Increased c-fos expression in nucleus of the solitary tract correlated with conditioned taste aversion to sucrose in rats

The basolateral amygdala-medial prefrontal cortex circuitry regulates behavioral flexibility during appetitive reversal learning

Environmental cues can become predictors of food availability through Pavlovian conditioning. Two forebrain regions important in this associative learning are the basolateral amygdala (BLA) and medial prefrontal cortex (mPFC). Recent work showed the BLA-mPFC pathway is activated when a cue reliably signals food, suggesting the BLA informs the mPFC of the cue's value. The current study tested this hypothesis by altering the value of 2 food cues using reversal learning and illness-induced devaluation paradigms. Rats that received unilateral excitotoxic lesions of the BLA and mPFC contralaterally placed, along with ipsilateral and sham controls, underwent discriminative conditioning, followed by reversal learning and then devaluation. All groups successfully discriminated between 2 auditory stimuli that were followed by food delivery (conditional stimulus [CS] +) or not rewarded (CS-), demonstrating this learning does not require BLA-mPFC communication. When the outcomes of the stimuli were reversed, the rats with disconnected BLA-mPFC (contralateral condition) showed increased responding to the CSs, especially to the rCS + (original CS-) during the first session, suggesting impaired cue memory recall and behavioral inhibition compared to the other groups. For devaluation, all groups successfully learned conditioned taste aversion; however, there was no evidence of cue devaluation or differences between groups. Interestingly, at the end of testing, the nondevalued contralateral group was still responding more to the original CS + (rCS-) compared to the devalued contralateral group. These results suggest a potential role for BLA-mPFC communication in guiding appropriate responding during periods of behavioral flexibility when the outcomes, and thus the values, of learned cues are altered. (PsycINFO Database Record (c) 2020 APA, all rights reserved).

Fos activation patterns related to acute ethanol and conditioned taste aversion in adolescent and adult rats

Studies in rats have revealed marked age differences in sensitivity to the aversive properties of ethanol, with a developmental insensitivity to ethanol aversion that is most pronounced during pre- and early adolescence, declining thereafter to reach the enhanced aversive sensitivity of adults. The adolescent brain undergoes significant transitions throughout adolescence, including in regions linked with drug reward and aversion; however, it is unknown how ontogenetic changes within this reward/aversion circuitry contribute to developmental differences in aversive sensitivity. The current study examined early adolescent (postnatal day [P]28-30) and adult (P72-74) Sprague-Dawley male rats for conditioned taste aversion (CTA) after doses of 0, 1.0, or 2.5 g/kg ethanol, and patterns of neuronal activation in response to ethanol using Fos-like immunohistochemistry (Fos+) to uncover regions where age differences in activation are associated with ethanol aversion. An adolescent-specific ethanol-induced increase in Fos+ staining was seen within the nucleus accumbens shell and core. An age difference was also noted within the Edinger-Westphal nucleus (EW) following administration of the lower dose of ethanol, with 1 g/kg ethanol producing CTA in adults but not in adolescents and inducing a greater EW Fos response in adults than adolescents. Regression analysis revealed that greater numbers of Fos+ neurons within the EW and insula (Ins) were related to lower consumption of the conditioned stimulus (CS) on test day (reflecting greater CTA). Some regionally specific age differences in Fos+ were noted under baseline conditions, with adolescents displaying fewer Fos+ neurons than adults within the prelimbic (PrL) cortex, but more than adults in the bed nucleus of the stria terminalis (BNST). In the BNST (but not PrL), ethanol-induced increases in Fos-immunoreactivity (IR) were evident at both ages. Increased ethanol-induced activity within critical appetitive brain regions (NAc core and shell) supports a role for greater reward-related activation during adolescence, possibly along with attenuated responsiveness to ethanol in EW and Ins in the age-typical resistance of adolescents to the aversive properties of ethanol.

Genetic Access to Gustatory Disgust-Associated Neurons in the Interstitial Nucleus of the Posterior Limb of the Anterior Commissure in Male Mice

Orofacial and somatic disgust reactions are observed in rats following intraoral infusion of not only bitter quinine (innate disgust) but also sweet saccharin previously paired with illness (learned disgust). It remains unclear, however, whether these innate and learned disgust reactions share a common neural basis and which brain regions, if any, host it. In addition, there is no established method to genetically access neurons whose firing is associated with disgust (disgust-associated neurons). Here, we examined the expression of cFos and Arc, two markers of neuronal activity, in the interstitial nucleus of the posterior limb of the anterior commissure (IPAC) of male mice that showed innate disgust and mice that showed learned disgust. Furthermore, we used a targeted recombination in active populations (TRAP) method to genetically label the disgust-associated neurons in the IPAC with YFP. We found a significant increase of both cFos-positive neurons and Arc-positive neurons in the IPAC of mice that showed innate disgust and mice that showed learned disgust. In addition, TRAP following quinine infusion (Quinine-TRAP) resulted in significantly more YFP-positive neurons in the IPAC, compared to TRAP following water infusion. A significant number of the YFP-positive neurons following Quinine-TRAP were co-labeled with Arc following the second quinine infusion, confirming that Quinine-TRAP preferentially labeled quinine-activated neurons in the IPAC. Our results suggest that the IPAC activity is associated with both innate and learned disgust and that disgust-associated neurons in the IPAC are genetically accessible by TRAP.

Role of anterior piriform cortex in the acquisition of conditioned flavour preference

Flavour aversion learning (FAL) and conditioned flavour preference (CFP) facilitate animal survival and play a major role in food selection, but the neurobiological mechanisms involved are not completely understood. Neuroanatomical bases of CFP were examined by using Fos immunohistochemistry to record neuronal activity. Rats were trained over eight alternating one-bottle sessions to acquire a CFP induced by pairing a flavour with saccharin (grape was CS+ in Group 1; cherry in Group 2; in Group 3, grape/cherry in half of animals; Group 4, grape/cherry in water). Animals were offered the grape flavour on the day immediately after the training and their brains were processed for c-Fos. Neurons evidencing Fos-like immunoreactivity were counted in the infralimbic cortex, nucleus accumbens core, and anterior piriform cortex (aPC). Analysis showed a significantly larger number of activated cells after learning in the aPC alone, suggesting that the learning process might have produced a change in this cortical region. Ibotenic lesions in the aPC blocked flavour-taste preference but did not interrupt flavour-toxin FAL by LiCl. These data suggest that aPC cells may be involved in the formation of flavour preferences and that the integrity of this region may be specifically necessary for the acquisition of a CFP.

