L-type Ca²âº channel activity determines modulation of GABA release by dopamine in the substantia nigra reticulata and the globus pallidus of the rat.

Modulation of L-type Ca²âº-channel function by dopamine is a major determinant of the rate of action potential firing by striatal medium spiny neurons. However, the role of these channels in modulating GABA release by nerve terminals in the basal ganglia is unknown. We found that depolarization-induced [³H]GABA release in both the substantia nigra reticulata and the external globus pallidus (GPe), was depressed by about 50% by either the selective L-channel dihydropyridine blocker nifedipine or the P/Q channel blocker ω-agatoxin TK. The effects of these blockers were additive and together eliminated about 90% of depolarization-induced [³H]GABA release. In addition, in the substantia nigra reticulata, dihydropyridines prevented both the stimulation of [³H]GABA release produced by dopamine D1 receptor activation and the inhibition caused by D4 receptor activation. In the GP nifedipine blocked the effects of D2 and A2(A) receptor coactivation as well as the effects of activating adenylyl cyclase with forskolin. ω-Agatoxin TK did not interfere with the action of these modulatory agents. The L-type Ca²âº-channel agonist BAYK 8644 stimulated GABA release in both substantia nigra reticulata and GP. Because dihydropyridine sensitivity is a key criterion to identify L-type Ca²âº-channel activity, our results imply that these channels are determinant of GABA release modulation by dopamine in striatonigral, striatopallidal and pallidonigral terminals.

Dopamine D4 receptor, but not the ADHD-associated D4.7 variant, forms functional heteromers with the dopamine D2S receptor in the brain.

Polymorphic variants of the dopamine D(4) receptor have been consistently associated with attention-deficit hyperactivity disorder (ADHD). However, the functional significance of the risk polymorphism (variable number of tandem repeats in exon 3) is still unclear. Here, we show that whereas the most frequent 4-repeat (D(4.4)) and the 2-repeat (D(4.2)) variants form functional heteromers with the short isoform of the dopamine D(2) receptor (D(2S)), the 7-repeat risk allele (D(4.7)) does not. D(2) receptor activation in the D(2S)-D(4) receptor heteromer potentiates D(4) receptor-mediated MAPK signaling in transfected cells and in the striatum, which did not occur in cells expressing D(4.7) or in the striatum of knockin mutant mice carrying the 7 repeats of the human D(4.7) in the third intracellular loop of the D(4) receptor. In the striatum, D(4) receptors are localized in corticostriatal glutamatergic terminals, where they selectively modulate glutamatergic neurotransmission by interacting with D(2S) receptors. This interaction shows the same qualitative characteristics than the D(2S)-D(4) receptor heteromer-mediated mitogen-activated protein kinase (MAPK) signaling and D(2S) receptor activation potentiates D(4) receptor-mediated inhibition of striatal glutamate release. It is therefore postulated that dysfunctional D(2S)-D(4.7) heteromers may impair presynaptic dopaminergic control of corticostriatal glutamatergic neurotransmission and explain functional deficits associated with ADHD.

Dopamine inhibits GABA transmission from the globus pallidus to the thalamic reticular nucleus via presynaptic D4 receptors.

