Reversal of sensorimotor gating deficits in Brattleboro rats by acute administration of clozapine and a neurotensin agonist, but not haloperidol: a potential predictive model for novel antipsychotic effects

Hippocampal chloride transporter KCC2 contributes to excitatory GABA dysregulation in the developmental rat model of schizophrenia

Recent studies have revealed an altered expression of NKCC1 and KCC2 in prefrontal cortex (PFC) and hippocampus of schizophrenic patients. Despite extensive considerations, the alteration of NKCC1 and KCC2 co-transporters at different stages of development has not been fully studied. Therefore, we evaluated the expression of these transporters in PFC and hippocampus at time points of four, eight, and twelve weeks in post-weaning social isolation rearing rat model. For this purpose, 23-25 days-old rats were classified into social- or isolation-reared groups. The levels of NKCC1 and KCC2 mRNA expression were evaluated at hippocampus or PFC regions at the time-points of four, eight, and twelve weeks following housing. Post-weaning isolation rearing decreased the hippocampal KCC2 mRNA expression level, but does not affect the NKCC1 mRNA expression. However, no significant difference was observed in the PFC mRNA levels of NKCC1 and KCC2 in the isolation-reared group compared to the socially-reared group during the course of modeling. Further, we assessed the therapeutic effect of selective NKCC1 inhibitor bumetanide (10 mg/kg), on improvement of prepulse inhibition (PPI) test on twelve weeks isolation-reared rats. Intraperitoneal administration of bumetanide (10 mg/kg) did not exert beneficial effects on PPI deficit. Our findings show that isolation rearing reduces hippocampal KCC2 expression level and may underlie hippocampal GABA excitatory. In addition, 10 mg/kg bumetanide is not effective in improving the reduced PPI of twelve weeks isolation-reared rats. Collectively, our findings show that hippocampal chloride transporter KCC2 contributes to excitatory GABA dysregulation in the developmental rat model of schizophrenia.

Ameliorating schizophrenia-like symptoms in vasopressin deficient male Brattleboro rat by chronic antipsychotic treatment

Due to its various function vasopressin has been associated with many psychiatric disorders, including schizophrenia. Our previous study confirmed that vasopressin-deficient (di/di) Brattleboro rat can be a good genetic model for schizophrenia. Our present aim was to confirm whether the treatment effects of marketed antipsychotics are similar in di/di rats to those seen in human schizophrenic patients. Chronic subcutaneous administration of aripiprazole (5 mg/kg), clozapine (1 mg/kg), haloperidol (0.1 mg/kg), olanzapine (0.3 mg/kg) or risperidone (0.25 mg/kg) was used for 15 days in control (+/+ Brattleboro) and di/di rats. Social discrimination, social avoidance and prepulse inhibition tests were conducted on day 1, 8 and 15 of the treatment. Vasopressin-deficient rats showed social memory- and sensorimotor gating deficit. All used antipsychotics successfully normalized the reduced prepulse inhibition of di/di animals. However, most were effective only after prolonged treatment. Aripiprazole, clozapine, and olanzapine normalized the social memory deficit, while the effects of haloperidol and risperidone were not unequivocal. All drugs reduced social interest to some extent both in control and in di/di animals, aripiprazole being the less implicated in this regard during the social avoidance test. The restoration of schizophrenia-like behavior by antipsychotic treatment further support the utility of the vasopressin-deficient Brattleboro rat as a good preclinical model. Reduced social interest might be a general side-effect of antipsychotics, and aripiprazole has the most favorable profile in this regard.

Antipsychotic drug-like facilitation of latent inhibition by a brain-penetrating neurotensin-1 receptor agonist

Latent inhibition (LI) is a measure of cognitive gating and refers to reduced conditioned learning when there is pre-exposure to the conditioned stimulus (CS) before it is paired with the unconditioned stimulus (US). Dysregulation of LI is associated with some neuropsychiatric disorders, including schizophrenia, and the ability to facilitate LI in rodents is a reasonably good predictive test for antipsychotic drugs. Converging evidence supports neurotensin-1 receptor (NTS1) agonists as novel drugs for schizophrenia. Therefore, we investigated the ability of a brain-penetrating, selective NTS1 agonist, PD149163, to facilitate LI in heterozygous Brattleboro rats, a strain that exhibits naturally low LI. Conditioned taste aversion to flavored water (FW; 0.1% saccharin) was induced by pairing it with malaise-inducing injections of lithium chloride (LiCl). Prior to LiCl-FW pairing, rats received subcutaneous injections of saline, or PD149163 (100 µg/kg or 200 µg/kg). Half the rats in each drug group had been allowed to drink FW the day before the LiCl-FW pairing (pre-exposed rats). Two days after pairing, the amount of FW each rat consumed was recorded. LI, defined as significantly greater FW drinking in the pre-exposed group compared with the non pre-exposed group, was exhibited only among rats that received 200 µg/kg of PD149163. These results further support NTS1 agonists as potentially novel drugs for the treatment of schizophrenia.

Antipsychotic-like effects of a neurotensin receptor type 1 agonist

Although neurotensin (NT) analogs are known to produce antipsychotic-like effects, the therapeutic possibility of a brain penetrant NTS1 agonist in treating psychiatric disorders has not been well studied. Here, we examined whether PD149163, a brain-penetrant NTS1-specific agonist, displays antipsychotic-like effects in C57BL/6J mice by investigating the effect of PD149163 on amphetamine-mediated hyperactivity and amphetamine-induced disruption of prepulse inhibition. In addition, we assessed the effect of PD149163 on glycogen synthase kinase-3 (GSK-3) activity, a downstream molecular target of antipsychotics and mood stabilizers, using phospho-specific antibodies. PD149163 (0.1 and 0.5mg/kg) inhibited amphetamine-induced hyperactivity in mice, indicating that NTS1 activation inhibits psychomotor agitation. PD149163 (0.5mg/kg) also increased prepulse inhibition, suggesting that NTS1 activation reduces prepulse inhibition deficits which often co-occur with psychosis in humans. Interestingly, PD149163 increased the inhibitory serine phosphorylation on both GSK-3α and GSK-3β in a dose- and time-dependent manner in the nucleus accumbens and medial prefrontal cortex of the mice. Moreover, PD149163 inhibited GSK-3 activity in the nucleus accumbens and medial prefrontal cortex in the presence of amphetamine. Thus, like most current antipsychotics and mood stabilizers, PD149163 inhibited GSK-3 activity in cortico-striatal circuitry. Together, our findings indicate that PD149163 may be a novel antipsychotic.

Oxytocin, Vasopressin, and the Motivational Forces that Drive Social Behaviors

The motivation to engage in social behaviors is influenced by past experience and internal state, but also depends on the behavior of other animals. Across species, the oxytocin (Oxt) and vasopressin (Avp) systems have consistently been linked to the modulation of motivated social behaviors. However, how they interact with other systems, such as the mesolimbic dopamine system, remains understudied. Further, while the neurobiological mechanisms that regulate prosocial/cooperative behaviors have been extensively examined, far less is understood about competitive behaviors, particularly in females. In this chapter, we highlight the specific contributions of Oxt and Avp to several cooperative and competitive behaviors and discuss their relevance to the concept of social motivation across species, including humans. Further, we discuss the implications for neuropsychiatric diseases and suggest future areas of investigation.

