| Inferior colliculus (IC) is a pivot along the central auditory passway and consists of dorsal cortex of the IC (ICd), central nucleus of the IC (ICc) and the external cortex of the IC (ICx). Learning-induced auditory plasticity can be evoked in IC neurons, and cholinergic modulation by activating muscarinic cholinergic receptors plays a key role, however, the cellular mechanism was unclear at the cellular level. Using whole-cell patch clamp recordings and biocytin staining, we explored the electrophysiological properties in neurons of IC subnuclei and the effects of artifically activateion of muscarinic receptors on the properties of IC neurons. Our results showed that:1.88 IC neurons were obtained from 25 mice, including 21 neurons from the ICd, 43 neurons from the ICc, and 24 neurons from the ICx. According to the responses induced by depolarizing current injection, three firing patterns including onset (6.8%), adapting (39.8%) and sustained (53.4%) were identified. Hyperpolarization-activated inward current (4) was observed in half of the recorded neurons. Sustained pattern occurred mostly in ICc and ICd (67.4% and 61.9%), and adapting occurred mostly in ICx neurons (75%). Action potential (AP) threshold and time constant showed significantly differences among neurons from ICd, ICc and ICx (p< 0.05).2. Carbachol (CCh) was used to investigate the effects of artificial activation of acetylcholine muscarinic receptor (mAChR) on electrophysiological properties.59 IC neurons were obtained from 22 mice. The firing patterns of neurons were classified as sustained (66.1%) and adapting (33.9%). Activation of mAChR led to excitation (62.7%,), inhibition (20.3%) and no effect (17%). The proportions of excitation, inhibition and no effect groups were 61.1%,16.7% and 22.2% in ICd,79.2%,12.5% and 8.3% in ICc, 41.2%,35.3% and 23.5% in ICx.3. Activation of mAChR together with current injection into the recorded neurons were used to mimic the protocol used in in vivo ertracellular recording.67.8% neurons showed persistent activities (PA).77.8% are in ICd,83.3% are in ICc and 35.3% are in ICx. Based on firing duration, two patterns of PA including long-lasting (90%) and self-terminating (10%) were identified. mAChR antagonist-atropine, as well as synaptic blockers-kynurenic acid (KA) and picrotoxin significantly decreased the PA frequency (p< 0.05). The effects of CCh on neurons with PA were mainly extitation which resulted in increase of firing rate and decrease in threshold, latency and time constant.These results suggested that:1. IC neurons showed differences in electrophysiological properties among subnuclei. The variance of responses may be related to the distinct types of neurons as well as the received projections and the distinct roles of the neurons in auditory processing.2. The distinct effects of CCh could be related to the distributions of mAChRs and synaptic input recevied by the recorded neurons.3. PA in the IC could be a result of interactions among muscarinic receptor, the intrinsic properties and neural circuits. The properties of PA in subnuclei of IC could be determined by mAChR distributions, intrinsic properties and synaptic innervations.This study provided insights for us to understand electrophysiology properties of IC neurons and mechanism of learing-induced auditory plasticity at cellular levels. |
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