Long-Term Modifications in the Strength of Excitatory Associative Inputs in the Piriform Cortex

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Abstract
Afferent olfactory information, in vivo and in vitro, can be rapidly adapted to through a metabotropic glutamate receptor (mGluR)–mediated attenuation of synaptic strength. Specific cellular and synaptic mechanisms underlying olfactory learning and habituation at the cortical level remain unclear. Through whole-cell recording, excitatory postsynaptic currents (EPSCs) were obtained from piriform cortex (PC) principal cells. Using a coincidental, pre- and postsynaptic stimulation protocol, long-term depression (LTD) in synaptic strength was induced at associative, excitatory synapses onto layer II pyramidal neurons of the mouse (P15-27) PC. LTD was mimicked and occluded by mGluR agonists and blocked by nonselective mGluR antagonist (RS)-α-methyl-4-sulfonophenylglycine (MSPG) but not by N-methyl-D-aspartic acid (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (APV). Analysis of the paired-pulse ratio, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/NMDA current ratio, and spontaneous EPSCs indicate that electrically induced LTD was mediated predominantly by postsynaptic mechanisms. Additionally, presynaptic mGluRs were involved in agonist-mediated synaptic depression. Immunohistochemical analysis supports the presence of multiple subclasses of mGluRs throughout the PC, with large concentrations of several receptors present in layer II. These observations provide further evidence of activity-dependent, long-term modification of associative inputs and its underlying mechanisms. Cortical adaptation at associative synapses provides an additional link between cortical olfactory processing and subcortical centers that influence behavior.