| Fear,refers to the subjective emotional states and the behavioral and physiological reaction of individuals who are perceiving danger or potential threats.Moderate fear is helpful for surviving,while excessive fear is detrimental for the daily life of the posttraumatic stress disorder(PTSD)or phobia patients.As such,it's necessary to distinguish from excessive fear.Pentametric A type gamma-aminobutyric acid receptor(GABA_AR)is the main ionotropic receptor of GABA which is the most ubiquitous inhibitory neurotransmitter in the central neural system(CNS).The GABA_ARs mediating phasic inhibition or tonic inhibition are different in aspects of their different subunit compositions,subcellular localizations and kinetic properties.The agonists of GABA_AR,like benzodiazepines,have long been in clinic in treating PTSD,however the side effect of these drugs are nonnegligible,so it's necessary to find a new GABA_AR pharmacological target.To date,?-containing GABA_AR[GABA_A(?)R]is the only class of GABA_AR shown to be extrasynaptically located,and it is also a group of non-benzodiazepine sensitive GABA_AR.Downregulation of the GABA_A(?)R is closely linked with negative valence related neuropsychiatric disorder,such as depression.Moreover,it has been reported that knock out of?subunit(encoded by Gabrd gene)impairs contextual fear extinction,however,whether it is involved in cued fear extinction and the underlying mechanisms are still elusive.Here,by using the Gabrd gene globally knock out(KO)mice and the wild type(WT)mice,we found that knock out of?subunit impaired the fear extinction without affecting fear learning,and such impairment had no gender differences.Then,the c-fos immunostaining results showed that fear extinction drastically increased medial frontal cortex(mPFC)neural activity in normal distinguished WT mice,while in KO mice mPFC,deleting?subunit led to a high basal neural activity thus avoiding a further activation of these neurons and vanishing the neural activity plasticity during behavioral training.By contrast,the c-fos expression patterns between genotypes had no difference in basolateral amygdala(BLA).Western blot(WB)and electrophysiological evidence showed that the GABA_A(?)R expressed abundantly in mPFC but not BLA of WT mice,such results were in parallel with c-fos expression pattern between WT and KO mice suggesting a role of GABA_A(?)R in regulation of mPFC neural activity.Then the further study revealed that knock out of?subunit increased neuronal excitability not only by weakening the tonic inhibition,but causing a loss of function of hyperpolarization activated cyclic nucleotide-gated channel(HCN channel)which was coupled with?subunit.Subsequently,by in vitro recording mPFC PNs,we observed that successful extinguishing of learned fear enhanced WT group neuronal excitability via suppressing HCN channel currents(I_h)and consequential decrement of median afterhyperpolarization(m AHP).As for the KO group,on the contrary,extinction training failed to cause the adaptational change of neuronal excitability due to the loss of the GABA_A(?)R-assisted dynamics of I_h.Although the disability of synaptic plasticity of mPFC PNs could also impair fear extinction,the synaptic transmissions in both genotypes of mice had no difference.Finally,AAV mediated local down-regulation of GABA_A(?)R in infralimbic cortex(IL),rather than in prelimbic cortex(PL),abolished fear extinction in WT mice.Likewise,reintroduction of GABA_A(?)R into IL of knockout mice sufficed to reverse the impaired plasticity of neuronal excitability and rescue the deficit of fear extinction,suggesting that the expression of GABA_A(?)R in IL and GABA_A(?)R enabled neuronal excitability plasticity were important to fear extinction.Taken together,we firstly uncovered the coupling between GABA_A(?)R and HCN channel,and revealed an unconventional role of GABA_A(?)R in facilitating fear extinction.These findings provide a potential targets and theoretical basis for treatment of PTSD and other related neuropsychiatric diseases. |
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