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ISSN : 1226-7155(Print)
ISSN : 2287-6618(Online)
International Journal of Oral Biology Vol.51 No.3 pp.68-79
DOI : https://doi.org/10.11620/IJOB.2026.51.3.68

Modulation of taste responses in parabrachial neurons by electrical stimulation of the dorsal motor nucleus of the vagus

Young Kyung Cho1,2*, Cheng-Shu Li3, Ki-Myung Chung1,2, Kyung-Nyun Kim1,2
1Department of Physiology and Neuroscience, College of Dentistry, Kangwon National University, Gangneung 25457, Republic of Korea
2Research Institute of Oral Sciences, Kangwon National University, Gangneung 25457, Republic of Korea
3Department of Anatomy, Southern Illinois University School of Medicine, Carbondale, IL 62901, United States
*Correspondence to: Young Kyung Cho, E-mail: ykcho04@kangwon.ac.krhttps://orcid.org/0000-0002-7367-8301
August 11, 2026 September 5, 2026 September 7, 2026

Abstract


The pontine parabrachial nucleus (PbN) serves as the second central relay in the gustatory pathway and is a critical site for the integration of taste and visceral information. The medullary dorsal motor nucleus of the vagus (DMV) is a key component of vago-vagal reflex circuits, and anatomical evidence suggests that DMV neurons project directly to the PbN. However, it remains unclear whether the DMV exerts a functional influence on taste-responsive PbN neurons. In the present study, we investigated this question by recording extracellular single-unit activity from gustatory PbN neurons in urethane-anesthetized hamsters. Taste responsiveness was established by applying sucrose, NaCl, citric acid, and quinine hydrochloride to the anterior tongue. Seventy-five of 80 responsive neurons (93.8%) were orthodromically excited by ipsilateral DMV stimulation. In addition, contralateral DMV stimulation activated 39 neurons (48.8%), of which 32 were excited and 7 were inhibited. No antidromic responses were observed, suggesting that a reciprocal PbN-to-DMV projection was weak or absent among the neurons examined. The effect of DMV activation on taste-evoked responses was examined in 20 PbN neurons that were excited and 7 that were inhibited by DMV stimulation. DMV train stimulation enhanced taste responses in all 20 excited neurons and suppressed taste responses in all 7 inhibited neurons without altering the preferred taste stimulus. These results demonstrate that gustatory PbN neurons receive convergent inputs from the DMV and that DMV activation bidirectionally modulates pontine taste processing, providing a potential neural substrate through which visceral signals influence gustatory responses.


Parabrachial nucleus , Dorsal nucleus of vagus nerve , Gustation , Vagal afferents , Electrophysiology

초록

    © The Korean Academy of Oral Biology. All rights reserved.

    This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    Introduction

    The parabrachial nucleus (PbN) in the pons serves as the second central relay in the rodent gustatory pathway, receiving taste information from the nucleus of the solitary tract (NST) and transmitting it to rostral gustatory centers including the ventral posteromedial nucleus of the thalamus (VPMpc), lateral hypothalamus (LH), central nucleus of the amygdala (CeA), and gustatory cortex [1,2]. Beyond its role as a gustatory relay, the PbN is also a critical site for the integration of gustatory and visceral sensory information [3,4]. Electrophysiological studies have demonstrated that a subset of taste-responsive neurons in the rat PbN is activated by gastric distension [3,5], and that intraduodenal infusion of lipid suppresses the overall magnitude of gustatory responses in the PbN of awake rats [6]. The distribution of gastric distension-responsive neurons extends throughout the rostrocaudal extent of the PbN, encompassing both its visceral and gustatory regions [7]. The importance of the PbN as an integrative site is further supported by studies in chronic decerebrate rats, which demonstrate that brain stem circuits are sufficient to regulate consumption of a sapid stimulus in response to gastric preload [8,9].

