Arturo Sierra

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.

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.

Expression of dopamine receptors in the subthalamic nucleus of the rat: characterization using reverse transcriptase-polymerase chain reaction and autoradiography.

We analysed the expression of dopamine receptor subtypes in the subthalamic nucleus by means of reverse transcriptase-polymerase chain reaction. We also studied, using autoradiography, all pharmacologically characterized dopamine receptors in four subregions of the subthalamic nucleus. For comparison, dopamine receptor subtypes were also evaluated in brain regions where they are more abundant and well characterized. The radioligands used were: [3H]SCH-23390, [3H]emonapride and [3H]2-dipropylamino-7-hydroxy-1,2,3,4-tetrahydronaphthalene for dopamine D1, D2 and D3 receptors, respectively; and [3H]YM-09151-2 in the presence of raclopride for dopamine D4 receptors. Finally, we also evaluated the effect of unilateral 6-hydroxydopamine injection into the medial forebrain bundle on dopamine receptor levels expressed in the ipsilateral subthalamic nucleus. The lesion was estimated by decrease in the binding of [3H]WIN-35428, a specific dopamine transporter label. D1, D2 and D3 receptor messenger RNAs and binding sites were present in the subthalamic nucleus, but no messenger RNA for D4 receptors was found, although specific binding sites for these receptors were observed. As compared to the intact side, the 6-hydroxydopamine lesion did not change D1 receptors, increased D2 receptors, and decreased D3 receptors and the dopamine transporter. The results suggest that postsynaptic D1, D2 or D3 receptors can mediate the effect of dopamine on subthalamic nucleus neuronal activity. D4 receptors would mediate exclusively presynaptic effects. These results reinforce the idea that dopamine receptors in the subthalamic nucleus may play an important role in the physiology of the basal ganglia and in the pathophysiology of Parkinsons disease.

Inhibitory control of the GABAergic transmission in the rat neostriatum by D2 dopamine receptors

The aim of the study was to determine the role of dopamine on the GABAergic input to striatal projection neurons. Accordingly, the effect of the activation of dopamine D2-like receptors on GABA-mediated depolarizing postsynaptic potentials evoked in striatal slices by local stimulation was studied. Conventional intracellular recording techniques were used to record the synaptic responses. The experiments were done in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM) and (+)-2-amino-5-phosphonovaleric acid (40 microM) to block the participation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate/kainate and N-methyl-D-aspartate receptors in the synaptic response. The GABAergic nature of the response was assessed by its potentiation by pentobarbital (50 microM) and by its elimination by bicuculline or picrotoxin. At 100 nM, a concentration already maximal, dopamine inhibited by 55% the GABAergic synaptic response. The inhibitory effect was totally blocked by the selective antagonist of D2-like receptors, sulpiride (100 nM). The dopamine inhibition was observed only in one-third of the studied neurons and was concentration dependent (IC50 = 14 nM). The inhibition was not associated with changes in the input resistance or any other membrane property. In addition, dopamine (50 nM) reduced the frequency but not the amplitude of spontaneous, bicuculline-sensitive depolarizing postsynaptic potentials. The D2-like receptor agonist quinpirole also dose-dependently (IC50 = 10 nM) inhibited the GABAergic synaptic response. As with dopamine, the inhibition did not change the membrane properties of the studied neurons. In addition, the quinpirole induced inhibition of the GABA response was accompanied by increased paired-pulse facilitation. The results indicate that D2-like receptors located on intrinsic GABAergic terminals in the rat striatum exert an inhibitory control of the GABAergic input to striatal projection neurons. The dopaminergic effect would be translated in facilitation of the firing of the neurons upon the arrival of the cortical input.

5-Hydroxytryptamine increases spontaneous activity of subthalamic neurons in the rat.

The effect of 5-hydroxytryptamine on the spontaneous activity of neurons of the subthalamic nucleus was examined by recording the extracellular unitary activity in an in vitro slice preparation. The most frequent response to 5-hydroxytryptamine (84% of 57 neurons tested) was an increase (twofold of basal at 10 microM) of the discharge frequency. The EC50 for the 5-hydroxytryptamine-induced effect was 1.8 +/- 0.5 microM (mean +/- SEM). The response was dose-dependently blocked by the serotoninergic antagonist mianserin and was not prevented by removal of calcium ions from the perfusing buffer. These results indicate that the serotoninergic input to the rat subthalamic nucleus exerts a postsynaptic excitatory action on most neurons of the nucleus.

Activation of D1 receptors stimulates accumulation of γ-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.

D-1 receptor mediated modulation of the release of γ-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.

l-DOPA stimulates the release of [3H]γ-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.

Appearance of EMG activity and motor asymmetry after unilateral lesions of the dopaminergic innervation to the subthalamic nucleus in the rat

The effects of unilateral injections of 6-hydroxydopamine into the subthalamic nucleus on muscular rigidity and on motor behavior were examined. Muscular rigidity was judged by the appearance of electromyographical activity in the gastrocnemius-soleus muscle and motor behavior by the turning induced by DL-methamphetamine. The unilateral injection of 6-hydroxydopamine produced the appearance of electromyographical activity ipsilateral to the injected side and ipsilateral turning in response to systemic administration of DL-methamphetamine. These results suggest that the dopaminergic innervation exerts a tonic inhibitory action on the spontaneous activity of the subthalamic neurons. The loss of this action would cause muscular rigidity and motor asymmetry.

M3 muscarinic receptors mediate cholinergic excitation of the spontaneous activity of subthalamic neurons in the rat

The effect of the muscarinic antagonist on carbachol-induced increase in spontaneous activity of neurons of the subthalamic nucleus was examined by recording the extracellular unitary activity in an in vitro slice preparation. Carbachol produced (98% of the 263 neurons tested) an increase (twofold of the basal at 500 nM) of the discharge frequency. The EC50 for the carbachol-induced effect was 375 +/- 8.7 nM (mean +/- SEM). The response was blocked by muscarinic antagonists in a dose dependent manner. However, the IC50 (94 +/- 3 nM) for the M3 antagonist 4-diphenyl acetoxy-N-methyl piperidine methobromide (4-DAMP) was considerably less than the other muscarinic antagonists (M1 antagonist pirenzepine, IC50 1340 +/- 110 nM; M2 antagonist AF-DX-116, IC50 6780 +/- 690 nM). These results suggest that the cholinergic input to the rat subthalamic nucleus exerts a postsynaptic excitatory action and this effect is likely mediated via muscarinic receptor type 3.