n-propyl-dihydrexidine on unconditioned behaviors in rats. The most striking results observed were large, dose-dependent decreases in locomotor activity (e.g., locomotor inhibition), and increases in vacuous chewing; yawning was also increased at the highest dose of (+/-)-N-n-propyl-dihydrexidine. The locomotor inhibition and yawning induced by (+/-)-N-n-propyl-dihydrexidine were blocked by pre-treatment with (-)-remoxipride (S(-)-3-bromo-N-((1-ethyl-2-pyrrolidinyl)-methyl)-2, 6-dimethoxybenzamide), a dopamine D2 receptor antagonist, but not by the dopamine D1 receptor antagonist (+)-SCH23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1 H-3-benzazepine). Vacuous chewing was decreased by both (-)-remoxipride and (+)-SCH23390. These data support the hypothesis that a subpopulation of post-synaptic dopamine D2 receptors has a critical role in decreases in locomotor activity and induction of vacuous chewing and yawning.

Neurobiology of L-DOPAergic systems

L-DOPA is proposed to be a neurotransmitter and/or neuromodulator in CNS. It is released probably from neurons, which may contain L-DOPA as an end-product, and/or from some compartment other than catecholamine-containing vesicles. The L-DOPA itself produces presynaptic and postsynaptic responses. All are stereoselective and most are antagonized by competitive antagonist. In striatum, L-DOPA is neuromodulator, mother of catecholamines, not only a precursor for dopamine but also a potentiator of children for presynaptic beta-adrenoceptors to facilitate dopamine release and postsynaptic D2 receptors, and ACh release inhibitor. All may cooperate for Parkinson's disease. Meanwhile, supersensitization of increase in L-glutamate release to nanomolar levodopa was seen in Parkinson's model rats, which may relate to dyskinesia or "on-off" during chronic therapy. In lower brainstem, L-DOPA tonically activates postsynaptic depressor sites of NTS and CVLM and pressor sites of RVLM. L-DOPA is probably a neurotransmitter of primary baroreceptor afferents terminating in NTS. GABA, the inhibitory neuromodulator for baroreflex in NTS, tonically functions to inhibit, via GABAA receptors, L-DOPA release and depressor responses to levodopa. Levodopa inversely releases GABA. L-DOPAergic monosynaptic relay from NTS to CVLM and from PHN to RVLM is suggested. Tonic L-DOPAergic baroreceptor-aortic nerve-NTS-CVLM relay seems to carry baroreflex information. Disturbance of neuronal activity to release L-DOPA in NTS, loss of the activity in CVLM, enhancement of the activity with decreased decarboxylation and increase in sensitivity to levodopa in RVLM may be involved in maintenance of hypertension in SHR. This is a story of "L-DOPAergic receptors" with extremely high affinity and low density.

The neurochemical effects on striatal dopamine turnover rate of N-stearyl dopamine after acute administration in rats

1. Considerable efforts have been made in order to develop autoreceptor selective agonists for the treatment of schizophrenia and hyperkinetic disorders. 2. Recent behavioural studies showed that the newly synthesized dopamine lipoamide, N-stearyl dopamine induced a strong hypomotility (-80%) in rats and mice. It is worth noting that this behavioural response was partially antagonized by dopaminergic antagonists, such as haloperidol and sulpiride, administered at doses that block DA autoreceptors. 3. In the present study the authors investigated the neurochemical changes induced by S-DA, in the striatum of the rat brain, in order to estimate a possible correlation between the above mentioned behavioural response and DA metabolism. 4. S-DA (10 or 100 mg/kg, i.p.) induced a significant decrease in DA turnover rate while it did not affect 5-HT metabolism in the striatum. 5. Considering that 5-DA induces a strong hypomotility, which can be partially antagonized by haloperidol or sulpiride administered at low doses, while also decreases the striatal DA turnover rate, it could be suggested that together these findings indicate that this DA lipoamide may be display the characteristics of an antipsychotic agent, acting on the "DA selective" autoreceptors.

Chronic clozapine selectively decreases prefrontal cortex dopamine as shown by simultaneous cortical, accumbens, and striatal microdialysis in freely moving rats

We used microdialysis to study the acute and chronic effects of clozapine on the metabolism of dopamine (DA) in terminal areas of the mesocortical, mesolimbic, and nigrostriatal systems simultaneously. In the acute experiment, groups of four rats received the following doses: 0 (vehicle), 10, 20, and 40 mg/kg of clozapine subcutaneously, which resulted in a dose-related increase in extracellular DA, 3,4-dihydroxyphenalacetic acid (DOPAC), and homovanillic acid (HVA) in the prefrontal cortex (PFC). In the nucleus accumbens (NAC) and striatum (STR), no significant changes were observed at any dose. In the chronic experiment, six rats received 20 mg/kg of clozapine and a control group received vehicle daily for 30 days. After 30 days of treatment, DA, DOPAC, and HVA were significantly lower in the PFC, and unchanged in the NAC or STR. The 30th clozapine injection failed to increase DA, DOPAC, or HVA in any of the three regions. We conclude that clozapine acted selectively on the mesocortical system, and that this may underlie clozapine's therapeutic, antipsychotic effect.

Cholecystokinin [corrected] octapeptide modulates dopamine release in rat striatum

The effect of cholecystokinin octapeptide sulfated (CCK8S) on the basal and electrically evoked release of [3H]dopamine ([3H]DA) in striatal slices from the rat brain was studied. Cholecystokinin octapeptide did not influence the basal release of [3H]DA. Field electrical stimulation (FES) (2 Hz) induced an increase of dopamine release from striatal slices, which was Ca2+ dependent and was abolished by tetrodotoxin, 10(-6) M. Cholecystokinin octapeptide (10(-9) M, 10(-8) M and 10(-7) M) dose dependently reduced the electrically evoked release of [3H]DA. This effect was antagonized by the CCK-A receptor antagonists loxiglumide (10(-7) M, 10(-6) M and 10(-5) M) or proglumide (10(-5) M, 10(-4) M and 10(-3) M). The results suggest that CCK receptors type A are involved in this effect of CCK8S in the striatum.

