The role of the mediodorsal thalamus in chemosensory preference and consummatory behavior in rats

Abstract

Experience plays a pivotal role in determining our food preferences. Consuming food generates odor–taste associations that shape our perceptual judgements of chemosensory stimuli, such as their intensity, familiarity, and pleasantness. The process of making consummatory choices relies on a network of brain regions to integrate and process chemosensory information. The mediodorsal thalamus is a higher-order thalamic nucleus involved in many experience-dependent chemosensory behaviors, including olfactory attention, odor discrimination, and the hedonic perception of flavors. Recent research has shown that neurons in the mediodorsal thalamus represent the sensory and affective properties of experienced odors, tastes, and odor–taste mixtures. However, its role in guiding consummatory choices remains unclear. To investigate the influence of the mediodorsal thalamus in the consummatory choice for experienced odors, tastes, and odor–taste mixtures, we pharmacologically inactivated the mediodorsal thalamus during 2-bottle brief-access tasks. We found that inactivation altered the preference for specific odor–taste mixtures, significantly reduced consumption of the preferred taste and increased within-trial sampling of both chemosensory stimulus options. Our results show that the mediodorsal thalamus plays a crucial role in consummatory decisions related to chemosensory preference and attention.

Introduction

Understanding the neural substrates guiding our choices of food and beverages is essential for unraveling the complex interplay between sensory perception, cognitive processing, and emotional evaluation that drives our consummatory behaviors. A fundamental experiential factor informing these choices is the perception of flavor (i.e. odor–taste mixtures). Sampling an odor–taste mixture initiates multisensory processes that generate robust associations between the odor and the taste’s quality and hedonic value (i.e. pleasantness or unpleasantness) (Stevenson et al. 1995; Sakai and Imada 2003; Prescott et al. 2004; Gautam and Verhagen 2010; Green et al. 2012). These experiences lead to preferences for odors sampled with pleasant tastes and the avoidance of odors sampled with unpleasant tastes (Fanselow and Birk 1982; Holder1991; Schul et al. 1996; Sakai and Yamamoto 2001; Gautam and Verhagen 2010; Green et al. 2012; McQueen et al. 2020).

The processing and integration of these chemosensory signals involve a complex network of brain regions (Samuelsen and Vincis 2021). The mediodorsal thalamus is a key player in this network, contributing to the perception of odors and odor–taste mixtures (Rousseaux et al. 1996; Sela et al. 2009; Tham et al. 2011; Small 2012). It forms extensive connections with numerous brain regions involved in chemosensory processing, including the piriform cortex, gustatory cortex, and basolateral amygdala (Price and Slotnick 1983; Kuroda et al. 1992; Ray and Price 1992; Shi and Cassell 1998). Its dense reciprocal connections with the orbitofrontal cortex—a region crucial for chemosensory based decision-making (Schoenbaum et al. 2011; Alcaraz et al. 2016)—highlights its role in cognitive functions related to working memory (Slotnick and Risser 1990; Han et al. 2013; Bolkan et al. 2017; Scott et al. 2020), sensory attention (Tham et al. 2009; Schmitt et al. 2017; Rikhye et al. 2018), and action–outcome associations (Slotnick and Kaneko 1981; Gaffan and Murray 1990; Corbit et al. 2003; Mitchell 2015). This centralized connectivity positions the mediodorsal thalamus as a key node in directing sensory attention and facilitating decision-making by providing feedforward information regarding stimulus type and value as well as cognitive feedback.

The mediodorsal thalamus is involved in numerous experience-dependent olfactory behaviors, such as odor attention (Plailly et al. 2008; Tham et al. 2011; Veldhuizen and Small 2011), odor discrimination (Eichenbaum et al. 1980; Sapolsky and Eichenbaum 1980; Staubli et al. 1987; Sela et al. 2009), and odor–reward associations (Oyoshi et al. 1996; Kawagoe et al. 2007; Courtiol and Wilson 2016). Recent findings have further illuminated the role of the mediodorsal thalamus in chemosensory processing, demonstrating that neurons represent the sensory and affective properties of distinct odors, tastes, and odor–taste mixtures (Fredericksen and Samuelsen 2022). Furthermore, pharmacological inactivation of the mediodorsal thalamus has been shown to disrupt working memory for olfactory-dependent foraging (Scott et al. 2020), and people with lesions in this area exhibit altered hedonic perception of odors and odor–taste mixtures (Rousseaux et al. 1996; Asai et al. 2008; Sela et al. 2009). These findings suggest a critical role for the mediodorsal thalamus in processing chemosensory information that informs consummatory choices.

Here, we employed a 2-bottle brief-access task to investigate the effect of mediodorsal thalamic inactivation on decisions to consume familiar odors, tastes, or odor–taste mixtures. Rats were given experience with 2 odor–taste mixtures: benzaldehyde-saccharin and isoamyl acetate-stevia. Notably, rats show a preference for these noncaloric sweeteners over water, with a preference for saccharin over stevia (Sclafani et al. 2010). Prior to each 2-bottle brief-access task, we administered bilateral infusions of either saline as a control, or NBQX disodium salt hydrate to inactivate the mediodorsal thalamus. NBQX is an AMPA/kainate glutamate receptor antagonist that blocks excitatory postsynaptic currents, thereby reducing excitatory neurotransmission in the targeted brain region (McDannald et al. 2004; Haney et al. 2010; Samuelsen et al. 2012). Our findings revealed that inactivation of the mediodorsal thalamus not only abolished the preference for benzaldehyde-saccharin but also significantly reduced the consumption of the saccharin taste alone. During inactivation, we also observed a significant increase in within-trial sampling of both solutions across all chemosensory modalities. These results further corroborate the role of the mediodorsal thalamus in consummatory behaviors related to chemosensory preferences and attention.

Materials and methods

Animals

All experimental procedures were performed according to university, state, and federal regulations regarding research animals. Procedures were approved by the University of Louisville Institutional Animal Care and Use Committee. Nine female Long-Evans rats (250–350 g; Charles Rivers) were single-housed and maintained on a 12/12-h light–dark cycle with ad libitum access to food and water unless otherwise specified. Consistent with prior experiments requiring head fixation in our lab and others (Jones et al. 2007; Fontanini et al. 2009; Samuelsen and Fontanini 2017; Fredericksen and Samuelsen 2022; Stocke and Samuelsen 2024), adult female rats were used because the size and strength of adult male rats significantly increase the risk of catastrophic headcap failure.

