# Cutaneous Allodynia of the Withers in Cattle: An Experimental In Vivo Neuroanatomical Preliminary Investigation of the Dichotomizing Sensory Neurons Projecting into the Reticulum and Skin of the Withers—A Case Study on Two Calves

## TL;DR

This study investigates whether sensory neurons in cattle can detect both internal and skin pain, which could explain a diagnostic test for a common cattle disease.

## Contribution

The study provides new neuroanatomical insights into sensory neuron convergence in cattle, using fluorescent tracers to explore pain mechanisms.

## Key findings

- No double-labeled neurons were found in the DRG, suggesting convergence of visceral and somatic inputs occurs centrally.
- Only a few neurons were labeled from the reticulum, while many were labeled from the skin.
- The convergence likely involves higher-order integrative centers, such as vagal pathways and brainstem nuclei.

## Abstract

Traumatic reticuloperitonitis, also known as “hardware disease”, is a common condition in cattle that can cause pain in the abdominal cavity. A clinical test called the Kalchschmidt pain test is used to detect this condition, based on the observation of skin hypersensitivity (cutaneous allodynia) at the withers. It is hypothesized that this reaction is caused by the convergence of visceral (from the reticulum) and somatic (from the skin) sensory information in the nervous system. This study investigated whether some sensory neurons located in the thoracic dorsal root ganglia (DRG) simultaneously send projections to both the reticulum and the skin of the withers, a condition known as dichotomization. Two retrograde fluorescent tracers were injected in two calves: Fast Blue (FB) into the reticulum and Diamidino Yellow (DY) into the skin of the withers. After 30 days, the DRG from T1 to T8 were examined under a fluorescence microscope. While many neurons were labeled from the skin, only a few were labeled from the reticulum, and no double-labeled neurons were found. These findings suggest that the convergence of visceral and somatic inputs likely occurs at a central level, rather than within individual sensory neurons, possibly involving vagal pathways and supra-spinal integration centers.

The presence of dichotomizing neurons in the dorsal root ganglia (DRG) of cattle, innervating both the reticulum and the withers, may indicate a pre-spinal convergence of visceral and cutaneous sensory information, i.e., that the DRG primary sensory neurons may elaborate the sensory information coming from two different anatomical areas before reaching the secondary sensory neurons within the spinal cord. This anatomical feature could be the underlying basis for the cutaneous allodynia observed in traumatic reticuloperitonitis, also known as the “Kalchschmidt pain test”. The aim of the study was to identify the DRG primary sensory neurons innervating the reticulum and the withers by using two different retrograde fluorescent tracers, Fast Blue (FB, affinity for cytoplasm) and Diamidino Yellow (DY, affinity for nucleus). In two anesthetized calves, FB and DY were injected into the reticulum and skin of the withers, respectively. At the end of the experimental period, the calves were deeply anesthetized and then euthanatized. The thoracic (T1–T8) DRG were collected and processed to obtain cryosections which were examined on a fluorescent microscope. A large number of neurons localized, especially in the T7 DRG, presented nuclei labeled with DY. On the contrary, only a few neurons localized exclusively in T6 and T7 DRG presented the cytoplasm labeled with FB. No neurons displayed FB and DY simultaneously within the cytoplasm and nucleus, respectively. The absence of double-labeled DRG neurons suggests that the convergence of visceral and somatic sensory inputs underlying the Kalchschmidt pain response likely does not occur at the level of individual DRG neurons. Rather, it may involve higher-order integrative centers, possibly including vagal pathways and brainstem nuclei which integrate the afferent information to coordinate respiratory movements of the diaphragm, intercostal muscles, and larynx. Although limited by the sample size, this case study provides a neuroanatomical basis for further investigation into central mechanisms of referred visceral pain in cattle.