Septal And Hippocampal Neurons Contribute To Auditory Relay And Fear Conditioning

Background:The hippocampus is a structure which located deep inside of the medial prefrontal cortex.It mainly includes the dentate gyrus,Hippocampus,subiculum,presubiculum,parasubiculum and entorhinal cortex.The main functions of the hippocampus are learning,memory,and spatial navigation.The generation of multiple memories depends on the hippocampus,such as spatial memory,declarative memory,context memory,and fear memory.At the same time,damage to the hippocampus will cause many mental and neurological diseases such as epilepsy and Alzheimer's disease.In addition,there are a large number of reports have reported that the hippocampus received various of sensory information inputs,such as visual,auditory,olfactory,tactile,and somatic sensations.The hippocampus has been considered to process auditory information.The research on hippocampal processing of auditory information has received more and more attention.At present,researches on hippocampal auditory information mainly focus on two aspects of sound-induced learning and memory models and auditory gating.But the property of the hippocampal responses to sound,the source of the sound responses,and the role of hippocampal sound responses in auditoy fear conditioning remains unclear.Therefore,this study mainly focuses on these three issues.Methods:The main methods used in this study were:in vivo electrophysiological recordings;immunohistochemical staining;pharmacological inactivation experiments;behavioral training experiments.First,with loose-patch recordings,we found that hippocampal(EC,CA3,CA1)neurons are mainly responsive to noise and are not tonotopically organized,and the auditory response was only occuring in awake state,not in anesthesia mice.Second,we used the immunohistochemical staining to label the morphological features of the recieding neurons.At the same time,the source of hippocampal auditory information is studied through pharmacological inactivation of specific brain regions combined with electrophysiological methods.Finally,the role of hippocampal sound responses in auditory fear conditioning are studied through behavioral training.Results:In this study,we used in vivo loose-patch recordings to investigate the response property of hippocampus neurons.With loose-patch recordings,we found that hippocampal(EC,CA3,CA1)neurons are mainly responsive to noise and are not tonotopically organized,and the auditory response was only occuring in awake state,not in anesthesia mice.Where is the auditory information from?We performed the loose-path recordings in(Primary auditory cortex,A1)and MS,our results showed that neurons in MS had the similar response characteristic compared those in the hippocampus,they only respond to noise stimulus not to tones,and only in awake state.However neurons in A1 not only respond to noise stimulus but also to tones,and the tonal respective field were observed.So we speculated that hippocampal auditory information was from MS,not A1.We compared the response latencies among these regions(MS,EC,CA3,CA1 and A1).We found that hippocampus neuronal latencies are shorter than those of A1 neurons,but longer than those of MS neurons,suggesting that hippocampal auditory information comes from the MS rather than A1.To examine whether auditory information to the hippocampus comes from the MS or A1,i.e.,from the non-lemniscal or lemniscal pathway,we injected lidocaine(a voltage-gated sodium channel blocker)to the ipsilateral A1 or MS via an implanted cannula,while loose-patch recordings were performed from neurons in the EC and CA3.After inactivation of A1,auditory responses and spontaneous discharges of EC and CA3 neurons 10 minutes after a lidocaine injection were almost the same as those recorded before that.Thus,hippocampal auditory inputs did not come from A1.However,after inactivation of MS,the raster-plot showed that the noise-evoked responses and spontaneous discharges of both EC and CA3 neurons could be completely inhibited by lidocaine.All recorded neurons in the EC and CA3 were significantly silenced.Therefore,hippocampal auditory inputs came from the MS.Our result showed that behavioral responses to auditory fear conditioning only occurred in awake mice not in anesthetized mice.Our electrophysiological and pharmacological experiments showed that the MS provided auditory input to the hippocampus,that anesthesia silenced the neurons in the MS and hippocampus,and that auditory fear conditioning could not be acquired by anesthetized mice.It is a question whether MS inactivation could block the acquisition of auditory fear conditioning.Our results indicate that MS inactivation induced mice amnesia,but A1 inactivation did not.Unexpectedly,MS inactivation blocked not only the noise-induced freezing response,but also the tone-induced one.It seemed that auditory fear conditioning behavior was independent of the frequency characteristics of the conditioned stimuli.We speculated that auditory fear conditioning behavior was associated with some cues but not with the specify frequency of conditioned stimuli.To further confirm this possibility,we devised another behavioral experiment.In this experiment,each mouse was conditioned to one kind of auditory stimulus(noise,2.5-,5-,10-,15-or 30-kHz tones)but tested with multiple sounds(noise,2.5-,5-,10-,15-and 30-kHz tones)24 h after the training.However,there were no significant differences in testing with different sounds and the sound testing order.These results indicate that the freezing response induced by auditory fear conditioning was independent of the frequency of sound used as CS.Conclusions:In the current studies,we had explored the auditory response properties of hippocampal neurons,the auditory pathway to the hippocampus,and the role of the hippocampal auditory responses in auditory fear conditioning by electrophysiological and behavioral assays combined with pharmacological manipulations.We had made four major findings.(1)Neurons in the hippocampus(EC,CA3 and CA1)responded only to noise burst stimuli in awake state.(2)Auditory fear conditioning was not associated with a specific frequency of sound.(3)Hippocampal auditory inputs came from the MS,not from Al.(4)MS inactivation impaired the acquisition of auditory fear conditioning.