Seizure protein 6 and its homolog seizure 6-like protein are physiological substrates of BACE1 in neurons

Background

The protease BACE1 (beta-site APP cleaving enzyme) is a major drug target in Alzheimer’s disease. However, BACE1 therapeutic inhibition may cause unwanted adverse effects due to its additional functions in the nervous system, such as in myelination and neuronal connectivity. Additionally, recent proteomic studies investigating BACE1 inhibition in cell lines and cultured murine neurons identified a wider range of neuronal membrane proteins as potential BACE1 substrates, including seizure protein 6 (SEZ6) and its homolog SEZ6L.

Methods and results

We generated antibodies against SEZ6 and SEZ6L and validated these proteins as BACE1 substrates in vitro and in vivo. Levels of the soluble, BACE1-cleaved ectodomain of both proteins (sSEZ6, sSEZ6L) were strongly reduced upon BACE1 inhibition in primary neurons and also in vivo in brains of BACE1-deficient mice. BACE1 inhibition increased neuronal surface levels of SEZ6 and SEZ6L as shown by cell surface biotinylation, demonstrating that BACE1 controls surface expression of both proteins. Moreover, mass spectrometric analysis revealed that the BACE1 cleavage site in SEZ6 is located in close proximity to the membrane, similar to the corresponding cleavage site in SEZ6L. Finally, an improved method was developed for the proteomic analysis of murine cerebrospinal fluid (CSF) and was applied to CSF from BACE-deficient mice. Hereby, SEZ6 and SEZ6L were validated as BACE1 substrates in vivo by strongly reduced levels in the CSF of BACE1-deficient mice.

Conclusions

This study demonstrates that SEZ6 and SEZ6L are physiological BACE1 substrates in the murine brain and suggests that sSEZ6 and sSEZ6L levels in CSF are suitable markers to monitor BACE1 inhibition in mice.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0134-z) contains supplementary material, which is available to authorized users.

N-aryl piperazine metabotropic glutamate receptor 5 positive allosteric modulators possess efficacy in preclinical models of NMDA hypofunction and cognitive enhancement

Impaired transmission through glutamatergic circuits has been postulated to play a role in the underlying pathophysiology of schizophrenia. Furthermore, inhibition of the N-methyl-d-aspartate (NMDA) subtype of ionotropic glutamate receptors (NMDAR) induces a syndrome that recapitulates many of the symptoms observed in patients with schizophrenia. Selective activation of metabotropic glutamate receptor subtype 5 (mGlu5) may provide a novel therapeutic approach for treatment of symptoms associated with schizophrenia through facilitation of transmission through central glutamatergic circuits. Here, we describe the characterization of two novel N-aryl piperazine mGlu5 positive allosteric modulators (PAMs): 2-(4-(2-(benzyloxy)acetyl)piperazin-1-yl)benzonitrile (VU0364289) and 1-(4-(2,4-difluorophenyl)piperazin-1-yl)-2-((4-fluorobenzyl)oxy)ethanone (DPFE). VU0364289 and DPFE induced robust leftward shifts in the glutamate concentration-response curves for Ca(2+) mobilization and extracellular signal-regulated kinases 1 and 2 phosphorylation. Both PAMs displayed micromolar affinity for the common mGlu5 allosteric binding site and high selectivity for mGlu5. VU0364289 and DPFE possessed suitable pharmacokinetic properties for dosing in vivo and produced robust dose-related effects in reversing amphetamine-induced hyperlocomotion, a preclinical model predictive of antipsychotic-like activity. In addition, DPFE enhanced acquisition of contextual fear conditioning in rats and reversed behavioral deficits in a mouse model of NMDAR hypofunction. In contrast, DPFE had no effect on reversing apomorphine-induced disruptions of prepulse inhibition of the acoustic startle reflex. These mGlu5 PAMs also increased monoamine levels in the prefrontal cortex, enhanced performance in a hippocampal-mediated memory task, and elicited changes in electroencephalogram dynamics commensurate with procognitive effects. Collectively, these data support and extend the role for the development of novel mGlu5 PAMs for the treatment of psychosis and cognitive deficits observed in individuals with schizophrenia.