Patterns of Spontaneous Local Network Activity in Developing Cerebral Cortex: Relationship to Adult Cognitive Function

Detecting neurodevelopμental disorders of cognition at the earliest possible stages could assist in understanding them mechanistically and ultimately in treating them. Finding early physiological predictors that could be visualized with functional neuroimaging would represent an important advance in this regard. We hypothesized that one potential source of physiological predictors is the spontaneous local network activity prominent during specific periods in development. To test this we used calcium imaging in brain slices and analyzed variations in the frequency and intensity of this early activity in one area, the entorhinal cortex (EC), in order to correlate early activity with level of cognitive function later in life. We focused on EC because of its known role in different types of cognitive processes and because it is an area where spontaneous activity is prominent during early postnatal development in rodent models of cortical development. Using rat strains (Long-Evans, Wistar, Sprague-Dawley and Brattleboro) known to differ in cognitive performance in adulthood we asked whether neonatal animals exhibit corresponding strain-related differences in EC spontaneous activity. Our results show significant differences in this activity between strains: compared to a high cognitive-performing strain, we consistently found an increase in frequency and decrease in intensity in neonates from three lower performing strains. Activity was most different in one strain considered a model of schizophrenia-like psychopathology. While we cannot necessarily infer a causal relationship between early activity and adult cognition our findings suggest that the pattern of spontaneous activity in development could be an early predictor of a developmental trajectory advancing toward sub-optimal cognitive performance in adulthood. Our results further suggest that the strength of dopaminergic signaling, by setting the balance between excitation and inhibition, is a potential underlying mechanism that could explain the observed differences in early spontaneous activity patterns.

Systemic administration of the neurotensin NTS₁-receptor agonist PD149163 improves performance on a memory task in naturally deficient male brown Norway rats

Agonists for the neurotensin NTS₁ receptor consistently exhibit antipsychotic effects in animal models without producing catalepsy, suggesting that NTS₁-receptor agonists may be a novel class of drugs to treat schizophrenia. Moreover, studies utilizing NTS₁ agonists have reported improvements in some aspects of cognitive functioning, including prepulse inhibition and learning procedures, which suggest an ability of NTS₁-receptor agonists to diminish neurocognitive deficits. The present study sought to assess both baseline delay-induced memory performance and the effects of NTS₁-receptor activation on learning and memory consolidation in male Long-Evans and Brown Norway rats using a delayed nonmatch-to-position task radial arm-maze task. In the absence of drugs, Brown Norway rats displayed a significant increase in spatial memory errors following 3-, 7-, and 24-hr delay, whereas Long-Evans rats exhibited an increase in spatial memory errors following only a 7-, and 24-hr delay. With Brown Norway rats, administration of PD149163 before or after an information trial significantly reduced errors during a retention trial after a 24 hr delay. Administration of the NTS(1/2)-receptor antagonist SR142948 prior to the information trial did not affect retention-trial errors. These data are consistent with previous findings that Brown Norway rats have natural cognitive deficits and that they may be useful for assessing putative antipsychotic drugs for cognitive efficacy. Moreover, the results of this study support previous findings suggesting that NTS₁-receptor agonists may improve some aspects of cognitive functioning.

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

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

Activation of neurotensin receptor 1 facilitates neuronal excitability and spatial learning and memory in the entorhinal cortex: beneficial actions in an Alzheimer's disease model

Neurotensin (NT) is a tridecapeptide distributed in the CNS, including the entorhinal cortex (EC), a structure that is crucial for learning and memory and undergoes the earliest pathological alterations in Alzheimer's disease (AD). Whereas NT has been implicated in modulating cognition, the cellular and molecular mechanisms by which NT modifies cognitive processes and the potential therapeutic roles of NT in AD have not been determined. Here we examined the effects of NT on neuronal excitability and spatial learning in the EC, which expresses high density of NT receptors. Brief application of NT induced persistent increases in action potential firing frequency, which could last for at least 1 h. NT-induced facilitation of neuronal excitability was mediated by downregulation of TREK-2 K(+) channels and required the functions of NTS1, phospholipase C, and protein kinase C. Microinjection of NT or NTS1 agonist, PD149163, into the EC increased spatial learning as assessed by the Barnes Maze Test. Activation of NTS1 receptors also induced persistent increases in action potential firing frequency and significantly improved the memory status in APP/PS1 mice, an animal model of AD. Our study identifies a cellular substrate underlying learning and memory and suggests that NTS1 agonists may exert beneficial actions in an animal model of AD.

Elucidating the role of neurotensin in the pathophysiology and management of major mental disorders

Neurotensin (NT) is a neuropeptide that is closely associated with, and is thought to modulate, dopaminergic and other neurotransmitter systems involved in the pathophysiology of various mental disorders. This review outlines data implicating NT in the pathophysiology and management of major mental disorders such as schizophrenia, drug addiction, and autism. The data suggest that NT receptor analogs have the potential to be used as novel therapeutic agents acting through modulation of neurotransmitter systems dys-regulated in these disorders.

Effects of the neurotensin NTS₁ receptor agonist PD149163 on visual signal detection in rats

Antipsychotic drugs provide limited efficacy for cognitive impairment in schizophrenia. Recent studies have found that the neurotensin NTS1 receptor agonist and putative atypical antipsychotic drug PD149163 reverses deficits in sensory-gating and novel object recognition, suggesting that this compound may have the potential to improve cognitive functioning in schizophrenia. The present study sought to extend these investigations by evaluating the effects of PD149163 on sustained attention using a visual signal detection operant task in rats. PD149163, the atypical antipsychotic drug clozapine, and the dopamine D2/3 receptor antagonist raclopride all significantly decreased percent "hit" accuracy, while none of these compounds altered "correct rejections" (compared to vehicle control). Clozapine and raclopride significantly increased response latency, while high doses of PD149163 and raclopride significantly increased trial omissions. Nicotine, which was tested as a positive control, significantly improved overall performance in this task and did not affect response latency or trial omissions. The present findings suggest that neurotensin NTS1 receptor agonists, like antipsychotic drugs, may inhibit sustained attention in this task despite having different pharmacological mechanisms of action.

The effect of leptin on prepulse inhibition in a developmental model of schizophrenia

Post-weaning social isolation is a developmental animal model of schizophrenia. Impairment of prepulse inhibition (PPI), possibly due to increased activity of the mesolimbic dopaminergic system, has frequently been reported in this model. There are some reports of increased level of leptin in schizophrenic patients. It has been shown that intracerebroventricular (ICV) injection of leptin decreases dopamine in the nucleus accumbens of rats. Here we investigated the effect of leptin on PPI impairment following social isolation. Five groups of Sprague-Dawley rats were reared post weaning in social or isolated conditions for 14 weeks. PPI was measured before treatment in week 12, and after ICV injection of vehicle or different doses of leptin (1, 5, and 10μg/5μl) in week 14. Results showed reduced PPI in untreated isolated compared to socially-reared rats in week 12 (p=0.009), but not in week 14 (p=0.45). Results also showed that leptin dose-dependently increased the basal PPI in isolated rats compared to vehicle, that was significant at a dose of 10μg (p=0.002). A considerable but non-significant effect of treatment with leptin on startle response (p=0.13) was seen. In conclusion, our results reveal that leptin significantly increases PPI in socially-isolated rats. The findings of this study suggest possible antipsychotic properties for leptin. We suggest further studies on the possible disruption of leptin signaling in schizophrenia, and also the possible interaction of leptin with therapeutic effects of second generation antipsychotics.

