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(±)-Tramadol, a widely used analgesic, is a racemate stimulating opioid receptors and inhibiting reuptake of noradrenaline and serotonin, that is, pharmacological principles previously shown to influence rat micturition.
We studied both (±)-tramadol and its enantiomers in conscious Sprague–Dawley rats undergoing continuous cystometry. The effects of these agents were compared to those of morphine (μ-opioid receptor agonist) and tested after pretreatment with naloxone (μ-opioid receptor antagonist). Cystometries were evaluated before and after intravenous (i.v.), intraperitoneal (i.p.) and intrathecal (i.t.) drug administrations.
The most conspicuous effects of i.v. (±)-tramadol (0.1–10 mg kg−1) was an increase in threshold pressure and an increase in micturition volume.
These effects were mimicked by (+)-tramadol (0.1–5 mg kg−1 i.v.), whereas (−)-tramadol (5 mg kg−1 i.v.) did not influence threshold pressure and micturition volume.
The effects of (±)-tramadol 5 mg kg−1 on micturition volume were blocked by pretreatment with naloxone 0.3 mg kg−1. Morphine (0.3–10 mg kg−1 i.p.) increased threshold pressure but did not significantly increase micturition volume in doses not resulting in overflow incontinence.
(±)-Tramadol 10 mg kg−1 increased urine production, an effect blocked by desmopressin 25 ng kg−1.
(±)-Tramadol effectively inhibits micturition in conscious rats by stimulating μ-opioid receptors. A synergy between opioid receptor stimulation and monoamine reuptake inhibition may contribute to the micturition effects.
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Overactive bladder is characterised by symptoms of urgency, with and without urge incontinence, usually with frequency and nocturia (Abrams et al., 2002). In Western Europe, the disorder was estimated to occur in nearly 17% of the population above 40 years of age (Milsom et al., 2001). Detrusor overactivity, the generic term for involuntary bladder contractions, is often the underlying condition. The most common drug treatment for overactive bladder is antimuscarinic drugs. However, side effects such as dry mouth and constipation remain a problem (Andersson et al., 2002), and alternative therapies are needed.
The micturition reflex can be influenced at spinal and supraspinal sites by interference with various neurotransmitters (de Groat & Yoshimura, 2001). Despite this, there are only few drugs with a defined action within the central nervous system, which have been used to treat disorders of micturition. These drugs include agonists at GABAB (γ-aminobutyric acid) receptors and antidepressants, which inhibit the reuptake of 5-hydroxytryptamine (5-HT) and noradrenaline (Andersson, 2002).
Tramadol is widely used as an analgesic. For moderate pain, it is as effective as morphine, without causing respiratory depression that is associated with opioids (Lehmann, 1997). Tramadol combines weak effects on opioid receptors with reuptake inhibition of 5-HT and noradrenaline (Table 1). The latter effects contribute to the analgesic action of (±) tramadol (Raffa et al., 1992; Sevcik et al., 1993; Raffa & Friderichs, 1996). Tramadol is a racemate, and opioid receptor activity and 5-HT reuptake inhibition are mainly associated with the (+)-tramadol enantiomer, whereas (−)-tramadol is a reuptake inhibitor of noradrenaline (Driessen & Reimann, 1992; Driessen et al., 1993; Raffa et al., 1993). Morphine is effective in blocking micturition reflexes within the central nervous system (Dray & Nunan, 1987b), and reuptake inhibitors of noradrenaline and/or 5-HT can influence micturition (Andersson, 2002). A combination of these principles may have interesting effects on micturition.
Table 1. Inhibition of opioid receptor binding and monoamine reuptake of morphine and tramadol, in assays using selective opioid receptor agonists and 5-HT and noradrenaline expressed as K1 (nM±s.e.m.)
| ||Opioid receptors||Monoamine reuptake|
The main aim of this study was to investigate the potential effect of (±)-tramadol on rat micturition. We studied the effects of intravenous (i.v.) (±)-tramadol in conscious rats who underwent continuous cystometry. In order to discriminate effects between the enantiomers, rats were also given (+)- and (−)-tramadol. In addition, (±)-tramadol was given intrathecally (i.t.), to investigate a potential spinal action of the unmetabolised compound. To evaluate the importance of μ-opioid receptor stimulation for the effects of (±)-tramadol, naloxone was given as a pretreatment to some rats. The effects of morphine were also studied, allowing a comparison of the cystometrical effects of (±)-tramadol and morphine. In some rats, desmopressin was given as a pretreatment to study a potential diuretic effect of (±)-tramadol.
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With (±)-tramadol given i.v. at 1, 5 and 10 mg kg−1, micturition volume exceeded estimated bladder capacity in all rats, compared to two out of 10 after saline. Then, estimation of residual volume and bladder capacity cannot be made. This phenomenon was abolished after pretreatment with desmopressin (Figure 1), as tramadol 10 mg kg−1 in these rats increased both estimated bladder capacity as well as micturition volume without a significant change in residual volume. Thus, (±)-tramadol does not impair efficiency of bladder emptying up to 10 mg kg−1. For the discussion below on experiments without desmopressin pretreatment, micturition volume is used to evaluate changes in bladder storage capacity. Thus, given that (±)-tramadol significantly increased micturition volume, it is concluded that (±)-tramadol increases rat bladder storage capacity.
