No abstract

No abstract

Sensory neurons innervating different tissues converge onto second-order neurons in the spinal cord. We examined whether inflammation or transient overexpression of nerve growth factor (NGF) in one tissue triggers hypersensitivity in referral sites. Thresholds to mechanical and thermal stimulation of the hindpaw, visceromotor responses to colorectal distension, and cystometrograms were performed in appropriate controls and mice with experimentally induced cystitis, inflammation of the hindpaw or front paw, or injection of viral vectors encoding NGF or green fluorescent protein (GFP). Cystitis and NGF but not GFP overexpression in the bladder triggered bladder hyperactivity associated with mechanical and thermal hypersensitivity in cutaneous referral sites and enhanced responses to colorectal distension. Hindpaw inflammation and injection of the NGF- but not GFP-encoding viral vector or front paw inflammation induced mechanical and thermal hyperalgesia in the affected hindpaw and increased responses to colorectal distension without altering the micturition reflex. In conclusion, sensitization of sensory pathways by inflammation or NGF contributes to the development of hypersensitivity in neighboring organs and cutaneous referral sites and provides a potential mechanism underlying the coexistence of pain syndromes in patients with functional diseases.

We studied sensitization of retrogradely labeled bladder sensory neurons and plasticity of P2X receptor function in a model of cystitis using patch-clamp techniques. Saline (control) or cyclophosphamide (CYP) was given intraperitoneally to rats on days 0, 2, and 4. On day 5, lumbosacral (LS, L6-S2) or thoracolumbar (TL, T12-L2) dorsal root ganglia were removed and dissociated. Bladders from CYP-treated rats showed partial loss of the urothelium and greater myeloperoxidase activity compared with controls. Bladder neurons from CYP-treated rats were increased in size (based on whole cell capacitance) compared with controls and exhibited lower activation threshold, increased action potential width, and greater number of action potentials in response to current injection or application of purinergic agonists. Most control LS bladder neurons (>85%) responded to ATP or α,β-metATP with a slowly desensitizing current; these agonists affected only half of TL neurons, producing predominantly fast/mixed desensitizing currents. CYP treatment increased the fraction of TL bladder neurons sensitive to purinergic agonists (>80%) and significantly increased current density in both LS and TL bladder neurons compared with control. Importantly, LS and TL neurons from CYP-treated rats showed a selective increase in the functional expression of heteromeric P2X 2/3 and homomeric P2X 3 receptors, respectively. Although desensitizing kinetics were slower in LS neurons from CYP-treated compared with control rats, recovery kinetics were similar. The present results demonstrate that bladder inflammation sensitizes and increases P2X receptor expression and/or function for both pelvic and lumbar splanchnic pathways, which contribute, in part, to the hypersensitivity associated with cystitis.

No abstract