    Anatomically, primary gustatory and vagal afferent fibers enter the medulla and terminate within distinct regions of the NST. Gustatory afferents from the tongue and oropharyngeal cavity are carried to the rostral NST by the facial (VII), glossopharyngeal (IX), and vagus (X) nerves, with terminals arranged in a topographic, rostral-to-caudal sequence [10-13]. General visceral afferents carried by the vagus nerve, by contrast, enter the dorsolateral medulla and travel to the solitary tract, terminating primarily in the intermediate NST, the area postrema, and the dorsal motor nucleus of the vagus (DMV) in a complex topographic pattern [14,15].

    The DMV occupies the vagal trigone of the floor of the fourth ventricle and constitutes a key component of vago-vagal reflex circuits [16-18]. In the rat, the DMV receives both direct vagal afferent projections and indirect vagal input relayed through NST interneurons. DMV neurons, in turn, project to the intrinsic ganglia of the viscera via vagal efferent fibers [19-21]. Importantly, not all neurons in the DMV project to the periphery. Two distinct cell types have been identified in the DMV following injection of horseradish peroxidase (HRP) into the cervical vagus nerve in both the hamster and cat: neurons labeled with HRP (vagal motor neurons) and neurons that remain unlabeled [22]. After vagotomy, the unlabeled cell population remains intact while the HRP-labeled neurons degenerate, indicating that the unlabeled cells do not project via the vagus nerve and may instead serve as interneurons or projection neurons to other brain regions [22].

    Evidence for a direct projection from the DMV to the PbN comes from both anterograde and retrograde tracing studies. An autoradiographic study in the cat demonstrated that cells in the DMV project to the PbN and the Kölliker–Fuse area [23]. Retrograde labeling studies in the rat also revealed labeled cells in the DMV following microinjection of WGA-HRP into the medial, superior cerebellar peduncle, and lateral subdivisions of the PbN [24], providing direct anatomical evidence that DMV neurons send axons to the PbN. Taken together, these findings suggest that visceral afferent information may reach the PbN not only via the NST but also through a direct DMV-to-PbN pathway.

    Despite this anatomical evidence, no electrophysiological study has directly examined whether the DMV exerts a functional influence on taste-responsive neurons in the PbN. The present study was therefore undertaken to address two questions: first, whether gustatory neurons in the PbN can be orthodromically activated by electrical stimulation of the DMV; and second, whether DMV activation modulates the taste-evoked responses of these neurons.

    Materials and Methods

    1. Animal surgery

    All experimental procedures were conducted in accordance with the guidelines on the use and care of laboratory animals set by the National Institutes of Health and approved by the Institutional Animal Care and Use Committee of Southern Illinois University at Carbondale. Young adult male Syrian golden hamsters (Mesocricetus auratus), weighing between 140 and 182 g (n = 37), were deeply anesthetized with urethane (1.7 g/kg, ip). Surgical procedures, including tracheotomy, stereotaxic mounting, and cerebellar aspiration to expose the brainstem, were performed as described previously [25,26]. Body temperature was maintained at 37 ± 1℃ with an electric heating pad.

    After exposing the floor of the fourth ventricle, a concentric bipolar stimulating electrode was lowered into the DMV bilaterally (coordinates: 0.5 mm anterior to the obex, 0.62–0.70 mm lateral to the midline, and 0.3 mm ventral to the brainstem surface). Stimulating electrodes were constructed from 33-gauge stainless steel tubing and 75-μm stainless steel wire, insulated with Epoxylite 6001 (Epoxylite) except at the tip.

    2. Single-unit recording and taste stimulation

    Extracellular single-unit recordings from gustatory PbN neurons were obtained using glass micropipettes filled with 2% Chicago Blue dye in 0.5 M sodium acetate, as described previously [26]. The mean recording coordinates were 4.0 ± 0.11 mm anterior to the obex and 1.4 ± 0.09 mm lateral to the midline. The taste responses of the PbN neurons were initially identified by a change in neural activity associated with the application of electrical shock (≤ 40 μA, 500-ms duration at 1/3 Hz) to the anterior tongue and confirmed by response to chemical stimulation of the anterior tongue. Taste stimuli (0.032 M sucrose, 0.032 M NaCl, 0.032 M quinine hydrochloride [QHCl], and 0.0032 M citric acid) were delivered to the anterior tongue via a gravity-flow system as described previously [25]. The stimulation sequence was a continuous flow initiated by the delivery of distilled water for 10 seconds, followed by 10 seconds of stimulus, followed by 10 seconds of distilled water rinse. Individual stimulations were separated by ≥ 2 minutes to avoid adaptation [27]. Each neuron was classified into one of four best-stimulus categories (sucrose-best, NaCl-best, citric acid-best, or QHCl-best) based on its largest response.

