Synaptic remodeling in the hypothalamus during obesity

Synapses are highly specialized sites that play a critical role in communication between neurons within the central nervous system. Morphological parameters such as the size, shape, and type of synapse have a direct correlation with neuronal function. However, synapses are dynamic structures that can undergo morphological modifications. This continual remodeling (e.g., synaptic scaling) of individual synaptic structures is an essential feature of synaptic plasticity in order to maintain optimal brain networks. While synaptic scaling is beneficial in the short term for homeostatic function, chronic changes in the synapse architecture can fundamentally alter network performance. In the context of obesity, studies evaluating synapse anatomy are limited. Moreover, the techniques employed, while useful, have been limited in scope by only evaluating a small region of interest or randomly selected synapses. To fully understand anatomical synaptic plasticity, it is critical to evaluate a large area within a brain nucleus. With this in mind, we implemented a large field of view high-resolution backscattered scanning electron microscopy (SEM) approach to characterize thousands of neuronal synapses from an individual animal during obesity. Six week old C57Bl/6J male mice were fed a high fat diet (HFD) or normal chow for 10 weeks (n=4/group), and brains were prepared for SEM imaging. We focused on the paraventricular nucleus of the hypothalamus (PVN) because it is a critical endocrine and autonomic region that has been implicated in obesity related diseases. We first acquired interactive zoomable maps of the PVN at a low magnification. This image was then used to navigate to the PVN and acquire a high-resolution and high magnification (~80,000x) image encompassing ~30-40% of the nucleus. Segmentation using ARIVIS software was then performed by manual identification and tracing of all neuronal synapses. Using this approach, we were able to identify and perform subsequent analysis on 44,501 individual synapses. Interestingly, the total number of synapses within the PVN was similar between normal chow and HFD fed animals (46±3 vs 39±4 synapses/μm 2 , normal chow vs HFD, p=0.22). Further sub-characterization also revealed no difference between diet groups including synapses with mitochondrial inclusion (14±1 vs 11±2 synapses/μm 2 , normal chow vs HFD, p=0.14) and those with an active zone (8±1 vs 5±1 synapses/μm 2 , normal chow vs HFD, p=0.13). Despite the similar number of synapses, the mean area of individual synapses was increased in response to HFD (0.458±0.003 vs 0.484±0.004 μm 2 , normal chow vs HFD, p<0.0001). In line with this, frequency distribution analysis also indicated a shift towards larger synapses within the PVN during obesity. Collectively, these findings highlight an SEM approach to image thousands of synapses within a single brain region and further indicate PVN synaptic alterations during obesity that may contribute to altered hypothalamic function. R01DK117007, R01HL141393, POST898956 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.