RESUMO
Recent findings from our laboratory demonstrated that the rostral nucleus of the solitary tract (rNST) retains some responsiveness to sugars in double-knock-out mice lacking either the T1R1+T1R3 (KO1+3) or T1R2+T1R3 (KO2+3) taste receptor heterodimers. Here, we extended these findings in the parabrachial nucleus (PBN) of male and female KO1+3 mice using warm stimuli to optimize sugar responses and employing additional concentrations and pharmacological agents to probe mechanisms. PBN T1R-independent sugar responses, including those to concentrated glucose, were more evident than in rNST. Similar to the NST, there were no "sugar-best" neurons in KO1+3 mice. Nevertheless, 1000 mm glucose activated nearly 55% of PBN neurons, with responses usually occurring in neurons that also displayed acid and amiloride-insensitive NaCl responses. In wild-type (WT) mice, concentrated sugars activated the same electrolyte-sensitive neurons but also "sugar-best" cells. Regardless of genotype, phlorizin, an inhibitor of the sodium-glucose co-transporter (SGLT), a component of a hypothesized alternate glucose-sensing mechanism, did not diminish responses to 1000 mm glucose. The efficacy of concentrated sugars for driving neurons broadly responsive to electrolytes implied an origin from Type III taste bud cells. To test this, we used the carbonic anhydrase (CA) inhibitor dorzolamide (DRZ), previously shown to inhibit amiloride-insensitive sodium responses arising from Type III taste bud cells. Dorzolamide had no effect on sugar-elicited responses in WT sugar-best PBN neurons but strongly suppressed them in WT and KO1+3 electrolyte-generalist neurons. These findings suggest a novel T1R-independent mechanism for hyperosmotic sugars, involving a CA-dependent mechanism in Type III taste bud cells.SIGNIFICANCE STATEMENT Since the discovery of Tas1r receptors for sugars and artificial sweeteners, evidence has accrued that mice lacking these receptors maintain some behavioral, physiological, and neural responsiveness to sugars. But the substrate(s) has remained elusive. Here, we recorded from parabrachial nucleus (PBN) taste neurons and identified T1R-independent responses to hyperosmotic sugars dependent on carbonic anhydrase (CA) and occurring primarily in neurons broadly responsive to NaCl and acid, implying an origin from Type III taste bud cells. The effectiveness of different sugars in driving these T1R-independent responses did not correlate with their efficacy in driving licking, suggesting they evoke a nonsweet sensation. Nevertheless, these salient responses are likely to comprise an adequate cue for learned preferences that occur in the absence of T1R receptors.
Assuntos
Papilas Gustativas , Paladar , Animais , Feminino , Masculino , Camundongos , Amilorida/farmacologia , Glucose , Camundongos Knockout , Cloreto de Sódio/farmacologia , Açúcares/farmacologia , Paladar/fisiologia , Papilas Gustativas/fisiologiaRESUMO
Strong evidence supports a major role for heterodimers of the type 1 taste receptor (T1R) family in the taste transduction of sugars (T1R2+T1R3) and amino acids (T1R1+T1R3), but there are also neural and behavioral data supporting T1R-independent mechanisms. Most neural evidence for alternate mechanisms comes from whole nerve recordings in mice with deletion of a single T1R family member, limiting conclusions about the functional significance and T1R independence of the remaining responses. To clarify these issues, we recorded single-unit taste responses from the nucleus of the solitary tract in T1R double-knockout (double-KO) mice lacking functional T1R1+T1R3 [KO1+3] or T1R2+T1R3 [KO2+3] receptors and their wild-type background strains [WT; C57BL/6J (B6), 129X1/SvJ (S129)]. In both double-KO strains, responses to sugars and a moderate concentration of an monosodium glutamate + amiloride + inosine 5'-monophosphate cocktail (0.1 M, i.e., umami) were profoundly depressed, whereas a panel of 0.6 M amino acids were mostly unaffected. Strikingly, in contrast to WT mice, no double-KO neurons responded selectively to sugars and umami, precluding segregation of this group of stimuli from those representing other taste qualities in a multidimensional scaling analysis. Nevertheless, residual sugar responses, mainly elicited by monosaccharides, persisted as small "sideband" responses in double-KOs. Thus other receptors may convey limited information about sugars to the central nervous system, but T1Rs appear critical for coding the distinct perceptual features of sugar and umami stimuli. The persistence of amino acid responses supports previous proposals of alternate receptors, but because these stimuli affected multiple neuron types, further investigations are necessary.NEW & NOTEWORTHY The type 1 taste receptor (T1R) family is crucial for transducing sugars and amino acids, but there is evidence for T1R-independent mechanisms. In this study, single-unit recordings from the nucleus of the solitary tract in T1R double-knockout mice lacking T1R1+T1R3 or T1R2+T1R3 receptors revealed greatly reduced umami synergism and sugar responses. Nevertheless, residual sugar responses persisted, mainly elicited by monosaccharides and evident as "sidebands" in neurons activated more vigorously by other qualities.
