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Department of Neurosurgery, Tufts Medical School and New England Medical Center, Boston, MA 02111.

1. Whole-cell patch clamp and optical recording techniques were applied to the same in vitro salamander olfactory bulb preparations to study the postsynaptic responses of single mitral/tufted cells in the context of the surrounding neural activity in which they are embedded. Mitral/tufted cells were identified by intracellular filling with biocytin. 2. Single mitral/tufted cells were under a tonic GABAA receptor-mediated inhibitory influence as revealed by the recording of bicuculline methiodide (BMI)/picrotoxin-sensitive inhibitory postsynaptic currents (IPSCs) in symmetrical chloride conditions at a holding potential of -70 mV. Depolarizing voltage steps (100 ms) applied to single cells or electrical stimulation of the olfactory nerve or medial olfactory tract evoked a prolonged increase in the frequency of GABAergic IPSCs. 3. The frequency of spontaneous and driven IPSCs was reduced with application of the glutamate receptor antagonists 6-cyano-2,3-dihydroxy-7-nitro-quionoxaline (CNQX) or 2-amino-5-phosphonopentanoic acid (AP5) whereas olfactory nerve- or medial olfactory tract-driven IPSC frequency was enhanced with removal of bathing Mg2+, indicating that GABAergic interneurones were driven by mitral/tufted cells at both non-NMDA and NMDA receptors. 4. Olfactory nerve or medial olfactory tract stimulation evoked widely distributed changes in fluorescence in preparations stained with the voltage-sensitive dye RH414. The optical response predominantly consisted of a decrease in fluorescence, indicative of depolarization. The presence of the dye did not obviously affect mitral/tufted cell postsynaptic responses. 5. BMI enhanced the amplitude and duration of optical signals related to depolarization within the bulb and in regions central to the bulb. In the presence of BMI, depolarizing activity appeared to spread hundreds of micrometres into regions of the bulb not activated in control conditions showing explicitly that GABAA receptors in the bulb participate in lateral inhibition. 6. CNQX and AP5 attenuated the optical signals within the bulb supporting the contention that in these conditions, optical signals arise mainly from granule cell dendritic activity. Furthermore, AP5 or removal of bath Mg2+ reduced or enlarged the spatial distribution of activity respectively, suggesting that in some cases the NMDA receptor may be involved in generating or stabilizing spatial patterns of activity. 7. It is concluded that in the salamander olfactory bulb, both GABAA- and glutamate receptor-mediated synaptic transmission shape the different temporal and spatial patterns of neural activity associated with olfactory coding.

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