Autism Paper

( <— for details of OD. Basically even before the mouse is born, Random activity in the visual cortex connects neurons to their neighbours. Then during critical period, connections get culled to create thin <.1mm strips of left/right/left/right etc. For left-eye-only mice, the left-eye strips invade the right eye territory. So it's a good test for plasticity — if an adult is able to restore even strips.)
^ Expression of the protein Lynx1 has been associated with the normal end of the critical period for synaptic plasticity in the visual system.

From Autism -- A 'Critical Period' disorder?

My comments in Bold Italic

3. Critical Period Mechanisms

Critical periods have been demonstrated in a variety of contexts [2]. Critical or sensitive periods exist for complex phenomena such as filial imprinting [53], acquisition of courtship song in birds [54, 55], sound localization [56], and fear extinction [57–59]. They also exist for primary sensory modalities and such as tonotopic map refinement in auditory cortex [60] and barrel formation [61] and tuning to whisker stimulation [62, 63] in rodent somatosensory cortex. One of the most mechanistically well-characterized critical periods is for ocular dominance (OD) plasticity in the mammalian visual cortex. Here, we will focus our discussion on the OD critical period because its underlying molecular and cellular mechanisms have been extensively dissected, making it the best model system for testing our hypothesis that critical periods may be abnormal in autism.

Abnormal visual input to one eye during infancy results in permanent loss of visual acuity, amblyopia (Greek for dull vision), if not corrected during childhood. If perturbation of vision occurs in adulthood, the visual impairments are significantly milder or absent [64]. This observation in humans inspired the development of a simple laboratory paradigm to test the existence of a critical period in animal models. David Hubel and Torsten Wiesel began investigating OD plasticity in a series of Nobel Prize winning experiments in the 1960s [65, 66].

They found that the closure of one eye (monocular deprivation) of a kitten during a specific time window early in postnatal life results in an experience-dependent loss of visual acuity in the deprived eye despite no physical damage to the eye itself [67]. This is due to a competitive invasion by the nondeprived eye into cortical territory previously responsive to the deprived eye. A functional loss of responsiveness to the deprived eye and an increase of responsiveness to the open eye are followed first by pruning and then regrowth of dendritic spines on cortical pyramidal neurons [68, 69]. Further structural reorganization takes place in the form of shrinking thalamocortical projections (OD columns) serving the deprived eye and expansion of those serving the open eye [70].

The ocular dominance critical period is present in all mammals tested so far, from humans to mice, and the duration of plasticity is in direct correlation to lifespan and brain weight [71]. The identification of rodents as models of amblyopia has made possible a fine dissection of the mechanisms underlying critical period expression. In particular, by taking advantage of genetically modified mouse models, a specific inhibitory circuit has been identified that controls the timing of OD plasticity [11]. Fine manipulation of inhibitory transmission is difficult in vivo, because enhancing inhibition silences the brain, while reducing inhibition easily induces epilepsy. With the generation of a mouse lacking only one of the two enzymes that synthesizes GABA (GAD65), researchers were able to titrate down the level of inhibition and test its role in the OD critical period [12]. Strikingly, the visual cortex of GAD65 knockout mice remains in an immature, precritical period state throughout life. At any age, functionally enhancing GABAergic transmission with benzodiazepine treatment triggers the opening of a normal-length critical period [72]. Historically, inhibitory neurotransmission was believed to develop postnatally to progressively restrict plasticity, but these key experiments proved GABA to actually be necessary for a normal OD critical period, prompting further investigation into the role of inhibition in brain plasticity.

Critical period should start at Day ~20 and end at day ~32. But GAD65 knockout mice can't make GABA, and this period never starts!

Inhibitory interneurons account for nearly 20% of cortical neurons and exhibit heterogeneous morphological and physiological characteristics [73]. Included in this large variety of inhibitory interneurons is a specific subset of GABAergic neurons that expresses the calcium-binding protein parvalbumin. Fast-spiking parvalbumin-positive basket cells (PV-cells) regulate critical period timing and plasticity [11, 74]. PV-cells develop with a late postnatal time course in anticipation of critical period onset across brain regions [75, 76]. In the visual cortex, PV-cells mature in an experience-dependent manner, and dark-rearing delays their maturation as well as critical period expression [77, 78]. On the other hand, overexpression of brain-derived neurotrophic factor (BDNF) promotes the maturation of PV-cells and speeds up the onset of the OD critical period [77, 79]. Moreover, Di Cristo et al. [80] have shown that premature cortical removal of polysialic acid (PSA), a carbohydrate polymer presented by the neural cell adhesion molecule (NCAM), results in a precocious maturation of perisomatic innervation of pyramidal cells by PV-cells, enhanced inhibitory synaptic transmission, and an earlier onset of OD plasticity. Recent results indicate that PV-cell maturation is surprisingly regulated by the Otx2 homeoprotein, an essential morphogen for embryonic head formation [78]. Otx2 is stimulated by visual experience to pass from the retina to visual cortex and selectively into PV-cells, thereby promoting their maturation and consequently activating OD critical period onset in the visual cortex.

