Insulin enhancement of opioid peptide transport across the blood-brain barrier and assessment of analgesic effect

Insulin crosses the blood-brain barrier (BBB) via receptor-mediated transcytosis and has been suggested to augment uptake of peripheral substances across the BBB. The δ-opioid receptor-selective peptide D-penicillamine^2,5 (DPDPE), a Met-enkephalin analog, produces analgesia via a central nervous system-derived effect. In vitro (K(cell), μ1·min^-1·mg^-1) and in situ (K(in), μl · min^-1 · g^-1) analyses of DPDPE transport (K(cell) = 0.56 ± 0.15; K(in) = 0.28 ± 0.03) revealed significant (P < .01) increases in DPDPE uptake by the BBB with 10 μM insulin (K(cell) = 1.61 ± 0.25; K(in) = 0.48 ± 0.04). In vitro cellular uptake was significantly increased (P < .05) at 1 μM insulin, whereas no significant uptake was observed with CTAP (a somatostatin opioid peptide analog) or sucrose (a paracellular diffusionary marker). No significant change in uptake was seen with DPDPE, CTAP, or sucrose in the presence of holo-transferrin (0-100 μM), indicating that the effect of insulin on DPDPE was not a generalized effect of receptor endocytosis. Insulin did not affect P-glycoprotein efflux, a mechanism that has shown affinity for DPDPE. A similar uptake of DPDPE into the brain (64% increase) was seen with the in situ brain perfusion model. Analgesic assessment revealed a significant decline in DPDPE (i.v.)-induced analgesia with increasing concentrations of insulin (i.v., i.c.v., s.c.) in a dose-dependent manner. Thus, insulin significantly increases DPDPE uptake across the BBB by a specific mechanism. The analgesic effect seen with DPDPE and insulin coadministration was shown to decrease, indicating that insulin reduces the analgesic effect within the central nervous system rather than at the BBB.