Distribution of GABAergic synaptic terminals on the dendrites of locust spiking local interneurones

Double-labelling and electron microscopy were used to assess the distribution of GABAergic synapses made onto the neurites of spiking local interneurones in the locust. The aims were to determine the sites of inputs mediating inhibition of the spiking local interneurones and to ascertain the relative abundance of such inputs. This information should allow us to understand better the integrative properties of these spiking local interneurones and the role of inhibition in shaping their receptive field properties or in fine tuning their spike-mediated outputs. Spiking interneurones in a midline population were labelled by intracellular injection of horseradish peroxidase after physiological characterisation. Colloidal gold immunocytochemistry was then used on ultrathin sections of these neurones with a polyclonal antibody raised against GABA. Most GABAergic (inhibitory) input synapses onto the interneurones are made on their ventral neurites, which also receive afferent (excitatory) inputs. These inhibitory inputs to the ventral neurites constitute 43% of the identifiable synapses. Relatively few GABAergic inputs were found onto the dorsal neurites, which are predominantly the sites of output synapses from these interneurones. These results suggest 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.