Acute and chronic administration of immunomodulators induces anorexia in Zucker rats

To investigate the possible involvement of leptin signaling in lipopolysaccharide (LPS) anorexia, we compared the anorectic effect of LPS in genetically obese (fa/fa) Zucker rats and in their lean (Fa/?) counterparts. The effects of interleukin-1beta (IL-1beta) and muramyl dipeptide (MDP) were also tested. LPS [100 microg/kg body weight (BW)], IL-1beta (2 microg/kg BW) and MDP (2.2 mg/kg BW) injected intraperitoneally (i.p.) at lights out reduced food intake similarly in obese and lean rats. LPS injection at 500 or 1000 microg/kg BW (i.p.) also reduced food intake and BW similarly in obese and lean rats, but obese regained BW faster than lean rats. LPS (2.45 microg or 9.8 microg/h/rat) administered chronically with i.p. implanted osmotic pumps reduced food intake similarly on experimental day 1, regardless of the genotype. After day 3, the lean rats' anorectic response and recovery were dose-dependent, whereas the anorectic response in obese rats was minimally affected by dose (significant dose effect only on day 3). Again, obese rats regained lost BW faster than lean rats. These results do not support a role for leptin as the sole mediator of anorexia induced by bacterial products (LPS and MDP) and IL-1beta.

Subdiaphragmatic vagal deafferentation fails to block the anorectic effect of hydroxycitrate

We investigated the neural mediation of the feeding suppression through orally administered hydroxycitrate (HCA) in male rats that were fed a high-glucose diet (about 48% glucose). Ten-day ad libitum food intake and body weight regain after previous body weight loss (13% of initial body weight) due to restrictive feeding were measured in rats with sham deafferentation (SHAM; n = 6), subdiaphragmatic vagal deafferentation (SDA; n = 7), and SDA plus celiac-superior mesenteric ganglionectomy (SDA/CGX; n = 9). HCA suppressed the 10-day cumulative food intake in all surgical groups and body weight regain in SDA and SDA/CGX groups. Independent of HCA, SDA and SDA/CGX rats consumed less food and gained less weight compared to SHAM rats. These results demonstrate that all vagal afferents from below the diaphragm and vagal efferents of the dorsal trunk as well as splanchnic nerves (afferents and efferents) are not necessary for the feeding-suppressive effect of HCA in this animal model. Vagal afferents, however, appear to play a role in the control of intake when a high-glucose diet is consumed after a period of restrictive feeding.

Evidence for a role of the 5-HT2C receptor in central lipopolysaccharide-, interleukin-1 beta-, and leptin-induced anorexia

We examined the role of serotonin (5-HT) and the 5-HT(1A) and 5-HT(2C) receptors in the anorectic effects of centrally administered lipopolysaccharide (LPS), interleukin-1 beta (IL-1 beta), and leptin. Food intake was measured in rats after intracerebroventricular (ICV) injections of LPS (20 ng), IL-1 beta (10 ng), or leptin (1 microg) at lights out, followed by intraperitoneal (IP) injections of either the 5-HT(1A) autoreceptor agonist 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT) (125 microg/kg) or the 5-HT(2C) receptor antagonist SB 242084 (0.3 mg/kg) at the onset of anorexia. SB 242084 significantly attenuated the food intake reduction caused by all compounds (all P<.01). IP 8-OH-DPAT attenuated ICV IL-1 beta-induced anorexia (P<.01). We also tested the involvement of the median raphe 5-HT(1A) receptors in peripheral LPS- and IL-1 beta-induced anorexia. Rats were injected intraperitoneally with either LPS (100 microg/kg) or IL-1 beta (2 microg/kg) at lights out, and 8-OH-DPAT (4 nmol) was administered directly into the median raphe nucleus at the onset of anorexia. Median raphe injections of 8-OH-DPAT significantly attenuated both IL-1 beta- and LPS-induced anorexia (both P<.01). These results implicate the 5-HT(2C) receptors in the mediation of central LPS-, IL-1 beta-, and leptin-induced anorexia. Our results also suggest that the midbrain raphe nuclei play a role in mediating the anorectic response to peripheral LPS and IL-1 beta.

