Effects Of D2 Receptor Activation Research Articles

Subtype-specific effects of dopaminergic D2 receptor activation on synaptic trains in layer V pyramidal neurons in the mouse prefrontal cortex.

In humans, prefrontal cortical areas are known to support executive functions. In mice, these functions are mediated by homologous regions in the medial prefrontal cortex (mPFC). Executive processes are critically dependent on optimal levels of dopamine (DA), but the cellular mechanisms of DA modulation are incompletely understood. Stable patterns of neuronal activity may be sensitive to frequency‐dependent changes in synaptic transmission. We characterized the effects of D2 receptor (D2R) activation on short‐term excitatory postsynaptic potential (EPSP) dynamics evoked at varying frequencies in the two subtypes of layer V pyramidal neurons in mouse mPFC. We isolated NMDA receptor and non‐NMDA receptor‐mediated components of EPSP trains evoked by stimulating fibers within layer V or layer I. All significant effects of D2 receptor activation were confined to type I (corticopontine) cells. First, we found that with layer I stimulation, D2R activation reduces the amplitude of NMDAR‐mediated EPSPs, with no effect on facilitation or depression of these responses at lower frequencies, but leading to facilitation with high frequency stimulation. Further, the non‐NMDA component also underwent synaptic depression at low frequencies. Second, with layer V stimulation, D2R activation had no effect on NMDA or non‐NMDA receptor‐mediated EPSP components. Overall, our results suggest that D2R activation may modulate memory functions by inhibiting ‘top‐down’ influences from apical tuft inputs activated at low frequencies, while promoting ‘top‐down’ influences from inputs activated at higher frequencies. These data provide further insight into mechanisms of dopamine's modulation of executive functions.

A2A-D2 receptor-receptor interaction modulates gliotransmitter release from striatal astrocyte processes.

Evidence for striatal A2A-D2 heterodimers has led to a new perspective on molecular mechanisms involved in schizophrenia and Parkinson's disease. Despite the increasing recognition of astrocytes' participation in neuropsychiatric disease vulnerability, involvement of striatal astrocytes in A2A and D2 receptor signal transmission has never been explored. Here, we investigated the presence of D2 and A2A receptors in isolated astrocyte processes prepared from adult rat striatum by confocal imaging; the effects of receptor activation were measured on the 4-aminopyridine-evoked release of glutamate from the processes. Confocal analysis showed that A2A and D2 receptors were co-expressed on the same astrocyte processes. Evidence for A2A-D2 receptor-receptor interactions was obtained by measuring the release of the gliotransmitter glutamate: D2 receptors inhibited the glutamate release, while activation ofA2A receptors, per se ineffective, abolished the effect ofD2 receptor activation. The synthetic D2 peptide VLRRRRKRVN corresponding to the receptor region involved in electrostatic interaction underlying A2A-D2 heteromerization abolished the ability of the A2A receptor to antagonize the D2 receptor-mediated effect. Together, the findings are consistent with heteromerization of native striatal astrocytic A2A-D2 receptors that via allosteric receptor-receptor interactions could play a role in the control of striatal glutamatergic transmission. These new findings suggest possible new pathogenic mechanisms and/or therapeutic approaches to neuropsychiatric disorders.

Evidence of D2 receptor expression in the nucleus incertus of the rat

The nucleus incertus (NI), located in the caudal brainstem, mainly consists of GABAergic neurons with widespread projections across the brain. It is the chief source of relaxin-3 in the mammalian brain and densely expresses corticotropin-releasing factor type 1 (CRF1) receptors. Several other neurotransmitters, peptides and receptors are reportedly expressed in the NI. In the present investigation, we show the expression of dopamine type-2 (D2) receptors in the NI by reverse transcriptase-polymerase chain reaction (RT-PCR), western blotting (WB) and immunofluorescence (IF). RT-PCR did not show expression of D3 receptors. D2 receptor short isoform (D2S)-like, relaxin-3, CRF1/2 receptor and NeuN immunoreactivity were co-expressed in the cells of the NI. Behavioural effects of D2 receptor activation by intra-NI infusion of quinpirole (a D2/D3 agonist) were evaluated. Hypolocomotion was observed in home cage monitoring system (LABORAS) and novel environment-induced suppression of feeding behavioural paradigms. Thus the D2 receptors expressed in the NI are likely to play a role in locomotion. Based on its strong bidirectional connections to the median raphe and interpeduncular nuclei, the NI was predicted to play a role in modulating behavioural activity and the present results lend support to this hypothesis. This is the first evidence of expression of a catecholamine receptor, D2-like immunoreactivity, in the NI.

