Dopamine D4 Receptor Is a Regulator of Morphine-Induced Plasticity in the Rat Dorsal Striatum

Long-term exposition to morphine elicits structural and synaptic plasticity in reward-related regions of the brain, playing a critical role in addiction. However, morphine-induced neuroadaptations in the dorsal striatum have been poorly studied despite its key function in drug-related habit learning. Here, we show that prolonged treatment with morphine triggered the retraction of the dendritic arbor and the loss of dendritic spines in the dorsal striatal projection neurons (MSNs). In an attempt to extend previous findings, we also explored whether the dopamine D4 receptor (D4R) could modulate striatal morphine-induced plasticity. The combined treatment of morphine with the D4R agonist PD168,077 produced an expansion of the MSNs dendritic arbors and restored dendritic spine density. At the electrophysiological level, PD168,077 in combination with morphine altered the electrical properties of the MSNs and decreased their excitability. Finally, results from the sustantia nigra showed that PD168,077 counteracted morphine-induced upregulation of μ opioid receptors (MOR) in striatonigral projections and downregulation of G protein-gated inward rectifier K+ channels (GIRK1 and GIRK2) in dopaminergic cells. The present results highlight the key function of D4R modulating morphine-induced plasticity in the dorsal striatum. Thus, D4R could represent a valuable pharmacological target for the safety use of morphine in pain management.

Dopamine D4 receptor subtype activation reduces the rat cardiac parasympathetic discharge

The dopaminergic system influences the heart rhythm by inhibiting the rat cardiac sympathetic and parasympathetic neurotransmissions through activation of D₂-like receptors (encompassing the D₂, D₃, and D₄ subtypes). Whereas D₂ receptor subtype activation results in cardiac sympatho-inhibition, the dopamine receptor subtypes involved in rat cardiac vago-inhibition remain unknown. Hence, this study investigated the specific functional role of the D₂-like receptor subtypes (D₂, D₃, and/or D₄) inhibiting the rat heart cholinergic drive. For this purpose, male Wistar rats were pithed and prepared for cardiac vagal stimulation. Bradycardic responses were obtained by electrical stimulation of vagal fibres (3, 6, 9 Hz; n = 100) or i.v. acetylcholine (ACh; 1, 5, 10 μg/kg; n = 15). Expression of D₂, D₃, and D₄ receptors was studied in left and right atrium samples by PCR (n = 4). Intravenous injections of quinpirole (D₂-like agonist; 1-30 μg/kg), but not of SFK-38393 (D₁-like agonist; 1-30 μg/kg), dose-dependently inhibited the vagally induced bradycardia. The vago-inhibition induced by quinpirole (which failed to affect the bradycardia to i.v. ACh) was unchanged after i.v. injections of the antagonists L-741,626 (D₂; 100 μg/kg) or SB-277011-A (D₃; 100 μg/kg), but it was abolished by L-745,870 (D₄; 100 μg/kg). mRNA levels of D₂, D₃, and D₄ receptor subtype were detected in the left and right rat atria. Our results suggest that the quinpirole-induced vagolytic effect involves prejunctional D₄ receptor subtypes, located in the left and right atria. This provides new evidence on the relevance of D₄ receptor modulating the heart parasympathetic control.

Characterizing fucoxanthin as a selective dopamine D3/D4 receptor agonist: Relevance to Parkinson's disease

Fucoxanthin and fucosterol are archetypal lipid components of edible brown algae that provide several health benefits. Lately, their protective role in Aβ1-42-induced cognitive dysfunction in animal models has been reported (Alghazwi et al., 2019; Oh et al., 2018). However, their role in the aminergic system and as a prime treatment approach for multifactorial neurodegenerative diseases still requires exploration. The main aims of the present study are to characterize the role of fucoxanthin and fucosterol in the aminergic pathway via in vitro human monoamine oxidase (hMAO) inhibition and cell-based functional G-protein coupled receptor (GPCR) assays and to underline their possible mechanisms of action via in silico molecular docking studies. Fucoxanthin displayed weak inhibition with IC50 values of 197.41 ± 2.20 and 211.12 ± 1.17 μM over two isoenzymes hMAO-A and hMAO-B, respectively. Fucosterol remained inactive up to 500 μM. In functional assay results, fucoxanthin showed a concentration-dependent agonist effect on dopamine D3 and D4 receptors. The half maximal effective concentration (EC50) of fucoxanthin for dopamine D3 and D4 receptors was 16.87 ± 3.41 and 81.87 ± 6.11 μM, respectively. For dopamine as a reference agonist, the EC50 values for these two receptors were 3.7 and 24 nM, respectively. Fucosterol showed no agonist activity on any of the tested receptors. Similarly, fucoxanthin showed a mild antagonist effect on dopamine D1 and tachykinin (NK1) receptor with inhibition of control agonist response by approximately 40% at 100 μM. Fucosterol displayed mild antagonist effects only on dopamine D1 and D4 receptors. In silico studies revealed potential mechanisms by which fucoxanthin binds to dopamine receptors to exert its agonist effects, including low binding energy and H-bond interactions with Ser196 and Thr115 at the D3 receptor and with Ser196 and Asp115 at the D4 receptor. Our results collectively suggest that fucoxanthin is a potential D3/D4 agonist for the management of neurodegenerative diseases, such as Parkinson's disease.

