Diminished gastric prokinetic response to ghrelin in a rat model of spinal cord injury

Abstract

Background: Patients with cervical or high-thoracic spinal cord injury (SCI) often present reduced gastric emptying and early satiety. Ghrelin provokes motility via gastric vagal neurocircuitry and ghrelin receptor agonists offer a therapeutic option for gastroparesis. We have previously shown that experimental high-thoracic injury (T3-SCI) diminishes sensitivity to another gastrointestinal peptide, cholecystokinin. This study tests the hypothesis that T3-SCI impairs the vagally mediated response to ghrelin.

Methods: We investigated ghrelin sensitivity in control and T3-SCI rats at 3-days or 3-weeks after injury utilizing: (i) acute (3-day post-injury) fasting and post-prandial serum levels of ghrelin; (ii) in vivo gastric reflex recording following intravenous or central brainstem ghrelin; and (iii) in vitro whole cell recording of neurons within the dorsal motor nucleus of the vagus (DMV).

Key results: The 2-day food intake of T3-SCI rats was reduced while fasting serum ghrelin levels were higher than in controls. Intravenous and fourth ventricle ghrelin increased in vivo gastric motility in fasted 3-day control rats but not fasted T3-SCI rats. In vitro recording of DMV neurons from 3-day T3-SCI rats were insensitive to exogenous ghrelin. For each measure, vagal responses returned after 3-weeks.

Conclusions and inferences: Hypophagia accompanying T3-SCI produces a significant and physiologically appropriate elevation in serum ghrelin levels. However, higher ghrelin levels did not translate into increased gastric motility in the acute stage of T3-SCI. We propose that this may reflect diminished sensitivity of peripheral vagal afferents to ghrelin or a reduction in the responsiveness of medullary gastric vagal neurocircuitry following T3-SCI.

Keywords: brainstem; gastric emptying; gastric stasis; gastrointestinal motility; vago-vagal reflexes.

Figure 1

Intravenous infusion of ghrelin provokes an increase in gastric contractions in surgical control but not acute T3-SCI rats. A) Representative original polygraph traces from fasted animals tested 3-days after control or T3-SCI surgery. B) Representative traces from fasted animals tested 3-weeks after control or T3-SCI surgery. Baseline motility index had no significant difference between control and T3-SCI in either 3-day or 3-week groups. C) Graphic summary of the significant increase in gastric contractions over baseline (100%, not depicted) following intravenous administration of ghrelin (2.5μmol•kg⁻¹) in 3-day control rats, while the ghrelin-induced increase from baseline was not significant in 3-day T3-SCI rats. Intravenous administration of ghrelin significantly increased gastric contractions above baseline in both 3-week control and 3-week T3-SCI rats. (* p<0.05).

Figure 2

Central (IV^th ventricle) administration of ghrelin increases gastric contractions in control but not T3-SCI rats 3-days following surgery. A) Representative traces from fasted animals following 10pmol. While there was no significant difference in baseline motility index between control and T3-SCI rats central application of ghrelin (3–100 pmol) significantly increased gastric contractions beginning at 10 pmol in 3-day control rats while there was no significant difference in gastric contractions of 3-day T3-SCI rats (* p<0.05; B).

Figure 3

The sensitivity to centrally administered ghrelin returns in 3-week T3-SCI rats. There remained no significant difference in the baseline motility index between 3-week control and 3-week T3-SCI rats. A) Representative traces from fasted animals following 100pmol. While there was no significant difference in baseline motility index between control and T3-SCI rats central application of ghrelin (3–100 pmol) increased gastric contractions but only significantly so at 100 pmol. (* p<0.05; B).

Figure 4

Graphical summary of the effects of ghrelin on sEPSCs in DMV neurons of surgical control and T3-SCI rats tested at 3-days (A) or 3-weeks (B). All values represent percent change from control recordings collected prior to bath perfusion with ghrelin (100nM). A) Perfusion of ghrelin significantly increased the sEPSC frequency of surgical control rats 3-days following surgery but had no effect on sEPSC amplitude while at 3-day following T3-SCI, ghrelin failed to increase both the frequency of sEPSC’s and the amplitude in DMV neurons. B) Ghrelin significantly increased sEPSC frequency in DMV neurons from of surgical control rats at 3-weeks after surgery and perfusion with ghrelin also significantly increased sEPSC frequency in 3-week T3-SCI rat DMV neurons. No significant differences were noted in sEPSC amplitudes from either group tested at 3-weeks. Individual data points were omitted from sEPSC amplitude graphs for clarity. (* p<0.05).