DISTRIBUTION OF SYNAPSES ON 2 ASCENDING INTERNEURONS CARRYING FREQUENCY-SPECIFIC INFORMATION IN THE AUDITORY-SYSTEM OF THE CRICKET - EVIDENCE FOR GABAERGIC INPUTS

Two identified cricket auditory interneurones, AN1 and AN2, were intra cellularly labelled with horseradish peroxidase following physiologica l characterisation. The neurones, which have some structural similarit ies, have their somata in the prothoracic ganglion and axons that proj ect to the brain. Although both carry auditory information, they have different response properties and participate in different types of ph onotactic behaviour. Ultrathin sections from selected regions of their prothoracic arborisations were examined in the electron microscope af ter postembedding immunostaining for the inhibitory transmitter GABA. In the prothoracic ganglion AN1 branches only in the medial ventral as sociation centre (mVAC) contralateral to the soma, and receives only i nput synapses. Twenty-seven percent of these were made by processes im munoreactive for GABA. AN2 branches not only in mVAC on both sides of the ganglion but also in several other areas. It makes output synapses from large diameter neurites in mVAC on both sides of the ganglion as well as from neurites in more posterior regions of the neuropile. Mos t input synapses are received onto branches in the contralateral mVAC where about 19% were made from GABA-immunoreactive processes, (C) 1994 Wiley-Liss, Inc.