Eckol as a Potential Therapeutic against Neurodegenerative Diseases Targeting Dopamine D₃/D₄ Receptors

The G protein-coupled receptor (GPCR) family of proteins comprises signaling proteins that mediate cellular responses to various hormones and neurotransmitters, and serves as a prime target for drug discovery. Towards our goal of discovering secondary metabolites from natural sources that can function as neuronal drugs, we evaluated the modulatory effect of eckol on various GPCRs via cell-based functional assays. In addition, we conducted in silico predictions to obtain molecular insights into the functional effects of eckol. Functional assays revealed that eckol had a concentration-dependent agonist effect on dopamine D₃ and D₄ receptors. The half maximal effective concentration (EC₅₀) of eckol for the dopamine D₃ and D₄ receptors was 48.62 ± 3.21 and 42.55 ± 2.54 µM, respectively, while the EC₅₀ values of dopamine as a reference agonist for these two receptors were 2.9 and 3.3 nM, respectively. In silico studies revealed that a low binding energy in addition to hydrophilic, hydrophobic, π⁻alkyl, and π⁻π T-shaped interactions are potential mechanisms by which eckol binds to the dopamine receptors to exert its agonist effects. Molecular dynamics (MD) simulation revealed that Phe346 of the dopamine receptors is important for binding of eckol, similar to eticlopride and dopamine. Our results collectively suggest that eckol is a potential D₃/D₄ agonist for the management of neurodegenerative diseases, such as Parkinson's disease.

Ultra-large library docking for discovering new chemotypes

Despite intense interest in expanding chemical space, libraries containing hundreds-of-millions to billions of diverse molecules have remained inaccessible. Here we investigate structure-based docking of 170 million make-on-demand compounds from 130 well-characterized reactions. The resulting library is diverse, representing over 10.7 million scaffolds that are otherwise unavailable. For each compound in the library, docking against AmpC β-lactamase (AmpC) and the D4 dopamine receptor were simulated. From the top-ranking molecules, 44 and 549 compounds were synthesized and tested for interactions with AmpC and the D4 dopamine receptor, respectively. We found a phenolate inhibitor of AmpC, which revealed a group of inhibitors without known precedent. This molecule was optimized to 77 nM, which places it among the most potent non-covalent AmpC inhibitors known. Crystal structures of this and other AmpC inhibitors confirmed the docking predictions. Against the D4 dopamine receptor, hit rates fell almost monotonically with docking score, and a hit-rate versus score curve predicted that the library contained 453,000 ligands for the D4 dopamine receptor. Of 81 new chemotypes discovered, 30 showed submicromolar activity, including a 180-pM subtype-selective agonist of the D4 dopamine receptor.

Intravitreally-administered dopamine D2-like (and D4), but not D1-like, receptor agonists reduce form-deprivation myopia in tree shrews

We examined the effect of intravitreal injections of D1-like and D2-like dopamine receptor agonists and antagonists and D4 receptor drugs on form-deprivation myopia (FDM) in tree shrews, mammals closely related to primates. In eleven groups (n = 7 per group), we measured the amount of FDM produced by monocular form deprivation (FD) over an 11-day treatment period. The untreated fellow eye served as a control. Animals also received daily 5 µL intravitreal injections in the FD eye. The reference group received 0.85% NaCl vehicle. Four groups received a higher, or lower, dose of a D1-like receptor agonist (SKF38393) or antagonist (SCH23390). Four groups received a higher, or lower, dose of a D2-like receptor agonist (quinpirole) or antagonist (spiperone). Two groups received the D4 receptor agonist (PD168077) or antagonist (PD168568). Refractions were measured daily; axial component dimensions were measured on day 1 (before treatment) and day 12. We found that in groups receiving the D1-like receptor agonist or antagonist, the development of FDM and altered ocular component dimensions did not differ from the NaCl group. Groups receiving the D2-like receptor agonist or antagonist at the higher dose developed significantly less FDM and had shorter vitreous chambers than the NaCl group. The D4 receptor agonist, but not the antagonist, was nearly as effective as the D2-like agonist in reducing FDM. Thus, using intravitreally-administered agents, we did not find evidence supporting a role for the D1-like receptor pathway in reducing FDM in tree shrews. The reduction of FDM by the dopamine D2-like agonist supported a role for the D2-like receptor pathway in the control of FDM. The reduction of FDM by the D4 receptor agonist, but not the D4 antagonist, suggests an important role for activation of the dopamine D4 receptor in the control of axial elongation and refractive development.

Elucidating the role of D4 receptors in mediating attributions of salience to incentive stimuli on Pavlovian conditioned approach and conditioned reinforcement paradigms

The power of drug-associated cues to instigate drug 'wanting' and consequently promote drug seeking is a corner stone of contemporary theories of addiction. Gambling disorder has recently been added to the pantheon of addictive disorders due to the phenomenological similarities between the diseases. However, the neurobiological mechanism that may mediate increased sensitivity towards conditioned stimuli in addictive disorders is unclear. We have previously demonstrated using a rodent analogue of a simple slot machine that the dopamine D4 receptor is critically engaged in controlling animals' attribution of salience to stimuli associated with reward in this paradigm, and consequently may represent a target for the treatment of gambling disorder. Here, we investigated the role of acute administration of a D4 receptor agonist on animals' responsivity to conditioned stimuli on both a Pavlovian conditioned approach (autoshaping) and a conditioned reinforcement paradigm. Following training on one of the two tasks, separate cohorts of rats (male and female) were administered a dose of PD168077 shown to be maximally effective at precipitating errors in reward expectancy on the rat slot machine task (10mg/kg). However, augmenting the activity of the D4 receptors in this manner did not alter behaviour on either task. These data therefore provide novel evidence that the D4 receptor does not alter incentive motivation in response to cues on simple behavioural tasks.

