Divergent response properties of layer-V neurons in rat primary auditory cortex

Layer-V pyramidal cells comprise a major output of primary auditory cortex (A1). At least two cell types displaying different morphology, projections and in vitro physiology have been previously identified in layer-V. The focus of the present study was to characterize extracellular receptive field properties of layer-V neurons to determine whether a similar breakdown of responses can be found in vivo. Recordings from 105 layer-V neurons revealed two predominant receptive field types. Thirty-two percent displayed strong excitatory V/U-shaped receptive field maps and spiking patterns with shorter stimulus-driven interspike intervals (ISIs), reminiscent of the bursting cells discussed in the in vitro literature. V/U-shaped maps remained relatively unchanged across the three sequential repetitions of the map run on each neuron. Neurons with V/U-shaped maps were also easily depolarized with extracellular current pulse stimulation. In contrast, 47% of the neurons displayed Complex receptive field maps characterized by weak and/or inconsistent excitatory regions and were difficult to depolarize with current pulses. These findings suggest that V/U-shaped receptive fields could correspond to previously described intrinsic bursting (IB) cells with corticotectal projections, and that neurons with Complex receptive fields might represent the regular spiking (RS) cells with their greater inhibitory input and corticocortical/corticostriatal projection pattern.