Autoradiographic characterization of neurotensin receptors in the entorhinal cortex of schizophrenic patients and control subjects

Dissecting the association between psychiatric disorders and neurological proteins: a genetic correlation and two-sample bidirectional Mendelian randomization study

OBJECTIVES

The role of neurological proteins in the development of bipolar disorder (BD) and schizophrenia (SCZ) remains elusive now. The current study aims to explore the potential genetic correlations of plasma neurological proteins with BD and SCZ.

METHODS

By using the latest genome-wide association study (GWAS) summary data of BD and SCZ (including 41,917 BD cases, 11,260 SCZ cases, and 396,091 controls) derived from the Psychiatric GWAS Consortium website (PGC) and a recently released GWAS of neurological proteins (including 750 individuals), we performed a linkage disequilibrium score regression (LDSC) analysis to detect the potential genetic correlations between the two common psychiatric disorders and each of the 92 neurological proteins. Two-sample Mendelian randomisation (MR) analysis was then applied to assess the bidirectional causal relationship between the neurological proteins identified by LDSC, BD and SCZ.

RESULTS

LDSC analysis identified one neurological protein, NEP, which shows suggestive genetic correlation signals for both BD (coefficient = -0.165, p value = 0.035) and SCZ (coefficient = -0.235, p value = 0.020). However, those association did not remain significant after strict Bonferroni correction. Two sample MR analysis found that there was an association between genetically predicted level of NEP protein, BD (odd ratio [OR] = 0.87, p value = 1.61 × 10^-6) and SCZ (OR = 0.90, p value = 4.04 × 10^-6). However, in the opposite direction, there is no genetically predicted association between BD, SCZ, and NEP protein level.

CONCLUSION

This study provided novel clues for understanding the genetic effects of neurological proteins on BD and SCZ.

Analysis of the transcriptional activity of genes of neuropeptides and their receptors in the blood of patients with thyroid pathology

The thyroid hormone plays a vital role in the development and maturation of the nervous system not only during prenatal and perinatal age but also in adults. “Peripheral marker hypothesis” revealed that gene expression changes in some regions of the brain are reflected into the peripheral blood lymphocytes. The objective of the study was to investigate changes in the gene expression profile of neuropeptides and their receptors in patients with different forms of thyroid pathology. One hundred fifty-three patients with thyroid pathology were enrolled in the study. They were divided into three groups: group 1 included 16 patients with postoperative hypothyroidism, group 2 included 65 patients with hypothyroidism resulting from autoimmune thyroiditis (AIT), and group 3 included 72 patients with AIT and elevated levels of anti-thyroglobulin (anti-Tg) and anti-thyroid peroxidase (anti-TPO) antibodies in the serum. We used a pathway-specific polymerase chain reaction (PCR) array (RT² Profiler™ PCR Array Human Neurotrophins & Receptors, QIAGEN, Germany) to identify and verify neuropeptides and receptors pathway-focused gene expression in 12 individuals that were randomly selected from each group using real-time PCR. Our research identified that patients with postoperative hypothyroidism had a considerably increased expression of NPY1R, NTSR1, and NPY4R. The patients with hypothyroidism caused by autoimmune thyroiditis had considerably lower expression of NTSR1, while the expression of NPY1R increased. The mRNA levels of NPY2R and PNOC increased in the patients with elevated levels of autoantibodies anti-Tg and anti-TPO in the serum, and mRNA levels of NPY1R and NTSR1 decreased in this group of patients.

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.

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.

Diverse roles of neurotensin agonists in the central nervous system

Neurotensin (NT) is a tridecapeptide that is found in the central nervous system (CNS) and the gastrointestinal tract. NT behaves as a neurotransmitter in the brain and as a hormone in the gut. Additionally, NT acts as a neuromodulator to several neurotransmitter systems including dopaminergic, sertonergic, GABAergic, glutamatergic, and cholinergic systems. Due to its association with such a wide variety of neurotransmitters, NT has been implicated in the pathophysiology of several CNS disorders such as schizophrenia, drug abuse, Parkinson's disease (PD), pain, central control of blood pressure, eating disorders, as well as, cancer and inflammation. The present review will focus on the role that NT and its analogs play in schizophrenia, endocrine function, pain, psychostimulant abuse, and PD.

