Late embryonic expression of AMPA receptor function in the CA1 region of the intact hippocampus in vitro

Studies in slices suggest that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepr opionic acid (AMPA) receptor-mediated synaptic currents are not present in CA1 (Cornu ammonis) pyramidal neurons at birth (P0). We have re-examined th is issue in the rat intact hippocampal formation (IHF) in vitro. Injections of biocytin or carbocyanine show that the temporo-ammonic, commissural and Schaffer collateral pathways are present at birth in the marginal zone of CA1. Electrical stimulation of these pathways evoked field excitatory posts ynaptic potentials (fEPSPs) in the marginal zone of CA1 from embryonic day 19 (E19) to postnatal day 9 (P9). These fEPSPs are mediated by synaptic AMP A receptors as they are reduced or completely blocked by: (i) tetrodotoxin; (ii) high divalent cation concentrations; (iii) the adenosine A1 receptor agonist CPA; (iv) anoxic episodes; (v) the selective AMPA receptor antagoni st 1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7,8-methylenedioxy-3,4-dihyd ro-5H-2,3-benzodiazepine (GYKI-53655) or the mixed AMPA-kainate receptor an tagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6-nitro-7-sulpham oylbenzo[f]quinoxaline-2,3-dione (NBQX). The amplitude of the fEPSPs is als o reduced by D(-)-2-amino-5-phosphonopentanoic acid (D-APV) and its duratio n is increased by bicuculline suggesting the participation of N-methyl-D-as partate (NMDA) acid GABA(A) (gamma-aminobutyric acid) receptors, Finally, A MPA receptor-mediated fEPSPs are also recorded in P0 slices, but they are s maller and more labile than in the IHF. Our results suggest that in embryon ic CA1 neurons, glutamate acting on AMPA receptors already provides a subst antial part of the excitatory drive and may play an important role in the a ctivity-dependent development of the hippocampus. Furthermore, the IHF may be a convenient preparation to investigate the properties of the developing hippocampus.