Within the piriform cortex, layer 2/3 pyramidal cells receive direct sensory input from M/T cells on their apical dendrites. Whereas olfactory information is encoded as a spatial map of activated M/T cells in the olfactory bulb,
odor representations in layer 2/3 of the piriform cortex are distributed Selleckchem BKM120 among spatially dispersed cell ensembles and lack stereotypy (Illig and Haberly, 2003, Rennaker et al., 2007 and Stettler and Axel, 2009). The mechanisms governing this transformation from a spatially segregated representation in the olfactory bulb to one that is highly distributed and nonstereotyped in the cortex are not well understood. Individual pyramidal cells in the piriform cortex are thought to receive converging input from M/T cells belonging to different glomeruli (Apicella et al., 2010, Davison and Ehlers, 2011, Miyamichi et al., 2011 and Wilson, 2001), and M/T cell axons from individual glomeruli project diffusely throughout the piriform cortex without obvious spatial patterning (Ghosh et al., 2011 and Sosulski et al., 2011). Although it is tempting to account for cortical odor responses PI3K inhibitor entirely by the convergence and divergence of direct olfactory bulb inputs, the dendrites of the piriform cortex pyramidal cells also receive extensive intracortical associational (ASSN) connections from excitatory neurons
within the piriform cortex and other cortical regions (Haberly, 2001, Haberly and Price, 1978 and Johnson et al., 2000). Although much effort has focused on elucidating how olfactory bulb afferent sensory inputs shape cortical odor representations, the contribution of intracortical excitatory circuits
to odor responses has been largely unexplored. In this study, we examine the relative contributions of sensory afferent input and intracortical connections to odor-driven excitatory synaptic transmission in the anterior piriform cortex (APC). We take advantage of the only differential expression of presynaptic GABAB receptors in APC to selectively silence intracortical synapses while leaving afferent sensory fibers unaffected. We show that intracortical connections in APC underlie the strength of odor-evoked excitatory synaptic transmission and expand the range of odors over which pyramidal cells can respond. Our results indicate that intracortical ASSN circuits make a major contribution to odor-evoked excitation, suggesting that odor representations in the piriform cortex cannot simply be accounted for by the convergence and divergence of M/T cell inputs. GABAB receptors are expressed on nerve terminals, and activation of presynaptic GABAB receptors causes a potent inhibition of neurotransmitter release from both pyramidal cells and local interneurons throughout the cortex (Bowery, 1993).