  NMDA receptors are ion channels activated by the neurotransmitter glutamate
in the mammalian brain and are important in synaptic function and plasticity,
but are also found in extrasynaptic locations and influence neuronal
excitability. There are different NMDA receptor subtypes which differ in their
single-channel conductance. Recently, synaptic plasticity has been studied in
mouse barrel cortex, the primary sensory cortex for input from the animal's
whiskers. Pharmacological data imply the presence of low-conductance NMDA
receptors in spiny stellate neurons of cortical layer 4, but of
high-conductance NMDA receptors in pyramidal neurons of layer 2/3. Here, to
obtain complementary electrophysiological information on the functional NMDA
receptors expressed in layer 4 and layer 2/3 neurons, single NMDA receptor
currents were recorded with the patch-clamp method. Both cell types were found
to contain high-conductance as well as low-conductance NMDA receptors. The
results are consistent with the reported pharmacological data on synaptic
plasticity, and with previous claims of a prominent role of low-conductance
NMDA receptors in layer 4 spiny stellate neurons, including broad integration,
amplification and distribution of excitation within the barrel in response to
whisker stimulation, as well as modulation of excitability by ambient
glutamate. However, layer 4 cells also expressed high-conductance NMDA
receptors. The presence of low-conductance NMDA receptors in layer 2/3
pyramidal neurons suggests that some of these functions may be shared with
layer 4 spiny stellate neurons.
