MEMBRANE AND SYNAPTIC PROPERTIES OF MITRAL CELLS IN SLICES OF RAT OLFACTORY-BULB

We have investigated the membrane properties and excitatory synaptic t ransmission of mitral cells in a slice preparation of rat olfactory bu lb. In response to intracellular injection of depolarizing current, mo st mitral cells showed several distinct membrane properties: (1) delay ed onset of firing (suggesting the presence of a type of potassium A c urrent); (2) subthreshold oscillation of the membrane potential; and ( 3) repetitive firing of clustered action potentials during prolonged t hreshold stimulation. Olfactory nerve (ON) stimulation evoked a long-l asting EPSP in most of the mitral cells. This long EPSP was completely blocked by combined application of NMDA and non-NMDA receptor antagon ists (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-evok ed EPSP was often preceded by a prespike, which was resistant to membr ane potential hyperpolarization at the soma. This fast prepotential ma y be indicative of an active response in the primary dendritic tufts o f the mitral cells. Stimulation of the lateral olfactory tract evoked an antidromic impulse followed by a short EPSP, which could also be el icited independently of an antidromic spike in the recorded cell. Sinc e the asymmetrical synapses so far observed on the mitral cells are al l from the ON, this antidromically evoked EPSP may reflect self-excita tion of a mitral cell by glutamate released from its own dendrites by antidromic impulse invasion, or/and lateral excitation by neighboring invaded dendrites.