Involvement of the Ryanodine Receptor in Morphologic Modification ofHermissendaType B Photoreceptors After In Vitro Conditioning

Abstract

We examined whether Ca2+induced Ca2+release through ryanodine receptors is involved in the conditioning of specific morphologic changes at the axon terminals of type B photoreceptors in the isolated circumesophageal ganglion of Hermissenda. Calcium chelation by bis(2-aminophenoxy) ethane- N,N,N′, N′-tetraacetic acid prevented the conformational change at the terminals after five paired presentations of light and vibration, which produce terminal branch contraction of B photoreceptors. Two ryanodine receptor blockers, dantrolene and micromolar concentrations of ryanodine, depressed the increase in excitability due to in vitro conditioning and the increase in intracellular Ca2+in response to membrane depolarization. Although the ability to increase intracellular Ca2+was depressed, synaptic transmission was preserved in the normal state from hair cells under dantrolene and ryanodine incubation. Ryanodine receptor blockers also prevented contraction at the B photoreceptor axon terminals. These results suggest that the ryanodine receptor has a crucial role in inducing the in vitro conditioning specific changes both physiologically and morphologically, including “focusing” at the B photoreceptor axon terminal.