    3. Electrical stimulation of the dorsal motor nucleus of the vagus

    After characterizing the taste response profile of each PbN neuron, rectangular pulses (0.5 ms, ≤ 0.1 mA) were delivered to the ipsilateral and then contralateral DMV to test for antidromic activation. The criteria for antidromic activation were constant latency and the ability to follow a paired-pulse stimulus at 100 Hz, confirmed by a collision test. Antidromic electrical activation defines a recorded neuron that projects an axon to the electrical stimulation site.

    Orthodromic responses were then examined by delivering single pulses (0.5 ms, 0.07 mA, 1/3 Hz) to each DMV electrode while recording ongoing activity. The stimulus intensity of 0.07 mA was chosen based on the fact that the DMV is approximately 300 μm thick in the hamster, and a current of 0.07 mA activates axons within a sphere of approximately 100 μm radius [28]. A peristimulus time histogram (PSTH, 1-ms bin width) was constructed from 50–200 stimulus pulses for each cell. After a PSTH was created following the DMV stimulation, the excitatory or inhibitory epoch was determined on each individual PSTH. The latency of an orthodromic response was defined as the time at which the onset of the response occurred after the delivery of electrical shock to the DMV (time 0).

    To examine whether DMV activation modulates taste responses, taste trials were repeated with and without concurrent train stimulation of the DMV (0.2 ms, 100 Hz) in neurons that were orthodromically activated. The shorter pulse width used for train stimulation (0.2 ms vs. 0.5 ms for single pulses) was adopted to minimize tissue damage and stimulus artifact, consistent with our previous studies [26,29]. For neurons excited by DMV stimulation, the stimulus intensity was set to 80% of the orthodromic activation threshold to prevent direct DMV-evoked spikes from contaminating the taste response [26]. For neurons inhibited by contralateral DMV stimulation, which showed no excitatory orthodromic response and thus lacked a defined excitatory threshold, train stimulation was applied at 0.1 mA, 0.2 ms at 100 Hz, consistent with the intensity used in previous studies [29,30]. This protocol was applied to 20 neurons that were excited and all 7 neurons that were inhibited by DMV stimulation. Electrical stimulation began at the onset of each taste trial and lasted for 20 seconds. Each test was separated by ≥ 10 minutes to allow recovery. For neurons excited by DMV stimulation, taste modulation was tested in a subset of 20 cells selected from the 75 excited neurons. These cells were chosen on the basis of adequate spontaneous or taste-evoked firing activity throughout the recording session. Neurons that lost stable spike isolation before the completion of all taste and stimulation trials were excluded from this analysis.

    4. Histology

    At the end of each experiment, recording sites were marked by depositing Chicago Blue dye via cathodal current (10 μA, 10 minutes), and DMV stimulating sites were marked by depositing iron via anodal current (10 μA, 30 seconds). Animals were perfused with 4% formalin containing 3% potassium ferrocyanide and ferricyanide. Brains were frozen-sectioned (40 μm), stained with neutral red, and recording and stimulating sites were plotted on standard hamster atlas sections [31].

    5. Data analysis

    The net taste response was calculated as the mean firing rate (spikes/s) during the first 5 seconds of chemical stimulation minus the mean baseline rate during the 5-seconds prerinse immediately preceding taste delivery. A taste response was considered significant if it exceeded 2.0 standard deviations (SD) above the baseline discharge. For orthodromic responses to DMV stimulation, individual PSTHs were analyzed as described previously [25,32]. An excitatory response was defined as an epoch of at least five consecutive 1-ms bins with a mean value > 2 SD above baseline. An inhibitory response was defined as ≥ 30 consecutive bins with a mean value < 50% of the baseline firing rate. Statistical comparisons were made using ANOVA, t-tests, and chi-square tests as appropriate. All values are reported as means ± standard error.

