PCP-induced Hyperlocomotion Research Articles

Discovery of dihydropyrazino-benzimidazole derivatives as metabotropic glutamate receptor-2 (mGluR2) positive allosteric modulators (PAMs)

A scaffold hopping strategy converted the known 1-[(1-methyl-1H-imidazol-2-yl)methyl]-4-phenylpiperidine core (1 and 2) by cyclization to a fused [6 + 5+6] membered heterocyclic mGluR2 PAM scaffold. Pharmacophore guided structure−activity relationship (SAR) studies resulted in a series of potent and metabolically stable mGluR2 PAMs. A representative optimized compound (95) having the most balanced profile, demonstrated efficacy in the PCP-induced hyper-locomotion model in mice that revealed the new chemotype being a promising PAM lead targeting mGluR2 receptors and providing support for further translational studies.

Effects of cariprazine on extracellular levels of glutamate, GABA, dopamine, noradrenaline and serotonin in the medial prefrontal cortex in the rat phencyclidine model of schizophrenia studied by microdialysis and simultaneous recordings of locomotor activity

RationaleAberrant glutamatergic, dopaminergic, and GABAergic neurotransmission has been implicated in schizophrenia. Cariprazine reverses the behavioral effects observed in the rat phencyclidine (PCP)-induced model of schizophrenia; however, little is known about its in vivo neurochemistry.ObjectivesThe study aims to compare the effects of cariprazine and aripiprazole on PCP-induced changes in the extracellular levels of glutamate, dopamine, serotonin, noradrenaline, and GABA in the rat medial prefrontal cortex (mPFC), and on locomotor activation.MethodsMicrodialysis was performed in awake rats with probes placed into the mPFC. Rats (n = 7/group) received vehicle (saline), cariprazine (0.05, 0.2, or 0.8 mg/kg), or aripiprazole (3 or 20 mg/kg) via gavage. After 60 min, 5 mg/kg PCP was administered intraperitoneally (i.p.). Samples were taken before drug administration, during pretreatment, and after PCP injection. Locomotor activity recording and microdialysis sampling occurred simultaneously.ResultsPCP treatment increased extracellular levels of all the neurotransmitters tested except GABA, for which there were no significant changes. Cariprazine and aripiprazole dose-dependently inhibited the PCP-induced increases of tested neurotransmitters. Overall effects were significant for higher cariprazine doses and both aripiprazole doses for glutamate and noradrenaline, for higher cariprazine doses and 20 mg/kg aripiprazole for dopamine, and for 0.8 mg/kg cariprazine and 20 mg/kg aripiprazole for serotonin and locomotor activity.ConclusionBoth cariprazine and aripiprazole dose-dependently attenuated PCP-induced hyperlocomotion and acute increases in glutamate, dopamine, noradrenaline, and serotonin levels in the mPFC; cariprazine was approximately 5-fold more potent than aripiprazole.

Antipsychotic-Like Efficacy of Dopamine D2 Receptor-Biased Ligands is Dependent on Adenosine A2A Receptor Expression.

Dopamine D2 receptor (D2R) activation triggers both G protein- and β-arrestin-dependent signaling. Biased D2R ligands activating β-arrestin pathway have been proposed as potential antipsychotics. The ability of D2R to heteromerize with adenosine A2A receptor (A2AR) has been associated to D2R agonist-induced β-arrestin recruitment. Accordingly, here we aimed to demonstrate the A2AR dependence of D2R/β-arrestin signaling. By combining bioluminescence resonance energy transfer (BRET) between β-arrestin-2 tagged with yellow fluorescent protein and bimolecular luminescence complementation (BiLC) of D2R/A2AR homomers and heteromers, we demonstrated that the D2R agonists quinpirole and UNC9994 could promote β-arrestin-2 recruitment only when A2AR/D2R heteromers were expressed. Subsequently, the role of A2AR in the antipsychotic-like activity of UNC9994 was assessed in wild-type and A2AR-/- mice administered with phencyclidine (PCP) or amphetamine (AMPH). Interestingly, while UNC9994 reduced hyperlocomotion in wild-type animals treated either with PCP or AMPH, in A2AR-/- mice, it failed to reduce PCP-induced hyperlocomotion or produced only a moderate reduction of AMPH-mediated hyperlocomotion. Overall, the results presented here reinforce the notion that D2R/A2AR heteromerization facilitates D2R β-arrestin recruitment, and furthermore, reveal a pivotal role for A2AR in the antipsychotic-like activity of the β-arrestin-biased D2R ligand, UNC9994.

