Phenazopyridine Inhibits Bladder Mechanosensory Signalling via the Bladder Lumen

Phenazopyridine (PAP) is an over-the-counter urinary analgesic that temporarily relieves dysuria, urgency, frequency, and pain during urinary tract infection (UTI). Sensitisation of bladder sensory nerves drives pathological bladder sensations, and systemic PAP suppresses afferent signalling during bladder distension. However, it remains unclear how PAP provides localised pain relief to the bladder without systemic analgesia or analgesia-associated side effects. Given the limited metabolism of PAP and its rapid excretion into the bladder following oral administration, we hypothesised that PAP may act locally within the bladder during urine storage, where it targets bladder-innervating sensory nerves following urothelial diffusion. To test this hypothesis, we used an ex vivo bladder preparation for sensory nerve recording to determine mechanosensory responses to bladder distension (0–50 mm Hg) following intravesical infusion of saline (0.9% NaCl, 100 μl/min) or PAP (100–300 μM) in mice (n = 13). Raw nerve responses were recorded and mechanosensory responses were determined as action potential firing in response to bladder filling, as previously described. Single mechanosensitive afferent units were characterised according to the activation threshold as either low-threshold (LT) or high-threshold (HT) units. Changes in the peak firing rate, total firing rate (area under the curve), and activation threshold were determined before and after PAP administration using Spike 2 software (Spike Software Solutions, Bournemouth, UK).