Endomorphin-2 immunoreactivity in the cervical dorsal horn of the rat spinal cord at the electron microscopic level

• Published in: Neuroscience 2002-01, Vol.113 (3), p.593-605
• Main Authors: Wang, Q.P, Zadina, J.E, Guan, J.-L, Kastin, A.J, Funahashi, H, Shioda, S
• Format: Article
• Language: English
• Subjects: Animals; axo-axonic; Biological and medical sciences; Central nervous system; Central neurotransmission. Neuromudulation. Pathways and receptors; Cervical Vertebrae; Fundamental and applied biological sciences. Psychology; granular vesicle; immunocytochemistry; Male; Microscopy, Immunoelectron; Oligopeptides - analysis; Oligopeptides - immunology; Posterior Horn Cells - chemistry; Posterior Horn Cells - ultrastructure; Presynaptic Terminals - chemistry; Presynaptic Terminals - ultrastructure; Rats; Rats, Wistar; Receptors, Opioid, mu - agonists; synapse; Synapses - chemistry; Synapses - ultrastructure; ultrastructure; Vertebrates: nervous system and sense organs
• ISSN: 0306-4522; 1873-7544
• DOI: 10.1016/S0306-4522(02)00153-7

Endomorphin-2 is a newly discovered endogenous opioid peptide with high affinity and selectivity for the μ-opioid receptor, and potent analgesic activity, particularly in the spinal cord. Using immunoelectron microscopy, we examined the ultrastructure of the endomorphin-2-like immunoreactive processes and their synaptic relationships in the spinal cord. Endomorphin-2-like immunopositive dense-cored vesicles were observed in many axon terminals, and, in a few cases, were observed together with immunonegative dense-cored vesicles. Immunopositive axons with or without myelination were also observed. The endomorphin-2-like immunoreactive axon terminals formed synapses with both immunopositive and immunonegative processes. Most synapses were asymmetrical, but symmetrical synapses were also found. Examples of axo-dendritic, axo-somatic and axo-axonic contacts were observed. This first demonstration of the ultrastructure and synaptic relationships of endomorphin-2-like immunoreactive axon terminals in the spinal cord dorsal horn provides morphological evidence that this peptide functions as a transmitter regulating pain processes.