Jian-You Lin

As is true in humans, no two rodents prefer precisely the same tastes. Furthermore, no one rodent keeps precisely the same taste preferences forever. Here, we have made use of this between- and within-animal variability to generate and test novel hypotheses about the stability and malleability of neural perceptual coding. We used a brief-access task (BAT) to reveal both individual differences in taste preferences and shifting preferences within individual rats (quantified in terms of lick bout lengths). We moved on to show that these phenomena are not simply random variation: first, by demonstrating that gustatory cortical (GC) taste response dynamics, the late part of which reflect palatability, match that individual rat’s BAT preferences (evaluated almost 2 weeks prior) better than canonical preference patterns; that is, individual differences in preferences reflect differences in neural taste processing. This match, however, links neural taste processing only to the most recent BAT session—GC palatability processing does not reflect performance in earlier BAT sessions. We hypothesized that any tasting experience might impact taste processing (i.e., not just BAT licking), and tested this hypothesis by adding a second session of GC taste-response recordings; palatability-epoch taste responses in this later session no longer matched the most recent pre-recording BAT session, demonstrating that responses following the first electrophysiology session had changed. Together, these data demonstrate that every tasting experience (regardless of the method of taste delivery) changes the rat’s processing of those tastes.

As is true in humans, no two rodents prefer precisely the same tastes. Furthermore, no one rodent keeps precisely the same taste preferences forever. Here, we have made use of this between- and within-animal variability to generate and test novel hypotheses about the stability and malleability of neural perceptual coding. We used a brief-access task (BAT) to reveal both individual differences in taste preferences and shifting preferences within individual rats (quantified in terms of lick bout lengths). We moved on to show that these phenomena are not simply random variation: first, by demonstrating that gustatory cortical (GC) taste response dynamics, the late part of which reflect palatability, match that individual rat’s BAT preferences (evaluated almost 2 weeks prior) better than canonical preference patterns; that is, individual differences in preferences reflect differences in neural taste processing. This match, however, links neural taste processing only to the most recent BAT session—GC palatability processing does not reflect performance in earlier BAT sessions. An implication of the fact that rats’ perceptual preferences shift across BAT sessions is that any tasting experience might impact taste processing, an implication that we tested by performing a second session of GC taste-response recordings—sessions in which tastes were delivered by intra-oral cannula to passive rats—and found that palatability-epoch taste responses in this later session no longer matched the most recent pre-recording BAT session. Together, these data demonstrate that every tasting experience (regardless of the method of taste delivery) changes the rat’s processing of those tastes.

Aversive taste behaviors, such as gaping, are commonly viewed as fixed, hard-wired responses important for ejecting potentially toxic tastes from the mouth. Yet, taste responses are highly susceptible to modulation by experience and context; for instance, conditioned taste aversion (CTA), a form of learning in which rats are made to respond aversively to a sweet taste after it has been paired with gastric malaise, can cause gaping to previously acceptable tastes. Here, we compare aversive responses to saccharin sodium (sacc CS) caused by repeated aversive experience (in the form of CTA training) to those caused by naturally aversive quinine HCl, and how these responses are aligned with gustatory cortical (GC) activity. Using simultaneous single-neuron ensemble recordings in GC and electromyography (EMG) recordings in the jaw-opener muscle in awake, freely-moving rats and a machine-learning based behavioral classifier, we not only identified two classes of gaping, but characterized how these classes reflect different mechanisms driving aversive behavioral outputs. The first gape class is taste-specific and emerges following GC ensemble dynamics associated with taste identity and palatability coding, leading to canonical gaping responses to naturally or learned-aversive stimuli. The second is rapid-onset (<200 ms following taste delivery), internal state-dependent gaping the animal exhibits to all tastes in a state of heightened vigilance. This generalized aversion is subsequently suppressed once palatability-related responses emerge in GC, such that only gapes to truly aversive tastes remain. Together, these findings demonstrate that gaping is not a unitary reflex but is instead driven by multiple aversion judgment systems active in different contexts.

