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The Journal of Neuroscience, September 10, 2003, 23(23):8392-8401

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Burst-Induced Synaptic Depression and Its Modulation Contribute to Information Transfer at Aplysia Sensorimotor Synapses: Empirical and Computational Analyses

Gregg A. Phares,¹ Evangelos G. Antzoulatos,¹ Douglas A. Baxter,² and John H. Byrne¹

¹W. M. Keck Center for the Neurobiology of Learning and Memory, and ²Center for Computational Biomedicine, Department of Neurobiology and Anatomy, The University of Texas-Houston Medical School, Houston, Texas 77030

The Aplysia sensorimotor synapse is a key site of plasticity for several simple forms of learning. Plasticity of this synapse has been extensively studied, albeit primarily with individual action potentials elicited at low frequencies. Yet, the mechanosensory neurons fire high-frequency bursts in response to even moderate tactile stimuli delivered to the skin. In the present study, we extend this analysis to show that sensory neurons also fire bursts in the range of 1-60 Hz in response to electrical stimuli similar to those used in behavioral studies of sensitization. Intracellular stimulation of sensory neurons to fire a burst of action potentials at 10 Hz for 1 sec led to significant homosynaptic depression of postsynaptic responses. The depression was transient and fully recovered within 10 min. During the burst, the steady-state depressed phase of the postsynaptic response, which was only 20% of the initial EPSP of the burst, still contributed to firing the motor neuron. To explore the functional contribution of transient homosynaptic depression to the response of the motor neuron, computer simulations of the sensorimotor synapse with and without depression were compared. Depression allowed the motor neuron to produce graded responses over a wide range of presynaptic input strength. In addition, enhancement of synaptic transmission throughout a burst increased motor neuron output substantially more than did preferential enhancement of the initial phase of a burst. Thus, synaptic depression increased the dynamic range of the sensorimotor synapse and can, in principle, have a profound effect on information processing.

Key words: burst; depression; modulation; facilitation; synaptic filtering; synaptic plasticity

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Received March 17, 2003; revised June 12, 2003; accepted June 25, 2003.

This article has been cited by other articles:

				D. Fioravante, R.-Y. Liu, A. K. Netek, L. J. Cleary, and J. H. Byrne Synapsin Regulates Basal Synaptic Strength, Synaptic Depression, and Serotonin-Induced Facilitation of Sensorimotor Synapses in Aplysia J Neurophysiol, December 1, 2007; 98(6): 3568 - 3580. [Abstract] [Full Text] [PDF]

				E. G. Antzoulatos and J. H. Byrne Long-Term Sensitization Training Produces Spike Narrowing in Aplysia Sensory Neurons J. Neurosci., January 17, 2007; 27(3): 676 - 683. [Abstract] [Full Text] [PDF]

				Y. Zhurov and V. Brezina Variability of motor neuron spike timing maintains and shapes contractions of the accessory radula closer muscle of Aplysia. J. Neurosci., June 28, 2006; 26(26): 7056 - 7070. [Abstract] [Full Text] [PDF]

				H. A. Mohamed, W. Yao, D. Fioravante, P. D. Smolen, and J. H. Byrne cAMP-response Elements in Aplysia creb1, creb2, and Ap-uch Promoters: IMPLICATIONS FOR FEEDBACK LOOPS MODULATING LONG TERM MEMORY J. Biol. Chem., July 22, 2005; 280(29): 27035 - 27043. [Abstract] [Full Text] [PDF]

				G. A. Phares and J. H. Byrne Analysis of 5-HT-Induced Short-Term Facilitation at Aplysia Sensorimotor Synapse During Bursts: Increased Synaptic Gain That Does Not Require ERK Activation J Neurophysiol, July 1, 2005; 94(1): 871 - 877. [Abstract] [Full Text] [PDF]

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