Biology subjects

Santin, J. M.

Publications and source records attributed to Santin, J. M..

9 recordsLinked to original sources

bioRxiv2026

Activity-dependent homeostatic synaptic plasticity widens the temperature range of synaptic transmission

The framework of homeostatic plasticity posits that neurons regulate cellular properties through feedback homeostasis to maintain activity during changes in the environment. However, when disturbances occur in wild animals they are often caused by environmental variables that induce their own acclimation effects, making it difficult to discern if activity-sensitive feedback plays a role in ecological settings. We addressed this problem using a natural activity perturbation, where frogs hibernate in cold water, leaving brainstem motor circuits that generate breathing inactive for long periods. We show here that motor inactivity, amid complex environmental variables in the hibernation environment, represents a key signal for increasing AMPA-glutamate receptors (AMPARs) on motoneurons. Homeostatic upregulation of AMPARs do not regulate neural activity per se but instead correspond with enhanced evoked transmission selectively at cool temperatures. The results show how homeostatic synaptic plasticity may allow animals to restart motor behavior after chronic inactivity encountered in the natural environment. More broadly, these results introduce the concept of homeostatic plasticity as a mechanism to shape thermal tolerance ranges of neural performance in ecological settings.

neuroscience↗

bioRxiv2026

Brain-derived ketone bodies can replace glucose to power neural function

The vertebrate brain is exquisitely sensitive to disruptions in glucose metabolism, and failure of adequate glucose delivery causes neurological dysfunction. Here, we discovered an animal with the capacity to defy this rule: We show that neural activity in frogs, animals with seemingly typical glucose demands, can stop metabolizing glucose by, in part, shifting to ketone bodies made exclusively within the brain after emergence from hibernation. This involves ketone body synthesis and transport from astrocytes to neurons to power synaptic transmission, along with the upregulation of gene expression that controls fatty acid catabolism and ketone body transport. Brain-derived ketone bodies also prevent decrements in activity that occur during hypoxia. These results provide insight into how frogs restart brain circuits following months of underwater hibernation when facing severe hypoxia and hypoglycemia that otherwise strongly impair neural performance in most animals. More broadly, they reveal the vertebrate brain has the capacity to serve as its own fuel reserve during the cessation of glucose metabolism, switching seamlessly to locally sourced ketone bodies while maintaining neural activity. This reframes glucose metabolism in the vertebrate brain not as a hard-wired necessity, but as a plastic trait that can in some cases be entirely abandoned. SignificanceThe brain relies on a continuous supply of glucose from the blood to support the large energy demands of neural activity. When glucose delivery is disrupted, neural activity collapses within minutes. Here, we demonstrate that hibernation induces a large capacity for frogs, animals with seemingly normal glucose needs, to operate neural circuits without glucose metabolism, replacing it ketone bodies produced exclusively within the brain. These results reveal that a brain-derived, non-glucose fuel reserve can power neural function in the absence of glucose delivery. These findings reframe neural activitys reliance on continuous glucose metabolism as a plastic trait, rather than a hard-wired constraint.