Time-dependent sensitization of antipsychotic effect in adolescent male and female rats

Many behavioral and biological effects of a psychoactive drug often undergo time-dependent change following even one single drug exposure. The present study examined whether one or two exposures of haloperidol, olanzapine or clozapine would also induce a time-dependent change in their behavioral effects in adolescent rats, and whether such a change vary between sexes. Adolescent Sprague-Dawley rats (<40days old) were first treated with one single injection of haloperidol (0.05 and 0.1mg/kg, sc), clozapine (10.0 and 20.0mg/kg, sc), 2 injections of olanzapine (1.0 and 2.0mg/kg, sc) or vehicle, and tested in a conditioned avoidance response (CAR) model or a PCP (3.20mg/kg, sc)-induced hyperlocomotion model to assess the drug’s antipsychotic-like behavioral effects. One or three weeks later, rats were challenged with the drug and their avoidance responses and the PCP-induced hyperlocomotion were re-assessed. One-trial haloperidol and 2-trial olanzapine induced a sensitization, while 1-trial clozapine induced a tolerance effect. The 1-trial haloperidol sensitization was significantly higher at the 3-week time point than at 1-week point, especially in the females. Clozapine tolerance in the conditioned avoidance response model also exhibited the time-dependent increase in both sex groups. Olanzapine sensitization in the PCP model showed a time-dependent change in a sex-dependent fashion. Overall, the time-dependent antipsychotic sensitization and tolerance can be demonstrated in adolescent animals. Many pharmacological (e.g. specific drugs, drug doses), individual (e.g. male versus female) and environmental (e.g. specific behavioral models) factors play a role in the modulation of the strength of antipsychotic sensitization and tolerance.

Sex differences in aripiprazole sensitization from adolescence to adulthood

The present study investigated the potential sex differences in repeated aripiprazole (ARI) treatment-induced behavioral sensitization from adolescence to adulthood, and to determine whether ARI sensitization can be transferred to olanzapine (OLZ) and/or clozapine (CLZ) using the conditioned avoidance response (CAR) and phencyclidine-induced (PCP) hyperlocomotion tests of antipsychotic activity. Male and female Sprague-Dawley adolescence rats (P46) were first treated with ARI (10mg/kg) for 5 consecutive days (P46–50) and tested for avoidance response and ARI-induced inhibition of PCP-induced hyperlocomotion. After they became adults (>P68), rats were challenged with ARI (1.5mg/kg, sc) (P70), OLZ (0.5mg/kg, sc; P73), CLZ (5mg/kg, sc; P76) and again with ARI (1.5mg/kg, sc; P84) and tested for avoidance response and ARI-induced inhibition of PCP-induced hyperlocomotion again. During the drug treatment period in adolescence, repeated ARI treatment suppressed avoidance response, inhibited the PCP-induced hyperlocomotion, and these effects were progressively increased across the 5-day period in both males and females, confirming the induction of ARI sensitization. On the challenge days, rats previously treated with ARI in adolescence also had significantly lower avoidance and lower PCP-induced hyperlocomotion than the previous vehicle rats, confirming the expression of ARI sensitization and its persistence into adulthood. More importantly, female rats made significantly more avoidances than males in both ARI and vehicle groups, indicating higher sensitivity to the acute and long-term effects of ARI. Further, on the OLZ and CLZ challenge days, prior ARI treatment seemed to increase sensitivity to OLZ exposure, however, this increase was not significant. Similarly, rats also showed an ARI sensitization to OLZ and CLZ on challenge days. Collectively, results from this experiment demonstrated a sex difference in response to ARI and enhanced inhibition of PCP-induced hyperlocomotion in animals that were pretreated with ARI as compared to controls.

Effects of Repeated Ropinirole Treatment on Phencyclidine-Induced Hyperlocomotion, Prepulse Inhibition Deficits, and Social Avoidance in Rats.

