Mapping study of the parabrachial taste-responsive area for the anterior tongue in the golden hamster

Regulation of vagally-evoked respiratory responses by the lateral parabrachial nucleus in the mouse

Vagal sensory inputs to the brainstem can alter breathing through the modulation of pontomedullary respiratory circuits. In this study, we set out to investigate the localised effects of modulating lateral parabrachial nucleus (LPB) activity on vagally-evoked changes in breathing pattern. In isoflurane-anaesthetised and instrumented mice, electrical stimulation of the vagus nerve (eVNS) produced stimulation frequency-dependent changes in diaphragm electromyograph (dEMG) activity with an evoked tachypnoea and apnoea at low and high stimulation frequencies, respectively. Muscimol microinjections into the LPB significantly attenuated eVNS-evoked respiratory rate responses. Notably, muscimol injections reaching the caudal LPB, previously unrecognised for respiratory modulation, potently modulated eVNS-evoked apnoea, whilst muscimol injections reaching the intermediate LPB selectively modulated the eVNS-evoked tachypnoea. The effects of muscimol on eVNS-evoked breathing rate changes occurred without altering basal eupneic breathing. These results highlight novel roles for the LPB in regulating vagally-evoked respiratory reflexes.

Retinorecipient areas in the common marmoset (Callithrix jacchus): An image-forming and non-image forming circuitry

The mammalian retina captures a multitude of diverse features from the external environment and conveys them via the optic nerve to a myriad of retinorecipient nuclei. Understanding how retinal signals act in distinct brain functions is one of the most central and established goals of neuroscience. Using the common marmoset (Callithrix jacchus), a monkey from Northeastern Brazil, as an animal model for parsing how retinal innervation works in the brain, started decades ago due to their marmoset's small bodies, rapid reproduction rate, and brain features. In the course of that research, a large amount of new and sophisticated neuroanatomical techniques was developed and employed to explain retinal connectivity. As a consequence, image and non-image-forming regions, functions, and pathways, as well as retinal cell types were described. Image-forming circuits give rise directly to vision, while the non-image-forming territories support circadian physiological processes, although part of their functional significance is uncertain. Here, we reviewed the current state of knowledge concerning retinal circuitry in marmosets from neuroanatomical investigations. We have also highlighted the aspects of marmoset retinal circuitry that remain obscure, in addition, to identify what further research is needed to better understand the connections and functions of retinorecipient structures.

Two neuronal groups for NaCl with differential taste response properties and topographical distributions in the rat parabrachial nucleus

It is crucial for animals to discriminate between palatable (safe) and aversive (toxic) tastants. The mechanisms underlying neuronal discrimination of taste stimuli remain unclear. We examined relations between taste response properties (spike counts, response duration, and coefficient of variation [CV]) and location of taste-sensitive neurons in the pontine parabrachial nucleus (PBN). Extracellular single units' activity in the PBN of Wistar rats was recorded using multibarrel glass micropipettes under urethane anesthesia. Forty taste-sensitive neurons were classified as NaCl (N)-best (n = 15), NaCl/HCl (NH)-best (n = 14), HCl (H)-best (n = 8), and sucrose (S)-best (n = 3) neurons. The net response to NaCl (15.2 ± 2.3 spikes/s) among the N-best neurons was significantly larger than that among the NH-best (4.5 ± 0.8 spikes/s) neurons. The response duration (4.5 ± 0.2 s) of the N-best neurons to NaCl was significantly longer than that of the NH-best (2.2 ± 0.3 s) neurons. These differences in the spike counts and the response durations between the two neuronal types in the PBN were similar to that previously reported in the rostral nucleus of the solitary tract (rNST). The CVs in the N-best and the NH-best neurons were significantly smaller in the PBN than those in the rNST. Histologically, most N-best neurons (12/13, 92%) were localized to the medial region, while NH-best neurons (11/13, 85%) were primarily found within the brachium conjunctivum. These results suggest that NaCl-specific taste information is transmitted by two distinct neuronal groups (N-best and NH-best), with different taste properties and locations within rNST to PBN tractography. Future studies on the higher order nuclei for taste could reveal more palatable and aversive taste pathways.

Internal biliary drainage superior to external biliary drainage in improving gut mucosa barrier because of goblet cells and mucin-2 up-regulation

Backgroud: Obstructive jaundice increases intestinal permeability, but the pathological mechanisms remain obscure, which results in debates about the necessity of performing preoperative biliary drainage in patients with obstructive jaundice. Mucin-2 (MUC2) and goblet cells regulated by bile acids play an important role in maintaining the function of intestinal mucosal barrier. The present study was to investigate the role of goblet cells and MUC2 in obstructive jaundice and evaluate the effect of biliary drainage on intestinal permeability.

STUDY DESIGN

We enrolled patients with malignant biliary obstruction and controls. We also did animal studies with four groups of rats: sham operation, obstructive jaundice, internal biliary drainage, and external biliary drainage. Histopathological analysis, biochemical measurement, and electron microscopy examination were done on pertinent samples.

RESULTS

Compared with the control group, the small intestinal mucosa was significantly damaged; goblet cells and MUC2 were significantly decreased and serum endotoxin level was significantly increased in patients and rats with obstructive jaundice. Biliary drainage, especially internal biliary drainage, significantly increased goblet cells and MUC2 and attenuated the damage of small intestinal mucosa.

CONCLUSIONS

In obstructive jaundice condition, goblet cells and MUC2 were reduced which were involved in the damage of intestinal mucosa barrier; biliary drainage increased goblet cells and MUC2, repaired mucosa layer and restored the intestinal mucosa barrier function.

Topographic organizations of taste-responsive neurons in the parabrachial nucleus of C57BL/6J mice: An electrophysiological mapping study

The activities of 178 taste-responsive neurons were recorded extracellularly from the parabrachial nucleus (PbN) in the anesthetized C57BL/6J mouse. Taste stimuli included those representative of five basic taste qualities, sweet, salty, sour, bitter and umami. Umami synergism was represented by all sucrose-best and sweet-sensitive sodium chloride-best neurons. Mediolaterally the PbN was divided into medial, brachium conjunctivum (BC) and lateral subdivisions while rostrocaudally the PbN was divided into rostral and caudal subdivisions for mapping and reconstruction of recording sites. Neurons in the medial and BC subdivisions had a significantly greater magnitude of response to sucrose and to the mixture of monopotassium glutamate and inosine monophosphate than those found in the lateral subdivision. In contrast, neurons in the lateral subdivision possessed a more robust response to quinine hydrochloride. Rostrocaudally no difference was found in the mean magnitude of response. Analysis on the distribution pattern of neuron types classified by their best stimulus revealed that the proportion of neuron types in the medial vs. lateral and BC vs. lateral subdivisions was significantly different, with a greater amount of sucrose-best neurons found medially and within the BC, and a greater amount of sodium chloride-, citric acid- and quinine hydrochloride-best neurons found laterally. There was no significant difference in the neuron-type distribution between rostral and caudal PbN. We also assessed breadth of tuning in these neurons by calculating entropy (H) and noise-to-signal (N/S) ratio. The mean N/S ratio of all neurons (0.43) was significantly lower than that of H value (0.64). Neurons in the caudal PbN had a significantly higher H value than in the rostral PbN. In contrast, mean N/S ratios were not different both mediolaterally and rostrocaudally. These results suggest that although there is overlap in taste quality representation in the mouse PbN, taste-responsive neurons still possessed a topographic organization.

