Effects of methyl-deficient diets on methionine and homocysteine metabolism in the pregnant rat

Although the importance of methyl metabolism in fetal development is well recognized, there is limited information on the dynamics of methionine flow through maternal and fetal tissues and on how this is related to circulating total homocysteine concentrations. Rates of homocysteine remethylation in maternal and fetal tissues on days 11, 19, and 21 of gestation were measured in pregnant rats fed diets with limiting or surplus amounts of folic acid and choline at two levels of methionine and then infused with L-[1-(13)C,(2)H(3)-methyl]methionine. The rate of homocysteine remethylation was highest in maternal liver and declined as gestation progressed. Diets deficient in folic acid and choline reduced the production of methionine from homocysteine in maternal liver only in the animals fed a methionine-limited diet. Throughout gestation, the pancreas exported homocysteine for methylation within other tissues. Little or no methionine cycle activity was detected in the placenta at days 19 and 21 of gestation, but, during this period, fetal tissues, especially the liver, synthesized methionine from homocysteine. Greater enrichment of homocysteine in maternal plasma than placenta, even in animals fed the most-deficient diets, shows that the placenta did not contribute homocysteine to maternal plasma. Methionine synthesis from homocysteine in fetal tissues was maintained or increased when the dams were fed folate- and choline-deficient methionine-restricted diets. This study shows that methyl-deficient diets decrease the remethylation of homocysteine within maternal tissues but that these rates are protected to some extent within fetal tissues.

Development aggravates the severity of skeletal muscle catabolism induced by endotoxemia in neonatal pigs

Accretion rates of muscle protein are elevated in normal neonates, but this anabolic drive decreases with maturation. As this change occurs, it is not known whether development also influences muscle protein catabolism induced by sepsis. We hypothesize that protein degradation in skeletal muscle induced by endotoxemia becomes more severe as the neonate develops. Fasted 7- and 26-day-old pigs were infused for 8 h with LPS (0 and 10 μg·kg(-1)·h(-1)), while plasma amino acids (AA), 3-methylhistidine (3-MH), and α-actin concentrations and muscle protein degradation signal activation were determined (n = 5-7/group/age). Plasma full-length α-actin was greater in 7- than 26-day-old pigs, suggesting a higher baseline protein turnover in neonatal pigs. LPS increased plasma total AA, 3-MH, and full-length and cleaved α-actin in 26- than in 7-day-old pigs. In muscle of both age groups, LPS increased AMPK and NF-κB phosphorylation, the abundances of activated caspase 3 and E-3 ligases MuRF1 and atrogin1, as well as the abundance of cleaved α-actin, suggesting activation of muscle proteolysis by endotoxin in muscle. LPS decreased Forkhead box 01 (Fox01) and Fox04 phosphorylation and increased procaspase 3 abundance in muscle of 26-day-old pigs despite the lack of effect of LPS on PKB phosphorylation. The results suggest that skeletal muscle in healthy neonatal pigs maintains high baseline degradation signal activation that cannot be enhanced by endotoxin, but as maturation advances, the effect of LPS on muscle protein catabolism manifests its severity.

Intermittent bolus feeding has a greater stimulatory effect on protein synthesis in skeletal muscle than continuous feeding in neonatal pigs

Orogastric tube feeding, using either continuous or intermittent bolus delivery, is common in infants for whom normal feeding is contraindicated. To compare the impact of different feeding strategies on muscle protein synthesis, after withholding food overnight, neonatal pigs received a complete formula orally as a bolus feed every 4 h or were continuously fed. Protein synthesis rate and translational mechanisms in skeletal muscle were examined after 0, 24, and 25.5 h. Plasma amino acid and insulin concentrations increased minimally and remained constant in continuously fed compared to feed-deprived pigs; however, the pulsatile meal feeding pattern was mimicked in bolus-fed pigs. Muscle protein synthesis was stimulated by feeding and the greatest response occurred after a bolus meal. Bolus but not continuous feeds increased polysome aggregation, the phosphorylation of protein kinase B, tuberous sclerosis complex 2, proline-rich Akt substrate of 40 kDa, eukaryotic initiation factor (eIF) 4E binding protein (4EBP1), and rp S6 kinase and enhanced dissociation of the 4EBP1 ·eIF4E complex and formation of the eIF4E ·eIF4G complex compared to feed deprivation (P < 0.05). Activation of insulin receptor substrate-1, regulatory associated protein of mammalian target of rapamycin, AMP-activated protein kinase, eukaryotic elongation factor 2, and eIF2α phosphorylation were unaffected by either feeding modality. These results suggest that in neonates, intermittent bolus feeding enhances muscle protein synthesis to a greater extent than continuous feeding by eliciting a pulsatile pattern of amino acid- and insulin-induced translation initiation.

Differential regulation of protein synthesis and mTOR signaling in skeletal muscle and visceral tissues of neonatal pigs after a meal

Protein synthesis (PS) increases after a meal in neonates, but the time course of the changes in PS in different tissues after a meal is unknown. We aimed to evaluate the changes in tissue PS, mammalian target of rapamycin complex 1 (mTORC1) activation, and proportion of ribosomal protein (rp) mRNAs in polysomes over 4 h after a bolus meal in neonatal pigs (n = 6/group; 5- to 7-d-old). The results show a more sustained increase in PS in glycolytic compared with mixed fiber type muscles and no changes in oxidative muscles. PS increased in liver, jejunum, and pancreas but not in kidney and heart. Feeding did not affect AMP-activated protein kinase or RAS-related GTP binding B activation. Phosphorylation of tuberous sclerosis complex 2, proline-rich Akt substrate of 40 kD, mTOR, eukaryotic initiation factor 4E binding protein, and rp S6 kinase 1 increased in all tissues after feeding. The proportion of mRNAs encoding rp S4 and S8 in liver polysomes increased within 30 min postfeeding. These results suggest that feeding stimulates mTORC1 signaling in muscle and viscera, but mTORC1 activation alone is not sufficient to stimulate PS in all tissues.

