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.     

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

The locations of taste-responsive areas within the brainstem parabrachial nucleus (PBN), an obligatory taste relay in the golden hamster (Mesocricetus auratus), were mapped in relation to cytoarchitectural boundaries. The PBN was systematically searched for multiunit neural activity in response to a taste mixture composed of 0.1 M sucrose, 0.03 M NaCl, and 0.1 M KCl applied to the anterior tongue. Taste responses were located exclusively in one of three subdivisions of the medial PBN, which is thought to be specialized for gustatory processing, and in one of six subdivisions of the lateral PBN, which is thought to be specialized for general visceral processing. Based on Nissl-stained material, both the medial and lateral PBN subdivisions in the hamster were similar to those reported for the rat PBN. The largest group of taste-responsive cells encompassed two-thirds of the central medial subdivision, while a smaller group of taste cells was exclusively located within the ventral lateral subdivision. The two taste-responsive subdivisions are separated by the superior cerebellar peduncle and contain diverse cell types. The finding that anterior tongue taste may be exclusively represented in circumscribed cytoarchitecturally defined parts of two PBN divisions suggests that taste information from the anterior tongue is required for both specific gustatory and general visceral functions.     

Lesions of the lateral parabrachial nuclei disrupt aversion learning induced by electrical stimulation of the area postrema

The research about the neural basis of taste aversion learning (TAL) has pointed out the area postrema (AP) as a fundamental structure implied in the processing of certain toxic stimuli. Likewise, recent studies demonstrated that electric stimulation of the AP is an efficient substitute of the aversive stimulus. The lateral parabrachial nucleus (PBN1), one of the subnuclei of the parabrachial complex, is the main anatomic rostral connection of the AP. In the experiment presented here, we demonstrate that TAL induced by electric stimulation of the AP is interrupted when the PBN1 is lesioned, thus giving support to the functional role of this anatomic system (AP-PBNI) in the codification of aversive stimuli processed by the AP.     

Induction of a brainstem correlate of conditioned taste aversion expression: role of the pontine parabrachial nucleus

Increases in Fos-like immunoreactivity (FLI) in the intermediate division of the nucleus of the solitary tract (iNTS) are seen following the expression of a conditioned taste aversion (CTA). In studies limited to behavioral assessment, the pontine parabrachial nucleus (PBN) has been demonstrated to play a critical role in the acquisition, but not the expression, of CTAs. To better define the role of the PBN in taste aversion learning, the present study examined the effects of PBN lesions on FLI in iNTS in animals with lesions placed either before or after CTA training. As is the case with behavioral expression of a CTA, timing of PBN lesions was found to be critical. Lesions placed prior to conditioning blocked evidence of conditioning, including both taste rejection and FLI in iNTS. Lesions placed after conditioning, but before testing, did not interfere with either taste rejection or FLI. These results support and extend prior claims that PBN is critical for CTA acquisition but not expression. They also demonstrate that input from PBN to iNTS is not necessary for the FLI seen there during CTA expression.     

C-fos expression in the rat brain after intraperitoneal injection of lithium chloride.

The distribution of evoked expression of the proto-oncogene c-fos was immunohistochemically examined in the rat brain after intraperitoneal injection of isotonic LiCl, which is commonly used to induce internal malaise in the conditioned taste aversion paradigm. C-fos-like immunoreactive neurones (c-fos neurones) were most densely observed in the central amygdaloid nucleus, external lateral subnucleus of the parabrachial nucleus (PBN), posteromedial and commissural parts of the nucleus of the tractus solitarius (NTS) and area postrema (AP). Experiments including vagotomy, intravenous injection of LiCl and lesions of the area postrema suggest that NTS neurones are activated via both sides of the vagus nerves, while AP neurones, humorally as well as neurally via the vagal nerve with a right side predominance. The activated NTS and AP neurones project mainly to the external lateral subnucleus of the PBN and lightly to the central lateral subnucleus of the PBN. These results are discussed in terms of the role of LiCl in the formation of conditioned taste aversion.     

