Circuit neuroscience: the road ahead

It is difficult to write about grand challenges in our field without pontificating or pretending to show a degree of certainty in assessing the field that I do not possess. I would rather comment on a few of the issues that particularly worry me. Therefore, this article is just a snapshot of our field now, as I see it, and encourage readers to read it as the opinion of just one of their colleagues. My comments are aimed at Circuit Neuroscience. What exactly is Circuit Neuroscience? As stated in the mission statement of Frontiers in Neural Circuits, I follow the definition of Circuit Neuroscience as the understanding of the computational function of neural circuits, linking this function with the circuit micro-structure. Within this field, I will address three different types of challenges: scientific, methodological and sociological ones

Neural ensembles: Role of intrinsic excitability and its plasticity

Synaptic connectivity defines groups of neurons that engage in correlated activity during specific functional tasks. These co-active groups of neurons form ensembles, the operational units involved in, for example, sensory perception, motor coordination and memory (then called an engram). Traditionally, ensemble formation has been thought to occur via strengthening of synaptic connections via long-term potentiation (LTP) as a plasticity mechanism. This synaptic theory of memory arises from the learning rules formulated by Hebb and is consistent with many experimental observations. Here, we propose, as an alternative, that the intrinsic excitability of neurons and its plasticity constitute a second, non-synaptic mechanism that could be important for the initial formation of ensembles. Indeed, enhanced neural excitability is widely observed in multiple brain areas subsequent to behavioral learning. In cortical structures and the amygdala, excitability changes are often reported as transient, even though they can last tens of minutes to a few days. Perhaps it is for this reason that they have been traditionally considered as modulatory, merely supporting ensemble formation by facilitating LTP induction, without further involvement in memory function (memory allocation hypothesis). We here suggest−based on two lines of evidence—that beyond modulating LTP allocation, enhanced excitability plays a more fundamental role in learning. First, enhanced excitability constitutes a signature of active ensembles and, due to it, subthreshold synaptic connections become suprathreshold in the absence of synaptic plasticity (iceberg model). Second, enhanced excitability promotes the propagation of dendritic potentials toward the soma and allows for enhanced coupling of EPSP amplitude (LTP) to the spike output (and thus ensemble participation). This permissive gate model describes a need for permanently increased excitability, which seems at odds with its traditional consideration as a short-lived mechanism. We propose that longer modifications in excitability are made possible by a low threshold for intrinsic plasticity induction, suggesting that excitability might be on/off-modulated at short intervals. Consistent with this, in cerebellar Purkinje cells, excitability lasts days to weeks, which shows that in some circuits the duration of the phenomenon is not a limiting factor in the first place. In our model, synaptic plasticity defines the information content received by neurons through the connectivity network that they are embedded in. However, the plasticity of cell-autonomous excitability could dynamically regulate the ensemble participation of individual neurons as well as the overall activity state of an ensemble

"Whole-body imaging of neural and muscle activity during behavior in hydra vulgaris: effect of osmolarity on contraction bursts"

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Yamamoto, W., & Yuste, R. "Whole-body imaging of neural and muscle activity during behavior in hydra vulgaris: effect of osmolarity on contraction bursts". Eneuro, (2020), doi: 10.1523/ENEURO.0539-19.2020.The neural code relates the activity of the nervous system to the activity of the muscles to the generation of behavior. To decipher it, it would be ideal to comprehensively measure the activity of the entire nervous system and musculature in a behaving animal. As a step in this direction, we used the cnidarian Hydra vulgaris to explore how physiological and environmental conditions alters simple contractile behavior and its accompany neural and muscle activity. We used whole-body calcium imaging of neurons and muscle cells and studied the effect of temperature, media osmolarity, nutritional state and body size on contractile behavior. In mounted Hydra preparations, changes in temperature, nutrition state or body size did not have a major effect on neural or muscle activity, or on contractile behavior. But changes in media osmolarity systematically altered contractile behavior and foot detachments, increasing their frequency in hypo-osmolar media solutions and decreasing it in hyperosmolar media. Similar effects were seen in ectodermal, but not in endodermal muscle. Osmolarity also bidirectionally changed the activity of contraction burst neurons, but did not affect the network of rhythmic potential neurons in the ectoderm. These findings show osmolarity-dependent changes in the activity of contraction burst neurons and ectodermal muscle, consistent with the hypothesis that contraction burst neurons respond to media hypo-osmolarity, activating ectodermal muscle to generate contraction bursts. This dedicated circuit could serve as an excretory system to prevent osmotic injury. This work demonstrates the feasibility of studying an entire neuronal and muscle activity in a behaving animal.This work was supported by the NSF (CRCNS 1822550). MBL research was supported in part by competitive fellowship funds from the H. Keffer Hartline, Edward F. MacNichol, Jr. Fellowship Fund, The E. E. Just Endowed Research Fellowship Fund, Lucy B. Lemann Fellowship Fund, and Frank R. Lillie Fellowship Fund Fellowship Fund of the Marine Biological Laboratory in Woods Hole, MA