Sepsis and development impede muscle protein synthesis in neonatal pigs by different ribosomal mechanisms

In muscle, sepsis reduces protein synthesis (MPS) by restraining translation in neonates and adults. Even though protein accretion decreases with development as neonatal MPS rapidly declines by maturation, the changes imposed by development on the sepsis-associated decrease in MPS have not been described. Pigs at 7 and 26 d of age were infused for 8 h with lipopolysaccharide (LPS, endotoxin, 0 and 10 μg · kg⁻¹ · h⁻¹). Fractional MPS rates and translation eukaryotic initiation factor (eIF) activation in muscle were examined (n = 5-7/group). The LPS-induced decrease in MPS was associated with reduced ribosomal and translational efficiency, whereas the age-induced decrease in MPS occurred by decreasing ribosome number. Abundances of mammalian target of rapamycin (mTOR) and S6 decreased, and that of the repressor eIF4E · 4E-binding protein 1 (4EBP1) association increased in 26-d-old pigs--compared with 7-d-old pigs. LPS decreased the abundance of the active eIF4E ·eIF4G association and the phosphorylation of eIF4G across ages, whereas the abundance of eIF4G declined and eIF2α phosphorylation increased with age. Therefore, when lacking anabolic stimulation, the decrease in MPS induced by LPS is associated with reduced ribosomal efficiency and decreased eIF4E ·eIF4G assembly, whereas that induced by development involves reduced ribosomal number, translation factor abundance, and increased eIF2α phosphorylation.

Differential effects of long-term leucine infusion on tissue protein synthesis in neonatal pigs

Leucine is unique among the amino acids in its ability to promote protein synthesis by activating translation initiation via the mammalian target of rapamycin (mTOR) pathway. Previously, we showed that leucine infusion acutely stimulates protein synthesis in fast-twitch glycolytic muscle of neonatal pigs but this response cannot be maintained unless the leucine-induced fall in amino acids is prevented. To determine whether leucine can stimulate protein synthesis in muscles of different fiber types and in visceral tissues of the neonate in the long-term if baseline amino acid concentrations are maintained, overnight fasted neonatal pigs were infused for 24 h with saline, leucine (400 micromol kg(-1) h(-1)), or leucine with replacement amino acids to prevent the leucine-induced hypoaminoacidemia. Changes in the fractional rate of protein synthesis and activation of mTOR, as determined by eukaryotic initiation factor 4E binding protein (4E-BP1) and S6 kinase 1 (S6K1) phosphorylation, in the gastrocnemius and masseter muscles, heart, liver, jejunum, kidney, and pancreas were measured. Leucine increased mTOR activation in the gastrocnemius and masseter muscles, liver, and pancreas, in both the absence and presence of amino acid replacement. However, protein synthesis in these tissues was increased only when amino acids were infused to maintain baseline levels. There were no changes in mTOR signaling or protein synthesis in the other tissues we examined. Thus, long-term infusion of leucine stimulates mTOR signaling in skeletal muscle and some visceral tissues but the leucine-induced stimulation of protein synthesis in these tissues requires sustained amino acid availability.

Stimulation of muscle protein synthesis by prolonged parenteral infusion of leucine is dependent on amino acid availability in neonatal pigs

The postprandial rise in amino acids, particularly leucine, stimulates muscle protein synthesis in neonates. Previously, we showed that a 1-h infusion of leucine increased protein synthesis, but this response was not sustained for 2 h unless the leucine-induced decrease in amino acids was prevented. To determine whether a parenteral leucine infusion can stimulate protein synthesis for a more prolonged, clinically relevant period if baseline amino acid concentrations are maintained, overnight food-deprived neonatal pigs were infused for 24 h with saline, leucine (400 mumol.kg(-1). h(-1)), or leucine with replacement amino acids. Amino acid replacement prevented the leucine-induced decrease in amino acids. Muscle protein synthesis was increased by leucine but only when other amino acids were supplied to maintain euaminoacidemia. Leucine did not affect activators of mammalian target of rapamycin (mTOR), i.e. protein kinase B, AMP-activated protein kinase, tuberous sclerosis complex 2, or eukaryotic elongation factor 2. There was no effect of treatment on the association of mTOR with regulatory associated protein of mammalian target of rapamycin (raptor), G-protein beta subunit-like protein, or rictor or the phosphorylation of raptor or proline-rich Akt substrate of 40 kDa. Phosphorylation of mTOR and its downstream targets, eukaryotic initiation factor (eIF) 4E binding protein and ribosomal protein S6 kinase, and the eIF4E . eIF4G association were increased and eIF2alpha phosphorylation was reduced by leucine and was not further altered by correcting for the leucine-induced hypoaminoacidemia. Thus, prolonged parenteral infusion of leucine activates mTOR and its downstream targets in neonatal skeletal muscle, but the stimulation of protein synthesis also is dependent upon amino acid availability.

Feeding rapidly stimulates protein synthesis in skeletal muscle of neonatal pigs by enhancing translation initiation

Food consumption increases protein synthesis in most tissues by promoting translation initiation, and in the neonate, this increase is greatest in skeletal muscle. In this study, we aimed to identify the currently unknown time course of changes in the rate of protein synthesis and the activation of factors involved in translation in neonatal muscle after a meal. After overnight food deprivation, 36 5- to 7-d-old piglets were administered a nutritionally complete bolus i.g. meal and were killed immediately before or 30, 60, 90, 120, or 240 min later. The increase in skeletal muscle protein synthesis peaked 30 min after the meal and this was sustained through 120 min, returning to baseline thereafter. The relative proportion of polysomes to nonpolysomes was higher only after 30 min. Protein kinase B phosphorylation peaked 30 min after feeding and returned to baseline by 90 min. The phosphorylation of mammalian target of rapamycin, eukaryotic initiation factor (eIF) 4E binding protein (4E-BP1), ribosomal protein S6, and eIF4G was increased within 30 min of feeding and persisted through 120 min, but all had returned to baseline by 240 min. The association of 4E-BP1.eIF4E was reduced and eIF4E.eIF4G increased 30 min after receiving a meal, remaining so for 120 min, before returning to baseline at 240 min. Thus, in neonates, food consumption rapidly increased skeletal muscle protein synthesis by enhancing translation initiation and this increase was sustained for at least 120 min after the meal but returned to baseline by 240 min after the feeding.