LP17: Dosage-dependent effect of D2 receptor activation on motor cortex plasticity in humans

LP17: Dosage-dependent effect of D2 receptor activation on motor cortex plasticity in humans

The Dopamine D2 Receptor Gene in Lamprey, Its Expression in the Striatum and Cellular Effects of D2 Receptor Activation

All basal ganglia subnuclei have recently been identified in lampreys, the phylogenetically oldest group of vertebrates. Furthermore, the interconnectivity of these nuclei is similar to mammals and tyrosine hydroxylase-positive (dopaminergic) fibers have been detected within the input layer, the striatum. Striatal processing is critically dependent on the interplay with the dopamine system, and we explore here whether D2 receptors are expressed in the lamprey striatum and their potential role. We have identified a cDNA encoding the dopamine D2 receptor from the lamprey brain and the deduced protein sequence showed close phylogenetic relationship with other vertebrate D2 receptors, and an almost 100% identity within the transmembrane domains containing the amino acids essential for dopamine binding. There was a strong and distinct expression of D2 receptor mRNA in a subpopulation of striatal neurons, and in the same region tyrosine hydroxylase-immunoreactive synaptic terminals were identified at the ultrastructural level. The synaptic incidence of tyrosine hydroxylase-immunoreactive boutons was highest in a region ventrolateral to the compact layer of striatal neurons, a region where most striatal dendrites arborise. Application of a D2 receptor agonist modulates striatal neurons by causing a reduced spike discharge and a diminished post-inhibitory rebound. We conclude that the D2 receptor gene had already evolved in the earliest group of vertebrates, cyclostomes, when they diverged from the main vertebrate line of evolution (560 mya), and that it is expressed in striatum where it exerts similar cellular effects to that in other vertebrates. These results together with our previous published data (Stephenson-Jones et al. 2011, 2012) further emphasize the high degree of conservation of the basal ganglia, also with regard to the indirect loop, and its role as a basic mechanism for action selection in all vertebrates.

Altered profile and D2-dopamine receptor modulation of high voltage-activated calcium current in striatal medium spiny neurons from animal models of Parkinson's disease

In the present work we analyzed the profile of high voltage-activated (HVA) calcium (Ca2+) currents in freshly isolated striatal medium spiny neurons (MSNs) from rodent models of both idiopathic and familial forms of Parkinson's disease (PD). MSNs were recorded from reserpine-treated and 6-hydroxydopamine (6-OHDA)-lesioned rats, and from DJ-1 and PINK1 (PTEN induced kinase 1) knockout (−/−) mice. Our analysis showed no significant changes in total HVA Ca2+ current. However, we recorded a net increase in the L-type fraction of HVA Ca2+ current in dopamine-depleted rats, and of both N- and P-type components in DJ-1−/− mice, whereas no significant change in Ca2+ current profile was observed in PINK1−/− mice. Dopamine modulates HVA Ca2+ channels in MSNs, thus we also analyzed the effect of D1 and D2 receptor activation. The effect of the D1 receptor agonist SKF 83822 on Ca2+ current was not significantly different among MSNs from control animals or PD models. However, in both dopamine-depleted rats and DJ-1−/− mice the D2 receptor agonist quinpirole inhibited a greater fraction of HVA Ca2+ current than in the respective controls. Conversely, in MSNs from PINK1−/− mice we did not observe alterations in the effect of D2 receptor activation. Additionally, in both reserpine-treated and 6-OHDA-lesioned rats, the effect of quinpirole was occluded by the selective L-type Ca2+ channel blocker nifedipine, while in DJ-1−/− mice it was mostly occluded by ω-conotoxin GVIA, blocker of N-type channels. These results demonstrate that both dopamine depletion and DJ-1 deletion induce a rearrangement in the HVA Ca2+ channel profile, specifically involving those channels that are selectively modulated by D2 receptors.

Dopamine D2 and Adenosine A2A Receptors Regulate NMDA-Mediated Excitation in Accumbens Neurons Through A2A–D2 Receptor Heteromerization

Bursting activity of striatal medium spiny neurons results from membrane potential oscillations between a down- and an upstate that could be regulated by G-protein-coupled receptors. Among these, dopamine D(2) and adenosine A(2A) receptors are highly enriched in striatal neurons and exhibit strong interactions whose physiological significance and molecular mechanisms remain partially unclear. More particularly, respective involvements of common intracellular signaling cascades and A(2A)-D(2) receptor heteromerization remain unknown. Here we show, by performing perforated-patch-clamp recordings on brain slices and loading competitive peptides, that D(2) and A(2A) receptors regulate the induction by N-methyl-D-aspartate of a depolarized membrane potential plateau through mechanisms relying upon specific protein-protein interactions. Indeed, D(2) receptor activation abolished transitions between a hyperpolarized resting potential and a depolarized plateau potential by regulating the Ca(V)1.3a calcium channel activity through interactions with scaffold proteins Shank1/3. Noticeably, A(2A) receptor activation had no effect per se but fully reversed the effects of D(2) receptor activation through a mechanism in which A(2A)-D(2) receptors heteromerization is strictly mandatory, demonstrating therefore a first direct physiological relevance of these heteromers. Our results show that membrane potential transitions and firing patterns in striatal neurons are tightly controlled by D(2) and A(2A) receptors through specific protein-protein interactions including A(2A)-D(2) receptors heteromerization.

Absence of coupling between D2 dopamine receptors and calcium channels in lactotrophs from cycling female rats.

