The parabrachial nucleus, a compact structure tucked into the dorsolateral pons, has long been recognized as a major relay station for alarm signals traveling from the body and spinal cord up to forebrain circuits. CGRP-expressing neurons in this region project densely to the central amygdala, the bed nucleus of the stria terminalis and other threat-processing hubs, and earlier work established that artificially activating these neurons produces powerful aversive states and widespread fear-like responses. What remained less clear was whether CGRP signaling in the parabrachial nucleus influences not just the intensity of fear, but the specific behavioral form that fear takes, an issue the new study set out to address directly.

To do so, the researchers employed a platform-based active avoidance paradigm, an experimental setup in which a rodent can terminate or avoid an aversive stimulus by moving onto a raised platform. Unlike classical Pavlovian fear-conditioning assays, which measure freezing as the primary output, active avoidance tasks capture the animal’s capacity to translate threat detection into goal-directed escape behavior. This distinction is crucial, because freezing and active avoidance are not simply different intensities of the same response; they represent distinct defensive strategies governed by partially separable neural circuits, and animals typically select between them based on factors such as the distance to the threat, the availability of escape routes and previous experience with the environment.

The central manipulation involved pharmacological or chemogenetic inhibition of CGRP signaling within the parabrachial nucleus. When CGRP activity was suppressed, the researchers observed a striking reorganization of defensive behavior: animals spent less time in the passive, immobile freezing posture that normally dominates their response to learned threat cues, and instead escalated their engagement with the active avoidance strategy, rapidly locating and mounting the escape platform. In other words, dampening CGRP did not blunt fear altogether, as one might naively predict if the peptide were simply a universal fear amplifier. Rather, it redirected the defensive response, channeling the animal’s motivational energy from one stereotyped strategy into another.

This dissociation carries substantial theoretical weight. Classical models of fear conditioning have often treated freezing as the canonical index of conditioned fear, so much so that a pharmacological intervention reducing freezing would conventionally be interpreted as anxiolytic or fear-reducing. The new results challenge that inference. Animals with inhibited parabrachial CGRP signaling were not less afraid; they were differently afraid, mobilizing an active coping strategy in place of an immobile one. The study therefore reinforces an increasingly influential view in behavioral neuroscience: that fear must be measured across a repertoire of adaptive responses, and that single-measure assays can systematically mischaracterize the effects of neural or pharmacological manipulations.

From a mechanistic standpoint, the findings suggest that CGRP neurons in the parabrachial nucleus act as a biasing signal within a distributed defensive decision network. These neurons are well positioned for such a role. They integrate interoceptive alarm signals, including those carried by the spinal trigeminal and lamina I spinothalamic pathways, and broadcast the resulting arousal state to forebrain structures that execute specific defensive programs. Projections to the central amygdala have been implicated in promoting passive defensive reactions, whereas circuits running through the ventrolateral periaqueductal gray and basal ganglia loops are more closely associated with active escape and avoidance. By altering the gain of CGRP signaling at the source, the manipulation appears to rebalance competition between these downstream effectors, tilting the system toward the active option.

The platform-based paradigm proved especially informative because it allowed within-subject quantification of both response types. Rather than relying on separate cohorts of freezing-conditioned and shuttle-box-trained animals, the researchers could observe individual animals distributing their behavior between immobility and platform escape across trials. This design revealed that the shift from freezing to avoidance was not a threshold artifact or a byproduct of altered shock sensitivity; instead, it reflected a genuine reweighting of strategy selection, consistent with a decision-level function for parabrachial CGRP rather than a purely sensory or motor role.

The translational implications are significant, particularly for post-traumatic stress disorder and other trauma- and anxiety-related conditions in which patients exhibit maladaptive defensive postures. Human PTSD is often characterized not only by exaggerated fear responses but by inflexibility in choosing among coping strategies, including a failure to engage active avoidance or escape behaviors that might reduce harm. If CGRP signaling in the parabrachial-amygdala pathway similarly constrains active coping in humans, drugs that modulate CGRP receptors, a class already developed and clinically validated for migraine prophylaxis through gepant compounds, could conceivably be repurposed or re-evaluated for their effects on defensive strategy selection. The new study does not establish such clinical effects, but it provides a rigorous animal-model foundation for asking the question.