Differences in effects of sultopride and sulpiride on dopamine turnover in rat brain

Sultopride and sulpiride are both chemically similar benzamide derivatives and selective antagonists of dopamine D2 receptors. However, these drugs differ in clinical properties. We compared the effects of sultopride and sulpiride on dopamine turnover in rats following the administration of these drugs alone or in combination with apomorphine. The administration of sultopride or sulpiride markedly accelerated dopamine turnover in the rat brain. The increase in the level of dopamine metabolites in the striatum was more marked in the sultopride-treated rats. Sulpiride affected the limbic dopamine receptors preferentially, whereas sultopride affected the striatal and the limbic dopamine receptors equally. A low dose of apomorphine induced a reduction in the concentration of dopamine metabolites in the striatum and the nucleus accumbens by approximately 55%, but not in the medial prefrontal cortex. Sultopride was more effective in preventing an apomorphine-induced reduction in dopamine metabolite levels. These results from rat experiments would model the pharmacological differences observed between sultopride and sulpiride in clinical use.

Baroreceptor-aortic nerve-mediated release of endogenous L-3,4-dihydroxyphenylalanine and its tonic depressor function in the nucleus tractus solitarii of rats

We have proposed that L-3,4-dihydroxyphenylalanine (L-DOPA) is a neurotransmitter and/or neuromodulator in the central nervous system [Misu Y. and Goshima Y. (1993) Trends pharmac. Sci. 14, 119-123]. This study aimed to explore whether or not endogenous L-DOPA, as a neurotransmitter candidate of the primary baroreceptor afferents, tonically functions to activate depressor neurons in the nucleus tractus solitarii of anesthetized rats. By parallel microdialysis in bilateral nucleus tractus solitarii areas, the basal L-DOPA release was in part inhibited by tetrodotoxin perfusion (1 microM) or Ca2+ deprivation, and was markedly reduced by alpha-methyl-p-tyrosine (200 mg/kg, i.p.), a tyrosine hydroxylase inhibitor. Forty to 100 mM K+ concentration-dependently released L-DOPA. Fifty millimoles K+ repetitively and constantly released L-DOPA. This release was Ca(2+)-dependent. Stimulation of the left aortic nerve (100 Hz, 8 V) repetitively and constantly released L-DOPA and this release was tetrodotoxin-sensitive. Phenylephrine i.v. infused produced L-DOPA release and reflex bradycardia, temporally associated with a rise and subsequent recovery of blood pressure. This release and bradycardia were abolished by denervation of the bilateral carotid sinus and aortic nerves. In addition, L-DOPA methyl ester, a competitive L-DOPA antagonist, when microinjected into depressor sites of the left nucleus tractus solitarii, antagonized depressor responses to mild stimulation (20 Hz, 3 V) of the ipsilateral aortic nerve. This antagonist alone, microinjected bilaterally, elicited a dose-dependent hypertension, which was abolished by alpha-methyl-p-tyrosine. Furthermore, by immunocytochemical analysis seven days after denervation of the left aortic nerve, tyrosine hydroxylase- and L-DOPA-, but not dopamine- and dopamine-beta-hydroxylase-immunoreactivities decreased in the ipsilateral nucleus tractus solitarii and dorsal motor vagus nucleus complex area. In the left ganglion nodosum, denervation decreased staining and number of L-DOPA-immunoreactive cells and staining of tyrosine hydroxylase-immunoreactive cells, but no modification of dopamine-immunoreactive cells was seen. Taken together with previous findings that L-DOPA itself is stereoselectively responsible for cardiovascular control in this nucleus, it is probable that L-DOPA is a neurotransmitter of the primary baroreceptor afferents terminating directly in depressor neurons and/or indirectly in some neurons within a microcircuit, including depressor neurons of the nucleus tractus solitarii. Endogenously released L-DOPA itself tonically functions to activate depressor neurons for regulation of blood pressure in the rat nucleus tractus solitarii.

Endogenously released L-dopa itself tonically functions to potentiate postsynaptic D2 receptor-mediated locomotor activities of conscious rats

A non-effective dose of exogenously applied L-dopa itself stereoselectively potentiates postsynaptic D2 receptor-mediated locomotor activities of rats. We further attempted to clarify whether or not endogenously released L-dopa tonically functions to potentiate activities of these receptors, simultaneously monitoring locomotor activities and basal release of L-dopa and dopamine during striatal microdialysis in conscious rats. Quinpirole (1 mg/kg, s.c.) alone, a selective D2 agonist, increased locomotor activities and decreased basal L-dopa and dopamine release 20-140 min after injection. Pretreatment with alpha-methyl-p-tyrosine (3 mg/kg, i.p.), a tyrosine hydroxylase inhibitor, decreased locomotor activities and further decreased L-dopa release without modification of dopamine release, compared to quinpirole alone, whereas 3-hydroxybenzylhydrazine (100 mg/kg, i.p.), a central dopa decarboxylase inhibitor, further increased locomotor activities and markedly increased L-dopa release without modification of dopamine release. Endogenously released L-dopa itself functions tonically to potentiate activities of postsynaptic D2 receptors relevant to locomotor movement of rats.