physiology↗

bioRxiv2026

Frogs uncouple neural activity from oxygen consumption after hibernation

AimAerobic metabolism supplies [~]90% of the ATP for neural activity. In frogs, activity has large aerobic needs typical of an average vertebrate, but surprisingly, can shift to using only glycolysis upon emergence from hibernation. We hypothesized that hibernation triggers a global reduction in the aerobic cost of neural function. MethodsWe simultaneously measured activity of the brainstem respiratory network via motor nerves and tissue oxygen partial pressure (pO2) in vitro from control and hibernated bullfrogs (4 weeks cold submergence; 4{degrees}C). To identify which functions differentially consume O2, we sequentially blocked activity and various cellular processes requiring activity-independent ion regulation and used the resulting tissue pO2 change ({Delta}pO2) as an index of O2 consumed. We further assessed how activity varies as a function of tissue pO2 and how O2 consumption varies across network activity levels. ResultsDespite similar network activity levels, we provide three lines of evidence that hibernation reduces its aerobic requirement. First, hibernators consume less oxygen for baseline activity. Second, network output remains stable from baseline to anoxia, while moderate hypoxia disrupts controls. Finally, accelerating activity does not enhance oxygen consumption as in controls, but aerobic metabolism ultimately increases during seizure-like activity. ConclusionHibernating frogs reduce aerobic needs for sustaining physiological levels of neural activity, revealing how they overcome the challenge of restarting motor circuits on the background of hypoxia during emergence from hibernation. More broadly, vertebrate neural circuits seemingly constrained by aerobic metabolism can exhibit substantial plasticity in the aerobic requirements for function. Practitioner pointsO_LIHibernation in bullfrogs reduces the oxygen consumed by neural activity while maintaining normal network output, demonstrating that the aerobic requirements of brain function are not fixed in the vertebrate brain. C_LIO_LIAfter hibernation, brainstem motor circuits maintain stable function from high levels of O2 to anoxia and do not increase O2 consumption when activity is elevated within the physiological range. C_LIO_LIThese findings reveal that vertebrate neural circuits can enter metabolic states requiring far less aerobic respiration, which may inform strategies for improving metabolic resilience in the brain during conditions of impaired oxygen delivery and other metabolic dysfunction. C_LI

physiology↗

bioRxiv2025

Hibernation improves neural performance during energy stress in regions across the central nervous system in the American bullfrog

Neuronal signaling requires high rates of ATP production via the oxidative metabolism of glucose. The American bullfrog is intriguing, as this species has typical brain energy requirements for an average vertebrate but modifies synaptic physiology and metabolism after hibernation to maintain function during hypoxia and ischemia. Given the importance of the respiratory system in restoring metabolic homeostasis during emergence from underwater hibernation, work to date has addressed this response in the brainstem respiratory network. Thus, metabolic plasticity has been interpreted as an adaptation used to restart respiratory motor behavior under hypoxic conditions during the transition from skin breathing to air breathing. It remains unclear whether these improvements are specific to the brainstem regions critical for breathing versus a global response within the central nervous system (CNS). To address this question, we recorded neural activity from the spinal cord, forebrain, and brainstem respiratory network in vitro. As expected, hypoxia disrupted the function of each network in control animals. After hibernation, each network improved its activity in hypoxia compared to controls. These results suggest that plasticity that improves neural function during energy stress following hibernation reflects a global response that may impact many behaviors controlled by the CNS and is not limited to regions involved in metabolic homeostasis.

neuroscience↗

bioRxiv2025

Homeostatic regulation of a motor circuit through temperature sensing rather than activity sensing

Homeostasis is a driving principle in physiology. To achieve homeostatic control of neural activity, neurons monitor their activity levels and then initiate corrective adjustments in excitability when activity strays from a set point. However, fluctuations in the brain microenvironment, such as temperature, pH, and other ions represent some of the most common perturbations to neural function in animals. Therefore, it is unclear if activity sensing is a universal strategy for different types of perturbations or if stability may arise by sensing specific environmental cues. Here we show the respiratory network of amphibians mounts a fast homeostatic response to restore motor function following inactivity caused by cooling over the physiological range. This response was not initiated by inactivity, but rather, by temperature. Compensation involved cold-activation of the noradrenergic system via mechanisms that involve inhibition of the Na+/K+ ATPase, causing {beta}-adrenoceptor signaling that enhanced network excitability. Thus, acute cooling initiates a modulatory response that opposes inactivity and enhances network excitability. As the nervous system of all animals is subjected to changes in the microenvironment, some circuits may have selected regulatory systems tuned to environmental variables in place of, or in addition to, activity-dependent control mechanisms. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=92 SRC="FIGDIR/small/632419v1_ufig1.gif" ALT="Figure 1"> View larger version (25K): org.highwire.dtl.DTLVardef@15d8cf5org.highwire.dtl.DTLVardef@d16920org.highwire.dtl.DTLVardef@4cb865org.highwire.dtl.DTLVardef@de266f_HPS_FORMAT_FIGEXP M_FIG C_FIG

neuroscience↗

bioRxiv2023

Compensatory changes in GABAergic inhibition are differentially expressed in the respiratory network to promote function following hibernation

