Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, C16
Integration strategies of pyramidal cells and Nkx2-1 interneurons in the early postnatal neocortex
P. G. Anastasiades1, S. Butt1
1. Dept. of Physiology, Anatomy & Genetics, University of Oxford, Oxford, United Kingdom.
Our current view of the mechanisms that underlie the integration and consolidation of neural network is that promiscuous connections, initially supported by spontaneous activity intrinsic to the network are subsequently pruned and sculpted by emergent sensory input. To probe this further we have completed a series of studies using photostimulation to investigate to what degree activity intrinsic to the neocortex is instructive in relation to pyramidal cell integration and whether or not interneuron populations with diverse mature characteristics adopt similar or divergent strategies. To map inputs we took advantage of laser scanning photostimulation (LSPS) in acute in vitro slices prepared from wildtype and Nkx2 1iCre;Z/EG mice. Mapping of pyramidal cells revealed that initial sources of input were diffuse, albeit slightly more concentrated from the immediate layer. Over time the input gradually got stronger and focused around the expected afferent input for the canonical circuit (Gilbert & Wiesel, 1983). In terms of layer 2/3 pyramidal cells this represented a relative reduction in layer 5 input (from 40.5% at P5-P8 to 28.6% at P17-P21; n>4), while perhaps more intriguingly in terms of layer 5 pyramidal cells it resulted in a pronounced shift toward later-born layer 2/3 input (increased from 27.1% input at P5-P8 to 45.9% at P17-P21). Using a modified LSPS protocol we were able to show that this change appears to be predicted by NMDA receptor-dependent inputs. Furthermore, analagous to the activation of silent synapses, evoking synchronous activity across cortical layers significantly enhanced input onto a subset of pyramidal cells prior to P12. Beyond this time point, input was immune to evoked activity in line with the fading of synchronised activity in vivo (Golshani et al., 2009) and the disappearance of NMDAR-only inputs observed using LSPS. To examine how locally-projecting GABAergic interneurons might assist in this process we recorded and mapped a genetically-defined population of Nkx2-1-derived interneurons over the corresponding timeframe. Our analysis revealed that there is a divergence in the strategies employed by fast-spiking (FS; n=19) and non-fast spiking/intrinsic bursting (NFS/IB; n=23) cells. The latter integrate early and do not develop further in terms of total synaptic input, although the layer distribution of afferent input is not static. In contrast FS cells which mediate lateral and feed-forward inhibition across all time points tested, bloom in terms of the scale of their input in parallel with the emergent pyramidal circuit. Thus it appears that layer 5 pyramidal cells and NFS/IB interneurons combine in the first few postnatal days to form a permissive network that acts to sculpt emergent connectivity intrinsic to the neocortex.
Where applicable, experiments conform with Society ethical requirements