Assuntos
Receptores Acoplados a Proteínas G/fisiologia , Transdução de Sinais/fisiologia , Núcleo Solitário/fisiologia , Papilas Gustativas/fisiologia , Percepção Gustatória/fisiologia , Aminoácidos/farmacologia , Animais , Açúcares da Dieta/farmacologia , Feminino , Aromatizantes/farmacologia , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Knockout , Receptores Acoplados a Proteínas G/genéticaRESUMO
The taste of l-glutamate and its synergism with 5'-ribonucleotides is thought to be primarily mediated through the T1R1+T1R3 heterodimer in some mammals, including rodents and humans. While knockout (KO) mice lacking either receptor subunit show impaired sensitivity to a range of monosodium glutamate (MSG) concentrations mixed with 2.5 mM inosine 5'-monophosphate (IMP) in amiloride, wild-type (WT) controls can detect this IMP concentration, hindering direct comparison between genotypes. Moreover, some residual sensitivity persists in the KO group, suggesting that the remaining subunit could maintain a limited degree of function. Here, C57BL/6J, 129X1/SvJ, and T1R1+T1R3 double KO mice ( n = 16 each to start the experiment) were trained in a two-response operant task in gustometers and then tested for their ability to discriminate 100 µM amiloride from MSG (starting with 0.6 M) and IMP (starting with 2.5 mM) in amiloride (MSG+I+A). Testing continued with successive dilutions of both MSG and IMP (in amiloride). The two WT strains were similarly sensitive to MSG+I+A ( P > 0.8). KO mice, however, were significantly impaired relative to either WT strain ( P < 0.01), although they were able to detect the highest concentrations. Thus, normal detectability of MSG+I+A requires an intact T1R1+T1R3 receptor, without regard for allelic variation in the T1R3 gene between the WT strains. Nevertheless, residual sensitivity by the T1R1+T1R3 KO mice demonstrates that a T1R-independent mechanism can contribute to the detectability of high concentrations of this prototypical umami compound stimulus.
Assuntos
Inosina Monofosfato/farmacologia , Receptores Acoplados a Proteínas G/genética , Glutamato de Sódio/farmacologia , Paladar/efeitos dos fármacos , Amilorida/farmacologia , Sistemas de Transporte de Aminoácidos , Animais , Proteínas de Arabidopsis , Condicionamento Operante/efeitos dos fármacos , Discriminação Psicológica/efeitos dos fármacos , Diuréticos/farmacologia , Feminino , Genótipo , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Knockout , Cloreto de Sódio/farmacologiaRESUMO
Peripheral gustatory neurons express P2X2 purinergic receptors and terminate in the rostral portion of the nucleus tractus solitarius (rNTS), but a relationship between the P2X2 terminal field and taste evoked activity has not been established. Additionally, a portion of somatosensory neurons from the trigeminal nerve, which are devoid of P2X2 expression, also terminate in the lateral rNTS. We hypothesized that P2X2 receptor expression on afferent nerve endings could be used as an anatomical tool for segregating gustatory from mechanosensory responsive regions in the mouse rNTS. C57BL/6 mice were used to record extracellular activity from neurons within the rNTS and the laterally adjacent reticular formation and trigeminal nucleus. Histological reconstruction of electrolytic lesions indicated that gustatory activity coincided with electrode tracks that traversed through P2X2 terminal fields. Gustatory recordings made more rostral in the rNTS had receptive fields located in the anterior oral cavity (AO), whereas gustatory recordings made more caudal in the rNTS had receptive fields located in the posterior oral cavity (PO). Mechanosensory neurons with AO receptive fields were recorded near the lateral border of the P2X2 terminal field and became numerous on electrode tracks made lateral to the P2X2 terminal field. In contrast, mechanosensory responses with PO receptive fields were recorded within the P2X2 terminal field along with gustatory activity and transitioned to mechanosensory only outside the P2X2 terminal field. Collectively, our results indicate that the lateral border of the P2X2 terminal field, demarcates a faithful "transition zone," where AO responses transition from gustatory to mechanosensory.
