Implications of variable synaptic weights for rate and temporal coding of cerebellar outputs
Curation statements for this article:-
Curated by eLife
eLife assessment
This useful study emphasizes some previously ignored aspects of synaptic communication between Purkinje neurons and their targets in the cerebellar nuclei. Reviewers felt that some aspects of the evidence were solid but that others were incomplete.
This article has been Reviewed by the following groups
Listed in
- Evaluated articles (eLife)
Abstract
Purkinje cell (PC) synapses onto cerebellar nuclei (CbN) neurons allow signals from the cerebellar cortex to influence the rest of the brain. PCs are inhibitory neurons that spontaneously fire at high rates, and many PC inputs are thought to converge onto each CbN neuron to suppress its firing. It has been proposed that PCs convey information using a rate code, a synchrony and timing code, or both. The influence of PCs on CbN neuron firing was primarily examined for the combined effects of many PC inputs with comparable strengths, and the influence of individual PC inputs has not been extensively studied. Here, we find that single PC to CbN synapses are highly variable in size, and using dynamic clamp and modeling we reveal that this has important implications for PC-CbN transmission. Individual PC inputs regulate both the rate and timing of CbN firing. Large PC inputs strongly influence CbN firing rates and transiently eliminate CbN firing for several milliseconds. Remarkably, the refractory period of PCs leads to a brief elevation of CbN firing prior to suppression. Thus, individual PC-CbN synapses are suited to concurrently convey rate codes and generate precisely timed responses in CbN neurons. Either synchronous firing or synchronous pauses of PCs promote CbN neuron firing on rapid time scales for nonuniform inputs, but less effectively than for uniform inputs. This is a secondary consequence of variable input sizes elevating the baseline firing rates of CbN neurons by increasing the variability of the inhibitory conductance. These findings may generalize to other brain regions with highly variable inhibitory synapse sizes.
Article activity feed
-
-
Author Response
eLife assessment
This useful study emphasizes some previously ignored aspects of synaptic communication between Purkinje neurons and their targets in the cerebellar nuclei. Reviewers felt that some aspects of the evidence were solid but that others were incomplete.
We think this is an extensive and complete study. The major issue that the reviewers raised is about the usage of high chloride internals in our recordings. We feel that this single issue does not really match the statement “others were incomplete”, which suggests that this study is incomplete in some way. Please note that in our complete revision we will respond to the issue of chloride by pointing out: (1) the advantages of using high chloride internals to determine the distribution of input sizes, (2) the challenges of estimating the relationship …
Author Response
eLife assessment
This useful study emphasizes some previously ignored aspects of synaptic communication between Purkinje neurons and their targets in the cerebellar nuclei. Reviewers felt that some aspects of the evidence were solid but that others were incomplete.
We think this is an extensive and complete study. The major issue that the reviewers raised is about the usage of high chloride internals in our recordings. We feel that this single issue does not really match the statement “others were incomplete”, which suggests that this study is incomplete in some way. Please note that in our complete revision we will respond to the issue of chloride by pointing out: (1) the advantages of using high chloride internals to determine the distribution of input sizes, (2) the challenges of estimating the relationship between input sizes for different chloride internals, (3) the previous studies that have established the relationship between input sizes and chloride levels at other synapses, and (4) additional simulations will be provided indicating that subtle changes in the input sizes would have minor quantitative effects on the influences of individual inputs, but would not affect the main conclusions of the paper.
Reviewer #1 (Public Review):
This manuscript explores physiological properties of Purkinje-to-nuclear synapses. The report provides largely incremental advances over what has already been discovered about this synaptic relationship. The main findings, as articulated by the authors, are that Purkinje-to-nuclear synaptic strength is variable, with a few very strong inputs to the cerebellar nuclei. They show that single inputs effectively inhibit nuclear firing and that the diversity of synaptic strength influences nuclear neuron responsivity to inputs by enhancing synaptic variance. In addition, while not necessarily surprising, it's nice to see that stronger inputs would have a stronger influence on a postsynaptic cell, both in terms of rates and temporal coding transfer. Overall, as it stands, the manuscript is not very scholarly, overstates the novelty of findings, and frames a straw-man. That said, buried in here are some potentially interesting observations.
