Mouse strain differences in selective serotonin reuptake inhibitors sensitivity correlates with serotonin transporter binding and function

OBJECTIVE Selective serotonin reuptake inhibitors(SSRIs) bind 5-HT transporters,leading to the accumulation of 5-HT and amelioration of depression.Although different mouse strain showed different sensitivity to SSRIs in mouse models of depression,the reason for these strain differences remains unclear.Here,therefore,in the present study,we examined immobility time and locomotor activity in two mouse strains,namely,C57BL/6 J and DBA/2 J mice,and the effects of the SSRIs fluoxetine.Furthermore,we analyzed 5-HT transporter binding and reuptake inhibition in both strains to explore their relationship with the immobility and locomotor activity effects of the three SSRIs in these two mouse strains.METHODS Strain differences in SSRI effects in the tail suspension test(TST) and forced swimming test(FST).To initiate our studies,we sought to confirm that SERT strain variation did not alter SERT protein expression,5-HT recognition,or uptake activity when expressed in C57BL/6 J and DBA/2 J mice.Radioligand binding assays were conducted to determine the affinity of the SSRIs for the 5-HT transporters in the two mouse strains.RESULTS SSRI citalopram dose-dependently reduced immobility time in both the FST and TST in DBA/2 J but not C57BL/6 J mouse strains,whereas fluoxetine showed opposite results.Paroxetine reduced immobility time similarly in both strains.The affinity of citalopram for the 5-HT transporter in DBA/2 J mice was 700-fold higher than that for in C57BL/6 J mice,whereas the affinity of fluoxetine in C57BL/6 J mice was 100-fold higher than that in the DBA/2 J mouse.Furthermore,High citalopram concentrations were required to [3 H]5-HT uptake in C57BL/6 J but not DBA/2 J mouse cortical synaptosomes,whereas fluoxetine also showed opposite results.CONCLUSION Immobility duration depends on 5-HT transporter binding levels,leading to apparent strain differences in immobility time in FST and TST.Furthermore,differences in 5-HT transporter binding may cause variations in SSRI responses on behaviors.SERT mutation mice maintained sensitivity to paroxetine,an antidepressant that is unaffected by the mouse mutation.Therefore,the background strain of these mice likely contributes to the acute behavioral actions of SSRIs in immobility time.These differences may help to explain some of the discrepancies in studies that used these strains of mice to examine the role of 5-HT in mouse models of depression.Future studies should investigate additional neural substrates and molecular mechanisms underlying strain variations in mouse models of depression to help identify genetic predispositions to this disorder in humans.

Role of PDE4D splice variants in the mediation of antidepressant and cognition-enhancing actions and its mechanisms

OBJECTIVE Phosphodiesterase 4(PDE4),specific for cyclicAMP(cAMP)-hydrolyzing,has four isoforms(PDE4A-D) with at least 25 splice variants. PDE4 inhibitors produce definite antidepressant-like and cognitive-enhancing effects. However,none of PDE4 inhibitors has yet been approved for clinical utility so far due to the concomitant side effects. The present research is to explore the splice variants of PDE4 D responsible for antidepressant-like and cognitive-enhancing effects of PDE4 inhibitors but not side effects. METHODS Long-form PDE4 Ds were silenced by the bilateral microinfusion of lentiviral vector containing mi RNAs(4Dmi R) into the prefrontal cortex(PFC),PDE4D4 or D5 was overexpressed by the bilateral microinfusion of lentiviral vector containing full c DNA into hippocampus. Antidepressant-like behaviors were measured by tail-suspension test(TST),forced swimming test(FST)and chronic unpredictable stress model. Cognitive behaviors were measured by the novel object recognition test(NOR) and Morris water maze test(MWM) in both normal mice and the mice with chronic unpredictable stress-induced memory deficits. The emetic potential was evaluated by the assessment of the anaesthetic reversal effect,a surrogate of the emesis test in non-vomiting species. The expressions of PDE4 isoforms/splice variants and cAMP level were examined by Western-blot and ELISA analysis. The dendritic complexity and spine density were assessed by Golgi staining. RESULTS(1)High and specific expression of EGFP(green,indicator of 4Dmi R expression) in PFC was observed under fluorescence microscopy.(2) 4Dmi R significantly down-regulated PDE4D4/5 splice variants,but not PDE4 A,PDE4 B or PDE4D1/2/3.(3) 4Dmi R treatments significantly increased cAMP signaling and dendritic complexity in PFC.(4) Rolipram and/or 4Dmi R treatments significantly decreased immobility in TST and FST.(5) Rolipram and/or 4Dmi R treatments reversed the depressive-like behaviors in chronically stressed mice,including the reduced sucrose preference,prolonged latency to novelty-suppressed feeding and increased immobility in FST.(6) Rolipram and/or 4Dmi R treatments significantly increased the recognition index in NOR task and both the entries and durations in MWM task.(7) Rolipram and/or 4Dmi R treatments reversed the memory deficits in chronically stressed mice,including the reduced the recognition index in NOR task and the decreased durations in MWM task.(8) Rolipram and/or 4DmiR treatments reversed the decreased cA MP signaling,dendritic complexity and spine density.(9) Rolipram or plus 4Dmi R treatment significantly decreased the duration of anaesthesia in the alpha2 adrenergic receptor-mediated anesthesia,but not 4Dmi R treatment alone.(10)Hippocampal overexpression of PDE4D5,but not PDE4D4,produced depressive-like and cognitive defect behaviors,which were reversed by rolipram.The measurements including cAMP signaling,dendritic complexity and in vivo hippocampal LTP,showed the same changes. CONCLUSION Long-form PDE4 Ds,especially the PDE4D5,are the major isoforms responsible for antidepressant-like and cognitive-enhancing effects with little side effects. The critical roles of long-form PDE4 Ds are mediated by their regulation of cAMP signaling pathway and neuroplasticity.

