A Shout Out to Immature Synapses. Focus on “Different Roles for AMPA and NMDA Receptors in Transmission at the Immature Retinogeniculate Synapse”

Abstract

A major topic of investigation in the field of developmental neuroscience is to understand how newly formed excitatory synapses receive and transmit information. It is now widely accepted that neural activity plays an important role in a number of developmental processes, including the refinement and establishment of orderly connections (Grubb and Thompson 2004). However, it remains unclear how many immature excitatory synapses (i.e., ones that utilize glutamate as a neurotransmitter) participate in the relay of activity. Contributing to this problem is the finding that immature glutamatergic synapses are comprised largely of N-methyl-D-aspartate (NMDA) receptors and lack AMPA receptors that mediate fast excitatory transmission. Such synapses are said to be “silent,” incapable of causing postsynaptic action potentials from resting levels because in addition to glutamate, the NMDA receptor requires membrane depolarization to relieve a voltage-dependent Mg 2 block. How then do silent synapses “speak” loudly enough to cause postsynaptic firing and thereby successfully relay information? In a recent article, Liu and Chen (2008) provide some answers to this question. They examine the synaptic responses and intrinsic properties of developing neurons by making use of an in vitro thalamic slice preparation that maintains the excitatory synaptic connections between retinal ganglion cells and relay cells of the dorsal lateral geniculate nucleus (LGN). In an elegant series of whole cell recording experiments, they show how a constellation of properties, including ligand-gated channel kinetics, receptor subunit composition, extended presence of neurotransmitter