Mark J. Zylka
Molecules and Mechanisms for Pain
We recently found that nociceptive (pain-sensing) circuits in mammals are highly organized at molecular and neuroanatomical levels. In our laboratory, we use molecular, genetic, electrophysiological and behavioral approaches to study these pain circuits in mice. Our ultimate goal is to identify new analgesics so that debilitating chronic pain conditions can be more effectively treated.
Neural circuit-based approaches:
We recently found a G protein-coupled receptor (GPCR or GPR) called Mrgprd (Mas-related GPR) which is expressed in a majority of all non- peptidergic neurons. Mrgprd is not expressed in CGRP+ neurons, nor is it expressed anywhere else in the brain or body.
To identify the tissues that Mrgprd-expressing
neurons innervate, we engineered knock-in mice that express a
membrane-tethered version of enhanced Green Fluorescent Protein (EGFPf)
from the Mrgprd locus (MrgprdΔEGFPf).
Surprisingly, we found that Mrgprd-expressing neurons only
innervate the epidermis of the skin (Fig. 2). Joints and internal
organs, were not innervated, suggesting pain signals are transmitted
from these tissues by molecularly-distinct circuits.
|Figure 2. Nerve
in glabrous skin.
Dorsal spinal cord. CGRP (red)
and Mrgprd (green) axons.
PKCγ interneurons (blue).
In addition to molecular differences, Mrgprd-expressing axons and CGRP+ axons terminate within different zones of the epidermis (Fig. 2). Mrgprd-expressing axons (green) also terminate beneath the red-labeled CGRP lamina in the dorsal spinal cord (Fig. 3). Taken together, these findings suggest peptidergic and non-peptidergic neurons might have unique functions and connectivity. Although studied for over 20 years, it is still not known why mammals have peptidergic and non- peptidergic circuits, both of which respond to noxious stimuli. Do these two molecularly different circuits have redundant or non-redundant functions in nociception? In our laboratory, we are trying to answer this fundamental question using a variety of approaches. As an example, we are making and studying "circuit knockout" mice. These mice are specifically missing either peptidergic or non-peptidergic neurons. We are studying the consequences of these ablations using molecular, electrophysiological and behavioral methodologies. We are also using genetically-encoded transneuronal tract tracers to better understand how these circuits interface with pain-related regions of the brain.
Awarded Transformative R01 grant from NIH (Sept. 2009)
Senior Editor of The Open Pain Journal (2010)