Lysophosphatidic acid (1-acyl-2-sn-glycero-3-phosphate, LPA) is a naturally occurring phospholipid, generated from activated and injured cells. LPA has growth factor-like activity and also mediates various cellular functions in many cell types (Anliker & Chun, 2004). Biological effects of LPA, which include Ca²⁺ mobilization, regulation of cytoskeleton, induction of mitogenesis and cell migration, are mediated by activation of cognate G protein-coupled receptors (LPA1, LPA2, LPA3 and LPA4; Anliker & Chun, 2004). The LPA1 receptor has been implicated in nociception at the spinal level (Renback et al., 2000). The objectives of this study were to investigate the expression of LPA receptors in rat lumbar spinal cord and dorsal root ganglia (DRG) using radioligand binding and RT-PCR techniques.

Experimental tissues were harvested from male Sprague-Dawley rats (180-220 g). Lumbar spinal cords were dissected and prepared for [³⁵S]GTPS binding experiments in cryostat sections and particulate preparations (n3; Leggett et al., 2004). Receptor expression was examined in lumbar spinal cord (dorsal and ventral halves) and L4-L6 DRG (n=4) by the RT-PCR technique.

Table 1: [³⁵S]GTPS binding (% binding in the absence of ligand)

[³⁵S]GTPS binding responses to LPA and the cannabinoid agonist HU-210 (Leggett et al., 2004), but not morphine, were significantly enhanced (*p<0.05, Student’s t-test) in the presence of bovine serum albumin (BSA, 0.2 %, Table 1). In the presence of BSA, LPA enhanced [³⁵S]GTPS in a concentration-dependent manner in rat lumbar spinal cord membranes (pEC₅₀ value 6.97 ± 0.10; R_max 130 ± 4). In tissue cryostat sections examined for [³⁵S]GTPS autoradiography, LPA (1 µM) and morphine (10 µM) significantly enhanced binding in the dorsal horn of the spinal cord (117 ± 5 % and 120 ± 2 % control, respectively, p<0.05, 1-way ANOVA with Dunnett’s post-hoc test).

RT-PCR analysis of extracts from the spinal cord and DRG indicated the presence of lpa1 mRNA in dorsal and ventral spinal cord and DRG, while lpa3 mRNA was only observed in the DRG.

The elevation of [³⁵S]GTPS in the dorsal spinal horn by LPA indicates the presence of LPA receptors in that region, although whether these receptors are \on primary afferent nerve terminals or cell bodies in that region awaits further investigation. Our results indicate the expression of LPA1 receptors in dorsal and ventral spinal cord and DRG, while LPA3 receptors appear to be limited to the DRG. In the absence of selective ligands for LPA receptors, it is not yet possible to identify which receptor(s) mediate the responses observed.

Anliker B & Chun J (2004) J. Biol. Chem. in press
Leggett JD et al. (2004) Br. J. Pharmacol. 141, 253-62
Renback K et al. (2000) Mol. Brain Res. 75, 350-354