BackgroundNeural circuits produce reliable activity patterns despite disturbances in the environment. For this to occur, neurons elicit synaptic plasticity during perturbations. However, recent work suggests that plasticity not only regulates circuit activity during disturbances, but these modifications may also linger to stabilize circuits during future perturbations. The implementation of such a regulation scheme for real-life environmental challenges of animals remains unclear. Amphibians provide insight into this problem in a rather extreme way, as circuits that generate breathing are inactive for several months during underwater hibernation and use compensatory plasticity to promote ventilation upon emergence. ResultsUsing ex vivo brainstem preparations and electrophysiology, we find that hibernation in American bullfrogs reduces GABAA receptor (GABAAR) inhibition in respiratory rhythm generating circuits and motor neurons, consistent with a compensatory response to chronic inactivity. Although GABAARs are normally critical for breathing, baseline network output at warm temperatures was not affected. However, when assessed across a range of temperatures, hibernators with reduced GABAAR signaling had greater activity at cooler temperatures, enhancing respiratory motor output under conditions that otherwise strongly depress breathing. ConclusionsHibernation reduces GABAAR signaling to promote robust respiratory output only at cooler temperatures. Although animals do not ventilate lungs during hibernation, we suggest this would be beneficial for stabilizing breathing when the animal passes through a large temperature range during emergence in the spring. More broadly, these results demonstrate that compensatory synaptic plasticity can increase the operating range of circuits in harsh environments, thereby promoting adaptive behavior in conditions that suppress activity.

physiology↗

bioRxiv2023

Plasticity in the functional properties of NMDA receptors improves motor circuit performance during severe energy stress

Brain energy stress leads to neuronal hyperexcitability followed by a rapid loss of function and cell death. In contrast, the frog brainstem switches into a state of extreme metabolic resilience that allows them to maintain motor function during hypoxia as they emerge from hibernation. NMDA receptors (NMDARs) are Ca2+-permeable glutamate receptors that contribute to the loss of homeostasis during hypoxia. Therefore, we hypothesized that hibernation leads to plasticity that reduces the role of NMDARs within neural networks to improve function during energy stress. To test this, we assessed a circuit with a large involvement of NMDAR synapses, the brainstem respiratory network of female bullfrogs, Lithobates catesbeianus. Contrary to our expectations, hibernation did not alter the role of NMDARs in generating network output, nor did it affect the amplitude, kinetics, and hypoxia sensitivity of NMDAR currents. Instead, hibernation strongly reduced NMDAR Ca2+ permeability and enhanced desensitization during repetitive stimulation. Under severe hypoxia, the normal NMDAR profile caused network hyperexcitability within minutes, which was mitigated by blocking NMDARs. After hibernation, the modified complement of NMDARs protected against hyperexcitability, as disordered output did not occur for at least one hour in hypoxia. These findings uncover state-dependence in the plasticity of NMDARs, whereby multiple changes to receptor function improve neural performance during energy stress without interfering with its normal role during healthy activity. Significance StatementNeural circuits lose homeostasis during severe energy stress, and NMDA-glutamate receptors play a major role in this response. In contrast, frogs have the remarkable capacity to use plasticity that improves circuit function from minutes to hours during hypoxia, likely as an adaptation to survive emergence from hibernation. We found this occurs, in part, through modification of NMDA receptors that renders them less permeable to Ca2+ and more likely to desensitize during high activity states. These NMDA receptor modifications do not influence normal network function but protect against hyperexcitability caused by hypoxia. This work points to endogenous plasticity mechanisms that improve network function during energy stress without altering circuit function when the brain is well-oxygenated.

