Hippocampal µ-opioid receptors on GABAergic neurons mediate stress-induced impairment of memory retrieval.

Stressful life events induce abnormalities in emotional and cognitive behaviour. The endogenous opioid system plays an essential role in stress adaptation and coping strategies. In particular, the µ-opioid receptor (µR), one of the major opioid receptors, strongly influences memory processing in that alterations in µR signalling are associated with various neuropsychiatric disorders. However, it remains unclear whether µR signalling contributes to memory impairments induced by acute stress. Here, we utilized pharmacological methods and cell-type-selective/non-cell-type-selective µR depletion approaches combined with behavioural tests, biochemical analyses, and in vitro electrophysiological recordings to investigate the role of hippocampal µR signalling in memory-retrieval impairment induced by acute elevated platform (EP) stress in mice. Biochemical and molecular analyses revealed that hippocampal µRs were significantly activated during acute stress. Blockage of hippocampal µRs, non-selective deletion of µRs or selective deletion of µRs on GABAergic neurons (µRGABA) reversed EP-stress-induced impairment of memory retrieval, with no effect on the elevation of serum corticosterone after stress. Electrophysiological results demonstrated that stress depressed hippocampal GABAergic synaptic transmission to CA1 pyramidal neurons, thereby leading to excitation/inhibition (E/I) imbalance in a µRGABA-dependent manner. Pharmaceutically enhancing hippocampal GABAA receptor-mediated inhibitory currents in stressed mice restored their memory retrieval, whereas inhibiting those currents in the unstressed mice mimicked the stress-induced impairment of memory retrieval. Our findings reveal a novel pathway in which endogenous opioids recruited by acute stress predominantly activate µRGABA to depress GABAergic inhibitory effects on CA1 pyramidal neurons, which subsequently alters the E/I balance in the hippocampus and results in impairment of memory retrieval.

Acute Stress Facilitates LTD Induction at Glutamatergic Synapses in the Hippocampal CA1 Region by Activating µ-Opioid Receptors on GABAergic Neurons.

Acute stress impairs recall memory through the facilitation of long-term depression (LTD) of hippocampal synaptic transmission. The endogenous opioid system (EOS) plays essential roles in stress-related emotional and physiological responses. Specifically, behavioral studies have shown that the impairment of memory retrieval induced by stressful events involves the activation of opioid receptors. However, it is unclear whether signaling mediated by µ-opioid receptors (µRs), one of the three major opioid receptors, participates in acute stress-related hippocampal LTD facilitation. Here, we examined the effects of a single elevated platform (EP) stress exposure on excitatory synaptic transmission and plasticity at the Schaffer collateral-commissural (SC) to CA1 synapses by recording electrically evoked field excitatory postsynaptic potentials and population spikes of hippocampal pyramidal neurons in anesthetized adult mice. EP stress exposure attenuated GABAergic feedforward and feedback inhibition of CA1 pyramidal neurons and facilitated low-frequency stimulation (LFS)-induced long-term depression (LTD) at SC-CA1 glutamatergic synapses. These effects were reproduced by exogenously activating µRs in unstressed mice. The specific deletion of µRs on GABAergic neurons (µRGABA) not only prevented the EP stress-induced memory impairment but also reversed the EP stress-induced attenuation of GABAergic inhibition and facilitation of LFS-LTD. Our results suggest that acute stress endogenously activates µRGABA to attenuate hippocampal GABAergic signaling, thereby facilitating LTD induction at excitatory synapses and eliciting memory impairments.

Sulfhydration of p66Shc at cysteine59 mediates the antioxidant effect of hydrogen sulfide.

AIMS: Mitochondrion is considered as the major source of intracellular reactive oxygen species (ROS). H2S has been reported to be an antioxidant, but its mechanism remains largely elusive. P66Shc is an upstream activator of mitochondrial redox signaling. The aim of this study was to explore whether the antioxidant effect of H2S is mediated by p66Shc. RESULTS: Application of exogenous H2S with its donor, NaHS, or overexpression of its generating enzyme, cystathionine ß-synthase, induced sulfhydration of p66Shc, but inhibited its phosphorylation caused by H2O2/D-galactose in SH-SY5Y cells or in the mice cortex. H2S also decreased mitochondrial ROS production and protected neuronal cells against stress-induced senescence. PKCßII and PP2A are the two key proteins to regulate p66Shc phosphorylation. Although H2S failed to affect the activities of these two proteins, it disrupted their association. Cysteine-59 resides in proximity to serine-36, the phosphorylation site of p66Shc. The C59S mutant attenuated the above-described biological function of H2S. INNOVATION: We revealed a novel mechanism for the antioxidant effect of H2S and its role in oxidative stress-related diseases. CONCLUSION: H2S inhibits mitochondrial ROS production via the sulfhydration of Cys-59 residue, which in turn, prevents the phosphorylation of p66Shc.