A role for silent synapses in the development of the pathway from layer 2/3 to 5 pyramidal cells in the neocortex.

The integration of neurons within the developing cerebral cortex is a prolonged process dependent on a combination of molecular and physiological cues. To examine the latter we used laser scanning photostimulation (LSPS) of caged glutamate in conjunction with whole-cell patch-clamp electrophysiology to probe the integration of pyramidal cells in the sensorimotor regions of the mouse neocortex. In the days immediately after postnatal day 5 (P5) the origin of the LSPS-evoked AMPA receptor (AMPAR)-mediated synaptic inputs were diffuse and poorly defined with considerable variability between cells. Over the subsequent week this coalesced and shifted, primarily influenced by an increased contribution from layers 2/3 cells, which became a prominent motif of the afferent input onto layer 5 pyramidal cells regardless of cortical region. To further investigate this particular emergent translaminar connection, we alternated our mapping protocol between two holding potentials (-70 and +40 mV) allowing us to detect exclusively NMDA receptor (NMDAR)-mediated inputs. This revealed distal MK-801-sensitive synaptic inputs that predict the formation of the mature, canonical layer 2/3 to 5 pathway. However, these were a transient feature and had been almost entirely converted to AMPAR synapses at a later age (P16). To examine the role of activity in the recruitment of early NMDAR synapses, we evoked brief periods (20 min) of rhythmic bursting. Short intense periods of activity could cause a prolonged augmentation of the total input onto pyramidal cells up until P12; a time point when the canonical circuit has been instated and synaptic integration shifts to a more consolidatory phase

Decoding the transcriptional basis for GABAergic interneuron diversity in the mouse neocortex.

The locally projecting GABAergic interneurons of the mammalian cerebral cortex are a highly heterogeneous population, whose malfunction or deficit has been implicated in a wide range of neurological disorders. However, the low incidence of the various distinct interneuron populations within the neocortex, combined with the lack of molecular or physiological markers specific to these subtypes, have hampered investigations into their function in the normal and dysfunctional brain. A number of research groups have begun to elucidate the developmental genetic mechanism that underpins this diversity in the mouse neocortex, spurred on by the knowledge that the temporal and spatial origin of an interneuron in the embryonic brain is predictive of its eventual intrinsic properties in the mature cortex. In this review we highlight a number of recent findings that strengthen our understanding of the transcription factor code that is at the heart of generating this diversity. Further understanding of this code will enable selective observation, targeting and manipulation of interneuron subtypes across both in vitro and in vivo systems

Cell-Type Specificity of Callosally Evoked Excitation and Feedforward Inhibition in the Prefrontal Cortex.

Excitation and inhibition are highly specific in the cortex, with distinct synaptic connections made onto subtypes of projection neurons. The functional consequences of this selective connectivity depend on both synaptic strength and the intrinsic properties of targeted neurons but remain poorly understood. Here, we examine responses to callosal inputs at cortico-cortical (CC) and cortico-thalamic (CT) neurons in layer 5 of mouse prelimbic prefrontal cortex (PFC). We find callosally evoked excitation and feedforward inhibition are much stronger at CT neurons compared to neighboring CC neurons. Elevated inhibition at CT neurons reflects biased synaptic inputs from parvalbumin and somatostatin positive interneurons. The intrinsic properties of postsynaptic targets equalize excitatory and inhibitory response amplitudes but selectively accelerate decays at CT neurons. Feedforward inhibition further reduces response amplitude and balances action potential firing across these projection neurons. Our findings highlight the synaptic and cellular mechanisms regulating callosal recruitment of layer 5 microcircuits in PFC

Studies of cortical connectivity using optical circuit mapping methods

An important consideration when probing the function of any neuron is to uncover the source of synaptic input onto the cell, its intrinsic physiology and efferent targets. Over the years, electrophysiological approaches have generated considerable insight into these properties in a variety of cortical neuronal subtypes and circuits. However, as researchers explore neuronal function in greater detail, they are increasingly turning to optical techniques to bridge the gap between local network interactions and behaviour. The application of optical methods has increased dramatically over the past decade, spurred on by the optogenetic revolution. In this review, we provide an account of recent innovations, providing researchers with a primer detailing circuit mapping strategies in the cerebral cortex. We will focus on technical aspects of performing neurotransmitter uncaging and channelrhodopsin-assisted circuit mapping, with the aim of identifying common pitfalls that can negatively influence the collection of reliable data

Non-canonical role for Lpar1-EGFP subplate neurons in early postnatal mouse somatosensory cortex

Subplate neurons (SPNs) are thought to play a role in nascent sensory processing in neocortex. To better understand how heterogeneity within this population relates to emergent function, we investigated the synaptic connectivity of <i>Lpar1-EGFP</i> SPNs through the first postnatal week in whisker somatosensory cortex (S1BF). These SPNs comprise of two morphological subtypes: fusiform SPNs with local axons, and pyramidal SPNs with axons that extend through the marginal zone. The former receive translaminar synaptic input up until the emergence of the whisker barrels; a timepoint coincident with significant cell death. In contrast, pyramidal SPNs receive local input from the subplate at early ages but then - during the later time window, acquire input from overlying cortex. Combined electrical and optogenetic activation of thalamic afferents identified that <i>Lpar1-EGFP</i> SPNs receive sparse thalamic innervation. These data reveal components of the postnatal network that interpret sparse thalamic input to direct the emergent columnar structure of S1BF