Atypical properties of several classes of antipsychotic drugs on the basis of differential induction of Fos-like immunoreactivity in the rat brain

The effect of amisulpride, olanzapine, quetiapine, and aripiprazole single administration on c-Fos expression in vasopressinergic and oxytocinergic neurons of the rat hypothalamic paraventricular nucleus

Antipsychotics, including amisulpride (AMI), quetiapine (QUE), aripiprazole (ARI), and olanzapine (OLA), are used to treat mental illnesses associated with psychotic symptoms. The effect of these drugs on c-Fos expression in vasopressinergic (AVP) and oxytocinergic (OXY) neurons was studied in the hypothalamic paraventricular nucleus (PVN) of rats. The presence of c-Fos in AVP and OXY perikarya was investigated in seven PVN cells segregations: the anterior (Ant), dorsal cup (Dc), wing-shaped (Wi), periventricular zone (Pe), circle-shaped core (Co) and shell of core (Sh), and the posterior (pPVN) after an acute treatment with AMI-20 mg/kg, QUE-15 mg/kg, ARI-10 mg/kg, and OLA-5 mg/kg/bw in rats. Ninety min after treatments, the animals were sacrificed by transcardial perfusion with fixative and the PVN area sliced into 35 μm thick coronal sections for immunohistochemistry. The c-Fos was processed by avidin-biotin-peroxidase complex intensified with nickel-enhanced 3,3'-diaminobenzidine tetrahydrochloride. Visualization of AVP- and OXY-synthesizing neurons was achieved by a fluorescent marker Alexa Flour 568. The c-Fos-AVP and c-Fos-OXY colocalizations were evaluated from c-Fos stained sections merged with AVP or OXY ones. AMI, QUE, ARI, and OLA, single administration distinctly increased the c-Fos expression in each of the PVN cells segregations. QUE induced the highest magnitude of activation of AVP and OXY neurons, while OLA and AMI had only moderate effects. Incontestable variabilities detected in c-Fos expression in PVN AVP and OXY neurons extend the knowledge of selected antipsychotics extra-striatal actions and may also be helpful in a presumption of their possible functional impact.

Effect of amisulpride, olanzapine, quetiapine, and aripiprazole single administration on c-Fos expression in vasopressinergic and oxytocinergic neurons of the rat hypothalamic supraoptic nucleus

OBJECTIVE

The goal of this study was to reveal the impact of four types of atypical antipsychotics including amisulpride (AMI), olanzapine (OLA), quetiapine (QUE), and aripiprazole (ARI), with different receptor-affinity profile and dissociation constant, on the activity of hypothalamic supraoptic nucleus (SON) vasopressinergic and oxytocinergic neurons.

METHODS

Male Sprague Dawley rats received a single injection of vehicle (VEH) (0.1 ml/100g), AMI (20 mg/kg), OLA (5 mg/kg), QUE (15 mg/kg/) or ARI (10 mg/kg). Ninety min after treatment, the animals were fixed by transcardial perfusion, the brains removed, and cryocut into serial coronal sections of 35 µm thickness. The sections were processed for c-Fos staining using an avidin-biotin-peroxidase complex and visualized by nickel intensified diaminobenzidine to reach black end product. Afterwards, the sections were exposed to vasopressin (AVP) and oxytocin (OXY) antibodies and the reaction product visualized by biotin-labeled fluorescent Alexa Fluor 568 dye. The data were evaluated from c-Fos and AVP or OXY merged sections.

RESULTS

The present study shows that all four antipsychotics applied induced c-Fos expression in the SON. With respect to the stimulation efficacy of the individual antipsychotics, estimated based on the quantity of c-Fos-labeled AVP and OXY neurons, could be a preferential action assigned to QUE over moderate effect of ARI and lower effect to OLA and reduced effect of AMI (VEH < AMI < OLA < ARI < QUE).

CONCLUSION

The present data for the first time provide an insight into the quantitative pattern of brain activity within the clusters of SON AVP and OXY cells in response to different atypical antipsychotics single treatment.

