Contributions of dopamine D1, D2, and D3 receptor subtypes to the disruptive effects of cocaine on prepulse inhibition in mice

The role of neuroactive steroids in tic disorders

Tics are sudden, repetitive movements or vocalizations. Tic disorders, such as Tourette syndrome (TS), are contributed by the interplay of genetic risk factors and environmental variables, leading to abnormalities in the functioning of the cortico-striatal-thalamo-cortical (CSTC) circuitry. Various neurotransmitter systems, such as gamma-aminobutyric acid (GABA) and dopamine, are implicated in the pathophysiology of these disorders. Building on the evidence that tic disorders are predominant in males and exacerbated by stress, emerging research is focusing on the involvement of neuroactive steroids, including dehydroepiandrosterone sulfate (DHEAS) and allopregnanolone, in the ontogeny of tics and other phenotypes associated with TS. Emerging evidence indicates that DHEAS levels are significantly elevated in the plasma of TS-affected boys, and the clinical onset of this disorder coincides with the period of adrenarche, the developmental stage characterized by a surge in DHEAS synthesis. On the other hand, allopregnanolone has garnered particular attention for its potential to mediate the adverse effects of acute stress on the exacerbation of tic severity and frequency. Notably, both neurosteroids act as key modulators of GABA-A receptors, suggesting a pivotal role of these targets in the pathophysiology of various clinical manifestations of tic disorders. This review explores the potential mechanisms by which these and other neuroactive steroids may influence tic disorders and discusses the emerging therapeutic strategies that target neuroactive steroids for the management of tic disorders.

Inversed Effects of Nav1.2 Deficiency at Medial Prefrontal Cortex and Ventral Tegmental Area for Prepulse Inhibition in Acoustic Startle Response

Numerous pathogenic variants of SCN2A gene, encoding voltage-gated sodium channel α2 subunit Nav1.2 protein, have been identified in a wide spectrum of neuropsychiatric disorders including schizophrenia. However, pathological mechanisms for the schizophrenia-relevant behavioral abnormalities caused by the variants remain poorly understood. Here in this study, we characterized mouse lines with selective Scn2a deletion at schizophrenia-related brain regions, medial prefrontal cortex (mPFC) or ventral tegmental area (VTA), obtained by injecting adeno-associated viruses (AAV) expressing Cre recombinase into homozygous Scn2a-floxed (Scn2afl/fl) mice, in which expression of the Scn2a was locally deleted in the presence of Cre recombinase. The mice lacking Scn2a in the mPFC exhibited a tendency for a reduction in prepulse inhibition (PPI) in acoustic startle response. Conversely, the mice lacking Scn2a in the VTA showed a significant increase in PPI. We also found that the mice lacking Scn2a in the mPFC displayed increased sociability, decreased locomotor activity, and increased anxiety-like behavior, while the mice lacking Scn2a in the VTA did not show any other abnormalities in these parameters except for vertical activity which is one of locomotor activities. These results suggest that Scn2a-deficiencies in mPFC and VTA are inversely relevant for the schizophrenic phenotypes in patients with SCN2A variants.

Dopaminergic signalling and behavioural alterations by Comt-Dtnbp1 genetic interaction and their clinical relevance

BACKGROUND AND PURPOSE

Cognitive and motor functions are modulated by dopaminergic signalling, which is shaped by several genetic factors. The biological effects of single genetic variants might differ depending on epistatic interactions that can be functionally multi-directional and non-linear.

EXPERIMENTAL APPROACH

We performed behavioural and neurochemical assessments in genetically modified mice and behavioural assessments and genetic screening in human patients with 22q11.2 deletion syndrome (22q11.2DS).

KEY RESULTS

Here, we confirm a genetic interaction between the Comt (catechol-O-methyltransferase, human orthologue: COMT) and Dtnbp1 (dystrobrevin binding protein 1, alias dysbindin, human orthologue: DTNBP1) genes that modulate cortical and striatal dopaminergic signalling in a manner not predictable by the effects of each single gene. In mice, Comt-by-Dtnbp1 concomitant reduction leads to a hypoactive mesocortical and a hyperactive mesostriatal dopamine pathway, associated with specific cognitive abnormalities. Like mice, in subjects with the 22q11.2DS (characterized by COMT hemideletion and dopamine alterations), COMT-by-DTNBP1 concomitant reduction was associated with analogous cognitive disturbances. We then developed an easy and inexpensive colourimetric kit for the genetic screening of common COMT and DTNBP1 functional genetic variants for clinical application.

CONCLUSIONS AND IMPLICATIONS

These findings illustrate an epistatic interaction of two dopamine-related genes and their functional effects, supporting the need to address genetic interaction mechanisms at the base of complex behavioural traits.

Interaction of Brain-Derived Neurotrophic Factor with the Effects of Chronic Methamphetamine on Prepulse Inhibition in Mice Is Independent of Dopamine D3 Receptors

The aim of the present study was to gain a better understanding of the role of brain-derived neurotrophic factor (BDNF) and dopamine D3 receptors in the effects of chronic methamphetamine (METH) on prepulse inhibition (PPI), an endophenotype of psychosis. We compared the effect of a three-week adolescent METH treatment protocol on the regulation of PPI in wildtype mice, BDNF heterozygous mice (HET), D3 receptor knockout mice (D3KO), and double-mutant mice (DM) with both BDNF heterozygosity and D3 receptor knockout. Chronic METH induced disruption of PPI regulation in male mice with BDNF haploinsufficiency (HET and DM), independent of D3 receptor knockout. Specifically, these mice showed reduced baseline PPI, as well as attenuated disruption of PPI induced by acute treatment with the dopamine receptor agonist, apomorphine (APO), or the glutamate NMDA receptor antagonist, MK-801. In contrast, there were no effects of BDNF heterozygosity or D3 knockout on PPI regulation in female mice. Chronic METH pretreatment induced the expected locomotor hyperactivity sensitisation, where female HET and DM mice also showed endogenous sensitisation. Differential sex-specific effects of genotype and METH pretreatment were observed on dopamine receptor and dopamine transporter gene expression in the striatum and frontal cortex. Taken together, these results show a significant involvement of BDNF in the long-term effects of METH on PPI, particularly in male mice, but these effects appear independent of D3 receptors. The role of this receptor in psychosis endophenotypes therefore remains unclear.

Antipsychotic drug efficacy correlates with the modulation of D1 rather than D2 receptor-expressing striatal projection neurons

Elevated dopamine transmission in psychosis is assumed to unbalance striatal output through D1- and D2-receptor-expressing spiny-projection neurons (SPNs). Antipsychotic drugs are thought to re-balance this output by blocking D2 receptors (D2Rs). In this study, we found that amphetamine-driven dopamine release unbalanced D1-SPN and D2-SPN Ca2+ activity in mice, but that antipsychotic efficacy was associated with the reversal of abnormal D1-SPN, rather than D2-SPN, dynamics, even for drugs that are D2R selective or lacking any dopamine receptor affinity. By contrast, a clinically ineffective drug normalized D2-SPN dynamics but exacerbated D1-SPN dynamics under hyperdopaminergic conditions. Consistent with antipsychotic effect, selective D1-SPN inhibition attenuated amphetamine-driven changes in locomotion, sensorimotor gating and hallucination-like perception. Notably, antipsychotic efficacy correlated with the selective inhibition of D1-SPNs only under hyperdopaminergic conditions-a dopamine-state-dependence exhibited by D1R partial agonism but not non-antipsychotic D1R antagonists. Our findings provide new insights into antipsychotic drug mechanism and reveal an important role for D1-SPN modulation.

