Quantitation of 5HT3 receptors in forebrain of serotonin transporter deficient mice

Summary. Mice deficient in the serotonin transporter (5HTT) display highly elevated extracellular 5HT levels. 5HT exerts ist effects via at least fourteen different cloned 5HT receptors located pre- and postsynaptically. In contrast to the other 5HT receptors, the 5HT3 receptor is a ionotropic receptor with ligand-gated cation channel function. Since G-protein-coupled 5HT receptors show extensive adaptive changes in 5HTT-deficient mice, we investigated whether 5HT3 receptors are also altered in these mice. Using quantitative autoradiography, we found that 5HT3 receptors are upregulated in frontal cortex (+46%), parietal cortex (+42%), and in stratum oriens of the CA3 region of the hippocampus (+18%) of 5HTT knockout mice. Changes in 5HT3 receptor mRNA expression, as determined by quantitative in situ hybridisation, were less pronounced. The adaptive changes of 5HT3 receptor expression constitute a part of the complex regulatory pattern of 5HT receptors in 5HTT knockout mice.     

Increased hippocampal DNA oxidation in serotonin transporter deficient mice

Summary. The serotonin transporter (5HTT) is the molecule responsible for the high-affinity reuptake of 5HT from the synaptic cleft. Mice lacking the 5HTT exhibit highly elevated extracellular concentrations of 5HT. We assessed whether the glutathione detoxification system is altered in 5HTT-deficient mice. While levels of reduced and oxidized glutathione were unchanged, glutathione metabolising enzymes showed a differential pattern of modulation. Glutathione peroxidase was reduced in frontal cortex, brainstem, and cerebellum of 5HTT-deficient mice, though not to a statistically significant extent, while a putative isoform of the detoxifying enzyme glutathione-S-transferase pi was decreased in a number of brain regions, especially in brainstem. At the level of the DNA, we found an increase of oxidative DNA adducts in the hippocampus of 5HTT-deficient mice. Given the importance of the hippocampus in learning and memory, this may be the most important neurochemical consequence of the absence of the 5HTT. Received January 7, 2002; accepted February 4, 2002     

Decreased anxiety in mice lacking the organic cation transporter 3

The organic cation transporter 3 (OCT3; synonymous: extraneuronal monoamine transporter, EMT, Slc22a3) encodes an isoform of the organic cation transporters and is expressed widely across the whole brain. OCTs are a family of high-capacity, bidirectional, multispecific transporters of organic cations. These also include serotonin, dopamine and norepinephrine making OCTs attractive candidates for a variety of neuropsychiatric disorders including anxiety disorders. OCT3 has been implicated in termination of monoaminergic signalling in the central nervous system. Interestingly, OCT3 mRNA is however also significantly up-regulated in the hippocampus of serotonin transporter knockout mice where it might serve as an alternative reuptake mechanism for serotonin. The examination of the behavioural phenotype of OCT3 knockout mice thus is paramount to assess the role of OCT3. We have therefore subjected mice lacking the OCT3 gene to a comprehensive behavioural test battery. While cognitive functioning in the Morris water maze test and aggression levels measured with the resident–intruder paradigm were in the same range as the respective control animals, OCT3 knockout animals showed a tendency of increased activity and were significantly less anxious in the elevated plus-maze test and the open field test as compared to their respective wild-type controls arguing for a role of OCT3 in the regulation of fear and anxiety, probably by modulating the serotonergic tone in limbic circuitries.     

The contribution of low-affinity transport mechanisms to serotonin clearance in synaptosomes

