Opioid modulation of taste responses in the nucleus of the solitary tract

Phenotyping neurons activated in the mouse brain during restoration of salt debt

Salt overconsumption contributes to hypertension, which is a major risk factor for stroke, heart and kidney disease. Characterising neuronal pathways that may control salt consumption is therefore important for developing novel approaches for reducing salt overconsumption. Here, we identify neurons within the mouse central amygdala (CeA), lateral parabrachial nucleus (LPBN), intermediate nucleus of the solitary tract (iNTS), and caudal NTS (cNTS) that are activated and display Fos immunoreactivity in mice that have consumed salt in order to restore a salt debt, relative to salt replete and salt depleted controls. Double-label immunohistochemical studies revealed that salt restoring mice had significantly greater densities of activated enkephalin neurons within the CeA and iNTS, while statistically significant changes within the LPBN and cNTS were not observed. Furthermore, within the CeA, restoration of salt debt conferred a significant increase in the density of activated calretinin neurons, while there was no change relative to control groups in the density of activated neurons that co-expressed protein kinase C delta (PKC-δ). Taken together, these studies highlight the importance of opioid systems within the CeA and iNTS in neuronal processes associated with salt restoration, and may aid the development of future pharmacological and other strategies for reducing salt overconsumption.

Inhibitory modulation of optogenetically identified neuron subtypes in the rostral solitary nucleus

Inhibition is presumed to play an important role in gustatory processing in the rostral nucleus of the solitary tract (rNST). One source of inhibition, GABA, is abundant within the nucleus and comes both from local, intrasolitary sources and from outside the nucleus. In addition to the receptor-mediated effects of GABA on rNST neurons, the hyperpolarization-sensitive currents, Ih and IA, have the potential to further modulate afferent signals. To elucidate the effects of GABAergic modulation on solitary tract (ST)-evoked responses in phenotypically defined rNST neurons and to define the presence of IA and Ih in the same cells, we combined in vitro recording and optogenetics in a transgenic mouse model. This mouse expresses channelrhodopsin 2 (ChR2) in GAD65-expressing GABAergic neurons throughout the rNST. GABA positive (GABA+) neurons differed from GABA negative (GABA-) neurons in their response to membrane depolarization and ST stimulation. GABA+ neurons had lower thresholds to direct membrane depolarization compared with GABA- neurons, but GABA- neurons responded more faithfully to ST stimulation. Both IA and Ih were present in subsets of GABA+ and GABA- neurons. Interestingly, GABA+ neurons with Ih were more responsive to afferent stimulation than inhibitory neurons devoid of these currents, whereas GABA- neurons with IA were more subject to inhibitory modulation. These results suggest that the voltage-gated channels underlying IA and Ih play an important role in modulating rNST output through a circuit of feedforward inhibition.

Potential effects of pleasant and cold stimuli on nausea and vomiting induced by disgusting tastes

Several pharmacological agents have disgusting tastes that are perceived strongly in the back of the mouth and may trigger nausea and vomiting (NV), resulting in poor adherence to medication schedules and negative impacts on clinical outcomes. Pleasant stimuli and cold temperature lessen the disgusting stimuli, lowering NV through different mechanisms. A pleasant stimulus can mask an unpleasant one, presumably through lateral inhibitory connections in the local neuronal circuit. Similarly, temperature deeply influences taste perception because the response to bitter as well as to salty and sour has been found to assume a reversed U-shaped form, being reduced by cooling to 18°C and enhanced by warming to 30-37°C. This Review describes the mechanisms by which pleasant and cold stimuli may mask emetogenic disgusting stimuli and identifies the potential clinical applications of cooling for inhibiting objectionable drug-related gustatory reactions. © 2015 Wiley Periodicals, Inc.

Controversies in fat perception

Nutritional fat is one of the most controversial topics in nutritional research, particularly against the background of obesity. Studies investigating fat taste perception have revealed several associations with sensory, genetic, and personal factors (e.g. BMI). However, neuronal activation patterns, which are known to be highly sensitive to different tastes as well as to BMI differences, have not yet been included in the scheme of fat taste perception. We will therefore provide a comprehensive survey of the sensory, genetic, and personal factors associated with fat taste perception and highlight the benefits of applying neuroimaging research. We will also give a critical overview of studies investigating sensory fat perception and the challenges resulting from multifaceted methodological approaches. In conclusion, we will discuss a multifactorial approach to fat perception to gain a better understanding of the underlying mechanisms that cause varying fat sensitivity which could be responsible for overeating. Such knowledge might be beneficial in new treatment strategies for obesity and overweight.