Chemosensory stimuli

Chemical stimuli were selected based on prior research involving the sensory preferences of rats (Aimé et al. 2007; Sclafani et al. 2010; Gautam and Verhagen 2012; Samuelsen and Fontanini 2017; Fredericksen et al. 2019; McQueen et al. 2020; Fredericksen and Samuelsen 2022). At the concentrations used here, the non-caloric sweeteners 0.1% saccharin (Sigma-Aldrich, S1002) and 0.1% stevia (Stevia Max, JG Group) are preferred to water, but saccharin is preferred to stevia (Sclafani et al. 2010). At the concentrations used here, the odorants 0.01% isoamyl acetate (Sigma-Aldrich, W205508) and 0.01% benzaldehyde (Sigma-Aldrich, W212709) are tasteless (Aimé et al. 2007; Gautam and Verhagen 2010; Samuelsen and Fontanini 2017). All stimuli were prepared with distilled water.

The 2-bottle brief-access task

Experiments utilized a computer-controlled 2-bottle brief-access apparatus directed by custom-written LabVIEW scripts (National Instruments, Austin, TX) (Fredericksen et al. 2019; McQueen et al. 2020). The apparatus featured a testing chamber, 2 motorized port doors, and a motorized stage for bottle positioning. A session began with the opening of the motorized port doors allowing access to 2 sipper tubes. Once opened, rats had a ~20-s contact window to engage in a trial by licking either bottle. If a bottle was contacted, the port doors remained open for an additional ~20 s. If no contact was made during the contact window, the port doors closed. The completion of each trial was followed by a 30-s intertrial interval.

Only the first 15 s of sampling was analyzed to ensure consistency across trials. The large sampling window afforded time for switching between ports within a trial. Licks were recorded using a grounded circuit. Signals greater than the maximum lick rate for a rat (10 Hz) were filtered out (Lin et al. 2013). The rat had to lick a spout at least 3 times to qualify as an engaged trial. Bottles were counterbalanced, and chemosensory stimuli were presented 10 times at each port for a total of 20 trials. This paradigm gave rats a limited amount of time in a fixed number of trials to drink from 2 simultaneously presented bottles containing different chemosensory stimuli. Rats were water regulated during training and experimental days, receiving 20–30 mL per day in addition to the liquid consumed during the 2-bottle brief-access task. Post-session water consumption was measured as the volume of water consumed in the home cage during the first hour after each session.

Data are presented as the number of total licks, licks per engaged trial, number of engaged trials, initiation time, number of lick clusters, average inter-lick interval, number of switches, proportion of switched trials, licks per cluster, cluster-switch ratio, volume of water consumed, preference ratio, and absolute change in preference ratio. A lick cluster was defined as a series of at least 2 licks with intervals of no more than 500 ms (Dwyer et al. 2017; López et al. 2023). A switch was defined by the end of a lick cluster on one bottle and the start of a new lick cluster on the other bottle. The inter-lick interval (ILI) was determined by calculating the time difference between consecutive licks within each lick cluster for each stimulus in every trial. The average ILI was then calculated for each stimulus in each trial for each animal, followed by averaging these values across all trials for each animal. The cluster-switch ratio was calculated as (Total # of clusters − Total # of switches) / (Total # of clusters + Total # of switches). A cluster-switch ratio of 1 indicates that no switches occurred, while a cluster-switch ratio of 0 indicates that all clusters were a result of switching from one bottle to the other. Thus, a lower cluster-switch ratio indicates that more clusters were dependent on switching between bottles. The preference ratio was calculated as (S1 − S2) / (S1 + S2), where S1 is the total number of licks for bottles containing stimulus 1, and S2 is the total number of licks for bottles containing stimulus 2. Thus, a positive preference ratio indicates a preference for stimulus 1, and a negative preference ratio indicates a preference for stimulus 2. The absolute change in preference ratio was calculated as the absolute difference between the preference ratio during the Saline and NBQX conditions.

Pre-surgery 2-bottle brief-access task training

Rats were placed on a water regulation regime and trained to drink water during a 2-bottle brief-access task. For the first 3 d, rats were habituated to the 2-bottle brief-access apparatus for 2 min before being presented with the choice of drinking distilled water from either port during 10 trials. After each session, rats were given a maximum of 20 mL of distilled water in their home cage overnight. Following this initial training, rats were given ad libitum access to water for 4 d before beginning the chemosensory experience and behavioral training protocol (Fig. 1A). First, rats were water restricted for 3 d receiving a maximum of 30 mL distilled water access in their home cage each day. To reduce the impact of neophobia (Barnett 1958; Corey 1978; Demattè et al. 2014), rats were given their primary experience with chemosensory stimuli in the home cage the day immediately prior to receiving it in the 2-bottle brief-access apparatus (Fig. 1A). Each 2-bottle brief-access session consisted of 20 trials. After each session, rats received one-hour access in their home cage to bottles of chemosensory stimuli or water. After the final chemosensory experience session, rats returned to ad libitum water in preparation for surgery.

Fig. 1.

Schematic outline of training and experimental sessions. (A) Rats were given 1-hour home-cage access to the 2 odors, 2 tastes, or the 2 odor–taste mixtures the day before the stimuli were presented in the 2-bottle brief-access task. In the 2-bottle brief-access task, rats chose between 2 lick ports of water on day 1, experienced benzaldehyde-saccharin and isoamyl acetate-stevia odor–taste mixtures on days 4–6, experienced benzaldehyde and isoamyl acetate odors on days 2 and 7, and experienced saccharin and stevia tastes on days 3 and 8. (B) Rats were trained to be head-restrained for 4 min before the 2-bottle brief-access task. Rats chose between 2 lick ports of water on day 1, odor–taste mixtures on days 2–4, odors on day 5, and tastes on day 6. (C) Rats were head-restrained and the mediodorsal thalamus was infused with saline on days 1–4 (Saline-1 condition) and days 9–12 (Saline-2 condition), and NBQX on days 5–8 (NBQX condition). Rats chose between 2 lick ports of water on days 1, 5, and 9, odor–taste mixtures on days 2, 6, and 10, odors on days 3, 7, and 11, and tastes on days 4, 8, and 12.

Bilateral cannula implantation surgery

Rats were anesthetized with isoflurane (induction: 5%, maintenance: 2–5%). Animals were secured in a stereotaxic frame and given subcutaneous injections of atropine sulfate (0.03 mg/kg), dexamethasone (0.2 mg/kg), and the analgesic buprenorphine HCl (0.03 mg/kg). The scalp was shaved, sterilized with 70% ethanol and povidone-iodine solution, and excised to reveal the skull. Small craniotomies were made to secure 5 anchoring screws (SMPPS0002, Micro Fasteners) into the skull. Two additional craniotomies were made for the placement of 26 ga guide cannulas (7 mm, Plastics One, Protech International, Inc.) into the mediodorsal thalamus (coordinates: 10° angle, AP: −3.3 mm, ML: ± 1.80 mm from bregma, DV: −5.20 mm from dura). Guide cannulas were implanted at a 10-degree angle to avoid the superior sagittal sinus and cemented to the skull with dental acrylic. Dummy stylets (7 mm, Plastics One, Protech International, Inc.) were inserted into the guide cannulas. A bolt was attached posteriorly to the guide cannulas to allow for head-restraint. Rats were allowed a recovery period of 7–13 d before behavioral training resumed.