Dysbindin-1A modulation of astrocytic dopamine and basal ganglia dependent behaviors relevant to schizophrenia

The mechanisms underlying the dichotomic cortical/basal ganglia dopaminergic abnormalities in schizophrenia are unclear. Astrocytes are important non-neuronal modulators of brain circuits, but their role in dopaminergic system remains poorly explored. Microarray analyses, immunohistochemistry, and two-photon laser scanning microscopy revealed that Dys1 hypofunction increases the reactivity of astrocytes, which express only the Dys1A isoform. Notably, behavioral and electrochemical assessments in mice selectively lacking the Dys1A isoform unraveled a more prominent impact of Dys1A in behavioral and dopaminergic/D2 alterations related to basal ganglia, but not cortical functioning. Ex vivo electron microscopy and protein expression analyses indicated that selective Dys1A disruption might alter intracellular trafficking in astrocytes, but not in neurons. In agreement, Dys1A disruption only in astrocytes resulted in decreased motivation and sensorimotor gating deficits, increased astrocytic dopamine D2 receptors and decreased dopaminergic tone within basal ganglia. These processes might have clinical relevance because the caudate, but not the cortex, of patients with schizophrenia shows a reduction of the Dys1A isoform. Therefore, we started to show a hitherto unknown role for the Dys1A isoform in astrocytic-related modulation of basal ganglia behavioral and dopaminergic phenotypes, with relevance to schizophrenia.

Cocaine Increases Sensorimotor Gating and is Related to Psychopathy

OBJECTIVE

Prepulse inhibition regulates sensorimotor gating and is a marker of vulnerability to certain disorders. We compared prepulse inhibition, psychopathy, and sensitivity to punishment and reward in patients with cocaine-related disorder without psychiatric comorbidities and a control group.

METHODS

This was an observational study of a sample of 22 male cases with cocaine-related disorder and 22 healthy male controls. We used the Psychiatric Research Interview for Substance and Mental Disorders and Mini International Neuropsychiatric Interview; the Sensitivity to Punishment and Sensitivity to Reward Questionnaire; and the Levenson Self-Report Psychopathy Scale and Hare Psychopathy Checklist-Revised. Prepulse inhibition was evaluated at 30, 60, and 120 ms.

RESULTS

Cocaine-related disorder group had a higher overall score (t = 12.556, p = .001) and primary psychopathy score (t = 3.750, p = .001) on Levenson Self-Report Psychopathy Scale, a higher score on both Hare Psychopathy Checklist-Revised factors, sensitivity to rewards (t = 3.076, p = .005) and prepulse inhibition at 30 ms (t = 2.859, p = .008).

CONCLUSIONS

Cocaine use in patients without psychiatric comorbidities seems to increase sensorimotor gating. Therefore, these patients likely have an increased sensitivity to rewards, causing them to focus more on cocaine-boosting stimuli, thus explaining the psychopathic traits of these individuals.

Disruption of prepulse inhibition is associated with compulsive behavior severity and nucleus accumbens dopamine receptor changes in Sapap3 knockout mice

Obsessive compulsive disorder (OCD) is associated with disruption of sensorimotor gating, which may contribute to difficulties inhibiting intrusive thoughts and compulsive rituals. Neural mechanisms underlying these disturbances are unclear; however, striatal dopamine is implicated in regulation of sensorimotor gating and OCD pathophysiology. The goal of this study was to examine the relationships between sensorimotor gating, compulsive behavior, and striatal dopamine receptor levels in Sapap3 knockout mice (KOs), a widely used preclinical model system for OCD research. We found a trend for disruption of sensorimotor gating in Sapap3-KOs using the translational measure prepulse inhibition (PPI); however, there was significant heterogeneity in both PPI and compulsive grooming in KOs. Disruption of PPI was significantly correlated with a more severe compulsive phenotype. In addition, PPI disruption and compulsive grooming severity were associated with reduced dopamine D1 and D2/3 receptor density in the nucleus accumbens core (NAcC). Compulsive grooming progressively worsened in Sapap3-KOs tested longitudinally, but PPI disruption was first detected in high-grooming KOs at 7 months of age. Through detailed characterization of individual differences in OCD-relevant behavioral and neurochemical measures, our findings suggest that NAcC dopamine receptor changes may be involved in disruption of sensorimotor gating and compulsive behavior relevant to OCD.

Prepulse Inhibition in Cocaine Addiction and Dual Pathologies

Cocaine addiction is frequently associated with different psychiatric disorders, especially schizophrenia and antisocial personality disorder. A small number of studies have used prepulse inhibition (PPI) as a discriminating factor between these disorders. This work evaluated PPI and the phenotype of patients with cocaine-related disorder (CRD) who presented a dual diagnosis of schizophrenia or antisocial personality disorder. A total of 74 men aged 18–60 years were recruited for this research. The sample was divided into four groups: CRD (n = 14), CRD and schizophrenia (n = 21), CRD and antisocial personality disorder (n = 16), and a control group (n = 23). We evaluated the PPI and other possible vulnerability factors in these patients by using different assessment scales. PPI was higher in the CRD group at 30 ms (F(3, 64) = 2.972, p = 0.038). Three discriminant functions were obtained which allowed us to use the overall Hare Psychopathy Checklist Revised score, reward sensitivity, and PPI at 30 ms to predict inclusion of these patients in the different groups with a success rate of 79.7% (42.9% for CRD, 76.2% for CRD and schizophrenia, 100% for CRD and antisocial personality disorder, and 91.3% in the control group). Despite the differences we observed in PPI, this factor is of little use for discriminating between the different diagnostic groups and it acts more as a non-specific endophenotype in certain mental disorders, such as in patients with a dual diagnosis.

Early life social experience affects adulthood fear extinction deficit and associated dopamine profile abnormalities in a rat model of PTSD

Individuals may develop fear extinction deficits after life-threatening traumatic events; such deficits indicate posttraumatic stress disorder (PTSD). Because the occurrence of this disorder differs among people who have experienced trauma, hidden underlying factors should be determined. Increasing evidence suggests the involvement of neuronal dysregulation of information processes or cognitive function during development. This neuronal dysregulation is caused by disturbances in dopamine (DA) transmission within the fear circuit, which comprises the medial prefrontal cortex (mPFC), amygdala, and hippocampus. Single prolonged stress (SPS) combined with an isolation rearing (IR) paradigm was used to randomly assign rats to four groups [social rearing-no SPS (SR-NS), SR-SPS, IR-NS, and IR-SPS], and their performance in prepulse inhibition (PPI) and on Pavlovian fear conditioning tests was assessed. Tissue DA levels and the expression of DA receptors (D1R and D2R) in the fear circuit were measured at the end of the experiment. Our results indicated that PPI deficits and fear extinction problems were specific to rats subjected to IR and SPS, respectively. Furthermore, IR-induced PPI deficits were not influenced by SPS, but SPS-induced fear extinction retrieval impairment could be adjusted according to previous IR experiences. Neurochemically, tissue DA levels and D1R expression in the mPFC and amygdala were nonspecifically reduced by IR and SPS, whereas D2R expression in the mPFC and amygdala was higher in IR-SPS than in SR-SPS rats. These findings suggest that early life experiences may influence fear responses in adulthood through a change in DA profiles within the fear circuit.

