Chronic Pain and Behavior: The Neurobiological Link Between Osteoarthritis and Aggression in Dogs

1. Osteoarthritis as a Chronic Pain State

Osteoarthritis is a progressive disorder characterized by cartilage degradation, subchondral bone remodeling, and chronic low-grade inflammation. Importantly, the pain associated with OA is not purely nociceptive but involves neuropathic and nociplastic components.

As the disease progresses, central sensitization develops, resulting in amplified pain processing within the central nervous system. In this state, pain perception becomes increasingly independent of peripheral tissue damage and instead reflects altered neural processing.

This transition from peripheral injury to central dysfunction is critical for understanding why behavioral changes may persist even when visible clinical signs appear mild.

2. Neurobiology of Pain-Induced Aggression

2.1 Shared Neural Circuits

Pain and defensive behavior share overlapping neural pathways, particularly within the periaqueductal gray (PAG), amygdala, and hypothalamus. These regions integrate sensory input with emotional and behavioral responses.

Chronic pain leads to sustained activation of these circuits, effectively priming the organism for defensive reactions.

2.2 Serotonin and Behavioral Regulation

Serotonin plays a central role in both pain modulation and impulse control. Chronic pain states are associated with dysregulation of serotonergic signaling, resulting in increased irritability and reduced behavioral inhibition.

These mechanisms are also closely linked to stress-related neurobiology, particularly involving cortisol regulation and chronic stress adaptation, as described in the neurobiology of chronic stress in dogs.

 

2.3 The HPA Axis and Stress Load

Chronic pain acts as a persistent physiological stressor, leading to prolonged activation of the hypothalamic–pituitary–adrenal (HPA) axis. Elevated cortisol levels and chronic stress exposure reduce resilience and lower thresholds for reactive behavior.

This aligns with broader concepts of stress-induced behavioral dysregulation observed in dogs.

2.4 Endogenous Opioid Dysfunction

The endogenous opioid system, responsible for modulating pain and emotional stability, may become dysregulated under chronic pain conditions. This reduces the organism’s ability to buffer both physical discomfort and emotional stress, increasing the likelihood of defensive responses.

3. Central Sensitization and Threat Amplification

Central sensitization results in heightened responsiveness of central neurons to sensory input. Clinically, this manifests as:

• Hyperalgesia – exaggerated response to painful stimuli

• Allodynia – pain in response to normally non-painful stimuli

For affected dogs, routine interactions such as touch or movement may be perceived as painful. The resulting aggressive response represents an adaptive attempt to avoid further discomfort.

This phenomenon reflects altered neural processing rather than intentional or learned aggression.

4. Clinical Presentation

Pain-induced behavioral changes are frequently misinterpreted. Common patterns include:

• Increased irritability during handling

• Defensive aggression when approached or touched

• Aggression associated with movement (e.g., rising, jumping)

• Reduced tolerance toward other animals

• General behavioral changes including withdrawal or reduced activity

These signs often overlap with conditions such as reactivity, which is further explored in reactivity in dogs from a neurological perspective.

 

5. Clinical Implications

Behavioral assessment without evaluation of pain is incomplete.

A comprehensive approach requires:

• detailed behavioral and medical history

• orthopedic and neurological examination

• consideration of chronic pain as a primary driver

Effective treatment must be multimodal and may include:

• pharmacological intervention

• physical rehabilitation

• environmental modification

• weight and dietary management

Importantly, behavior modification alone is insufficient when underlying pain is present.

6. Conclusion

Chronic pain, particularly from osteoarthritis, represents a fundamental but often overlooked factor in canine behavioral disorders.

The interaction between pain, neurochemical regulation, and emotional processing creates a physiological state in which defensive aggression becomes more likely. This relationship is grounded in shared neural circuitry and stress-related mechanisms rather than purely learned behavior.

Recognizing pain as a primary driver of behavioral change is essential for accurate diagnosis and effective intervention.

Aggression in such cases should be understood not as a behavioral failure, but as a manifestation of underlying biological distress.

References

• Vázquez-León,  P., Miranda-Páez, A., Valencia-Flores, K., & Sánchez-Castillo, H.  (2021). Defensive and Emotional Behavior Modulation by Serotonin in the  Periaqueductal Gray. International Association for the Study of Pain .

• Yarns, B. C., et al. (2022). At the intersection of anger, chronic pain, and the brain: A mini-review. Neuroscience & Biobehavioral Reviews, 135, 104558 .

• (2025). Pain in plain sight: Recognizing chronic pain in the exam room. dvm360 .

• (2025). Pain Management. American College of Veterinary Surgeons .

• NeuroVet. (2024). Neuropathic Pain Syndromes .

• Bruehl,  S., Burns, J. W., Chung, O. Y., & Chont, M. (2008). Pain-related  effects of trait anger expression: neural substrates and the role of  endogenous opioid mechanisms. Neuroscience & Biobehavioral Reviews, 33(3), 475–491 .

• (2025). Pain and Pain Management in Pets. MSD Veterinary Manual .