ABSTRACT
Serotonin (5-HT)-induced long-term facilitation (LTF) of the Aplysia sensorimotor synapse depends on enhanced gene expression and protein synthesis, but identification of the genes whose expression and regulation are necessary for LTF remains incomplete. In this study, we found that one such gene is synapsin, which encodes a synaptic vesicle-associated protein known to regulate short-term synaptic plasticity. Both synapsin mRNA and protein levels were increased by 5-HT. Upregulation of synapsin protein occurred in presynaptic sensory neurons at neurotransmitter release sites. To investigate the molecular mechanisms underlying synapsin regulation, we cloned the promoter region of Aplysia synapsin, and found that the synapsin promoter contained a cAMP response element (CRE), raising the possibility that the transcriptional activator CRE-binding protein 1 (CREB1) mediates 5-HT-induced regulation of synapsin. Indeed, binding of CREB1 to the synapsin promoter was increased following treatment with 5-HT. Furthermore, increased acetylation of histones H3 and H4 and decreased association of histone deacetylase 5 near the CRE site are consistent with transcriptional activation by CREB1. RNA interference (RNAi) targeting synapsin mRNA blocked the 5-HT-induced increase in synapsin protein levels and LTF; in the absence of 5-HT treatment, basal synapsin levels were unaffected. These results indicate that the 5-HT-induced regulation of synapsin levels is necessary for LTF and that this regulation is part of the cascade of synaptic events involved in the consolidation of memory.
Subject(s)
Long-Term Potentiation/physiology , Serotonin/metabolism , Synapses/metabolism , Synapsins/metabolism , Acetylation , Animals , Aplysia/physiology , Cyclic AMP Response Element-Binding Protein/genetics , Cyclic AMP Response Element-Binding Protein/metabolism , Gene Expression , Gene Expression Regulation , Histones/genetics , Histones/metabolism , Promoter Regions, Genetic , Serotonin/genetics , Synapses/genetics , Synapsins/geneticsABSTRACT
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.
Subject(s)
Aplysia/physiology , Long-Term Synaptic Depression/physiology , Motor Neurons/physiology , Neural Inhibition/physiology , Neurons, Afferent/physiology , Synaptic Transmission/physiology , Action Potentials/physiology , Animals , Computer Simulation , Electric Stimulation , Excitatory Postsynaptic Potentials/physiology , Models, Neurological , Neural Networks, Computer , Synapses/physiologyABSTRACT
The 5-HT-induced synaptic plasticity of Aplysia sensorimotor synapses has typically been probed by firing a single presynaptic spike. In this study, 5-HT-induced synaptic plasticity was probed with brief bursts of spikes (10 Hz, 1 s), which are more behaviorally relevant stimuli. Because such bursts provide a greater challenge to the release machinery than single spikes, their use may reveal additional aspects of synaptic modulation, and, in particular, the role of extracellular signal-regulated protein kinase (ERK), which has recently been implicated in several examples of short- and long-term synaptic plasticity. Excitatory postsynaptic currents (EPSCs) were characterized by their amplitudes. In addition, two kinetic measurements, time to peak and decay time constant, were determined for the initial and last EPSCs of each burst. Application of 5-HT produced a uniform increase in gain by facilitating each EPSC elicited during a burst of spikes without affecting the kinetics of the initial or last EPSC. These data suggest that short-term facilitation during a burst is mediated largely by processes such as those that affect the size of the releasable pool or rate of vesicle mobilization rather than by an increase in the duration of the presynaptic action potential. An ERK cascade inhibitor (U0126) had no effect on the 5-HT-mediated facilitation of either the initial EPSC or EPSCs elicited late in the burst.