The μ-opioid receptor agonist DAMGO presynaptically suppresses solitary tract-evoked input to neurons in the rostral solitary nucleus

Taste processing in the rostral nucleus of the solitary tract (rNST) is subject to modulatory influences including opioid peptides. Behavioral pharmacological studies suggest an influence of μ-opioid receptors in rNST, but the underlying mechanism is unknown. To determine the cellular site of action, we tested the effects of the μ-opioid receptor agonist DAMGO in vitro. Whole cell patch-clamp recordings were made in brain stem slices from GAD67-GFP knockin mice expressing enhanced green fluorescent protein (EGFP) under the control of the endogenous promoter for GAD67, a synthetic enzyme for GABA. Neuron counts showed that ∼36% of rNST neurons express GABA. We recorded monosynaptic solitary tract (ST)-evoked currents (jitter ≤ 300 μs) in both GAD67-EGFP-positive (GAD67+) and GAD67-EGFP-negative (GAD67-) neurons with equal frequency (25/31; 22/28), but the inputs to the GAD67+ neurons had significantly smaller paired-pulse ratios compared with GAD67- neurons. DAMGO (0.3 μM) significantly suppressed ST-evoked currents in both cell types (mean suppression = 46 ± 3.3% SE), significantly increased the paired-pulse ratio of these currents, and reduced the frequency of spontaneous miniature excitatory postsynaptic currents but did not diminish their amplitude, indicating a presynaptic site of action. Under inhibitory amino acid receptor blockade, DAMGO was significantly more suppressive in GAD67+ neurons (59% reduction) compared with GAD67- neurons (35% reduction), while the reverse was true in normal artificial cerebrospinal fluid (GAD67+: 35% reduction; GAD67-: 57% reduction). These findings suggest that DAMGO suppresses activity in rNST neurons predominantly via a presynaptic mechanism, and that this effect may interact significantly with tonic or evoked inhibitory activity.

Opiate agonists and antagonists modulate taste perception in opiate-maintained and recently detoxified subjects

Heroin addicts consume large quantities of refined sugars. This study investigated the effect of opiate use and antagonism on sweet taste in opiate-maintained drug users and detoxified former chronic opiate users, using a within-subject design. Seven opiate users received methadone and seven buprenorphine maintenance. Six detoxified subjects received naltrexone. Sucrose recognition thresholds and measurements of pleasantness and intensity were determined before and four hours after 1) a single dose of methadone or buprenorphine or 2) naltrexone. Control data were taken from a cohort of healthy volunteers including smokers. All measures of sweet and salt taste perception were significantly greater in opiate users and recently detoxified subjects compared to control subjects, with the exception of sweet pleasantness, which returned to control level after detoxification. Acute methadone administration reduced salt thresholds and unpleasantness to control levels. Increased sweet thresholds and salt unpleasantness in detoxified subjects were reversed by acute opioid antagonism, returning to control levels. These results suggest that opiate use and antagonism alters taste perception. Some of the alterations reverse on detoxification (sweet pleasantness), and others can be reversed by opioid antagonism (sweet threshold, salt unpleasantness). Changes in taste perception may underlie altered consumption of refined sugars in opiate users.

μ-Opioid modulation in the rostral solitary nucleus and reticular formation alters taste reactivity: evidence for a suppressive effect on consummatory behavior

The neural control of feeding involves many neuromodulators, including the endogenous opioids that bind μ-opioid receptors (MORs). Injections of the MOR agonist, Damgo, into limbic and hypothalamic forebrain sites increase intake, particularly of palatable foods. Indeed, forebrain Damgo injections increase sucrose-elicited licking but reduce aversive responding (gaping) to quinine, suggesting that MOR activation may enhance taste palatability. A μ-opioid influence on taste reactivity has not been assessed in the brain stem. However, MORs are present in the first-order taste relay, the rostral nucleus of the solitary tract (rNST), and in the immediately subjacent reticular formation (RF), a region known to be essential for consummatory responses. Thus, to evaluate the consequences of rNST/dorsal RF Damgo in this region, we implanted rats with intraoral cannulas, electromyographic electrodes, and brain cannulas aimed at the ventral border of the rNST. Licking and gaping elicited with sucrose, water, and quinine were assessed before and after intramedullary Damgo and saline infusions. Damgo slowed the rate, increased the amplitude, and decreased the size of fluid-induced lick and gape bouts. In addition, the neutral stimulus water, which typically elicits licks, began to evoke gapes. Thus, the current results demonstrate that μ-opioid activation in the rNST/dorsal RF exerts complex effects on oromotor responding that contrast with forebrain effects and are more indicative of a suppressive, rather than a facilitatory effect on ingestion.