Post-surgery head-restraint and 2-bottle brief-access training

Rats were placed on a water regulation schedule where they received access to 30 mL of water each day in their home cage for 3 d. To prepare them for the pharmacological inactivation experiment, rats underwent 6 d of head-restraint training without infusion. During each session, rats were head restrained for 4 min, placed in the 2-bottle brief-access apparatus, and allowed 2 min to habituate before beginning a 20-trial 2-bottle brief-access task. Stimuli provided during the 6 head-restraint training sessions consisted of water on day 1, odor–taste mixtures on days 2–4, odors on day 5, and tastes on day 6 (Fig. 1B). After each session, rats were given 20 mL of water in their home-cage. Following the final head-restraint training session, rats received access to 30 mL of distilled water in in their home cage for 3 d before beginning the 12-d pharmacological inactivation experiment (Fig. 1C).

Pharmacological inactivation of the mediodorsal thalamus during 2-bottle brief-access task

Rats underwent the same procedure for each 2-bottle brief-access behavioral session. Rats were head-restrained, the dummy stylets were removed, and 33 ga injection cannulas (7 mm, Plastics One, Protech International, Inc.) were inserted through guide cannula into the mediodorsal thalamus. Each injection cannula was connected to polyethylene (PE)-50 tubing (A-M Systems), which was attached to 10 µL Hamilton syringes backfilled with mineral oil. A dual syringe pump (Pump 11 Elite, Harvard Apparatus) was used to infuse 400 nL of sterile saline or 400 nL of NBQX disodium salt hydrate (Sigma-Aldrich, N183) at a rate of 200 nL per minute. Two additional minutes were allotted for the diffusion of fluid into the brain before the injection cannulas were removed, and the dummy stylets were replaced (as in [McDannald et al. 2004; Haney et al. 2010; Samuelsen et al. 2012]). The rats were then moved from the head-restraint box to the test chamber of the 2-bottle brief-access apparatus, where they had 2 min to acclimate before starting the task. Chemosensory stimuli were counterbalanced and presented 10 times at each port for a total of 20 trials. The 4 chemosensory choice tasks were consistent across the 3 consecutive infusion conditions: Day-1) water vs. water, Day-2) benzaldehyde-saccharin vs. isoamyl acetate-stevia, Day-3) benzaldehyde vs. isoamyl acetate, and Day-4) saccharin vs. stevia (Fig. 1C). Saline was infused into the mediodorsal thalamus for the first 4 chemosensory choice task sessions (Saline-1), followed by 4 d of NBQX inactivation of the mediodorsal thalamus during the 4 chemosensory choice task sessions (NBQX). The second round of saline infusion sessions (Saline-2) was performed to examine whether consummatory behaviors were impacted by the previous NBQX sessions or the repeated experience in the 2-bottle brief-access task. One rat was unable to complete the second saline sessions due to a head-cap failure.

Histology

After completing the pharmacological inactivation experiment, rats were anesthetized with a mixture of ketamine, xylazine, and acepromazine (100, 5.2, and 1 mg/kg) and 400 nL of biotinylated dextran amine (BDA; Invitrogen, D1956) was bilaterally infused (200 nL/minute) into the mediodorsal thalamus. Two additional minutes were allotted for the diffusion of the fluid into the brain before removing the injection cannulas and replacing the dummy stylets. Rats were then transcardially perfused with cold 0.1 M phosphate buffer saline (PBS) and cold 4% paraformaldehyde. Brains were removed, post-fixed for 24 h, cryoprotected in 30% sucrose, and cut into 70 µm thick sections. Sections were washed 3 times (10 min) in PBS before being incubated in 1:100 Alexa Fluor488-conjugated streptavidin (Invitrogen, S11223) in PBS with 0.2% Triton X-100 for 2 h in the dark. Next, sections were washed in a PBS and 0.2% Triton X-100 solution for 10 min, then washed twice (10 min) in PBS, followed by 3 washes (10 min) in 0.1M phosphate buffer (PB). Sections were incubated for 30 min in solution 0.1 M PB with DAPI (Invitrogen, D1306). Finally, sections were washed 3 times (10 min) with 0.1 M PB and mounted with Fluormount-G medium (SouthernBiotech). Only animals with cannula placement within the mediodorsal thalamus were included in the study (Fig. 2).

Fig. 2.

Representative image and schematic representation of targeted infusion in the mediodorsal thalamus. (A) Prior to perfusion, biotinylated dextran amine (BDA, green) was bilaterally infused into the mediodorsal thalamus to estimate the spread of saline and NBQX infusions. Sections were stained with DAPI (blue). (B) Schematic representation of the rat brain atlas showing the placement of the cannula tips (cyan circles) and the BDA spread (gray shade) within the mediodorsal thalamus for each rat. Only rats with cannula placement within the mediodorsal thalamus were included in the study. CL, centrolateral thalamus; CM, central medial thalamus; IMD, intermediodorsal thalamus; LHb, lateral habenula; MHb, medial habenula; MD, mediodorsal thalamus; PC, paracentral thalamus; PVP, paraventricular thalamus, posterior.

Experimental design and statistical analysis

Our primary hypothesis was that inactivation of the mediodorsal thalamus would significantly alter consummatory behaviors for chemosensory stimuli. Given this hypothesis, we used a priori tests to directly test the expected differences between conditions rather than using post hoc tests that depend on interactions. Planned comparisons allow for a more controlled approach by reducing the risk of inflating the Type I error rate that may occur with post hoc analyses. The decision to use a priori tests is supported by existing literature on the role of the mediodorsal thalamus in decision-making and chemosensory behaviors (Rousseaux et al. 1996; Asai et al. 2008; Sela et al. 2009; Chakraborty et al. 2016).

We fit the data to a 2-way repeated-measures analysis of variance (ANOVA) model to account for the repeated measures design, where each rat was exposed to both the saline and drug conditions across the 4 types of stimuli. The 2 factors in the repeated-measures ANOVA were Condition (saline vs. drug) and Stimulus Type (water, odor–taste mixtures, odors, and tastes). The model tested for the main effects of Condition and Stimulus Type, as well as the interaction between these factors. A priori comparisons between the saline and drug conditions for each stimulus type were conducted regardless of the interaction term results. This approach allowed us to test our specific hypotheses directly without relying on post hoc tests that are dependent on interaction effects (Dean and Voss 1999; Maxwell et al. 2017). By using a 2-way repeated-measures ANOVA and predefining our comparisons, we were able to efficiently analyze the data while maintaining control over Type I error rates and focusing on our primary research questions.

