The onset of the therapeutic response to antidepressant treatment exhibits a characteristic delay. Animal models sensitive to chronic, but not acute, antidepressant treatment are greatly needed for studying antidepressant mechanisms. We initially assessed four inbred mouse strains for their behavioral response to chronic treatment with the selective-serotonin reuptake inhibitor fluoxetine (0, 5, 10 mg/kg/day in drinking water), which is used for the treatment of mood and anxiety disorders. Only the highly anxious BALB/c strain exhibited sensitivity to fluoxetine in the forced swim test. Additionally, fluoxetine reduced locomotion in C57BL/6 and 129SvEv, but not BALB/c and DBA/2, strains. We then evaluated the effects of subchronic (B4 days) and chronic (B24 days) fluoxetine treatment (0, 10, 18, 25 mg/kg/day) on measures of anxiety and depression in BALB/c mice. Anxiety measures were obtained using the open field and noveltyinduced hypophagia tests. Antidepressant effects were evaluated using the forced swim test. We found 18 mg/kg/day of chronic fluoxetine to be active in all three paradigms; subchronic treatment had no effect. Anxiety-related measures were reduced by 18 mg/kg/ day. In the forced swim test, 10 and 18 mg/kg/day increased swimming and reduced immobility. Here we present several novel effects of chronic, but not subchronic, antidepressant treatment.

Aquaporins are a family of water channels found in animals, plants, and microorganisms. A subfamily of aquaporins, the aquaglyceroporins, are permeable for water as well as certain solutes such as glycerol, lactate, and urea. Here we show that the brain contains two isoforms of AQP9--an aquaglyceroporin with a particularly broad substrate specificity--and that the more prevalent of these isoforms is expressed in brain mitochondria. The mitochondrial AQP9 isoform is detected as an approximately 25 kDa band in immunoblots. This isoform is likely to correspond to a new AQP9 mRNA that is obtained by alternative splicing and has a shorter ORF than the liver isoform. Subfractionation experiments and high-resolution immunogold analyses revealed that this novel AQP9 isoform is enriched in mitochondrial inner membranes. AQP9 immunopositive mitochondria occurred in astrocytes throughout the brain and in a subpopulation of neurons in the substantia nigra, ventral tegmental area, and arcuate nucleus. In the latter structures, the AQP9 immunopositive mitochondria were located in neurons that were also immunopositive for tyrosine hydroxylase, as demonstrated by double labeling immunogold electron microscopy. Our findings suggest that mitochondrial AQP9 is a hallmark of astrocytes and midbrain dopaminergic neurons. In physiological conditions, the flux of lactate and other metabolites through AQP9 may confer an advantage by allowing the mitochondria to adjust to the metabolic status of the extramitochondrial cytoplasm. We hypothesize that the complement of mitochondrial AQP9 in dopaminergic neurons may relate to the vulnerability of these neurons in Parkinson's disease.

Histone modification, which affects the rate of transcription without altering DNA sequence, occurs in response to various psychiatric drugs and in several models of psychiatric disease. As increases in histone acetylation have been seen after treatment with antidepressants, we investigated whether directly increasing histone acetylation using a histone deacetylase inhibitor would have antidepressant effects. We administered sodium butyrate (NaB, 100 mg/kg, i.p.) to mice acutely (3 injections over 24 hours) or chronically (twice daily for 21 days) and subjected them to a number of behavioral tests of antidepressant response. This dose of NaB had no effect on overall locomotor activity after either acute or chronic treatment. Acutely treated mice showed an increase in immobility in the forced-swim test (FST) and an increase in latency to consume in the novel environment of the novelty-induced hypophagia (NIH) paradigm, an anxiogenic effect. The effect of NaB on anxiety did not generalize to another test, the elevated zero maze, where it had no effect. Chronic treatment with NaB had no effect on latency to consume in the NIH or immobility in the FST. However, this dose did alter histone acetylation in the hippocampus. While H4 acetylation increased in the hippocampus 30 min following acute NaB, chronic treatment caused a decrease in AcH4. There were no changes in AcH3 following either treatment. While changes in chromatin structure may be involved in the mechanism of action of antidepressant drugs, these data suggest that increasing histone acetylation pharmacologically is not sufficient to produce antidepressant effects.

Serotonin 2C receptors (5-HT 2C R) have been shown to undergo post-transcriptional RNA editing. This modification affects the affinity, coupling and constitutive activity of the receptor. In vivo, manipulations such as stress or antidepressant administration dramatically modify the pattern of 5-HT 2C R mRNA editing, suggesting that this phenomenon might be involved in the pathophysiology of stress-related disorders. Indeed, alterations of 5-HT 2C R mRNA editing have been observed in depressed patients. Thus, the recent development of mice expressing either the non-edited (5-HT 2C R-INI) or the fully-edited form of 5-HT 2C R (5-HT 2C R -VGV) provides a novel opportunity to investigate the relevance of this phenomenon in the context of stress-related disorders. We observed that both 5-HT 2C R-INI and 5-HT 2C R-VGV mice exhibit exaggerated anxiety-like behaviors in the elevated plus maze paradigm. This phenotype was observed when the INI or VGV mutations were present in mice on a BALB/c background, as well as non-significant trends in the same direction in mice on a C57BL/6J background. In animal models of antidepressant-like activity, the absence of editing of 5-HT 2C R mRNA (5-HT 2C R-INI) induced an increase in the time spent immobile in the forced-swim test (FST) and tail suspension test (TST). Complete editing of 5-HT2C receptor mRNA (5-HT 2C R-VGV) induced antidepressant-like behavior in the FST and TST, as reflected by a significant decrease in time spent immobile. These phenotypes were unrelated to alterations in locomotor activity in both 5-HT 2C R-INI and -VGV. In the TST, these phenotypes were accompanied by a decrease and an increase in response to desipramine in 5-HT 2C R-INI and -VGV, respectively. These data constitute the first in vivo demonstration of a role for 5-HT 2C R mRNA editing in anxiety-and depression-related behaviors.

The transcription factor cAMP response element-binding protein (CREB) has been implicated in the pathophysiology of depression as well as in the efficacy of antidepressant treatment. However, altering CREB levels appears to have differing effects on anxiety-and depression-related behaviors, depending on which brain region is examined. Furthermore, many manipulations of CREB lead to corresponding changes in other CREB family proteins, and the impact of these changes has been largely ignored. To further investigate the region-specific importance of CREB in depression-related behavior and antidepressant response, we used Creb loxP/loxP mice to localize CREB deletion to the hippocampus. In an assay sensitive to chronic antidepressant response, the novelty-induced hypophagia procedure, hippocampal CREB deletion, did not alter the response to chronic antidepressant treatment. In contrast, mice with hippocampal CREB deletion responded to acute antidepressant treatment in this task, and this accelerated response was accompanied by an increase in hippocampal neurogenesis. Upregulation of the CREB-family protein cAMP response-element modulator (CREM) was observed after CREB deletion. Viral overexpression of the activator isoform of CREM, CREM, in the hippocampus also resulted in an accelerated response to antidepressants as well as increased hippocampal neurogenesis. This is the first demonstration of CREM within the brain playing a role in behavior and specifically in behavioral outcomes following antidepressant treatment. The current results suggest that activation of CREM may provide a means to accelerate the therapeutic efficacy of current antidepressant treatment.

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