Inhibitory synapse formation
Chemical synapses are the key connective elements in neuronal networks. They are not only crucial for information processing but their plasticity also endows the brain with its outstanding capacity for adaptation to the environment. Understanding synapses, and how they are formed, is therefore a fundamentally important task in neuroscience. So far, most studies have focused on the formation of glutamatergic synapses while the formation of the other major type of synapses in the brain, which use the inhibitory transmitter gamma aminobutyric acid (GABA), remains less examined, in spite of the fact that GABAergic synapses form 10-20% of all synapses in the brain and are indispensable for proper and stable functioning of in the brain.
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Figure 1: GABAergic axons (green) make inhibitory synapses of dendrites of CA1 pyramidal neurons (red).
Basic information, well known for glutamatergic synapses, such as the specific pre- and postsynaptic protein complexes that are present and the time course by which proteins are recruited to new synapses, is not yet well understood for GABAergic synapses. Also, many molecular tools such as specific promoters or fluorescent synaptic proteins are designed specifically for glutamatergic and have yet to be developed for GABAergic synapses. This makes studying GABAergic synapse formation more challenging, but perhaps also more exciting.
Glutamatergic synapses on excitatory neurons in the hippocampus are almost exclusively located on dendritic spines. It has been shown that dendrites play an active role in the formation of new glutamatergic synapses by growing out small protrusions which make contact with presynaptic axons and boutons. In contrast to glutamatergic synapses, GABAergic synapses are usually not located on spines, but directly on the dendritic shaft.
Methods
We use high-resolution two-photon imaging to examine the formation of GABAergic synapses in the CA1 area of organotypical hippocampal cultures. We make use of GAD65-GFP mice, in which a subset of GABAergic interneurons express GFP (López-Bendito et al., 2004). CA1 pyramidal neurons are filled with Alexa Fluor 594 through a patch pipette.
Results
We found that GABAergic synapses are formed via a fundamentally different process than glutamatergic spine synapses. Outgrowing dendritic protrusions can distinguish between potential presynaptic partners. Whereas contacts with glutamatergic boutons could be long-lasting, contacts with GABAergic boutons were always short-lived and resulted in retraction of the dendritic protrusions. Similarly, contacts made by GABAergic axonal protrusions were always transient. This strongly suggests that axonal and dendritic protrusions do not mediate the formation of GABAergic synapses. New GABAergic contacts were formed exclusively at locations where GABAergic axons and postsynaptic dendrite are already in close proximity: by the appearance of new boutons at preexisting axon-dendrite crossings (Figure 1).
Our findings imply that GABAergic axons in a mature network can make new synapses only with postsynaptic partners that are in their immediate neighborhood, which is in marked contrast to the way glutamatergic synapses are made. This puts significant structural constraints on the generation and plasticity of GABAergic and glutamatergic synapses which will be important to be kept in mind when trying to understand development and plasticity of the intricate neural networks of the brain.
Open questions
It is presently not known what determines when and where new GABAergic boutons appear. We would like to know how the formation of GABAergic synapses is influenced by (network) activity. Another future goal is to determine the molecular players underlying these processes.
