GABA-IMMUNOREACTIVITY IN PROCESSES PRESYNAPTIC TO THE TERMINALS OF AFFERENTS FROM A LOCUST LEG PROPRIOCEPTOR

Individually labelled sensory neurons from the femoral chordotonal org an, a proprioceptor at the femoro-tibial joint of a locust hindleg, we re analysed by intracellular recording, and by electron microscopical immunocytochemistry to reveal the arrangement of their input and outpu t synapses and to determine whether the input synapses were GABAergic. Intracellular recordings from these sensory neurons show spikes super imposed on a barrage of synaptic potentials during movements of the fe moro-tibial joint. These synaptic inputs can be mimicked by GABA. Inpu t synapses are made onto the vesicle-containing terminals of afferents and are often closely associated with the output synapses. By contras t, the axons of the afferents in the neuropil have no vesicles and nei ther make nor receive synapses. The input synapses to the afferent ter minals are made from processes typically a few microns in diameter, wh ereas the output synapses are made onto much smaller processes of only 0.1-0.2 mum. Input synapses at which an afferent terminal is the only postsynaptic element are common. Where the synapse is dyadic the seco nd postsynaptic element does not usually appear to be a chordotonal af ferent. The output synapses from the afferent terminals are usually dy adic. At 78% of the input synapses, the presynaptic neurite showed imm unoreactivity to a GABA antibody, supporting the physiological evidenc e that the presynaptic effects can be mediated by the release of GABA. The remaining (22%) immunonegative synapses are intermingled with tho se showing GABA immunoreactivity, but their putative transmitter is un known. These morphological observations suggest that the presynaptic c ontrol of the chordotonal afferents. is largely mediated by GABAergic neurons, but because other types of neuron also appear to be involved, presynaptic modulation may be more complex than has yet been revealed by the physiology.