On the selection of mice for haloperidol response and non-response

A genetic locus in 7p12.2 associated with treatment resistant schizophrenia

Approximately 30% of patients with schizophrenia are treatment resistant (TRS), i.e. have persistent psychotic symptoms despite adequate trials of at least two antipsychotic drugs (APDs). Most TRS patients are candidates for clozapine treatment which is underutilized because of its side effects and difficulty in identifying TRS. We conducted a genome-wide association study (GWAS) of 79 TRS and 95 non-treatment resistant (NTRS) Caucasian schizophrenia patients to identify possible biomarkers for TRS, which might also provide insight into the pathobiology of TRS. The single nucleotide polymorphism, rs2237457, located in 7p12.2, a region reported to have imprinted inheritance, was found to have the lowest p value in an allelic association test (unadjusted p = 5.53 × 10(-6)). Haploview disclosed a 30 kb block flanking this SNP within GRB10, 70 kb upstream of l-dopa decarboxylase (DDC), an enzyme which is rate-limiting in the synthesis of trace amines and neurotransmitters implicated in schizophrenia and the action of APDs. This SNP or haplotype was identified as an exclusive cis-acting eQTL for DDC in human dorsolateral prefrontal cortex by BrainCloud®. A replication sample genotyped for this SNP produced a weaker result, but in the same direction. After combining the two samples, rs2237457 remained significantly associated with TRS (unadjusted p = 5.66 × 10(-7) in recessive mode; 9.42 × 10(-5) in allelic association). If replicated in an independent sample, rs2237457 may provide a biomarker to identify a significant proportion of Caucasian TRS. The results implicate trace amines and their synthesis in the pathophysiology of TRS.

Forward genetic approaches to understanding complex behaviors

Assigning function to genes has long been a focus of biomedical research.Even with complete knowledge of the genomic sequences of humans, mice and other experimental organisms, there is still much to be learned about gene function and control. Ablation or overexpression of single genes using knockout or transgenic technologies has provided functional annotation for many genes, but these technologies do not capture the extensive genetic variation present in existing experimental mouse populations. Researchers have only recently begun to truly appreciate naturally occurring genetic variation resulting from single nucleotide substitutions,insertions, deletions, copy number variation, epigenetic changes (DNA methylation,histone modifications, etc.) and gene expression differences and how this variation contributes to complex phenotypes. In this chapter, we will discuss the benefits and limitations of different forward genetic approaches that capture the genetic variation present in inbred mouse strains and present the utility of these approaches for mapping QTL that influence complex behavioral phenotypes.

Gene networks and haloperidol-induced catalepsy

The current study examined the changes in striatal gene network structure induced by short-term selective breeding from a heterogeneous stock for haloperidol response. Brain (striatum) gene expression data were obtained using the Illumina WG 8.2 array, and the datasets from responding and non-responding selected lines were independently interrogated using a weighted gene coexpression network analysis (WGCNA). We detected several gene modules (groups of coexpressed genes) in each dataset; the membership of the modules was found to be largely concordant, and a consensus network was constructed. Further validation of the network topology showed that using approximately 35 samples is sufficient to reliably infer the transcriptome network. An in-depth analysis showed significant changes in network structure and gene connectivity associated with the selected lines; these changes were validated using a bootstrapping procedure. The most dramatic changes were associated with a gene module richly annotated with neurobehavioral traits. The changes in network connectivity were concentrated in the links between this module and the rest of the network, in addition to changes within the module; this observation is consistent with recent results in protein and metabolic networks. These results suggest that a network-based strategy will help identify the genetic factors associated with haloperidol response.

Targeting pain-depressed behaviors in preclinical assays of pain and analgesia: drug effects on acetic acid-depressed locomotor activity in ICR mice

AIMS

Pain depresses expression of many behaviors, and one goal of analgesic treatment is to restore pain-depressed behaviors. Assays that focus on pain-depressed behaviors may contribute to preclinical assessment of candidate analgesics.

MAIN METHODS

This study compared effects of the mu opioid receptor agonist morphine (an acknowledged analgesic), the dopamine receptor antagonist haloperidol (a non-analgesic sedative), the adenosine receptor antagonist caffeine (a non-analgesic stimulant) and the neurokinin-1 receptor antagonist CJ 11,974-01 (a candidate analgesic) on acetic acid-induced writhing (a traditional pain-stimulated behavior) and acetic acid-induced suppression of locomotor activity (a pain-depressed behavior) in male ICR mice. Drug effects on non-depressed (baseline) locomotor activity were also examined.

KEY FINDINGS

I.P. administration of acetic acid (0.18-1%) was equipotent in stimulating writhing and depressing locomotor activity. Morphine blocked both acid-induced stimulation of writhing and depression of locomotion, although it was 56-fold less potent in the assay of acid-depressed locomotion. Haloperidol and CJ 11,974-01 decreased acid-stimulated writhing, but failed to block acid-induced depression of locomotion. Caffeine had no effect on acid-stimulated writhing or acid-depressed locomotor activity, although it did increase non-depressed locomotion. Thus, morphine was the only drug to block both acid-stimulated writhing and acid-depressed locomotion.

SIGNIFICANCE

Complementary assays of pain-stimulated and pain-depressed behaviors may improve the predictive validity of preclinical studies that assess candidate analgesic drugs. The low potency of morphine to block acid-induced depression of locomotion suggests that locomotor activity may be a relatively insensitive measure for studies of pain-depressed behavior.