More broadly, the work exemplifies a shift in how learning and memory research frames threat-related behavior. Instead of treating fear as a unitary internal state read out through a single behavioral channel, contemporary neuroscience increasingly emphasizes a menu of genetically and anatomically specified defensive circuits whose relative activation determines the observable strategy. CGRP neurons in the parabrachial nucleus now appear to be one of the clearest molecularly identifiable control points for that selection process. By demonstrating that inhibition of this specific neuropeptide population reconfigures, rather than diminishes, defensive behavior, the study offers both a conceptual lesson, that fear research must account for strategy switching, and a practical one, that assays capturing active coping are indispensable for interpreting interventions aimed at threat-related circuitry. As attention turns to how upstream inputs and downstream targets partition the freezing-versus-avoidance decision, the parabrachial CGRP system is likely to remain a focal point for understanding how brains choose how to be afraid.

Subject of Research: Role of CGRP neurons in the parabrachial nucleus in selecting between freezing and active avoidance defensive strategies

Article Title: CGRP inhibition in the PBN shifts defensive strategy from freezing to active avoidance in a platform-based active avoidance paradigm

Article References: CGRP inhibition in the PBN shifts defensive strategy from freezing to active avoidance in a platform-based active avoidance paradigm. (n.d.). https://doi.org/10.1038/s41539-026-00453-3

Image Credits: AI Generated

DOI: 10.1038/s41539-026-00453-3

Keywords: CGRP, parabrachial nucleus, active avoidance, freezing, fear conditioning, defensive behavior, threat processing, amygdala, neuropeptides, behavioral neuroscience, PTSD, platform-based avoidance paradigm

Thermoregulation is essential because mammals must keep their internal body temperature within a narrow range despite changing conditions outside the body. When an animal becomes cold, heat loss must be limited and heat production must increase. The body can constrict blood vessels in the skin, activate brown adipose tissue, induce skeletal muscle shivering and drive the animal towards a warmer environment. Cold can also alter feeding and energy use, while pleasant or mildly cool temperatures may engage reward circuits and influence dopamine release. How the brain combines these different reactions has remained unclear. The new work places the parabrachial nucleus at the centre of this puzzle, showing that a defined group of neurons responds to cold and links sensory information with physiological, behavioural, metabolic and affective adaptations.

To identify these cells, the researchers used activity-dependent genetic labelling in mice. This approach marks neurons that become active during a particular experience, allowing scientists to examine and manipulate the same population later. When mice were exposed to cold conditions, neurons in the parabrachial nucleus were labelled according to their activity. The resulting PB^Cold population showed rapid activation when temperatures fell, but the response did not quickly disappear. Instead, the neurons remained active across a broad range of cold stimuli, indicating that they may encode persistent environmental cooling rather than simply registering the instant at which temperature begins to drop. This sustained activity is important because thermoregulatory responses must continue for as long as the threat of heat loss remains.

The parabrachial nucleus is strategically positioned within the brain’s sensory and homeostatic networks. It receives information associated with the body’s internal state and external sensory conditions and can communicate with multiple downstream regions that control autonomic and behavioural functions. The study’s results indicate that PB^Cold neurons are primary recipients of cold sensory information in the brain. Their activity provides a possible neural explanation for how one environmental signal can produce many simultaneous outcomes. A drop in temperature can prompt brown fat to generate heat, blood vessels in the tail to constrict, muscles to shiver and the animal to seek shelter. Rather than treating these responses as independent circuits operating in parallel, the findings suggest that PB^Cold neurons help coordinate them as parts of a unified cold-defence programme.

The researchers tested this idea by silencing PB^Cold neurons, both temporarily and permanently. The consequences reached across several biological systems. When these neurons were inhibited, mice showed impaired activation of brown adipose tissue thermogenesis, a form of heat production that burns stored energy without requiring muscle contraction. Tail vasoconstriction was also disrupted, weakening a key mechanism that reduces heat loss from the body’s surface. Somatic responses were affected as well: the animals displayed impaired skeletal muscle shivering, another major source of heat when the environment becomes cold. Together, these results show that PB^Cold neurons are not simply associated with cold sensation. They are necessary for the effective deployment of multiple heat-preserving and heat-generating mechanisms.

