Immunity to Nerve Growth Factor Prevents Afferent Plasticity Following Urinary Bladder Hypertrophy

A neurologic basis for the overactive bladder

OBJECTIVES

The functions of the lower urinary tract (LUT) to store and periodically release urine are dependent on neural circuits in the brain and spinal cord. This paper reviews the central neural control of micturition and how disruption of this control can lead to bladder overactivity and incontinence.

METHODS

Neuroanatomic, electrophysiologic, and pharmacologic techniques have provided information about the neural circuitry and the neurotransmitters involved in the central nervous control of voiding. Experimental models of neural injury, including spinal cord transection, cerebral infarction, and localized brain lesions, have been studied to identify the mechanisms contributing to the neurogenic overactive bladder.

RESULTS

Normal storage of urine is dependent on 1) spinal reflex mechanisms that activate sympathetic and somatic pathways to the urethral outlet and 2) tonic inhibitory systems in the brain that suppress the parasympathetic excitatory outflow to the urinary bladder. Voiding is mediated by inhibition of sympathetic-somatic pathways and activation of a spinobulbospinal parasympathetic reflex pathway passing through a micturition center in the rostral pons. Damage to the brain can induce bladder overactivity by reducing suprapontine inhibition. Damage to axonal pathways in the spinal cord leads to the emergence of primitive spinal bladder reflexes triggered by C-fiber bladder afferent neurons. The C-fiber afferent neurotoxin capsaicin, administered intravesically, has been useful in treating certain types of neurogenic bladder overactivity.

CONCLUSIONS

The central nervous mechanisms controlling the LUT are organized in the brain and spinal cord as simple on-off switching circuits that are under voluntary control. Damage to central inhibitory pathways or sensitization of peripheral afferent terminals in the bladder can unmask primitive voiding reflexes that trigger bladder overactivity.

The role of nerve growth factor in a model of visceral inflammation

There is growing evidence that nerve growth factor may be an important mediator of the sensory disorders associated with inflammation. This hypothesis was tested in a rat model of cystitis. In this model, an experimental inflammation is created in anaesthetized rats with an irritant chemical. Within 1 h, bladder reflexes, activated by the sensory innervation of this viscus, become exaggerated, mimicking the disorders seen in humans with chronic cystitis. The development of this hyper-reflexia following experimental inflammation was quantified using the technique of repeated cystometrograms. By several measures, bladder reflex excitability increased about three-fold after 5 h. Firstly, the study investigated whether inflammatory changes can be prevented by pharmacological antagonism of nerve growth factor. A synthetic fusion protein was used, consisting of the extracelluar domain of the high-affinity nerve growth factor receptor, trkA, coupled to the Fc portion of an immunoglobulin. Previous work has shown that this molecule can sequester nerve growth factor and reduce its bioavailability both in vitro and in vivo. Treatment of animals with the fusion molecule at 1 mg/kg, immediately before inflammation of the bladder, largely, and very significantly, prevented the expected increases in reflex excitability of this organ. Pretreatment with a related fusion protein, capable of sequestering the neurotrophin brain-derived neurotrophic factor and neurotrophin-4/5, but not nerve growth factor, was without effect. Similarly, a control fusion molecule, without neurotrophin-sequestering capacity, did not reduce the inflammation-induced hyper-reflexia. Systemic treatment with the nerve growth factor-sequestering molecule, but not control molecules, partially and significantly reversed established inflammatory changes, by about 30-60%, depending on outcome measure. The nerve growth factor-sequestering protein had no significant effects on bladder reflex excitability in the uninflamed state. It was also without significant effect on capsaicin-induced contractions of the urinary bladder. Administration of exogenous nerve growth factor into the lumen of the urinary bladders of normal anaesthetized rats produced a rapid and marked bladder hyper-reflexia similar to that seen with experimental inflammation. These findings are consistent with other circumstantial evidence that nerve growth factor may interact with visceral sensory systems. Together, the data strongly suggest that nerve growth factor produced in inflamed tissues is a critical mediator of the sensory disorders associated with inflammation.