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Currents Contributing to Decision Making in Neurons B31/B32 of Aplysia

¹Leslie and Susan Gonda (Goldschmied) Multidisciplinary Brain Research Center, ²Mina and Everard Goodman Faculty of Life Sciences, Bar Ilan University, Ramat Gan, Israel; and ³Center for Neurobiology and Behavior and ⁴Department of Psychiatry, Columbia University, New York, New York

Submitted 28 August 2007; accepted in final form 18 November 2007

Biophysical properties of neurons contributing to the ability of an animal to decide whether or not to respond were examined. B31/B32, two pairs of bilaterally symmetrical Aplysia neurons, are major participants in deciding to initiate a buccal motor program, the neural correlate of a consummatory feeding response. B31/B32 respond to an adequate stimulus after a delay, during which time additional stimuli influence the decision to respond. B31/B32 then respond with a ramp depolarization followed by a sustained soma depolarization and axon spiking that is the expression of a commitment to respond to food. Four currents contributing to decision making in B31/B32 were characterized, and their functional effects were determined, in current- and voltage-clamp experiments and with simulations. Inward currents arising from slow muscarinic transmission were characterized. These currents contribute to the B31/B32 depolarization. Their slow activation kinetics contribute to the delay preceding B31/B32 activity. After the delay, inward currents affect B31/B32 in the context of two endogenous inactivating outward currents: a delayed rectifier K⁺ current (I_K-V) and an A-type K⁺ current (I_K-A), as well as a high-threshold noninactivating outward current (I_maintained). Hodgkin-Huxley kinetic analyses were performed on the outward currents. Simulations using equations from these analyses showed that I_K-V and I_K-A slow the ramp depolarization preceding the sustained depolarization. The three outward currents contribute to braking the B31/B32 depolarization and keeping the sustained depolarization at a constant voltage. The currents identified are sufficient to explain the properties of B31/B32 that play a role in generating the decision to feed.