Major classes of sensory neurons to the urinary bladder

A novel in vitro bladder preparation was used to examine effect of various stimuli (stretch, von Frey hair compression, stroking of receptive fields, applications of chemical stimuli to the mucosa) on electrophysiological recordings from guinea pig bladder afferents in vitro. Several functionally distinct classes of bladder sensory neurons were distinguished. These include stretch-sensitive afferents—muscle mechanoreceptors which behaved as “in-series tension receptors” and tension-mucosal mechanoreceptors, which could be activated by stretch, mucosal stroking with light von Frey hair (0.1–2 mN) and by hypertonic solutions (1 M mannitol and 490–850 mM NaCl) applied locally to their receptive fields in the mucosa. In addition, we have recorded stretch-insensitive afferents—mucosal mechanoreceptors and chemoreceptors. The non-selective P2X/P2Y purinoreceptor antagonist, PPADS (30 μM) did not affect stretch-induced firing by low threshold muscle mechanoreceptors but significantly inhibited α,β-methylene ATP (30 μM)-induced contractions and associated afferent firing. Transduction by low threshold stretch-sensitive muscle mechanoreceptors does not appear to involve exocytotic synaptic transmission since it occurs in Ca2+-free (with 1mM EDTA and 6 mM Mg2+) Krebs solution. The data suggest that the endogenous transmitter ATP is not involved in mechanotransduction by this specific class of low threshold muscle mechanoreceptors in the guinea pig bladder; rather they appear to transduce mechanical stimuli directly, possibly via stretch-activated ion channels. Mechanisms of activation of other classes of mechanoreceptors and chemoreceptors remain to be established.