Assuntos
Mecanorreceptores/metabolismo , Receptores Purinérgicos P2X2/metabolismo , Núcleo Solitário/metabolismo , Animais , Camundongos , Camundongos Endogâmicos C57BLRESUMO
The intriguing observation that leptin decreases sweet-evoked peripheral gustatory responses has aroused much interest (Kawai K, Sugimoto K, Nakashima K, Miura H, Ninomiya Y. 2000. Leptin as a modulator of sweet taste sensitivities in mice. Proc Natl Acad Sci U S A. 97(20):11044-11049.) due to its implied importance in controlling appetite. The effects of this anorexic hormone, however, appear more conditional than originally believed. In this issue of Chemical Senses, a careful study by Glendinning and colleagues, find no effects of leptin on sweet-evoked chorda tympani responses, whereas an equally careful study by Meredith and colleagues, find decreased release of ATP and increased release of 5-HT from taste buds in response to sweet stimuli.
Assuntos
Cálcio/metabolismo , Leptina/sangue , Leptina/farmacologia , Neurotransmissores/metabolismo , Edulcorantes/farmacologia , Papilas Gustativas/efeitos dos fármacos , Papilas Gustativas/metabolismo , Paladar/efeitos dos fármacos , Paladar/fisiologia , Língua/metabolismo , Animais , Feminino , MasculinoRESUMO
Taste processing in the rostral nucleus of the solitary tract (rNST) is subject to modulatory influences including opioid peptides. Behavioral pharmacological studies suggest an influence of µ-opioid receptors in rNST, but the underlying mechanism is unknown. To determine the cellular site of action, we tested the effects of the µ-opioid receptor agonist DAMGO in vitro. Whole cell patch-clamp recordings were made in brain stem slices from GAD67-GFP knockin mice expressing enhanced green fluorescent protein (EGFP) under the control of the endogenous promoter for GAD67, a synthetic enzyme for GABA. Neuron counts showed that â¼36% of rNST neurons express GABA. We recorded monosynaptic solitary tract (ST)-evoked currents (jitter ≤ 300 µs) in both GAD67-EGFP-positive (GAD67+) and GAD67-EGFP-negative (GAD67-) neurons with equal frequency (25/31; 22/28), but the inputs to the GAD67+ neurons had significantly smaller paired-pulse ratios compared with GAD67- neurons. DAMGO (0.3 µM) significantly suppressed ST-evoked currents in both cell types (mean suppression = 46 ± 3.3% SE), significantly increased the paired-pulse ratio of these currents, and reduced the frequency of spontaneous miniature excitatory postsynaptic currents but did not diminish their amplitude, indicating a presynaptic site of action. Under inhibitory amino acid receptor blockade, DAMGO was significantly more suppressive in GAD67+ neurons (59% reduction) compared with GAD67- neurons (35% reduction), while the reverse was true in normal artificial cerebrospinal fluid (GAD67+: 35% reduction; GAD67-: 57% reduction). These findings suggest that DAMGO suppresses activity in rNST neurons predominantly via a presynaptic mechanism, and that this effect may interact significantly with tonic or evoked inhibitory activity.
Assuntos
Ala(2)-MePhe(4)-Gly(5)-Encefalina/farmacologia , Potenciais Evocados , Neurônios GABAérgicos/efeitos dos fármacos , Inibição Neural , Neurotransmissores/farmacologia , Receptores Opioides mu/agonistas , Núcleo Solitário/fisiologia , Animais , Neurônios GABAérgicos/fisiologia , Camundongos , Potenciais Pós-Sinápticos em Miniatura , Núcleo Solitário/citologia , Núcleo Solitário/efeitos dos fármacos , Sinapses/efeitos dos fármacos , Sinapses/fisiologiaRESUMO
Consummatory responses to taste stimuli are modulated by visceral signals processed in the caudal nucleus of the solitary tract (cNST) and ventrolateral medulla. On the basis of decerebrate preparations, this modulation can occur through local brain stem pathways. Among the large number of neuropeptides and neuromodulators implicated in these visceral pathways is neuropeptide Y (NPY), which is oftentimes colocalized in catecholaminergic neurons themselves implicated in glucoprivic-induced feeding and satiety. In addition to the cNST and ventrolateral medulla, noradrenergic and NPY receptors are found in circumscribed regions of the medullary reticular formation rich in preoromotor neurons. To test the hypothesis that NPY may act as a neuromodulator on preoromotor neurons, we recorded the effects of bath application of NPY and specific Y1 and Y2 agonists on currents elicited from electrical stimulation of the rostral (taste) NST in prehypoglossal neurons in a brain stem slice preparation. A high proportion of NST-driven responses were suppressed by NPY, as well as Y1 and Y2 agonists. On the basis of paired pulse ratios and changes in membrane resistance, we concluded that Y1 receptors influence these neurons both presynaptically and postsynaptically and that Y2 receptors have a presynaptic locus. To test the hypothesis that NPY may act in concert with norepinephrine (NE), we examined neurons showing suppressed responses in the presence of a Y2 agonist and demonstrated a greater degree of suppression to a Y2 agonist/NE cocktail. These suppressive effects on preoromotoneurons may reflect a satiety pathway originating from A2 neurons in the caudal brain stem.