Phencyclidine (PCP), a noncompetitive N-methyl d-aspartate (NMDA) receptor antagonist, provides the most complete pharmacologic model of schizophrenia in humans and animals. Acute PCP causes hyperlocomotion, disrupts prepulse inhibition (PPI), and increases social avoidance in rats. We have previously shown that repeated treatment with the dopamine (DA) D2-like receptor agonists, quinpirole or ropinirole, prevents agonist-induced PPI disruption. In the present study, we examined whether repeated ropinirole treatment similarly attenuates the effects of PCP in a more complete model of schizophrenia symptoms and examined the effect of repeated D2-like agonist treatment on locomotion, PPI, and social interaction after acute PCP challenge. The acute effect of PCP (3.0 or 6.0 mg/kg) on locomotor activity was examined to establish a minimum effective dose. Thereafter, the effect of PCP challenge (3.0 mg/kg) on locomotor activity, PPI, and social interaction was assessed in adult male rats before or 7-10 days after termination of repeated daily treatment with ropinirole (0.1 mg/kg) or saline vehicle (0.1 ml/kg) for 28 days. Repeated ropinirole treatment attenuates PCP-induced hyperlocomotion, PPI deficits, and social avoidance. These findings suggest that repeated ropinirole treatment might affect a final common pathway that is vulnerable to both PCP- and dopamine agonist-induced behavioral disruption, thereby providing an alternative approach to block the effects of PCP.

Differential control of dopamine ascending pathways by serotonin2B receptor antagonists: New opportunities for the treatment of schizophrenia

Recent studies suggest that the central serotonin2B receptor (5-HT2BR) could be an interesting pharmacological target for treating neuropsychiatric disorders related to dopamine (DA) dysfunction, such as schizophrenia. Thus, the present study was aimed at characterizing the role of 5-HT2BRs in the control of ascending DA pathway activity. Using neurochemical, electrophysiological and behavioral approaches, we assessed the effects of two selective 5-HT2BR antagonists, RS 127445 and LY 266097, on in vivo DA outflow in DA-innervated regions, on mesencephalic DA neuronal firing, as well as in behavioral tests predictive of antipsychotic efficacy and tolerability, such as phencyclidine (PCP)-induced deficit in novel object recognition (NOR) test, PCP-induced hyperlocomotion and catalepsy. Both RS 127445 (0.16 mg/kg, i.p.) and LY 266097 (0.63 mg/kg, i.p.) increased DA outflow in the medial prefrontal cortex (mPFC). RS 127445, devoid of effect in the striatum, decreased DA outflow in the nucleus accumbens, and potentiated haloperidol (0.1 mg/kg, s.c.)-induced increase in mPFC DA outflow. Also, RS 127445 decreased the firing rate of DA neurons in the ventral tegmental area, but had no effect in the substantia nigra pars compacta. Finally, both RS 127445 and LY 266097 reversed PCP-induced deficit in NOR test, and reduced PCP-induced hyperlocomotion, without inducing catalepsy. These results demonstrate that 5-HT2BRs exert a differential control on DA pathway activity, and suggest that 5-HT2BR antagonists could represent a new class of drugs for improved treatment of schizophrenia, with an ideal profile of effects expected to alleviate cognitive and positive symptoms, without eliciting extrapyramidal symptoms.

Design and synthesis of a novel 2-oxindole scaffold as a highly potent and brain-penetrant phosphodiesterase 10A inhibitor

Highly potent and brain-penetrant phosphodiesterase 10A (PDE10A) inhibitors based on the 2-oxindole scaffold were designed and synthesized. (2-Oxo-1,3-oxazolidin-3-yl)phenyl derivative 1 showed the high P-glycoprotein (P-gp) efflux (efflux ratio (ER)=6.2) despite the potent PDE10A inhibitory activity (IC50=0.94nM). We performed an optimization study to improve both the P-gp efflux ratio and PDE10A inhibitory activity by utilizing structure-based drug design (SBDD) techniques based on the X-ray crystal structure with PDE10A. Finally, 1-(cyclopropylmethyl)-4-fluoro-5-[5-methoxy-4-oxo-3-(1-phenyl-1H-pyrazol-5-yl)pyridazin-1(4H)-yl]-3,3-dimethyl-1,3-dihydro-2H-indol-2-one (19e) was identified with improved P-gp efflux (ER=1.4) and an excellent PDE10A inhibitory activity (IC50=0.080nM). Compound 19e also exhibited satisfactory brain penetration, and suppressed PCP-induced hyperlocomotion with a minimum effective dose of 0.3mg/kg by oral administration in mice.