As PV cells mature they produce GABA and trigger the critical period ON.
Dark rearing slows this down, therefore visual experience must feed back into this system.

Visual experience stimulates Otx2 which matures PV cells.
Too much BDNF speeds maturation, so plasticity window opens&closes earlier.

What is not clear to me is what causes the window to close. Or what is the difference between the PRE-window state and the POST-window state.

PV-cells receive direct thalamic input and also connect to each other in large networks across brain regions by chemical synapses and gap junctions [81, 82]. Moreover, PV-cells form numerous synapses onto the somata of pyramidal cells, which in turn enrich these sites with GABAA receptors containing the α1-subunit [11, 70, 74, 78, 83]. This makes PV-cells perfectly situated to detect changes in sensory input, to regulate the spiking of excitatory pyramidal cells, and to synchronize brain regions [84–86]. Manipulations that disrupt this specific circuit will disrupt the OD critical period [87]. Recent studies have made much progress regarding the origin and fate determination of cortical interneurons [88]. In particular, progenitors of PV-cells derive from the medial ganglionic eminence with a relatively late birth date, and their differentiation and migration into specific cortical layers can be regulated by homeoproteins like Lhx6 [88, 89], or excitatory projection neurons [90]. Although the closure of the OD critical period is tightly regulated, transplanting immature GABAergic cells into the visual cortex can reallow OD plasticity later in life [91]. This second sensitive period only emerges once the newly transplanted GABAergic cells reach a critical maturation stage of connectivity. This further supports a key role of inhibition in the timing of experience-dependent circuit refinement.

^ this seems to be suggesting that at some point the life cycle of a GABA-producing cell, it sends out "start plasticity" messages
Does it also send out "stop plasticity" messages later? I don't think so… that seems to be accomplished by some kind of experiential feedback loop. [102] shows that lack of stimulus slows down closing of the window.

GABAergic = GABA-producing

Once the critical period is initiated, plasticity is only possible for a set length of time, and then the critical period closes [92]. Several functional and structural brakes on plasticity have been identified in recent years [93]. Disruption of these brakes in the adult brain allows critical periods to reopen and neuronal circuits to be reshaped. In the case of OD plasticity, this means that monocular deprivation in adulthood would induce a shift in responsiveness to the nondeprived eye and cause a loss of acuity in the deprived hemisphere. Interestingly these brakes share a common theme of regulating E/I balance, and particularly the GABAergic system. Locally reducing inhibition in adulthood restores plasticity in visual cortical circuits [94, 95]. Treatment with the antidepressant drug fluoxetine also reopens plasticity, potentially by altering inhibitory transmission and increasing BDNF levels [96, 97]. Finally, knocking out lynx1, an endogenous prototoxin that promotes desensitization of the nicotinic acetylcholine receptor (nAchR), extends the critical period into adulthood [98]. Lynx1 likely modulates E/I balance because treatment with diazepam in lynx1 knockout mice abolishes adult plasticity by restoring this balance to normal adult levels.

So fluoxetine (a.k.a Prozac) reopens plasticity
Knocking out lynx1 reopens plasticity

Structural factors also restrict remodeling of circuits with the closure of critical periods. For example, PV-cells become increasingly enwrapped in perineuronal nets (PNN) of extracellular matrix with the progression of the critical period, and enzymatic removal of these nets or disruption of their formation restores plasticity in adulthood [78, 99, 100]. In addition, the maturation of myelination throughout the layers of the visual cortex, as measured by myelin basic protein (MBP) levels, increases as the critical period closes [101]. Myelin signaling through Nogo receptors (NgRs) limits plasticity in adulthood, and genetic or pharmacological disruption of this receptor allows persistent OD plasticity later in life [101, 102].

So, enzymatic removal of PNNs restores plasticity in adulthood
Disrupting Myelin signalling through NgRs allows persistent OD plasticity later in life <— I think this is what Valproate/HDACi do!

