Increasing the density of the D2L receptor and manipulating the receptor environment are required to evidence the partial agonist properties of aripiprazole

Zacopride stimulates 5-HT4 serotonin receptors in the human atrium

Zacopride (4-amino-5-chloro-2-methoxy-N-(quinuclidin-3-yl)-benzamide) is a potent agonist in human 5-HT4 serotonin receptors in vitro and in the gastrointestinal tract. Zacopride was studied as an antiemetic drug and was intended to treat gastric diseases. Zacopride has been speculated to be useful as an antiarrhythmic agent in the human ventricle by inhibiting cardiac potassium channels. It is unknown whether zacopride is an agonist in human cardiac 5-HT4 serotonin receptors. We tested the hypothesis that zacopride stimulates human cardiac atrial 5-HT4 serotonin receptors. Zacopride increased the force of contraction and beating rate in isolated atrial preparations from mice with cardiac-specific overexpression of human 5-HT4 serotonin receptors (5-HT4-TG). However, it was inactive in wild-type mouse hearts (WT). Zacopride was as effective as serotonin in raising the force of contraction and beating rate in atrial preparations of 5-HT4-TG. Zacopride raised the force of contraction in human right atrial preparations (HAP) in the absence and presence of the phosphodiesterase III inhibitor cilostamide (1 µM). The positive inotropic effect of zacopride in HAP was attenuated by either 10 µM tropisetron or 1 µM GR125487, both of which are antagonists at 5-HT4 serotonin receptors. These data suggest that zacopride is also an agonist at 5-HT4 serotonin receptors in the human atrium.

Selective Activation of M1 Muscarinic Receptors Attenuates Human Colon Cancer Cell Proliferation

M3 muscarinic receptor (M3R) activation stimulates colon cancer cell proliferation, migration, and invasion; M3R expression is augmented in colon cancer and ablating M3R expression in mice attenuates colon neoplasia. Several lines of investigation suggest that in contrast to these pro-neoplastic effects of M3R, M1R plays an opposite role, protecting colon epithelial cells against neoplastic transformation. To pursue these intriguing findings, we examined the relative expression of M1R versus M3R in progressive stages of colon neoplasia and the effect of treating colon cancer cells with selective M1R agonists. We detected divergent expression of M1R and M3R in progressive colon neoplasia, from aberrant crypt foci to adenomas, primary colon cancers, and colon cancer metastases. Treating three human colon cancer cell lines with two selective M1R agonists, we found that in contrast to the effects of M3R activation, selective activation of M1R reversibly inhibited cell proliferation. Moreover, these effects were diminished by pre-incubating cells with a selective M1R inhibitor. Mechanistic insights were gained using selective chemical inhibitors of post-muscarinic receptor signaling molecules and immunoblotting to demonstrate M1R-dependent changes in the activation (phosphorylation) of key downstream kinases, EGFR, ERK1/2, and p38 MAPK. We did not detect a role for drug toxicity, cellular senescence, or apoptosis in mediating M1R agonist-induced attenuated cell proliferation. Lastly, adding M1R-selective agonists to colon cancer cells augmented the anti-proliferative effects of conventional chemotherapeutic agents. Collectively, these results suggest that selective M1R agonism for advanced colon cancer, alone or in combination with conventional chemotherapy, is a therapeutic strategy worth exploring.

Maternal treatment with aripiprazole prevents the development of a valproic acid-induced autism-like phenotype in juvenile male mice

