The control of urinary storage and voiding is hierarchically organized within the central nervous system (CNS) and peripheral nervous system. The periaqueductal gray (PAG) and Barrington's nucleus (Barr) are areas located in the brainstem which play a critical role in lower urinary tract (LUT) control [1]. Barr is known to have strong structural and functional connectivity with the PAG [2]. Altered CNS processing has been suggested to contribute to LUT symptoms, such as urgency urinary incontinence. Previous studies have demonstrated that PAG can be divided into clusters based on intrinsic functional connectivity patterns using functional MRI (fMRI) [3]. The aim of this study was to investigate the interaction between PAG clusters and Barr during a strong desire to void using 7 Tesla (7T) fMRI.

Eight female subjects, including two individuals meeting the criteria for overactive bladder, underwent fMRI scans during a strong desire to void. PAG was intrinsically subdivided into clusters using the Louvain module detection algorithm, which is an established tool to subdivide brain regions into functional subregions based of functional connectivity measured by fMRI. This resulted in three clusters per subject, visualized in cluster maps (Fig. 1.A). To parcellate the PAG based on interaction with Barr, the mean fMRI signal of Barr was correlated with the fMRI signal of every voxel in PAG, resulting in correlation maps (Fig. 1.B). PAG voxels were then divided into three groups based on their correlation coefficients: the strongest negative, the weakest, and the strongest positive 1/3th of correlation coefficients per subject (Fig. 1.C). Spatial organization of clusters resulting from intrinsic parcellation using Louvain module detection algorithm and the grouping of the PAG correlation map based on the functional connectivity with Barr were then compared for each participant. This is done by calculating the correlation between both parcellation approaches per subject. A permutation test was used to assess statistical significance.

In all subjects, the division of the correlation map into the three groups exhibited a significantly similar spatial organization compared to the PAG clusters identified using the Louvain module detection algorithm (p<0.05) after false discovery rate correction (q=0.05).

The findings suggest that functionally distinct clusters within the PAG demonstrate unique interactions with Barr during a strong desire to void. The observation that both intrinsic and Barr-connectivity-based parcellation methods of the PAG result in comparable spatial distributions indicates that these clusters may represent fundamental functional units within the PAG. These results indicate that brainstem structures can be reliably parcellated using the Louvain module detection algorithm, highlighting its potential to uncover functional subregions within the PAG. Data from OAB patients during a strong desire to void show the same pattern as healthy adults. It should be investigated whether this is the same at low bladder volumes with no conscious LUT sensations.

This study provides evidence that functional clusters within the PAG exhibit distinct interactions with Barr during strong desire to void. Most neuroimaging research treat the PAG as one homogeneous region, while animal studies show the PAG is involved in different steps along the bi-directional communication between the LUT and higher cortical areas to coordinate micturition. It is therefore essential to gain access to refined methods that enable evaluation in functional subregions of the PAG. These insights enhance our understanding of the functional organization of brainstem nuclei involved in LUT control and could aid in the functional assessment of these regions in humans with unprecedented detail. Such advancements are crucial for deciphering the mechanisms underlying existing therapeutic strategies and for developing new treatment approaches for LUT disorders.

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