Distribution of input and output synapses on the central branches of bushcricket and cricket auditory afferent neurones: Immunocytochemical evidence for GABA and glutamate in different populations of presynaptic boutons

In order to investigate the synapses on the terminals of primary auditory a fferents in the bushcricket and cricket, these were impaled with microelect rodes and after physiological characterisation, injected intracellularly wi th horseradish peroxidase. The tissue was prepared for electron microscopy, and immunocytochemistry for gamma-aminobutyric acid (GABA) and glutamate w as carried out an ultrathin sections by using a post-embedding immunogold t echnique. The afferent terminals received many input synapses. Between 60-6 5% of these were made by processes immunoreactive for GABA and approximatel y 25% from processes immunoreactive for glutamate. The relative distributio n of the different classes of input were analysed fi om serial section reco nstruction of terminal afferent branches. Inputs from GABA and glutamate-im munoreactive processes appeared to be scattered at random over the terminal arborisation of the afferents both with respect to each other and to the a rchitecture of the terminals. They were, however, always found close to the output synapses. The possible roles of presynaptic inhibition in the audit ory afferents is discussed in the context of the auditory responses of the animals. J. Comp. Neurol. 403:281-294, 1999. (C) 1999 Wiley-Liss, Inc.