Autism spectrum disorder (ASD) describes a heterogeneous group of neurodevelopmental conditions characterized by deficits in social communication and repetitive behaviors. Aripiprazole (APZ) is an atypical antipsychotic that can safeguard mice against autism-like behavior induced by valproic acid (VPA). In the present study, we examined the effects of maternal treatment with APZ (10 mg/kg) in juvenile mice prenatally exposed to VPA on neurodevelopmental behaviors, social interactions, communication, and working memory, as well as synaptophysin (SYP), synaptosomal-associated protein, 25 kDa (SNAP-25) and microtubule-associated protein 2 (MAP-2) expression in the medial prefrontal cortex (mPFC) and cell viability in the hippocampus. In addition, to evaluate possible APZ interference with the anticonvulsant properties of VPA on pentylenetetrazole (PTZ)-induced seizures were evaluated. Maternal treatment with APZ significantly prevented body weight loss, self-righting, eye-opening, social interactions, social communication, and working memory deficits in mice prenatally exposed to VPA. Additionally, the decrease in the SYP, SNAP-25, and MAP-2 expressions in the mPFC and cell death in the hippocampus was prevented by APZ. Furthermore, APZ (10 mg/kg) did not interfere with the anticonvulsant effect of VPA (15 mg/kg) in animals with PTZ-induced seizures. These findings indicate that maternal treatment with APZ in pregnant mice exposed to VPA protects animals against the ASD-like behavioral phenotype, and this effect may be related, at least in part, to synaptic plasticity and neuronal protection in the PFC and hippocampus. APZ may serve as an effective pharmacological therapeutic target against autistic behaviors in the VPA animal model of ASD, which should be further investigated to verify its clinical relevance.

Dopamine D2L receptor density influences the recruitment of β-arrestin2 and Gi1 induced by antiparkinsonian drugs

INTRODUCTION

Brain imaging studies have highlighted that the density of dopamine D₂ receptors markedly fluctuates across the stages of Parkinson's disease and in response to pharmacological treatment. Moreover, receptor density constitutes a molecular determinant for the signaling profile of D₂ receptor ligands. We therefore hypothesized that variations in receptor expression could influence D₂ receptor response to antiparkinsonian drugs, most notably with respect to the recruitment bias between G_i1 and β-arrestin2.

METHODS

The recruitment bias of dopamine, pramipexole, ropinirole, and rotigotine was examined using a nanoluciferase-based biosensor for probing the interactions of the D_2L receptor with either G_i1 or β-arrestin2. The characterization of the functional selectivity of these D₂ receptor agonists was performed at two distinct D_2L receptor densities by taking advantage of a cell model carrying an inducible system that enables the overexpression of the D_2L receptor when exposed to doxycycline.

RESULTS

A high receptor density oriented the balanced signaling profile of dopamine towards a preferential recruitment of G_i1. It also moderated the marked G_i1 and β-arrestin2 biases of pramipexole and rotigotine, respectively. At variance, the G_i1 bias of ropinirole appeared as not being influenced by D_2L receptor density.

CONCLUSIONS

Taken together, these observations highlight receptor density as a key driver of the signaling transducer recruitment triggered by antiparkinsonian agents. Moreover, given the putative beneficial properties of β-arrestin2 in promoting locomotion, this study provides molecular insights that position the arrestin-biased ligand rotigotine as a putatively more beneficial D₂ receptor agonist for the treatment of early and late Parkinson's disease.

Receptor density influences ligand-induced dopamine D2L receptor homodimerization

Chronic treatments with dopamine D2 receptor ligands induce fluctuations in D2 receptor density. Since D2 receptors tend to assemble as homodimers, we hypothesized that receptor density might influence constitutive and ligand-induced homodimerization. Using a nanoluciferase-based complementation assay to monitor dopamine D2L receptor homodimerization in a cellular model enabling the tetracycline-controlled expression of dopamine D2L receptors, we observed that increasing receptor density promoted constitutive dopamine D2L receptor homodimerization. Receptor full agonists promoted homodimerization, while antagonists and partial agonists disrupted dopamine D2L receptor homodimers. High receptor densities enhanced this inhibitory effect only for receptor antagonists. Taken together, our findings indicate that both receptor density and receptor ligands influence dopamine D2L receptor homodimerization, albeit excluding any strict correlation with ligands' intrinsic activity and highlighting further complexity to dopaminergic pharmacology.

Novel opioid-neurotensin-based hybrid peptide with spinal long-lasting antinociceptive activity and a propensity to delay tolerance development

The behavioral responses exerted by spinal administration of the opioid-neurotensin hybrid peptide, PK23, were studied in adult male rats. The antinociceptive effect upon exposure to a thermal stimulus, as well as tolerance development, was assessed in an acute pain model. The PK23 chimera at a dose of 10 nmol/rat produced a potent pain-relieving effect, especially after its intrathecal administration. Compared with intrathecal morphine, this novel compound was found to possess a favourable side effect profile characterized by a reduced scratch reflex, delayed development of analgesic tolerance or an absence of motor impairments when given in the same manner, though some animals died following barrel rotation as a result of its i.c.v. administration (in particular at doses higher than 10 nmol/rat). Nonetheless, these results suggest the potential use of hybrid compounds encompassing both opioid and neurotensin structural fragments in pain management. This highlights the enormous potential of synthetic neurotensin analogues as promising future analgesics.

