Age- and sex-related changes in cortical and striatal nitric oxide synthase in the Q175 mouse model of Huntington's disease.

In Huntington's disease (HD), corticostriatal and striatopallidal projection neurons preferentially degenerate as a result of mutant huntingtin expression. Pathological deficits in nitric oxide (NO) signaling have also been reported in corticostriatal circuits in HD, however, the impact of age and sex on nitrergic transmission is not well characterized. Thus, we utilized NADPH-diaphorase (NADPH-d) histochemistry and qPCR assays to assess neuronal NO synthase (nNOS) activity/expression in aged male and female Q175 heterozygous mice. Compared to age-matched controls, male Q175 mice exhibited reductions in NADPH-d staining in the motor cortex at 21, but not, 16 months of age. Comparisons across genotypes showed that striatal NADPH-d staining was significantly decreased at both 16 and 21 months of age. Comparisons within sexes in 21 month old mice revealed a decrease in striatal NADPH-d staining in males, but no changes were detected in females. Significant correlations between cortical and striatal NADPH-d staining deficits were also observed in males and females at both ages. To directly assess the role of constitutively active NOS isoforms in these changes, nNOS and endothelial NOS (eNOS) mRNA expression levels were examined in R6/2 (3 month old) and Q175 (11.5 month old) mice using qPCR assays. nNOS transcript expression was decreased in the cortex (40%) and striatum (54%) in R6/2 mice. nNOS mRNA down-regulation in striatum of Q175 animals was more modest (19%), and no changes were detected in cortex. eNOS expression was not changed in the cortex or striatum of Q175 mice. The current findings point to age-dependent deficits in nNOS activity in the HD cortex and striatum which appear first in the striatum and are more pronounced in males. Together, these observations and previous studies indicate that decreases in nitrergic transmission progress with age and are likely to contribute to corticostriatal circuit pathophysiology particularly in male patients with HD.

Biomarker Analysis of Orally Dosed, Dual Active, Matrix Metalloproteinase (MMP)-2 and MMP-9 Inhibitor, AQU-118, in the Spinal Nerve Ligation (SNL) Rat Model of Neuropathic Pain.

There is an unmet medical need for the development of non-addicting pain therapeutics with enhanced efficacy and tolerability. The current study examined the effects of AQU-118, an orally active inhibitor of metalloproteinase-2 (MMP-2) and MMP-9, in the spinal nerve ligation (SNL) rat model of neuropathic pain. Mechanical allodynia and the levels of various biomarkers were examined within the dorsal root ganglion (DRG) before and after oral dosing with AQU-118. The rats that received the SNL surgery exhibited significant mechanical allodynia as compared to sham controls. Animals received either vehicle, positive control (gabapentin), or AQU-118. After SNL surgery, the dorsal root ganglion (DRG) of those rats dosed with vehicle had elevated messenger RNA (mRNA) expression levels for MMP-2, IL1-ß & IL-6 and elevated protein levels for caspase-3 while exhibiting decreased protein levels for myelin basic protein (MBP) & active IL-ß as compared to sham controls. Rats orally dosed with AQU-118 exhibited significantly reduced mechanical allodynia and decreased levels of caspase-3 in the DRG as compared to vehicle controls. Results demonstrate that oral dosing with the dual active, MMP-2/-9 inhibitor, AQU-118, attenuated mechanical allodynia while at the same time significantly reduced the levels of caspase-3 in the DRG.

Characterization of behavioral and neuromuscular junction phenotypes in a novel allelic series of SMA mouse models.