All statistical analyses were performed using GraphPad Prism (GraphPad Software, Inc.). Repeated-measures comparisons across control conditions (i.e. non-infusion, Saline-1, and Saline-2) were performed using a mixed-effects model analysis because one rat did not complete the Saline-2 condition due to a head-cap failure. Overall comparisons between infusion conditions (i.e. Saline and NBQX) were tested with a 2-tailed paired t-test. Two-way repeated-measures ANOVA were used to compare the effects of infusion condition and stimulus category on the number of total licks, licks per engaged trial, number of engaged trials, initiation time, number of switches, preference ratios, number of lick clusters, average inter-lick interval, licks per cluster, cluster-switch ratio, and post-session water consumption. The Dunn–Sidak correction was used to correct for multiple planned (a priori) comparisons. A Kruskal–Wallis test was used to compare the absolute change in preference ratio between stimulus categories. Post hoc analyses were performed using Tukey HSD tests to correct for familywise error. A χ² test (P < 0.05) was used to compare the overall proportion of switched trials between infusion conditions and the proportion of trials that a switch occurred from one stimulus to the other (e.g. switch from stevia to saccharin or switch from saccharin to stevia). Fisher’s exact test with Dunn–Sidak correction for familywise errors was used to compare the proportion of switched trials within each stimulus category.

Results

The purpose of this study was to determine the role of the mediodorsal thalamus in consummatory choices between similarly experienced chemosensory stimuli. We pharmacologically inactivated the mediodorsal thalamus during a 2-bottle brief-access task to investigate the area’s role in the consummatory choice of experienced odors, tastes, and odor–taste mixtures. Figure 1 shows the behavioral paradigm schedule. First, rats were given experience with odors, tastes, and odor–taste mixtures and trained to drink from the 2-bottle brief-access apparatus (Fig. 1A). Following surgical implantation of guide cannulas into the mediodorsal thalamus, rats underwent head-restraint training without infusion while being retrained in the 2-bottle brief-access task (Fig. 1B). The pharmacological inactivation experiment consisted of 12 sessions divided into 3 infusion conditions (Fig. 1C). The choice between chemosensory stimuli in the 2-bottle brief-access task was consistent across each 4-d condition. The first day was water vs. water, the second day was benzaldehyde-saccharin vs. isoamyl acetate-stevia (odor–taste mixture stimuli), the third day was isoamyl acetate vs. benzaldehyde (odor stimuli), and the fourth day was saccharin vs. stevia (taste stimuli). The first 4-d condition consisted of head-restraint with infusion of saline (i.e. Saline-1), the second 4-d condition was head-restraint with the infusion of NBQX disodium salt hydrate (i.e. NBQX), and the third 4-d condition was head-restraint with the infusion of saline (i.e. Saline-2). Figure 2 shows a representative example of the spread of bilateral BDA infusion within the mediodorsal thalamus and the 9 bilateral cannula locations with BDA spread for 8 of the 9 rats.

The saline sessions before (Saline-1 condition) and after (Saline-2 condition) the NBQX sessions enabled us to investigate whether infusion of NBQX had an enduring effect on preferences and whether a greater amount of experience in the 2-bottle brief-access task influenced consummatory choices. We specifically examined whether there were any significant differences in total licks, licks per engaged trial, and the number of engaged trials across the non-infusion, Saline-1, and Saline-2 conditions.

Repeated-measures comparisons across these 3 conditions were performed using a mixed-effects model analyses because one rat was unable to complete the Saline-2 condition due to a head-cap failure. We found no significant differences in the total licks across the 3 conditions for odor–taste mixture [F (2,15) = 0.560, P = 0.583], odor [F (2,15) = 0.987, P = 0.396], or taste sessions [F (2,15) = 0.101, P = 0.904]. Next, we performed 2-way repeated-measures mixed-effects analyses to examine the effects of condition on licks per engaged trial within each stimulus category. For odor–taste mixtures, there was no effect of condition [F (2,16) = 1.946, P = 0.175] and no interaction [F (2, 14) = 0.759, P = 0.759], but there was a main effect of odor–taste mixture [F (1,8) = 19.03, P = 0.002]. For odors, there was no main effect of condition [F (2,16) = 0.649, P = 0.537], odor stimuli [F (1,8) = 2.961, P = 0.124], and no interaction [F (2, 14) = 0.609, P = 0.558]. For tastes, there was no main effect of condition [F (2,16) = 0.049, P = 0.952] and no interaction [F (2, 14) = 0.844, P = 0.451], but there was a main effect of taste stimuli [F (1,8) = 17.45, P = 0.003]. We also examined whether task engagement was consistent across these 3 conditions. A mixed-effects model analysis revealed that the total number of engaged trials was similar across conditions and found no significant differences for odor–taste mixture [F (2,15) = 0.695, P = 0.514], odor [F (2,15) = 0.956, P = 0.407], or taste sessions [F (2,15) = 0.497, P = 0.618].

These results indicate that the different control conditions did not alter consummatory behaviors. In fact, the only significant effects were dependent upon stimulus category, which reflect differences in preference between the chemosensory stimuli. Importantly, the consistency across conditions indicates that neither the infusion of saline into the mediodorsal thalamus, nor the greater amount of experience in the 2-bottle brief-access task significantly affected consummatory behaviors. Therefore, the results of the 2 saline infusion conditions were averaged (i.e. Saline) for further analyses.