Prepulse Inhibition of the Startle Reflex as a Predictor of Vulnerability to Develop Locomotor Sensitization to Cocaine

Prepulse inhibition (PPI) of the startle reflex is a measure of sensory-motor synchronization. A deficit in PPI has been observed in psychiatric patients, especially those with schizophrenia and vulnerable subjects, since the neural bases of this disorder are also involved in the regulation of PPI. Recently, we have reported that baseline PPI levels in mice can predict their sensitivity to the conditioned reinforcing effects of cocaine in the conditioned place preference (CPP) paradigm. Mice with a low PPI presented a lower sensitivity to the conditioned rewarding effects of cocaine; however, once they acquired conditioned preference with a higher dose of the drug, a more persistent associative effect of cocaine with respect to environmental cues was evident in these animals when compared with High-PPI mice. Therefore, we proposed that the PPI paradigm can determine subjects with a higher vulnerability to the effects of cocaine. Developing locomotor sensitization after pre-exposure to cocaine is considered an indicator of transitioning from recreational use to a compulsive consumption of the drug. Thus, the aim of the present study was to evaluate whether subjects with a low PPI display a higher locomotor sensitization induced by cocaine. First, male and female OF1 mice were classified as High- or Low-PPI according to their baseline PPI levels. Subsequently, the motor effects induced by an acute dose of cocaine (Experiments 1 and 2) and the development of locomotor sensitization induced by pre-exposure to this drug (Experiments 3 and 4) were recorded using two apparatuses (Ethovision and actimeter). Low-PPI mice presented low sensitivity to the motor effects of an acute dose of cocaine, but a high increase of activity after repeated administration of the drug, thus suggesting a great developed behavioral sensitization. Differences after pretreatment with cocaine vs. saline were more pronounced among Low-PPI subjects than among High-PPI animals. These results endorse our hypothesis that the PPI paradigm can detect subjects who are more likely to display behaviors induced by cocaine and which can increase the risk of developing a cocaine use disorder. Herein, we further discuss whether a PPI deficit can be considered an endophenotype for cocaine use disorder.

Dopamine Receptor Blockade Attenuates Purinergic P2X4 Receptor-Mediated Prepulse Inhibition Deficits and Underlying Molecular Mechanisms

Sensorimotor gating refers to the ability to filter incoming sensory information in a stimulus-laden environment and disruption of this physiological process has been documented in psychiatric disorders characterized by cognitive aberrations. The effectiveness of current pharmacotherapies for treatment of sensorimotor gating deficits in the patient population still remains controversial. These challenges emphasize the need to better understand the biological underpinnings of sensorimotor gating which could lead to discovery of novel drug targets for therapeutic intervention. Notably, we recently reported a role for purinergic P2X4 receptors (P2X4Rs) in regulation of sensorimotor gating using prepulse inhibition (PPI) of acoustic startle reflex. P2X4Rs are ion channels gated by adenosine-5′-triphosphate (ATP). Ivermectin (IVM) induced PPI deficits in C57BL/6J mice in a P2X4R-specific manner. Furthermore, mice deficient in P2X4Rs [P2X4R knockout (KO)] exhibited PPI deficits that were alleviated by dopamine (DA) receptor antagonists demonstrating an interaction between P2X4Rs and DA receptors in PPI regulation. On the basis of these findings, we hypothesized that increased DA neurotransmission underlies IVM-mediated PPI deficits. To test this hypothesis, we measured the effects of D1 and D2 receptor antagonists, SCH 23390 and raclopride respectively and D1 agonist, SKF 82958 on IVM-mediated PPI deficits. To gain mechanistic insights, we investigated the interaction between IVM and dopaminergic drugs on signaling molecules linked to PPI regulation in the ventral striatum. SCH 23390 significantly attenuated the PPI disruptive effects of IVM to a much greater degree than that of raclopride. SKF 82958 failed to potentiate IVM-mediated PPI disruption. At the molecular level, modulation of D1 receptors altered IVM’s effects on dopamine and cyclic-AMP regulated phosphoprotein of 32 kDa (DARPP-32) phosphorylation. Additionally, IVM interacted with the DA receptors antagonists and SKF 82958 in phosphorylation of Ca²⁺/calmodulin kinase IIα (CaMKIIα) and its downstream target, neuronal nitric oxide synthase (nNOS). Current findings suggest an involvement for D1 and D2 receptors in IVM-mediated PPI disruption via modulation of DARPP-32, CaMKIIα and nNOS. Taken together, the findings suggest that stimulation of P2X4Rs can lead to DA hyperactivity and disruption of information processing, implicating P2X4Rs as a novel drug target for treatment of psychiatric disorders characterized by sensorimotor gating deficits.

Baseline prepulse inhibition of the startle reflex predicts the sensitivity to the conditioned rewarding effects of cocaine in male and female mice

RATIONALE

Prepulse inhibition (PPI) of the startle reflex is a model of pre-attentional inhibitory function. The dopamine baseline in the nucleus accumbens plays a key role in PPI regulation as well as in the rewarding effects of cocaine.

OBJECTIVES

The aim of this study was to evaluate the predictive ability of PPI to identify the more vulnerable mice of both sexes to the conditioned rewarding effects of cocaine.

METHODS

Male and female OF1 mice were first tested in the PPI paradigm to classify them as high or low PPI. Afterwards, they were evaluated in the conditioned place preference (CPP) paradigm induced by cocaine (1, 6 and 12 mg/kg). Moreover, the D1R and D2R protein expressions in the striatum of high and low PPI animals were analysed by Western blot.

RESULTS

Only high-PPI mice acquired CPP induced by low doses of cocaine (1 and 6 mg/kg), while the low-PPI mice needed a higher dose of cocaine (12 mg/kg) to acquire the CPP, but once mice were conditioned, males did not extinguish the conditioned preference and females reinstated the preference with lower doses of cocaine than their control counterparts. Low-PPI animals, especially females, showed higher basal levels of D2R than those with a higher PPI.

CONCLUSIONS

Low-PPI mice presented a lower sensitivity to the conditioned rewarding effects of cocaine, but once they were conditioned with a higher dose, they displayed a stronger, perseverant conditioned preference. The predictive capacity of PPI to detect the more vulnerable mice to the conditioned effects of cocaine is discussed.

Effects of immune activation during early or late gestation on schizophrenia-related behaviour in adult rat offspring

Maternal exposure to infectious agents during gestation has been identified as a significant risk factor for schizophrenia. Using a mouse model, past work has demonstrated that the gestational timing of the immune-activating event can impact the behavioural phenotype and expression of dopaminergic and glutamatergic neurotransmission markers in the offspring. In order to determine the inter-species generality of this effect to rats, another commonly used model species, the current study investigated the impact of a viral mimetic Poly (I:C) at either an early (gestational day 10) or late (gestational day 19) time-point on schizophrenia-related behaviour and neurotransmitter receptor expression in rat offspring. Exposure to Poly (I:C) in late, but not early, gestation resulted in transient impairments in working memory. In addition, male rats exposed to maternal immune activation (MIA) in either early or late gestation exhibited sensorimotor gating deficits. Conversely, neither early nor late MIA exposure altered locomotor responses to MK-801 or amphetamine. In addition, increased dopamine 1 receptor mRNA levels were found in the nucleus accumbens of male rats exposed to early gestational MIA. The findings from this study diverge somewhat from previous findings in mice with MIA exposure, which were often found to exhibit a more comprehensive spectrum of schizophrenia-like phenotypes in both males and females, indicating potential differences in the neurodevelopmental vulnerability to MIA exposure in the rat with regards to schizophrenia related changes.

Dorsal striatum D1-expressing neurons are involved with sensorimotor gating on prepulse inhibition test

Prepulse inhibition (PPI) is a behavioral test in which the startle reflex response to a high-intensity stimulus (pulse) is inhibited by the prior presentation of a weak stimulus (prepulse). The classic neural circuitry that mediates startle response is localized in the brainstem; however, recent studies point to the contribution of structures involved in higher cognitive functions in regulating the sensorimotor gating, particularly forebrain regions innervated by dopaminergic nuclei. The aim of the present study was to verify the role of dorsal striatum (DS) and dopaminergic transmitting mediated by D1 and D2 receptors on PPI test in rats. DS inactivation induced by muscimol injection did not affect PPI (%PPI and startle response), although it impaired the locomotor activity and caused catalepsy. Infusion of D1-like antagonist SCH23390 impaired %PPI but did not disturb the startle response and locomotor activity evaluated immediately after PPI test. D2 antagonist microinjection (sulpiride) did not affect %PPI and startle response, but impaired motor activity. These results point to an important role of DS, probably mediated by direct basal ganglia pathway, on modulation of sensorimotor gating, in accordance with clinical studies showing PPI deficits in schizophrenia, Tourette syndrome, and compulsive disorders - pathologies related to basal ganglia dysfunctions.

