Selective D4 Receptor Agonist Research Articles

Targeting hypersensitive corticostriatal terminals in restless legs syndrome.

The first aim was to demonstrate a previously hypothesized increased sensitivity of corticostriatal glutamatergic terminals in the rodent with brain iron deficiency (BID), a pathogenetic model of restless legs syndrome (RLS). The second aim was to determine whether these putative hypersensitive terminals could constitute a significant target for drugs effective in RLS, including dopamine agonists (pramipexole and ropinirole) and α2 δ ligands (gabapentin). A recently introduced in vivo optogenetic-microdialysis approach was used, which allows the measurement of the extracellular concentration of glutamate upon local light-induced stimulation of corticostriatal glutamatergic terminals. The method also allows analysis of the effect of local perfusion of compounds within the same area being sampled for glutamate. BID rats showed hypersensitivity of corticostriatal glutamatergic terminals (lower frequency of optogenetic stimulation to induce glutamate release). Both hypersensitive and control glutamatergic terminals were significant targets for locally perfused pramipexole, ropinirole, and gabapentin, which significantly counteracted optogenetically induced glutamate release. The use of selective antagonists demonstrated the involvement of dopamine D4 and D2 receptor subtypes in the effects of pramipexole. Hypersensitivity of corticostriatal glutamatergic terminals can constitute a main pathogenetic mechanism of RLS symptoms. Selective D4 receptor agonists, by specifically targeting these terminals, should provide a new efficient treatment with fewer secondary effects. Ann Neurol 2017;82:951-960.

Neuregulin and Dopamine D4 Receptors Contribute Independently to Depotentiation of Schaffer Collateral LTP by Temperoammonic Path Stimulation.

Prior studies have found that dopamine (DA), acting at D4 receptors, and neuregulin (NRG), likely acting at ErbB4 receptors, are involved in a form of depotentiation of long-term potentiation (LTP) at Schaffer collateral (SC) synapses in the hippocampus. Furthermore, DA and NRG actions are intertwined in that NRG induces DA release. We previously found that low-frequency stimulation (LFS) of temperoammonic (TA) inputs to area CA1 also depotentiates previously established SC LTP through a complex signaling pathway involving endocannabinoids, GABA, adenosine, and mitogen-activated protein kinases (MAPKs), but not glutamate. In the present studies, we found that TA-induced SC depotentiation in hippocampal slices from Sprague-Dawley albino rats also involves activation of both D4 receptors and NRG-activated ErbB receptors, but that the roles of these two modulator systems are independent with D4 receptor antagonism failing to alter chemical depotentiation by NRG1β. Furthermore, a selective D4 receptor agonist was unable to depotentiate SC LTP when administered alone, suggesting that D4 receptor activation is necessary but not sufficient for TA-induced SC depotentiation. Chemical depotentiation by NRG1β was inhibited by a Pan-ErbB antagonist and by picrotoxin (PTX), an antagonist of GABA-A receptors (GABAARs), indicating that NRG likely promotes SC depotentiation via effects on GABA and interneurons. These findings have implications for understanding the role of DA and NRG in cognitive dysfunction associated with neuropsychiatric illnesses.

Role of dopamine D4 receptors in copulatory behavior: Studies with selective D4 agonists and antagonists in male rats

Dopamine influences the anticipatory and consummatory phases of sexual behavior, by acting on receptors of the D2 family (D2, D3 and D4) and in particular of the D2 subtype, although evidence for a role of D4 receptors in erectile function and copulatory behavior is also available. In order to clarify such a role of D4 receptors, the effect of selective D4 receptor agonists and antagonists on copulatory behavior of sexually potent male rats in classic copulation tests with a receptive female, was compared with that of apomorphine and haloperidol, a classic dopamine receptor agonist and antagonist, respectively. PD-168,077 (0.05–0.2mg/kg) and ABT-724 (0.01–0.04mg/kg), two selective D4 receptor agonists, given subcutaneously, improved dose-dependently copulatory behavior as shown by the decrease of mount frequency and post ejaculatory interval induced by PD-168,077, and of mount frequency, ejaculation latency, post ejaculatory and inter intromission intervals induced by ABT-724, and by the increase of ejaculation frequency and copulatory efficacy induced by both drugs. Conversely, L-745,870 (1–5mg/kg), a selective D4 receptor antagonist, given intraperitoneally, impaired dose-dependently copulatory behavior, as shown by the increase in intromission and ejaculation latencies, mount frequency, post ejaculatory interval and the decrease in ejaculation frequency and copulatory efficacy induced by this drug. L-745,870 (5mg/kg) administered before PD-168,077 (0.2mg/kg) or ABT-724 (0.04mg/kg), also abolished completely the facilitatory effects of both PD-168,077 and ABT-724 on sexual behavior. These results confirm the involvement of D4 receptors in specific aspects of male rat copulatory behavior that overlap only partially with those influenced by apomorphine and haloperidol.