A number of mouse models for spinal muscular atrophy (SMA) have been genetically engineered to recapitulate the severity of human SMA by using a targeted null mutation at the mouse Smn1 locus coupled with the transgenic addition of varying copy numbers of human SMN2 genes. Although this approach has been useful in modeling severe SMA and very mild SMA, a mouse model of the intermediate form of the disease would provide an additional research tool amenable for drug discovery. In addition, many of the previously engineered SMA strains are multi-allelic by design, containing a combination of transgenes and targeted mutations in the homozygous state, making further genetic manipulation difficult. A new genetic engineering approach was developed whereby variable numbers of SMN2 sequences were incorporated directly into the murine Smn1 locus. Using combinations of these alleles, we generated an allelic series of SMA mouse strains harboring no, one, two, three, four, five, six or eight copies of SMN2. We report here the characterization of SMA mutants in this series that displayed a range in disease severity from embryonic lethal to viable with mild neuromuscular deficits.

Global Rhes knockout in the Q175 Huntington's disease mouse model.

Huntington's disease (HD) results from an expansion mutation in the polyglutamine tract in huntingtin. Although huntingtin is ubiquitously expressed in the body, the striatum suffers the most severe pathology. Rhes is a Ras-related small GTP-binding protein highly expressed in the striatum that has been reported to modulate mTOR and sumoylation of mutant huntingtin to alter HD mouse model pathogenesis. Reports have varied on whether Rhes reduction is desirable for HD. Here we characterize multiple behavioral and molecular endpoints in the Q175 HD mouse model with genetic Rhes knockout (KO). Genetic RhesKO in the Q175 female mouse resulted in both subtle attenuation of Q175 phenotypic features, and detrimental effects on other kinematic features. The Q175 females exhibited measurable pathogenic deficits, as measured by MRI, MRS and DARPP32, however, RhesKO had no effect on these readouts. Additionally, RhesKO in Q175 mixed gender mice deficits did not affect mTOR signaling, autophagy or mutant huntingtin levels. We conclude that global RhesKO does not substantially ameliorate or exacerbate HD mouse phenotypes in Q175 mice.

The novel triple reuptake inhibitor JZAD-IV-22 exhibits an antidepressant pharmacological profile without locomotor stimulant or sensitization properties.

Triple reuptake inhibitors (TRIs) that block the dopamine transporter (DAT), norepinephrine transporter, and serotonin transporter are being developed as a new class of antidepressant that may have better efficacy and fewer side effects compared with traditional antidepressants. We describe a novel TRI, 2-[4-(4-chlorophenyl)-1-methylpiperidin-3-ylmethylsulfanyl]-1-(3-methylpiperidin-1-yl)-ethanone (JZAD-IV-22), that inhibits all three monoamine transporters with approximately equal potency in vitro. (+/-)-1-(3,4-dichlorophenyl)-3-azabicyclo-[3.1.0]hexane hydrochloride (DOV 216,303), a TRI shown to be an effective antidepressant in a clinical trial, shows reuptake inhibition similar to that of JZAD-IV-22 in vitro. Furthermore, both JZAD-IV-22 and DOV 216,303 increase levels of dopamine, norepinephrine, and serotonin in the mouse prefrontal cortex when administered by peripheral injection. JZAD-IV-22 and DOV 216,303 exhibited antidepressant-like efficacy in the mouse forced-swim and tail-suspension tests at doses that increased neurotransmitter levels. Because development of DAT inhibitors could be hindered by abuse liability, both JZAD-IV-22 and DOV 216,303 were compared in two assays that are markers of abuse potential. Both JZAD-IV-22 and DOV 216,303 partially substituted for cocaine in a drug discrimination assay in rats, and high doses of DOV 216,303 produced locomotor sensitization in mice. JZAD-IV-22 showed no evidence of sensitization at any dose tested. These results demonstrate that JZAD-IV-22 is a TRI with antidepressant-like activity similar to that of DOV 216,303. The striking feature that distinguishes the two TRIs is that locomotor sensitization, a common underlying feature of drugs of abuse, is seen with DOV 216,303 but is completely lacking in JZAD-IV-22. These findings may have implications for the potential for abuse liability in humans.

Identification of pyridazino[4,5-b]indolizines as selective PDE4B inhibitors.

Substituted pyridazino[4,5-b]indolizines were identified as potent and selective PDE4B inhibitors. We describe the structure-activity relationships generated around an HTS hit that led to a series of compounds with low nanomolar affinity for PDE4B and high selectivity over the PDE4D subtype.

