The role of the external lateral parabrachial subnucleus in flavor preferences induced by predigested food administered intragastrically

Danger and distress: Parabrachial-extended amygdala circuits

Our understanding of the role of the parabrachial nucleus (PBN) has evolved as technology has advanced, in part due to cell-specific studies and complex behavioral assays. This is reflected in the heterogeneous neuronal populations within the PBN to the extended amygdala (EA) circuits which encompass the bed nucleus of the stria terminalis (BNST) and central amygdala (CeA) circuitry, as they differentially modulate aspects of behavior in response to diverse threat-like contexts necessary for survival. Here we review how the PBN→CeA and PBN→BNST pathways differentially modulate fear-like behavior, innate and conditioned, through unique changes in neurotransmission in response to stress-inducing contexts. Furthermore, we hypothesize how in specific instances the PBN→CeA and PBN→BNST circuits are redundant and in part intertwined with their respective reciprocal projections. By deconstructing the interoceptive and exteroceptive components of affect- and stress related behavioral paradigms, evidence suggests that the PBN→CeA circuit modulates innate response to physical stimuli and fear conditioning. Conversely, the PBN→BNST circuit modulates distress-like stress in unpredictable contexts. Thereby, the PBN provides a pathway for alarming interoceptive and exteroceptive stimuli to be processed and relayed to the EA to induce stress-relevant affect. Additionally, we provide a framework for future studies to detail the cell-type specific intricacies of PBN→EA circuits in mediating behavioral responses to threats, and the relevance of the PBN in drug-use as it relates to threat and negative reinforcement. This article is part of the special Issue on 'Neurocircuitry Modulating Drug and Alcohol Abuse'.

Place preferences induced by electrical stimulation of the external lateral parabrachial subnucleus in a sequential learning task: Place preferences induced by NLPBe stimulation

It is known that electrical stimulation of the external lateral parabrachial nucleus (NLPBe) can sustain concurrent taste and place learning. Place preferences can be learned through different procedures. Previous studies demonstrated that electrical stimulation of the PBNLe can generate aversive and preference place learning using concurrent procedures. In the concurrent procedure, the animals can move freely in the maze, and intracranial electrical stimulation is associated with their voluntary stay in one of the two maze compartments. However, the rewarding properties of most stimuli, whether natural or drugs of abuse, have usually been investigated using the sequential procedure, in which animals are confined while receiving the unconditioned stimulus and then undergo a choice test without stimulation in a later phase. This study examined whether this stimulation can sustain place preference learning in sequential tasks. Results demonstrated that place preferences can also be induced by the electrical stimulation of the NLBe using sequential procedures. These findings suggest that the NLPBe may form part of a brain reward axis that shares certain characteristics with those observed in the processing of natural rewarding agents and especially of drugs of abuse.

Differential rewarding effects of electrical stimulation of the lateral hypothalamus and parabrachial complex: Functional characterization and the relevance of opioid systems and dopamine

BACKGROUND

Since the discovery of rewarding intracranial self-stimulation by Olds and Milner, extensive data have been published on the biological basis of reward. Although participation of the mesolimbic dopaminergic system is well documented, its precise role has not been fully elucidated, and some authors have proposed the involvement of other neural systems in processing specific aspects of reinforced behaviour.

AIMS AND METHODS

We reviewed published data, including our own findings, on the rewarding effects induced by electrical stimulation of the lateral hypothalamus (LH) and of the external lateral parabrachial area (LPBe) - a brainstem region involved in processing the rewarding properties of natural and artificial substances - and compared its functional characteristics as observed in operant and non-operant behavioural procedures.

RESULTS

Brain circuits involved in the induction of preferences for stimuli associated with electrical stimulation of the LBPe appear to functionally and neurochemically differ from those activated by electrical stimulation of the LH.

INTERPRETATION

We discuss the possible involvement of the LPBe in processing emotional-affective aspects of the brain reward system.

