Distinct stages of synapse elimination are induced by burst firing of CA1 neurons and differentially require MEF2A/D

Experience and activity refine cortical circuits through synapse elimination, but little is known about the activity patterns and downstream molecular mechanisms that mediate this process. We used optogenetics to drive individual mouse CA1 hippocampal neurons to fire in theta frequency bursts to understand how cell autonomous, postsynaptic activity leads to synapse elimination. Brief (1 hr) periods of postsynaptic bursting selectively depressed AMPA receptor (R) synaptic transmission, or silenced excitatory synapses, whereas more prolonged (24 hr) firing depressed both AMPAR and NMDAR EPSCs and eliminated spines, indicative of a synapse elimination. Both synapse silencing and elimination required de novo transcription, but only silencing required the activity-dependent transcription factors MEF2A/D. Burst firing induced MEF2A/D-dependent induction of the target gene Arc which contributed to synapse silencing and elimination. This work reveals new and distinct forms of activity and transcription-dependent synapse depression and suggests that these processes can occur independently.

Experience-Dependent and Differential Regulation of Local and Long-Range Excitatory Neocortical Circuits by Postsynaptic Mef2c

Development of proper cortical circuits requires an interaction of sensory experience and genetic programs. Little is known of how experience and specific transcription factors interact to determine the development of specific neocortical circuits. Here, we demonstrate that the activity-dependent transcription factor, Myocyte enhancer factor-2C (Mef2c), differentially regulates development of local versus long-range excitatory synaptic inputs onto layer 2/3 neurons in the somatosensory neocortex in vivo. Postnatal, postsynaptic deletion of Mef2c in a sparse population of L2/3 neurons suppressed development of excitatory synaptic connections from all local input pathways tested. In the same cell population, Mef2c deletion promoted the strength of excitatory inputs originating from contralateral neocortex. Both the synapse promoting and synapse suppressing effects of Mef2c deletion required normal whisking experience. These results reveal a role of Mef2c in experience-dependent development of specific sensory neocortical circuits.

Fragile X mental retardation protein regulates synaptic and behavioral plasticity to repeated cocaine administration

Repeated cocaine exposure causes persistent, maladaptive alterations in brain and behavior, and hope for effective therapeutics lies in understanding these processes. We describe here an essential role for fragile X mental retardation protein (FMRP), an RNA-binding protein and regulator of dendritic protein synthesis, in cocaine conditioned place preference, behavioral sensitization, and motor stereotypy. Cocaine reward deficits in FMRP-deficient mice stem from elevated mGluR5 (or GRM5) function, similar to a subset of fragile X symptoms, and do not extend to natural reward. We find that FMRP functions in the adult nucleus accumbens (NAc), a critical addiction-related brain region, to mediate behavioral sensitization but not cocaine reward. FMRP-deficient mice also exhibit several abnormalities in NAc medium spiny neurons, including reduced presynaptic function and premature changes in dendritic morphology and glutamatergic neurotransmission following repeated cocaine treatment. Together, our findings reveal FMRP as a critical mediator of cocaine-induced behavioral and synaptic plasticity.