Evidence that the anorexia induced by lipopolysaccharide is mediated by the 5-HT2C receptor

Rats consistently reduce their food intake following injections of bacterial lipopolysaccharides (LPS). Because inhibition of serotonergic (5-HT) activity by 8-OH-DPAT (5-HT(1A) activation) attenuates LPS-induced anorexia, we conducted a series of studies to examine whether other 5-HT-receptors are involved in the mediation of peripheral LPS-induced anorexia. In all experiments, rats were injected with LPS (100 microg/kg body weight [BW] ip) at lights out (hour 0). Antagonists were administered peripherally at hour 4, shortly after the onset of anorexia, which presumably follows the enhanced cytokine production after LPS. Food intake was then recorded during the subsequent 2 h or longer. 5-HT receptor antagonists cyanopindolol and SB 224289 (5-HT(1B)), ketanserin (5-HT(2A)), RS-102221 (5-HT(2C)), and metoclopramide (5-HT(3)) failed to attenuate LPS-induced anorexia. In contrast, both ritanserin (5-HT(2A/C)-receptor antagonist) (0.5 mg/kg BW) and SB 242084 (5-HT(2C)) (0.3 mg/kg BW) attenuated LPS-induced anorexia at doses that did not alter food intake in non-LPS-treated rats (all P<.01). Our results suggest that at least part of the anorexia following peripheral LPS administration is mediated through an enhanced 5-HT-ergic activity and the 5-HT(2C) receptor.

Hypothalamic implants of dilute estradiol fail to reduce feeding in ovariectomized rats

To investigate further the site where estradiol (E(2)) inhibits food intake, we tested the effects on feeding of subcutaneous and intrahypothalamic implants of 10% E(2) benzoate in cholesterol (CHOL) or CHOL alone. E(2) was implanted subcutaneously in Silastic tubes, and intrahypothalamically via bilateral 29-gauge cannulas into the paraventricular nucleus (PVN) or the medial preoptic area (MPA) of ovariectomized (OVX) Sprague-Dawley and Long-Evans rats. Three-day implant periods followed 3-day baseline periods. Rats were allowed ad libitum access to chow and tap water, and food intake and body weight were measured each day. Subcutaneous 10% E(2) implants in Sprague-Dawley rats reduced food intake 21% on Day 2 and 34% on Day 3 (P's<.01) and decreased 3-day body weight gain 11 g (P<.05). In contrast, 10% E(2) implants in the PVN of Sprague-Dawley rats did not change food intake or body weight. Implants of 10% or 20% E(2) in the MPA also failed to decrease food intake. MPA implants of 10% E(2) decreased body weight gain 8 g (P<.05), but MPA implants of 20% E(2) decreased weight gain only 4 g (P>.05). To determine whether the strain of rat affected our negative results on food intake, we implanted 10% E(2) into the PVN of Long-Evans rats. Again, PVN E(2) did not decrease food intake significantly in comparison to the pretest baseline. PVN E(2) did, however, decrease body weight gain 5 g and decreased food intake 6% compared to rats with implants of CHOL (both P<.05), but these effects appeared to be due to an increase in feeding in the CHOL group in comparison to their baseline. Finally, CHOL and E(2) implants did not impair the responsivity of the PVN because acute implants of norepinephrine (NE) into the PVN of E(2)- or CHOL-treated Long-Evans rats significantly increased food intake. Our results do not support the hypothesis that E(2)'s actions in either the PVN or the MPA are sufficient to account for its inhibitory effects on feeding.