Evaluation of Functional Selectivity of Haloperidol, Clozapine, and LASSBio-579, an Experimental Compound With Antipsychotic-Like Actions in Rodents, at G Protein and Arrestin Signaling Downstream of the Dopamine D2 Receptor

LASSBio-579, an N-phenylpiperazine antipsychotic lead compound, has been previously reported as a D₂ receptor (D₂R) ligand with antipsychotic-like activities in rodent models of schizophrenia. In order to better understand the molecular mechanism of action of LASSBio-579 and of its main metabolite, LQFM 037, we decided to address the hypothesis of functional selectivity at the D₂R. HEK-293T cells transiently coexpressing the human long isoform of D₂ receptor (D₂LR) and bioluminescence resonance energy transfer (BRET)-based biosensors were used. The antagonist activity was evaluated using different concentrations of the compounds in the presence of a submaximal concentration of dopamine (DA), after 5 and 20 min. For both signaling pathways, haloperidol, clozapine, and our compounds act as DA antagonists in a concentration-dependent manner, with haloperidol being by far the most potent, consistent with its nanomolar D₂R affinity measured in binding assays. In our experimental conditions, only haloperidol presented a robust functional selectivity, being four- to fivefold more efficient for inhibiting translocation of β-arrestin-2 (β-arr2) than for antagonizing Gi activation. Present data are the first report on the effects of LASSBio-579 and LQFM 037 on the β-arr2 signaling pathway and further illustrate that the functional activity could vary depending on the assay conditions and approaches used.

Antidopaminergic medication in healthy subjects provokes subjective and objective mental impairments tightly correlated with perturbation of biogenic monoamine metabolism and prolactin secretion

OBJECTIVES

Off-label prescription of antipsychotics to patients without psychotic symptoms has become a routine matter for many psychiatrists and also some general practitioners. Nonetheless, little is known about the possibly detrimental effects of antidopaminergic medications on general psychopathology, subjective mental state, or a possible association with physiological parameters in nonpsychotic individuals.

METHODS

In this randomized, single-blinded study, groups of healthy volunteers (n=18) received low doses of reserpine, aripiprazole, haloperidol, or placebo on 7 successive days. Relevant physiological parameters (plasma prolactin, concentrations of catecholamine metabolites in plasma, and 24-hour urine) and each subject's mental state (Positive and Negative Syndrome Scale, Hamilton Rating Scale for Depression, visual analogue scale, Beck Depression Inventory II) were assessed at the start and end of the trial.

RESULTS

Of the three active treatments, only reserpine caused a significant increase in some plasma- and urine-catecholamine metabolites, but all three medications evoked objective and subjective changes in general psychopathology scores, which correlated with individual increases in plasma homovanillic acid concentrations. Both objective and subjective impairments were significantly more pronounced in the subgroup with greatest increase of plasma prolactin. Subjects experiencing the most pronounced side effects under haloperidol, which compelled them to drop out, showed significantly higher prolactin concentration increases than those who tolerated haloperidol well.

CONCLUSION

We found consistent associations between altered markers of dopamine transmission and several objective and subjective mental impairments in healthy volunteers after 1 week's treatment with antidopaminergic medications. These findings should draw attention to a more intensive risk-benefit evaluation in cases of off-label prescription of antipsychotic medications.

Waterborne aripiprazole blunts the stress response in zebrafish

Here we provide, at least to our knowledge, the first evidence that aripiprazole (APPZ) in the water blunts the stress response of exposed fish in a concentration ten times lower than the concentration detected in the environment. Although the mechanism of APPZ in the neuroendocrine axis is not yet determined, our results highlight that the presence of APPZ residues in the environment may interfere with the stress responses in fish. Since an adequate stress response is crucial to restore fish homeostasis after stressors, fish with impaired stress response may have trouble to cope with natural and/or imposed stressors with consequences to their welfare and survival.

