Presynaptic control of sensory input on the mammalian olfactory system

Presynaptic inhibition modulates signal transmission§in the olfactory pathway by several mechanisms, a§major one being mediated by GABAB receptors, which§suppress presynaptic calcium influx and subsequent§transmitter release from the receptor neuron§terminal. Presynaptic inhibition could function to§limit the strength of olfactory input to the central§nervous system, as well as provide a substrate for§centrifugal control of odorant representations early§in the olfactory system. Odorant and electrical§stimulation combined with in vivo pharmacology were§used to characterize the functional determinants of§GABAB-mediated presynaptic. The results of this work§showed that tonic inhibition is a major determinant§of receptor input strength; and were dependent on§glutamatergic transmission from second-order neurons§in the glomerular layer. When the behavioral state of§the awake animal was altered, odorant-evoked§responses were affected. Taken together these results§suggest that tonic inhibition via top-down,§centrifugal control plays a major role in modulating§the magnitude of sensory input to the brain as a§function of behavioral state.