In Vitro and In Vivo Characterization of the Alkaloid Nuciferine

Rationale

The sacred lotus (Nelumbo nucifera) contains many phytochemicals and has a history of human use. To determine which compounds may be responsible for reported psychotropic effects, we used in silico predictions of the identified phytochemicals. Nuciferine, an alkaloid component of Nelumbo nucifera and Nymphaea caerulea, had a predicted molecular profile similar to antipsychotic compounds. Our study characterizes nuciferine using in vitro and in vivo pharmacological assays.

Methods

Nuciferine was first characterized in silico using the similarity ensemble approach, and was followed by further characterization and validation using the Psychoactive Drug Screening Program of the National Institute of Mental Health. Nuciferine was then tested in vivo in the head-twitch response, pre-pulse inhibition, hyperlocomotor activity, and drug discrimination paradigms.

Results

Nuciferine shares a receptor profile similar to aripiprazole-like antipsychotic drugs. Nuciferine was an antagonist at 5-HT_2A, 5-HT_2C, and 5-HT_2B, an inverse agonist at 5-HT₇, a partial agonist at D_2, D₅ and 5-HT₆, an agonist at 5-HT_1A and D₄ receptors, and inhibited the dopamine transporter. In rodent models relevant to antipsychotic drug action, nuciferine blocked head-twitch responses and discriminative stimulus effects of a 5-HT_2A agonist, substituted for clozapine discriminative stimulus, enhanced amphetamine induced locomotor activity, inhibited phencyclidine (PCP)-induced locomotor activity, and rescued PCP-induced disruption of prepulse inhibition without induction of catalepsy.

Conclusions

The molecular profile of nuciferine was similar but not identical to that shared with several approved antipsychotic drugs suggesting that nuciferine has atypical antipsychotic-like actions.

The Impact of Selective Dopamine D2, D3 and D4 Ligands on the Rat Gambling Task

Gambling is an addictive disorder with serious societal and personal costs. To-date, there are no approved pharmacological treatments for gambling disorder. Evidence suggests a role for dopamine in gambling disorder and thus may provide a therapeutic target. The present study therefore aimed to investigate the effects of selective antagonists and agonists of D2, D3 and D4 receptors in a rodent analogue of the Iowa gambling task used clinically. In this rat gambling task (rGT), animals are trained to associate different response holes with different magnitudes and probabilities of food pellet rewards and punishing time-out periods. As in the Iowa gambling task, the optimal strategy is to avoid the tempting high-risk high-reward options, and instead favor those linked to smaller per-trial rewards but also lower punishments, thereby maximizing the amount of reward earned over time. Administration of those selective ligands did not affect decision making under the rGT. Only the D4 drug had modest effects on latency measures suggesting that D4 may contribute in some ways to decision making under this task.

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.

Evaluation of N-phenyl homopiperazine analogs as potential dopamine D3 receptor selective ligands

A series of N-(2-methoxyphenyl)homopiperazine analogs was prepared and their affinities for dopamine D2, D3, and D4 receptors were measured using competitive radioligand binding assays. Several ligands exhibited high binding affinity and selectivity for the D3 dopamine receptor compared to the D2 receptor subtype. Compounds 11a, 11b, 11c, 11f, 11j and 11k had K(i) values ranging from 0.7 to 3.9 nM for the D3 receptor with 30- to 170-fold selectivity for the D3 versus D2 receptor. Calculated logP values (logP=2.6-3.6) are within the desired range for passive transport across the blood-brain barrier. When the binding and the intrinsic efficacy of these phenylhomopiperazines was compared to those of previously published phenylpiperazine analogues, it was found that (a) affinity at D2 and D3 dopamine receptors generally decreased, (b) the D3 receptor binding selectivity (D2:D3 K(i) value ratio) decreased and, (c) the intrinsic efficacy, measured using a forskolin-dependent adenylyl cyclase inhibition assay, generally increased.

Dopamine D4 receptor transmission in the prefrontal cortex controls the salience of emotional memory via modulation of calcium calmodulin-dependent kinase II

Dopamine (DA) signaling in the medial prefrontal cortex (mPFC) plays a critical role in the processing of emotional information and memory encoding. Activation of DA D4 receptors within the prelimbic (PLC) division of the mPFC bidirectionally modulates emotional memory by strongly potentiating the salience of normally nonsalient emotional memories but blocking the acquisition of suprathreshold emotionally salient fear memories. Previous in vitro studies have shown that activation of cortical DA D4 receptors can bidirectionally modulate levels of α-calcium calmodulin-dependent kinase II (α-CaMKII), a molecule essential for learning and memory. Using an olfactory fear conditioning procedure in rats combined with microinfusions into the mPFC, we examined the potential role of D4 receptor-mediated control of emotional memory salience through signaling via CaMKII, cAMP/protein kinase A (PKA), and protein phosphatase-1 (PP1) signaling. We report that CaMKII blockade prevents the ability of intra-mPFC DA D4 receptor activation to potentiate the salience of subthreshold fear memory. In contrast, blockade of either cAMP/PKA or PP1 signaling pathways rescued the blockade of suprathreshold fear memory via intra-mPFC D4 receptor activation. Our results demonstrate that modulation of emotional memory salience via intra-mPFC DA D4 receptor transmission depends upon downstream signaling via CaMKII, cAMP/PKA, and PP1 substrates.