Involvement of the neurotensin receptor 1 in the behavioral effects of two neurotensin agonists, NT-2 and NT69L: lack of hypothermic, antinociceptive and antipsychotic actions in receptor knockout mice

Neurotensin (NT) is a neuropeptide implicated in the pathophysiology of schizophrenia and in mediating the efficacy of antipsychotic drugs. NT is also involved in the regulation of body temperature and pain sensitivity. Using neurotensin receptor 1 (NTR1) knockout (KO) and wild-type (WT) mice, these studies evaluated the involvement of NTR1 in the behavioral responses produced by peripheral administration of NT agonists (NT-2 and NT69L). Animals were characterized in paradigms designed to assess hypothermia, antinociception, and antipsychotic-like effects. Under basal conditions, there were no phenotypic differences between NTR1 KO and WT mice. In WT mice, both NTR1 agonists decreased core body temperature (active doses in mg/kg, i.p., for NT-2 and NT69L, respectively: 1 and 3), increased tail withdrawal latencies (1 and 3), produced decreased spontaneous climbing (0.1, 0.3, 1 and 1, 3, 10) and reversed apomorphine-induced climbing (0.3, 1 and 1, 3). In contrast, none of the effects of either agonist were present in KO mice. These results suggest that NTR1: (1) does not play a major role in the control of basal thermoregulation, nociception or psychomotor stimulation in mice (barring possible developmental plasticity), (2) does mediate these behavioral responses to NT agonists, and (3) may play a role in the potential antipsychotic effects of these agonists.

Effects of repeated dizocilpine treatment on adult rat behavior after neonatal lesions of the entorhinal cortex

Disturbed cortical development is implicated in some psychiatric diseases, e.g. in schizophrenia. Additionally, N-methyl-d-aspartate (NMDA) receptor antagonists like ketamine or phencyclidine have been reported to exacerbate schizophrenic symptoms. We here investigated the effects of neonatal entorhinal cortex (EC) lesions on adult rat behavior before and after repeated high-dose treatment with the NMDA antagonist dizocilpine, in order to combine these etiopathogenetical factors in an animal model. Bilateral neonatal (postnatal day 7) lesions were induced by microinjection of ibotenic acid (1.3 microg/0.2 microl PBS) into the EC. Naive and sham-lesioned rats served as controls. Adult rats were tested for behavioral flexibility on a cross maze, for locomotor activity in the open field and for sensorimotor gating using prepulse inhibition (PPI) of startle. Rats were then treated with dizocilpine (0.5 mg/kg b.i.d. for 7 days) and retested 1 week after withdrawal using the same behavioral tests as before. PPI was additionally measured after acute low-dose challenge with dizocilpine (0.15 mg/kg). EC lesions reduced behavioral flexibility as shown by impaired switching between spatial (allocentric) and non-spatial (egocentric) maze strategies. High-dose dizocilpine treatment disturbed switching to the egocentric strategy in all groups, which added to the effect of EC lesions. Neonatal EC lesions did not alter locomotor activity or PPI, but high-dose dizocilpine treatment reduced motor activity of all groups without changing PPI. The combination of neonatal EC lesions and adult dizocilpine treatment does not lead to super-additive effects on behavior. However, both treatments may serve to model certain aspects of psychiatric symptoms.

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.

Towards understanding the schizophrenia code: an expanded convergent functional genomics approach

Identifying genes for schizophrenia through classical genetic approaches has proven arduous. Here, we present a comprehensive convergent analysis that translationally integrates brain gene expression data from a relevant pharmacogenomic mouse model (involving treatments with a psychomimetic agent - phencyclidine (PCP), and an anti-psychotic - clozapine), with human genetic linkage data and human postmortem brain data, as a Bayesian strategy of cross validating findings. Topping the list of candidate genes, we have three genes involved in GABA neurotransmission (GABRA1, GABBR1, and GAD2), one gene involved in glutamate neurotransmission (GRIA2), one gene involved in neuropeptide signaling (TAC1), two genes involved in synaptic function (SYN2 and KCNJ4), six genes involved in myelin/glial function (CNP, MAL, MBP, PLP1, MOBP and GFAP), and one gene involved in lipid metabolism (LPL). These data suggest that schizophrenia is primarily a disorder of brain functional and structural connectivity, with GABA neurotransmission playing a prominent role. These findings may explain the EEG gamma band abnormalities detected in schizophrenia. The analysis also revealed other high probability candidates genes (neurotransmitter signaling, other structural proteins, ion channels, signal transduction, regulatory enzymes, neuronal migration/neurite outgrowth, clock genes, transcription factors, RNA regulatory genes), pathways and mechanisms of likely importance in pathophysiology. Some of the pathways identified suggest possible avenues for augmentation pharmacotherapy of schizophrenia with other existing agents, such as benzodiazepines, anticonvulsants and lipid modulating agents. Other pathways are new potential targets for drug development. Lastly, a comparison with our earlier work on bipolar disorder illuminates the significant molecular overlap between schizophrenia and bipolar disorder.