Overproduction of Neurons Is Correlated with Enhanced Cortical Ensembles and Increased Perceptual Discrimination

Brains vary greatly in neuronal number and density, even across individuals within the same species, yet it remains unclear whether such variation leads to differences in brain function or behavior. By imaging cortical activity of a mouse model in which neuronal production is moderately enhanced in utero, we find that animals with more cortical neurons also develop enhanced functional correlations and more distinct neuronal ensembles in primary visual cortex. These mice also have sharper orientation discrimination in their visual behavior. These results unveil a correlation between neuronal ensembles and behavior and suggest that neuronal number is linked to functional modularity and perceptual discrimination of visual cortex. By experimentally linking differences in neuronal number and behavior, our findings could help explain how evolutionary and developmental variability of individual and species brain size may lead to perceptual and cognitive differences

A custom two-photon and second-harmonic microscope

The introduction of two-photon microscopy has revolutionized life sciences by enabling long-term imaging of living preparations in highly scattering tissue while minimizing photodamage. At the same time, commercial two-photon microscopes are expensive and this has prevented the widespread distribution of this technique to the biological community. As an alternative to commercial systems, we provide an update of our efforts designing custom-built two-photon instruments by modifying the Olympus Fluoview laser scanning confocal microscope. With the newer version of our instrument we modulate the intensity of the laser beam using a Pockel's cell in arbitrary spatiotemporal patterns, perform simultaneous optical imaging and optical stimulation experiments and also can combine them with second harmonic generation measurements

Imaging voltage in neurons

In the last decades, imaging membrane potential has become a fruitful approach to study neural circuits, especially in invertebrate preparations with large, resilient neurons. At the same time, particularly in mammalian preparations, voltage imaging methods suffer from poor signal to noise and secondary side effects, and they fall short of providing single-cell resolution when imaging of the activity of neuronal populations. As an introduction to these techniques, we briefly review different voltage imaging methods (including organic fluorophores, SHG chromophores, genetic indicators, hybrid, nanoparticles, and intrinsic approaches) and illustrate some of their applications to neuronal biophysics and mammalian circuit analysis. We discuss their mechanisms of voltage sensitivity, from reorientation, electrochromic, or electro-optical phenomena to interaction among chromophores or membrane scattering, and highlight their advantages and shortcomings, commenting on the outlook for development of novel voltage imaging methods

Las nuevas neurotecnologías y su impacto en la ciencia, medicina y sociedad

El presente discurso fue leído el 19 de diciembre de 2019 en el Paraninfo de la Universidad de Zaragoza, 150 años después de que Santiago Ramón y Cajal se incorporara a las aulas de esta universidad. LOS RECIENTES AVANCES en neurotecnología e inteli- gencia artificial están permitiendo un acceso mayor y más rápido a la información acumulada en el cerebro de animales y personas. El esfuerzo científico mundial, que ha provocado la creación de la Iniciativa Internacional del Cerebro, y el desarrollo de redes neu- ronales cada vez más potentes realizado por la industria tecnoló- gica están impulsando unas nuevas neurotecnologías que podrían marcar el comienzo de una revolución en la neurociencia que nos permitirá descifrar las bases científicas de nuestras mentes y facili- tará la comprensión y la obtención de novedosos tratamientos para las enfermedades mentales y neurológicas. Pero, al mismo tiempo, estas tecnologías, combinadas con la inteligencia artificial, podrían usarse para descifrar y manipular procesos mentales y para aumen- tar cognitivamente a las personas conectándolas a las interfaces cerebro-computadora, alterando lo que significa ser humano..

Two-photon imaging with diffractive optical elements

Two-photon imaging has become a useful tool for optical monitoring of neural circuits, but it requires high laser power and serial scanning of each pixel in a sample. This results in slow imaging rates, limiting the measurements of fast signals such as neuronal activity. To improve the speed and signal-to-noise ratio of two-photon imaging, we introduce a simple modification of a two-photon microscope, using a diffractive optical element (DOE) which splits the laser beam into several beamlets that can simultaneously scan the sample. We demonstrate the advantages of DOE scanning by enhancing the speed and sensitivity of two-photon calcium imaging of action potentials in neurons from neocortical brain slices. DOE scanning can easily improve the detection of time-varying signals in two-photon and other non-linear microscopies