Inhibition of the tyrosine phosphatase STEP61 restores BDNF expression and reverses motor and cognitive deficits in phencyclidine-treated mice.

Brain-derived neurotrophic factor (BDNF) and STriatal-Enriched protein tyrosine Phosphatase 61 (STEP61) have opposing functions in the brain, with BDNF supporting and STEP61 opposing synaptic strengthening. BDNF and STEP61 also exhibit an inverse pattern of expression in a number of brain disorders, including schizophrenia (SZ). NMDAR antagonists such as phencyclidine (PCP) elicit SZ-like symptoms in rodent models and unaffected individuals, and exacerbate psychotic episodes in SZ. Here we characterize the regulation of BDNF expression by STEP61, utilizing PCP-treated cortical culture and PCP-treated mice. PCP-treated cortical neurons showed both an increase in STEP61 levels and a decrease in BDNF expression. The reduction in BDNF expression was prevented by STEP61 knockdown or use of the STEP inhibitor, TC-2153. The PCP-induced increase in STEP61 expression was associated with the inhibition of CREB-dependent BDNF transcription. Similarly, both genetic and pharmacologic inhibition of STEP prevented the PCP-induced reduction in BDNF expression in vivo and normalized PCP-induced hyperlocomotion and cognitive deficits. These results suggest a mechanism by which STEP61 regulates BDNF expression, with implications for cognitive functioning in CNS disorders.

Partial mGlu₅ Negative Allosteric Modulators Attenuate Cocaine-Mediated Behaviors and Lack Psychotomimetic-Like Effects.

Cocaine abuse remains a public health concern for which pharmacotherapies are largely ineffective. Comorbidities between cocaine abuse, depression, and anxiety support the development of novel treatments targeting multiple symptom clusters. Selective negative allosteric modulators (NAMs) targeting the metabotropic glutamate receptor 5 (mGlu5) subtype are currently in clinical trials for the treatment of multiple neuropsychiatric disorders and have shown promise in preclinical models of substance abuse. However, complete blockade or inverse agonist activity by some full mGlu5 NAM chemotypes demonstrated adverse effects, including psychosis in humans and psychotomimetic-like effects in animals, suggesting a narrow therapeutic window. Development of partial mGlu5 NAMs, characterized by their submaximal but saturable levels of blockade, may represent a novel approach to broaden the therapeutic window. To understand potential therapeutic vs adverse effects in preclinical behavioral assays, we examined the partial mGlu5 NAMs, M-5MPEP and Br-5MPEPy, in comparison with the full mGlu5 NAM MTEP across models of addiction and psychotomimetic-like activity. M-5MPEP, Br-5MPEPy, and MTEP dose-dependently decreased cocaine self-administration and attenuated the discriminative stimulus effects of cocaine. M-5MPEP and Br-5MPEPy also demonstrated antidepressant- and anxiolytic-like activity. Dose-dependent effects of partial and full mGlu5 NAMs in these assays corresponded with increasing in vivo mGlu5 occupancy, demonstrating an orderly occupancy-to-efficacy relationship. PCP-induced hyperlocomotion was potentiated by MTEP, but not by M-5MPEP and Br-5MPEPy. Further, MTEP, but not M-5MPEP, potentiated the discriminative-stimulus effects of PCP. The present data suggest that partial mGlu5 NAM activity is sufficient to produce therapeutic effects similar to full mGlu5 NAMs, but with a broader therapeutic index.