Activation of the hypothalamic-pituitary-adrenal axis in lithium-induced conditioned taste aversion learning

Intraperitoneal injections (ip) of lithium chloride at large doses induce c-Fos expression in the brain regions implicated in conditioned taste aversion (CTA) learning, and also activate the hypothalamic-pituitary-adrenal (HPA) axis and increase the plasma corticosterone levels in rats. A pharmacologic treatment blunting the lithium-induced c-Fos expression in the brain regions, but not the HPA axis activation, induced CTA formation. Synthetic glucocorticoids at conditioning, but not glucocorticoid antagonist, attenuated the lithium-induced CTA acquisition. The CTA acquisition by ip lithium was not affected by adrenalectomy regardless of basal corticosterone supplement, but the extinction was delayed in the absence of basal corticosterone. Glucocorticoids overloading delayed the extinction memory formation of lithium-induced CTA. ip lithium consistently induced the brain c-Fos expression, the HPA activation and CTA formation regardless of the circadian activation of the HPA axis. Intracerebroventricular (icv) injections of lithium at day time also increased the brain c-Fos expression, activated the HPA axis and induced CTA acquisition. However, icv lithium at night, when the HPA axis shows its circadian activation, did not induce CTA acquisition nor activate the HPA axis, although it increased the brain c-Fos expression. These results suggest that the circadian activation of the HPA axis may affect central, but not peripheral, effect of lithium in CTA learning in rats, and the HPA axis activation may be necessary for the central effect of lithium in CTA formation. Also, glucocorticoids may be required for a better extinction; however, increased glucocorticoids hinder both the acquisition and the extinction of lithium-induced CTA.

Circadian activation of the hypothalamic-pituitary-adrenal axis may affect central, but not peripheral, effect of lithium in conditioned taste aversion learning in rats

Activation of the hypothalamic-pituitary-adrenal (HPA) axis has been implicated in conditioned taste aversion (CTA) learning induced by lithium chloride. This study investigated if circadian activation of the HPA axis affects the lithium-induced CTA formation. The pairing of conditioned stimulus (sucrose) and unconditioned stimulus (lithium chloride) was performed at night (shortly after light-off) when the HPA activity shows its circadian increase. Intraperitoenal injection of lithium chloride (0.15M, 3ml/kg or 12ml/kg) at night induced CTA formation and the HPA axis activation and increased c-Fos expression in both the parabrachial nucleus (PBN) and the nucleus tractus of solitarius (NTS) in a dose dependent manner. However, intracerebroventricular lithium (0.6M, 5µl) at night failed to induce CTA or the HPA axis activation, although it increased c-Fos expression in the PBN and NTS. Results suggest that circadian activation of the HPA axis may affect central, but not peripheral, effect of lithium in CTA formation, and the lithium-induced c-Fos expression in brain regions may not be effective to induce CTA unless it is coupled with the HPA axis activation. It is concluded that the HPA axis activation may play an important role mediating not only peripheral but also central effect of lithium in CTA formation.

Fear memory formation can affect a different memory: fear conditioning affects the extinction, but not retrieval, of conditioned taste aversion (CTA) memory

The formation of fear memory to a specific stimulus leads to subsequent fearful response to that stimulus. However, it is not apparent whether the formation of fear memory can affect other memories. We study whether specific fearful experience leading to fear memory affects different memories formation and extinction. We revealed that cued fear conditioning, but not unpaired or naïve training, inhibited the extinction of conditioned taste aversion (CTA) memory that was formed after fear conditioning training in rats. Fear conditioning had no effect on retrieval of CTA memory but specifically impaired its extinction. Extinguished fear memory, after fear extinction training, had no effect on future CTA memory extinction. Fear conditioning had no effect on CTA memory extinction if CTA memory was formed before fear conditioning. Conditioned taste aversion had no effect on fear conditioning memory extinction. We conclude that active cued fear conditioning memory can affect specifically the extinction, but not the formation, of future different memory.

Increase of glucocorticoids is not required for the acquisition, but hinders the extinction, of lithium-induced conditioned taste aversion

Lithium chloride at doses sufficient to induce conditioned taste aversion (CTA) causes c-Fos expression in the paraventricular nucleus and increases the plasma level of corticosterone with activation of the hypothalamic-pituitary-adrenal axis. This study was conducted to define the role of glucocorticoid in the acquisition and extinction of lithium-induced CTA. In experiment 1, Sprague-Dawley rats received dexamethasone (2mg/kg) or RU486 (20mg/kg) immediately after 5% sucrose access, and then an intraperitoneal injection of isotonic lithium chloride (12ml/kg) was followed with 30min interval. Rats had either 1 or 7 days of recovery period before the daily sucrose drinking tests. In experiment 2, rats were conditioned with the sucrose-lithium pairing, and then received dexamethasone or vehicle at 30min before each drinking test. In experiment 3, adrenalectomized (ADX or ADX+B) rats were subjected to sucrose drinking tests after the sucrose-lithium pairing. Dexamethasone, but not RU486, pretreatment blunted the formation of lithium-induced CTA memory. Dexamethasone prior to each drinking test suppressed sucrose consumption and prolonged the extinction of lithium-induced CTA. Sucrose consumption was significantly suppressed not only in ADX+B rats but also in ADX rats during the first drinking session; however, a significant decrease was found only in ADX rats on the fourth drinking session. These results reveal that glucocorticoid is not a necessary component in the acquisition, but an important player in the extinction, of lithium-induced CTA, and suggest that a pulse increase of glucocorticoid may hinder the extinction memory formation of lithium-induced CTA.

