Aberrant development of excitatory circuits to inhibitory neurons in the primary visual cortex after neonatal binocular enucleation

The development of GABAergic interneurons is important for the functional maturation of cortical circuits. After migrating into the cortex, GABAergic interneurons start to receive glutamatergic connections from cortical excitatory neurons and thus gradually become integrated into cortical circuits. These glutamatergic connections are mediated by glutamate receptors including AMPA and NMDA receptors and the ratio of AMPA to NMDA receptors decreases during development. Since previous studies have shown that retinal input can regulate the early development of connections along the visual pathway, we investigated if the maturation of glutamatergic inputs to GABAergic interneurons in the visual cortex requires retinal input. We mapped the spatial pattern of glutamatergic connections to layer 4 (L4) GABAergic interneurons in mouse visual cortex at around postnatal day (P) 16 by laser-scanning photostimulation and investigated the effect of binocular enucleations at P1/P2 on these patterns. Gad2-positive interneurons in enucleated animals showed an increased fraction of AMPAR-mediated input from L2/3 and a decreased fraction of input from L5/6. Parvalbumin-expressing (PV) interneurons showed similar changes in relative connectivity. NMDAR-only input was largely unchanged by enucleation. Our results show that retinal input sculpts the integration of interneurons into V1 circuits and suggest that the development of AMPAR- and NMDAR-only connections might be regulated differently.

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

ABSTRACTSubplate neurons (SPNs) are a transient neuronal population shown to play a key role in nascent sensory processing relaying thalamic information to the developing cerebral cortex. However there is little understanding of how heterogeneity within this population relates to emergent function. To address this question we employed optical and electrophysiological technologies to investigate the synaptic connectivity of SPNs defined by expression of the Lpar1-EGFP transgene through the first postnatal week in primary whisker somatosensory cortex (S1BF) in mouse. Our data identify that the Lpar1-EGFP SPNs represent two morphological subtypes: (1) transient, fusiform SPNs with axons largely restricted to the subplate zone; (2) pyramidal SPNs with axon collaterals that traverse the overlying cortex to extend through the marginal zone. Laser scanning photostimulation of caged glutamate was used to determine columnar glutamatergic and GABAergic input onto both of these SPN subtypes. These experiments revealed that the former receive translaminar input from more superficial cortical layers up until the emergence of the whisker barrels (~postnatal (P)5). In contrast, pyramidal SPNs only receive local input from the adjacent subplate network at early ages but then at later ages can acquire varied input from the overlying cortex. Combined electrical stimulation of the ventral posterior nucleus of the thalamus and optogenetic activation of thalamic afferents in thalamocortical slice preparations revealed that Lpar1-EGFP SPNs only receive sparse thalamic innervation during early postnatal development. Taken together, these data reveal two components of the postnatal network that interpret sparse thalamic input to direct the emergent columnar structure of neonatal somatosensory cortex.

Transient coupling between subplate and subgranular layers to L1 neurons before and during the critical period

Cortical layer 1 (L1) contains a diverse population of interneurons which can modulate processing in superficial cortical layers but the intracortical sources of synaptic input to these neurons and how these inputs change over development is unknown. We here investigated the changing intracortical connectivity to L1 in primary auditory cortex (A1) in slices of mouse A1 across development using laser-scanning photostimulation. Before P10 L1 cells receive most excitatory input from within L1, L2/3, L4 and L5/6 as well as the subplate. Excitatory inputs from all layers increase and peak during P10-P16, the peak of the critical period. Inhibitory inputs followed a similar pattern. Functional circuit diversity in L1 emerges after P16. In adult, L1 neurons receive ascending inputs from superficial L2/3 and subgranular L5/6, but only few inputs from L4. A subtype of L1 neurons, NDNF+ neurons, follow a similar pattern, suggesting that transient hyperconnectivity is a universal feature of developing cortical circuits. Our results demonstrate that deep excitatory and superficial inhibitory circuits are tightly linked in early development and might provide a functional scaffold for the layers in between. These results suggest that early thalamic driven spontaneous and sensory activity in subplate can be relayed to L1 from the earliest ages on, that the critical period is characterized by high transient columnar hyperconnectivity, and that in particular circuits originating in L5/6 and subplate might play a key role.

