Investigating the Effect of Physiological Need States on Palatability and Motivation Using Microstructural Analysis of Licking

Abstract

The study of consummatory responses during food intake represents a unique opportunity to investigate the physiological, psychological and neurobiological processes that control ingestive behavior. Recording the occurrence and temporal organization of individual licks across consumption, also called lickometry, yields a rich data set that can be analyzed to dissect consummatory responses into different licking patterns. These patterns, divided into trains of licks separated by pauses, have been used to deconstruct the many influences on consumption, such as palatability evaluation, incentive properties, and post-ingestive processes. In this review, we describe commonly used definitions of licking patterns and how various studies have defined and measured these. We then discuss how licking patterns can be used to investigate the impact of different physiological need states on processes governing ingestive behavior. We also present new data showing how licking patterns are changed in an animal model of protein appetite and how this may guide food choice in different protein-associated hedonic and homeostatic states. Thus, recording lick microstructure can be achieved relatively easily and represents a useful tool to provide insights, beyond the measurement of total intake, into the multiple factors influencing ingestive behavior. HIGHLIGHTS • Analysis of lick microstructure provides useful information on incentive and hedonic processes underlying ingestive behavior • Different physiological need states, such as hunger, impact licking patterns • Protein appetite increases palatability of protein-containing food, which may be used to orient food-related behaviors

Protein Appetite Drives Macronutrient-Related Differences in Ventral Tegmental Area Neural Activity

“…Following these forced choice trials, rats were presented with 20 trials in which both bottles were available at the same time (free choice trials) to confirm the existence of protein preference in the PR group ( Murphy et al, 2018 ; Naneix et al, 2020 ). In free choice trials, PR rats significantly licked more for casein than for maltodextrin ( Fig.…”

“…Here, we use in vivo fiber photometry to record the activity of VTA neurons during consumption of isocaloric protein- and carbohydrate-containing solutions in an animal model of protein preference ( Murphy et al, 2018 ; Naneix et al, 2020 , 2021 ). We find that, in PR animals, protein consumption is associated with elevated neural activation, relative to carbohydrate consumption.…”

Age dependent effects of protein restriction on dopamine release

“…These results suggest that in protein restricted animals, 'phasic' bursting activity of dopamine mesolimbic neurons evokes greater dopamine release, relative to control animals. As phasic responses of mesolimbic dopamine neurons are well known to convey important information about primary reinforcers like food and associated stimuli [16,19,23,63,65], an increase in NAc dopamine levels in response to salient events may alter motivated behaviors like food preferences [11,13] and increase the rewarding properties of protein-enriched food in restricted/deprived animals [12]. Interestingly, protein restriction did not alter the phasic/tonic ratio either in NAc or dorsal striatum, suggesting that protein restriction at adulthood may induce a more general increase in capacity of terminals for dopamine release rather than by enhancing the contrast between different dopamine signaling modes [57,66].…”

“…Furthermore, protein deficiency and severe protein malnutrition are especially detrimental during development and early life when demand is highest [5][6][7]. Numerous species, including humans and rodents, regulate their food intake and food-related behaviors to avoid protein deficiency [8][9][10][11][12][13]. Increasing evidence implicates broad hypothalamic and limbic circuits in the regulation of protein appetite [10,[13][14][15].…”

Protein appetite drives macronutrient-related differences in ventral tegmental area neural activity

“…Here, we use in vivo fiber photometry to record the activity of VTA neurons during consumption of isocaloric protein-and carbohydrate-containing solutions in an animal model of protein preference (Murphy et al, 2018;Naneix et al, 2019Naneix et al, , 2020. We find that, in protein-restricted animals, protein consumption is associated with elevated neural activation, relative to carbohydrate consumption.…”