WAY-100635 has high selectivity for serotonin 5-HT1A versus dopamine D4 receptors

The serotonin 5-HT(1A) receptor antagonist WAY-100635 was recently reported to have potent agonist properties at dopamine D(4) receptors (Chemel et al., 2006, Psychopharmacology 188, 244-251.). Herein WAY-100635 (pK(i) at human (h) serotonin 5-HT(1A) receptors=9.51; pK(i) at dopamine hD(4.4) receptors=7.42) stimulated [(35)S]GTPgammaS incorporation in membranes of Chinese Hamster Ovary cells expressing dopamine hD(4.4) receptors with only moderate potency and modest efficacy (pEC(50)=6.63; E(max)=19% of dopamine). Moreover, in antagonism experiments, WAY-100635 had a much lower potency at dopamine hD(4.4) receptors (pK(B)=7.09), than at serotonin h5-HT(1A) receptors (pK(B)=9.47). These data demonstrate that WAY-100635 has high selectivity for serotonin h5-HT(1A)versus dopamine hD(4.4) receptors.

Dopamine/Serotonin Receptor Ligands. 16.1Expanding Dibenz[d,g]azecines to 11- and 12-Membered Homologues. Interaction with Dopamine D1−D5Receptors

Oxygenated 7-methyl-5,6,7,8,9,14-hexahydrodibenz[d,g]azecines are potent dopamine receptor antagonists, preferentially at D1/D5. We synthesized the hydroxylated, methoxylated, and chlorinated 11-membered and 12-membered homologues of these 10-membered heterocycles. Their affinities for the human cloned D1-D5 receptors (radioligand binding) and functionalities (calcium assay) were measured. Enlarging the dibenzazecines to the corresponding dibenzazacycloundecenes and dibenzazacyclododecenes generally maintains the high antagonistic affinity for D1/D5 but also leads to a compound with a clozapine-like binding profile due to additional affinity for D4.

F15063, a potential antipsychotic with dopamine D2/D3 antagonist, 5-HT1A agonist and D4 partial agonist properties: (IV) duration of brain D2-like receptor occupancy and antipsychotic-like activity versus plasma concentration in mice

F15063 (N-[(2,2-dimethyl-2,3-dihydro-benzofuran-7-yloxy)ethyl]-3-(cyclopent-1-enyl)-benzylamine fumarate salt) is a novel potential antipsychotic with dopamine D(2)/D(3) blocking properties and agonist activity at 5-HT(1A) and D(4) receptors. The pertinent parameter for pharmacological activity of antipsychotics appears to be central D2-like receptor occupancy. However, its duration is not necessarily correlated with drug plasma levels, on which clinical dosing regimens are often based. Thus, we compared in mice the duration of actions of F15063 and haloperidol to (1) inhibit apomorphine-induced climbing and sniffing (behavioural measures of D2-like receptor antagonism) and (2) occupy D2-like receptors in vivo in the striatum and olfactory tubercles (inhibition of [(3)H]nemonapride binding). Finally, we measured plasma levels of F15063. D2-like receptor occupancy in the striatum remained elevated at 1, 4 and 8 h postadministration, with both F15063 (ID(50): 7.1, 3.6 and 16.5 mg/kg p.o., respectively) and the typical antipsychotic, haloperidol (ID(50): 1.4, 0.52 and 0.53 mg/kg p.o., respectively). This was paralleled by a protracted inhibition of apomorphine-induced climbing (ED(50): 0.9, 2.8 and 3.6 mg/kg p.o., and 0.21, 0.37 and 0.87 mg/kg p.o., respectively, for F15063 and haloperidol). In contrast, after administration of 10 mg/kg p.o. of F15063, its plasma levels decreased rapidly: 15.2, 2.1 and 0.6 ng/ml, 1, 4 and 8 h after administration, respectively. A similar pattern of results was observed when F15063 and haloperidol were administered i.p. and s.c., respectively. To summarise, the time-course of D2-like receptor occupancy and inhibition of apomorphine-climbing (and sniffing) behaviours was similarly long lasting with F15063 and haloperidol. In addition, the durations of action of F15063 and haloperidol in a behavioural model of antipsychotic-like activity were closely correlated to their occupancy of central D2-like receptors, and much longer than their presence in plasma.

Dopaminergic regulation of inhibitory and excitatory transmission in the basolateral amygdala-prefrontal cortical pathway

Projections from the basolateral amygdala (BLA) and dopamine (DA) input from the ventral tegmental area (VTA) converge in the medial prefrontal cortex (mPFC), forming a neural circuit implicated in certain cognitive and emotional processes. However, the role that DA plays in modulating activity in the BLA-mPFC pathway is unknown. The present study investigated the mechanisms by which DA modulates BLA-evoked changes in mPFC neural activity, using extracellular single-unit recordings in urethane-anesthetized rats. BLA stimulation evoked two distinct types of responses in separate populations of mPFC neurons: monosynaptic, excitatory responses and, more commonly, inhibition of spontaneous firing. Stimulation of the VTA or local iontophoretic application of DA attenuated BLA-evoked inhibition of PFC neuron firing. Administration of selective DA receptor agonists revealed that these effects were mediated by D2 and D4 (but not D1) receptors. In addition, VTA stimulation or DA application attenuated BLA-evoked firing of a separate population of mPFC neurons in a frequency-dependent manner; firing evoked by higher-frequency stimulation of the BLA was less inhibited than that evoked by single-pulse stimulation. Attenuation of BLA-evoked firing was also induced by of D1 (but not D2 or D4) receptor agonists. These data indicate that dissociable DA receptor mechanisms regulate the balance of excitatory and inhibitory transmission in BLA-mPFC circuits, biasing toward an increase in the excitatory influence that the BLA exerts over populations of mPFC neurons. These findings may have important implications for understanding the pathophysiology underlying emotional and cognitive disturbances present in disorders such as depression and drug addiction.