Assuntos
Tronco Encefálico/efeitos dos fármacos , Neurônios/efeitos dos fármacos , Receptores de Neuropeptídeo Y/metabolismo , Transmissão Sináptica/efeitos dos fármacos , Paladar/efeitos dos fármacos , Animais , Neuropeptídeo Y/farmacologia , Ratos , Ratos Sprague-Dawley , Receptores de Neuropeptídeo Y/agonistas , Receptores de Neuropeptídeo Y/antagonistas & inibidoresRESUMO
The rostral nucleus of the solitary tract (rNST), the initial CNS site for processing gustatory information, is comprised of two major cell types, glutamatergic excitatory and GABAergic inhibitory neurons. Although many investigators have described taste responses of rNST neurons, the phenotypes of these cells were unknown. To directly compare the response characteristics of both inhibitory and noninhibitory neurons, we recorded from mice expressing Channelrhodopsin-2 (ChR2) under the control of GAD65, a synthetic enzyme for GABA. We observed that chemosensitive profiles of GABAergic taste neurons (G+TASTE) were similar to non-GABA taste neurons (G-TASTE) but had much lower response rates. We further observed a novel subpopulation of GABA cells located more ventrally in the nucleus that were unresponsive to taste stimulation (G+UNR), suggesting pathways for inhibition initiated by centrifugal sources. This preparation also allowed us to determine how optogenetic activation of the rNST GABA network impacted the taste responses of G-TASTE neurons. Activating rNST inhibitory circuitry suppressed gustatory responses of G-TASTE neurons across all qualities and chemosensitive types of neurons. Although the tuning curves of identified G-TASTE were modestly sharpened, the overall shape of response profiles and the ensemble pattern remained highly stable. These neurophysiological effects were consistent with the behavioral consequences of activating GAD65-expressing inhibitory neurons using DREADDs. In a brief-access licking task, concentration-response curves to both palatable (sucrose, maltrin) and unpalatable (quinine) stimuli were shifted to the right when GABA neurons were activated. Thus, the rNST GABAergic network is poised to modulate taste intensity across the qualitative and hedonic spectrum.
Assuntos
Núcleo Solitário , Paladar , Animais , Channelrhodopsins/genética , Neurônios GABAérgicos/fisiologia , Camundongos , Quinina/farmacologia , Sacarose/farmacologia , Paladar/fisiologiaRESUMO
The neural control of feeding involves many neuromodulators, including the endogenous opioids that bind µ-opioid receptors (MORs). Injections of the MOR agonist, Damgo, into limbic and hypothalamic forebrain sites increase intake, particularly of palatable foods. Indeed, forebrain Damgo injections increase sucrose-elicited licking but reduce aversive responding (gaping) to quinine, suggesting that MOR activation may enhance taste palatability. A µ-opioid influence on taste reactivity has not been assessed in the brain stem. However, MORs are present in the first-order taste relay, the rostral nucleus of the solitary tract (rNST), and in the immediately subjacent reticular formation (RF), a region known to be essential for consummatory responses. Thus, to evaluate the consequences of rNST/dorsal RF Damgo in this region, we implanted rats with intraoral cannulas, electromyographic electrodes, and brain cannulas aimed at the ventral border of the rNST. Licking and gaping elicited with sucrose, water, and quinine were assessed before and after intramedullary Damgo and saline infusions. Damgo slowed the rate, increased the amplitude, and decreased the size of fluid-induced lick and gape bouts. In addition, the neutral stimulus water, which typically elicits licks, began to evoke gapes. Thus, the current results demonstrate that µ-opioid activation in the rNST/dorsal RF exerts complex effects on oromotor responding that contrast with forebrain effects and are more indicative of a suppressive, rather than a facilitatory effect on ingestion.