Regional and source-based patterns of [11C]-(+)-PHNO binding potential reveal concurrent alterations in dopamine D2 and D3 receptor availability in cocaine-use disorder

Dopamine type 2 and type 3 receptors (D₂R/D₃R) appear critical to addictive disorders. Cocaine-use disorder (CUD) is associated with lower D₂R availability and greater D₃R availability in regions primarily expressing D₂R or D₃R concentrations, respectively. However, these CUD-related alterations in D₂R and D₃R have not been concurrently detected using available dopaminergic radioligands. Furthermore, receptor availability in regions of mixed D₂R/D₃R concentration in CUD remains unclear. The current study aimed to extend investigations of CUD-related alterations in D₂R and D₃R availability using regional and source-based analyses of [¹¹C]-(+)-PHNO positron emission tomography (PET) of 26 individuals with CUD and 26 matched healthy comparison (HC) participants. Regional analysis detected greater binding potential (BP_ND) in CUD in the midbrain, consistent with prior [¹¹C]-(+)-PHNO research, and lower BP_ND in CUD in the dorsal striatum, consistent with research using non-selective D₂R/D₃R radiotracers. Exploratory independent component analysis (ICA) identified three sources of BP_ND (striatopallidal, pallidonigral, and mesoaccumbens sources) that represent systems of brain regions displaying coherent variation in receptor availability. The striatopallidal source was associated with estimates of regional D₂R-related proportions of BP_ND (calculated using independent reports of [¹¹C]-(+)-PHNO receptor binding fractions), was lower in intensity in CUD and negatively associated with years of cocaine use. By comparison, the pallidonigral source was associated with estimates of regional D₃R distribution, was greater in intensity in CUD and positively associated with years of cocaine use. The current study extends previous D₂R/D₃R research in CUD, demonstrating both lower BP_ND in the D₂R-rich dorsal striatum and greater BP_ND in the D₃R-rich midbrain using a single radiotracer. In addition, exploratory ICA identified sources of [¹¹C]-(+)-PHNO BP_ND that were correlated with regional estimates of D₂R-related and D₃R-related proportions of BP_ND, were consistent with regional differences in CUD, and suggest receptor alterations in CUD may also be present in regions of mixed D₂R/D₃R concentration.

Role of purinergic P2X4 receptors in regulating striatal dopamine homeostasis and dependent behaviors

Purinergic P2X4 receptors (P2X4Rs) belong to the P2X superfamily of ion channels regulated by ATP. We recently demonstrated that P2X4R knockout (KO) mice exhibited deficits in sensorimotor gating, social interaction, and ethanol drinking behavior. Dopamine (DA) dysfunction may underlie these behavioral changes, but there is no direct evidence for P2X4Rs' role in DA neurotransmission. To test this hypothesis, we measured markers of DA function and dependent behaviors in P2X4R KO mice. P2X4R KO mice exhibited altered density of pre-synaptic markers including tyrosine hydroxylase, dopamine transporter; post-synaptic markers including dopamine receptors and phosphorylation of downstream targets including dopamine and cyclic-AMP regulated phosphoprotein of 32 kDa and cyclic-AMP-response element binding protein in different parts of the striatum. Ivermectin, an allosteric modulator of P2X4Rs, significantly affected dopamine and cyclic AMP regulated phosphoprotein of 32 kDa and extracellular regulated kinase1/2 phosphorylation in the striatum. Sensorimotor gating deficits in P2X4R KO mice were rescued by DA antagonists. Using the 6-hydroxydopamine model of DA depletion, P2X4R KO mice exhibited an attenuated levodopa (L-DOPA)-induced motor behavior, whereas ivermectin enhanced this behavior. Collectively, these findings identified an important role for P2X4Rs in maintaining DA homeostasis and illustrate how this association is important for CNS functions including motor control and sensorimotor gating. We propose that P2X4 receptors (P2X4Rs) regulate dopamine (DA) homeostasis and associated behaviors. Pre-synaptic and post-synaptic DA markers were significantly altered in the dorsal and ventral striatum of P2X4R KO mice, implicating altered DA neurotransmission. Sensorimotor gating deficits in P2X4R KO mice were rescued by DA antagonists. Ivermectin (IVM), a positive modulator of P2X4Rs, enhanced levodopa (L-DOPA)-induced motor behavior. These studies highlight potential interactions between P2X4Rs and DA system.

Animal models of tic disorders: a translational perspective

Tics are repetitive, sudden movements and/or vocalizations, typically enacted as maladaptive responses to intrusive premonitory urges. The most severe tic disorder, Tourette syndrome (TS), is a childhood-onset condition featuring multiple motor and at least one phonic tic for a duration longer than 1 year. The pharmacological treatment of TS is mainly based on antipsychotic agents; while these drugs are often effective in reducing tic severity and frequency, their therapeutic compliance is limited by serious motor and cognitive side effects. The identification of novel therapeutic targets and development of better treatments for tic disorders is conditional on the development of animal models with high translational validity. In addition, these experimental tools can prove extremely useful to test hypotheses on the etiology and neurobiological bases of TS and related conditions. In recent years, the translational value of these animal models has been enhanced, thanks to a significant re-organization of our conceptual framework of neuropsychiatric disorders, with a greater focus on endophenotypes and quantitative indices, rather than qualitative descriptors. Given the complex and multifactorial nature of TS and other tic disorders, the selection of animal models that can appropriately capture specific symptomatic aspects of these conditions can pose significant theoretical and methodological challenges. In this article, we will review the state of the art on the available animal models of tic disorders, based on genetic mutations, environmental interventions as well as pharmacological manipulations. Furthermore, we will outline emerging lines of translational research showing how some of these experimental preparations have led to significant progress in the identification of novel therapeutic targets for tic disorders.

Microinjection of CART (cocaine- and amphetamine-regulated transcript) peptide into the nucleus accumbens inhibits the cocaine-induced upregulation of dopamine receptors and locomotor sensitization

Repeated exposure to addictive drugs enhances dopamine receptor (DR) signaling and the ultimate phosphorylation of the cyclic adenosine 5'-monophosphate (cAMP)-response element-binding protein (CREB)-regulated cocaine- and amphetamine-regulated transcript (CART) expression in the nucleus accumbens (NAcc). These effects are known to contribute to the expression of behavioral sensitization. CART peptides are neuropeptides that modulate drug reward and reinforcement. The present experiments investigated the effects of CART 55-102 microinjection into the NAcc on (1) the phosphorylation of CREB, (2) cAMP/protein kinase A (PKA) signaling and (3) extracellular signal-regulated kinase (ERK) phosphorylated kinase signaling. Here, we show that repeated microinjections into the NAcc of CART 55-102 peptides (1.0 or 2.5μg, 0.5μl/side) attenuates cocaine-induced enhancements of D1R, D2R and D3R phosphorylation in this sites. Furthermore, the microinjection of CART 55-102 followed by repeated injections of cocaine (15mg/kg) dose-dependently blocked the enhancement of cAMP levels, PKA activity and pERK and pCREB levels on the fifth day of cocaine administration. The cocaine-induced locomotor activity and behavioral sensitization in rats were also inhibited by the 5-day-microinjection of CART peptides. These results suggest that the phosphorylation of CREB by cocaine in the NAcc was blocked by the CART 55-102 peptide via the inhibition of D1R and D2R stimulation, D3R phosphorylation, cAMP/PKA signaling and ERK phosphorylated kinase signaling. These effects may have played a compensatory inhibitory role in the behavioral sensitization of rats that received microinjections of CART 55-102.

