Distribution and Pharmacological Properties of the GABAA/ Benzodiazepine/Chloride Ionophore Receptor Complex in the Brain of the Fish Anguilla anguilla

In the present study, we characterized the distribution and the pharmacological properties of the different components of the GABAA receptor complex in the brain of the eel (Anguilla anguilla). Benzodiazepine recognition sites labeled "in vitro" with 3H]flunitrazepam (3H]FNT) were present in highest concentration in the optic lobe and in lowest concentration in the medulla oblongata and spinal cord. A similar distribution was observed in the density of gamma-3H]aminobutyric acid (3H]GABA) binding sites. GABA increased the binding of 3H]FNT in a concentration-dependent manner, with a maximal enhancement of 45% above the control value, and, vice versa, diazepam stimulated the binding of 3H]GABA to eel brain membrane preparations. The density of benzodiazepine and GABA recognition sites and their reciprocal regulation were similar to those observed in the rat brain. In contrast, the binding of the specific ligand for the Cl- ionophore, t-35S]butylbicyclophosphorothionate (35S]TBPS), to eel brain membranes was lower than that found in the rat brain. In addition, 35S]TBPS binding in eel brain was less sensitive to the inhibitory effects of GABA and muscimol and much more sensitive to the stimulatory effect of bicuculline, when compared with 35S]TBPS binding in the rat brain. Moreover, the uptake of 36Cl- into eel brain membrane vesicles was only marginally stimulated by concentrations of GABA or muscimol that significantly enhanced the 36Cl- uptake into rat brain membrane vesicles. Finally, intravenous administration of the beta-carboline inverse agonist 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylic acid methyl ester (20 mg/kg) and of the chloride channel blocker pentylenetetrazole (80 mg/kg) produced convulsions in eels that were antagonized by diazepam at doses five to 20 times higher than those required to produce similar effects in rats. The results may indicate a different functional activity of the GABA-coupled chloride ionophore in the fish brain as compared with the mammalian brain.