Presynaptic inhibitory muscarinic receptors modulating [3H] acetylcholine release in the rat urinary bladder

The occurrence of presynaptic muscarinic receptors inhibiting the release of acetylcholine (ACh) from nerve terminals was investigated in the rat urinary bladder. Strips from the extratrigonal area were preincubated with [3H]choline and stimulated at 0.2 Hz. Both [3H]ACh and [3H]choline content were measured in the tissue. The uptake of tritiated choline was prevented by hemicholinium-3. The field stimulation at 2 Hz (360 shocks) produced a release of tritium. Most of the induced outflow was found to be [3H]ACh. Both tetrodotoxin treatment and calcium omission from the medium prevented such an evoked-outflow of tritium. When two electrical stimulations (S1 and S2) at 2 Hz (360 shocks) were carried out at 45-min intervals, an S2/S1 ratio of 0.82 was found. Physostigmine reduced the evoked-release of [3H]ACh whereas atropine increased it in a concentration-dependent manner. Atropine antagonized the inhibitory effect of physostigmine, so that the S2/S1 ratio did not vary significantly from control experiments. Both carbachol and muscarine strongly decreased the [3H]ACh evoked outflow. Muscarine increased the spontaneous outflow of tritium also. These findings suggest that the urinary bladder of the rat is equipped with presynaptic inhibitory muscarinic receptors modulating ACh release from cholinergic postganglionic neurons.