Rapid-Onset, Short-Duration Induction of Colorectal Contractions in Anesthetized, Adult, Male Rats

Abstract

Substantial clinical and preclinical evidence indicates that transient receptor potential vanilloid 1 (TRPV1) receptors are expressed on terminals of colorectal chemoreceptors and mechanoreceptors and are involved in various rectal hypersensitivity disorders with common features of colorectal overactivity. These stimulatory properties of TRPV1 receptors on colorectal function suggested that brief stimulation of TRPV1 might provide a means of pharmacologically activating the colorectum to induce defecation in patients with an "unresponsive" colorectum. The current studies explored the basic features of TRPV1 receptor-induced contractions of the colorectum in anesthetized rats with and without acute spinal cord injury (aSCI). Cumulative concentration-response curves to intrarectal (IR) capsaicin (CAP) solutions (0.003%-3.0%) were performed in anesthetized aSCI and spinal intact rats. CAP produced an "inverted U," cumulative concentration-response curve with a threshold for inducing colorectal contractions at 0.01% and a peak response at 0.1% and slight decreases in responses up to 3%. Decreases in responses with concentrations >0.1% are due to a rapid desensitization (i.e., ≤30 minutes) of TRPV1 receptors to each successive dose. Desensitization appeared fully recovered within 24 hours in spinal intact rats. Colorectal contractions were completely blocked by atropine, indicating a reflexogenic activation of parasympathetic neurons, and responses were completely unaffected by a neurokinin 2 receptor antagonist, indicating that release of neurokinin A from afferent terminals and subsequent direct contractions of the smooth muscle was not involved. IR administration of three other TRPV1 receptor agonists produced similar results as CAP. SIGNIFICANCE STATEMENT: Individuals with spinal cord injury often lose control of defecation. Time-consuming bowel programs using digital stimulation of the rectum are used to empty the bowel. This study shows that intrarectal administration of the transient receptor potential vanilloid 1 (TRPV1) receptor agonist, capsaicin, can induce rapid-onset, short-duration colorectal contractions capable of inducing defecation in spinal cord injured and intact rats. Therefore, TRPV1 agonists show promise as potential therapeutics to induce defecation in individuals with neurogenic bowel.

Fig. 1.

Physiograph tracings of blood pressure and CRP in response to cumulative doses of CAP administered intrarectally at 5-minute intervals. The concentration of CAP is shown below the arrows and is expressed as a % concentration in ethanol/water vehicle. (A) In a spinal intact rat, CAP (0.01%–0.3%) evoked a rapid increase in CRP. At 1% CAP, the response was delayed and smaller and no response was seen at 3% CAP. (B) In the acute spinal cord injured rat, CAP (0.01%–0.03%) evoked a rapid increase in CRP. At higher doses of 0.1%–3%, the response was either markedly reduced or absent. No obvious drug-induced changes in blood pressure were observed.

Fig. 2.

CRP responses to cumulative doses of IR CAP. Complete desensitization to repeated IR administration of high dose CAP in aSCI rats. Cumulative doses of CAP were administered at 5-minute intervals. The % concentration of is shown on the horizontal axis. CAP increased (A) CRP and (B) CRP AUC in both intact and aSCI groups (pressure: [F(7,214) = 11.44, P < 0.0001; AUC: F(7,208) = 9.697, P < 0.0001]. *Indicates a statistical difference from vehicle (0), Tukey’s multiple comparison test. No group difference was observed in CRP [F(1,214) = 0.2400, P = 0.6247], but a difference was observed in the colorectal AUC [F(1,208) = 4.064, P = 0.0451] with no dose matched difference between intact and aSCI. The vehicle (0) groups include three doses of ethanol (0.03%, 2.85%, and 28.5% ethanol in water). Intact: N = 31, aSCI N = 36 for 0; intact N = 9–12, and aSCI N = 10–13 for each concentration of CAP. Complete desensitization was observed to a high dose (1% or 3%) of CAP (second dose) administered 1 hour after the first high dose administration (first dose). Both the peak (C) CRP and (D) CRP AUC responses were nearly abolished. *Indicates P < 0.05 compared with the previous response to CAP, Student’s paired t test, n = 7.

Fig. 3.

Effects of IR CAP on MAP and heart rate in anesthetized intact and aSCI rats. CAP (0.3%–1%) transiently increased MAP by less than 15% in aSCI [F(7,206) = 3.616, P = 0.0011], but not in intact rats. *Indicates a difference from vehicle (0) within group. A significant interaction [F(7,206) = 2.688, P = 0.0110] and difference between intact and aSCI groups [F(1,206) =14.60, P = 0.0002] was observed. No difference in heart rate was observed with dose [F(7,212) = 1515, P = 0.1635] or between groups [F(1,212) = 2.390, P = 0.1236]. Vehicle, (0) N = 26; CAP, N = 11–14/group.

Fig. 4.

Physiograph tracings of CRP in response to TRPV1 agonists, OLDA, OEA, and nonivamide. All three agonists evoked a rapid increase in CRP. The wide black arrow indicates when a cotton plug (0.1–0.15 g weight saturated in ∼0.1–0.2 ml solution) was placed into the rectum. The concentration of each dose is shown near the arrow at the horizontal axis. The black downward arrows indicate when the cotton plug was removed. *The colorectal balloon catheter was expelled.

Fig. 5.

CAP-induced increase in CRP is mediated via muscarinic receptor activation in an aSCI rat. (A) Physiographs are shown from a single rat that was administered the drugs in the sequence shown. Atropine (0.5 mg/kg) blocks the CRP increase induced by bethanechol and blocks the CAP (0.1%–1%)-induced CRP activity. In the presence of atropine, the NK2 agonist, LMN-NKA, still induces a smooth muscle-mediated increase in CRP. (B) CAP-induced increase in CRP is not mediated via NK2R activation in an aSCI rat. Physiographs are shown from a single rat that was administered the drugs in the sequence shown. The NK2R agonist, LMN-NKA, induced a robust CRP response that was blocked by the NK2R antagonist, GR159897 (1 mg/kg). Administration of GR159897 did not block the CAP-induced increase in CRP. The concentrations of CAP are expressed as a % concentration.

Fig. 6.

Representative colorectal pressure traces showing desensitization during repeated IR administration of CAP in three aSCI rats. CAP was administered over 2 minutes via a cotton plug soaked with 0.2 ml of CAP solution at time = 0, 10 minutes, and 30 minutes. Upward pointing arrows indicate when the CAP-saturated plug was administered, and downward arrows indicate removal of the plug. (A) CRP following repeated administration of 0.01% CAP. (B) CRP following repeated administration of 0.1% CAP. (C) CRP following repeated administration of 1% CAP.