Assuntos
Analgésicos Opioides/farmacologia , Comportamento Consumatório/efeitos dos fármacos , Ala(2)-MePhe(4)-Gly(5)-Encefalina/farmacologia , Receptores Opioides mu/agonistas , Formação Reticular/efeitos dos fármacos , Núcleo Solitário/efeitos dos fármacos , Paladar/efeitos dos fármacos , Analgésicos Opioides/administração & dosagem , Análise de Variância , Animais , Ingestão de Alimentos/efeitos dos fármacos , Eletromiografia , Ala(2)-MePhe(4)-Gly(5)-Encefalina/administração & dosagem , Infusões Parenterais , Injeções , Masculino , Atividade Motora/efeitos dos fármacos , Quinina/administração & dosagem , Ratos , Ratos Sprague-Dawley , Receptores Opioides mu/metabolismo , Formação Reticular/metabolismo , Núcleo Solitário/metabolismo , Sacarose/administração & dosagem , Fatores de TempoRESUMO
OBJECTIVES: Breathlessness is the most significant symptom in those dying of COVID-19. Historically, though, it has often been palliated poorly at end of life. The aim of this work was to assess whether breathlessness in patients dying from COVID-19 was being managed appropriately. METHODS: A multicentre, retrospective analysis of clinical data was undertaken. Patients who had died of COVID-19 across three acute hospitals over a 2-month period were included. Those already prescribed background opioids and those who died in intensive care were excluded. Data were collected from clinical notes, where available. RESULTS: 71 patients from 18 wards (3 hospitals) were included. The median total dose of opioid and midazolam given in the last 24 hours of life (continuous subcutaneous infusion ± 'as required' medication) was 33 mg (14-55) and 15 mg (6-26), respectively. 37 patients (52%) were prescribed continuous subcutaneous infusions. There were 426 recorded respiratory rates of at least 25 breaths per minute, for which an opioid or benzodiazepine was given in 113 (27%) of instances. CONCLUSIONS: Less than a third of episodes of breathlessness, as measured by respiratory rate, were palliated with anticipatory medicines. Specific palliative care guidelines for COVID-19 are necessary but may not always be followed.
RESUMO
The nucleus of the solitary tract is a potential site for taste-visceral interactions. Connections from the caudal, visceral area of the nucleus (cNST) to the rostral, gustatory zone (rNST) have been described, but the phenotype of cells giving rise to the projection(s) and their distribution among rNST subdivisions are unknown. To determine these characteristics of the intrasolitary pathway, we injected pan-neuronal and floxed AAV viruses into the cNST of mice expressing cre in glutamatergic, GABAergic, or catecholaminergic neurons. Particular attention was paid to the terminal field distribution in rNST subdivisions by simultaneously visualizing P2X2 localized to gustatory afferent terminals. All three phenotypically identified pathways terminated in rNST, with the density greatest for glutamatergic and sparsest for catecholaminergic projections, observations supported by retrograde tracing. Interestingly, cNST neurons had more prominent projections to rNST regions medial and ventral to P2X2 staining, i.e., the medial and ventral subdivisions. In addition, GABAergic neurons projected robustly to the lateral subdivision and adjacent parts of the reticular formation and spinal trigeminal nucleus. Although cNST neurons also projected to the P2X2-rich central subdivision, such projections were sparser. These findings suggest that cNST visceral signals exert stronger excitatory and inhibitory influences on local autonomic and reflex pathways associated with the medial and ventral subdivisions compared to weaker modulation of ascending pathways arising from the central subdivision and ultimately destined for the forebrain.
Assuntos
Neurônios/fisiologia , Núcleo Solitário/citologia , Animais , Catecolaminas/fisiologia , Feminino , Ácido Glutâmico/fisiologia , Imuno-Histoquímica , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Vias Neurais/fisiologia , Neurônios Aferentes/fisiologia , Prosencéfalo/citologia , Prosencéfalo/fisiologia , Receptores Purinérgicos P2X2/metabolismo , Paladar/fisiologia , Ácido gama-Aminobutírico/fisiologiaRESUMO
The current investigation used double labeling for NADPHd and Fos-like immunoreactivity to define the relationship between nitric oxide synthase-containing neural elements and taste-activated neurons in the nucleus of the solitary tract (NST) and subjacent reticular formation (RF). Stimulation of awake rats with citric acid and quinine resulted in significant increases in the numbers of double-labeled neurons in both the NST and RF, suggesting that some medullary gustatory neurons utilize nitric oxide (NO) as a transmitter. Overall, double-labeled neurons were most numerous in the caudal reaches of the gustatory zone of the NST, where taste neurons receive inputs from the IXth nerve, suggesting a preferential role for NO neurons in processing gustatory inputs from the posterior oral cavity. However, double-labeled neurons also exhibited a preferential distribution depending on the gustatory stimulus. In the NST, double-labeled neurons were most numerous in the rostral central subnucleus after either stimulus but had a medial bias after quinine stimulation. In the RF, after citric acid stimulation, there was a cluster of double-labeled neurons with distinctive large soma in the parvicellular division of the lateral RF, subjacent to the rostral tip of NST. In contrast, in response to quinine, there was a cluster of double-labeled neurons with much smaller soma in the intermediate zone of the medial RF, a few hundred micrometers caudal to the citric acid cluster. These differential distributions of double-labeled neurons in the NST and RF suggest a role for NO in stimulus-specific gustatory autonomic and oromotor reflex circuits.