Pharmacological evaluation of a novel phosphodiesterase 10A inhibitor in models of antipsychotic activity and cognition

In this study, we report the pharmacological effects of a novel PDE10A inhibitor, SEP-39. SEP-39 is a potent (1.0nM) inhibitor of human PDE10A in vitro, with good selectivity (>16000-fold) against other PDEs. In an in vivo occupancy study, the RO50 value was determined to be 0.7mg/kg (p.o.), corresponding to plasma and brain exposures of 28ng/mL and 43ng/g, respectively. Using microdialysis, we show that 3mg/kg (p.o.) SEP-39 significantly increased rat striatal cGMP concentrations. Furthermore, SEP-39 inhibits PCP-induced hyperlocomotion at doses of 1 and 3mg/kg (p.o.) corresponding to 59–86% occupancy. At similar doses in a catalepsy study, the time on the bar was increased but the maximal effect was less than that seen with haloperidol. In an EEG study, 3 and 10mg/kg (p.o.) SEP-39 suppressed REM intensity and increased the latency to REM sleep. We also demonstrate the procognitive effects of SEP-39 in the rat novel object recognition assay. These effects appear to require less PDE10A inhibition than the reversal of PCP-induced hyperlocomotion or EEG effects, as improvements in recognition index were seen at doses of 0.3mg/kg and above. Our data demonstrate that SEP-39 is a potent, orally active PDE10A inhibitor with therapeutic potential in a number of psychiatric indications.

Evolution and synthesis of novel orally bioavailable inhibitors of PDE10A

The design and synthesis of highly potent, selective orally bioavailable inhibitors of PDE10A is reported. Starting with an active compound of modest potency from a small focused screen, we were able to evolve this series to a lead molecule with high potency and selectivity versus other PDEs using structure-based design. A systematic refinement of ADME properties during lead optimization led to a lead compound with good half-life that was brain penetrant. Compound 39 was highly potent versus PDE10A (IC50=1.0 nM), demonstrated high selectivity (>1000-fold) against other PDEs and was efficacious when dosed orally in a rat model of psychosis, PCP-induced hyperlocomotion with an EC50 of 1 mg/kg.

Repeated administration of aripiprazole produces a sensitization effect in the suppression of avoidance responding and phencyclidine-induced hyperlocomotion and increases D2 receptor-mediated behavioral function.

The present study investigated how repeated administration of aripiprazole (a novel antipsychotic drug) alters its behavioral effects in two behavioral tests of antipsychotic activity and whether this alteration is correlated with an increase in dopamine D2 receptor function. Male adult Sprague-Dawley rats were first repeatedly tested with aripiprazole (3, 10 and 30 mg/kg, subcutaneously (sc)) or vehicle in a conditioned avoidance response (CAR) test or a phencyclidine (PCP) (3.20 mg/kg, sc)-induced hyperlocomotion test daily for five consecutive days. After 2-3 days of drug-free retraining or resting, all rats were then challenged with aripiprazole (1.5 or 3.0 mg/kg, sc). Repeated administration of aripiprazole progressively increased its inhibition of avoidance responding and PCP-induced hyperlocomotion. More importantly, rats previously treated with aripiprazole showed significantly lower avoidance response and lower PCP-induced hyperlocomotion than those previously treated with vehicle in the challenge tests. An increased sensitivity to quinpirole (a selective D2/3 agonist) in prior aripiprazole-treated rats was also found in the quinpirole-induced hyperlocomotion test, suggesting an enhanced D2/3-mediated function. These findings suggest that aripiprazole, despite its distinct receptor mechanisms of action, induces a sensitization effect similar to those induced by other antipsychotic drugs and this effect may be partially mediated by brain plasticity involving D2/3 receptor systems.

Repeated effects of the neurotensin receptor agonist PD149163 in three animal tests of antipsychotic activity: Assessing for tolerance and cross-tolerance to clozapine

