Cannabinoid agonists stimulate [3H]GABA release in the globus pallidus of the rat when G(i) protein-receptor coupling is restricted: role of dopamine D2 receptors.

The motor effects of cannabinoids in the globus pallidus appear to be caused by increases in interstitial GABA. To elucidate the mechanism of this response, we investigated the effect of the selective cannabinoid type 1 receptor (CB1) cannabinoid agonist arachidonyl-2-chloroethylamide (ACEA) on [(3)H]GABA release in slices of the rat globus pallidus. ACEA had two effects: concentrations between 10(-8) and 10(-6) M stimulated release, whereas higher concentrations (IC(50) approximately 10(-6) M) inhibited it. Another cannabinoid agonist, WIN-55,212-2, also had bimodal effects on release. Studies of cAMP production indicate that under conditions of low G(i/o), availability the coupling of CB1 receptors with G(i/o) proteins can be changed into CB1:G(s/olf) coupling; therefore, we determined the effects of conditions that limit G(i/o) availability on [(3)H]GABA release. Blockers of G(i/o) protein interactions, pertussis toxin and N-ethylmaleimide, transformed the inhibitory effects of ACEA on GABA release into stimulation. It also has been suggested that stimulation of D2 receptors can reduce G(i/o) availability. Blocking D2 receptors with sulpiride [(S)-5-aminosulfonyl-N-[(1-ethyl-2-pyrrolidinyl)methyl]-2-methoxybenzamidersqb] or depleting dopamine with reserpine inhibited the ACEA-induced stimulation of release. Thus, the D2 dependence of stimulation is consistent with the proposal that D2 receptors reduce G(i/o) proteins available for binding to the CB1 receptor. In summary, CB1 receptor activation has dual effects on GABA release in the globus pallidus. Low concentrations stimulate release through a process that depends on activation of dopamine D2 receptors that may limit G(i/o) protein availability. Higher concentrations of cannabinoid inhibit GABA release through mechanisms that are independent of D2 receptor activation.

6-OHDA-induced hemiparkinsonism and chronic L-DOPA treatment increase dopamine D1-stimulated [(3)H]-GABA release and [(3)H]-cAMP production in substantia nigra pars reticulata of the rat.

It has been proposed that striatonigral GABAergic transmission in the substantia nigra reticulata (SNr) is enhanced during Parkinson's disease and subsequent L-DOPA treatment. To evaluate this proposal we determined the effects of activating dopamine D1 receptors on depolarization induced [(3)H]-GABA release and on [(3)H]-cAMP accumulation in slices of SNr of rats with unilateral 6-OHDA lesions with and without l-DOPA treatment. Denervation increased depolarization induced D1-stimulated [(3)H]-GABA release, while repeated L-DOPA treatment further enhanced this response. Both also enhanced the effects of forskolin on [(3)H]-cAMP production and [(3)H]-GABA release, while neither modified the stimulating effects of 8-Br-cAMP on the release. These results shown that, after 6-OHDA lesions and l-DOPA treatment, cAMP signaling is enhanced. Furthermore, the results suggest that activation of sites in the signaling cascade downstream of cAMP synthesis is not required to increase release.

Adenosine A2A receptor stimulation decreases GAT-1-mediated GABA uptake in the globus pallidus of the rat.

We examined modulation of [(3)H]GABA uptake in slices of the rat globus pallidus because stimulation of adenosine A(2A) receptors increases extracellular GABA in this structure. Pharmacological analysis showed that GAT-1 is the main transporter present in these slices. Both adenosine and the A(2A) agonist CGS 21680 reduced GABA uptake. Antagonist ZM 241385 prevented these effects. Agents that increase protein kinase A activity like forskolin and 8-bromo-cAMP also inhibited GABA uptake. The inhibition of uptake produced by these substances and by CGS 21680 was prevented by the protein kinase A blocker H-89. The protein phosphatase blocker okadaic acid reduced uptake; this effect and the response to CGS 21680 were not additive. The effective concentrations of adenosine (EC(50)=15.2microM) are within the range measured in the interstitial fluid under some physiological conditions. Thus, inhibition of uptake may be important in increasing interstitial GABA during endogenous adenosine release.

