Intrinsic electrophysiology of neurons in thalamorecipient layers of developing rat auditory cortex

During early postnatal life, several critical events contribute to the func tional development of rat sensory neocortex. Thalamocortical innervation of sensory cortex is completed during the first postnatal week and extrathala mic innervation develops over the first several weeks. In auditory cortex, acoustic-evoked potentials first occur in week 2 and develop most rapidly o ver weeks 2-3. Thus, rapid functional maturation of cortical circuits in se nsory cortex occurs during the second and third postnatal weeks. The electr ophysiological properties of cortical neurons that receive afferent inputs during this time may play an important role in development and function. In this study we examined the intrinsic electrophysiology, including spiking patterns, of neurons in layers II/III and IV of auditory cortex during post natal weeks 2 and 3. Many neurons displayed characteristics consistent with previous descriptions of response classes (regular spiking, fast spiking, intrinsic bursting). In addition, we identified two groups, Rectifying and On-spiking neurons, that were characterized by (i) brief spike trains in re sponse to maintained intracellular depolarizations, and (ii) striking outwa rd rectification upon depolarization. Unusually brief spike trains (1-2 spi kes) and short spike latencies (<10 ms) further distinguished On-spiking fr om Rectifying cells. Biocytin labeling demonstrated that On-spiking and Rec tifying cells could be either pyramidal or nonpyramidal neurons. The intrin sic physiology of these cell groups may play an important role in auditory cortex function. (C) 1999 Elsevier Science B.V. All rights reserved.