The ontogeny of apomorphine-induced alterations of neostriatal dopamine release: effects on spontaneous release

The effects of apomorphine (0.05, 0.1, and 1.0 mg/kg, s.c.) on the extracellular levels of dopamine and the dopamine metabolite 3,4-dihydroxyphenylacetic acid were studied through the use of in vivo microdialysis in the neostriatum of developing and adult rats. Fifteen-minute samples were collected from urethane-anesthetized rats 5, 10-11, 21-22, and 35-36 days old and adults and quantified by HPLC with electrochemical detection. Apomorphine attenuated extracellular levels of dopamine in all age groups, suggesting that the dopamine autoreceptor modulating release in the neostriatum is functional by 5 days of age. A dose-response effect of apomorphine on extracellular dopamine was observed in all age groups except at 10-11 days of age. Extracellular levels of 3,4-dihydroxyphenylacetic acid were also significantly decreased in all age groups, consistent with the hypothesis that synthesis-modulating dopamine autoreceptors in the neostriatum are functional by 5 days of age. Apomorphine had a significantly greater effect on extracellular 3,4-dihydroxyphenylacetic acid levels at the 0.05 and 0.1 mg/kg doses in the 5- and 10-11-day-old age groups compared with the other ages. Absolute levels of extracellular dopamine were significantly attenuated at 5 days of age compared with the other ages, and absolute levels of extracellular 3,4-dihydroxyphenylacetic acid monotonically increased with age.

Low and high dose bromocriptine have different effects on striatal dopamine release: an in vivo study

We wished to determine if low and high doses of bromocriptine produce distinct patterns of dopamine release and metabolism. Accordingly, we administered bromocriptine (0, 2.5, 5, and 10 mg/kg, IP) to rats and monitored extracellular concentrations of dopamine and dopamine metabolites in the corpus striatum with the technique of cerebral microdialysis. Extracellular dopamine levels increased following administration of 2.5 and 5 mg/kg bromocriptine. In contrast, dopamine levels decreased following 10 mg/kg bromocriptine. Dopamine metabolite levels decreased 45 minutes following all doses of bromocriptine. Bromocriptine administration had no effect on the levels of 5HIAA, the major serotonin metabolite. These findings with high dose bromocriptine fit the predicted profile of a dopamine D2 receptor agonist. The delayed decrease in dopamine metabolites at all bromocriptine doses is consistent with the known dopamine synthesis inhibiting action of bromocriptine. In contrast, the increased dopamine release observed following low and medium doses of bromocriptine is not readily explainable by current theories of bromocriptine action which predict decreased dopamine release and therefore decreased striatal extracellular dopamine levels with both high and low-doses of bromocriptine. Our findings indicate that bromocriptine has a complex pharmacological action that extends beyond simple agonism at dopamine D2 receptors.

Is L-dopa an endogenous neurotransmitter?

Since the 1960s, L-3,4-dihydroxyphenylalanine (L-dopa), a precursor of dopamine, has been thought to occur in the cytoplasm of catecholaminergic neurones. L-Dopa is traditionally believed to be an inert amino acid that exerts actions and effectiveness in Parkinson's disease via its conversion to dopamine by L-aromatic amino acid decarboxylase. In contrast to this generally accepted idea, Yoshimi Misu and Yoshio Goshima propose, in this Viewpoint article, that L-dopa itself is an endogenous neurotransmitter or neuromodulator in the CNS. This hypothesis is mainly based on the findings that L-dopa is released in a transmitter-like manner and that exogenously applied levodopa produces some responses.

Beginning-of-dose motor deterioration following the acute administration of levodopa and apomorphine in Parkinson's disease

Six Parkinsonian patients on long term levodopa therapy complained of short-lived deterioration of Parkinsonian symptoms immediately after levodopa intake. After withdrawal of the drug overnight, and following an oral challenge with levodopa/carbidopa (250/25) in all six cases, and with subcutaneous apomorphine (3 mg) in two, deterioration below base line levels of disability were observed which would not be explained by loss of sleep benefit. This occurred 10-20 minutes after levodopa challenge and lasted for 10-20 minutes. The latency and duration of this phenomenon were shorter with apomorphine but the characteristics were similar. This phenomenon may be due to an inhibitory effect of levodopa acting via presynaptic dopamine receptors.

Do autoreceptors mediate dopamine agonist--induced yawning and suppression of exploration? A critical review

The hypothesis that stimulation of dopamine autoreceptors is the mechanism by which dopamine agonists induce yawning and suppression of exploration is critically examined. It is shown that the relation between reduced extracellular dopamine levels, assessed by microdialysis, and behavioural effects of dopamine agonists, a dopamine synthesis inhibitor and a granule storage blocker is highly inconsistent. The time-course and duration of the behavioural effects of dopamine agonists differ from the reduction of extracellular dopamine. Amphetamine cotreatment is shown to increase dopamine levels, while yawning and suppression of exploration can still be induced. The data strongly indicate that autoreceptors are not the mediators of these behavioural effects. It is proposed that postsynaptic receptors mediate dopamine agonist induced yawning and suppression of exploration. Evidence is also presented showing that yawning and suppression of exploration are not functionally equivalent.

Effects of acute and chronic systemic administration of some typical antipsychotic drugs on turnover of dopamine and potassium ion-induced release of dopamine in the striatum of the rat in vivo

Chronoamperometry was used to measure the dose dependency of antipsychotic drug (spiperone, haloperidol and chlorpromazine)-induced increases in depolarization-stimulated release of dopamine in the striatum of the rat in vivo. The dose-response curves were found to be at least biphasic (small doses increased release and large doses inhibited release) and different in shape from dose-response curves for increases in the turnover of dopamine, suggesting that the two processes may not be related. The threshold dose to induce increased depolarization-stimulated release of dopamine correlated with the values in the literature for doses required to block stereotype induced by apomorphine and amphetamine and doses sufficient to cause a maximal increase in release correlated with the doses required for the induction of catalepsy. In addition, the ratio of the doses required to obtain half-maximal and maximal increases in stimulated release of dopamine, matched values in the literature of the ratio of doses required to ameliorate psychotic symptoms, whereas the doses required to reverse the increase induced by antipsychotic drugs did not. Chronic administration of haloperidol resulted in alterations in the dose-response curves for both the release and turnover of DA. Chronic administration caused a reversal of the effect of acute administration in the small dose range (inhibition as opposed to enhancement of release), a decrease in the maximum magnitude of release of dopamine obtained and an overall shift to the right of the dose-response curve. Chronic administration of haloperidol decreased turnover, relative to animals receiving the drug acutely and no shift in the position of the dose-response curve was observed.

