Electrophysiological evidence for glutamate as a vomeronasal receptor cell neurotransmitter

Bipolar receptor cells in the vomeronasal organ send axonal projections to the accessory olfactory bulb where they synapse with mitral cell dendrites. Although the nature of the synapse is thought to be excitatory, the neurotransmitter(s) involved has not yet been identified. Electrophysiological recordings of single neurons in the mitral cell layer of the AOB in response to vomeronasal nerve stimulation were conducted to characterize the synaptic response and the underlying neurotransmitter substance. Extracellular activity was recorded in vivo (whole animal) and in vitro (AOB slice) from female rats. In vivo, the predominant response to stimulation of the VNO was excitation. In many instances in the whole animal preparation, the excitation was followed by an inhibitory response. Attempts to block the excitatory response by ejecting kynurenic acid in close proximity to the mitral cell being recorded were not successful. Since this failure may have been due to inability of the antagonist to reach its presumed site of action at the dendrite, further recordings were carried out in vitro. In the AOB slice preparation, the predominant response to stimulation of the VN nerve endings was excitation. Superfusion of the non-NMDA antagonist, CNQX, into the medium resulted in a reduction of the orthodromic excitation in 5 of 8 cells. The NMDA antagonist, AP-5, was found to blunt orthodromic excitation in 1 of 4 cells. These results suggest that the excitatory response evoked in mitral cells followng stimulation of the VN nerve is mediated by glutamate.