| Primary auditory cortex(AI)is the highest center in central auditory system.Learning-induced auditory plasticity can be evoked in AI neurons,and cholinergic modulation plays a key role.However,the cellular mechanism of cholinergic modulation remains unclear.Using in vitro whole cell patch clamp recordings and biocytin staining,we recorded the electrophysiological properties,and studied the effects of carbochol on AI pyramidal neurons.Here were the results:1.137 AI pyramidal neurons were obtained from 40 mice.According to firing patterns,two groups of pyramidal neurons including regular spiking(RS)(62%,n=85)and intrinsically bursting(IB)(38%,n=52)were identified.The distributions of RS and IB neurons in each layer were as follows:layer ?/?:RS(73.7%,14/19),IB(26.3%,5/19);layer ?/?:RS(57.1%,16/28),IB(42.9%,12/28);layer ?:RS(58.6%,41/70),IB(41.4%,29/70);layer ?:RS(70.0%,14/20),IB(30.0%,6/20).IB neurons were mostly found in layer ?/? and layer ?,layer ? had lower activation threshold(Athr)than those in other layers(p<0.05,independent t-test).The Athr of pyramidal neurons in layer ?/?is significantly higher than that of neurons in layer ?(RS,p<0.01;IB,p<0.01;independent t-test).2.Carbachol(CCh)was used to study the effects of activating cholinergic receptors on electrophysiological properties of AI pyramidal neurons.104 AI pyramidal neurons were studied.80 neurons(76.9%)showed depolariztion of resting membrane potential(RMP),decrease of threshold(Ethr)and Athr(p<0.01,paired t-test),and increase of input resistance(Rin)and sag current(p<0.01,paired t-test).14 neurons(13.5%)showed hyperpolarization of RMP,and decrease of Rin(p<0.05,paired t-test)in layer ?.Meanwhile,CCh has no effects on RMP of 10 neurons(9.6%)(4 in layer ?/?,4 in layer?/? and 2 in layer ?).The results showed that activation of cholinergic receptors can either excite or inhibit AI pyramidal neurons;the excitatory effect may be related to the suppression of voltage dependent potassium channels or activation of calcium activated non-specific cation current(CAN);potassium outflow through calcium activated potassium channels or transmitter released by inhibitory internurons may result in the inhibitory effect.3.Mimicing the protocol used in in vivo extracelluar recordings,we tested the effects of activating cholinergic receptors together with positive current injection on the recorded neurons.A combination of cholinergic receptors activation and suprathreshold depolarization led to persistent activity(PA)in 37.8%(31/82)recorded neurons,among which 83.9%(26/31)were RS neurons,and 16.1%(5/31)were IB neurons.The amplitude of afterdepolarization(ADP)following positive current injection was 8.7 ± 3.6 mV in RS neurons,and 7.1 ± 3.5 mV in IB neurons(p>0.05,independent t-test).PA persisted under blockade of glutamate and GABA transmission(kynurenic acid,KA,2 mM;picrotoxin,0.1 mM)(6/6),However,PA was abolished with muscarinic receptors antagonish atropine(6/6).The results indicated the ability of AI pyramidal cells to generate PA which was dependent on activation of mAChRs on recorded neurons.PA may underline the possible mechanism of learning-induced auditory plasticity in AI.Overall,this study provides insights for us to understand the cellular mechanism of cholinergic modulation in central auditory system. |
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