DISTRIBUTION OF GABAERGIC SYNAPTIC TERMINALS ON THE DENDRITES OF LOCUST SPIKING LOCAL INTERNEURONS

Double-labelling and electron microscopy were used to assess the distr ibution of GABAergic synapses made onto the neurites of spiking local interneurones in the locust. The aims were to determine the sites of i nputs mediating inhibition of the spiking local interneurones and to a scertain the relative abundance of such inputs. This information shoul d allow us to understand better the integrative properties of these sp iking local interneurones and the role of inhibition in shaping their receptive field properties or in fine tuning their spike-mediated outp uts. Spiking interneurones in a midline population were labelled by in tracellular injection of horseradish peroxidase after physiological ch aracterisation. Colloidal gold immunocytochemistry was then used on ul trathin sections of these neurones with a polyclonal antibody raised a gainst GABA. Most GABAergic (inhibitory) input synapses onto the inter neurones are made on their ventral neurites, which also receive affere nt (excitatory) inputs. These inhibitory inputs to the ventral neurite s constitute 43% of the identifiable synapses. Relatively few GABAergi c inputs were found onto the dorsal neurites, which are predominantly the sites of output synapses from these interneurones. These results s uggest that much synaptic integration takes place in the ventral field of branches and that GABA-mediated presynaptic inhibitory control of spike-mediated outputs from the dorsal neurites is unlikely to occur. (C) 1993 Wiley-Liss, Inc.