INVOLVEMENT OF SPHINGOMYELIN HYDROLYSIS AND THE MITOGEN-ACTIVATED PROTEIN-KINASE CASCADE IN THE DELTA(9)-TETRAHYDROCANNABINOL-INDUCED STIMULATION OF GLUCOSE-METABOLISM IN PRIMARY ASTROCYTES

The effects of cannabinoids on metabolic pathways and signal transduct ion systems were studied in primary cultures of rat astrocytes. Delta( 9)-Tetrahydrocannabinol (THC), the major active component of marijuana , increased the rate of glucose oxidation to CO2 as well as the rate o f glucose incorporation into phospholipids and glycogen. These effects of THC were mimicked by the synthetic cannabinoid HU-210, and prevent ed by forskolin, pertussis toxin, and the CB1 receptor antagonist SR 1 41716. THC did not affect basal cAMP levels but partially antagonized the forskolin-induced elevation of intracellular cAMP concentration. T HC stimulated p42/p44 mitogen-activated protein kinase (MAPK) activity , Raf-1 phosphorylation, and Raf-1 translocation to the particulate ce ll fraction. In addition, the MARK inhibitor PD 098095 and the phospho inositide 3-kinase inhibitors wortmannin and LY 294002 were able to an tagonize the THC-induced stimulation of glucose oxidation to CO2, phos pholipid synthesis and glycogen synthesis. The possible involvement of sphingomyelin breakdown in the metabolic effects of THC was studied s ubsequently. THC produced a rapid stimulation of sphingomyelin hydroly sis that was concomitant to an elevation of intracellular ceramide lev els. This effect was prevented by SR 141716. Moreover, the cell-permea ble ceramide analog D-erythro-N-octanoylsphingosine, as well as exogen ous sphingomyelinase, were able in turn to stimulate MARK activity, to increase the amount of Raf-1 bound to the particulate cell fraction, and to stimulate glucose metabolism. The latter effect was prevented b y PD 098059 and was not additive to that exerted by THC. Results thus indicate that THC produces a cannabinoid receptor-mediated stimulation of astrocyte metabolism that seems to rely on sphingomyelin hydrolysi s and MAPK stimulation.