Delta 9-tetrahydrocannabinol inhibits arachidonic acid acylation of phospholipids and triacylglycerols in guinea pig cerebral cortex slices.
We reported earlier that delta 9-tetrahydrocannabinol (THC), the main psychoactive ingredient in marihuana, increased markedly the level of unesterified arachidonic acid (AA) in guinea pig cerebral cortex slices prelabeled with [14C]AA. The purpose of the present study was to clarify the mechanism underlying THC-enhanced mobilization of AA. We could find little data to support an involvement for phospholipase A2 in this response. For example, the levels of lysophosphatidylcholine or lysophosphatidylethanolamine were not elevated after incubation with THC. A role for phosphoinositidase C-initiated lipolytic pathways was excluded, because neither basal nor acetylcholine-stimulated inositol phosphate formation was altered by THC. When we prelabeled slices with [14C]stearate or [3H]glycerol, THC did not elevate levels of unesterified [14C]stearate, nor did we observe significant changes in the phospholipids that were labeled with either precursor. These findings were in marked contrast to the previously reported reductions in [14C]AA-labeled phosphatidylinositol after exposure of prelabeled brain slices to THC; moreover, they suggested that the THC-induced effects on brain lipid metabolism in vitro were rather specific for AA. We show here that, when the acylation of brain lipids with AA was measured by addition of [3H]AA in the presence and absence of THC at zero time and incubation for 1 hr at 37 degrees, THC elicited marked, dose-dependent, and saturable reductions in esterified [3H]AA levels. The reductions in incorporation were balanced by increases in unesterified [3H]AA. The IC50 for the effect was on the order of 8 microM, and a maximal response occurred at 32 microM. We observed that the THC-induced suppression in acylation of the phospholipids by radiolabeled AA was up to 5-fold greater than the THC-elicited loss of AA from slices prelabeled before exposure to THC. The largest inhibitions of acylation were in phosphatidylinositol; the suppression of radioactivity in this phospholipid accounted for over 50% of the rise in unesterified [3H]AA. The radioactivity incorporated in triacylglycerols were also reduced markedly by THC. In contrast, the incorporation of radioactivity in phosphatidylcholine remained unaffected by THC. Taken together, these findings suggest that THC mobilizes AA by inhibiting acylation of certain lipids with AA, particularly phosphatidylinositol and triacylglycerol, rather than by liberating fatty acids by lipolysis. Comparison of the effects of several primary cannabinoids on lipid acylation with [3H]AA revealed that there was no relationship between the potencies of cannabinoids in inhibiting the incorporation of [3H]AA into membrane lipids and their psychoactive potencies in vivo; moreover, the stereoisomers of THC were equipotent.(ABSTRACT TRUNCATED AT 400 WORDS)
Reichman
M
M
Department of Pharmacology, University of Wisconsin Medical School, Madison 53706.
Nen
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Hokin
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D000478:Alkylation; D000818:Animals; D016718:Arachidonic Acid / Q000378:metabolism*; D002250:Carbon Radioisotopes; D002540:Cerebral Cortex / Q000033:anatomy & histology / Q000187:drug effects* / Q000378:metabolism; D013759:Dronabinol / Q000494:pharmacology*; D006168:Guinea Pigs; D008297:Male; D010741:Phospholipases A / Q000378:metabolism; D054467:Phospholipases A2; D010743:Phospholipids / Q000378:metabolism*; D013228:Stearates / Q000378:metabolism; D013329:Structure-Activity Relationship; D014280:Triglycerides / Q000494:pharmacology*
D002250:Carbon Radioisotopes; D010743:Phospholipids; D013228:Stearates; D014280:Triglycerides; D016718:Arachidonic Acid; D013759:Dronabinol; D010741:Phospholipases A; D054467:Phospholipases A2