Although many studies assert that the serotonin (5-HT) transporter (SERT) is the predominant mechanism controlling extracellular 5-HT concentrations, accumulating evidence suggests that low affinity, high capacity transport mechanisms may contribute more to 5-HT clearance than previously thought. The goal of this study was to quantify the contributions of SERT relative to other mechanisms in clearing extracellular 5-HT concentrations ranging from 50 nM to 1 μM in synaptosomes prepared from wild-type and SERT knockout mice using rotating disk electrode voltammetry. SERT inhibitors combined with decynium-22 (D-22), a blocker of several low-affinity transporters, blocked all uptake of 5-HT into synaptosomes. We found that SERT is responsible for the majority of synaptosomal uptake only at relatively low 5-HT concentrations, but comprises a diminishing proportion of 5-HT clearance when extracellular 5-HT increases above 100 nM. The effect of D-22 was similar in wild-type and SERT knockout synaptosomes. Thus, there was no evidence of upregulation of low-affinity mechanisms in knockout mice across the concentrations of 5-HT tested. These are surprising results, in light of the prevailing view that SERT is the primary uptake mechanism for extracellular 5-HT at physiological concentrations. We conclude that non-SERT mediated 5-HT uptake is substantial even at modest 5-HT concentrations. These findings, in conjunction with other studies, have important implications for understanding serotonergic disorders and may explain the variable efficacy and stability of patients' responses to antidepressants, such as the selective serotonin reuptake inhibitors.     

Influence of chronic amphetamine treatment and acute withdrawal on serotonin synthesis and clearance mechanisms in the rat ventral hippocampus

Amphetamine withdrawal in both humans and rats is associated with increased anxiety states, which are thought to contribute to drug relapse. Serotonin in the ventral hippocampus mediates affective behaviors, and reduced serotonin levels in this region are observed in rat models of high anxiety, including during withdrawal from chronic amphetamine. This goal of this study was to understand the mechanisms by which reduced ventral hippocampus serotonergic neurotransmission occurs during amphetamine withdrawal. Serotonin synthesis (assessed by accumulation of serotonin precursor as a measure of the capacity of in vivo tryptophan hydroxylase activity), expression of serotonergic transporters, and in vivo serotonergic clearance using in vivo microdialysis were assessed in the ventral hippocampus in adult male Sprague Dawley rats at 24 h withdrawal from chronic amphetamine. Overall, results showed that diminished extracellular serotonin at 24 h withdrawal from chronic amphetamine was not accompanied by a change in capacity for serotonin synthesis (in vivo tryptophan hydroxylase activity), or serotonin transporter expression or function in the ventral hippocampus, but instead was associated with increased expression and function of organic cation transporters (low‐affinity, high‐capacity serotonin transporters). These findings suggest that 24 h withdrawal from chronic amphetamine reduces the availability of extracellular serotonin in the ventral hippocampus by increasing organic cation transporter‐mediated serotonin clearance, which may represent a future pharmacological target for reversing anxiety states during drug withdrawal.     

Local inhibition of organic cation transporters increases extracellular serotonin in the medial hypothalamus

In the rat dorsomedial hypothalamus (DMH), serotonin (5-HT) concentrations are altered rapidly in response to acute stressors. The mechanism for rapid changes in 5-HT concentrations in the DMH is not clear. We hypothesize that the mechanism involves corticosteroid-induced alterations in the uptake of 5-HT from extracellular fluid through the action of corticosterone-sensitive organic cation transporters (OCTs). To determine if OCTs affect the clearance of 5-HT from the extracellular fluid compartment within the medial hypothalamus (MH), the OCT blocker, decynium 22 (0, 10, 30, or 100 microM), was perfused into the MH via a microdialysis probe, and dialysate 5-HT concentrations were measured at 20 min intervals. In addition, home cage behavior was measured both before and after drug administration. Inhibition of OCTs in the MH resulted in a reversible dose-dependent increase in extracellular 5-HT concentration. Increases in extracellular 5-HT concentrations were associated with increases in grooming behavior in rats treated with the highest concentration of decynium 22. No other behavioral responses were observed following administration of any concentration of decynium 22. These findings are consistent with the hypothesis that OCTs in the MH play an important role in the regulation of serotonergic neurotransmission and specific behavioral responses. Because the MH plays an important role in the neuroendocrine, autonomic, and behavioral responses to stress-related stimuli, these data lead to new questions regarding the role of interactions between corticosterone and corticosterone-sensitive OCTs in stress-induced 5-HT accumulation within the MH as well as the physiological and behavioral consequences of these interactions.     

