Science Topics – 128

Mammalian sensory cortex shows remarkable plasticity during development, which enables neural responses or circuit morphology to change or reorganize in response to inputs from the outside world, and is thought to be the neural basis for learning and memory. The plasticity is strongest during a certain period of time, called “critical period”. The precise mechanism of the critical period, together with its initiation is not known. Based on a series of experiments since 2007, we have postulated a hypothetical model showing how the critical period is initiated. Cortical inputs from thalamus are relayed to layer 4(L4) then to L2/3, and so on. L4-L2/3 synapse is thought to be important for critical period plasticity, which exhibit Hebbian STDP, in which “L4 followed by L2/3” firing causes strengthening, while “L2/3 followed by L4” firing causes weakening of the synapses involved. We found the Hebbian STDP appears after P15, together with the maturation of L4 GABAergic circuit producing fast feedforward inhibition that regulates “L4 followed byL2/3” firing. Before that, L4-L2/3 and thalamocortical synapses exhibit distinct forms of STDP subserving circuit formation and reorganization of thalamocortical and L4-L2/3 network. Thus, we believe that after underlying circuit is accomplished, expression of Hebbian STDP together with maturation of GABA circuit regulating L4-L2/3 firing sequence, the critical period plasticity is initiated.

Fumitaka Kimura and Chiaki Itami A hypiothetical model concerning how spike-timing-dependent plasticity contributes to neural circuit formation and initiation of the critical period in barrel cortex Journal of Neuroscience in press (2019)

＜Figure Legends＞
Chronological sequence from the formation of thalamocortical projection and retraction from L2/3, formation of L4 to L2/3 projection, leading to the initiation of the critical period of map plasticity during development. Thalamic (VB) neurons form synaptic connection with cortical plate/cortical cells through all-LTP STDP until P5. After P7, differentiation of L4, L2/3 from cortical plate is over, then, the thalamocortical synapses to L2/3 switch to all-LTD STDP, which helps retraction from overshooted projection to L2/3. Thalamus to L4 lost STDP stabilizing its projection (P7~). L4 starts to make synapses with L2/3 from P6-P7 through all-LTP STDP. After P12, latency for thalamocortical projections to L4 PV (+) GABA becomes shortened, which enables fast feedforward inhibition to L4 and L2/3, guaranteeing “L4 followed by L2/3” firing. Simultaneously L4 to L2/3 synapses switch to Hebbian STDP with LTP and LTD, thus, critical period is initiated.

¹ Department of Molecular Neuroscience, Osaka University Graduate School of Medicine,Japan
² Department of Physiology, Faculty of Medicine, Saitama Medical University, Japan