Periods of natural torpor or hibernation are characterized by markedly reduced metabolic and neural activity, yet animals typically display an intense NREM sleep bout with elevated Delta EEG power immediately upon arousal. This rebound has long been considered an indication that sleep pressure continues to accumulate even during hypometabolic states. However, while some studies support a homeostatic origin of the increase Delta power, others argue that it may simply reflect a non-specific reactivation of cortical networks upon rewarming. A comparable, pronounced Delta increase was also reported in rats, a non-hibernating species, following a pharmacologically induced synthetic torpor. In rats, sleep deprivation following synthetic torpor has also been shown to promote Tau dephosphorylation, suggesting that the physiological events following the return to normothermia may interact with sleep–wake processes. These observations prompted us to assess whether the post–synthetic torpor Delta power increase in this model represents a homeostatic response attributable to sleep loss during the hypometabolic bout. Twelve adult male Sprague–Dawley rats were implanted under general anaesthesia with electrodes for EEG, nuchal and diaphragm EMG recording, a thermistor to measure deep brain temperature, a catheter for arterial pressure recording and heart rate determination; and a microcannula targeting the Raphe Pallidus (RPa). Synthetic torpor was induced by repeated microinjections (100 nl) of the GABA-A receptor agonist muscimol (1 mM) into the RPa, at an ambient temperature of 15 ± 1°C. After six hours of hypothermia, animals were either allowed to recover spontaneously—a condition during which they typically engage in prolonged sleep (n = 6) or kept awake by gentle handling for 6 h (n = 6). Rats subjected to sleep deprivation displayed a robust elevation in NREM sleep Delta power at the beginning of the recovery period (281±13%), which was significantly larger (p < 0.05) than that observed in non-sleep deprived animals (237±19%). The stronger Delta rebound after sleep deprivation supports the hypothesis that the NREM sleep bout which follows synthetic torpor is due to the buildup of a sleep debt during the hypometabolic state. This work was funded by the Italian Ministry of University and Research, and the University of Bologna. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

Preclinical data from the peculiar animal model of "synthetic torpor" (ST), a reversible hypothermic condition resembling natural torpor but pharmacologically induced in rats (a nonhibernating mammal), suggest that in hypothermic conditions, the neuroprotective effects of melatonin are strongly enhanced. As a protection technique, during aortic-arch surgery, patients are induced a hypothermia similar to ST. Since in ST, systemic melatonin was found particularly high, our aim was to assay serum melatonin in patients (N = 8) undergoing aortic surgery, either during hypothermia or in the following four recovery days. Serum markers of blood-brain barrier (BBB) integrity (the astrocytic protein S100B) and neuronal damage (neuronal-specific enolase [NSE]) were also measured. Results show that, in contrast to what observed in ST, in hypothermic patients, melatonin was dramatically reduced with respect to the preanesthesia level, slowly recovering during the postsurgery period. Also, S100B and NSE were raised during surgery, indicating a compromised BBB integrity and some ongoing neuronal damage, though both markers returned closer to preanesthesia levels within the recovery period. Together, the present results show that, in aortic-arch surgery, hypothermic patients totally lack the systemic melatonin peak that was observed in ST, the BBB was temporarily compromised, and some acute neuronal damage occurred. A main implication of this work is that, exploiting the low BBB efficiency that makes easier to reach brain parenchyma, by administering melatonin during the hypothermic stage of the surgery, the observed gap could be filled, possibly triggering the neuroprotective mechanism seen in preclinical observations and leading to better neurological outcomes for this surgical procedure.

Healthcare-associated infections (HAIs) pose significant risks, leading to increased morbidity, mortality, and costs, exacerbated by multi-drug-resistant microorganisms. This study aimed to evaluate pharmacological prophylaxis targeting sympathetic reflex control of immunity to mitigate systemic infections, offering a novel approach to combating HAIs. The study included animal experiments and a retrospective analysis of orthopedic surgery patients in Romagna, Italy. Young female pigs were intravenously inoculated with Escherichia coli (E. coli) and divided into two groups: propranolol-treated (non-selective β-blocker; 3 mg/kg; 3x/day orally) and vehicle-treated, starting two days before infection. Parameters such as bacteraemia, serum cytokines, biochemical profile, blood count, lactate, glycemia, and flow cytometry were assessed. Additionally, a retrospective analysis of 92,649 orthopedic surgery hospitalizations (2017-2022) examined the association of non-selective and selective β1-blockers with HAI development using conditional logistic regression. Propranolol-treated pigs exhibited a disinhibited immune response to systemic infection, clearing circulating bacteria much earlier than vehicle-treated animals. The retrospective analysis showed that patients on non-selective beta-blockers had a 71.7% reduced risk of developing HAIs, while those on selective β1-blockers had an 18% higher risk. These findings suggest that targeting sympathetic reflex control of immunity via pharmacological prophylaxis may reduce HAIs in surgical patients.