Dopamine D4 receptor excitation of lateral habenula neurons via multiple cellular mechanisms.

Glutamatergic lateral habenula (LHb) output communicates negative motivational valence to ventral tegmental area (VTA) dopamine (DA) neurons via activation of the rostromedial tegmental nucleus (RMTg). However, the LHb also receives a poorly understood DA input from the VTA, which we hypothesized constitutes an important feedback loop regulating DA responses to stimuli. Using whole-cell electrophysiology in rat brain slices, we find that DA initiates a depolarizing inward current (I(DAi)) and increases spontaneous firing in 32% of LHb neurons. I(DAi) was also observed upon application of amphetamine or the DA uptake blockers cocaine or GBR12935, indicating involvement of endogenous DA. I(DAi) was blocked by D4 receptor (D4R) antagonists (L745,870 or L741,742), and mimicked by a selective D4R agonist (A412997). I(DAi) was associated with increased whole-cell conductance and was blocked by Cs+ or a selective blocker of hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channel, ZD7288. I(DAi) was also associated with a depolarizing shift in half-activation voltage for the hyperpolarization-activated cation current (Ih) mediated by HCN channels. Recordings from LHb neurons containing fluorescent retrograde tracers revealed that I(DAi) was observed only in cells projecting to the RMTg and not the VTA. In parallel with direct depolarization, DA also strongly increased synaptic glutamate release and reduced synaptic GABA release onto LHb cells. These results demonstrate that DA can excite glutamatergic LHb output to RMTg via multiple cellular mechanisms. Since the RMTg strongly inhibits midbrain DA neurons, activation of LHb output to RMTg by DA represents a negative feedback loop that may dampen DA neuron output following activation.

Regulation of Inhibitory Synapses by Presynaptic D4 Dopamine Receptors in Thalamus

Dopamine (DA) receptors are the principal targets of drugs used in the treatment of schizophrenia. Among the five DA receptor subtypes, the D(4) subtype is of particular interest because of the relatively high affinity of the atypical neuropleptic clozapine for D(4) compared with D(2) receptors. GABA-containing neurons in the thalamic reticular nucleus (TRN) and globus pallidus (GP) express D(4) receptors. TRN neurons receive GABAergic afferents from globus pallidus (GP), substantia nigra pars reticulata (SNr), and basal forebrain as well as neighboring TRN neuron collaterals. In addition, TRN receives dopaminergic innervations from substantia nigra pars compacta (SNc); however, the role of D(4) receptors in neuronal signaling at inhibitory synapses is unknown. Using whole cell recordings from in vitro pallido-thalamic slices, we demonstrate that DA selectively suppresses GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) evoked by GP stimulation. The D(2)-like receptor (D(2,3,4)) agonist, quinpirole, and selective D(4) receptor agonist, PD168077, mimicked the actions of DA. The suppressive actions of DA and its agonists were associated with alterations in paired pulse ratio and a decrease in the frequency of miniature IPSCs, suggesting a presynaptic site of action. GABA(A) receptor agonist, muscimol, induced postsynaptic currents in TRN neurons were unaltered by DA or quinpirole, consistent with the presynaptic site of action. Finally, DA agonists did not alter intra-TRN inhibitory signaling. Our data demonstrate that the activation of presynaptic D(4) receptors regulates GABA release from GP efferents but not TRN collaterals. This novel and selective action of D(4) receptor activation on GP-mediated inhibition may provide insight to potential functional significance of atypical antipsychotic agents. These findings suggest a potential heightened TRN neuron activity in certain neurological conditions, such as schizophrenia and attention deficit hyperactive disorders.

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 D4 Receptors Regulate GABAA Receptor Trafficking via an Actin/Cofilin/Myosin-dependent Mechanism

