Early and late components of AMPA-receptor mediated field potentials in hippocampal slices

Field EPSPs were recorded from the CA1 region of hippocampal slices under conditions in which components of the responses other than those generated by AMPA-type glutamate receptors were blocked. Laminar profile analysis indicated that the resultant potentials had separable phases: an early and fast stage followed by a late and slow stage. The location of the fast response was sensitive to stimulation position in the stratum radiatum; i.e., distal stimulation elicited maximum, negative going potentials in the distal stratum radiatum while proximal stimulation recordings were maximal in the proximal segment. The distribution of the late component was largely independent of stimulation electrode position in stratum radiatum. Current source density analysis revealed that the late response had a source in the most distal dendrites (stratum moleculare) and a sink near the cell body layer. It was not accompanied by evident changes in membrane conductance and had a decay time constant similar to the membrane time constant. Stimulation of the afferents to the basal dendrites of the pyramidal cells also resulted in a late response which again had a source in stratum moleculare. These results strongly suggest that the magnitude and waveform of the late component of the AMPA receptor mediated field potential reflects the biophysical properties of the most distal branches of the dendritic arborization. The laminar analyses also show that the late potential is minimal in the mid-stratum radiatum and thus suggest that this site is most appropriate for investigations concerned with the waveform of the fast component of AMPA receptor mediated synaptic response.