Characterization of the quantitative trait locus for haloperidol-induced catalepsy on distal mouse chromosome 1

We report here the confirmation of the quantitative trait locus for haloperidol-induced catalepsy on distal chromosome (Chr) 1. We determined that this quantitative trait locus was captured in the B6.D2-Mtv7a/Ty congenic mouse strain, whose introgressed genomic interval extends from approximately 169.1 to 191.3 Mb. We then constructed a group of overlapping interval-specific congenic strains to further break up the interval and remapped the locus between 177.5 and 183.4 Mb. We next queried single nucleotide polymorphism (SNP) data sets and identified three genes with nonsynonymous coding SNPs in the quantitative trait locus. We also queried two brain gene expression data sets and found five known genes in this 5.9-Mb interval that are differentially expressed in both whole brain and striatum. Three of the candidate quantitative trait genes were differentially expressed using quantitative real-time polymerase chain reaction analyses. Overall, the current study illustrates how multiple approaches, including congenic fine mapping, SNP analysis and microarray gene expression screens, can be integrated both to reduce the quantitative trait locus interval significantly and to detect promising candidate quantitative trait genes.

Targeting Pain-Suppressed Behaviors in Preclinical Assays of Pain and Analgesia: Effects of Morphine on Acetic Acid-Suppressed Feeding in C57BL/6J Mice

Pain increases the rate, frequency, or intensity of some behaviors (eg, withdrawal responses) and suppresses other behaviors (eg, feeding). Our laboratories are developing assays to test analgesic drug candidates using measurements of pain-suppressed rather than pain-elicited behaviors. Such assays may model important aspects of clinical pain and provide a means for distinguishing true analgesics from drugs that produce motor impairment. The present study compared effects of the mu opioid analgesic morphine and the nonanalgesic neuroleptic haloperidol on intraperitoneal acetic acid-induced writhing (a pain-elicited behavior) and suppression of feeding behavior (a pain-suppressed behavior). In feeding studies, C57BL/6J mice were given access to a dish containing 8 mL Ensure(trade mark) liquid food (0-100% in water) during daily sessions (7.5-120 min). Levels of consumption were dependent on both Ensure concentration and session duration. Intraperitoneal injection of acetic acid (0.10-0.56%) produced a time- and concentration-dependent decrease in Ensure consumption. Morphine (1 mg/kg) prevented both acid-induced writhing and acid-induced suppression of feeding, whereas the dopamine antagonist haloperidol inhibited writhing without preventing acid-induced suppression of feeding. The effects of morphine were time-dependent, selective for acid-suppressed feeding, and naltrexone-reversible. These results suggest that assays of pain-suppressed behaviors may complement assays of pain-elicited behaviors in preclinical studies of candidate analgesics.

PERSPECTIVE

This paper presents a new preclinical strategy for assessing pain and analgesia in mice that is congruent with current methods of pain assessment in the clinic. This strategy may therefore be a useful complement to more traditional procedures for assessing pain and analgesia.

Overexpression of dopamine D2 receptors reduces alcohol self-administration

The mechanism(s) underlying predisposition to alcohol abuse are poorly understood but may involve brain dopamine system(s). Here we used an adenoviral vector to deliver the dopamine D2 receptor (DRD2) gene into the nucleus accumbens of rats, previously trained to self-administer alcohol, and to assess if DRD2 levels regulated alcohol preference and intake. We show that increases in DRD2 (52%) were associated with marked reductions in alcohol preference (43%), and alcohol intake (64%) of ethanol preferring rats, which recovered as the DRD2, returned to baseline levels. In addition, this DRD2 overexpression similarly produced significant reductions in ethanol non-preferring rats, in both alcohol preference (16%) and alcohol intake (75%). This is the first evidence that overexpression of DRD2 reduces alcohol intake and suggests that high levels of DRD2 may be protective against alcohol abuse.

Effect of genetic cross on the detection of quantitative trait loci and a novel approach to mapping QTLs

A genome-wide scan was conducted in two F(2) intercrosses, C57BL/6J (B6)xDBA/2J (D2) and BALB/cJ (C)xLP/J (LP), for three different phenotypes: basal locomotor activity, ethanol-induced locomotor activity, and haloperidol-induced catalepsy. For basal activity, significant quantitative trait loci (QTLs, LOD> or =4.3) were detected on chromosomes 9 and 19 for the CxLP intercross and chromosome 1 for the B6xD2 intercross. Significant QTLs for ethanol-induced activation were detected on chromosome 6 for the CxLP intercross, and on chromosomes 1 and 2 for the B6xD2 intercross. For haloperidol-induced catalepsy, significant QTLs were detected on chromosome 14 (two different QTLs) in the CxLP intercross, and chromosomes 1 and 9 in the B6xD2 intercross. These data illustrate the importance of the genetic cross for QTL detection. Finally, the data reported here, and elsewhere, are also used to demonstrate a novel approach to QTL detection and localization.

Nonhuman behavioral models in the genetics of disturbed behavior

The development of the association method in which genetic markers match quantitative traits had led to quantitative trait loci (QTL) interval mapping. The association method has been extensively used in animal behavior genetics. Animal research allows more suitable linkage studies and detailed assessment of cellular and subcellular components of the central nervous system that may play a crucial role in the development susceptibility to behavioral disorders. Moreover, experimental designs in the laboratory setting allow genotype x environment interactions to be controlled, thus possibly providing more information on the role of nongenetic factors in gene expression. Experimental results are discussed which indicate that animal studies will provide a sort of test for hypotheses arising in clinical settings, allowing gene-product and product-behavior pathways to be examined at molecular levels when the gene accounts for a very small amount of genetic variance. In such a perspective, new molecular biology approaches and behavior genetics in nonhuman species could provide useful tools in the assessment of the genetic as well as nongenetic factors that lead to psychopathology.