Neurotensin is an endogenous neuropeptide closely associated with the mesolimbic dopaminergic system and shown to possess antipsychotic-like effects. In particular, acute neurotensin receptor activation can inhibit conditioned avoidance response (CAR), attenuate phencyclidine (PCP)-induced prepulse inhibition (PPI) disruptions, and reverse PCP-induced hyperlocomotion. However, few studies have examined the long term effects of repeated neurotensin receptor activation and results are inconsistent. Since clinical administration of antipsychotic therapy often requires a prolonged treatment schedule, here we assessed the effects of repeated activation of neurotensin receptors using an NTS1 receptor selective agonist, PD149163, in 3 behavioral tests of antipsychotic activity. We also investigated whether reactivity to the atypical antipsychotic clozapine was altered following prior PD149163 treatment. Using both normal and prenatally immune activated rats generated through maternal immune activation with polyinosinic:polycytidylic acid, we tested PD149163 in CAR, PCP (1.5mg/kg)-induced PPI disruption, and PCP (3.2mg/kg)-induced hyperlocomotion. For each paradigm, rats were first repeatedly tested with vehicle or PD149163 (1.0, 4.0, 8.0mg/kg, sc) along with vehicle or PCP for PPI and hyperlocomotion tests, then challenged with PD149163 after 2 drug-free days. All rats were then challenged with clozapine (5.0mg/kg, sc). During the repeated test period, PD149163 exhibited antipsychotic-like effects in all three models. On the PD149163 challenge day, prior drug treatment only caused a tolerance effect in CAR. This tolerance in CAR was transferrable to clozapine, as it enhanced clozapine tolerance in the same group of animals. Although no tolerance effect was seen in the PD149163 challenge for the PCP-induced hyperlocomotion test, the clozapine challenge showed increased sensitivity in groups previously exposed to repeated PD149163 treatment. Our findings suggest that repeated exposure to NTS1 receptor agonists can induce a dose-dependent tolerance and cross-tolerance to clozapine to some of its behavioral effects but not others.

Asenapine sensitization from adolescence to adulthood and its potential molecular basis

Asenapine is a new antipsychotic drug that induces a long-lasting behavioral sensitization in adult rats. The present study investigated the developmental impacts of adolescent asenapine treatment on drug sensitivity and on 3 proteins implicated in the action of antipsychotic drugs (i.e. brain-derived neurotrophic factor (BDNF), dopamine D2 receptor, and ΔFosB) in adulthood. Male adolescent Sprague–Dawley rats (postnatal days, P 43–48) were first treated with asenapine (0.05, 0.10 or 0.20mg/kg, sc) and tested in the conditioned avoidance or PCP (2.0mg/kg, sc)-induced hyperlocomotion tasks for 5 days. After they became adults (∼P 76), asenapine sensitization was assessed in a single avoidance or PCP-induced hyperlocomotion challenge test with all rats being injected with asenapine (0.10mg/kg, sc). Rats were then sacrificed 1 day later and BDNF, D2 and ΔFosB in the prefrontal cortex, striatum and hippocampus were examined using Western blotting. In adolescence, repeated asenapine treatment produced a persistent and dose-dependent inhibition of avoidance response, spontaneous motor activity and PCP-induced hyperlocomotion. In the asenapine challenge test, adult rats treated with asenapine (0.10 and 0.20mg/kg) in adolescence made significantly fewer avoidance responses and showed a stronger inhibition of spontaneous motor activity than those previously treated with saline. However, no group difference in the levels of BDNF, D2 and ΔFosB expression was found. These findings suggest that although adolescent asenapine treatment for a short period of time induces a robust behavioral sensitization that persists into adulthood, such a long-term effect is not likely to be mediated by BDNF, D2 and ΔFosB.

Effect of environmental cues on the behavioral efficacy of haloperidol, olanzapine, and clozapine in rats.