Interactions between adenosine A(2a) and dopamine D2 receptors in the control of [(3)H]GABA release in the globus pallidus of the rat.

The interactions between adenosine A(2A) receptors and dopamine D2 receptors on the modulation of depolarization-evoked [(3)H]-gamma-amino-butyric-acid release (GABA) were examined in slices of the globus pallidus of the rat. The stimulation of release caused by activation of A(2A) receptors was blocked when dopaminergic influences were eliminated with three independent methods: a) antagonism of D2 receptors with sulpiride; b) alkylation of these receptors with N-ethoxycarbonyl-2-ethoxy-1, 2-dihydroquinoline (EEDQ); c) depletion of dopamine with reserpine. In turn, activation of A(2A) receptors modified the response to stimulation of D2 receptors: the EC(50) for quinpirole increased nearly one thousand times when A(2A) receptors were stimulated. Antagonism of A(2A) receptors in the absence of added agonists inhibited [(3)H] GABA release indicating receptor occupancy by endogenous adenosine. The dopamine dependence and the large effects of activating A(2A) receptors on the potency of dopaminergic agonists clarify some of the therapeutic properties of A(2A) antagonists in parkinsonian animals and patients.

Dopamine D4 receptors inhibit depolarization-induced [3H]GABA release in the rat subthalamic nucleus.

We explored the role of dopamine D4 receptors on [3H]GABA release in the subthalamic nucleus. [3H]GABA release was evoked by high K+ in slices of the nucleus. The selective dopamine D4 receptor agonist PD168,077 (N-[[4-(2-cyanophenyl)-1-piperazynil]methyl]-3-methyl-benzamide) inhibited GABA release with greater potency (EC50=3.2 nM) than quinpirole (EC50=200 nM). SKF 21297 (6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide), a dopamine D1-like receptor agonist, had no effect. L-745,870 (3-[[4-(4-chlorophenyl)piperazin-1-yl]methyl]-1-1H-pyrollo[2,3-b] pyridine), a selective dopamine D4 receptor antagonist, reverted the quinpirole inhibition with greater potency (IC50=8.7 nM) than that of the dopamine D2/D3 receptor antagonist sulpiride and raclopride (IC50=4804 and 788 nM, respectively). Both methylphenidate and methamphetamine, dopamine reuptake blockers, inhibited by 30% high K(+)-evoked GABA release; the inhibition was blocked by L-745,870. These results show that dopamine D4 receptors modulate GABA release in the subthalamic nucleus. The results would explain how agents that increase interstitial dopamine like methylphenidate and amphethamine might control locomotor hyperactivity seen in disorders of dopamine D4 receptors.

Activation of dopamine D4 receptors modulates [3H]GABA release in slices of the rat thalamic reticular nucleus.

The thalamic reticular nucleus (nRt) is innervated by dopaminergic projections from the sustantia nigra compacta (SNc) and is rich in dopamine D4 receptors, however, the functional effects of dopamine on this structure are unknown. We examined whether the D1 receptor agonist SKF 38393, or the D2 class receptor agonist quinpirole, modify depolarization evoked Ca(2+)-dependent [3H]GABA release. SKF 38393 was without effects, whereas quinpirole inhibited [3H]GABA release with an IC50 of 81 +/- 33 nM. Dose-dependence determinations of agonists (quinpirole and PD 168, 077) and antagonists (L-745,870, U-101958, clozapine and raclopride) with different affinities for different D2 class subtype receptors showed that a D4 receptor mediates quinpirole inhibition. We used methylphenidate, an agent that acts by increasing interstitial dopamine, to determine whether endogenous dopamine modulates [3H]GABA release. Methylphenidate inhibited [3H]GABA release showing that the nRt contains sufficient endogenous dopamine to activate D4 receptors. This inhibition was completely reversed by selectively blocking D4 receptors with L-745,870 or U-101958 indicating that the catecholamine receptors that modulate GABA release are D4 receptors. Given the importance of the nRt in the control of attention, sensory processing and the generation of rhythmic activity during slow wave sleep, it is possible that abnormal nRt function may generate some of the manifestations of the disorders of dopaminergic transmission.