Consumption varies across the stages (metestrus, diestrus, proestrus, estrus) of a rat's estrous cycle, changing in ways that might be expected to reflect, in part, a direct impact of hormones on taste palatability. Evidence regarding this hypothesis has been mixed, however, and critical within-subject experiments comparing consumption of multiple tastes with distinct valences across all estrous phases have been few. Here, we assayed female rats' licking of palatable (saccharin, sucrose, NaCl) and aversive (quinine-HCl, citric acid) tastes in brief-access trials, while tracking their estrous cycles through vaginal cytology. We observed sucrose palatability to be high at metestrus, the same phase at which the palatability of the aversive citric acid was low. These patterns were consistent across tastes of similar palatability, despite vast differences between the substances' receptor mechanisms and central impacts. Together, these results reveal a general (i.e., independent of particular tastant identity) magnification of palatability—higher than average for palatable tastes and lower for aversive tastes—centered largely on metestrus. We tested whether this phenomenon reflects lateral hypothalamic (LH) estradiol processing, by infusing LH with an estrogen receptor blocker (ICI182, 780) across five consecutive tasting sessions. Control infusions replicated the metestrus magnification of palatability; as predicted, ICI infusions blocked this effect, but estrogen receptor inhibition failed to render preferences “cycle free,” instead delaying the palatability magnification until diestrus. In summary, the estrous cycle directly mediates taste palatability in a manner involving hypothalamic actions of estradiol, but this effect is only one of several impacting consumption across the estrous cycle.

As is true in humans, no two rodents prefer precisely the same tastes, and no one rodent shows precisely the same taste preference pattern forever. Here, we make use of this between- and within-animal variability to generate and test novel insights about the stability and malleability of neural perceptual coding. We used a brief-access task (BAT) to reveal both between-rat individual differences in taste preferences and shifting preferences within individual rats (quantified in terms of lick bout lengths). We moved on to show that these phenomena are not simply random variation: first, we demonstrated that gustatory cortical (GC) taste response dynamics, the late part of which have been shown to reflect palatability, match that individual rat’s BAT preferences (evaluated almost 2 weeks prior) better than canonical preference patterns, revealing that the individual differences reflect differences in neural taste processing; this enhanced correspondence, however, links neural taste processing only to the most recent BAT session—GC palatability processing does not reflect performance in earlier BAT sessions. The finding that rats’ perceptual preferences shift across BAT sessions carries with it the implication that any tasting experience might change these preferences. We tested this implication by performing a second session of GC taste-response recordings—sessions in which tastes were delivered by intra-oral cannula to passive rats—and found that palatability-epoch taste responses in this later session no longer matched the most recent pre-recording BAT session. Together, these data demonstrate that every tasting experience (regardless of the method of taste delivery) changes the rat’s perceptual/neural processing of those tastes.

Food intake varies across the stages of a rat’s estrous cycle. It is reasonable to hypothesize that this cyclic fluctuation in consumption reflects an impact of hormones on taste palatability/preference, but evidence for this hypothesis has been mixed, and critical within-subject experiments in which rats sample multiple tastes during each of the four main estrous phases (metestrus, diestrus, proestrus, and estrus) have been scarce. Here, we assayed licking for pleasant (sucrose, NaCl, saccharin) and aversive (quinine-HCl, citric acid) tastes each day for 5-10 days while tracking rats’ estrous cycles through vaginal cytology. Initial analyses confirmed the previously-described increased consumption of pleasant stimuli 24-48 hours following the time of high estradiol. A closer look, however, revealed this effect to reflect a general magnification of palatability—higher than normal preferences for pleasant tastes and lower than normal preferences for aversive tastes—during metestrus. We hypothesized that this phenomenon might be related to estradiol processing in the lateral hypothalamus (LH), and tested this hypothesis by inhibiting LH estrogen receptor activity with ICI _182,780 during tasting. Control infusions replicated the metestrus magnification of palatability pattern; ICI infusions blocked this effect as predicted, but failed to render preferences “cycle free,” instead delaying the palatability magnification until diestrus. Clearly, estrous phase mediates details of taste palatability in a manner involving hypothalamic actions of estradiol; further work will be needed to explain the lack of a flat response across the cycle with hypothalamic estradiol binding inhibited, a result which perhaps suggests dynamic interplay between brain regions or hormones. Significance Statement: Consummatory behaviors are impacted by many variables, including naturally circulating hormones. While it is clear that consumption is particularly high during the stages following the high-estradiol stage of the rodent’s estrous (and human menstrual) cycle, it is as of yet unclear whether this phenomenon reflects cycle stage-specific palatability (i.e., whether pleasant tastes are particularly delicious, and aversive tastes particularly disgusting, at particular phases). Here we show that palatability is indeed modulated by estrous phase, and that this effect is governed, at least in part, by the action of estradiol within the lateral hypothalamus. These findings shed light on the mechanisms underlying the adverse impact on human welfare due to irregularities observed across the otherwise cyclic menstrual process.