Axonal Conduction Velocity in CA1 Area of Hippocampus is Reduced in Mouse Models of Alzheimer's Disease.

The timing of action potentials arrival at synaptic terminals partially determines integration of synaptic inputs and is important for information processing in the CNS. Therefore, axonal conduction velocity (VC) is a salient parameter, influencing the timing of synaptic inputs. Even small changes in VC may disrupt information coding in networks requiring accurate timing. We recorded compound action potentials in hippocampal slices to measure VC in three mouse models of Alzheimer's disease. We report an age-dependent reduction in VC in area CA1 in two amyloid-ß precursor protein transgenic mouse models, line 41 and APP/PS1, and in a tauopathy model, rTg4510.

Transcriptional Assessment of Striatal mRNAs as Valid Biomarkers of Disease Progression in Three Mouse Models of Huntington's Disease.

BACKGROUND: Huntington's disease (HD) is a progressive neurodegenerative disorder that prominently affects the basal ganglia, leading to affective, cognitive, behavioral, and motor decline. The primary site of neuron loss in HD is the striatal part of the basal ganglia, with GABAergic medium size spiny neurons (MSNs) being nearly completely lost in advanced HD. OBJECTIVE: Based on the hypothesis that mutant huntingtin (mHTT) protein injures neurons via transcriptional dysregulation, we set out to establish a transcriptional profile of HD disease progression in the well characterized transgenic mouse model, R6/2, and two Knock-in models (KI); zQ175KI (expressing mutant mouse/human chimeric Htt protein) and HdhQ200 HET KI (carrying one allele of expanded mouse CAG repeats). METHODS: In this study, we used quantitative PCR (qPCR) to evaluate striatal mRNA levels of markers of neurotransmission, neuroinflammation, and energy metabolism. RESULTS: After analyzing and comparing transcripts from pre-symptomatic and symptomatic stages, markers expressed in the basal ganglia MSNs, which are typically involved in maintaining normal neurotransmission, showed a genotype-specific decrease in mRNA expression in a pattern consistent with human studies. In contrast, transcripts associated with neuroinflammation and energy metabolism were mostly unaffected in these animal models of HD. CONCLUSION: Our results show that transcripts linked to neurotransmission are significantly reduced and are consistent with disease progression in both zQ175KI and R6/2 transgenic mouse models.

Differences in Synaptic Dysfunction Between rTg4510 and APP/PS1 Mouse Models of Alzheimer's Disease.

Genetically modified mice have provided insights into the progression and pathology of Alzheimer's disease (AD). Here, we have examined two mouse models of AD: the rTg4510 mouse, which overexpresses mutant human Tau gene, and the APP/PS1 mouse, which overexpresses mutant human genes for amyloid precursor protein and presenilin 1. Both models exhibit deficits in hippocampal function, but comparative analyses of these deficits are sparse. We used extracellular field potential recordings in hippocampal slices to study basal synaptic transmission (BST), paired-pulse facilitation (PPF), and long-term potentiation (LTP) at the Schaffer collateral-CA1 pyramidal cell synapses in both models. We found that 6-7, but not 2-3-month-old rTg4510 mice exhibited reduced pre-synaptic activation (fiber volley (FV) amplitude, ∼50%) and field excitatory post-synaptic potential (fEPSP) slope (∼40%) compared to wild-type controls. In contrast to previous reports, BST, when controlled for FV amplitude, was not altered in rTg4510. APP/PS1 mice (2-3 mo and 8-10 mo) had unchanged FV amplitude compared to wild-type controls, while fEPSP slope was reduced by ∼34% in older mice, indicating a deficit in BST. PPF was unchanged in 8-10-month-old APP/PS1 mice, but was reduced in 6-7-month-old rTg4510 mice. LTP was reduced only in older rTg4510 and APP/PS1 mice. Our data suggest that BST deficits appear earlier in APP/PS1 than in rTg4510, which exhibited no BST deficits at the ages tested. However, FV and synaptic plasticity deficits developed earlier in rTg4510. These findings highlight fundamental differences in the progression of synaptic pathology in two genetically distinct models of AD.

