Immunohistochemical Localization of RF-Amide and GABAA Receptors in the Hypostome and Tentacles of Hydra vulgaris

Abstract

This study examined the distribution and localization of the neuropeptide arginine phenylalanine-amide (RF-amide) and GABAA receptors in the hypostomal and tentacular nerve net of the cnidarian polyp Hydra vulgaris. Immunohistochemical techniques were used to visualize two distinct anti-GABAA α-1 subunit positivereceptor ring structures, nerve cell bodies, neurites and epithelial cells, as well as an anti-RF-amide positive nerve ring, nerve cell bodies and neurites throughout the ectoderm of the hypostome and tentacles of ablated hypostomes. One proximal anti- GABAA positive-receptor ring, exclusively labeling ectodermal effector cells, coinciding with an anti-α-tubulin- positive proximal nerve ring labeling nerve fibers is described. Another anti-GABAA receptor- positive ring, labeling nerve cell bodies and fibers that contributed to the anti-α-tubulin- positive distal nerve ring is also reported. Anti-GABAA receptors were also found on ectodermal effector and epithelial cells in the apex of the hypostome, in battery cell components in tentacles and at tentacle insertions, suggesting involvement of GABAA receptors in both tentacle and contraction pulse control. Anti-RF-amide positive nerve fibers and nerve cell bodies of the proximal nerve ring and ubiquitously in bi-polar and tri-polar nerve cells and nerve fibers throughout the hypostome as well as in ganglion cells, sensory cells and neurites of the tentacle were identified. This study provides the first evidence of the localized distribution of anti-GABAA α-1 subunit receptors in the nerves and effector cells of the hypostome of H. vulgaris. It also provides the first evidence of an intimate association between anti-GABAA receptor and anti-RF-amide among the effector and neuronal cells, suggesting a mutual interaction in the neuro-effector systems controlling hydra’s behavior.