FUNCTIONAL-ORGANIZATION OF RAT OLFACTORY-BULB GLOMERULI REVEALED BY OPTICAL IMAGING

The functional organization and synaptic physiology of olfactory bulb glomeruli were studied in rat in vitro slice preparations stained with the voltage-sensitive dye RH-155. Optical signals were recorded with a 100-element photodiode array at high temporal resolution, Pharmacolo gical and ionic manipulations were used to investigate synaptic respon ses to stimulation of the olfactory nerve layer (ONL), ONL stimulation evoked a sodium-mediated compound action potential that propagated ac ross the ONL and invaded individual glomeruli, This presynaptic volley evoked calcium-dependent synaptic responses the amplitudes of which w ere largest within the glomerular layer (GL); smaller amplitude respon ses were recorded in deeper layers of the olfactory bulb. Synaptic res ponses in the Gt were attenuated by the non-NMDA ionotropic glutamate receptor antagonist CNQX; the residual component was suppressed by the NMDA glutamate receptor antagonist AP-5. The GABA(A) receptor antagon ist bicuculline methiodide had little effect, whereas the GABA(B) rece ptor agonist baclofen dramatically attenuated ONL-evoked synaptic resp onses. The effects of baclofen were reversed by the GABA(B) receptor a ntagonist CGP35348. Paired-pulse depression of ONL-evoked synaptic res ponses in the GL was partially reversed by CGP35348, These findings su ggest that olfactory nerve axons release glutamate to activate both NM DA and non-NMDA receptors on GL neurons, that GABA(A) receptor-mediate d inhibition has little effect on these responses, and that GABA(B) re ceptor-mediated inhibition may act presynaptically on olfactory nerve axons to modulate their inputs to olfactory bulb neurons.