A role for cyclooxygenase-2 in lipopolysaccharide-induced anorexia in rats

Because nonselective cycloooxygenase (COX) inhibition attenuated anorexia after lipopolysaccharide (LPS) administration, we tested the ability of resveratrol (2.5, 10, and 40 mg/kg) and NS-398 (2.5, 10, and 40 mg/kg), selective inhibitors of the two COX isoforms COX-1 and -2, respectively, to attenuate LPS (100 microg/kg ip)-induced anorexia. NS-398 (10 and 40 mg/kg) administered with LPS at lights out attenuated LPS-induced anorexia, whereas resveratrol at all doses tested did not. Because prostaglandin (PG) E(2) is considered the major metabolite synthesized by COX, we measured plasma and cerebrospinal fluid (CSF) PGE(2) levels after LPS administration. LPS induced a time-dependent increase of PGE(2) in CSF but not in plasma. NS-398 (5, 10, and 40 mg/kg) blocked the LPS-induced increase in CSF PGE(2), whereas resveratrol (10 mg/kg) did not. These results support a role of COX-2 in mediating the anorectic response to peripheral LPS and point at PGE(2) as a potential neuromodulator involved in this response.

A role for serotonin in lipopolysaccharide-induced anorexia in rats

Rats consistently reduce their food intake following injection of bacterial lipopolysaccharides (LPS). Because LPS increases CNS serotonin (5-HT) turnover, and because increases in CNS 5-HT turnover are associated with a decrease in food intake, we conducted a series of studies to examine 5-HT's potential role in LPS-induced anorexia. Chronic CNS 5-HT depletion by cisterna magna (CM) administration of 5,7-dihydroxytryptamine (5,7-DHT) failed to attenuate LPS-induced (100 microg/kg, ip) anorexia. In subsequent experiments, LPS was injected at lights out (hour 0) and [8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT)] or N-CBZ-[(8beta)-1,6-dimethylergolin-8-yl]methylamine (metergoline) was injected at hour 5 - the time when LPS-treated rats become anorectic. Food intake was measured during the subsequent 2 h. In LPS-treated rats, 8-OH-DPAT (62.5, 125, or 250 microg/kg, sc) injection increased food intake. In a 2 x 2 factorial arrangement of LPS and 8-OH-DPAT, 125 microg/kg 8-OH-DPAT increased food intake significantly more in LPS-treated rats than in non-LPS-treated rats (significant LPS x 8-OH-DPAT interaction). In LPS-treated rats, 1 and 5 mg/kg metergoline significantly enhanced food intake. However, in a 2 x 2 arrangement of LPS and metergoline, 1 mg/kg metergoline failed to increase food intake in LPS and non-LPS-treated rats in two separate trials. The ability of the 5-HT(1A) receptor agonist 8-OH-DPAT to attenuate LPS-induced anorexia in rats supports a role of 5-HT in LPS-induced anorexia.

Inhibition of TNF-alpha production contributes to the attenuation of LPS-induced hypophagia by pentoxifylline

Cytokines such as tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) are assumed to mediate anorexia during bacterial infections. To improve our understanding of the role that these two cytokines serve in mediating infection during anorexia, we investigated the ability of pentoxifylline (PTX), a potent inhibitor of TNF-alpha production, to block the anorectic effects of the bacterial products lipopolysaccharide (LPS) and muramyl dipeptide (MDP) in rats. Intraperitoneally injected PTX (100 mg/kg body wt) completely eliminated the anorectic effect of intraperitoneally injected LPS (100 microg/kg body wt) and attenuated the anorectic effect of a higher dose of intraperitoneally injected LPS (250 microg/kg body wt). Concurrently, PTX pretreatment suppressed low-dose LPS-induced TNF-alpha production by more than 95% and IL-1beta production 39%, as measured by ELISA. Similarly, high-dose LPS-induced TNF-alpha production was reduced by approximately 90%. PTX administration also attenuated the tolerance that is normally observed with a second injection of LPS. In addition, PTX pretreatment attenuated the hypophagic effect of intraperitoneally injected MDP (2 mg/kg body wt) but had no effect on the anorectic response to intraperitoneally injected recombinant human TNF-alpha (150 ug/kg body wt). The results suggest that suppression of TNF-alpha production is sufficient to attenuate LPS- and MDP-induced anorexia. This is consistent with the hypothesis that TNF-alpha plays a major role in the anorexia associated with bacterial infection.