Dietary fibers reduce food intake by satiation without conditioned taste aversion in mice

It is well known that intake of dietary fiber (DF) potently decreases food intake and feelings of hunger and/or promotes satiety ratings. However, the mechanisms explaining these effects are not well characterized. This work was performed to determine which of satiation and/or satiety mechanisms provoke the decrease of food intake induced by DF in mice. We tested in an intra-group protocol a low-viscosity (LV, fructo-oligosaccharide), a viscous (VP, guar gum) and a high-viscosity (HV, mixture of guar gum and fructo-oligosaccharide) preload. These were given to mice by intra-gastric gavage. It appeared that viscous preloads such as VP and HV reduced the daily energy intake by 14% and 21% respectively. The strong effect of HV was mainly due to a large decrease of meal size (by 57%) and meal duration (by 65%) with no effect on ingestion rate during the first 30 min after administration. Therefore, the DF-induced decrease of energy intake was due to a satiation mechanism. This is further supported by a 3-fold increased sensitization of neurons in the nucleus of the solitary tract as observed by c-Fos protein immunolabelling. No compensation of food intake was observed during the rest of the day, a phenomenon that may be explained by the fact that metabolic rate remained high despite the lower food intake. We have also shown that the DF-induced inhibition of food intake was not paired with a conditioned taste aversion. To conclude, this work demonstrates that DF inhibits food intake by increasing satiation during ~1h after administration.

Taste neophobia and c-Fos expression in the rat brain

Taste neophobia refers to a reduction in consumption of a novel taste relative to when it is familiar. To gain more understanding of the neural basis of this phenomenon, the current study examined whether a novel taste (0.5% saccharin) supports a different pattern of c-Fos expression than the same taste when it is familiar. Results revealed that the taste of the novel saccharin solution evoked more Fos immunoreactivity than the familiar taste of saccharin in the basolateral region of the amygdala, central nucleus of the amygdala, gustatory portion of the thalamus, and the gustatory insular cortex. No such differential expression was found in the other examined areas, including the bed nucleus of stria terminalis,medial amygdala, and medial parabrachial nucleus. The present results are discussed with respect to a forebrain taste neophobia system.

Mitogen-activated protein kinase in the amygdala plays a critical role in lithium chloride-induced taste aversion learning

The intracellular mitogen-activated protein kinase (MAPK) pathway in the brain is necessary for the formation of a variety of memories including conditioned taste aversion (CTA) learning. However, the functional role of MAPK activation in the amygdala during lithium chloride (LiCl)-induced CTA learning has not been established. In the present study, we investigated if local microinjection of SL327, a MAPK kinase inhibitor, into the rat amygdala could alleviate LiCl-induced CTA learning. Our results revealed that acute administration of a high dose of LiCl (0.15M, 12 ml/kg, i.p.) rapidly increased the level of phosphorylated MAPK (pMAPK)-positive cells in the central nucleus of the amygdala (CeA) and nucleus of the solitary tract (NTS) of rats as measured by immunohistochemistry. Local microinjection of SL327 (1 μg/0.5 μl/hemisphere) into the CeA 10 min before LiCl administration decreased both the strength of LiCl-induced CTA paired with 0.125% saccharin and the level of LiCl-induced pMAPK-positive cells in the CeA, but not in the NTS. Our data suggest that the intracellular signaling cascade of the MAPK pathway in the CeA plays a critical role in the processing of visceral information induced by LiCl for CTA learning.

c-Fos Expression in the Nucleus of the Solitary Tract in Response to Salt Stimulation in Rats

Salt signals in tongue are relayed to the nucleus of the solitary tract (NST). This signaling is very important to determine whether to swallow salt-related nutrition or not and suggests some implications in discrimination of salt concentration. Salt concentration-dependent electrical responses in the chorda tympani and the NST were well reported. But salt concentration-dependency and spatial distribution of c-Fos in the NST were not well established. In the present study, NaCl signaling in the NST was studied in urethane-anesthetized rats. The c-Fos immunoreactivity in the six different NST areas along the rostral-caudal axis and six subregions in each of bilateral NST were compared between applications of distilled water and different concentrations of NaCl to the tongue of experimental animals. From this study, salt stimulation with high concentration (1.0 M NaCl) induced significantly higher c-Fos expression in intermediate NST and dorsal-medial and dorsal-middle subregions of the NST compared to distilled water stimulation. The result represents the specific spatial distribution of salt taste perception in the NST.

Learning through the taste system

Taste is the final arbiter of which chemicals from the environment will be admitted to the body. The action of swallowing a substance leads to a physiological consequence of which the taste system should be informed. Accordingly, taste neurons in the central nervous system are closely allied with those that receive input from the viscera so as to monitor the impact of a recently ingested substance. There is behavioral, anatomical, electrophysiological, gene expression, and neurochemical evidence that the consequences of ingestion influence subsequent food selection through development of either a conditioned taste aversion (CTA) (if illness ensues) or a conditioned taste preference (CTP) (if nutrition). This ongoing communication between taste and the viscera permits the animal to tailor its taste system to its individual needs over a lifetime.