The globus pallidus sends a significant GABAergic projection to the thalamic reticular nucleus. Because pallidal neurons express D4-dopamine receptors, we have explored their presence on pallidoreticular terminals by studying the effect of dopamine and D4-receptor agonists on the GABAergic transmission in the thalamic reticular nucleus. We made whole-cell recordings of inhibitory postsynaptic currents (IPSCs) and miniature inhibitory postsynaptic currents (mIPSCs) in the thalamic reticular neurons. Dopamine consistently reduced the IPSCs. The effect of dopamine was associated with paired-pulse facilitation, indicating a presynaptic location of the receptors. The effect of dopamine was also measured on the mIPSCs, reducing their frequency but not affecting their amplitude, which also suggests a presynaptic site of action. The selective D4-receptor agonist PD 168,077 also reduced the IPSCs, which was also associated with paired-pulse facilitation. In addition, this agonist reduced the frequency of the mIPSCs with no effect on their amplitude. The D4-receptor antagonist L-745,870 totally blocked the effect of the D4-receptor agonist, indicating the specificity of its effect. To verify the location of the receptors on the pallidal terminals, these were eliminated by injecting kainic acid into the globus pallidus. Kainic acid produced a drastic (80%) fall in the globus pallidus neuronal population. In this condition, the effect of the activation of D4 receptors both on the IPSCs and mIPSCs was prevented, thus indicating that the location of the receptors was on the pallidal terminals. Our results demonstrate that dopamine controls the activity of the thalamic reticular neurons by regulating the inhibitory input from the globus pallidus.

Dopamine D(1) receptor facilitation of depolarization-induced release of gamma-amino-butyric acid in rat striatum is mediated by the cAMP/PKA pathway and involves P/Q-type calcium channels.

Transmission in the "direct" pathway through the basal ganglia, which has an important role in the control of motor movement, is markedly facilitated by the concurrent activation of dopamine D(1) receptors. Consistent with this, Ca(2+)-dependent, depolarization-induced release of [(3)H]-GABA from striatal slices from rats pretreated with reserpine was greatly increased in the presence of 1 microM SKF 38393, a dopamine D(1)-like receptor agonist. The effect of SKF 38393 was mimicked by 1 mM 8-bromo-cyclic AMP (Br-cAMP) and inhibited by the protein kinase A (PKA) inhibitor H-89, mean inhibition 92% +/- 4% with 10 microM H-89 (n = 3). The effects of SKF 38393 and Br-cAMP were not additive. The stimulatory effects of SKF 38393 and Br-cAMP were practically abolished in the presence of the histamine H(3) receptor agonist immepip (1 microM). The depolarization-induced release of [(3)H]-GABA in the presence of SKF 38393 was not significantly inhibited by 5 microM nimodipine, an L-type Ca(2+) channel blocker, or by 0.3 microM omega-conotoxin MVIIA, a selective blocker of N-type channels. However, preincubation of the slices with 0.95 microM omega-agatoxin TK, a P/Q-type channel blocker, followed by washing before changing to a depolarizing medium containing SKF 38393, resulted in a marked inhibition of the stimulated release of [(3)H]-GABA, mean 68% +/- 4% (n = 3). These observations provide evidence that dopamine D(1) agonist facilitation of the depolarization-induced release of GABA from striatal terminals is mediated by the cAMP/PKA pathway and involves mainly P/Q-type Ca(2+) channels.

L-DOPA inhibits depolarization-induced [3H]GABA release in the dopamine-denervated globus pallidus of the rat: the effect is dopamine independent and mediated by D2-like receptors.

The effect of L-DOPA on [(3)H]GABA release in slices of globus pallidus from 6-OHDA-lesioned rats was studied. Release was evoked by high (15 mM) K(+). The lesion reduced dopamine content and dopamine synthesized from L-DOPA. The inhibition of DOPA decarboxylase blocked dopamine synthesis. Endogenous dopamine released by high K(+) inhibited [(3)H]GABA release in normal but not in lesioned slices. L-DOPA inhibited (IC(50) = 0.44 microM) evoked [(3)H]GABA release. The inhibition was via D2-like receptors but not mediated by dopamine. The turning behavior induced by L-DOPA methyl ester (25 mg/kg, i.p.) was not abolished by the DOPA decarboxylase inhibitor 3-hydroxybenzylhydrazine but in this condition it was abolished by sulpiride. Results suggest that L-DOPA acting as D2-like agonist inhibits GABA release in the rat globus pallidus and induces turning behavior in rats with unilateral lesions of the dopamine innervation. L-DOPA could control Parkinson's disease symptoms acting not only as dopamine precursor but also by itself.

