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Comparison of NPY Y1 Receptors in the Spines of Ovariectomized and Ovary Intact Rats

Introduction Neuropeptide Y (NPY) is a 36-amino acid neuropeptide that acts as a neurotransmitter in the brain and in the autonomic nervous system of humans. In the autonomic system it is produced mainly by neurons of the sympathetic nervous system and serves as a strong vasoconstrictor. Recently, NPY has been shown to increase during stress and in patients with chronic back pain. NPY has the potential to innervate 5 subtypes of receptors Y1-Y5. The Y1 receptor has been shown to play a role in cell survival and contribute to pain hypersensitivity. In addition, Y1 receptors are also thought to contribute to bone loss leading to disc degeneration in osteoporosis. The goal of our study was identify the location of the Y1 receptor in the disc of osteoporotic animals and to compare those to ovary intact control. Methods Eight OVX animals and eight ovary intact control animals were followed weekly for 8 weeks. Four animals in each group were sacrificed at 4 and 8 weeks and blood and spines were harvested. Spines were decalcified and processed for histological analysis. Routine hematoxylin and eosin staining along with immunhistochemical analysis of the spine Y1 receptors. Results The results showed the intervertebral discs in the control group appeared normal. The nucleus pulposus contained abundant notochordal cells, surrounded by large zones of extracellular matrix, and the cartilage endplates were hyaline cartilage composed of chondrocytes. In the OVX animals, the discs showed degenerative changes, where the nucleus pulposus appeared reduced in size and comprised relatively few, chondrocyte-like cells. Mucoid degeneration could also be seen. An increased number of small chondrocytes appeared in the inner layer of the annulus. Bony tissues that contained bone marrow, hematopoietic lineage cells and mineralized bone, became more obvious in the deep zone of middle cartilage endplate. Immunohistochemical analysis for Y1 receptors showed positive staining in the outer annulus in the control with defined staining of chondrocyte cells. Y1 receptor staining was diminished in the OVX animals. Higher levels of circulating NPY was found in the plasma of OVX animals and may contribute to internalization of the Y1 receptor as a compensatory mechanism. Conclusions Y1 receptors are located in the outer portion of the annulus of rats. The portion of cellular staining differed between the OVX and intact ovary control animals. More experiments are needed to clarify the role of NPY in the normal and osteoporotic spine.