Alpha 2-adrenoceptors mediate the inhibition of cholinergic transmission in parasympathetic ganglia of the rabbit urinary bladder.

Intracellular recordings were made from neurons of vesical parasympathetic ganglia (VPG) isolated from the rabbit urinary bladder and maintained, in vitro. Bath-application of norepinephrine (NE, 500 nM-5 microM) caused a hyperpolarizing response at the postsynaptic membrane of VPG neurons in a concentration-dependent manner. NE blocked the action potential elicited by an orthodromic stimulation of preganglionic (pelvic) nerve fibers. At a relatively low concentration (5-100 nM), NE depressed the fast excitatory postsynaptic potential (EPSP), without producing the hyperpolarization. NE (100 nM) produced 49 +/- 17% (N = 5) decrease in the amplitude of the fast EPSP. NE did not depress the acetylcholine (ACh) potential produced by iontophoretic application of ACh to the ganglion cells. NE did not affect the amplitude of the miniature EPSP, while it reduced the frequency of miniature EPSPs. These results suggest that NE inhibits the nicotinic transmission in the rabbit VPG, probably reducing the ACh release from presynaptic nerve terminals. Epinephrine (1 microM) was more potent than NE (1 microM) in producing the hyperpolarization as well as the blockade of the fast EPSP amplitude. Isoproterenol was ineffective as an agonist for these inhibitory adrenoceptors. Clonidine mimicked the effect of NE on the fast EPSP. Yohimbine and idazoxan antagonized both the inhibition of the fast EPSP and the hyperpolarization produced by NE. These results suggest that alpha 2-adrenoceptors are responsible for the inhibition of the neuronal activity in parasympathetic ganglia of the rabbit urinary bladder. Immunohistochemical study demonstrated the presence of tyrosine hydroxylase (TH)-labelled neuronal elements in the VPG. They were a small proportion of principal neurons, their dendrites, and many varicose fibers.(ABSTRACT TRUNCATED AT 250 WORDS)