Non-peptidergic primary afferents are presynaptic to neurokinin-1 receptor immunoreactive lamina I projection neurons in rat spinal cord

Saeed, Ribeiro-Da-Silva, Ribeiro-Da-Silva Alfredo

Description

Pain-related (nociceptive) information is carried from the periphery to the dorsal horn of the spinal cord mostly by two populations of small diameter primary afferents, the peptidergic and the non-peptidergic. The peptidergic population expresses neuropeptides, such as substance P and calcitonin gene-related peptide, while the non-peptidergic fibers are devoid of neuropeptides, express the purinergic receptor P2X3, and bind the isolectin B4 (IB4). Although it has been known for some time that in rat the peptidergic afferents terminate mostly in lamina I and outer lamina II and non-peptidergic afferents in inner lamina II, the extent of the termination of the latter population in lamina I was never investigated as it was considered as very minor. Because our preliminary evidence suggested otherwise, we decided to re-examine the termination of non-peptidergic afferents in lamina I, in particular with regards to their innervation of projection neurons expressing substance P receptors (NK-1r). We used retrograde labeling of neurons from the parabrachial nucleus combined with lectin IB4 binding and immunocytochemistry. Samples were examined by confocal and electron microscopy. Results By confocal microscopy, we studied the termination of non-peptidergic afferents in lamina I using IB4 binding and P2X3 immunoreactivity as markers, in relation to CGRP immunoreactivy, a marker of peptidergic afferents. The number of IB4 or P2X3-labeled fibers in lamina I was higher than previously thought, although they were less abundant than CGRP-labeled afferents. There were very few fibers double-labeled for CGRP and either P2X3 or IB4. We found a considerable number of IB4-positive fiber varicosities in close apposition to NK-1r-positive lamina I projection neurons, which were distinct from peptidergic varicosities. Furthermore, we confirmed at the ultrastructural level that there were bona fide synapses between P2X3-immunoreactive non-peptidergic boutons and neurokinin-1 receptor-positive lamina I dendrites. Conclusions These results indicate the presence of direct innervation by non-peptidergic nociceptive afferents of lamina I projection neurons expressing NK-1r. Further investigations are needed to better understand the role of these connections in physiological conditions and chronic pain states.

Informations

• Publié par: biomed
• Publié le: 01 janvier 2012
• Nombre de lectures: 2
• Langue: English
• Poids de l'ouvrage: 1 Mo

Extrait

Saeed and RibeirodaSilvaMolecular Pain2012,8:64 http://www.molecularpain.com/content/8/1/64

MOLECULAR PAIN

R E S E A R C HOpen Access Nonpeptidergic primary afferents are presynaptic to neurokinin1 receptor immunoreactive lamina I projection neurons in rat spinal cord 1,2 1,2,3,4* Abeer W Saeedand Alfredo RibeirodaSilva

Abstract Background:Painrelated (nociceptive) information is carried from the periphery to the dorsal horn of the spinal cord mostly by two populations of small diameter primary afferents, the peptidergic and the nonpeptidergic. The peptidergic population expresses neuropeptides, such as substance P and calcitonin generelated peptide, while the nonpeptidergic fibers are devoid of neuropeptides, express the purinergic receptor P2X3, and bind the isolectin B4 (IB4). Although it has been known for some time that in rat the peptidergic afferents terminate mostly in lamina I and outer lamina II and nonpeptidergic afferents in inner lamina II, the extent of the termination of the latter population in lamina I was never investigated as it was considered as very minor. Because our preliminary evidence suggested otherwise, we decided to reexamine the termination of nonpeptidergic afferents in lamina I, in particular with regards to their innervation of projection neurons expressing substance P receptors (NK1r). We used retrograde labeling of neurons from the parabrachial nucleus combined with lectin IB4 binding and immunocytochemistry. Samples were examined by confocal and electron microscopy. Results:By confocal microscopy, we studied the termination of nonpeptidergic afferents in lamina I using IB4 binding and P2X3 immunoreactivity as markers, in relation to CGRP immunoreactivy, a marker of peptidergic afferents. The number of IB4 or P2X3labeled fibers in lamina I was higher than previously thought, although they were less abundant than CGRPlabeled afferents. There were very few fibers doublelabeled for CGRP and either P2X3 or IB4. We found a considerable number of IB4positive fiber varicosities in close apposition to NK1rpositive lamina I projection neurons, which were distinct from peptidergic varicosities. Furthermore, we confirmed at the ultrastructural level that there were bona fide synapses between P2X3immunoreactive nonpeptidergic boutons and neurokinin1 receptorpositive lamina I dendrites. Conclusions:These results indicate the presence of direct innervation by nonpeptidergic nociceptive afferents of lamina I projection neurons expressing NK1r. Further investigations are needed to better understand the role of these connections in physiological conditions and chronic pain states.

Introduction Unmyelinated C afferents comprise approximately 70% of all primary afferents fibers [1]. The majority of such fibers transmit nociceptive information from the periph ery to the spinal dorsal horn and have been divided into two main populations, the peptidergic and the non peptidergic [2]. The peptidergic population synthesizes

* Correspondence: [e-mail] 1 Department of Pharmacology and Therapeutics, McGill University, Montreal, Quebec H3G 1Y6, Canada 2 Alan Edwards Centre for Research on Pain, McGill University, Montreal, Quebec H3A 2B2, Canada Full list of author information is available at the end of the article

neuropeptides, such as substance P (SP) and calcitonin gene related peptide (CGRP), expresses the high affinity nerve growth factor receptor, trkA, and terminates mainly in lamina I and outer lamina II of the spinal dor sal horn [35]. The nonpeptidergic unmyelinated affer ents, although devoid of neuropeptides, express the purinergic P2X3 receptor, bind the plant isolectin B4 (IB4), express receptors for glial cellderived neuro trophic factor (GDNF) and terminate mainly in inner lamina II of the spinal dorsal horn [68]. These two populations can also be differentiated based on the ultra structural properties of their central terminals in the

© 2012 Saeed and RibeirodaSilva; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.