Roles for pain modulatory cells during micturition and continence

We studied how the nervous system selects between noxious stimulus-evoked withdrawals and micturition, movements that are necessary for survival but use overlapping muscles and therefore cannot occur simultaneously. In lightly anesthetized rats, micturition was favored, because noxious stimulation never interrupted micturition, whereas withdrawals were suppressed during voiding. Neurons in the ventromedial medulla (VMM) are a major source of descending antinociceptive signals. To test whether VMM neurons support withdrawal suppression during micturition, the discharge of VMM neurons was recorded during continence and micturition. VMM cells that were inhibited (M-inh) or excited (M-exc) during micturition were observed. M-inh cells were excited by noxious cutaneous stimulation and thus are likely nociception facilitating, whereas M-exc cells were inhibited by noxious heat and are likely nociception inhibiting. The excitation of nociception-inhibiting M-exc and inhibition of nociception-facilitating M-inh cells predicts suppression of withdrawals during micturition. M-exc cells were typically silent before micturition, whereas most M-inh cells fired before micturition, suggesting that these cells may also play a preparatory role for micturition. To test this idea, we examined manipulations that either advanced or delayed the onset of micturition. Hypothalamic stimulation and noxious paw heat advanced micturition while exciting M-inh cells and inhibiting M-exc cells. In contrast, colorectal distension, a stimulus that delays micturition, inhibited M-inh cells and excited M-exc cells. These results suggest a model in which, during continence, VMM M-inh cells facilitate and M-exc cells inhibit bladder afferents, advancing micturition onset when M-inh cells are activated and delaying onset when M-exc cells are activated.