Amygdalofugal influence on processing of taste information in the nucleus of the solitary tract of the rat

Previous studies have shown that corticofugal input to the first central synapse of the ascending gustatory system, the nucleus of the solitary tract (NST), can alter the way taste information is processed. Activity in other forebrain structures, such as the central nucleus of the amygdala (CeA), similarly influence activation of NST taste cells, although the effects of amygdalofugal input on neural coding of taste information is not well understood. The present study examined responses of 110 NST neurons to 15 taste stimuli before, during, and after electrical stimulation of the CeA in rats. The taste stimuli consisted of different concentrations of NaCl (0.03, 0.1, 0.3 M), sucrose (0.1, 0.3, 1.0 M), citric acid (0.005, 0.01 M), quinine HCl (0.003, 0.03 M), and 0.03 M MSG, 0.1 M KCl, as well as 0.1 M NaCl, 0.01 M citric acid, and 0.03 M MSG mixed with 10 muM amiloride. In 66% of NST cells sampled (73/110) response rates to the majority of effective taste stimuli were either inhibited or augmented. Nevertheless, the magnitude of effect across stimuli was often differential, which provides a neurophysiological mechanism to alter neural coding. Subsequent analysis of across-unit patterns showed that amygdalofugal input plays a role in shaping spatial patterns of activation and could potentially influence the perceptual similarity and/or discrimination of gustatory stimuli by altering this feature of neural coding.

Changes in mouse mu opioid receptor Exon 7/8-like immunoreactivity following food restriction and food deprivation in rats

Opioid agonists and antagonists respectively increase and decrease food intake. That selective mu opioid antagonists are more effective than antisense probes directed against the mu opioid receptor (MOR-1) gene in reducing deprivation-induced feeding suggests a role for isoforms. Both food restriction and deprivation alter protein and mRNA levels of opioid peptides and receptors. Antisera directed against Exon 4 of the MOR-1-like immunoreactivity (LI) (Exon 4) clone or directed against mouse Exons 7/8 (mE7/8-LI) revealed high levels of immunoreactivity in brain nuclei related to feeding behavior. Therefore, the present study assessed MOR-1LI and mE7/8-LI in hypothalamic and extrahypothalamic sites in rats exposed to ad libitum feeding, food restriction (2, 7, 14 days), or food deprivation (24, 48 h). MOR-1-LI displayed robust reactivity, but was insensitive to food restriction or deprivation. mE7/8-LI, both in terms of cell counts and relative optical density, was significantly and selectively increased in the dorsal and ventral parvocellular subdivisions of the hypothalamic paraventricular nucleus in food-restricted (14 days) rats, but all other restriction or deprivation regimens were ineffective in other hypothalamic nuclei. In contrast, significant and site-specific decreases in relative optical density in the rostral part of the nucleus tractus solitarius (NTS) were observed in food-restricted (2, 7 days) or food-deprived (24, 48 h) animals, but these regimens were ineffective in the other extrahypothalamic sites. This study indicates the sensitivity of this mE7/8-LI probe in the hypothalamic parvocellular paraventricular nucleus and rostral NTS to food restriction and deprivation in rats.

Characteristics of rostral solitary tract nucleus neurons with identified afferent connections that project to the parabrachial nucleus in rats

Afferent information derived from oral chemoreceptors is transmitted to second-order neurons in the rostral solitary tract nucleus (rNST) and then relayed to other CNS locations responsible for complex sensory and motor behaviors. Here we investigate the characteristics of rNST neurons sending information rostrally to the parabrachial nucleus (PBN). Afferent connections to these rNST-PBN projection neurons were identified by anterograde labeling of the chorda tympani (CT), glossopharyngeal (IX), and lingual (LV) nerves. We used voltage- and current-clamp recordings in brain slices to characterize the expression of both the transient A-type potassium current, IKA and the hyperpolarization-activated inward current, Ih, important determinants of neuronal repetitive discharge characteristics. The majority of rNST-PBN neurons express IKA, and these IKA-expressing neurons predominate in CT and IX terminal fields but were expressed in approximately half of the neurons in the LV field. rNST-PBN neurons expressing Ih were evenly distributed among CT, IX and LV terminal fields. However, expression patterns of IKA and Ih differed among CT, IX, and LV fields. IKA-expressing neurons frequently coexpress Ih in CT and IX terminal fields, whereas neurons in LV terminal field often express only Ih. After GABAA receptor block all rNST-PBN neurons responded to afferent stimulation with all-or-none excitatory synaptic responses. rNST-PBN neurons had either multipolar or elongate morphologies and were distributed throughout the rNST, but multipolar neurons were more often encountered in CT and IX terminal fields. No correlation was found between the biophysical and morphological characteristics of the rNST-PBN projection neurons in each terminal field.

