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 presentation 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 intracellular calcium. Two previously demonstrated sources of calcium are influx through voltage-dependent channels, and release of calcium from intracellular stores through the IP3 receptor channel. Both modeling studies and identification of memory-related genes using RNA fingerprinting suggest that a third source of calcium, release from intracellular stores through the ryanodine receptor, may be involved in classical conditioning. We describe here an experiment suggesting that this third source of calcium is necessary for the cellular changes underlying associative memory storage. Paired presentations of a light stimulus with a turbulence stimulus resulted in a significant increase in input resistance. Unpaired presentations of light and turbulence did not produce a significant increase in input resistance. A third group of nervous systems first was incubated in dantrolene to block release of calcium through the ryanodine receptor, and then received paired training. There was no change in input resistance for this group. The effect of dantrolene on light adaptation of the photoreceptor was assessed by measuring the generator potential of a second light pulse presented some number of seconds after a first light pulse. The results show that at interpulse intervals of 5 s, 10 s and 20 s, the generator potential of the dantrolene group is significantly greater than that of the control group. These results suggest a role for the ryanodine receptor in both a cellular correlate of classical conditioning and light adaptation.