Differential agonist profile of the enantiomers of 3-PPP at striatal dopamine autoreceptors: dependence on extracellular dopamine

The effects of the enantiomers of 3-hydroxyphenyl-N-n-propylpiperidine (3-PPP) at dopamine (DA) synthesis modulating autoreceptors, measured as DOPA accumulation after decarboxylase inhibition, were assessed in vivo and in rat striatal slices. In vivo, (+)-3-PPP inhibited DOPA accumulation in the striatum, nucleus accumbens, and medial prefrontal cortex, whereas (-)-3-PPP either increased (striatal) or had no effect (accumbens, prefrontal cortex), on DOPA accumulation. In vitro, both (+)- and (-)-3-PPP reduced basal DOPA accumulation with a similar order of potency (apparent EC50 = 2.1 and 1.0 microns, respectively) and maximal effect, although they were less potent than the D2 DA receptor agonist quinpirole (EC50 = 0.15 microM). The inhibition of tyrosine hydroxylation was also observed in slices obtained from reserpine-pretreated rats and was blocked by the selective D2 DA antagonist (-)-sulpiride. This suggests that 3-PPP inhibition of DOPA accumulation was mediated directly by stimulation of DA D2 receptors. Increasing the amount of extracellular DA by depolarizing slices with 30 mM K+ did not alter the qualitative effects of either quinpirole or (+)-3-PPP. However, the stimulation of DA autoreceptors by (-)-3-PPP was no longer apparent under conditions of elevated extracellular DA. Under these depolarizing conditions, (-)-3-PPP actually antagonized the inhibitory effect afforded by either quinpirole or pergolide. A similar switch in profile was observed with transdihydrolisuride (TDHL). The data support the notion that (-)-3-PPP and TDHL are partial agonists at synthesis modulating DA autoreceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of administration of dopamine D2 agonist quinpirole on exploratory locomotion

Injections of the dopamine D2 agonist quinpirole (LY 171555) into the nucleus accumbens reduced exploratory locomotion in a dose-dependent manner. Injections of the dopamine D1 agonist SKF 38393 had no effect on exploratory locomotion. The results are consistent with observations from recent electrophysiological and behavioral experiments which suggest a presynaptic action of the D2 agonist. It is proposed that quinpirole activates D2 receptors on the axon terminals of glutamatergic hippocampal-accumbens neurons that are associated with exploratory locomotion.

Effect of intra-amygdala dopaminergic grafts on methamphetamine-induced locomotor activity, extracellular dopamine and dopamine metabolite overflow: a comparison with the effect of intra-accumbens grafts

The effects of 6-hydroxydopamine lesions of the ventral tegmental area and following intra-amygdala or intra-accumbens dopaminergic (DAergic) grafts on methamphetamine (MAP)-induced locomotor activity were investigated in rats. Intra-accumbens DAergic grafts from rat embryos restored the locomotor hyperactivity response to MAP 5 weeks after grafting, while intra-amygdala grafts did not restore responses by 10 weeks after grafting. Biochemical measurements of extracellular DAergic activity in the amygdala (AMY) by in vivo microdialysis after grafting showed no significant change in the basal levels of dopamine (DA) and partial restoration of metabolite levels. MAP induced an increase of DA efflux and a decrease in dihydroxyphenylacetic acid without a significant change in homovanillic acid, which is the same pattern of response seen in control animals. These biochemical changes are similar to those seen previously after intra-accumbens grafts. The results show that restoration of DAergic activity in the AMY in the presence of DAergic denervation of accumbens does not have an effect on MAP-induced locomotion.

Selective cross-activation/inhibition of second messenger systems and the reduction of age-related deficits in the muscarinic control of dopamine release from perifused rat striata

Possible alterations in muscarinic cholinergic (mACh) signal transduction in senescence were studied in rat neostriata. Acetylcholine (ACh) activation of striatal muscarinic heteroreceptors by carbachol or oxotremorine enhances K(+)-evoked release of dopamine from perifused striata of 6- but not 24-month-old rats. Present experiments determined the effects of simultaneous activation or activation/inhibition of more than one second messenger on K(+)-evoked release of DA from perifused striatal slices from these age groups. Combinations of carbachol (500 microns), which stimulates inositol-1,4,5-bisphosphate (IP3) production and inhibits cyclic AMP production, with oxotremorine (500 microns), which inhibits cyclic AMP production, in the presence of 30 mM KCl (in a modified Krebs-Ringer medium) reduced the age-related reduction in mAChR enhancement of DA release (analyzed by HPLC coupled to electrochemical detection; 5 min fractions were collected on ice in perchloric acid; flow rate 120 microliters/min). Combinations of these agonists with the putative second messenger arachidonic acid (10 microM), also enhanced K(+)-evoked release of DA in the striatal tissue from the 24-month group. IP3 activation was lower in the striatal tissue from old animals than those from young under all conditions, but cross-activation/inhibition actually may have lowered the IP3 threshold necessary for enhanced DA release to occur. In a subsequent experiment, pre-loading striatal tissue from young animals with either carbachol or oxotremorine under basal release conditions reduced the responding when the basal release medium was switched to one containing 30 mM KCl and combinations of the agonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioral and biochemical effects of intra-accumbens dopaminergic grafts