Adenosine A1 receptors control dopamine D1-dependent [(3)H]GABA release in slices of substantia nigra pars reticulata and motor behavior in the rat.

Abnormalities in dopaminergic control of basal ganglia function play a key role in Parkinson's disease. Adenosine appears to modulate the dopaminergic control in striatum, where an inhibitory interaction between adenosine and dopamine receptors has been demonstrated. However the interaction has not been established in substantia nigra pars reticulata (SNr) where density of both receptors is high. Here we have explored the interaction between A1/D1 receptors in SNr. In SNr slices, SKF 38393, a selective D1 receptor agonist, produced a stimulation of depolarization-induced Ca(2+)-dependent [(3)H]GABA release that was inhibited by adenosine. The adenosine inhibition was abolished by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective adenosine A1 receptor antagonist. DPCPX per se enhanced GABA release, indicating inhibition of the release by endogenous adenosine. When D1 receptors were blocked with SCH 23390 or the slices were depleted of dopamine, the effect of DPCPX was suppressed, showing that activation of dopamine receptors was necessary for the adenosine inhibition. In normal slices, 2-chloro-n(6)-cyclopentyladenosine (CCPA), a selective A1 agonist, inhibited GABA release, but the inhibition was prevented by the blockade of D1 receptors with SCH 23390. Superperfusion with 8-bromo-cAMP produced a stimulation of GABA release that was not blocked by CCPA: this finding indicates that the blockade of D1 effects caused by activation of A1 receptors is specific. To see if these actions on GABA release were correlated with changes in motor behavior we studied the effect of unilateral intranigral injections of modifiers of adenosine A1 and dopamine D1 receptors in rats challenged with systemic methamphetamine. Both the A1 agonist CCPA and the D1 antagonist SCH 23390 produced ipsilateral turning whereas the A1 antagonist DPCPX caused contralateral turning. These motor effects are consistent with the findings on GABA release. The results indicate the presence of an inhibitory A1/D1 receptor interaction in SNr. The inhibition exerted by A1 adenosine receptors on GABAergic striatonigral transmission would be due exclusively to blockade of the facilitation resulting from activation of D1 dopamine receptors. The data permit to better understand the action of adenosine antagonists in the treatment of Parkinson's disease.

Intrapallidal dopamine restores motor deficits induced by 6-hydroxydopamine in the rat.

To explore whether dopamine deficits in the globus pallidus have a role in generating the motor symptoms of Parkinson's disease, we examined the effects of selective intrapallidal administration of dopamine or its antagonists in rats unilaterally lesioned with 6-hydroxydopamine into the medial forebrain bundle. Either the turning behavior induced by apomorphine or the deficit in the performance of a skilled forelimb-reaching task was used as assay for drug action. Microinjection of either the D2 receptor antagonist, sulpiride, or the D1 receptor antagonist, SCH-23390, into the dopamine-denervated pallidum significantly reduced apomorphine induced turning. In animals trained to perform a skilled forelimb-reaching task, 6-OHDA lesions caused a marked motor deficit in the contralateral forelimb. Intrapallidal dopamine applied either intermittently or continuously, restored up to 50% of the motor performance. Continuous application promoted a motor recovery that outlasted dopamine administration. These results show that lack of dopamine in the GP plays an important role in generating the motor symptoms caused by lesion of dopaminergic pathways. Moreover, motor recovery was produced by selectively injecting dopamine into the globus pallidus.

Histamine H(3) receptor-mediated inhibition of depolarization-induced, dopamine D(1) receptor-dependent release of [(3)H]-gamma-aminobutryic acid from rat striatal slices.