(±)-Tramadol increased threshold pressure. Given the data shown in Figure 2-4, threshold pressure can be increased by ∼100% by (±)-, (+)-tramadol and morphine, before overflow incontinence is observed. Changes in threshold pressure, that is, pressure immediately before micturition, reflect changes in the urethra and/or bladder. During the continent storage phase, maximal urethral pressure exceeds bladder pressure. Since there was no significant change in basal bladder pressure in the presence of (±)-tramadol, the increase in threshold pressure could imply a change in sensitivity to, or reaction of, bladder wall tension receptors. Neither morphine nor (±)-, (+)- or (−)-tramadol decreased micturition pressure (except for (±)-tramadol 5 mg kg−1). For morphine, this is consistent with results after i.t. administration, on micturition pressure in conscious rats (Igawa et al., 1993) and urethral resistance in anaesthetised dogs (Drenger et al., 1986). Hence, (±)-tramadol probably does not impair urethral closure. As shown in Figure 2, relative increases in micturition volume and threshold pressure seem to reach a plateau at 5 and 1 mg kg−1 (±)-tramadol, respectively. These doses are analogous with those resulting in analgesia (Raffa & Friderichs, 1996).
Opioid receptor-mediated inhibition of micturition can be caused by stimulation of μ- and δ-opioid receptors (Dray & Metsch, 1984a; Hisamitsu & de Groat, 1984; Kontani & Kawabata, 1988; Shimizu et al., 2000). Thus, administration of opioid receptor active drugs (i) systemically (Sillen & Rubenson, 1986; Kontani & Kawabata, 1988), (ii) intrathecally (Dray & Metsch, 1984a; Hisamitsu & de Groat, 1984; Durant & Yaksh, 1988; Igawa et al., 1993) and (iii) intracerebroventricularly (Hisamitsu & de Groat, 1984; Sillen & Rubenson, 1986; Dray & Nunan, 1987b; Kontani et al., 1989) inhibits micturition. The main site for inhibitory effects via opioid receptor stimulation is likely to be within the central nervous system (Dray & Metsch, 1984b). However, the peripherally active opioid receptor agonist, loperamide, exerted a dose-dependent inhibitory action on induced bladder contractions (Berggren et al., 1992). Furthermore, naloxone may in vitro facilitate electrically induced contractile activity of rat bladder strips (Berggren et al., 1991). Thus, it cannot be completely excluded that peripheral opioid receptor stimulation influences micturition.
Pretreatment with naloxone, μ-opioid receptor antagonist, abolished the effects of (±)-tramadol on micturition volume and attenuated the effects on threshold pressure. This suggests that μ-opioid receptor activation plays a major role for (±)-tramadol. The spinal cord is probably essential for the effects of μ-opioid receptor stimulation on micturition, as inhibition of isovolumetric rat bladder contractions via systemic morphine, was abolished by i.t. naloxone (Dray & Metsch, 1984a) and isovolumetric rat bladder contractions were more effectively attenuated by morphine given i.t. vs i.c.v. at equal doses (Dray & Nunan, 1987b). In the rat spinal cord, opioid receptors are concentrated in the superficial dorsal horn (Coggeshall & Carlton, 1997), where bladder afferents merge into the spinal cord (Yoshimura & de Groat, 1997). In this region, μ-opioid receptors have a predominant distribution, but δ- and κ-opioid receptors are also present, and κ-opioid receptors may prevail in the lumbosacral region (Coggeshall & Carlton, 1997) where they may control urethral sphincter activity (de Groat & Yoshimura, 2001). The major importance of spinal μ-opioid receptor stimulation was demonstrated as (i) the minute amount of 0.5 μg morphine given i.t. resulted in overflow incontinence in conscious rats (Igawa et al., 1993), and (ii) a μ-opioid receptor agonist given i.t. was more potent than a δ-opioid receptor agonist given i.t. to inhibit isovolumetric bladder contractions in anaesthetised rats. Stimulation of spinal κ-opioid receptors was ineffective (Dray & Metsch, 1984a; Dray & Nunan, 1987a).
(±)-Tramadol given i.t. (10–50 μg) was ineffective for micturition control, probably because of the low opioid receptor activity of unmetabolised (±)-tramadol (Frink et al., 1996). The effects of 200 μg, corresponding to ∼1 mg kg−1 may be because of systemic actions. Metabolites of (±)-tramadol–(±)-M1 and also the metabolite (±)-M5–have higher μ-opioid receptor affinity, but also higher δ- and κ-opioid receptor affinity, compared to unmetabolised (±)-tramadol (Table 1). These metabolites are probably responsible for the μ-opioid receptor-mediated actions of (±)-tramadol.
