Immunochemical Localization of GABAA Receptor Subunits in the Freshwater Polyp Hydra vulgaris (Cnidaria, Hydrozoa)

gamma-aminobutyric acid (GABA) receptors, responding to GABA positive allosteric modulators, are present in the freshwater polyp Hydra vulgaris (Cnidaria, Hydrozoa), one of the most primitive metazoans to develop a nervous system. We examined the occurrence and distribution of GABAA receptor subunits in Hydra tissues by Western Blot and immunohistochemistry. Antibodies against different GABAA receptor subunits were used in Hydra membrane preparations. Unique protein bands, inhibited by the specific peptide, appeared at 35, 60, 50 and 52 kDa in membranes incubated with alpha3, beta1, gamma3 or delta antibodies, respectively. Immunohistochemical screening of whole mount Hydra preparations revealed diffuse immunoreactivity to alpha3, beta1 or gamma3 antibodies in tentacles, hypostome, and upper part of the gastric region; immunoreactive fibers were also present in the lower peduncle. By contrast, delta antibodies revealed a strong labeling in the lower gastric region and peduncle, as well as in tentacles. Double labeling showed colocalization of alpha3/beta1, alpha3/gamma3 and alpha3/delta immunoreactivity in granules or cells in tentacles and gastric region. In the peduncle, colocalization of both alpha3/beta1 and alpha3/gamma3 immunoreactivity was found in fibers running horizontally above the foot. These data indicate that specific GABAA receptor subunits are present and differentially distributed in Hydra body regions. Subunit colocalization suggests that Hydra GABA receptors are heterologous multimers, possibly sub-serving different physiological activities.