Analysis of synaptic events in the mouse accessory olfactory bulb with current source-density techniques.

The accessory olfactory bulb of the mouse was studied by current source-density analysis of field potentials to determine the laminar and temporal distribution of synaptic currents evoked by electrical stimulation of the vomeronasal organ. The one-dimensional current source-density analysis revealed two major spatially and temporally distinct inward membrane currents (sinks): one in the glomerular layer and the other in the external plexiform layer. The glomerular layer sink preceded the external plexiform layer sink by a mean of 5.5 ms. Local infusions of the broad-spectrum excitatory amino acid antagonist, kynurenate, into the accessory olfactory bulb blocked the external plexiform layer sink without an obvious effect on the glomerular layer sink. The selective non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione produced a dose-dependent blockade of the external plexiform layer sink, whereas the selective N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovalerate was without effect. These results, taken together with the cytoarchitecture of the accessory olfactory bulb, suggest that the glomerular layer sink results mainly from synaptic excitation evoked in the glomerular dendritic branches of mitral cells by the vomeronasal afferent fibres and the external plexiform layer sink mainly from non-N-methyl-D-aspartate receptor-mediated synaptic excitation in the peripheral processes of granule cells via the mitral to granule cell dendrodendritic synapse.