Beach, Thomas G.

AbstractBackgroundThe brainstem nucleus incertus (NI) or ‘uncertain nucleus’, was originally described by Streeter in 1903 as a midline region in the floor of the fourth ventricle of the human brain with an ‘unknown’ function. A century later, the NI has been shown to play a key role in memory formation in rodents, through direct and indirect inhibition of the hippocampus; and the anatomy of the NI and its projections throughout the brain have been systematically mapped in rats, mice, and primates, but not in humans. In these novel studies, we mapped the human NI using the neuropeptide, relaxin‐3 (RLN3), as neurochemical marker. In rodents, RLN3 is produced mostly in NI neurons with some subsidiary positive cells in the periaqueductal gray and an area dorsal to the substantia nigra.MethodsSerial, free‐floating coronal sections of formalin‐fixed postmortem tissue were prepared from the pons of an adult female (aged 96 years) without a history of dementia or other neurological disorders, provided by the Banner Brain and Body Donation Program, Sun City, Arizona, USA. Using immunohistochemistry, we first identified the NI by mapping the distribution of microtubule‐associated protein‐2 (MAP2) and RLN3. The specificity of the RLN3 antibody used was verified in a dot‐blot using native RLN3 peptide. We also investigated the presence of markers detected in the NI of rats ‐ glutamic acid dehydrogenase‐65/67 (GAD65/67) and corticotropin‐releasing factor type 1 receptor (CRF1); and used a phosphorylated‐tau (AT8) to determine if the NI displayed tau accumulation.ResultsDetection of MAP2‐immunoreactivity revealed the neuronal distribution throughout the pons. RLN3‐immunoreactivity was detected in neurons in the dorsal, anterior‐medial region, revealing the broad anatomy of the human NI. Neurons within this same area were also positive for GAD65/67‐ and CRF1‐immunoreactivity, in line with previous studies; and AT8‐immunoreactivity was also detected.ConclusionsRLN3 is present in neurons of the human NI and aspects of its anatomy are shared across species, including the macaque. Accumulation of phosphorylated‐tau in the NI suggests its possible involvement in the pathology of Alzheimer’s disease and related dementias. Further comparative studies are required to better understand the neurochemical anatomy of the human NI.

Abstract Falls are a public health concern for older adults, particularly those with cognitive impairments, like mild cognitive impairment (MCI) and Alzheimer’s disease (AD) dementia. Executive function (EF) is linked to falls in older adults, but little is known about the relationship between individual EF assessments and falls in individuals with varying cognitive statuses. Using cross-sectional secondary data analyses with binary logistic regression, we examined the associations between EF assessments (Trail Making Test (ΔTMT), Digit Span (DS), Stroop test, Clock Drawing Test, and phonemic/semantic fluency tests) and falls (fall history and subsequent falls) in 466 participants (aged 66-103) with either intact cognition, MCI, or AD dementia. Surprisingly, a significant interaction effect emerged with AD dementia and EF assessments (ΔTMT, DS, and semantic fluency), revealing that individuals with AD dementia with worse EF had a lower probability of prior fall history. These findings were influenced by a low percentage of reported falls in people with severe AD dementia. Further stratified analysis of the interaction effects showed that DS and semantic fluency were significantly associated with fall history in people without AD dementia. While some EF assessments were individually associated with falls, interpretations should be made cautiously, especially considering the appropriateness of the assessments for specific populations. The counterintuitive result may stem from factors specific to severe AD dementia. In conclusion, this study uncovers complex interactions between cognitive status, EF assessments, and fall history, and the potential for further investigation to enhance our understanding of falls in individuals with severe AD dementia.