Vasopressin V1a and V1b Receptors: From Molecules to Physiological Systems

The neurohypophysial hormone arginine vasopressin (AVP) is essential for a wide range of physiological functions, including water reabsorption, cardiovascular homeostasis, hormone secretion, and social behavior. These and other actions of AVP are mediated by at least three distinct receptor subtypes: V1a, V1b, and V2. Although the antidiuretic action of AVP and V2 receptor in renal distal tubules and collecting ducts is relatively well understood, recent years have seen an increasing understanding of the physiological roles of V1a and V1b receptors. The V1a receptor is originally found in the vascular smooth muscle and the V1b receptor in the anterior pituitary. Deletion of V1a or V1b receptor genes in mice revealed that the contributions of these receptors extend far beyond cardiovascular or hormone-secreting functions. Together with extensively developed pharmacological tools, genetically altered rodent models have advanced the understanding of a variety of AVP systems. Our report reviews the findings in this important field by covering a wide range of research, from the molecular physiology of V1a and V1b receptors to studies on whole animals, including gene knockout/knockdown studies.

Predictive animal models of mania: hits, misses and future directions

Mania has long been recognized as aberrant behaviour indicative of mental illness. Manic states include a variety of complex and multifaceted symptoms that challenge clear clinical distinctions. Symptoms include over-activity, hypersexuality, irritability and reduced need for sleep, with cognitive deficits recently linked to functional outcome. Current treatments have arisen through serendipity or from other disorders. Hence, treatments are not efficacious for all patients, and there is an urgent need to develop targeted therapeutics. Part of the drug discovery process is the assessment of therapeutics in animal models. Here we review pharmacological, environmental and genetic manipulations developed to test the efficacy of therapeutics in animal models of mania. The merits of these models are discussed in terms of the manipulation used and the facet of mania measured. Moreover, the predictive validity of these models is discussed in the context of differentiating drugs that succeed or fail to meet criteria as approved mania treatments. The multifaceted symptomatology of mania has not been reflected in the majority of animal models, where locomotor activity remains the primary measure. This approach has resulted in numerous false positives for putative treatments. Recent work highlights the need to utilize multivariate strategies to enable comprehensive assessment of affective and cognitive dysfunction. Advances in therapeutic treatment may depend on novel models developed with an integrated approach that includes: (i) a comprehensive battery of tests for different aspects of mania, (ii) utilization of genetic information to establish aetiological validity and (iii) objective quantification of patient behaviour with translational cross-species paradigms.

Clozapine and PD149163 elevate prepulse inhibition in Brown Norway rats

Unmedicated schizophrenia patients exhibit deficits in prepulse inhibition (PPI) of the acoustic startle response. Similar deficits can be induced in rodents via a variety of manipulations and these deficits can be reversed by antipsychotics. Brown Norway (BN) rats exhibit natural PPI deficits under certain parametric conditions. We treated BN rats with haloperidol or clozapine to determine if the BN rat is a useful animal model with predictive validity for the effects of antipsychotics. In addition, we also tested PD149163, a neurotensin-1 receptor agonist, which has been shown to exhibit antipsychotic-like effects in several other animal models. BN rats received subcutaneous injections of either saline or one of two doses of haloperidol (0.5 mg/kg, 1.0 mg/kg), clozapine (7.5 mg/kg, 10 mg/kg) or PD149163 (1.0 mg/kg, 2.0 mg/kg). PPI was measured in startle chambers 30 min after injection. Systemic clozapine and PD149163 but not haloperidol facilitated PPI in BN rats (p < .001). This drug response profile suggests that the BN rat may be useful for detecting atypical antipsychotics and antipsychotics with novel mechanisms of action. The results also add to the evidence suggesting that PD149163 may have antipsychotic properties.

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.

Neurobiology of sociability

Sociability consists of behaviors that bring animals together and those that keep animals apart. Remarkably, while the neural circuitry that regulates these two "faces" of sociability differ from one another, two neurohormones, oxytocin (Oxt) and vasopressin (Avp), have been consistently implicated in the regulation of both. In this chapter the the structure and function of the Oxt and Avp systems, the ways in which affiliative and aggressive behavior are studied and the roles of Oxt and Avp in the regulation of sociability will be briefly reviewed. Finally, work implicating Oxt and Avp in sociability in humans, with a focus on neuropsychiatric disorders will be highlighted.

Startle response related genes

The startle reaction (also known as the startle response, the startle reflex, or the alarm reaction) is the psychological and physiological response to a sudden unexpected stimulus, such as a flash of light, a loud noise (acoustic startle reflex), or a quick movement near the face. Abnormalities of startle response have been observed in many stress-related mental disorders, such as schizophrenia and post-traumatic stress disorder (PTSD). However, the molecular mechanisms of startle in stress-associated conditions--for example, whether the startle reaction is associated with any gene variance--is still unknown. In this paper, we will carry out a systematic review by retrieving, assessing, and combining, when applicable, individual studies investigating association of the molecular variation of candidate gene with the startle response. The systematic review is based on the search for numerous publications using the keywords "startle gene" on September 15, 2010 using PubMed, which comprises more than 20 million citations for biomedical literature from MEDLINE and life science journals. A total of 486 publications regarding genes associated with startle have been obtained and reviewed here. There are fewer than 20 publications associating genes with the startle response between 1979, when the first valuable paper was published, and 1999. However, publications have dramatically increase from 2001 and reaches over 70 in 2009. We have characterized them into three categories: startle-associated gene studies in humans, in animals, as well as in both human and animals. This review of research strategy may provide the information for identifying a biomarker for startle response, with the objective of translating research into clinical utility: diagnosis and treatment of stress-induced mental disorders.

The neurotensin-1 receptor agonist PD149163 inhibits conditioned avoidance responding without producing catalepsy in rats

Agonists for neurotensin (NT)-1 receptors have produced antipsychotic-like effects in many animals, including reversal of prepulse inhibition deficits and psychostimulant-induced increases in spontaneous activity. The present study sought to provide a basic assessment of the putative antipsychotic effects of PD149163 in rats using a two way conditioned avoidance response task, which is highly validated for screening antipsychotic drugs, and an inclined grid assessment, which is used to assess extrapyramidal side effect liability. PD149163 (0.0625-8.0 mg/kg) significantly suppressed conditioned avoidance responding (CAR) following administration of a 1.0 or 8.0 mg/kg dose. PD149163 failed to significantly increase catalepsy scores. The typical antipsychotic drug haloperidol (0.01-1.0 mg/kg) significantly suppressed CAR at a 0.1, 0.3, and 1.0 mg/kg dose, and a significant increase in catalepsy scores was found at the 1.0 mg/kg dose. The atypical antipsychotic drug clozapine (2.5-10.0 mg/kg) also produced a significant inhibition of CAR, which occurred following administration of a 10.0 mg/kg dose. Clozapine failed to significantly increase catalepsy scores. Finally, D-amphetamine (1.0 mg/kg), serving as a negative control, failed to suppress CAR or increase catalepsy scores. These data further suggest that PD149163 may have atypical antipsychotic-like properties.

