In the mammalian brain, synapse development is regulated by the excitatory input that neurons receive. The molecular mechanisms that lead from excitation to the establishment of excitatory synapses are quite well understood, but less is known about the mechanisms that modulate the development of inhibitory synapses. Greenberg and

colleagues have now demonstrated the involvement of the activity- regulated transcription factor NPAS4 in the modification of both types of synapse.

The authors used DNA micro-arrays and other means to identify transcription factors that are regu-lated specifically by Ca2+ influx into neurons during the period of devel-opment in which inhibitory synapses are maturing. They found just one, NPAS4, and went on to investigate whether this protein has a role in the regulation of synapses.

As a marker of inhibitory synapses, the authors used the presence of opposed GAD65 (a GABA-producing enzyme) puncta at the presynaptic membrane and GABAA-γ2 (a GABAA receptor subu-nit) puncta at the postsynaptic mem-brane. Immunostaining revealed that knocking down NPAS4 with RNA interference (RNAi) in dissociated, immature rat hippocampal cultures significantly reduced the colocaliza-tion of the two types of puncta (and, by proxy, the number of inhibitory synapses), whereas overexpressing NPAS4 had the opposite effect. The density of the GABAA-γ2 puncta was most affected, and so the authors inferred that NPAS4 regulates inhibi-tory synapses through effects on postsynaptic specializations. By con-trast, manipulation of NPAS4 levels

had no effect on the development of excitatory synapses.

The authors also investigated the effects of NPAS4 on synapses in functional circuits in rat organotypic hippocampal slices. Acute removal of Npas4 from these cells using a Cre recombinase system increased the interval between miniature inhibitory postsynaptic currents, indicating that there was a decrease in inhibitory inputs onto these neurons; conversely, it decreased the interval between miniature excitatory postsynaptic currents, indicating that there was an increase in the number and/or strength of excitatory synapses. Overexpression of NPAS4 had the opposite effect on both types of current.

These results indicate that NPAS4 regulates the number of inhibitory synapses in the developing brain and has an important role in modulating the strength and number of both inhibitory and excitatory synapses in response to excitatory input in the mature nervous system. Further characterization of the genes that are regulated by NPAS4 will improve our understanding of how neuronal activity modulates synaptic function.

Craig Nicholson

ORIGINAL RESEARCH PAPER Lin, Y. et al. Activity-dependent regulation of inhibitory synapse development by Npas4. Nature 24 Sep 2008 (doi:10.1038/nature0731945)

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Altering your inhibitions

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NATuRe RevIewS | neuroscience vOlume 9 | NOvemBeR 2008

Nature Reviews Neuroscience | AOP, published online 15 october 2008; doi:10.1038/nrn2520