The disruption extended beyond automatic physiology. Mice with silenced PB^Cold neurons also showed altered cold-avoidance behaviour, suggesting that the neural population helps translate temperature information into decisions about where to move. This behavioural response is crucial because avoiding cold can be more efficient than producing heat after exposure has already occurred. The neurons also influenced cold-induced hyperphagia, the increase in food intake that can accompany exposure to low temperatures. Eating more provides fuel for energy-intensive thermogenesis, linking appetite to thermal demand. In addition, silencing the cells affected dopamine release in response to rewarding cool stimuli. This finding connects the cold-sensing circuit to affective and motivational systems, indicating that the parabrachial pathway may help determine whether a temperature is experienced as threatening, tolerable or rewarding.

The importance of the circuit became especially clear under severe cold. Animals in which PB^Cold neurons were permanently silenced were less able to mount appropriate defensive responses and experienced compromised survival in extreme low-temperature conditions. This result elevates the cells from one component of a sensory pathway to a critical part of the machinery that protects the organism from environmental danger. The researchers also performed the opposite experiment. Activating PB^Cold neurons promoted warmth-seeking behaviour, increased food intake and raised energy expenditure. In other words, stimulating the population was sufficient to reproduce several hallmarks of a cold-response state, even without simply relying on the animal’s normal sensory experience. The bidirectional effects strengthen the conclusion that these neurons actively organize cold adaptation.

The study further examined the molecular identity of the PB^Cold population. Gene-expression profiling identified Grp and Trhr as highly specific markers for subsets of these neurons. The markers were not completely efficient, meaning that they label only portions of the full PB^Cold population, but their specificity could make them useful tools for studying the circuit in greater detail. Grp encodes gastrin-releasing peptide, a signalling molecule involved in neural communication, while Trhr encodes the thyrotropin-releasing hormone receptor, a receptor that can influence neuronal activity and endocrine-related pathways. The presence of these molecular signatures provides a route towards separating functionally distinct subgroups and determining whether particular neurons specialize in thermogenesis, vasoconstriction, shivering, feeding, avoidance or reward.

The findings also raise broader questions about how the brain represents temperature. A sensory signal is not necessarily a simple numerical readout of the environment. The sustained activity of PB^Cold neurons suggests that the nervous system may maintain a continuing representation of cooling, enabling downstream circuits to adjust their responses as long as thermal conditions demand it. At the same time, the broad effects of manipulating these neurons show that the representation is connected to competing priorities, including survival, energy availability and behavioural choice. Understanding this architecture could eventually help explain why disorders affecting temperature regulation often involve multiple symptoms at once. It may also illuminate how the brain balances the energetic cost of generating heat with the need to preserve body temperature.

By identifying PB^Cold neurons as a central hub for cold-induced homeostatic responses, the study adds a major piece to the neural map of thermal regulation. The work shows that a relatively defined population in the parabrachial nucleus can receive cold-related sensory information and coordinate responses ranging from brown-fat activation and shivering to appetite, environmental avoidance and dopamine signalling. The discovery does not reduce thermoregulation to a single circuit; rather, it reveals how diverse downstream systems can be organized around a shared sensory command. In an increasingly detailed picture of the brain’s control over the body, PB^Cold neurons emerge as a vital interface between the outside world and the internal fight to stay warm.

Subject of Research: Cold-activated parabrachial nucleus neurons and their role in coordinating thermoregulatory, behavioural, metabolic and affective responses.

Article Title: Parabrachial neurons orchestrate cold-induced homeostatic responses

Article References: Jung, S., Kondaurova, A., Lee, M. et al. Parabrachial neurons orchestrate cold-induced homeostatic responses. Nature Metabolism (2026). https://doi.org/10.1038/s42255-026-01565-1

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s42255-026-01565-1

Keywords: thermoregulation, cold sensation, parabrachial nucleus, PB^Cold neurons, brown adipose tissue, shivering, vasoconstriction, cold avoidance, hyperphagia, dopamine, neural circuits, homeostasis