Two types of inhibitory influences target different groups of taste-responsive cells in the nucleus of the solitary tract of the rat

Electrical stimulation of the chorda tympani nerve (CT; innervating taste buds on the rostral tongue) is known to initiate recurrent inhibition in cells in the nucleus of the solitary tract (NTS, the first central relay in the gustatory system). Here, we explored the relationship between inhibitory circuits and the breadth of tuning of taste-responsive NTS neurons. Initially, NTS cells with evoked responses to electrical stimulation of the CT (0.1 ms pulses; 1 Hz) were tested with each of four tastants (0.1 M NaCl, 0.01 M HCl, 0.01 M quinine and 0.5 M sucrose) in separate trials. Next, the CT was electrically stimulated using a paired-pulse (10-2000 ms interpulse interval; blocks of 100 trials) paradigm. Forty-five (30 taste-responsive) of 51 cells with CT-evoked responses (36 taste-responsive) were tested with paired pulses. The majority (34; 75.6%) showed paired-pulse attenuation, defined as fewer evoked spikes in response to the second (test) pulse compared with the first (conditioning) pulse. A bimodal distribution of the peak of paired-pulse attenuation was found with modes at 10 ms and 50 ms in separate groups of cells. Cells with early peak attenuation showed short CT-evoked response latencies and large responses to relatively few taste stimuli. Conversely, cells with late peak attenuation showed long CT-evoked response latencies and small taste responses with less selectivity. Results suggest that the breadth of tuning of an NTS cell may result from the combination of the sensitivities of peripheral nerve inputs and the recurrent influences generated by the circuitry of the NTS.

Activation of delta-opioid receptors reduces excitatory input to putative gustatory cells within the nucleus of the solitary tract

The rostral nucleus of the solitary tract (NST) is the first central relay in the gustatory pathway and plays a key role in processing and modulation of gustatory information. Here, we investigated the effects of opioid receptor agonists and antagonists on synaptic responses of the gustatory parabrachial nuclei (PbN)-projecting neurons in the rostral NST to electrical stimulation of the solitary tract (ST) using whole cell recordings in the hamster brain stem slices. ST-evoked excitatory postsynaptic currents (EPSCs) were significantly reduced by met-enkephalin (MetE) in a concentration-dependent fashion and this effect was eliminated by naltrexone hydrochloride, a nonselective opioid receptor antagonist. Bath application of naltrindole hydrochloride, a selective delta-opioid receptor antagonist, eliminated MetE-induced reduction of EPSCs, whereas CTOP, a selective mu-opioid receptor antagonist had no effect, indicating that delta-opioid receptors are involved in the reduction of ST-evoked EPSCs induced by MetE. SNC80, a selective delta-opioid receptor agonist, mimicked the effect of MetE. The SNC80-induced reduction of ST-evoked EPSCs was eliminated by 7-benzylidenenaltrexone, a selective delta1-opioid receptor antagonist but not by naltriben mesylate, a selective delta2-opioid receptor antagonist, indicating that delta1-opioid receptors mediate the reduction of ST-evoked EPSCs induced by SNC80. Single-cell reverse transcriptase-polymerase chain reaction analysis revealed the presence of delta1-opioid receptor mRNA in cells that responded to SNC80 with a reduction in ST-evoked EPSCs. Moreover, Western blot analysis demonstrated the presence of 40-kDa delta-opioid receptor proteins in the rostral NST tissue. These results suggest that postsynaptic delta1-opioid receptors are involved in opioid-induced reduction of ST-evoked EPSCs of PbN-projecting rostral NST cells.

