Induction of c-fos expression in spinal neurons by nociceptive and nonnociceptive stimulation of LUT.

Expression of c-fos gene in spinal neurons was detected with an immunocytochemical technique to study the spinal processing of nociceptive and nonnociceptive input from the lower urinary tract (LUT) of the urethan-anesthetized rat. Two preparations were used to activate afferent pathways in the LUT: 1) the urinary bladder was exposed through an abdominal incision, and saline or 1% acetic acid solution was infused directly into the bladder lumen and expelled through the urethra; and 2) the bladder was catheterized through the urethra, and the urethral outlet was ligated to allow distension and reflex contractions to occur under isovolumetric conditions. The first preparation mimicked the physiological changes occurring during normal voiding, whereas the second preparation generated high (presumably nociceptive) pressures when the bladder contracted against a closed outlet. The results indicate that distension-induced voiding increased c-fos expression largely in the region of the sacral parasympathetic nucleus (52% of the total number of cells/L6 section) and the dorsal commissure (25% of the total number of cells), whereas nociceptive stimuli markedly increased the number of c-fos-positive cells in the dorsal commissure (3.5 x increase above the number induced by distension, representing 50% of the total number of cells/L6 section). Bladder contractions against a closed outlet elicited a distribution of c-fos-positive cells similar to that induced by chemical irritation. Drugs that suppressed bladder reflexes did not reduce c-fos induced by distension, indicating that voiding reflexes do not contribute to c-fos expression.