Background

Inhalational anesthetics are known to disrupt PDZ2 domain–mediated protein–protein interactions of the postsynaptic density (PSD)-95 protein. The aim of this study is to investigate the underlying mechanisms in response to early isoflurane exposure on synaptic PSD-95 PDZ2 domain disruption that altered spine densities and cognitive function. The authors hypothesized that activation of protein kinase-G by the components of nitric oxide (NO) signaling pathway constitutes a mechanism that prevents loss of early dendritic spines and synapse in neurons and cognitive impairment in mice in response to disruption of PDZ2 domain of the PSD-95 protein.

Methods

Postnatal day 7 mice were exposed to 1.5% isoflurane for 4 h or injected with 8 mg/kg active PSD-95 wild-type PDZ2 peptide or soluble guanylyl cyclase activator YC-1 along with their respective controls. Primary neurons at 7 days in vitro were exposed to isoflurane or PSD-95 wild-type PDZ2 peptide for 4 h. Coimmunoprecipitation, spine density, synapses, cyclic guanosine monophosphate–dependent protein kinase activity, and novel object recognition memory were assessed.

Results

Exposure of isoflurane or PSD-95 wild-type PDZ2 peptide relative to controls causes the following. First, there is a decrease in PSD-95 coimmunoprecipitate relative to N-methyl-d-aspartate receptor subunits NR2A and NR2B precipitate (mean ± SD [in percentage of control]: isoflurane, 54.73 ± 16.52, P = 0.001; and PSD-95 wild-type PDZ2 peptide, 51.32 ± 12.93, P = 0.001). Second, there is a loss in spine density (mean ± SD [spine density per 10 µm]: control, 5.28 ± 0.56 vs. isoflurane, 2.23 ± 0.67, P < 0.0001; and PSD-95 mutant PDZ2 peptide, 4.74 ± 0.94 vs. PSD-95 wild-type PDZ2 peptide, 1.47 ± 0.87, P < 0.001) and a decrease in synaptic puncta (mean ± SD [in percentage of control]: isoflurane, 41.1 ± 14.38, P = 0.001; and PSD-95 wild-type PDZ2 peptide, 50.49 ± 14.31, P < 0.001). NO donor or cyclic guanosine monophosphate analog prevents the spines and synapse loss and decline in the cyclic guanosine monophosphate–dependent protein kinase activity, but this prevention was blocked by soluble guanylyl cyclase or protein kinase-G inhibitors in primary neurons. Third, there were deficits in object recognition at 5 weeks (mean ± SD [recognition index]: male, control, 64.08 ± 10.57 vs. isoflurane, 48.49 ± 13.41, P = 0.001, n = 60; and female, control, 67.13 ± 11.17 vs. isoflurane, 53.76 ± 6.64, P = 0.003, n = 58). Isoflurane-induced impairment in recognition memory was preventable by the introduction of YC-1.

Conclusions

Activation of soluble guanylyl cyclase or protein kinase-G prevents isoflurane or PSD-95 wild-type PDZ2 peptide–induced loss of dendritic spines and synapse. Prevention of recognition memory with YC-1, a NO-independent activator of guanylyl cyclase, supports a role for the soluble guanylyl cyclase mediated protein kinase-G signaling in countering the effects of isoflurane-induced cognitive impairment.

Editor’s Perspective
What We Already Know about This Topic
  • The PDZ2 domain of the postsynaptic density-95 protein clusters together ion channels and downstream signal transduction proteins at synapses

  • Inhalational anesthetics disrupt PDZ2 domain–mediated protein–protein interactions resulting in impaired synaptogenesis and N-methyl-d-aspartate receptor–dependent synaptic plasticity

  • The downstream signaling pathways linking the PDZ2 domain to synaptogenesis and synaptic plasticity and their response to anesthetics are incompletely understood

What This Article Tells Us That Is New
  • In neonatal mice, a 4-h-long exposure to isoflurane disrupted the interactions between the NR2A/2B subunits of the N-methyl-d-aspartate receptor and the PDZ2 domain of the postsynaptic density-95 protein

  • This disruption led to a decrease in cyclic guanosine monophosphate–dependent protein kinase activity, to impaired synaptogenesis, and to persistent cognitive dysfunction

  • In vivo and in vitro experiments using pharmacologic activators and inhibitors revealed that activation of protein kinase-G by components of the NO signaling pathway constitute downstream signaling mechanisms that prevent these effects

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