Tissue methionine cycle activity and homocysteine metabolism in female rats: impact of dietary methionine and folate plus choline

Impaired transfer of methyl groups via the methionine cycle leads to plasma hyperhomocysteinemia. The tissue sources of plasma homocysteine in vivo have not been quantified nor whether hyperhomocysteinemia is due to increased entry or decreased removal. These issues were addressed in female rats offered diets with either adequate or excess methionine (additional methyl groups) with or without folate and choline (impaired methyl group transfer) for 5 wk. Whole body and tissue metabolism was measured based on isotopomer analysis following infusion with either [1-(13)C,methyl-(2)H3]methionine or [U-(13)C]methionine plus [1-(13)C]homocysteine. Although the fraction of intracellular methionine derived from methylation of homocysteine was highest in liver (0.18-0.21), most was retained. In contrast, the pancreas exported to plasma more of methionine synthesized de novo. The pancreas also exported homocysteine to plasma, and this matched the contribution from liver. Synthesis of methionine from homocysteine was reduced in most tissues with excess methionine supply and was also lowered in liver (P<0.01) with diets devoid of folate and choline. Plasma homocysteine concentration (P<0.001) and flux (P=0.001) increased with folate plus choline deficiency, although the latter still represented <12% of estimated tissue production. Hyperhomocysteinemia also increased (P<0.01) the inflow of homocysteine into most tissues, including heart. These findings indicate that a full understanding of hyperhomocysteinemia needs to include metabolism in a variety of organs, rather than an exclusive focus on the liver. Furthermore, the high influx of homocysteine into cardiac tissue may relate to the known association between homocysteinemia and hypertension.

Leucine stimulates protein synthesis in skeletal muscle of neonatal pigs by enhancing mTORC1 activation

Skeletal muscle in the neonate grows at a rapid rate due in part to an enhanced sensitivity to the postprandial rise in amino acids, particularly leucine. To elucidate the molecular mechanism by which leucine stimulates protein synthesis in neonatal muscle, overnight-fasted 7-day-old piglets were treated with rapamycin [an inhibitor of mammalian target of rapamycin (mTOR) complex (mTORC)1] for 1 h and then infused with leucine for 1 h. Fractional rates of protein synthesis and activation of signaling components that lead to mRNA translation were determined in skeletal muscle. Rapamycin completely blocked leucine-induced muscle protein synthesis. Rapamycin markedly reduced raptor-mTOR association, an indicator of mTORC1 activation. Rapamycin blocked the leucine-induced phosphorylation of mTOR, S6 kinase 1 (S6K1), and eukaryotic initiation factor (eIF)4E-binding protein-1 (4E-BP1) and formation of the eIF4E.eIF4G complex and increased eIF4E.4E-BP1 complex abundance. Rapamycin had no effect on the association of mTOR with rictor, a crucial component for mTORC2 activation, or G protein beta-subunit-like protein (GbetaL), a component of mTORC1 and mTORC2. Neither leucine nor rapamycin affected the phosphorylation of AMP-activated protein kinase (AMPK), PKB, or tuberous sclerosis complex (TSC)2, signaling components that reside upstream of mTOR. Eukaryotic elongation factor (eEF)2 phosphorylation was not affected by leucine or rapamycin, although current dogma indicates that eEF2 phosphorylation is mTOR dependent. Together, these in vivo data suggest that leucine stimulates muscle protein synthesis in neonates by enhancing mTORC1 activation and its downstream effectors.

Fed levels of amino acids are required for the somatotropin-induced increase in muscle protein synthesis

Chronic somatotropin (pST) treatment in pigs increases muscle protein synthesis and circulating insulin, a known promoter of protein synthesis. Previously, we showed that the pST-mediated rise in insulin could not account for the pST-induced increase in muscle protein synthesis when amino acids were maintained at fasting levels. This study aimed to determine whether the pST-induced increase in insulin promotes skeletal muscle protein synthesis when amino acids are provided at fed levels and whether the response is associated with enhanced translation initiation factor activation. Growing pigs were treated with pST (0 or 180 microg x kg(-1) x day(-1)) for 7 days, and then pancreatic-glucose-amino acid clamps were performed. Amino acids were raised to fed levels in the presence of either fasted or fed insulin concentrations; glucose was maintained at fasting throughout. Muscle protein synthesis was increased by pST treatment and by amino acids (with or without insulin) (P<0.001). In pST-treated pigs, fed, but not fasting, amino acid concentrations further increased muscle protein synthesis rates irrespective of insulin level (P<0.02). Fed amino acids, with or without raised insulin concentrations, increased the phosphorylation of S6 kinase (S6K1) and eukaryotic initiation factor (eIF) 4E-binding protein 1 (4EBP1), decreased inactive 4EBP1.eIF4E complex association, and increased active eIF4E.eIF4G complex formation (P<0.02). pST treatment did not alter translation initiation factor activation. We conclude that the pST-induced stimulation of muscle protein synthesis requires fed amino acid levels, but not fed insulin levels. However, under the current conditions, the response to amino acids is not mediated by the activation of translation initiation factors that regulate mRNA binding to the ribosomal complex.

Stimulation of muscle protein synthesis by somatotropin in pigs is independent of the somatotropin-induced increase in circulating insulin

Chronic treatment of growing pigs with porcine somatotropin (pST) promotes protein synthesis and doubles postprandial levels of insulin, a hormone that stimulates translation initiation. This study aimed to determine whether the pST-induced increase in skeletal muscle protein synthesis was mediated through an insulin-induced stimulation of translation initiation. After 7-10 days of pST (150 microg x kg(-1) x day(-1)) or control saline treatment, pancreatic glucose-amino acid clamps were performed in overnight-fasted pigs to reproduce 1) fasted (5 microU/ml), 2) fed control (25 microU/ml), and 3) fed pST-treated (50 microU/ml) insulin levels while glucose and amino acids were maintained at baseline fasting levels. Fractional protein synthesis rates and indexes of translation initiation were examined in skeletal muscle. Effectiveness of pST treatment was confirmed by reduced urea nitrogen and elevated insulin-like growth factor I levels in plasma. Skeletal muscle protein synthesis was independently increased by both insulin and pST. Insulin increased the phosphorylation of protein kinase B and the downstream effectors of the mammalian target of rapamycin, ribosomal protein S6 kinase, and eukaryotic initiation factor (eIF)4E-binding protein-1 (4E-BP1). Furthermore, insulin reduced inactive 4E-BP1.eIF4E complex association and increased active eIF4E.eIF4G complex formation, indicating enhanced eIF4F complex assembly. However, pST treatment did not alter translation initiation factor activation. We conclude that the pST-induced stimulation of skeletal muscle protein synthesis in growing pigs is independent of the insulin-associated activation of translation initiation.

