Electrophysiological effects of three groups of glutamate metabotropic receptors in rat piriform cortex.

1. The effects of three metabotropic glutamate receptor (mGluR) agonists were tested in two pathways of rat piriform cortex. The group I, II and III mGluR agonists used were RS-3,5-dihydroxyphenenylglycine (DHPG) (10-100 microM), (2S,1'S,2'S)-2-Carboxycyclopropyl (L-CCG) (20-100 microM) and L(+)-2-amino-4-phosphonobutyric acid (L-AP4) (5-500 microM), respectively. 2. The effects of the three groups of agonists on synaptic transmission in the two piriform cortex pathways also were examined. All three agonists reduced the amplitude of the monosynaptic EPSPs generated by stimulation of the lateral olfactory tract (LOT) or of the association fiber pathway (ASSN). This was always accompanied by an increase in paired pulse facilitation. 3. Group I and II mGluR agonists had similar synaptic effects on the two pathways, while the group III mGluR agonist suppressed the LOT pathway more than the association pathway. 4. The group II and III mGluR agonists had no effect on passive membrane properties of pyramidal neurons. Group I agonists depolarized the pyramidal neuron membrane potential, and enhanced both membrane resistance and noise. 5. Our data suggest that all three types of mGluRs modulate synaptic transmission in both of these pathways in piriform cortex. Only group I agonists alter post-synaptic membrane properties, while all three types of receptor regulate synaptic transmission. Groups I and II are equally potent in the LOT and association fiber pathways, while group III receptors are more potent in the LOT than the association fiber pathways.