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1 Neurobiology, Ludwig-Maximilians University of Munich, Munich, Germany
2 Vollum Institute, Oregon Health & Science University, Portland, Oregon, United States
* To whom correspondence should be addressed. E-mail: achim.klug{at}uni-muenchen.de.
One of the hallmarks of auditory neurons in vivo is spontaneous activity that occurs even in the absence of any sensory stimuli. Sound evoked bursts of discharges are thus embedded within this background of random firing. The calyx of Held synapse has been characterized in vitro as a fast relay that reliably fires at high stimulus frequencies (up to 800 Hz). However, inherently due to the preparation method, spontaneous activity is absent in studies using brain stem slices. We first determine in vivo spontaneous firing rates of MNTB principal cells from Mongolian gerbils and then reintroduce this random firing to in vitro gerbil brain stem synapses at near physiological temperature. After conditioning synapses with afferent fiber stimulation at Poisson averaged rates of 20, 40, and 60 Hz, we observed a number of differences in the properties of synaptic transmission between conditioned and unconditioned synapses. Accordingly, current-clamp, dynamic-clamp, and loose-patch recordings revealed a number of action potential failures at the postsynaptic cell during high frequency stimulation trains, although the initial onset of evoked activity was still transmitted with higher fidelity. We thus propose that some in vivo auditory synapses are in a tonic state of reduced EPSC amplitudes as a consequence of high spontaneous spiking, and this in vivo like conditioning has important consequences for the encoding of signals throughout the auditory pathway.
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