The Electrophysiological Properties And Mechanism Of Cholinergic Regulation Of The Pyramidal Neurons In The V Layer Of The Primary Auditory Cortex In Developing Mouse

Auditory cortex(AC)is the highest auditory center along the auditory pathway for coding and processing of acoustic information.The layer V(Layer,L5)of the primary auditory cortex(AI)plays an important role in processing and integrating acoustic information.In vivo studies have shown that AI can undergo frequency-specific auditory plasticity through learning and experience,which is modulated by the cholinergic system through muscarinic acetylcholine receptors(mAChRs,M receptors).The existing studies focus mainly on adult animals and there is a lack of studies on the intrinsic properties of AI-L5 pyramidal neurons in developing mouse.The cellular mechanisms of mAChR mediated auditory plasticity are not clear.Therefore,in this study,whole-cell patch-clamp recording and pharmacological methods were used to study the intrinsic properties of AI-L5 pyramidal neurons and the cellular mechanism of mAChR activation in developing mouse.The results are as follows:1.In total,357 pyramidal neurons were recorded from AI L5 in brain slices of three developmental stages in developing mouse(Mus musculus)(postnatal ten days(P10)-P14,n=185;P15-P18,n=78;P19-P29,n=94).Our results showed that the input resistance and the time constant of L5 pyramidal neurons decreased with the increasing of development time during P10-P18 and reached steady state at P19-P29.The resting membrane potentials were more hyper-polarized with the increasing of development time during P10-P18 and reached steady state at P19-P29.The resting membrane potentials were significantly lower than that in P10-P14,the input resistance of AI-L5 pyramidal neurons in P19-P29 mouse was significantly lower than that in P10-P14,the activation threshold is significantly greater than that in P10-P14,and the threshold intensity was significantly higher than that in P10-P14(all p<0.01).2.The effects of activating cholinergic receptors on the electrophysiological properties of pyramidal neurons were studied using carbachol(CCh).The results showed that the activation of mAChR significantly enhanced the excitability of AI-L5 pyramidal neurons in mouse of all tested group.Specifically,the resting membrane potential were significantly depolarized,the input impedance were significantly increased,the activation threshold were significantly diminished,and the threshold intensity were significantly decreased(all p<0.01).3.Our earlier study showed that the activation of cholinergic receptors combined with depolarization current can induce persistent activity(PA)in pyramidal neurons,which is characterized by increasing spiking after the current injection.This study shows that PA was induced equally in the three developmental stages of mouse(69%,63%,63.6%),and there were no significant differences in the proportion of PA among the groups.Meanwhile,there were no significant difference in the firing frequency of PA and the plateau potential among the groups.4.In order to determine the subtypes of mAChR involved in generating PA,blockers of M1 to M4 were used to study their effects on electrophysiological properties and PA in L5 pyramidal neurons of the three developmental stages.It was found that there were different effects on basic electrophysiological properties of neurons after blocking different muscarinic receptor subtypes.The PA was completely blocked after blocking muscarinic receptor M1 and M3,respectively.The firing frequency of PA were significantly reduced(p<0.01)after blocking muscarinic receptor M2,and there is no significant difference on the firing frequency of PA after blocking muscarinic receptor M4.To sum up,the excitability is higher on AI-L5 pyramidal neurons of mouse in crucial period,which may be due to the development of synaptic is dynamically in crucial period,which need high excitability of neurons to respond to the acoustic information easily,that the animal can better surviving.Pharmacological studies indicated that muscarinic receptors M1,M2 and M3 are involved in generating PA,while muscarinic receptor M4 didn't involve in generating PA.This may be related to the different muscarinic receptor subtypes coupling with different G proteins,which play a role in the interaction of second messengers differently.