Inhibitory mechanisms in the rabbit olfactory bulb: dendrodendritic mechanisms.

The unitary responses of mitral and granular layer cells and olfactory bulb field potentials following stimulation of the lateral olfactory tract (LOT), olfactory nerves and anterior limb of the anterior commissure (AAC) were analyzed. The hyperpolarization that followed antidromic invasion of mitral cells was shown to result from a synaptically evoked inhibitory potential. The analysis of the LOT evoked field potential indicated that the initial negative deflection represented antidromic invasion of mitral cell somata and dendrites. The results suggest that the second negative deflection fo the field potential represented a synaptically evoked depolarization of granule cell dendrites, while the succeeding positive deflection represented the synaptically evoked inhibition of mitral cell dendrites and somata. A conditioning LOT stimulus inhibited the intracellularly recorded hyperpolarization of mitral cells and abolished the field potential evoked by a test LOT stimulus. In addition, cells in the granular layer that responded to LOT stimulation were inhibited by conditioning stimuli supplied to the LOT, AAC or olfactory nerves. The results are consistent with and lend support to the dendrodendritic hypothesis advanced by Rallet al. and suggest that the cells in the granular layer which are activated by the dendrites of mitral cells aregranule cells.