Figure 2.

LTD unmasks cocaine-generated silent synapses. A, Diagram showing experimental timeline, in which the silent synapses assessment was ∼1 h after the last cocaine session on training day 5 (operationally defined as withdrawal day 0). B, C, Example traces (B) and summary (C) showing that on withdrawal day 0, NAcSh MSNs from cocaine-trained rats exhibited prolonged decay kinetics of NMDAR EPSCs compared with saline-trained rats. D, E, Example EPSCs evoked at −70 or +50 mV (insets) over 100 consecutive trials in the minimum stimulation assay from recordings on withdrawal day 0 after saline (D) or cocaine (E) self-administration. F, Summary showing a comparably low percentage of silent synapses in NAcSh MSNs between saline- and cocaine-trained rats on withdrawal day 0. G, Example NAcSh dendrites from saline-trained rats (top) and cocaine-trained rats (bottom) on withdrawal day 0. H, Summary showing that on withdrawal day 0, spine densities of NAcSh MSNs in saline- and cocaine-trained rats were both higher than age-matched naive control rats (indicated by the dashed line; control: 1.6 ± 0.1, n/m = 54/15; cocaine: 2.0 ± 0.1, n/m = 34/8; t₍₈₆₎ = 5.4, p < 0.01). I, Schematic illustration depicting the hypothesized two-step process of cocaine-induced generation of silent synapses, including the first step of generation of synapses containing both AMPARs and NMDARs during cocaine experience, and the second step of internalization of AMPARs from newly generated synapses after short-term withdrawal. J–O, Example EPSCs and trials in the minimum stimulation assay and summary of failure rates after a mild LTD induction in NAcSh MSNs from naive rats (J, K), cocaine-trained rats on withdrawal day 0 (L, M), or cocaine-trained rats on withdrawal day 1 (N, O). P, Summary showing that a mild LTD induction selectively increased the failure rate of AMPAR EPSCs in NAcSh MSNs from cocaine-trained rats on withdrawal day 0, but not on withdrawal day 1 or in drug-naive rats. Q, Summary showing in cocaine-trained rats that the low percentage of NAcSh silent synapses on withdrawal day 0 was increased by LTD induction to a level comparable to that on withdrawal day 1. R, S, Self-administration results of rats whose electrophysiological and morphologic results are presented in B–Q. *p < 0.05; **p < 0.01; ns, no significance.