Early antipsychotic exposure affects serotonin and dopamine receptor binding density differently in selected brain loci of male and female juvenile rats

BACKGROUND

Antipsychotic drugs (APDs) were developed to treat schizophrenia in adults; however they have been increasingly prescribed (mostly "off-label") for children and adolescents. This study aimed to investigate the effects of aripiprazole, olanzapine and risperidone on the binding of serotonin (5-HT) and dopamine receptors in juvenile rat brain regions that are involved in antipsychotic efficacy.

METHODS

Male and female rats were treated orally with aripiprazole (1mg/kg), olanzapine (1mg/kg), risperidone (0.3mg/kg) or vehicle 3 times/day starting from postnatal day 23 (±1day) for 20 days. Quantitative autoradiography was performed to examine the receptor binding densities.

RESULTS

Olanzapine significantly decreased 5-HT2A (5-HT2AR) and 5-HT2C receptor (5-HT2CR) binding in the prefrontal cortex (PFC), cingulate cortex (Cg) and nucleus accumbens (NAc) of both male and female rats. In the caudate putamen (CPu), olanzapine attenuated 5-HT2AR binding in both genders, and reduced 5-HT2CR binding in male rats. Olanzapine increased D2 receptor (D2R) binding in the NAcS of male rats, but decreased it in females. Olanzapine increased D1 receptor (D1R) binding in the Cg, while aripiprazole decreased D1R binding in the PFC of males. Aripiprazole significantly reduced 5-HT2AR binding in the male PFC. Risperidone decreased 5-HT2AR binding in the PFC of female rats, while attenuating D1R binding in the PFC and Cg of males. However, APDs have no effects on the binding of serotonin and dopamine transporters.

CONCLUSION

This study revealed that aripiprazole, olanzapine and risperidone affected 5-HT2AR, 5-HT2CR, 5-HTT, D1R and D2R bindings differently in the brains of juvenile male and female rats.

Partial agonism and fast dissociation of LASSBio-579 at dopamine D2 receptor

In an attempt to better understand the molecular mechanism of action of the antipsychotic lead LASSBio-579 and of its main metabolite LQFM 037, the aim of this work was to evaluate their intrinsic activity and binding kinetics at the dopamine D2 receptor. In transfected HEK cells expressing the D2L receptor under an inducible promoter, LASSBio-579 and LQFM 037, but not clozapine, behaved as weak partial agonists in [(35)S]-GTPγS binding assays performed in optimized conditions previously shown to evidence the partial agonist profile of aripiprazole. Besides, data obtained in radioligand competition assays on rat striatal membranes suggested a rapid association to and dissociation from the D2-like receptors. Using the kinetic rate index based on the strategy of the dual-point competition association assay, we showed that our compounds share a similar kinetic profile with clozapine, distinct from the typical antipsychotic haloperidol. These two characteristics could contribute to the atypical-like profile observed after administration of LASSBio-579 to rodents, in models of positive and negative symptoms of schizophrenia.

Pretreatment or Posttreatment with Aripiprazole Attenuates Methamphetamine-induced Stereotyped Behavior in Mice

Aripiprazole is a third-generation atypical antipsychotic and a dopamine D₂ receptor partial agonist. In the present study, we investigated whether a single administration of aripiprazole to mice, either as a pretreatment or as a posttreatment, would affect stereotypy induced by methamphetamine (METH). Pretreatment of male ICR mice with aripiprazole (1 or 10 mg/kg, i.p.) attenuated the incidence of METH-induced stereotypical behavior in a dose-dependent manner. Pretreatment of mice with 1 mg/kg aripiprazole produced an increase in the locomotor activity in mice treated with METH compared with mice treated with vehicle plus METH and with 10 mg/kg aripiprazole plus METH. This increase in locomotion is indicative of a rightward shift in the dose–response curve for METH, consistent with a shift in the type of stereotypical behavior observed from biting to sniffing. Aripiprazole posttreatment, after METH-induced stereotypical behavior, was fully expressed and also significantly attenuated overall stereotypy in an aripiprazole dose-dependent manner. These data suggest that the antagonism of METH effects by aripiprazole should be investigated as a potential treatment for acute METH overdose.