    Results

    1. Histology

    Data were collected from 37 hamsters; 8 animals were excluded due to misplacement of the stimulating electrodes outside the DMV, leaving 29 animals for analysis. Representative histological sections showing the recording and stimulating sites are illustrated in Fig. 1. The recording site in Fig. 1A is located in the medial PbN (MPB) and the stimulating site in Fig. 1B is located in the lateral portion of the DMV.

    2. Taste response characteristics of parabrachial nucleus neurons

    A total of 80 taste-responsive neurons were recorded from the PbN of 29 hamsters. Each PbN taste neuron was tested for its response to the four basic taste stimuli and classified into one of four best-stimulus categories. Of the 80 neurons, 21 were NaCl-best, 12 were sucrose-best, 22 were citric acid-best, and 25 were QHCl-best. The taste response profiles of all 80 cells are shown in Fig. 2, with cells within each best-stimulus group arranged in order of decreasing response magnitude to their best stimulus. An example of a single-unit recording from a citric acid-best PbN neuron, along with its PSTH, is shown in Fig. 3. It depicts 30 seconds of the extracellularly recorded neural activity during taste trials consisting of 10 seconds of distilled water prerinse, followed by 10 seconds of the taste stimulus, and then by 10 seconds of poststimulus distilled water rinse. The y-axis scale bar for the raw traces in Fig. 3 is not provided, as the amplitude of extracellularly recorded action potentials depends on the amplification gain applied during recording rather than on any physiological property of the neuron.

    The overall spontaneous activity of the 80 PbN neurons ranged from 0 to 17.9 spikes/s, with a mean of 4.04 ± 0.45 spikes/s. This was comparable to that reported previously for hamster PbN neurons (3.92 ± 0.38 spikes/s; t = 0.221, df = 179, p = 0.825) [32] and significantly higher than that of NST neurons recorded in prior studies (2.12 ± 0.27 spikes/s; t = 3.905, df = 208, p < 0.001) [33]. Since only 5 neurons failed to respond to ipsilateral DMV stimulation, comparisons between DMV-responsive and non-responsive groups were based on contralateral DMV stimulation results. Spontaneous firing rates did not differ significantly between contralateral DMV-responsive neurons (mean = 3.96 ± 0.62 spikes/s) and non-responsive neurons (mean = 4.13 ± 0.66 spikes/s; t = 0.192, df = 78, p = 0.849). There was also no significant difference in taste response magnitude between contralateral DMV-responsive and non-responsive groups (F[1,312] = 0.022, p = 0.882), nor in the distribution of best-stimulus categories between the two groups (χ2 = 5.469, df = 3, p = 0.140).

    3. Orthodromic responses of parabrachial nucleus taste neurons to dorsal motor nucleus of the vagus stimulation

    All 80 taste-responsive PbN cells were tested with bilateral DMV stimulation. No antidromic responses were observed in any of the neurons examined, indicating that a PbN-to-DMV projection was not detected among the cells sampled. However, 78 of 80 neurons (97.5%) were orthodromically activated following ipsilateral and/or contralateral DMV stimulation.

    Stimulation of the ipsilateral DMV orthodromically activated 75 of 80 (93.8%) PbN neurons. All ipsilateral responses were excitatory. The five non-responsive cells consisted of 1 NaCl-best, 1 citric acid-best, 1 QHCl-best, and 2 sucrose-best neurons. The mean activation latency following ipsilateral DMV stimulation was 11.2 ± 0.54 ms (range: 6–24 ms), and the mean duration of the evoked excitatory response was 27.5 ± 8.9 ms (range: 5–680 ms). Several characteristic response patterns were observed in PbN neurons following DMV stimulation, as illustrated in Fig. 4: a simple excitatory burst (Fig. 4A and 4B), excitation followed by suppression (Fig. 4C and 4D), a double excitatory peak (Fig. 4E and 4F), and a prolonged excitatory response (Fig. 4G and 4H). Responses followed by suppression were more frequent after ipsilateral stimulation (29 of 75 cells) than after contralateral stimulation (6 of 32 cells). Double-peak responses were similarly more common after ipsilateral (24 of 75 cells) than contralateral stimulation (3 of 32 cells).