Terminal field specificity of forebrain efferent axons to brainstem gustatory nuclei

Rostral forebrain structures like the gustatory cortex (GC), bed nucleus of the stria terminalis (BNST), central nucleus of the amygdala (CeA), and lateral hypothalamus (LH) send projections to the nucleus of solitary tract (NST) and the parabrachial nucleus (PBN) that modulate taste-elicited responses. However, the proportion of forebrain-induced excitatory and inhibitory effects often differs when taste cell recording changes from the NST to the PBN. The present study investigated whether this descending influence originates from a shared or distinct population of forebrain neurons. Under electrophysiological guidance, the retrograde tracers fast blue (FB) and fluorogold (FG) or green (GFB) and red (RFB) fluorescent latex microbeads were injected iontophoretically or by pressure pulses (10 ms at 20 psi) into the taste-responsive regions of the NST and the ipsilateral PBN in six rats. Seven days later, the animals were euthanized and tissue sections containing the LH, CeA, BNST, and GC were processed for co-localization of FB and FG or GFB and RFB. The results showed that the CeA is the major source of input to the NST (82.3+/-7.6 cells/section) and the PBN (76.7+/-11.5), compared to the BNST (31.8+/-4.5; 37.0+/-4.8), the LH (35.0+/-5.4; 33.6+/-5.7), and the GC (27.5+/-4.0; 29.0+/-4.6). Of the total number of retrogradely labeled cells, the incidence of tracer co-localization was 17+/-3% in the GC, 17+/-2% in the CeA, 15+/-3% in the BNST and 16+/-1% in the LH. Thus, irrespective of forebrain source the majority of descending input to the gustatory NST and PBN originates from distinct neuronal populations. This arrangement provides an anatomical substrate for differential modulation of taste processing in the first and second central relays of the ascending gustatory system.

Local circuit input to the medullary reticular formation from the rostral nucleus of the solitary tract

The intermediate reticular formation (IRt) subjacent to the rostral (gustatory) nucleus of the solitary tract (rNST) receives projections from the rNST and appears essential to the expression of taste-elicited ingestion and rejection responses. We used whole cell patch-clamp recording and calcium imaging to characterize responses from an identified population of prehypoglossal neurons in the IRt to electrical stimulation of the rNST in a neonatal rat pup slice preparation. The calcium imaging studies indicated that IRt neurons could be activated by rNST stimulation and that many neurons were under tonic inhibition. Whole cell patch-clamp recording revealed mono- and polysynaptic projections from the rNST to identified prehypoglossal neurons. The projection was primarily excitatory and glutamatergic; however, there were some inhibitory GABAergic projections, and many neurons received excitatory and inhibitory inputs. There was also evidence of disinhibition. Overall, bath application of GABA(A) antagonists increased the amplitude of excitatory currents, and, in several neurons, stimulation of the rNST systematically decreased inhibitory currents. We have hypothesized that the transition from licks to gapes by natural stimuli, such as quinine monohydrochloride, could occur via such disinhibition. We present an updated dynamic model that summarizes the complex synaptic interface between the rNST and the IRt and demonstrates how inhibition could contribute to the transition from ingestion to rejection.

Naltrexone treatment in clomiphene resistant women with polycystic ovary syndrome

BACKGROUND

Endogenous opiates may affect various aspects of reproductive and metabolic function in patients with polycystic ovary syndrome (PCOS). This study evaluated long-term inhibition of the opioid system using naltrexone in clomiphene citrate (CC)-resistant women with PCOS.

METHODS

A group of 30 infertile females with PCOS were evaluated; all subjects were obese, hyperandrogenic and hyperinsulinemic; 16 patients were amenorrhic and 14 were oligomenorrhic. All subjects received natrexone (50 mg p.o. daily) for 6 months. Patients who did not ovulate after 12 weeks of naltrexone monotherapy, also received CC (starting at 50 mg/day for 5 days and, for non-responders, increasing it up to 150 mg/day).

RESULTS

Of the 30 women, 3 ovulated during naltrexone monotherapy and 19 of the remaining 27 ovulated during naltrexone + CC therapy. There were no conceptions during naltrexone monotherapy, but 9 of 27 women (33.3%) conceived during naltrexone + CC; there was one missed abortion at 9 weeks, one preterm delivery at 34 weeks and seven term live births. Naltrexone therapy was also followed by significant reductions in BMI, fasting serum insulin, luteinizing hormone (LH), LH/follicle-stimulating hormone ratio and testosterone.

CONCLUSIONS

In this preliminary trial, naltrexone improved endocrine and metabolic function in women with CC-resistant PCOS. Furthermore, naltrexone restored CC sensitivity in the majority of subjects, resulting in a significant number of pregnancies.