F15063, a compound with D2/D3 antagonist, 5-HT 1A agonist and D4 partial agonist properties. III. Activity in models of cognition and negative symptoms

BACKGROUND AND PURPOSE

The D(2)/D(3) receptor antagonist, D(4) receptor partial agonist, and high efficacy 5-HT(1A) receptor agonist F15063 was shown to be highly efficacious and potent in rodent models of activity against positive symptoms of schizophrenia. However F15063 induced neither catalepsy nor the 'serotonin syndrome'. Here, we evaluated its profile in rat models predictive of efficacy against negative symptoms/cognitive deficits of schizophrenia.

EXPERIMENTAL APPROACH

F15063, given i.p., was assessed in models of behavioural deficits induced by interference with the NMDA/glutamatergic (phencyclidine: PCP) or cholinergic (scopolamine) systems.

KEY RESULTS

Through 5-HT(1A) activation, F15063 partially alleviated (MED: 0.04 mg kg(-1)) PCP-induced social interaction deficit between two adult rats, without effect by itself, underlining its potential to combat negative symptoms. At doses above 0.16 mg kg(-1), F15063 reduced interaction by itself. F15063 (0.16 mg kg(-1)) selectively re-established PCP-impaired 'cognitive flexibility' in a reversal learning task, suggesting potential against adaptability deficits. F15063 (0.04-0.63 mg kg(-1)) also reversed scopolamine-induced amnesia in a juvenile-adult rat social recognition test, indicative of a pro-cholinergic influence. Activity in this latter test is consistent with its D(4) partial agonism, as it was blocked by the D(4) antagonist L745,870. Finally, F15063 up to 40 mg kg(-1) did not disrupt basal prepulse inhibition of startle reflex in rats, a marker of sensorimotor gating.

CONCLUSIONS AND IMPLICATIONS

The balance of D(2)/D(3), D(4) and 5-HT(1A) receptor interactions of F15063 yields a promising profile of activity in models of cognitive deficits and negative symptoms of schizophrenia.

F15063, a compound with D2/D3 antagonist, 5-HT 1A agonist and D4 partial agonist properties. II. Activity in models of positive symptoms of schizophrenia

BACKGROUND AND PURPOSE

F15063 is a high affinity D(2)/D(3) antagonist, D(4) partial agonist, and high efficacy 5-HT(1A) agonist, with little affinity (40-fold lower than for D(2) receptors) at other central targets. Here, the profile of F15063 was evaluated in models of positive symptoms of schizophrenia and motor side-effects.

EXPERIMENTAL APPROACH

Rodent behavioural tests were based on reversal of hyperactivity induced by psychostimulants and on measures of induction of catalepsy and 'serotonin syndrome'.

KEY RESULTS

F15063 potently (ED(50)s: 0.23 to 1.10 mg kg(-1) i.p.) reversed methylphenidate-induced stereotyped behaviors, blocked d-amphetamine and ketamine hyperlocomotion, attenuated apomorphine-induced prepulse inhibition (PPI) deficits, and was active in the conditioned avoidance test. In mice, it reversed apomorphine-induced climbing (ED(50)=0.30 mg kg(-1) i.p.). F15063, owing to its 5-HT(1A) agonism, did not produce (ED(50)>40 mg kg(-1) i.p.) catalepsy in rats and mice, a behavior predictive of occurrence of extra-pyramidal syndrome (EPS) in man. This absence of cataleptogenic activity was maintained upon sub-chronic treatment of rats for 5 days at 40 mg kg(-1) p.o. Furthermore, F15063 did not induce the 'serotonin syndrome' in rats (flat body posture and forepaw treading: ED(50) >32 mg kg(-1) i.p.).

CONCLUSIONS AND IMPLICATIONS

F15063 conformed to the profile of an atypical antipsychotic, with potent actions in models of hyperdopaminergic activity but without inducing catalepsy. These data suggest that F15063 may display potent antipsychotic actions with low EPS liability. This profile is complemented by a favourable profile in rodent models of negative symptoms and cognitive deficits of schizophrenia (companion paper).

Discovery of 3-methyl-N-(1-oxy-3',4',5',6'-tetrahydro-2'H-[2,4'-bipyridine]-1'-ylmethyl)benzamide (ABT-670), an orally bioavailable dopamine D4 agonist for the treatment of erectile dysfunction

The goal of this study was to identify a structurally distinct D(4)-selective agonist with superior oral bioavailability to our first-generation clinical candidate 1a (ABT-724) for the potential treatment of erectile dysfunction. Arylpiperazines such as (heteroarylmethyl)piperazine 1a, benzamide 2, and acetamides such as 3a,b exhibit poor oral bioavailability. Structure-activity relationship (SAR) studies with the arylpiperidine template provided potent partial agonists such as 4d and 5k that demonstrated no improvement in oral bioavailability. Further optimization with the (N-oxy-2-pyridinyl)piperidine template led to the discovery of compound 6b (ABT-670), which exhibited excellent oral bioavailability in rat, dog, and monkey (68%, 85%, and 91%, respectively) with comparable efficacy, safety, and tolerability to 1a. The N-oxy-2-pyridinyl moiety not only provided the structural motif required for agonist function but also reduced metabolism rates. The SAR study leading to the discovery of 6b is described herein.