^ NOTE (102) says OD critical period is 20-32 days

In addition to reopening plasticity, disruption of these brakes also may allow recovery from early deprivation-induced loss of function, like amblyopia. In order to test this, animals are subjected to long-term monocular deprivation spanning the critical period. This results in permanent amblyopia, even if the deprived eye is reopened in adulthood and allowed to receive visual input. Significantly, some of the manipulations described above allow recovery of acuity, including enzymatic degradation of PNNs [103], disruption of NgR signaling [102], administration of fluoxetine [96], and enhanced cholinergic signaling by lynx1 knockdown or treatment with acetylcholinesterase inhibitors [98]. Treatment with drugs like fluoxetine and acetylcholinesterase inhibitors offers particularly promising therapeutic potential because they are already FDA-approved for human use. As the mechanisms behind the closure of critical periods are explored, more light will be shed on potential interventions that could reopen plasticity or reset abnormal critical periods by restoring the brain to a more juvenile-like state.

**^ 4 techniques evaluated by experiment
1) enzymatic degradation of PNNs [103]
2) disruption of NgR signaling [102]
3) administration of fluoxetine [96],
4a) enhanced cholinergic signaling by lynx1 knockdown
4b) … or treatment with acetylcholinesterase inhibitors [98] **

To read:

How generally might these same mechanisms apply to critical periods in other parts of the brain? Interestingly, recent evidence has shown that similar mechanisms may exist in other brain regions. For example, the maturation of PV-cells in the barrel cortex peaks during the critical period for whisker tuning [75]. Furthermore, whisker trimming exclusively during this critical period in mice results in decreased PV expression and reduced inhibitory transmission in vitro [104]. In the zebra finch, brain regions dedicated to singing exhibit progressive PNN formation around PV-cells with a time course that parallels the critical period [105]. The maturity of the song correlates with the percentage of PV-cells that are enwrapped in PNNs, and this can be manipulated with experience by altering exposure to tutor song. In rodent auditory cortex, spectrally limited noise exposure prevents the closure of the critical period for regions of auditory cortex that selectively respond to those interrupted frequencies, and PV-cell number is also reduced in those regions [106]. In the rodent, conditioned fear can be eliminated during early life but is protected from erasure in adulthood [57]. A developmental progression of PNN formation around PV-cells coincides with this switch and enzymatic degradation of PNNs allows juvenile-like fear extinction in adulthood [58, 59], similar to the reopening of OD plasticity in the adult visual cortex [99].

While evidence that very distinct critical periods may share a common role for PV-cells and PNNs is promising, such findings are still largely correlative and will require further cellular and molecular dissection in the future. In light of these findings, it is interesting to note that at least nine different mouse models of autism share a common disruption of PV-cells [58, 59]. In relation to what we know about the importance of inhibitory transmission to critical period regulation, it is quite interesting to consider the evidence that inhibition, or E/I balance in general, is disrupted in neurodevelopmental disorders such as autism. A summary of the key evidence supporting the notion of E/I imbalance in autism is presented below.

Moreover, HDAC2, but not HDAC1, negatively regulates memory formation and synaptic plasticity [33]. "Regulating Critical Period Plasticity: Insight from the Visual System to Fear Circuitry for Therapeutic Interventions"

^ although this paper is interested in restoring plasticity for fear response, it lists different mechanisms for plasticity:

  • PNNs
  • Lynx family
  • Myelin-related Nogo receptor signaling

And different drugs for restoring it:

  • SSRI e.g. fluoxetine a.k.a. Prozac
  • acetylcholinesterase inhibitor (AchEI) e.g. Nicotine
  • HDAC inhibitors e.g. Valproate, TSA

Recent studies using rodent visual cortex have identified multiple structural and functional molecular “brakes” that actively limit plasticity and close the critical period in the adult brain (8, 21). Structural brakes include PNNs (22), myelin-related inhibitory signaling mediated by Nogo receptor (23), and paired immunoglobulin-like receptor B expression (PirB) (24). Functional brakes, such as the nicotinic receptor binding protein Lynx1 act upon excitatory-inhibitory balance within local circuits (25). Importantly, lifting these brakes can induce critical period plasticity in adulthood…

[25] tells us that Lynx1 knockout enhances nicotinic acetylcholine receptor signaling.

