Sodium-coupled glucose cotransporters contribute to hypothalamic glucose sensing

Specialized neurons within the hypothalamus have the ability to sense and respond to changes in ambient glucose concentrations. We investigated the mechanisms underlying glucose-triggered activity in glucose-excited neurons, using primary cultures of rat hypothalamic neurons monitored by fluorescence calcium imaging. We found that 35% (738 of 2,139) of the neurons were excited by increasing glucose from 3 to 15 mmol/l but only 9% (6 of 64) of these glucose-excited neurons were activated by tolbutamide, suggesting the involvement of a ATP-sensitive K+ channel-independent mechanism. alpha-Methylglucopyranoside (alpha MDG; 12 mmol/1), a nonmetabolizable substrate of sodium glucose cotransporters (SGLTs), mimicked the effect of high glucose in 67% of glucose-excited neurons, and both glucose- and alpha MDG-triggered excitation were blocked by Na+ removal or by the SGLT inhibitor phloridzin (100 nmol/l). In the presence of 0.5 mmol/l glucose and tolbutamide, responses could also be triggered by 3.5 mmol/l alpha MDG, supporting a role for an SGLT-associated mechanism at low as well as high substrate concentrations. Using RT-PCR, we detected SGLT1, SGLT3a, and SGLT3b in both cultured neurons and adult rat hypothalamus. Our findings suggest a novel role for SGLTs in glucose sensing by hypothalamic glucose-excited neurons.