CUTANEOUS ACTIVATION OF THE INHIBITORY L30 INTERNEURONS PROVIDES A MECHANISM FOR REGULATING ADAPTIVE GAIN-CONTROL IN THE SIPHON WITHDRAWAL REFLEX OF APLYSIA

The functional role of inhibition in the neural network underlying the siphon withdrawal response (SWR) of Aplysia was assessed by examining a recurrent circuit comprised of identified inhibitory interneurons ( L30s), and excitatory interneurons (L29s). We previously showed that a ctivity-dependent potentiation of the L30 inhibitory synapse onto L29 can regulate the net excitatory input elicited by tactile siphon stimu lation onto siphon motor neurons (LFS cells) (Fischer and Carew, 1993a ). To explore the functional significance of L30 potentiated inhibitio n, we have examined how a behaviorally relevant stimulus that activate s the L30 interneurons modulates the SWR circuit. Utilizing a reduced preparation, we show that weak tactile stimulation of the tail strongl y activates the L30s, and leads to significant potentiation of the L30 synapse. Next, we demonstrate that similar weak tail stimulation prod uces significant inhibition of siphon tap-evoked responses in both L29 interneurons and LFS motor neurons. We further show that this form of inhibition is transient, having a time course of approximately 60 sec . Finally, we directly tested the role of the L30s in mediating this f orm of inhibition by hyperpolarizing two (of three) L30 interneurons d uring tail stimulation. L30 inactivation significantly attenuated tail stimulation-induced inhibition of siphon-evoked input to both L29 int erneurons and LFS motor neurons. Based on these results, we suggest th at L30-potentiated inhibition may have an important adaptive role in o ptimizing the signal-to-noise ratio for activation of the SWR circuit by providing stabilization of SWR responsiveness under a wide range of environmental conditions.