Guanfacine Normalizes the Overexpression of Presynaptic α-2A Adrenoceptor Signaling and Ameliorates Neuropathic Pain in a Chronic Animal Model of Type 1 Diabetes

Abstract

Background: Diabetes is associated with several complications, including neuropathic pain, which is difficult to manage with currently available drugs. Descending noradrenergic neurons possess antinociceptive activity; however, their involvement in diabetic neuropathic pain remains to be explored. Methods: To infer the regulatory role of this system, we examined as a function of diabetes, the expression and localization of alpha-2A adrenoceptors (α2-AR) in the dorsal root ganglia and key regions of the central nervous system, including pons and lumbar segment of the spinal cord using qRT-PCR, Western blotting, and immunofluorescence-based techniques. Results: The data revealed that presynaptic synaptosomal-associated protein-25 labeled α2-AR in the central and peripheral nervous system of streptozotocin diabetic rats was upregulated both at the mRNA and protein levels. Interestingly, the levels of postsynaptic density protein-95 labeled postsynaptic neuronal α2-AR remained unaltered as a function of diabetes. These biochemical abnormalities in the noradrenergic system of diabetic animals were associated with increased pain sensitivity as typified by the presence of thermal hyperalgesia and cold/mechanical allodynia. The pain-related behaviors were assessed using Hargreaves apparatus, cold-plate and dynamic plantar aesthesiometer. Chronically administered guanfacine, a selective α2-AR agonist, to diabetic animals downregulated the upregulation of neuronal presynaptic α2-AR and ameliorated the hyperalgesia and the cold/mechanical allodynia in these animals. Conclusion: Together, these findings demonstrate that guanfacine may function as a potent analgesic and highlight α2-AR, a key component of the descending neuronal autoinhibitory pathway, as a potential therapeutic target in the treatment of diabetic neuropathic pain.

Activation of 5-HT1A Receptors Normalizes the Overexpression of Presynaptic 5-HT1A Receptors and Alleviates Diabetic Neuropathic Pain

“…in STZ-induced T1DMin the lumbar segment of the spinal cord (LSSC, segment L1-L4) and dorsal root ganglia (DRG) of the spinal nerves tissue sections. Similar to what we observed before [ 27 ], the T1DM LSSC tissue sections revealed Nissl body loss and pyknosis, eosinophilic cytoplasm, ischemic neuronal cells, and glial cells with cellular edema ( Figure 3 B), which were not observed in those of control rats ( Figure 3 A). Furthermore, 8-OH-DPAT treatment improved the LSSC cellular structure but did not completely reverse it to the control LSSC histology ( Figure 3 C).…”

“…Similar to what we observed before [ 27 ], the H&E staining of the T1DM DRG tissue sections showed notable neuronophagia with basophilic, vacuolar-like defects and pigmentary inclusions, and satellitosis ( Figure 3 E) that was absent in the control DRG tissue section ( Figure 3 D). Moreover, treatment of T1DM rats with 8-OH-DPAT resulted in an improvement in the DRG tissue structure ( Figure 3 F).…”

“…injection of streptozotocin (STZ, 55 mg/kg). We have recently shown that diabetic rats developed hyperglycemia, exhibited decreased body weight, increased feed/water consumption and lowered nociceptive sensitivity in response to thermal (heat/cold) and mechanical stimuli when compared to those of control rats [ 27 ].…”

“…We recently showed that SNAP-25 is a biochemical marker for presynaptic LSSC tissues, while PSD-95 is for postsynaptic LSSC tissues [ 27 ]. Double-immunofluorescent staining with 5-HT1AR and SNAP-25 ( Figure 5 C) specific antibodies demonstrated expression and co-localization of these proteins in the LSSC presynaptic tissues.…”

“…Although 5-HT1AR and α2-AR may be regarded as targets for therapeutic drugs, there is a lack of potential clinical candidates that have a high degree of selectivity towards either presynaptic and postsynaptic α2-AR or 5-HT1AR. Guanfacine, a selective α2A-AR agonist, and 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT), a selective 5-HT1AR agonist, have demonstrated therapeutic efficacy in models of pain [ 26 , 27 ]. Among 5-HT receptors, 5-HT1AR is characterized as antinociceptive [ 28 ].…”