    Stimulation of the contralateral DMV orthodromically activated 39 of 80 (48.8%) PbN neurons: 32 (40.0%) showed excitatory responses and 7 (8.8%) showed inhibitory responses. The mean activation latency of the 32 excited neurons following contralateral DMV stimulation was 25.1 ± 1.1 ms (range: 11–35 ms), significantly longer than after ipsilateral DMV stimulation (t = 12.750, df = 105, p < 0.0001). The mean duration of excitation following contralateral DMV stimulation (45.5 ± 18.9 ms) did not differ significantly from that following ipsilateral stimulation (t = 0.982, df = 105, p = 0.329). Thirty-six cells (45%) were activated bilaterally, including 6 that showed inhibitory responses to contralateral DMV stimulation. The remaining inhibited cell did not respond to ipsilateral DMV stimulation.

    The 7 PbN neurons inhibited by contralateral DMV stimulation had a mean latency of 26.3 ± 4.3 ms (range: 14–48 ms) and a mean inhibitory duration of 51.7 ± 8.6 ms (range: 31–91 ms). These cells consisted of 2 sucrose-best, 3 NaCl-best, 1 citric acid-best, and 1 QHCl-best neuron. For neurons with low spontaneous firing rates, inhibitory responses to DMV stimulation were difficult to detect from spontaneous activity alone. In such cases, single-pulse DMV stimulation was delivered while driving the neuron with taste stimulation. When taste stimulation was applied simultaneously to elevate neuronal firing, clear inhibitory responses became evident with the same number of stimulus pulses (Fig. 5A and 5B), confirming that these neurons received inhibitory input from the DMV.

    4. Dorsal motor nucleus of the vagus stimulation modulates taste responses of parabrachial nucleus neurons

    To determine whether DMV input influences gustatory processing, we compared taste responses of PbN neurons with and without concurrent train stimulation of the DMV. Twenty neurons that were excited by DMV stimulation (4 sucrose-best, 6 NaCl-best, 5 citric acid-best, and 5 QHCl-best) were tested. Taste responses were compared between the control condition (taste alone) and the DMV stimulation condition, in which train stimulation of the DMV was delivered concurrently with the taste stimulus. DMV train stimulation enhanced taste-evoked responses in all 20 neurons without altering their best-stimulus category. Representative examples are shown in Fig. 6A and 6B. The mean net taste response was significantly higher during DMV stimulation (22.7 ± 2.36 spikes/s) than in the control condition (15.74 ± 1.59 spikes/s; t = 8.299, df = 28, p < 0.001). Taste responses recovered to control levels when retested 10 minutes after DMV stimulation (15.2 ± 1.6 spikes/s; Fig. 6C).

    All 7 neurons that were inhibited by contralateral DMV stimulation were also tested with train stimulation of the contralateral DMV during taste trials. DMV activation suppressed taste-evoked responses in all 7 neurons. The mean net taste response was significantly reduced during DMV stimulation (11.7 ± 2.60 spikes/s) compared with the control condition (16.2 ± 3.23 spikes/s; t = 5.288, df = 6, p < 0.005). Taste responses returned to control levels 10 minutes after DMV stimulation (15.16 ± 2.45 spikes/s; Fig. 5C).

    Taken together, these results demonstrate that DMV activation modulates taste responses of PbN neurons in a manner consistent with the type of input each neuron receives from the DMV: excitatory input enhances taste responses, whereas inhibitory input suppresses them.

    Discussion

    The present study examined whether the DMV exerts a functional influence on taste-responsive neurons in the PbN. The principal findings are: (1) the vast majority of gustatory PbN neurons (93.8%) were orthodromically activated by ipsilateral DMV stimulation, with exclusively excitatory responses; (2) contralateral DMV stimulation activated approximately half of the neurons tested, with predominantly excitatory but also some inhibitory responses; and (3) DMV activation modulated taste-evoked responses of PbN neurons in a manner consistent with the type of input received—excitatory input enhanced taste responses, whereas inhibitory input suppressed them. These results provide direct electrophysiological evidence that visceral signals from the DMV converge onto gustatory neurons in the PbN and modulate their responsiveness to taste stimulation.

