Involvement of forebrain glucose-monitoring neurons in taste information processing: electrophysiological and behavioral studies.

The ventromedial hypothalamic nucleus (VMH), the nucleus accumbens (NAcc) and the mediodorsal and ventrolateral prefrontal (orbitofrontal) cortices (mdPFC and OBF, respectively), key structures of the forebrain limbic circuitry, are known to play important roles in various mechanisms of the homeostatic regulation (Oomura, 1980; Swanson and Mogenson, 1981; Neafsey, 1990). In previous studies, special chemoneurons, the so called ‘glucose-sensitive’ (GS) and ‘glucose-receptor’ (GR) neural cells have been discovered in the above regions, and these glucose-monitoring (GM) neurons, in addition to specifically change in activity in response to increase of the extracellular glucose concentration, have also been shown to be involved in various processes of the organization of feeding (Oomura et al., 1964; Oomura, 1980; Karadi et al., 1995, 2002). Despite an increasing amount of related data, however, very little is known yet about simultaneous ‘endogenous’ and ‘exogenous’ gustatory chemosensitivities of these GM cells. To elucidate the involvement of GM units in an integrative processing of complex chemical information, microelectrophysiological as well as behavioral investigations have been performed. Extracellular single neuron activity of the VMH, NAcc, mdPFC and OBF was recorded during (i) microelectrophoretic administration of D-glucose and other chemicals and (ii) gustatory stimulations as well. Recently conducted behavioral–biochemical experiments provided evidence for that the pancreatic β-cell destroying streptozotocin (STZ) (Like and Rossini, 1976) specifically damages the GM neurons if applied locally to these structures (Egyed et al., 2000; Karadi et al., 2000). In our other line of present studies, taste reactivity and taste associated learning ability were tested in STZ treated or control rats.