The blood-brain barrier as a regulatory interface in the gut-brain axes.

The blood-brain barrier (BBB) prevents the unrestricted movement of peptides and proteins between the brain and blood. However, some peptides and regulatory proteins can cross the BBB by saturable and non-saturable mechanisms. Leptin and insulin each cross the BBB by their own transporters. Impaired transport of leptin occurs in obesity and accounts for peripheral resistance; that is, the condition wherein an obese animal loses weight when given leptin directly into the brain but not when given leptin peripherally. Leptin transport is also inhibited in starvation and by hypertriglyceridemia. Since hypertriglyceridemia occurs in both starvation and obesity, we have postulated that the peripheral resistance induced by hypertriglyceridemia may have evolved as an adaptive mechanism in response to starvation. Insulin transport is also regulated. For example, treatment of mice with lipopolysaccharide (LPS) increases insulin transport across the BBB by about threefold. Since many of the actions of CNS insulin oppose those of peripheral insulin and since LPS releases proinflammatory cytokines, enhanced transport of insulin across the BBB could be a mechanism which promotes insulin resistance in sepsis. The brain endothelial cells which comprise the BBB secrete many substances including cytokines. Such secretion can be stimulated from one side of the BBB with release into the other side. For example, it appears that adiponectin can inhibit release of interleukin-6 from brain endothelial cells. Overall, the BBB represents an important interface in mediating gut-brain axes.