This proposal seeks to continue research investigating the role that an interneuron, the S-cell, contributes to a novel non-associative learning process, intrinsic sensitization. Changes in S-cell activity have been observed during intrinsic sensitization, a hypothesized type of learning observed for the first time in experiments performed during this fellowship. The S-cell is already known to be critical to other forms of learning in the leech and there is evidence that these changes in S- cell activity are due to serotonergic modulation. However, intrinsic sensitization has been shown to be a serotonin-independent learning process. Therefore, I propose to: (1) Examine whether intrinsic sensitization is dependent on S-cell activity by performing laser axotomies of the S-cell to specifically lesion this interneuron pathway; (2) Observe if intrinsic sensitization induces changes in the response properties of the S-cell (e.g. change in excitability and/or number of initiation sites) and compare these changes to those known to occur in the S-cell during other forms of learning. These experiments will contribute to efforts towards understanding the cellular basis of learning-induced neural plasticity.