DISTRIBUTION OF INPUT SYNAPSES FROM PROCESSES EXHIBITING GABA-LIKE ORGLUTAMATE-LIKE IMMUNOREACTIVITY ONTO TERMINALS OF PROSTERNAL FILIFORMAFFERENTS IN THE LOCUST

The locust prosternum carries a population of long filiform hairs that are very sensitive to air currents. The sensory afferent neurones tha t innervate the hairs make strong monosynaptic connections with an ide ntified intersegmental interneurone (A4I1) which is known to contact m otor neurones that supply muscles controlling wing angle during flight . In order discover how the synapse between the afferents and interneu rone A4I1 might be modulated, the afferents were labelled intracellula rly by backfilling with horseradish peroxidase to reveal their central terminals which lie in the prothoracic ganglion. A postembedding immu nogold method was used to make a quantitative assessment of the preval ence of immunoreactivity for GABA and glutamate in processes presynapt ic to the afferent terminals. In one afferent neurone, where 77 synaps es were examined, 40 (52%) of the presynaptic processes were immunorea ctive for GABA. When adjacent sections through the same terminal branc hes were labelled with the two antibodies, it was demonstrated that GA BA- and glutamate-like immunoreactivity was present in different popul ations of presynaptic processes. A series of 110 ultrathin sections wa s cut through one set of afferent terminal branches and alternate grid s were stained with GABA and glutamate antibodies. From these sections , the terminals were reconstructed and the position of 35 input and 21 output synapses mapped. Of the 35 input synapses, 18 (51%) were immun oreactive for GABA, 14 (40%) were immunoreactive for glutamate and 3 ( 9%) were unlabelled by either antibody. On these terminals, the differ ent classes of input synapses appeared to be intermingled at random wi th the output synapses made by the afferent, and no pattern governing synapse distribution could be discerned. (C) 1994 Wiley-Liss, Inc.