6-Hydroxydopamine (6-OHDA) lesions in the ventral tegmental area (VTA) in rats prevented the hyperactivity response to methamphetamine in an open field. Transplantation of mesencephalic dopaminergic cells, obtained from rat embryos, into the nucleus accumbens (NAC) of 6-OHDA-lesioned animals restored the hyperactivity 4 weeks after grafting. By microdialysis of the NAC in freely moving rats no significant differences in baseline concentration of dopamine (DA) among the 3 groups (control, lesioned, grafted) were observed. However, after methamphetamine administration, DA increased significantly during the first 80 min in control animals, during the first 40 min in grafted animals, but did not increase in lesioned animals. On the other hand baseline concentrations of dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) decreased to one sixth to one third of those of controls in 6-OHDA-lesioned animals, and they did not respond to methamphetamine. After grafting, however, DOPAC and HVA restored to control levels and responded to methamphetamine with decreases as was observed in control animals. Data suggest that grafts not only restore the ability to release DA but also improve DA metabolism in the NAC. This might be a reason for recovery of locomotor activity.

Yawning and suppression of exploration induced by dopamine agonists: no relation to extracellular striatal levels of dopamine

The present study was aimed at testing the hypothesis that yawning and suppression of exploration induced by low doses of dopamine agonists in the rat are caused by a reduction of synaptic dopamine levels. The decrease in extracellular levels of dopamine in the corpus striatum induced by alpha-methyl-p-tyrosine (alpha MPT, 50-200 mg/kg IP), reserpine (2-5 mg/kg SC) and apomorphine (APO, 0.05 mg/kg SC) was measured in microdialysis experiments. Reserpine and alpha MPT reduced the dopamine levels to the same extent as APO. Exploratory behaviour was suppressed by APO, but not by alpha MPT (50 and 100 mg/kg) when tested in a separate experiment. Reserpine (2 mg/kg) suppressed exploration after 4 hr, but not after 3 hr. Changes in extracellular levels of dopamine were tested simultaneously with changes in yawning in another group of rats implanted with guide cannulae for microdialysis probes. There was a discrepancy in the time-course for the induction of yawning as compared to the changes in extracellular dopamine levels after APO (0.05 mg/kg) as well as after pergolide (0.02 mg/kg SC). Yawning appeared before and lasted shorter than the decrease in dopamine. The time-courses for APO-induced suppression of exploration and yawning were similar. The dose-response curve for APO-induced yawning was not changed by alpha MPT (200 mg/kg), while the suppression of exploration induced by APO, but not by pergolide, was enhanced by pretreatment with alpha MPT. The results show that yawning and suppression of exploration induced by dopamine agonists are not related to changes in extracellular levels of dopamine. It is proposed that these behaviours may be mediated by postsynaptic receptors.

Reduction of motor behavioural deficits in senescence via chronic prolactin or estrogen administration: time course and putative mechanisms of action

The effects of chronic estrogen (E2), rat prolactin (rPRL), modified ovine prolactin (mPRL) administration on motor behavior (inclined screen performance) and striatal dopamine (DA) (D2subtype) receptor concentrations were examined in senescent (greater than 24 months of age) female rats, mPRL possesses no lactotrophic activity. Administration of either E2 or rPRL was effective in improving both inclined screen performance (increased time that the animal could remain on the screen by 95 and 413 s, respectively, compared to highest pre-injection performance) and striatal D2 receptor concentrations (14 and 20% respectively). These were indications, however, from separate analyses that improvements in inclined screen performance were seen prior to any increases in striatal D2 receptor concentrations. These early performance increases seemed instead to be the result of improved muscarinic receptor control over striatal DA autoreceptor function. Later improvements in inclined screen performance (at 6-7 days after the E2 injections were begun) were more dependent on increased striatal DA receptor concentrations. A second set of experiments which involved the injection of E2 into senescent male as well as female rats indicated that there were no sex differences in improvements in inclined screen performance, and that once the E2 injections were discontinued, performance returned to preadministration levels. The results are discussed in terms of two important processes that may be involved in mediating enhanced inclined screen performance following E2 administration: (1) enhancement of muscarinic receptor regulation of DA autoreceptor function; and (2) increases in striatal DA receptor density.

Neurotensin counteracts apomorphine-induced inhibition of dopamine release as studied by microdialysis in rat neostriatum

Microdialysis in the neostriatum of the halothane-anesthetized male rats was used to study the effect of neurotensin on the release of dopamine and its metabolites in the absence or presence of systemic apomorphine treatment. Perfusate levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were assayed by high-performance liquid chromatography in combination with electrochemical detection. Perfusion with neurotensin (1000 nM but not 10 nM) increased the dialysate levels of dopamine without affecting those of DOPAC and HVA. Systemic treatment with apomorphine (0.05 and 0.5 mg/kg, s.c.) reduced the dialysate levels of dopamine, DOPAC and HVA in a dose-related way. Neurotensin (10 nM but not 1 nM) counteracted the inhibitory effect of apomorphine on dialysate levels of dopamine without affecting those of DOPAC and HVA. The results indicate a facilitatory effect of neurotensin on dopamine release in rat neostriatum. It is suggested that activation of neurotensin receptors may cause a reduction in the affinity of dopamine autoreceptors, since the low dose of neurotensin is able to counteract the inhibitory effect of apomorphine on dopamine release.

