Inhibitory neurons have been reported to utilize AMPARs that lack GluR2 subunits (McDonald, 1996) and thus are calcium-permeable (Seeburg, 1993). If the shafts exhibiting augmented GluR1 immunoreactivity following the paired presentation of stimuli do belong to inhibitory interneurons, then this change is likely to exert a potent influence upon the postsynaptic membrane potential and allow influx of calcium, independent of NMDAR activity. In both the cortex (Hull et al., 2009) and LA (Rainnie et al., 1991; Bissiere et al., 2003; Kodirov et al., 2006; Shaban et al., 2006), the excitatory afferents targeting inhibitory interneurons exert potent feed-forward inhibition upon the spiny excitatory neurons. In the LA, this pathway is active, even in the naïve state, but even more, following paired conditioning, according to the results of the present study. Following the paired presentation of stimuli, an increase in the feed-forward inhibition may occur specifically upon the pathways in the LA that were not simultaneously activated by CS and US inputs, and, in this way, increase the signal-to-noise ratio for the pathway subserving the CS and also contribute toward maintaining an overall homeostasis of LA excitability.
