Activity-regulated somatostatin expression reduces dendritic spine density and lowers excitatory synaptic transmission via postsynaptic somatostatin receptor 4.

Neuronal activity regulates multiple aspects of the morphological and functional development of neural circuits. One mechanism by which it achieves this is through regulation of gene expression. In a screen for activity-induced genes, we identified somatostatin (SST), a neuropeptide secreted by the SST subtype of interneurons. Using real time quantitative PCR and ELISA, we showed that persistent elevation of neuronal activity increased both the gene expression and protein secretion of SST over a relatively prolonged time course of 48 h. Using primary hippocampal neuronal cultures, we found that SST treatment for 1 day significantly reduced the density of dendritic spines, the morphological bases of excitatory synapses. Furthermore, the density of pre- and postsynaptic markers of excitatory synapses was significantly lowered following SST treatment, whereas that of inhibitory synapses was not affected. Consistently, SST treatment reduced the frequency of miniature excitatory postsynaptic currents, without affecting inhibition. Finally, lowering the endogenous level of SST receptor subtype 4 in individual hippocampal pyramidal neurons significantly blocked the effect of SST in reducing spine density and excitatory synaptic transmission in a cell autonomous fashion, suggesting that the effect of SST in regulating excitatory synaptic transmission is mainly mediated by SST receptor subtype 4. Together, our results demonstrated that activity-dependent release of SST reduced the density of dendritic spines and the number of excitatory synapses through postsynaptic activation of SST receptor subtype 4 in pyramidal neurons. To our knowledge, this is the first demonstration of the long term effect of SST on neuronal morphology.

The kinase activity of EphA4 mediates homeostatic scaling-down of synaptic strength via activation of Cdk5.

Neurons within a network have the ability to homeostatically scale-down their excitatory synaptic strength under conditions of persistent neuronal activity elevation, a process pivotal to neural circuit stability. How this homeostatic regulation is achieved at the molecular level in developing neural circuits, which face gradually elevated neuronal activity as part of circuit wiring, is not well-understood. Using dissociated hippocampal neuronal cultures, we identified a critical and cell autonomous role for the receptor tyrosine kinase EphA4 in mediating activity-induced homeostatic down-regulation of excitatory synaptic strength. Reducing the endogenous level of EphA4 in individual neurons by RNAi effectively blocked activity-induced scaling-down of excitatory synaptic strength, while co-transfection of RNAi resistant EphA4 rescued this effect. Furthermore, interfering with EphA4 forward signaling using EphA4-Fc blocked activity-induced homeostatic synaptic scaling-down, while direct activation of EphA4 with its ligand EphrinA1 weakened excitatory synaptic strength. Up- or down-regulating EphA4 function in individual neurons also did not affect the density of excitatory synapses. The kinase activities of EphA4 and its downstream effector Cdk5 were both required for homeostatic synaptic scaling, as overexpression of EphA4 with constitutively active kinase activity reduced excitatory synaptic strength, while interfering with either the kinase activity of EphA4 or Cdk5 blocked activity-induced synaptic scaling. Consistently, the activities of EphA4 and Cdk5 increased significantly during global and persistent activity elevation. Together, our work demonstrated that the kinase activity of EphA4, via activation of downstream Cdk5 activity, mediates the scaling-down of excitatory synaptic strength under conditions of global activity elevation.