Current-source density analysis in the rat olfactory bulb: Laminar distribution of kainate/AMPA- and NMDA-receptor-mediated currents

The one-dimensional current-source density method was used to analyze lamin ar field potential profiles evoked in rat olfactory bulb slices by stimulat ion in the olfactory nerve (ON) layer or mitral cell layer (MCL) and to ide ntify the field potential generators and the characteristics of synaptic ac tivity in this network. Single pulses to the ON evoked a prolonged (greater than or equal to 400 ms) sink (S1(ON)) in the glomerular layer (GL) with c orresponding sources in the external plexiform layer (EPL) and MCL and a re latively brief sink (S2(ON)) in the EPL, reversing in the internal plexifor m and granule cell layers. These sink/source distributions suggested that S 1(ON) and S2(ON) were generated in the apical dendrites of mitral/tufted ce lls and granule cells, respectively. The kainate/AMPA-receptor antagonist C NQX (10 mu M) reduced the early phase of S1(ON), blocked S2(ON), and reveal ed a low amplitude, prolonged sink at the location of S2(ON) in the EPL. Re duction of Mg2+, in CNQX, enhanced both the CNQX-resistant component of S1( ON) and the EPL sink. This EPL sink reversed below the MCL, suggesting it w as produced in granule cells. The NMDA-receptor antagonist APV (50 mu M) re versibly blocked the CNQX-resistant field potentials in all layers. Single pulses were applied to the MCL to antidromically depolarize the dendrites o f mitral/tufted cells. In addition to synaptic currents of granule cells, a low-amplitude, prolonged sink (S1(mcl)) was evoked in the GL. Correspondin g sources were in the EPL, suggesting that S1(mcl) was generated in the glo merular dendritic tufts of mitral/tufted cells. Both S1(mcl) and the granul e cell currents were nearly blocked by CNQX (10 mu M) but enhanced by subse quent reduction of Mg2+ these currents were blocked by APV. S1(mcl) also wa s enhanced by gamma-aminobutyric acid-A-receptor antagonists applied to sta ndard medium; this enhancement was reduced by APV. ON activation produces p rolonged excitation in the apical dendrites of mitral/tufted cells, via kai nate/AMPA and NMDA receptors, providing the opportunity for modulation and integration of sensory information at the first level of synaptic processin g in the olfactory system. Granule cells respond to input from the lateral dendrites of mitral/tufted cells via both kainate/AMPA and NMDA receptors; however, in physiological concentrations of extracellular Mg2+, NMDA-recept or activation does not contribute significantly to the granule cell respons es. The glomerular sink evoked by antidromic depolarization of mitral/tufte d cell dendrites suggests that glutamate released from the apical dendrites of mitral/tufted cells may excite the same or neighboring mitral/tufted ce ll dendrites.