Sex-specific brain deficits in auditory processing in an animal model of cocaine-related schizophrenic disorders

Cocaine is a psychostimulant in the pharmacological class of drugs called Local Anesthetics. Interestingly, cocaine is the only drug in this class that has a chemical formula comprised of a tropane ring and is, moreover, addictive. The correlation between tropane and addiction is well-studied. Another well-studied correlation is that between psychosis induced by cocaine and that psychosis endogenously present in the schizophrenic patient. Indeed, both of these psychoses exhibit much the same behavioral as well as neurochemical properties across species. Therefore, in order to study the link between schizophrenia and cocaine addiction, we used a behavioral paradigm called Acoustic Startle. We used this acoustic startle paradigm in female versus male Sprague-Dawley animals to discriminate possible sex differences in responses to startle. The startle method operates through auditory pathways in brain via a network of sensorimotor gating processes within auditory cortex, cochlear nuclei, inferior and superior colliculi, pontine reticular nuclei, in addition to mesocorticolimbic brain reward and nigrostriatal motor circuitries. This paper is the first to report sex differences to acoustic stimuli in Sprague-Dawley animals (Rattus norvegicus) although such gender responses to acoustic startle have been reported in humans (Swerdlow et al. 1997 [1]). The startle method monitors pre-pulse inhibition (PPI) as a measure of the loss of sensorimotor gating in the brain's neuronal auditory network; auditory deficiencies can lead to sensory overload and subsequently cognitive dysfunction. Cocaine addicts and schizophrenic patients as well as cocaine treated animals are reported to exhibit symptoms of defective PPI (Geyer et al., 2001 [2]). Key findings are: (a) Cocaine significantly reduced PPI in both sexes. (b) Females were significantly more sensitive than males; reduced PPI was greater in females than in males. (c) Physiological saline had no effect on startle in either sex. Thus, the data elucidate gender-specificity to the startle response in animals. Finally, preliminary studies show the effect of cocaine on acoustic startle in tandem with effects on estrous cycle. The data further suggest that hormones may play a role in these sex differences to acoustic startle reported herein.

Inhibition of 5α-reductase attenuates behavioral effects of D1-, but not D2-like receptor agonists in C57BL/6 mice

Converging lines of evidence point to the involvement of neurosteroids in the regulation of dopamine (DA) neurotransmission and signaling, yet the neurobiological bases of this link remain poorly understood. We previously showed that inhibition of steroid 5α-reductase (5αR), the key rate-limiting enzyme in neurosteroidogenesis, attenuates the behavioral effects of non-selective DA receptor agonists in rats, including stereotyped responses and sensorimotor gating deficits, as measured by the prepulse inhibition (PPI) of the acoustic startle reflex. Since previous findings suggested that the role of DA D(1)- and D(2)-like receptor families in behavioral regulation may exhibit broad interspecies and interstrain variations, we assessed the impact of 5αR blockade on the behavioral effects of DAergic agonists in C57BL/6 mice. The prototypical 5αR inhibitor finasteride (FIN; 25-50 mg/kg, intraperitoneally, IP) dose-dependently countered the PPI deficits and the enhancement of rearing responses induced by the full D(1)-like receptor agonist SKF-82958 (0.3 mg/kg, IP); however, FIN did not significantly affect the hyperlocomotive and startle-attenuating effects of SKF-82958. Whereas the D(2)-like receptor agonist quinpirole (QUIN; 0.5 mg/kg, IP) did not induce significant changes in PPI, the combination of this agent and FIN surprisingly produced marked gating and startle deficits. In contrast with previous data on rats, FIN did not affect the reductions of startle reflex and PPI produced by the non-selective DAergic agonist apomorphine (APO; 0.5 mg/kg, IP). These findings collectively indicate that, in C57BL/6 mice, 5αR differentially modulates the effects of D(1)- and D(2)-like receptor agonists in behavioral regulation.

Phosphodiesterase 4 inhibition enhances the dopamine D1 receptor/PKA/DARPP-32 signaling cascade in frontal cortex

RATIONALE

Alteration of dopamine neurotransmission in the prefrontal cortex, especially hypofunction of dopamine D1 receptors, contributes to psychotic symptoms and cognitive deficit in schizophrenia. D1 receptors signal through the cAMP/PKA second messenger cascade, which is modulated by phosphodiesterase (PDE) enzymes that hydrolyze and inactivate cyclic nucleotides. Though several PDEs are expressed in cortical neurons, the PDE4 enzyme family (PDE4A-D) has been implicated in the control of cognitive function. The best studied isoform, PDE4B, interacts with a schizophrenia susceptibility factor, disrupted in schizophrenia 1 (DISC1).

OBJECTIVES

We explore the control of mouse frontal cortex dopamine D1 receptor signaling and associated behavior by PDE4.

RESULTS

Inhibition of PDE4 by rolipram induced activation of cAMP/PKA signaling in cortical slices and in vivo, leading to the phosphorylation of DARPP-32 and other postsynaptic and presynaptic PKA-substrates. Rolipram also enhanced DARPP-32 phosphorylation invoked by D1 receptor activation. Immunohistochemical studies demonstrated PDE4A, PDE4B, and PDE4D expression in DARPP-32-positive neurons in layer VI of frontal cortex, most likely in D1 receptor-positive, glutamatergic corticothalamic pyramidal neurons. Furthermore, the ability of rolipram treatment to improve the performance of mice in a sensorimotor gating test was DARPP-32-dependent.

CONCLUSIONS

PDE4, which is co-expressed with DARPP-32 in D1 receptor-positive cortical pyramidal neurons in layer VI, modulates the level of D1 receptor signaling and DARPP-32 phosphorylation in the frontal cortex, likely influencing cognitive function. These biochemical and behavioral actions of PDE4 inhibitors may contribute to the hypothesized antipsychotic actions of this class of compounds.

Genetic models of sensorimotor gating: relevance to neuropsychiatric disorders

Sensorimotor gating, or the ability of a sensory event to suppress a motor response, can be measured operationally via prepulse inhibition (PPI) of the startle response. PPI is deficient in schizophrenia patients as well as other neuropsychiatric disorders, can be measured across species, and has been used widely as a translational tool in preclinical neuropharmacological and genetic research. First developed to assess drug effects in pharmacological and developmental models, PPI has become one of the standard behavioral measures in genetic models of schizophrenia and other neuropsychiatric disorders that exhibit PPI deficits. In this chapter we review the literature on genetic models of sensorimotor gating and discuss the utility of PPI as a tool in phenotyping mutant mouse models. We highlight the approaches to genetic mouse models of neuropsychiatric disease, discuss some of the important caveats to these approaches, and provide a comprehensive table covering the more recent genetic models that have evaluated PPI.

Reversal of scopolamine-induced disruption of prepulse inhibition by clozapine in mice

Prepulse inhibition (PPI) of the acoustic startle reflex refers to the reduction of the startle response to an intense acoustic pulse stimulus when it is shortly preceded by a weak non-startling prepulse stimulus and provides a cross-species measure of sensory-motor gating. PPI is typically impaired in schizophrenia patients, and a similar impairment can be induced in rats by systemic scopolamine, a muscarinic cholinergic receptor antagonist that can evoke a range of cognitive and psychotic symptoms in healthy humans that are commonly referred to as the "anti-muscarinic syndrome" resembling some clinical features of schizophrenia. Scopolamine-induced PPI disruption has therefore been proposed as an anti-muscarinic animal model of schizophrenia, but parallel investigations in the mouse remain scant and the outcomes are mixed and often confounded by an elevation of startle reactivity. Here, we distinguished the PPI-disruptive and the confounding startle-enhancing effects of scopolamine (1 and 10mg/kg, i.p.) in C57BL/6 wild-type mice by showing that the latter partly stemmed from a shift in spontaneous baseline reactivity. With appropriate correction for between-group differences in startle reactivity, we went on to confirm that the PPI-disruptive effect of scopolamine could be nullified by clozapine pre-treatment (1.5mg/kg, i.p.) in a dose-dependent manner. This is the first demonstration that scopolamine-induced PPI disruption is sensitive to atypical antipsychotic drugs. In concert with previous data showing its sensitivity to haloperidol the present finding supports the predictive validity of the anti-muscarinic PPI disruption model for both typical and atypical antipsychotic drug action.