A precision oncology approach to the pharmacological targeting of mechanistic dependencies in neuroendocrine tumors.

We introduce and validate a new precision oncology framework for the systematic prioritization of drugs targeting mechanistic tumor dependencies in individual patients. Compounds are prioritized on the basis of their ability to invert the concerted activity of master regulator proteins that mechanistically regulate tumor cell state, as assessed from systematic drug perturbation assays. We validated the approach on a cohort of 212 gastroenteropancreatic neuroendocrine tumors (GEP-NETs), a rare malignancy originating in the pancreas and gastrointestinal tract. The analysis identified several master regulator proteins, including key regulators of neuroendocrine lineage progenitor state and immunoevasion, whose role as critical tumor dependencies was experimentally confirmed. Transcriptome analysis of GEP-NET-derived cells, perturbed with a library of 107 compounds, identified the HDAC class I inhibitor entinostat as a potent inhibitor of master regulator activity for 42% of metastatic GEP-NET patients, abrogating tumor growth in vivo. This approach may thus complement current efforts in precision oncology.

Disruption of ShcA signaling halts cell proliferation--characterization of ShcC residues that influence signaling pathways using yeast.

Shc adapter proteins are thought to regulate cellular proliferation, differentiation and apoptosis by activating the SOS-Grb2-RAS-MAPK signaling cascade. Using the small hairpin RNA (shRNA) technique, we found that decreasing ShcA mRNA reduced the proliferative ability of HEK293 mammalian culture cells. We then recapitulated phosphorylation-dependent Shc-Grb2 complex formation in Saccharomyces cerevisiae. Immunoprecipitation followed by Western analysis demonstrated that activated TrkB, composed of the intracellular domain of TrkB fused to glutathione S-transferase (GST-TrkB(ICD)), promoted the association of ShcC and Grb2 in yeast. The Ras-recruitment system (RRS), in which a myristoylated (Myr)-bait and son of sevenless (hSOS)-prey are brought together to complement the defective Ras-cAMP pathway in a thermosensitive cdc25H mutant yeast strain, was used to validate a phenotypic assay. Yeast cells transformed with both Myr-ShcC and hSOS-Grb2 (referred to as scheme 1) or Myr-Grb2 and hSOS-ShcC (scheme 2) did not grow at non-permissive temperature; the additional transformation of GST-TrkB(ICD) enabled growth. GST-TrkB(ICD) also enabled growth with hSOS-Grb2 and either Myr-ShcA or Myr-SHP2. Mutational analysis of TrkB showed that its kinase activity was essential for complementation, while its docking site for Shc proteins was not. Mutational analysis of ShcC showed that the PTB and SH2 domains were not essential for complementation but phosphorylation at Y304 in the CH1 domain was. Phosphorylation at Y304 could not be substituted by an acidic amino acid. The RRS provides a genetic system to probe Shc proteins and potentially identify member specific protein partners and pharmacological reagents.

Selective phosphodiesterase (PDE)-4 inhibitors: a novel approach to treating memory deficit?

Phosphodiesterase-4 (PDE4) belongs to an important family of proteins that regulates the intracellular level of cyclic adenosine monophosphate (cAMP). Several lines of evidence indicate that targeting PDE4 with selective inhibitors may offer novel strategies in the treatment of age-related memory impairment and Alzheimer's disease. The rationale for such an approach stems from preclinical studies indicating that PDE4 inhibitors can counteract deficits in long-term memory caused by pharmacological agents, aging or overexpression of mutant forms of human amyloid precursor proteins. In addition to their pro-cognitive and pro-synaptic plasticity properties, PDE4 inhibitors are potent neuroprotective, neuroregenerative and anti-inflammatory agents. Based on the fact that Alzheimer's disease is a progressive neurodegenerative disorder that is characterised by cognitive impairment, and that neuroinflammation is now recognised as a prominent feature in Alzheimer's pathology, we have concluded that targeting PDE4 with selective inhibitors may offer a novel therapy aimed at slowing progression, prevention and, eventually, therapy of Alzheimer's disease.