Sulfur amino acid metabolism in pregnancy: the impact of methionine in the maternal diet

Animal studies show that the balance of methionine relative to other amino acids in the maternal diet is critical, as fetal growth is not only retarded by diets that are deficient but also by those containing excess. Diets with an inappropriate balance of methionine can adversely affect both short-term reproductive function and the long-term physiology of the offspring. The catabolism of unused methionine increases the demand for glycine and may cause a deficiency. High levels of methionine may also perturb intracellular S-adenosyl methionine pools and have an effect on the methylation of DNA and proteins. Excess methionine in the diet may also indirectly influence fetal development through the production of homocysteine or by the perturbation of endocrine functions. The metabolic interactions among dietary methionine, folic acid, and choline mean that other diet components can also change the methionine requirement.

Quercetin glucosides are completely hydrolyzed in ileostomy patients before absorption

Flavonoids, dietary components in vegetables, fruits and beverages, may protect against coronary heart disease, stroke and cancer. However, the bioavailability of these compounds is questionable. A previous study in ileostomy patients of the most abundant flavonoid, quercetin, suggested a 52% absorption of its major dietary forms, monoglucoside (QMG) and diglucoside (QDG), from an onion meal. However, this was based on indirect measurements after acid hydrolysis. Because human intestinal Caco-2 cell monolayers showed minimal absorption of the glucosides, we repeated the study in ileostomy patients, using molecularly specific analytical methodology for the intact glucosides and quercetin. The onion meal had high concentrations of both QMG and QDG with only trace amounts of quercetin. The intake of QMG and QDG in four patients ranged from 10.9 to 51.6 mg. No QMG or QDG was detected in the ileostomy fluid. In contrast, the amounts of the aglycone quercetin were substantial, 2.9-11.3 mg. This corresponded to 19.5-35.2% of total quercetin glucosides ingested, implying absorption of 64.5-80.7%. These findings suggest a different interpretation than that from the previous study, i.e., that both QMG and QDG are efficiently hydrolyzed in the small intestine by beta-glucosidases to quercetin, most of which is then absorbed.

Face-selective neurons during passive viewing and working memory performance of rhesus monkeys: evidence for intrinsic specialization of neuronal coding

The functional organization of prefrontal cortex (PFC) is a central issue in cognitive neuroscience. Previous physiological investigations have often failed to reveal specialization within the PFC. However, these studies have generally not been designed to examine this issue. Methodological issues such as statistical criteria for specificity, the number of neurons sampled, the extent of cortex sampled, and the number, location and nature of the stimuli used are among the variables that need to be considered in evaluating the results of studies on functional localization. In the present study, we have examined neurons in macaque monkeys trained to fixate while viewing visual stimuli, including faces, or to use them as memoranda on a working memory task. Visual responses of over 1500 neurons were recorded throughout a wide expanse of the PFC (areas 12, 9, 46, 8 and 45). Neurons were considered selective for faces if the best response to a face was over twice as strong as that to any of a wide variety of non-face stimuli. Full electrode track reconstructions in three monkeys revealed in each that neurons which met this criterion were concentrated almost exclusively in three distinct subregions within the projection region of the temporal lobe visual areas. We further show that for all neurons, the most visually selective neurons (for faces, objects or color patterns) were also the most concentrated in the temporal lobe recipient PFC. Similar face selectivity, regional specialization, and delay or delay-like activity were observed in monkeys whether trained on memory tasks or not, which suggests that these are naturally occurring properties of prefrontal neurons. These results confirm neuronal and regional specialization for information processing in PFC and elucidate how heretofore unexamined experimental variables have a strong influence on the detection of regional specialization.

Areal segregation of face-processing neurons in prefrontal cortex

A central issue in cognitive neuroscience concerns the functional architecture of the prefrontal cortex and the degree to which it is organized by sensory domain. To examine this issue, multiple areas of the macaque monkey prefrontal cortex were mapped for selective responses to visual stimuli that are prototypical of the brain's object vision pathway-pictorial representations of faces. Prefrontal neurons not only selectively process information related to the identity of faces but, importantly, such neurons are localized to a remarkably restricted area. These findings suggest that the prefrontal cortex is functionally compartmentalized with respect to the nature of its inputs.

Ontogenic and glucocorticoid-accelerated expression of rat 14 kDa bile acid-binding protein

BACKGROUND

A 14 kDa protein is a major rat ileal cytosolic bile acid-binding protein (14 kDa I-BABP). This report describes the normal and glucocorticoid-accelerated postnatal expression of 14 kDa I-BABP. METHODS. 14 kDa I-BABP and its mRNA were detected by antiserum and antisense cRNA probe, respectively, in ileum and ovary from 7-, 14-, 21-, 28-, and 35-day-old rats.

RESULTS

A positive histochemical reaction with 14 kDa I-BABP antiserum was found in cytosol of ileal enterocytes and ovarian luteal cells at 21 and 35 days of age, respectively. Likewise, Northern blot analysis indicated that the cRNA probe hybridized to a single transcript of 500 bp in total cellular RNA prepared from the ileum of 21-day-old and ovary of 35-day-old rats. Corticosteroid treatment resulted in a histochemical reaction in ileum of 14-day-old animals as compared to the appearance of this reaction in 21-day-old control littermates. Similarly, Northern blot analysis detected the earlier appearance of mRNA levels in corticosteroid-treated animals (11 days vs. 17 days in control animals).

CONCLUSIONS

14 kDa I-BABP and its mRNA are normally expressed in rat ileum at 17 days of postnatal life. 14 kDa I-BABP is expressed considerably later in rat ovary. Corticosteroid treatment results in precocious expression of ileal 14 kDa I-BABP.

