The pharmacology of excitatory transmission in the rat olfactory cortex slice

The pharmacology of excitatory transmission in slices of olfactory cortex of the rat, perfused with solution containing picrotoxin, has been studied by assessing the effects of cis-2,3-piperidine dicarboxylate, a nonselective antagonist of excitatory amino acid receptors, 2-amino-5-phosphonopentanoate, a selective antagonist of N-methyl-D-aspartate (NMDA) receptors and 2-amino-4-phosphonobutyrate (APB) and baclofen, which act at APB and GABAB sites, respectively, on evoked surface field potentials. Monosynaptic excitatory transmission was monitored by measuring the amplitude of the N'a'-wave, evoked on stimulation of the lateral olfactory tract, whilst di-/polysynaptic excitatory transmission was evaluated by calculating the areas of the potentials evoked on direct stimulation of the superficial and deep-lying association fibre systems. On the basis of the effects of the drugs in this and earlier studies, it is concluded that: (i) transmission at the lateral olfactory tract-pyramidal cell synapse is mediated by kainate/quisqualate but not NMDA receptors and is regulated by inhibitory APB receptors, located on the tract terminals; (ii) NMDA receptors are involved in mediating excitatory transmission at the synapses of superficial association fibres with the proximal apical dendrites of pyramidal cells with inhibitory APB receptors playing a regulatory role; (iii) transmission at synapses of association fibres with basal dendrites of pyramidal cells, is mediated in part by NMDA receptors with (presynaptic?) GABAB receptors exerting a strong inhibitory influence. These proposed roles of NMDA receptors have been confirmed in experiments in which the effects of magnesium ions on field potentials evoked in slices perfused in magnesium-free solution were monitored.