Role of acetylcholine and muscarinic receptors in serotonin-induced bronchoconstriction in the mouse

For the murine trachea, it has been reported that constriction evoked by serotonin (5-HT) is largely dependent on acetylcholine (ACh) released from the epithelium, owing to the sensitivity of the 5-HT response to epithelium removal, sensitivity to atropine, and insensitivity to tetrodotoxin (Moffatt et al., 2003). Consistent with this assumption, the respiratory epithelium contains ACh, its synthesizing enzyme, and the high-affinity choline transporter CHT1 (Reinheimer et al., 1996; Pfeil et al., 2003; Proskocil et al., 2004). Recently, we demonstrated that ACh can be released from non-neuronal cells by corticosteroid-sensitive polyspecific organic cation transporters (OCTs), which are also expressed by airway epithelial cells (Lips et al., 2005). Hence, we proposed that 5-HT evokes release of ACh from epithelial cells via OCTs and that this epithelial-derived ACh induces bronchoconstriction. We tested this hypothesis in a well-established model of videomorphometric analysis of bronchial diameter in precision-cut murine lung slices utilizing epithelium removal to assess the role of the epithelium, OCT mouse knockout (KO) strains to assess the role of OCT isoforms, and muscarinic receptor M2/M3 double-KO mice to assess the cholinergic component of 5-HT induced bronchoconstriction, as bronchi of this strain are entirely unresponsive to cholinergic stimulation(Struckmann et al., 2003).     

Subregion‐specific decreases in hippocampal serotonin transporter protein expression and function associated with endophenotypes of depression

Stress influences the development of depression, and depression is associated with structural and functional changes in the hippocampus. The current study sought to determine whether chronic corticosteroid (CORT) treatment influences serotonin transporter (5‐HTT) protein expression and function in the CA1, CA3, and dentate gyrus (DG) subregions of the hippocampus. Male CD‐1 mice were subcutaneously injected with CORT at a dose of 20 mg/kg once daily for 3 weeks. Behavioral state was assessed using sucrose preference, physical state of the coat, forced swimming test, and tail suspension test. We then determine 5‐HTT protein expression and synaptosomal 5‐HT uptake in the CA1, CA3 and DG subregions. CORT treatment induced anhedonia and behavioral despair, two core endophenotypes of clinical depression; 5‐HTT protein expression levels and synaptosomal 5‐HT uptake were both decreased in a subregion‐specific manner, with the greatest decrease observed in the DG, a moderate decrease in the CA3, and the CA1 showed no apparent change. In addition, a reduction in tissue mass was detected in the DG following the CORT treatment. These data indicate that subregion‐specific decreases in hippocampal 5‐HTT protein expression and function are associated with endophenotypes of depression. © 2014 Wiley Periodicals, Inc.     

Loss of brain-derived neurotrophic factor gene allele exacerbates brain monoamine deficiencies and increases stress abnormalities of serotonin transporter knockout mice

To study the neurochemical and behavioral effects of altered brain-derived neurotrophic factor (BDNF) expression on a brain serotonin system with diminished serotonin transport capability, a double-mutant mouse model was developed by interbreeding serotonin transporter (SERT) knockout mice with BDNF heterozygous knockout mice (BDNF +/-), producing SERT -/- x BDNF +/- (sb) mice. Prior evidence implicates serotonin and SERT in anxiety and stress responses. Some studies have shown that BDNF supports serotonergic neuronal development, leading to our hypothesis that reduced BDNF availability during development might exaggerate the consequences of absent SERT function. In the present study, brain serotonin and 5-hydroxyindol acetic acid concentrations in male sb mice were significantly reduced in the hippocampus and hypothalamus compared with wild-type control SB mice, BDNF-deficient Sb mice, and serotonin transporter knockout sB mice. The sb mice had significantly increased anxiety-like behaviors compared with SB, Sb, and sB mice as measured on the elevated plus maze test. These sb mice also had significantly greater increases in plasma adrenocorticotrophic hormone than mice with other genotypes after a stressful stimulus. Analysis of neuronal morphology showed that hypothalamic and hippocampal neurons exhibited 25-30% reductions in dendrites in sb mice compared with SB control mice. These findings support the hypothesis that genetic changes in BDNF expression interact with serotonin and other circuits that modulate anxiety and stress-related behaviors. Thus, this double-mutant mouse model should prove valuable in studying other gene x gene consequences for brain plasticity as well as in evaluating epistatic interactions of BDNF and serotonin transporter gene polymorphisms in neuropsychiatric disorders.     

