Proceedings of The Physiological Society
Cardiff University (2009) Proc Physiol Soc 17, PC04
Maturation of excitatory synaptic transmission occurs more slowly in mouse primary visual cortex than in barrel cortex
C. E. Cheetham1, K. D. Fox1
1. School of Biosciences, Cardiff University, Cardiff, United Kingdom.
The development and plasticity of cortical circuits has been widely studied in both whisker barrel cortex (BC) and primary visual cortex (VC)(1). During repetitive stimulation, postsynaptic responses undergo short-term synaptic plasticity. Synapses generally switch from strong depression to weaker depression or facilitation between the second and fifth postnatal weeks (2). Although the timing of developmental critical periods differs between BC and VC (1), a comparison of the development of synaptic properties in BC and VC has not been made. We made whole cell recordings of layer 2/3 (L2/3) pyramidal neurons at 35 - 37°C in acute brain slices from postnatal day (P)12-P138 C57BL/6 mice, and applied short stimulus trains to presynaptic axons. Statistics were calculated from paired-pulse ratios (PPR) at 20 Hz (mean±S.E.M.) using t-tests unless stated. For horizontal connections within L2/3, PPR increased dramatically between P12 and P28 in both BC (0.96±0.08 to 1.13±0.03) and VC (0.89±0.06 to 1.21±0.05; P<0.001 for age, P=0.86 for area, two-way ANOVA). In the L4 to L2/3 pathway, there was also a marked increase in PPR in BC between P12 and P28 (0.90±0.09 to 1.11±0.08, P=0.045), which was blocked by total bilateral whisker deprivation (0.94±0.04, P=0.74 vs P12). In contrast, VC synapses exhibited similar short-term plasticity at P12 and P28 (0.93±0.05 to 0.96±0.05, P=0.73), but underwent a significant increase in PPR between P28 and P43 (to 1.10±0.04, P=0.029). No further changes were seen in either area up to P138 (P=0.86, one-way ANOVA), indicating that synaptic maturation is complete by P28 in BC, and P43 in VC. Short-term synaptic dynamics are thought to reflect presynaptic release probability: lower release probability results in greater facilitation (3). Therefore, we compared release probability in the L4 to L2/3 pathway at P28, when BC has matured but VC has not, by measuring the rate of use-dependent blockade of NMDARs (N-methyl-D-aspartate receptors) by MK-801. Use-dependent blockade occurred more slowly in BC than in VC (% block after 100 trials: VC 17±4%, BC 31±5%, P<0.001 for cortical area and trial number, two-way ANOVA), indicating a lower release probability in BC than VC. Our data show that synapses in primary sensory cortex continue to mature until six weeks of age, and highlight the interplay between development and plasticity. Maturation involves a reduction in presynaptic release probability and concomitant increase in short-term facilitation. In the L4 to L2/3 pathway, maturation occurs two weeks later in VC than in BC, in keeping with the relative timing of developmental critical periods in the two areas. Greater short-term facilitation at mature synapses will enhance the temporal fidelity of signal processing at higher stimulus frequencies.
Where applicable, experiments conform with Society ethical requirements