| Research Abstract |
Recent progress in the studies of the olfactory system makes it one of useful sensory model systems for understanding neural mechanisms for the information processing. In the olfactory bulb, the primary center of the olfactory system, glomeruli are regarded as important functional units in the transmission of odorant signals and in processing the olfactory information, but have been believed to be composed by only a small number of neuronal types and thus to be simple in their neuronal and synaptic organization. However, accumulating morphological data reveal that each type of neurons might further consist of several different subpopulations, indicating that the organization of glomeruti might not be so simple as it was believed.
We analyzed the neuronal organization of the rat OB immunocytochemically and found the chemical and structural heterogeneity of the periglomerular cells, proposing to classify them into two types, type 1 and 2. Type 1 periglomerular cells send their intraglomerular dendrites both into the olfactory nerve (ON) zone and non-ON zone, whereas type 2 periglomerular cells send their intraglomerular dendrites only into the non-ON zone. Type 1 cells might include GABAergic neurons and type 2 cells apparently include CB-IR and CR-IR neurons. The differences in dendritic projection of two categories of periglomerular cells, especially their prominent differences in the synaptic inputs from the olfactory nerve terminals, lead us to speculate their possible functional differences in the local circuit. Type 1 cells are likely to receive activation from ON afferents at the same time when projection neurons receive them, whereas type 2 cells are activated via synapses from projection neurons only after projection neurons are activated. As both types make many synapses, including reciprocal ones, with projection neurons, these differences in synaptic connections might cause various physiological responses of projection neurons to single odorant molecules.
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