Distribution of organic cation transporter 3, a corticosterone‐sensitive monoamine transporter,in the rat brain

Organic cation transporter 3 (OCT3) is a high‐capacity, low‐affinity transporter that mediates bidirectional, sodium‐independent transport of dopamine, norepinephrine, epinephrine, serotonin, and histamine. OCT3‐mediated transport is directly inhibited by corticosterone, suggesting a potential role for the transporter in mediating some of the effects of stress and glucocorticoids on monoaminergic neurotransmission. To elucidate the importance of OCT3 in clearance of extracellular monoamines in the brain, we used immunohistochemical techniques to describe the distribution of OCT3‐like‐immunoreactive (OCT3‐ir) cells throughout the rostrocaudal extent of adult male rat brains. OCT3‐ir cell bodies were widely distributed throughout the brain, with the highest densities observed in the superior and inferior colliculi, islands of Calleja, subiculum, lateral septum, lateral and dorsomedial hypothalamic nuclei, and granule cell layers of the main and accessory olfactory bulbs, the cerebellum, and the retrosplenial granular cortex. OCT3‐ir cells and/or fibers were also observed in circumventricular organs, and OCT3‐ir ependymal cells were observed in the linings of all cerebral ventricles. The widespread distribution of OCT3‐ir cell bodies, including regions receiving dense monoaminergic projections, suggests an important role for this transporter in regulating extracellular concentrations of monoamines in the rat brain and is consistent with the hypothesis that corticosterone‐induced inhibition of OCT3‐mediated transport may contribute to effects of acute stress or corticosterone on monoaminergic neurotransmission. J. Comp. Neurol. 512:529–555, 2009. © 2008 Wiley‐Liss, Inc.     

Molecular mechanism of histamine clearance by primary human astrocytes

Histamine clearance is an essential process for avoiding excessive histaminergic neuronal activity. Previous studies using rodents revealed the predominant role of astrocytes in brain histamine clearance. However, the molecular mechanism of histamine clearance has remained unclear. We detected histamine N‐methyltransferase (HNMT), a histamine‐metabolizing enzyme, in primary human astrocytes and the astrocytes of human brain specimens. Immunocytochemical analysis and subcellular fractionation assays revealed that active HNMT localized to the cytosol, suggesting that histamine transport into the cytosol is crucial for histamine inactivation. We showed that primary human astrocytes transported histamine in a time‐dependent manner. Kinetics analysis showed that two low‐affinity transporters were involved in histamine transport. Histamine uptake by primary human astrocytes was not dependent on the extracellular Na⁺/Cl^? concentration. Histamine is reported to be a substrate for three low‐affinity and Na⁺/Cl^?‐independent transporters: organic cation transporter 2 (OCT2), OCT3, and plasma membrane monoamine transporter (PMAT). RT‐PCR analysis revealed that OCT3 and PMAT were expressed in primary human astrocytes. Immunohistochemistry confirmed OCT3 and PMAT expression in the astrocytes of human brain specimens. Drug inhibition assays and gene knockdown assays revealed the major contribution of PMAT and the minor contribution of OCT3 to histamine transport. The present study demonstrates for the first time that the molecular mechanism of histamine clearance is by primary human astrocytes. These findings might indicate that PMAT, OCT3 and HNMT in human astrocytes play a role in the regulation of extraneuronal histamine concentration and the activities of histaminergic neurons.     

Constitutive plasma membrane monoamine transporter (PMAT,Slc29a4) deficiency subtly affects anxiety‐like and coping behaviours