Effects of repeated administration of low doses of apomorphine in three behavioural models in the rat

A low dose of the dopamine (DA) receptor agonist apomorphine (APO 0.05 mg/kg) was given repetitively and the effects were tested in three different behavioural models: reduction of spontaneous locomotion, induction of yawning and decrease in water intake in water-deprived animals. The APO-induced suppression of exploration and decrease in water intake were not affected by a previous injection of APO given 1 or 3 hours before the test dose of APO. There was a small, but significant, decrease in the induction of yawning by a previous dose of APO given 1 hour or 30 min before the test dose. However, pretreatment with APO 3 hours before the test dose did not diminish the yawning response. It is suggested that the dopaminergic mechanisms mediating APO induced yawning are different from those mediating decrease in water intake and suppression of exploration. The results are also discussed in relation to the proposed efficiency of low doses of DA agonists in the treatment of various neurological and psychiatric disorders.

Comparative study of sulpiride and haloperidol on dopamine turnover in the rat brain

The effects of the neuroleptics, sulpiride and haloperidol, on dopamine (DA) turnover were compared following the acute and chronic administration of these drugs alone or in combination with levodopa or apomorphine. In the acute treatment, the increase in DA metabolites in the striatum and nucleus accumbens was more marked in the haloperidol-treated rats than in the sulpiride-treated rats. Following the additional administration of levodopa, however, the potency of the neuroleptics in elevating DA metabolites was reversed. A low dose of apomorphine induced a marked reduction in the striatal DA metabolite levels by approximately 50%. When rats were pretreated with the neuroleptics, haloperidol was more effective in preventing an apomorphine-induced reduction in DA metabolites. On repeated administration of the neuroleptics, a tolerance occurred in the striatum and nucleus accumbens, but not in the prefrontal cortex. This differential development of tolerance was observed in the different brain regions and with the different drugs administered. These results suggest that the pharmacological mechanism of sulpiride on DA turnover differs from that of haloperidol.

Haloperidol given chronically decreases basal dopamine in the prefrontal cortex more than the striatum or nucleus accumbens as simultaneously measured by microdialysis

Simultaneous microdialysis was performed in the prefrontal cortex, striatum, and the caudal region of the nucleus accumbens using implanted guide shafts with removable microdialysis probes. Extracellular dopamine (DA), dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) were measured in response to acute and chronic haloperidol, an antipsychotic drug. Six rats received haloperidol (0.5 mg/kg, IP) while changes in DA and its metabolites were monitored for six hours. Then the microdialysis probes were removed and daily injections of haloperidol were given for 28 more days. Next the probes were reinserted, and a final haloperidol challenge was given. A control group of six rats was treated the same way but with saline. The first injection of haloperidol increased extracellular DA, DOPAC and HVA proving increased turnover in all three regions. A month later, after chronic treatment, basal DA and its metabolites decreased in just the cortex. DOPAC and HVA also decreased in the cortex and striatum, suggesting a decrease in turnover at these two sites. None of these changes occurred in the accumbens. The last haloperidol challenge after chronic treatment no longer increased extracellular DA in any of the three sites, but increased production of metabolites was still detectable. This suggests a lack of DA responsiveness in all three sites. In summary, chronic haloperidol affected each of the regions differently; DA metabolites decreased in the STR and PFC; basal DA decreased in the PFC.

Mesocortical dopaminergic neurons. 2. Electrophysiological consequences of terminal autoreceptor activation

Measurement of drug- and stimulation-induced changes in the electrical excitability of dopaminergic terminals was employed to assess the effects of stimulation of dopamine terminal autoreceptors in the prefrontal cortex in urethane-anesthetized rats. Systemic or local administration of amphetamine decreased, whereas systemic administration of haloperidol increased the excitability of prefrontal cortical dopaminergic terminals of ventral tegmental area dopaminergic neurons. Mesoprefrontal dopaminergic terminal excitability was also responsive to spontaneous and stimulation-induced alterations in the rate of impulses reaching the terminal fields. These results are comparable to those previously reported for nigrostriatal and mesoaccumbens dopaminergic neurons, and are discussed with regard to the operational characteristics of autoinhibition in the mesocortical dopaminergic system.

Yawning and suppression of exploration in amphetamine-treated rats, incompatibility with the autoreceptor hypothesis

The hypothesis that yawning and suppression of exploration, induced by low doses of dopamine agonists, are mediated by stimulation of dopamine autoreceptors was tested by studying the influence of amphetamine on these behavioural models and on extracellular levels of dopamine sampled by microdialysis. Behaviour was measured in a holeboard apparatus. A low dose of amphetamine (0.2 mg/kg) caused slight activation of habituated rats. The same dose of amphetamine completely counteracted the decrease in extracellular dopamine levels caused by pergolide (0.02 mg/kg) and, instead, elevated the dopamine levels to 300% above baseline. The same or higher doses of amphetamine (0.5-1.0 mg/kg) did not completely antagonise suppression of exploration or yawning induced by apomorphine (0.05 mg/kg) or pergolide (0.02 mg/kg). The results suggest that both yawning and suppression of exploration induced by low doses of dopamine agonists are not mediated by dopamine autoreceptors, since these behaviours could be elicited when the extracellular levels of dopamine were elevated above baseline. The alternative hypothesis that these behaviours are mediated by sensitive post-synaptic receptors is suggested. It was also found that combined treatment with SCH 23390 (0.05 mg/kg) and amphetamine (2 mg/kg) induced yawning, which further supports the new hypothesis.