PIP3EA and PD-168077, two selective dopamine D4 receptor agonists, induce penile erection in male rats: site and mechanism of action in the brain

PIP3EA (2-[4-(2-methoxyphenyl)piperazin-1-yl-methyl]imidazo[1,2-a]pyridine) and PD-168077 (N-[4-2-cyanophenylpiperazin-1-ylmethyl]-3-methylbenzamide maleate), two selective dopamine D4 agonists, administered systemically, intracerebroventricularly or into the paraventricular nucleus of the hypothalamus induce penile erection in male Sprague-Dawley rats. A U-inverted dose-response curve was found with either compound when given subcutaneously (1-100 microg/kg) or intracerebroventricularly (0.1-20 microg/rat), but not into the paraventricular nucleus (10-200 ng/rat). The pro-erectile effect of PIP3EA and of PD-168077 occurs concomitantly with an increased nitric oxide (NO) production in the paraventricular nucleus, as measured by the increased concentration of nitrites and nitrates found in the dialysate obtained from the paraventricular nucleus by intracerebral microdialysis. These effects of PIP3EA and PD-168077 were reduced by L-745,870 (3-[4-(4-chlorophenyl)piperazin-1-ylmethyl]-1H-pyrrolo[2,3-b]pyridine trihydrochloride), a selective dopamine D4 receptors antagonist, by omega-conotoxin, a blocker of voltage-dependent Ca2+ channels of the N-type, by S-methyl-thiocitrulline, a neuronal nitric oxide synthase inhibitor, and by d(CH2)5Tyr(Me)2-Orn8-vasotocin, an oxytocin receptor antagonist, given into the lateral ventricles, but not into the paraventricular nucleus. Comparison of the dose-response curves of PIP3EA and PD-168077 revealed that PIP3EA is as potent as PD-168077 when given into the paraventricular nucleus, but more potent when given systemically. However, both compounds are less efficacious (e.g. induce a lower number of penile erection episodes) than apomorphine, a classical mixed dopamine receptor agonist, irrespective of the route of administration. These results confirm previous findings showing that central D4 receptors mediate penile erection and show that dopamine D4 receptor agonists act in the paraventricular nucleus to facilitate penile erection by increasing central oxytocinergic neurotransmission.

Acute clozapine suppresses synchronized pyramidal synaptic network activity by increasing inhibition in the ferret prefrontal cortex

Recent studies have indicated that impaired neural circuitry in the prefrontal cortex is a prominent feature of the neuropathology of schizophrenia. Clozapine is one of the most effective antipsychotic drugs used for this debilitating disease. Despite its effectiveness, the mechanism by which clozapine acts on prefrontal cortical circuitry remains poorly understood. In this study, in vitro multiple whole cell recordings were performed in slices of the ferret prefrontal cortex. Clozapine, which effectively inhibited the spontaneous synchronized network activities in the prefrontal neurons, achieved the suppressive effect by decreasing the recurrent excitation among pyramidal neurons and by enhancing the inhibitory inputs onto pyramidal cells through a likely network mechanism. Indeed, under the condition of disinhibition, the depressing effects were reversed and clozapine enhanced the recurrent excitation. These results suggest that the therapeutic actions of clozapine in alleviating the positive symptoms of schizophrenia are achieved, at least partially, through the readjustment of synaptic balance between the excitation and inhibition in the prefrontal cortical circuitry.

WAY-100635 is a potent dopamine D4 receptor agonist

RATIONALE AND OBJECTIVES

WAY-100635 is a prototypical 5-HT1A receptor antagonist and has been used widely as a pharmacological probe to investigate the distribution and function of 5-HT1A receptors. Results from our studies suggested that WAY-100635 was potently inducing effects unrelated to its 5-HT1A receptor affinity. In the present work, we evaluated the in vitro pharmacology of this compound at two D2-like receptor subtypes.

METHOD

The functional properties and binding affinities of WAY-100635 were evaluated in HEK 293 cells stably expressing dopamine D2L or D4.4 receptors.

RESULTS

Initial screens performed by the NIMH Psychoactive Drug Screening Program indicated that WAY-100635 displayed 940, 370, and 16 nM binding affinities at D2L, D3, and D4.2 receptors, respectively. Subsequent saturation analyses demonstrated that the Kd of [3H]WAY-100635 at D4.2 receptors was 2.4 nM, only tenfold higher than 5-HT1A. WAY-100635 and its major metabolite, WAY-100634, were potent agonists in HEK-D4.4 cells (EC50=9.7+/-2.2 and 0.65+/-0.2 nM, respectively). WAY-100635 behaved as a full agonist, and WAY-100634 was a nearly full agonist. In HEK-D2L cells, WAY-100635 weakly antagonized the effects of 300 nM quinpirole. Subsequent radioligand binding studies confirmed that WAY-100635 possesses high affinity for D4.4 receptors but binds weakly to D2L receptors (3.3+/-0.6 and 420+/-11 nM, respectively).

CONCLUSIONS

This study demonstrates that WAY-100635 is not a "selective" 5-HT1A receptor antagonist, as previously reported, and conclusions drawn from studies that employed WAY-100635 as a selective 5-HT1A antagonist may need to be reevaluated.

2-[(4-phenylpiperazin-1-yl)methyl]imidazo(di)azines as selective D4-ligands. Induction of penile erection by 2-[4-(2-methoxyphenyl)piperazin-1-ylmethyl]imidazo[1,2-a]pyridine (PIP3EA), a potent and selective D4 partial agonist

A series of novel 2-[(4-phenylpiperazin-1-yl)methyl]imidazoazines and aza-analogues were prepared and screened at selected dopamine, serotonin, and adrenergic receptor subtypes. 2-Substituted imidazopyridines and pyridazines presented high affinities and selectivities for D4 dopamine receptors. Whereas functional experiments indicated neutral antagonists or weak partial agonist effects for most of the target compounds, the 2-methoxyphenyl substituted 2-piperazinylmethylimidazopyridine 3c (PIP3EA) displayed substantial agonist efficacy in mitogenesis experiments and GTPgammaS binding tests, resulting in EC50 values of 3.0 (46%) and 4.5 nM (57%), respectively. Our D4 agonist 3c induced penile erection in vivo when administered to rats. This effect was inhibited by L-745,870 a D4 selective antagonist, confirming the mechanistic pathway.