Parvalbumin neurons in the entorhinal cortex of subjects diagnosed with bipolar disorder or schizophrenia

BACKGROUND

Growing evidence indicates that the entorhinal cortex (ECx) might be affected in schizophrenia (SZ) and bipolar disorder (BD). To test whether distinct interneuronal subpopulations might be altered, numbers of parvalbumin-immunoreactive (PVB-IR) neurons were measured in the ECx of BD and SZ subjects. These neurons play a pivotal role within ECx intrinsic circuits.

METHODS

Numbers, numerical density, and soma size of PVB-IR neurons were measured in the ECx of normal control (n = 16), BD (n = 10), and SZ (n = 10) subjects. The volume of the ECx was measured in Nissl-stained sections.

RESULTS

In BD, decreases of total numbers (p = .02) and numerical densities (p = .01) of PVB-IR neurons were detected in the ECx. Within distinct subregions, reductions were detected in the superficial layers of the lateral (p = .02), intermediate (p = .04), and caudal (p = .01) ECx. In SZ, total numbers and numerical densities were not altered. A reduction of soma size was present in the intermediate ECx (p = .01). Volume was unaffected in either disorder.

CONCLUSIONS

In BD, a decrease of PVB-IR neurons may alter intrinsic inhibitory networks within the superficial layers of the ECx. The likely consequence is a disruption of integration and transfer of information from the cerebral cortex to the hippocampus.

Involvement of neuropeptide systems in schizophrenia: human studies

Neuropeptides are heterogeneously distributed throughout the digestive, circulatory, and nervous systems and serve as neurotransmitters, neuromodulators, and hormones. Neuropeptides are phylogenetically conserved and have been demonstrated to regulate numerous behaviors. They have been hypothesized to be pathologically involved in several psychiatric disorders, including schizophrenia. On the basis of preclinical data, numerous studies have sought to examine the role of neuropeptide systems in schizophrenia. This chapter reviews the clinical data, linking alterations in neuropeptide systems to the etiology, pathophysiology, and treatment of schizophrenia. Data for the following neuropeptide systems are included: arginine-vasopressin, cholecystokinin (CCK), corticotropin-releasing factor (CRF), interleukins, neuregulin 1 (NRG1), neurotensin (NT), neuropeptide Y (NPY), opioids, secretin, somatostatin, tachykinins, thyrotropin-releasing hormone (TRH), and vasoactive intestinal peptide (VIP). Data from cerebrospinal fluid (CSF), postmortem and genetic studies, as well as clinical trials are described. Despite the inherent difficulties associated with human studies (including small sample size, variable duration of illness, medication status, the presence of comorbid psychiatric disorders, and diagnostic heterogeneity), several findings are noteworthy. Postmortem studies support disease-related alterations in several neuropeptide systems in the frontal and temporal cortices. The strongest genetic evidence supporting a role for neuropeptides in schizophrenia are those studies linking polymorphisms in NRG1 and the CCKA receptor with schizophrenia. Finally, the only compounds that act directly on neuropeptide systems that have demonstrated therapeutic efficacy in schizophrenia are neurokinin receptor antagonists. Clearly, additional investigation into the role of neuropeptide systems in the etiology, pathophysiology, and treatment of schizophrenia is warranted.

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.

Neurobiologic basis of nicotine addiction and psychostimulant abuse: a role for neurotensin?

