The distribution of GABA sensitivity on crayfish muscle receptor organs.

1. The effectiveness of inhibition of gamma-aminobutyric acid (GABA) at various points on crayfish abdominal muscle receptor organs (MROs) was investigated by iontophoretic methods that have a spatial resolution of 8 mum. 2. The MROs in the rostral abdominal segments, which receive inhibitory innervation, were always strongly inhibited by focal iontophoretic application of GABA; MROs in the 6th abdominal segment, which do not receive inhibitory innervation, were usually not strongly inhibited. 3. In the rostral MROs, the magnitude of GABA current necessary to abolish stretch induced impulses was least for dendritic points; it increased as a function of distance from the dendrites, over an area extending from the receptor muscle to a point on the axon 200 mum from the cell body. 4. Outward currrents from NaCl-filled micropipettes had no effect on impulse production when applied to the dendritic region of rostral MROs, but could produce transient inhibition of impulses when applied to the axon. 5. It is suggested that a high degree of GABA sensitivity is localized to the rostral MRO dendrites, where inhibitory synapses are most numerous.