Disruption of orofacial movement topographies in congenic mutants with dopamine D5 but not D4 receptor or DARPP-32 transduction 'knockout'

The role of D(1)-like [D(1), D(5)] and D(2)-like [D(2), D(3), D(4)] dopamine receptors and dopamine transduction via DARPP-32 in topographies of orofacial movement was assessed in restrained mice with congenic D(4) vs. D(5) receptor vs. DARPP-32 'knockout'. D(4) and DARPP-32 mutants evidenced no material phenotype; also, there were no alterations in topographical responsivity to either the selective D(2)-like agonist RU 24213 or the selective D(1)-like agonist SK and F 83959. In contrast, D(5) mutants evidenced an increase in spontaneous vertical jaw movements, which habituated more slowly than in wildtypes, and a decrease in horizontal jaw movements; topographical responsivity to SK and F 83959 and RU 24213 was unaltered. D(5) receptors regulate distinct topographies of vertical and horizontal jaw movement in an opposite manner. In assuming that the well-recognised role of the D(1)-like family in regulating orofacial movements involves primarily D(1) receptors, a role for their D(5) counterparts may have been overlooked.

1-Aryl-3-(4-pyridine-2-ylpiperazin-1-yl)propan-1-one Oximes as Potent Dopamine D4 Receptor Agonists for the Treatment of Erectile Dysfunction

A new series of dopamine D4 receptor agonists, 1-aryl-3-(4-pyridinepiperazin-1-yl)propanone oximes, was designed through the modification of known dopamine D4 receptor agonist PD 168077. Replacement of the amide group with a methylene-oxime moiety produced compounds with improved stability and efficacy. Structure-activity relationsips (SAR) of the aromatic ring linked to the N-4-piperazine ring confirmed the superiority of 2-pyridine as a core for D4 agonist activity. A two-methylene linker between the oxime group and the N-1-piperazine ring displayed the best profile. New dopamine D4 receptor agonists, exemplified by (E)-1-(4-chlorophenyl)-3-(4-pyridin-2-ylpiperazin-1-yl)propan-1-one O-methyloxime (59a) and (E)-1-(3-chloro-4-fluorophenyl)-3-(4-pyridin-2-ylpiperazin-1-yl)propan-1-one O-methyloxime (64a), exhibited favorable pharmacokinetic profiles and showed oral bioavailability in rat and dog. Subsequent evaluation of 59a in the rat penile erection model revealed in vivo activity, comparable in efficacy to apomorphine. Our results suggest that the oximes provide a novel structural linker for 4-arylpiperazine-based D4 agonists, possessing leadlike quality and with potential to develop a new class of potent and selective dopamine D4 receptor agonists.

Synthesis and biological investigations of dopaminergic partial agonists preferentially recognizing the D4 receptor subtype

Aminomethyl-substituted biaryls bearing a pyrazole or triazole moiety were synthesized and investigated for dopamine and serotonin receptor binding. The N-arylpyrazoles 3b,f,g and 4 revealed Ki values in the subnanomolar range (0.28-0.70 nM) for the dopamine D4 receptor subtype. Employing both mitogenesis and GTPgammaS assays, ligand efficacy was evaluated indicating partial agonist properties. Interestingly, the tetrahydropyrimidine 4 (FAUC 2020) displayed significant intrinsic selectivity for D2(long) over D2(short).

Dopamine D4 receptor signaling in the rat paraventricular hypothalamic nucleus: Evidence of natural coupling involving immediate early gene induction and mitogen activated protein kinase phosphorylation

The dopamine D4 receptor has been investigated for its potential role in several CNS disorders, notably schizophrenia and more recently, erectile dysfunction. Whereas studies have investigated dopamine D4 receptor-mediated signaling in vitro, there have been few, if any, attempts to identify dopamine D4 receptor signal transduction pathways in vivo. In the present studies, the selective dopamine D4 agonist PD168077 induces c-Fos expression and extracellular signal regulated kinase (ERK) phosphorylation in the hypothalamic paraventricular nucleus (PVN), a site known to regulate proerectile activity. The selective dopamine D4 receptor antagonist A-381393 blocked both c-Fos expression and ERK1/2 phosphorylation produced by PD168077. In addition, PD168077-induced ERK1/2 phosphorylation was prevented by SL327, an inhibitor of ERK1/2 phosphorylation. Interestingly, treatment with A-381393 alone significantly reduced the amount of Fos immunoreactivity as compared to basal expression observed in vehicle-treated controls. Dopamine D4 receptor and c-Fos coexpression in the PVN was observed using double immunohistochemical labeling, suggesting that PD168077-induced signaling may result from direct dopamine D4 receptor activation. Our results demonstrate functional dopamine D4 receptor expression and natural coupling in the PVN linked to signal transduction pathways that include immediate early gene and MAP kinase activation. Further, the ability of the selective dopamine D4 antagonist A-381393 alone to reduce c-Fos expression below control levels may imply the presence of a tonic dopamine D4 receptor activation under basal conditions in vivo. These findings provide additional evidence that the PVN may be a site of dopamine D4 receptor-mediated proerectile activity.