Originally, uptake‐mediated termination of monoamine (e.g., serotonin and dopamine) signalling was believed to only occur via high‐affinity, low‐capacity transporters (“uptake₁”) such as the serotonin or dopamine transporters, respectively. Now, the important contribution of a second low‐affinity, high‐capacity class of biogenic amine transporters has been recognised, particularly in circumstances when uptake₁ transporter function is reduced (e.g., antidepressant treatment). Pharmacologic or genetic reductions in uptake₁ function can change locomotor, anxiety‐like or stress‐coping behaviours. Comparable behavioural investigations into reduced low‐affinity, high‐capacity transporter function are lacking, in part, due to a current dearth of drugs that selectively target particular low‐affinity, high‐capacity transporters, such as the plasma membrane monoamine transporter. Therefore, the most direct approach involves constitutive genetic knockout of these transporters. Other groups have reported that knockout of the low‐affinity, high‐capacity organic cation transporters 2 or 3 alters anxiety‐like and stress‐coping behaviours, but none have assessed behaviours in plasma membrane monoamine transporter knockout mice. Here, we evaluated adult male and female plasma membrane monoamine transporter wild‐type, heterozygous and knockout mice in locomotor, anxiety‐like and stress‐coping behavioural tests. A mild enhancement of anxiety‐related behaviour was noted in heterozygous mice. Active‐coping behaviour was modestly and selectively increased in female knockout mice. These subtle behavioural changes support a supplemental role of plasma membrane monoamine transporter in serotonin and dopamine uptake, and suggest sex differences in transporter function should be examined more closely in future investigations.     

Serotonin fibre sprouting and increase in serotonin transporter immunoreactivity in the CA1 area of hippocampus in a triple transgenic mouse model of Alzheimer’s disease

Alzheimer’s disease (AD) is a neurodegenerative disease that deteriorates cognitive functions and associated brain regions such as the hippocampus, being the primary cause of dementia. Serotonin (5‐HT) is widely present in the hippocampus, being an important neurotransmitter involved in learning and memory. Although recent evidence suggests alterations in 5‐HT neurotransmission in AD, it is not clear how hippocampal 5‐HT innervation is modified. Here, we studied hippocampal 5‐HT innervation by analysing: (i) the expression, density and distribution of 5‐HT transporter (SERT)‐immunoreactive fibres; (ii) the specific morphological characteristics of serotonergic fibres and their relation to amyloid plaques; and (iii) the total number of 5‐HT neurons within the raphe nuclei in triple transgenic mouse model of AD. We used quantitative light microscopy immunohistochemistry comparing transgenic and non‐transgenic animals of different ages (3, 6, 9, 12 and 18 months). The transgenic animals showed a significant increase in SERT fibres in the hippocampus in a subfield‐, strata‐ and age‐specific manner. The increase in SERT fibres was specific to the CA1 stratum lacunosum‐moleculare. An increase in SERT fibres in transgenic animals was observed at 3 months (by 61%) and at 18 months (by 74%). No changes, however, were found in the total number of raphe 5‐HT neurons at any age. Our results indicate that triple transgenic mice display changes in the expression of SERT and increased SERT fibres sprouting, which may account for imbalanced serotonergic neurotransmission associated with (or linked to) AD cognitive impairment.     

Exploring the relationship between serotonin and brain-derived neurotrophic factor: analysis of BDNF protein and extraneuronal 5-HT in mice with reduced serotonin transporter or BDNF expression

Serotonin (5-HT) has been proposed to promote neuronal plasticity during the treatment of mood and anxiety disorders and following neurodegenerative insult by altering the expression of critical genes including brain-derived neurotrophic factor (BDNF). In this study, mice with constitutive reductions in the serotonin transporter (SERT) or BDNF were investigated to further assess the functional relationship between serotonin neurotransmission and BDNF expression. Using a modified extraction procedure and a commercial enzyme-linked immunosorbant assay, 50% decreases in BDNF protein in hippocampus, frontal cortex and brain stem were confirmed in 4-month-old mice lacking one copy of the BDNF gene (BDNF^+/−). By contrast, 4-month-old male and female mice with partial (SERT^+/−) or complete (SERT^−/−) reductions in SERT expression showed no differences in BDNF protein levels compared to SERT^+/+ mice, although male SERT knockout mice of all genotypes had higher BDNF levels in hippocampus, frontal cortex, and brain stem than female animals. Microdialysis also was performed in BDNF^+/− mice. In addition to other phenotypic aspects suggestive of altered serotonin neurotransmission, BDNF^+/− mice show accelerated age-related degeneration of 5-HT forebrain innervation. Nevertheless, extracellular 5-HT levels determined by zero net flux microdialysis were similar between BDNF^+/+ and BDNF^+/− mice in striatum and frontal cortex at 8–12 months of age. These data illustrate that a 50% decrease in BDNF does not appear to be sufficient to cause measurable changes in basal extracellular 5-HT concentrations and, furthermore, that constitutive reductions in SERT expression are not associated with altered BDNF protein levels at the ages and in the brain regions examined in this study.     