Fos expression induced by olanzapine and risperidone in the central extended amygdala

The extended amygdala has been proposed to play an essential role in cognitive and affective processes and in neuropsychiatric disorders. In the present study, we examined the induction of Fos-like nuclei in the central amygdaloid nucleus (CeA), sublenticular extended amygdala (SLEA), interstitial nucleus of the posterior limb of the anterior commissure (IPAC), and bed nucleus of the stria terminalis (BSTL) of rodents to improve the knowledge regarding the pharmacological profile, therapeutic efficacy, and side-effects of olanzapine, an atypical antipsychotic drug and risperidone, a mixed atypical/typical antipsychotic drug in the rat brain. In addition, we evaluated the induction of Fos-like-nuclei in areas connected with these structures such as prefrontal cortex (PFCx), and nucleus accumbens shell, and in other important areas including the lateral septum and caudate-putamen that are involved in the therapeutic efficacy or side-effects of antipsychotic drugs. Fos-like-immunoreactivity induced by olanzapine and risperidone was compared with that by the atypical antipsychotic clozapine and typical antipsychotic haloperidol. Regarding the extended amygdala, and similarly to clozapine, olanzapine (5-10 mg/kg) and, with a lower efficacy, risperidone (1-3 mg/kg), induced Fos-like-nuclei in CeA, IPAC, SLEA, and BSTL. Both these drugs increased the induction of Fos-like-nuclei in PFCx, nucleus accumbens shell, lateral septum, and caudate-putamen. On the contrary, the increase of Fos-like-nuclei in the extended amygdala by haloperidol was restricted to IPAC only. These findings, consistent with the important role of extended amygdala in neuropsychiatric disorders characterized by affective disturbances, showed that olanzapine and risperidone, contrary to haloperidol, preferentially activated Fos-expression in these brain areas.

c-Fos expression response to olanzapine, amisulpride, aripiprazole, and quetiapine single administration in the rat forebrain: Effect of a mild stress preconditioning

Antipsychotics have been shown to stimulate different forebrain areas, whereas some of them are sensitive to stress. In the present study, effect of a single administration of olanzapine (OLA), amisulpride (AMI), aripiprazole (ARI), and quetiapine (QUE) on the activity of cells in the striatal dorsolateral (stDL) area, the periventricular zone (peVZ), the septal ventrolateral (seVL) nucleus, and the accumbens nucleus shell (shACC) and core (coACC) was investigated in male rats preconditioned with a mild stress complex (CMS) for 20 days. The objective of the study was to extend the anatomical-functional knowledge on the mechanism of selected antipsychotics with the goals: 1) to analyze the ability of the selected antipsychotics to induce c-Fos protein expression in the above mentioned forebrain structures and to map the pattern of their topography and 2) to find out whether longer-lasting mild stress preconditioning may modify the impact of the selected antipsychotics on the activity of cells in the forebrain areas in adult rats. Ten groups of rats were used. CMS complex contained five stressors: cage crowding, air-puff noising, wet bedding, predator stress, and forced swimming. AMI (20 mg/kg), OLA (5 mg/kg), QUE (15 mg/kg), and ARI (10 mg/kg/b.w.) were administered intraperitoneally and 90 min later the animals transcardially perfused by fixative. c-Fos was visualized by ABC complex. In unstressed animals, OLA and ARI elevated c-Fos expression in all areas studied, AMI and QUE in all areas except stDL, seVL and coACC, shACC FL-2 (shACC posterior level), respectively. CMS potentiated the effect of AMI in coACC, and QUE in shACC FL-2 and suppressed the effect of AMI in peVZ, and ARI in peVZ and seVL. The present data provide new insights into activity of cells in response to CMS challenge, which might be helpful in understanding the diverse clinical effects of atypical antipsychotics.

Effects of aripiprazole on caffeine-induced hyperlocomotion and neural activation in the striatum

Aripiprazole is an antipsychotic that acts as a partial agonist at dopamine D2 receptors. In addition to its antipsychotic activity, this compound blocks the effects of some psychostimulant drugs. It has not been verified, however, if aripiprazole interferes with the effects of caffeine. Hence, this study tested the hypothesis that aripiprazole prevents caffeine-induced hyperlocomotion and investigated the effects of these drugs on neural activity in the striatum. Male Swiss mice received injections of vehicle or antipsychotic drugs followed by vehicle or caffeine. Locomotion was analyzed in a circular arena and c-Fos protein expression was quantified in the dorsolateral, dorsomedial, and ventrolateral striatum, and in the core and shell regions of nucleus accumbens. Aripiprazole (0.1, 1, and 10 mg/kg) prevented caffeine (10 mg/kg)-induced hyperlocomotion at doses that do not change basal locomotion. Haloperidol (0.01, 0.03, and 0.1 mg/kg) also decreased caffeine-induced hyperlocomotion at all doses, although at the two higher doses, this compound reduced basal locomotion. Immunohistochemistry analysis showed that aripiprazole increases c-Fos protein expression in all regions studied, whereas caffeine did not alter c-Fos protein expression. Combined treatment of aripiprazole and caffeine resulted in a decrease in the number of c-Fos positive cells as compared to the group receiving aripiprazole alone. In conclusion, aripiprazole prevents caffeine-induced hyperlocomotion and increases neural activation in the striatum. This latter effect is reduced by subsequent administration of caffeine. These results advance our understanding on the pharmacological profile of aripiprazole.

