Developmental activity and function of layer 5 cortical circuits following early postnatal reorganization

The National Institute of Neurological Disorders and Stroke awarded President and Fellows of Harvard College $151,352 on August 27, 2026, under the Extramural Research Programs in the Neurosciences and Neurological Disorders program (CFDA 93.853) to investigate developmental activity and function of layer 5 cortical circuits following early postnatal reorganization.

The research examines how inhibitory somatostatin-expressing cortical interneurons develop targeting specificity for distinct excitatory neuron populations in the primary visual cortex across key postnatal developmental timepoints (postnatal days 10, 15, and 30) in mice. The study tests the hypothesis that these interneurons initially target intratelencephalic neurons and later shift their targeting to extratelencephalic neurons following eye-opening, potentially driven by changes in excitatory drive. The work employs two-photon calcium imaging to characterize activity patterns of layer 5 interneurons and excitatory populations and investigates whether spontaneous waves of neural activity prior to eye-opening influence this developmental switch.

The award funds research at Harvard Medical School in Boston, Massachusetts, with a period of performance ending February 28, 2029. This project grant targets a fundamental gap in understanding circuit development implicated in neurodevelopmental disorders including autism, epilepsy, and schizophrenia.

Abstract

The precise formation of circuits in the cerebral cortex, involving both excitatory pyramidal neurons (PNS) and inhibitory interneurons (INS), is crucial for proper brain function, with disruptions in this process being implicated in neurodevelopmental disorders such as autism, epilepsy, and schizophrenia. Recent work from the fishell lab has revealed that the connectivity of neurons in layer 5 (L5), a critical region for top-down information processing, follows a highly specific targeting pattern that arises during postnatal development. Specifically, in primary visual cortex (V1), L5 is comprised of two distinct excitatory populations-intratelencephalic (IT) and extratelencephalic (ET) neurons-both of which are modulated by inhibitory somatostatin-expressing cortical interneurons (SST cins). Notably, specific subtypes of SST cins, such as CHRNA2 and CALB2, selectively target ET populations in adulthood. However, IT remains unclear whether these targeting patterns are innate or develop over time. Preliminary findings suggest a novel hypothesis: SST cins may initially target IT neurons and later shift their targeting specificity to ET neurons after eye-opening (P14) in mice, implying a dynamic developmental switch. This transition could be influenced by changes in excitatory drive, considering that retinotopic maps in V1 are shaped by spontaneous waves of activity both prior to and during eye-opening, while feedforward excitation from visual experience appears necessary for the proper maturation of visual responses. The goal of this proposal is to investigate the mechanisms driving this developmental switch and its functional implications. To do this, the study will (1) examine activity patterns of L5 SST cins, IT, and ET populations across key developmental timepoints (P10, P15, P30) using 2-photon calcium imaging, (2) determine whether the switch in SST CIN targeting influences activity within L5 circuits or the ability of the animal to detect visual stimulus through use of targeted expression of KIR2.1 in SST cins which prevents the switch in synaptic targeting, and (3) assess whether visual experience is necessary for the proper development of these circuits by rearing mice in darkness and analyzing the effects on L5 activity and circuit formation. By elucidating the activity-dependent processes that regulate the development and refinement of cortical circuits, this project aims to improve our understanding of how disruptions in these processes contribute to neurodevelopmental disorders.