| Summary: | 碩士 === 國立陽明大學 === 神經科學研究所 === 102 === Pharmacology studies have implicated the role of Norepinephrine (NE) in the emotional memory via β-adrenoceptor (β-AR). By in situ X-gal staining of brains obtained from Adra2c-KO/lacZ-KI (Adra2c-KO) mouse model, α2C-adrenoceptor (α2C-AR) is mostly expressed in the limbic brain. We therefore would like to investigate whether other adrenoceptor (AR) subtypes are also involved in emotion regulation and the impact of α2C-AR deficiency in fear conditioning; a behavioral paradigm of emotion.
Pavlovian Fear Conditioning paradigm employed in the present study is composed of (i) a training phase (acquisition); (ii) the first memory retrieval phase (day 2 and day 30 post-training) and (iii) the second memory retrieval phase (day 65, mice were subjected to an immobilization stress for 6 days before the behavioral test). Freezing behavior (% freezing) was used as an index of context- and cue-association fear memory. The behavior results indicate:
(1) According to the first memory retrieval test, α2C-AR deficient mice retain equivalent ability in maintaining both context- and cue-association fear memory as WT (C57BL/6) mice tested on day 2 and day 30 post-training. And α2C-AR deficient mice exhibit better cue-association fear memory than control mice at day 30 post-training; WT: 80% vs. KO: 91%. Significant genotype difference exists.
(2) After the second memory retrieval test, both WT and Adra2c-KO mice causes a significant reduction in context-associated freezing; WT: 27% vs. KO: 25%. However, no significant difference was noted between genotypes.
(3) Chronic Stress causes a significant reduction in cue-associated freezing for both genotypes; WT: 43% vs. KO: 27%. Significant genotype difference exists.
Further, focus on the serum level of corticosteroid (CORT) and the glucocorticoid receptor (GR) expression in the limbic brain after chronic stress:
(4) At the baseline condition, the serum [CORT] (CORT concentration) of α2C-AR deficient mice is lower than WT mice. Significant genotype difference exists. (baseline level of [CORT], WT: 11 µg/dl vs. KO: 7 µg/dl)
(5) The deficiency of α2C-AR affects the serum [CORT] regulation under chronic stress condition at day 5. Lower serum [CORT] was found in Adra2c-KO mice compared with WT mice; WT: 27 µg/dl vs. KO: 19 µg/dl. Significant genotype difference exists.
(6) Chronic stress affected amygdala’s glucocorticoid receptor (GR) protein expression level in WT mice, but was not found in Adra2c-KO mice. (the decreased ratio after chronic stress condition, WT: 0.21 vs. KO: 0.06 )
(7) The protein level of GR decreased after chronic stress in hypothalamus (HTM) for both genotypes. Significant genotype difference exists. (normalized ratio, WT: 0.63 vs. KO: 0.97 )
To find out the possible signaling pathway of α2C-AR deficient mice:
(8) Significant genotype difference of ERK1/2 and p-ERK1/2 protein level exists in the preliminary immunohistochemistry (IHC) data, obtained from (dorsal and ventral) hippocampus and amygdala in both genotypes.
(9) The phosphorelation ratio of p-ERK2 increaseded after chronic stress in medial prefrontal cortex (normalized p-ERK2 phosphorelate ratio, WT: +0.12; KO: +1.05) and amygdala (normalized p-ERK2 phosphorelate ratio, WT: +0.1; KO: +0.36). And significant genotypic difference is observed in the basal level of p-ERK2 phosphorelation activity in mPFC and AG. (the decreased normalized phosphorelate ratio of p-ERK2 under baseline condition , mPFC: -0.89; AG: -0.3)
To sum up, the genotypic differences observed in the behavioral and the stress hormone CORT level of the α2C-AR deficient mice. Which, might caused by the protein level difference of GR and the p-ERK1/2 expression in the limbic brain area.
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