Psychological stress can induce not only frequent urination but also 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.
A previous study reported that a representative stress response, the sympatho-adrenomedullary (SA) system, is activated by centrally administered (±)-epibatidine (EP) [1], an agonist of nicotinic acetylcholine receptors (nAChRs).  Although it is already reported that EP centrally inhibits the rat micturition reflex [2], brain mechanisms for the inhibitory effect are not clarified yet.  In this study, we investigated how centrally administered EP inhibits the micturition reflex focusing on the dependence on the SA system, brain nAChR subtypes and brain receptors of GABA, an inhibitory neurotransmitter, 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 cystometry (12 ml/h saline infusion) and to collect blood samples, respectively.  Two hours after the surgery, cystometry 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.  We also confirmed effects of centrally pretreated mecamylamine (MEC, a non-selective antagonist of nAChRs, 100 or 300 nmol in 5 µl saline/rat, icv) on the EP (1 nmol/rat, icv)-induced responses. 
(2) In some experiments, acute bilateral adrenalectomy (ADX) was performed before insertion of catheters described in (1).  After the ADX, hydrocortisone was administered (5 mg/kg, im) to maintain levels of glucocorticoid.  EP administration (1 nmol/rat, icv), cystometry and collection of blood samples were performed as described in (1).  
(3) Effects of central pretreatment with methyllycaconitine (MLA, an alpha7 nAChR antagonist, 30 or 100 nmol in 5 µl saline/rat, icv), dihydro-beta-erythroidine (DHbE, an alpha4beta2 nAChR antagonist, 100 or 300 nmol in 5 µl saline/rat, icv) or SR95531 (SR, a GABAA antagonist, 0.1 nmol in 5 µl saline/rat, icv) on the EP (1 nmol/rat, icv)-induced responses were also investigated.

(1) Centrally administered EP dose-dependently prolonged ICI and elevated plasma NA and Ad without altering MVP compared to the vehicle-treated group (Fig. 1A).  These EP-induced changes were significantly attenuated by central pretreatment with MEC (data not shown).
(2) The EP-induced ICI prolongation was not affected by ADX, which abolished the EP-induced elevation of plasma NA and Ad (Fig. 1B).  
(3) Central pretreatment with MLA or SR significantly attenuated the EP-induced ICI prolongation (Fig. 2A and 2B), respectively, while DHbE showed no significant effect on the EP-induced response (data not shown).

Our present data indicate that EP centrally inhibits the micturition reflex through brain nAChRs as shown by centrally administered EP-induced ICI prolongation and by MEC-induced attenuation of the EP-induced response.  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 ICI prolongation.  Since centrally administered EP had no effect on MVP, an urodynamic parameter of bladder efferent activity, EP might depress inputs to the micturition center, thereby inhibiting the micturition reflex.  In the central nervous system, alpha7 and alpha4beta2 nAChRs are the most predominant subtypes of nAChRs and these subtypes are reported to enhance the release of various neurotransmitters including GABA, an inhibitory neurotransmitter, in the brain [3].  In this study, MLA and SR, but not DHbE, centrally attenuated the EP-induced ICI prolongation.  Therefore, stimulation of brain alpha7 nAChRs might inhibit the micturition reflex through these receptors-mediated enhancement of GABA release and stimulation of brain GABAA receptors.

Centrally administered EP can inhibit the rat micturition reflex through brain alpha7 nAChRs and brain GABAA receptors, independently of the SA outflow modulation.  Thus, brain alpha7 nAChRs could be a new target for alleviation of psychological stress-induced exacerbation of urinary bladder dysfunction such as OAB and BPS/IC.

Shimizu T, Yokotani K. Brain cyclooxygenase and prostanoid TP receptors are involved in centrally administered epibatidine-induced secretion of noradrenaline and adrenaline from the adrenal medulla in rats. Eur J Pharmacol. 2009;606:77-83.
Lee SJ, Nakamura Y, de Groat WC. Effect of (+/-)-epibatidine, a nicotinic agonist, on the central pathways controlling voiding function in the rat. Am J Physiol Regul Integr Comp Physiol. 2003;285:R84-R90.
Gotti C, Zoli M, Clementi F. Brain nicotinic acetylcholine receptors: native subtypes and their relevance. Trends Pharmacol Sci. 2006;27:482-491.