Silent synapses in neural plasticity: Current evidence

7Silent synapses, defined as structural specializations for neurotransmissi on that do not produce a physiological response in the receiving cell, may occur frequently in neural circuits. Their recruitment to physiological eff ectiveness may be an important component of circuit modification. In severa l nervous systems, evidence from electrophysiological and optophysiological measurements has established a strong case for the existence of silent syn apses and for their emergence as active synapses with appropriate stimulati on. During normal development and aging, synapses of individual neurons cha nge in number, and many of these may be functionally silent at certain stag es of their developmental trajectory. Changes in their status may contribut e to shaping the properties of neural pathways during development, often in response to neural activity. In general, it is often difficult to distingu ish physiological emergence of pre-established silent synapses from develop mental maturation or de novo formation of new synapses. Several possible me chanisms for silent synapses and their recruitment are reviewed. These incl ude incompletely assembled synapses that lack structural components, insuff icient availability of key presynaptic proteins, and nonfunctional postsyna ptic receptors, or presence of receptors that do not mediate a postsynaptic response except under specific conditions (conditionally silent synapses). The available silent synapses can often be rapidly activated, and converse ly, active synapses appear to be rapidly silenced in many instances. These properties enable silent synapses to participate in short-term facilitation and depression. In addition, they may contribute to long-term facilitation and potentiation, especially during development.