Large-scale phenome analysis defines a behavioral signature for Huntington's disease genotype in mice.

Rapid technological advances for the frequent monitoring of health parameters have raised the intriguing possibility that an individual's genotype could be predicted from phenotypic data alone. Here we used a machine learning approach to analyze the phenotypic effects of polymorphic mutations in a mouse model of Huntington's disease that determine disease presentation and age of onset. The resulting model correlated variation across 3,086 behavioral traits with seven different CAG-repeat lengths in the huntingtin gene (Htt). We selected behavioral signatures for age and CAG-repeat length that most robustly distinguished between mouse lines and validated the model by correctly predicting the repeat length of a blinded mouse line. Sufficient discriminatory power to accurately predict genotype required combined analysis of >200 phenotypic features. Our results suggest that autosomal dominant disease-causing mutations could be predicted through the use of subtle behavioral signatures that emerge in large-scale, combinatorial analyses. Our work provides an open data platform that we now share with the research community to aid efforts focused on understanding the pathways that link behavioral consequences to genetic variation in Huntington's disease.

Phosphodiesterase 10A Inhibition Improves Cortico-Basal Ganglia Function in Huntington's Disease Models.

Huntington's disease (HD) symptoms are driven to a large extent by dysfunction of the basal ganglia circuitry. HD patients exhibit reduced striatal phoshodiesterase 10 (PDE10) levels. Using HD mouse models that exhibit reduced PDE10, we demonstrate the benefit of pharmacologic PDE10 inhibition to acutely correct basal ganglia circuitry deficits. PDE10 inhibition restored corticostriatal input and boosted cortically driven indirect pathway activity. Cyclic nucleotide signaling is impaired in HD models, and PDE10 loss may represent a homeostatic adaptation to maintain signaling. Elevation of both cAMP and cGMP by PDE10 inhibition was required for rescue. Phosphoproteomic profiling of striatum in response to PDE10 inhibition highlighted plausible neural substrates responsible for the improvement. Early chronic PDE10 inhibition in Q175 mice showed improvements beyond those seen with acute administration after symptom onset, including partial reversal of striatal deregulated transcripts and the prevention of the emergence of HD neurophysiological deficits. VIDEO ABSTRACT.

Differential effects of regulator of G protein signaling (RGS) proteins on serotonin 5-HT1A, 5-HT2A, and dopamine D2 receptor-mediated signaling and adenylyl cyclase activity.

Regulator of G protein signaling (RGS) proteins function as GTPase accelerating proteins (GAP) for Galpha subunits, attenuating G-protein-coupled receptor signal transduction. The present study tested the ability of members of different subfamilies of RGS proteins to modulate both G-protein-dependent and -independent signaling in mammalian cells. RGS4, RGS10, and RGSZ1 significantly attenuated Galphai-mediated signaling by 5-HT1A, but not by dopamine D2, receptor-expressing cells. Additionally, RGS4 and RGS10 significantly inhibited forskolin-stimulated cAMP production in both cell lines. In contrast, RGS2, RGS7, and RGSZ1 had no effect on forskolin-stimulated cAMP production in these cells. RGS2 and RGS7 significantly decreased Galphaq-mediated signaling by 5-HT2A receptors, confirming that the RGS4 and RGS10 effects on forskolin-stimulated cAMP production were specific, and not simply due to overexpression. Interestingly, similar expression levels of RGS4 protein resulted in greater inhibition of G-protein-independent cAMP production compared to G-protein-dependent GAP activity. Our results suggest specificity and selectivity of RGS proteins on G-protein-dependent and -independent signaling in mammalian cells.