Addiction to psychostimulant drugs such as nicotine, amphetamine, and cocaine is a serious public health problem for which there is a paucity of accepted forms of pharmacotherapy. Nicotine dependence has become more frequently associated with psychiatric illness in recent decades, and patients who have schizophrenia are at highest risk and have the poorest prognosis for stopping their addiction. Possible mechanisms for this association include self-medication, with nicotine attenuating attentional deficits and negative symptoms. Neurotensin has been postulated to be an endogenous neuroleptic, and the performance of neurotensin analogues in animal models of addiction makes such compounds intriguing candidates for treatment of addiction in high-risk psychiatric populations.

Neurotensin agonists: possible drugs for treatment of psychostimulant abuse

Although many neuropeptides have been implicated in the pathophysiology of psychostimulant abuse, the tridecapeptide neurotensin holds a prominent position in this field due to the compelling literature on this peptide and psychostimulants. These data strongly support the hypothesis that a neurotensin agonist will be clinically useful to treat the abuse of psychostimulants, including nicotine. This paper reviews the evidence for a role for neurotensin in stimulant abuse and for a neurotensin agonist for its treatment.

Neuropeptides involved in the pathophysiology of schizophrenia and major depression

The present review summarizes the findings on the role of neuropeptides in the pathophysiology of schizophrenia and major depression. Several neuropeptides as vasopressin and endorphins in particular, beta-endorphin and gamma-type endorphins, cholecystokinin (CCK), neurotensin, somatostatin and Neuropeptide Y have been implicated in schizophrenia. During the last decade, however, few attempts to explore the significance of most of these and other neuropeptides in the pathophysiology of the disease or their therapeutic potential are found in the literature. An exception is neurotensin, which exerts neuroleptic-like effects in animal studies, while CSF, brain and blood studies are inconclusive. Things are different in major depression. Here much attention is paid to the endocrine abnormalities found in this disorder in particular the increased activity of the hypothalamic-pituitary-adrenal (HPA) axis. Neuropeptides as corticotropin-releasing hormone (CRH), vasopressin and corticosteroids are implicated in the symptomatology of this disorder. As a consequence much work is going on investigating the influence of CRH and corticosteroid antagonists or inhibitors of the synthesis of corticosteroids as potential therapeutic agents. This review emphasizes the role of vasopressin in the increased activity of the HPA axis in major depression and suggests exploration of the influence of the now available non-peptidergic vasopressin orally active V1 antagonists.

Neurotensin receptor binding abnormalities in the entorhinal cortex in schizophrenia and affective disorders

BACKGROUND

Convergent evidence from in vivo and in vitro studies of schizophrenia have implicated the mesial temporal lobe as a primary site of pathological change in this disorder. We have previously reported decreased neurotensin receptor density in layer II of the intermediate entorhinal cortex (ERC) in schizophrenia, a finding seen elsewhere but not seen in more caudal ERC.

METHODS

To study neuroanatomic and diagnostic specificity, we measured the density of neurotensin receptors in the intermediate and caudal ERC and hippocampal formation of schizophrenic, affective disorder control subjects, and normal control subjects. Slide-based radioligand binding was used to perform these studies.

RESULTS

Not only schizophrenic but also affective disorder subjects had decreased neurotensin receptor density in layer II of the intermediate ERC. Affective disorder subjects had significantly decreased neurotensin receptor density in layers V/VI of the intermediate ERC, and schizophrenic subjects trended in the same direction.

CONCLUSIONS

These findings demonstrate region-specific changes in neurotensin receptor binding levels in the mesial temporal lobe; however, there is no clear diagnostic specificity for these changes, because they were seen to varying degrees in both schizophrenia and affective disorders.