Area postrema lesions attenuate LiCl-induced c-Fos expression correlated with conditioned taste aversion learning

Lesions of the area postrema (AP) block many of the behavioral and physiological effects of lithium chloride (LiCl) in rats, including formation of conditioned taste aversions (CTAs). Systemic administration of LiCl induces c-Fos immunoreactivity in several brain regions, including the AP, nucleus of the solitary tract (NTS), lateral parabrachial nucleus (latPBN), supraoptic nucleus (SON), paraventricular nucleus (PVN), and central nucleus of the amygdala (CeA). To determine which of these brain regions may be activated in parallel with the acquisition of LiCl-induced CTAs, we disrupted CTA learning in rats by ablating the AP and then quantified c-Fos-positive cells in these brain regions in sham- and AP-lesioned rats 1 h following LiCl or saline injection. Significant c-Fos induction after LiCl was observed in the CeA and SON of AP-lesioned rats, demonstrating activation independent of an intact AP. LiCl-induced c-Fos was significantly attenuated in the NTS, latPBN, PVN and CeA of AP-lesioned rats, suggesting that these regions are dependent on AP activation. Almost all of the lesioned rats showed some damage to the subpostremal NTS, and some rats also had damage to the dorsal motor nucleus of the vagus; this collateral damage in the brainstem may have contributed to the deficits in c-Fos response. Because c-Fos induction in several regions was correlated with magnitude of CTA acquisition, these regions are implicated in the central mediation of lithium effects during CTA learning.

c-Fos induction by a 14 T magnetic field in visceral and vestibular relays of the female rat brainstem is modulated by estradiol

There is increasing evidence that high magnetic fields interact with the vestibular system of humans and rodents. In rats, exposure to high magnetic fields of 7 T or above induces locomotor circling and leads to a conditioned taste aversion if paired with a novel taste. Sex differences in the behavioral responses to magnetic field exposure have been found, such that female rats show more locomotor circling and enhanced conditioned taste aversion compared to male rats. To determine if estrogen modulates the neural response to high magnetic fields, c-Fos expression after 14 T magnetic field exposure was compared in ovariectomized rats and ovariectomized rats with estradiol replacement. Compared to sham exposure, magnetic field exposure induced significantly more c-Fos positive cells in the nucleus of the solitary tract and the parabrachial, medial vestibular, prepositus, and supragenualis nuclei. Furthermore, there was a significant asymmetry in c-Fos induction between sides of the brainstem in several regions. In ovariectomized rats, there was more c-Fos expressed in the right side compared to left side in the locus coeruleus and parabrachial, superior vestibular, and supragenualis nuclei; less expression in the right compared to left side of the medial vestibular; and no asymmetry in the prepositus nucleus and the nucleus of the solitary tract. Chronic estradiol treatment modulated the neural response in some regions: less c-Fos was induced in the superior vestibular nucleus and locus coeruleus after estradiol replacement; estradiol treatment eliminated the asymmetry of c-Fos expression in the locus coeruleus and supragenualis nucleus, created an asymmetry in the prepositus nucleus and reversed the asymmetry in the parabrachial nucleus. These results suggest that ovarian steroids may mediate sex differences in the behavioral responses to magnetic field exposure at the level of visceral and vestibular nuclei of the brainstem.

A combined method of laser capture microdissection and X-Gal histology to analyze gene expression in c-Fos-specific neurons

c-Fos is a member of the activator protein 1 family that regulates transcription of target genes. c-Fos is transiently induced in specific regions of the brain after a variety of external stimuli including learning and memory formation. Analysis of gene expression in c-Fos-expressing cells of the brain may help identify target genes that play important roles in synaptic strength or neuronal morphology. In the present study, we developed a combined method of laser capture microdissection and 5-bromo-4-chloro-3-indoly-beta-D-galactopyranosidase (X-Gal) histology to analyze gene expression in stimulus-induced c-Fos-positive cells. Using transgenic mice carrying a c-fos-lacZ fusion gene, c-Fos-positive cells were easily identified by measuring of beta-galactosidase (beta-Gal) activity. To establish the fidelity of the reporter transgene, the time course of endogenous c-Fos and the c-fos-lacZ transgene expression in the amygdala induced by LiCl administration was investigated by immunohistochemistry and X-Gal staining. LiCl increased the numbers of c-Fos- and beta-Gal-positive cells in the central and basolateral amygdala of the transgenic mice. To ensure that RNA was preserved in X-Gal stained tissue sections, different fixations were examined, with the conclusion that ethanol fixation was best for both RNA preservation and X-Gal staining quality. Finally, in combining X-Gal staining, single-cell LCM and RT-PCR, we confirmed mRNA expression of endogenous c-fos and beta-actin genes in LiCl-induced beta-Gal-positive cells in the CeA, cortex and hippocampus. Combining LCM and transgenic reporter genes provides a powerful tool with which to investigate tissue- or cell-specific gene expression.

Expression of AP-1 family transcription factors in the amygdala during conditioned taste aversion learning: role for Fra-2

Conditioned taste aversion (CTA) learning occurs after the pairing of a novel taste with a toxin (e.g. sucrose with LiCl). The immediate early gene c-Fos is necessary for CTA learning, but c-Fos alone cannot be sufficient for consolidation. The expression of other AP-1 proteins from the Fos- and Jun-families may also be required shortly after conditioning for CTA consolidation. To screen for the expression of AP-1 transcription factors within small subregions, RT-PCR analysis was used after laser capture microdissection of the amygdala. Rats were infused intraorally with 5% sucrose (6 ml/6 min) or injected with LiCl (12 ml/kg, 0.15 M, i.p.) or given sucrose paired with LiCl (sucrose/LiCl), or not treated; 1 h later their brains were dissected. The lateral (LA), basolateral (BLA), and central (CeA) subnuclei of the amgydala of single 5 microm sections from individual rats were dissected using the Arcturus PixCell II system. Semi-quantitative RT-PCR showed the consistent presence of c-Fos, Fra-2, c-Jun, and JunD in the amygdala. In situ hybridization confirmed that c-Fos and Fra-2 mRNA expression was increased in the CeA after LiCl and sucrose/LiCl treatment. Immunohistochemistry for Fra-2 revealed high baseline levels of Fra-2 protein in the BLA and CeA, but also an increase in Fra-2 in the BLA and CeA after LiCl and sucrose/LiCl treatment. The similarity of response in LiCl and sucrose/LiCl treated groups might reflect activation by LiCl in both groups. To control for the effects of LiCl, rats were tested in a learned safety experiment. Fra-2 and c-Fos were examined in response to sucrose/LiCl in rats with prior familiarity with sucrose compared to rats without prior exposure to sucrose. The familiar (pre-exposure) group showed a significantly decreased number of Fra-2-positive cells compared with the novel group in the BLA, but not in the CeA. Because pre-exposure to sucrose attenuates CTA learning, a decreased cellular response in pre-exposed rats suggests a specific correlation with CTA learning. Changes in Fra-2 and c-Fos expression in the BLA and CeA at the time of conditioning, together with constitutive expression of c-Jun and JunD, may contribute to CTA learning.