neuroscience↗

bioRxiv2022

A requirement for correctly scaled synapses in the regulation of motor circuit output

Neural networks tune synaptic and cellular properties to produce stable activity. One form of homeostatic regulation involves scaling the strength of synapses up or down in a global and multiplicative manner to oppose activity disturbances. In American bullfrogs, excitatory synapses scale up to regulate breathing motor function after inactivity in hibernation, connecting homeostatic compensation to motor behavior. In traditional models of homeostatic synaptic plasticity, inactivity is thought to increase synaptic strength via mechanisms that involve reduced Ca2+ influx through voltagegated channels. Therefore, we tested whether pharmacological inactivity and inhibition of voltage-gated Ca2+ channels are sufficient to drive synaptic compensation in this system. For this, we chronically exposed ex vivo brainstem preparations containing the intact respiratory network to tetrodotoxin (TTX) to stop activity and nimodipine to block L-type Ca2+ channels. We show that hibernation and TTX similarly increased motoneuron synaptic strength and that hibernation occluded the response to TTX. In contrast, block of L-type Ca2+ channels did not alter mean synaptic strength but disrupted the apparent multiplicative scaling of synaptic compensation typically observed in response to hibernation. Thus, inactivity drives up synaptic strength through mechanisms that do not rely on reduced L-type channel function, while Ca2+ signaling associated with the hibernation environment independently regulates the balance of synaptic weights. Altogether, these results point to multiple feedback signals for shaping synaptic compensation that gives rise to proper network function during environmental challenges in vivo.

neuroscience↗

bioRxiv2019

Molecular Profiling to Infer Neuronal Cell Identity: Lessons from small ganglia of the Crab Cancer borealis

Understanding circuit organization depends on identification of cell types. Recent advances in transcriptional profiling methods have enabled classification of cell types by their gene expression. While exceptionally powerful and high throughput, the ground-truth validation of these methods is difficult: if cell type is unknown, how does one assess whether a given analysis accurately captures neuronal identity? To shed light on the capabilities and limitations of solely using transcriptional profiling for cell type classification, we performed two forms of transcriptional profiling - RNA-seq and quantitative RT-PCR, in single, unambiguously identified neurons from two small crustacean networks: the stomatogastric and cardiac ganglia. We then combined our knowledge of cell type with unbiased clustering analyses and supervised machine learning to determine how accurately functionally-defined neuron types can be classified by expression profile alone. Our results demonstrate that expression profile is able to capture neuronal identity most accurately when combined with multimodal information that allows for post-hoc grouping so analysis can proceed from a supervised perspective. Solely unsupervised clustering can lead to misidentification and an inability to distinguish between two or more cell types. Therefore, our study supports the general utility of cell identification by transcriptional profiling, but adds a caution: it is difficult or impossible to know under what conditions transcriptional profiling alone is capable of assigning cell identity. Only by combining multiple modalities of information such as physiology, morphology or innervation target can neuronal identity be unambiguously determined.\n\nSIGNIFICANCE STATEMENTSingle cell transcriptional profiling has become a widespread tool in cell identification, particularly in the nervous system, based on the notion that genomic information determines cell identity. However, many cell type classification studies are unconstrained by other cellular attributes (e.g., morphology, physiology). Here, we systematically test how accurately transcriptional profiling can assign cell identity to well-studied anatomically- and functionally-identified neurons in two small neuronal networks. While these neurons clearly possess distinct patterns of gene expression across cell types, their expression profiles are not sufficient to unambiguously confirm their identity. We suggest that true cell identity can only be determined by combining gene expression data with other cellular attributes such as innervation pattern, morphology, or physiology.

neuroscience↗