Effect of sensory stimuli on huddling behavior of pigs

Two experiments involving pigs at 1, 3, and 8 d of age were conducted to 1) compare huddling between littermates and nonlittermates, 2) study the ability of pigs to distinguish an anesthetized piglet from a piglet-shaped object, and 3) explore the importance of physical contact between pigs on huddling behavior. Experiments were conducted in an enclosed rectangular aluminum test chamber having pressure sensors beneath floor panels to detect test pig location. Test objects were placed on a platform at one end of the chamber and test pig location was monitored during a 45 min trial. Experiment 1 involved a total of 45 pigs (5 pigs/treatment on d 1, 2, and 3). The results indicate that, regardless of age (P > .05), when either a littermate or a nonlittermate occupied the platform, average location of test pigs that "settled" (ceasing to move for 7 min or more) was closer to the platform (P < .01), time spent near the platform was greater (P < .01), and movement about the chamber was less (P < .01) than when the platform was empty. No differences (P > .05) were observed between littermate and nonlittermate stimuli for these variables. During Exp. 2, the platform was covered with wire mesh. A total of 98 pigs were used in the study. Treatments were a cage containing 1) no object (n = 24), 2) a wooden block (n = 25), 3) a pig-shaped latex casting (n = 24), or 4) an anesthetized 8- to 10-d-old pig (n = 25). Pig age and treatment did not affect the percentage of time in each trial that pigs spent within 23.5 cm of the cage or the percentage of pigs settling within 23.5 cm of the cage. These studies show that pigs huddle similarly with littermates and nonlittermates and that physical contact with another piglet but not visual recognition of another piglet affects piglet huddling.

The effect of thermal environment and age on neonatal pig behavior

Three experiments were conducted to evaluate the ability of a radiant environment and the presence of a littermate to attract pigs during the first 3 d of age. The effect of stimuli on pig movement was studied in an enclosed rectangular aluminum test chamber containing four similar sections that were heated independently. In Exp. 1, all sections were at 34.8 degrees C to evaluate the chamber for biases of where pigs located themselves at 1 (n = 24) and 2 d (n = 26) of age. More (P < .025) pigs settled (e.g., no movement for 7 min) in end sections than in middle sections. Age did not affect time to settle or settling location. The effect on pig location of heating one chamber end section to either 23, 40, 48, 56, or 64 degrees C and leaving the remaining sections unheated (24 degrees C) was determined in Exp. 2. Settling of pigs at 1 (n = 50) and 2 d (n = 50) of age was affected by temperature (P < .001) but not by age. The minimum distance between average pig location and the heated section occurred at 48 degrees C. Experiment 3 involved 15 pigs each at 1 and 3 d during a 1-h trial to compare the relative pig attraction to 1) a heated chamber end section at 44.4 degrees C when remaining sections were at 23.5 degrees C, 2) an anesthetized littermate in an end section when all sections were at 24.1 degrees C, or 3) a choice test involving a 45.5 degrees C end section and an anesthetized littermate in the opposite end section with three unheated sections at 23.7 degrees C. Average distance between the test animal and the heated section was greater (P < .01) than that between the test animal and an anesthetized pig. Pigs that were allowed a choice preferred to lie near an anesthetized littermate in a cold section rather than alone in a 45 degrees C section (P < .01), and they were less (P < .005) active when an anesthetized littermate was present in the chamber. Although radiant heat effectively attracted pigs, heat was less attractive than an anesthetized littermate. The greater attraction of pigs to a littermate than to radiant heat may explain why pigs remain near the sow and littermates during d 1 and 2 after birth.

