Role of alpha2-adrenoceptors and glutamate mechanisms in the external urethral sphincter continence reflex in rats

Abstract

Purpose: We investigated the role of alpha(2)-adrenoceptors and glutamate mechanisms in the urethral continence reflex in response to abdominal pressure increases.

Materials and methods: Under urethane anesthesia external urethral sphincter electromyogram activity was evaluated in spinal cord transected (T8-T9) female rats during lower abdominal wall compression before and after intravenous application of test drugs. The effects of the N-methyl-D-aspartate glutamate receptor antagonist MK-801 (Sigma) or the alpha(2)-adrenoceptor agonist medetomidine (Tocris Cookson, Ellisville, Missouri) (each 0.03, 0.3 and 3 mg/kg intravenously) on external urethral sphincter activity were examined. A 0.3 mg/kg intravenous dose of the alpha(2)-adrenoceptor antagonist idazoxan (Sigma) was then administered before or after the application of 1 mg/kg MK-801 intravenously. In addition, 0.3 mg/kg idazoxan were administered intravenously following the application of 1 mg/kg of the serotonin/norepinephrine reuptake inhibitor duloxetine (Kemprotec, Middlesbrough, United Kingdom) intravenously.

Results: MK-801 and medetomidine dose dependently decreased external urethral sphincter activity. Idazoxan significantly increased external urethral sphincter activity by 64% but the increase in activity after idazoxan was abolished by MK-801. On the other hand, idazoxan did not reverse the inhibitory effects of MK-801. In addition, idazoxan significantly potentiated the duloxetine effects on external urethral sphincter activity by 120%.

Conclusions: These results indicate that 1) glutamate is a major excitatory neurotransmitter in the urethral continence reflex response to abdominal pressure increases, 2) alpha(2)-adrenoceptor activation suppresses external urethral sphincter activity, probably via presynaptic inhibition of glutamate release and 3) the effects of serotonin/norepinephrine reuptake inhibitors are enhanced by alpha(2)-adrenoceptor inhibition. Therefore, alpha(2)-adrenoceptor antagonists could be beneficial for treating stress urinary incontinence.

FIGURE 1

Representative traces of intravescial pressures and EUS-electromyogram. EUS activity during lower abdominal wall compression for 10 seconds (bar) was recorded three times at 2 minute intervals before (control) and 10 min after i.v. administration of test drugs. Abdominal compression induced increases in EUS-electromyogram activity, and cumulative application of medetomidine (Med; 0.03, 0.3 and 3 mg/kg) dose-dependently decreased the EUS activity during abdominal compression. In addition, after-discharges of EUS-electromyogram were often observed following cessation of abdominal compression, most obviously after the first compression. EUS activity was completely eliminated by α-bungarotoxin (0.4 mg/kg) at the end of experiment (lowest trace).

FIGURE 2

Effects of cumulative application of MK-801, a NMDA glutamate receptor antagonist, on EUS activity and blood pressure. A; higher single doses (0.3 and 3 mg/kg) of MK-801 (MK) significantly decreased the EUS activity during abdominal compression by 70% (*P<0.05) and 91% (**P<0.01) compared with pre-drug control, respectively. B; the highest dose (3 mg/kg) of MK-801 significantly decreased the mean blood pressure compared with control (**P<0.01).

FIGURE 3

Effects of medetomidine, an α₂-AR agonist, on EUS activity and blood pressures. A; Cumulative application of medetomidine (Med; 0.03, 0.3 and 3 mg/kg) significantly decreased the EUS activity during abdominal compression dose-dependently by 27% (*P<0.05), 54% (**P<0.01) and 82% (**P<0.01) compared with pre-drug control, respectively. B; Cumulative application of medetomidine did not change the mean blood pressure.

FIGURE 4

Relationship between the effects of idazoxan, an α₂-AR antagonist, and MK-801, a NMDA glutamate receptor antagonist, on EUS activity. A; Idazoxan (Ida; 0.3 mg/kg) significantly increased the EUS activity during abdominal compression by 64% compared with pre-drug control (*P<0.05). The increased EUS activity after idazoxan was then eliminated and reduced to the level lower than control (*P<0.05) after subsequent application of MK-801 (MK; 1 mg/kg). B; MK-801 significantly decreased the EUS activity by 74% compared with pre-drug control (**P<0.01), but the decreased EUS activity by MK-801 was not changed by subsequent application of idazoxan.

FIGURE 5

Effects of idazoxan, an α₂-AR antagonist, in combination with duloxetine, a 5-HT/NE reuptake inhibitor, on EUS activity. A; duloxetine (Dul; 1 mg/kg) significantly increased the EUS activity during abdominal compression by 45% compared with pre-drug control (*P<0.05), and the increased EUS activity by duloxetine was significantly augmented by idazoxan (Ida; 0.3 mg/kg) by 52% compared with duloxetine alone (*P<0.05) or by 120% compared with pre-duloxetine control (**P<0.01). B; Consecutive application of duroxetine and idazoxan did not change the mean blood pressure.

FIGURE 6

Hypothetical schema of α₂-adrenoceptor (AR) and glutamatergic (Glu) mechanisms in the control of external urethral sphincter (EUS) continence reflex induced by abdominal compression. Glutamate is the major excitatory neurotransmitter, and α₂-AR activation suppresses EUS activity probably via α₂-AR-mediated presynaptic inhibition of glutamate release. In addition, the effects of serotonin (5-HT)/norepinephrine (NE) reuptake inhibitors can be enhanced by α₂-AR antagonists that suppress α₂-AR activation induced by increased NE.