Field potential and intracellular potential studies of the olfactory bulb in the carp: evidence for a functional separation of the olfactory bulb into lateral and medial subdivisions

Abstract

1. Field potentials in the olfactory bulb and intracellular potentials from mitral cells were analyzed in the carp. Electrical shocks were applied to a part of the input (the lateral or medial bundle of the olfactory nerve: l-ON or m-ON respectively) or output pathways (lateral or medial olfactory tract: LOT or MOT respectively) of the olfactory bulb in order to activate the olfactory bulb partially. 2. When shocks were applied to the regions described above, the distributions of the C2-wave component (which reflects the synaptic depolarization of the peripheral dendrites of granule cells) of field potentials were different between the lateral and medial parts of the olfactory bulb. It was suggested that the patterns of excitatory synaptic inputs to peripheral dendrites of the granule-cell population are different between the two parts of the olfactory bulb. 3. The distributions of the C3- and C4-wave components (which reflect the synaptic depolarization of somata and deep dendrites of granule cells by volleys in centrifugal nerve fibers to the olfactory bulb) of field potentials were similar between the two parts of the olfactory bulb. It was suggested that the patterns of excitatory synaptic inputs from the centrifugal fibers to somata and deep dendrites of the granule-cell population are similar between the two parts of the bulb. 4. Mitral cells activated antidromically by LOT shocks or synaptically by l-ON shocks were located mainly in the lateral part of the olfactory bulb. On the other hand, mitral cells activated antidromically by MOT shocks or synaptically by m-ON shocks were located mainly in the medial part of the bulb. 5. Mitral cells, showing IPSPs in response to LOT, MOT and l-ON shocks, but not to m-ON shocks, were located mainly in the lateral part of the olfactory bulb. On the other hand, mitral cells, showing IPSPs to MOT and m-ON shocks, but not to LOT and l-ON shocks, were located mainly in the medial part of the bulb. Such a spatial distribution of mitral cells showing IPSPs was in accordance with that expected from the spatial distribution of the field potentials. 6. These results suggest that the olfactory bulb of the carp can be functionally separated into two subdivisions (the lateral and medial parts); the activities of neurons in the one part exert little influence on neurons in the other part. 7. From these results we suppose that the olfactory system of the carp is composed of two separate systems; in the lateral olfactory system, the lateral part of the olfactory bulb receives inputs mainly from the lateral bundle of the olfactory nerve and sends outputs to the LOT, while in the medial olfactory system, the medial part of the olfactory bulb receives inputs mainly from the medial bundle of the olfactory nerve and sends outputs to the MOT.