1. A study was made of the regulation of [(3)H]-gamma-aminobutyric acid ([(3)H]-GABA) release from slices of rat striatum by endogenous dopamine and exogenous histamine and a histamine H(3)-agonist. Depolarization-induced release of [(3)H]-GABA was Ca(2+)-dependent and was increased in the presence of the dopamine D(2) receptor family antagonist, sulpiride (10 microM). The sulpiride-potentiated release of [(3)H]-GABA was strongly inhibited by the dopamine D(1) receptor family antagonist, SCH 23390 (1 microM). Neither antagonist altered basal release. 2. The 15 mM K(+)-induced release of [(3)H]-GABA in the presence of sulpiride was inhibited by 100 microM histamine (mean inhibition 78+/-3%) and by the histamine H(3) receptor-selective agonist, immepip, 1 microM (mean inhibition 81+/-5%). The IC(50) values for histamine and immepip were 1.3+/-0.2 microM and 16+/-2 nM, respectively. The inhibitory effects of histamine and immepip were reversed by the H(3) receptor antagonist, thioperamide, 1 microM. 3. The inhibition of 15 mM K(+)-induced [(3)H]-GABA release by immepip was reversed by the H(3) receptor antagonist, clobenpropit, K(d) 0.11+/-0.04 nM. Clobenpropit alone had no effect on basal or stimulated release of [(3)H]-GABA. 4. Elevated K(+) caused little release of [(3)H]-GABA from striatal slices from reserpinized rats, unless the D(1) partial agonist, R(+)-SKF 38393, 1 microM, was also present. The stimulated release in the presence of SKF 38393 was reduced by 1 microM immepip to the level obtained in the absence of SKF 38393. 5. These observations demonstrate that histamine H(3) receptor activation strongly inhibits the dopamine D(1) receptor-dependent release of [(3)H]-GABA from rat striatum; primarily through an interaction at the terminals of GABA neurones.

Regulated release of serotonin from axonal growth cones isolated from the fetal rat brain.

In the present work we propose an hypothetical model related to a molecular recognizing system for serotonin in isolated growth cone particles. This model is supported by previous results from our laboratory plus new ones which show that growth cones release serotonin tonically and such release can be stimulated by potassium in a calcium-dependent manner. The present results, together with other author's data, suggest a physiological basis for the putative role of serotonin as a trophic factor during nervous system development.

D2 receptor-mediated inhibition of GABA release by endogenous dopamine in the rat globus pallidus.

Attempting to better understand the role of the dopaminergic innervation in the rat globus pallidus, we examined here whether or not endogenous dopamine modulates the release of [3H]GABA in superfused pallidal slices. The superfusion medium contained elevated (15 mM) potassium. The release of endogenous dopamine was induced by the dopamine releaser drug, methamphetamine. Methamphetamine (100 microM) inhibited by 46% the release of [3H]GABA. Methamphetamine inhibition was completely blocked by reserpinization of the rats. It was also completely blocked by the D2 dopamine receptor antagonist sulpiride (10 microM). Sulpiride alone caused a 105% increase in GABA release. The increase was not observed in slices from reserpinized rats. Quinpirole (10 microM), a D2 dopamine receptor agonist, inhibited (43%) [3H]GABA release. The results suggest that endogenous dopamine exerts an inhibitory effect on GABA release in the rat globus pallidus. The effect is mediated by D2 receptors presumably located on striatopallidal axon terminals.

Reciprocal interaction between glutamate and dopamine in the pars reticulata of the rat substantia nigra: a microdialysis study.

We studied the interactions between glutamate and dopamine in the pars reticulata of the substantia nigra by using microdialysis in unanaesthetized rats. Increased extracellular levels of glutamate in the pars reticulata were obtained by microinjecting the muscarinic agonist carbachol into the ipsilateral subthalamic nucleus. The increase of glutamate levels was followed by increments in extracellular levels of dopamine and GABA. Increased levels of the three neurotransmitters were also observed during the administration of N-methyl-D-aspartate through the microdialysis probe. The increase in glutamate and GABA caused by N-methyl-D-aspartate was blocked by SCH 23390, a selective D1 antagonist. However, the D1 antagonist did not prevent the increase in dopamine levels. The selective D1 agonist SKF 38393, added to the microdialysis probe, increased the levels of the three neurotransmitters. However, after the lesion of the subthalamic nucleus with kainic acid, SKF 38393 increased only the level of GABA but not those of glutamate and dopamine. In addition, the lesion of the subthalamic nucleus produced a drastic (80%) fall in the extracellular levels of glutamate. These data suggest that glutamate, through N-methyl-d-aspartate receptors, stimulates the release of dopamine from dopaminergic dendrites present in the substantia nigra pars reticulata, and that dopamine in turn stimulates the release of glutamate and GABA. Both effects are mediated by D1 dopamine receptors present on subthalamonigral and striatonigral axon terminals, respectively.

