Calcitonin gene-related peptide is a key neurotransmitter in the neuro-immune axis

Abstract

The question of how the neural and immune systems interact in host defense is important, integrating a system that senses the whole body with one that protects. Understanding the mechanisms and routes of control could produce novel and powerful ways of promoting and enhancing normal functions as well as preventing or treating abnormal functions. Fragmentation of biological research into specialities has resulted in some failures in recognizing and understanding interactions across different systems and this is most striking across immunology, hematology, and neuroscience. This reductionist approach does not allow understanding of the in vivo orchestrated response generated through integration of all systems. However, many factors make the understanding of multisystem cross-talk in response to a threat difficult, for instance the nervous and immune systems share communication molecules and receptors for a wide range of physiological signals. But, it is clear that physical, hard-wired connections exist between the two systems, with the key link involving sensory, unmyelinated nerve fibers (c fibers) containing the neuropeptide calcitonin gene-related peptide (CGRP), and modified macrophages, mast cells and other immune and host defense cells in various locations throughout the body. In this review we will therefore focus on the induction of CGRP and its key role in the neuroimmune axis.

Keywords: C fibers; CGRP; TNF-alpha; TRPV1; gut-brain axis; neuroimmunology.

Figure 1

Pathways of CGRP release via TRPV1 activation. TRPV1 is activated by many external factors associated with pain but also regular sensation below pain thresholds. These include the well characterized responses to heat, pH, and damage, as well as to toxins, immune challenges and membrane potential changes brought about by physiological and pathological processes. Endogenous and/or intracellular activation can also occur through the actions of PIP2 and Anandomide. Any activation of TRPV1 results in release of CGRP from the neuron. This released CGRP then acts on its specific receptor on cells immediately adjacent to the neuron, which in the case of sensory c fibers can be lymphocytes, DCs, mast cells, macrophages or cells within the hemopoietic microenvironment of the bone marrow (not shown). The range of activities stimulated by CGRP are listed below each cell type.