L-?, ?-Diamino acids facilitate GABA induction of larval metamorphosis in a gastropod mollusc (Haliotis rufescens)

Abstract

1. l-α, ω-diamino acids and their derivatives facilitate the chemically induced settlement and metamorphosis of larvae ofHaliotis rufescens, a marine gastropod. 2. Organic inducers of settlement and metamorphosis used in this study include γ-aminobutyric acid (GABA), muscimol, and the macromolecular fraction of a cell-free extract of the naturally recruiting crustose red alga,Lithothamnium californicum. An inorganic inducer of settlement and metamorphosis, ionic potassium, is also used. 3. When the inducing compound is GABA, muscimol, or a macromolecular component ofL. californicum, the presence of a diamino acid facilitator converts a sigmoid concentration-response curve to a hyperbolic one, and shifts the curve to the left (Figs. 1, 3 and 4). The Hill coefficient in the absence of facilitator reveals significant positive cooperativity for induction by GABA; increasing concentration of facilitator progressively lowers the Hill coefficient (Fig. 2). When induction is activated directly by ionic potassium (Fig. 5), facilitators have significantly less effect. 4. A number ofl-α, ω-diamino acids (includingl-α, β-diaminopropionic acid (l-DAPA),l-α, γ-diaminobutyric acid (l-DABA),l-ornithine, andl-lysine) all facilitate the induction of larval settlement and metamorphosis at micromolar concentrations (Fig. 6). Of a number of other amino acids and their derivatives tested, only thel-lysine derivatives poly-l-lysine andl-(−)-α-amino-e-caprolactam are active as facilitators. Facilitatory activity is dependent on both of the amino groups, and the carboxyl group, of the diamino acids (Table 1); thel-stereochemical configuration also is essential for this activity (Fig. 7). 5. The benzodiazepines flurazepam, diazepam, and flunitrazepam, facilitators of GABA binding in certain mammalian GABAergic neurons, do not facilitate induction of larval settlement and metamorphosis by GABA. 6. The diamino acidl-lysine does not facilitate induction by substances that can not themselves induce settlement (Figs. 8, 9). 7. Allosteric regulation of chemosensory receptors controlling larval metamorphosis is suggested by the results of this study.