Histamine H3 receptor activation selectively inhibits dopamine D1 receptor-dependent [3H]GABA release from depolarization-stimulated slices of rat substantia nigra pars reticulata.

The release of [3H]GABA from slices of rat substantia nigra pars reticulata induced by increasing extracellular K+ from 6 to 15 mM in the presence of 10 microM sulpiride was inhibited by 73 +/- 3% by 1 microM SCH 23390, consistent with a large component of release dependent upon D1 receptor activation. The histamine H3 receptor-selective agonist immepip (1 microM) and the non-selective agonist histamine (100 microM) inhibited [3H]GABA release by 78 +/- 2 and 80 +/- 2%, respectively. The inhibition by both agonists was reversed by the H3 receptor antagonist thioperamide (1 microM). However, in the presence of 1 microM SCH 23390 depolarization-induced release of [3H]GABA was not significantly decreased by 1 microM immepip. In rats depleted of dopamine by pretreatment with reserpine, immepip no longer inhibited control release of [3H]GABA, but in the presence of 1 microM SKF 38393, which produced a 7 +/- 1-fold stimulation of release, immepip reduced the release to a level not statistically different from that in the presence of immepip alone. Immepip (1 microM) also inhibited the depolarization-induced release of [3H]dopamine from substantia nigra pars reticulata slices, by 38 +/- 3%. The evidence is consistent with the proposition that activation of histamine H3 receptors leads to the selective inhibition of the component of depolarization-induced [3H]GABA release in substantia nigra pars reticulata slices which is dependent upon D1 receptor activation. This appears to be largely an action at the terminals of the striatonigral GABA projection neurons, which may be enhanced by a partial inhibition of dendritic [3H]dopamine release.

D-1 receptor mediated modulation of the release of gamma-aminobutyric acid by endogenous dopamine in the basal ganglia of the rat.

1. Presynaptic D1 receptors are present on GABAergic terminals of neostriatal projections. 2. By activating these receptors, exogenous dopamine enhances the release of GABA. 3. Here the authors have explored whether endogenous dopamine was also able to activate the receptors, thus enhancing GABA release. 4. The effect of methamphetamine, a dopamine releaser, on the release of tritiated GABA was studied in slices of substantia nigra pars reticulata, entopeduncular nucleus and caudate-putamen, targets of the striatal projections. 5. Methamphetamine enhanced the release of the label. However the enhancement required an intact dopaminergic innervation, since it was lost in slices isolated from rats with 6-hydroxydopamine-induced lesions of the dopaminergic nigrostriatal system. 6. The activation of the receptors by endogenous dopamine was also judged by the effect of the selective D1 antagonist SCH 23390 in potassium depolarized slices. By preventing activation of the receptors by dopamine released as result of depolarization, the antagonist reduced GABA release. In 6-OHDA lesioned slices, no reduction was observed, even though the slices were also depolarized. 7. The results indicate that endogenous dopamine enhances GABA release from striatal terminals in the pars reticulata of the substantia nigra, entopeduncular nucleus and caudate-putamen. This would facilitate GABAergic neurotransmission. 8. The study suggests that the function of DA in the basal ganglia is widespread, modulating not only the firing of the striatal efferent neurons but also the transmission of the fired impulses across synapses in the target nuclei of these neurons.