Lysine deficiency alters diet selection without depressing food intake in rats

Under states of protein deficiency, the dietary limiting amino acid, rather than protein content, can act as the dietary stimulus to control diet selection. If fact, threonine-deficient rats will alter their diet selection patterns solely on the basis of very small changes (0.009 g/100 g) in the dietary threonine concentration. In these studies, we assessed whether lysine-deficient rats will also alter their diet selection patterns on the basis of small changes in dietary Lys concentration. In all experiments, growing rats were adapted to diets in which the protein fraction (purified amino acids or wheat gluten) was limiting in Lys. They were then given a choice between the adaptation diet (AD) diet and a slightly more deficient diet. Rats that were adapted to a Lys-deficient diet (0.25 g Lys/100 g) selected their AD over diets containing as little as 0.01% less Lys (P < 0.01) within 5 d. To determine how deficient rats must be before they alter their selection patterns, rats were adapted to diets containing various levels of Lys, i.e., 2 levels below the requirement for growth and 2 levels above the requirement for growth, but below the requirement for maximal nitrogen retention. Only rats adapted to diets containing Lys below their requirement for growth selected their AD over a diet containing 0.05% less Lys (P < 0.005). Finally, to determine whether rats will alter their selection to whole protein-based diets, rats were adapted to 25% wheat gluten diets supplemented with 0.03-0.21% Lys. Rats selected the AD over a diet containing as little as 0.09% less supplemental Lys by d 4 of the trial (P < 0.05). We conclude that rats are sensitive to changes as small as 0.01% in dietary Lys concentration, but that sensitivity requires prior adaptation to Lys-deficient diets.

The effect of farrowing crate heat lamp location on sow and pig patterns of lying and pig survival

Three experiments were conducted to study sow and pig behavior during the 1st 3 d after birth and pig survival during the 1st 2 wk after farrowing. In Exp. 1, 23 sows were housed in conventional farrowing crates that were divided into five sections: a .5- x 1.5-m front creep section and the remaining area divided into four sections, .75 x 1.05 m each. Air temperature was maintained at 19 degrees C, and a 250-W heat lamp was placed at the right side of the front creep in Treatment 1 (T1), or in the creep at the right side of the sow for Treatment 2 (T2). The percentage of pigs within 8 cm of the sow's trunk was not affected by treatment, but it decreased (P < .001) from 61.8 +/- 3.4% on d 1 to 28.1 +/- 3.5% on d 3. As the percentage of pigs near the sow decreased, the percentage of pigs within the section containing the heat lamp increased (T1, P < .05; T2, P < .10). Experiment 2 involved 15 sows and litters housed as in Exp. 1, except that heat lamps were not provided, and average air temperature was 27.3 +/- .2 degrees C during behavioral observations. Even though the portion of the litter near the sow decreased (P < .001) from d 1 to d 3 (d 1, 57.0 +/- 3.4%; d 2, 42.9 +/- 3.3%; d 3, 31.7 +/- 3.3%), pigs did not concentrate in any specific section as they moved away from the sow. The average number of pigs within the front creep section (Section 1) for the 3-d period was less than (P < .01) the number in any other crate section. Experiment 3 involved 147 sows and tested the effect of solid creep floor covering on pig survival for each of the heat lamp locations used in Exp. 1. Neither heat lamp location nor floor covering affected pig survival. During the 1st 3 d of life, pigs tend to lie near the sow regardless of heat lamp location or air temperature. Heat lamp position and floor covering under the lamp do not affect pig survival.

Vagal and splanchnic afferents are not necessary for the anorexia produced by peripheral IL-1beta, LPS, and MDP

We investigated the extrinsic gut neural mediation of the suppression of food intake in male Sprague-Dawley rats induced by peripheral intraperitoneal administration of 2 microg/kg interleukin-1beta (IL-1beta), 100 microg/kg bacterial lipopolysaccharide (LPS), and 2 mg/kg muramyl dipeptide (MDP). Food intake during the first 3 and 6 h of the dark cycle was measured in rats with subdiaphragmatic vagal deafferentation (n = 9), celiac superior mesenteric ganglionectomy (n = 9), combined vagotomy and ganglionectomy (n = 9), and sham deafferentation (n = 9). IL-1beta, LPS, and MDP suppressed food intake at 3 and 6 h in all surgical groups. The results demonstrate that neither vagal nor nonvagal afferent nerves from the upper gut are necessary for the feeding-suppressive effects of intraperitoneal IL-1beta, LPS, or MDP in the rat and suggest that peripheral administration of immunomodulators produces anorexia via a humoral pathway.

