Availability of chemosensory receptors is down-regulated by habituation of larvae to a morphogenetic signal.

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Abstract
Larvae of the gastropod mollusc Haliotis rufescens are induced to settle from the plankton and metamorphose in response to exogenous .gamma.-aminobutyric acid (GABA) and a number of GABA-mimetic compounds, including a GABA-mimetic inducer uniquely associated with the surfaces of the naturally recruiting algae. Previous evidence has shown that recognition of these inducers is mediated by specialized chemosensory receptors on the larval epithelium and that transduction of the morphogenetic signal then is mediated by cAMP and excitatory depolarization. We demonstrate here the specific and saturable labeling of a population of larval receptors with the GABA analog .beta.-(p-chlorophenyl)-[3H]GABA ([3H]baclofen); identification of these labeled receptors with those controlling metamorphosis is suggested by four independent criteria: (i) the effectiveness of GABA and its close structural analogs to induce metamorphosis is closely correlated with the effectiveness of these compounds to compete for binding to this receptor; (ii) the naural inducer purified from the recruiting algae cmpetes for binding to this receptor; (iii) (-)-[3H]baclofen specifically bound to the receptors is shed from the larvae after .apprxeq. 20 hr, at the time of corresponding to the metamorphic abscission and shedding of sensory cilia and other structures from the larvae; and (iv) the availability of the receptors for labeling and the ability of the larvae to respond to GABA and GABA analogs can be down-regulated in parallel by habituation of the larvae early in their development. These down-regulated larvae are fully capable of settlement and metamorphosis in response to agents that elevate intracellular cAMP or depolarize the chemosensory membrane, confirming that down-regulation is confined to the receptors, with no effect on the postreceptor pathway. The results reported here thus suggest that the sensitivity of marine invertebrate larvae to morphogenetic stimuli from the environment can be down-regulated by reduction in the number of chemosensory receptors available for interaction with the molecules that induce settlement and metamorphosis. In this respect, chemosensory receptors for environmental and morphogenetic signals are demonstrated biochemically to respond to habituation in a similar manner to neuronal and hormonal receptors.