Suppression of bladder reflex activity in chronic spinal cord injured cats by activation of serotonin 5-HT_1A receptors

Abstract

The effects of 8-OH-DPAT (5-HT_1A receptor agonist) and WAY100635 (5-HT_1A receptor antagonist) on reflex bladder activity were investigated in α-chloralose anesthetized or conscious chronic spinal cord injured cats. The results were similar in both anesthetized and conscious animals. Cystometrograms revealed that 8-OH-DPAT (0.5 mg/kg, s.c.) significantly increased the bladder volume threshold for eliciting a large amplitude micturition contraction, but only slightly reduced the amplitude of the contractions and did not alter the small amplitude pre-micturition contractions. 8-OH-DPAT also reduced the amplitude of isovolumetric bladder contractions. The inhibitory effect of 8-OH-DPAT was reversed by WAY100635 (0.5 mg/kg) or blocked by pre-treatment with WAY100635. Reflex bladder contractions evoked by tactile stimulation of the perigenital region were not altered by 8-OH-DPAT. These results suggest that the inhibitory effect of 8-OH-DPAT is mediated by an action on interneuronal pathways in the spinal cord or on the C-fiber afferent limb of the spinal micturition reflex and not on bladder smooth muscle or the efferent limb of the reflex pathway. Drugs that activate 5-HT_1A receptors might be useful in treating detrusor overactivity after spinal cord injury.

Introduction

The functions of the lower urinary tract to store and periodically release urine are dependent on a complex neural control system in the brain and spinal cord that utilizes multiple neurotransmitters to regulate the activity of three sets of peripheral nerves: lumbosacral parasympathetic and sympathetic nerves innervating the bladder and urethral smooth muscle and pudendal nerves innervating the striated muscle of the external urethral sphincter (de Groat et al., 1993, Morrison et al., 2005). Immunohistochemical, anatomical and pharmacological experiments have provided evidence that serotonin (5-hydroxytryptamine, 5-HT) is involved in the central neural pathways controlling the lower urinary tract (de Groat, 2002, Thor, 2003). Dahlström and Fuxe, 1964, Dahlström and Fuxe, 1965 discovered that sympathetic and parasympathetic nuclei throughout the spinal cord exhibited a dense collection of 5-HT containing nerve fibers that originate in the raphe nuclei in the caudal brain stem. Subsequent transneuronal tracing studies revealed that raphe neurons in the brain stem of the rat are labeled after injection of pseudorabies virus into either the urinary bladder (Nadelhaft and Vera, 1995, Sugaya et al., 1997), urethra (Vizzard et al., 1995), or the external urethral sphincter (Nadelhaft and Vera, 1996).

Iontophoretic application of 5-HT from micropipettes in close proximity to spinal autonomic neurons in the cat revealed that 5-HT excited thoracic sympathetic preganglionic neurons (PGN) (de Groat and Ryall, 1967) but inhibited excitatory amino acid-induced firing of bladder parasympathetic preganglionic neurons in the sacral spinal cord (Ryall and de Groat, 1972). Intravenous administration of 5-HT precursors (5-hydroxytryptophan, 5-HTP and tryptophan) in cats depressed reflex bladder activity and depressed reflex discharges on bladder parasympathetic nerves elicited by bladder distension or electrical stimulation of bladder afferent nerves (de Groat et al., 1981). Conversely, 5-HTP enhanced the reflex firing in the sympathetic hypogastric nerves elicited by electrical stimulation of bladder afferent axons. Electrical or chemical stimulation of the raphe nuclei in the cat also inhibited reflex bladder activity indicating that endogenously released 5-HT has an inhibitory effect on bladder function (McMahon and Spillane, 1982, Morrison and Spillane, 1986, Chen et al., 1993, Sugaya et al., 1998).

Multiple 5-HT receptors (5-HT₁, 5-HT₂ and 5-HT₃) which have been identified in the dorsal horn, the sacral parasympathetic nucleus (SPN) and the sphincter motor nucleus of the lumbosacral spinal cord (Monroe and Smith, 1983, Marlier et al., 1991, Pubols et al., 1992, Thor et al., 1993, de Groat, 2002, Burgard et al., 2003) could contribute to the serotonergic control of the lower urinary tract. 5-methoxydimethyltryptamine (5-MeODMT), an agonist that activates 5-HT_1A and 5-HT₂ receptors depressed the parasympathetic reflex to the bladder elicited by electrical stimulation of bladder afferent axons, but enhanced a somato-bladder parasympathetic reflex elicited by stimulation of pudendal nerve afferent axons (Thor et al., 1990). 5-MeODMT also enhanced reflexes in the hypogastric and pudendal nerves. In cats in which the bladder was irritated by intravesical infusion of dilute acetic acid, 5MeODMT or 8-hydroxy-2-di-N-propylaminotetralin (8-OH-DPAT), a 5-HT_1A receptor agonist, increased bladder capacity and decreased the frequency of bladder contractions during continuous cystometrograms (Thor et al., 2002). The effects of the drugs were antagonized by WAY100635, a 5-HT_1A receptor antagonist. These experiments raised the possibility that bulbospinal projections from serotonergic raphe neurons in the brain stem activate a spinal 5-HT_1A receptor mechanism that promotes urinary continence by inhibiting the parasympathetic outflow to the bladder.

In the present experiments, we examined the role of 5-HT_1A receptors in the spinal pathways controlling bladder function by studying the effects of a 5-HT_1A receptor agonist (8-OH-DPAT) and antagonist (WAY100635) on reflex bladder activity in conscious or anesthetized chronic spinal cord injured cats. In cats with an intact spinal cord, reflex bladder activity is triggered by A-δ bladder afferent axons and is mediated by a spinobulbospinal reflex pathway passing through a coordinating center in the rostral pons (the pontine micturition center) (de Groat and Ryall, 1969, de Groat, 1975). However, after spinal cord injury, neurogenic detrusor overactivity is mediated by spinal reflex pathways activated by C-fiber bladder afferents (de Groat et al., 1990, Cheng et al., 1999). Because C-fiber bladder afferents may be involved in the generation of various types of bladder disorders and serotonergic drugs may be useful in the treatment of these disorders, it was of interest to determine if 8-OH-DPAT could inhibit the C-fiber afferent evoked micturition reflex or the micturition reflex elicited by tactile stimulation of somatic afferents in the perigenital region in conscious or anesthetized chronic spinal cord injured (SCI) cats. A preliminary description of these observations has appeared in an abstract (Miscik et al., 2003).