Agonist and antagonist effects of aripiprazole on D₂-like receptors controlling rat brain dopamine synthesis depend on the dopaminergic tone

BACKGROUND

The atypical antipsychotic drug aripiprazole binds with high affinity to a number of G protein coupled receptors, including dopamine D₂ receptors, where its degree of efficacy as a partial agonist remains controversial.

METHODS

We examined the properties of aripiprazole at D₂-like autoreceptors by monitoring the changes of dopamine synthesis in adult rat brain striatal minces incubated ex vivo. The effects of the dopaminergic tone on the properties of aripiprazole were assayed by comparing a basal condition (2 mM K(+), low dopaminergic tone) and a stimulated condition (15 mM K(+), where dopamine release mimics a relatively higher dopaminergic tone). We also used 2 reference compounds: quinpirole showed a clear agonistic activity and preclamol (S-(-)-PPP) showed partial agonism under both basal and stimulated conditions.

RESULTS

Aripiprazole under the basal condition acted as an agonist at D₂-like autoreceptors and fully activated them at about 10 nM, inhibiting dopamine synthesis similarly to quinpirole. Higher concentrations of aripiprazole had effects not restricted to D₂-like autoreceptor activation. Under the stimulated (15 mM K(+)) condition, nanomolar concentrations of aripiprazole failed to decrease dopamine synthesis but could totally block the effect of quinpirole.

CONCLUSIONS

Under high dopaminergic tone, aripiprazole acts as a D₂-like autoreceptor antagonist rather than as an agonist. These data show that, ex vivo, alteration of dopaminergic tone by depolarization affects the actions of aripiprazole on D₂-like autoreceptors. Such unusual effects were not seen with the typical partial agonist preclamol and are consistent with the hypothesis that aripiprazole is a functionally selective D₂R ligand.

Functionally selective dopamine D₂, D₃ receptor partial agonists

Dopamine D2 receptor-promoted activation of Gα(o) over Gα(i) may increase synaptic plasticity and thereby might improve negative symptoms of schizophrenia. Heterocyclic dopamine surrogates comprising a pyrazolo[1,5-a]pyridine moiety were synthesized and investigated for their binding properties when low- to subnanomolar K(i) values were determined for D(2L), D(2S), and D3 receptors. Measurement of [(35)S]GTPγS incorporation at D(2S) coexpressed with G-protein subunits indicated significant bias for promotion of Gα(o1) over Gα(i2) coupling for several test compounds. Functionally selective D(2S) activation was most striking for the carbaldoxime 8b (Gα(o1), pEC50 = 8.87, E(max) = 65%; Gα(i2), pEC50 = 6.63, E(max) = 27%). In contrast, the investigated 1,4-disubstituted aromatic piperazines (1,4-DAPs) behaved as antagonists for β-arrestin-2 recruitment, implying significant ligand bias for G-protein activation over β-arrestin-2 recruitment at D(2S) receptors. Ligand efficacy and selectivity between D(2S) and D3 activation were strongly influenced by regiochemistry and the nature of functional groups attached to the pyrazolo[1,5-a]pyridine moiety.

Repeated aripiprazole treatment causes dopamine D2 receptor up-regulation and dopamine supersensitivity in young rats

Aripiprazole is a second-generation antipsychotic that is increasingly being prescribed to children and adolescents. Despite this trend, little preclinical research has been done on the neural and behavioral actions of aripiprazole during early development. In the present study, young male and female Sprague-Dawley rats were pretreated with vehicle, haloperidol (1 mg/kg), or aripiprazole (10 mg/kg) once daily on postnatal days (PD) 10-20. After 1, 4, or 8 days (i.e. on PD 21, PD 24, or PD 28), amphetamine-induced locomotor activity and stereotypy, as well as dorsal striatal D2 receptor levels, were measured in separate groups of rats. Pretreating young rats with aripiprazole or haloperidol increased D2 binding sites in the dorsal striatum. Consistent with these results, dopamine supersensitivity was apparent when aripiprazole- and haloperidol-pretreated rats were given a test day injection of amphetamine (2 or 4 mg/kg). Increased D2 receptor levels and altered behavioral responding persisted for at least 8 days after conclusion of the pretreatment regimen. Contrary to what has been reported in adults, repeated aripiprazole treatment caused D2 receptor up-regulation and persistent alterations of amphetamine-induced behavior in young rats. These findings are consistent with human clinical studies showing that children and adolescents are more prone than adults to aripiprazole-induced side effects, including extrapyramidal symptoms.

