The distribution of GABA sensitivity on crayfish muscle receptor organs.

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Abstract
The effectiveness of inhibition by GABA at various points on crayfish abdominal muscle receptor organs (MRO) was investigated by iontophoretic methods that have a spatial resolution of 8 .mu.m. The MRO in the rostral abdominal segments which receive inhibitory innervation, were always strongly inhibited by focal iontophoretic application of GABA; MRO in the 6th abdominal segment which do not receive inhibitory innervation, were usually not strongly inhibited. In the rostral MRO, 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 .mu.m from the cell body. Outward currents from NaCl-filled micropipettes had no effect on impulse production when applied to the dendritic region of rostral MRO, but could produce transient inhibition of impulses when applied to the axon. A high degree of GABA sensitivity is probably localized to the rostral MRO dendrites, where inhibitory synapses are most numerous.