Activation of lateral hypothalamus-projecting parabrachial neurons by intraorally delivered gustatory stimuli

The present study investigated a subpopulation of neurons in the mouse parabrachial nucleus (PbN), a gustatory and visceral relay area in the brainstem, that project to the lateral hypothalamus (LH). We made injections of the retrograde tracer Fluorogold (FG) into LH, resulting in fluorescent labeling of neurons located in different regions of the PbN. Mice were stimulated through an intraoral cannula with one of seven different taste stimuli, and PbN sections were processed for immunohistochemical detection of the immediate early gene c-Fos, which labels activated neurons. LH projection neurons were found in all PbN subnuclei, but in greater concentration in lateral subnuclei, including the dorsal lateral subnucleus (dl). Fos-like immunoreactivity (FLI) was observed in the PbN in a stimulus-dependent pattern, with the greatest differentiation between intraoral stimulation with sweet (0.5 M sucrose) and bitter (0.003 M quinine) compounds. In particular, sweet and umami-tasting stimuli evoked robust FLI in cells in the dl, whereas quinine evoked almost no FLI in cells in this subnucleus. Double-labeled cells were also found in the greatest quantity in the dl. Overall, these results support the hypothesis that the dl contains direct a projection to the LH that is activated preferentially by appetitive compounds; this projection may be mediated by taste and/or postingestive mechanisms.

Sweet-bitter and umami-bitter taste interactions in single parabrachial neurons in C57BL/6J mice

We investigated sweet-bitter and umami-bitter mixture taste interactions by presenting sucrose or umami stimuli mixed with quinine hydrochloride (QHCl) while recording single-unit activity of neurons in the parabrachial nucleus (PbN) of urethane-anesthetized C57BL/6J mice. A total of 70 taste-responsive neurons were classified according to which stimulus evoked the greatest net response (36 sucrose-best, 19 NaCl-best, 6 citric acid-best, and 9 QHCl-best). Although no neurons responded best to monopotassium glutamate (MPG) or inosine 5'-monophosphate (IMP), the combination of these two stimuli evoked a synergistic response (i.e., response > 120% of the sum of the component responses) in all sucrose-best and some NaCl-best neurons (n = 43). Adding QHCl to sucrose or MPG + IMP resulted in suppression of the response (responses to mixture < responses to the more effective component) in 41 of 43 synergistic neurons. Neurons showing QHCl suppression were classified into two types: an "MS1" type (n = 27) with suppressed responses both to sucrose and MPG + IMP and an "MS2" type (n = 14) that showed suppressed responses only to sucrose. No neuron displayed suppressed responses to MPG or IMP alone. The suppression ratio (1 - mixture response/sucrose or MPG + IMP response) of sucrose and MPG + IMP in MS1 neurons had a weak positive correlation (r = 0.36). The pattern of reconstructed recording sites of neuron types suggested chemotopic organization in the PbN. Although a peripheral basis for QHCl suppression has been demonstrated, our results suggest that convergence in the PbN plays a role in shaping responses to taste mixtures.

Gustatory neural responses to umami stimuli in the parabrachial nucleus of C57BL/6J mice

Umami is considered to be the fifth basic taste quality and is elicited by glutamate. The mouse is an ideal rodent model for the study of this taste quality because of evidence that suggests that this species, like humans, may sense umami-tasting compounds as unique from other basic taste qualities. We performed single-unit recording of taste responses in the parabrachial nucleus (PbN) of anesthetized C57BL/6J mice to investigate the central representation of umami taste. A total of 52 taste-responsive neurons (22 sucrose-best, 19 NaCl-best, 5 citric acid-best, and 6 quinine-best) were recorded from stimulation period with a large panel of basic and umami-tasting stimuli. No neuron responded best to monopotassium glutamate (MPG) or inosine 5'-monophosphate (IMP), suggesting convergence of input in the central nervous system. Synergism induced by an MPG-IMP mixture was observed in all sucrose-best and some NaCl-best neurons that possessed strong sensitivity to sucrose. In more than half of sucrose-best neurons, the MPG-IMP mixture evoked stronger responses than those elicited by their best stimulus. Furthermore, hierarchical cluster analysis and multidimensional analysis indicated close similarity between sucrose and the MPG-IMP mixture. These results strongly suggest the mixture tastes sweet to mice, a conclusion consistent with previous findings that show bidirectional generalization of conditioned taste aversion between sucrose and umami mixtures, and suppression of taste responses to both sucrose and mixtures by the antisweet polypeptide gurmarin in the chorda tympani nerve. The distribution pattern of reconstructed recording sites of specific neuron types suggested chemotopic organization in the PbN.

Subnuclear organization of parabrachial efferents to the thalamus, amygdala and lateral hypothalamus in C57BL/6J mice: a quantitative retrograde double labeling study

The present study investigated the subnuclear organization of collateralized efferent projection patterns from the mouse parabrachial nucleus (PbN), the second taste relay in rodents, to higher gustatory centers, including the ventroposteromedial nucleus of the thalamus (VPMpc), central nucleus of the amygdala (CeA) and lateral hypothalamus (LH). We made injections of the retrograde tracer red and green latex microspheres into the VMPpc and CeA (VPMpc-CeA group), VMPpc and LH (VPMpc-LH group) or CeA and LH (CeA-LH group, n=6 for each group). Injections into these areas preferentially resulted in retrograde labeling in the ipsilateral PbN in all groups. Cells projecting to the VPMpc, CeA, and LH were generally found in all subnuclei, but were differentially distributed. VPMpc-projecting cells predominated in gustatory-related subnuclei, CeA-projecting neurons predominated in the external lateral (el) subnucleus, and concentrated labeling was observed in the dorsal lateral subnucleus (dl) following LH injection. Double-labeled neurons were found for all groups, almost entirely ipsilaterally and primarily in the medial (m), waist area (wa), ventral lateral (vl) and el subnuclei. These results suggest that PbN neurons in different subdivisions have different projection and collateralization patterns to the VPMpc, CeA and LH. Functional implications of these projections are discussed with an emphasis on their roles in taste.

Orosensory deprivation alters taste-elicited c-Fos expression in the parabrachial nucleus of neonatal rats

In the present study we examined the effects of neonatal orosensory deprivation on taste-elicited gustatory activity in the rat parabrachial nucleus (PBN) using the functional anatomical marker c-Fos. Animals in three groups (GG, GO and GM) received gastric cannula implantation surgery on postnatal day 9 (P9). Animals in the fourth group (MR) did not receive any surgery. GG rats were fed by infusion of artificial milk directly into the stomach. GO rats were fed by intraoral infusion of artificial milk. GM and MR rats were reared by their mother with free access to mother's milk, water and rat chow. Rats from all groups were similar in body weight and length by P21. On P21 rats in all groups were intraorally presented with 0.5M sucrose solution and the brains were extracted and processed for c-Fos immunohistochemistry. Taste-elicited c-Fos expression in both the gustatory waist area, and the external lateral subnucleus of the PBN in rats in the GG group was significantly more robust than in the other three groups. These findings suggest a substantial alteration in orosensory-evoked neuronal response in this nucleus, due to sensory or motor deprivation during a critical developmental stage.

Focus on HTR2C: A possible suggestion for genetic studies of complex disorders

HTR2C is one of the most relevant and investigated serotonin receptors. Its role in important brain structures such as the midbrain, the lateral septal complex, the hypothalamus, the olfactory bulb, the pons, the choroid plexus, the nucleus pallidus, the striatum and the amygdala, the nucleus accumbens and the anterior cingulated gyrus candidate it as a promising target for genetic association studies. The biological relevance of these brain structures is reviewed by way of the focus on HTR2C activity, with a special attention paid to psychiatric disorders. Evidence from the genetic association studies that dealt with HTR2C is reviewed and discussed alongside the findings derived from the neuronatmic investigations. The reasons for the discrepancies between these two sets of reports are discussed. As a result, HTR2C is shown to play a pivotal role in many different psychiatric behaviors or psychiatric related disrupted molecular balances, nevertheless, genetic association studies brought inconsistent results so far. The most replicated association involve the feeding behavior and antipsychotic induced side effects, both weight gain and motor related: Cys23Ser (rs6318) and -759C/T (rs3813929) report the most consistent results. The lack of association found in other independent studies dampens the clinical impact of these reports. Here, we report a possible explanation for discrepant findings that is poorly or not at all usually considered, that is that HTR2C may exert different or even opposite activities in the brain depending on the structure analyzed and that mRNA editing activity may compensate possible genetically controlled functional effects. The incomplete coverage of the HTR2C variants is proposed as the best cost-benefit ratio bias to fix. The evidence of brain area specific HTR2C mRNA editing opens a debate about how the brain can differently modulate stress events, and process antidepressant treatments, in different brain areas. The mRNA editing activity on HTR2C may play a major role for the negative association results.

