2.1. Anatomy and Physiology of Gastrointestinal Motility
The movements of the GI tract are different for each region. They also show marked differences in relation to prematurity
[23][24].
Gastric motility is ruled by myoelectrical activity that consists of rhythmically occurring transmembrane potential variations, called slow waves, that are independent of the presence of motor activity
[25]. These waves that slowly travel through the GI tract are defined motor migratory complexes (MMCs) and occur every 2–4 h with the purpose of clearing the tract of undigested material, mucus, and debris
[26]. MMCs initially occur due to vagal input and the release of motilin in the duodenum. Their propagation, on the other hand, is coordinated by enteric neurons. MMCs occur in humans only during periods of fasting and in the small intestine as opposed to other species. Phasic contraction is related to a different kind of electrical activity, called electrical response activity, characterized by spikes superimposed on gastric slow waves. The origin of electrical activity is found on the greater curve, approximately in the proximal corpus, from where the slow waves propagate towards the pylorus
[26][27].
Different motor patterns occur in the proximal and distal stomach. In the proximal stomach, receptive relaxation and accommodation occur, which are both mediated by neurons in the brainstem via vagal reflexes. The distal stomach exhibits different motor patterns during feeding and fasting. In the feeding state, the distal stomach grinds and mixes. Extrinsic neurons are not essential for this contractile activity, but it can be modulated by vagal pathways.
Intestinal motor activity is also correlated with gestational age and motor development, which is defined by phases I, II, and III of the migration of the motor complex between 37 GW and term age
[28]. In particular, the frequency of duodenal contractions, the number of duodenal contractions per impulse, and the peak intraluminal pressure of duodenal motility in preterm versus full-term infants are different
[29]. Clustered phasic contractions are more frequent but of shorter duration and lower amplitude; duodenal clusters are less common and antro-duodenal coordination is more immature in preterm infants
[30].
A role of insulin on gut motility has also been supported by Shulman et al., who demonstrated significantly reduced gastric residuals within the first month in preterm infants treated with oral insulin compared to untreated peers
[15].
2.2. The Enteric Nervous System
By 12 GW, the fetal colon has a dense neuronal network in the myenteric plexus with expression of excitatory neurotransmitter and synaptic markers. Instead, the markers of inhibitory neurotransmitters appear no earlier than 14 GW. However, electrical train stimulation of internodal strands did not evoke activity in the the enteric nervous system (ENS) of 12- or 14-GW tissues
[31].
Onset of evoked electrical activity in the human fetal ENS appears at approximately 16 GW. Such activity appears to coincide with increases in gene expression of various ion channels known to modulate enteric action potentials. The temporal development of several neural subtypes and enteric glia occurs between 12 and 16 GW
[32][33]. The ENS consists of three parts: enteric neurons, enteric glial cells (EGCs), and interstitial cells of Cajal (ICCs). They form two major ganglionated structures and functional subunits—submucosal plexus (or Meissner’s plexus) and myenteric plexus (or Auerbach’s plexus). Meissner’s plexus is located in the connective tissue of submucosa, and innervates muscularis mucosae, intestinal neuroendocrine cells, glandular epithelium and submucosal blood vessels, while Auerbach’s plexus is located between the circular and longitudinal muscle layers and is associated with the contractility of the circular and longitudinal muscles
[34][35].
Cdh19 is a direct target of Sox10 during early sacral NCC migration towards the hindgut and forms cadherin–catenin complexes which interact with the cytoskeleton in migrating cells
[36]. The migration phase is followed by the proliferation of NCCs which will form millions of ENS cells. The NCCs then assemble into groups (myenteric ganglia or submucosal ganglia) and then differentiate into a range of enteric neurons and glia cells and form complex neuronal networks necessary for controlled intestinal activity
[37][38][39].
Diverse populations of fibroblast-like interstitial cells are present in the adult gut. Loss or dysfunction of these cells has been linked to a wide variety of GI disorders. This broad group of cells comprises various subpopulations of Kit-positive ICCs and fibroblast-like cells expressing PDGFRα. ICCs located at the level of the myenteric plexus (ICC-MY) mediate slow waves, the electrical events that time the occurrence of phasic contractions
[40][41], while evidence is accumulating that ICC and PDGFRα-expressing cells located within and surrounding GI muscle bundles serve as intermediaries in both excitatory and inhibitory neuromuscular transmission.
2.3. The Microbiota-Gut–Brain Axis
During the early postnatal period, infants undergo consistent gut development, which is parallel and interdependent, even if not always synchronous, with brain development. In the first years of life, the gut experiences huge changes of the resident microbiota and a substantial maturation of both the enterocytes and of the ENS. Meanwhile, the brain and the central nervous system (CNS) grow rapidly, both in terms of brain volume and neural function.
