Contribution of excitatory amino acids to morphine-induced metabolic alterations

Previous studies have indicated that excitatory amino acids are involved in the analgesic and addictive properties of morphine. However, their role in the morphine-induced alterations in glucose metabolism is not known. This study assessed the contribution of NMDA receptor activation to the morphine-induced hormonal and metabolic alterations in conscious unrestrained chronically catheterized rats. Whole body glucose flux was assessed with a primed constant intravenous infusion of [3-³H]glucose in rats pretreated with the NMDA-receptor antagonist MK-801 (0.25 mg/kg, intraarterial) or an equal volume (1.5 ml) of sterile saline (0.9%) administered 15 min prior to i.c.v. injection of H₂O (Con; 5 μl) or morphine sulfate (80 μg). No significant alterations were noted in metabolic and hormonal parameters of H₂O injected rats. i.c.v. morphine increased the plasma glucose concentration (60%), hepatic glucose production (R(a); 60%) and whole body glucose utilization (R(d); 53%), but did not alter the glucose metabolic clearance rate (MCR). MK-801 alone resulted in transient hyperglycemia (25%), stimulation of glucose R(a) (60%) and glucose R(d) (53%), and a significant (30%) increase in MCR. MK-801 pretreatment blunted the morphine-induced hyperglycemia and the increased glucose R(a) and R(d). Morphine increased the plasma concentration of epinephrine (4-fold), norepinephrine (2-fold) and corticosterone (67%); however, no alterations in plasma insulin and glucagon were detected. MK-801 pretreatment, blunted the morphine-induced increase in corticosterone and norepinephrine, and elicited a significant rise in insulin concentrations. These results indicate that activation of the NMDA receptors contributes to the morphine-induced hyperglycemia and hormonal alterations. Furthermore, this response appears partially mediated by activation of sympathetic outflow and suppression of insulin release, which is blunted by inhibition of NMDA receptors.