The GABAergic innervation of the stretch receptor neurons of the crayfish Orconectes limosus has been investigated by means of light- and electron microscope immunocytochemistry using an antibody to GABA. Both whole-mount preparations and post-embedding semithin sections revealed a massive GABAergic innervation of both the slowly and the fast adapting receptor neurons. The stretch receptor organ is supplied by one principle GABA-immunoreactive axon, which gives off several branches that innervate the receptor neurons. Cell body, initial axon segment and dendritic region of the sensory neurons are covered by numerous GABA-immunoreactive varicose fibers. Electron microscopy revealed that the GABA-immunoreactive varicosities establish specialized synaptic contacts with the sensory neurons. The functional significance of the occurrence of GABA-immunoreactive varicosities on the different parts of the sensory neurons is discussed. The results support the physiological and pharmacological evidence that GABA is a transmitter substance of the efferent inhibitory neurons which innervate the crayfish stretch receptor neurons.
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