The role of metabotropic glutamate receptors and cortical adaptation in habituation of odor-guided behavior

Decreases in behavioral investigation of novel stimuli over time may bemediated by a variety of factors including changes in attention, internalstate, and motivation. Sensory cortical adaptation, a decrease in sensorycortical responsiveness over prolonged stimulation, may also play a role. Inolfaction, metabotropic glutamate receptors on cortical afferent pre-synapticterminals have been shown to underlie both cortical sensory adaptation andhabituation of odor-evoked reflexes. The present experiment examined whetherblockade of sensory cortical adaptation through bilateral infusion of thegroup III metabotropic glutamate receptor antagonistcyclopropyl-4-phosphonophenylglycine (CPPG) into the anterior piriform cortexcould reduce habituation of a more complex odor-driven behavior such asinvestigation of a scented object or a conspecific. The results demonstratethat time spent investigating a scented jar, or a conspecific, decreases overthe course of a continuous 10 minute trial. Acute infusion of CPPG bilaterallyinto the anterior piriform cortex significantly enhanced the time spentinvestigating the scented jar compared to investigation time in control rats,without affecting overall behavioral activity levels. Infusions into the brainoutside of the piriform cortex were without effect. CPPG infusion into thepiriform cortex also produced an enhancement of time spent investigating aconspecific, although this effect was not significant.