Psychological stress plays an important role in the induction of frequent urination and exacerbation of bladder dysfunction including overactive bladder (OAB) and bladder pain syndrome/interstitial cystitis (BPS/IC).  Psychological stress-related information is conveyed to the brain, and then the brain recruits neuronal and neuroendocrine systems for adaptation to stressful conditions.  However, the brain pathophysiological mechanisms underlying psychological stress-induced effects on bladder function are still unclear.
Previously, we reported that the sympatho-adrenomedullary (SA) system, a representative response to stressful conditions, is activated by centrally administered epibatidine (EP) [1], an agonist of nicotinic acetylcholine receptors (nAChRs).  Although stimulation of brain nAChRs is reported to inhibit the micturition reflex [2], brain mechanisms for the inhibitory effect are not clarified yet.  In this study, therefore, we investigated effects of centrally administered EP on the micturition reflex concerning with their dependence on the SA system in rats.

Urethane anesthetized (0.8 g/kg, ip) male Wistar rats (300-400 g) were used.
(1) Catheters were inserted into the bladder dome and the femoral artery to perform cystometrograms (CMG, 12 ml/h saline infusion) and to collect blood samples, respectively.  Two hours after the surgery, CMG was started to evaluate intercontraction intervals (ICI) and maximal voiding pressure (MVP).  One hour after the start, EP (0.3 or 1 nmol/rat) or vehicle [2.5 µl N,N-dimethylformamide (DMF)/rat] was intracerebroventricularly (icv) administered.  Plasma noradrenaline (NA) and adrenaline (Ad) levels were measured at just before and at 5 min after the icv administration.  (2) Catheters were inserted into the bladder dome and the femoral vein to perform CMG and to administer EP intravenously (iv), respectively.  Three hours after the surgery, EP (1 nmol/rat) or vehicle (1.25% DMF/saline, 200 µl/rat) was iv administered.  CMG was performed as described in (1).  (3) In some experiments, acute bilateral adrenalectomy (ADX) was performed before the catheters insertion described in (1).  After the ADX, hydrocortisone was administered (5 mg/kg, im) to maintain levels of glucocorticoid.  EP administration (1 nmol/rat, icv), CMG and collection of blood samples were performed as described in (1).  (4) Effects of central pretreatment with mecamylamine (MEC, an antagonist of nAChRs, 100 or 300 nmol in 5 µl saline/rat, icv) on the EP (1 nmol/rat, icv)-induced responses were also investigated.

(1) Centrally administered EP (0.3 or 1 nmol/rat, icv) dose-dependently prolonged ICI and elevated plasma NA and Ad without altering MVP compared to the vehicle-treated group (Fig. 1A).  (2) Systemic administration of EP (1 nmol/rat, iv) showed no significant effect on ICI or MVP compared to the vehicle-treated group (data not shown).  (3) The EP (1 nmol/rat, icv)-induced ICI prolongation was not affected by ADX, which abolished the EP-induced elevation of plasma NA and Ad (Fig. 1B).  (4) Pretreatment with MEC (100 or 300 nmol/rat, icv) dose-dependently suppressed the EP (1 nmol/rat, icv)-induced ICI prolongation and elevation of plasma NA and Ad (Fig. 2).

Our present data indicate that EP centrally inhibits the micturition reflex as shown by centrally, but not systemically, administered EP-induced prolongation of ICI.  Although NA and Ad generally induce urinary storage, the EP-induced inhibition seems to be independent of the EP-induced activation of the central SA outflow because ADX, which abolished the EP-induced elevation of plasma NA and Ad, had no effect on the EP-induced prolongation of ICI.  Since centrally administered EP had no effect on MVP, EP might depress inputs to the micturition center, thereby inhibiting the micturition reflex.  EP might centrally inhibit the micturition reflex through brain nAChRs as evidenced by MEC-induced suppression of the EP-induced ICI prolongation.  Activation of nAChRs is reported to enhance the release of various neurotransmitters including GABA, glutamate, acetylcholine, serotonin, NA and dopamine in the brain [3], therefore, further studies are necessary to clarify mechanisms for brain nAChRs-mediated inhibition of the micturition reflex focusing on these neurotransmitters.

Stimulation of brain nAChRs can activate mechanisms that inhibit the micturition reflex, and these mechanisms are independent of the SA outflow modulation.  Thus, brain nAChRs could be a new target for alleviation of psychological stress-induced exacerbation of urinary bladder dysfunction such as OAB and BPS/IC.

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