Stereochemical and neuroanatomical selectivity of pramipexole effects on sensorimotor gating in rats

BACKGROUND

In rats, prepulse inhibition (PPI) of acoustic startle is disrupted by systemic administration of dopaminergic agonists, such as the dopamine D3 receptor (D3R)-preferential agonist pramipexole (PPX). PPX has D3R-active (S) and -inactive (R) stereoisomers. Here, we tested the neuroanatomical and stereochemical selectivity of PPX effects on PPI.

METHODS

(S)-PRA or (R)-PRA (0, 0.47, 1.42, 4.73 μmol/kg) was injected sc 15 min prior to PPI testing in adult male Sprague Dawley rats. In separate rats, (S)-PPX (0, 3, 10 μg/0.5μl/side, ic) was infused into the nucleus accumbens (NAc), caudodorsal striatum (CS), or olfactory tubercle/Islands of Calleja (ICj) 15 min prior to PPI testing. D3R expression in these brain regions was assessed using quantitative rt-PCR. The PPI-disruptive effects of systemic (S)-PPX were also tested after pretreatment with the D3R-selective antagonist, U99194 (10mg/kg).

RESULTS

Systemic administration of PPX stereoisomers demonstrated a dose-dependent effect of (S)-PPX on PPI, while (R)-PPX had no effect on PPI. PPX decreased PPI when infused into the NAc and ICj, but not the CS. Quantitative rt-PCR revealed D3R expression in ICj>NAc>CS. The PPI-disruptive effects of PPX were prevented by U99194.

CONCLUSION

The PPI-reducing effects of PPX are stereospecific for the D3R-active (S)-isomer, neuroanatomically preferential for the D3R-rich ventral vs. D3R poor caudodorsal striatum, and prevented by pharmacologic D3R blockade. These findings are consistent with the conclusion that PPX disrupts PPI via stimulation of mesolimbic D3Rs.

Acute cocaine induces fast activation of D1 receptor and progressive deactivation of D2 receptor striatal neurons: in vivo optical microprobe [Ca2+]i imaging

Cocaine induces fast dopamine increases in brain striatal regions, which are recognized to underlie its rewarding effects. Both dopamine D1 and D2 receptors are involved in cocaine's reward but the dynamic downstream consequences of cocaine effects in striatum are not fully understood. Here we used transgenic mice expressing EGFP under the control of either the D1 receptor (D1R) or the D2 receptor (D2R) gene and microprobe optical imaging to assess the dynamic changes in intracellular calcium ([Ca(2+)](i)) responses (used as marker of neuronal activation) to acute cocaine in vivo separately for D1R- versus D2R-expressing neurons in striatum. Acute cocaine (8 mg/kg, i.p.) rapidly increased [Ca(2+)](i) in D1R-expressing neurons (10.6 ± 3.2%) in striatum within 8.3 ± 2.3 min after cocaine administration after which the increases plateaued; these fast [Ca(2+)](i) increases were blocked by pretreatment with a D1R antagonist (SCH23390). In contrast, cocaine induced progressive decreases in [Ca(2+)](i) in D2R-expressing neurons (10.4 ± 5.8%) continuously throughout the 30 min that followed cocaine administration; these slower [Ca(2+)](i) decreases were blocked by pretreatment with a D2R antagonist (raclopride). Since activation of striatal D1R-expressing neurons (direct-pathway) enhances cocaine reward, whereas activation of D2R-expressing neurons suppresses it (indirect-pathway) (Lobo et al., 2010), this suggests that cocaine's rewarding effects entail both its fast stimulation of D1R (resulting in abrupt activation of direct-pathway neurons) and a slower stimulation of D2R (resulting in longer-lasting deactivation of indirect-pathway neurons). We also provide direct in vivo evidence of D2R and D1R interactions in the striatal responses to acute cocaine administration.

Psychomotor stimulation by dopamine D₁-like but not D₂-like agonists in most mouse strains

Many neurological and psychiatric disorders are treated with dopamine modulators. Studies in mice may reveal genetic factors underlying those disorders or responsiveness to various treatments, and species and strain differences both complicate the use of mice and provide valuable tools. We evaluated psychomotor effects of the dopamine D₁-like agonist R-6-Br-APB and the dopamine D₂-like agonist quinelorane using a locomotor activity procedure in 15 mouse strains (inbred 129S1/SvImJ, 129S6/SvEvTac, 129X1/SvJ, A/J, BALB/cByJ, BALB/cJ, C3H/HeJ, C57BL/6J, CAST/EiJ, DBA/2J, FVB/NJ, SJL/J, SPRET/EiJ, outbred Swiss Webster, and CD-1) and Sprague-Dawley rats, using groups of both females and males. Both D₁ and D₂ stimulation produced hyperactivity in the rats, and surprisingly, only two mouse strains were similar in that regard (C3H/HeJ, SPRET/EiJ). In contrast, the majority of mouse strains exhibited hyperactivity only with D₁ stimulation, whereas D₂ stimulation had no effect or decreased activity. BALB substrains, A/J and FVB/NJ mice showed only decreased activity after either D₁ or D₂ stimulation. CAST/EiJ mice exhibited hyperactivity exclusively with D₂ stimulation. Sex differences were observed but no systematic trend emerged: For example, of the five strains in which a main factor of sex was identified for the stimulant effects of the D₁ agonist, responsiveness was greatest in females in three of those strains and in males in two of those strains. These results should aid in the selection of mouse strains for future studies in which D₁ or D₂ responsiveness is a necessary consideration in the experimental design.

Multiple receptors contribute to the behavioral effects of indoleamine hallucinogens

Serotonergic hallucinogens produce profound changes in perception, mood, and cognition. These drugs include phenylalkylamines such as mescaline and 2,5-dimethoxy-4-methylamphetamine (DOM), and indoleamines such as (+)-lysergic acid diethylamide (LSD) and psilocybin. Despite their differences in chemical structure, the two classes of hallucinogens produce remarkably similar subjective effects in humans, and induce cross-tolerance. The phenylalkylamine hallucinogens are selective 5-HT(2) receptor agonists, whereas the indoleamines are relatively non-selective for serotonin (5-HT) receptors. There is extensive evidence, from both animal and human studies, that the characteristic effects of hallucinogens are mediated by interactions with the 5-HT(2A) receptor. Nevertheless, there is also evidence that interactions with other receptor sites contribute to the psychopharmacological and behavioral effects of the indoleamine hallucinogens. This article reviews the evidence demonstrating that the effects of indoleamine hallucinogens in a variety of animal behavioral paradigms are mediated by both 5-HT(2) and non-5-HT(2) receptors.