Effect of acute asenapine treatment on Fos expression in the forebrain structures under normal conditions and mild stress preconditioning in the rat

Asenapine (ASE) is a novel atypical antipsychotic drug approved for the treatment of schizophrenia and bipolar disorder. Stress is an inseparable part of the human life, which may interfere with the therapeutic effect of different drugs. The aim of the present study was: (1) to delineate the quantitative and qualitative profiles of the ASE effect on Fos expression in the striatum, septum, nucleus accumbens, and the prefrontal cortex and (2) to find out whether a chronic unpredictable variable mild stress (CMS) preconditioning may modify the effect of acute ASE treatment. Stress paradigms included restrain, social isolation, crowding, swimming, and cold. The animals were exposed to CMS for 21 days and on the 22nd day received an injection of vehicle (saline 300 μl/rat s.c.) or ASE (0.3mg/kg s.c.). They were sacrificed 90 min after the treatments. Fos protein was visualized by avidin biotin peroxidase (ABC). Four groups of animals were investigated: controls+vehicle, controls+ASE, CMS+vehicle, and CMS+ASE. The number of Fos labeled neurons was calculated per total investigated area, which was selective for each structure, and also recalculated per unified sector. ASE treatment induced significant and very similar increase of the Fos expression in both ASE control and ASE CMS animals in comparison with saline control and CMS ones. Moreover, ASE induced regional differences in the number of Fos-positive neurons. In both ASE groups most pronounced response in the number of Fos profiles occurred in the dorsolateral striatum, ventrolateral septum, shell of the nucleus accumbens, and the medial prefrontal cortex. Mild Fos response was seen in the dorsomedial and ventromedial striatum and core of the nucleus accumbens. No response was seen in the dorsolateral septum. The present paper demonstrates for the first time the character of the Fos distribution in the forebrain structures induced by acute ASE treatment as well as ASE response to 21 days CMS preconditioning. The study provides an important comparative background that may help in the further understanding of the effect of ASE on the brain activation as well as its responsiveness to CMS challenges.

Quetiapine and aripiprazole signal differently to ERK, p90RSK and c-Fos in mouse frontal cortex and striatum: role of the EGF receptor

Background

Signaling pathways outside dopamine D2 receptor antagonism may govern the variable clinical profile of antipsychotic drugs (APD) in schizophrenia. One postulated mechanism causal to APD action may regulate synaptic plasticity and neuronal connectivity via the extracellular signal-regulated kinase (ERK) cascade that links G-protein coupled receptors (GPCR) and ErbB growth factor signaling, systems disturbed in schizophrenia. This was based upon our finding that the low D2 receptor affinity APD clozapine induced initial down-regulation and delayed epidermal growth factor receptor (EGFR or ErbB1) mediated activation of the cortical and striatal ERK response in vivo distinct from olanzapine or haloperidol. Here we map whether the second generation atypical APDs aripiprazole and quetiapine affect the EGFR-ERK pathway and its substrates p90RSK and c-Fos in mouse brain, given their divergent agonist and antagonist properties on dopaminergic transmission, respectively.

Results

In prefrontal cortex, aripiprazole triggered triphasic ERK phosphorylation that was EGFR-independent but had no significant effect in striatum. Conversely quetiapine did not alter cortical ERK signaling but elevated striatal ERK levels in an EGFR-dependent manner. Induction of ERK by aripiprazole did not affect p90RSK signaling but quetiapine decreased RSK phosphorylation within 1-hour of administration. The transcription factor c-Fos by comparison was a direct target of ERK phosphorylation induced by aripiprazole in cortex and quetiapine in striatum with protein levels in temporal alignment with that of ERK.

Conclusions

These data indicate that aripiprazole and quetiapine signal to specific nuclear targets of ERK, which for quetiapine occurs via an EGFR-linked mechanism, possibly indicating involvement of this system in its action.