We hypothesized that inactivating the mediodorsal thalamus would significantly decrease consumption and task engagement due to its known involvement in olfactory attention and in processing the hedonic value of odors as well as odor–taste mixtures (Rousseaux et al. 1996; Asai et al. 2008; Plailly et al. 2008; Sela et al. 2009; Tham et al. 2009, 2011; Veldhuizen and Small 2011; Schmitt et al. 2017; Rikhye et al. 2018). First, we compared the total licks between the 2 infusion conditions and found that rats performed significantly fewer licks during the NBQX condition than the Saline condition (Saline: 1000.11 ± 87.51 vs. NBQX: 819.39 ± 119.76, t₍₈₎ = 3.064, P = 0.016) (Fig. 3A). Next, we used a 2-way repeated-measures ANOVA to determine whether the number of total licks differed between the infusion conditions during the four 2-bottle brief-access sessions (i.e. water, odor–taste mixtures, odors, or tastes) (Fig. 3B). There were significant main effects of infusion condition [F (1,8) = 9.388, P = 0.016] and stimulus category [F (3,24) = 12.37, P < 0.001], but no significant interaction [F (3,24) = 1.837, P = 0.170]. Despite the lack of a significant interaction, planned comparisons between infusion conditions were conducted to explore the potential differences in specific stimulus categories, based on our hypothesis that inactivating the mediodorsal thalamus would differentially affect the consumption of different types of stimuli. A priori comparisons revealed a significant difference in the total licks for tastes (Saline: 1286.83 ± 101.64 vs. NBQX: 901.44 ± 150.14, P < 0.01), but not water (Saline: 771.78 ± 100.19 vs. NBQX: 721.56 ± 170.84), odor–taste mixtures (Saline: 1062.89 ± 96.75 vs. NBQX: 965.56 ± 122.91), or odors (Saline: 878.94 ± 98.96 vs. NBQX: 689.00 ± 117.63). Contrary to our expectations, these data indicate that inactivation of the mediodorsal thalamus led to a significant decrease in the sampling of taste stimuli but had little impact on the overall consumption of water, odor–taste mixtures, or odors.

Fig. 3.

Inactivation of the mediodorsal thalamus reduces overall consumption. (A) Rats performed significantly fewer licks (± SEM) overall during the NBQX condition (white bars) than the Saline condition (gray bars). (B) During the NBQX condition, rats performed significantly fewer licks for tastes, but not for water, odor–taste mixtures, or odors. **P < 0.01.

To better understand how inactivation of the mediodorsal thalamus influenced the consummatory choice between chemosensory stimuli, we performed 2-way repeated measures analyses examining the effects of infusion condition on licks per engaged trial within each stimulus category (Fig. 4). For the choice between odor–taste mixtures, there was a significant main effect for infusion condition [F (1,8) = 7.895, P = 0.023] and between benzaldehyde-saccharin and isoamyl acetate-stevia [F (1,8) = 9.785, P = 0.014], but no significant interaction [F (1,8) = 0.1904, P = 0.674]. A priori comparisons showed that during the Saline condition, rats sampled significantly more benzaldehyde-saccharin than isoamyl acetate-stevia (41.42 ± 2.07 vs. 24.88 ± 2.60, P < 0.05). However, there was no difference between the 2 odor–taste mixtures during the NBQX condition (32.95 ± 5.79 vs. 19.61 ± 2.60) (Fig. 4A). For the odors, there was a significant main effect between infusion condition [F (1,8) = 10.89, P = 0.011], but no effect between benzaldehyde and isoamyl acetate [F (1,8) = 1.504, P = 0.255] and no significant interaction [F (1,8) = 0.9674, P = 0.354] (Fig. 4B). A priori comparisons found no difference between the 2 odors in either the Saline (34.99 ± 4.59 vs. 23.35 ± 4.49) or NBQX condition (21.57 ± 3.39 vs. 18.67 ± 4.88). For tastes, there were significant main effects of infusion condition [F (1,8) = 28.90, P < 0.001] and between saccharin and stevia [F (1,8) = 6.995, P = 0.030], but no interaction [F (1,8) = 2.222, P = 0.174] (Fig. 4C). A priori comparisons revealed that rats sampled significantly more saccharin than stevia during both the Saline (44.98 ± 3.27 vs. 28.36 ± 2.91, P < 0.01) and NBQX conditions (30.37 ± 4.84 vs. 19.94 ± 2.97, P < 0.05). However, rats sampled significantly less saccharin during the NBQX condition compared to the Saline condition (44.98 ± 3.27 vs. 30.37 ± 4.84, P < 0.01) (Fig. 4C). These data show that rats preferred to consume saccharin-containing over stevia-containing stimuli during the Saline condition. During inactivation of the mediodorsal thalamus, rats still preferred saccharin to stevia but consumed significantly less of it. However, rats no longer preferred to consume the odor–taste mixture of benzaldehyde-saccharin over isoamyl acetate-stevia when the mediodorsal thalamus was inactivated with NBQX.

Fig. 4.

Inactivation of the mediodorsal thalamus alters consummatory behaviors. (A) Rats performed significantly more licks per engaged trial (± SEM) of benzaldehyde-saccharin (dark blue bars) than isoamyl acetate-stevia (dark orange bars) during the Saline condition, but showed no preference for sampling either odor–taste mixture during the NBQX condition. (B) Rats performed equal licks per engaged trial for benzaldehyde (blue bars) and isoamyl acetate (orange bars) odors during both Saline and NBQX conditions. (C) Rats performed significantly more licks per engaged trial of saccharin (light blue bars) than stevia (light orange bars) taste during both Saline and NBQX conditions. Rats performed significantly fewer licks per engaged trial for saccharin during the NBQX condition than during the Saline condition. **P < 0.01. *P < 0.05.

Next, we examined the impact of mediodorsal thalamus inactivation on stimulus preference (Fig. 5). A preference ratio indicates which of the 2 stimuli were sampled more during each 2-bottle choice; a positive ratio indicates a preference for stimuli containing benzaldehyde-saccharin or its components, and a negative ratio indicates a preference for stimuli containing isoamyl acetate-stevia or its components. There was no difference in the mean preference ratios between infusion conditions for any of the stimulus categories (Fig. 5A–C). However, visual inspection of individual animal’s preference ratios indicated that some scores greatly changed between conditions. Therefore, we calculated the absolute difference in preference ratio between infusion conditions for each stimulus category (Fig. 5D). This measures the change in the preference ratio when the mediodorsal thalamus was inactivated. The results of a Kruskal–Wallis test revealed a significant difference between the 3 stimulus categories (H (2) = 7.51, P = 0.023). Post hoc analyses showed that the absolute change in preference ratio was significantly smaller for taste stimuli (0.13 ± 0.03) compared to odors (0.40 ± 0.11, P < 0.05), but not odor–taste mixtures (0.32 ± 0.07, P > 0.05) (Fig. 5D). These results indicate that the taste preference remained consistent during inactivation of the mediodorsal thalamus.

Fig. 5.

Taste preference is the least impacted by mediodorsal thalamus inactivation. Preference ratios (± SEM) did not significantly differ across conditions for (A) odor–taste mixture stimuli, (B) odor stimuli, or (C) taste stimuli. (D) The absolute difference in preference ratio (± SEM) was significantly smaller for tastes (white bar) compared to odors (light gray bar), but not odor–taste mixtures (dark gray bar), indicating preference was most consistent for tastes. *P < 0.05.