    1. Convergence of gustatory and visceral input in the parabrachial nucleus

    The high proportion of gustatory PbN neurons activated by DMV stimulation (93.8%) is consistent with the known role of the PbN as a site of gustatory-visceral integration [3,4]. An earlier electrophysiological study demonstrated convergent gustatory and vagal afferent input onto neurons in the caudal MPB, and showed that co-activation of these neurons by the simultaneous arrival of both input sources frequently resulted in additive interactions [4]. The higher proportion observed in the present study may reflect several factors. First, that study used only 0.3 M NaCl as the gustatory stimulus and may therefore have missed neurons that respond preferentially to other taste qualities [4]. Second, electrical stimulation of the DMV, which directly activates neurons projecting to the PbN [23,24], may recruit a broader population of PbN neurons than cervical vagal stimulation, which must relay through the NST before reaching the PbN. Third, species differences between rats and hamsters cannot be excluded.

    The proportion of PbN taste neurons coactivated by DMV stimulation was also notably higher than that reported for gastric distension (~35%) [3] or intraduodenal lipid infusion [6]. This difference likely reflects the nature of the stimulation: electrical stimulation of the DMV simultaneously activates inputs derived from multiple visceral receptor populations and compartments across the gastrointestinal tract, whereas gastric distension and intraduodenal lipid infusion selectively engage mechanoreceptors or chemoreceptors within specific visceral compartments [3,6]. A concern with electrical stimulation is the possibility that current spread may have activated nearby structures, including fibers of passage or the adjacent NST. We cannot entirely exclude this possibility; however, if current spread to the NST were responsible, the latency of PbN responses should approximate the known antidromic latency from the PbN to the NST (~4.7 ms) [1]. Instead, the mean response latency following ipsilateral DMV stimulation was 11.2 ms—substantially longer, and consistent with a polysynaptic pathway via the DMV rather than direct NST activation. These findings should therefore be interpreted as evidence for functional DMV-to-PbN connectivity, while acknowledging that the specific synaptic pathway has not been anatomically verified in the present study.

    2. Response characteristics: ipsilateral dominance and contralateral inhibition

    Responses of PbN gustatory neurons to DMV stimulation were predominantly excitatory, consistent with Hermann and Rogers [4], who also observed that the majority of PbN units showed excitatory responses to convergent gustatory and vagal stimulation. The excitatory dominance is also consistent with studies showing that vagal stimulation excited neurons in other brainstem targets, including the hypothalamus [34,35]. In contrast, gastric distension and intraduodenal lipid infusion generally suppressed gustatory responses in the PbN [3,6]. These differences may reflect the fact that specific visceral stimuli selectively activate inhibitory pathways—for example, those signaling satiety—whereas electrical stimulation of the DMV activates a broader and more heterogeneous population of projection neurons.

    A notable finding was that inhibitory responses were observed exclusively following contralateral DMV stimulation. Six of the 7 inhibited neurons also received excitatory input from the ipsilateral DMV, indicating that the ipsilateral and contralateral pathways from the DMV to the PbN are functionally distinct. This asymmetry suggests that the pathway from the contralateral DMV to the PbN crosses the midline, possibly at the level of the NST or pons. Consistent with this interpretation, approximately 59% of NST neurons and 33% of PbN neurons communicate with their contralateral counterparts [1], suggesting that crossed brainstem pathways are a general organizational feature of the gustatory system in the hamster. GABAergic and glutamatergic inputs to the DMV are differentially organized [36], and it is possible that DMV projection neurons with different targets receive distinct synaptic inputs that shape their output. Future studies using cell-type-specific tracing or optogenetic approaches could directly test these circuit-level hypotheses.

