State-dependent neuronal activities of glutamatergic and GABAergic neurons in the rat insular cortex during a cue-guided lever manipulation task involving saccharin reward

The insular cortex (IC) plays a pivotal role in processing gustatory information. Compared to those of glutamatergic neurons, the activities of GABAergic neurons in the IC during feeding behaviors are less well understood. If the IC GABAergic neurons have same role as other sensory cortices, they will suppress redundant neuronal activities. To elucidate GABAergic neuronal activities and their correlation with glutamatergic neuronal activity during feeding behaviors, we recorded extracellular unit activities in the rat IC during a cue-guided lever manipulation task in which saccharin solution was provided as a reward. Neurons were classified into fast-spiking-like (FSL) and non-FSL neurons using spike waveforms and firing rates. Compared with non-FSL neurons, a larger population of FSL neurons changed their firing rates during the task. When firing increases in FSL neurons were observed, 45-50% of simultaneously recorded non-FSL neurons showed increased firing rates. In contrast, during decreases in the firing rates of FSL neurons, only 10-31% of simultaneously recorded non-FSL neurons exhibited decreased firing rates. Cross-correlation analysis between FSL and non-FSL neurons revealed that the pairs showing changes in the firing rate in the same direction tended to exhibit greater synchronized activity than those showing distinct firing rate changes. These results suggest that GABAergic FSL and glutamatergic neurons in the IC discretely fire depending on the processing of taste information. These distinct activities may contribute not only to taste detection but also to preceding information processing such as expectation and anticipation of the taste.