The GABA(A) receptor-mediated inhibitory transmission in prefrontal cortex (PFC) is implicated in cognitive processes such as working memory. Our previous study has found that GABA(A)R current is subject to the regulation of dopamine D(4) receptors, a PFC-enriched neuromodulator critically involved in various mental disorders associated with PFC dysfunction. In this study, we have investigated the cellular mechanism underlying D(4) modulation of GABA(A)Rs. We found that the density of surface clusters of GABA(A)R beta2/3 subunits was reduced by D(4), suggesting that the D(4) reduction of GABA(A)R current is associated with a decrease in functional GABA(A)Rs at the plasma membrane. Moreover, the D(4) reduction of GABA(A)R current was blocked by the actin stabilizer phalloidin and was occluded by the actin destabilizer latrunculin, suggesting that D(4) regulates GABA(A)R trafficking via an actin-dependent mechanism. Cofilin, a major actin depolymerizing factor whose activity is strongly increased by dephosphorylation at Ser(3), provides the possible link between D(4) signaling and the actin dynamics. Because myosin motor proteins are important for the transport of vesicles along actin filaments, we also tested the potential involvement of myosin in D(4) regulation of GABA(A)R trafficking. We found that dialysis with a myosin peptide, which competes with endogenous myosin proteins for actin-binding sites, prevented the D(4) reduction of GABA(A)R current. These results suggest that D(4) receptor activation increases cofilin activity presumably via its dephosphorylation, resulting in actin depolymerization, thus causing a decrease in the myosin-based transport of GABA(A)R clusters to the surface.

Selective dopamine D4 receptor agonist (A-412997) improves cognitive performance and stimulates motor activity without influencing reward-related behaviour in rat

Current therapies for attention deficit hyperactivity disorder comprise psychostimulants, which block the dopamine transporter and/or stimulate the release of dopamine, leading to a global elevation in extrasynaptic dopamine. These drugs are, however, associated with a series of unwanted side effects such as insomnia, anorexia, headache, stomach problems and potential drug abuse. Recent evidence suggests that the dopamine D4 receptor may represent a selective dopamine target that could mediate cognitive as well as striatal motor processes. In this study we compare the effects of a selective D4 receptor agonist, A-412997, with methylphenidate or amphetamine in preclinical models of efficacy versus abuse liability. Both methylphenidate and A-412997 improved a temporally induced deficit in the rat novel object recognition task at doses 10-fold lower than those stimulating activity. In both cases, procognitive doses were associated with elevated extracellular levels of dopamine and acetylcholine in the medial prefrontal cortex. In contrast to amphetamine, A-412997 did not mediate reward-related behaviour in the conditioned place preference paradigm, a preclinical rodent test used to assess potential abuse liability. Collectively, these data suggest that selective activation of the D4 receptor may represent a target for the treatment of attention deficit hyperactivity disorder without the potential drug abuse liability associated with current psychostimulant therapies.

Control of the Subthalamic Innervation of the Rat Globus Pallidus by D2/3 and D4 Dopamine Receptors

The effects of activating dopaminergic D(2/3) and D(4) receptors during activation of the subthalamic projection to the globus pallidus (GP) were explored in rat brain slices using the whole cell patch-clamp technique. Byocitin labeling and both orthodromic and antidromic activation demonstrated the integrity of some subthalamopallidal connections in in vitro parasagittal brain slices. Excitatory postsynaptic currents (EPSCs) that could be blocked by CNQX and AP5 were evoked onto pallidal neurons by local field stimulation of the subthalamopallidal pathway in the presence of bicuculline. Bath application of dopamine and quinpirole, a dopaminergic D(2)-class receptor agonist, reduced evoked EPSCs by about 35%. This effect was only partially blocked by sulpiride, a D(2/3) receptor antagonist. The sulpiride-sensitive reduction of the subthalamopallidal EPSC was associated with an increase in the paired-pulse ratio (PPR) and a reduction in the frequency but not the mean amplitude of spontaneous EPSCs (sEPSCs), indicative of a presynaptic site of action, which was confirmed by variance-mean analysis. The sulpiride-resistant EPSC reduction was mimicked by PD 168,077 and blocked by L-745,870, selective D(4) receptor agonist and antagonist, respectively, suggesting the involvement of D(4) receptors. The reduction of EPSCs produced by PD 168,077 was not accompanied by changes in PPR or the frequency of sEPSCs; however, it was accompanied by a reduction in mean sEPSC amplitude, indicative of a postsynaptic site of action. These results show that dopamine modulates subthalamopallidal excitation by presynaptic D(2/3) and postsynaptic D(4) receptors. The importance of this modulation is discussed.

