Submitted on August 28, 2007
Accepted on November 18, 2007
Currents Contributing to Decision-Making in Neurons B31/B32 of Aplysia
Itay Hurwitz1, Amit Ophir2, Alon Korngreen1, John Koester3, and Abraham J. Susswein4*
1 Leslie and Susan Gonda (Goldschmied) Multidisciplinary Brain Research Center, Bar Ilan University, Ramat Gan, Israel; Mina and Everard Goodman Faculty of Life Sciences, Bar Ilan University, Israel
2 Mina and Everard Goodman Faculty of Life Sciences, Bar Ilan University, Israel
3 Center for Neurobiology and Behavior, Columbia University, New York, New York, United States; Department of Psychiatry, Columbia University, New York, New York, United States
4 Leslie and Susan Gonda (Goldschmied) Multidisciplinary Brain Research Center, Bar Ilan University, Ramat Gan, Israel; Mina and Everard Goodman Faculty of Life Sciences, Bar Ilan University, Israel; Center for Neurobiology and Behavior, Columbia University, New York, New York, United States
* To whom correspondence should be addressed. E-mail: avy{at}mail.biu.ac.il.
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 (IK-V) and an A-type K+ current (IK-A), as well as a high-threshold non-inactivating outward current (Imaintained). Hodgkin-Huxley kinetic analyses were performed on the outward currents. Simulations using equations from these analyses showed that IK-V and IK-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.
