Author Response
Reviewer #2 (Public Review):
The current work was basically a follow-up of a previous study in juvenile mice, and the results were also very similar to the juvenile results (Sommeijer et al., 2017). One possible interpretation of the results is that the lack of OD plasticity in adult V1 and dLGN was caused by an early blockade of the development of the inhibitory circuit in dLGN, which retains the dLGN in an immature stage till adulthood. The authors indeed claimed in the discussion that the 2-day OD shift is intact in juvenile dLGN and V1 in KO mice, and provided evidence in supplementary figure that GABAergic and cholinergic synapse amount are similar between WT and KO mice. However, the 7-day OD shift is indeed defected in juvenile V1 and dLGN in KO mice (Sommeijer et al., 2017), and it is possible that this early functional deficit didn't induce a structural remodeling in adulthood. To better support the author's claim that the lack of adult V1 OD plasticity is specifically due to reduced dLGN synaptic inhibition, the author should generate conditional KO mice that dLGN synaptic inhibition was only interfered in adulthood.
In order to address this point it is important to discuss the plasticity deficits in dLGN and V1 separately.
Concerning V1 plasticity: We have previously shown that brief MD during the standard critical period induces an OD shift in V1 of mice lacking thalamic synaptic inhibition in dLGN (Sommeijer et al, 2017). OD plasticity induced by brief MD is a hallmark of critical period plasticity in V1, and it thus seems unlikely that critical period onset in V1 is defective or that development of V1 is halted in an immature state that does not support OD plasticity in thalamus-specific GABRA1 deficient mice.
The observed plasticity deficit during the critical period was limited to the second stage of the OD shift in V1, which requires long-term monocular deprivation. The straightforward explanation for this result and our current findings is that both during the critical period and in adulthood, the second stage of OD plasticity in V1 induced by long-term monocular deprivation requires thalamic plasticity or inhibition. The proposed alternative, that lack of thalamic synaptic inhibition during development results in a possible lack of structural change in V1 that would cause a lifelong deficiency selectively affecting OD plasticity induced by long-term monocular deprivation, is not impossible but requires many more assumptions.
Concerning dLGN plasticity: The simplest explanation for the observed lack of OD plasticity in dLGN is that it is a direct consequence of the absence of synaptic inhibition in the KO mice. However, an alternative explanation could indeed be that dLGN is kept in an immature (pre-critical period-like) state due to the developmental absence of synaptic inhibition. This situation would be analogous to that in V1 of GAD65 deficient mice (which have reduced GABA release), in which OD plasticity cannot be induced by brief monocular deprivation during the critical period or in adulthood (Fagiolini and Hensch, 2000). Because this deficit can be reversed by treating the mice with benzodiazepines (positive allosteric modulators of GABA receptors) at any age, it is thought that development of V1 in GAD65 mice is halted in a pre-critical period-like state until inhibition is strengthened. We cannot exclude that something similar occurs in dLGN of mice lacking thalamic synaptic inhibition, although we did not observe any changes in hallmarks of dLGN maturity, such as reduced receptive field size (Fig. 1C), and increased cholinergic and inhibitory bouton densities (Suppl. Fig. 1).
However, if the analogy with the developmental deficit in V1 of GAD65 deficient mice is valid, the reduced plasticity is still likely to be a direct consequence of reduced inhibition. In GAD65 deficient mice, long-term monocular deprivation during the critical period causes a full OD shift, showing that no additional deficits (besides reduced inhibition) limit OD plasticity in V1 of these mice (Fagiolini and Hensch, 2000). And, as already mentioned, increasing inhibition rescues OD plasticity in GAD65 KO mice. Thus, the immature state of V1 in these mice is probably a situation in which inhibition tone is too low to support efficient OD plasticity. In dLGN, knocking out GABRA1 at a later age could therefore also create a situation in which inhibition is too low to support thalamic OD plasticity, which is not different from the situation in which the gene is inactivated at birth. Only if lack of synaptic inhibition in thalamus affects another, unknown developmental process that is of importance later in life to support OD plasticity in dLGN, the proposed experiment would result in a different outcome. We are not convinced that this scenario is likely enough to justify repeating most of this study, but now using mice in which GABRA1 is inactivated in dLGN through bilateral AAV-cre injections.
Independently of the exact cause of the plasticity deficit in dLGN, our results make clear that a cortical plasticity deficit in adulthood can have a thalamic origin, which we believe is an important insight that is highly relevant.
- The authors found that in juveniles, dLGN OD shift is dependent on V1 feedback, but not in adults. However, a recent work showed that the effects of V1 silencing on dLGN OD plasticity could differ with various starting points and duration of the V1 silencing and MD (Li et al., 2023). Could the authors provide more details of the MD and V1 silencing for an in-depth discussion?
We would be happy to include some discussion about this interesting new paper in a revised manuscript. Some of the results may appear to contradict our findings. Most strikingly, the study by Li et al does not find evidence for OD plasticity in dLGN of 60-day old mice after 7 days of monocular deprivation. This seems to be at odds with the current work and with that of (Jaepel et al 2017) and (Huh et al. 2020). However, in the (Li et al, 2022) study, only the binocular neurons which responded to both contralateral and ipsilateral stimulus were included to measure the OD. This has important consequences for assessing OD and its plasticity. To illustrate: if dLGN neurons are monocularly responsive to the contralateral eye and become binocular after deprivation of the contralateral eye, they are excluded from analysis before deprivation but included after. This would cause an underestimation of the size of this OD shift. In our experiments, all dLGN neurons with receptive fields that were within 30o degrees away from the center of the visual field were included in the analysis, potentially explaining the different outcome of the studies.
Also, Li et al observed that an OD shift in dLGN was still present after silencing V1 at p24. This observation is not necessarily at odds with our observation that the OD shift reduces at p30 upon silencing V1, as we find that the ODI does not return to normal but remains slightly lower (though not significantly so). Moreover, the age and the duration of deprivation were different and as mentioned before, analysis was performed differently.
Interestingly, an excitotoxic lesion of V1 was found to alter OD in dLGN during development and affect OD plasticity in dLGN at various ages in the work of Li et al. This suggests that continuous crosstalk between thalamus and cortex during development guides plasticity, possibly optimizing thalamocortical and corticothalamic connections. The continued absence of corticothalamic feedback is likely to have a much larger impact on dLGN plasticity than the acute silencing we performed.
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Huh CYL, Abdelaal K, Salinas KJ, Gu D, Zeitoun J, Figueroa Velez DX, Peach JP, Fowlkes CC, Gandhi SP. Long-term Monocular Deprivation during Juvenile Critical Period Disrupts Binocular Integration in Mouse Visual Thalamus. J Neurosci. 2020 Jan 15;40(3):585-604. doi: 10.1523/JNEUROSCI.1626-19.2019
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Li N, Liu Q, Zhang Y, Yang Z, Shi X, Gu Y. Cortical feedback modulates distinct critical period development in mouse visual thalamus.. iScience. 2022 Dec 7;26(1):105752.
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