Search

Dorsal raphe (DR) and median raphe (MR) 5-HT neurons are two distinct sub-systems known to be regulated by 5-HT 1A and 5-HT 1B auto-receptors. Whether the auto-receptors in each sub-system are functionally altered in depressive-like state remains unknown. The present study is aimed to study a specific circuit (DR-ventral hippocampus and MR-dorsal hippocampus) within each sub-system to investigate changes in receptor sensitivity in the pathogenesis of depression. A mouse model of depression was developed through the social defeat paradigm, and was then treated with fluoxetine (FLX). 5-HT 1A auto-receptor in the neuronal cell body (DR or MR) and 5-HT 1B auto-receptor in the axonal terminal (ventral or dorsal hippocampus) were directly targeted by local perfusion of antagonists (5-HT 1A : WAY100635; 5-HT 1B : GR127935) through reverse microdialysis. Time courses of dialysate 5-HT measured at the axonal terminal were subsequently determined for each circuit. At baseline, 5-HT 1A and 5-HT 1B antagonists dose-dependently increased dialysate 5-HT, with sub-circuit specificity. In the depressive-like state, greater increases in dialysate 5-HT were observed only in the DR-ventral hippocampus circuit following local delivery of both antagonists, which were then fully restored following the FLX treatment. In contrast, no changes were observed in the MR-dorsal hippocampus circuit. Our results demonstrate differential changes in sensitivities of 5-HT 1A and 5-HT 1B auto-receptors in the DR-ventral hippocampus and MR-dorsal hippocampus circuits. 5-HT 1A and 5-HT 1B auto-receptors in the DR-ventral hippocampus circuit are sensitized in the depressive-like state. Taken together, these results suggest that the DR sub-system maybe the neural substrate mediating depressive phenotypes., Competing Interests: Declaration of Competing Interest The authors have declared that no competing interests exist., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Corticosterone (CORT) has long been shown to modulate 5-HT system, and to alter hippocampal functions in various physiological and pathological conditions. However, CORT-elicited changes in the hippocampal 5-HT transmission and the immobility phenotype had not been fully addressed. The current study sought to explore effects of acute CORT subcutaneously injected at 10, 20, 40 mg/kg on the extracellular 5-HT in the hippocampus and the immobility time in male CD-1 mice. Following an injection of CORT or vehicle, time course of the extracellular 5-HT in the hippocampal CA3 was determined using in vivo microdialysis. The immobility time was measured at 0 min, 60 min, 120 min, 180 min in the forced swimming test (FST) and the tail suspension test (TST), respectively. Results showed that the vehicle, used to dissolve CORT, did not affect the dialysate 5-HT, nor the immobility time, by comparing the pre- and post-injection. CORT was found to dose-dependently increase the dialysate 5-HT and decrease the immobility time when compared to their vehicle-treated controls. The peak increase in the dialysate 5-HT and the decrease in the immobility time were both obtained at the 120 min following the CORT injection. Furthermore, a negative correlation was detected between the immobility time and the peak increase in the dialysate 5-HT. Our results indicated that acute CORT injection elicits antidepressant-like actions on the hippocampal 5-HT and the immobility time. The study suggested that hippocampal 5-HT responses may be one of the neurochemical bases for the immobility phenotype following CORT injection., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Mouse strain differences in immobility and in sensitivity to antidepressants have been observed in the forced swimming test (FST) and the tail suspension test (TST). However, the neurotransmitter systems and neural substrates that contribute to these differences remain unknown. To investigate the role of the hippocampal serotonin transporter (5-HTT), we measured baseline immobility and the immobility responses to fluoxetine (FLX) in the FST and the TST in male CD-1, C57BL/6, DBA and BALB/c mice. We observed strain differences in baseline immobility time, with CD-1 mice showing the longest and DBA mice showing the shortest. In contrast, DBA and BALB/c mice showed the highest sensitivity to FLX, whereas CD-1 and C57BL/6 mice showed the lowest sensitivity. Also we found strain differences in both the total 5-HTT protein level and the membrane-bound 5-HTT level (estimated by V max) as follows: DBA>BALB/c>CD-1=C57BL/6. The uptake efficiency of the membrane-bound 5-HTT (estimated by 1/K m) was highest in DBA and BALB/c mice and lowest in CD-1 and C57BL/6 mice. A correlation analysis of subregions within the hippocampus revealed that immobility time was negatively correlated with V max and positively correlated with K m in the hippocampus. Therefore a higher uptake capacity of the membrane-bound 5-HTT in the hippocampus was associated with lower baseline immobility and greater sensitivity to FLX. These results suggest that alterations in hippocampal 5-HTT activity may contribute to mouse strain differences in the FST and the TST.