Torpor is a state used by several mammals to survive harsh winters and avoid predation, characterized by a drastic reduction in metabolic rate followed by a decrease in body temperature, heart rate, and many physiological variables. During torpor, all organs and systems must adapt to the new low-energy expenditure conditions to preserve physiological homeostasis. These adaptations may be exploited in a translational perspective in several fields. Recently, many features of torpor were shown to be mimicked in non-hibernators by the inhibition of neurons within the brainstem region of the Raphe Pallidus. The physiological resemblance of this artificial state, called synthetic torpor, with natural torpor has so far been described only in physiological terms, but no data have been shown regarding the induced morphological changes. Here, we show the first description of the ultrastructural changes in the liver, kidney, lung, skeletal muscle, and testis induced by a 6-hours inhibition of Raphe Pallidus neurons in a non-hibernating species, the rat.

Healthcare-associated infections (HAIs) pose significant risks, leading to increased morbidity, mortality, and costs, exacerbated by multi-drug-resistant microorganisms. This study aimed to evaluate pharmacological prophylaxis targeting sympathetic reflex control of immunity to mitigate systemic infections, offering a novel approach to combating HAIs. The study included animal experiments and a retrospective analysis of orthopedic surgery patients in Romagna, Italy. Young female pigs were intravenously inoculated with Escherichia coli (E. coli) and divided into two groups: propranolol-treated (non-selective β-blocker; 3mg/kg; 3x/day orally) and vehicle-treated, starting two days before infection. Parameters such as bacteraemia, serum cytokines, biochemical profile, blood count, lactate, glycemia, and flow cytometry were assessed. Additionally, a retrospective analysis of 92,649 orthopedic surgery hospitalizations (2017–2022) examined the effects of non-selective and selective β1-blockers on HAIs development using conditional logistic regression. Propranolol-treated pigs exhibited a disinhibited immune response to systemic infection, clearing circulating bacteria much earlier than vehicle-treated animals. The retrospective analysis showed that patients on non-selective beta-blockers had a 71.7% reduced risk of developing HAIs, while those on selective β1-blockers had an 18% higher risk. These findings suggest that targeting sympathetic reflex control of immunity via pharmacological prophylaxis may reduce HAIs in surgical patients.

The Raphe Pallidus (RPa) is a brainstem nucleus containing sympathetic premotor neurons that control thermogenesis and modulate cardiovascular function. It receives inputs from various hypothalamic areas, including the Lateral Hypothalamus (LH), a heterogeneous region intricately involved in several autonomic and behavioral functions. A key subpopulation of neurons in the LH expresses orexin/hypocretin, a neuropeptide which is crucially involved in the regulation of the wake–sleep states and feeding behavior. The RPa receives orexinergic projections from the LH and orexinergic signalling in the RPa has been shown to enhance thermogenesis in the anaesthetized rat, but only in the presence of an already existing thermogenic drive, without significantly affecting cardiovascular function. The present work was aimed at exploring the effects on thermoregulation and autonomic function and the possible role in the modulation of the wake–sleep states and feeding behavior of orexin injection in the RPa in the free-behaving rat. In order to assess the influence of an already present thermogenic drive on orexinergic signalling in the RPa, animals were studied at three different ambient temperatures (Ta, 10°C, 24°C, and 32°C). We found that orexin injection into the RPa variably affected the wake–sleep states, autonomic functions, motor activity, and feeding behavior, at the different Tas. In particular, in the first post-injection hour, we observed an increase in wakefulness, which was large at Ta 24°C and Ta 10°C and rather mild at Ta 32°C. Deep brain temperature was increased by orexin injection at Ta 10°C, but not at either Ta 24°C or Ta 32°C. Moreover, an increase in mean arterial blood pressure occurred at Ta 24°C, which was probably masked by the high baseline levels at Ta 10°C and was completely absent at Ta 32°C. Finally, an enhancement in feeding behavior was observed at Ta 24°C and 10°C only. In accordance with what observed in anaesthetized rats, orexinergic signalling in the RPa seems to be ineffective in the absence of any thermogenic drive. Moreover, the effects observed on the wake–sleep states and feeding behavior introduce the RPa as a novel player in the central neural network promoting wakefulness and feeding.

