Background
AQP9 is a water channel that facilitates the diffusion of water and also of glycerol and monocarboxylates (for review see Badaut et Regli, 129(4), 969, Neuroscience 2004). Recent reports have shown that the expression of AQP9, is negatively regulated by insulin in hepatocytes. Brain AQP9 is expressed in catecholaminergic neurons and in astrocytes. Interestingly, insulin has been shown to cross the blood-brain barrier and bind specific receptors in the brain, which may be implicated in energy balance. Based on the above evidence, we examined whether brain AQP9 can be regulated by insulin.
Methods and Results
This hypothesis has been tested in the rat, in brain stem slices in vitro and in streptozotocin (STZ) treated animals in vivo; a diabetes model in the rat. The use of brainstem culture eliminates the influence of circulating insulin. By double immunolabeling experiments, we show the presence of Tyrosine Hydroxylase-positive neurons exhibiting AQP9 staining in brain stem slices. In these slices, there is a significant decrease in AQP9 levels 6 h after treatment with 2 mM of insulin (41% of the control, P<0.05, ANOVA, n=6) measured by Western blot and AQP9 levels return to control values at 24 hours. In STZ treated rats (diabetic rats), the level of AQP9 expression is significantly increased in the liver (138% of control, p<0.05). In the brain, AQP9 is also significantly increased in the nucleus of solitary tract (240% of control rats, P <0.005, ANOVA, n=4), in the locus coereuleus (180%, P<0.05, ANOVA, n=4) and in the subtantia nigra (200% of control rats, P<0.005, ANOVA, n=8). In contrast, AQP9 expression is not modified in the cerebellum where AQP9 is expressed on astrocytes. AQP9 immunoreactivity is significantly increased in dopaminergic neurons in the subtantia nigra of STZ rats (126 versus 36 arbitrary units for control rats, P<0.005, n=3, ANOVA). In the cerebellum, no significant difference was observed between control and STZ rats.
Conclusion
These results show that in the brain AQP9 expression on catecholaminergic neurons is regulated by the systemic insulin. This result supports the hypothesis that AQP9 may be implicated in brain metabolism as a channel for glycerol and monocarboxylates.