Neuromodulatory action of dopamine in the nucleus accumbens: an in vivo intracellular study

Intracellular recordings were made from neurons in the nucleus accumbens in situ to determine how dopamine produces the selective neuromodulatory action in the accumbens observed in previous studies. Electrical stimulation of the basolateral nucleus of the amygdala was found to produce monosynaptically evoked depolarizing and hyperpolarizing postsynaptic potential sequences in a large proportion of the accumbens neurons sampled. Dopamine applied iontophoretically or released endogenously by stimulation of the ventral tegmental area produced consistent membrane depolarization and an increase in membrane conductance but not an increase in spontaneous activity of the accumbens neurons. Stimulation of the ventral tegmental area with trains of 10 pulses at 10 Hz prior to stimulation of the amygdala produced 8-58% reduction in the amplitude of the depolarizing postsynaptic potential but no change in the late hyperpolarizing postsynaptic potential. Although attenuation of the depolarizing postsynaptic potential amplitude from ventral tegmental area stimulation was often accompanied by membrane depolarization, it appeared that the two responses were not causally related. The effect of ventral tegmental area stimulation on the evoked depolarizing postsynaptic potential and the membrane potential were blocked by haloperidol indicating the involvement of dopamine. Iontophoretically applied dopamine produced responses similar to ventral tegmental area stimulation with two exceptions: (i) iontophoretically applied dopamine produced consistently stronger maximal attenuation of the depolarizing postsynaptic potential than did ventral tegmental area stimulation; and (ii) iontophoretically applied dopamine always attenuated both the depolarizing postsynaptic potential and hyperpolarizing postsynaptic potential whereas ventral tegmental area stimulation produced selective attenuation of the depolarizing postsynaptic potential only. These electrophysiological results are complementary to those from pharmacological experiments and suggest that one of several physiological functions of dopamine in the nucleus accumbens is a neuromodulatory one involving presynaptic action on non-dopaminergic terminals.

Alterations in muscarinic control of striatal dopamine autoreceptors in senescence: a deficit at the ligand-muscarinic receptor interface?

Research has indicated that the release of striatal dopamine (DA) is controlled by inhibitory DA autoreceptors which are in turn regulated by inhibitory muscarinic inhibitory cholinergic heteroreceptors (HTRs) located in close vicinity to the autoreceptors. Muscarinic activation enhances K+-evoked release of DA from striatal slices from mature but not senescent rats. Since it has been shown that age-dependent declines in Ca2+ mediated acetylcholine release can be restored by the ionophore A23187, it was of interest to determine if age-related decrements in Ca2+ mobilization might contribute to the alterations in muscarinic control of the striatal DA autoreceptors seen in senescence. Cross-cut striatal tissue slices obtained from two age-groups (6 and 24 months) of Wistar rats were superfused with a modified Krebs-Ringer medium containing 2.5 mM KCl. After a 30-min equilibration period, a 5-min baseline fraction was collected. The medium was then switched to one which contained 30 mM KCl and, depending upon the experiment, the muscarinic agonists carbachol, or oxotremorine or the Ca2+ mobilizing agents A23187 or inositoltrisphosphate (IP3) and enhancement of K+-evoked release of DA was examined. Six 5-min fractions were collected. DA release was determined by HPLC coupled to electrochemical detection. Results indicated that although deficits were seen in oxotremorine and carbachol enhancement of K+-evoked release of DA, these decrements were not observed when either A23187 or IP3 were utilized to enhance the K+-evoked release of DA.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related decrements in the muscarinic enhancement of K+-evoked release of endogenous striatal dopamine: an indicator of altered cholinergic-dopaminergic reciprocal inhibitory control in senescence

Previous experiments have indicated that the release of striatal dopamine (DA) is controlled by inhibitory DA autoreceptors which are mediated by inhibitory cholinergic heteroreceptors (HTRs). Activation of the HTRs by muscarinic or nicotine agonists potentiates the K+-evoked release of DA from the striatum. Present experiments were carried out to determine if this relationship is altered as a function of aging. Cross-cut striatal tissue slices obtained from 3 age-groups (6, 12-18 and 24 months) Wistar rats were superfused with a modified Krebs-Ringer basal release medium containing 2.5 mM KCl. After a 30-min equilibration period, a 5-min baseline fraction was collected from each chamber. The medium was then switched to one containing 30 mM KCl, and depending upon the experiment, 1 of 4 concentrations of a particular muscarinic (oxotremorine, pilocarpine, carbachol or bethanecol) or nicotinic (nicotine) agonist. In some experiments DA autoreceptor function was assessed directly with haloperidol. Six 5-min fractions were taken during depolarization. DA release was assessed using high performance liquid chromatography coupled to electrochemical detection. Results indicated that the efficacy of the muscarinic agonists was reduced in an age-dependent manner with the oldest age groups showing the smallest enhancement. The age at which the decline was seen was dependent on the muscarinic agonist that was applied. Deficits were seen as early as 12 months when full agonists (e.g. carbachol) were applied, but did not appear until 18 months when partial agonists (e.g. oxotremorine) were applied. These age-related alterations were not seen when haloperidol or nicotine were used to enhance the K+-evoked release of DA.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Transmitter-like release of endogenous 3,4-dihydroxyphenylalanine from rat striatal slices

Biphasic electrical field stimulation (0.5-5 Hz, 2 ms, 25 V, 3 min) and high K+ (10-30 mM, 5 min) released endogenous 3,4-dihydroxyphenylalanine (DOPA) from superfused rat striatal slices. Characteristics of the DOPA release were compared with those of 3,4-dihydroxyphenylethylamine (dopamine, DA). Electrical stimulation at 2 Hz evoked DOPA and DA over similar time courses. alpha-Methyl-p-tyrosine (0.2 mM) markedly reduced release of DOPA but not of DA. Maximal release (0.3 pmol) of DOPA was obtained at 2 Hz and at 15 mM K+. The impulse-evoked release of DOPA and DA was completely tetrodotoxin (0.3 microM) sensitive and Ca2+ dependent and the 15 mM K+-evoked release was also Ca2+ dependent. On L-[3,5-3H]tyrosine (1 microM) superfusion, high K+ (15 and 60 mM) released DOPA and DA together with concentration-dependent decreases in tyrosine 3-monooxygenase (EC 1.14.16.2) activity as indicated by [3H]H2O formation, followed by concentration-dependent increases after DOPA and DA release ended. These findings suggest that striatal DOPA is released by a Ca2+-dependent excitation-secretion coupling process similar to that involved in transmitter release.