Further characterization of the predictive validity of the Brattleboro rat model for antipsychotic efficacy

Our laboratory and others have reported that Brattleboro (BRAT) rats, a Long Evans (LE) strain with a single gene mutation, have inherent deficits in prepulse inhibition (PPI) homologous to those observed in schizophrenia patients and that these deficits are reversed by antipsychotic drugs (APDs). To further evaluate the potential predictive validity of BRAT rat PPI for APDs, we compared the effects of acute subcutaneous administration of the typical APD chlorpromazine to that of three psychotropic drugs without antipsychotic efficacy, the antidepressant imipramine, the anxiolytic diazepam and the anticonvulsant mood stabilizer valproic acid on male and female BRAT rat PPI. Male and female BRAT rats exhibited baseline (saline treatment) PPI that was not different from each other (21.1% and 21.3%, respectively) and low compared with those historically exhibited by LE rats (approximately 59%). Chlorpromazine facilitated PPI in male and female BRAT rats, whereas imipramine, diazepam, and valproic acid had no significant effect on PPI. These results suggest that PPI in the BRAT rat responds specifically to drugs with APD efficacy but not psychotropic drugs of different therapeutic families.

Sensorimotor gating in NTS1 and NTS2 null mice: effects of d-amphetamine, dizocilpine, clozapine and NT69L

Pre-pulse inhibition (PPI) of the acoustic startle reflex is deficient in patients with schizophrenia. This deficiency is mimicked in mice by the use of the psychotomimetic drugs d-amphetamine and dizolcipine. Antipsychotic drugs such as clozapine are used to treat schizophrenic patients and are also administered to mice to prevent PPI disruption. Neurotensin (NT) produces antipsychotic-like effects when injected into rodent brain through its effects at NT subtype 1 (NTS1) and 2 (NTS2) receptors. We hypothesized that the NT receptor agonist (NT69L) would prevent PPI disruption in mice challenged with d-amphetamine (10 mg kg(-1)) and dizocilpine (1 mg kg(-1)). We investigated the role of NTS1 and NTS2 in PPI using wild-type (WT), NTS1 (NTS1(-/-)) and NTS2 (NTS2(-/-)) knockout mice, via its disruption by psychotomimetic drugs, as well as the ability of clozapine and NT69L to block these PPI disruptions. There were no differences in baseline PPI across the three genotypes. d-Amphetamine and dizocilpine disrupted PPI in WT and NTS2(-/-) mice but not in NTS1(-/-) mice. In WT mice, clozapine (1 mg kg(-1)) and NT69L (1 mg kg(-1)) significantly blocked d-amphetamine-induced disruption of PPI. Similarly, in WT mice, clozapine significantly blocked dizocilpine-induced PPI disruption, but NT69L did not. In NTS2(-/-) mice clozapine blocked d-amphetamine-but not dizocilpine-induced PPI disruption, while NT69L blocked both d-amphetamine- and dizocilpine-induced PPI disruption. Our results indicate that NTS1 seems essential for d-amphetamine and dizocilpine disruption of PPI. Additionally, this report provides support to the hypothesis that NT analogs could be used as novel antipsychotic drugs.

Nitric oxide synthase inhibitors improve prepulse inhibition responses of Wistar rats

INTRODUCTION

Cognitive and attentional deficits in schizophrenia include impairment of the sensorimotor filter as measured by prepulse inhibition (PPI). In this way, the study of animals that naturally present low PPI responses could be a useful approach for screening new antipsychotic drugs. Several pieces of evidence suggest that dopamine and nitric oxide (NO) can modulate PPI but their role in those animals is unknown.

OBJECTIVES

The aim of this study was to investigate the role of dopamine and NO in Wistar rats with naturally low PPI response.

METHODS

Male Wistar rats with low PPI responses received an i.p. injection of the antipsychotics haloperidol (0.1, 0.3 or 1mg/kg) or clozapine (0.5, 1.5 or 5mg/kg), the anxiolytic diazepam (1 or 3mg/kg) or the NO synthase (NOS) inhibitors, N(G)- nitro-l-arginine (l-NOARG; 40mg/kg, acutely or sub-chronically) or 7-Nitroindazole (7-NI; 3, 10 or 30mg/kg). All animals were submitted to the PPI test 1h after injection. Striatal and cortical dopamine, DOPAC, and noradrenaline levels of rats with low PPI responses were compared to rats with normal PPI responses.

RESULTS

We found increased levels of catecholamines on the striatum and prefrontal cortex of Wistar rats with low PPI. In these animals, both antipsychotics, typical and atypical, and NOS inhibitors significantly increased PPI.

CONCLUSION

Taken together, our findings suggest that the low PPI phenotype may be driven by an overactive catecholamine system. Additionally, our results corroborate the hypothesis of dopamine and NO interaction on PPI modulation and suggest that Wistar rats with low PPI may represent an interesting non-pharmacological model to evaluate new potential antipsychotics.

Effects of neurotensin-2 receptor deletion on sensorimotor gating and locomotor activity

Endogenous neurotensin (NT) has been implicated in brain processes relevant to schizophrenia as well as the therapeutic effects of antipsychotic drugs (APDs) used to treat this disorder. Converging evidence suggests that NT1 receptors mediate the antipsychotic-like effects of NT, such as prepulse inhibition (PPI) elevation. However, the role of NT2 receptors in these effects is not known. To investigate the contribution of NT2 receptors to the regulation of PPI, we measured baseline PPI and acoustic startle response (ASR), in male and female wild type (WT) and NT2 knockout (KO) mice. For comparison, we also measured locomotor activity. Baseline PPI was significantly elevated in both male (P<0.01) and female (P<0.01) NT2 KO compared to WT mice, while ASR was significantly decreased in KO mice of both genders (P<0.01). In contrast, female but not male KO mice exhibited significantly less baseline ambulations (P<0.05). These data support the regulation of baseline PPI, ASR and locomotor activity by endogenous NT acting at the NT2 receptor. Further studies investigating the role of NT2 receptors in the modulation of APD-like effects are warranted.