    3. Dorsal motor nucleus of the vagus activation modulates taste responses of parabrachial nucleus neurons

    A key finding of the present study is that activation of the DMV altered taste-evoked responses of PbN neurons. In neurons that received excitatory input from the DMV, concurrent DMV train stimulation significantly enhanced taste responses across all four taste qualities tested, without altering best-stimulus category. Conversely, in neurons that received inhibitory input from the contralateral DMV, DMV stimulation significantly suppressed taste-evoked responses. These effects closely parallel the pattern of modulation reported for forebrain inputs to the PbN, including those from the LH, CeA, VPMpc, nucleus accumbens, and bed nucleus of the stria terminalis [26,29,30,32]. The fact that DMV-evoked modulation was not taste-quality specific is also consistent with these prior studies, which showed that centrifugal modulation of PbN neurons by forebrain stimulation generally affects responses to all taste stimuli rather than selectively targeting specific taste qualities [26,29,30,32]. The enhancement of taste responses by excitatory DMV input is consistent with the earlier observation by Hermann and Rogers [4] that combining gustatory and vagal afferent stimulation produced additive or supra-additive activation of PbN neurons. Such enhancement of gustatory sensitivity and inhibitory modulation by DMV stimulation may reflect a mechanism by which satiety signals attenuate taste responsiveness at the pontine level, consistent with the suppressive effects of gastric distension and postprandial signals on PbN gustatory responses [3,6]. Further studies are needed to address this issue.

    4. Physiological implications

    In addition to a gustatory nucleus of the taste pathway [1,2], the PbN has long been recognized as a critical brainstem site for the integration of gustatory and visceral information [3,4]. Recent studies have substantially expanded this view, identifying the PbN as a broad-spectrum interoceptive hub that processes visceral signals related to satiety, pain, and homeostatic challenge, and relays this information to forebrain circuits [37-40]. Within this framework, the present findings suggest that the DMV represents one route through which visceral state information reaches gustatory PbN neurons. Because the DMV itself receives visceral afferent input—both directly from vagal afferents and indirectly via the NST—and projects to the PbN [23,24], it is well positioned to relay the current status of gastrointestinal activity to pontine taste circuits. The modulation of PbN taste responses by DMV activation demonstrated here may thus provide a neural substrate through which visceral signals, such as those signaling gastric fullness or intestinal nutrient content, influence the processing of taste information in the central gustatory pathway. Such modulation could contribute to the dynamic regulation of food palatability and intake that occurs in response to changes in internal state.

    In conclusion, the present study demonstrates that electrical stimulation of the DMV orthodromically activates the majority of taste-responsive neurons in the hamster PbN and modulates their taste-evoked responses. These results extend prior evidence for gustatory-visceral convergence in the PbN and suggest the DMV as a source of visceral modulatory input to pontine gustatory circuits. Further studies using selective chemical stimulation or optogenetic approaches will be needed to determine the specific neurotransmitters and pathways mediating these effects and to clarify the functional significance of DMV-to-PbN signaling under physiologically relevant conditions.

    Funding

    This work was supported in part by NIDCD grant DC006623 to C-S Li and by Kangnung National University Research grant 2007-0033 to Young Kyung Cho.

    Conflicts of Interest

    Ki-Myung Chung and Kyung-Nyun Kim are editorial board members of the journal, but were not involved in the review process of this manuscript. Otherwise, there is no conflict of interest to declare.

    Figure

    IJOB-51-3-68_F1.jpg

    Photomicrographs of representative recording and stimulating sites in the hamster brain. (A) Coronal section through the pons showing a recording site (arrow) in the medial parabrachial nucleus (MPB), marked with Chicago Blue dye, located medial to the superior cerebellar peduncle (scp) and dorsolateral to the mesencephalic trigeminal nucleus (Me5) at the level where the locus coeruleus (LC) is most evident. (B) Coronal section through the medulla of the same animal showing the tip of the stimulating electrode (arrowhead) within the dorsal motor nucleus of the vagus (DMV, arrows), along with the electrode penetration track. The stimulating site is located dorsal to the hypoglossal nucleus (12), medial to the ventrolateral nucleus of the solitary tract (SolVL), and ventral to the medial nucleus of the solitary tract (SolM), at the level where the area postrema (AP) is most prominent. Scale bar = 500 μm in both (A) and (B).