Iminodibenzyl class antipsychotics for schizophrenia: a systematic review and meta-analysis of carpipramine, clocapramine, and mosapramine

BACKGROUND

We conducted a meta-analysis of the iminodibenzyl antipsychotics carpipramine, clocapramine, and mosapramine, which are classified as second-generation antipsychotics (SGAs) for schizophrenia treatment.

METHODS

We searched data that had been published in PubMed, the Cochrane Library databases, PsycINFO, CiNii, and the Japan Medical Abstracts Society up to August 29, 2014. Randomized controlled trials that compared iminodibenzyl antipsychotics with other antipsychotics in patients with schizophrenia were included. Odds ratios and standardized mean differences were evaluated.

RESULTS

We included four randomized controlled trials on carpipramine (number of patients [n]=290), six on clocapramine (n=1,048), and five on mosapramine (n=986) in the meta-analysis. There were no significant differences in the response rates or in the discontinuation rates either between carpipramine and the other pooled antipsychotics or between clocapramine and the other pooled antipsychotics. On the Positive and Negative Syndrome Scale, mosapramine's positive subscale scores were superior to those of the other pooled antipsychotics (standard mean of difference =-0.22); however, on that same scale, there were no significant differences in total scores, negative scores, general subscale scores, response rates, or the discontinuation rates between mosapramine and the other pooled antipsychotics. Furthermore, the incidences of extrapyramidal symptoms and of hyperprolactinemia were significantly greater with mosapramine than with the other pooled antipsychotics.

CONCLUSION

The pharmacological profiles of carpipramine and clocapramine, which are classified as SGAs, were similar to those of first-generation antipsychotics because there were no significant differences in efficacy and safety outcomes. However, mosapramine was associated with a greater risk of extrapyramidal symptoms and hyperprolactinemia than the other SGAs were, although it may be beneficial for the improvement of positive symptoms.

Efficacy and tolerability of blonanserin in 48 patients with intractable schizophrenia

Takaki M, Okahisa Y, Kodama M, Mizuki Y, Sakamoto S, Ujike H, Uchitomi Y. Efficacy and tolerability of blonanserin in 48 patients with intractable schizophrenia.

Background: Blonanserin is effective for the treatment of schizophrenia in Korea and Japan.

Methods: We administered blonanserin to 48 Japanese patients with schizophrenia for whom other atypical antipsychotics were not sufficiently effective or tolerated.

Results: Previous antipsychotics were replaced with blonanserin because of its effectiveness (54.2%; 26/48) or tolerability (45.8%; 22/48). Blonanserin was more effective in 65.4% (17/26) of the and better tolerated in 95.5% (21/22) of the patients. Of 48 patients, 33 continued blonanserin for 1 year. The mean Clinical Global Impression of Severity scores improved from 4.60 to 2.48. The mean Global Assessment of Functioning score improved from 29.8 to 51.7. Nineteen patients (39.6%; 19/48) had a social role. The reasons for discontinuation of blonanserin were ineffectiveness against psychosis (27.1%; 13/48) or intolerability (4.2%; 2/48). The ratio of discontinuation for intolerability versus ineffectiveness was 0.15, which was the lowest among atypical psychotics.

Conclusions: Blonanserin may be effective and safe for the treatment of intractable schizophrenia.

Neuroanatomical substrates of the disruptive effect of olanzapine on rat maternal behavior as revealed by c-Fos immunoreactivity

Olanzapine is one of the most widely prescribed atypical antipsychotic drugs in the treatment of schizophrenia. Besides its well-known side effect on weight gain, it may also impair human parental behavior. In this study, we took a preclinical approach to examine the behavioral effects of olanzapine on rat maternal behavior and investigated the associated neural basis using the c-Fos immunohistochemistry. On postpartum days 6-8, Sprague-Dawley mother rats were given a single injection of sterile water or olanzapine (1.0, 3.0 or 5.0mg/kg, sc). Maternal behavior was tested 2h later, after which rats were sacrificed and brain tissues were collected. Ten brain regions that were either implicated in the action of antipsychotic drugs and/or in the regulation of maternal behavior were examined for c-Fos immunoreactivity. Acute olanzapine treatment dose-dependently disrupted various components of maternal behavior (e.g., pup retrieval, pup licking, nest building, crouching) and increased c-Fos immunoreactivity in the medial prefrontal cortex (mPFC), nucleus accumbens shell and core (NAs and NAc), dorsolateral striatum (DLSt), ventral lateral septum (LSv), central amygdala (CeA) and ventral tegmental area (VTA), important brain areas generally implicated in the incentive motivation and reward processing. In contrast, olanzapine treatment did not alter c-Fos in the medial preoptic nucleus (MPN), ventral bed nucleus of the stria terminalis (vBST) and medial amygdala (MeA), the core brain areas directly involved in the mediation of rat maternal behavior. These findings suggest that olanzapine disrupts rat maternal behavior primarily by suppressing incentive motivation and reward processing via its action on the mesocorticolimbic dopamine systems, other limbic and striatal areas, but not by disrupting the core processes involved in the mediation of maternal behavior in particular.