Internal body state influences topographical plasticity of sensory representations in the rat gustatory cortex

Primary sensory cortices are remarkably organized in spatial maps according to specific sensory features of the stimuli. These cortical maps can undergo plastic rearrangements after changes in afferent ("bottom-up") sensory inputs such as peripheral lesions or passive sensory experience. However, much less is known about the influence of "top-down" factors on cortical plasticity. Here, we studied the effect of a visceral malaise on taste representations in the gustatory cortex (GC). Using in vivo optical imaging, we showed that inducing conditioned taste aversion (CTA) to a sweet and pleasant stimulus induced plastic rearrangement of its cortical representation, becoming more similar to a bitter and unpleasant taste representation. Using a behavior task, we showed that changes in hedonic perception are directly related to the maps plasticity in the GC. Indeed imaging the animals after CTA extinction indicated that sweet and bitter representations were dissimilar. In conclusion, we showed that an internal state of malaise induces plastic reshaping in the GC associated to behavioral shift of the stimulus hedonic value. We propose that the GC not only encodes taste modality, intensity, and memory but extends its integrative properties to process also the stimulus hedonic value.

Taste aversion learning induced c-fos expression in the nucleus of the solitary tract after spontaneous flavor intake: role of the inter-stimulus interval

Taste aversion learning (TAL) can be induced by associating a flavor intake with the immediate or delayed (30 min) intragastric administration of a noxious substance, e.g., hypertonic NaCl. The objective of this study was to analyze the induction of c-Fos immunoreactivity in the intermediate nucleus of the solitary nucleus (iNST) after acquisition of a contiguous or delayed TAL, offering the flavor for voluntary consumption in both cases. The behavioral results obtained indicate that, although the learning was established under both experimental conditions, an increase in c-Fos induction was only produced in the group that learned by means of a non-delayed TAL. Immunohistochemical analyses revealed the participation of different brain structures in these two TAL modalities. Thus, the nucleus of the solitary tract may be involved in the TAL procedure in which voluntary flavor intake and intragastric administration of the noxious visceral stimulus are contiguous but not in delayed TAL, which would depend on other anatomical circuits that do not include the iNST.

D-Cycloserine enhances conditioned taste aversion learning in rats

Conditioned taste aversion (CTA) is a form of associative learning in which the pairing of a taste with a toxin causes an animal to avoid the taste. NMDA receptor mediated neurotransmission has been implicated in CTA, but the role of the NMDA receptor glycine-binding site has not been examined. To examine the effects on CTA of the glycinergic NMDA receptor agonist D-cycloserine, rats received D-cycloserine (15 mg/kg, i.p.) or vehicle 15 min before 10-min access to 0.125% saccharin, followed by a low dose of LiCl (19 mg/kg, i.p.). CTA was measured with 24-h, 2-bottle preference tests between water and saccharin. Vehicle-treated rats formed a mild CTA that rapidly extinguished, while d-cycloserine-treated rats formed a stronger CTA that extinguished slowly. The effect of d-cycloserine was specific to the NMDA receptor glycine-binding site, because pretreatment with HA-966 (6 mg/kg), a partial glycinergic agonist, blocked enhancement by D-cycloserine. Three follow-up experiments suggest that the enhancement of CTA was not due to an aversive effect of D-cycloserine. First, saccharin paired with D-cycloserine (15 mg/kg) alone did not induce a CTA, although a higher dose (30 mg/kg) did significantly lower saccharin preference. Second, pretreatment with D-cycloserine did not increase the duration of "lying-on-belly" behavior induced by LiCl. Third, pretreatment with D-cycloserine did not increase c-Fos induction by either LiCl or vehicle injection in central visceral relays (the nucleus of the solitary tract, the parabrachial nucleus, the central nucleus of the amygdala, the supraoptic nucleus, and the paraventricular nucleus). These results confirm the participation of NMDA receptor, and specifically the glycine-binding site of NMDA receptor, in CTA learning.

Aldosterone-sensitive neurons in the nucleus of the solitary tract: bidirectional connections with the central nucleus of the amygdala

The HSD2 (11-beta-hydroxysteroid dehydrogenase type 2-expressing) neurons in the nucleus of the solitary tract (NTS) of the rat are aldosterone-sensitive and have been implicated in sodium appetite. The central nucleus of the amygdala (CeA) has been shown to modulate salt intake in response to aldosterone, so we investigated the connections between these two sites. A prior retrograde tracing study revealed only a minor projection from the HSD2 neurons directly to the CeA, but these experiments suggested that a more substantial projection may be relayed through the parabrachial nucleus. Small injections of cholera toxin beta subunit (CTb) into the external lateral parabrachial subnucleus (PBel) produced both retrograde cell body labeling in the HSD2 neurons and anterograde axonal labeling in the lateral subdivision of the CeA. Also, injections of either CTb or Phaseolus vulgaris leucoagglutinin into the medial subdivision of the CeA labeled a descending projection from the amygdala to the medial NTS. Axons from the medial CeA formed numerous varicosities and terminals enveloping the HSD2 neurons. Complementary CTb injections, centered in the HSD2 subregion of the NTS, retrogradely labeled neurons in the medial CeA. These bidirectional projections could form a functional circuit between the HSD2 neurons and the CeA. The HSD2 neurons may represent one of the functional inputs to the lateral CeA, and their activity may be modulated by a return projection from the medial CeA. This circuit could provide a neuroanatomical basis for the modulation of salt intake by the CeA.