The role of neurotensin in the pathophysiology of schizophrenia and the mechanism of action of antipsychotic drugs

It has become increasingly clear that schizophrenia does not result from the dysfunction of a single neurotransmitter system, but rather pathologic alterations of several interacting systems. Targeting of neuropeptide neuromodulator systems, capable of concomitantly regulating several transmitter systems, represents a promising approach for the development of increasingly effective and side effect-free antipsychotic drugs. Neurotensin (NT) is a neuropeptide implicated in the pathophysiology of schizophrenia that specifically modulates neurotransmitter systems previously demonstrated to be dysregulated in this disorder. Clinical studies in which cerebrospinal fluid (CSF) NT concentrations have been measured revealed a subset of schizophrenic patients with decreased CSF NT concentrations that are restored by effective antipsychotic drug treatment. Considerable evidence also exists concordant with the involvement of NT systems in the mechanism of action of antipsychotic drugs. The behavioral and biochemical effects of centrally administered NT remarkably resemble those of systemically administered antipsychotic drugs, and antipsychotic drugs increase NT neurotransmission. This concatenation of findings led to the hypothesis that NT functions as an endogenous antipsychotic. Moreover, typical and atypical antipsychotic drugs differentially alter NT neurotransmission in nigrostriatal and mesolimbic dopamine (DA) terminal regions, and these effects are predictive of side effect liability and efficacy, respectively. This review summarizes the evidence in support of a role for the NT system in both the pathophysiology of schizophrenia and the mechanism of action of antipsychotic drugs.

Glutamate and its role in psychiatric illness

Glutamate, a dicarboxylic amino acid, is the most abundantly active neurotransmitter in the mammalian brain; it is also the principal excitatory neurotransmitter in the cerebral cortex. As our knowledge of this neurotransmitter deepens, it is increasingly being implicated in the pathophysiology of mental illness. This review begins by examining the physiology of glutamate and its receptors. Its role in memory, movement, perception and neuronal development is discussed. The development of the glutamate hypothesis of schizophrenia is traced, and the emerging lines of evidence for attenuated function of the N-methyl-D-aspartate receptor in schizophrenia are examined. For ease of discussion, these are divided into pharmacological, post-mortem, imaging, platelet and genetic studies. Interactions between glutamate and other neurotransmitters are discussed, as are possible mechanisms by which such altered receptor activity might result in the clinical expression of schizophrenia. The possible role of glutamate in major depression and bipolar disorder is explored. The review concludes by highlighting the importance of avoiding a reductionist approach to the pathophysiology of any mental illness. Copyright 2001 John Wiley & Sons, Ltd.

Neurotensin: peptide for the next millennium

Neurotensin is an endogenous tridecapeptide neurotransmitter (pGlu-Leu-Tyr-Glu-Asn-Lys-Pro-Arg-Arg-Pro-Try-Ile-Leu-OH) that was discovered by Carraway and Leeman in bovine hypothalami in the early 1970s. Since then this peptide has been the subject of a multitude of articles detailing discoveries related to its activity, receptors, localization, synthesis, and interactions with other systems. This review article does not intend to summarize again all the history of this fascinating peptide and its receptors, since this has been done quite well by others. The reader will be directed to these other reviews, where appropriate. Instead, this review attempts to provide a summary of current knowledge about neurotensin, why it is an important peptide to study, and where the field is heading. Special emphasis is placed on the behavioral studies, particularly with reference to agonists, antagonists, and antisense studies, as well as, the interaction of neurotensin with other neurotransmitters.

The high affinity neurotensin receptor gene (NTSR1): comparative sequencing and association studies in schizophrenia

Neurotensin and its high affinity receptor (NTSR1) localise within dopaminergic neurones in the mesocortical, mesolimbic and nigrostriatal systems and it is now clear that neurotensin can selectively modulate dopaminergic neurotransmission. This has led to the hypothesis that altered neurotensin function contributes to the pathogenesis of schizophrenia and other psychoses. This hypothesis has been supported circumstantially by a number of lines of evidence. (1) Central administration of neurotensin produces effects similar to those produced by the peripheral administration of atypical antipsychotics. (2) Observations of low levels of neurotensin in the CSF of schizophrenics. (3) Reduced numbers of neurotensin receptors in the brains of schizophrenics. Given the above link between neurotensin and dopamine, and the evidence implicating altered neurotensin function in psychosis, we have postulated that DNA sequence variation in neurotensin or its receptors might be associated with schizophrenia. In keeping with this hypothesis, an association has recently been reported between schizophrenia and the gene encoding the neurotensin high affinity receptor (NTSR1). However, caution is required because the associated marker, a tetranucleotide repeat, is located 3 kb away from the 3' end of the gene and there is no evidence that it is functional. Therefore, as a follow-up to our earlier work on neurotensin, we have now sought to test the hypothesis that DNA sequence variants that alter the structure or expression of the NTSR1 gene (VAPSEs) are associated with schizophrenia. However, while we found 14 novel sequence variants in 28 probands with psychosis, none resulted in an amino acid change, and neither direct nor indirect association studies suggested these are involved in susceptibility to schizophrenia.

