Excitatory effects of bombesin receptors in urinary tract of normal and diabetic rats in vivo

Abstract

Aims

Bombesin receptors (BB receptors) and bombesin related peptides are expressed in the lower urinary tract of rodents. Here we investigated whether in vivo activation of BB receptors can contract the urinary bladder and facilitate micturition in sham rats and in a diabetic rat model of voiding dysfunction.

Material and methods

In vivo cystometry experiments were performed in adult female Sprague–Dawley rats under urethane anesthesia. Diabetes was induced by streptozotocin (STZ; 65 mg/kg, i.p.) injection. Experiments were performed 9 and 20 weeks post STZ-treatment. Drugs included neuromedin B (NMB; BB1 receptor preferring agonist), and gastrin-releasing peptide (GRP; BB2 receptor preferring agonist).

Key findings

NMB and GRP (0.01–100 μg/kg in sham rats; 0.1–300 μg/kg in STZ-treated rats, i.v.) increased micturition frequency, bladder contraction amplitude and area under the curve dose dependently in both sham and STZ-treated rats. In addition, NMB (3, 10 μg/kg i.v.) triggered voiding in > 80% of STZ-treated rats when the bladder was filled to a sub-threshold voiding volume. NMB and GRP increased mean arterial pressure and heart rate at the highest doses, 100 and 300 μg/kg.

Significance

Activation of bombesin receptors facilitated neurogenic bladder contractions in vivo. Single applications of agonists enhanced or triggered voiding in sham rats as well as in the STZ-treated rat model of diabetic voiding dysfunction. These results suggest that BB receptors may be targeted for drug development for conditions associated with poor detrusor contraction such as an underactive bladder condition.

Introduction

Several patient populations such as the elderly and diabetic experience voiding dysfunction characterized by the inability to completely void urine from the bladder during micturition. This results in elevated post-void residual urine volumes and symptoms of frequency, nocturia, and urinary tract infections. This condition has recently been recognized as an underactive bladder and represents a highly unmet medical need and a major focus area for basic research and drug development (Miyazato et al., 2013). Current therapies, such as bethanechol and distigmine which increase bladder contractions via activation of muscarinic receptors, have limited efficacy and tolerability due to severe side effects associated with the use of parasympathomimetics (Barendrecht et al., 2007). New therapies with agents that contract the bladder with minimal side effects are needed.

The bombesin receptors (BB receptors) are G-protein coupled receptors and are divided into three subtypes: bombesin receptor 1, 2, and 3. BB1 receptors and BB2 receptors are preferentially activated by the endogenous peptides neuromedin B (NMB) and gastrin-releasing peptide (GRP), respectively. No naturally occurring ligand has been identified for BB3 receptor. NMB and GRP along with their receptors are distributed throughout the brain, spinal cord and peripheral tissues and are involved in a variety of functions ranging from control of smooth muscle contractions, gastric acid secretions, glucoregulation, thermoregulation, satiety, pain and itch (review (Jensen et al., 2008)). In the lower urinary tract, BB receptors and/or bombesin related peptides are expressed in the bladder, urethra and pelvic ganglia of rat and/or guinea-pig (Dalsgaard et al., 1983, Ghatei et al., 1985, Keast and Chiam, 1994, Kilgore et al., 1993, Panula, 1986, Radziszewski et al., 1996, Radziszewski et al., 2011, Watts and Cohen, 1991) and in intramural ganglia of the human male urinary bladder (Dixon et al., 1997). Functional studies have shown that activation of these receptors in the bladder (Falconieri Erspamer et al., 1988, Kullmann et al., 2008, Maggi et al., 1992, Rouissi et al., 1991, Watts and Cohen, 1991) and urethra (Radziszewski et al., 2011) produces smooth muscle contraction. It is not known whether the BB receptors are involved in micturition.

Patients with diabetes experience bladder dysfunction characterized by increased capacity, poor emptying and overflow incontinence, characteristics of an underactive bladder. In fact, diabetic cystopathy has been recognized as one of the leading causes of underactive bladder condition (Daneshgari et al., 2009, Gomez et al., 2011, Miyazato et al., 2013). The STZ-treated diabetic rat model has been extensively used as a rodent model of the voiding dysfunction encountered in diabetic patients. In this model, there are time-dependent morphological and functional changes in myogenic and neurogenic components of the bladder and urethra that affect voiding and storage phases and result in a large capacity bladder and increased residual volume (Daneshgari et al., 2006a, Daneshgari et al., 2006b, Daneshgari et al., 2009, Liu and Daneshgari, 2005, Liu and Daneshgari, 2006, Malmgren et al., 1989). In the present study we used the STZ-treated diabetic rat as a model of voiding dysfunction to test the effectiveness of BB receptor agonists to improve voiding.