Fine structural survey of the intermediate subnucleus of the nucleus tractus solitarii and its glossopharyngeal afferent terminals

The intermediate subnucleus of the nucleus tractus solitarii (imNTS) receives somatosensory inputs from the soft palate and pharynx, and projects onto the nucleus ambiguus, thus serving as a relay nucleus for swallowing. The ultrastructure and synaptology of the rat imNTS, and its glossopharyngeal afferent terminals, have been examined with cholera toxin-conjugated horseradish peroxidase (CT-HRP) as an anterograde tracer. The imNTS contained oval or ellipsoid-shaped, small to medium-sized neurons (18.2 x 11.4 microm) with little cytoplasm, few cell organelles and an irregularly shaped nucleus. The cytoplasm often contained one or two nucleolus-like stigmoid bodies. The average number of axosomatic terminals was 1.8 per profile. About 83% of them contained round vesicles and formed asymmetric synaptic contacts (Gray's type I), while about 17% contained pleomorphic vesicles and formed symmetric synaptic contacts (Gray's type II). The neuropil contained small or large axodendritic terminals, and about 92% of them were Gray's type I. When CT-HRP was injected into the nodose ganglion, many labeled terminals were found in the imNTS. All anterogradely labeled terminals contacted dendrites but not somata. The labeled terminals were usually large (2.69+/-0.09 mum) and exclusively of Gray's type I. They often contacted more than two dendrites, were covered with glial processes, and formed synaptic glomeruli. A small unlabeled terminal occasionally made an asymmetric synaptic contact with a large labeled terminal. The large glossopharyngeal afferent terminals and the neurons containing stigmoid bodies characterized the imNTS neurons that received pharyngeal afferents.

Stimulus-induced increase of taste responses in the hamster chorda tympani by repeated exposure to 'novel' tastants

Variations in amplitude of responses of the chorda tympani to repeated application of various novel tastants were measured in familiarized and control groups of adult hamsters. Three groups of 10 hamsters were pre-exposed to 5 mM dulcin, 50 mM potassium L-glutamate (KGlu) or 1 mM 5'guanosine monophosphate (5'GMP). In the fourth group, the tongue was rinsed with 5'GMP for 20 min just prior to recording from the chorda tympani. The tastants were novel to the fifth group (naïve control). A series of 17 stimuli was repeated six times and responses were quantified relative to the initial response of each of the 50 hamsters. The responses of the chorda tympani increased with repetition in the control group. In contrast, no increase in amplitude of response to the pre-exposed tastants or to stimuli with qualitatively related tastes was observed in the group familiarized with either KGlu or 5'GMP. These results indicate that the response of the chorda tympani depends on previous exposure to a tastant. The sensitivity of taste cells appears to be modulated, possibly by stimulus-induced supplementary receptors.

A role for prefrontal cortex in the extinction of a conditioned taste aversion

This study used immunohistochemical methods to determine if the medial prefrontal cortex (mPFC) is involved in the extinction of a conditioned taste aversion (CTA). As rats reached 90% reacceptance of a tastant (saccharin: SAC) that had previously been associated with lithium chloride-induced malaise, c-Fos protein expression increased dramatically as compared to animals with active CTAs, animals without CTAs (i.e., explicitly unpaired CS-US exposures) or animals drinking SAC for the first time. These data indicate a role for mPFC (prelimbic and infralimbic cortex) in the formation of a CTA extinction memory.

Afferent and efferent connections of the parvicellular subdivision of iNTS: defining a circuit involved in taste aversion learning

Conditioned taste aversion (CTA) expression is associated with strong increases in Fos-like immunoreactivity (FLI) in a region of the brainstem identified as the parvicellular subdivision of the intermediate nucleus of the solitary tract (iNTSpc). To identify the projections to and from cells in iNTSpc which display strong FLI in response to expression of a CTA, anterograde and retrograde tract tracing was used. When appropriate, tract tracing was combined with double labeling for FLI in animals which received CTA training as well as tracer injections and were re-exposed to the CS taste. With respect to afferent projections, iNTSpc receives a strong, direct, ipsilateral projection from amygdala and the distribution of the fiber terminals yields a striking match to that of cells expressing FLI after CTA expression. As for efferent projections, these cells in iNTSpc are characterized by a mixed, rather than homogeneous, projection pattern. Targets of these cells include pons and forebrain as well as local medullary sites, all of which are known to be involved in gastrointestinal function. Thus, activation of these cells may provide a circuit through which gastrointestinal/visceral responses are coordinated as a component of the conditioned aversion.

Nω-nitro-l-arginine methyl ester attenuates lithium-induced c-Fos, but not conditioned taste aversion, in rats

Lithium chloride (LiCl) at doses sufficient to induce conditioned taste aversion (CTA) causes c-Fos expression in the relevant brain regions and activates the hypothalamic-pituitary-adrenal (HPA) axis. It has been suggested that nitric oxide (NO) in the central nervous system may play a role not only in the activation of HPA axis but also in CTA learning, and that LiCl may activate the brain NO system. To determine the role of NO in lithium-induced CTA, we examined the lithium-induced CTA, brain c-Fos expression, and plasma corticosterone level with Nomega-nitro-L-arginine methyl ester (L-NAME) pretreatment. Intraperitoneal L-NAME (30 mg/kg) given 30 min prior to LiCl significantly decreased lithium-induced c-Fos expression in the brain regions implicated in CTA learning, such as the hypothalamic paraventricular nucleus (PVN), central nucleus of amygdala (CeA), and nucleus tractus of solitarius. However, either the lithium-induced CTA acquisition or the increase in plasma corticosterone was not attenuated by l-NAME pretreatment. These results suggest that NO may be involved in lithium-induced neuronal activation of the brain regions, but not in the CTA acquisition or the HPA axis activation.