Rats are known to use a 22-kHz ultrasonic vocalisation as a distress call to warn of danger to other members of their group. We monitored 22-kHz ultrasonic vocalisation emissions in rats (lean and obese) as part of a sleep deprivation study to detect the eventual presence of stress during the procedure. Unexpectedly, we detected ultrasonic vocalisation emission during rapid eye movement (REM) sleep, but not during non-REM (NREM) sleep, in all the rats. The event occurs during the expiratory phase and can take place singularly or as a train. No difference was detected in the number or duration of these events in lean versus obese rats, during the light versus the dark period, and after sleep deprivation. As far as we know, this is the first report showing that rats can vocalise during REM sleep.

Fluorescent Neuronal Cells v2 is a collection of fluorescence microscopy images and the corresponding ground-truth annotations, designed to foster innovative research in the domains of Life Sciences and Deep Learning. This dataset encompasses three image collections wherein rodent neuronal cell nuclei and cytoplasm are stained with diverse markers to highlight their anatomical or functional characteristics. Specifically, we release 1874 high-resolution images alongside 750 corresponding ground-truth annotations for several learning tasks, including semantic segmentation, object detection and counting. The contribution is two-fold. First, thanks to the variety of annotations and their accessible formats, we anticipate our work will facilitate methodological advancements in computer vision approaches for segmentation, detection, feature extraction, unsupervised and self-supervised learning, transfer learning, and related areas. Second, by enabling extensive exploration and benchmarking, we hope Fluorescent Neuronal Cells v2 will catalyze breakthroughs in fluorescence microscopy analysis and promote cutting-edge discoveries in life sciences.

Data sharing not applicable - no new data generated

Neuronal Tau protein hyperphosphorylation (PPtau) is a hallmark of tauopathic neurodegeneration. However, a reversible brain PPtau occurs in mammals during either natural or “synthetic” torpor (ST), a transient deep hypothermic state that can be pharmacologically induced in rats. Since in both conditions a high sleep pressure builds up during the regaining of euthermia, the aim of this work was to assess the possible role of post-ST sleep in PPtau dephosphorylation. Male rats were studied at the hypothermic nadir of ST, and 3–6 h after the recovery of euthermia, after either normal sleep (NS) or total sleep deprivation (SD). The effects of SD were studied by assessing: (i) deep brain temperature (Tb); (ii) immunofluorescent staining for AT8 (phosphorylated Tau) and Tau-1 (non-phosphorylated Tau), assessed in 19 brain structures; (iii) different phosphorylated forms of Tau and the main cellular factors involved in Tau phospho-regulation, including pro- and anti-apoptotic markers, assessed through western blot in the parietal cortex and hippocampus; (iv) systemic factors which are involved in natural torpor; (v) microglia activation state, by considering morphometric variations. Unexpectedly, the reversibility of PPtau was more efficient in SD than in NS animals, and was concomitant with a higher Tb, higher melatonin plasma levels, and a higher frequency of the microglia resting phenotype. Since the reversibility of ST-induced PPtau was previously shown to be driven by a latent physiological molecular mechanism triggered by deep hypothermia, short-term SD soon after the regaining of euthermia seems to boost the possible neuroprotective effects of this mechanism.

The observation of neurogenic fever resulting from subarachnoid hemorrhage (SAH) in animal models is a useful tool for the interpretation of its pathophysiology in humans, which is still a major challenge in the management of neurocritical patients. This systematic review aims to identify the prognostic factors and pathophysiological elements that determine the onset of neurogenic fever and its severity in animal models. In addition, our study aims to analyze which pharmacological treatments are most effective. All the articles available in Pubmed, Embase, and the Biological Science Collection until August 2021 concerning in vivo experimental studies on SAH animal models, including full texts and abstracts written in English and Italian, were considered. The risk of bias was assessed with SYRCLE's Risk of Bias tool. In total, 81 records were retrieved; after excluding duplicates, 76 records were potentially relevant. A total of 64 articles was excluded after title and abstract screening. The remaining 12 studies were evaluated as full texts, and 6 other studies were excluded (SAH-induced animal studies without a body temperature assessment). In one study, body temperature was measured after SAH induction, but the authors did not report temperature recording. Therefore, only five studies met the search criteria. The high methodological heterogeneity (different animal species, different temperature measurement methods, and different methods of the induction of bleeding) prevented meta-analysis. Synthesis methodology without meta-analysis (SWiM) was used for data analysis. The total number of animals used as controls was 87 (23 rabbits, 32 mice, and 32 rats), while there were 130 animals used as interventions (54 rabbits, 44 mice, and 32 rats). The presence of blood in the subarachnoid space, particularly red blood cells, is responsible for neurogenic fever; the role of hemoglobin is unclear. The mechanism is apparently not mediated by prostaglandins. The autonomic nervous system innervating brown adipose tissue is undoubtedly implicated in the onset of neurogenic fever. The activation of the central adenosine-1 receptor is effective in controlling the temperature of animals with neurogenic fever (by inhibiting thermogenesis of brown adipose tissue).

