Yet another more interesting explanation for the difference observed among the studies is that a substantial proportion of spines in the LA receive inputs from sources other than the thalamus or cortex, the two sources for which GluR1 immunoreactivity at synapses had been assessed by the HRP-DAB method (Farb and LeDoux, 1997, 1999). If spines that receive input from sources other than thalamus or cortex express only low levels of GluR1, then our PEG procedure, which did not preselect for glutamergic axons of any particular source, would yield a lower proportion of GluR1-immunoreactive spines (~20%).
