Use of the H3 receptor antagonist radioligand [3H]-A-349821 to reveal in vivo receptor occupancy of cognition enhancing H3 receptor antagonists

Background and purpose:

The histamine H₃ receptor antagonist radioligand [³H]-A-349821 was characterized as a radiotracer for assessing in vivo receptor occupancy by H₃ receptor antagonists that affect behaviour. This model was established as an alternative to ex vivo binding methods, for relating antagonist H₃ receptor occupancy to blood levels and efficacy in preclinical models.

Experimental approach:

In vivo cerebral cortical H₃ receptor occupancy by [³H]-A-349821 was determined in rats from differences in [³H]-A-349821 levels in the isolated cortex and cerebellum, a brain region with low levels of H₃ receptors. Comparisons were made to relate antagonist H₃ receptor occupancy to blood levels and efficacy in a preclinical model of cognition, the five-trial inhibitory avoidance response in rat pups.

Key results:

In adult rats, [³H]-A-349821, 1.5 µg·kg⁻¹, penetrated into the brain and cleared more rapidly from cerebellum than cortex; optimally, [³H]-A-349821 levels were twofold higher in the latter. With increasing [³H]-A-349821 doses, cortical H₃ receptor occupancy was saturable with a binding capacity consistent with in vitro binding in cortex membranes. In studies using tracer [³H]-A-349821 doses, ABT-239 and other H₃ receptor antagonists inhibited H₃ receptor occupancy by [³H]-A-349821 in a dose-dependent manner. Blood levels of the antagonists corresponding to H₃ receptor occupancy were consistent with blood levels associated with efficacy in the five-trial inhibitory avoidance response.

Conclusions and implications:

When employed as an occupancy radiotracer, [³H]-A-349821 provided valid measurements of in vivo H₃ receptor occupancy, which may be helpful in guiding and interpreting clinical studies of H₃ receptor antagonists.     

Internal jugular phlebectasia as an incidental finding in cervical spine surgery

Idiopathic internal jugular phlebectasia, occurs either unilaterally or bilaterally affecting the internal jugular vein is a rare congenital variation often diagnosed during childhood. It usually presents with a benign swelling over the lateral side of neck on the affected side, seen on exertion. A-30-year old male was operated for anterior cervical dissectomy from right lateral approach and was diagnosed per-operatively as internal jugular phlebectasia. The surgery was abandoned at this stage on the advice of cardiothoracic surgeon to investigate the patient for the secondary etiological factors for internal jugular vein dilatation. The patient was reassured without any active intervention for the phlebectasia and cervical dissectomy was performed in the second surgery through the lateral approach from left side. This case is presented in view of rarity and suggested that during preoperative workup the nearby structures like carotid sheath should be evaluated by magnetic resonance imaging to avoid such per-operative surprises.     

Visual attention in children with migraine: a controlled comparative study

The aim of this study was to evaluate the visual attention of children with migraine and compare it with a control group. Thirty migrainous children and 30 controls without headache were subjected to a visual attention assessment with Trail Making Tests (TMT) A/B, Letter Cancellation Test, and the Brazilian computerized test Visual Attention Test, third edition. The migraine group was evaluated after 2 days without headache. The migraine group had an inferior performance compared with the control group on TMT A ( P  = 0.03) and B ( P  = 0.001), and more errors on tasks 1 ( P  = 0.032) and 2 ( P  = 0.015) of the Visual Attention Test, presenting difficulty with selective and alternate attention. Attention is a neurological function that depends on structures such as the brainstem, cerebral cortex and the limbic system and on neurotransmitters such as dopamine and noradrenaline. The neurochemical aspects involved in the physiopathology of migraine and attention mechanisms probably predispose these children to visual attention deficits.     

