Previous studies showed that approximately 60–70% of the LA spines forming synapses with thalamic or cortical afferents were GluR1-immunoreactive (Farb and LeDoux, 1997, 1999). This difference in proportion of GluR1 immunoreactivity raises the possibility that PEG, as opposed to the horseradish peroxidase-diaminobenzidine (HRP-DAB) that was used in previous studies, resulted in greater failure to detect GluR1 immunoreactivity. However, the rate of detection of GluR2/3 within spines by the two procedures is comparable (60–70% for both) (Farb and LeDoux, 1997, 1999; Radley et al., 2007), so the idea that PEG is less efficient than HRP-DAB for detecting synaptic proteins is certainly not generalizable. Another possibility is that the lower level of GluR1 detected in the current study reflects differential sensitivity of the two GluR subunits to the tissue fixation condition or occurred because the other studies included the intercalated, GABAergic neurons that express high levels of GluR1.