Regulators of G-protein signaling 4: modulation of 5-HT1A-mediated neurotransmitter release in vivo.

Regulators of G-protein signaling (RGS) play a key role in the signal transduction of G-protein-coupled receptors (GPCRs). Specifically, RGS proteins function as GTPase accelerating proteins (GAPs) to dampen or "negatively regulate" GPCR-mediated signaling. Our group recently showed that RGS4 effectively GAPs Galpha(i)-mediated signaling in CHO cells expressing the serotonin-1A (5-HT(1A)) receptor. However, whether a similar relationship exists in vivo has yet to be identified. In present studies, a replication-deficient herpes simplex virus (HSV) was used to elevate RGS4 mRNA in the rat dorsal raphe nuclei (DRN) while extracellular levels of 5-HT in the striatum were monitored by in vivo microdialysis. Initial experiments conducted with noninfected rats showed that acute administration of 8-OH-DPAT (0.01-0.3 mg/kg, subcutaneous [s.c.]) dose dependently decreased striatal levels of 5-HT, an effect postulated to result from activation of somatodendritic 5-HT(1A) autoreceptors in the DRN. In control rats receiving a single intra-DRN infusion of HSV-LacZ, 8-OH-DPAT (0.03 mg/kg, s.c.) decreased 5-HT levels to an extent similar to that observed in noninfected animals. Conversely, rats infected with HSV-RGS4 in the DRN showed a blunted neurochemical response to 8-OH-DPAT (0.03 mg/kg, s.c.); however, increasing the dose to 0.3 mg/kg reversed this effect. Together, these findings represent the first in vivo evidence demonstrating that RGS4 functions to GAP Galpha(i)-coupled receptors and suggest that drug discovery efforts targeting RGS proteins may represent a novel mechanism to manipulate 5-HT(1A)-mediated neurotransmitter release.

Genetic deletion of transglutaminase 2 does not rescue the phenotypic deficits observed in R6/2 and zQ175 mouse models of Huntington's disease.

Huntington's disease (HD) is an autosomal dominant, progressive neurodegenerative disorder caused by expansion of CAG repeats in the huntingtin gene. Tissue transglutaminase 2 (TG2), a multi-functional enzyme, was found to be increased both in HD patients and in mouse models of the disease. Furthermore, beneficial effects have been reported from the genetic ablation of TG2 in R6/2 and R6/1 mouse lines. To further evaluate the validity of this target for the treatment of HD, we examined the effects of TG2 deletion in two genetic mouse models of HD: R6/2 CAG 240 and zQ175 knock in (KI). Contrary to previous reports, under rigorous experimental conditions we found that TG2 ablation had no effect on either motor or cognitive deficits, or on the weight loss. In addition, under optimal husbandry conditions, TG2 ablation did not extend R6/2 lifespan. Moreover, TG2 deletion did not change the huntingtin aggregate load in cortex or striatum and did not decrease the brain atrophy observed in either mouse line. Finally, no amelioration of the dysregulation of striatal and cortical gene markers was detected. We conclude that TG2 is not a valid therapeutic target for the treatment of HD.

Chemistry and behavioral studies identify chiral cyclopropanes as selective α4ß2-nicotinic acetylcholine receptor partial agonists exhibiting an antidepressant profile.

Despite their discovery in the early 20th century and intensive study over the last 20 years, nicotinic acetylcholine receptors (nAChRs) are still far from being well understood. Only a few chemical entities targeting nAChRs are currently undergoing clinical trials, and even fewer have reached the marketplace. In our efforts to discover novel and truly selective nAChR ligands, we designed and synthesized a series of chiral cyclopropane-containing α4ß2-specific ligands that display low nanomolar binding affinities and excellent subtype selectivity while acting as partial agonists at α4ß2-nAChRs. Their favorable antidepressant-like properties were demonstrated in the classical mouse forced swim test. Preliminary ADMET studies and broad screening toward other common neurotransmitter receptors were also carried out to further evaluate their safety profile and eliminate their potential off-target activity. These highly potent cyclopropane ligands possess superior subtype selectivity compared to other α4ß2-nAChR agonists reported to date, including the marketed drug varenicline, and therefore may fully satisfy the crucial prerequisite for avoiding adverse side effects. These novel chemical entities could potentially be advanced to the clinic as new drug candidates for treating depression.