B-HT 920 stimulates postsynaptic D2 dopamine receptors in the normal rat: electrophysiological and behavioral evidence

The putative autoreceptor-selective dopamine (DA) agonist B-HT 920 was tested using electrophysiological and behavioral models thought to reflect actions at postsynaptic D2 DA receptors. Direct iontophoretic application of B-HT 920 onto nucleus accumbens neurons caused a current-dependent inhibition of firing which could be attenuated by pretreatment with alpha-methyl-p-tyrosine (to deplete DA) and reinstated (enabled) by concurrent administration of the selective D1 DA receptor agonist SKF 38393. These findings suggest that, like other selective D2 DA receptor agonists, the postsynaptic effects of B-HT 920 require concurrent stimulation of D1 DA receptors. Behavioral indices of postsynaptic D2 DA receptor stimulation (stereotyped sniffing and rearing) were also evident following combined treatment with B-HT 920 and SKF 38393. Moreover, similar "low-level" stereotyped behaviors were also observed when B-HT 920 was administered alone following pretreatment with the alpha-2 adrenoceptor antagonists idazoxane and piperoxane, suggesting that alpha-2 agonist actions of B-HT 920, in some way, mask the expression of D2 receptor-mediated stereotyped responses. When B-HT 920 was combined with SKF 38393 following pretreatment with idazoxane, both the intensity and form (continual licking and gnawing) of stereotyped behavior was enhanced. Taken together, these electrophysiological and behavioral findings indicate that B-HT 920 possesses the properties of a selective D2 DA receptor agonist and cannot be considered as a DA autoreceptor-selective compound.

7-[3-(4-[2,3-Dimethylphenyl]piperazinyl)propoxy]-2(1H)-quinolinone (OPC-4392), a presynaptic dopamine autoreceptor agonist and postsynaptic D2 receptor antagonist

7-[3-(4-[2,3-dimethylphenyl]piperazinyl)propoxy]-2(1H)-quinolinone (OPC-4392), was synthesized in our laboratories and compared with apomorphine, 3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP) and dopamine antagonists in a series of tests designed to characterize dopamine receptor activation and inhibition. The assertion that OPC-4392 acts as an agonist at presynaptic dopamine autoreceptors is supported by the following behavioral and biochemical observations: OPC-4392, 3-PPP and apomorphine inhibited the reserpine-induced increase in DOPA accumulation in the forebrain of mice and in the frontal cortex, limbic forebrain and striatum of rats. In addition, the gamma-butyrolactone (GBL)-induced increase in DOPA accumulation in the mouse forebrain was also inhibited by OPC-4392, 3-PPP and apomorphine. Haloperidol antagonized the inhibitory effect of OPC-4392 in both instances. The inhibitory effect of OPC-4392 on GBL-induced DOPA accumulation lasted for at least 8 hours after oral administration to mice, while that of 3-PPP and apomorphine disappeared in 4 hours after subcutaneous injection. OPC-4392 failed to increase spontaneous motor activity in reserpinized mice, enhance spontaneous ipsilateral rotation in rats with unilateral striatal kainic acid (KA) lesions, induce contralateral rotation in rats with unilateral striatal 6-hydroxydopamine (6-OHDA) lesions and inhibit 14C-acetylcholine (Ach) release stimulated by 20 mM KCl in rat striatal slices. In addition, OPC-4392 appears to block postsynaptic D2 receptors since OPC-4392, as well as dopamine antagonists, was able to inhibit stereotyped behavior and climbing behavior induced by apomorphine in mice, displace the 3H-spiroperidol binding to rat synaptosomal membranes in vitro and reverse the inhibitory effect of apomorphine on Ach release in rat striatal slices. These results suggest that OPC-4392 acts as a dopamine agonist at presynaptic autoreceptors related to dopamine synthesis and acts as dopamine antagonist at postsynaptic D2 receptors.

Autoinhibition of histamine synthesis mediated by presynaptic H3-receptors

The regulation of histamine synthesis was studied on rat brain slices or synaptosomes labeled with L-[3H]histidine. Depolarization by increased extracellular K+ concentration enhanced by about twofold the [3H]histamine formation in slices of cerebral cortex. This stimulation was also observed, although to a lesser extent, in synaptosomes from cerebral cortex and slices from the posterior hypothalamus where most histaminergic cell-bodies are located, suggesting that it may occur in nerve endings as well as in perikarya. In the presence of exogenous histamine in increasing concentrations the K+-induced stimulation was progressively reduced by up to 60-70%. The effect of exogenous histamine appears to be receptor-mediated as shown by its saturable character, high pharmacological specificity and competitive reversal by histamine antagonists. The EC50 value of histamine for synthesis reduction (0.34 +/- 0.03 microM) was similar to its EC50 value for release inhibition known to be mediated by H3-receptors. In addition, whereas mepyramine and tiotidine, two potent antagonists at H1- and H2-receptors, respectively, were poorly effective, the H3-receptor antagonists burimamide and impromidine reversed the histamine effect in an apparently competitive manner. These effects were observed in slices of cerebral cortex or posterior hypothalamus as well as in cortical synaptosomes. Furthermore, even in the absence of added histamine, H3-receptor antagonists enhanced the depolarization-induced stimulation of [3H]histamine synthesis, indicating a participation of released endogenous histamine in the synthesis control process. The potencies of H3-receptor antagonists were similar to those of these agents at presynaptic autoreceptors controlling [3H]histamine release. It is concluded that H3-receptors control not only release but also synthesis of histamine at the level of nerve endings and also, presumably, of perikarya. A relationship between the two regulatory processes, possibly via intracellular calcium, seems likely but remains to be investigated at the molecular level.