Molecular cloning, tissue distribution, and expression of a 14-kDa bile acid-binding protein from rat ileal cytosol

A cDNA clone encoding the major intestinal cytosolic 14-kDa bile acid-binding protein (14-kDa I-BABP) was isolated from a rat ileal lambda gt22A library following immunoscreening using a monospecific antiserum raised against a 14-kDa polypeptide found in the rat ileal cytosol. One clone of 516 bp encoded a 128-amino acid protein with a predicted molecular mass of 14,544 Da. The deduced amino acid sequence of 14-kDa I-BABP showed 100% homology to rat intestinal 15-kDa protein (I-15P) and 72% homology to porcine 15-kDa gastrotropin, whereas comparison of I-BABP to rat 14-kDa fatty acid-binding proteins of liver, intestine, and heart revealed homologies of 44%, 25%, and 28%, respectively. Northern blot analysis revealed a single transcript of approximately 0.5 kb in ileum and ovary; however, the abundance of I-BABP mRNA was much greater in ileum than in ovary. No transcript was seen in RNA extracted from stomach, jejunum, colon, liver, adrenal, brain, heart, kidney, or testis. Transfection of the I-BABP cDNA into COS-7 cells resulted in the expression of a 14-kDa protein that was identical to the ileal cytosolic I-BABP as determined by immunoblotting. Photoaffinity labeling of expressed 14-kDa protein was saturable with respect to increasing concentrations of 7,7-azo[3H]taurocholate (Km, 83.3 microM; Vmax, 6.7 pmol/mg per 5 min). Taurocholate inhibited 7,7-azotaurocholate labeling by > 96% with lesser inhibition by taurochenodeoxycholate (83.1%), chenodeoxycholate (74.6%), cholate (50.5%), and progesterone (38.5%), whereas oleic acid and estradiol did not inhibit binding.

Functional synergism between putative gamma-aminobutyrate-containing neurons and pyramidal neurons in prefrontal cortex

The responses of putative gamma-aminobutyratergic interneurons (fast-spiking) and pyramidal (regular-spiking) cell pairs were compared in monkeys performing visual and memory-guided oculomotor tasks. Both fast- and regular-spiking neurons had similar receptive fields, indicating that gamma-aminobutyratergic interneurons carry a specific informational signal, as opposed to providing nonspecific modulation. However, the responses of the pairs were inverted and the timing of excitatory and inhibitory responses appeared to be phased, a property consistent with gamma-aminobutyrate-mediated shaping of receptive fields. These observations (i) provide evidence that interneurons and pyramidal cells can be differentiated in vivo and (ii) begin to elucidate the role of gamma-aminobutyratergic mechanisms in cognition.

Viewing preferences of rhesus monkeys related to memory for complex pictures, colours and faces

In order to determine the preferences of rhesus monkeys for visual stimuli, their eye movements were measured in response to presentations of complex pictures, fields of uniform colour, and of faces using the scleral search coil technique. The monkeys (n = 4) controlled both the onset and offset of the stimuli by the direction of their gaze. Each stimulus was presented 4 times, with 0 or 2, and 36 or 38 trials between successive presentations. Several trends were apparent in their scanning behaviour: (1) all 4 monkeys spent more time looking at pictures and faces compared to colour fields. As individuals, they differed in their overall propensity in looking at visual stimuli: monkeys that spent the most (or least) time looking at pictures spent the most (or least) time looking at colour fields. (2) Although the monkeys appeared to prefer pictures and faces to colour fields as measured by gaze duration, preferences for individual pictures, faces and colour fields were not evident. (3) Memory for recently presented stimuli substantially affected gaze duration which was significantly longer for the first compared to the second presentation of the pictures and faces, and memory was estimated to influence gaze duration over as many as 38 intervening trials. These effects were not significant in the case of colour fields. (4) There were no significant differences either in the average latencies to initiate eye movements or the number of saccades on the first and second presentations of pictures, colors or faces for the 4 monkeys. However, the average latencies to the first eye movement within a trial were longer for colour fields than for pictures for all 4 monkeys. Individual monkeys differed substantially in their mean latencies for the initiation of the first eye movement within a trial, which ranged from 235 ms to 414 ms in the two extreme cases. (5) At the presentation of faces, the monkeys tended to make saccades to major facial features, and only occasionally to the perimeter of faces. We conclude, firstly, that patterns of eye movements in monkeys reflect their natural predilection for sampling novel stimuli and that monkeys are motivated to view visual arrays. Secondly, that the protracted viewing of novel stimuli and long latencies to initiate saccades contrasts with short latency neuronal responses recorded in inferior temporal cortex. These behavioural and neurophysiological data suggest that neuronal activity specific to novel stimuli and to faces is not a consequence of oculomotor responses to these stimuli.

Timed photoaffinity labeling and characterization of bile acid binding and transport proteins in rat ileum

Rat ileal enterocytes were radiolabeled by flash photolysis with a photolabile derivative of taurocholate (7,7-azo-[3H]TC) and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Maximal labeling of the bile acid binding proteins (BABPs) was achieved between 15 and 90 s. When enterocytes were pulsed with 7,7-azo-[3H]TC for 2 min, and then 0.5 mM TC was added to chase the radiolabel, the radioactivity in the BABPs was displaced by 50% after 2 min. The 99-kDa brush-border membrane (BBM) protein had the highest initial labeling rate, followed by 43-kDa actin, 35- and 14-kDa cytosolic proteins, 54-kDa basolateral membrane (BLM) protein, 59-kDa BLM-associated protein, and 20-kDa microsomal protein. When a mixed microsomal and cytosolic fraction was photolabeled with 7,7-azo-[3H]TC and then separated, the 20-kDa microsomal protein was labeled. However, if the microsomal fraction alone was photolabeled, the 20-kDa protein was not labeled, suggesting this protein required a cytosolic cofactor for labeling. Using Triton X-114 phase separation and EDTA extraction, the BABPs were separated into amphiphilic integral membrane proteins (99- and 54-kDa proteins) and hydrophilic proteins (14-, 35-, 43-, and 59-kDa proteins). From these data, a model is proposed for transcellular bile acid transport in rat ileal enterocytes.

Dissociation of object and spatial processing domains in primate prefrontal cortex

Areas and pathways subserving object and spatial vision are segregated in the visual system. Experiments show that the primate prefrontal cortex is similarly segregated into object and spatial domains. Neurons that code information related to stimulus identity are dissociable, both by function and region, from those that code information related to stimulus location. These findings indicate that the prefrontal cortex contains separate processing mechanisms for remembering "what" and "where" an object is.