This review provides us with an opportunity to more clearly summarize what is new in our findings. Our study builds upon Person and Raman (2012) and other studies, and makes a number of important advances. (1) We provide a much more extensive characterization of input sizes (n=157) than previous studies, and show that the distribution of input sizes is skewed, with the largest inputs almost 100 times larger than the smallest inputs. This distribution is clearly different from that of Person and Raman (2012), where the estimation of unitary PC input sizes was based on small sample sizes from a broad range of age (n=30, P13-29 animals). The high Cl- concentration internal we used in our recordings provides us with superior stability and sensitivity in detecting such variability in input size. (2) We show for the first time that the distribution of input sizes becomes more skewed in juvenile animals than in young animals, suggesting that PC-CbN synapses are modified by plasticity mechanisms during development. (3) Our dynamic clamp approach is based on the skewed distribution of input sizes we observed, and the Purkinje cell firing patterns we recorded in vivo, whereas Person and Raman (2012) primarily focused their dynamic clamp studies on 40 uniform sized inputs (even though they recognized that there are also somewhat larger inputs), with their firing interspike intervals drawn from Gaussian distributions (which lack refractory periods and do not represent realistic PCs firing patterns). We also complement our dynamic clamp studies with simulations using an integrate-and-fire model that does a good job of replicating our dynamic clamp studies. This allowed us to more thoroughly explore the effects of different size input that would not be practical with dynamic clamp studies. (4) We show that individual PC inputs powerfully regulate the rate and timing of CbN neuron firing, without requiring a high degree of PC synchrony. (5) We further show that timing control by PCs leads to strong inhibition of CbN firing and, surprisingly, a brief elevation prior to the inhibition. This result from the refractory period of PCs, which generate a disinhibition period prior to the inhibition, and is shaped by the firing statistics of PC inputs. If such an elevation prior to inhibition was observed in vivo, it could be misinterpreted as excitation of CbN neurons by other inputs (e.g., mossy fiber collaterals) preceding the PC inputs. (6) We show that the total inhibitory conductance and the coefficient of variation (CV) of this conductance are both important factors in controlling the firing rate of CbN neurons. Having variable input sizes or synchronized inputs all lead to higher CV of the inhibitory conductance and therefore higher firing rates. (7) We show that all different-sized PC inputs transmit a robust rate code that simply depend on their sizes. (8) Our study helps to resolve a long-standing controversy in the field. Some thought that PC synchrony is an effective way of controlling CbN neuron firing, while others doubted the physiological relevance of PC synchrony. Here we show that a single large input is functionally equivalent to many small, perfectly synchronized inputs, which can influence the rate and timing of CbN firing as previously proposed (Person and Raman, 2012a), but without requiring a high degree of PC synchrony. We also suggest that a high degree of synchrony is not a prerequisite for an appreciable influence, because synchronizing a few large inputs can have large effects on CbN neuron firing. We strived to be fair and thorough, and we think that the study is scholarly. Prior to the initial submission, we sought advice from experts in the field, Indira Raman and Nicolas Brunel, and their input was very helpful in this regard. We will revise the manuscript to more clearly articulate what has been done previously, and what aspects of our study are new.
Reviewer #2 (Public Review):
In this manuscript, the authors address how cerebellar Purkinje cells (PC) control the firing of nuclear cells (CbN), the output stage of the cerebellar. They used patch-clamp recordings in acute cerebellar slices, and combined dynamic clamp with simulations of nuclear cell firing rate.
This article addresses one of the most fundamental unresolved question of the cerebellar physiology: how inhibitory PCs control the output stage of the cerebellum?
They first described a developmental evolution of the that PC-CbN synapses. Inhibitory synaptic weights become highly variable after three weeks of age, with a group of very large PC inputs. They used dynamic clamp to examine the influence of these variable inputs on CbN firing rate. They demonstrate that while all input size affect CbN discharge, larger ones can stop them for a few milliseconds. Using a distribution of variable input size, they showed that increasing the variability of PC inputs favor CbN discharge, while increasing the magnitude of a constant inhibitory conductance decrease their firing rate. By varying the frequency of PC inputs, they suggest that CbNs faithfully transmit rate code, but larger inputs are more effective to decrease their firing rate. Finally, addressing how synchrony of variable PC inputs influence CbN discharge, dynamic clamp studies and simulations showed that input synchronization enhance firing, but driven by the total charge of the inhibitory input.
The keystone observations that PC inputs are highly variable is very interesting and convincing and open new questions about PC-CbN plasticity. More importantly the combination of dynamic clamp and simulations is a real strength of the study, allowing the authors to test many combinations of inputs in real cells and extrapolating their hypotheses in silico. Weaknesses result from the assumptions made on the construction of the distribution of inputs and the many different conditions explored. The organization of the article could be difficult to read for a non-specialist of cerebellar physiology.