Behavioral effects of dopamine receptor inactivation in the caudate-putamen of preweanling rats: role of the D2 receptor

RATIONALE

Inactivating dopamine (DA) receptors in the caudate-putamen (CPu) attenuates basal and DA agonist-induced behaviors of adult rats while paradoxically increasing the locomotor activity of preweanling rats.

OBJECTIVE

The purpose of this study was to determine (a) whether D1 or D2 receptor inactivation is responsible for the elevated locomotion shown by preweanling rats and (b) whether DA receptor inactivation produces a general state in which any locomotor-activating drug will cause a potentiated behavioral response.

METHODS

Dimethyl sulfoxide (DMSO) or N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was bilaterally infused into the CPu on postnatal day (PD) 17. In experiment 1, DA receptors were selectively protected from EEDQ-induced alkylation by pretreating rats with D1 and/or D2 antagonists. On PD 18, rats received bilateral microinjections of the DA agonist R(-)-propylnorapomorphine into the dorsal CPu, and locomotor activity was measured for 40 min. In subsequent experiments, the locomotion of DMSO- and EEDQ-pretreated rats was assessed after intraCPu infusions of the selective DA agonists SKF82958 and quinpirole, the partial agonist terguride, or after systemic administration of nonDAergic compounds.

RESULTS

Experiment 1 showed that EEDQ's ability to enhance the locomotor activity of preweanling rats was primarily due to the inactivation of D2 receptors. Consistent with this finding, only drugs that directly or indirectly stimulated D2 receptors produced a potentiated locomotor response in EEDQ-treated rats.

CONCLUSIONS

These results show that DA receptor inactivation causes dramatically different behavioral effects in preweanling and adult rats, thus providing additional evidence that the D2 receptor system is not functionally mature by the end of the preweanling period.

Support for 5-HT2C receptor functional selectivity in vivo utilizing structurally diverse, selective 5-HT2C receptor ligands and the 2,5-dimethoxy-4-iodoamphetamine elicited head-twitch response model

There are seemingly conflicting data in the literature regarding the role of serotonin (5-HT) 5-HT2C receptors in the mouse head-twitch response (HTR) elicited by the hallucinogenic 5-HT2A/2B/2C receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI). Namely, both 5-HT2C receptor agonists and antagonists, regarding 5-HT2C receptor-mediated Gq-phospholipase C (PLC) signaling, reportedly attenuate the HTR response. The present experiments tested the hypothesis that both classes of 5-HT2C receptor compounds could attenuate the DOI-elicited-HTR in a single strain of mice, C57Bl/6J. The expected results were considered in accordance with ligand functional selectivity. Commercially-available 5-HT2C agonists (CP 809101, Ro 60-0175, WAY 161503, mCPP, and 1-methylpsilocin), novel 4-phenyl-2-N,N-dimethyl-aminotetralin (PAT)-type 5-HT2C agonists (with 5-HT2A/2B antagonist activity), and antagonists selective for 5-HT2A (M100907), 5-HT2C (SB-242084), and 5-HT2B/2C (SB-206553) receptors attenuated the DOI-elicited-HTR. In contrast, there were differential effects on locomotion across classes of compounds. The 5-HT2C agonists and M100907 decreased locomotion, SB-242084 increased locomotion, SB-206553 resulted in dose-dependent biphasic effects on locomotion, and the PATs did not alter locomotion. In vitro molecular pharmacology studies showed that 5-HT2C agonists potent for attenuating the DOI-elicited-HTR also reduced the efficacy of DOI to activate mouse 5-HT2C receptor-mediated PLC signaling in HEK cells. Although there were differences in affinities of a few compounds at mouse compared to human 5-HT2A or 5-HT2C receptors, all compounds tested retained their selectivity for either receptor, regardless of receptor species. Results indicate that 5-HT2C receptor agonists and antagonists attenuate the DOI-elicited-HTR in C57Bl/6J mice, and suggest that structurally diverse 5-HT2C ligands result in different 5-HT2C receptor signaling outcomes compared to DOI.