Comprehensive behavioral and molecular characterization of a new knock-in mouse model of Huntington's disease: zQ175.

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by motor, cognitive and psychiatric manifestations. Since the mutation responsible for the disease was identified as an unstable expansion of CAG repeats in the gene encoding the huntingtin protein in 1993, numerous mouse models of HD have been generated to study disease pathogenesis and evaluate potential therapeutic approaches. Of these, knock-in models best mimic the human condition from a genetic perspective since they express the mutation in the appropriate genetic and protein context. Behaviorally, however, while some abnormal phenotypes have been detected in knock-in mouse models, a model with an earlier and more robust phenotype than the existing models is required. We describe here for the first time a new mouse line, the zQ175 knock-in mouse, derived from a spontaneous expansion of the CAG copy number in our CAG 140 knock-in colony [1]. Given the inverse relationship typically observed between age of HD onset and length of CAG repeat, since this new mouse line carries a significantly higher CAG repeat length it was expected to be more significantly impaired than the parent line. Using a battery of behavioral tests we evaluated both heterozygous and homozygous zQ175 mice. Homozygous mice showed motor and grip strength abnormalities with an early onset (8 and 4 weeks of age, respectively), which were followed by deficits in rotarod and climbing activity at 30 weeks of age and by cognitive deficits at around 1 year of age. Of particular interest for translational work, we also found clear behavioral deficits in heterozygous mice from around 4.5 months of age, especially in the dark phase of the diurnal cycle. Decreased body weight was observed in both heterozygotes and homozygotes, along with significantly reduced survival in the homozygotes. In addition, we detected an early and significant decrease of striatal gene markers from 12 weeks of age. These data suggest that the zQ175 knock-in line could be a suitable model for the evaluation of therapeutic approaches and early events in the pathogenesis of HD.

Dissociation between duration of action in the forced swim test in mice and nicotinic acetylcholine receptor occupancy with sazetidine, varenicline, and 5-I-A85380.

RATIONALE: Nicotinic acetylcholine receptor (nAChR) agonists, partial agonists, and antagonists have antidepressant-like effects in rodents and reduce symptoms of depression in humans. OBJECTIVES: The study determined whether the antidepressant-like effect of the nAChR ß2* partial agonist sazetidine-A (sazetidine) in the forced swim test was due to activation or desensitization of ß2* nAChRs. The study also determined if sazetidine's behavioral responses in the forced swim test corresponded to ß2* nAChRs receptor occupancy and drug bioavailability. RESULTS: Acute antidepressant-like effects in the forced swim test were seen with sazetidine and the full ß2* agonist 5-I-A8350 (BALB/cJ mice) and the less selective ß2* partial agonist varenicline in C57BL/6J but not BALB/cJ mice. The role of ß2* nAChRs was confirmed by results showing: (1) reversal of sazetidine's antidepressant-like effects in the forced swim test by nAChR antagonists mecamylamine and dihydro-ß-erythroidine; (2) absence of sazetidine's effect in mice lacking the ß2 subunit of the nAChR; and (3) a high correspondence between behaviorally active doses of sazetidine and ß2* receptor occupancy. ß2* receptor occupancy following acute sazetidine, varenicline, and 5-I-A8350 lasted beyond the duration of action in the forced swim test. Sazetidine's long lasting receptor occupancy did not diminish behavioral efficacy in the forced swim test following repeated dosing. CONCLUSIONS: Results demonstrate that activation of a small population of ß2* nAChRs (10-40%) is sufficient to elicit sazetidine's antidepressant-like actions without producing tolerance and suggest that ligands that activate ß2* nAChRs would be promising targets for the development of a new class of antidepressant.