Analysis of the gene structure of the human (SLC22A3) and murine (Slc22a3) extraneuronal monoamine transporter

Summary. The organic cation transporter 3 (OCT3), also termed as extraneuronal monoamine transporter (EMT), is known to be expressed in glial cells where it is responsible for the uptake of catecholamines and neurotoxic organic cations such as 1-methyl-4-phenylpyridinium (MPP⁺). We have now analyzed the structure of the human and murine OCT3 gene. The coding regions of both genes consist of 11 exons and 10 introns. All exon-intron junctions contain fully conserved gt/ag consensus splice sites. The human introns are without exception larger than their murine counterparts. In both genes, the introns, apart from intron 1, are located at the same position. Mouse and human exons have the same size with exception of exon 1 which is 15 bp larger in the human gene. The organization of the human OCT3 gene also shows pronounced similarities with other genes of human organic cation transporters such as those for hOCT1, hOCTN2, hORCTL3, and hORCTL4. The genes of these transporters share about the same exon-intron structure and exon sizes, indicating that the genes may have evolved from a common anchestor gene through duplication. Knowledge of the human gene structure of the OCT3 should enable investigations of possible polymorphisms and their involvement in e.g. psychiatric disorders; and knowledge of the mouse exon-intron organization is essential for generating a knock-out mouse which should help to recognize the physiological importance of the OCT3. Received May 26, 2000; accepted June 21, 2000     

Glutamate transporter type 3 knockout reduces brain tolerance to focal brain ischemia in mice

Excitatory amino-acid transporters (EAATs) transport glutamate into cells under physiologic conditions. Excitatory amino-acid transporter type 3 (EAAT3) is the major neuronal EAAT and also uptakes cysteine, the rate-limiting substrate for synthesis of glutathione. Thus, we hypothesize that EAAT3 contributes to providing brain ischemic tolerance. Male 8-week-old EAAT3 knockout mice on CD-1 mouse gene background and wild-type CD-1 mice were subjected to right middle cerebral artery occlusion for 90 minutes. Their brain infarct volumes, neurologic functions, and brain levels of glutathione, nitrotyrosine, and 4-hydroxy-2-nonenal (HNE) were evaluated. The EAAT3 knockout mice had bigger brain infarct volumes and worse neurologic deficit scores and motor coordination functions than did wild-type mice, no matter whether these neurologic outcome parameters were evaluated at 24 hours or at 4 weeks after brain ischemia. The EAAT3 knockout mice contained higher levels of HNE in the ischemic penumbral cortex and in the nonischemic cerebral cortex than did wild-type mice. Glutathione levels in the ischemic and nonischemic cortices of EAAT3 knockout mice tended to be lower than those of wild-type mice. Our results suggest that EAAT3 is important in limiting ischemic brain injury after focal brain ischemia. This effect may involve attenuating brain oxidative stress.     

Reduced programmed cell death in brains of serotonin transporter knockout mice

Serotonin (5-HT) is known to reduce apoptosis and in rodent models of brain ischemia. Modulation of programmed cell death during neural development was assessed in early postnatal brains of serotonin transporter (5-HTT) knockout mice, characterized by elevated extracellular 5-HT levels. The number of apoptotic cells visualized at postnatal day-1 (P1) by ISEL+ or TUNEL staining was significantly reduced in the striatum, thalamus/hypothalamus, cerebral cortex and hippocampus of 5-HTT knockout mice, compared to wild type and heterozygote mice, with differences displaying an increasing fronto-caudal gradient and regional specificity. These findings underscore 5-HT roles in the regulation of programmed cell death during brain development, and spur interest into pharmacological interventions aimed at relieving pathological apoptosis by potentiating serotoninergic neurotransmission.     