Assuntos
Neurônios/enzimologia , Óxido Nítrico Sintase/metabolismo , Formação Reticular/enzimologia , Transdução de Sinais/fisiologia , Núcleo Solitário/enzimologia , Paladar/fisiologia , Animais , Imuno-Histoquímica , Masculino , NADH Desidrogenase/metabolismo , Vias Neurais/enzimologia , Óxido Nítrico/metabolismo , Proteínas Proto-Oncogênicas c-fos/metabolismo , Ratos , Ratos Sprague-Dawley , Formação Reticular/citologia , Núcleo Solitário/citologiaRESUMO
This article summarizes research findings from 6 experts in the field of taste and feeding that were presented at the 2015 American Society for Parenteral and Enteral Nutrition Research Workshop. The theme was focused on the interaction of taste signals with those of a postingestive origin and how this contributes to regulation of food intake through both physiological and learning processes. Gastric bypass results in exceptional loss of fat mass and increases in circulating levels of key gut peptides, some of which are also expressed along with their cognate receptors in taste buds. Changes in taste preference and food selection in both bariatric surgery patients and rodent models have been reported. Accordingly, the effects of this surgery on taste-related behavior were examined. The conservation of receptor and peptide signaling mechanisms in gustatory and extraoral tissues was discussed in the context of taste responsiveness and the regulation of metabolism. New findings detailing the features of neural circuits between the caudal nucleus of the solitary tract (NST), receiving visceral input from the vagus nerve, and the rostral NST, receiving taste input, were discussed, as was how early life experience with taste stimuli and learned associations between flavor and postoral consequences of nutrients can exert potent and long-lasting effects on feeding.
Assuntos
Papilas Gustativas/fisiologia , Animais , Cirurgia Bariátrica , Glicemia/metabolismo , Congressos como Assunto , Modelos Animais de Doenças , Preferências Alimentares , Hormônios Gastrointestinais/sangue , Humanos , Ratos , Receptores Acoplados a Proteínas G/metabolismo , PaladarRESUMO
Numerous studies suggest an essential role for the intermediate (IRt) and parvocellular (PCRt) reticular formation (RF) in consummatory ingestive responses. Although the IRt and PCRt contain a large proportion of neurons with projections to the oromotor nuclei, these areas of the RF are heterogeneous with respect to neurotransmitter phenotypes. Glutamatergic, GABAergic, cholinergic, and nitrergic neurons are all found in the PCRt and IRt, but the projections of neurons with these phenotypes to the motor trigeminal (mV) and hypoglossal nucleus (mXII) has not been fully evaluated. In the present study, after small injections of Fluorogold (FG) into mV and mXII, sections were processed immunohistochemically to detect retrogradely labeled FG neurons in combination with the synthetic enzymes for nitric oxide (nitric oxide synthase) or acetylcholine (choline acetyltransferase) or in situ hybridization for the synthetic enzyme for GABA (GAD65/67) or the brainstem vesicular transporter for glutamate (VGLUT2). In three additional cases, FG injections were made into one motor nucleus and cholera toxin (subunit b) injected in the other to determine the presence of dual projection neurons. Premotor neurons to mXII (pre-mXII) were highly concentrated in the IRt. In contrast, there were nearly equal proportions of premotor-trigeminal neurons (pre-mV) in the IRt and PCRt. A high proportion of pre-oromotor neurons were positive for VGLUT2 (pre-mXII: 68%; pre-mV: 53%) but GABAergic projections were differentially distributed with a greater projection to mV (25%) compared to mXII (8%). Significant populations of cholinergic and nitrergic neurons overlapped pre-oromotor neurons, but there was sparse double-labeling (<10%). The IRt also contained a high proportion of neurons that projected to both mV and MXII. These different classes of premotor neurons in the IRt and PCRt provide a substrate for the rhythmic activation of lingual and masticatory muscles.