The interdependency of the two developmental processes is mediated through the so-called gut–brain axis, which constitutes a bidirectional communication between the gut and the brain, made of several interrelated components
[42]. In recent years, the role of microbiota in the gut–brain axis has been studied extensively, so that the GBA is now commonly referred to as the microbiota-GBA (MGBA)
[43]. Gut microbiota alterations have been directly linked to physical and mental health through a series of mechanisms which are yet to be fully elucidated
[44].
The connection between gut and brain during early development involves both top-down and bottom-up mechanisms, with signals from the brain modulating GI functions, and GI-derived molecules influencing brain processes through different pathways. Even if the features of the MGBA in health and disease have been thoroughly described, the knowledge of the exact mechanisms through which the gut and the brain communicate in young infants is, at present, limited.
The mechanisms involved in the interplay between the brain and the gut include neural, endocrine, immune, and metabolic mediators
[45].
Neural connections include the CNS, the ENS, and the autonomic nervous system (ANS). The ENS receives inputs from the brain and, in turn, provides ascending information through neural circuits. The ANS comprises sympathetic and parasympathetic nerves: the sympathetic system exerts mainly an inhibitory influence on the gut, while the vagus nerve (VN) appears to be able to sense hormones, cytokines, and metabolites from the GI tract, leading to afferent signals to the brain. The development and myelination of the VN is not complete at birth and continues until adolescence, with a peak in the myelination rate observed in the first months of life, when consumption of human milk, and human-milk-derived bioactive components, is the highest
[46]. The ANS is connected to the limbic system of the brain, whose main components are the hippocampus, the amygdala, and the limbic cortex, and which is responsible for a variety of brain processes in health and disease
[45].
As for humoral components of the MGBA, they mainly consist in the hypothalamic–pituitary–adrenal axis (HPA), the enteroendocrine system and the immune system. Stress responses are modulated through the HPA, via the release of corticosterone, adrenaline, and noradrenaline. The enteroendocrine cells (EECs) in the gut produce GI hormones such as ghrelin, glucagon-like peptide (GLP-1), cholecystokinin, and peptide YY (PYY); these hormones regulate food intake and satiety and are also involved in the regulation of emotions and mood
[42].
The resident immune cells in the brain, known as the microglia, act as a neuroimmune component of the MGBA, by finely tuning neurogenesis and synaptogenesis. The maturation and function of the microglia appear to be related to changes in the gut microbiota, and also influenced by SCFAs
[47], which are end-products of bacterial fermentation of dietary fibers and resistant starch in the colon. Furthermore, immune signaling molecules, whose production is also driven by gut microbiota, can play a role in the MGBA, by binding to VN receptors or by crossing the blood–brain barrier. In this respect, recent studies have suggested a potential role of bacterial peptidoglycans (PGNs), which are components of the bacterial cell wall, released not only by exogenous bacteria, but also by the resident gut microbiota. It has been proposed that PGNs could enter the systemic circulation and reach the developing brain, where PGN sensing molecules are expressed abundantly during specific time windows of perinatal development, thus potentially exerting a direct effect on brain function and development
[48].
Finally, a central role in the MGBA is played by metabolic mediators, including tryptophan metabolites such as serotonin (5-hydroxytryptamine, 5-HT), melatonin, SCFAs, and other neurotransmitters. 5-HT is involved in most branches of the MGBA; for instance, it acts as a neurotransmitter both in the CNS and ENS, and 5-HT receptors have a critical function in the HPA
[49]. The production of host 5-HT in the gut is regulated by microbiota, as indigenous spore-forming bacteria, derived from mouse and human microbiota, have been shown to promote 5-HT biosynthesis from enterochromaffin cells in the colon, thus impacting GI motility and homeostasis
[50]. SCFAs (acetate, propionate, and butyrate) are thought to influence gut–brain communication through different potential pathways, either directly or indirectly. Once produced by colonic fermentation, SCFAs can exert a direct effect on the intestinal mucosal immunity and can modulate the integrity and function of the gut barrier. Furthermore, by interacting with the EECs, SCFAs promote a direct gut–brain signaling through the secretion of gut hormones, such as GLP-1 and PYY, and other metabolic mediators such as γ-aminobutyric acid and 5-HT. In addition, they can induce systemic inflammation through differentiation of T regulatory cells and secretion of interleukins and can probably also act centrally in the CNS by modulating neuroinflammation
[51].
There are several factors which could have a specific impact on the MGBA development in early life; one of the most relevant might be prematurity, as preterm birth interrupts the physiological growth and development of both the GI tract and the nervous system and leads to a certain degree of microbial dysbiosis. Despite that, at present there are no studies specifically designed to describe the unique features of the MGBA in preterm infants
[52]. In addition, nutrition during sensitive developmental time windows is thought to have a major impact on the microbiota-gut–brain crosstalk
[53][54], either through an effect of single nutrients (for example, milk fat globule membranes
[55], human milk oligosaccharides
[56], or through the well-known benefits of exclusive human milk feeding compared to other feeding sources
[57].