Characterization of parabrachial subnuclei in mice with regard to salt tastants: possible independence of taste relay from visceral processing

We examined whether salt taste and/or abdominal illness were dealt within different subnuclei in the parabrachial nucleus (PBN) in mice, using retrograde tracing methods and c-Fos-like immunoreactivity (FLI) detection procedures. Some PBN subnuclei have distinct functions and receive various sensory inputs from the nucleus of the solitary tract (NTS) and other areas and relay them to the higher order nuclei such as the thalamus. The afferent-dependent pattern of FLI has been investigated in the PBN. However, it is unclear in which PBN subnuclei the tastants induce c-Fos, or whether PBN subnuclei process taste inputs separately from other inputs, or integrate them. After the tracer injections into the thalamic taste relay, the retrograde labeled cells revealed the taste relay cells in the PBN at the boundary with the superior cerebellar peduncle of both the inner part of the external lateral subnucleus and the medial subnucleus and in the waist area. On the other hand, NaCl intake induced intense FLI in the dorsal lateral subnucleus, whereas LiCl intake yielded intense FLI in both the dorsal lateral subnucleus and the outer part of the external lateral subnucleus. Thus, the present findings that subnuclei relaying taste information to the thalamus do not yield FLI in response to salt taste and abdominal illness indicate that they lack FLI yielding pathways or that they are independent from the subnuclei processing salt taste and visceral information via c-Fos in mice.

Blocking glutamate receptors in the waist area of the parabrachial nucleus decreases taste reactivity behaviors in conscious rats

The "waist" area (W) of the parabrachial nucleus contains neurons that receive orosensory input and play a role in the initiation of oromotor behaviors. Immunohistochemical data indicate that neurons in W receive glutamatergic input and express glutamate receptors, but a behavioral role for glutamate neurotransmission within W has not been investigated. To determine the role of specific glutamate receptors in taste reactivity behaviors, glutamate receptor blockers were delivered into W by reverse microdialysis during intraoral infusion of 0.1 M sodium chloride, 0.1 M sucrose, 0.03 M hydrochloric acid, and 0.003 M quinine hydrochloride. Blocking alpha-amino-3-hydroxy-5-methyl-isoxazolepropionate (AMPA)/kainate ionotropic glutamate receptors in W with 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX) reduced ingestive taste reactivity behaviors to each tastant by 72-85% compared with baseline levels (P's < 0.05). Blocking N-methyl-D-aspartate receptors as well as type 1 and group III metabotropic glutamate receptors had minor effects on taste reactivity responses to the tastants. These data provide strong evidence for a behavioral role of glutamatergic neurotransmission in W in conscious rats.

Gustatory neural circuitry in the hamster brain stem

The nucleus of the solitary tract (NST) and the parabrachial nuclei (PbN) are the first and second central relays for the taste pathway, respectively. Taste neurons in the NST project to the PbN, which further transmits taste information to the rostral taste centers. Nevertheless, details of the neural connections among the brain stem gustatory nuclei are obscure. Here, we investigated these relationships in the hamster brain stem. Three electrode assemblies were used to record the activity of taste neurons extracellularly and then to electrically stimulate these same areas in the order: left PbN, right PbN, and right NST. A fourth electrode, a glass micropipette, was used to record from gustatory cells in the left NST. Results showed extensive bilateral communication between brain stem nuclei at the same level: 1) 10% of 96 NST neurons projected to the contralateral NST and 58% received synaptic input from the contralateral NST; and 2) 12% of 43 PbN neurons projected to the contralateral PbN and 21% received synaptic input from the contralateral PbN. Results also showed extensive communication between levels: 1) as expected, the majority of 119 NST neurons, 82%, projected to the ipsilateral PbN, but 85% of the 20 NST neurons tested received synaptic input from the ipsilateral PbN, as did 59% of 22 NST neurons that did not project to the PbN; and 2) although few, 3%, of 119 NST cells projected to the contralateral PbN and 38% received synaptic input from the contralateral PbN. These results demonstrated that taste neurons in the NST not only project to, but also receive descending input from the bilateral PbN and that gustatory neurons in the NST and PbN also communicate with the corresponding nucleus on the contralateral side.

Chronic prevention of mu-opioid receptor (MOR) G-protein coupling in the pontine parabrachial nucleus persistently decreases consumption of standard but not palatable food

RATIONALE

Acute pharmacological studies implicate mu-opioid receptors (MORs) in the parabrachial nucleus (PBN) of the brainstem in modulating eating. The long-term effects of preventing the cellular function of parabrachial MORs on food consumption remain to be elucidated.

OBJECTIVES

To determine whether (1) chronic inhibition of MOR-mediated G-protein coupling in the PBN of rats would persistently reduce eating and (2) food properties dictate the effects of MOR blockade.

MATERIALS AND METHODS

We microinfused the irreversible MOR antagonist, beta-funaltrexamine (beta-FNA) into the lateral PBN and measured the intake of standard and calorically dense palatable chow for 1 week. First, rats were given standard chow for 20 h daily and a calorically dense palatable chow for 4 h during the day. We infused the agonist, [D: -Ala(2), N-Me-Phe(4), Glycinol(5)]-Enkephalin (DAMGO), 1 week after beta-FNA to probe the acute effects of exogenous stimulation of MORs on palatable food intake. [(35)S]GTPgammaS autoradiography quantified regional loss of MOR cellular function. Next, we measured the actions of beta-FNA on food intake in rats given only standard or palatable chow for 1 week.

RESULTS

One infusion of beta-FNA persistently decreased consumption of standard but not palatable chow, regardless of feeding regimen. beta-FNA also blocked DAMGO-stimulated palatable chow intake, prevented DAMGO-stimulated G-protein coupling in the central and external lateral subnuclei of the PBN, and decreased coupling in the medial PBN. beta-FNA did not affect kappa-opioid receptors.

CONCLUSIONS

MORs in the lateral PBN serve a physiological role in stimulating consumption of standard food. Properties of the diet, such as high palatability or caloric density, may override the influence of inhibiting MOR function.

Medial parabrachial nucleus neurons modulate d-fenfluramine-induced anorexia through 5HT2C receptors

We previously reported that lesions of the medial parabrachial nucleus (PBN) enhanced d-fenfluramine (DFEN)-induced anorexia; a finding that suggests these lesions may potentiate the release of serotonin (5HT) or increase the postsynaptic action of 5HT. In the present study, we used SB 206553 (a 5HT2B/2C receptor antagonist) or m-CPP (a 5HT2C/1B receptor agonist) in a standard behavioral procedure (deprivation-induced feeding) to further explore the role of the medial PBN in drug-induced anorexia. In Experiment 1, DFEN (0 or 1.0 mg/kg) was given alone or in combination with SB 206553 (2.0 or 5.0 mg/kg). In Experiment 2, we investigated the food-suppressive effects of m-CPP (0.5, 1.0 or 2.0 mg/kg). The results of Experiment 1 show that SB 206553, while having no influence on the performance of control subjects, attenuated (2.0 mg/kg) or abolished (5 mg/kg) the potentiating effect of the lesions on DFEN-induced anorexia. In Experiment 2, m-CPP induced a suppression of food intake in nonlesioned animals that was significantly potentiated in rats with medial PBN lesions. These results are consistent with the hypothesis that medial PBN neurons mediate anorexia through 5HT2C receptors.