The effects of stimulus novelty and familiarity on neuronal activity in the amygdala of monkeys performing recognition memory tasks

The function of the amygdala in behavioural responses to novel stimuli and its possible function in recognition memory were investigated by recording the responses of 659 amygdaloid neurons in monkeys performing recognition memory and visual discrimination tasks. The aim was to determine the contribution of the amygdala in the encoding of familiarity and therefore its role in supporting memory-related neuronal mechanisms in the basal forebrain. The responses of three groups of neurons reflected different forms of memory. One group (n = 10) responded maximally to novel stimuli and significantly less so to the same stimuli when they were familiar. The calculated memory spans of these neurons were in the range of 2-10 intervening trials, and this short-term retention of information may reflect the operation of a neural mechanism encoding memory for the recency of stimulus presentation. Two other groups responded to the sight of particular categories of familiar stimuli: to foods (n = 6) or to faces (n = 10). The responses of some of these stimulus-selective neurons declined with repeated presentations of foods (3/4 tests) and faces (2/6 tests). The activity of these latter two groups of neurons may be involved in behavioural responses to familiar visual stimuli, particularly when such stimuli have affective or motivational significance. We conclude that the neurophysiological data provide evidence of amygdaloid mechanisms for the recognition of recently seen visual stimuli. However, these amygdaloid mechanisms do not appear to be sufficient to support the performance of long-term recognition memory tasks without additional and complementary functions carried out by other ventromedial temporal, prefrontal and diencephalic structures which also project to the basal forebrain.

Effect of antiserum to a 99 kDa polypeptide on the uptake of taurocholic acid by rat ileal brush border membrane vesicles

A 99 kDa polypeptide in rat ileal brush border membrane (BBM), regarded as a component of the active bile acid transport system on account of photoaffinity labeling, has been purified by affinity chromatography and preparative gel electrophoresis and utilized as an immunogen for raising polyclonal antibody. Immune serum, but not preimmune serum, specifically recognized a single band of 99 kDa protein on immunoblots of ileal and renal BBM. In contrast, no reactivity was observed with proteins in jejunal BBM. This polyclonal antibody, compared with preimmune serum and anticytosolic bile acid binding protein (14 kDa) serum, significantly inhibited the Na+ dependent uptake of [3H] taurocholate by BBM vesicles (p less than 0.01). [14C] D-glucose uptake by BBM vesicles was not influenced by the immune serum (p less than 0.01). Thus, these studies provide further support for the specific role of a 99 kDa protein in ileal BBM bile acid transport.

Characterization of a novel 14 kDa bile acid-binding protein from rat ileal cytosol

A 14 kDa polypeptide in rat ileal cytosol has been identified as the major intestinal cytosolic bile acid-binding protein (I-BABP) by photoaffinity labeling with the radiolabeled 7,7-azo derivative of taurocholate (7,7-azo-TC). To further characterize I-BABP, the protein was purified by lysylglycocholate Sepharose 4B affinity and DE-52 anion-exchange chromatography. The purified I-BABP contained a single 14 kDa band on SDS-PAGE. The 14 kDa protein showed a 26-fold increase in binding affinity for [3H]7,7-azo-TC compared to cytosolic protein. Immunoblotting of protein fractions separated by affinity chromatography showed that neither liver fatty acid binding protein (L-FABP) nor intestinal fatty acid binding protein (I-FABP) bind to the affinity column and that the 14 kDa protein which bound to the column and was subsequently eluted with detergent did not cross-react with anti-L-FABP or anti-I-FABP. The 14 kDa protein labeled with [3H]7,7-azo-TC was radioimmunoprecipitated from cytosol by rabbit antiserum raised against purified I-BABP. I-BABP was shown to have a blocked N-terminus; however, its mixed internal sequence generated from cyanogen bromide-cleaved protein and amino acid composition indicated that it was related to (although clearly distinct from) both I-FABP and L-FABP. These studies have isolated a 14 kDa bile acid-binding protein from rat ileal cytosol which is immunologically and biochemically distinct from I-FABP and L-FABP.

Identification of the 14 kDa bile acid transport protein of rat ileal cytosol as gastrotropin

The 14 kDa bile acid binding protein of rat ileal cytosol (I-BABP), previously shown to be the major intracellular transporter of bile acids in enterocytes, was purified by affinity chromatography and gel electrophoresis. Enzymatic digestion of I-BABP which had been electroblotted to nitrocellulose led to the recovery and sequence analysis of four peptides representing 47 residues of sequence (approximately 35% of the full sequence). All the peptide sequences displayed high levels of identity (greater than 60%) and homology (greater than 80%) to the sequences of porcine and canine gastrotropin. This high level of homology together with other features of I-BABP identify it as rat gastrotropin, establishing gastrotropin as the major intracellular bile acid carrier of rat enterocytes.

The effects of visual stimulation and memory on neurons of the hippocampal formation and the neighboring parahippocampal gyrus and inferior temporal cortex of the primate

The activity of 736 single neurons was recorded from the hippocampal formation (HF), the rhinal cortex (RH), the medial and anterior inferior temporal cortex (TE), or areas TF and TH of the parahippocampal gyrus (PHG) of monkeys during the performance of a delayed matching to sample task. The results indicate differences between the areas in their contributions to sensory processing and memory. Of the neurons, 55% responded to either the first (S1) and/or the second (S2) of the two successively presented visual stimuli. The proportion of responsive neurons and the proportion of neurons that responded selectively on the basis of shape or color (but not size) were significantly higher in areas TE + RH than in HF + PHG. The responses to S1 differed from those to S2 for 18% of the total sample: of these differentially responsive neurons, 66% of the TE + RH neurons responded more strongly to S1 (the sample presentation, allowing stimulus acquisition), whereas 71% of the HF + PHG neurons responded more strongly to S2 (the match/nonmatch comparison, when the behavioral decision could be made). Of 239 TE + RH neurons recorded during the delayed matching task or when objects were shown, 12% displayed evidence of memory for the previous occurrence of stimuli by responding strongly to the first, but significantly less strongly to subsequent presentations of visual stimuli that were novel or had not been seen recently. In contrast, none (0%) of 328 neurons so tested in HF and PHG had a response that declined significantly on stimulus repetition. For six (86%) of seven TE + RH neurons tested, the decrement in response persisted even after distraction by intervening presentations of other stimuli. Further evidence of information storage was found for 7 (33%) of 21 neurons for which responses to the first presentations of unfamiliar objects were significantly greater than to the first presentations of very familiar objects, even though the familiar objects had not been seen for greater than 15 min.

