Oxytocin Neuron Activation Eases Arthritis and Pain in Rat Study

Activating the brain’s oxytocin system eased both joint inflammation and pain-related behavior in rats with a chronic inflammatory form of arthritis, according to a study published in Frontiers in Endocrinology. Researchers at the University of Occupational and Environmental Health in Kitakyushu, Japan, used two complementary approaches: chemogenetic activation of oxytocin neurons in genetically engineered rats and repeated administration of a non-peptide oxytocin receptor agonist called LIT-001 in ordinary Wistar rats. Both interventions reduced arthritis severity, joint swelling, and mechanical hypersensitivity in a rat model of adjuvant arthritis, a well-established model of chronic inflammatory pain.

Oxytocin is a neuropeptide synthesized primarily in the paraventricular nucleus and supraoptic nucleus of the hypothalamus. It is released from the posterior pituitary into the systemic circulation, where it plays essential roles in parturition and lactation, but it also acts within the central nervous system and contributes to pain modulation. Oxytocin neurons originating in the paraventricular nucleus project to the spinal dorsal horn and brainstem, where they influence nociceptive processing, and activation of the paraventricular nucleus or the oxytocin system has produced antinociceptive effects in animal models of neuropathic, inflammatory, and visceral pain. Accumulating evidence also points to a role for the oxytocin system in regulating inflammatory responses, with altered oxytocin receptor expression reported in inflamed tissues and immune cells.

The team applied a chemogenetic technique known as DREADDs, short for Designer Receptors Exclusively Activated by Designer Drugs, to the adjuvant arthritis model. They used OXT-hM3Dq-mCherry transgenic rats, in which oxytocin neurons carry an engineered receptor that can be selectively activated by the drug clozapine-N-oxide, or CNO. Adjuvant arthritis was induced on Day 0 by intradermal injection of heat-killed Mycobacterium tuberculosis suspended in liquid paraffin into the base of the tail. Arthritis severity was then scored daily on a four-point scale per limb, summed across all four limbs to a maximum of 16, with assessments performed independently by two observers.

In the first experiment, CNO was given daily during the established phase of arthritis, from Days 15 to 17, at a dose of 1 mg/kg subcutaneously. Body weight and arthritis index were measured daily, and a separate group of animals was euthanized on Day 17 for histological analysis of the hind limbs. The arthritis index rose in both groups until Day 16, but scores continued to climb in the vehicle-treated group while they declined from Day 17 onward in the CNO-treated group. As a result, arthritis scores were significantly lower in the CNO-treated group from Day 22 to Day 25, with P values ranging from 0.034 to 0.024. No significant difference in body weight was observed between the groups.

Histological examination of the ankle joints revealed the difference in tissue damage. In the vehicle-treated group, the researchers observed synovial hyperplasia, inflammatory cell infiltration into the bone marrow cavity, pannus formation within the joint space, apparent destruction of the articular cartilage, and bone erosion. In the CNO-treated group, these pathological changes appeared less pronounced, with less prominent synovial hyperplasia and inflammatory infiltration, less extensive pannus formation, and relatively well-preserved articular cartilage and overall joint architecture. The authors emphasized that this evaluation was descriptive, that no validated semi-quantitative histological scoring system was used, and that the assessment was not performed in a blinded manner, so these findings should be interpreted cautiously.

A second experiment tested whether earlier intervention would work better. CNO was administered during the onset phase, from Days 10 to 12, to transgenic rats with adjuvant arthritis. Ankle swelling was assessed by measuring the mediolateral and anteroposterior diameters of the ankle joint with calipers on Day 10 before treatment and again on Day 13, assuming an elliptical shape to estimate the ankle joint cross-sectional area. Mechanical hypersensitivity was assessed with the Randall–Selitto test, in which increasing mechanical pressure is applied to the dorsal surface of the right hind paw and the force at which the animal shows an escape response or vocalization is recorded as the mechanical nociceptive threshold, with a cut-off value of 450 g to avoid tissue damage.

The onset-phase treatment produced clear effects. The arthritis index was significantly lower in the CNO-treated group than in the vehicle-treated group on Day 13, with P < 0.001. The ankle joint cross-sectional area was significantly smaller in the CNO-treated group on Day 13, with P = 0.008, indicating reduced ankle swelling. In the vehicle-treated group, the nociceptive threshold was significantly lower on Day 13 than on Day 10, with P < 0.001, but no significant change was observed between Days 10 and 13 in the CNO-treated group, meaning that the treatment prevented the decline in mechanical nociceptive thresholds. The authors noted that because the observation periods differed between the onset- and established-phase experiments, the magnitude and durability of the effects between these intervention periods cannot be directly compared, and the onset-phase experiment did not include long-term histological assessment.