Convergence of vagal and gustatory afferent input within the parabrachial nucleus of the rat

Our previous anatomical and electrophysiological studies demonstrated that first-order hepatic and gustatory afferents project to separate regions of the solitary nucleus (NST) and no intra-NST interaction of these two sensory systems could be demonstrated. However, iontophoretic injections of horseradish peroxidase into physiologically identified zones of the NST revealed that both of these regions send overlapping projections to the immediately subjacent parvocellular reticular formation as well as the postero-medial parabrachial nucleus (PBN). The present electrophysiological studies demonstrate that an interstitial zone of neurons in the caudal, medial PBN, indeed, receive convergent input from second-order gustatory and vagal afferents. Co-activation of these PBN units by the simultaneous arrival of both input sources frequently resulted in an additive interaction of evoked activity. PBN units lateral and caudal to this zone responded to vagal stimulation only, while units in the anterior and extreme medial portion of the PBN only responded to gustatory stimulation. By virtue of the efferent projections of the PBN, one might speculate that the convergence of information at this locus may, eventually, play a role in directing long term feeding behavior patterns such as learned taste aversion as well as the more transient changes in taste preference with visceral loading.     

Establishing aversive, but not safe, taste memories requires lateralized pontine-cortical connections

Aversive and safe taste memory processing is dramatically disrupted by bilateral lesions of the pontine parabrachial nucleus (PBN). To determine how such lesions affect patterns of neuronal activation in forebrain, lesions were combined with assessment of cFos-like immunoreactivity (FLI) in insular cortex (IC) and amygdala after conditioned taste aversion (CTA) training. Increases in FLI in amygdala and IC, which are normally seen following novel (versus familiar) CS-US pairing, were eliminated after PBN lesions. This suggests that PBN lesions prevent transmission of critical CS and US information to forebrain regions for the processing of both aversive and safe taste memories. Unilateral asymmetrical lesions of PBN and IC blocked CTA acquisition as well as normal patterns of FLI in amygdala after novel CS-US pairing, an effect not seen when unilateral lesions were confined to a single hemisphere. The crossed-disconnection experiments provide compelling evidence that functional interactions between PBN and IC are required for CTA acquisition, but not for safe taste memory formation and retrieval. The dissociation between effects of the different types of lesions on safe and aversive taste memories supports emerging evidence that the neural underpinnings of the two types of taste learning differ.     

Localization of parabrachial nucleus neurons projecting to the thalamus or the amygdala in the cat using horseradish peroxidase.

Topographical localization of parabrachial nucleus (PBN) neurons projecting directly to the thalamus or the amygdala was examined in the cat by the horseradish peroxidase (HRP) method. After HRP injection in the central nucleus of the amygdala, PBN neurons labeled with the enzyme were seen ipsilaterally in the ventral portion of the lateral PBN as well as in the medial PBN. When the HRP injections were centered on the parvocellular portion of the posteromedial ventral nucleus of the thalamus (VPMpc), HRP-labeled neurons were observed ipsilaterally in the dorsal portion of the lateral PBN as well as in the medial PBN. Within the medial PBN, the distribution of neurons projecting to the amygdala overlapped that of neurons projecting to VPMpc; the cell bodies of the former neurons, however, tended to be more elongated than the latter, and the mean of the average soma diameters of the former was significantly larger than the latter. On the other hand, in the lateral PBN no significant differences were noted between the means of the average soma diameters of neurons projecting to VPMpc and those projecting to the amygdala. The PBN neurons in the cat were presumed to transmit gustatory and general visceral information ipsilaterally to the thalamic taste region and the limbic areas in the basal forebrain.     

cFos induction during conditioned taste aversion expression varies with aversion strength

Fos-like immunoreactivity (FLI) can indicate the location of neurons activated following expression of conditioned taste aversion (CTA). After one conditioning trial FLI has been identified in the intermediate nucleus of the solitary tract (iNTS) with little expression in other brain regions. The present study assessed the effect of increasing aversion strength on the magnitude and anatomical distribution of FLI during CTA expression. When animals received three rather than one conditioning trial, significant FLI was seen not only in the iNTS but also in the parabrachial nucleus (PBN), and the central nucleus of the amygdala (CNA), regions thought to be important in taste aversion learning.     

Organization of parabrachial nucleus efferents to the thalamus and amygdala in the golden hamster.