Recent evidence suggests that an important mechanism underlying the inhibition of PRL secretion by dopamine in the anterior pituitary is a direct inhibition of current through voltage-gated calcium channels. An alternative mechanism involves the activation of G protein-coupled potassium channels by D2 receptor activation, subsequent hyperpolarization of the lactotroph membrane, and an indirect inhibition of calcium influx as spontaneous electrical activity is reduced. Using patch voltage clamp methods, we have reexamined the effect of D2 receptor activation on calcium currents (ICa) in pituitary cells from normal cycling female rats and in GH4Cl pituitary tumor cells expressing cloned D2 receptors. Furthermore, we have examined secretory responses using a single cell immunoblot method. Dopamine (0.1-10 microM) failed to significantly inhibit ICa in either GH4Cl cells or normal female lactotrophs. Similarly, the D2 agonist quinpirole (20-100 microM) did not reduce ICa in lactotrophs. No responses to D2 agonists were seen when barium was substituted for calcium or when experiments were performed using the nystatin-permeabilized patch technique to avoid loss of intracellular macromolecules. Quinpirole also failed to inhibit ICa in lactotrophs isolated from lactating female rats. We have thus far been unable to observe a significant inhibition of ICa by activation of D2 receptors. PRL secretion assessed by immunoblotting methods was dramatically inhibited by quinpirole at normal (5 mM) extracellular K+. However, in elevated (50 mM) K+ that depolarizes the cells and activates calcium channels, quinpirole produced only a very modest inhibition of secretion. We conclude that direct inhibition of ICa by D2 receptor activation is not a major mechanism underlying the dopaminergic inhibition of PRL, secretion in normal female lactotrophs.

Absence of coupling between D2 dopamine receptors and calcium channels in lactotrophs from cycling female rats

Recent evidence suggests that an important mechanism underlying the inhibition of PRL secretion by dopamine in the anterior pituitary is a direct inhibition of current through voltage-gated calcium channels. An alternative mechanism involves the activation of G protein-coupled potassium channels by D2 receptor activation, subsequent hyperpolarization of the lactotroph membrane, and an indirect inhibition of calcium influx as spontaneous electrical activity is reduced. Using patch voltage clamp methods, we have reexamined the effect of D2 receptor activation on calcium currents (ICa) in pituitary cells from normal cycling female rats and in GH4Cl pituitary tumor cells expressing cloned D2 receptors. Furthermore, we have examined secretory responses using a single cell immunoblot method. Dopamine (0.1-10 microM) failed to significantly inhibit ICa in either GH4Cl cells or normal female lactotrophs. Similarly, the D2 agonist quinpirole (20-100 microM) did not reduce ICa in lactotrophs. No responses to D2 agonists were seen when barium was substituted for calcium or when experiments were performed using the nystatin-permeabilized patch technique to avoid loss of intracellular macromolecules. Quinpirole also failed to inhibit ICa in lactotrophs isolated from lactating female rats. We have thus far been unable to observe a significant inhibition of ICa by activation of D2 receptors. PRL secretion assessed by immunoblotting methods was dramatically inhibited by quinpirole at normal (5 mM) extracellular K+. However, in elevated (50 mM) K+ that depolarizes the cells and activates calcium channels, quinpirole produced only a very modest inhibition of secretion. We conclude that direct inhibition of ICa by D2 receptor activation is not a major mechanism underlying the dopaminergic inhibition of PRL, secretion in normal female lactotrophs.

Potassium Channel Blockers Inhibit D2 Dopamine, but Not A1Adenosine, Receptor‐Mediated Inhibition of Striatal Dopamine Release

D2 dopamine autoreceptors and A1 adenosine heteroreceptors inhibit the evoked release of dopamine from rat striatum. We examined the role of potassium channels in this modulation by determining the effects of two potassium channel blockers, 4-aminopyridine and tetraethylammonium, on the modulation of electrically stimulated release of endogenous dopamine from rat striatal slices. Maximally effective concentrations of the D2 dopamine receptor agonist N-0437 (10 nM) and of adenosine (50 microM) caused a 30% inhibition of evoked dopamine overflow, and their effects were additive. When coperfused with N-0437, both 4-aminopyridine and tetraethylammonium blocked the inhibition caused by N-0437 in a dose-dependent manner. 4-Aminopyridine was approximately three orders of magnitude more potent than tetraethylammonium, with complete blockade occurring at 3 microM and 1 mM, respectively. Binding experiments confirmed that neither 4-aminopyridine nor tetraethylammonium was a direct-acting D2 dopamine receptor antagonist at the concentration necessary to block the release-modulatory effect of D2 receptor activation. In contrast, the inhibitory modulation produced by adenosine was not affected by 4-aminopyridine (30 microM) or tetraethylammonium (1 mM). These results suggest that D2 dopamine and A1 adenosine receptors inhibit dopamine release in the striatum by different mechanisms. D2 dopamine autoreceptor action appears to involve potassium channels, whereas A1 adenosine receptor action does not.