Selective suppression of afferent but not intrinsic fiber synaptic transmission by 2-amino-4-phosphonobutyric acid (AP4) in piriform cortex

Differences in the glutaminergic modulation of afferent and intrinsic fiber synaptic transmission in piriform (olfactory) cortex were investigated using extracellular and intracellular recording techniques in a transverse slice preparation. 2-Amino-4-phosphonobutyric acid (AP4) strongly suppressed synaptic potentials evoked by afferent fiber stimulation in layer 1a, while having a much weaker effect on synaptic potentials evoked by intrinsic fiber stimulation in layer 1b. Both the racemic mixture and l-(+)-enantiometer of AP4 showed this differential effect. Suppression of afferent fiber synaptic potentials was accompanied by an increase in paired pulse facilitation, suggesting a pre-synaptic mechanism, while intrinsic fiber synaptic potentials showed little change in facilitation. Previous work has shown that cholinergic modulation in piriform cortex appears selective for intrinsic fiber synapses. The present data describes a pre-synaptic glutaminergic modulation complementary to the cholinergic modulation.