Previous studies have reported that context can powerfully modulate the inhibitory effect of an antipsychotic drug on phencyclidine (PCP)-induced hyperlocomotion (a behavioral test used to evaluate putative antipsychotic drugs). The present study investigated the experimental conditions under which environmental stimuli exert their influence through associative conditioning processes. Experiment 1 examined the extent to which previous antipsychotic treatment in the home cages affected a drug's ability to inhibit PCP-induced hyperlocomotion in novel motor activity test apparatus. Five days of repeated haloperidol (0.05 mg/kg, subcutaneously) and olanzapine (2.0 mg/kg, subcutaneously) treatment in the home cages still potentiated their inhibition of PCP-induced hyperlocomotion (i.e. sensitization) assessed in a new environment, whereas the clozapine (10.0 mg/kg, subcutaneously) treatment enhanced the development of clozapine tolerance, indicating a lack of environmental modulation of antipsychotic efficacy. Experiment 2 assessed the impact of different numbers of antipsychotic administrations (e.g. 4, 2 or 0), in either the home environment or test environment, on a drug's ability to inhibit PCP-induced hyperlocomotion. Repeated administration of clozapine (5.0 mg/kg, subcutaneously) or olanzapine (1.0 mg/kg, subcutaneously) for 4 consecutive days, irrespective of where these treatments occurred, led to a similar level of inhibition of PCP-induced hyperlocomotion. However, 4-day haloperidol (0.03 mg/kg, subcutaneously) treatment in the test apparatus led to significantly higher inhibition than a 4-day home-cage treatment. Thus, more exposures to the test environment under the influence of haloperidol (but not clozapine or olanzapine) caused a stronger inhibition than fewer exposures, indicating a strong environmental modulation. Collectively, these findings suggest that previous antipsychotic treatment in one environment could alter later antipsychotic-like response assessed in a different environment under certain test conditions. Therefore, whether the circumstances surrounding antipsychotic drug administration have a powerful effect on the expression of antipsychotic-like efficacy is dependent on specific experimental and drug treatment factors.

Long-lasting sensitization induced by repeated risperidone treatment in adolescent Sprague-Dawley rats: a possible D2 receptor mediated phenomenon?

Risperidone use in children and adolescents for the treatment of various neuropsychiatric disorders (e.g., schizophrenia, autism, disruptive behavior, etc.) has increased substantially in recent decades. However, its long-term effect on the brain and behavioral functions is not well understood. The present study investigated how a short-term risperidone treatment in adolescence impacts antipsychotic response in adulthood in the conditioned avoidance response and phencyclidine (PCP)-induced hyperlocomotion tests. Male adolescent Sprague-Dawley rats (postnatal days [P] 40-44 or 43-48) were first treated with risperidone (0.3, 0.5, or 1.0 mg/kg, subcutaneously (sc)) and tested in the conditioned avoidance or PCP (3.2 mg/kg, sc)-induced hyperlocomotion model daily for five consecutive days. After they became adults (~P 76-80), they were challenged with risperidone (0.3 mg/kg, sc) to assess their sensitivity to risperidone reexposure. A quinpirole (a D2/3 receptor agonist, 1.0 mg/kg, sc)-induced hyperlocomotion test was later conducted to assess the risperidone-induced functional changes in D2 receptor. In the risperidone challenge test in adulthood, adult rats previously treated with risperidone in adolescence made significantly fewer avoidance responses and exhibited significantly lower PCP-induced hyperlocomotion than those previously treated with vehicle. They also appeared to be more hyperactive than the vehicle-pretreated ones in the quinpirole-induced hyperlocomotion test. Prepulse inhibition of acoustic startle or fear-induced 22 kHz ultrasonic vocalizations in adulthood was not altered by adolescence risperidone treatment. Adolescent risperidone exposure induces a long-term increase in behavioral sensitivity to risperidone that persists into adulthood. This long-lasting change might be due to functional upregulation of D2-mediated neurotransmission.

Adult response to olanzapine or clozapine treatment is altered by adolescent antipsychotic exposure: A preclinical test in the phencyclidine hyperlocomotion model

This study examined how repeated olanzapine (OLZ) or clozapine (CLZ) treatment in adolescence alters sensitivity to the same drug in adulthood in the phencyclidine (PCP) hyperlocomotion model. Male adolescent Sprague-Dawley rats (postnatal day (P) 44-48) were first treated with OLZ (1.0 or 2.0 mg/kg, subcutaneously (sc)) or CLZ (10.0 or 20.0 mg/kg, sc) and tested in the PCP (3.2 mg/kg, sc)-induced hyperlocomotion model for five consecutive days. Then a challenge test with OLZ (0.5 mg/kg) or CLZ (5.0 mg/kg) was administered either during adolescence (~P 51) or after the rats matured into adults (~P 76 and 91). During adolescence, repeated OLZ or CLZ treatment produced a persistent inhibition of PCP-induced hyperlocomotion across the five test days. In the challenge test during adolescence, rats previously treated with OLZ did not show a significantly stronger inhibition of PCP-induced hyperlocomotion than those previously treated with vehicle (VEH). In contrast, those previously treated with CLZ showed a weaker inhibition than the VEH controls. When assessed in adulthood, the enhanced sensitivity to OLZ and the decreased sensitivity to CLZ were detected on ~P 76, even on ~P 91 in the case of OLZ. These findings suggest that adolescent OLZ or CLZ exposure can induce long-term alterations in antipsychotic response that persist into adulthood.