Gustatory hedonic value: potential function for forebrain control of brainstem taste processing

Among well-nourished populations, eating beyond homeostatic needs when presented with caloric-dense palatable food evidences the assertion that an increasing proportion of consumption is driven by pleasure, not just by the need for calories. This presents a major health crisis because the affective component of foods constitutes a behavioral risk factor that promotes over consumption [Sorensen, L.B., Moller, P., Flint, A., Martens, M., Raben, A., 2003. Effect of sensory perception of foods on appetite and food intake: a review of studies on humans. Int. J. Obes. Relat. Metab. Disord. 27, 1152-1166; Yeomans, M.R., Blundell, J.E., Leshem, M., 2004. Palatability: response to nutritional need or need-free stimulation of appetite? Br. J. Nutr. 92 (Suppl. 1), S3-S14]. Overweight or obese individuals have an increased risk of developing hypertension, stroke, heart disease, chronic musculoskeletal problems, type-2 diabetes, and certain types of cancers [Hill, J.O., Catenacci, V., Wyatt, H.R., 2005. Obesity: overview of an epidemic. Psychiatr. Clin. N. Am. 28, 1-23, vii]. The etiology of obesity is complex involving genetic, metabolic, and behavioral factors, but ultimately results from long-term energy imbalance. Evidence indicates that learned and some forms of unlearned control of ingestive behavior driven by palatability (i.e. hedonic value) are critically dependent on reciprocal interactions between brainstem gustatory nuclei and the ventral forebrain. This review discusses the current understanding of centrifugal control of taste processing in subcortical gustatory nuclei and the potential role of such modulation in hedonic responding.

The melanocortin antagonist AgRP (83-132) increases appetitive responding for a fat, but not a carbohydrate, reinforcer

Consumption of a diet high in fat is a risk factor for a number of health problems, including obesity, type 2 diabetes and cardiovascular disease. Considerable pharmacological, genetic, and molecular evidence suggests that the hypothalamic melanocortin system plays a critical role in the control of food intake and body weight and, specifically, in fat ingestion. Administration of a melanocortin antagonist, agouti-related peptide (AgRP) (83-132) selectively increases intake of pure fat and high-fat mixed diets. Here, we examined possible mechanisms for this fat-specific effect of AgRP (83-132). In Experiment 1, we determined that intracerebroventricular administration of AgRP (83-132) selectively increased operant responding for a peanut oil, but not a sucrose, reinforcer when tested under a progressive ratio schedule. Experiment 2 employed a Pavlovian conditioning paradigm, in which icv AgRP enhanced appetitive responding toward stimuli that had previously been paired with peanut oil and reduced responding toward stimuli previously paired with sucrose, in the absence of consumption of either macronutrient. Finally, in Experiment 3, we tested the hypothesis that the MC system acts in anticipation of a fat consumption and found that hypothalamic AgRP mRNA was slightly, though not significantly, elevated in an environment predicting fat availability relative to one predicting carbohydrate availability. Collectively, these data indicate that, in addition to increasing free intake of dietary fats, AgRP (83-132) promotes responding for the opportunity to consume a fat reinforcer, as well as appetitive responding to fat-paired stimuli in the absence of ingestive stimulation. These results suggest a possible role for AgRP in the increased fat intake associated with obesity.

Behavioral and neural responses to gustatory stimuli delivered non-contingently through intra-oral cannulas

The act of eating requires a decision by an animal to place food in its mouth. The reasons to eat are varied and include hunger as well as the food's expected reward value. Previous studies of tastant processing in the rat primary gustatory cortex (GC) have used either anesthetized or awake behaving preparations that yield somewhat different results. Here we have developed a new preparation in which we explore the influences of intra-oral and non-contingent tastant delivery on rats' behavior and on their GC neural responses. We recorded single-unit activity in the rat GC during two sequences of tastant deliveries, PRE and POST, which were separated by a waiting period. Six tastants ranging in hedonic value from sucrose to quinine were delivered in the first two protocols called 4TW and L-S. In the third one, the App L-S protocol, only hedonically positive tastants were used. In the 4TW protocol, tastants were delivered in blocks whereas in the two L-S protocols tastants were randomly interleaved. In the 4TW and L-S protocols the probability of ingesting tastants in the PRE sequence decreased exponentially with the trial number. Moreover, in both protocols this decrease was greater in the POST than in the PRE sequence likely because the subjects learned that unpleasant tastants were to be delivered. In the App L-S protocol the decrease in ingestion was markedly slower than in the other protocols, thus supporting the hypothesis that the decrease in appetitive behavior arises from the non-contingent intra-oral delivery of hedonically negative tastants like quinine. Although neuronal responses in the three protocols displayed similar variability levels, significant differences existed between the protocols in the way the variability was partitioned between chemosensory and non-chemosensory neurons. While in the 4TW and L-S protocols the former population displayed more changes than the latter, in the App L-S protocol variability was homogeneously distributed between the two populations. We posit that these tuning changes arise, at least in part, from compounds released upon ingestion, and also from differences in areas of the oral cavity that are bathed as the animals ingest or reject the tastants.