Further neurochemical and behavioural investigation of Brattleboro rats as a putative model of schizophrenia

Brattleboro (BRAT) rats are a mutant variant of the Long-Evans (LE) strain deficient in the neurohormone vasopressin. BRAT rats show behavioural alterations relevant to schizophrenia. In particular, BRAT rats show deficits in prepulse inhibition (PPI) and alterations in various measures of cognition. The aim of this study was to replicate the reported PPI deficits in BRAT rats and its reversal by antipsychotic drugs and to investigate other behavioural and neurochemical characteristics. Acoustic startle reactivity, PPI, spontaneous and amphetamine-induced locomotor activity (LMA) and ex-vivo steady state neurochemistry were measured in male homozygous BRAT rats and LE rats. The effects of antipsychotics on PPI deficits were also determined. Relative to LE, BRAT rats showed enhanced startle reactivity, hyperactivity to a novel environment, PPI deficits and decreased levels of dopamine and DOPAC (dihydroxyphenylacetic acid) in the frontal cortex. BRAT and LE rats showed similar levels of hyperactivity following amphetamine (0.26 mg/kg s.c.). PPI deficits were attenuated by acute clozapine (5-10 mg/kg s.c.), risperidone (0.1-1 mg/kg i.p.), haloperidol (0.1-0.5 mg/kg p.o.) and less robustly by olanzapine (0.3-3 mg/kg s.c.). Chronic administration of clozapine (5 mg/kg s.c., once daily) attenuated baseline hyperactivity and elevated PPI of both strains. Clozapine concentrations were higher in BRAT brains compared with LE rats. These data confirm the reported PPI deficit in BRAT rats and its reversal by antipsychotic drugs, suggesting BRAT rats may represent a potential model for identifying novel antipsychotic drugs.

Chronic NT69L potently prevents drug-induced disruption of prepulse inhibition without causing tolerance

NT69L is a neurotensin receptor agonist with antipsychotic-like activity. NT69L blocks apomorphine-induced climbing in rats with no effect on stereotypic behavior, attenuates d-amphetamine-induced hyperactivity, and blocks pharmacologically induced disruption of prepulse inhibition (PPI) of the startle response. Repeated administration of NT69L results in tolerance to some, but not to all of its effects. Because schizophrenic patients require long-term treatment, chronic (21-day) administration of NT69L was tested in PPI with comparisons to chronic haloperidol and clozapine treatment. Sprague-Dawley rats received acute or 21 daily, subcutaneous injections of NT69L (1.0mg/kg). On days 1 and 21 the NT69L injection was followed 30 min later by treatment with either saline; the dopamine agonist, d-amphetamine (5.0mg/kg); or the serotonin 5-HT(2A) psychotomimetic receptor agonist [1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane] DOI (0.5mg/kg). Experiments were repeated with either haloperidol (1mg/kg) or clozapine (20mg/kg) in place of NT69L. Acute injection of NT69L significantly blocked d-amphetamine and DOI disruption of PPI. As with the acute injection, 21 daily administrations of NT69L also blocked d-amphetamine- and DOI-induced disruption of PPI. The data show that animals do not develop tolerance to the antipsychotic-like effects of NT69L when tested in the PPI of the startle response. The persistent efficacy of NT69L with chronic treatment provides further support for the therapeutic use of neurotensin (NT) agonists to treat schizophrenia and possibly other disorders that are characterized by PPI deficits. The modulatory role of NT69L on the dopaminergic and serotonergic neurotransmission systems both of which are implicated in the pathophysiology of schizophrenia is discussed.

Promise and pitfalls of animal models of schizophrenia

Animal models are indispensible tools for advancing understanding of the cause of any given disease and developing new treatments. Developing animal models for schizophrenia presents formidable challenges owing to the distinctively human nature of the symptoms that define it and the thus-far-obscured underlying biological mechanisms. Nevertheless, progress has been and continues to be made in this important field of endeavor. This article discusses the challenges facing investigators who seek to develop and use animal models for translational research in schizophrenia and the responses that have emerged to those challenges, as well as the likely pathways that will lead to future progress.

The effects of glycine transporter I inhibitor, N-methylglycine (sarcosine), on ketamine-induced alterations in sensorimotor gating and regional brain c-Fos expression in rats

Reduced N-methyl-D-aspartate receptor (NMDAR) function may contribute to the pathogenesis of schizophrenia. Sarcosine, a potent glycine transporter inhibitor, can increase synaptic glycine and then promote NMDAR function. We assessed the antipsychotic potential of sarcosine by comparing the abilities of sarcosine and clozapine to restore the prepulse inhibition (PPI) deficit, hyperlocomotion and regional brain c-Fos expression changes caused by an NMDAR antagonist, ketamine. Four groups of rats were given acute injections, including saline+saline, saline+30 mg/kg ketamine, 100mg/kg sarcosine+30 mg/kg ketamine, and 15 mg/kg clozapine+30 mg/kg ketamine. Both sarcosine and clozapine reversed the ketamine-induced PPI deficit and hyperlocomotion. They both did not change ketamine-induced increase in c-Fos expression in the prefrontal cortex and nucleus accumbens. However, in the olfactory bulb, sarcosine, but not clozapine, significantly reduced the ketamine-induced increase in c-Fos expression. Our animal study demonstrated that sarcosine may have antipsychotic potential.

The acute and subchronic effects of a brain-penetrating, neurotensin-1 receptor agonist on feeding, body weight and temperature

The neurotensin-1 (NT1) receptor has been implicated in mediating a number of important neurotensin effects. We have found that PD149163, a selective, brain-penetrating, NT1 receptor agonist, produces a number of therapeutic-like preclinical effects after peripheral administration including pro-cognitive, antipsychotic and anxiolytic effects. In this study, we investigated PD149163's effect on food intake and thermal regulation, two physiological processes thought to be mediated by NT1 receptors. Brown Norway rats and leptin-deficient mice (ob/ob) mice were administered subcutaneous PD149163 (0, 0.1, 0.25, or 1 mg/kg) for ten consecutive days. Weight and 24-h food intake were measured in mice and rats and core body temperature was also measured in rats. PD149163 significantly decreased food intake in rats and ob/ob mice and no tolerance was demonstrated to this effect over the course of the study. PD149163-treated animals exhibited weight loss compared to saline-treated animals. PD149163 produced hypothermia as expected but this effect did show tolerance over the course of the study, unlike feeding. The results suggest that NT1 receptor agonists are candidates for treatment of obesity and that somewhat different mechanisms are involved in NT1-induced feeding regulation and temperature regulation.

Sensorimotor gating in neurotensin-1 receptor null mice

BACKGROUND

Converging evidence has implicated endogenous neurotensin (NT) in the pathophysiology of brain processes relevant to schizophrenia. Prepulse inhibition of the startle reflex (PPI) is a measure of sensorimotor gating and considered to be of strong relevance to neuropsychiatric disorders associated with psychosis and cognitive dysfunction. Mice genetically engineered to not express NT display deficits in PPI that model the PPI deficits seen in schizophrenia patients. NT1 receptors have been most strongly implicated in mediating the psychosis relevant effects of NT such as attenuating PPI deficits. To investigate the role of NT1 receptors in the regulation of PPI, we measured baseline PPI in wildtype (WT) and NT1 knockout (KO) mice. We also tested the effects of amphetamine and dizocilpine, a dopamine agonist and NMDA antagonist, respectively, that reduce PPI as well as the NT1 selective receptor agonist PD149163, known to increase PPI in rats.

