*5.1. Neuro Fibres and Neurotransmitters*

Important work over the past years has been performed to understand the mechanisms by which endometriotic lesions induce pain, the primary symptom of patients. Much of this effort has focused on understanding the extent and type of nerves present in endometriotic lesions compared to surrounding tissues. The presence of nerves in endometriotic lesions has been confirmed. In humans, murine models and rat models, ectopic endometrium implants develop sympathetic, parasympathetic, and sensory nerve fibres [24,25,38]. An imbalance in the distribution of sensitive and sympathetic nerve fibres in peritoneal EM lesions in favour of the sensitive nerve fibres of 4:1 has been observed [38]. In the peritoneum of patients without EM, on the other hand, the ratio of sensitive nerve fibres to sympathetic nerve fibres is 1:5. An inverse ratio of the nerve fibres to one another is evident in EM-aged tissue. In line with these data, similar results were also found in intestinal EM [39]. The sympathetic innervation in the intestinal wall, in particular, speaks for influence on the pathogenesis mechanisms, especially since patients with intestinal infection often complain about functional intestinal disorders. The total nerve fibre density is reduced in the periphery of the peritoneal lesion (an area more than 4 mm away from the actual endometrial lesion), most likely due to a reduced number of sympathetic nerves fibres [38].

The neurotransmitters norepinephrine, adenosine, neuropeptide Y (NPY), substance P (SP), vasoactive intestinal peptide (VIP), and endogenous opioids of the different nerve fibre types exert different effects on inflammatory processes by binding to specific receptors of the immune cells. Furthermore, nerve fibres seem to play an immunomodulatory role [40,41]. The expression of neurotransmitter receptors is not evenly distributed among immune cells but appears to be dependent on the microenvironment [42–44]. Nerve fibres communicate with immune cells in a synapse-like manner and thus modulate immune cell function [45]. Both efferent (sympathetic and parasympathetic) and afferent (sensitive) nerve fibres influence immune cells through the local release of neurotransmitters [40].

A recent study [46] showed that sensory nerve-derived neuropeptides SP and CGRP facilitate epithelial–mesenchymal transition (EMT), fibroblast-to-myofibroblast transdifferentiation (FMT) and further turn stromal cells into smooth muscle cells (SMCs) in EM, yielding increased collagen production, elevated cellular contractility, and eventually fibrosis. Neutralization of their respective receptors, such as NK1R, RAMP-1 and CRLR, however, abrogates these processes. More remarkably, they showed that lesional nerve fibre density correlated with the lesional expression levels of the receptors, with the extent of lesional fibrosis as well as the severity of pain in EM patients [46]. These data provide strong evidence that sensory nerve fibres play a potent facilitatory role in expediting the development and fibrogenesis of endometriotic lesions.

More and more data indicate that the peritoneal EM lesions in particular lead to the release of neurotrophic substances into the peritoneal fluid. Significantly increased NGF and NT-3 levels [47] and also estrogen levels [27] have been detected, especially in patients with peritoneal lesions. In vitro analyses using a neuronal growth assay also demonstrated the neuromodulatory properties of EM at the functional level. Peritoneal fluids from patients with and without EM were incubated with sensitive and sympathetic ganglia. The incubation with peritoneal fluid from patients with peritoneal EM showed a significantly

increased sprouting of sensitive nerve fibres, but lower sprouting of sympathetic ones. The peritoneal fluid of patients without EM, however, showed the exact opposite: sympathetic nerve fibres were induced, while sensitive nerve fibres were inhibited. This reflects the results of the changes associated with peritoneal EM and demonstrates serious changes in peritoneal innervation, which are caused by EM, lead to far-reaching changes in the entire peritoneal milieus, and may be the cause of the complex symptoms of the patients [48]. Interactions between endometrial lesions, nerve fibres, and immune cells are considered to be essential factors in these changes.