Parabrachial inputs to Fos-immunoreactive neurons in the lateral central nucleus of amygdala activated by hypotension: a light and electron microscopic study in the rat

Morphological features and functional implications of projections of the parabrachial nucleus to the central nucleus of the amygdala were investigated in the rat. The anatomical study was based on injections of the tracers horseradish peroxidase and biotinylated dextran amine. An extremely dense concentration of labeled fibers was found in the lateral and lateral capsular subdivisions of the central nucleus of the amygdala, originating mainly from the external lateral and ventral lateral subnuclei of the parabrachial nucleus. The parabrachial fibers exhibited the morphological characteristic of forming dense pericellular terminal arborizations. The functional implications of this pathway in cardiovascular functions were verified using Fos protein induction in response to hypotension induced by continuous intravenous administration of hydralazine-hydrochloride. In this paradigm, Fos immunoreactivity was found to be confined to the lateral and lateral capsular subdivisions of the central nucleus of the amygdala. Double immunostaining methods were used to visualize, at the electron microscopic level, terminals labeled by biotinylated dextran amine and Fos cell labeling. With this approach, we were able to confirm that Fos-immunoreactive neurons in the central nucleus of the amygdala receive axosomatic terminals from the parabrachial nucleus. The present findings point out that parabrachial inputs to the central nucleus of the amygdala play a relevant role in regulating cardiovascular function.

Hypothalamic regulation of pancreatic secretion is mediated by central cholinergic pathways in the rat

The vago-vagal reflex plays an important role in mediating pancreatic secretion evoked by cholecystokinin and non-cholecystokinin-dependent luminal factors. We hypothesize that the vago-vagal reflex mediating pancreatic secretion in the rat is under central control and regulated by cholinergic pathways in the hypothalamus. To test this hypothesis, we demonstrated that chronic decerebration decreased basal pancreatic enzyme secretion from 318 +/- 12 to 233 +/- 9 mg h-1 and reduced the net increase in pancreatic secretion stimulated by intraduodenal infusion of 5 % peptone and hypertonic NaCl by 54 % and 45 %, respectively. Intracerebroventricular administration of methscopolamine (MSCP, 50 nmol (5 mul)-1), a blood-brain barrier-impermeant cholinergic muscarinic receptor antagonist, evoked results similar to those achieved by chronic decerebration. To localize the sites of action, we demonstrated that microinjection of MSCP (20 nmol) into the lateral hypothalamic nucleus or the paraventricular nucleus resulted in inhibition of both basal pancreatic protein secretion and luminally stimulated pancreatic secretion by 48 % and 52 %, respectively. Intracerebroventricular injection of hemicholinium-3 at doses known to deplete the endogenous ACh store produced similar inhibitory results. In addition, microinjection of ACh (5 pmol) or the muscarinic M1 receptor agonist McN-A-343 (30 ng) into the lateral hypothalamic nucleus increased pancreatic secretion over basal levels by 46 % and 40 %, respectively. Selective lesions of lateral septal cholinergic neurons decreased basal pancreatic secretion and inhibited peptone-induced pancreatic secretion by 30 %. Destruction of the lateral parabrachial nucleus produced a 44 % inhibition of peptone-induced pancreatic section. Finally, microinjection of glutamate into the lateral septum or the lateral parabrachial nucleus stimulated vagal pancreatic efferent nerve firings from a basal level of 0 +/- 0.5 impulses (30 s)-1 to 4.5 +/- 0.5 and 14 +/- 2 impulses (30 s)-1, respectively, and pancreatic protein output increased 50 % and 84 % over basal levels. Administration of MSCP to the paraventricular nucleus eliminated these effects. These observations suggest that cholinergic neurons of the lateral septum and lateral parabrachial nucleus regulate pancreatic secretion. Further, cholinergic input from the lateral parabrachial nucleus to the hypothalamus plays a major role in the modulation of vagal pancreatic efferent nerve activity and pancreatic secretion evoked by the vago-vagal reflex.

Distinct regional distributions of NK1 and NK3 neurokinin receptor immunoreactivity in rat brainstem gustatory centers

Tachykinins and their receptors are present in gustatory centers, but little is known about tachykinin function in gustation. In this study, immunohistochemical localization of substance P and two centrally prevalent neurokinin receptors, NK1 and NK3, was carried out in the rostral nucleus of the solitary tract and the caudal parabrachial nucleus to evaluate regional receptor/ligand correspondences. All three proteins showed regional variations in labeling density that correlated with distinct sites in gustatory centers. In the rostral nucleus of the solitary tract, the relative densities of substance P and NK1 receptors varied in parallel across subnuclei, with both being moderate to dense in the dorsocentral, chemoresponsive zone. NK3 receptors had a distinct distribution in the caudal half of this zone, suggesting a unique role in processing taste input from the posterior tongue. In the caudal parabrachial nucleus, substance P and NK1 receptor immunoreactivities were dense in the pontine taste area, while NK3 receptor labeling was sparse. The external medial subnucleus had substantial NK3 receptor and substance P labeling, but little NK1 receptor immunoreactivity. These findings suggest that distinct tachykinin ligand/neurokinin receptor combinations may be important in local processing of information within brainstem gustatory centers.

Dual separate pathways for sensory and hedonic aspects of taste

It is proposed that in the gustatory system there exist separate sensory and hedonic (reward-aversion) representations in each of the primary structures in which processing of gustatory stimuli occurs. Anatomical and physiological data are used to determine putative separate sensory and hedonic representations in the nucleus of the solitary tract, parabrachial complex, gustatory thalamus, and cortical gustatory areas. In the nucleus of the solitary tract, the sensory representation is located in the rostralmost part of the nucleus, and the hedonic representation most probably in the intermediate parts. In the parabrachial complex, the sensory representation is located in the central medial and ventral lateral subnuclei, and in the waist area, and the hedonic representation in the inner division of the external lateral subnucleus and in the external medial subnucleus. In the rodent gustatory thalamic relay, the sensory representation occurs in the dorsal lateral parts of the nucleus, and the hedonic representation in the ventromedial parts. In rodent gustatory insular cortex, the sensory representation is found in anterior parts of the gustatory area, and the hedonic representation caudal to the sensory representation. The function of the separate sensory and hedonic representations is discussed in relation to the conditioned taste aversion paradigm.

Stimulation of Specific Regions of the Parabrachial Nucleus Elicits Ingestive Oromotor Behaviors in Conscious Rats

The "waist" area and external subnuclei of the parabrachial nucleus (PBN) have been implicated in the processing of gustatory information, yet their behavioral roles are not clearly defined. In the current study, areas within and surrounding the PBN were stimulated while oromotor behaviors were monitored in conscious rats. Electrical and chemical (100 mM glutamate) stimulation of the waist area increased ingestive oromotor behaviors over baseline (p<.01). Stimulation of external PBN subnuclei and areas medial and ventral to the PBN failed to cause a behavioral change. These data support the hypothesis that the waist area of the PBN constitutes part of the neural substrate involved in eliciting oromotor behaviors in response to taste input.

Effects of gustatory nerve transection and regeneration on quinine-stimulated Fos-like immunoreactivity in the parabrachial nucleus of the rat

The distribution of quinine-stimulated Fos-like immunoreactivity (FLI) in several subdivisions of the parabrachial nucleus (PBN) known to be responsive to gustatory stimulation was examined in rats in which the chorda tympani nerve (CT) and/or glossopharyngeal nerve (GL) was transected (Experiment 1) and in rats in which the GL was transected with regeneration promoted or prevented (Experiment 2). We confirmed previous findings in the literature by demonstrating that rats intraorally infused with 3 mM quinine showed a robust population of FLI in the waist area and the external lateral (EL) and external medial (EM) subdivisions of the PBN (Yamamoto et al. [1994] Physiol Behav 56:1197-1202; Travers et al., [ 1999] Am J Physiol 277:R384-R394). In the waist area, only GL transection significantly decreased the number of FLI-neurons elicited by intraoral infusion of quinine compared with water-stimulated controls. In the external subdivisions neither neurotomy affected the number of FLI-neurons. The effect of GL transection in the waist area was enduring for rats in which the GL did not regenerate (up to 94 days), but regeneration of the GL after 52 days restored quinine-stimulated FLI to control values. In these same GL-transected animals, there were parallel decreases in the number of gapes elicited by intraoral quinine stimulation that recovered, but only subsequent to regeneration of the GL. These data provide support for the role of the waist area in the brainstem processing that underlies oromotor rejection behaviors and also help substantiate the hypothesis that the CT and GL are relatively specialized with regard to function. Moreover, when the quinine-induced pattern of neural activity in the second central gustatory relay, as assessed by FLI, is substantially altered by the loss of peripheral gustatory input from the GL, it can be restored upon regeneration of the nerve.