Activation of D1 receptors stimulates accumulation of gamma-aminobutyric acid in slices of the pars reticulata of 6-hydroxydopamine-lesioned rats.

D1 dopamine receptors are present on terminals of striatal neurons to the pars reticulata of the substantia nigra in the rat. Here we have studied the effect of the activation of these receptors on the synthesis of gamma-aminobutyric acid (GABA) in slices of the pars reticulata of the substantia nigra isolated from 6-hydroxydopamine-lesioned rats. The synthesis was judged by the accumulation of GABA after inhibiting GABA transaminase with aminooxyacetic acid. Both dopamine and SCH 23390, a D1 agonist, stimulated the synthesis. The effect of both compounds was blocked by SCH 23390, a D1 antagonist, but not by sulpiride, a D2 antagonist. In the absence of receptor activation, the synthesis was very slow. The results suggest a trophic influence of dopamine upon the synthesis of GABA via D1 receptors.

L-dopa stimulates the release of [3H]gamma-aminobutyric acid in the basal ganglia of 6-hydroxydopamine lesioned rats.

L-DOPA stimulated the K(+)-induced [3H]GABA (gamma-aminobutyric acid) release from slices of substantia nigra pars reticulata, entopeduncular nucleus, globus pallidus and caudate-putamen isolated from the ipsilateral side of 6-hydroxydopamine-lesioned rats, but the release from ipsilateral subthalamic slices was not affected. In substantia nigra, L-DOPA stimulation (EC50 = 1 microM) of [3H]GABA release was dose-dependently blocked (IC50 = 0.1 microM for the stimulation caused by 10 microM L-DOPA) by the D1 antagonist SCH 23390, but was not affected by (-)-sulpiride, a D2 antagonist. SCH 23390 also blocked the stimulation in the other nuclei. The DOPA decarboxylase inhibitor NSD-1015 (500 microM) did not prevent the stimulation induced by L-DOPA in all of the studied nuclei. The results suggest that L-DOPA is able to activate D1 receptors located on the terminals of striatal projections via the dopamine formed by a decarboxylation mediated by an NSD-1015-resistant enzyme. Activation of the presynaptic D1 receptors results in stimulation of GABA release.

Activation of D1 dopamine receptors stimulates the release of GABA in the basal ganglia of the rat.

Here we have explored whether dopamine is able to modulate the release of gamma-aminobutyric acid (GABA) from striatal terminals to substantia nigra pars reticulata, entopeduncular nucleus, globus pallidus and caudate-putamen. The type of dopamine receptors involved was assessed by the blocking effect of either SCH 23390 (D1 antagonist) or (-)-sulpiride (D2 antagonist) of the dopamine effect. Dopamine stimulated (EC50 3.2 microM) the depolarization-induced release of [3H]GABA from slices isolated from all of the above mentioned nuclei. SCH 23390 dose-dependently blocked the dopamine stimulation, but (-)-sulpiride did not show any blocking effect. The results suggest that dopamine via D1 receptors modulates the release of GABA from striatal GABAergic terminals.

Presynaptic modulation of the release of GABA by GABAA receptors in pars compacta and by GABAB receptors in pars reticulata of the rat substantia nigra.

The effect of GABA agonists and antagonists on K+-stimulated [3H]GABA release was studied to assess how presynaptic GABA receptors modulate GABA release. The release was affected in a quite different manner in the pars compacta and in the pars reticulata. Muscimol markedly inhibited the release from the pars compacta but had no effect on the release from the pars reticulata. Baclofen inhibited the release from the pars reticulata without affecting the release from the pars compacta. Bicuculline itself facilitated the release from the pars compacta but inhibited the release from the pars reticulata. Picrotoxin facilitated the release from the pars compacta and had no effect in the pars reticulata. The results suggest that the release of GABA from GABAergic terminals in the substantia nigra of the rat brain is modulated by GABAA autoreceptors in the pars compacta and by GABAB receptors in the pars reticulata.