Does neurotensin mediate the effects of antipsychotic drugs?

The possibility that the neuropeptide neurotensin (NT) may function as an endogenous antipsychotic compound was first hypothesized almost two decades ago. Since that time, considerable effort has been directed towards determining whether NT neurons mediate the effects of antipsychotic drugs (APDs). The anatomic, biochemical, behavioral, and clinical relevance of this hypothesis is reviewed. Although the majority of the available evidence is indirect, the availability of several NT receptor (NTR) antagonists have now made possible the direct examination of the involvement of the NT system in the mechanism of action of APDs. Preliminary studies in our laboratory demonstrate the ability of a selective NTR antagonist to block the effects of APDs in two models of sensory motor gating deficits characteristic of schizophrenia. These data, taken together with a compelling series of studies demonstrating that increases of NT/neuromedin N mRNA expression and NT content in the nucleus accumbens and striatum after chronic administration of APDs are predictive of clinical efficacy and extrapyramidal side effects, respectively, provide direct preclinical evidence for a role of the NT system in the clinical efficacy of APDs. Although effects of selective NTR antagonists in normal volunteers or schizophrenic patients have not been studied, and nonpeptidergic NTR agonists have not yet been identified, these cumulative results provide the groundwork for the use of NT-ergic compounds in the treatment of schizophrenia.

Cell biology of the hippocampal formation in schizophrenia

The hippocampal formation (HF) has been a centerpiece of neuropathologic investigations of schizophrenia. Numerous MRI studies have demonstrated a slight bilateral reduction in HF volume. Reports of reduced N-acetyl aspartate measured with in vivo proton spectroscopy suggest that neuronal pathology exists. However, morphometric data from postmortem studies have not revealed a clear change in HF size, and recent studies of neuronal number and of cytoarchitecture have been largely negative. Evidence of glial proliferation is consistently absent. The most reproducible positive anatomic finding in postmortem HF has been reduced size of neuronal cell bodies. Studies of gene transcription have provided replicable evidence of decreased expression of mRNAs for synaptophysin, GAP-43, cholecystokinin, and non-NMDA glutamate receptor subunits (GLU R 1 and 2), particularly in CA 3-4. These data about the cellular and molecular biology of the HF in schizophrenia are different from that found in a number of conditions associated with hippocampal damage, including excitotoxicity, epilepsy, alcoholism, Alzheimer's disease, steroid neurotoxicity, and normal aging. Notwithstanding the real possibility that the data are epiphenomena of chronic illness, the findings may implicate a unique cellular defect in schizophrenia--a genetic variation affecting the plasticity of HF circuitry and connectivity. Directions for further research are proposed.

The neuroanatomy and neurochemistry of schizophrenia

Substantial progress, in part owing to recent refinements in methodology, has been made in unraveling the anatomic correlates of schizophrenia. Subtle pathomorphologic changes, distinct from those of well-known degenerative brain disorders, have been observed. Neurochemical characterization has illuminated the nature of these morphologic abnormalities and has pointed to complex dysregulation of neurotransmitters and G proteins. New biochemical hypotheses such as the glutamate hypothesis have replaced and revitalized more established concepts in the neurochemistry of schizophrenia.

Retrosplenial/presubicular continuum in primates: a developmental approach in fetal macaques using neurotensin and parvalbumin as markers