Oral and gastric input to the parabrachial nucleus of the rat

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.

Medial versus lateral parabrachial nucleus lesions in the rat: effects on mercaptoacetate-induced feeding and conditioned taste aversion

The two experiments of the present study examined the influence of bilateral electrophysiologically-guided ibotenic acid lesions of the medial (gustatory) and lateral (viscerosensory) subdivisions of the parabrachial nucleus (PBN) on lipoprivic feeding and on the acquisition of a conditioned taste aversion. In Experiment 1, mercaptoacetate (0, 400, 600, or 800 micromol/kg) failed to enhance food intake in normal rats maintained and tested on standard laboratory chow. In the same procedure, rats with lesions of the medial or lateral PBN consumed less food during baseline but nonetheless were sensitive to the orexigenic action of mercaptoacetate. In Experiment 2, both types of PBN lesions prevented acquisition of a conditioned taste aversion induced by the oral self administration of lithium chloride. The results suggest that PBN neurons essential for conditioned taste aversion are not involved in the mercaptoacetate-induced feeding of rats maintained and tested on standard laboratory chow.

Lateral parabrachial nucleus lesions in the rat: neophobia and conditioned taste aversion

The present study investigated the hypothesis that the conditioned taste aversion (CTA) deficit consequent to lesions of the lateral parabrachial nucleus (LPBN) may be due to a disruption of neophobia. In Experiment 1, subjects were tested with one of three taste stimuli (alanine, saccharin, or quinine) and two nontaste stimuli (capsaicin and almond odor). Ibotenic acid lesions of the LPBN eliminated neophobia to alanine and saccharin but had no influence on the neophobic response to quinine, capsaicin, or almond odor. In Experiment 2, all the LPBN-lesioned (LPBNX) rats failed to develop a CTA. These results do not support the experimental hypothesis. Not only was the lesion-induced disruption of neophobia restricted to taste stimuli, the deficit was selective within that category. It is already known that LPBNX rats are unable to acquire conditioned aversions to capsaicin as well as alanine. Thus, the absence of a conditioned ingestional aversion in LPBNX rats is not predicated upon the absence of a neophobic response to the target stimulus. The present results, although exposing a stimulus selective disruption of neophobia, suggest that this deficit is independent of, rather than responsible for, the absence of conditioned ingestional aversions in rats with LPBN lesions.

Projections of the parabrachial nucleus in the old world monkey

The efferent projections of the pontine parabrachial nucleus (PBN) were examined in the Old World monkey (Macaca fascicularis) using tritiated amino acid autoradiography and horseradish peroxidase histochemistry. Parabrachiofugal fibers ascended to the forebrain along three pathways: the central tegmental tract, the ventral ascending catecholaminergic pathway, and a pathway located on the midline between the medial longitudinal fasciculi. The PBN projected heavily to the central nucleus of the amygdala and the lateral division of the bed nucleus of the stria terminalis and moderately to the ventral tegmental area and the substantia nigra. Light terminal label also was present within the dorsomedial, ventromedial, lateral, supramammillary, and infundibular nuclei of the hypothalamus and the annular nucleus and the dorsal raphe nucleus within the brain stem. The overall pattern of terminal label was similar to that previously reported for nonprimate species, but several differences were notable. In monkey the projection to the ventrobasal thalamus did not coincide with the region that contains gustatory-responsive neurons. In rats, these parabrachiothalamic fibers convey gustatory activity but in the monkey these fibers may carry visceral afferent information. The projections from the PBN to the hypothalamus in the monkey were neither as widespread nor as intense as in the rat, and the monkey lacks a projection from the PBN to the frontal and insular cortices.

Lateral parabrachial nucleus lesions in the rat: aversive and appetitive gustatory conditioning

Previous research involving tests of innate preferences and aversions shows that bilateral ibotenic acid lesions of the visceral neurons located in the lateral parabrachial nucleus of the pons selectively disrupt consumption of those gustatory stimuli whose intake is augmented or restricted by their postoral consequences. The present study examined the performance of the same experimental subjects in learned preference and aversion tasks. The lesioned rats failed to develop a conditioned taste aversion (Experiment 1), a conditioned flavor preference (Experiment 2), and a conditioned aversion to the oral trigeminal stimulus, capsaicin (Experiment 3). The pattern of results from both types of taste-guided behaviors (innate and learned) suggests that excitotoxic lesions of the lateral parabrachial nucleus diminish sensitivity to gastrointestinal feedback which, in the present experiments, precludes aversive and appetitive associative learning.

Lateral parabrachial nucleus lesions in the rat: long-and short-duration gustatory preference tests

The present study reports two experiments that evaluated the influence of bilateral ibotenic acid lesions of the viscerosensory neurons in the lateral parabrachial nucleus (LPBN) on intake of four prototypical taste stimuli (sucrose, sodium chloride, citric acid, and quinine hydrochloride). In the 24-h, two-bottle tests of Experiment 1, rats with lesions of the LPBN were severely impaired in their concentration-dependent consumption of sucrose, displayed a mild disturbance of sodium chloride intake, and drank normal amounts of citric acid and quinine hydrochloride. These lesion-induced deficits were less pronounced when assessed with the 15-min, 1-bottle tests of Experiment 2. The results suggest that destruction of the viscerosensory neurons within the LPBN disrupt the processing of gastrointestinal feedback.

The parabrachial nucleus and conditioned taste aversion

The parabrachial nucleus (PBN) surrounds the brachium conjunctivum in the dorsolateral pons. Although composed of numerous subnuclei, the PBN is typically organized into medial and lateral subdivisions according to their location relative to the brachium. In rodents, the medial PBN is part of the central gustatory system, whereas the lateral PBN is a component of the visceral sensory system. Lesions of the PBN disrupt conditioned taste aversion, a critically important learning mechanism that prevents the repeated ingestion of toxic food. Relevant neurobehavioral literature is reviewed to elucidate the role of the PBN in taste aversion learning.

Differential projections from gustatory responsive regions of the parabrachial nucleus to the medulla and forebrain

The present study combined extracellular electrophysiology with anterograde and retrograde tracing techniques to determine efferent projections from taste responsive sites within the parabrachial nucleus (PBN). Taste activity was recorded from two distinct regions of the PBN, the waist region consisting of the ventrolateral (VL) and central medial (CM) subnuclei, and the external region, consisting of the external medial (EM) and external lateral (EL) subnuclei. Ascending and descending projections from these two regions differed. Small biotinylated dextran injections placed in taste responsive sites in the waist area produced a prominent descending projection to the medullary parvocellular reticular formation, a projection nearly non-existent from the external region. Differences in ascending projections were more subtle. Projections to the thalamus were bilateral in all cases, however, the waist region had a larger ipsilateral thalamic projection than the external region and the external region had a larger contralateral projection compared to the waist. Central nucleus of amygdala (CNA) projections from the waist area were primarily from posterior tongue responsive sites in VL and terminated in the central medial and lateral CNA subnuclei; external region projections were distributed to the capsular region of CNA. Both the external and waist region projected to substantia innominata (SI). Different efferent projections from the two gustatory responsive regions of the PBN may reflect functional specialization of PBN subnuclei. Descending projections from orally responsive sites in the waist area project to the lateral parvocellular reticular formation, a region implicated in brainstem circuitry underlying consummatory components of ingestive function. The external region, contains cells responsive to pain and oral aversive stimuli, but does not apparently contribute directly to local brainstem functions. Rather, forebrain pathways appear critical to the expression of external region functions.