Introduction: Hyperphosphorylated Tau protein (PPTau) is the hallmark of tauopathic neurodegeneration. During "synthetic torpor" (ST), a transient hypothermic state which can be induced in rats by the local pharmacological inhibition of the Raphe Pallidus, a reversible brain Tau hyperphosphorylation occurs. The aim of the present study was to elucidate the - as yet unknown - molecular mechanisms underlying this process, at both a cellular and systemic level. Methods: Different phosphorylated forms of Tau and the main cellular factors involved in Tau phospho-regulation were assessed by western blot in the parietal cortex and hippocampus of rats induced in ST, at either the hypothermic nadir or after the recovery of euthermia. Pro- and anti-apoptotic markers, as well as different systemic factors which are involved in natural torpor, were also assessed. Finally, the degree of microglia activation was determined through morphometry. Results: Overall, the results show that ST triggers a regulated biochemical process which can dam PPTau formation and favor its reversibility starting, unexpectedly for a non-hibernator, from the hypothermic nadir. In particular, at the nadir, the glycogen synthase kinase-β was largely inhibited in both regions, the melatonin plasma levels were significantly increased and the antiapoptotic factor Akt was significantly activated in the hippocampus early after, while a transient neuroinflammation was observed during the recovery period. Discussion: Together, the present data suggest that ST can trigger a previously undescribed latent and regulated physiological process, that is able to cope with brain PPTau formation.

The ability to induce a hypothermia resembling that of natural torpor would be greatly beneficial in medical and non-medical fields. At present, two procedures based on central nervous pharmacological manipulation have been shown to be effective in bringing core body temperature well below 30 °C in the rat, a non-hibernator: the first, based on the inhibition of a key relay in the central thermoregulatory pathway, the other, based on the activation of central adenosine A1 receptors. Although the role of mitochondria in the activation and maintenance of torpor has been extensively studied, no data are available for centrally induced hypothermia in non-hibernators. Thus, in the present work the respiration rate of mitochondria in the liver and in the kidney of rats following the aforementioned hypothermia-inducing treatments was studied. Moreover, to have an internal control, the same parameters were assessed in a well-consolidated model, i.e., mice during fasting-induced torpor. Our results show that state 3 respiration rate, which significantly decreased in the liver of mice, was unchanged in rats. An increase of state 4 respiration rate was observed in both species, although it was not statistically significant in rats under central adenosine stimulation. Also, a significant decrease of the respiratory control ratio was detected in both species. Finally, no effects were detected in kidney mitochondria in both species. Overall, in these hypothermic conditions liver mitochondria of rats remained active and apparently ready to be re-activated to produce energy and warm up the cells. These findings can be interpreted as encouraging in view of the finalization of a translational approach to humans.

