Differential distribution of diacylglycerol lipase‐alpha and N‐acylphosphatidylethanolamine‐specific phospholipase d immunoreactivity in the superficial spinal dorsal horn of rats

Abstract

It is generally accepted that the endocannabinoid system plays important roles in spinal pain processing. Although it is documented that cannabinoid-1 receptors are strongly expressed in the superficial spinal dorsal horn, the cellular distribution of enzymes that can synthesize endocannabinoid ligands is less well studied. Thus, using immunocytochemical methods at the light and electron microscopic levels, we investigated the distribution of diacylglycerol lipase-alpha (DGLα) and N-acylphosphatidylethanolamine-specific phospholipase D (NAPE-PLD), enzymes synthesizing the endocannabinoid ligands, 2-arachidonoylglycerol (2-AG) and anandamide, respectively. Positive labeling was revealed only occasionally in axon terminals, but dendrites displayed strong immunoreactivity for both enzymes. However, the dendritic localization of DGLα and NAPE-PLD showed a remarkably different distribution. DGLα immunolabeling in dentrites was always revealed at membrane compartments in close vicinity to synapses. In contrast to this, dendritic NAPE-PLD labeling was never observed in association with synaptic contacts. In addition to dendrites, a substantial proportion of astrocytic (immunoreactive for GFAP) and microglial (immunoreactive for CD11b) profiles were also immunolabeled for both DGLα and NAPE-PLD. Glial processes immunostained for DGLα were frequently found near to synapses in which the postsynaptic dendrite was immunoreactive for DGLα, whereas NAPE-PLD immunoreactivity on glial profiles at the vicinity of synapses was only occasionally observed. Our results suggest that both neurons and glial cells can synthesize and release 2-AG and anandamide in the superficial spinal dorsal horn. 2-AG can primarily be released by postsynaptic dendrites and glial processes adjacent to synapses, whereas anandamide can predominantly be released from non-synaptic dendritic and glial compartments.

Cannabinoids to treat spinal cord injury

“…3; Garcia-Ovejero et al, 2009). Anandamide may be produced by neurons and possibly microglial cells, that express NAPE-PLD in the intact dorsal horn (Hegyi et al, 2012) with an increase in the first days after injury (authors unpublished observations).…”

“…Accordingly, the enzymatic machinery for 2-AG formation and degradation is also present in the spinal cord: diacylglycerol lipase alpha (DAGL-α), a major synthesizing enzyme for 2-AG ( Fig. 1) is strongly expressed in the dorsal horn, the intermediolateral nucleus and laminae VIII and IX, moderately in the rest of gray matter and with lower intensity in white matter oligodendrocytes and subpial astrocytes (Garcia-Ovejero et al, 2009;Nyilas et al, 2009;Hegyi et al, 2012). Some authors report also that a notable proportion of dorsal horn DAGL-α belongs to microglia and astrocytes (Hegyi et al, 2012).…”

“…1) is strongly expressed in the dorsal horn, the intermediolateral nucleus and laminae VIII and IX, moderately in the rest of gray matter and with lower intensity in white matter oligodendrocytes and subpial astrocytes (Garcia-Ovejero et al, 2009;Nyilas et al, 2009;Hegyi et al, 2012). Some authors report also that a notable proportion of dorsal horn DAGL-α belongs to microglia and astrocytes (Hegyi et al, 2012). Non-injured spinal cords also express monoacylglycerol lipase (MAGL), the main degradative enzyme for 2-AG ( Fig.…”

“…The enzymes responsible for the synthesis and degradation of anandamide, N-acylphosphatidilethanolamine-phospholipase D (NAPE-PLD) and fatty acid amidohydrolase (FAAH), respectively, are also expressed in the non-injured spinal cord (Cravatt et al, 2004;Garcia-Ovejero et al, 2009;Sagar et al, 2010;Hegyi et al, 2012;Fullhase et al, 2013). NAPE-PLD in the dorsal horn may be associated with dendrites, astrocytes and microglia (Hegyi et al, 2012).…”

“…NAPE-PLD in the dorsal horn may be associated with dendrites, astrocytes and microglia (Hegyi et al, 2012). Other N-acyl ethanolamines (oleoylethanolamine, palmitoylethanolamine) that share the same enzymatic machinery with anandamide, are also produced in the normal cord and are modulated after SCI (Garcia-Ovejero et al, 2009;Hama et al, 2014), but they lack affinity for cannabinoid receptors.…”