Supraspinal connections and termination patterns of the parabrachial complex determined by the biocytin anterograde tract-tracing technique in the rat

We have re-evaluated, using the anterograde tracer biocytin, supraspinal efferent projections from the parabrachial complex (PBN) to gain new information about the nature of its connections and nerve terminal patterns. We selectively injected biocytin into the 3 main regions of the nucleus (lateral PBN, medial PBN and Kölliker-Fuse nucleus). We observed distinct groups of ascending and descending fibres of different calibre from the PBN running throughout the brain and reaching many brain areas involved in the regulation of autonomic function. Here we detected labelled bouton-like terminals and fibres with en-passage varicosities. The ascending efferents from the lateral PBN mainly reached the reticular, raphe and thalamic nuclei, the zona incerta (ZI), central nucleus of the amygdala (CeA) and lateral area of the periaqueductal grey (PAG). Thin descending efferents reached the ventral region of the solitary tract nucleus (STN). The ascending efferents from the medial PBN were seen in the raphe nuclei, reticular nuclei, ventral and lateral areas of the PAG, thalamic nuclei, and in the medial and lateral nuclei of the amygdala. Descending efferents were seen in the STN and in some reticular nuclei. The ascending projections from the Kölliker-Fuse targeted the ventral area of PAG, CeA, ZI, lateral hypothalamic area, ventromedial thalamic nucleus and, with only a few terminals, the ipsi and contralateral reticular area. A large number of descending efferents reached STN, caudal and paragigantocellular reticular nuclei. The higher sensitivity of biocytin compared with other types of markers allowed us to determine more effectively the distribution, nature and extent of the supraspinal PBN connections. This suggested that in several nerve circuits the PBN probably plays a more important role than previously thought.

Differences in the intrinsic membrane characteristics of parabrachial nucleus neurons processing gustatory and visceral information

Whole-cell current-clamp recordings were made from neurons in the rat parabrachial nucleus (PBN) in three rostro-caudal brain slices. During recording the neurons were located in one of four quadrants of the PBN. Successful recordings were obtained from neurons in three of these quadrants termed the dorsolateral (DL), dorsomedial (DM) and ventromedial (VM) quadrants. Recordings were made of the intrinsic membrane properties and repetitive discharge characteristics of 58 neurons in the DL, 60 neurons in the DM, and 54 neurons in the VM-quadrants. The input resistance of the neurons in the DL quadrant was significantly lower and the membrane time constant significantly shorter than that of the neurons in the DM- and VM-quadrants. The mean action potential duration of the VM-quadrant neurons was significantly longer than that of both DL- and DM-quadrant neurons. The discharge frequency in response to a 1500 ms 100 pA current pulse of the DL quadrant neurons was significantly lower than that of the neurons in the other two quadrants. The latency of action potential initiation following a 100 pA depolarizing current pulse was significantly longer for DL quadrant neurons compared to neurons in the other two quadrants. Neurons were divided into groups based on their response to a long depolarizing current pulse immediately preceded by a hyperpolarizing current pulse. In all three rostro-caudal slices of the PBN, the largest populations of neurons were in Group II and Group III. The results demonstrate that neurons in different locations in the PBN have different membrane and repetitive discharge properties. These different PBN locations receive inputs from the visceral and gustatory regions of the NST. It is possible therefore that the differences in properties of the PBN neurons may relate to the type of sensory information that they process.

Increase of the aversive value of taste stimuli following ibotenic acid lesion of the central amygdaloid nucleus in the rat

Male Sprague-Dawley rats received bilateral ibotenic acid lesions of the central amygdaloid nucleus (CeA) and were compared to sham-lesioned rats in their response to different concentrations of saccharin and quinine solutions. In two-bottle choice test situation, the lesioned rats exhibited a lower saccharin preference at concentrations of 2.5; 7.5 and 25 mM, while their aversion towards quinine and the highest concentration of saccharin (50 mM) was increased. In a one-bottle test, the lesioned rats showed consistent decreases in their consumption of 2.5 and 7.5 mM saccharin solutions whereas their intake of 0.9 mM solution of saccharin was equal to that of the sham-lesioned rats. The lesion of the CeA had no significant effects on the acquisition of conditioned taste aversion. There was less postoperative weight gain in lesioned rats as compared to sham-lesioned animals but the lesion had no significant effect on daily water intake. These findings suggest that the CeA plays an important role in the normal response to exteroceptive food stimuli via modulation of the aversive value of taste stimuli. The results are discussed in the context of an interaction between the CeA and the lateral hypothalamus (LH) in the modulation of palatability and feeding behavior.

Anterior and posterior oral cavity responsive neurons are differentially distributed among parabrachial subnuclei in rat

The responses of single parabrachial nucleus (PBN) neurons were recorded extracellularly to characterize their sensitivity to stimulation of individual gustatory receptor subpopulations (G neurons, n = 75) or mechanical stimulation of defined oral regions (M neurons, n = 54) then localized to morphologically defined PBN subdivisions. Convergence from separate oral regions onto single neurons occurred frequently for both G and M neurons, but converging influences were more potent when they arose from nearby locations confined to the anterior (AO) or posterior oral cavity (PO). A greater number of G neurons responded optimally to stimulation of AO than to PO receptor subpopulations, and these AO-best G neurons had higher spontaneous and evoked response rates but were less likely to receive convergent input than PO-best G neurons. In contrast, proportions, response rates, and convergence patterns of AO- and PO-best M neurons were more comparable. The differential sensitivity of taste receptor subpopulations was reflected in PBN responses. AO stimulation with NaCl elicited larger responses than PO stimulation; the converse was true for QHCl stimulation. Within the AO, NaCl elicited a larger response when applied to the anterior tongue than to the nasoincisor duct. Hierarchical cluster analysis of chemosensitive response profiles suggested two groups of PBN G neurons. One group was composed of neurons optimally responsive to NaCl (N cluster); the other to HCl (H cluster). Most N- and H-cluster neurons were AO-best. Although they were more heterogenous, all but one of the remaining G neurons were unique in responding best or second-best to quinine and so were designated as quinine sensitive (Q+). Twice as many Q+ neurons were PO- compared with AO-best. M neurons were scattered across PBN subdivisions, but G neurons were concentrated in two pairs of subdivisions. The central medial and ventral lateral subdivisions contained both G and M neurons but were dominated by AO-best N-cluster G neurons. The distribution of G neurons in these subdivisions appeared similar to distributions in most previous studies of PBN gustatory neurons. In contrast to earlier studies, however, the external medial and external lateral-inner subdivisions also contained G neurons, intermingled with a comparable population of M neurons. Unlike cells in the central medial and ventral lateral subnuclei, nearly every neuron in the external subnuclei was PO best, and only one was an N-cluster cell. In conclusion, the present study supports a functional distinction between sensory input from the AO and PO at the pontine level, which may represent an organizing principle throughout the gustatory neuraxis. Furthermore, two morphologically distinct pontine regions containing orosensory neurons are described.

Ascending and descending projections from the rostral nucleus of the solitary tract originate from separate neuronal populations

Anterograde studies have shown that neurons within the rostral (gustatory) nucleus of the solitary tract project to the parabrachial nucleus, as well as to sites within the medulla including the reticular formation and caudal nucleus of the solitary tract. In order to determine the degree to which the same neurons contribute to both projections, injections of retrograde tracers were made simultaneously into both the parabrachial nuclei and medullary reticular formation of the rat. Only a small proportion of neurons were double labeled. Consistent with studies in hamster, labeled neurons projecting to the parabrachial nuclei in rat consisted of both stellate and elongate neurons, concentrated within the central subdivision of the rostral nucleus of the solitary tract. Injections into the medullary reticular formation also labeled both stellate and elongate neurons but these were concentrated in the ventral subdivision of the nucleus. The results of the present study demonstrate that different populations of neurons in the nucleus of the solitary tract contribute to ascending and descending pathways. This suggest a possible functional specialization within the nucleus of the solitary tract for those neurons whose output eventually reaches the forebrain compared to those neurons with local connections.

