Impulse activity in presynaptic dendrites: Analysis of mitral cells in the isolated turtle olfactory bulb

Impulse activity has been reported in neuronal dendrites in several regions of the central nervous system, where it is believed to assist in boosting transmission of signals from remote dendritic sites to the cell body. We have studied this activity in the dendrites of mitral cells in an isolated preparation of the turtle olfactory bulb. Intracellular recordings have been obtained from mitral cells responding to single volleys in the olfactory nerves or lateral olfactory tract. In addition to the large somatic spike, a small fast prepotential (FPP) was present in nearly all cells in response to an orthodromic volley in the olfactory nerves, but it was never seen in antidromic responses from the lateral olfactory tract. Collision tests using antidromic and orthodromic volleys showed that the FPP does not propagate into the axon. Hyperpolarizing current injections caused delay and blocking of the soma spike with little effect on the FPP response. These and other tests provided evidence to localize the FPP in the dendrites and to distinguish it from injury potentials and from spikes in the axon hillock or axonal initial segment. These results suggest that one function of the impulse in mitral cell dendrites is the classical one of boosting transmission of synaptic responses from the glomerular tuft to the cell body. In addition, it is well established that mitral cell dendrites are presynaptic to the dendrites of interneurons within the bulb and that these connections provide pathways for recurrent inhibition of the mitral cells. It therefore appears that the dendritic impulse in mitral cells acts as a booster for local dendritic synaptic output. These results provide further evidence for the multiple state-dependent input-output functions of cells with presynaptic dendrites.