One possibility for the reduced sampling during the NBQX condition is a decrease in task engagement. Therefore, we examined whether the number of trials in which the rat chooses to engage differed between conditions. A 2-way repeated measures ANOVA verified that there was no significant main effect between Saline and NBQX infusion conditions [F (1,8) = 0.815, P = 0.393] or interaction [F (3,24) = 1.935, P = 0.151], but there was a main effect between stimulus categories [F (3,24) = 4.927, P = 0.008]. However, a priori comparisons found no significant difference in the number of engaged trials between conditions for any stimulus category (Fig. 6A). These findings indicate that the rats consistently participated in the 2-bottle brief-access task regardless of infusion condition or chemosensory choice.

Fig. 6.

Inactivation of the mediodorsal thalamus does not alter task engagement or motivation. (A) There was no significant difference in the number of engaged trials (± SEM) between Saline (gray bars) and NBQX (white bars) conditions for water, odor–taste mixtures, odors, or tastes. (B) There was a significant main effect of stimulus category in the initiation time (± SEM), but no significant differences across stimulus categories between the infusion conditions. (C) There was no significant difference in the inter-lick interval (ILI) (± SEM) across stimulus categories between the infusion conditions. (D) There was no significant difference in the number of lick clusters (± SEM) across stimulus categories between the infusion conditions. (E) Rats drank significantly more water (± SEM) during the first hour in the home cage following 2-bottle brief-access task with the NBQX condition than with the Saline condition. (F) Rats drank significantly more water in the home cage after receiving odors and tastes in the 2-bottle brief-access task with the NBQX condition than with the Saline condition. There was no difference for odor–taste mixtures or water. ***P < 0.001. **P < 0.01. *P < 0.05.

Inactivation of the mediodorsal thalamus could have also impacted the motivation or ability to initiate a trial. As an alternative assessment of task engagement, we measured the time it took to engage in a trial by determining the duration until the first spout contact (i.e. initiation time) (Fig. 6B). The results of a 2-way repeated-measures ANOVA showed no significant main effect of infusion condition [F (1,8) = 0.008, P = 0.931] or interaction [F (3,24) = 0.447, P = 0.721]. However, there was a significant main effect of initiation time for stimulus category [F (3,24) = 5.636, P = 0.005]. A priori comparisons showed no significant differences between the infusion conditions for any chemosensory category.

Next, we examined whether inactivation altered the inter-lick interval, a measure of motivation and motor control (Fig. 6C). The results of a 2-way repeated-measures ANOVA showed no significant main effect of infusion condition [F (1,8) = 0.970, P = 0.353], stimulus category [F (3,24) = 2.089, P = 0.128], or interaction [F (3,24) = 1.202, P = 0.330]. We also examined whether the number of clusters, defined as bouts where the rat licked at least 2 times within 500 ms, was affected (Fig. 6D). The results of a 2-way repeated-measures ANOVA showed no significant main effect of infusion condition [F (1,8) = 2.158, P = 0.180], stimulus category [F (3,24) = 0.117, P = 0.949], or interaction [F (3,24) = 2.252, P = 0.108]. We then compared the licks per cluster between the 2 infusion conditions and found that overall rats performed significantly fewer licks per cluster during the NBQX condition than the Saline condition (Saline: 27.38 ± 2.40 vs. NBQX: 18.54 ± 2.90, t₍₈₎ = 5.820, P < 0.001). The results of a 2-way repeated-measures ANOVA showed a significant main effect of infusion condition [F (1,8) = 33.88, P < 0.001] and stimulus category [F (3,24) = 10.54, P < 0.001], but no interaction effect [F (3,24) = 2.676, P = 0.070]. Post hoc analyses were not performed because there was no significant interaction between infusion and stimulus category, and this was not a preplanned comparison. Together, these results indicate that the inactivation of the mediodorsal thalamus did not significantly alter motivation, the ability to initiate trials, or motor control of licking but did reduce the overall number of licks per lick cluster.

Our results show that rats initiated and engaged in a similar number of trials regardless of infusion condition but consumed significantly less during the inactivation of the mediodorsal thalamus. One possible explanation is that inactivation made the rats less thirsty. If this were the case, we would expect similar amounts of water consumption post-session. To investigate this, we measured the volume of water that rats consumed in the home cage for 1 h following the 2-bottle brief-access task and compared across Saline and NBQX infusion conditions (Fig. 6E). We found that rats drank significantly more water after the NBQX condition compared to the Saline condition (Saline: 6.29 ± 0.55 vs. NBQX: 7.83 ± 0.42, t₍₈₎ = 3.153, P = 0.014). We then used a 2-way repeated-measures ANOVA to determine whether post-session water consumption differed between the infusion conditions across the stimulus categories tested within the 2-bottle brief-access task (Fig. 6F). There was a significant main effect of infusion condition [F (1,8) = 9.938, P = 0.014], main effect of stimulus category [F (3,24) = 3.243, P = 0.040], and a significant interaction between condition and stimulus category [F (3,24) = 3.030, P = 0.049]. Post hoc analyses revealed that rats drank significantly more water in the home cage after the NBQX condition for odors (Saline: 6.06 ± 0.56 vs. NBQX: 8.78 ± 0.60, P < 0.001) and tastes (Saline: 6.17 ± 0.72 vs. NBQX: 8.22 ± 0.66, P < 0.01), but not after odor–taste mixtures (Saline: 7.06 ± 0.84 vs. NBQX: 7.67 ± 0.41, P > 0.05) or water (Saline: 5.89 ± 0.40 vs. NBQX: 6.67 ± 0.53, P > 0.05). Consumption amounts were not consistently reduced across all stimulus categories for the NBQX condition (Fig. 6F), suggesting that the inactivation of the mediodorsal thalamus did not decrease overall thirst.

Another possibility is that increased switching between ports within each 15-second trial window reduced the amount of time rats could sample from the spouts. To test this, we examined whether the proportion of engaged trials in which rats sampled from both ports differed between Saline and NBQX infusion conditions (Fig. 7A). A χ² test revealed a significant difference between infusion conditions in the proportion of trials in which rats switched between ports (Saline: 26.2% vs. NBQX: 53.5%, χ² = 134.1, P < 0.001). A priori comparisons revealed that the proportion of switched trials was significantly higher during the NBQX condition than the Saline condition for water (Saline: 29.5% vs. NBQX: 48.1%, Fisher’s exact test, P < 0.001), odor–taste mixtures (Saline: 26.1% vs. NBQX: 49.4%, Fisher’s exact test, P < 0.001), odors (Saline: 29.1% vs. NBQX: 57.8%, Fisher’s exact test, P < 0.001), and tastes (Saline: 20.8% vs. NBQX: 58.3%, Fisher’s exact test, P < 0.001). These results indicate that inactivating the mediodorsal thalamus leads to a higher proportion of trials where rats sample from both bottles.