While gustation in the hamster has been extensively studied at the behavioral and physiological level, very little is known about the central anatomy of the taste system. The purpose of this study was to trace the connections of the parabrachial nucleus (PBN) in the golden Syrian hamster (Mesocricetus auratus) using wheat germ agglutinin-conjugated horseradish peroxidase. The PBN is the site of the second central synapse for the ascending gustatory system and receives taste afferents from the nucleus of the solitary tract. Following large injections into the PBN, anterogradely transported label was seen in the lateral hypothalamus, dorsal thalamus, bed nucleus of the stria terminalis, and amygdala. The anatomy of the two primary targets, the ventral posteromedial thalamus and central nucleus of the amygdala, is described based on Nissl-stained material, and acetylcholinesterase and NADH dehydrogenase histochemistry. Injections into these two regions revealed different patterns of efferents within the PBN. Following injections into the thalamus, retrogradely labelled cell bodies were distributed throughout the PBN subdivisions bilaterally, but concentrated in the central medial (CM) and external lateral (EL) subdivisions. Following injections into the amygdala, retrogradely labelled cell bodies were primarily in the ipsilateral PBN EL, while anterogradely transported label was distributed throughout much of the ipsilateral PBN. The majority of CM efferents projecting to the thalamus were elongate cells, whereas the majority of CM efferents to the amygdala were round-oval cells. These results indicate that the ascending central gustatory system changes from a serial pathway (nucleus of the solitary tract-PBN) to a parallel organization consisting of two major projections, the parabrachio-thalamo-cortical and parabrachio-amygdaloid pathways.     

Conditioned taste aversion and amygdala lesions in the rat: a critical review

Studies using permanent lesions implicate the amygdala, a recipient of gustatory and viscerosensory information, in taste aversion learning. Reviewing this literature with respect to the location of the lesions and the quality of the behavioral methodology reveals little, if any, involvement of the medial amygdala or central nucleus in conditioned taste aversion. Although a disruption is found following damage to the basolateral region, the attenuated conditioned taste aversion appears to be a consequence of a lesion-induced impairment of neophobia rather than an association formation deficit. The key to understanding the functional significance of the basolateral amygdala in conditioned taste aversion reduces, we believe, to determining the role of this structure in gustatory neophobia.     

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.     

Electron microscopy of classically stained astrocytes

The ascending connections of the lateral lemniscus were studied in the cat and squirrel monkey (Saimiri sciureus). In both species, the central nucleus of the inferior colliculus receives a massive projection from the lateral lemniscus. Only a few lemniscal fibers were found to terminate in the external nucleus of the inferior colliculus. The commissure of the lateral lemniscus originates from the dorsal nucleus of the lateral lemniscus and projects to the contralateral dorsal nucleus and the contralateral central nucleus of the inferior colliculus. No lemniscal fibers were seen ascending in the inferior brachium or terminating in the principal division of the medial geniculate body. A bundle of fibers was observed, however, which passed medial to the inferior brachium and terminated in the magnocellular or internal division of the medial geniculate. The bundle degenerated after lesions confined to the lateral lemniscus and is probably identical with the central acoustic tract of earlier workers. Evidence is presented that the fibers of the bundle are of spinal origin. Since the lemniscal fibers which ascend to the thalamus appear to be non-auditory, it is suggested that the inferior colliculus is an obligatory relay station in the classical auditory system and that the inferior brachium is the only ascending pathway of the system to project upon the thalamus.     

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.     

Organization of taste-evoked activity in the hamster parabrachial nucleus.

Small groups of 3-4 neighboring taste neurons in the hamster parabrachial nucleus (PBN) were either generalists (42%) or specialists (58%). Generalists responded well to sucrose, NaCl, KCl, and their mixture but specialists responded to only one or two stimuli. PBN temporal response patterns resembled chorda tympani patterns, indicating that they were preserved across two synapses. Generalist neuron-groups were distributed throughout the central medial (CM) subdivision of PBN. Sucrose-sensitive neuron-groups, which responded best to mixtures, were restricted to rostral CM, whereas most NaCl-best neuron groups were in caudal CM. These results suggest a multifaceted functional organization of taste sensibilities in the PBN.     

