Membrane and synaptic properties of mitral cells in slices of rat olfactory bulb

We have investigated the membrane properties and excitatory synaptic transmission of mitral cells in a slice preparation of rat olfactory bulb. In response to intracellular injection of depolarizing current, most mitral cells showed several distinct membrane properties: (1) delayed onset of firing (suggesting the presence of a type of potassium A current); (2) subthreshold oscillation of the membrane potential; and (3) repetitive firing of clustered action potentials during prolonged threshold stimulation. Olfactory nerve (ON) stimulation evoked a long-lasting EPSP in most of the mitral cells. This long EPSP was completely blocked by combined application of NMDA and non-NMDA receptor antagonists (20 mu M CNQX and 100 mu M APV), confirming that glutamate is the neurotransmitter at the synapses from ON to mitral cells. The ON-evoked EPSP was often preceded by a prespike, which was resistant to membrane potential hyperpolarization at the soma. This fast prepotential may be indicative of an active response in the primary dendritic tufts of the mitral cells. Stimulation of the lateral olfactory tract evoked an antidromic impulse followed by a short EPSP, which could also be elicited independently of an antidromic spike in the recorded cell. Since the asymmetrical synapses so far observed on the mitral cells are all from the ON, this antidromically evoked EPSP may reflect self-excitation of a mitral cell by glutamate released from its own dendrites by antidromic impulse invasion, or/and lateral excitation by neighboring invaded dendrites.