Fig. 7.

Inactivation of the mediodorsal thalamus increases within-trial switching between stimuli. (A) The proportion of switched trials was significantly higher during NBQX condition than Saline condition across all stimulus categories. (B) The mean number of switches (± SEM) was significantly higher during the NBQX condition than Saline condition for odor–taste mixtures, odors, and tastes, but not for water. (C) The proportion of switches to the benzaldehyde-saccharin mixture or its components was significantly higher than to the isoamyl acetate-stevia mixture or its components in the Saline condition. (D) In the Saline condition, but not the NBQX condition, the proportion of switches to the benzaldehyde-saccharin mixture was higher than to the isoamyl acetate-stevia mixture. (E) There was no difference between switches to benzaldehyde and switches to isoamyl acetate. (F) In the Saline condition, but not the NBQX condition, the proportion of switches to saccharin was higher than to stevia. ***P < 0.001. *P < 0.05.

Given the length of a trial, it is possible for rats to perform multiple switches within a single trial, resulting in an increase in the total number of switches. We compared the total number of switches between Saline and NBQX infusion conditions and found that rats switched between the 2 ports significantly more during inactivation (Saline: 5.53 ± 1.85 vs. NBQX: 15.06 ± 5.240, t₍₈₎ = 2.769, P = 0.035). A 2-way repeated-measures ANOVA showed a significant main effect of infusion condition [F (1,8) = 6.473, P = 0.035] and a significant interaction between condition and stimulus category [F (3,24) = 4.373, P = 0.014], but no main effect of stimulus category [F (3,24) = 1.641, P = 0.206] (Fig. 7B). A priori comparisons revealed that rats switched between bottles significantly more often during the NBQX condition for odor–taste mixtures (Saline: 5.39 ± 2.13 vs. NBQX: 14.78 ± 5.94, P < 0.001), odors (Saline: 5.56 ± 1.90 vs. NBQX: 18.22 ± 5.99, P < 0.001), and tastes (Saline: 4.22 ± 1.61 vs. NBQX: 16.67 ± 5.55, P < 0.001), but not for water (Saline: 6.94 ± 2.06 vs. NBQX: 10.56 ± 4.33, P > 0.05).

Next, we investigated whether the switching behavior of rats was based on the stimuli. Specifically, we examined whether there was a difference between the proportion of switches to bottles containing the benzaldehyde-saccharin mixture or its components and the proportion of switches to bottles containing the isoamyl acetate-stevia mixture or its components (Fig. 7C). A χ² test revealed a significant difference between switches to benzaldehyde-saccharin components and switches to isoamyl acetate-stevia components (χ² = 16.46, P < 0.001). Post hoc analyses showed that the proportion of switches to the benzaldehyde-saccharin components (58.4%) was significantly higher than switches to the isoamyl acetate-stevia components during the Saline condition (41.6%, Fisher’s exact test, P < 0.001), but not different during the NBQX condition (51.9% vs. 48.1%, P > 0.1).

We further examined the proportion of switches within each stimulus category. A χ² test revealed a significant difference between switches to the benzaldehyde-saccharin mixture and switches to the isoamyl acetate-stevia mixture (χ² = 16.41, P < 0.001) (Fig. 7D). Post hoc analyses revealed that the proportion of switches to the benzaldehyde-saccharin mixture (64.5%) was significantly higher than switches to the isoamyl acetate-stevia mixture during the Saline condition (35.5%, Fisher’s exact test, P < 0.001), but not different during the NBQX condition (47.4% vs. 52.6%, P > 0.1). A χ² test revealed no difference between switches to benzaldehyde and switches to isoamyl acetate (χ² = 0.96, P = 0.810) (Fig. 7E). There was a significant difference between switches to saccharin and switches to stevia (χ² = 9.41, P = 0.024) (Fig. 7F). Post hoc analyses revealed that the proportion of switches to the saccharin (60.0%) was significantly higher than switches to stevia mixture during the Saline condition (40.0%, Fisher’s exact test, P < 0.001), but not different during the NBQX condition (55.3% vs. 44.7%, P > 0.05).

Together, the results from this experiment demonstrate that inactivation of the mediodorsal thalamus significantly decreased overall consumption and altered consummatory choices. Despite this reduction in consumption, rats maintained consistent task engagement and initiation times, indicating that motivation and motor control were not significantly affected. Additionally, inactivation led to a significant increase in the number of switches between chemosensory stimuli, but not water, suggesting that the increased switching behavior is chemosensory-dependent.

Discussion

In this study, we used a 2-bottle brief-access task to investigate the role of the mediodorsal thalamus in making consummatory choices among familiar odors, tastes, and odor–taste mixtures. We found that inactivation of the mediodorsal thalamus significantly reduced the consumption of the preferred saccharin taste, eliminated the preference for benzaldehyde-saccharin over isoamyl acetate-stevia, and resulted in more frequent within-trial switching between stimuli. These changes in consummatory behavior were not due to decreased task engagement or disruption of motor control, as rats participated in a similar number of trials with comparable trial initiation times and inter-lick intervals. Our results suggest that disrupting the activity of the mediodorsal thalamus may alter the hedonic perception of chemosensory stimuli or lead to attentional deficits or indecision. Our findings are consistent with previous literature that demonstrates the involvement of the mediodorsal thalamus in both hedonic value and sensory attention (Rousseaux et al. 1996; Asai et al. 2008; Plailly et al. 2008; Sela et al. 2009; Tham et al. 2009, 2011; Veldhuizen and Small 2011; Schmitt et al. 2017; Rikhye et al. 2018).

Since prior experience plays a crucial role in shaping decisions about what to eat, we gave rats experience with odor–taste mixtures containing tastes with the same quality but slightly different hedonic values. At the concentrations used here, both noncaloric sweeteners are preferred over water, but saccharin is preferred to stevia (Sclafani et al. 2010). Our results confirmed the saccharin preference and showed that rats also prefer a mixture of benzaldehyde-saccharin over one of isoamyl acetate-stevia. Numerous animal and human studies have demonstrated that experience with odor–taste mixtures leads to associations between the odor and the paired taste’s quality and hedonic value (Fanselow and Birk 1982; Holder 1991; Stevenson et al. 1995, 1998; Schul et al. 1996; Sakai and Yamamoto 2001; Prescott et al. 2004; Gautam and Verhagen 2010; Green et al. 2012; Blankenship et al. 2019; McQueen et al. 2020). Therefore, we expected that rats would prefer to consume benzaldehyde over isoamyl acetate, but we found no significant odor preference. This could be due to the similar quality and positive hedonic value of the tastes, combined with the time-sensitive environment of the task. The fixed number of trials with a limited sampling period may have outweighed the consequence of sampling the “wrong” less preferred odor.