METHODS

Baseline PPI and acoustic startle response were measured in WT and NT1 KO mice. After baseline testing, mice were tested again after receiving intraperatoneal (IP) saline or one of three doses of amphetamine (1.0, 3.0 and 10.0 mg/kg), dizocilpine (0.3, 1.0 and 3.0 mg/kg) and PD149163 (0.5, 2.0 and 6.0 mg/kg) on separate test days.

RESULTS

Baseline PPI and acoustic startle response in NT1 KO mice were not significantly different from NT1 WT mice. WT and KO mice exhibited similar responses to the PPI-disrupting effects of dizocilpine and amphetamine. PD149163 significantly facilitated PPI (P < 0.004) and decreased the acoustic startle response (P < 0.001) in WT but not NT1 KO mice.

CONCLUSIONS

The data does not support the regulation of baseline PPI or the PPI disruptive effects of amphetamine or dizocilpine by endogenous NT acting at the NT1 receptor, although they support the antipsychotic potential of pharmacological activation of NT1 receptors by NT1 agonists.

The brattleboro rat displays a natural deficit in social discrimination that is restored by clozapine and a neurotensin analog

Cognitive deficits in schizophrenia are a major source of dysfunction for which more effective treatments are needed. The vasopressin-deficient Brattleboro (BRAT) rat has been shown to have several natural schizophrenia-like deficits, including impairments in prepulse inhibition and memory. We investigated BRAT rats and their parental strain, Long-Evans (LE) rats, in a social discrimination paradigm, which is an ethologically relevant animal test of cognitive deficits of schizophrenia based upon the natural preference of animals to investigate conspecifics. We also investigated the effects of the atypical antipsychotic, clozapine, and the putative antipsychotic, PD149163, a brain-penetrating neurotensin-1 agonist, on social discrimination in these rats. Adult rats were administered saline or one of the three doses of clozapine (0.1, 1.0, or 10 mg/kg) or PD149163 (0.1, 0.3, or 1.0 mg/kg), subcutaneously. Following drug administration, adult rats were exposed to a juvenile rat for a 4-min learning period. Animals were then housed individually for 30 min and then simultaneously exposed to the juvenile presented previously and a new juvenile for 4 min. Saline-treated LE rats, but not BRAT rats, exhibited intact social discrimination as evidenced by greater time spent exploring the new juvenile. The highest dose of clozapine and the two highest doses of PD149163 restored social discrimination in BRAT rats. These results provide further support for the utility of the BRAT rat as a genetic animal model relevant to schizophrenia and drug discovery. The potential of neurotensin agonists as putative treatments for cognitive deficits of schizophrenia was also supported.

Clozapine administration modifies neurotensin effect on synaptosomal membrane Na+, K+ -ATPase activity

Na+, K+-ATPase is inhibited by neurotensin, an effect which involves the peptide high affinity receptor (NTS1). Neurotensin effect on cerebral cortex synaptosomal membrane Na+, K+-ATPase activity of rats injected i.p. with antipsychotic clozapine was studied. Whereas 3.5 x 10(-6) M neurotensin decreased 44% Na+, K+-ATPase activity in the controls, the peptide failed to modify enzyme activity 30 min after a single 3.0, 10.0 and 30.0 mg/kg clozapine dose. Neurotensin decreased Na+, K+-ATPase activity 40 or 20% 18 h after 3.0 or 5.6 mg/kg clozapine administration, respectively, and lacked inhibitory effect 18 h after 17.8 and 30.0 mg/kg clozapine doses. Results indicated that the clozapine treatment differentially modifies the further effect of neurotensin on synaptosomal membrane Na+, K+-ATPase activity according to time and dose conditions employed. Taken into account that clozapine blocks the dopaminergic D2 receptor, findings obtained favor the view of an interplay among neurotensinergic receptor, dopaminergic D2 receptor and Na+, K+-ATPase at synaptic membranes.

New topics in vasopressin receptors and approach to novel drugs: role of the vasopressin receptor in psychological and cognitive functions

Arginine vasopressin (AVP) is a neurohypophyseal peptide best known as an antidiuretic hormone. AVP receptors have been classified into three subtypes: V1a, V1b, and V2 receptors. V1a receptor (V1aR) and V1b receptor (V1bR) are widely distributed in the central nervous system, including the septum, cortex, hippocampus, and hypothalamus. Clinical studies have demonstrated an involvement of AVP in psychiatric disorders. In the present study, we examined the performance of V1aR or V1bR knockout (KO) mice compared to wild-type (WT) mice in behavioral tests. V1aR and V1bR KO mice exhibited deficits of social behavior and prepulse inhibition in comparison to WT mice. Moreover, V1aR KO mice exhibited reduced anxiety-like behavior and impairment of spatial learning. These results suggest that V1aR and V1bR play an important role in psychological and cognitive functions.

The neurotensin-1 receptor agonist PD149163 blocks fear-potentiated startle

Preliminary evidence suggests that the neuropeptide, neurotensin (NT) may regulate fear/anxiety circuits. We investigated the effects of PD149163, a NT1 receptor agonist, on fear-potentiated startle (FPS). Sprague Dawley rats were trained to associate a white light with a mild foot shock. In one experiment, animals were treated with either subcutaneous vehicle or PD149163 (0.01, 0.1 or 1.0 mg/kg) 24 h after training. Twenty minutes later their acoustic startle response in the presence or absence of the white light was tested. In a second experiment, saline and 1.0 mg/kg PD149163 were tested using a separate group of rats. In the first experiment, PD149163 produced a non-significant decrease in baseline acoustic startle at all three doses. As expected, saline-treated rats exhibited significant FPS. An ANOVA of percentage FPS revealed no significant effect of treatment group overall but the high dose group did not display FPS strongly suggesting an FPS effect at this dose. This finding was confirmed in the second experiment where the high dose of PD149163 reduced percent FPS relative to saline (P < 0.05). These data suggest that systemically administered NT1 agonists modulate the neural circuitry that regulates fear and anxiety to produce dose-dependent anxiolytic-like effects on FPS.

The reversal of amphetamine-induced locomotor activation by a selective neurotensin-1 receptor agonist does not exhibit tolerance

RATIONALE

Neurotensin-1 (NT1) receptor agonists have been proposed as putative antipsychotic drugs. Recently, brain-penetrating NT analogs produced by stability-enhancing modification of the smallest NT fragment, NT(8-13), have demonstrated antipsychotic-like efficacy after acute systemic injection in several preclinical animal tests predictive for antipsychotic efficacy. However, the evidence regarding the persistence versus tolerance of these effects after repeated administration is ambiguous. Previous studies have used compounds that nonselectively activated both NT1 and NT2 receptors or used continuous slow, central infusion of doses rather than daily acute administration, both factors which may have contributed to the ambiguity in the literature regarding the emergence of tolerance.

OBJECTIVES

To determine if tolerance develops to the antipsychotic-like effects of NT1 receptor agonists, we investigated the effects of subchronic daily systemic administration of PD149163, a brain-penetrating NT analog with selectivity for the NT1 receptor, on amphetamine-induced locomotor activation, a classic preclinical test of antipsychotic efficacy.