The Functional and Neurobiological Properties of Bad Taste

The gustatory system serves as a critical line of defense against ingesting harmful substances. Technological advances have fostered the characterization of peripheral receptors and have created opportunities for more selective manipulations of the nervous system, yet the neurobiological mechanisms underlying taste-based avoidance and aversion remain poorly understood. One conceptual obstacle stems from a lack of recognition that taste signals subserve several behavioral and physiological functions which likely engage partially segregated neural circuits. Moreover, although the gustatory system evolved to respond expediently to broad classes of biologically relevant chemicals, innate repertoires are often not in register with the actual consequences of a food. The mammalian brain exhibits tremendous flexibility; responses to taste can be modified in a specific manner according to bodily needs and the learned consequences of ingestion. Therefore, experimental strategies that distinguish between the functional properties of various taste-guided behaviors and link them to specific neural circuits need to be applied. Given the close relationship between the gustatory and visceroceptive systems, a full reckoning of the neural architecture of bad taste requires an understanding of how these respective sensory signals are integrated in the brain.

Relevance of the nucleus of the solitary tract, gelatinous part, in learned preferences induced by intragastric nutrient administration

Food preferences have been investigated in Wistar rats utilizing a learned concurrent flavor preference behavioral procedure. Previous studies have demonstrated that the perivagal administration of neurotoxin capsaicin disrupts the learning of preferences induced by intragastric administration of rewarding nutrients (pre-digested milk). The vagus nerve projects almost exclusively towards the nucleus of the solitary tract (NST), a brain medullary gateway for visceral signals. The objective of this study was to investigate the participation of the lateral portion of the dorsomedial region, the gelatinous subnucleus (SolG), in the learning of a concurrent preference task. Results show that unlike neurologically intact animals, which learn this task correctly, animals lesioned in the gelatinous part of NST manifest a disruption of discrimination learning. Thus, intakes of the flavored stimulus paired with predigested liquid diet and of the flavored stimulus paired with physiological saline were virtually identical. However, SolG- and sham-lesioned groups consumed similar total amounts of both flavors. These findings suggest that SolG, as a relay of the vagus nerve, along with its anatomical projection, the external lateral parabrachial subnucleus (LPBe), may constitute an anatomical axis that is important in the induction of concurrent flavor/side preferences. It also appears to be relevant in other behavioral processes that require rapid processing of information from the upper gastrointestinal tract.

Disruption of re-intake after partial withdrawal of gastric food contents in rats lesioned in the gelatinous part of the nucleus of the solitary tract

Sensory information from the upper gastrointestinal tract is critical in food intake regulation. Signals from different levels of the digestive system are processed to the brain, among other systems, via the vagus nerve, which mainly projects towards the nucleus of the solitary tract (NST). The objective of this study was to analyze the participation of the gelatinous part (SolG) of the NST in short-term food intake. One-third of the stomach food content was withdrawn at 5 min after the end of a meal, and food was then available ad libitum for different time periods. SolG-lesioned and control animals ingested a similar amount of the initial liquid meal, but the former consumed significantly smaller amounts and failed to compensate for the food deficit, whereas the controls re-ingested virtually the same amount as extracted. These data suggest that the SolG, as in the case of related anatomical structures such as the vagus nerve or external lateral parabrachial subnucleus, may be relevant in particular circumstances that require the rapid processing of vagal-related food intake adjustment associated to the upper gastrointestinal tract.

Tolerance to repeated rewarding electrical stimulation of the parabrachial complex

The parabrachial complex has been related to various rewarding behavioral processes. As previously shown, electrical stimulation of the lateral parabrachial external (LPBe) subnucleus induces opiate-dependent concurrent place preference. In this study, two groups of animals (and their respective controls) were subjected to sessions of rewarding brain stimulation daily or on alternate days. The rats stimulated every other day maintained a consistent preference for the place associated with the brain stimulation. However, as also found in the Insular Cortex, there was a progressive decay in the initial place preference of animals receiving daily stimulation. These data suggest that the rewarding effects induced by electrical stimulation of LPBe subnucleus may be subject to tolerance. These findings are discussed with respect to other anatomical areas showing reward decay and to the reinforcing effects induced by various electrical and chemical rewarding agents.

