[Serotonergic neurons in the median raphe nucleus mediate anxiety- and depression-like behavior].

Serotonin (5-hydroxtryptamine, 5-HT), one of the central neurotransmitters, is the most important modulator for emotion regulation, sensory processing, cognitive control, etc. The serotonergic neurons are limited in amount and mainly distributed in the dorsal raphe nucleus (DR) and the median raphe nucleus (MR) in the midline of the brain stem. Previous studies mainly focused on the function of 5-HT neurons in the DR, but little is known about 5-HT neurons in MR. In the present study, with Pet1-Cre transgenic mice and DREADDs technology, we specifically activated or silenced 5-HT neurons in the MR, and aimed to explore their roles in anxiety- and depressive-like behaviors. The results showed that silencing 5-HT neurons in the MR decreased anxiety-like behaviors in the open field and elevated plus maze tasks. Inhibition of 5-HT neurons in the MR decreased depressive-like behaviors in the sucrose preference and forced swim test, while activation of 5-HT neurons in the MR enhanced depressive-like behaviors in the sucrose preference test. These results suggest that the 5-HT neurons in the MR play a key role in regulating anxiety- and depression-like behaviors.

[Optogenetic activation of dorsal hippocampal astrocytic Rac1 blocks the learning of associative memory].

Rac1 belongs to the family of Rho GTPases, and plays important roles in the brain function. It affects the cell migration and axon guidance via regulating the cytoskeleton and cellular morphology. However, the effect of its dynamic activation in regulating physiological function remains unclear. Recently, a photoactivatable analogue of Rac1 (PA-Rac1) has been developed, allowing the activation of Rac1 by the specific wavelength of light in living cells. Thus, we constructed recombinant adeno-associated virus (AAV) of PA-Rac1 and its light-insensitive mutant PA-Rac1-C450A under the control of the mouse glial fibrillary acidic protein (mGFAP) promoter to manipulate Rac1 activity in astrocytes by optical stimulation. Primary culture of hippocampal astrocytes was infected with the recombinant AAV-PA-Rac1 or AAV-PA-Rac1-C450A. Real-time fluorescence imaging showed that the cell membrane of the astrocyte expressing PA-Rac1 protruded near the light spot, while the astrocyte expressing PA-Rac1-C450A did not. We injected AAV-PA-Rac1 and AAV-PA-Rac1-C450A into dorsal hippocampus to investigate the role of the activation of Rac1 in regulating the associative learning. With optical stimulation, the PA-Rac1 group, rather than the PA-Rac1-C450A group, showed slower learning curve during the fear conditioning compared with the control group, indicating that activating astrocytic Rac1 blocks the formation of contextual memory. Our data suggest that the activation of Rac1 in dorsal hippocampal astrocyte plays an important role in the associative learning.

ß-Arrestin-biased signaling mediates memory reconsolidation.

A long-standing hypothesis posits that a G protein-coupled signaling pathway mediates ß-adrenergic nervous system functions, including learning and memory. Here we report that memory retrieval (reactivation) induces the activation of ß1-adrenergic ß-arrestin signaling in the brain, which stimulates ERK signaling and protein synthesis, leading to postreactivation memory restabilization. ß-Arrestin2-deficient mice exhibit impaired memory reconsolidation in object recognition, Morris water maze, and cocaine-conditioned place preference paradigms. Postreactivation blockade of both brain ß-adrenergic Gs protein- and ß-arrestin-dependent pathways disrupts memory reconsolidation. Unexpectedly, selective blockade of the Gs/cAMP/PKA signaling but not the ß-arrestin/ERK signaling by the biased ß-adrenergic ligands does not inhibit reconsolidation. Moreover, the expression of ß-arrestin2 in the entorhinal cortex of ß-arrestin 2-deficient mice rescues ß1-adrenergic ERK signaling and reconsolidation in a G protein pathway-independent manner. We demonstrate that ß-arrestin-biased signaling regulates memory reconsolidation and reveal the potential for ß-arrestin-biased ligands in the treatment of memory-related disorders.

[Neurons in NAc core and BLA are activated during cocaine context-associated reward memory retrieval in mice].

The intense associative memories that develop between cocaine-paired contexts and rewarding stimuli make addiction hard to cure by contributing to cocaine seeking and relapse. So it's of great importance to examine the neurobiological basis of addiction memory. Cocaine conditioned place preference (CPP) used in this study is a form of Pavlovian conditioning which can establish associations between drug and contextual factors. c-Fos and Zif268 are commonly used immediate early gene (IEG) makers to identify neurons that are activated after a stimulus or behavioral conditioning. This study was designed to reveal neuronal c-Fos, Zif268 expression pattern in 10 brain regions following cocaine context-associated reward memory retrieval in mice, combining animal behavioral study and immunofluorescence method. C57BL/6 mice were randomly divided into 3 groups: Saline retrieval, Cocaine retrieval, and No retrieval of cocaine groups. Cocaine retrieval and No retrieval of cocaine underwent CPP training (one side paired with cocaine, and the other side with saline) except that No retrieval of cocaine group didn't undergo CPP test. Saline retrieval group received saline injections (i.p) on both sides. The results showed that: Neuronal c-Fos, Zif268 protein expression levels in nucleus accumbens (NAc) core both were elevated in Cocaine retrieval group compared with those in Saline retrieval (Control) group during cocaine context-associated reward memory retrieval. Zif268 protein expression level in basolateral amygdala (BLA) was also elevated in Cocaine retrieval group compared with that in control mice. Elevation was not seen in other regions such as hippocampus, prefrontal cortex (PFC). Thus, NAc core and BLA were activated during cocaine context-associated reward memory retrieval. The results suggest that neurons that are activated in NAc core and BLA are crucial basis of cocaine context-associated reward memory.

[Voluntary wheel running enhances cell proliferation and expression levels of BDNF, IGF1 and WNT4 in dentate gyrus of adult mice].

Adult hippocampal neurogenesis plays important roles in learning, memory and mood regulation. External factors, such as physical exercise, have been found to modulate adult hippocampal neurogenesis. Voluntary running enhances cell proliferation in subgranular zone (SGZ) and increases the number of new born neurons in rodents, but underlying mechanisms are not fully understood. In this study, we used BrdU assay to identify proliferating cells in 2-month-old C57BL/6 mice after 15 days of voluntary wheel running test. mRNA and protein levels for several neural factors in dentate gyrus, Ammon's horn, and cortex were also analyzed by RT-qPCR and Western blot assay after 15 days of voluntary wheel running. Our data show that voluntary wheel running for 15 days elevated the number of proliferation cells in dentate gyrus and significantly up-regulated the mRNA levels of Bdnf, Igf1 and Wnt4. The protein levels of BDNF and IGF1 in dentate gyrus were also increased after voluntary wheel running. These results indicate that the increase of adult hippocampal neurogenesis caused by voluntary wheel running for 15 days might be through up-regulating BDNF, IGF1 and WNT4 in dentate gyrus.