Hardingham, N. R. | Search bioRxiv

bioRxiv2023

Interdependence of primary and secondary somatosensory cortices for plasticity and texture discrimination learning

Feedforward and feedback pathways are important for transfer and integration of information between sensory cortical areas. Here we find that two closely connected cortical areas, the primary (S1) and secondary somatosensory cortices (S2) are both required for mice to learn a whisker-dependent texture discrimination. Increased inhibition in either area (using excitatory DREADDs expressed in inhibitory interneurones) prevents learning. We find that learning the discrimination produces structural plasticity of dendritic spines on layer 2/3 pyramidal neurones in vibrissae S1 that is restricted to the basal dendrites and leaves dendritic spines on apical dendrites unchanged. As S2 projects to the apical dendrites of S1 neurones, we tested whether S2 affects LTP-induction in S1. We found that feedback projections from S2 to S1 gates LTP on feedforward pathways within S1. These studies therefore demonstrate the interdependence of S1 and S2 for learning and plasticity in S1. HIGHLIGHTSO_LIBoth primary (S1) and secondary (S2) somatosensory cortices are necessary for whisker based texture discrimination learning C_LIO_LIS2 feedback connections to S1 gate LTP at feedforward pathways in S1 C_LIO_LIS1 undergoes structural plasticity of pre-existing spines during learning C_LIO_LIS1 learning induced plasticity and LTP occurs on basal but not apical dendrites C_LI

neuroscience↗

bioRxiv2022

Hebbian and homeostatic plasticity mechanisms are segregated in sub-types of layer 5 neuron in the visual cortex

Cortical layer 5 contains two major types of projection neuron known as IB (intrinsic bursting) cells that project sub-cortically and RS (regular spiking) cells that project between cortical areas. We studied the plasticity properties of RS and IB cells in the visual cortex during the critical period for ocular dominance plasticity in mice. RS neurons exhibited synaptic depression in response to both dark exposure (DE) and monocular deprivation (MD), and their homeostatic recovery from depression was dependent on TNF. In contrast, IB cells demonstrated opposite responses to DE and MD, potentiating to DE and depressing to MD. IB cells potentiation depended on CaMKII-autophosphorylation and not TNF. IB cells showed mature synaptic properties at the start of the critical period while RS cells matured during the critical period. Together with observations in somatosensory cortex, these results suggest that differences in RS and IB plasticity mechanisms are a general cortical property. Significance StatementThe neocortex contains cells that project to different locations in the brain. In this study we show that neurons projecting to different target locations exhibit different synaptic plasticity mechanisms. Cortically projecting cells show synaptic depression and homeostatic up-regulation, subcortically projecting cells show classical Hebbian potentiation. This is important because it implies that the way a cortical neuron responds to experience and encodes information depends on the neuronal subcircuits in which it is embedded. We show that ignoring this distinction leads to erroneous conclusions regarding plasticity time-course and significance. These findings constitute an important step toward understanding how learning and memory is organized within subcircuits in the cerebral cortex.

neuroscience↗