Mechanisms by which the serotonergic system inhibits micturition in rats

Abstract

Aims

Serotonergic neurons and amino acid neurons are involved in the central nervous control of lower urinary tract function. We investigated the role of the serotonergic system in the central regulation of micturition, as well as the relationship between serotonergic neurons and amino acid neurons in the lumbosacral cord of rats.

Main methods

Under urethane anesthesia, bladder and urethral activity were recorded before and after intrathecal injection of serotonin (5-hydroxytryptamine: 5-HT), a 5-HT_2A receptor antagonist (ketanserin: KET), or KET + 5-HT by isovolumetric cystometry and measurement of the urethral pressure in intact rats and rats with hypogastric nerve transection (HGNT). Amino acid levels in the lumbosacral cord were also measured after intrathecal injection of 5-HT in intact rats.

Key findings

In intact rats, intrathecal injection of 5-HT transiently abolished rhythmic bladder contractions, decreased the maximal bladder contraction pressure, and increased the intravesical baseline pressure and the urethral baseline pressure. Intrathecal injection of KET + 5-HT also transiently abolished rhythmic bladder contractions. In HGNT rats, intrathecal injection of 5-HT transiently abolished rhythmic bladder contractions and increased the urethral baseline pressure. Intrathecal injection of 5-HT decreased the level of glycine in the lumbosacral cord.

Significance

The serotonergic system may be involved in blocking the afferent pathway of the micturition reflex, increasing sympathetic activity, and secondary promotion of urethral contraction through inhibition of glycinergic neurons in the lumbosacral cord. 5-HT_2A receptors may be involved in these effects on the bladder and urethra. Therefore, the serotonergic system may play a role of the maintenance of urine storage.

Introduction

Serotonergic neurons are located in the raphe nuclei of the brainstem and project their axons widely throughout the brain and spinal cord, including the dorsal horns of the spinal cord, the sympathetic nuclei, the parasympathetic nuclei, and the urethral sphincter motor nucleus (Onuf's nucleus) (Bowker et al., 1981, Thor et al., 1993, Helton et al., 1995, Xu et al., 2007). Onuf's nucleus has a major association with serotonin (5-hydroxytryptamine: 5-HT). Compared with other parts of the ventral horn in the sacral spinal cord, Onuf's nucleus shows dense staining for 5-HT in nerve terminals and also contains 5-HT_{1A, B, C}, 5-HT_{2A, B, C} and 5-HT_5A receptors (Thor et al., 1993, Helton et al., 1995, Xu et al., 2007). Various studies have shown that 5-HT and its receptors are involved in the central control of lower urinary tract function (Helton et al., 1995, Sugaya et al., 1998, de Groat and Yoshimura, 2001, de Groat, 2002, Thor, 2003: Ramage 2006). Electrical stimulation of the brainstem raphe nuclei, where 5-HT-containing neurons are located, inhibits rhythmic bladder contraction (Sugaya et al. 1998). S-norfluoxetine, a selective 5-HT reuptake inhibitor, was reported to increase bladder capacity and sphincter activity (Fuller et al., 1992, Wong et al., 1992). Therefore, 5-HT neurons appear to inhibit bladder activity. Among the various 5-HT receptors, 5-HT_2A receptors have been identified throughout the entire spinal cord, with a high level of expression in the sympathetic preganglionic cells, dorsal horn, and motoneurons (including the Onuf's nucleus) (Doly et al. 2004). Intravenous and intrathecal administration of a 5-HT_2A receptor agonist increases electromyographic activity in the external urethral sphincter (Mbaki and Ramage 2008).

Some amino acids are known to be important neurotransmitters and are involved in the central mechanisms regulating micturition and urine storage (Shapiro 1997). For example, glutamate is a major excitatory neurotransmitter that facilitates the micturition reflex (Mayer and Westbrook 1987). While γ-aminobutyric acid (GABA) is an important inhibitory neurotransmitter in the central nervous system and it inhibits the micturition reflex at the level of the lumbosacral cord (Igawa et al. 1993). Glycine is another important inhibitory neurotransmitter, and higher concentrations of glycine are found in the spinal cord than in supraspinal regions (Elekes et al. 1986). Therefore, both amino acid neurons and 5-HT neurons have a role in the central regulation of lower urinary tract function. However, the relations between these neurons involved in the regulation of lower urinary tract function are unknown. In order to clarify the role of 5-HT neurons in the central nervous control of lower urinary truct function, as well as the relations between 5-HT neurons and amino acid neurons, we examined the effects of intrathecal injection of 5-HT or a 5-HT_2A receptor antagonist (ketanserin: KET) on bladder and urethral activity in intact rats and rats with hypogastric nerve transection (HGNT rats). Amino acid levels in the lumbosacral cord were also investigated.