Differential effects of naloxone on rewarding electrical stimulation of the central nucleus of the amygdala and parabrachial complex in a place preference study

The central nucleus of the amygdala (CeA) is considered to be involved in different affective, sensory, regulatory, and acquisition processes. This study analyzed whether electrical stimulation of the PB-CeA system induces preferences in a concurrent place preference (cPP) task, as observed after stimulation of the parabrachial-insular cortex (PB-IC) axis. It also examined whether the rewarding effects are naloxone-dependent. The results show that electrical stimulation of the CeA and external lateral parabrachial subnucleus (LPBe) induces consistent preference behaviors in a cPP task. However, subcutaneous administration of an opiate antagonist (naloxone; 4mg/ml/kg) blocked the rewarding effect of the parabrachial stimulation but not that of the amygdala stimulation. These results are interpreted in the context of multiple brain reward systems that appear to differ both anatomically and neurochemically, notably with respect to the opiate system.

Rewarding effects of the electrical stimulation of the parabrachial complex: taste or place preference?

The lateral parabrachial complex has been related to various emotional-affective processes. It has been shown that electrical stimulation of the external Lateral Parabrachial (LPBe) nucleus can induce reinforcing effects in place preference and taste discrimination tasks but does not appear to support self-stimulation. This study examined the relative relevance of place and taste stimuli after electrical stimulation of the LPBe nucleus. A learning discrimination task was conducted that simultaneously included both sensory indexes (taste and place) in order to determine the preference of animals for one or the other. After a taste stimulus reversal task, the rewarding effect of stimulation was found to be preferentially associated with place. These results are discussed in the context of the rewarding action and biological constraints induced by different natural and artificial reinforcing agents.

Effects of intragastric administration of predigested nutrients on food intake, body weight and taste acceptability: Potential relevance of the cephalic/neural phase of digestion

In this study we analyzed the effect of the intragastric administration of partially digested and natural nutrients on subsequent food intake, body weight and flavor acceptability in rats. The results showed that enterally administered natural nutrients reduced the subsequent ingestion of food to a greater degree compared with the same nutrients in partially digested form. This greater reduction does not appear to be due to a higher nutritional effect of the former, because the body weight of both groups of animals was similar. Animals intragastrically administered with partially digested nutrients developed an acceptance response to a previously paired flavored stimulus, in contrast to animals receiving natural nutrients under the same conditions. These results are interpreted in terms of the cephalic phase of digestion and may be relevant to the treatment of clinical symptoms associated with enteral feeding.

Lesions of the lateral parabrachial area block the aversive component and induced-flavor preference for the delayed intragastric administration of nutrients in rats: Effects on subsequent food and water intake

The aim of this study was to examine the function of the lateral parabrachial area (LPB) in relation to the intragastric administration of nutrients. The consumption of flavors associated with intragastric nutrient administration and the subsequent food and water intake were measured in rats with lesions in the LPB. The results showed that bilateral LPB lesions prevented development of aversions and induced flavor preference when there was a delay between the presentation of a flavor and the intragastric administration of nutrients. However, these lesions did not disrupt development of the aversive process when there was no delay between the presentations. Likewise, the LPB lesions increased subsequent food intake when there was a delay but not when there was no delay between the presentations. In contrast, the water intake was reduced in both situations. These results are interpreted in terms of a dual visceral system for processing the intragastric effects of foods.

Gustatory and homeostatic functions of the rodent parabrachial nucleus

Previous studies demonstrate that lesions to the rodent parabrachial nucleus (PBN) disrupt the formation of gustatory-postingestive associations, while preserving gustatory and viscerosensory functions. This suggests that the rodent PBN functions essentially as an integrative circuit, supporting the conditioning of tastants to postingestive factors. In the case of primates, however, anatomical studies have failed to demonstrate gustatory projections from medullary nuclei to PBN. It should therefore be inferred that the primate PBN lacks the associative functions assigned to its rodent counterpart. Moreover, the ability of rodent midbrain dopaminergic systems to respond to the activation of palatable tastants depends on the integrity of the gustatory PBN. However, recent studies demonstrate that caloric palatable compounds do not require taste signaling to produce elevated brain dopamine levels. This raises the possibility that, in rodents, PBN neurons are important for the detection of postingestive effects of nutrients that occur independently of gustatory input. If confirmed, such function would assign non-associative roles to the rodent PBN, approximating its functional organization to its primate counterpart. We are currently testing this possibility by monitoring the behavioral responses to caloric glucose solutions in sweet-blind mice having sustained bilateral lesions to the PBN. Preliminary results indicate that the rodent PBN regulates nutrient intake even when no gustatory inputs are involved. This favors the assignment of non-gustatory, homeostatic functions to the rodent PBN during feeding, a concept that brings an additional perspective on the rodent versus primate functional discrepancy associated with the anatomy of this pontine nucleus.

