Aims: 

It is generally accepted that the endocannabinoid system, acting as a retrograde modulator of synaptic transmission, plays important roles in pain processing at various levels of the central nervous system including the superficial spinal dorsal horn. Although it is documented that cannabinoid receptors 1 are strongly expressed in the spinal dorsal horn, the cellular distribution of enzymes that synthesize endocannabinoid ligands is less well studied. Thus, we investigated the expression of diacylglycerol lipase-alpha (DGL-alpha) and N-acyl-phosphatidyl-ethanolamine-hydrolyzing phospholipase D (NAPE-PLD), enzymes synthesizing the major endocannabinoid ligands: 2-arachidonoylglycerol (2-AG) and anandamide, respectively, in the spinal dorsal horn.

Methods: 

The distribution of DGL-alpha and NAPE-PLD was analyzed by using immunocytochemical methods at light and electron microscopic levels.

Results: 

Postsynaptic dendrites showed strong immunoreactivity for both enzymes, but positive labeling was revealed only occasionally in axon terminals. The dendritic localization of DGL-alpha and NAPE-PLD showed remarkable differences. In double stained sections, a substantial proportion of DGL-alpha positive puncta established close appositions with axon terminals of nociceptive primary afferents and putative excitatory spinal neurons. In ultrastructural studies, these puncta were found to be attached to dendritic membranes in close vicinity to asymmetric synaptic contacts. In contrast, NAPE-PLD positive puncta were mostly revealed on dendritic membrane compartments further away from synapses. In addition, both enzymes showed a remarkably strong labeling on astrocytes and microglial cells.

Conclusion: 

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 be released by postsynaptic dendritic compartments adjacent to asymmetric synapses. It is likely that 2-AG and anandamide can also be released from dendritic and glial compartments that are not associated with synapses.