Scientific Understanding of Consciousness
Cortical Inhibition Regulated by Sensory Experience
Nature 531, 371–375 (17 March 2016)
Sensory experience regulates cortical inhibition by inducing IGF1 in VIP neurons
R. Mardinly, et.al.
Department of Molecular and Cellular Biology, University of California Berkeley, 205 Life Sciences Addition, Berkeley, California 94720, USA
Department of Neurobiology, Harvard Medical School, 220 Longwood Ave, Boston, Massachusetts 02115, USA
FM Kirby Neurobiology Center, Boston Children’s Hospital, 3 Blackfan Circle, Boston, Massachusetts 02115, USA
Inhibitory neurons regulate the adaptation of neural circuits to sensory experience, but the molecular mechanisms by which experience controls the connectivity between different types of inhibitory neuron to regulate cortical plasticity are largely unknown. Here we show that exposure of dark-housed mice to light induces a gene program in cortical vasoactive intestinal peptide (VIP)-expressing neurons that is markedly distinct from that induced in excitatory neurons and other subtypes of inhibitory neuron. We identify Igf1 as one of several activity-regulated genes that are specific to VIP neurons, and demonstrate that IGF1 functions cell-autonomously in VIP neurons to increase inhibitory synaptic input onto these neurons. Our findings further suggest that in cortical VIP neurons, experience-dependent gene transcription regulates visual acuity by activating the expression of IGF1, thus promoting the inhibition of disinhibitory neurons and affecting inhibition onto cortical pyramidal neurons.
To explore how sensory experience affects gene expression in VIP neurons, we examined this process in the visual cortex of adult mice that were housed in standard conditions, in complete darkness (that is, dark-housed), or dark-housed and then exposed to light for increasing amounts of time. Light deprivation for as little as 12 h drives robust gene expression after light exposure, and increasing durations of dark-housing accentuate the gene induction response irrespective of the phase of the circadian rhythm. To purify RNA selectively from VIP-expressing and other inhibitory neuron subtypes, we generated mice that were heterozygous for alleles of either Vip-cre, Sst-cre or Pv-cre, and were also heterozygous for the Rpl22-HA (RiboTag) allele, which expresses a haemagglutinin (HA)-tagged ribosomal protein specifically in Cre-expressing neurons. For purposes of comparison, we also purified ribosome-bound RNA from excitatory and inhibitory neurons, labelled by Emx1-cre or Gad2-cre.
By quantitative real-time PCR (qPCR), we find that messenger RNAs for cell-type-specific marker genes are highly enriched in the appropriate samples and that light exposure induces the expression of early-response genes in each Cre line. To quantify experience-induced gene expression at a genome-wide level, we performed RNA-seq on RNA isolated from the dark-housed/light-exposed RiboTag-mice. This analysis identified genes which exhibited reproducible changes in expression levels in response to visual stimulation in at least one Cre line (n = 602) and thus allowed us to ask how levels of these experience-regulated genes are correlated across the different neuronal subtypes compared to non-regulated genes (n = 13,678). We found that the expression of experience-regulated genes is remarkably dissimilar across different neuronal subtypes when compared to genes that are not regulated by sensory experience (irrespective of differences in the number or expression levels of experience-regulated genes. While unique subsets of experience-responsive genes were identified in each neuronal subtype, VIP neurons are the most responsive to sensory stimulation and possess an experience-induced gene expression program that is markedly distinct from the other neuronal subtypes analysed. This suggests that in VIP neurons the experience-dependent gene program may have a unique function in adapting the cortex’s neural circuits to sensory experience.
We hypothesized that experience-regulated genes that are specifically expressed and selectively regulated in VIP neurons are likely to have important functions in regulating the synaptic connectivity onto VIP neurons. Thus, we first identified the mRNAs that are specifically enriched in each subtype and cross-referenced these genes with the list of experience-regulated genes. This analysis identified 31 genes that are both cell-type-specific and experience-regulated, 11 of which are specific to VIP neurons. Notably, secreted molecules are significantly over-represented in this gene set (GO-term ‘Secreted’ P = 0.002) and each type of neuron has its own set of cell-type-specific experience-regulated secreted factors, including four experience-induced secreted molecules that are specific to VIP neurons (Igf1, Crh, Prok2, Fbln2).
We next performed fluorescent in situ hybridization (FISH) on sections of visual cortices of dark-housed/light-exposed mice to quantify the percentage of cells that co-express an inhibitory subtype marker and the respective secreted factor. Of the four secreted factors, Igf1 is the one factor that is expressed in the vast majority of VIP neurons, and whose expression is highly enriched in these neurons.
Cortical inhibition regulates ocular dominance (OD) plasticity and visual acuity, and hyper-activation of VIP neurons drives a form of adult cortical plasticity.
To test whether the effect of Igf1 knockdown is experience-dependent, we next monocularly deprived mice for a brief period of time, beginning at the peak time of ocular dominance plasticity. After four days of monocular deprivation, we recorded visual-evoked potentials from the visual cortex contralateral to the deprived eye and quantified the C:I ratio upon stimulation at low spatial frequency as well as the visual acuity upon stimulation of the deprived eye. Brief monocular deprivation led to a reduction in the C:I ratio in mice injected with AAVs expressing either control or Igf1 shRNA; this is a consequence of the reduction in the contralateral response. Notably, when we tested visual acuity after brief monocular deprivation, both Igf1 and control shRNA injected mice exhibited similar levels of amblyopia (that is, loss of visual acuity) in the deprived eye, despite the higher visual acuity in the Igf1 shRNA injected mice that were not monocularly deprived. These findings indicate that VIP neuron-derived IGF1 regulates visual acuity in an experience-dependent manner and may function as a sensory-dependent brake on cortical plasticity. The observation that in response to sensory experience IGF1 in VIP neurons controls inhibition, taken together with the previous finding that experience induces BDNF in excitatory neurons to regulate excitatory–inhibitory balance, suggests a model in which each type of neuron within a cortical circuit expresses a unique set of experience-induced secreted factors that control specific synaptic inputs onto the neuron and plasticity within a neural circuit.
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