Cyclophosphamide is a chemotherapeutic drug that is effective in the treatment of various cancers. However, this drug induces severe chemical cystitis in animals and humans. It is well established that this cystitis induces bladder overactivity, characterized by decreased bladder capacity and increased micturition frequency [20
]. Thus, we used cyclophosphamide to induce OAB model in rats.
Bladder filling and storage processes require accommodation to increasing urine volumes at low intravesical pressure with appropriate sensation, still sphincter remains closed during increasing in intra-abdominal pressure, and involuntary bladder contraction not appear [21
]. For these reasons, normal detrusor function allows bladder filling during the storage phase, with little or no change in bladder pressure [1
]. In contrast, OAB is the observation of involuntary detrusor contraction, and both increased bladder pressure and time [21
]. The present study showed that contraction pressure and time were significantly increased after cyclophosphamide injection, indicating that cyclophosphamide deteriorated bladder function and as a result, induced OAB.
The PMC play an important role in the control of urinary bladder function. Activation of PMC neurons induces bladder contractions and relaxation of the bladder neck and external urethral sphincter, which results in micturition. Two regions associated with the PMC are PAG and MPA of the hypothalamus [5
]. The PAG-PMC projection is considered to take part in the micturition reflex. The vlPAG acts as a central sensorimotor integrative relay of the micturition reflex, via the reception of sensory information concerning bladder fullness and the direct projection to the PMC [6
]. In addition, the PMC is densely innervated by the MPA [6
]. The MPA sends projections to the PMC that synapse on neurons directly though projections to the spinal cord [5
]. Bladder filling information from sensory interneurons in the lumbar spinal cord finally reaches to PMC in order to empty the bladder at appropriate time [22
]. Electrical or chemical stimulation on the lower urinary tract was shown to cause changes in neuronal activity in the micturition centers, such as the PMC, PAG, MPA, and spinal cord [22
]. Bon et al. [23
] reported that injection of cyclophosphamide caused cystitis and increased number of c-Fos-positive cells in the PMC. Noxious stimulation of deep structures, such as muscle, joint, and viscera provoked a significant increase in the number of c-Fos-positive neurons in the vlPAG [24
]. Chung et al. [25
] indicated that chemical irritation on bladder provoked c-Fos expression in the PAG and MPA of the rats. Acute or chronic bladder irritation increased immediate early gene expression in the spinal neurons [7
]. In the present results, expressions of c-Fos in the neuronal voiding centers (MPA, vlPAG, PMC, spinal cord [L4
] regions) were significantly increased after cyclophosphamide injection, indicating that induction of the OAB activated neurons in the voiding centers.
The exact mechanisms by which NO in the neuronal voiding centers modulates micturition are not fully understood. However, because NO regulates components of the lower urinary tract such as the bladder and urethra, NO levels in the neuronal voiding centers may also control the lower urinary tract status [26
]. Gene expression and neurotransmitter synthesis in the brain can be altered by pain, peripheral irritation, and inflammation [26
]. In the present results, expressions of NOS in the neuronal voiding centers (MPA, vlPAG, PMC, spinal cord [L4
] regions) were significantly increased after cyclophosphamide injection, indicating that induction of the OAB enhanced NO levels in the neuronal voiding centers.
As shown in this study, we suggest the relation of cyclophosphamide-induced expressions of c-Fos and NO in the neuronal voiding centers with cyclophosphamide-induced OAB symptoms. Enhanced expressions of c-Fos and NOS in the voiding centers activate neurons in the voiding centers and trigger OAB symptoms.
Increasing of NO production by iNOS exerts detrimental effect on bladder, such as overactive bladder [11
]. Up-regulation of iNOS was detected after cyclophosphamide injection, lipopolysaccharide instillation, and during urinary obstruction [28
]. iNOS expression is closely implicated in the bladder inflammation and injury [11
]. Inhibition of iNOS activity in OAB animal model significantly attenuated the increase of bladder size and bladder contraction number and suppressed bladder fibrosis [29
]. In the present results, iNOS expression in the bladder was significantly increased after cyclophosphamide injection, indicating that cyclophosphamide induced inflammation of bladder.
Change of α1
-adrenoceptors expression in human blood vessels, spinal cord, ganglia, and nerve terminals has been postulated to contribute to the pathogenesis of lower urinary tract symptoms [30
]. Activation of α1
-adrenoceptors in bladder causes OAB symptoms, such as outflow obstruction, bladder instability, and frequent micturition [31
]. Thus, the beneficial effect of α1
-adrenergic receptor antagonists is a class effect in micturition function. Treatment with α1
-adrenergic receptor antagonists relieves bladder obstruction, and may control voiding function by acting at bladder wall and spinal cord [32
]. In human research, daytime frequency, nocturia, and incontinence episodes were significantly decreased with α1
-adrenergic receptor antagonists [33
]. Moreover, α1
-adrenergic receptor antagonists significantly increased bladder capacity and decreased frequency [34
]. In the present results, OAB-evoked increase of contraction pressure and time in the bladder was significantly suppressed by tamsulosin treatment. This suppressing effect of tamsulosin on contraction pressure and time can be ascribed to antagonizing effect of tamsulosin at α1
-adrenergic receptors in the detrusor muscle or at prejunctional α1
-adrenoceptors. This inhibition of tamsulosin may induce alteration of NO level in the lower urinary tract. The present study showed that iNOS protein expression in the bladder was significantly decreased by tamsulosin treatment. These results reveal that tamsulosin suppressed bladder inflammation, and as a result, alleviated the cyclophosphamide-induced OAB symptoms.
During the storage phase, continence is maintained by inhibition of the parasympathetic system and by activation of the sympathetic system, through the acting of α1
-adrenergic receptors, leads to compression of urethral sphincter and bladder. Conversely, during voiding, the PMC inhibits the sympathetic system and activates the parasympathetic system, resulting in a sustained relaxation of urethral sphincter and bladder [16
]. In the present results, antagonizing effect of tamsulosin on α1
-adrenergic receptors suppressed the OAB-induced expressions of c-Fos and NOS in the neuronal voiding centers. The inhibitory effects of tamsulosin on the expressions of c-Fos and NOS appeared most potently at the 0.01 mg/kg dose. These results show that tamsulosin may exert inhibitory effect on neuronal activation in the voiding centers and delay triggering of OAB symptoms.
From the present results, α1-adrenergic receptor antagonist tamsulosin may overcome OAB-induced micturition dysfunction through inhibition of neuronal voiding centers, thus we raise the possibility that tamsulosin is effective therapeutic modality for ameliorating the symptoms of OAB.