Elsevier

Neuroscience

Volume 106, Issue 4, 31 October 2001, Pages 833-842
Neuroscience

Presynaptic localization of the carboxy-terminus epitopes of the μ opioid receptor splice variants MOR-1C and MOR-1D in the superficial laminae of the rat spinal cord

https://doi.org/10.1016/S0306-4522(01)00317-7Get rights and content

Abstract

Opioids inhibit nociceptive transmission at the level of the spinal cord, possibly through inhibition of neurotransmitter release by presynaptic μ opioid receptors (MORs) thus preventing the activation of ascending pathways and the perception of pain. Most nociceptive primary afferents are unmyelinated fibers containing peptides such as substance P and/or calcitonin gene-related peptide. However, few terminals contain both substance P and MOR. Recently, we identified new carboxy-terminal MOR splice variants that are localized in the superficial laminae of the dorsal horn. We now report the precise cellular distribution of two of these MOR-1 variants, MOR-1C (exon 7/8/9 epitope) and MOR-1D (exon 8/9 epitope), at the ultrastructural level. In the superficial laminae of the dorsal horn, the majority of the labeling of MOR-1C and MOR-1D was found in unmyelinated axons. This distribution contrasts with that of MOR-1 (exon 4 epitope), in which labeling is equally found in dendrites and soma, as well as in axons. The presence of dense core vesicles in many of the MOR-1C-like immunoreactive terminals implies that this splice variant might be involved in presynaptic inhibition of transmitter release from peptide-containing afferents to the dorsal horn. Consistent with this finding, confocal microscopy analyses showed that many MOR-1C profiles in laminae I–II also contained calcitonin gene-related peptide, whereas fewer MOR-1 profiles contained either substance P or calcitonin gene-related peptide in this same region.

From these findings we suggest that there are differential distributions of MOR-1 splice variants as well as distinct peptide colocalizations in the dorsal horn.

Section snippets

Perfusion and tissue processing

The procedures used in these studies were approved by the Institutional Animal Care and Use Committee of Cornell University Medical College and of the Memorial Sloan Kettering Cancer Center. All efforts were made to prevent animal suffering and to use the minimum number of animals needed to make sound scientific conclusions. For electron microscopy studies, male Sprague–Dawley rats (300–400 g; n=3; Charles River Laboratories, Wilmington, MA, USA) were deeply anesthetized with sodium

MOR-1C and MOR-1D distributions in the superficial dorsal horn

The light microscopic distribution of MOR-1C-LI and MOR-1D-LI examined in this study is illustrated in Fig. 1. In the spinal cord (Fig. 1A), MOR-1C was mainly found in the superficial laminae where it was located primarily in punctate profiles (Fig. 1B) that we show correspond to axon terminals at the ultrastructural level. We also observed MOR-1C-LI thin beaded processes in laminae V–VI and in lamina X (Abbadie et al., 2000c).

The superficial laminae of the spinal cord contained the highest

Discussion

In the superficial laminae of the dorsal horn, the majority of MOR-1C and MOR-1D-LI profiles were myelinated axons, unmyelinated axons or axon terminals. This distribution contrasts with that of MOR-1-LI which is frequently found in dendrites and soma, as well as in axons, in the same region. We also found that one third of MOR-1C-LI terminals contain dense core vesicles, indicating that this splice variant might play a role in presynaptic inhibition of neurotransmitter release from

Conclusion

We showed that two distinct splice variants of MOR-1, MOR-1C and MOR-1D, were localized in myelinated axons, unmyelinated axons or axon terminals in the superficial laminae of the dorsal horn. This distribution contrasts with that of MOR-1-LI which in the same region is frequently found in dendrites and soma as well as in axons. We also found that one third of MOR-1C-LI terminals contain dense core vesicles, indicating that this splice variant might play a role in presynaptic inhibition of

Acknowledgements

We thank Theresa Zhou, Sarita Sharma and Alla Goldberg for technical assistance. This work was supported, in part, by grants (DA02615 and DA7242) and a Senior Scientist Award to G.W.P. (DA00220) from the National Institute on Drug Abuse, a core grant from the National Cancer Institute to MSKCC (CA08748) and a grant from the National Institute of Dental and Craniofacial Research (DE 12640) to S.A.A.

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    Present address: Neurological Sciences Institute, Oregon Health Sciences University, 1120 North West 20th Avenue, Portland, OR 97209, USA.

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