Small changes in essential amino acid concentrations alter diet selection in amino acid-deficient rats

The rat's sensitivity to changes in the dietary limiting amino acid concentration (LAA) was examined on the basis of dietary selection. Rats were adapted to purified low protein basal (Basal) diets in which threonine (Thr) was the LAA (0.188-0.212% wt/wt of diet). In Experiment 1, rats made a clear selection for their adaptation diet over a diet containing 0.012% less threonine after 2-3 d of choice. Rats made no clear dietary selection when given a choice between their adaptation diet and a diet containing 0.012% more threonine. Experiment 2 was conducted to examine the rat's sensitivity to small decreases in the LAA concentration. Rats adapted to a 0.200% Thr-Basal diet clearly responded to decreases as small as 0.009% in the concentration of threonine and selected against the more deficient diet when given a choice between it and the 0.200% Thr-Basal adaptation diet. Because plasma and brain amino acid concentrations are important for detection of other amino acid deficiencies, these variables were measured to determine whether they were affected by such small changes in dietary amino acid concentration. In Experiments 3 and 4, rats were adapted to the 0.200% Thr-Basal diet and then fed 0.188, 0.200 or 0.212% Thr-Basal diets for 6 h, or 0.188 and 0.212% Thr-Basal for 54 h. Amino acid concentrations in plasma, prepiriform cortex and anterior cingulate cortex were not significantly different among treatments. Norepinephrine concentration in the prepiriform cortex was not affected by dietary treatment. We conclude that small decreases in LAA concentration can cause selection against the more deficient diet, but that detection of such deficiencies does not require significant changes in plasma and brain amino acid concentrations.

Ovariectomy and estradiol affect postingestive controls of sucrose licking

Ovariectomy (OVX) has been shown to increase, and estradiol replacement to decrease, meal size in rats. Because little is known about how estradiol influences meals, we conducted two experiments to examine the effects of OVX and beta-estradiol 3-benzoate (EB) replacement on the microstructure of licking behavior. In both experiments, patterns of licking were analyzed in adult female Sprague-Dawley rats during an 0.8 M sucrose test meal. In Experiment 1, meal microstructure was determined preOVX and 10-12 days postOVX. Rate of licking following OVX was not changed during min 1 of the meal, but was significantly faster during min 2-4 of the meal (p < 0.03). The numbers of bursts (runs of licks separated by 250-500 ms) and numbers of clusters (runs of licks separated by > 500 ms) were significantly increased during min 2-4 (p < 0.05). In Experiment 2, OVX rats received EB replacement. Rate of licking after EB replacement was not changed during min 1 of the meal, but was significantly slower during the remainder of the meal (min 2-4, min 5-7, and min 8-10). Burst size, cluster size, and interburst interval were less after EB replacement during min 5-7 of the test meal (all p < 0.05). Because both OVX and EB replacement failed to alter the rate of licking during min 1, estrogen did not appear to alter the palatability of sucrose. OVX and EB replacement did appear to affect a postingestive mechanism(s) that is engaged within 2-4 min of meal onset.

