Norepinephrine inhibits calcium action potential through alpha 2-adrenoceptors in rabbit vesical parasympathetic neurons

Neural control of the lower urinary tract

This article summarizes anatomical, neurophysiological, pharmacological, and brain imaging studies in humans and animals that have provided insights into the neural circuitry and neurotransmitter mechanisms controlling the lower urinary tract. The functions of the lower urinary tract to store and periodically eliminate urine are regulated by a complex neural control system in the brain, spinal cord, and peripheral autonomic ganglia that coordinates the activity of smooth and striated muscles of the bladder and urethral outlet. The neural control of micturition is organized as a hierarchical system in which spinal storage mechanisms are in turn regulated by circuitry in the rostral brain stem that initiates reflex voiding. Input from the forebrain triggers voluntary voiding by modulating the brain stem circuitry. Many neural circuits controlling the lower urinary tract exhibit switch-like patterns of activity that turn on and off in an all-or-none manner. The major component of the micturition switching circuit is a spinobulbospinal parasympathetic reflex pathway that has essential connections in the periaqueductal gray and pontine micturition center. A computer model of this circuit that mimics the switching functions of the bladder and urethra at the onset of micturition is described. Micturition occurs involuntarily in infants and young children until the age of 3 to 5 years, after which it is regulated voluntarily. Diseases or injuries of the nervous system in adults can cause the re-emergence of involuntary micturition, leading to urinary incontinence. Neuroplasticity underlying these developmental and pathological changes in voiding function is discussed.

Reduction of the N-type calcium current by noradrenaline in neurones of rabbit vesical parasympathetic ganglia

1. Intracellular and single-electrode voltage-clamp recordings were made from neurones of vesical parasympathetic ganglia (VPG) isolated from the rabbit urinary bladder. 2. Noradrenaline (NA, 0.5-5 microM) shortened the duration of the action potentials and depressed the amplitudes of both spike after-hyperpolarization and after-current. 3. Voltage-dependent calcium currents (ICa) were recorded by using microelectrodes filled with 2 M-caesium chloride in a superfusing solution containing tetraethylammonium (TEA, 50 mM) and tetrodotoxin (TTX, 500 nM). Noradrenaline (0.5-5 microM) depressed both the ICa and the tail current evoked by depolarizing voltage jumps from -100 to -50 mV to -30 to +20 mV. 4. Substitution of barium for calcium also produced an inward current (IBa) with no obvious tail current. Noradrenaline (1 microM) reduced the magnitude of the IBa without affecting the voltage dependence of the current-voltage relationship for IBa. 5. Yohimbine (1 microM), but not prazosin (1 microM) or propranolol (1 microM), antagonized the NA-induced inhibition of the IBa. UK 14304, a potent alpha 2-adrenoceptor agonist, mimicked NA in depressing the IBa. 6. The transient low-threshold (T), the transient high-threshold (N) and the slowly inactivating high-threshold (L) calcium currents co-existed in VPG neurones. 7. Noradrenaline reduced the IBa evoked at clamp potentials more positive than -20 mV from holding potentials near the resting membrane potential (-70 to -50 mV). Under these conditions, the IBa consisted primarily of N- and L-current components. In contrast, NA had no effect on the isolated T- and L-currents. It is concluded that NA selectively inhibits the N-type calcium channels by an action at alpha 2-adrenoceptors in the rabbit VPG neurones.

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)