Rapid toxin-induced gustatory conditioning in rats: separate and combined effects of systemic injection or intraoral infusion of lithium chloride

The present experiment examined the individual and combined effects of systemic injection and oral ingestion of lithium chloride (LiCl) on both within and across session shifts in palatability. Male rats fitted with intraoral cannulae received two conditioning days in which they were injected with either LiCl or sodium chloride (NaCl) and were then presented with brief intraoral infusions of a sucrose plus LiCl or NaCl solution. The individual taste reactivity responses during the intraoral infusions were videotaped and later analyzed for response frequency. Forty-eight hours after the second conditioning day the same sucrose plus salt solution was presented again in the absence of any injection. The present results demonstrate that systemic injections of LiCl result in profound within session and across session decreases in ingestive responding accompanied by increased active and passive aversive responses. Animals receiving LiCl by injection as well as ingestion demonstrated an exaggerated response. Rats which received LiCl only through intraoral infusions produced the same pattern of decreased ingestive responding to the sucrose plus salt flavored tastant on each test trial suggesting little or no across session conditioning effects. A two process model by which animals may regulate toxicosis is discussed.

Dynamic processing of taste aversion extinction in the brain

While substantial advances have been made in discovering how the brain learns and remembers, less is known about how the brain discards information, reorganizes information, or both. These topics are not only relevant to normal brain functioning but also speak to pathologies in which painful memories do not wane but are evoked time and again (e.g., post-traumatic stress disorder; PTSD). Here, we measured brain activity (as indicated by the regional expression of c-Fos protein) in rats during acquisition and throughout extinction of a conditioned taste aversion (CTA). We compared that brain activity with animals that had intact CTA memories or those that experienced an explicitly unpaired (EU) conditioned stimulus (CS; saccharin, SAC) and unconditioned stimulus (US; lithium chloride, LiCl). The data show a dynamic and nonuniform pattern of c-Fos protein expression in brain nuclei known to mediate gustation and CTAs. In particular, brainstem nuclei (e.g., nucleus of the solitary tract; NTS) and the basolateral nucleus of the amygdala (BLA) are active early as CTAs are formed and as extinction of the learned response begins. Later in the extinction process, the BLA reduces c-Fos expression relative to nonextinguished controls. Finally, as almost full reacceptance of the taste is achieved, the gustatory neocortex (GNC) expresses enhanced levels of c-Fos protein. Thus, extinction of a CTA is not represented by a simple reversal of the c-Fos activity evoked by CTA conditioning. Rather, the data demonstrate that extinction of conditioned responses is a dynamic process in which the activity levels of particular nuclei along the brain's taste pathway change depending on the extent to which the conditioned response has been extinguished.

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.

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.

The diverse roles of specific GLP-1 receptors in the control of food intake and the response to visceral illness

Intracerebroventricular administration of glucagon-like peptide-1 (7-36) amide (GLP-1) reduces food intake and produces symptoms of visceral illness, such as a conditioned taste aversion (CTA). The central hypothesis of the present work is that separate populations of GLP-1 receptors mediate the anorexia and taste aversion associated with GLP-1 administration. To test this hypothesis, we first compared the ability of various doses of GLP-1 to induce anorexia or CTA when administered into either the lateral or fourth ventricle. Lateral and fourth ventricular GLP-1 resulted in reduction of food intake at similar doses, whereas only lateral ventricular GLP-1 resulted in a CTA. Such data indicate that both hypothalamic and caudal brainstem GLP-1 receptors are likely to participate in the ability of GLP-1 to reduce food intake. We also hypothesized that the site that must mediate the ability of GLP-1 to induce visceral illness is in the central nucleus of the amygdala (CeA). Administration of 0.2 or 1.0 microg of GLP-1 (7-36) but not the inactive GLP-1 (9-36) resulted in a strong CTA with no accompanying anorexia. In addition, bilateral CeA administration of 2.5 microg of a GLP-1 receptor antagonist before intraperitoneal administration of the toxin lithium chloride resulted in a diminished CTA. Together, these data indicate that separate GLP-1 receptor populations mediate the multiple responses to GLP-1. These results indicate that GLP-1 is a flexible system that can be activated under various circumstances to alter the ingestion of nutrients and/or produce other visceral illness responses, depending on the ascending pathways of the GLP-1 system that are recruited.

Vasopressin content in select brain regions during extinction of a conditioned taste aversion

Previous studies have shown that low levels of vasopressin during extinction of conditioned taste avoidance are associated with a faster extinction, that fluid deprivation differentially alters vasopressin levels in various neural areas, and that extinction of conditioned taste avoidance is accelerated in fluid deprived male rats. The following study was designed to identify areas of the brain in which vasopressin levels are different in fluid deprived and nondeprived males during extinction of conditioned taste avoidance. Arginine vasopressin content was determined by radioimmunoassay in the paraventricular nucleus (PVN), medial amygdala (AMe), bed nucleus of the stria terminalis (BNST), nucleus tractus solitarius (NTS), medial septum (MS), lateral septum (LS), and insular cortex (IC) of unconditioned nondeprived males and conditioned males that were maintained on a 23-h fluid deprivation schedule or that were nondeprived. Vasopressin content in the PVN of deprived and nondeprived males differed during extinction. Based on comparisons with unconditioned nondeprived males, this difference was due to an elevation in the vasopressin content of the nondeprived but not the deprived males. These results raise the possibility that a vasopressinergic system in the PVN plays a critical role in the differential extinction rate of fluid deprived and nondeprived males, which will need to be verified by manipulating vasopressin levels in this brain site during extinction of a conditioned taste avoidance.

Quinine and citric acid elicit distinctive Fos-like immunoreactivity in the rat nucleus of the solitary tract

The present experiment investigated Fos-like immunoreactivity (FLI) in the nucleus of the solitary tract (NST) after intraoral infusions of 0.1 M citric acid, 0.3 M NaCl, and 0.3-30 mM quinine monohydrochloride (QHCl) in awake, behaving rats. Increases in QHCl concentration produced increases in the numbers of FLI-labeled neurons in the rostral part of the intermediate (i(r)) and rostral (r) NST, but the topographic distribution of FLI was consistent across QHCl concentrations and distinctive compared with effects of citric acid. Quinine elicited FLI concentrated in the medial third of the nucleus; acid elicited more broadly distributed FLI concentrated farther laterally. Surprisingly, in contrast to QHCl and citric acid, NaCl produced FLI that was indistinguishable from that produced by water. Although the functional significance of these patterns is unknown, citric acid and QHCl are nonpreferred stimuli but produced different oromotor behaviors. QHCl (30 mM) elicited approximately 3.2 times as many gapes as citric acid (0.1 M), and acid elicited more ingestive responses. Parallel differences in FLI expression suggest that different NST regions may have distinctive roles in triggering oromotor behaviors.

