there are few experimental systems thatexamine the mechanism of liking something one dislikes,and further research on the mechanism of increased pref-erence by bitter taste experience may lead to the elucida-tion of a complementary mechanism
What is unique about the study is that it is looking at a change and not just a mechanism for likes and dislikes taste-wise
The decrease in aversiveness may be dueto fewer bitter taste receptors
Interesting that change in natural aversion to bitterness could be because the taste cannot be sensed as well
Furthermore, the VTA dopamine neurons pro-jects directly to the CeA, especially to the medial part ofthe CeA (CeM), where Prkcd-positive neurons are lessabundant and Sst-positive neurons are more abundant[15, 37]. Therefore, it is also possible that umami infor-mation is relayed to Sst-positive neurons in the CeA viaVTA dopamine neurons
Dopamine is associated with pleasure and motivation, so its connection to taste pathways fits into the system
The reason for this is unclear.One can speculate that there may be some interactionbetween sodium and umami signals
Uncertainty in unexpected results between MSG and MPG solutions, possible interactions between sodium and umami signals
It would be an interesting future study toexamine whether there is a critical period to induce suchexperience-dependent changes in taste preference
Next step to take would be establishing a period of time where exposure induces changes in taste preferences
ittle crossover effect ofdifferent taste qualitie
Exposure to one taste quality does not affect preference for another
Our results are consistent with this body ofevidence and showed that prolonged exposure to umamiand bitter in the post-weaning juvenile period alsoincreases the preference for the exposed taste
Results from study support hypothesis and previous research on the modification of taste preference through experience
Taken together, both Prkcd-positive andSst-positive neurons are not unique populations to respondto a particular tastant, but are composed of mixed cells thatrespond to negative and positive tastants, although there isa bias in the tendency of the responding tastant
Cellular populations respond to a mix of negative and positive tastants with some bias in tendency
Interestingly, one-third of sweet-response and one-fifthof umami-response Sst-positive neurons also respondedto umami and sweet tastants, respectively, both of whichare thought to be attractive tastants
Interesting or unexpected finding that some Sst-positive neurons responded to both umami and sweet
In Prkcd-positive neurons, the largest popula-tions (19.7%) responded to bitter tastant (Fig. 2B), as wasreported in previous Fos-labeling studies
As expected, the largest populations of Prkcd-positive neurons responded to the bitter
one intriguing possibility is that there are cell-type specificresponses to the negative and positive taste qualities withinthe CeA, and neuronal activity changes occur in this circuitmay lead to the modification of outcome behavior towardthe tastant
Hypothesis for neural activity mechanisms in connection to taste quality response
We next sought to deter-mine the areas of the brain that display neuronal activityassociated with these behavioral changes
In addition to behavioral changes, study is interested in neuronal activity associated with them
. In contrast, the Bitter groupconsumed as much bitter solution as water (p = 0.7539;Fig. 1F) and exhibited bitter preference ratio around50% (p = 0.8361; Fig. 1G), which was significantly highcompared with that of the other groups
Exposure to bitter flavors led to a higher consumption of the bitter solution and less aversion
Six-week-old naïve and prolonged taste exposed micewere acclimated to test bottles for 3–4 days beforethe stimulation
Habituation period lasted 3-4 days, not long enough to see long-term effects in preference
We found that MPG experience for 3 weeksenhanced umami intake similar to MSG experience
Indicates that the experiments with MSG reflect umami effect and not just sodium effects
both the Water and Umami groups contactedthe umami bottle longer duration than the water bot-tle
Interesting that more contact was made (exploration) but intake preference for whatever they had been consuming previously still existed
The Umami group exhib-ited a significant increase in intake of umami solutioncompared with water, whereas the Water and Bittergroups showed no difference in water and umami intake
As expected, more exposure to umami flavor led to more preference for intake of that solution
To investigate the influence of prolonged experience ofumami and bitter tastants on taste preference, mice werereared with ad libitum water (Water group), umami solu-tion (Umami group), or bitter solution (Bitter group) for3 weeks in the immediate post-weaning period
Experimental variable was the solution provided for feeding: water (control), umami, or bitter
The intake and access duration were analyzed usingpaired t-test. The preference ratio and access ratio wereanalyzed by one-way ANOVA followed by Tukey’s posthoc test and one sample t-test. The Fos FISH cell count-ing data were analyzed by unpaired t-test
Stastistical analysis was done with t-tests, one-way ANOVA, and Tukey's post-hoc test
