Gating of the micturition reflex by tonic activation of bladder cold receptors in the cat.

Aims: To determine whether C afferents can modify the gating of the Adelta micturition reflex in order to identify the neuronal site of interaction of the two afferent systems.

Methods: Adult female cats, anaesthetized with alpha-chloralose, had their bladder and urethra catherized through a slit in the proximal urethra. Micturition threshold volume was assessed by cystometry and bladder efferent activity recorded simultaneously. The bladder was filled at a slow rate (1.2-3.5 ml/min) with either body-warm saline (control) or menthol solution (0.06 mM) or by cold saline (4 degrees C).

Results: Of 14 trial sessions in 5 animals, the threshold volume of the Adelta micturition reflex was consistently reduced by menthol infusions from a control median (md) value of 16.8 to 10.2 ml (P < 0.01). The threshold pressure was also somewhat decreased from md 0.7 to 0.5 kPa (P < 0.05), while the peak pressure or pressure slope did no differ in two situations. Similar results were obtained with slow cold infusions into the bladder (nine sessions in three animals). The threshold volume decreased from md 19.8 to 17.4 ml (P < 0.05). The bladder reflex response to slow menthol or cold infusions had the typical features of an Adelta micturition reflex in that the efferent activity was largely abolished by the bladder Adelta mechanoreceptor unloading.

Conclusions: Gradual tonic activation of bladder cold receptors lowers the threshold volume of the ordinary Adelta micturition, pointing to a segmental spinal mechanism for the gating of the micturition reflex.