Benign prostatic hyperplasia (BPH) is a main cause of lower urinary tract symptoms (LUTS), including storage LUTS such as urinary frequency and urgency, in men.  Although BPH-induced bladder outlet obstruction (BOO) has been accepted as a major mechanism inducing storage LUTS, an increasing number of clinical studies demonstrate that asymptomatic prostatic inflammation is involved in not only the development of histological BPH but also the emergence of male LUTS, suggesting that prostatic inflammation could be another potential mechanism inducing storage LUTS in BPH patients. Previous studies showed that a rat model of formalin-induced prostatic inflammation exhibits bladder overactivity as evidenced by frequent urination after 4 weeks of injection.[1]  However, since an adequate chronic animal model is necessary for further studies, here we aimed to investigate whether the formalin-induced prostatic inflammation in a rat model can lead to long-lasting bladder overactivity and changes in bladder afferent neuron excitability.

Male SD rats were used and prostatic inflammation was induced by formalin (5%; 50 µl per lobe) injection into bilateral ventral lobes of the prostate. (1) Metabolic cage: Eight rats were used. Before, 1 week, 4 weesk, and 8 weeks after formalin injection, voiding behaviour was evaluated in metabolic cages. (2) Continuous CMG: Twenty rats were randomly divided into four groups: Control, 1-week, 4-week and 8-week groups. Continuous cystometrograms (CMG) in a conscious condition were recorded on different time points to measure intercontraction intervals (ICI) and voided volume per micturition. (3) Tissue inflammation: After continuous CMG, ventral lobes of the prostate and the bladder were removed to perform HE staining to evaluate inflammation levels. (4) Characterization of afferent neuron excitability: Twenty rats were randomly divided into four groups for patch clamp recordings. L6-S1 dorsal root ganglia (DRG) were removed at different time points, and whole-cell patch clamp recordings were performed on dissociated bladder afferent neurons labelled by FLuoro-gold (FG) injected into the bladder wall, to examine the electrical properties of neurons.

(1) Metabolic cage: Compared to before formalin injection, rats exhibited a significant (P<0.05) increase in micturition episodes/24h and decrease in voided volume per micturition after formalin injection at each time point (micturition episodes/24h: 21.7±3.3 [1 week], 21.5±3.7 [4 weeks], and 21.3±3.6 [8 weeks] v.s. 16.7±2.1 [before]; voided volume per micturition (mL): 0.40±0.10 [1 week], 0.41±0.09 [4 weeks], and 0.41±0.14 [8 weeks] v.s. 0.46±0.13 [before]).
(2) Continuous CMG: Compared to control rats, formalin-treated rats with prostatic inflammation exhibited a significantly (P<0.05) higher number of non-voiding contractions per void and shorter ICI (Fig 1).
(3) Tissue inflammation:  Hematoxylin and eosin staining showed that there were regular shaped acini and intact basement membrane in the prostate tissue of control rats, whereas stromal infiltration of mast cells and lymphocytes and irregular shaped acini were found in prostate tissues of formalin-induced rats. Bladder tissues from any of four groups did not show inflammatory cell accumulation or epithelial formation changes. 
(4) Afferent neuron excitablity: In patch clamp recordings, capsaicin-sensitive bladder afferent neurons from rats with prostatic inflammation had significantly (P<0.05) lower thresholds for spike activation (-19±18mV [1 week], -22±1.7mV [4 week] and -21±1.8mV [8 weeks], respectively) compared to control rats (-19.5±1.1mV) (Fig. 2). The number of action potentials of bladder afferent neurons during an 800 ms depolarizing pulse was significantly increased after prostatic inflammation at every time point compared to control rats (Fig. 2).

These results indicate that: (1) formalin prostatic injection can induce chronic inflammation only in the prostate but not in the bladder, (2) formalin induced prostatic inflammation can result in long-lasting bladder overactivity as evidenced by reduced voided volume per micturition and ICI, (3) prostatic inflammation can induce long-lasting hyperexcitability of capsaicin-sensitive C-fiber bladder afferent neurons. Thus, it is assumed that intra-prostatic injection of formalin can induce chronic prostatic inflammation and result in bladder overactivity via sensitization of C-fiber bladder afferent pathways.

Formalin-induced prostatic inflammation can induce long-lasting bladder overactivity in rats evident as frequent micturition and increased non-voiding contractions in association of bladder afferent neuron hyperexcitability. Clinically, chronic prostatic inflammation might contribute to storage LUTS in BPH patients through affecting bladder afferent neuron excitability, and this long-lasting model of prostatic inflammation would be useful for the study of inflammation-related aspects of male LUTS pathophysiology.

Mizoguchi, S., Mori, K., Wang, Z. et al.: Effects of Estrogen Receptor beta Stimulation in a Rat Model of Non-Bacterial Prostatic Inflammation. Prostate, 77: 803, 2017