MATERIALS AND METHODS

Sprague-Dawley rats were pretreated with eight consecutive daily subcutaneous (SC) injections of PD149163 or saline. On the ninth day, rats received a pair of SC injections consisting of PD149163 or saline, followed by amphetamine (0.5 mg/kg) or saline. Locomotor activity was then measured in photobeam-equipped cages.

RESULTS

The results indicated that repeated daily administration of PD149163 was able to antagonize amphetamine's locomotor-activating effect comparable to that of the first dose, despite that repeated administration of PD149163 produced an increase in baseline locomotor activity not seen after the first dose.

CONCLUSIONS

The results do not support the development of tolerance for the acute antipsychotic-like effect of NT1 agonists and thus lend support to the contention that NT1 agonists are viable candidates as putative novel antipsychotic drugs.

Realistic expectations of prepulse inhibition in translational models for schizophrenia research

INTRODUCTION

Under specific conditions, a weak lead stimulus, or "prepulse", can inhibit the startling effects of a subsequent intense abrupt stimulus. This startle-inhibiting effect of the prepulse, termed "prepulse inhibition" (PPI), is widely used in translational models to understand the biology of brainbased inhibitory mechanisms and their deficiency in neuropsychiatric disorders. In 1981, four published reports with "prepulse inhibition" as an index term were listed on Medline; over the past 5 years, new published Medline reports with "prepulse inhibition" as an index term have appeared at a rate exceeding once every 2.7 days (n=678). Most of these reports focus on the use of PPI in translational models of impaired sensorimotor gating in schizophrenia. This rapid expansion and broad application of PPI as a tool for understanding schizophrenia has, at times, outpaced critical thinking and falsifiable hypotheses about the relative strengths vs. limitations of this measure.

OBJECTIVES

This review enumerates the realistic expectations for PPI in translational models for schizophrenia research, and provides cautionary notes for the future applications of this important research tool.

CONCLUSION

In humans, PPI is not "diagnostic"; levels of PPI do not predict clinical course, specific symptoms, or individual medication responses. In preclinical studies, PPI is valuable for evaluating models or model organisms relevant to schizophrenia, "mapping" neural substrates of deficient PPI in schizophrenia, and advancing the discovery and development of novel therapeutics. Across species, PPI is a reliable, robust quantitative phenotype that is useful for probing the neurobiology and genetics of gating deficits in schizophrenia.

Endogenous neurotensin is involved in estrous cycle related alterations in prepulse inhibition of the acoustic startle reflex in female rats

Ovarian hormones regulate prepulse inhibition (PPI) of the acoustic startle reflex. Results from studies in intact female rodents investigating sex, estrous cycle and ovarian hormone regulation of PPI are inconsistent. In experiment #1, we investigated whether PPI in female rats is influenced by the time of day of testing and the estrous cycle stage of the rat. PPI was examined across the day of proestrus (P) and diestrus 1 (D1) in female rats and compared to males. PPI in males and P females was significantly higher than in D1 females. PPI in males and D1 females was significantly affected by the time of day of testing with PPI being reduced in the afternoon and evening compared to morning. PPI in P females was not significantly affected by the time of day of testing. Previous studies have demonstrated estrous cycle regulation of central nervous system neurotensin (NT) neurons and peripherally administered NT receptor agonists regulate PPI in a manner similar to antipsychotic drugs. Experiment #2 of this study was designed to examine whether endogenous NT is involved in estrous cycle regulation of PPI. The NT receptor antagonist SR 142948A reduced the high levels of PPI during D1 and P. In contrast, when tested at a time of day in which PPI was low in D1 females, administration of both the typical antipsychotic drug haloperidol and the NT receptor antagonist significantly increased PPI. These data support an effect of time of day and estrous cycle stage on PPI in female rats. The estrous cycle variations in PPI are mediated in part by endogenous NT.

Neurotensin agonists: potential in the treatment of schizophrenia

Neurotensin (NT) is a neuropeptide that, for decades, has been implicated in the biology of schizophrenia. It is closely associated with, and is thought to modulate, dopaminergic and other neurotransmitter systems involved in the pathophysiology of various neuropsychiatric diseases, including schizophrenia. This review outlines the neurochemistry and function of the NT system and the data implicating its role in schizophrenia. The data suggest that NT receptor agonists have the potential to be used as novel therapeutic agents for the treatment of schizophrenia, with the added benefits of (i) not causing weight gain, an adverse effect that is problematic with some of the currently used atypical antipsychotic drugs; and (ii) helping patients to stop smoking, a behaviour that is highly prevalent in those with schizophrenia.

The effects of chronic administration of established and putative antipsychotics on natural prepulse inhibition deficits in Brattleboro rats

We previously reported that vasopressin deficient Brattleboro (BRAT) rats exhibit deficits in prepulse inhibition (PPI) of the startle reflex that are consistent with PPI deficits exhibited by patients with schizophrenia and other neuropsychiatric disorders. Preliminary evidence indicates that this may be the basis of a predictive model for antipsychotic drug efficacy. Here we report the effects of acute and chronic administration of established and putative antipsychotics on these PPI deficits. BRAT rats, compared to their derivative strain, Long Evans rats, exhibited significantly decreased PPI and startle habituation consistent with patients with schizophrenia and other neuropsychiatric disorders. The second generation antipsychotics, risperidone and clozapine as well as a neurotensin agonist (PD149163) increased BRAT rat PPI, whereas saline, the typical antipsychotic, haloperidol, and a vasopressin analog (1-desamino-D-arginine vasopressin) did not. Similar to their effects in humans, chronic administration of antipsychotic drugs produced stronger effects than acute administration. These results further support the BRAT rat as a model of sensorimotor gating deficits with predictive validity for antipsychotics. The model appears to be able to differentiate first generation from second generation antipsychotics, identify putative antipsychotics with novel mechanisms (i.e., peptides) and reasonably model the therapeutic time course of antipsychotic drugs in humans.

Upregulation of striatal dopamine-2 receptors in Brattleboro rats with prepulse inhibition deficits

BACKGROUND

Brattleboro rats (BRATs) have natural deficits in prepulse inhibition (PPI) of the startle response similar to those exhibited by schizophrenia patients, which are reversed by antipsychotics. We sought to determine whether they also have increases in striatal dopamine-2 (D2) receptors found in some studies examining the brains of schizophrenia patients.

METHODS

Several days after startle testing, the brains of BRAT and Long Evans (LE) rats were removed, and D1 and D2 receptor levels were measured by autoradiography.

RESULTS

PPI was lower in BRATs consistent with previous reports. D2, but not D1, receptor binding was significantly higher in the nucleus accumbens shell and the dorsomedial caudate of BRAT compared with LE rats, consistent with some findings in schizophrenia patients. Furthermore, individual rat PPI was inversely correlated with D2 binding density.

CONCLUSIONS

These findings suggest that the dopamine system in BRATs is dysregulated and these abnormalities may contribute to the PPI deficits observed in these rats.

