Competition and Homeostasis of Excitatory and Inhibitory Connectivity in the Adult Mouse Visual Cortex

Integrating EM and Patch-seq data: Synaptic connectivity and target specificity of predicted Sst transcriptomic types

Neural circuit function is shaped both by the cell types that comprise the circuit and the connections between those cell types ¹ . Neural cell types have previously been defined by morphology ^{2, 3} , electrophysiology ^{4, 5} , transcriptomic expression ^6-8 , connectivity ^9-13 , or even a combination of such modalities ^14-16 . More recently, the Patch-seq technique has enabled the characterization of morphology (M), electrophysiology (E), and transcriptomic (T) properties from individual cells ^17-20 . Using this technique, these properties were integrated to define 28, inhibitory multimodal, MET-types in mouse primary visual cortex ²¹ . It is unknown how these MET-types connect within the broader cortical circuitry however. Here we show that we can predict the MET-type identity of inhibitory cells within a large-scale electron microscopy (EM) dataset and these MET-types have distinct ultrastructural features and synapse connectivity patterns. We found that EM Martinotti cells, a well defined morphological cell type ^{22, 23} known to be Somatostatin positive (Sst+) ^{24, 25} , were successfully predicted to belong to Sst+ MET-types. Each identified MET-type had distinct axon myelination patterns and synapsed onto specific excitatory targets. Our results demonstrate that morphological features can be used to link cell type identities across imaging modalities, which enables further comparison of connectivity in relation to transcriptomic or electrophysiological properties. Furthermore, our results show that MET-types have distinct connectivity patterns, supporting the use of MET-types and connectivity to meaningfully define cell types.

Inhibition, but not excitation, recovers from partial cone loss with greater spatiotemporal integration, synapse density, and frequency

Neural circuits function in the face of changing inputs, either caused by normal variation in stimuli or by cell death. To maintain their ability to perform essential computations with partial inputs, neural circuits make modifications. Here, we study the retinal circuit’s responses to changes in light stimuli or in photoreceptor inputs by inducing partial cone death in the mature mouse retina. Can the retina withstand or recover from input loss? We find that the excitatory pathways exhibit functional loss commensurate with cone death and with some aspects predicted by partial light stimulation. However, inhibitory pathways recover functionally from lost input by increasing spatiotemporal integration in a way that is not recapitulated by partially stimulating the control retina. Anatomically, inhibitory synapses are upregulated on secondary bipolar cells and output ganglion cells. These findings demonstrate the greater capacity for inhibition, compared with excitation, to modify spatiotemporal processing with fewer cone inputs.

Lee et al. find partial cone loss triggers inhibition, but not excitation, to increase spatiotemporal integration, recover contrast gain, and increase synaptic release onto retinal ganglion cells. Natural images filtered by cone-loss receptive fields perceptually match those of controls. Thus, inhibition compensates for fewer cones to potentially preserve perception.

Pten loss results in inappropriate excitatory connectivity

Pten mutations are associated with autism spectrum disorder. Pten loss of function in neurons increases excitatory synaptic connectivity, contributing to an imbalance between excitation and inhibition. We aimed to determine whether Pten loss results in aberrant connectivity in neural circuits. We compared postnatally generated wild-type and Pten knockout granule neurons integrating into the dentate gyrus using a variety of methods to examine their connectivity. We found that postsynaptic Pten loss provides an advantage to dendritic spines in competition over a limited pool of presynaptic boutons. Retrograde monosynaptic tracing with rabies virus reveals that this results in synaptic contact with more presynaptic partners. Using independently excitable opsins to interrogate multiple inputs onto a single neuron, we found that excess connectivity is established indiscriminately from among glutamatergic afferents. Therefore, Pten loss results in inappropriate connectivity whereby neurons are coupled to a greater number of synaptic partners.

β-Catenin in the Adult Visual Cortex Regulates NMDA-Receptor Function and Visual Responses

The formation, plasticity and maintenance of synaptic connections is regulated by molecular and electrical signals. β-Catenin is an important protein in these events and regulates cadherin-mediated cell adhesion and the recruitment of pre- and postsynaptic proteins in an activity-dependent fashion. Mutations in the β-catenin gene can cause cognitive disability and autism, with life-long consequences. Understanding its synaptic function may thus be relevant for the treatment of these disorders. So far, β-catenin's function has been studied predominantly in cell culture and during development but knowledge on its function in adulthood is limited. Here, we show that ablating β-catenin in excitatory neurons of the adult visual cortex does not cause the same synaptic deficits previously observed during development. Instead, it reduces NMDA-receptor currents and impairs visual processing. We conclude that β-catenin remains important for adult cortical function but through different mechanisms than during development.

All for One But Not One for All: Excitatory Synaptic Scaling and Intrinsic Excitability Are Coregulated by CaMKIV, Whereas Inhibitory Synaptic Scaling Is Under Independent Control

Neocortical circuits use a family of homeostatic plasticity mechanisms to stabilize firing, including excitatory and inhibitory synaptic scaling and homeostatic intrinsic plasticity (Turrigiano and Nelson, 2004). All three mechanisms can be induced in tandem in cultured rat neocortical pyramidal neurons by chronic manipulations of firing, but it is unknown whether they are coinduced by the same activity-sensors and signaling pathways, or whether they are under independent control. Calcium/calmodulin-dependent protein kinase type IV (CaMKIV) is a key sensory/effector in excitatory synaptic scaling that senses perturbations in firing through changes in calcium influx, and translates this into compensatory changes in excitatory quantal amplitude (Ibata et al., 2008; Goold and Nicoll, 2010). Whether CaMKIV also controls inhibitory synaptic scaling and intrinsic homeostatic plasticity was unknown. To test this we manipulated CaMKIV signaling in individual neurons using dominant-negative (dn) or constitutively-active (ca) forms of nuclear-localized CaMKIV and measured the induction of all three forms of homeostatic plasticity. We found that excitatory synaptic scaling and intrinsic plasticity were bidirectionally coinduced by these manipulations. In contrast, these cell-autonomous manipulations had no impact on inhibitory quantal amplitude. Finally, we found that spontaneous firing rates were shifted up or down by dnCaMKIV or caCaMKIV, respectively, suggesting that uncoupling CaMKIV activation from activity generates an error signal in the negative feedback mechanism that controls firing rates. Together, our data show that excitatory synaptic scaling and intrinsic excitability are tightly coordinated through bidirectional changes in the same signaling pathway, whereas inhibitory synaptic scaling is sensed and regulated through an independent control mechanism.SIGNIFICANCE STATEMENT Maintaining stable function in highly interconnected neural circuits is essential for preventing circuit disorders, and is accomplished through a set of negative feedback mechanisms that sense and compensate for perturbations in activity. These "homeostatic" mechanisms can target synaptic excitation, synaptic inhibition, and intrinsic excitability, but whether they are independently controlled is not known. We find that synaptic excitation and intrinsic excitability are coregulated in individual neurons through CaMKIV signaling, which is tightly controlled by neuronal activity. In contrast, synaptic inhibition is unaffected by changes in firing or CaMKIV signaling in individual neurons. These results show that circuit stability is controlled both through cell-autonomous mechanisms that regulate some aspects of excitability, as well as circuit-level mechanisms that adjust inhibition.