To complement the chemogenetic approach, the researchers also tested LIT-001, a non-peptide oxytocin receptor agonist, in ordinary Wistar rats with adjuvant arthritis. The drug was given intraperitoneally at 10 mg/kg once daily from Days 10 to 12, with vehicle-treated animals serving as controls. Arthritis scores were significantly lower in the LIT-001-treated group on Day 13, with P < 0.001. The ankle joint cross-sectional area was significantly smaller in the LIT-001-treated group on Day 13, with P = 0.004, and the nociceptive threshold was significantly lower on Day 13 than on Day 10 in the vehicle-treated group, with P = 0.039, but no significant change was observed between Days 10 and 13 in the LIT-001-treated group. The authors noted that the effects of LIT-001 were directionally similar to those observed after chemogenetic activation of oxytocin neurons, although the two interventions were tested in different experimental settings and should not be directly compared.

Immunohistochemical analyses added a window into central neural responses. Brain tissues were collected on Day 14 from the LIT-001 experiment, and the numbers of FosB-immunoreactive cells, a marker of neuronal activation, were quantified in the paraventricular nucleus, the supraoptic nucleus, and the dorsal raphe nucleus. The number of FosB-immunoreactive cells in the paraventricular nucleus was significantly greater in the LIT-001-treated group than in the vehicle-treated group, with P = 0.008. In the supraoptic nucleus, no significant difference was observed, with P = 0.108. In the dorsal raphe nucleus, the number of FosB-immunoreactive cells was significantly lower in the LIT-001-treated group, with P = 0.019, and the number of detectable tryptophan hydroxylase-immunoreactive cells, which mark serotonergic neurons, was also significantly lower, with P = 0.005.

The authors offered several possible interpretations while acknowledging important limitations. The increase in FosB expression in the paraventricular nucleus may reflect altered neuronal activity associated with the improvement of inflammatory responses and pain-related behaviors following oxytocin receptor stimulation, since that nucleus sends projections to the spinal dorsal horn and brainstem where it participates in pain modulation and autonomic regulation. The reductions in the dorsal raphe nucleus may reflect decreased neuronal activity secondary to reduced peripheral inflammation and diminished nociceptive input, or alternatively, oxytocin signaling may directly influence brainstem monoaminergic circuits, given that projections from the paraventricular nucleus to the dorsal raphe and the presence of oxytocin receptors within the dorsal raphe have been demonstrated previously. The authors also noted that they did not directly assess circulating or local oxytocin levels, oxytocin receptor expression in joint tissues, or the brain penetration and sites of action of LIT-001, that oxytocin receptor antagonists and local pharmacological interventions were not used, and that the observed effects cannot be definitively attributed to oxytocin receptor-dependent mechanisms. Animals were not randomly assigned to treatment groups, evaluators were not blinded, and a CNO-treated non-DREADD control group was not included, so potential DREADD-independent effects of CNO cannot be completely excluded. Behavioral specificity tests such as the open field or elevated plus maze were not performed, so potential confounding effects of motor impairment, sedation, or altered anxiety-related behavior cannot be ruled out. Further studies are needed to determine whether these findings have clinical relevance to inflammatory arthritis in humans.

Subject of Research: Chemogenetic and pharmacological activation of the oxytocin system in a rat model of adjuvant arthritis

Article Title: Chemogenetic and pharmacological activation of the oxytocin system ameliorates adjuvant arthritis in rats

Article References: Hashimoto, W., Suzuki, H., Fujihara, H., Ueno, H., Fukushima, H., Ariyoshi, R., Tsukamoto, M., Kawasaki, M., Arashi, T., Horishita, T., Sakai, A., Ueta, Y., & Maruyama, T. (2026). Chemogenetic and pharmacological activation of the oxytocin system ameliorates adjuvant arthritis in rats. Frontiers in Endocrinology, 17, Article 1941424. https://doi.org/10.3389/fendo.2026.1941424

Image Credits: AI Generated

DOI: 10.3389/fendo.2026.1941424

Keywords: oxytocin, adjuvant arthritis, chemogenetics, DREADDs, LIT-001, chronic inflammatory pain, mechanical hypersensitivity, paraventricular nucleus, dorsal raphe nucleus, joint inflammation, rat model, neuroendocrinology