Electrical intracerebral stimulation of the area postrema on taste aversion learning

The structural characteristics of the area postrema, its anatomical connections, participation in the detection of emesis-provoking substances and the effects of area postrema lesions on taste aversion learning acquisition, are all factors which speak in favor of a role as a chemoreceptor zone involved in the detection of aversive agents which act as effective inducers of taste aversion learning. The feasibility of substituting electrical intracerebral stimulation of the area postrema for the aversive stimulus was investigated in a taste aversion learning paradigm. In Expt. 1, 0.1-ms rectangular pulses of 50 Hz, delivered intermittently or continously for 4 h after a 15-min delay following ingestion of the gustatory stimulus, produced reliable learning. Expt. 2 showed the learning thus induced to reflect all the characteristics features attributed to taste aversion learning: one-trial learning, long interstimulus delay and cue-consequence specificity. These results suggest that the area postrema could participate in the detection of the aversive consequences of particular taste aversion learning-inducing agents.     

Morphine- and cocaine-induced c-Fos levels in Lewis and Fischer rat strains

Lewis (LEW) and Fischer 344 (F344) rat strains have been reported to differ in their sensitivity to the rewarding and aversive effects of both cocaine and morphine. Specifically, LEW rats self-administer morphine and cocaine to a greater extent than F344 rats, while LEW (compared to F344) rats are more sensitive to the aversive effects of cocaine but less sensitive to the aversive effects of morphine. Consistent with assessments of the rewarding effects of morphine and cocaine in these two strains, LEW rats have lower basal, and generally higher drug-induced, activity in brain regions associated with reward. Although the brain areas that mediate the aversive effects of drugs are becoming better defined, no studies have compared the activation of these areas by aversion-inducing drugs in the LEW and F344 strains. As such, the relationship between the ability of drugs to activate these aversion-associated brain areas and to induce a conditioned taste aversion (CTA) in these strains is unknown. To explore this relationship, LEW and F344 rats were injected with saline or doses of morphine or cocaine (32 mg/kg for both drugs) that have been shown to generate differential taste aversion learning in these strains. All animals were subsequently tested for c-Fos expression in areas of the brain associated with aversion learning (the lateral and medial parabrachial nucleus, intermediate and caudal nucleus tractus solitarius and area postrema), reward (the shell of the nucleus accumbens) and locomotion (the core of the nucleus accumbens and the caudate putamen). The present results indicated that patterns of morphine- and cocaine-induced c-Fos within CTA-associated, but not reward- or locomotor-associated, brain regions paralleled the differential behavioral sensitivities of LEW and F344 rats to these drugs within CTA learning. Analyses with other drugs that do and do not induce aversions differentially would further assess the role of these brain areas in aversion learning, in general, and in strain-dependent differences, in particular.     

Medial versus lateral parabrachial nucleus lesions in the rat: effects on cholecystokinin- and D-fenfluramine-induced anorexia

The two major components of the pontine parabrachial nucleus (PBN), the medial (gustatory) and lateral (visceral) subdivisions, have been implicated in a variety of ingestive behaviors. The present study examined the influence of bilateral ibotenic acid lesions of the medial or lateral PBN on the anorectic effects of two systemically administered drug treatments. In Experiment 1, 24-h food-deprived rats where injected with sulfated cholecystokinin (26-33) (CCK; 0, 4.0, or 8.0 microg/kg) and then given 60 min access to food. In Experiment 2, the influence of D-fenfluramine (DFEN; 0, 0.5, 1.0, or 2.0 mg/kg) on deprivation-induced feeding was examined in the same rats using the same behavioral procedure as in Experiment 1. Lesions of the lateral PBN abolished CCK-, but not DFEN-induced anorexia whereas lesions of the medial PBN augmented DFEN-, but had no influence on CCK-induced anorexia. The results suggest that the satiating effects of CCK and DFEN are mediated through different mechanisms involving, respectively, visceral and orosensory processing.     

Accessory optic system of rhesus monkey.

The accessory optic system of the rhesus monkey (Macaca mulatta) was investigated using the silver method of de Olmos-Ingram to determine the course and distribution of its degenerating fibers following retinal evisceration. Serial Niss 1 sections were used to relate the axonal degeneration to the brain stem cytoarchitecture. It is found that this system consists of a dorsal and a lateral terminal nucleus together with a superior fasciculus (posterior fibers). The retinal fibers within this superior fasciculus originate primarily from the contralateral and some from ipsilateral retina. These fibers leave the superior quadrigeminal brachium to course ventrally and anteriorly over the caudolateral aspects of the medial geniculate, the inferior brachium, and the dorsolateral portion of the cerebral peduncle to terminate within the dorsal and lateral terminal nuclei.