Ryanodine receptor modulation of in vitro associative learning in Hermissenda crassicornis

Classical conditioning of the mollusc, Hermissenda crassicornis, is a model system used to study cellular correlates of associative learning. Paired p resentation of light and turbulence, but not unpaired presentations, causes Hermissenda to contract its foot in response to light alone. Intracellular recordings from the type B photoreceptors of the Hermissenda eye reveal a learning specific increase of input resistance, and a reduction of voltage- dependent potassium currents, both of which depend on an elevation of intra cellular calcium. Two previously demonstrated sources of calcium are influx through voltage-dependent channels, and release of calcium from intracellu lar stores through the IP3 receptor channel. Both modeling studies and iden tification of memory-related genes using RNA fingerprinting suggest that a third source of calcium, release from intracellular stores through the ryan odine receptor, may be involved in classical conditioning. We describe here an experiment suggesting that this third source of calcium is necessary fo r the cellular changes underlying associative memory storage. Paired presen tations of a light stimulus with a turbulence stimulus resulted in a signif icant increase in input resistance. Unpaired presentations of light and tur bulence did not produce a significant increase in input resistance. A third group of nervous systems first was incubated in dantrolene to block releas e of calcium through the ryanodine receptor, and then received paired train ing. There was no change in input resistance for this group. The effect of dantrolene on light adaptation of the photoreceptor was assessed by measuri ng the generator potential of a second light pulse presented some number of seconds after a first light pulse. The results show that at interpulse int ervals of 5 s, 10 s and 20 s, the generator potential of the dantrolene gro up is significantly greater than that of the control group. These results s uggest a role for the ryanodine receptor in both a cellular correlate of cl assical conditioning and light adaptation. (C) 1999 Elsevier Science B.V. A ll rights reserved.