Activity-dependent regulation of neural networks: the role of inhibitory synaptic plasticity in adaptive gain control in the siphon withdrawal reflex of Aplysia.(The Future of Aquatic Research in Space: Neurobiology, Cellular and Molecular Physiology)

Neural networks can be dynamic systems that enable organisms to adapt to, and learn about, complex and variable environments. The broad objective of our research program is to understand the nature of this type of adaptive process by examining the functional significance of neural plasticity observed at both cellular and network levels during adaptive behavioral modifications. Our primary goal is to characterize the role of inhibitory modulation in network function and plasticity, by focusing upon a recurrent synaptic circuit formed by the L29 excitatory interneurons and the L30 inhibitory interneurons within the siphon withdrawal reflex (SWR) network of Aplysia californica. Recurrent inhibitory circuits such as this are a common feature of motor networks, providing a mechanism for rapid control of behavioral output. Moreover, intrinsic and extrinsic modulation of recurrent inhibitory circuitry can endow motor networks with a high degree of flexibility and an enhanced capability for adaptive modification. By focusing upon identified inhibitory elements within a well-defined circuit with direct behavioral relevance, and performing experiments ranging from cellular to behavioral levels of analysis, we are beginning to define a functional role for inhibitory processing in mediating adaptive gain control of the SWR in response to changing environmental conditions.

Our cellular experiments have demonstrated that dynamic interactions between the L29s and L30s can significantly regulate reflex input to the LFS-type siphon motor neurons (MNs). The L29s (5 total) and L30s (3 total) are reciprocally interconnected by both chemical and electrical synapses: the L29s directly excite the L30s which, in turn, directly inhibit the L29s. The L29 excitatory interneurons, which are known to act as facilitatory neuromodulatory neurons (Hawkins et al., 1981), also provide substantial excitatory input to LFS MNs; a single L29 can account for as much as 50% of…