Immunocytochemical evidence for the GABAergic innervation of the stretch receptor neurons in crayfish

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.