Aims: 

The glucose-monitoring (GM) neurons in the forebrain are known to play important roles in the central feeding control, nevertheless, our knowledge is insufficient yet about the complex functional attributes of these chemosensory cells.

Methods: 

Series of microelectrophysiological, neurochemical-behavioral, and fMRI experiments have been conducted in laboratory rats and macaques. Single neuron activity was recorded in various forebrain areas in anesthesized rats and alert rhesus monkeys by means of the multibarreled microelectrophoretic technique. Furthermore, feeding and metabolic consequences of a single microinjection of streptozotocin (STZ) into these same forebrain regions were studied in the laboratory rat. In addition, repeated i.v. glucose injection induced brain activation was examined by fMRI in the primate.

Results: 

About 20–25% of all neurons tested changed in activity in response to the microelectroosmotic administration of glucose. Gustatory stimulations elicited firing rate changes in 1/3 of all cells, and the GM neurons were more likely to be influenced by taste stimuli than the glucose-insensitive units. Phasic and tonic activity changes were recorded during intragastric infusions, and neuronal responses to intragastrically delivered glucose and monosodium L-glutamate, especially those of the GM cells, were often altered by preceding administration of sweet and umami gustatory stimuli. The GM neuron destroying STZ microinjections in these forebrain regions resulted in glucose intolerance, metabolic alterations, and taste perception deficits as well. Repeated i.v. glucose injections in the monkey led to changes of fMRI activation-deactivation patterns in the hypothalamus, amygdala, nucleus accumbens and orbitofrontal cortex, structures with high proportion of the GM cells.

Conclusion: 

Our findings indicate that both endogenous and exogenous chemosensory signals converge in the limbic forebrain. The GM neurons here integrate multiple information, and thus, appear to play significant role in adaptive mechanisms of the central regulation of homeostasis.

Support: 

Ajinomoto 51064/2009, OTKA K 68431, and HAS