Synaptic scaling is a form of homeostatic plasticity that scales synaptic strengths up or down to compensate for prolonged changes in activity. It has been controversial whether this plasticity is expressed presynaptically, postsynaptically, or both. Here we describe in detail the homeostatic changes that take place at excitatory synapses in visual cortical cultures after 1 or 2 d of activity blockade.

A fundamental property of neuronal circuits is the ability to adapt to altered sensory inputs. It is well established that the functional synaptic changes underlying this adaptation are reflected by structural modifications in excitatory neurons. In contrast, the degree to which structural plasticity in inhibitory neurons accompanies functional changes is less clear. Here, we use two-photon imaging to monitor the fine structure of inhibitory neurons in mouse visual cortex after deprivation induced by retinal lesions. We find that a subset of inhibitory neurons carry dendritic spines, which form glutamatergic synapses. Removal of visual input correlates with a rapid and lasting reduction in the number of inhibitory cell spines. Similar to the effects seen for dendritic spines, the number of inhibitory neuron boutons dropped sharply after retinal lesions. Together, these data suggest that structural changes in inhibitory neurons may precede structural changes in excitatory circuitry, which ultimately result in functional adaptation following sensory deprivation.

Homeostatic plasticity of excitatory synapses plays an important role in stabilizing neuronal activity, but the mechanism of this form of plasticity is incompletely understood. In particular, whether the locus of expression is presynaptic or postsynaptic has been controversial. Here we show that the expression locus depends on the time neurons have spent in vitro. In visual cortical cultures < or =14 days in vitro (DIV), 2 days of TTX treatment induced an increase in miniature excitatory postsynaptic current (mEPSC) amplitude onto pyramidal neurons, without affecting mEPSC frequency. However, in cultures > or =18 DIV, the same TTX treatment induced a large increase in mEPSC frequency, whereas the amplitude effect was reduced. The increased mEPSC frequency was associated with an increased density of excitatory synapses and increased presynaptic vesicle release in response to electrical stimulation. This indicates a shift from a predominantly postsynaptic response to TTX in < or =14 DIV cultures, to a coordinated pre- and postsynaptic response in > or =18 DIV cultures. This shift was not specific for cortical cultures because a similar shift was observed in cultured hippocampal neurons. Culturing neurons from older animals showed that the timing of the switch depends on the time the neurons have spent in vitro, rather than their postnatal age. This temporal switch in expression locus can largely reconcile the contradictory literature on the expression locus of homeostatic excitatory synaptic plasticity in central neurons. Furthermore, our results raise the intriguing possibility that the expression mechanism of homeostatic plasticity can be tailored to the needs of the network during different stages of development or in response to different challenges to network function.

Repetitive transcranial magnetic stimulation (rTMS) is used as a therapeutic tool in neurology and psychiatry. While repetitive magnetic stimulation (rMS) has been shown to induce plasticity of excitatory synapses, it is unclear whether rMS can also modify structural and functional properties of inhibitory inputs. Here we employed 10-Hz rMS of entorhinohippocampal slice cultures to study plasticity of inhibitory neurotransmission on CA1 pyramidal neurons. Our experiments reveal a rMS-induced reduction in GABAergic synaptic strength (2–4 h after stimulation), which is Ca2+-dependent and accompanied by the remodelling of postsynaptic gephyrin scaffolds. Furthermore, we present evidence that 10-Hz rMS predominantly acts on dendritic, but not somatic inhibition. Consistent with this finding, a reduction in clustered gephyrin is detected in CA1 stratum radiatum of rTMS-treated anaesthetized mice. These results disclose that rTMS induces coordinated Ca2+-dependent structural and functional changes of specific inhibitory postsynapses on principal neurons.