Long-lasting recovery of psychotic-like symptoms in isolation-reared rats after chronic but not acute treatment with the cannabinoid antagonist AM251

In this work we investigated the ability of AM251 to reverse schizophrenia-like symptoms produced by a neurodevelopmental animal model based on a social isolation procedure. First, we assessed the validity of our isolation-rearing protocol and, as expected, isolation-reared rats showed hyperlocomotion in a novel environment, cognitive impairment in the novel object recognition (NOR) test and a significant increase in the number of aggressive behaviours in the social interaction test compared to group-housed controls. This behavioural picture was associated with a reduction in CB₁ receptor/G protein coupling in specific brain areas as well as reduced c-Fos immunoreactivity in the prefrontal cortex and caudate putamen. In this model, chronic but not acute treatment with the CB₁ receptor antagonist AM251 counteracted isolation-induced cognitive impairment in the NOR test and aggressive behaviours in the social interaction test. This behavioural recovery was accompanied by the rescue of CB₁ receptor functionality and c-Fos levels in all brain regions altered in isolation-reared rats. Moreover, chronic AM251 also increased c-Fos immunoreactivity in the nucleus accumbens, as previously demonstrated for antipsychotic drugs. Interestingly, the behavioural recovery due to chronic AM251 administration persisted until 10 d after discontinuing the treatment, indicating a long-lasting effect of the cannabinoid antagonist on psychotic-like symptoms.

Efficacy and tolerability of Blonanserin in the patients with schizophrenia: a randomized, double-blind, risperidone-compared trial

OBJECTIVES

The objective of this study was to evaluate the efficacy and tolerability of blonanserin for the treatment of Korean patients with schizophrenia using a double-blind risperidone-compared design.

METHODS

Patients aged 18 to 65 years with schizophrenia were randomly assigned to blonanserin or risperidone treatment for 8 weeks. The efficacy was assessed using the mean change in Positive and Negative Syndrome Scale score total scores from baseline to week 8. Safety assessments included monitoring of vital signs, a physical examination, laboratory tests, and adverse events.

RESULTS

Of 206 randomly enrolled patients, 103 receiving blonanserin and 103 receiving risperidone were included in the analysis. In this study, noninferiority between blonanserin and risperidone was demonstrated. The mean change in the Positive and Negative Syndrome Scale total score at the final evaluation time point was -23.48 +/- 19.73 for the blonanserin group and -25.40 +/- 18.38 for the risperidone group. Adverse events, which occurred less frequently in the blonanserin than in the risperidone group, included dysarthria (P = 0.0288), dizziness (P = 0.0139), increased alanine aminotransferase and aspartate aminotransferase (P = 0.0095 and P = 0.0032, respectively), and increased level blood prolactin (P = 0.0012). On the other hand, the adverse events that occurred more frequently in the blonanserin than in the risperidone group was hand tremor (P = 0.0006).

CONCLUSIONS

Blonanserin was effective in the treatment of Korean patients with schizophrenia compared with risperidone and was more tolerable with a better safety profile, particularly with respect to prolactin elevation. These findings suggest that blonanserin is useful in the treatment of schizophrenia.