Dopamine receptor mediation of the exploratory/hyperactivity effects of modafinil

Modafinil (2-((diphenylmethyl)sulfinyl)acetamide) is described as an atypical stimulant and is a putative cognition enhancer for schizophrenia, but the precise mechanisms of action remain unclear. Receptor knockout (KO) mice offer an opportunity to identify receptors that contribute to a drug-induced effect. Here we examined the effects of modafinil on exploration in C57BL/6J mice, in dopamine drd1, drd2, drd3, and drd4 wild-type (WT), heterozygous (HT), and KO mice, and in 129/SJ mice pretreated with the drd1 antagonist SCH23390 using a cross-species test paradigm based on the behavioral pattern monitor. Modafinil increased activity, specific exploration (rearing), and the smoothness of locomotor paths (reduced spatial d) in C57BL/6J and 129/SJ mice (increased holepoking was also observed in these mice). These behavioral profiles are similar to that produced by the dopamine transporter inhibitor GBR12909. Modafinil was ineffective at increasing activity in male drd1 KOs, rearing in female drd1 KOs, or reducing spatial d in all drd1 KOs, but produced similar effects in drd1 WT and HT mice as in C57BL/6J mice. Neither dopamine drd2 nor drd3 mutants attenuated modafinil-induced effects. Drd4 mutants exhibited a genotype dose-dependent attenuation of modafinil-induced increases in specific exploration. Furthermore, the drd1 KO effects were largely supported by the SCH23390 study. Thus, the dopamine drd1 receptor appears to exert a primary role in modafinil-induced effects on spontaneous exploration, whereas the dopamine drd4 receptor appears to be important for specific exploration. The modafinil-induced alterations in exploratory behavior may reflect increased synaptic dopamine and secondary actions mediated by dopamine drd1 and drd4 receptors.

Working memory span capacity improved by a D2 but not D1 receptor family agonist

Patients with schizophrenia exhibit poor working memory (WM). Although several subcomponents of WM can be measured, evidence suggests the primary subcomponent affected in schizophrenia is span capacity (WMC). Indeed, the NIMH-funded MATRICS initiative recommended assaying the WMC when assessing the efficacy of a putative therapeutic for FDA approval. Although dopamine D1 receptor agonists improve delay-dependent memory in animals, evidence for improvements in WMC due to dopamine D1 receptor activation is limited. In contrast, the dopamine D2-family agonist bromocriptine improves WMC in humans. The radial arm maze (RAM) can be used to assess WMC, although complications due to ceiling effects or strategy confounds have limited its use. We describe a 12-arm RAM protocol designed to assess whether the dopamine D1-family agonist SKF 38393 (0, 1, 3, and 10 mg/kg) or bromocriptine (0, 1, 3, and 10 mg/kg) could improve WMC in C57BL/6N mice (n=12) in cross-over designs. WMC increased and strategy usage decreased with training. The dopamine D1 agonist SKF 38393 had no effect on WMC or long-term memory. Bromocriptine decreased WMC errors, without affecting long-term memory, consistent with human studies. These data confirm that WMC can be measured in mice and reveal drug effects that are consistent with reported effects in humans. Future research is warranted to identify the subtype of the D2-family of receptors responsible for the observed improvement in WMC. Finally, this RAM procedure may prove useful in developing animal models of deficient WMC to further assess putative treatments for the cognitive deficits in schizophrenia.

Substance use in adolescence and psychosis: clarifying the relationship

Adolescence is a time of exploration of the self, and this exploration may involve the use of alcohol and drugs. Sadly, for some, adolescence also marks the first signs of a psychosis. The temporal proximity between the onset of substance use and of psychosis has been the cause of much debate. Here we review the association of alcohol, cannabis, stimulants, and other drugs with psychosis, and we conclude that the use of cannabis and the amphetamines significantly contributes to the risk of psychosis.

The effects of pramipexole on prepulse inhibition and locomotor activity in C57BL/6J mice

Pramipexole (PRA) is a preferential D3R agonist that, in rats and humans, modifies prepulse inhibition (PPI) of the acoustic startle reflex, an operational measure of sensorimotor gating. The ability to use similar PPI measures across species, and the relative ease of genetic manipulations in mice, suggests that molecular studies of the D3R regulation of sensorimotor gating might be best pursued in mice. Here, we evaluate the effects of PRA on PPI and locomotion in C57BL/6J mice, the background strain for many gene knockout mouse models. Male C57BL/6J mice were tested for PPI and locomotor activity after injection of PRA. No significant effects of PRA on PPI were observed at any dose (0.1-10.0 mg/kg), but a significant reduction in startle magnitude was observed after 10 mg/kg PRA. In contrast, the D1/2 agonist, apomorphine (5 mg/kg) significantly reduced PPI in these mice. At doses of PRA that did not alter startle magnitude (0.3, 1, 3 mg/kg), significant decreases in the amount of locomotor and investigatory behavior were observed. Distinct from findings in rats and humans, it seems that either: (i) PRA does not activate D3Rs in C57BL/6J mice, or (ii) D3R agonists are not sufficient to alter PPI in this mouse strain.

The effects of kappa-opioid receptor ligands on prepulse inhibition and CRF-induced prepulse inhibition deficits in the rat

RATIONALE

Kappa-opioid receptor (KOR) agonists produce dysphoria and psychotomimesis in humans. KORs are enriched in the prefrontal cortex and other brain regions that regulate mood and cognitive function. Dysregulation of the dynorphin/KOR system has been implicated in the pathogenesis of schizophrenia, depression, and bipolar disorder. Prepulse inhibition of the acoustic startle reflex (PPI), a sensorimotor gating process, is disrupted in many psychiatric disorders.

OBJECTIVES

The present study determined whether KOR ligands alter PPI in rats.

RESULTS

Utilizing a range of doses of the synthetic KOR agonists (+/-) U50,488, (-) U50,488, and U69,593 and the naturally occurring KOR agonist, Salvinorin A, we demonstrate that KOR activation does not alter PPI or startle reactivity in rats. Similarly, selective KOR blockade using the long-acting antagonist nor-binaltorphimine (nor-BNI) was without effect. In contrast to KOR ligands, MK-801 and quinpirole produced deficits in PPI. Stress and corticotropin-releasing factor (CRF) decrease PPI levels. The dynorphin/KOR system has been suggested to be a key mediator of various behavioral effects produced by stress and CRF. We therefore examined the contribution of KORs to CRF-induced alterations in PPI. Intracerebroventricular infusion of CRF decreased PPI. Administration of nor-BNI failed to affect the CRF-evoked disruption in PPI.

CONCLUSIONS

Together, these results provide no evidence of a link between the dynorphin/KOR system and deficits in sensory gating processes. Additional studies, however, examining whether dysregulation of this opioid system contributes to cognitive deficits and other behavioral abnormalities associated with psychiatric disorders are warranted.

Transgenic mice in the study of drug addiction and the effects of psychostimulant drugs

The first transgenic models used to study addiction were based upon a priori assumptions about the importance of particular genes in addiction, including the main target molecules of morphine, amphetamine, and cocaine. This consequently emphasized the importance of monoamine transporters, opioid receptors, and monoamine receptors in addiction. Although the effects of opiates were largely eliminated by mu opioid receptor gene knockout, the case for psychostimulants was much more complex. Research using transgenic models supported the idea of a polygenic basis for psychostimulant effects and has associated particular genes with different behavioral consequences of psychostimulants. Phenotypic analysis of transgenic mice, especially gene knockout mice, has been instrumental in identifying the role of specific molecular targets of addictive drugs in their actions. In this article, we summarize studies that have provided insight into the polygenic determination of drug addiction phenotypes in ways that are not possible with other methods, emphasizing research into the effects of psychostimulant drugs in gene knockouts of the monoamine transporters and monoamine receptors.