Rats on a macronutrient self-selection diet eat most meals from a single food cup

Computerized meal pattern analysis was performed on female Sprague-Dawley rats (n = 10). Rats were housed for 14 days in cages adapted for macronutrient selection. Rats selected their diet from vitamin and mineral supplemented, semipurified sources of carbohydrate, fat and protein, in three individual food cups. For analysis of meal patterns, minimum meal size was > 50 mg, and mean minimum intermeal interval was 7.9 min. Daily energy intake averaged 289.0 kJ, with 75% occurring during the dark cycle. Energy intake was 28% carbohydrate, 50% fat and 22% protein. Of 12.3 daily meals, 56% were from one, 35% from two, and only 9% of the meals were from three food cups. Seventy percent of the time carbohydrate was the first meal of the dark cycle. The average number of meals per day eaten from carbohydrate, fat, and protein were not significantly different (6.9, 6.2 and 5.6, respectively, p > 0.05). Energy intake for a meal was greatest when fat was eaten (18.35 kJ), than when either carbohydrate (8.68 kJ) or protein (8.97 kJ) was eaten. Meal duration was 7.03 min for carbohydrate, 3.75 min for fat, and 7.47 min for protein. These results provide evidence that rats on a macronutrient self-selection diet eat most meals from a single macronutrient source.

Timing and dose of amino acids injected into prepyriform cortex influence food intake

The effects of time before feeding and dose of dietary-limiting amino acids (DLAA) injected into the prepyriform cortex (PPC) on intake of amino acid-imbalanced diets were evaluated. Intake of imbalanced diet was increased from approximately 50% to approximately 75% of baseline when an optimal amount of DLAA (1 nmol L-isoleucine or 2 nmol L-threonine) was injected immediately prior to feeding. Injections made several hours prior to feeding were more effective, increasing intake of imbalanced diets to approximately 85% of baseline. Delivering two half-optimal doses of DLAA, several hours apart, increased intake of imbalanced diet only to the same level as a single injection of the optimal dose immediately prior to feeding. The increase in intake of a threonine-imbalanced diet after injecting 2 nmol threonine 6 h prior to feeding was abolished if an additional 2 nmol threonine was injected immediately prior to feeding. It appears that it is the sum of the changes in tissue DLAA concentrations in the PPC that are recognized and influence food intake when amino acid imbalanced diets are fed.

ICS 205-930 and feeding responses to amino acid imbalance: a peripheral effect?

Serotonin3 (5-HT3) receptor antagonists (ICS 205-930 and MDL 72222) have been shown to block or ameliorate the anorectic response of the rat to amino acid imbalanced (IMB) diets. Two experiments were conducted to determine whether the effects of these antagonists are mediated through central or peripheral 5-HT3 receptors. In Experiment One, ICS 205-930 (ICS) was injected centrally, into either the lateral ventricle (doses: 0.3 pmol to 10 nmol), or the cisterna magna (62 nmol). The intake of rats fed an isoleucine IMB diet was not affected by these injections. In Experiment Two, rats received an IP injection of either saline, ICS, or a quaternized derivative of ICS (Q-ICS) that should not cross the blood-brain barrier. Both ICS- and Q-ICS-injected rats ate significantly more (p less than 0.05) IMB diet than saline-injected rats. Intake of IMB diet was not different (p greater than 0.4) between ICS and Q-ICS groups. From these results, it appears that ICS restores intake of IMB through a peripheral component.

Distribution of dietary limiting amino acid injected into the prepyriform cortex

Diffusion or metabolism of the dietary limiting amino acid (DLAA) in the prepyriform cortex (PPC) may account for the time lag between injection of the DLAA into the PPC and the increase in intake of an amino acid-imbalanced diet. Results from the injection of [3H]Leu +/- [14C]Thr (DLAA) into the PPC indicated rapid (less than 15 min) and limited diffusion (85-90% of recovered label was less than or equal to 1 mm from the injection site). 3H and 14C decreased in the trichloroacetic acid (TCA)-soluble fraction and increased in the TCA-insoluble fraction during the first 1.5 h and remained constant in the TCA-insoluble fraction 1.5-6 h after injection. An increase (approximately 50%) in 3H in the TCA-insoluble fraction was found less than or equal to 30 min after injection of the DLAA. There was no affect of the DLAA on 3H in the TCA-soluble fraction. These results indicated that a change in metabolism within the PPC may be responsible for the delay in onset of the feeding response after injection of the DLAA into the PPC.