The A kinase anchor protein AKAP150 recruits the cAMPdependent protein kinase (PKA) to dendritic spines. Here we show that in AKAP150 (AKAP5) knock-out (KO) mice frequency of miniature excitatory post-synaptic currents (mEPSC) and inhibitory post-synaptic currents (mIPSC) are elevated at 2 weeks and, more modestly, 4 weeks of age in the hippocampal CA1 area versus litter mate WT mice. Linear spine density and ratio of AMPAR to NMDAR EPSC amplitudes were also increased. Amplitude and decay time of mEPSCs, decay time of mIPSCs, and spine size were unaltered. Mice in which the PKA anchoring C-terminal 36 residues of AKAP150 are deleted (D36) showed similar changes. Furthermore, whereas acute stimulation of PKA (2-4 h) increases spine density, prolonged PKA stimulation (48 h) reduces spine density in apical dendrites of CA1 pyramidal neurons in organotypic slice cultures. The data from the AKAP150 mutant mice show that AKAP150-anchored PKA chronically limits the number of spines with functional AMPARs at 2-4 weeks of age. However, synaptic transmission and spine density was normal at 8 weeks in KO and D36 mice. Thus AKAP150-independent mechanisms correct the aberrantly high number of active spines in juvenile AKAP150 KO and D36 mice during development.Dendritic spines receive most of the excitatory input in mammalian neurons (1, 2) and compartmentalize signaling that regulates the synaptic response (3, 4). Several protein kinases and phosphatases, including CaMKII, cAMP-dependent protein kinase (PKA), 3 PKC, and protein phosphatase PP2A and PP2B (calcineurin) (5) play prominent roles in synaptic plasticity. Synaptic activity translates into their activation, thereby affecting spine stability, morphology, and postsynaptic responses (6 -13). Spines are dynamically gained and lost during synaptogenesis (9, 14 -16) and in response to neuronal activity (17-19).AKAP150 is a product of the Akap5 gene in mice and is the main AKAP that recruits PKA to postsynaptic sites (20 -25). It binds with its very C terminus to the N-terminal dimerization domain of the regulatory RII subunits of PKA (26,27). It also binds PKC (28), PP2B (29,30), and the SH3-GK region of the postsynaptic scaffolding proteins SAP97 and PSD-95 (31, 32). PSD-95 binds with its PDZ domains to the C-terminal ((S/ T)X(I/V)) motif of NMDA-type glutamate receptor (NMDAR) NR2 subunits (33). PSD-95 also binds to the C terminus of stargazin (␥2) and its homologues ␥3, ␥4, and ␥8, which associate with AMPAR-type glutamate receptor (AMPAR) for postsynaptic localization in conjunction with . SAP97 directly binds to the AMPAR GluR1 subunit via a PDZ-C-terminal interaction (37) to link AKAP150 and thereby PKA, PKC,. Phosphorylation of GluR1 on Ser-845 by PKA can increase channel activity (42) and accumulation of GluR1-containing AMPAR at the postsynaptic site during hippocampal LTP (43, 44) (see also Refs. 45,46). The N terminus of AKAP150 also binds F-actin, cadherin, adenylyl cyclases, and PIP 2 and targets AKAP150 to dendritic spines and the central region binds PSD-...