Cocaine- and amphetamine-regulated transcript peptide immunoreactivity in the brain of the CCK-1 receptor deficient obese OLETF rat

Cocaine- and amphetamine-regulated transcript (CART) peptide is expressed in brain areas involved in homeostatic regulation and reward. CART has been shown to reduce food intake, but the underlying mechanisms and the relevance of this effect on obesity yet remain unknown. Therefore, we used immunohistochemistry to investigate the expression of CART peptide in various brain regions of the obese Otsuka Long Evans Tokushima Fatty (OLETF) rats lacking the CCK-1 receptor. Analysis revealed that whereas the distribution of CART-peptide immunoreactive neurons and axonal networks was identical in OLETF rats and lean controls, the intensity of CART immunoreactivity was significantly reduced in the rostral part of the nucleus accumbens (p < 0.01), the basolateral complex of the amygdala (p < 0.05) and the rostro-medial nucleus of the solitary tract (p < 0.001) of the OLETF rats. These areas are involved in reward and integration of taste and viscerosensory information and have been previously associated with altered functions in this strain. The findings suggest that in addition to previously described deficits in peripheral satiety signals and augmented orexigenic regulation, the anorectic effect of CART peptide may also be diminished in OLETF rats.

Rapid acquisition of conditioned flavor preferences in rats

Rats learn to prefer flavors paired with the post-oral effects of glucose. The present study examined how rapidly they acquire this preference. In Experiment 1, food-restricted rats were given repeated three-session training/testing cycles: one 30-min session with a CS+ flavor paired with intragastric (IG) infusion of 16% glucose, another session with a CS- flavor paired with IG water, and a third session with a choice between the flavors with their infusates. The rats preferred the CS+ (69%) in the first choice session, and preference increased across the six cycles to 86%. These data demonstrate that the post-oral reinforcing action of glucose is potent enough to support one-trial learning. In Experiment 2, two groups of rats were trained in the same way, with the CS+ flavor paired with IG infusion of 16% glucose or 7.1% corn oil emulsion, but tests were conducted under extinction conditions, with both CS+ and CS- flavors paired with IG water. Significant preference for the CS+ was acquired more rapidly with glucose (71% CS+ in test 1) than with oil (69% CS+ in test 4). Consistent with previous work, the post-oral stimulation by glucose was more potent than that of isocaloric oil emulsion in conditioning preferences. The last experiment examined the acquisition rate for a flavor-taste conditioned preference. Rats were trained with a CS+ flavor mixed into an 8% fructose + 0.2% saccharin solution and a CS- flavor in 0.2% saccharin. The same three-session training/testing cycles were used, and in the tests the flavors were presented in saccharin. A significant 74% preference for the CS+ flavor was apparent by the second test. Together these studies show that the acquisition of flavor preferences, whether based on flavor-taste or flavor-nutrient associations, can be quite rapid.

Learned flavor preferences induced by intragastric administration of rewarding nutrients: role of capsaicin-sensitive vagal afferent fibers

Learned flavor preferences can be established after intragastric nutrient administration by two different behavioral procedures, concurrent and sequential. In a concurrent procedure, two flavored stimuli are offered separately but at the same time on a daily basis: one stimulus is paired with the simultaneous intragastric administration of partially digested food and the other with physiological saline. In sequential learning, the two stimuli are presented during alternate sessions. Neural mechanisms underlying these learning modalities have yet to be fully elucidated. The aim of this study was to examine the role of vagal afferent fibers in the visceral processing of rewarding nutrients during concurrent (experiment 1) and sequential (experiment 2) flavor preference learning in Wistar rats. For this purpose, capsaicin, a neurotoxin that destroys slightly myelinated or unmyelinated sensory axons, was applied to the subdiaphragmatic region of the esophagus to selectively damage most of the vagal afferent pathways that originate in the gastrointestinal system. Results showed that capsaicin [1 mg of capsaicin dissolved in 1 ml of vehicle (10% Tween 80 in oil)] blocked acquisition of concurrent but not sequential flavor preference learning. These results are interpreted in terms of a dual neurobiological system involved in processing the rewarding effects of intragastrically administered nutrients. The vagus nerve, specifically capsaicin-sensitive vagal afferent fibers, would only be essential in concurrent flavor preference learning, which requires rapid processing of visceral information.

