Inhibition of Olfactory Receptor Neuron Input to Olfactory Bulb Glomeruli Mediated by Suppression of Presynaptic Calcium Influx

1 Department of Biology, Boston University, Boston, Massachusetts; 2 Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut; and 3 Department of Anatomy and Neurobiology and Program in Neuroscience, University of Maryland School of Medicine, Baltimore, Maryland Submitted 17 March 2005; accepted in final form 18 May 2005 We investigated the cellular mechanism underlying presynaptic regulation of olfactory receptor neuron (ORN) input to the mouse olfactory bulb using optical-imaging techniques that selectively report activity in the ORN presynaptic terminal. First, we loaded ORNs with calcium-sensitive dye and imaged stimulus-evoked calcium influx in a slice preparation. Single olfactory nerve shocks evoked rapid fluorescence increases that were largely blocked by the N-type calcium channel blocker -conotoxin GVIA. Paired shocks revealed a long-lasting suppression of calcium influx with 40% suppression at 400-ms interstimulus intervals and a recovery time constant of 450 ms. Blocking activation of postsynaptic olfactory bulb neurons with APV/CNQX reduced this suppression. The GABA B receptor agonist baclofen inhibited calcium influx, whereas GABA B antagonists reduced paired-pulse suppression without affecting the response to the conditioning pulse. We also imaged transmitter release directly using a mouse line that expresses synaptopHluorin selectively in ORNs. We found that the relationship between calcium influx and transmitter release was superlinear and that paired-pulse suppression of transmitter release was reduced, but not eliminated, by APV/CNQX and GABA B antagonists. These results demonstrate that primary olfactory input to the CNS can be presynaptically regulated by GABAergic interneurons and show that one major intracellular pathway for this regulation is via the suppression of calcium influx through N-type calcium channels in the presynaptic terminal. This mechanism is unique among primary sensory afferents. Address for reprint requests and other correspondence: M. Wachowiak, Dept. of Biology, Boston University, 5 Cummington St. Boston, MA 02215 (E-mail: dmattw{at}bu.edu )