Axonal projection patterns of neurons in the secondary gustatory nucleus of channel catfish

The second gustatory nucleus of teleost fishes receives ascending fibers from the primary gustatory center in the medulla and sends efferent fibers to several nuclei in the inferior lobe of the diencephalon. Similar to the corresponding parabrachial nucleus in birds and mammals, the secondary gustatory nucleus of catfish consists of several cytoarchitectonically distinct subnuclei which receive input from different portions of the primary gustatory nuclei. However, it is unclear how the subnuclear organization relates to the processing of gustatory information in the hindbrain and the subsequent transmission of that information to the forebrain. To determine whether cells within different subnuclei of the secondary gustatory nucleus of channel catfish project to different diencephalic targets, single cells were intracellularly labeled with biocytin. Three subnuclei have been identified in the secondary gustatory nucleus: a medial subnucleus spanning most of the rostrocaudal extent of the nucleus, a central subnucleus and a dorsal subnucleus, the latter two located in the rostrolateral portion of the complex. Cells throughout the secondary gustatory nucleus typically possessed similar collateral projections to several nuclei in the inferior lobe, although four of the six cells filled in the medial subnucleus projected only to nucleus centralis. The only apparent subnucleus-specific projection pattern involved cells at the rostral edge of the secondary gustatory nucleus and in the secondary visceral nucleus. Axons of these cells terminated only in restricted portions of nucleus lobobulbaris. These results suggest that efferents from different subnuclei of the secondary gustatory nucleus of catfish, like those of the parabrachial nucleus of birds and mammals, do not possess simple, topographical projections to target nuclei in the diencephalon.

Distribution of brainstem projections from spinal lamina I neurons in the cat and the monkey

The distribution of terminal projections in the brainstem from lamina I neurons in the spinal dorsal horn was investigated with the anterograde tracer Phaseolus vulgaris-leucoagglutinin in the cat and the cynomolgus monkey. Iontophoretic injections made with physiological guidance were restricted to lamina I or to laminae I-III in the cervical (C6-8) or lumbar (L6-7) enlargement. The distribution of terminal labeling was essentially identical in the cat and the monkey, although consistently of greater intensity in the monkey. Terminations were observed in the solitary nucleus, the dorsomedial medullary reticular formation, the entire rostrocaudal extent of the ventrolateral medulla, the locus coeruleus, the subcoerulear region and the Kölliker-Fuse nucleus, the lateral and medial portions of the parabrachial nucleus, the cuneiform nucleus, the ventrolateral and lateral portions of the periaqueductal gray, and the intercollicular nucleus. Lamina I terminations were generally bilateral in the medulla but more dense contralaterally in the pons and mesencephalon. The density and laterality of labeling in the medulla varied between cases independently from that in the pons and mesencephalon, suggesting that the lamina I projections to these regions may originate from different subsets of neurons. A clear topographic organization was observed only in the lateral column of the periaqueductal gray, where lumbar lamina I terminations were found caudal to cervical terminations. These observations indicate that spinal lamina I neurons project to a variety of brainstem sites involved in autonomic (cardiovascular, respiratory) and homeostatic processing and the control of behavioral state. These projections provide an afferent substrate for spino-bulbo-spinal somatoautonomic reflex arcs activated by nociceptive, thermoreceptive activity and for a spino-bulbo-hypothalamic relay of such activity by cells in the caudal ventrolateral medulla. These observations support the general concept that lamina I projections distribute modality-selective sensory information relevant to the physiological status and maintenance of the tissues and organs of the entire organism.

Gustatory function in the parabrachial nuclei: implications from lesion studies in rats

In rodents, third order gustatory neurons reside in the parabrachial nuclei of the dorsal pons. Lesions in this area of the brain have a variety of consequences on taste-related behaviors. Some behaviors are severely impaired, such as the expression of either conditioned taste aversion or depletion-induced sodium appetite. Other taste-based behaviors are less affected or not influenced at all. Although the lesion-behavior approach possesses serious methodological limitations, the constellation of findings from studies employing this experimental strategy in the PBN has promising implications. Foremost among these is the suggestion that the neural circuitry subserving performance in some of these taste-guided behavioral paradigms is dissociable. This paper critically reviews this body of behavioral research and discusses the conceptual ramifications of the results.

Trigeminal projections to the peribrachial region in the muskrat

The anterograde and retrograde transport of wheat germ agglutinin-horseradish peroxidase was used to study the trigeminoperibrachial pathway in the muskrat after injections of tracer into either the medullary dorsal horn or the dorsolateral pons. After injections into the medullary dorsal horn, labeled fibers ascended into the ipsilateral dorsolateral pons via the spinal trigeminal tract, within the neuropil of the trigeminal sensory complex and within the reticular formation adjacent to the spinal trigeminal nucleus. At caudal levels of the ipsilateral peribrachial area, dense terminal-like label distributed in the Kölliker-Fuse nucleus continued into the lateral parabrachial nucleus. At intermediate levels ipsilaterally, the Kölliker-Fuse nucleus again was labeled densely, as were areas analogous to the external lateral and external medial subnuclei of the parabrachial nucleus in the rat. A thin band of label along the ventral spinocerebellar tract outlined an unlabeled area in the central portion of the lateral parabrachial nucleus. Rostrally near the pontomesencephalic junction, the area designated the superior lateral subnucleus in the hamster was labeled, while sparser label was present more dorsally. Contralateral to the injections, caudal and intermediate levels of the peribrachial area contained only scant reaction product. However, the rostral area of the superior lateral subnucleus was labeled densely via fibers ascending in the trigeminothalamic tract. Injections made just rostral to the obex and either centered in or including the dorsal or ventral paratrigeminal nuclei produced similar labeling at caudal and intermediate levels of the peribrachial area. An exception, however, was that the caudal medial parabrachial nucleus was also labeled after the dorsal paratrigeminal injection. Also, only scant label was found in the rostral third of the dorsolateral pons on either side after these injections. Both trigeminothalamic and trigeminolemniscal pathways were labeled contralaterally after these injections. These trigeminal projections to the dorsolateral pons were compared to the projections from the nucleus tractus solitarii and the ventrolateral medulla. Numerous trigeminal neurons were labeled retrogradely after injections of wheat germ agglutinin-horseradish peroxidase into the dorsolateral pons. In the medullary dorsal horn, they were found almost exclusively in laminae I and V. Labeled neurons in lamina I were especially prominent in rostral ventral levels of the medullary dorsal horn. Labeled cells in lamina I were continuous with others found in the displaced band of substantia gelatinosa at the interface of the subnucleus caudalis and subnucleus interpolaris, as well as with those found in the ventral and dorsal paratrigeminal nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

Gustatory and tactile stimulation of the posterior tongue activate overlapping but distinctive regions within the nucleus of the solitary tract

Both the gustatory and somatosensory systems provide necessary sensory input for the initiation and control of oromotor behaviors. Behavioral studies indicate that somatosensory input from the posterior tongue (PT) is important in initiating swallowing, whereas PT taste input is particularly important in gustatory rejection reflexes. However, there have been few studies of the central representation of PT gustatory or tactile responses. In the present study, electrophysiological multi-unit recording techniques were used to map the location of PT-mediated taste and tactile responses in the nucleus of the solitary tract (NST) of the rat. A stimulation technique that allows taste stimuli to be introduced directly and specifically into the papillae trenches was used to optimally activate PT taste receptors located within the circumvallate (CV) and foliate (FOL) papillae. The results demonstrated that non-PT responsive sites dominated the rostral half of the rostral division of NST (rNST), while PT-responsive sites dominated the caudal half. Some PT-responsive sites extended into the caudal NST. Both gustatory and tactile stimuli were effective at 28% of PT-responsive locations (taste-tactile sites), whereas at the remaining locations, only tactile stimulation was effective (tactile-only sites). Although these two types of PT-responsive sites exhibited some anatomical overlap, their distributions were distinctive, with taste-tactile sites restricted medially and the laterally located tactile-only sites offset caudally. On the other hand, responses arising from stimulation of the CV and FOL exhibited no anatomical organization, i.e., responses to stimulation of both papillae were coexistensive. On average, of the four tastants used (0.01 M Na saccharin, 0.3 M NaCl, 0.01 M quinine hydrochloride, 0.03 M HCl), HCl was the most effective stimulus for both the CV and FOL. The present results delimit the regions of the NST that provide a substrate for the gustatory and somatosensory limbs of PT-mediated oromotor reflexes.

