Immunocytochemical techniques were used to examine alterations in the expression of neuronal nitric oxide synthase (NOS) in bladder pathways following acute and chronic irritation of the urinary tract of the rat. Chemical cystitis was induced by cyclophosphamide (CYP) which is metabolized to acrolein, an irritant eliminated in the urine. Injection of CYP (n = 10, 75 mg/kg, i.p.) 2 hours prior to perfusion (acute treatment) of the animals increased Fos-immunoreactivity (IR) in neurons in the dorsal commissure, dorsal horn, and autonomic regions of spinal segments (L1-L2 and L6-S1) which receive afferent inputs from the bladder, urethra, and ureter. Fos-IR in the spinal cord was not changed in rats receiving chronic CYP treatment (n = 15, 75 mg/kg, i.p., every 3rd day for 2 weeks). In control animals and in animals treated acutely with CYP, only small numbers of NOS-IR cells (0.5-0.7 cell profiles/sections) were detected in the L6-S1 dorsal root ganglia (DRG). Chronic CYP administration significantly (P < or = .002) increased bladder weight by 60% and increased (7- to 11-fold) the numbers of NOS-immunoreactive (IR) afferent neurons in the L6-S1 DRG. A small increase (1.5-fold) also occurred in the L1 DRG, but no change was detected in the L2 and L5 DRG. Bladder afferent cells in the L6-S1 DRG labeled by Fluorogold (40 microliters) injected into the bladder wall did not exhibit NOS-IR in control animals; however, following chronic CYP administration, a significant percentage of bladder afferent neurons were NOS-IR: L6 (19.8 +/- 4.6%) and S1 (25.3 +/- 2.9%). These results indicate that neuronal gene expression in visceral sensory pathways can be upregulated by chemical irritation of afferent receptors in the urinary tract and/or that pathological changes in the urinary tract can initiate chemical signals that alter the chemical properties of visceral afferent neurons.