Behavioral and neural responses to gustatory stimuli delivered non-contingently through intra-oral cannulas

Abstract

The act of eating requires a decision by an animal to place food in its mouth. The reasons to eat are varied and include hunger as well as the food's expected reward value. Previous studies of tastant processing in the rat primary gustatory cortex (GC) have used either anesthetized or awake behaving preparations that yield somewhat different results. Here we have developed a new preparation in which we explore the influences of intra-oral and non-contingent tastant delivery on rats' behavior and on their GC neural responses. We recorded single unit activity in the rat GC during two sequences of tastant deliveries, PRE and POST, which were separated by a waiting period. Six tastants ranging in hedonic value from sucrose to quinine were delivered in the first two protocols called 4TW and L-S. In the third one, the App L-S protocol, only hedonically positive tastants were used. In the 4TW protocol, tastants were delivered in blocks whereas in the two L-S protocols tastants were randomly interleaved.In the 4TW and L-S protocols the probability of ingesting tastants in the PRE sequence decreased exponentially with the trial number. Moreover, in both protocols this decrease was greater in the POST than in the PRE sequence likely because the subjects learned that unpleasant tastants were to be delivered. In the App L-S protocol the decrease in ingestion was markedly slower than in the other protocols, thus supporting the hypothesis that the decrease in appetitive behavior arises from the noncontingent intra-oral delivery of negatively hedonic tastants like quinine.Although neuronal responses in the three protocols displayed similar variability levels, significant differences existed between the protocols in the way the variability was partitioned between chemosensory and non-chemosensory neurons. While in the 4TW and L-S protocols the former population displayed more changes than the latter, in the App L-S protocol variability was homogeneously distributed between the two populations. We posit that these tuning changes arise,

Central role for the insular cortex in mediating conditioned responses to anticipatory cues

“…Responses to food delivery and food consumption were expected, because they are consistent with the role in taste processing that is traditionally assigned to the IC in rodents. Indeed, the rodent IC is generally considered to be the site of the primary gustatory cortex across species (19,(33)(34)(35). Although evidence for responses to anticipatory cues is less in line with classical reports on the function of the IC in rodents, the evidence is entirely consistent with more recent evidence suggesting the involvement of the IC in processing the anticipation of food and reward in general (18,24,25).…”

Speed and accuracy of taste identification and palatability: impact of learning, reward expectancy, and consummatory licking

“…10E, see also (52)]. Therefore, the use of high QHCl concentrations is not an ideal stimulus to uncover pure taste coding mechanisms, since it will not only induce an aversive (bitter) taste sensation, but will also activate neural pathways that reduce consummatory licking and induce an overall state of fear [or punishment (41)]. If a suprathreshold QHCl concentration is to be used, then attention must be paid to control for different oromotor responses (24) evoked by QHCl vs. other tastants (e.g., sucrose) before attributing that neuronal changes are driven only by reward and punishment mechanisms, instead of oromotor differences (31,35,50).…”

Associatively Learned Representations of Taste Outcomes Activate Taste-Encoding Neural Ensembles in Gustatory Cortex

“…Indeed, the majority of neurons recorded in GC do not appear to be selective for chemosensory taste properties. As few as 14% of cells using averaging techniques (Yasoshima and Yamamoto, 1998) and as many as 41% using more sensitive temporal analyses (Katz et al, 2001; Soares et al, 2007) selectively encoded taste information. Thus, such broad tuning as well as the integration of somatosensory and taste information inherent in the region may limit the sensitivity of IEG ensemble analysis in GC.…”