Blonanserin: a review of its use in the management of schizophrenia

Oral blonanserin (Lonasen) is an atypical antipsychotic agent indicated for use in patients with schizophrenia in Japan and Korea. It is effective in the treatment of patients with schizophrenia, providing short- and long-term efficacy against both the positive and negative symptoms of the disorder in several randomized and noncomparative trials. Notably, in two randomized, double-blind trials of 8 weeks' duration, blonanserin was noninferior to haloperidol or risperidone for primary endpoints, although it appeared to be better than haloperidol in improving negative symptoms. Blonanserin is generally well tolerated and appears to have an acceptable profile in terms of bodyweight gain. Potential tolerability benefits of the drug in short-term trials included fewer extrapyramidal symptoms than haloperidol and fewer reports of prolactin level increases or hyperprolactinaemia than risperidone. Nevertheless, extrapyramidal symptoms and hyperprolactinaemia were among the most common adverse reactions associated with blonanserin in noncomparative long-term studies. Further prospective and long-term comparative studies are required in order to definitively position blonanserin with respect to other antipsychotic agents. In the meantime, available clinical data suggest that blonanserin is an effective and generally well tolerated option for the short-term treatment of schizophrenia and for those requiring longer-term therapy.

The effects of apomorphine and D-amphetamine on striatal c-Fos expression in Sprague-Dawley and Long Evans rats and their F1 progeny

We previously reported that Sprague-Dawley (SD) rats are significantly more sensitive than Long Evans (LE) rats to disruption of prepulse inhibition (PPI) of the startle reflex by the dopamine agonists, apomorphine (APO) and D-amphetamine (AMPH). This susceptibility is inherited through F1 (SD x LE) and N2 backcross (F1 x SD) generations via an orderly pattern (SD>N2>F1>LE). Here we examined systemic APO (0.5 mg/kg) and AMPH (4.5 mg/kg) modulation of neural activity in four regions of the striatum suspected to be involved in the dopaminergic regulation of PPI - dorsolateral (dlCPu) and medial (mCPu) caudate/putamen and core (NACc) and medial shell (NACms) regions of nucleus accumbens - under conditions that mimicked those used to assess PPI. Immunohistochemical quantification of c-Fos protein expression was used as the surrogate measure of neural activity in SD and LE rats and their F1 crosses. Vehicle-treatment showed significant regional differences in Fos expression, particularly between the dlCPu and the other three areas, but no strain-related differences were observed. Three of four brain areas examined (dlCPu, mCPu and NACc) exhibited drug-induced changes in Fos expression--APO decreased and AMPH increased Fos expression in each region. The aggregate effect across these three regions revealed Fos expression to be significantly greater in LE compared to SD rats for both drugs, with F1 rats intermediate. This pattern of inheritance (LE>F1>SD) reveals an inverse relationship between striatal Fos expression and PPI sensitivity for these drugs; and a positive relationship with reported heritable differences in D2-linked G-protein binding in the CPu and NACc, and with locomotor activation/suppression by AMPH and APO.

Dissociation between in vivo occupancy and functional antagonism of dopamine D2 receptors: comparing aripiprazole to other antipsychotics in animal models

The novel antipsychotic aripiprazole requires high (>90%) striatal D2 receptor occupancy (D2RO) to be clinically active, but despite its high D2RO it does not show extrapyramidal symptoms. While most antipsychotics are active at nearly 65% D2RO, they show motor side effects when D2RO exceeds 80%. We investigated this discrepancy between D2RO, 5HT2 receptor occupancy (5-HT2RO) and in vivo functional activity of aripiprazole in comparison to haloperidol (typical) and risperidone (atypical) in animal models. All three drugs showed dose-dependent D2RO. While risperidone clearly showed higher 5-HT2RO than D2RO, aripiprazole and haloperidol showed higher D2RO than 5-HT2RO at all doses. Haloperidol and risperidone induced catalepsy at doses producing >80% D2RO, while aripiprazole despite higher D2RO (>90%) induced no catalepsy. Haloperidol and risperidone's ED50 values for inhibition of conditioned avoidance response (CAR) and amphetamine-induced locomotor activity (AIL) corresponded to approximately 60% D2RO. In contrast, aripiprazole showed a significant dissociation; while it blocked AIL at similar D2RO, a 23-fold higher dose (86% D2RO) was required to inhibit CAR. FOS expression in shell region of the nucleus accumbens was significant for all drugs at D2ROs that were effective in CAR. However, in the core region of the nucleus accumbens and dorsolateral striatum, aripiprazole differed from the others in that despite high D2RO it induced low FOS. Haloperidol and risperidone showed dose/occupancy-dependent prolactin elevations, while aripiprazole did not. Across models, haloperidol and risperidone show similar occupancy-functional antagonism of the D2 system, while aripiprazole shows a clear dissociation. Partial agonism of aripiprazole offers a good explanation for this dissociation and provides a framework for understanding occupancy-functional relationships of partial D2 agonist antipsychotics.