Taste and tactile responsiveness of neurons in the posterior diencephalon of the channel catfish

Many teleosts possess an enlargement of the ventral diencephalon called the inferior lobe. In ostariophysine species (e.g., carps and catfishes), this region receives ascending fibers from the primary and secondary gustatory centers in the hindbrain. Extracellular unit activity was recorded from identified nuclei in the inferior lobe of the channel catfish to characterize taste and tactile responsiveness from the different nuclei associated with gustatory projections. Taste responses (to amino acids and nucleotides) were recorded from units in the nucleus centralis (nCLI), nucleus lobobulbaris (caudal portion--nLB, rostrolateral portion--rl nLB, and parvicellular portion--nLBp), and lateral thalamic nucleus (nLT), supporting the proposed gustatory role for these nuclei. Tactile responsiveness was distinct between different nuclei in the caudal inferior lobe. Units from the nCLI and nLB had lower spontaneous activity than those from other nuclei, and typically had receptive fields including the whole extraoral body surface, ipsilaterally. Units from the rl nLB and nLBp had receptive fields, often including both oral and extraoral surfaces, bilaterally, but rl nLB receptive fields typically included the whole body, while those from nLBp units were often restricted to the head and mouth. The apparent electrophysiological distinction between these nuclei, combined with their different connectivity patterns, suggest that the gustatory nuclei in the inferior lobe of channel catfish are involved in various different sensory processing mechanisms.

Hypothalamic melanin-concentrating hormone and alpha-neoendorphin-immunoreactive neurons project to the medial part of the rat parabrachial area

Neurons in the middle and posterior parts of the lateral hypothalamus project to the parabrachial area, and in particular to the gustatory relay-station located in the medial part of this area. In the present study we have examined some of the neuropeptide immunoreactivities of the lateral hypothalamus neurons that project to the gustatory region of the parabrachial area. By coupling retrograde transport and immunohistochemistry, we found that 50-60% of medial parabrachial area-projecting cells located in the juxta-capsular region of the posterior lateral hypothalamus are labeled by rat melanin-concentrating hormone antiserum, while 28% of the retrogradely labeled neurons located in the perifornical lateral hypothalamus are visualized with alpha-neoendorphin antiserum. Moreover, a large number of terminals distributed throughout the parabrachial nucleus are immunoreactive to melanin-concentrating hormone or alpha-neoendorphin antisera. These immunoreactivities are not co-localized within the same lateral hypothalamic neurons. The potential role of these peptidergic projections in the reward mechanisms elicited in the medial parabrachial area and in the control of palatability is discussed.

Efferent projections from the parabrachial nucleus demonstrated with the anterograde tracer Phaseolus vulgaris leucoagglutinin

Efferent projections from the parabrachial complex (PBN) were studied in the rat using the anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L). Projections to the hypothalamus (ventromedial, dorsomedial, paraventricular, and supraoptic nuclei) originate primarily in the lateral PBN (1PBN). The amygdalar central nucleus (ACE) receives strong projections from all parts of the PBN although the external 1PBN projects primarily to the lateral ACE. Whereas the projections to the lateral bed nucleus of the stria terminalis, median preoptic nucleus, diagonal band of Broca, and lateral preoptic area originate primarily from the 1PBN, those to the insular cortex arise from the medial PBN (mPBN). The mPBN projects to the ventral posteromedial thalamus and the 1PBN and mPBN project to the zona incerta. Descending projections from the mPBN and Kölliker-Fuse area target the commissural nucleus tractus solitarius (NTS); the mPBN projects to the more rostral NTS. Similarly, the caudal parvicellular reticular formation (RF) receives projections from the mPBN and 1PBN, whereas input to the rostral RF arises from the former. All compartments of the PBN project to the ventrolateral medulla, although the projections arising from the 1PBN are densest. Finally, the raphe nuclei and periaqueductal gray receive some projections from most PBN divisions. These pathways provide a potential means whereby autonomic information can be relayed through the PBN to other structures important in regulating autonomic functions.

Physiological and morphological characteristics of spinal neurons projecting to the parabrachial region of the cat

Neurons in the lumbosacral, superficial spinal dorsal horn in the cat were recorded extra- and intracellularly, using dorsal root stimulation as a search stimulus. Isolated neurons were tested for antidromic activation from the contra- and ipsilateral parabrachial region. Seventy-one nociceptive-specific neurons, 11 innocuous cooling neurons, and 8 multireceptive neurons were antidromically activated from the lateral parabrachial region. The receptive fields and response properties were typical of other lamina I and lamina II neurons, in that the receptive fields were usually discrete and relatively small, and the responses ranged from sluggish and decrementing to brisk and augmenting with afterdischarge. The conduction velocity to the parabrachial region averaged 3.7 m/sec for the nociceptive-specific neurons, 3.9 m/sec for the innocuous cooling neurons, and 13.5 m/sec for the multireceptive neurons. Intracellularly labeled neurons were mostly medium to large Waldeyer-like neurons in lamina I. Some had axon collaterals that distributed varicosities in laminae I, II, and V. These data indicate that a slowly conducting nociceptive-specific and thermoreceptive pathway exists between the superficial dorsal horn and the parabrachial region at the pontine-midbrain junction.

Convergence of oral and extraoral information in the superior secondary gustatory nucleus of the channel catfish

Neurons within the superior secondary gustatory nucleus (nGS) of the channel catfish were examined electrophysiologically for responses to mechanical and chemical stimulation of neural peripheral receptive fields (RFs). Of the 28 single units sampled, 18 had mechanosensory RFs on the extraoral epithelium, two had RFs within the oropharyngeal cavity, and eight had RFs that included both oral and extraoral surfaces. RF sizes varied from approximately 2 cm2 on the ipsilateral lips and barbels to the whole body surface, bilaterally. No obvious correlation existed between RF pattern and recording location within the nGS. Eight of the mechanosensory nGS units also responded to amino acid taste stimuli with thresholds from micromolar to millimolar concentrations. The convergence of oral and extraoral information within the nGS determined electrophysiologically was corroborated anatomically by HRP labeling experiments. Restricted HRP injections into each of the primary gustatory nuclei of the medulla, the vagal (VL) and facial (FL) lobes, labeled fibers that appeared to terminate diffusely throughout the nGS, and injections into different portions of the nGS retrogradely labeled cells in both the FL and VL. The present electrophysiological and neuroanatomical data distinguish the convergent gustatory representation within the nGS of the catfish from the highly specific somatotopic and viscerotopic sensory maps previously identified in the FL and VL, respectively.

Information processing in mammalian gustatory systems

The taste system has multiple functions that are carried by three cranial nerves. It is now apparent that these functions cannot be accommodated by a single coding mechanism for taste quality. A current view emphasizes the likely existence of coding channels activated by specific sets of receptors.

The sense of taste: neurobiology, aging, and medication effects

The sense of taste is an oral chemical sense in mammals that is involved in the choice of foods. Initial transduction of taste stimuli occurs in taste buds, which are distributed in four discrete fields in the oral cavity. Medications can affect the taste buds and ion channels in taste-bud cell membranes involved in stimulus transduction. The sense of taste gradually declines with aging, with bitter taste most affected. Neural circuits that mediate taste in primates include cranial nerves VII, IX, and X, the solitary nucleus in the brain stem, the ventroposteromedial nucleus of the thalamus, and the insular-opercular cortex. The central taste pathways process taste information about sweet, salty, sour, and bitter stimuli serially and in parallel. Medications associated with "metallic" dysgeusia and taste losses affect the taste system via unknown mechanisms.