Astragaloside IV suppressed hippocampal GABAergic synaptic transmission and enhanced memory through EGR-1 mediated BDNF/TrkB signaling pathway in mice

Background: Astragaloside IV (ASIV) is one of the saponins isolated from Astragalus membranaceus, a widely used traditional Chinese medicine and a health product sold all over the world. However, so far, the effect of ASIV on GABAergic synaptic transmission has not been elucidated yet. In the present study, the effect of ASIV on memory and hippocampal GABAergic synaptic transmission was investigated in wild type and early growth response protein 1 (EGR-1) knockout mice. Methods: Behavioral tests including radial-arm maze test and shuttle-box test, liquid chromatography-tandem mass spectrometry, western blotting analysis, quantitative PCR, electrophysiological recording, and electron microscopy were used in this study. Results: ASIV was shown to enhance the learning and memory of mice in behavioral tests, such as radial-arm maze test and shuttle-box test. It significantly reduced the concentration of GABA, the expression of glutamate decarboxylase 2 (GAD65) as well as the ratio of inhibitory synapses in mouse hippocampus, which was accompanied with a suppression of hippocampal spontaneous inhibitory postsynaptic currents. ASIV administration decreased the expression of EGR-1, brain-derived neurotrophic factor (BDNF) and tyrosine receptor kinase B (TrkB) in the hippocampus. Furthermore, blockage of BDNF/TrkB signaling with K-252a abrogated the effect of ASIV on GAD65 expression. When EGR-1 was knocked out, the promotive effects of ASIV on learning and memory, as well as the inhibitory effects on GABAergic synaptic transmission and GAD65, BDNF and TrkB expression, were abolished. In addition, ASIV was found to down-regulate the pre-existing EGR-1 baseline to better adapt to the learning stimuli. Conclusions: Together, these results demonstrated a novel role of ASIV in enhancing memory and reducing hippocampal GABAergic synaptic transmission through EGR-1 mediated BDNF/TrkB signaling pathway in mice.

Mu-opioids suppress GABAergic synaptic transmission onto orbitofrontal cortex pyramidal neurons with subregional selectivity

The orbitofrontal cortex (OFC) plays a critical role in evaluating outcomes in a changing environment. Administering opioids to the OFC can alter the hedonic reaction to food rewards and increase their consumption in a subregion specific manner. However, it is unknown how mu-opioid signalling influences synaptic transmission in the OFC. Thus, we investigated the cellular actions of mu-opioids within distinct subregions of the OFC. Using in-vitro patch clamp electrophysiology in brain slices containing the OFC, we found that the mu-opioid agonist, DAMGO produced a concentration-dependant inhibition of GABAergic synaptic transmission onto medial OFC (mOFC), but not lateral OFC (lOFC) neurons. This effect was mediated by presynaptic mu-opioid receptor activation of local parvalbumin (PV+)-expressing interneurons. The DAMGO-induced suppression of inhibition was long-lasting and not reversed upon washout of DAMGO, or by application of the mu-opioid receptor antagonist, CTAP, suggesting an inhibitory long-term depression (iLTD) induced by an exogenous mu-opioid. We show that LTD at inhibitory synapses is dependent on downstream cAMP/PKA signaling, which differs between the mOFC and lOFC. Finally, we demonstrate that endogenous opioid release triggered via moderate physiological stimulation can induce LTD. Taken together, these results suggest that presynaptic mu-opioid stimulation of local PV+ interneurons induces a long-lasting suppression of GABAergic synaptic transmission, which depends on subregional differences in mu-opioid receptor coupling to the downstream cAMP/PKA intracellular cascade. These findings provide mechanistic insight into the opposing functional effects produced by mu-opioids within the OFC.Significance StatementConsidering that both the OFC and the opioid system regulate reward, motivation, and food intake; understanding the role of opioid signaling within the OFC is fundamental for a mechanistic understanding of the sequelae for several psychiatric disorders. This study makes several novel observations. First, mu-opioids induce a long-lasting suppression of inhibitory synaptic transmission onto OFC pyramidal neurons in a regionally selective manner. Secondly, mu-opioids recruit PV+ inputs to suppress inhibitory synaptic transmission in the mOFC. Thirdly, the regional selectivity of mu-opioid action of endogenous opioids is due to the efficacy of mu-opioid receptor coupling to the downstream cAMP/PKA intracellular cascades. These experiments are the first to reveal a cellular mechanism of opioid action within the OFC.