Tau is a key protein in neurons, where it affects the dynamics of the microtubule system. The hyperphosphorylation of Tau (PP-Tau) commonly leads to the formation of neurofibrillary tangles, as it occurs in tauopathies, a group of neurodegenerative diseases, including Alzheimer's. Hypothermia-related accumulation of PP-Tau has been described in hibernators and during synthetic torpor (ST), a torpor-like condition that has been induced in rats, a non-hibernating species. Remarkably, in ST PP-Tau is reversible and Tau de-phosphorylates within a few hours following the torpor bout, apparently not evolving into pathology. These observations have been limited to the brain, but in animal models of tauopathies, PP-Tau accumulation also appears to occur in the spinal cord (SpCo). The aim of the present work was to assess whether ST leads to PP-Tau accumulation in the SpCo and whether this process is reversible. Immunofluorescence (IF) for AT8 (to assess PP-Tau) and Tau-1 (non-phosphorylated Tau) was carried out on SpCo coronal sections. AT8-IF was clearly expressed in the dorsal horns (DH) during ST, while in the ventral horns (VH) no staining was observed. The AT8-IF completely disappeared after 6 h from the return to euthermia. Tau-1-IF disappeared in both DH and VH during ST, returning to normal levels during recovery. To shed light on the cellular process underlying the PP-Tau pattern observed, the inhibited form of the glycogen-synthase kinase 3β (the main kinase acting on Tau) was assessed using IF: VH (i.e., in motor neurons) were highly stained mainly during ST, while in DH there was no staining. Since tauopathies are also related to neuroinflammation, microglia activation was also assessed through morphometric analyses, but no ST-induced microglia activation was found in the SpCo. Taken together, the present results show that, in the DH of SpCo, ST induces a reversible accumulation of PP-Tau. Since during ST there is no motor activity, the lack of AT8-IF in VH may result from an activity-related process at a cellular level. Thus, ST demonstrates a newly-described physiological mechanism that is able to resolve the accumulation of PP-Tau and apparently avoid the neurodegenerative outcome.

Each of the wake-sleep states is characterized by specific changes in autonomic activity and bodily functions. The goal of such changes is not always clear. During non-rapid eye movement (NREM) sleep, the autonomic outflow and the activity of the endocrine system, the respiratory system, the cardiovascular system, and the thermoregulatory system seem to be directed at increasing energy saving. During rapid eye movement (REM) sleep, the goal of the specific autonomic and regulatory changes is unclear, since a large instability of autonomic activity and cardiorespiratory function is observed in concomitance with thermoregulatory changes, which are apparently non-functional to thermal homeostasis. Reciprocally, the activation of thermoregulatory responses under thermal challenges interferes with sleep occurrence. Such a double-edged and reciprocal interaction between sleep and thermoregulation may be favored by the fact that the central network controlling sleep overlaps in several parts with the central network controlling thermoregulation. The understanding of the central mechanism behind the interaction between sleep and thermoregulation may help to understand the functionality of thermoregulatory sleep-related changes and, ultimately, the function(s) of sleep. © 2021 American Physiological Society. Compr Physiol 11:1591-1604, 2021.

Tau protein is of primary importance for neuronal homeostasis and when hyperphosphorylated (PP-Tau), it tends to aggregate in neurofibrillary tangles, as is the case with tauopathies, a class of neurodegenerative disorders. Reversible PP-Tau accumulation occurs in the brain of hibernating rodents and it was recently observed in rats (a non-hibernator) during synthetic torpor (ST), a pharmacological-induced torpor-like condition. To date, the expression of PP-Tau in the rat enteric nervous system (ENS) is still unknown. The present study immunohistochemically investigates the PP-Tau expression in the myenteric plexus of the ileum and colon of normothermic rats (CTRL) and during ST, focusing on the two major subclasses of enteric neurons, i.e., cholinergic and nitrergic. Results showed that both groups of rats expressed PP-Tau, with a significantly increased percentage of PP-Tau immunoreactive (IR) neurons in ST vs. CTRL. In all rats, the majority of PP-Tau-IR neurons were cholinergic. In ST rats, the percentage of PP-Tau-IR neurons expressing a nitrergic phenotype increased, although with no significant differences between groups. In addition, the ileum of ST rats showed a significant decrease in the percentage of nitrergic neurons. In conclusion, our findings suggest an adaptive response of ENS to very low core body temperatures, with changes involving PP-tau expression in enteric neurons, especially the ileal nitrergic subpopulation. In addition, the high presence of PP-Tau in cholinergic neurons, specifically, is very interesting and deserves further investigation. Altogether, these data strengthen the hypothesis of a common cellular mechanism triggered by ST, natural hibernation and tauopathies occurring in ENS neurons.

Physiological regulation can be defined as the integrated neural control mechanism underlying somatic, autonomic, and neuroendocrine activity. Such an integration occurs mostly at hypothalamic level. Physiological regulation is intrinsically heterogeneous between non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. During NREM sleep, it is aimed at the maintenance of body homeostasis at a low level of energy expenditure. During REM sleep, the cardiorespiratory instability and the thermoregulatory impairment are indicative of a derangement of the integrative neural regulation of physiological functions. However, the finding that osmoregulation is maintained during REMS suggests that this derangement does not affect the whole hypothalamus.