Differential responsiveness of neocortical areas in the parieto-occipital region to low-intensity stimulation in vitro.

1. Extracellular recording techniques were used to map the responses to low-intensity stimuli across rat neocortical slices prepared from the parieto-occipital region. Intracellular recordings were obtained in regions of particular interest. After electrophysiological recording, slices were fixed in paraformaldehyde (4%) and Nissl-stained to allow identification of cytoarchitectonically discrete areas. 2. In most neocortical areas of the parieto-occipital region (Par1, HL, Fr1, Fr2), low-intensity stimuli applied to layer V evoked only short monophasic field excitatory postsynaptic potentials (fEPSPs) in layer II/III. In Oc2 (secondary visual cortex, rostral part), however, a long-lasting potential (up to 450 ms) consistently followed the short monophasic response seen in the other areas. 3. The late response observed in Oc2 was characterized by its inter-stimulus variability, polyphasic appearance and N-methyl-D-aspartate (NMDA)-receptor dependence. Intracellular recordings obtained from layer II/III pyramidal neurons located in Oc2 revealed short-latency, monophasic early excitatory postsynaptic potentials (eEPSPs) followed by long-lasting excitatory postsynaptic potentials (eEPSPs) followed by long-lasting excitatory postsynaptic potentials (EPSPs), which often triggered several action potentials 4. Recordings with two extracellular electrodes, one positioned directly above the stimulation electrode and the other at a variable distance (but also in layer II/III), suggested that the corresponding excitation spreads across the slice, but does not cross the cytoarchitectonic borders of Oc2. 5. Our data indicate that the electrophysiological responses to low-intensity stimuli differ within the parieto-occipital region of the rat neocortex. The type of response elicited depends on the cytoarchitectonic area in which the stimulus is applied. The results raise the question of whether further differences between neocortical subregions, with respect to basic electrophysiological properties, exist.