Cannabinoid CB1 and CB2 receptors regulate the micturition reflex at levels of the lower urinary tract (LUT) and peripheral/spinal neural pathways [1]. Although brain CB receptors are involved in neuromodulation [2], their role in micturition reflex regulation remains unclear. We therefore investigated whether brain cannabinoid receptor modulation affects basal micturition and a neuropeptide bombesin (BB)-induced central facilitation of the micturition reflex in rats.

In urethane-anesthetized (0.8 g/kg, ip) male Wistar rats (315-390 g), a bladder catheter was inserted from the bladder dome for cystometry. Two hours after the surgery, cystometry (saline instillation at 12 ml/h) was started to evaluate intercontraction interval (ICI) and maximal voiding pressure (MVP). One hour later, ACEA (CB1 agonist, 30 and 100 nmol/rat), HU308 (CB2 agonist, 30, 100 and 300 nmol/rat), or BB (0.003, 0.01 and 0.03 nmol/rat) was administered intracerebroventricularly (ICV) at 2 or 3 doses in each rat at 90-min interval. For BB experiments, N,N-dimethylformamide (DMF) was ICV pre-treated 60 min before BB administration. Evaluations of ICI and MVP were continued 90 min after the last administration. To examine pretreatment effects on the BB-induced response, ACEA (30 or 100 nmol/rat), HU308 (300 nmol/rat) or DMF was ICV administered 60 min before BB administration (0.03 nmol/rat, ICV). In addition, immunohistochemical staining for CB1 and CB2 receptors was performed in brain sections from an intact rat focusing on the periaqueductal gray (PAG), a key supraspinal site for micturition control [3].

ACEA alone had no effect on ICI or MVP (data not shown), but ACEA pretreatment attenuated BB (0.03 nmol/rat, ICV)-induced ICI shortening (Fig. 1A-1B). This attenuation was abolished by AM4113 pre-treatment (CB1 antagonist, 100 nmol/rat, ICV) (data not shown). HU308 alone showed no effect on ICI or MVP (data not shown), and HU308 pre-treatment did not alter the BB-induced ICI shortening (Fig. 2). In the PAG, CB1, but not CB2, receptor immunoreactivity was detected (Fig. 3).

Because ACEA or AM4113 alone did not alter ICI or MVP, brain CB1 receptors do not tonically regulate the micturition reflex under physiological conditions. By contrast, ACEA attenuated the BB-induced ICI shortening, and this effect was blocked by AM4113, indicating that brain CB1 receptor activation suppresses the BB-induced central facilitation of the micturition reflex. In the brain, postsynaptic excitation can trigger release of endogenous CB receptor ligands, which retrogradely activate presynaptic CB1 receptors, thereby inhibiting neurotransmitter release [2]. These mechanisms are involved in on-demand inhibition of neurotransmission to prevent excess neural excitation. Therefore, brain CB1 receptors may act as a brake that limits excessive facilitation of the micturition reflex induced by BB through on-demand inhibition of neurotransmission. In contrast, the lack of HU308 on basal ICI and MVP or the BB-induced response indicates little involvement of brain CB2 receptors in the micturition reflex regulation.
Because ACEA-induced attenuation appeared within 30 min after BB administration, the responsible CB1 receptor sites may be periventricular regions including the PAG, which is connected to the third and fourth ventricles and projects to the pontin micturition center [3]. The widespread CB1 receptor immunoreactivity observed in the PAG supports the idea that ACEA inhibits BB-induced neurotransmission via presynaptic CB1 receptors in the PAG, thereby attenuating facilitation of the micturition reflex.

Brain CB1 receptors can play a brake-like role during excess facilitation of the micturition reflex induced by BB, but brain CB2 receptors have little effect on regulation of the micturition reflex in rats. Therefore, brain CB1 receptors can be novel therapeutic targets for patients with LUT dysfunctions attributed to neurogenic bladder overactivity.

Hedlund P, Gratzke C. The endocannabinoid system - a target for the treatment of LUTS? Nat Rev Urol. 2016; 13: 463-470.
Kendall DA, Yudowski GA. Cannabinoid Receptors in the Central Nervous System: Their Signaling and Roles in Disease. Front Cell Neurosci. 2017; 10: 294.
Holstege G. The emotional motor system and micturition control. Neurourol Urodyn. 2010; 29: 42-48.