Six minutes after the start of imaging,mice were given 100 μL of bitter (1 mM quinine), sweet(50 mM sucrose), and umami (100 mM monosodiumglutamate and 10 mM disodium inosinate) taste solutionalternately with neutral-taste water at 2-min intervals forthree trials
Exposure to three taste qualities: bitter, sweet, and umami
More than a weekafter viral injection, a second surgery was performed toimplant a customized 0.6-mm-diameter gradient index(GRIN) lens probe
Don't fully understand purpose of separate surgeries for viral injection and then lens probe implantation
intake of less than 0.1 g during the 15-min test wereexcluded from the data
Could this also indicate something else is affecting the rat's behavior? Especially since they were deprived, it would be unusual for them to not drink more than that
A preference ratio was calculatedas the ratio of the taste solution (umami or bitter) intaketo the total (taste solution and water) intake. An accessratio was determined as the ratio of the access durationin the taste bottle to the total access duration in two bot-tles
Measured response via preference ratio and access ratio
The two-bottle test was performed with the bottle posi-tions switched for 2 days to avoid side preference
Important to avoid as much bias as possible within the experimental set up
two-bottle test following 19–21 h ofwater deprivation
Water deprivation increased motivation to drink during the two-bottle test
Mice in the prolonged taste exposure groups had ad libi-tum access to food and one of the following taste solu-tions instead of water beginning at 4 weeks of age,immediately after weaning, for 3 week
Exposure to taste solutions (umami or bitter) began at 4 weeks of age immediately after weaning
Adult male C57BL/6J mice
Subjects are male mice. Is it easier to care for mice belonging to just one sex or are males behaviorally easier to work with?
establishing a behavioral paradigm for experience-dependent plasticity in the taste preference for attractive(umami) and aversive (bitter) taste quality, and examinedthe neuronal correlates in mice using in vivo calciumimaging and fluorescence in situ hybridization in multi-ple brain regions
Purpose of the study to provide more insights into taste preferences and how this correlates to neuronal activation
information about eachtaste quality has a discrete pathway from its taste recep-tors to the corresponding neuronal taste circuits
Interesting how each of the five taste qualities has a distinct signaling pathway, such so that impairing one would likely not implicate the other taste qualities
These findingssuggest that both attractive and aversive valence of tastesignals can be subject to influence from previous expe-riences; however, the neuronal mechanisms underlyingthese observations are not well understood
Shows gap in research in specifically understanding the mechanisms behind the preferences exhibited
in rats and mice, expo-sure to sour and bitter substances before and after wean-ing leads to a significant preference for those substancesin adulthood
Previous evidence for preference for substances exposed to before weaning in rats supporting direction of this study
For exam-ple, human newborn infants exhibit affective behav-iors such as lip sucking, elevation of the corners of themouth, and rhythmic tongue protrusions when exposedto sweet or umami solutions [6, 7]. They also show aver-sive responses such as nose wrinkling and grimacingwhen exposed to bitter or sour solutions. Rats and micealso exhibit affective behaviors such as rhythmic and lat-eral tongue protrusions when administered with sweet orumami solutions [7–9]. They also show aversive behav-iors, such as gasping, chin rubbing, and handshaking,when exposed to quinine solution
Humans and rats show comparable behaviors in response to exposure to either sweet/umami or bitter/sour
leading to heightened preferences several years after children’slast exposure to the formula for foods containing similar flavor notes
The later preferences (past infancy) of the participants of this specific study was not explored
Forty-seven mothers who were formula feeding their infants were recruited during 2006–2009
Subjects were human mothers and their infants, decent sample size
Infantsrandomized to feed ePHF for 3 or 8 months ate more of the savory broth (p=0.002 and0.005, respectively), and consumed it at a faster rate relative to the plain broth
Most important finding from study. Longer exposure (3-8 months) to ePHF led to more savory brother being eaten and at a quicker rate, supporting hypothesis
one controlgroup fed the control cow-milk formula (CMF) for 8 months, and three groups fed ePHF for 1, 3,or 8 months and CMF otherwise
Length of time consuming ePHF (1, 3, or 8 months) was the experimental variable
Table 1
Table 1 shows no significant differences in the characteristics of infants and mothers measured, indicating no glaring confounding variables or other patterns
This formula has pronounced bitter,sour, and savory tastes
Flavor profile of ePHF include taste not typically enjoyed by young children
More knowledge of thetype of flavor learning that is occurring during formula feeding and communication of thisknowledge to the caregiver is needed.