Transient Subgranular Hyperconnectivity to L2/3 and Enhanced Pairwise Correlations During the Critical Period in the Mouse Auditory Cortex

During the critical period, neuronal connections are shaped by sensory experience. While the basis for this temporarily heightened plasticity remains unclear, shared connections introducing activity correlations likely play a key role. Thus, we investigated the changing intracortical connectivity in primary auditory cortex (A1) over development. In adult, layer 2/3 (L2/3) neurons receive ascending inputs from layer 4 (L4) and also receive few inputs from subgranular layer 5/6 (L5/6). We measured the spatial pattern of intracortical excitatory and inhibitory connections to L2/3 neurons in slices of mouse A1 across development using laser-scanning photostimulation. Before P11, L2/3 cells receive most excitatory input from within L2/3. Excitatory inputs from L2/3 and L4 increase after P5 and peak during P9–16. L5/6 inputs increase after P5 and provide most input during P12–16, the peak of the critical period. Inhibitory inputs followed a similar pattern. Functional circuit diversity in L2/3 emerges after P16. In vivo two-photon imaging shows low pairwise signal correlations in neighboring neurons before P11, which peak at P15–16 and decline after. Our results suggest that the critical period is characterized by high pairwise activity correlations and that transient hyperconnectivity of specific circuits, in particular those originating in L5/6, might play a key role.

Thalamocortical And Intracortical Inputs Differentiate Layer-Specific Mouse Auditory Corticocollicular Neurons

Long-range descending projections from the auditory cortex play key roles in shaping response properties in the inferior colliculus. The auditory corticocollicular projection is massive and heterogeneous, with axons emanating from cortical layers 5 and 6, and plays a key role in directing plastic changes in the inferior colliculus. However, little is known about the cortical and thalamic networks within which corticocollicular neurons are embedded. Here, laser scanning photostimulation glutamate uncaging and photoactivation of channelrhodopsin-2 were used to probe the local and long-range network differences between pre-identified mouse layer 5 and layer 6 auditory corticocollicular neurons in vitro. Layer 5 corticocollicular neurons were found to vertically integrate supragranular excitatory and inhibitory input to a substantially greater degree than their layer 6 counterparts. In addition, all layer 5 corticocollicular neurons received direct and large thalamic inputs from channelrhodopsin-2 labeled thalamocortical fibers whereas such inputs were less common in layer 6 corticocollicular neurons. Finally, a new low calcium/synaptic blockade approach to separate direct from indirect inputs using laser photostimulation was validated. These data demonstrate that layer 5 and 6 corticocollicular neurons receive distinct sets of cortical and thalamic inputs, supporting the hypothesis that they have divergent roles in modulating the inferior colliculus. Furthermore, the direct connection between the auditory thalamus and layer 5 corticocollicular neurons reveals a novel and rapid link connecting ascending and descending pathways.

Neonatal Hypoxia–Ischemia Causes Functional Circuit Changes in Subplate Neurons

Neonatal hypoxia–ischemia (HI) in the preterm human results in damage to subcortical developing white matter and cognitive impairments. Subplate neurons (SPNs) are among the first-born cortical neurons and are necessary for normal cerebral development. While moderate or severe HI at P1 in rats leads to SPN loss, it is unclear if HI, esp. forms not associated with overt cell loss lead to altered SPN circuits. Thus, we used two HI models with different severities in P1 rats. Cauterization of the common carotid artery (CCA) causes a largely transient and thus milder ischemia (HI-Caut) while CCA ligation causes more severe ischemia (HI-Lig). While HI-Lig caused subplate damage, HI-Caut did not cause overt histological damage on the light microscopic level. We used laser-scanning photostimulation (LSPS) in acute thalamocortical slices of auditory cortex during P5–10 to study the functional connectivity of SPNs. Both HI categories resulted in hyperconnectivity of excitatory and inhibitory circuits to SPNs. Thus, alterations on the circuit level are present in the absence of cell loss. Our results show that SPN circuits are uniquely susceptible to HI. Given the key developmental role of SPNs, our results suggest that altered SPN circuits might underlie the abnormal development of cortical function after HI.

Functional topographic organization of the motor reticulothalamic pathway

The thalamic reticular nucleus (TRN) is a thin layer of GABAergic cells lying rostral and lateral to the dorsal thalamus, and its projection to thalamic relay cells (i.e., the reticulothalamic pathway) strongly inhibits these cells. In an attempt to extend earlier studies of reticulothalamic connections to sensory thalamic nuclei, we used laser-scanning photostimulation to study the reticulothalamic projections to the main motor thalamic relays, the ventral anterior and lateral (VA and VL) nuclei, as well as to the nearby central lateral (CL) thalamic nucleus. VA/VL and the earlier studied somatosensory thalamic nuclei are considered “core” nuclei with topographic thalamocortical projections, whereas CL is thought to be a “matrix” nucleus with diffuse thalamocortical projections. We found that the TRN input footprints to VA/VL and CL are spatially localized and topographic and generally conform to the patterns established earlier for the TRN projections to sensory thalamic relays. These remarkable similarities suggest similar organization of reticulothalamic pathways and TRN regulation of thalamocortical communication for motor and sensory systems and perhaps also for core and matrix thalamus. Furthermore, we found that VA/VL and CL shared overlapping TRN input regions, suggesting that CL may also be involved in the relay of motor information.