#23 from above takes us to: "Experience-Driven Plasticity of Visual Cortex Limited by Myelin and Nogo Receptor"

Nogo-66 Receptor = NgR
Critical period remains open for NgR-knockout mice

"physiological NgR signaling from myelin-derived Nogo, MAG, and OMgp consolidates the neural circuitry established during experience-dependent plasticity. […] NgR signaling limits […] axonal regeneration."

Previous investigations have revealed a critical role for parvalbumin-positive γ-aminobutyric acid (GABA)–ergic neurons in timing the critical period.

^ i.e. PV cells (which are GABA-producing) regulate timing the critical period.

Chondroitin sulfate proteoglycans (CSPGs) are astrocyte-and neuron-derived axon-outgrowth inhibitors that have also been implicated in OD plasticity. Infusion of chondroitinase ABC into spinal cord–injured animals cleaves glycosaminoglycan chains and promotes a degree of regeneration and functional recovery (29) comparable to that of Nogo/NgR antagonism (8, 11).


//PNNs play a critical role in the closure of critical periods and their digestion allows the reopening of these critical periods in the adult brain.PNNs play a critical role in the closure of critical periods and their digestion allows the reopening of these critical periods in the adult brain. They are largely negatively charged and composed of CSPGs (chondroitin sulfate proteoglycans), molecules that play a key role in development and plasticity during postnatal development and in the adult.//

PNNs are composed of a condensed matrix of chondroitin sulfate proteoglycans, molecules that consist of a core protein and a glycosaminoglycan (GAG) chain. The CS-GAG chains associated with PNNs differs from those found floating in the extra-cellular matrix in a noncondensed form. PNNs are composed of brevican, neurocan, versican, aggrecan, phosphacan, hyaluronan, tenascin-R and various link proteins. The CSPGs aggrecan, versican, neurocan, brevican, and phosphacan are bound to hyaluronan. PNNs found in both the brain and the spinal cord have the same composition.[4] Chondroitinase ABC (ChABC), a bacterial enzyme routinely used to digest CSPGs, works by catalyzing the removal of the CS-GAG chains of CSPGs.[1]

//In the cortex and other subcortical areas, PNNs preferentially surround GABAergic interneurons containing the calcium-binding protein parvalbumin.[5] The onset of the critical period corresponds closely to the emergence of parvalbumin-positive cells. Parvalbumin-positive cells synapse onto α1-subunit-containing GABAA receptors. The α1-subunit-containing GABAA receptors have been shown to be the only GABAA receptors that drive cortical plasticity.[2] For this reason, PNNs were first thought to have a strong role in the closure of the critical period.//

Although genetic and pharmacological manipulation of cortical inhibition supports a model in which parvalbumin-positive inhibitory neurons initiate the critical period for OD plasticity (31, 32), glutamatergic synapses also contribute substantially to OD plasticity (33). Both the incomplete extent of OD plasticity restoration by chondroitinase treatment and the GABA-restricted CSPG distribution led us to consider whether more widely distributed neurite-inhibiting mechanisms might participate in OD plasticity. As the vast majority of cortical neurons express NgR (Fig. 1B), we considered whether NgR-mediated myelin inhibition of neurite outgrowth contributes to closing the critical period.

As the NgR ligand, Nogo-A, accounts for a considerable fraction of myelin inhibition of neurite outgrowth (9, 34, 35)…

Absence of NgR protein does not alter GAD65, parvalbumin, or tPA immunoreactivity in the visual cortex of P60 mice (Fig. 3, A to E), which suggests that NgR functions independently or downstream of these proteins to regulate plasticity.

^ suggests NgR knockout is independent brake on plasticity from GABA/PV-cell system

[…]our results support a model in which myelin and NgR function independently.

//The current study provides genetic evidence for the hypothesis that myelination consolidates neural circuitry by suppressing plasticity in the mature brain. Specifically, NgR and Nogo-A/B are required for maturation-dependent restrictions on OD plasticity to monocular deprivation in the visual cortex. However, myelin is not the only limit on cortical plasticity, because CSPGs are also known to have a role in the visual cortex (30), and certain measures of OD persist in the adult mouse (19, 21-23). Dark rearing delays the maturation of GABAergic neurons and the deposition of CSPGs into perineuronal nets (30) but does alter the maturation of intracortical myelination, which is controlled by developmental determinants not dependent on visual experience. Thus, at least two distinct inhibitors limit OD plasticity to the critical period, and eliminating either one is sufficient to facilitate plasticity. //

PROZAC paper:

Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License