Studies On The Electrophysiologic Mechanism Underlying Of Response Patterns To Echolocation Behavior-related Sound Stimuli In Inferior Collicular Neurons Of CF-FM Bat

The response patterns of the central auditory neurons to sound stimuli are both their basic properties and the information carriers which are used to represent the specific sound meaning. So, the study on the response patterns to sound stimuli is helpful for us to understand the mechanisms underlying the auditory behavior and the sound information processing. Our previous studies in the inferior colliculus (IC) of the leaf-nosed bat, Hipposideros armiger (a CF-FM bat) found that the response of IC neurons to the CF-FM stimulus could be classified into two types, i.e. single-on (SO) and double-on (DO) responses, which were called the SO and DO neurons, respectively. These two types of the neurons have different auditory response properties and they possibly play different roles in echo location and other auditory behaviors. However, it is still not clear how these two patterns were shaped and what the neural mechanism they have. In order to probe the mechanisms of these two patterns, we did further study on the IC of Pratt’s roundleaf bat, Hipposideros pratti.The experiment was performed on the 12 Pratt’s roundleaf bat, Hipposideros pratti. The sound stimulus signals i.e. CF, FM and CF-FM used in the experiment, which mimic the echolocation signal emitted by this bat and is called the behavior-related sound stimulus were generated through the electronic instruments. The responses of the neurons to sound stimuli and the changes in the membrane potentials of the neurons were recorded by in vivo extracellular and intracellular recordings. Total of 145 neurons were recorded by extracellular recording. Among them,104 neurons were examined with CF, FM and CF-FM sounds. These neurons according to their responses to CF-FM stimulus were classified into the SO (N=68,65.4%) and DO (N=36,34.6%) responders or neurons. In order to probe the mechanisms of these two patterns, we did further study on the IC of Pratt’s roundleaf bat (Hipposideros pratti) using in vivo intracellular recording. Total of 171 neurons were recorded. Among them,126 neurons were examined with CF, FM and CF-FM sounds and they according to the responses of the neurons to CF-FM stimulus were also classified into the SO (N=86,68.3%) and DO (N=40,31.7%) responders or neurons. The results showed that most SO neurons (N=56,65.1%) had a post-spike hyperpolarization (PSH) of different durations that followed behind AP firing elicited by sole CF, FM and CF-FM sounds, especially for the longer PSH duration (71.9 ± 74.6 ms, P<0.01) elicited by CF sound compared to DO neurons. While among the DO neurons, the responses elicited by sole CF, FM and CF component of CF-FM sound in the minority (N=10,25%) of them were also followed by a PSH, especially for the shorter PSH duration (29.3 ± 35.2 ms, P<0.01) elicited by CF sound and CF component compared to SO neurons. These data suggested that the PSH directly participated in the formation of SO and DO neurons, and the PSH elicited by the CF component was the main synaptic mechanism underlying the SO and DO response patterns. Because most SO neurons had longer PSH duration elicited by CF component than DO neurons (P<0.01), the excitatory inputs elicited by FM component are under the range of PSH duration, which possibly depress the responses of SO neurons to the FM component and SO response pattern was shaped. For DO neurons, owing to the shorter PSH duration elicited by CF component, the excitatory response elicited by FM component happened behind the ending of PSH, which couldn’t depress the responses to the FM component and DO response pattern was shaped. Considering that the roles of CF and FM components of CF-FM are respectively for obtaining information of bat’s flight speed and for obtaining information of target detail and distance between bat and target during bat’s echolocation, it is suggested that the SO and DO neurons have different functions in the echolocation. Therefore, we infer that the SO neurons, which only discharged to the CF component of CF-FM sounds, might participate in acquiring the relative flying velocity information for Doppler-shift compensation and target locking during hunting. Moreover, the DO neurons, which discharge to both CF and FM components, might participate in acquiring information of wing-beat velocity, range and fine detail or target feature.