GABA immunoreactivity in processes presynaptic to the locust wing stretch receptor neuron

Studies on the locust Locusta migratoria (Leitch et al. [1998] Soc. Neurosci. Abstr. 624.10) suggest that gamma-aminobutyric acid (GABA)ergic interneurons, activated via muscarinic receptors, may be involved in modulation of transmitter release at synapses between the forewing stretch receptor (fSR) and wing depressor motoneurons. To help elucidate the role GABAergic interneurons may play in modulation of transmitter release from fSR terminals, the proportion and distribution of GABA-immunoreactive (GABA-IR) inputs to the fSR were analysed using double-labelling (horseradish peroxidase and GABA immunocytochemistry) and electron microscopy. Forty-three percent of synaptic inputs to the fSR were from GABA-IR profiles, the majority of which were located on lateral branches. The highest proportion (57%) of inputs to the fSR, however, were from non-IR processes containing either clear spherical vesicles or mixed clear and dense-cored vesicles. Outputs from the fSR to GABA-IR profiles were also found, although their number was relatively low (7%). Networks were identified in which both the fSR and its non-IR postsynaptic partner received input from the same GABA-IR neuron. Such an arrangement would allow both pre- and postsynaptic inhibition of fSR afferent outputs, for example at fSR/motoneuron synapses. These observations demonstrate that the fSR does receive presynaptic inputs from GABA-IR neurons, thus providing morphological support for pharmacological and electrophysiological findings that GABAergic neurons are involved in the presynaptic modulation of the fSR. Nevertheless, modulation at this synapse may be more intricate and involve other, as yet unidentified, neurotransmitters released from non GABA-IR presynaptic processes and also muscarinic receptors located on the fSR itself.