Monoaminergic compensation in the neuropeptide Y deficient mouse brain

Neuropeptide Y (NPY) is an important central regulator of food consumption and energy expenditure via the hypothalamus. NPY containing neurons have a broad central distribution and are often colocalized with norepinephrine (NE). However, NPY deficient mice do not exhibit any substantial changes in food consumption, body weight or body composition when compared to wild type mice. Since NE and serotonin (5HT) are also important regulators of appetite and metabolism, we evaluated these systems in NPY deficient mice. Brain sections from NPY deficient and wild type mice were labeled with either (3)H-nisoxetine for the NE transporter (NET) or (3)H-citalopram for the 5HT transporter (SERT). Tyrosine hydroxylase expression was evaluated by radioimmunohistochemistry. Brain monoamines and metabolites were evaluated using HPLC. NPY deficient mice exhibited a substantial decrease in NET binding in most brain regions examined. NET binding was less than 50% of control binding in the cerebral cortex and subregions of the thalamus with the greatest decrease seen in the hypothalamus. In contrast, more modest and regionally variable changes were observed in the SERT binding with decreases in regions such as the accessory olfactory nucleus, glomerular layer of the olfactory bulb and the CA1 region of the hippocampus. Measurement of NE and 5HT content as well as the primary metabolites revealed increased NE turnover and decreased 5HT content in the hypothalamus. Therefore, developmental compensation by the NE and 5HT systems may contribute to the absence of a body weight phenotype in NPY deficient mice.     

Brain serotonin transporter is associated with cognitive‐affective biases in healthy individuals

Cognitive affective biases describe the tendency to process negative information or positive information over the other. These biases can be modulated by changing extracellular serotonin (5‐HT) levels in the brain, for example, by pharmacologically blocking and downregulating the 5‐HT transporter (5‐HTT), which remediates negative affective bias. This suggests that higher levels of 5‐HTT are linked to a priority of negative information over positive, but this link remains to be tested in vivo in healthy individuals. We, therefore, evaluated the association between 5‐HTT levels, as measured with [¹¹C]DASB positron emission tomography (PET), and affective biases, hypothesising that higher 5‐HTT levels are associated with a more negative bias. We included 98 healthy individuals with measures of [¹¹C]DASB binding potential (BP_ND) and affective biases using The Emotional Faces Identification Task by subtracting the per cent hit rate for happy from that of sad faces (EFIT_AB). We evaluated the association between [¹¹C]DASB BP_ND and EFIT_AB in a linear latent variable model, with the latent variable (5‐HTT_LV) modelled from [¹¹C]DASB BP_ND in the fronto‐striatal and fronto‐limbic networks implicated in affective cognition. We observed an inverse association between 5‐HTT_LV and EFIT_AB (β = −8% EFIT_AB per unit 5‐HTT_LV, CI = −14% to −3%, p = .002). These findings show that higher 5‐HTT levels are linked to a more negative bias in healthy individuals. High 5‐HTT supposedly leads to high clearance of 5‐HT, and thus, a negative bias could result from low extracellular 5‐HT. Future studies must reveal if a similar inverse association exists in individuals with affective disorders.     

Release and uptake of catecholamines in the bed nucleus of the stria terminalis measured in the mouse brain slice

The release and clearance of electrically evoked catecholamine (CA) in the ventral portion of the bed nucleus of the stria terminalis (BSTV) in mouse brain slices was evaluated with fast-scan cyclic voltammetry at carbon-fiber microelectrodes (CFME). Uptake in this region was observed to be markedly slower than in the caudate putamen (CPu). Clearance rates were reduced in the BSTV in both norepinephrine transporter knockout (NET KO) and dopamine transporter knockout (DAT KO) mice when compared to results in wild-type (WT) mice. However, uptake was faster in the BSTV in both the DAT and NET KO mice than in the CPu of DAT KO mice. This indicates that both transporters play a role in CA clearance in the BSTV. The transporters restrict extracellular CA to the general area of the BSTV, as revealed by the diminished signal as the CFME is moved sequentially further and further from the site where CA release is evoked. However, in slices from the DAT KOs and NET KOs, CA release could be observed outside of the BSTV region during such experiments. These results show that the low rate uptake in the BSTV facilitates extrasynaptic diffusion of catecholamine, but that uptake still regulates and limits the range of the transmitter to the region. Slower clearance from the extracellular fluid allows the released CA to act as a volume transmitter and diffuse to distant sites within the region to exert its neurochemical action.