Associative Training Results in Persistent Reductions in a Calcium-Activated Potassium Current in Hermissenda Type B Photoreceptors

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Type B photoreceptors from the eyes of associatively trained Hermissenda were found to have significantly greater light responses and net input resistances than Type B cells from comparably trained random control animals on retention days following conditioning. These differences were still apparent when the contribution of a fast, rapidly inactivating K+ current (IA) to the photoresponse was minimized by either depolarization-induced inactivation or block by 4-aminopyridine ions (4-AP). The training-produced differences in Type B cell light responses were abolished, however, by treatments that reduced the contribution of a calcium-activated K+ current (IK-Ca) to the light response. Under voltage-clamp conditions in which IK-Ca was isolated from other components of outward current, it was found to be selectively reduced by associative training. The associative reduction of IK-Ca could not be attributed to training-produced reductions in the voltage-dependent calcium current (ICa) of Type B cells. ICa was instead enhanced by associative training. Measurements of light-activated ionic currents failed to reveal significant training-related differences in the magnitude of the transient, inward sodium current (INa-light). A slower component of light-induced inward current was greater for Type B cells from associatively trained animals, however. Previous results suggest that this slower component may arise from a light-induced reduction of a calcium-activated K+ current. Collectively, the results indicate that training-produced reductions in a calcium-activated K+ current are primarily responsible for the enhanced photoresponses of Type B cells from conditioned animals.

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