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., (Copyright © 2014 Wiley Periodicals, Inc.)

The serotonin transporter (5-HTT) regulates the extracellular concentration of serotonin, influencing neurotransmission. Evidence suggests that 5-HTT is altered during depression, but the precise changes in 5-HTT expression in the pathogenesis and treatment of depression are not clear. We investigated the protein expression of hippocampal 5-HTT in CD-1 mice exposed to unpredictable chronic mild stress for 10 continuous weeks. Following 6 weeks of the stress procedure, the mice were separated into anhedonic and non-anhedonic groups, which were then treated with fluoxetine (FLX, 10mg/kg/day, i.p.) for 4 weeks. Behavioral state and therapeutic efficacy of the drug treatment were assessed using sucrose preference, physical state of the coat and body weight. Our results show that changes in hippocampal 5-HTT protein expression correlated with stress-induced behavioral states. Decreases in 5-HTT expression were associated with the stress-induced anhedonic state, whereas increases were associated with the stress-induced non-anhedonic state. Following FLX treatment, the changes in 5-HTT expression were reversed in a subpopulation of anhedonic mice, i.e., the treatment-responsive anhedonic mice. The treatment did not alter the changes in the treatment-resistant anhedonic mice or in the non-anhedonic mice. The data indicate that down-regulation of hippocampal 5-HTT protein expression is a signature change associated with anhedonia, a key endophenotype of clinical depression. Differential changes in 5-HTT expression may contribute to variations in the susceptibility to anhedonia., (Copyright © 2013 Elsevier B.V. All rights reserved.)

Hippocampal responses to selective 5-HT reuptake inhibitor (SSRI) have long been studied. However, its sub-regional involvements in mediating SSRI's pharmacological effects have not been fully addressed. The current study sought to investigate neurochemical, neurobiological and neurobehavioral changes in response to direct fluoxetine perfusion into the ventral and dorsal sub-regions of the hippocampus in C57BL/6 mice. Following fluoxetine perfusion, time courses of dialysate 5-HT, 5-HT transporter (5-HTT) protein (total, membrane and cytoplasmic fractions), locomotion, and immobility times in the forced swim test (FST) and tail suspension test (TST) were determined. At baseline, 5-HT uptake efficiency assessed by the no-net-flux microdialysis, and 5-HTT protein were measured as well. Results show that fluoxetine dose-dependently increased dialysate 5-HT, lowered membrane 5-HTT protein and increased cytoplasmic fraction without changing the total level, decreased immobility times in both the FST and TST, with greater responses all detected in the ventral sub-region compared to the dorsal sub-region. Fluoxetine didn't affect locomotor activity, ruling out the possibility that fluoxetine's effects on immobility maybe due to alteration in locomotion. Besides, lower 5-HT uptake efficiency and lower membrane 5-HTT protein level were found in the ventral sub-region at baseline. Together, the sub-regional differences at baseline and in responses to fluoxetine added powerful evidence to support the existence of two distinct 5-HT sub-systems in the hippocampus, with greater changes to fluoxetine detected in the ventral sub-system. [Display omitted] • Fluoxetine dose-dependently increases hippocampal 5-HT, with greater responses detected in the ventral sub-region. • The Fluoxetine decreases membrane-bound 5-HTT and increases cytoplasmic 5-HTT without altering the total level in both sub-regions. • Fluoxetine elicits dose-dependent decreases in immobility time, with greater changes found in the ventral sub-region. • At baseline, lower 5-HT uptake efficiency and lower membrane-bound 5-HTT protein level were found in the ventral sub-region. [ABSTRACT FROM AUTHOR]