Assuntos
Mapeamento Encefálico , Vias Neurais/metabolismo , Neurônios/metabolismo , Neurotransmissores/metabolismo , Formação Reticular/metabolismo , Núcleos do Trigêmeo/metabolismo , Acetilcolina/metabolismo , Animais , Comportamento Consumatório/fisiologia , Comportamento Alimentar/fisiologia , Ácido Glutâmico/metabolismo , Nervo Hipoglosso/citologia , Nervo Hipoglosso/metabolismo , Imuno-Histoquímica , Hibridização In Situ , Masculino , Vias Neurais/citologia , Neurônios/citologia , Neurotransmissores/classificação , Ácido Nítrico/metabolismo , Ratos , Ratos Sprague-Dawley , Formação Reticular/citologia , Distribuição Tecidual , Núcleos do Trigêmeo/citologia , Ácido gama-Aminobutírico/metabolismoRESUMO
The process by which the mammalian nervous system represents the features of a sapid stimulus that lead to a perception of taste quality has long been controversial. The labeled-line (sparse coding) view differs from the across-neuron pattern (ensemble) counterpoint in proposing that activity in a given class of neurons is necessary and sufficient to generate a specific taste perception. This article critically reviews molecular, electro-physiological, and behavioral findings that bear on the issue. In the peripheral gustatory system, the authors conclude that most qualities appear to be signaled by labeled lines; however, elements of both types of coding characterize signaling of sodium salts. Given the heterogeneity of neuronal tuning functions in the brain, the central coding mechanism is less clear. Both sparse coding and neuronal ensemble models remain viable possibilities. Furthermore, temporal patterns of discharge could contribute additional information. Ultimately, until specific classes of neurons can be selectively manipulated and perceptual consequences assessed, it will be difficult to go beyond mere correlation and conclusively discern the validity of these coding models.
Assuntos
Vias Aferentes/fisiologia , Neurônios/fisiologia , Percepção/fisiologia , Transdução de Sinais/fisiologia , Paladar/fisiologia , Vias Aferentes/citologia , Animais , Comportamento Animal/fisiologia , Sistema Nervoso Central/citologia , Sistema Nervoso Central/fisiologia , Humanos , Fibras Nervosas/fisiologia , Neurônios/classificação , Sistema Nervoso Periférico/citologia , Sistema Nervoso Periférico/fisiologia , Ratos , Papilas Gustativas/citologia , Papilas Gustativas/fisiologiaRESUMO
Projections to the parabrachial nucleus (PBN) from the nucleus of the solitary tract (NST) carry afferent signals from both the oral cavity and gastrointestinal tract. Although physiological studies suggest the convergence of oral and gastrointestinal sensory signals in the parabrachial nucleus, anatomical studies have emphasized the segregation of these pathways. To more precisely determine the anatomical relationship between gastric distension and oral afferent representation in PBN, small deposits of two anterograde tracers were made into the NST under physiological guidance in the same rat. Gastric terminations were dense and separate from taste projections in the rostral portion of the external lateral and dorsal lateral subnuclei. Gustatory projections were densest and separate from gastric terminations in the ventral lateral and central medial subnuclei of the caudal waist region, but were intermingled with gastric projections in these subnuclei and the external subnuclei at slightly more rostral levels. Patterns of segregation and overlap often appeared as 'patches' within or across subnuclear boundaries. In a second set of experiments, physiological evidence for overlap in PBN was evaluated from single unit extracellular responses evoked by gastric distension and orosensory (taste and orotactile) stimulation. Neurophysiological recordings verified that a small proportion of single cells within the waist and external subnuclei could be activated by both gastric and orotactile stimulation. The anatomical experiments further revealed intranuclear projections from the caudal NST injections that extended rostrally to sites at which responses to oral stimulation had been recorded. Although existing physiological data suggest such interactions are more limited than those in PBN, these anatomical data suggest that gastric/oral interactions may also exist in the NST.
Assuntos
Boca/inervação , Vias Neurais/citologia , Ponte/citologia , Núcleo Solitário/citologia , Estômago/inervação , Fibras Aferentes Viscerais/citologia , Potenciais de Ação/fisiologia , Animais , Masculino , Boca/fisiologia , Vias Neurais/fisiologia , Estimulação Física , Ponte/fisiologia , Terminações Pré-Sinápticas/fisiologia , Terminações Pré-Sinápticas/ultraestrutura , Ratos , Ratos Sprague-Dawley , Núcleo Solitário/fisiologia , Estômago/fisiologia , Fibras Aferentes Viscerais/fisiologiaRESUMO
PURPOSE: The purpose of this investigation was to determine the effects of low-level laser (LLL) irradiation on neural regeneration in surgically created defects in the rabbit inferior alveolar nerve. STUDY DESIGN: Five adult female New Zealand White rabbits underwent bilateral exposure of the inferior alveolar nerve. A 6-mm segment of nerve was resected, and the nerve gap was repaired via entubulation by using a Gore-Tex conduit. The experimental side received 10 postoperative LLL treatments with a 70-mW gallium-aluminum-arsenide diode at 4 sites per treatment. At 15 weeks after surgery, the nerve segments were harvested bilaterally and prepared for light microscopy. Basic fuchsin and toluidine blue were used to highlight myelinated axons. The segments were examined histomorphometrically by using computer analysis to determine mean axonal diameter, total fascicular surface area, and axonal density along the repair sites. RESULTS: Gross examination of all nerves showed intact neural bundles with variable degrees of osseous remodeling. Light microscopic evaluation revealed organized regenerated neural tissue in both groups with more intrafascicular perineural tissue in the control group. Histomorphometric evaluation revealed increased axonal density in the laser treated group as compared with the control. CONCLUSIONS: LLL irradiation may be a useful noninvasive adjunct to promote neuronal wound healing in surgically created defects repaired with expanded polytetrafluoroethylene entubulation.