    IJOB-51-3-68_F2.jpg

    Taste response profiles of all 80 parabrachial nucleus (PbN) neurons. Responses are expressed as net taste responses (spikes/s), calculated as the mean firing rate during the first 5 s of chemical stimulation minus the mean baseline rate during the 5-s prerinse. Each neuron was assigned to one of four best-stimulus categories based on the tastant that elicited the largest net response. Cells are arranged along the abscissa according to their best-stimulus category: cells 1–21 (NaCl-best), 22–33 (sucrose-best), 34–55 (citric acid-best), and 56–80 (quinine hydrochloride [QHCl]-best). Within each category, cells are ordered by decreasing magnitude of response to their best stimulus. Mean baseline activity (response to distilled water, dH2O) is shown at the bottom. Note that baseline values represent spontaneous firing during the prerinse period and are shown separately from the net taste responses. Black bars indicate the two neurons that did not respond to either ipsilateral or contralateral dorsal motor nucleus of the vagus stimulation.

    IJOB-51-3-68_F3.jpg

    Single-unit recording from a taste-responsive parabrachial nucleus neuron. The top four traces show 30-seconds raw recordings in response to NaCl, sucrose, quinine hydrochloride (QHCl), and citric acid, respectively. Artifacts (triangles) indicate the opening and closing of the solenoid controlling stimulus delivery. This neuron responded best to citric acid. The fifth trace shows the response to citric acid after filtering out background activity and solenoid artifacts. Bottom: peristimulus time histogram (PSTH) showing the spike frequency in response to citric acid, derived from the filtered trace. The x-axis of the PSTH (0–30 seconds) applies to all traces above. The presence of single-unit spikes is confirmed by the consistent waveform shape shown in the inset.

    IJOB-51-3-68_F4.jpg

    Representative peristimulus time histograms (PSTHs) illustrating the types of excitatory responses of parabrachial nucleus (PbN) taste neurons to dorsal motor nucleus of the vagus (DMV) stimulation. Four response patterns are shown: simple excitatory burst (A, B); excitation followed by suppression (C, D); double excitatory peak (E, F); and prolonged excitatory response (G, H). Electrical pulses were delivered to the DMV at time = 0. Each PSTH was accumulated over 50–200 stimulus sweeps at 1/3 Hz.

    IJOB-51-3-68_F5.jpg

    Peristimulus time histograms (PSTHs) illustrating inhibitory responses and suppression of taste responses in parabrachial nucleus (PbN) neurons to contralateral dorsal motor nucleus of the vagus (DMV) stimulation. Both neurons had low spontaneous firing rates (0.05 and 0.8 spikes/s). (A, B) PSTHs created with DMV stimulation alone (200 sweeps): no suppression is detectable due to the very low baseline firing. PSTHs created with simultaneous chemical stimulation of the tongue and DMV stimulation (200 sweeps): clear inhibitory responses are evident when taste stimulation elevates neuronal firing. The taste stimuli used were mixtures of effective tastants for each neuron. Electrical pulses were delivered to the DMV at time = 0. (C) Mean taste responses (± standard error) of all 7 inhibited neurons before (open bars), during (gray bars), and after (dark gray bars) contralateral DMV train stimulation. Taste-evoked responses were significantly suppressed during DMV stimulation and recovered to control levels after stimulation was terminated.

    *p < 0.005.

    IJOB-51-3-68_F6.jpg

    Enhancement of taste responses in parabrachial nucleus (PbN) neurons by dorsal motor nucleus of the vagus (DMV) train stimulation. (A, B) Taste response profiles of two representative PbN neurons before (control), during (ES), and after (recovery) train stimulation of the DMV. Each histogram reflects 20 seconds of activity. (C) Mean taste responses (± standard error) of all 20 neurons tested before (open bars), during (gray bars), and after (dark gray bars) DMV train stimulation. The overall taste response was significantly enhanced during DMV stimulation.

    ES, electrical stimulation; C, citric acid; N, NaCl; Q, quinine hydrochloride; S, sucrose.

    *p < 0.001.

    Table

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