Neurogenic and myogenic properties of pan-colonic motor patterns and their spatiotemporal organization in rats

Abstract

Background and aims: Better understanding of intrinsic control mechanisms of colonic motility will lead to better treatment options for colonic dysmotility. The aim was to investigate neurogenic and myogenic control mechanisms underlying pan-colonic motor patterns.

Methods: Analysis of in vitro video recordings of whole rat colon motility was used to explore motor patterns and their spatiotemporal organizations and to identify mechanisms of neurogenic and myogenic control using pharmacological tools.

Results: Study of the pan-colonic spatiotemporal organization of motor patterns revealed: fluid-induced or spontaneous rhythmic propulsive long distance contractions (LDCs, 0.4-1.5/min, involving the whole colon), rhythmic propulsive motor complexes (RPMCs) (0.8-2.5/min, dominant in distal colon), ripples (10-14/min, dominant in proximal colon), segmentation and retrograde contractions (0.1-0.8/min, prominent in distal and mid colon). Spontaneous rhythmic LDCs were the dominant pattern, blocked by tetrodotoxin, lidocaine or blockers of cholinergic, nitrergic or serotonergic pathways. Change from propulsion to segmentation and distal retrograde contractions was most prominent after blocking 5-HT3 receptors. In the presence of all neural blockers, bethanechol consistently evoked rhythmic LDC-like propulsive contractions in the same frequency range as the LDCs, indicating the existence of myogenic mechanisms of initiation and propulsion.

Conclusions: Neurogenic and myogenic control systems orchestrate distinct and variable motor patterns at different regions of the pan-colon. Cholinergic, nitrergic and serotonergic pathways are essential for rhythmic LDCs to develop. Rhythmic motor patterns in presence of neural blockade indicate the involvement of myogenic control systems and suggest a role for the networks of interstitial cells of Cajal as pacemakers.

Figure 1. Propulsive motor patterns.

Spatiotemporal maps created from video recordings of motor patterns of the whole rat colon. The colon as well as the in and outflow tubes are filled with PBS and the fluid column in the outflow tube determines an intraluminal pressure of 5 cm H₂O in the experiments described in this figure and all other figures. a. In a quiescent colon, fluid-infusion (at white arrow, see methods) induced a long distance contraction (LDC) which was followed without further stimulus by rhythmic LDCs. Ripples followed the LDCs in the proximal colon. Boxed area = Video S1 “Induced and spontaneous LDC”. The white band preceding the first LDC is the distention caused by fluid infusion. The white areas preceding spontaneous LDCs are relaxations preceding the contraction. b. Interrupted LDCs showed a relaxation at about 1/3 down the colon length. The interruption (relaxation) was seen as a white spot on the black contraction. The boxed LDC is shown in Video S2 “Interrupted LDC”. c. A tandem contraction (see text) is shown followed by a spontaneous LDC. Boxed area = Video S3 “Tandem contraction” d. Five rhythmic LDCs are shown together with RPMCs. Ripples were seen in the proximal colon. The boxed area is shown in Video S4 “Motor complexes and LDC”.

Figure 2. Influence of a sustained contraction/distention on the LDC.

Spatiotemporal maps created from video recordings of motor patterns of the whole rat colon. a. A sustained contraction was present in the mid colon. LDCs started in the proximal colon and were interrupted by the sustained contraction but proceeded normally at the distal end of the sustained contraction. In addition, three contractions were seen that started at the distal end of the sustained contraction. They fall under the definition of RPMCs. b. A sustained distention, preceded and followed by a narrow ring contraction, was present in the mid colon. The LDCs in this experiment were exceptionally long in duration. The LDCs did not proceed after the contraction. In between the LDCs, RPMCs were initiated at the distal side of the sustained contraction.

Figure 3. Effect of TTX on LDCs.

Spatiotemporal maps created from video recordings of motor patterns of the whole rat colon. All arrows indicate infusion of PBS into proximal colon. a. Control LDCs followed by addition of TTX, added 1 min before the start of panel b. b. TTX abolished induced and rhythmic LDCs. Fluid infusion occurred at arrows. Rhythmic contractions remained in the proximal colon and a few propagated into the mid colon. c. Control LDCs (different animal). d. TTX was added 7 min before the start of panel d. TTX abolished induced and rhythmic LDCs. Rhythmic motor activity remained in the proximal colon. e. Carbachol, 2×10⁻⁶ M, was added 10 min before start of panel e. Carbachol, in the presence of TTX induced rhythmic LDC-like activity as well as retrograde contractions in the distal colon.

Figure 4. Lidocaine and bethanechol.

Spatiotemporal maps created from video recordings of motor patterns of the whole rat colon. All arrows indicate infusion of PBS into proximal colon. a. Control LDCs with distal RPMCs. b. Lidocaine abolished LDCs but rhythmic contractile activity remained up to 6/min. Two retrograde contractions were seen. Lidocaine was added 3 min before start of panel b. c. Addition of bethanechol (2×10⁻⁶ M) introduced LDC-like activity as well as retrograde contractions in the distal colon. Fluid infusion effects were variable, sometimes without effect (first arrow), sometimes inducing a propulsive contraction. Bethanechol was added 14 min before start of panel c.

Figure 5. Atropine and L-NNA.

Spatiotemporal maps created from video recordings of motor patterns of the whole rat colon. All arrows indicate infusion of PBS into proximal colon. a. Control LDCs with distal RPMCs. b. After addition of atropine the LDCs were getting smaller and disappeared after 30 min. Atropine was added 3 min before start of panel b. c. Control LDCs with distal RPMCs. d. Typical LDCs were not seen after blockade of nitric oxide synthesis. Strong rhythmic contractile activity was always present showing various and varying patterns at frequencies that were higher compared to control activity. L-NNA was added 8 min before start of panel d.

Figure 6. Effect of the 5-HT₃ antagonist granisetron.

Spatiotemporal maps created from video recordings of motor patterns of the whole rat colon. a. Control activity consisted of a tandem contraction and two interrupted LDCs accompanied by distal antegrade RPMCs (rat 1). b. After addition of granisetron (3.8×10⁻⁶ M, added 30 min before start of panel b), LDC activity was abolished, RPMCs were disrupted and segmentation activity was induced in the mid and distal colon (rat 1). c. LDCs and related motor patterns were abolished 20 min after addition of granisetron (3.8×10⁻⁶ M, rat 2) for a period of 5 min, thereafter segmentation activity emerged (not shown).d. In the presence of granisetron, bethanechol (2×10⁻⁶ M, added 10 min before start of trace), evoked rhythmic LDC- like pan-colonic propulsive contractions with mid- and distal colon antegrade and bi-directional contractions (rat 2). e. Rhythmic retrograde contractions in distal colon became dominant after 30 min in the presence of granisetron and bethanechol (2×10⁻⁶ M, added 20 min before start of trace) (rat 2).