There are differences between the enantiomers of the metabolites in affinity for opioid receptors, for example, (+)-M1 has an affinity approximately 1 : 2–10 and (−)-M1 has an affinity approximately 1 : 200–300, for μ-, δ- and κ-opioid receptors, compared to morphine (Table 1). Nevertheless, (−)-M1 has a μ-opioid receptor affinity roughly equal to that of unmetabolised (+)-tramadol (Frink et al., 1996). Yet, (−)-tramadol was ineffective for micturition control. Thus, either metabolites of (−)-tramadol were not generated in sufficient amounts at a time when drug effects were evaluated, or (−)-tramadol metabolites were insufficiently active on opioid receptors to influence rat micturition. In support of the latter alternative, (±)-tramadol is rapidly metabolised (Lintz et al., 1981). As an inhibitor of noradrenaline reuptake, (−)-tramadol has an inhibitory potency similar to that of fluoxetine, a selective inhibitor of noradrenaline reuptake (Frink et al., 1996). This action may be of importance for the analgesic effect of (±)-tramadol (Raffa & Friderichs, 1996). It also represents a potential mechanism for micturition control, since a descending noradrenergic pathway from locus coeruleus has a facilitating effect on micturition (de Groat & Yoshimura, 2001), and in the bladder both α- and β-adrenoceptor may influence contractility (Andersson, 1993). However, noradrenaline reuptake inhibition was ineffective for cat micturition (Katofiasc et al., 2002) and (−)-tramadol was ineffective for rat micturition. Taken together, these results suggest that (+)-tramadol is the effective enantiomer.
The importance of μ-opioid receptors for the effects of (±)-tramadol on micturition is consistent with the naloxone-induced blockade of the (±)-tramadol-mediated increase in micturition volume. Furthermore, both (±)-tramadol and morphine increased threshold pressure. Yet, in contrast to the significant increase in bladder storage capacity after (±)-tramadol 5–10 mg kg−1 i.v., without signs of overflow incontinence, morphine did not significantly increase bladder storage capacity in doses not resulting in overflow incontinence. In anaesthetised rats, morphine i.v. at 0.1 mg kg−1 doubled the bladder capacity (Kontani & Kawabata, 1988). Also in anaesthetised rats, morphine at 1–2 mg kg−1 i.v. abolished micturition (Kontani & Kawabata, 1988; Shimizu et al., 2000). Thus, the effects of (±)-tramadol, and (+)-tramadol, cannot solely be explained by interaction with μ-opioid receptors. Within the brain, both μ- and δ-opioid receptors may influence micturition control (Hisamitsu & de Groat, 1984; Dray et al., 1985; de Groat, 1990). Despite that (+)-M1 is approximately 30 times more potent at μ- than δ-opioid receptors (Frink et al., 1996), it cannot be excluded that δ-opioid receptor activation in the brain may influence the outcome.
5-HT receptor modulation is important for micturition control (de Groat, 2002). Reuptake inhibition of 5-HT due to (+)-tramadol, which is approximately 40 times less potent than the selective 5-HT reuptake inhibitor fluoxetine (Frink et al., 1996), may therefore be of importance. However, the selective 5-HT reuptake inhibitor citalopram was ineffective in isovolumetric bladder contractions (Testa et al., 1999). Still, activation of supraspinal opioid receptors influences micturition via mechanisms modulated through a descending 5-HT containing system. Thus, the inhibitory action of morphine given i.c.v., but not i.t., was attenuated by (i) 5-HT depletion in the CNS and (ii) i.t. methysergide (5-HT receptor antagonist) (Dray & Nunan, 1987b). Speculatively, effects of (±)-tramadol on micturition, via supraspinal μ-opioid receptor activation, may be in synergy with (+)-tramadol-mediated 5-HT reuptake inhibition.
Effects on urine production
That micturition volume was larger than estimated residual volume in all rats given high doses of (±)- and (+)-tramadol, an effect not observed in saline controls with and without desmopressin pretreatment, is considered a diuretic effect. Since the same effect was observed with (±)- and (+)-tramadol as well as morphine, but not (−)-tramadol, involvement of opioid receptors is suggested. Desmopressin, a vasopressin analogue that reduces urine production in rats (Vavra et al., 1974), abolished the diuretic effect of (±)-tramadol, for example, mean MV-BV was below zero and no rats had a larger micturition volume than estimated bladder capacity.
Stimulation of κ-opioid receptors in the rat hypothalamus is known to stimulate diuresis by suppressing vasopressin release (Leander et al., 1987; Brooks et al., 1993). The (+)-M1 metabolite of (±)-tramadol, has affinity for the κ-opioid receptor (Frink et al., 1996; Lai et al., 1996), which may explain the diuretic effect. Though stimulation of κ-opioid receptors in the kidney may attenuate the response to vasopressin (Slizgi & Ludens, 1982), experiments with Brattleboro rats discourage a renal site of action (Abrahams et al., 1986). To our knowledge, (±)-tramadol does not have a diuretic effect in humans. This might be because of a difference between primates and rodents, for example, observed on brain κ- and μ-opioid receptor distribution in rats and monkeys (Mansour et al., 1988).