Learning depends on more than the association of paired stimuli in a vacuum. For example, benign experience with a taste stimulus weakens future conditioned taste aversions (CTA) to that taste—a phenomenon known as latent inhibition (Lubow and Moore, 1959). Our work has revealed a similar phenomenon in rodents whereby benign experience with tastes significantly enhances later CTA to a novel taste (Flores et al., 2016; Flores et al., 2018). This phenomenon suggests that taste experience broadly alters future taste processing in gustatory cortex (GC). To delve into this possibility, we previously have used in-vivo electrophysiology in female Long-Evans rats, recording GC activity during benign taste experience. These recordings revealed an increase in the discriminability of GC responses to both familiar and novel tastes following taste experience (Flores et al., 2022). Here, we build upon these earlier results to present a potential mechanism for this increased discriminability and salience—a reduction of cortical response variability of single-neuron responses, and enhanced coherence of ensemble responses, to the novel taste. We go on to demonstrate that the differences between pre- and post-CTA responses are greater following benign taste experience. These results suggest that benign taste experience impacts GC responses towards a novel taste pre-and post-CTA conditioning by increasing taste coding reliability at both the single-neuron and ensemble levels. Significance Statement: Animals and humans readily learn the consequences of consuming a specific taste and react by changing their behaviors. However, those changes often vary and can be influenced by previous experience. For example, our previous work has shown that inconsequential taste experiences increase the probability of a strong taste aversion. The work presented here evaluates how benign taste experience can impact cortical processing dynamics using in-vivo electrophysiology in freely behaving rats. We report that seemingly inconsequential taste experience impacts cortical plasticity. This unravels a new area of chemosensory research and may shed light on what differentiates the neural dynamics of strong and weak taste aversion learning.

Experience impacts learning and perception. Familiarity with stimuli that later become the conditioned stimulus (CS) in a learning paradigm, for instance, reduces the strength of that learning—a fact well documented in studies of conditioned taste aversion (CTA; De La Casa & Lubow, 1995; Lubow, 1973; Lubow & Moore, 1959a). Recently, we have demonstrated that even experience with “incidental” (i.e., non-CS) stimuli influences CTA learning: Long Evans rats pre-exposed to salty and/or sour tastes later learn unusually strong aversions to novel sucrose (Flores, Moran, Bernstein, & Katz, 2016), and exhibit enhanced sucrose-responsiveness after learning in gustatory cortex (GC; Flores et al., 2018). These findings suggest that incidental taste experience may change spiking responses that have been shown to underlie the processing of tastes in GC. Here, we test this hypothesis, evaluating whether GC neuron spiking responses change across three days of taste exposure. Our results demonstrate that the discriminability of GC ensemble taste responses increases with this familiarization. Analysis of single-neuron responses recorded across multiple sessions reveals that taste exposure not only enriches identity and palatability information in taste-evoked activity, but also enhances the discriminability of even novel tastes. These findings demonstrate that “mere” familiarization with incidental episodes of tasting changes the neural spiking responses of taste processing and provide specific insight into how such taste experience may impact later learning.

Taste palatability is centrally involved in consumption decisions—we ingest foods that taste good and reject those that don't. Gustatory cortex (GC) and basolateral amygdala (BLA) almost certainly work together to mediate palatability-driven behavior, but the precise nature of their interplay during taste decision-making is still unknown. To probe this issue, we discretely perturbed (with optogenetics) activity in rats’ BLA→GC axons during taste deliveries. This perturbation strongly altered GC taste responses, but while the perturbation itself was tonic (2.5 s), the alterations were not—changes preferentially aligned with the onset times of previously-described taste response epochs, and reduced evidence of palatability-related activity in the ‘late-epoch’ of the responses without reducing the amount of taste identity information available in the ‘middle epoch.’ Finally, BLA→GC perturbations changed behavior-linked taste response dynamics themselves, distinctively diminishing the abruptness of ensemble transitions into the late epoch. These results suggest that BLA ‘organizes’ behavior-related GC taste dynamics.