Dopamine D1 and D4 receptor subtypes differentially modulate recurrent excitatory synapses in prefrontal cortical pyramidal neurons

Although dopamine (DA) effects in the prefrontal cortex (PFC) have been studied extensively, the function of steady-state ambient levels of DA in the regulation of afferent excitatory transmission in PFC pyramidal neurons remains relatively unexplored. Using intracellular sharp-electrode and whole-cell recordings combined with intracellular labeling in brain slices, we found that D1/D5 receptor blockade did not alter synaptic responses in the PFC, but D1/D5 receptor activation consistently enhanced recurrent synaptic excitation in the majority of pyramidal neurons tested. In contrast, D4 receptor blockade resulted in an evoked complex multiple spike discharge pattern that contained both early and late (presumably multisynaptic) components of the evoked response that is contingent upon the preservation of axon collaterals of the neuron under study. Moreover, GABAergic interneurons were found to play a role in both responses; blockade of GABA(a)-mediated inhibition caused bath application of DA to convert monosynaptic excitatory postsynaptic potentials (EPSPs) to complex spike bursts riding on the late component of the EPSP. On the other hand, during the blockade of GABA(a)-mediated conductances, administration of a D4 receptor antagonist failed to facilitate evoked multiple spike discharge. Morphological analysis of axon collaterals of labeled neurons revealed that neurons in which the D4 receptor blockade induced the putative polysynaptic response had axon collaterals that were largely preserved. These data suggest that DA exerts a bidirectional modulation of PFC pyramidal neurons in brain slices provided that local network connections with interneurons are preserved, with D4 receptors under tonic stimulation by ambient low levels of DA, whereas D1/D5 receptors activated upon phasic DA input.

Activation of dopamine D4 receptors induces synaptic translocation of Ca2+/calmodulin-dependent protein kinase II in cultured prefrontal cortical neurons

One of the important targets of dopamine D4 receptors in prefrontal cortex (PFC) is the multifunctional Ca2+/calmodulin-dependent protein kinase II (CaMKII). In the present study, we investigated the effect of D4 receptor activation on subcellular localization of CaMKII. We found that activation of D4 receptors, but not D2 receptors, induced a rapid translocation of alpha-CaMKII from cytosol to postsynaptic sites in cultured PFC neurons. Activated CaMKII (Thr286 phospho-CaMKII) was also redistributed to postsynaptic sites after D4 receptor stimulation. The translocation was blocked by inhibiting the phospholipase C/inositol 1,4,5-trisphosphate receptor/Ca2+ signaling. Point mutation of the calmodulin binding site (Ala302), but not the autophosphorylation site (Thr286), of alpha-CaMKII prevented the D4-induced CaMKII translocation. Moreover, D4 receptors failed to induce CaMKII translocation in the presence of an actin stabilizer, and D4 activation reduced the binding of CaMKII to F-actin. Concomitant with the synaptic accumulation of alpha-CaMKII in response to D4 receptor activation, a D4-induced increase in the CaMKII phosphorylation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor glutamate receptor 1 (GluR1) subunits and the amplitude of AMPA receptor-mediated excitatory postsynaptic currents was also observed. Thus, our results show that D4 receptor activation induces the synaptic translocation of CaMKII through a mechanism involving Ca2+/calmodulin and F-actin, which facilitates the regulation of synaptic targets of CaMKII, such as AMPA receptors.

A-412997 is a selective dopamine D4 receptor agonist in rats

A-412997 (2-(3',4',5',6'-tetrahydro-2'H-[2,4'] bipyridinyl-1'-yl)-N-m-tolyl-acetamide) is a highly selective dopamine D4 receptor agonist that binds with high affinity to rat dopamine D4 and human dopamine D4.4 receptors (Ki=12.1 and 7.9 nM, respectively). In contrast to the dopamine D4 receptor agonists PD168077 and CP226269, A-412997 showed a better selectivity profile and no affinity <1000 nM for other dopamine receptors or any other proteins in a panel of seventy different receptors and channels. In functional assays using calcium flux, A-412997 was a potent full agonist at rat dopamine D4 receptors (28.4 nM, intrinsic activity=0.83) and did not activate rat dopamine D2L receptors, unlike CP226269. Dopamine D4 receptor selective agonists have been shown to induce penile erection in rats by central mechanisms. A-412997 induces penile erection in a conscious rat model (effective dose=0.1 micromol/kg, s.c.) with comparable efficacy as the nonselective D2-like agonist, apomorphine. When dosed systemically, A-412997 crossed the blood brain barrier rapidly and achieved significantly higher levels than PD168077. A-412997 is a highly selective dopamine D4 receptor agonist and a useful tool to understand the role of dopamine D4 receptors in rat models of central nervous system processes and disease.

A-412997, a selective dopamine D4 agonist, improves cognitive performance in rats

The recent development of a highly selective dopamine D4 receptor agonist, A-412997 (2-(3',4',5',6'-tetrahydro-2'H-[2,4'] bipyridinyl-1'-yl)-N-m-tolyl-acetamide), has provided a pharmacological tool with which to conduct systematic investigations into the putative role for dopamine D4 receptors in the central nervous system. These present studies evaluated the potential cognitive enhancing properties of A-412997 in rat models of ADHD (5-trial repeated acquisition inhibitory avoidance in Spontaneous Hypertensive Rat pups) and short-term memory (Social Recognition), in comparison with the less selective dopamine D4 receptor agonists PD168077 and CP226269. A-412997 showed significant dose-dependent efficacy in both models. PD168077 repeatedly improved acquisition in the 5-trial inhibitory avoidance model but failed to reach significance at any dose tested, although significantly improved social recognition was observed (albeit less potent than A-412997). CP226269 showed a significant enhancement in the 5-trial inhibitory avoidance model. These results support a role for the dopamine D4 receptor subtype in cognition.