The present experiments using a freely moving conscious rat model permit the measurement of colonic motility in rats in the physiological fed status. The results demonstrated that ghrelin given ICV and IP stimulates gastrointestinal motility indicated by shortened colonic transit time. In addition we found that the NPY type 1 receptor is primarily involved in the ghrelin induced modulation of fasted motor activity of the colon.
There is convincing evidence that the most effective appetite stimulating peptides, NPY and ghrelin, act in the CNS and the periphery to simulate not only feeding but also GI function such as gastric acid secretion and gastric motility [
1,
8‐
10]. Stomach derived ghrelin, first described in 1999 by Kojima et al., is the first peripheral orexigenic peptide identified [
4]. Ghrelin was identified as endogenous ligand for the GH secretagogue receptor (GHS R) and a peripheral metabolic signal informing the brain about stomach nutrient load [
17,
24]. Physiological studies suggest a functional relationship of ghrelin and NPY within the brain. It has been demonstrated that peripherally (i.v.) and central (ICV) administered ghrelin increases the expression of the immediate early gene c-
fos, a marker of neuronal activity, in the arcuate nucleus and the PVN in awake fed rats [
25]. Furthermore, exogenous ghrelin increases mRNA levels for NPY into the arcuate nucleus and simulates food intake through hypothalamic NPY
1 receptors [
14,
16,
26]. Further Fujino et al. have demonstrated that ICV pretreatment with neuropeptide Y antiserum completely blocked ghrelin induced gastric and duodenal motoractivity [
9]. Taken together these data suggest that there is an anatomical interaction and functional relationship between ghrelin and NPY within the brain. Six recognized subtypes of neuropeptide Y receptors have been described (NPY
1 to NPY
6). Two of these, NPY
1- and NPY
2 receptors, are found in high density in the hypothalamus. There is compelling evidence that, in particular, NPY
1 and NPY
2 receptors are involved in the CNS regulation of gastrointestinal function. [
1,
8,
27] For this reason, we focused on neuropeptide Y
1 and Y
2 receptor pathways in the present study and did not investigate the role of neuropeptide Y receptor subtypes Y
4 and Y
5 which are also expressed in the hypothalamus and are also involved in the autonomic control of feeding behavior and GI function. In the present study pretreatment with the NPY
1 receptor antagonist, BIBP-3226, blocked stimulation of colonic motility induced by systemic microinjection of exogenous ghrelin (ICV and IP). In our hands BIIE-2046, which is a selective antagonist of the NPY Y
2 receptor, failed to affect the ghrelin induced induction of fasted motor activity. It was previously described that knocking out NPY significantly decreases ghrelin stimulated feeding [
17,
28]. In this context Fujino et al. have recently demonstrated that the ghrelin induced fasted gastroduodenal motor activity in rats is blocked by ICV injection of GHS-R antagonist as well as NPY antiserum [
9]. The results presented by Fujino et al. also suggest that the vagal pathway may mediate the action of centrally administered ghrelin on gastroduodenal motility [
9]. Thus we can speculate that central NPY pathways, e.g. centrally NPY receptor activation, are the primary downstream mediator of circulating ghrelin. This interpretation is consistent with neuroanatomical and physiological facts: Neuropeptide Y works at two sites, locally within the arcuate nucleus to inhibit POMC neuronal activity and at afferent-terminal sites, in particular the paraventricular nucleus of the hypothalamus. Guan et al. have shown that neuropeptide Y- and ghrelin like immunoreactive (LI) neurons within the arcuate nucleus could influence each other by complex synaptic transmissions [
29]. Furthermore Cowley et al. have demonstrated that ghrelin stimulated the activity of arcuate neuropeptide Y-LI neurons and mimicked the effect of neuropeptide Y in the PVN [
15]. Compelling evidence showed that NPY projections from the arcuate nucleus (ARC) to the PVN are involved in the CNS regulation of food intake and other physiological functions of the organism, e.g. digestive function, by neuroendocrine and autonomic pathways [
17,
18]. For example NPY released from ARC neurons activates NPY-Y
1 receptors in the hypothalamus, e.g. the PVN, and results in the stimulation of GI motor function [
18]. Furthermore arcuate NPYergic neurons have been thought to regulate feeding behavior by NPY receptor subtypes Y1 and Y5 in the PVN and adjacent areas [
17]. Pretreatment with a Y1, but not other receptor antagonist markedly inhibited ghrelin-induced feeding, pointing to NPY receptor Y1 as one of the downstream pathways [
9,
17]. With regard to the characteristic physiological feature that peripheral ghrelin does not cross the blood-brain barrier in rodents it is important to note that the arcuate nucleus is the only hypothalamic structure located outside the brain-blood barrier [
30]. Thus we can speculate that circulating ghrelin modulates gastrointestinal motility
via activation of hypothalamic, in particular by using NPYergic pathways via activation of NPY-Y1 receptors, in the arcuate nucleus. This hypothesis is in good agreement with our observation that the effect of peripherally (IP) administered ghrelin on colonic motility is blocked by ICV pretreatment with the specific NPY
1 receptor antagonist, BIBP-3226. The NPY
2 receptor antagonist BIIE-0246 injected in the 3
rd ventricle at the equipotent dose as BIBP-3226 was not effective to antagonize the ghrelin effect on GI motility significantly. This data suggests that ghrelin unfolds a stimulatory effect on colonic motility primarily by acting on central NPY
1 and not via NPY
2 receptors. This interpretation is confirmed by the observation that Y
1 receptors acts rather postsynaptically and the Y
2 receptor rather presynaptically [
31,
32] The question of whether neuropeptide Y
4 or Y
5 receptors in the CNS are involved in the CNS control of gastrointestinal function should be examined in future studies.