Learned preferences induced by electrical stimulation of a food-related area of the parabrachial complex: Effects of naloxone

Electrical stimulation of the External Lateral Parabrachial Subnucleus (LPBe), a food-related area, induced behavioral preferences for associated stimuli in a taste discrimination learning task. Although this stimulation appeared to be ineffective to elicit standard lever press self-stimulation, it induced place preference for one of two training compartments of a rectangular maze in which animals (adult male Wistar rats) received concurrent electrical brain stimulation. In subjects that consistently showed a preference behavior in different trials, administration of the opioid antagonist naloxone (4 mg/ml/kg) blocked concurrent learning when the test was made in a new maze but not in the same maze in which animals had learned the task. These results are discussed in terms of the possible participation of the LPBe subnucleus in different natural and artificial brain reward systems.

An orexigenic role for mu-opioid receptors in the lateral parabrachial nucleus

The pontine parabrachial nucleus (PBN) has been implicated in regulating ingestion and contains opioids that promote feeding elsewhere in the brain. We tested the actions of the selective mu-opioid receptor (mu-OR) agonist [d-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO) in the PBN on feeding in male rats with free access to food. Infusing DAMGO (0.5-4.0 nmol/0.5 microl) into the lateral parabrachial region (LPBN) increased food intake. The hyperphagic effect was anatomically specific to infusions within the LPBN, dose and time related, and selective for ingestion of chow compared with (nonnutritive) kaolin. The nonselective opioid antagonist naloxone (0.1-10.0 nmol intra-PBN) antagonized DAMGO-induced feeding, with complete blockade by 1.0 nmol and no effect on baseline. The highly selective mu-opioid antagonist d-Phe-Cys-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; 1.0 nmol) also prevented this action of DAMGO, but the kappa-antagonist nor-binaltorphimine did not. Naloxone and CTAP (10.0 nmol) decreased intake during scheduled feeding. Thus stimulating mu-ORs in the LPBN increases feeding, whereas antagonizing these sites inhibits feeding. Together, our results implicate mu-ORs in the LPBN in the normal regulation of food intake.

Lateral parabrachial lesions impair lithium chloride-induced aversive responses but not saccharin-induced flavor preference

Behavioral taste-guided experiments, as well as molecular studies employing c-FLI expression in response to aversive/appetitive unconditioned stimulus, have strongly suggested a visceral role for the lateral parabrachial subnuclei (lPB). The main objective in the present study was to further evaluate the functional role of the lPB in lithium chloride-induced behavioral/physiological responses. We employed a lesion/behavioral experimental strategy combining a lithium chloride-induced place aversion procedure together with the simultaneous evaluation of behavioral ("Lying on Belly", "LOB") and physiological (body temperature) responses elicited by the toxin. Data showed that lPB-lesioned animals failed to avoid the chamber previously paired with lithium chloride. Moreover, "LOB", and not hypothermia, in response to lithium chloride was impaired in parabrachial lesioned animals. Finally, all the animals were tested in a free discriminative flavor-preference task induced by saccharin, a non-caloric reinforcer, which precludes visceral feedback as essential in acquiring the learned response. As expected, both control and lesioned animals developed a clear flavor-preference to the flavor previously paired with saccharin, which shows normal gustatory and associative processing in lPB-lesioned animals. This study extends previous results on the functional visceral role of lPB subnuclei by providing alternative behavioral evidence other than taste-guided behavior, that the lPB is pivotal in visceral processing. Present data are discussed in the context of the visceral hypothesis that holds that the lPB is critically involved in processing post-oral visceral feedback.