Repeated asenapine treatment produces a sensitization effect in two preclinical tests of antipsychotic activity

Among several commonly used atypical antipsychotic drugs, olanzapine and risperidone cause a sensitization effect in the conditioned avoidance response (CAR) and phencyclidine (PCP)-induced hyperlocomotion paradigms – two well established animal tests of antipsychotic drugs, whereas clozapine causes a tolerance effect. Asenapine is a novel antipsychotic drug recently approved for the treatment of schizophrenia and manic disorders. It shares several receptor binding sites and behavioral features with other atypical antipsychotic drugs. However, it is not clear what type of repeated effect (sensitization or tolerance) asenapine would induce, and whether such an effect is transferrable to other atypicals. In this study, male adult Sprague-Dawley rats were first repeatedly tested with asenapine (0.05, 0.10 or 0.20 mg/kg, sc) for avoidance response or PCP (3.20 mg/kg, sc)-induced hyperlocomotion daily for 5 consecutive days. After 2–3 days of retraining/drug-free recovery, they were then challenged with asenapine (0.10 mg/kg, sc), followed by olanzapine (0.50 mg/kg, sc) and clozapine (2.50 mg/kg, sc). During the 5-day drug test period (the induction phase), repeated asenapine treatment progressively increased its inhibition of avoidance response and PCP-induced hyperlocomotion in a dose-dependent fashion. On the asenapine and olanzapine challenge tests (the expression phase), rats previously treated with asenapine still showed significantly lower avoidance response and lower PCP-induced hyperlocomotion than those previously treated with vehicle. An increased reactivity to clozapine challenge in prior asenapine-treated rats was also found in the PCP-induced hyperlocomotion test. These findings suggest that asenapine is capable of inducing a sensitization effect and a cross-sensitization to olanzapine and clozapine (to a lesser extent). Because the behavioral profile of asenapine in both tests is similar to that of olanzapine, but different from that of clozapine, we suggest that asenapine resembles olanzapine to a greater extent than clozapine in its therapeutic and side effect profiles.

The atypical antipsychotic clozapine induces 5‐HT2ARmediated signaling and behavioral events in a beta‐arrestin2‐ independent but Akt‐dependent manner

The G protein‐coupled serotonin 2A receptor (5‐HT2AR) is a prominent target for atypical antipsychotic drugs, such as clozapine. Although clozapine is known to inhibit 5‐HT2AR mediated G protein coupling, it differs from classic GPCR antagonists in that it also induces 5‐HT2AR internalization and activates Akt signaling via a 5‐HT2AR‐mediated event. The cognate neurotransmitter at the 5‐HT2AR, serotonin, also induces 5‐HT2AR internalization and Akt phosphorylation, events which require the interaction between the receptor and the intracellular scaffolding and regulatory protein, beta‐arrestin2 (βarr2). Moreover, the 5‐HT2AR/βarr2/Akt interaction is critical for serotonin‐induced, 5‐HT2AR‐mediated behavioral responses in mice. Therefore, we hypothesized that clozapine also utilizes a βarr2‐mediated mechanism to induce 5‐HT2AR signaling and regulatory events, as well as its behavioral effects in mice. Here, we demonstrate that, unlike serotonin, clozapine‐mediates 5‐ HT2AR internalization and Akt phosphorylation independent of receptor interactions with βarr2. Moreover, clozapine‐mediated suppression of MK‐801 and PCP‐induced hyperlocomotion is βarr2 independent, although it is dependent upon Akt. These results demonstrate that serotonin and clozapine utilize differential mechanisms to in the induction of similar events downstream of the 5‐HT2AR. The further elucidation of these mechanisms in vivo may allow for the development of more effective atypical antipsychotic drugs.Funding from NIDA: DA025158