This information is definitely relevant to childrearing and advising parents on nutrition and introduction of varied flavors being beneficial
establishing a sensitive period forflavor preference during infancy
While there does seem to be a significant effect of exposure (and duration of exposure), this study so far does not fully establish a critical period for when an infant is most sensitive to the exposure and/or at what point is no longer sensitive to the exposure
These results implicatebreastfeeding, and a varied diet for the nursing mother, in acceptance of varied foods atweaning and beyond and suggest that breastfeeding mothers may have to eat a food for atleast 3 months in order for their nursing infants to experience and learn to like the taste ofthat food
This is the major implication that study draws from its results
infants do not need solid food—they can learn frommother’s milk
Really interesting that their tastes can be influenced by flavor in the milk and not just directly from food they consume and allows for exposure to more kinds of food to occur before solid food-eating begins
The adaptive reason for this age-relatedplasticity may reflect the importance of infants becoming familiar with and accepting offlavors that their mothers consume, which are transmitted through breast milk
This might have to do with adapting to the environment the mother is raising the child is and adjusting preferences based on what food will be available in that specific environment is beneficial to survival
The amount of exposure to the flavor of this formula required to shift the hedonic tonefor glutamate in a food appears to be about 3 months
Minimum length of time of exposure to see this specific effect on preference months later is between 1 and 3 months based on the experimental groups
How many months infants fed ePHF influenced their acceptance of a savory-flavored brothat weaning
This is the main finding from the study: longer exposure to ePHF increased acceptance of savory broth later
The 1M ePHF experimental groupand the control group did not differ
Based on Figure 2, they did differ just not statistically significantly
Figure 2. Intake
Figure present for broth intake but not for rate of consumption even though both were indicated to show significant differences between experimental and control
Although relationships existbetween food habits in childhood and later in life3, these data do not address how and whenexperiences with food flavors affect later preferences
Highlights knowledge of food preferences that study builds on to specifically investigate how food flavor exposure affects later preferences within some kind of critical period
Figure 1. Experimental groups
Noticed in this figure that the group that was 8 months on ePHF started at 2 weeks of age compared to the other groups that started at 6 weeks. Wonder why this was done, especially since it could impact the length of a potential critical period for developing acceptance of flavor
Dependent measures included total intake (g) and rate (g/min) of each feed and mothers’rating of their infant’s enjoyment of the broth during each test session
Dependent variable was response to broth, including total intake of broth, rate of eating broth, and parental rating of enjoyment
Separate analyses of variance (ANOVA) were conducted with experimental group as thebetween-subjects factor and witha priori contrasts specified to focus on differences betweenexperimental groups and the control group
Analyzed data using ANOVA and Bonferroni corrections to understand significance of results
mothers rated their infant’s enjoymentof the broth on a 9-point scale (9=extreme like)
Interesting to get parent perspective on measuring infant enjoyment, less objective but helpful in understanding typical behavior in infant (i.e. infant might not typically eat a lot but eats more in experiment, even if it's lower than average)
The amount consumed was determined by weighing each bottle immediately beforeand after each feed
Enjoyment of broth was experimentally associated with greater amount of broth consumed by the infant
When infants were 8.5 months, the dyad was tested at Monell on 2 separate days
All groups were measured for response at 8.5 months regardless of length of ePHF exposure, tested twice to have separate exposure to plain vs savory broth
Feeding ended when the child rejected the food at least three consecutivetimes, exhibiting such rejection behaviors as turning his or her head away, pushing the bottleaway, or crying, or becoming playful.
Specific behaviors led to ending of feeding, rejection limit seems arbitrary but also reasonable
randomized into one of four groups (Fig. 1): infants fed cow-milk-basedformula (CMF; Enfamil; Mead Johnson Nutritionals, Evansville, IN; CMF Control, n=13)for 8 months or fed ePHF (Nutramigen; Mead Johnson Nutritionals; Evansville, IN) for 1(1M ePHF, n=11), 3 (3M ePHF, n=11), or 8 (8M ePHF, n=11) months and CMF otherwise
Four randomized groups in study: no ePHF (control), ePHF for 1 month, ePHF for 3 months, or ePHF for 8 months
infants whose parents chose to feedthem ePHF (usually because of colic) are more accepting of savory, bitter, and sour tastesthan were infants whose parents chose to feed them cow’s milk formula (CMF)
Prior evidence that exposure to ePHF leads to infants being more accepting of savory, bitter, and sour tastes (hypothesis for randomized version of trial in this study)
distinct flavor of extensively hydrolyzed protein hydrolysate formula(ePHF)
ePHF is used as a way to experiment with exposure to a strong, savory flavor
The absence of a robust experimental paradigm, like that employed for other sensorysystems (e.g., vision5, audition/language6) and other animals7,8 has inhibited progress inunderstanding whether human flavor programming exhibits age-related changes
Explains why there is a gap in research and less attention placed on taste in comparison to other sensory systems
This study aimed todetermine the effects of duration of exposure during the sensitive period on a food containing anexemplar of the savory flavor.
General research question and goal of study to identify a sensitive (critical) period for savory taste