Hippocampus and medial temporal cortex: neuronal activity related to behavioural responses during the performance of memory tasks by primates

Neuronal activity was recorded in the hippocampal formation, the parahippocampal gyrus and medial inferotemporal cortex of monkeys performing memory tasks. In a modified delayed matching to sample task in which 2 sequentially presented stimuli were compared on each trial, a match condition required a right panel press, whereas a non-match condition required a left panel press. The activity of 336/736 (45.7%) neurons was related to the behavioural responses (left or right panel presses) in this task. The incidence of response-related activity was 57.4% in cortical areas adjacent to the rhinal sulcus plus medial inferotemporal cortex, and 40.2% for the hippocampal formation. For 58.9% of these response-related neurons, the activity change associated with the behavioural response was greater than that during presentation of the sensory stimuli, though neurons commonly responded (33.2% of all recorded neurons) to both sensory and motor events. The activity of 198 neurons (26.9%) differed between go-left and go-right trials; such neurons were found in all areas but were nearly twice as common in the posterior as in the anterior hippocampal formation. The importance of visual stimuli for the response-related neuronal activity was examined during the performance of a delayed alternation task without visual cues indicating direction of response. The response-related activity of 8 neurons recorded during the delayed alternation and the delayed matching tasks was similar in both tasks, indicating that memory for the behavioural responses influences the activity of the response-related neurons. In order to test the effects of stimulus familiarity and non-spatial responses on medial temporal neurons, recognition memory and visual discrimination tasks requiring lick responses were performed. The activity of 2/375 (0.5%) neurons was related to the lick responses; 3/68 neurons in the inferomedial temporal cortex responded on the basis of stimulus novelty and none reflected their reinforcement value. It is concluded that the hippocampal formation, the parahippocampal gyrus and medial inferotemporal cortex all have a role in the utilisation of sensory, mnemonic and motor information underlying the selection of spatially-directed behavioural responses.

Identification of cytosolic and microsomal bile acid-binding proteins in rat ileal enterocytes

Studies were performed to determine the subcellular fractions and proteins involved in the intracellular transport of bile acids in rat ileal cells. The photolabile derivative 7,7-azo-taurocholate inhibited the Na(+)-dependent uptake of taurocholate into rat ileal enterocytes reversibly in the dark and irreversibly following photolysis. When photolabeled cells were submitted to subcellular fractionation, greatest radioactivity was found in the soluble protein (SP) fraction with decreasing radioactivity in the brush-border-(BBM), basolateral-(BLM), mitochondria-(MT), microsome-(MC), and Golgi-(GO) enriched fractions. Following trichloroacetic acid precipitation, delipidation, and correction for loss of marker enzyme activity, protein bound radioactivity was in SP greater than BBM greater than MC greater than BLM greater than GO greater than MT. When photolabeled cells were first fractionated and then submitted to sodium dodecyl sulfate-polyacrylamide gel electrophoresis, a 99-kDa polypeptide was associated with BBM, 54- and 59-kDa polypeptides with BLM, 14-, 35-, 43-, 59-, and 68-kDa polypeptides with SP and a 20-kDa polypeptide with MC fractions. Immunoprecipitation with known antisera identified the 68-kDa polypeptide as albumin and the 43-kDa polypeptide as actin. No precipitation on the 14-kDa polypeptide was noted with anti-hepatic and anti-intestinal fatty acid-binding proteins. No precipitation of the 35-kDa polypeptide occurred with antibody to the hepatic cytosolic bile acid-binding protein. These studies reveal a previously unrecognized 20-kDa microsomal, and 14- and 35-kDa cytosolic bile acid-binding polypeptides which may be involved in the transcellular movement of bile acids.

Modern approaches to bile acid transport proteins

New techniques for studying solute uptake across epithelial membranes in the gut are providing a clearer picture of bile acid handling. In the small intestine, liver, and other sites, membrane proteins that mediate active bile acid transport are key players. Better understanding of these transporters may lead to elucidation of disorders of bile handling and better treatment of such disorders.

Learning and memory is reflected in the responses of reinforcement-related neurons in the primate basal forebrain

Certain basal forebrain neurons encode the learned reinforcement value of objects: they respond differentially to visual stimuli that signal availability of fruit juice (positively reinforcing) or saline (negatively reinforcing) obtained by lick responses in visual discrimination tasks. In this report we describe the rapid, learning-related changes in the responses of these neurons during the acquisition and reversal of the reinforcement contingency of a visual discrimination reversal task. The same neurons also responded differentially to novel and familiar stimuli in 2 recognition memory tasks, in which monkeys applied the learned rule that lick responses to novel stimuli elicited saline and responses to familiar stimuli elicited juice. These differential responses to novel and familiar stimuli thus reflected the reinforcement value of the stimuli. A single presentation of a novel or a familiar stimulus was sufficient to elicit a differential response which was maintained even when the stimulus had not been seen recently. The maintenance of the differential response indicates that these neurons are influenced by a durable memory for the stimuli, estimated to be 30 trials on average. These differential neurons were recorded in the substantia innominata, the diagonal band of Broca, and a periventricular region of the basal forebrain. The responses of the reinforcement-related neurons in these 3 regions were similar in most respects. These results support the conclusion that basal forebrain neurons respond to sensory stimuli that, through learning of different contingencies, signal the availability of reinforcement. We suggest that the properties of learning and memory reflected in these neuronal responses are due to afferent pathways from ventromedial regions of the prefrontal and temporal cortices and the amygdala, and that the responses of these neurons provide an enabling mechanism that facilitates the operation of diverse cortical regions in which specific sensory, motor, or mnemonic functions take place.

Neuronal responses related to reinforcement in the primate basal forebrain

In the present study neurones recorded in the substantia innominata, the diagonal band of Broca and a periventricular region of the basal forebrain responded differentially to stimuli signalling the availability of fruit juice or saline obtained by making lick responses in two different visual discrimination tasks. The activity of certain neurones reflected the rewarding nature of stimuli used to signal the availability of juice in the tasks, responding to the sight and delivery of both foods and syringes used to deliver juice in tests in which behavioural responses were irrelevant. The activity of other neurones reflected aversion, responding to task stimuli signalling availability of saline and to syringes used to deliver saline to the mouth. In another task an auditory cue that signalled the availability of juice elicited neuronal responses. These neurones also responded to a tone cue used to signal the onset of the trial, and during certain mouth and arm movements which the monkey used to obtain reinforcement. The responses of these differential neurones were similar in most respects in all 3 regions of the basal forebrain. Thus these neurones respond to a range of visual and auditory stimuli that monkeys have learned can be used to obtain reinforcement, but not on the basis of sensory properties such as shape or colour of the stimuli. We conclude that the reinforcement-related nature of the neuronal signal from the basal forebrain could be used to facilitate processing in cortical regions, optimising the functioning of sensory, motor and association cortices, thus increasing the probability of responding appropriately to learned environmental contingencies. We suggest that the properties of these neurones are due to afferent inputs from ventromedial regions of the prefrontal and temporal cortices and amygdala.