Dopamine Presynaptically Depresses Fast Inhibitory Synaptic Transmission via D4 Receptor-Protein Kinase A Pathway in the Rat Dorsolateral Septal Nucleus

The lateral septal nucleus receives a diffuse dopaminergic input originating from the ventral tegmental area of the brain stem. We examined whether dopamine (DA) modulates synaptic transmission in the slice preparation of the rat dorsolateral septal nucleus (DLSN). Bath application (10-15 min) of DA (30 muM) markedly depressed the amplitude of fast and slow inhibitory postsynaptic potentials (IPSPs) in DLSN neurons, while it produced only a minor depression of the amplitude of excitatory postsynaptic potentials (EPSPs) obtained in the presence of bicuculline. DA (30 muM) depressed the monosynaptic fast IPSP to approximately 50% of control, but did not depress the inward current (I(GABA)) induced by exogenous gamma-aminobutyric acid (GABA). DA decreased the frequency of miniature fast IPSPs (m-fIPSPs) without significantly changing their amplitude. PD 168077, a selective D4 receptor agonist, depressed the fast and slow IPSPs but not the EPSP and decreased the frequency of m-fIPSPs. Both DA and PD 168077 increased the paired-pulse ratio of the monosynaptic fast IPSP. The inhibitory effect of DA on the fast IPSP was significantly attenuated by L-741,742, an antagonist at D4 receptors, but not by SCH 23390 and sulpiride, a D1-like and a D2-like receptor antagonist, respectively. N-ethylmaleimide, a blocker of pertussis toxin (PTX)-sensitive G protein (G(i/o)), attenuated the DA-induced depression of the fast IPSP. N-[2-((p-bromocinnamyl) amino)ethyl]-5-isoquinoline sulfonamide, a protein kinase A (PKA) inhibitor, attenuated the DA-induced depression of the fast IPSP. These results suggest that DA inhibits spontaneous and evoked release of GABA via the D4 receptor-G(i)-protein-PKA system in DLSN neurons.

Central mechanisms regulating penile erection in conscious rats: the dopaminergic systems related to the proerectile effect of apomorphine.

Apomorphine has been used as a pharmacological probe of dopaminergic receptors in a variety of central nervous system disorders. The utility of apomorphine as an agent for the treatment of erectile dysfunction has also been demonstrated clinically. Apomorphine is a nonselective dopaminergic receptor agonist with potent binding affinity (Ki) of 101, 32, 26, 2.6, and 10 nM for D1, D2, D3, D4, and D5, respectively. When administered either subcutaneously (s.c.) or intracerebroventricularly (i.c.v.), apomorphine fully evoked penile erections in conscious rats with maximum effect at 0.1 micromol/kg s.c. and 3 nmol/rat i.c.v., respectively. Apomorphine was less efficacious when injected intrathecally (i.t.) to L4-L6 spinal levels (50% at 30-100 nmol/rat i.t.). Penile erection facilitated by apomorphine was blocked by haloperidol and clozapine (i.p. and i.c.v.) but not by domperidone (a peripherally acting dopaminergic receptor antagonist). In this model using conscious rats, penile erection was significantly induced by quinpirole (D2-D3-D4 receptor agonist), but not by R(+)-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol (SKF38393) and R(+)-6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzapine (SKF81297) (D1 receptor agonists), or a D2 receptor agonist R-5,6-dihydro-N,N-dimethyl-4H-imidazo[4,5,1-ij]quinolin-5-amine (PNU-95666E). The role of D4 receptors in penile erection was demonstrated using selective D4 receptor agonists [(4-phenylpiperazinyl)-methyl]benzamide (PD168077) and 5-fluoro-2-[[4-(2-pyridinyl)-1-piperazinyl]methyl]-1H-indole (CP226269), whether administered systemically (s.c.) or locally in the brain (i.c.v.). The ability of apomorphine to activate D3 receptors in relation to its proerectile activity remains to be elucidated by use of selective subtype agonists. These results suggest that the proerectile action of apomorphine in rats is mediated at supraspinal levels and that this effect is not mimicked by a D2 receptor agonist but associated with activation of D4 receptors.

Characterization of the role of medial prefrontal cortex dopamine receptors in cocaine-induced locomotor activity.

Medial prefrontal cortex (mPFC) dopamine (DA) modulates the motor-stimulant response to cocaine. The present study examined the specific mPFC DA receptor subtypes that mediate this behavioral response. Intra-mPFC injection of the DA D2-like receptor agonist quinpirole blocked cocaine-induced motor activity, an effect that was prevented by coadministration of the D2 receptor antagonist sulpiride. Intra-mPFC injection of the selective D4 receptor agonist PD 168,077 or the selective D1 receptor agonist SKF 81297 did not alter the motor-stimulant response to cocaine. Finally, it was found that an intermediate dose of quinpirole, which only attenuated cocaine-induced motor activity, was not altered by SKF 81297 coadministration, suggesting a lack of synergy between mPFC D1 and D2 receptors. These results suggest that D2 receptor mechanisms in the mPFC are at least partly responsible for mediating the acute motor-stimulant effects of cocaine.