Shepherd, C. E. | bioRxiv Science

bioRxiv2025

Characterisation of putative retrotrapezoid nucleus (RTN) chemoreceptor neurons in the adult human brainstem

The retrotrapezoid nucleus (RTN) of rodents is located ventral to the facial motor nucleus (7N) and consists of acid-sensitive neurons that activate breathing and mediate the central component of the ventilatory response to hypercapnia. In rodents, RTN neurons can be histologically identified by the presence of paired-like homeobox 2B positive nuclei (Phox2b+) and the absence of cytoplasmic choline acetyltransferase (ChAT-) and tyrosine hydroxylase (TH-). Up to 50% of rodent RTN neurons synthesise galanin, and 88% express pituitary adenylate cyclase activating polypeptide (PACAP). The human RTN (hRTN) has not been mapped to date. This study aimed to map the location and cytoarchitecture of the adult hRTN and compare the findings to the homologies of rodents, macaques and human infants. Formalin-fixed, paraffin-embedded tissue blocks from two adult cases, spanning the medulla-pons, were serially sectioned (10{micro}m thick) and every four in thirty sections was assayed for immunohistochemistry for ChAT, or double-labelled Phox2b/TH, Phox2b/galanin and Phox2b/PACAP, followed by analysis using QuPath software. hRTN neurons, identified as Phox2b+/TH-/ChAT-, were located ventral to 7N and lateral to the superior olive, overlapped with the C1 or A5 catecholaminergic population and extended rostrocaudally from Obex +13 to +17 mm. In the parafacial area, 90% of Phox2b immunoreactive (-ir) neurons are hRTN neurons, totaling around 5000 bilaterally, and were surrounded by numerous TH-ir fibers. Galanin- and PACAP-ir was identified in 43% and 39% of Phox2b-ir parafacial neurons, respectively. This is the first study to characterise and quantitatively map the adult human RTN using a series of neurochemical markers.

neuroscience↗

bioRxiv2025

Region-specific variations in the cerebrovasculature underlie disease progression in Parkinson's disease

Parkinsons disease is a progressive neurodegenerative disorder characterised by motor dysfunction, dopaminergic neuronal loss in the substantia nigra and abnormal accumulation of -synuclein Lewy bodies. Research suggests that the cerebrovascular system plays a role in fluid dynamics, waste clearance, and removal of abnormal proteins. Imaging studies show that this waste clearance system, known as the glymphatic system, is disrupted in Parkinsons disease, highlighting its involvement in the disease. This immunohistochemical human brain tissue study quantified changes in the cerebrovascular system (perivascular space, string vessels, pericytes, aquaporin-4 and astrocytes) in Parkinsons disease (n=18) cases with variable disease durations (median=14, range= 19) compared to age and post-mortem matched (P >0.05) control cases (n=7). Analysis was carried out in brain regions variably affected by cell loss (substantia nigra) and protein deposition (substantia nigra and medial temporal cortex). The occipital cortex was included, as this region is not affected by cell loss or protein deposition. Group differences were analysed and the relationship with protein deposition (Lewy body stage, amyloid score, neurofibrillary tangle score) was assessed. Although total astrocyte density did not change (P >0.05), Parkinsons disease cases exhibited reduced aquaporin-4 in astrocytic endfeet and enlargement of the arteriolar and venular perivascular space. Significant changes in the capillary network were also observed with increased string vessel formation (P <0.001) and pericyte loss (P <0.001), changes likely to impact blood flow and its regulation. The formation of string vessels significantly correlated with disease duration (P <0.05), especially in the occipital cortex. The occipital cortex demonstrated the greatest decreases in pericytes (P <0.001) and aquaporin-4 mislocalisation (P <0.05), while changes in pericyte density were also significant in the substantia nigra. In contrast, these changes were not significant in the medial temporal cortex despite protein deposition in this region. Although no Lewy pathology was detected in the occipital cortex, there was a positive relationship between Lewy body stage and perivascular space size (Rho =0.6, P <0.05). These findings reveal progressive, region-specific alterations in the cellular components of the glymphatic system and vascular integrity in Parkinsons disease. Notably, the correlation between string vessel formation and disease duration, even in a region unaffected by protein deposition, suggests that vascular changes may play an important role in disease progression. These results emphasize the need for further investigation into the interplay between regional vascular changes and Parkinsons disease progression, which may offer novel insights for therapeutic strategies.

neuroscience↗