In spite of numerous hodological and neuropsychological studies emphasizing the multimodal connections and integrative functions of the retrosplenial cortex in primates, the precise fate of its caudoventral extent and the composition of the merging area with the hippocampal formation remain a matter of debate. We reported previously how the anlage of the retrosplenial cortex merges with the immature presubicular zone in the fetal rhesus monkey at the end of the first trimester of gestation. In the present study, this caudal area was further defined on a chemoarchitectonic basis, particularly during the late prenatal and perinatal stages, which correspond to the development of the cingulate sulcus and temporal gyri, and the differentiation of the retrosplenial/subicular complex. Neurotensin (NT), a pyramidal cell marker in the limbic cortex, and parvalbumin (PV), a marker of a subset of inhibitory local circuit neurons in the hippocampal formation, were used as immunocytochemical markers. According to distinct chemoarchitectural patterns, (1) areas 29 l and 29 m of the retrosplenial cortex formed a triangle-shaped ventral expansion which merged with a similar but dorsal expansion of the pre/parasubicular fields. A temporal extension of area 29 m down to area TH could not be detected. The pre/parasubiculum contributed with area 29 m to the lateral bank of the calcarine sulcus as far as the most caudal extent of the hippocampal formation. (2) The lamina principalis interna of the presubiculum was well individualized and did not appear as a simple horizontal shift of adjoining fields. (3) NT and PV displayed a distinct temporal profile of development. NT was already expressed in the pyramidal cells of the prospective retrosplenial cortex and ventral hippocampal formation at E47 (term 165 days). Major pathways of the hippocampal formation and retrosplenial cortex (fimbria, fornix, angular and cingulum bundles) were progressively labeled indicating early developing projections. A large set of NT-positive afferents reached the retrosplenial cortex between E114 and E120. Their laminar distribution was compatible with a thalamic or a subicular origin. (4) The development of PV expression was delayed until the last quarter of gestation, supporting its proposal as a signal of functional onset. The developmental fate and the particular connections of the presubiculum suggest that its functional importance should be further investigated during infancy and adulthood.

Abnormal cholecystokinin mRNA levels in entorhinal cortex of schizophrenics

Limbic cortical regions, including anterior cingulate cortex (ACC), prefrontal cortex (PFC) and entorhinal cortex (ERC), have been implicated in the neuropathology of schizophrenia. Glutamate projection neurons connect these limbic cortical regions to each other, as well as to the terminal fields of the striatal/accumbens dopamine neurons. Subsets of these glutamate projection neurons, and of the GABA interneurons in cortex, contain the neuropeptide cholecystokinin (CCK). In an effort to study the limbic cortical glutamate projection neurons and GABA interneurons in schizophrenia, we have measured CCK mRNA with in situ hybridization histochemistry in postmortem samples of dorsolateral (DL)PFC, ACC and ERC of seven schizophrenics, nine non-psychotic suicides and seven normal controls. CCK mRNA is decreased in ERC (especially layers iii vi) and subiculum in schizophrenics relative to controls. Cellular analysis indicates that there is a decrease in density of CCK mRNA in labelled neurons. In so far as ERC CCK mRNA is not reduced in rats treated chronically with haloperidol, this decrease in schizophrenics does not appear to be related to neuroleptic treatment. In contrast, in DLPFC, where schizophrenics do not differ from normals, the suicide victims have elevated CCK mRNA (especially in layers v and vi), and increased cellular density of CCK mRNA, relative to both normals and schizophrenics. These results lend further support for the involvement of ERC and hippocampus in schizophrenia, suggesting that neurons that utilize CCK may be particularly important. Similarly, an increase in CCK mRNA levels in the PFC of suicides adds to a growing body of evidence implicating this structure in this pathological state. In so far as CCK is co-localized with GABA or glutamate in cortical neurons, both of these neuronal populations need to be studied further in schizophrenia and suicide.

Characterization of the genomic structure, promoter region, and a tetranucleotide repeat polymorphism of the human neurotensin receptor gene

In the present study, we have cloned the human neurotensin receptor (NTR) gene, determined its structure, demonstrated that its promoter is functional in transfection experiments, and identified the start site of transcription and a tetranucleotide repeat polymorphism that locates at less than 3 kilobase pairs from the gene. The gene contains three introns, all in the coding regions. Several differences in genomic clones and previously characterized cDNA sequences are reconciled. The 5' regulatory region, which is rich in presumptive transcription factors, can drive luciferase expression in transfected CHO-K1 cells. Stepwise 5' deletions identify a positive modulator between -782 and -1309 and a negative modulator between -1309 and -1563. Southern blot analyses demonstrate a single copy gene for the NTR. The tetranucleotide repeat polymorphism is highly informative with at least 23 alleles and might serve as a very useful marker for genetic study of the relationship between the NTR and neuropsychiatric disorders.