Regulation of receptor-mediated settlement and metamorphosis in larvae of a gastropod mollusc (Haliotis rufescens)
Larvae of the marine gastropod mollusc Haliotis rufescens can be induced to undergo metamorphosis by γ-aminobutyric acid (GABA) and stereochemically related compounds. This inductive response exhibits homotropic positive cooperativity, and is subject to both up-regulation and down-regulation. Up-regulation (facilitation, or enhancement) is brought about in a concentration-dependent manner by diamino acids such as L-diaminopropionic acid (DAPA), and is characterized by decreased Hill coefficients (nH) and concentration requirements (C50) for inducing compounds. Up-regulation does not require the simultaneous presence of facilitating and inducing compounds, and the up-regulated state can persist for days. Larvae are capable of being up-regulated 2 days before they are capable of undergoing settlement and metamorphosis. Down-regulation (desensitization, or habituation) can be brought about by exposure of pre-competent larvae to inducing molecules 4 days prior to the attainment of metamorphic competence; it is then slowly reversible. These biological findings are discussed in the light of recent binding studies indicating that up-regulation may be mediated at a site other than the GABA receptor, whereas down-regulation may result from reduction in the number of these receptors. The existence of multiple, interacting chemosensory receptors or transducers that control the induction of metamorphosis is suggested by these results.
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Document Type: Research Article
Publication date: 01 September 1986
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