Role of spinal metabotropic glutamate receptor 5 in pudendal inhibition of the nociceptive bladder reflex in cats

Abstract

This study examined the role of spinal metabotropic glutamate receptor 5 (mGluR5) in the nociceptive C-fiber afferent-mediated spinal bladder reflex and in the inhibtion of this reflex by pudendal nerve stimulation (PNS). In α-chloralose-anesthetized cats after spinal cord transection at the T9/T10 level, intravesical infusion of 0.25% acetic acid irritated the bladder, activated nociceptive C-fiber afferents, and induced spinal reflex bladder contractions of low amplitude (<50 cmH2O) and short duration (<20 s) at a smaller bladder capacity ∼80% of saline control capacity. PNS significantly (P < 0.01) increased bladder capacity from 85.5 ± 10.1 to 137.3 ± 14.1 or 148.2 ± 11.2% at 2T or 4T stimulation, respectively, where T is the threshold intensity for PNS to induce anal twitch. MTEP {3-[(2-methyl-4-thiazolyl)ethynyl]pyridine; 3 mg/kg iv, a selective mGluR5 antagonist} completely removed the PNS inhibition and significantly (P < 0.05) increased bladder capacity from 71.8 ± 9.9 to 94.0 ± 13.9% of saline control, but it did not change the bladder contraction amplitude. After propranolol (3 mg/kg iv, a β1/β2-adrenergic receptor antagonist) treatment, PNS inhibition remained but MTEP significantly (P < 0.05) reduced the bladder contraction amplitude from 18.6 ± 2.1 to 6.6 ± 1.2 cmH2O and eliminated PNS inhibition. At the end of experiments, hexamethonium (10 mg/kg iv, a ganglionic blocker) significantly (P < 0.05) reduced the bladder contraction amplitude from 20.9 ± 3.2 to 8.1 ± 1.5 cmH2O on average demonstrating that spinal reflexes were responsible for a major component of the contractions. This study shows that spinal mGluR5 plays an important role in the nociceptive C-fiber afferent-mediated spinal bladder reflex and in pudendal inhibition of this spinal reflex.

Keywords: bladder; cat; glutamate; neuromodulation; pudendal.

Fig. 1.

Effect of spinal cord transection (SCT) at the T9/T10 level on reflex bladder activity. A: cystometrograms (CMGs) during saline infusion before and after SCT or during 0.25% acetic acid (AA) infusion after SCT. Infusion rate = 1 ml/min. B: summarized results from 12 cats. The downward white arrow in A marks the contraction that is used to determine the bladder capacity after SCT. The bladder capacity is normalized to the measurement during saline infusion before SCT. *Significantly different (paired t-test). Infusion rate = 1–3 ml/min.

Fig. 2.

Hexamethonium (Hx) reduced the amplitude of spinal reflex bladder contractions. Hx (10 mg/kg iv) was administered during isovolumetric contractions (A) and significantly reduced the bladder contraction amplitude in both MTEP {3-[(2-methyl-4-thiazolyl)ethynyl]pyridine}-pretreated and propranolol+MTEP-pretreated cats (B). *Significant (P < 0.05) difference (paired t-test); n = 6 cats in each treatment group.

Fig. 3.

Effect of MTEP on spinal reflex bladder activity induced by AA irritation. A: bladder activity during CMG or under isovolumetric conditions before and after MTEP (3 mg/kg iv) treatment. B: MTEP had no effect on the maximal amplitude of isovolumetric contractions. C: MTEP significantly increased bladder capacity. The downward white arrow on CMG traces in A indicates where the bladder capacity is measured. *Significant (P < 0.05) difference (paired t-test); n = 6 cats.

Fig. 4.

Effect of MTEP on spinal reflex bladder activity induced by AA irritation in propranolol (3 mg/kg iv)-pretreated cats. A: bladder activity during CMG or under isovolumetric conditions before and after MTEP (3 mg/kg iv) treatment. B: MTEP significantly reduced maximal amplitude of isovolumetric contractions. C: MTEP significantly increased bladder capacity. The downward white arrow on CMG traces in A indicates where the bladder capacity is measured. *Significant (P < 0.05) difference (paired t-test); n = 6 cats.

Fig. 5.

Inhibition of spinal reflex bladder contractions by pudendal nerve stimulation (PNS) is eliminated after MTEP (3 mg/kg iv) treatment. A: before MTEP treatment. B: after MTEP treatment. The downward white arrow on CMG traces indicates where the bladder capacity is measured. The black bar under pressure trace indicates PNS duration. C: summarized result. *Significant (P < 0.01, posttest) difference from control by 1-way ANOVA (P < 0.0001, F = 32.27, treatment df = 3, individual df = 5). #Significant (P < 0.001, posttest) difference before and after MTEP treatment by 2-way ANOVA (between groups: P = 0.0003, F = 19.4, df = 1; interaction: P < 0.0001, F = 13.33, df = 3).

Fig. 6.

In propranolol-pretreated cats, inhibition of spinal reflex bladder contractions by PNS is eliminated after MTEP (3 mg/kg iv) treatment. A: before MTEP treatment. B: after MTEP treatment. The downward white arrow on CMG traces indicates where the bladder capacity is measured. The black bar under pressure trace indicates PNS duration. C: summarized result. *Significant (P < 0.05, posttest) difference from control by 1-way ANOVA (P = 0.0006, F = 10.33, treatment df = 3, individual df = 5).

Fig. 7.

Possible sites where MTEP acts to modulate: 1) the spinal micturition reflex activated by C-fiber afferents (shown at right) and 2) inhibition of bladder activity elicited by stimulation of pudendal afferent nerves (shown at left). The surpraspinal micturition reflex activated by Aδ afferents and removed by SCT is also at right. Hx and propranolol block transmission, respectively, in bladder ganglia and at the sympathetic neuroeffector junction.