FG 7142 specifically reduces meal size and the rate and regularity of sustained feeding in female rats: evidence that benzodiazepine inverse agonists reduce food palatability

Benzodiazepine receptor inverse agonists reduce food intake in males, but their actions in females, in whom stress-related eating disorders are more common, as well as their behavioral mode of action remain unclear. The consummatory effects of benzodiazepine receptor ligands have alternately been hypothesized to reflect changes in the hedonic evaluation of food or secondary effects of anxiety-related or cognitive properties. To test the anorectic mode of action of benzodiazepine inverse agonists, the effects of FG 7142 on feeding microstructure were studied in nondeprived female Wistar rats (n=32). Microstructure analysis used a novel meal definition that recognizes prandial drinking. On pharmacologically synchronized diestrus I, rats were pretreated (-30 min dark onset) with the benzodiazepine partial inverse agonist FG 7142 (i.p. 0, 3.75, 7.5, 15 mg/kg) in a between-subjects design. FG 7142 delayed the onset of (16-541%), decreased the amount eaten (36-52%) and drunk (63-87%), and reduced the time spent drinking (59-87%) within the first nocturnal meal. Dose-dependent incremental anorexia continued 6 h into the dark cycle, whereas FG 7142 did not suppress the quantity, duration or rate of drinking past the first meal. Treated rats ate smaller meals (17-42%) of normal duration. This reflected that FG 7142 slowed feeding within meals (9-38%) by decreasing the regularity and maintenance of feeding from pellet-to-pellet. FG 7142 did not influence postprandial satiety; meal frequency and inter-meal intervals were unaffected. FG 7142 anorexia was blocked by the benzodiazepine receptor antagonist flumazenil in a 2:1 molar ratio (n=17 rats). The very early, nonspecific (+10 min), but not subsequent (2.5, 4.5 h) feeding-specific phase, of FG 7142 anorexia was mirrored by anxiogenic-like behavior in FG 7142-treated (7.5 mg/kg) female rats (n=48) in the elevated plus-maze. Thus, benzodiazepine receptor inverse agonists preferentially lessen the maintenance of feeding in female rats, effects opposite to those of palatable food.

Taste-evoked Fos expression in nitrergic neurons in the nucleus of the solitary tract and reticular formation of the rat

The current investigation used double labeling for NADPHd and Fos-like immunoreactivity to define the relationship between nitric oxide synthase-containing neural elements and taste-activated neurons in the nucleus of the solitary tract (NST) and subjacent reticular formation (RF). Stimulation of awake rats with citric acid and quinine resulted in significant increases in the numbers of double-labeled neurons in both the NST and RF, suggesting that some medullary gustatory neurons utilize nitric oxide (NO) as a transmitter. Overall, double-labeled neurons were most numerous in the caudal reaches of the gustatory zone of the NST, where taste neurons receive inputs from the IXth nerve, suggesting a preferential role for NO neurons in processing gustatory inputs from the posterior oral cavity. However, double-labeled neurons also exhibited a preferential distribution depending on the gustatory stimulus. In the NST, double-labeled neurons were most numerous in the rostral central subnucleus after either stimulus but had a medial bias after quinine stimulation. In the RF, after citric acid stimulation, there was a cluster of double-labeled neurons with distinctive large soma in the parvicellular division of the lateral RF, subjacent to the rostral tip of NST. In contrast, in response to quinine, there was a cluster of double-labeled neurons with much smaller soma in the intermediate zone of the medial RF, a few hundred micrometers caudal to the citric acid cluster. These differential distributions of double-labeled neurons in the NST and RF suggest a role for NO in stimulus-specific gustatory autonomic and oromotor reflex circuits.

