GABA immunoreactive elements in the sensory system of the earthworm Eisenia fetida (Annelida, Clitellata)

The presence of γ-aminobutyric acid (GABA) immunoreactive elements was observed in thesensory system of Eisenia fetida (Annelida, Clitellata). Among the primary sensory cells a high number of labelled cells was found in the epithelium. Using whole-mount preparations and multispectral recording, the number and the distribution pattern of the immunopositive cells were determined in different body segments. Our morphological analysis revealed four typical types of stained primary sensory cells, which could be responsible for the different role of the GABA mediated sensory functions. The peripheral processes of the primary sensory cells ramifying at the border of epithelium and muscular layer produced a basiepidermal network where GABA-positive fibres and their connections were observed. The central processes of the labelled cells projected directly to the ventral nerve cord (VNC) through the segmental nerves to form the ventrolateral and ventromedial sensory longitudinal axon bundles (SLABs). Inhibitory GABA sensory inputs could influence indirectly the activity of the giant motoneurons through the ventrolateral giant axons, and thus the contraction of the body organizing withdrawal and escape reflexes. Applying ultrastructural investigations the synaptic connections of GABA-immunoreactive structures were identified both in the basiepidermal plexus, in the segmental nerves, and in the SLABs of VNC suggesting multistep regulatory effect of GABA in sensory processing of earthworms.