Receptor subtypes involved in callosally-induced postsynaptic potentials in rat frontal agranular cortex in vitro

Summary

A slice preparation of rat frontal agranular cortex preserving commissural inputs has been used for intracellular recording from layer V pyramidal cells, in order to characterize the synaptic potentials induced by stimulation of the corpus callosum and to reveal the subtypes of amino acid receptors involved. Stimulation of the corpus callosum induced EPSPs followed by early IPSPs with a peak latency of 30 ± 2 ms and late IPSPs with a peak latency of 185 ± 18 ms. Reversal potentials for early and late IPSPs were −75 ± 5 mV (early) and −96 ± 5 mV (late). Late IPSPs were more dependent on extracellular K⁺ concentration. The early IPSPs were blocked by GABA_A antagonists, bicuculline and picrotoxin, whereas the late IPSPs were reduced by the GABA_B antagonist, phaclofen. CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), an antagonist of non-NMDA (N-methyl-D-aspartate) receptors, suppressed both EPSPs and late IPSPs at 5 µM. Early IPSPs remained at this concentration but were suppressed by 20 µM CNQX. In Mg²⁺-free solution, EPSPs were larger and more prolonged than in control solution. These enhanced EPSPs persisted after 5 to 20 µM CNQX, but were reduced in amplitude, and their onset was delayed by 3.6 ± 0.8 ms. The remaining EPSPs were suppressed by 50 µM APV (DL-2-amino-5-phosphono-valeric acid), an antagonist of NMDA receptors. In Mg²⁺-free solution containing 5 to 20 µM CNQX, the late IPSPs were not diminished. The remaining late IPSPs were suppressed by APV or by phaclofen. By contrast, the amplitude of early IPSPs was not affected by APV in Mg²⁺-free solution containing 5 µM CNQX. These results show that stimulation of the corpus callosum can induce GABA_A and GABA_B dependent IPSPs and NMDA and non-NMDA dependent excitation. It is suggested that these four types of amino acid-based transmission are conveyed by intracortical pathways with different characteristics.