During the inactivation of the mediodorsal thalamus, we found that rats still preferred the saccharin taste but sampled significantly less of it. Additionally, the previous preference for the benzaldehyde-saccharin mixture over the isoamyl acetate-stevia mixture was abolished. These changes in consummatory behavior were likely related to the reduced sampling time caused by the increase in within-trial switching. Previous studies suggest several possibilities for the increased switching observed during inactivation of the mediodorsal thalamus. One possibility is that inactivating the mediodorsal thalamus altered the perceived value of the chemosensory stimuli, resulting in rats switching between ports to repeatedly “test” stimuli (Rousseaux et al. 1996; Asai et al. 2008; Sela et al. 2009; Tham et al. 2011). However, our results suggest this is not the case for taste stimuli, as rats still preferred saccharin over stevia. The change in mixture preference suggests that inactivation of the mediodorsal thalamus may have altered the perceptual value of the odor–taste mixtures. Future studies employing odor–taste mixtures with opposite hedonic values would help to elucidate the role of the mediodorsal thalamus in the hedonic evaluation of chemosensory stimuli.

Another possible explanation for increased switching behaviors is that inactivation of the mediodorsal thalamus disrupts sensory attention, resulting in the inability to focus on one sensory stimulus over another (Tham et al. 2009; Veldhuizen and Small 2011; Rikhye et al. 2018). The mediodorsal thalamus is known to be involved in directing attention towards relevant sensory stimuli (Plailly et al. 2008; Schmitt et al. 2017). For example, during an olfactory attention task, neural coupling increases between the piriform cortex and mediodorsal thalamus, as well as the mediodorsal thalamus and orbitofrontal cortex (Plailly et al. 2008). Studies of attention deficit hyperactivity disorder (ADHD) have revealed that individuals with ADHD exhibit increased brain activity in the mediodorsal thalamus during a conscious resting state (Tian et al. 2008). This suggests that individuals with ADHD may be processing more sensory information, consistent with their symptoms of inattention and distraction by environmental stimuli. These studies suggest that abnormal activity in the mediodorsal thalamus, whether overactive or inactive, can cause attentional issues and that a balanced relationship between the cortex and thalamus is needed for optimal attention. Future studies that require rats to attend to a small stimulus sample for a set period to trigger the delivery of more would help determine whether the mediodorsal thalamus is necessary for chemosensory attention during a 2-bottle brief-access task.

A third possibility is that inactivation of the mediodorsal thalamus impairs decision-making by disrupting the ability to stop sampling options and pick the best one (Chakraborty et al. 2016). Chakraborty and colleagues examined the role of the mediodorsal thalamus in decision-making when reward values changed. They found that monkeys with mediodorsal thalamus lesions failed to efficiently update their behavior following a reversal in reward value. The authors noted the similarity of this behavioral deficit to the effects seen with orbitofrontal cortex lesions (Walton et al. 2011). However, while orbitofrontal cortical lesions caused monkeys to reselect an option often chosen in the past, mediodorsal thalamic lesions caused monkeys to continually switch between all options, never learning to persist in choosing the updated best option. The observed switching behavior following mediodorsal thalamus inactivation in our study aligns with these findings.

In our study, inactivation of the mediodorsal thalamus increased switching behavior during choices between chemosensory stimuli but did not alter switching during choices between water and water. This suggests that rats do not continue switching between options when the stimuli are identical, although it remains unclear whether this is due to stimulus identity or value. Future studies could investigate these possibilities by inactivating the mediodorsal thalamus while presenting 2 mixtures with the same taste but different odors: both mixtures would have the same hedonic value but would differ in chemical identity. An increase in switching behavior would indicate that differences in chemical identity, rather than hedonic value, cause the failure to persist with a singular sampling choice. Alternatively, if the amount of switching between stimuli does not increase, it would suggest the equivalent hedonic values of the stimuli make it unnecessary to seek other sampling options. This experimental endeavor may help provide answers to individuals suffering from lesions of the mediodorsal thalamus who are experiencing altered food perceptions or a reduced desire to eat (Rousseaux et al. 1996; Asai et al. 2008; Sela et al. 2009).

It is important to acknowledge that infusion of NBQX could potentially affect areas beyond the mediodorsal thalamus. The most likely region affected is the habenular complex, located just dorsal to the mediodorsal thalamus. The habenular complex, particularly the lateral habenula, plays a key role in adapting to changes in tasks or context contingencies (Mizumori and Baker 2017). Studies have shown that the lateral habenula is crucial for flexible decision-making based on subjective costs and reward magnitudes (Stopper and Floresco 2014; Baker et al. 2019; Trusel et al. 2019; Post et al. 2022). Specifically, inactivation of the lateral habenula in rats disrupts their ability to distinguish or prioritize rewards based on subjective costs such as effort or risk, but they are still able choose between rewards of different magnitudes (Stopper and Floresco 2014). The potential spread of NBQX into the lateral habenula in our experimental paradigm could have impacted the decision-making processes based on the subjective cost of time to drink within our brief-access task, and the lack of risk in that there were no negative stimuli presented. However, as with our study, it is important to consider that drug infusions intended to inactivate the lateral habenula may also have spread to the mediodorsal thalamus. This potential spread could mean that some of their observed effects might be due to unintended inactivation of the mediodorsal thalamus. Future studies using optogenetics or DREADDs to specifically target neurons in distinct regions could help disentangle these effects and provide more precise insights into the specific contributions of each area.

In conclusion, our findings implicate the mediodorsal thalamus in the consummatory choice of experienced chemosensory stimuli. Our results show that the inactivation of the mediodorsal thalamus eliminated the preference between odor–taste mixtures, significantly reduced the consumption of the preferred taste, and increased within-trial switching between chemosensory stimuli. These findings indicate that the mediodorsal thalamus plays a significant role in consummatory behaviors related to the hedonic value of chemosensory stimuli and sensory attention. Future research will aim to elucidate the neural mechanisms underlying its involvement in decisions that guide consummatory behavior.

Author contributions

KEG and CLS designed the research. KEG performed the research. KEG and CLS analyzed the data. KEG and CLS wrote the paper. All authors reviewed and approved the final manuscript.

Funding

This work was supported by the National Institute of Deafness and Other Communication Disorders at the National Institutes of Health (R01-DC018273; CLS).

Conflict of interest

The authors declare no conflict of interest.

Data availability

The data that supports the findings of this article will be shared on reasonable request to the corresponding author.