Modeling the positive symptoms of schizophrenia in genetically modified mice: pharmacology and methodology aspects

In recent years, there have been huge advances in the use of genetically modified mice to study pathophysiological mechanisms involved in schizophrenia. This has allowed rapid progress in our understanding of the role of several proposed gene mechanisms in schizophrenia, and yet this research has also revealed how much still remains unresolved. Behavioral studies in genetically modified mice are reviewed with special emphasis on modeling psychotic-like behavior. I will particularly focus on observations on locomotor hyperactivity and disruptions of prepulse inhibition (PPI). Recommendations are included to address pharmacological and methodological aspects in future studies. Mouse models of dopaminergic and glutamatergic dysfunction are then discussed, reflecting the most important and widely studied neurotransmitter systems in schizophrenia. Subsequently, psychosis-like behavior in mice with modifications in the most widely studied schizophrenia susceptibility genes is reviewed. Taken together, the available studies reveal a wealth of available data which have already provided crucial new insight and mechanistic clues which could lead to new treatments or even prevention strategies for schizophrenia.

Dopamine is necessary for cue-dependent fear conditioning

Dopamine (DA) is implicated in many behaviors, including motor function, cognition, and reward processing; however, the role of DA in fear processing remains equivocal. To examine the role of DA in fear-related learning, dopamine-deficient (DD) mice were tested in a fear-potentiated startle paradigm. DA synthesis can be restored in DD mice through administration of 3, 4-dihydroxy-l-phenylalanine (l-Dopa), thereby permitting the assessment of fear processing in either a DA-depleted or -replete state. Fear-potentiated startle was absent in DD mice but could be restored by l-Dopa administration immediately after fear conditioning. Selective viral-mediated restoration of DA synthesis within the ventral tegmental area fully restored fear learning in DD mice, and restoration of DA synthesis to DA neurons projecting to the basolateral amygdala restored short-term memory but not long-term memory or shock sensitization. We also demonstrate that the DA D(1) receptor (D(1)R) and D(2)-like receptors are necessary for cue-dependent fear learning. These findings indicate that DA acting on multiple receptor subtypes within multiple target regions facilitates the stabilization of fear memory.

Using prepulse inhibition to detect functional D3 receptor antagonism: effects of WC10 and WC44

Prepulse inhibition of startle (PPI) is an operational measure of sensorimotor gating that is impaired in schizophrenia. Treatment with mixed dopamine D2/D3 antagonists diminishes schizophrenia symptoms, and opposes dopamine agonist-induced PPI deficits in rats. There are reasons to believe that functional D3 receptor antagonists might offer more favorable therapeutic profiles compared to current antipsychotics. However, D3-related drug discovery is hampered by the absence of assays sensitive to D3-mediated (antipsychotic) properties in vivo. Here, we characterized two putative D3-active compounds - WC10 and WC44 - in a PPI-based screening assay, comparing the sensitivity of test compounds to oppose PPI deficits induced by the mixed D1/D2-like agonist apomorphine vs. the preferential D3 agonist pramipexole in rats. WC10, WC44 (0, 1, 3, 10 mg/kg, each), and the preferential D2 antagonist L741,626 (0, 1 mg/kg) were studied, in combination with apomorphine (0, 0.5 mg/kg), or pramipexole (0, 1 mg/kg). L741,626 prevented apomorphine-, but not pramipexole-induced PPI deficits. WC10, but not WC44, prevented apomorphine-induced PPI deficits; both compounds opposed pramipexole-induced PPI deficits, suggesting functional D3 and D1/D2 antagonist profiles for WC10, and functional D3 receptor antagonism for WC44. This assay may be valuable for detecting predominantly D3 vs. D2 receptor-linked mechanisms of action in vivo.

Prepulse inhibition and genetic mouse models of schizophrenia

Mutant mouse models related to schizophrenia have been based primarily on the pathophysiology of schizophrenia, the known effects of antipsychotic drugs, and candidate genes for schizophrenia. Sensorimotor gating deficits in schizophrenia patients, as indexed by measures of prepulse inhibition of startle (PPI), have been well characterized and suggested to meet the criteria as a useful endophenotype in human genetic studies. PPI refers to the ability of a non-startling "prepulse" to inhibit responding to the subsequent startling stimulus or "pulse." Because of the cross-species nature of PPI, it has been used primarily in pharmacological animal models to screen putative antipsychotic medications. As techniques in molecular genetics have progressed over the past 15 years, PPI has emerged as a phenotype used in assessing genetic mouse models of relevance to schizophrenia. In this review, we provide a selected overview of the use of PPI in mouse models of schizophrenia and discuss the contribution and usefulness of PPI as a phenotype in the context of genetic mouse models. To that end, we discuss mutant mice generated to address hypotheses regarding the pathophysiology of schizophrenia and candidate genes (i.e., hypothesis driven). We also briefly discuss the usefulness of PPI in phenotype-driven approaches in which a PPI phenotype could lead to "bottom up" approaches of identifying novel genes of relevance to PPI (i.e., hypothesis generating).

Enhanced latent inhibition in dopamine receptor-deficient mice is sex-specific for the D1 but not D2 receptor subtype: implications for antipsychotic drug action

Latent inhibition (LI) is reduced learning to a stimulus that has previously been experienced without consequence. It is an important model of abnormal allocation of salience to irrelevant information in patients with schizophrenia. In rodents LI is abolished by psychotomimetic drugs and in experimental conditions where LI is low in controls, its expression is enhanced by antipsychotic drugs with activity at dopamine (DA) receptors. It is however unclear what the independent contributions of DA receptor subtypes are to these effects. This study therefore examined LI in congenic DA D1 and D2 receptor knockout (D1 KO and D2 KO) mice. Conditioned suppression of drinking was used as the measure of learning in the LI procedure. Both male and female DA D2 KO mice showed clear enhancement of LI reproducing antipsychotic drug effects in the model. Unexpectedly, enhancement was also seen in D1 KO female mice but not in D1 KO male mice. This sex-specific pattern was not replicated in locomotor or motor coordination tasks nor in the effect of DA KOs on baseline learning in control groups indicating some specificity of the effect to LI. These data suggest that the dopaminergic mechanism underlying LI potentiation and possibly antipsychotic action may differ between the sexes, being mediated by D2 receptors in males but by both D1 and D2 receptors in females. These data suggest that the DA D1 receptor may prove an important target for understanding sex differences in the mechanisms of action of antipsychotic drugs and in the aetiology of aberrant salience allocation in schizophrenia.

Neurophysiological endophenotypes across bipolar and schizophrenia psychosis

The search for liability genes of the world's 2 major psychotic disorders, schizophrenia and bipolar disorder I (BP-I), has been extremely difficult even though evidence suggests that both are highly heritable. This difficulty is due to the complex and multifactorial nature of these disorders. They encompass several intermediate phenotypes, some overlapping across the 2 psychotic disorders that jointly and/or interactively produce the clinical manifestations. Research of the past few decades has identified several neurophysiological deficits in schizophrenia that frequently occur before the onset of psychosis. These include abnormalities in smooth pursuit eye movements, P50 sensory gating, prepulse inhibition, P300, mismatch negativity, and neural synchrony. Evidence suggests that many of these physiological deficits are distinct from each other. They are stable, mostly independent of symptom state and medications (with some exceptions) and are also observed in non-ill relatives. This suggests a familial and perhaps genetic nature. Some deficits are also observed in the BP-I probands and to a lesser extent their relatives. These deficits in physiological measures may represent the intermediate phenotypes that index small effects of genes (and/or environmental factors). The use of these measures in genetic studies may help the hunt for psychosis liability genes and clarify the extent to which the 2 major psychotic disorders share etio-pathophysiology. In spite of the rich body of work describing these neurophysiological measures in psychotic disorders, challenges remain: Many of the neurophysiological phenotypes are still relatively complex and are associated with low heritability estimates. Further refinement of these physiological phenotypes is needed that could identify specific underlying physiological deficits and thereby improve their heritability estimates. The extent to which these neurophysiological deficits are unique or overlap across BP-I and schizophrenia is unclear. And finally, the clinical and functional consequences of the neurophysiological deficits both in the probands and their relatives are not well described.