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

Cholinergic modulation of neurons in the gustatory region of the nucleus of the solitary tract

The rostral portion of the nucleus of the solitary tract (rNST) is an obligatory relay for gustatory afferent input on its way to the forebrain. Previous studies have demonstrated excitation of rNTS neurons by glutamate and substance P and inhibition by gamma-aminobutyric acid (GABA) and met-enkephalin (ENK). Despite the existence of cholinergic neurons and putative terminals within the rNTS, there are no data on the effects of acetylcholine (ACh) on rNTS processing. Here, we use patch-clamp recording of rNTS neurons in vitro to examine ACh-mediated responses and voltage-gated conductances in these cells. Results revealed (1) intrinsic voltage-gated inhibition via activation of voltage-gated potassium A-channels (I(A)), found almost exclusively in the medial rNTS, and hyperpolarization-activated potassium/sodium channels (I(h)), found more frequently in the lateral rNST; and (2) ligand-gated inhibition via activation of muscarinic m2 ACh receptors (mAChRs) linked to inward rectifier potassium channels (K(ir)) evenly distributed throughout the rNTS, a mechanism dependent on cholinergic inputs. Muscarinic responses were blocked by AFDX-116, a selective m2 mAChR antagonist, and by BaCl2, an antagonist of K(ir) channels. In addition, many rNTS neurons exhibited excitation via alpha7 and non-alpha7 nicotinic AChRs. Non-alpha7 nAChRs, blocked by 10 microM mecamylamine, occurred more frequently in the lateral rNTS. In contrast, alpha7 nAChRs, blocked by 20 nM methyllcaconitine, were evenly distributed across the nucleus. As previously reported for voltage-activated conductances, none of these currents was related to neuronal morphology. These voltage- and ligand-dependent inhibitory mechanisms would be expected to contribute to the modulation of gustatory processing through the NST.

Endogenous opiates and behavior: 2003

This paper is the 26th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2003 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Endogenous opioids and feeding behavior: a 30-year historical perspective

This invited review, based on the receipt of the Third Gayle A. Olson and Richard D. Olson Prize for the publication of the outstanding behavioral article published in the journal Peptides in 2002, examines the 30-year historical perspective of the role of the endogenous opioid system in feeding behavior. The review focuses on the advances that this field has made over the past 30 years as a result of the timely discoveries that were made concerning this important neuropeptide system, and how these discoveries were quickly applied to the analysis of feeding behavior and attendant homeostatic processes. The discoveries of the opioid receptors and opioid peptides, and the establishment of their relevance to feeding behavior were pivotal in studies performed in the 1970s. The 1980s were characterized by the establishment of opioid receptor subtype agonists and antagonists and their relevance to the modulation of feeding behavior as well as by the use of general opioid antagonists in demonstrating the wide array of ingestive situations and paradigms involving the endogenous opioid system. The more recent work from the 1990s to the present, utilizes the advantages created by the cloning of the opioid receptor genes, the development of knockout and knockdown techniques, the systematic utilization of a systems neuroscience approach, and establishment of the reciprocity of how manipulations of opioid peptides and receptors affect feeding behavior with how feeding states affect levels of opioid peptides and receptors. The role of G-protein effector systems in opioid-mediated feeding responses, which was the subject of the prize-winning article, is then reviewed.

An immunocytochemical mapping of methionine-enkephalin-arg6-gly7-leu8 in the human brainstem

Using an indirect immunoperoxidase technique, we studied the distribution of immunoreactive fibers and cell bodies containing methionine-enkephalin-Arg(6)-Gly(7)-Leu(8) in the adult human brainstem. Immunoreactive cell bodies were found in the reticular formation of the medulla oblongata (in which we observed the highest density of immunoreactive cell bodies) and the pons, the solitary nucleus, the hypoglossal nucleus, the medial and spinal vestibular nuclei, the lateral cuneate nucleus, the nucleus prepositus, the central gray of the pons and mesencephalon, the central and pericentral nuclei of the inferior colliculus, the superior colliculus, ventral to the superior olive and in the midline region of the pons and mesencephalon. The highest density of immunoreactive fibers containing methionine-enkephalin-Arg(6)-Gly(7)-Leu(8) was found in the spinal trigeminal nucleus, the central gray and the reticular formation of the medulla oblongata, pons and mesencephalon, the solitary nucleus, the spinal vestibular nucleus, the dorsal accessory olivary nucleus, the raphe obscurus, the substantia nigra and in the interpeduncular nucleus. The widespread distribution of immunoreactive structures containing methionine-enkephalin-Arg(6)-Gly(7)-Leu(8) in the human brainstem indicates that this neuropeptide might be involved in several physiological mechanisms, acting as a neurotransmitter and/or neuromodulator.