Neuronal responses related to the novelty and familarity of visual stimuli in the substantia innominata, diagonal band of Broca and periventricular region of the primate basal forebrain

This study examined correlates of memory processes in neuronal activity recorded from the substantia innominata, the diagonal band of Broca and the periventricular region or the basal forebrain of monkeys performing recognition and/or visual discrimination tasks. Two types of neurons were found that responded differentially to stimuli on the basis of their novelty or their familiarity. Neurons (5/572) in the periventricular region rostral to the thalamus and caudal to the anterior commissure responded to familiar stimuli with increases in firing rate. Neurons in the substantia innominata (16/1058) and in the diagonal band (14/489) responded maximally to novel stimuli, with smaller responses to reappeared presentations of these stimuli. The properties of these two types of neurons were similar in three respects: (1) the magnitude of the differential response to novel and familiar presentations of stimuli were largest for stimuli presented on successive trials, and were attenuated for stimuli that had not been seen for some intervening trials; (2) both types of neurons responded to highly familiar stimuli as if they were novel if they had not been recently seen; (3) both types responded to two- and three-dimensional stimuli; and were recorded in rhesus monkeys trained on the recognition tasks and in untrained cynomolgus monkeys. An injection of HRP into the periventricular region of one monkey resulted in retrograde labeling of ventromedial regions of the prefrontal, cingulate and temporal cortices, of the amygdala, medial thalamus, supramammillary region of the midbrain. These data indicate that information about the novelty, familiarity or recency of presentation of visual stimuli is reflected in the responses of some basal forebrain neurons. Neurons in the substantia innominata and diagonal band of Broca could not be distinguished using the battery of applied tests, suggesting functional mechanisms common to both regions. The results of the anatomical experiment suggest that ventromedial limbic cortical and subcortical regions projecting to or through the periventricular region may be important for the transmission of information about visual stimuli and for memory function.

Identification and comparison of bile acid-binding polypeptides in ileal basolateral membrane

Bile acid-binding polypeptides were examined using basolateral membrane vesicles and enterocytes isolated from rat ileum. The uptake of a photolabile taurocholate derivative, (7,7,-azo-3 alpha,12 alpha-dihydroxy-5 beta[3 beta-3H]cholan-24-oyl)-2- aminoethanesulfonate,7,7-azo-TC, in ileal vesicles preloaded with paraaminohippurate (PAH) was stimulated with respect to uptake in unpreloaded vesicles. The PAH-transstimulated uptake of 7,7-azo-TC was inhibited by taurocholate and vice versa. Irradiation of membrane vesicles in the presence of 7,7-azo-TC irreversibly inhibited PAH-transstimulated taurocholate uptake. Photoaffinity labeling of basolateral membrane vesicles directly with [3H] 7,7-azo-TC and separation of proteins by SDS-PAGE revealed incorporation of radioactivity into several polypeptides. Photoaffinity labeling of vesicles in the presence of taurocholate inhibited the labeling of 54,000 and 59,000 mol. wt. polypeptides. The efflux of taurocholate from ileal enterocytes was cis-inhibited by 7,7-azo-TC and transstimulated by PAH. Irradiation of enterocytes in the presence of 7,7-azo-TC inhibited taurocholate efflux greater than the presence of 7,7-azo-TC in the dark. When enterocytes that were irradiated in the presence of [3H] 7,7-azo-TC were fractionated and the resultant basolateral membrane fraction was subjected to SDS-PAGE, incorporation of radioactivity into the 54,000 and 59,000 mol. wt. polypeptides was seen. In contrast, when the brush-border membrane fraction was subjected to SDS-PAGE, greatest incorporation of radioactivity was seen in the previously described 99,000 mol. wt. polypeptide. These studies suggest that 7,7-azo-TC shared transporters with natural bile acid and identified polypeptides that may be involved in bile acid transport across the basolateral membrane and differ from that seen in the brush-border membrane of the ileal epithelial cell.

An analysis of imaging studies and liver function tests to detect hepatic neoplasia

A retrospective study was conducted to determine the sensitivity, specificity, and accuracy of scintigraphy, ultrasound, and CT scanning in conjunction with biochemical tests in the detection of liver neoplasia. Sixty-three patients with metastatic liver disease and 45 patients with nonmalignant liver disease received a total of 46 liver/spleen scans, 61 ultrasounds, and 49 CT scans. The sensitivities of liver/spleen scan, ultrasound, and CT scan were 72, 73, and 81%, respectively; the specificities were 86, 94, and 83%, respectively; and, the accuracies were 78, 84, and 82%, respectively. No statistically significant difference in specificity, sensitivity, or accuracy was seen between the three imaging studies. The serum alkaline phosphatase was significantly more accurate than total bilirubin and SGOT in detecting liver metastasis. However, 19 and 13% of the malignant and nonmalignant groups, respectively, had normal biochemical tests. The sensitivity, specificity, and accuracy of the imaging studies in the presence of an abnormal alkaline phosphatase, SGOT, or total bilirubin were not significantly different than in the presence of normal biochemical tests. These results suggest that other factors such as cost, reproducibility, and availability of skilled interpreters should be considered in the selection of imaging studies for the detection of neoplastic liver disease.

Neuronal evidence that inferomedial temporal cortex is more important than hippocampus in certain processes underlying recognition memory

Amnesia has been reported to result from combined damage to the amygdala, hippocampus and inferomedial temporal cortex in man and monkey. Evidence is presented that neuronal activity in the monkey inferomedial temporal cortex reflects memory for the previous occurrence of visual stimuli: 26 (15%) of 173 single units responded more strongly to first than to subsequent presentations of unfamiliar stimuli. No such responses were found for neurones recorded in the hippocampus and subicular cortex. The findings suggest that the inferomedial temporal cortex plays a central role in processes necessary for recognition memory.