Introduction and Objective

Chronic pelvic pain (CPP) is a bothersome and highly prevalent symptom present in many chronic disorders of the pelvis. Nociceptive sensitization in the central¹ and peripheral nervous systems² has been identified as a key element underlying the generation and maintenance of CPP symptoms, including visceral pain, urgency, and urinary frequency. The resident glial cells in sensory ganglia, namely satellite glial cells (SGCs), effectively isolate individual ganglionic neurons and regulate neuronal activity.^3-6 We previously showed that activation of Gq-G protein coupled receptors (Gq-GPCR) in SGCs produced strong analgesia in vivo, which reversed inflammation-induced hind paw hypersensitivity.⁷ This study aims to test the hypothesis that activating satellite glial Gq-GPCR signaling pathways in lumbosacral sensory ganglia alleviates visceral hypersensitivity and therefore modulates micturition in vivo.

Methods

To evaluate the analgesic potential of satellite glial Gq-GPCR signaling, a vascular endothelial growth factor (VEGF_A)-induced mouse model of CPP was utilized.⁸ Urine VEGF has been identified as one of the urinary biomarkers of urological chronic pelvic pain syndrome in patients.⁹ We have previously shown that repeated intravesical instillations of VEGF_A (6.41 nM in 100 μL), a mouse VEGF recombinant protein, in adult C57BL/6 mice led to increased sensory and cholinergic nerve density in the bladder wall, visceral mechanical hyperalgesia, and nociceptor upregulation in lumbosacral sensory ganglia associated with visceral hypersensitivity.¹⁰ Von Frey tests, spontaneous voiding spot assay, and awake cystometry recordings were used to assess animals’ visceral sensitivity and micturition patterns (Figure 1).

To selectively activate Gq-GPCR signaling pathways in lumbosacral sensory SGCs, Gq-coupled Designer Receptors Exclusively Activated by Designer Drugs (Gq-DREADD, also called hM3Dq) were expressed in glial acidic fibrillary protein (GFAP⁺) expressing glia using Gfap-hM3Dq transgenic mice or targeted adeno-associated viral vector delivery. Clozapine N-oxide (CNO), a small molecule hM3Dq agonist, was administered via intraperitoneal injection to selectively activate Gq-GPCR signaling in GFAP⁺ glia in awake and free-moving mice. Glial signaling-driven changes in visceral mechanical sensitivity and micturition functions were compared in the same animals between with and without CNO administration and between mice expressing functional hM3Dq in SGCs and their wild-type littermate controls.

Results

CNO-induced, hM3Dq-mediated Gq-GPCR activation in lumbosacral SGCs significantly decreased visceral mechanical sensitivity in both Gfap-hM3Dq transgenic mice (Figure 2, A) and C57BL6/J mice receiving intrathecal injection of adeno-associated viral vector 8-GFAP-hM3Dq vectors (Figure 2, B). Such analgesic effect effectively reversed visceral hypersensitivity in VEGF-instilled animals with CPP symptoms (Figure 2) in a dose-dependent manner. Moreover, CNO-induced Gq-GPCR activation in GFAP⁺ glia led to higher urinary output, shorter intermicturition intervals, and decreased micturition bladder pressure compared to those parameters recorded from the same animals prior to CNO administration (Figure 3), suggesting that Gq-GPCR activation in peripheral GFAP⁺ glia enhances micturition frequency and bladder contractions in vivo.

Conclusions

Our data demonstrated that Gq-GPCR activation of sensory glia modulates visceral sensitivity and voiding functions in vivo. Our data suggest the therapeutic potential of manipulating peripheral glial G-GPCR signaling in alleviating visceral pain and enhancing micturition in vivo. Further studies are warranted to investigate the molecular mechanisms underlying the glial modulation of micturition in the peripheral nervous system.

Conflicts of Interest

Dr. Xie has no conflict of interest or disclosure regarding this research activity.