We thank the reviewer for their kind comments. We will revise the manuscript to clarify the assumptions made to construct the distribution of input sizes. We will do our best to revise the manuscript to make it easier for a non-specialist to read.
-
eLife assessment
This useful study emphasizes some previously ignored aspects of synaptic communication between Purkinje neurons and their targets in the cerebellar nuclei. Reviewers felt that some aspects of the evidence were solid but that others were incomplete.
-
Reviewer #1 (Public Review):
This manuscript explores physiological properties of Purkinje-to-nuclear synapses. The report provides largely incremental advances over what has already been discovered about this synaptic relationship. The main findings, as articulated by the authors, are that Purkinje-to-nuclear synaptic strength is variable, with a few very strong inputs to the cerebellar nuclei. They show that single inputs effectively inhibit nuclear firing and that the diversity of synaptic strength influences nuclear neuron responsivity to inputs by enhancing synaptic variance. In addition, while not necessarily surprising, it's nice to see that stronger inputs would have a stronger influence on a postsynaptic cell, both in terms of rates and temporal coding transfer. Overall, as it stands, the manuscript is not very scholarly, …
Reviewer #1 (Public Review):
This manuscript explores physiological properties of Purkinje-to-nuclear synapses. The report provides largely incremental advances over what has already been discovered about this synaptic relationship. The main findings, as articulated by the authors, are that Purkinje-to-nuclear synaptic strength is variable, with a few very strong inputs to the cerebellar nuclei. They show that single inputs effectively inhibit nuclear firing and that the diversity of synaptic strength influences nuclear neuron responsivity to inputs by enhancing synaptic variance. In addition, while not necessarily surprising, it's nice to see that stronger inputs would have a stronger influence on a postsynaptic cell, both in terms of rates and temporal coding transfer. Overall, as it stands, the manuscript is not very scholarly, overstates the novelty of findings, and frames a straw-man. That said, buried in here are some potentially interesting observations.
-
Reviewer #2 (Public Review):
In this manuscript, the authors address how cerebellar Purkinje cells (PC) control the firing of nuclear cells (CbN), the output stage of the cerebellar. They used patch-clamp recordings in acute cerebellar slices, and combined dynamic clamp with simulations of nuclear cell firing rate.
This article addresses one of the most fundamental unresolved question of the cerebellar physiology: how inhibitory PCs control the output stage of the cerebellum?
They first described a developmental evolution of the that PC-CbN synapses. Inhibitory synaptic weights become highly variable after three weeks of age, with a group of very large PC inputs. They used dynamic clamp to examine the influence of these variable inputs on CbN firing rate. They demonstrate that while all input size affect CbN discharge, larger ones can …Reviewer #2 (Public Review):
In this manuscript, the authors address how cerebellar Purkinje cells (PC) control the firing of nuclear cells (CbN), the output stage of the cerebellar. They used patch-clamp recordings in acute cerebellar slices, and combined dynamic clamp with simulations of nuclear cell firing rate.
This article addresses one of the most fundamental unresolved question of the cerebellar physiology: how inhibitory PCs control the output stage of the cerebellum?
They first described a developmental evolution of the that PC-CbN synapses. Inhibitory synaptic weights become highly variable after three weeks of age, with a group of very large PC inputs. They used dynamic clamp to examine the influence of these variable inputs on CbN firing rate. They demonstrate that while all input size affect CbN discharge, larger ones can stop them for a few milliseconds. Using a distribution of variable input size, they showed that increasing the variability of PC inputs favor CbN discharge, while increasing the magnitude of a constant inhibitory conductance decrease their firing rate. By varying the frequency of PC inputs, they suggest that CbNs faithfully transmit rate code, but larger inputs are more effective to decrease their firing rate. Finally, addressing how synchrony of variable PC inputs influence CbN discharge, dynamic clamp studies and simulations showed that input synchronization enhance firing, but driven by the total charge of the inhibitory input.The keystone observations that PC inputs are highly variable is very interesting and convincing and open new questions about PC-CbN plasticity. More importantly the combination of dynamic clamp and simulations is a real strength of the study, allowing the authors to test many combinations of inputs in real cells and extrapolating their hypotheses in silico. Weaknesses result from the assumptions made on the construction of the distribution of inputs and the many different conditions explored. The organization of the article could be difficult to read for a non-specialist of cerebellar physiology.
-
