Presenting Author:

Sandra Hackelberg, Ph.D.

Principal Investigator:

Daniela Menichella, M.D.

Department:

Neurology, Ken and Ruth Davee Department

Keywords:

neuropathy, pain, diabetes, DRG, DREADD, hyperexcitability

Location:

Third Floor, Feinberg Pavilion, Northwestern Memorial Hospital

B105 - Basic Science

Probing diabetic DRG neuron hyperexcitablity with DREADD receptors

Neuropathic pain is a devastating complication affecting about 25 % of diabetes patients. It is characterized by hyperactivity of nociceptors in the dorsal root ganglia, leading to the activation of pain pathways in the absence of appropriate stimuli. Neuronal damage is further evidenced by distal small fiber degeneration. Despite the high prevalence of diabetic neuropathy and the accompanying impairment of patient quality of life, there is little knowledge on the mechanisms of neuronal damage and disease progression. As a consequence, treatment is ineffective and limited to symptom mitigation. In this regard, strict glycemic control remains the only effective treatment option for the prevention of disease progression. Pain management frequently involves the use opioids, but these are both ineffective and problematic in long term use. Nociceptor hyperexcitability is thought not only to be a consequence of damage leading to pain symptoms, but also a pivotal part of the insult leading to progression of neuronal degeneration. Thus, the prevention and mitigation of nociceptor hyperexcitability is central both to the prevention of disease progression and symptom management. Designer receptors exclusively activated by designer drugs (DREADD) are a powerful tool modulate the activity of neuronal circuits. DREADD receptors are G-protein coupled receptors that have been modified to be selectively activated by the systemically inert designer drug clozapine-N-oxide (CNO). Here, we have utilized inhibitory and excitatory DREADDs to modulate the excitability of dorsal root ganglion (DRG) neurons in vitro and in vivo in the high fat diet mouse model of type II diabetes. For targeting of nociceptors, transgenic mice for the inhibitory hM4 DREADD and the excitatory hM3 DREADD were crossed with Nav1.8-tdTomato-Cre mice. The population of Nav1.8 DRG consists of C-nociceptors (>90%), as well as low-threshold C-mechanoreceptors and a lower percentage of Aδ nociceptors and Aβ afferents. Electrophysiological recordings in vitro confirmed the inhibitory and excitatory modulation of Nav.1.8-tdTomato DRG neurons by DREADD activation. Moreover, activation of inhibitory DREADD receptors in vivo reversed small fiber degeneration and mechanical allodynia in von Frey behavioral tests. In contrast, in vivo activation of excitatory DREADD accelerated the onset of small fiber degeneration. This study reveals for the first time a critical role of Nav 1.8 nociceptors hyper-excitability in the pathogenesis of neuropathic pain and small fiber neuropathy in diabetes. Furthermore, these observations will add to our understanding of how changes in nociceptors excitability contribute to the progression of small fiber neuropathy in PDN, which is a critical barrier to progression for effective and disease modifying treatment of this currently intractable and widespread affliction.