S1-2 The 5-HT6 Receptor-Related Mechanism for the Cognition-Enhancing Properties of Hypidone Hydrochloride(YL-0919),A Novel Protential Antidepressant

Hypidone hydrochloride(YL-0919),the 5-HT1A/6 agonists and 5-HT reuptake inhibitor,is a novel potent antidepressant with original chemical structure.Previous studies confirmed that YL-0919 has significant antidepressant-and anxiolytic-like effects.Compared with first-line antidepressants,YL-0919 possesses rapid-onset and cognition-enhancing advantages without causing sexual disorders.Recently,it has been found that it has high affinity with 5-HT6 receptor.Objective:To study the target characteristics of YL-0919 to 5-HT6 receptors,and to explore the relationship between the 5-HT6 receptor and the cognition-enhancing,antidepressant/anxiolytic-like effects of YL-0919 and targeting mechanisms.Methods:The radioligand binding inhibition test and[35S]-GTPγS binding assay were used to evaluate the binding affinity of YL-0919 to 5-HT6 receptor in rat striatum,transient CHO cell line and stable Hela cell lines.Novel object recognition(NOR),Morris water maze(MWM)and step-down test(SD)were used to evaluate the cognition-enhancing activity of YL-0919,and the selective 5-HT6 receptor antagonist SB271046 was used to evaluate the relationship between behavioral improvement caused by YL-0919 and 5-HT6 receptor activation.To study the 5-HT6 receptor related mechanisms of YL-0919,the competitive immunofluorescence assay were used to examine the cAMP level in h5-HT6 receptor-expressed in the Hela cells Results:①Radioligand competitive binding experiments showed that YL-0919 had high binding affinity with 5-HT6 receptors in the rat striatum,the CHO cells transiently expressed the h5-HT6 receptor and the Hela cells stably expressed the h5-HT6 receptor,with Ki of 10.72,14.76 and 28.12 nM respectively;[35S]-GTPγS showed full agonist characteristics of YL-0919 in striatum and cells,with EC50 of 71.23,64.73 and 52.92 nM respectively,and the maximum efficiency(Emax)reached 100%which is the same to the 5-HT6 receptor agonist WAY208466,suggesting that YL-0919 is a full 5-HT6 receptor agonist.②Cognitive-related behavioral tests showed that subchronic oral administration of YL-0919(1.25~2.5 mg·kg-1)could significantly increase the recognition index in NOR,the entries and duration in the target quadrant,the entries crossing the platform in WMW,shortened the first time crossing the platform in MWM and the step-down latency in SD,suggesting the cognitionenhancing effects of YL-0919;compared with Vilazodone,the partial agonist of 5-HT1A receptor and 5-HT reuptake inhibitor,which of no such functions;Further study showed that 5-HT6 receptor antagonist SB271046(10 mg·kg-1)completely blocked the cognition-enhancing effects of YL-0919 without affecting the cognitive activity itself,suggesting that 5-HT6 receptor activation might be its underlying mechanisms;③Mechanism study found that YL-0919 could significantly increase cAMP levels in the Hela cells stably-expressed the h5-HT6 receptor,which could be dose-dependent blocked by SB271046.Conclusion:YL-0919 is a full agonist of 5-HT6 receptor.YL-0919 showed significant cognition-enhancing effects in various kinds of animal models,and its underlying important mechanism might be activating 5-HT6 receptor.In addition,enhancing downstream cAMP-CREB signaling pathway of 5-HT6 receptor might at least partially mediate the above process.Moreover,5-HT6 receptor activation might also be one of the mechanisms of antidepressant-and anxiolytic-like effects of YL-0919.In conclusion,this study confirmed the 5-HT6 receptor-related mechanisms of YL-0919,the 1.1 types of antidepressants,laying the experimental foundation for developing novel antidepressants with cognition-enhancing effects.