Assuntos
Terapia com Luz de Baixa Intensidade , Nervo Mandibular/efeitos da radiação , Regeneração Nervosa/efeitos da radiação , Traumatismos do Nervo Trigêmeo , Animais , Traumatismos dos Nervos Cranianos/radioterapia , Feminino , Terapia com Luz de Baixa Intensidade/instrumentação , Terapia com Luz de Baixa Intensidade/métodos , Politetrafluoretileno , Coelhos , Estatísticas não Paramétricas , StentsRESUMO
It has been demonstrated that temporal features of spike trains can increase the amount of information available for gustatory processing. However, the nature of these temporal characteristics and their relationship to different taste qualities and neuron types are not well-defined. The present study analyzed the time course of taste responses from parabrachial (PBN) neurons elicited by multiple applications of "sweet" (sucrose), "salty" (NaCl), "sour" (citric acid), and "bitter" (quinine and cycloheximide) stimuli in an acute preparation. Time course varied significantly by taste stimulus and best-stimulus classification. Across neurons, the ensemble code for the three electrolytes was similar initially but quinine diverged from NaCl and acid during the second 500 ms of stimulation and all four qualities became distinct just after 1s. This temporal evolution was reflected in significantly broader tuning during the initial response. Metric space analyses of quality discrimination by individual neurons showed that increases in information (H) afforded by temporal factors was usually explained by differences in rate envelope, which had a greater impact during the initial 2s (22.5% increase in H) compared to the later response (9.5%). Moreover, timing had a differential impact according to cell type, with between-quality discrimination in neurons activated maximally by NaCl or citric acid most affected. Timing was also found to dramatically improve within-quality discrimination (80% increase in H) in neurons that responded optimally to bitter stimuli (B-best). Spikes from B-best neurons were also more likely to occur in bursts. These findings suggest that among PBN taste neurons, time-dependent increases in mutual information can arise from stimulus- and neuron-specific differences in response envelope during the initial dynamic period. A stable rate code predominates in later epochs.
Assuntos
Neurônios/fisiologia , Percepção Gustatória/fisiologia , Potenciais de Ação , Animais , Masculino , Ratos , Tempo de ReaçãoRESUMO
There is growing evidence of heterogeneity among responses to bitter stimuli at the peripheral, central and behavioral levels. For instance, the glossopharyngeal (GL) nerve and neurons receiving its projections are more responsive to bitter stimuli than the chorda tympani (CT) nerve, and this is particularly true for some bitter stimuli like PROP & cycloheximide that stimulate the GL to a far greater extent. Given this information, we hypothesized that cutting the GL would have a greater effect on behavioral avoidance of cycloheximide and PROP than quinine and denatonium, which also stimulate the CT, albeit to a lesser degree than salts and acids. Forty male SD rats were divided into four surgery groups: bilateral GL transection (GLX), chorda tympani transection (CTX), SHAM surgery, and combined transection (CTX + GLX). Postsurgical avoidance functions were generated for the four bitter stimuli using a brief-access test. GLX significantly compromised avoidance compared to both CTX and SHAM groups for all stimuli (p < .02), while CTX and SHAM groups did not differ. Contrary to our hypothesis, GLX had a greater effect on quinine than cycloheximide (mean shift of 1.02 vs. 0.27 log10 units). Moreover, combined CTX + GLX transection shifted the concentration-response function further than GLX alone for every stimulus except cycloheximide (ps < .03), suggesting that the GSP nerve is capable of maintaining avoidance of this stimulus to a large degree. This hypothesis is supported by reports of cycloheximide-responsive cells with GSP-innervated receptive fields in the NST and PBN.