Neurotensin agonists as an alternative to antipsychotics

Neurotensin (NT) is a 13 amino acid neuropeptide that is found in the central nervous system and in the gastrointestinal tract. In brain, this peptide is prominently associated anatomically with dopaminergic, as well as other neurotransmitter systems. Based on animal studies, already decades old, researchers have hypothesised that NT receptor agonists will have antipsychotic properties in patients. However, to date no one has obtained a non-peptide NT receptor agonist. Therefore, there has been great interest in obtaining peptide analogues of NT, that, unlike NT resist degradation by peptidases and cross the blood-brain barrier, yet have the pharmacological characteristics of native NT, for therapeutic use in the treatment of schizophrenia, as well as other neuropsychiatric diseases such as Parkinson's disease and addiction to psychostimulants. In this review, we present the rationale for development of NT receptor agonists for treatment of certain central nervous system diseases, as well as a review of those peptide agonists that are in early stages of development.

Bioactive analogs of neurotensin: focus on CNS effects

Neurotensin (NT) is a 13-amino acid neuropeptide found in the central nervous system and in the gastrointestinal tract. It is closely associated anatomically with dopaminergic and other neurotransmitter systems, and evidence supports a role for NT agonists in the treatment of various neuropsychiatric disorders. However, NT is readily degraded by peptidases, so there is much interest in the development of stable NT agonists, that can be injected systemically, cross the blood-brain barrier (BBB), yet retains the pharmacological characteristics of native NT for therapeutic use in the treatment of diseases such as schizophrenia, Parkinson's disease and addiction.

Neurotensin: role in psychiatric and neurological diseases

Neurotensin (NT), an endogenous brain-gut peptide, has a close anatomical and functional relationship with the mesocorticolimbic and neostriatal dopamine system. Dysregulation of NT neurotransmission in this system has been hypothesized to be involved in the pathogenesis of schizophrenia. Additionally, NT containing circuits have been demonstrated to mediate some of the mechanisms of action of antipsychotic drugs, as well as the rewarding and/or sensitizing properties of drugs of abuse. NT receptors have been suggested to be novel targets for the treatment of psychoses or drug addiction.

Psychosis pathways converge via D2high dopamine receptors

The objective of this review is to identify a target or biomarker of altered neurochemical sensitivity that is common to the many animal models of human psychoses associated with street drugs, brain injury, steroid use, birth injury, and gene alterations. Psychosis in humans can be caused by amphetamine, phencyclidine, steroids, ethanol, and brain lesions such as hippocampal, cortical, and entorhinal lesions. Strikingly, all of these drugs and lesions in rats lead to dopamine supersensitivity and increase the high-affinity states of dopamine D2 receptors, or D2High, by 200-400% in striata. Similar supersensitivity and D2High elevations occur in rats born by Caesarian section and in rats treated with corticosterone or antipsychotics such as reserpine, risperidone, haloperidol, olanzapine, quetiapine, and clozapine, with the latter two inducing elevated D2High states less than that caused by haloperidol or olanzapine. Mice born with gene knockouts of some possible schizophrenia susceptibility genes are dopamine supersensitive, and their striata reveal markedly elevated D2High states; suchgenes include dopamine-beta-hydroxylase, dopamine D4 receptors, G protein receptor kinase 6, tyrosine hydroxylase, catechol-O-methyltransferase, the trace amine-1 receptor, regulator of G protein signaling RGS9, and the RIIbeta form of cAMP-dependent protein kinase (PKA). Striata from mice that are not dopamine supersensitive did not reveal elevated D2High states; these include mice with knockouts of adenosine A2A receptors, glycogen synthase kinase GSK3beta, metabotropic glutamate receptor 5, dopamine D1 or D3 receptors, histamine H1, H2, or H3 receptors, and rats treated with ketanserin or aD1 antagonist. The evidence suggests that there are multiple pathways that convergetoelevate the D2High state in brain regions and that this elevation may elicit psychosis. This proposition is supported by the dopamine supersensitivity that is a common feature of schizophrenia and that also occurs in many types of genetically altered, drug-altered, and lesion-altered animals. Dopamine supersensitivity, in turn, correlates with D2High states. The finding that all antipsychotics, traditional and recent ones, act on D2High dopamine receptors further supports the proposition.

Disruption of the prepulse inhibition of the startle reflex in vasopressin V1b receptor knockout mice: reversal by antipsychotic drugs

In the present study, we investigated whether mice lacking the arginine vasopressin (AVP) V1b receptor (V1bR) exhibit deficits of prepulse inhibition (PPI) of the startle reflex, reminiscent of the sensorimotor gating deficits observed in a large majority of schizophrenic patients. V1bR knockout (KO) mice displayed significantly reduced levels of PPI of the startle reflex. In addition to PPI deficits, V1bR KO mice showed increased acoustic startle response. However, acoustic startle response was not significantly correlated to the PPI of the startle reflex in V1bR KO mice. V1bR KO mice also showed a decrease in basal levels of extracellular dopamine (DA) in the medial prefrontal cortex, which is thought to be an important brain region for PPI. Moreover, PPI deficits observed in the V1bR KO mice are significantly reversed by atypical antipsychotics such as risperidone and clozapine but not by a typical neuroleptic haloperidol, like in schizophrenic patients. By contrast, we did not observe any significant differences between V1bR KO mice and wild-type mice in the open-field, light/dark, elevated plus maze, and forced swimming tests. The results of the present study indicate that V1bR may be involved in the regulation of PPI of the startle reflex. The V1bR has been considered an important molecular target for the development of antipsychotic drugs.

Characterization of the effects of corticotropin-releasing factor on prepulse inhibition of the acoustic startle response in Brown Norway and Wistar-Kyoto rats

Sensori-motor gating, as assessed by prepulse inhibition of the startle response is diminished in patients with schizophrenia. We have previously shown that inbred Brown Norway (BN) rats display significantly less prepulse inhibition of the acoustic startle response than inbred Wistar-Kyoto (WKY) rats, and that prepulse inhibition is decreased by central administration of the neuropeptide, corticotropin-releasing factor (CRF) in both strains. The present study was conducted to establish whether peripheral administration of CRF alters prepulse inhibition, whether a low, threshold dose for decreasing prepulse inhibition is the same in the two rat strains, and whether central administration of a CRF receptor antagonist enhances prepulse inhibition in the BN strain. CRF-induced behavioral activation was also examined to determine whether the two rat strains are differentially sensitive to a behavioral effect of CRF that does not involve the startle response. In each experiment, BN rats showed significantly less prepulse inhibition than WKY rats. Subcutaneous administration of CRF had no affect on startle amplitude or prepulse inhibition of the startle response in either rat strain. In BN, but not in WKY rats, low-dose CRF (0.3 microg) decreased prepulse inhibition. However, doses of CRF that did not alter prepulse inhibition in the WKY strain, did result in behavioral activation. No dose of CRF tested affected baseline startle amplitude. Central administration of the CRF receptor antagonist, astressin had no effect on prepulse inhibition or startle amplitude in either rat strain. Central administration of the CRF receptor antagonist, D-Phe CRF (12-41) had no effect on prepulse inhibition in WKY rats, resulted in a only a small, non-significant increase in prepulse inhibition in BN rats, while it decreased startle amplitude. The results suggest that CRF reduces prepulse inhibition of the acoustic startle response independently of effects on the pituitary-adrenal axis, and that endogenous CRF has at most, a minor role in the low prepulse inhibition found in BN rats.