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.

Conditioned taste aversion and c-Fos expression in cholestatic rats

Rats in which a ligation of the bile duct (BDL) was paired with a saccharin taste developed a persistent conditioned taste aversion in both preference and taste reactivity tests. All BDL animals regardless of pairing had increased c-Fos-like immunoreactivity (FLI) in the area postrema and the nucleus of the solitary tract. This FLI may reflect the illness associated with BDL, but there was no evidence of conditioned FLI.

Conditioned taste aversion as a learning and memory paradigm

Conditioned taste aversion (CTA) is a well established learning and memory paradigm in rats and mice that is considered to be a special form of classical conditioning. Rodents--as well as many other species including man--learn to associate a novel taste (CS) with nausea (US), and as a consequence avoid drinking fluid with this specific taste. In contrast to other types of classical conditioning, even CS-US intervals lasting several hours lead to an aversion to the gustatory CS. With increasing CS-US delay duration, however, the aversion against the CS gradually decreases. Mice differ from rats in their reaction to the CS as well as the US. They tolerate a much higher concentration of saccharin and they do not show any clear signs of nausea when injected with the US. Advantages of this task are its relative independence of motor behavior, well described pathways for the CS and partly the US, and the wealth of available anatomical and pharmacological data implying several brain structures (e.g. parabrachial nucleus, amygdala, insular cortex), neurotransmitters and their receptors (e.g. cholinergic system, NMDA-receptors), and cellular processes (e.g. expression of immediate early genes, Ras-MAP kinase signaling pathway, CREB phosphorilation, protein tyrosine phosphorilation, protein synthesis) in CTA. The CTA paradigm has also been successfully used to phenotype mouse mutants.

Does Parkinson's disease have an immunological basis? The evidence and its therapeutic implications

Parkinson's disease (PD) is an age-related neurodegenerative movement disorder of unknown aetiology. Immune abnormalities have been described in PD including the occurrence of autoantibodies against neuronal structures and high numbers of microglia cells expressing the histocompatibility glycoprotein human leucocyte antigen-DR in the substantia nigra. An infectious cause for PD has been discussed for years. Disturbed cellular and humoral immune functions in peripheral blood of patients with PD have been also reported. An elevated gammadelta(+) T cell population and increased immunoglobulin G immunity in CSF to heat shock proteins have been found in PD. Cytokines and apoptosis-related proteins were elevated in the striatum in patients with PD. Activated glial cells may participate in neuronal cell death in PD by providing toxic substances. We may conclude that the immune system is involved in the pathogenesis of PD. However, we are not able to determine whether the disturbances described above constitute a primary or secondary phenomenon. Immunomodulatory agents may have important applications in the development of new therapies for PD.

Impaired conditioned taste aversion learning in spinophilin knockout mice

Plasticity in dendritic spines may underlie learning and memory. Spinophilin, a protein enriched in dendritic spines, has the properties of a scaffolding protein and is believed to regulate actin cytoskeletal dynamics affecting dendritic spine morphology. It also binds protein phosphatase-1 (PP-1), an enzyme that regulates dendritic spine physiology. In this study, we tested the role of spinophilin in conditioned taste aversion learning (CTA) using transgenic spinophilin knockout mice. CTA is a form of associative learning in which an animal rejects a food that has been paired previously with a toxic effect (e.g., a sucrose solution paired with a malaise-inducing injection of lithium chloride). Acquisition and extinction of CTA was tested in spinophilin knockout and wild-type mice using taste solutions (sucrose or sodium chloride) or flavors (Kool-Aid) paired with moderate or high doses of LiCl (0.15 M, 20 or 40 mL/kg). When sucrose or NaCl solutions were paired with a moderate dose of LiCl, spinophilin knockout mice were unable to learn a CTA. At the higher dose, knockout mice acquired a CTA but extinguished more rapidly than wild-type mice. A more salient flavor stimulus (taste plus odor) revealed similar CTA learning at both doses of LiCl in both knockouts and wild types. Sensory processing in the knockouts appeared normal because knockout mice and wild-type mice expressed identical unconditioned taste preferences in two-bottle tests, and identical lying-on-belly responses to acute LiCl. We conclude that spinophilin is a candidate molecule required for normal CTA learning.

Chorda tympani nerve stimulation evokes Fos expression in regionally limited neuron populations within the gustatory nucleus of the solitary tract

The distribution of neurons in the rostral nucleus of the solitary tract (rNST) that respond to gustatory input from the anterior tongue was visualized by Fos protein immunohistochemistry following electrical stimulation of the chorda tympani (CT) nerve in rats. Maps of Fos-immunoreactive (Fos-ir) neurons were compared with the distribution of CT afferent terminal fields labeled by transganglionic transport of rhodamine-dextran in a separate group of animals. The primary concentration of Fos-ir neurons localized in register with the major terminal fields of CT afferent fibers, in the central third of the rostral 1.0 mm of the NST ipsilateral to the stimulated nerve. A similar correspondence in location and degree of labeling of Fos-ir neurons and afferent terminals was observed in the ipsilateral dorsal spinal trigeminal complex (Sp5) pars caudalis, near the obex, and the Sp5 pars oralis near the rostral pole of the rNST. Thus, the magnitude of Fos upregulation in brainstem targets of the CT nerve having chemosensory or nociceptive function, was proportional to the relative density of the CT afferent input. This correspondence, and the absence of labeling in neurons known to be one additional synapse away from the afferent input within gustatory or oral reflex pathways, suggests that the cell map obtained represents mainly neurons that are directly activated via primary afferent synapses from CT fibers. The availability of a method to histochemically identify a population of putative second-order taste neurons will facilitate analysis of the cellular/molecular properties of these neurons and of synaptic circuitry in the rNST.