The Role of 5-HT1A Agonist Buspirone in Pain Regulation in Postcolitis Rats

Background

In recent years, increasing clinical and experimental evidence has shown that the brain’s serotonin (5-HT) system plays a significant role in the pathogenesis of gastrointestinal disorders, such as inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS). These conditions are often accompanied by visceral hypersensitivity and chronic abdominal pain. The Raphe Magnus (RMg) and Dorsal Raphe (DR) nuclei are key structures in central pain regulation, and their activity changes in intestinal pathology have been widely studied. However, the mechanisms underlying their neuroplastic changes remain unclear. Given the important role of 5-HT1A receptors in pain regulation and raphe nucleus neuronal activity, this study aimed to investigate whether 5-HT1A receptor-dependent visceral and somatic pain processing is altered after colitis.

Research Source

This study was conducted by Olga A. Lyubashina, Boris M. Sushkevich, and Ivan B. Sivachenko from the Pavlov Institute of Physiology at the Russian Academy of Sciences. It was published in 2025 in the European Journal of Neuroscience and supported by the Russian Science Foundation (Grant No. 23-25-00151).

Research Process

1. Experimental Animals and Colitis Model

The study used adult male Wistar rats, divided into healthy control and postcolitis recovery groups. The colitis model was induced by trinitrobenzenesulfonic acid (TNBS), which was administered into the colon via the anus to induce inflammation. Postcolitis recovery group rats were tested 29-60 days after inflammation resolution to ensure complete recovery.

2. Anesthesia and Surgical Preparation

Before the experiment, rats were anesthetized with a mixture of urethane and α-chloralose via intraperitoneal injection. Surgical preparations included cannulation of the femoral artery and vein for blood pressure monitoring and intravenous drug administration, as well as tracheal intubation to maintain respiration. The rat’s head was fixed in a stereotaxic apparatus, and the skull was drilled to expose the RMg and DR nuclei.

3. Neuronal Recording

Insulated tungsten microelectrodes were used to record the electrical activity of RMg and DR neurons. The electrodes were precisely inserted into the target nuclei using a microdrive, and the signals were amplified, filtered, and displayed in real-time using computer software. Blood pressure and respiration were continuously monitored to ensure anesthesia depth and experimental stability.

4. Visceral and Somatic Noxious Stimulation

Visceral noxious stimulation was achieved through colorectal distension (CRD), using a balloon rapidly inflated to 80 mmHg and maintained for 30 seconds. Somatic noxious stimulation was achieved through tail squeezing (TS), using surgical forceps to firmly squeeze the tail for 30 seconds. Both stimuli caused significant changes in blood pressure and respiration, indicating their noxious nature.

5. Experimental Protocol

Before the experiment, rats were allowed to rest for at least 45 minutes. Neuronal activity was recorded for 180 seconds, including 60 seconds of baseline activity, 30 seconds of stimulation, and 90 seconds of recovery. CRD and TS were separated by at least 5 minutes to ensure neuronal activity returned to baseline. Neuronal responses to CRD and TS were recorded before and after intravenous administration of Buspirone (0.1, 0.5, 2.0, and 4.0 mg/kg).

6. Data Analysis

Neuronal activity data were analyzed offline using Spike 2 software, with spike sorting based on waveform shape and amplitude. Baseline and stimulation-induced activities of each neuron were calculated and expressed as spikes per second. Neuronal responses to stimuli were expressed as a percentage of baseline activity, with changes exceeding 10% considered significant. Blood pressure responses were assessed by comparing pre- and post-stimulation levels.

Key Findings

1. Changes in RMg Neurons After Colitis

In healthy control rats, low doses of Buspirone (0.1 and 0.5 mg/kg) inhibited RMg neuronal responses to CRD, while high doses (2 and 4 mg/kg) enhanced RMg neuronal responses to both CRD and TS. However, in postcolitis recovery rats, all doses of Buspirone inhibited RMg neuronal responses to CRD and TS, indicating significant changes in pain responses after colitis.

2. Changes in DR Neurons After Colitis

In healthy control rats, both low and high doses of Buspirone inhibited DR neuronal responses to CRD and TS. However, in postcolitis recovery rats, the inhibitory effect of Buspirone on DR neuronal responses to TS was significantly weakened, indicating altered pain responses after colitis.

3. Changes in Blood Pressure Responses

In healthy control rats, high doses of Buspirone significantly inhibited the blood pressure drop induced by CRD and TS. However, in postcolitis recovery rats, the inhibitory effect of Buspirone on blood pressure responses was significantly weakened, indicating altered systemic pain responses after colitis.

Conclusion

This study revealed significant changes in 5-HT1A receptor-dependent pain regulation mechanisms after colitis. In healthy rats, Buspirone modulated pain responses in RMg and DR neurons by activating 5-HT1A autoreceptors and heteroreceptors. However, after colitis, this regulatory mechanism was altered, leading to reduced pain responses in RMg neurons and enhanced pain responses in DR neurons. These changes may be related to the development of visceral and somatic hypersensitivity after colitis.

Research Highlights

1. Novel Research Methods: This study used microelectrode recording technology to precisely record RMg and DR neuronal responses to visceral and somatic noxious stimuli, combined with systemic blood pressure responses, providing a comprehensive evaluation of Buspirone’s analgesic effects.
2. Important Findings: The study found significant changes in 5-HT1A receptor-dependent pain regulation mechanisms after colitis, offering new insights into postcolitis pain hypersensitivity.
3. Potential Applications: The results suggest that the use of Buspirone for pain treatment after colitis may need reevaluation. Future research could further explore the role of 5-HT1A receptors in chronic pain.

Additional Valuable Information

The study also found that the regulatory effects of Buspirone on RMg and DR neurons exhibited sex differences, with female rats showing greater sensitivity to Buspirone. This finding provides new clues for future research on the role of sex differences in pain regulation.