Peripheral Sensory Neuropathy in Nile Grass Rat Model of Type-2 Diabetes

Jyoti Singh
Peripheral sensory neuropathy (PSN) is a chronic neurological disease affecting over half of type-2 diabetes (T2D) patients worldwide. It presents with multiple devastating, often progressive symptoms such as numbness, altered sensitivity to temperature, tactile allodynia, hyperalgesia, and paresthesia. PSN typically affects the feet first and gradually progresses to hands in a 'stockingglove' pattern. These symptoms manifest primarily due to dysfunction of distal peripheral sensory nerves;
more » ... sensory nerves; however, pathologic alterations may also occur at their dorsal root ganglion (DRG) neuron (DRGN) somata. Although animal models have been used to elucidate mechanisms underlying T2D-PSN pathogenesis, none of these models displays the full spectrum of the human T2D complications suggesting the requirement of a new model. Male African Nile grass rats (NGRs) serve as a potential T2D model as they show human-like T2D progression when fed with a high calorie, low fiber ('chow') diet. It has been published that chow-fed NGRs develop insulinresistance, hyperinsulinemia, increased body weight and pancreatic β-cell dysfunction among other human T2D outcomes. Therefore, I hypothesized that T2D NGRs also develop PSN. To address this, I initially demonstrated in 12-24 months-old T2D NGRs that these animals display hallmark PSN pathophysiological features. First, I demonstrated with immunohistochemistry retraction of the sensory nerve fibers in the epidermal skin of T2D NGR hind foot. Next, using Von Frey filaments and a Hargreaves apparatus, I found in behavioral tests that the plantar hind paw has decreased sensitivity to mechanical and heat stimuli. Using further immunohistochemical approaches, I demonstrated that the diverse DRGN subpopulations are still viable in T2D while their mRNA and protein levels are upregulated for the voltage-gated sodium channel (Nav) variants Nav 1.7 and Nav 1.9, but not Nav1.8. Also, a significant portion of satellite glial 'helper' cells that iii surround DRGNs are activated and DRGs were infiltrated with macrophages, both processes indicating trauma and inflammation. In a further set of experiments, I established the 'skin-nerve' preparation that has so far been used only for other rat and mouse species, for studying neurophysiological properties of sensory saphenous nerve fibers innervating the NGR dorsal foot skin. I first demonstrated the viability of this 'ex-vivo' preparation by manually probing the dermis side of the isolated skin patch via mechanical stimulation with a rod to evoke single-fiber action potentials (SFAPs) from different receptive field (RF) points and determine their adaptation behavior. With subsequent electrical stimulation, I determined the threshold and latency of activation of the SFAP and identified it as either a Aδ-or C-fiber based on nerve conduction velocity. Next, SFAPs evoked by both rod and electrical stimulation at one RF point were compared with those evoked with Von Frey filament stimulation whose mechanical excitation thresholds were determined. It turned out that SFAP responses evoked with Von Frey filaments were from different fibers and therefore they were grouped as 'mechanosensitive fibers'. In T2D NGRs, the amplitude and threshold of electricallyevoked SFAPs were decreased for both Aδ-and C-fibers and mechanosensitive fibers also had a lower mechanical threshold. In summary, the above human-like PSN pathological features established NGRs as a novel model to study T2D-PSN. NGRs serve as a potential model that, unlike any other existing model, has a unique advantage to investigate the onset and early progression of T2D-PSN and therefore provides an opportunity to develop novel approaches for effective therapeutic interventions. iv PREFACE The thesis is an original work by Jyoti Singh. The research project, of which this thesis is a part,
doi:10.7939/r3-bqgs-7b30 fatcat:ycc7pjyhevh7dfswmdxlqy3e5a