Comparison of pulmonary and pleural responses of rats and hamsters to inhaled refractory ceramic fibers

The present study was designed to determine whether pleural fiber burdens or subchronic pleural fibroproliferative and inflammatory changes can help explain the marked interspecies differences in pleural fibrosis and mesothelioma that are observed following long-term inhalation of RCF-1 ceramic fibers by rats and hamsters. Fischer 344 rats and Syrian golden hamsters were exposed to RCF-1 for 4 h per day, 5 days per week, for 12 consecutive weeks. Lung and pleural fiber burdens were characterized during and after exposure. For all time points, approximately 67% of fibers associated with lung tissues from both rats and hamsters were longer than 5 microns in length. In comparison, fibers longer than 5 microns recovered from the pleural compartment, following a 12-week exposure and 12 weeks of recovery, accounted for 13% (hamsters) and 4% (rats) of the distribution. In the 12 weeks after the cessation of exposure, the number of fibers longer than 5 microns in length remained constant in the hamster at approximately 150 fibers per cm2 pleura. This was 2 to 3 times the corresponding fiber surface density in the rat. Significant pulmonary and pleural inflammation was detected at all time points and for both species. DNA synthesis by pleural mesothelial cells was quantified by bromodeoxyuridine uptake following 3 days of labeling. Labeling indices were higher in hamsters than in rats, both for RCF-1-exposed and filtered air-control animals and was highest for the parietal surface of the pleura. Significantly greater collagen deposition was measured in the visceral pleura of hamsters 12 weeks post-exposure but was not significantly elevated in rats. These findings demonstrate that subchronic inhalation exposure to RCF-1 induces pleural inflammation, mesothelial-cell turnover, pleural fibrosis, and an accumulation of fibers with a length greater than 5 microns in the hamster. The accumulation of long fibers in the pleural space may contribute to the pathology observed in the hamster following chronic inhalation of RCF- 1, whereas the presence of short, thin fibers may play a role in the acute-phase biological response seen in both species.      

Both K-ras and H-ras protooncogene mutations are associated with Harderian gland tumorigenesis in B6C3F1 mice exposed to isoprene for 26 weeks

Isoprene is the 2-methyl analog of 1,3-butadiene, a genotoxic and carcinogenic compound in rats and mice. Male B6C3F1 mice were exposed to 0, 2200 or 7000 ppm isoprene by inhalation (6 h/day; 5 days/week) for 26 weeks. Following a 26-week recovery period, an increased incidence of Harderian gland (HG) neoplasms was observed at both concentrations. The present study was designed to characterize genetic alterations in the K-ras and H-ras protooncogenes in HG neoplasms. Mutations in K-ras and H-ras were identified by single-strand conformational analysis and direct sequencing of polymerase chain reaction (PCR) amplified DNA, isolated from paraffin-embedded sections of HG neoplasms. A higher frequency of ras mutations, in particular K- ras mutations, was detected in isoprene-induced neoplasms than in 1,3- butadiene-induced or control HG neoplasms. All of the isoprene-induced HG neoplasms exhibited activated K-ras (60%) or H-ras (40%) mutations. In contrast, ras mutations were detected in 69% of HG neoplasms from 1,3-butadiene exposed mice (14% K-ras and 55% H-ras) and in 56% of HG neoplasms obtained from control B6C3F1 mice (8% K-ras and 48% H-ras). The predominant mutations in isoprene-induced HG neoplasms, but not in previously or newly analysed 1,3-butadiene-induced HG neoplasms, consisted of A-->T transversions (CAA-->CTA) at K-ras codon 61 (15/30) and C-->A transversions (CAA-->AAA) at H-ras codon 61 (8/30). Two- thirds of the K-ras CTA mutations were detected in HG neoplasms from the 2200 ppm exposure group while one-third was present in the 7000 ppm group. Isoprene-induced HG neoplasms with K-ras or H-ras mutations had an elevated proliferating cell nuclear antigen (PCNA) index, compared to spontaneous HG neoplasms without ras mutations. The high frequency and specificity of the ras mutation profile suggest that ras protooncogene activation contributes to isoprene-induced HG tumorigenesis.      

Metabolism of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) by human cytochrome P4501B1

Cytochrome P4501B1 (CYP1B1) is the most recently identified member of the dioxin-inducible CYP1 family. CYP1B1 is constitutively expressed in most human tissues, including colon and breast, and can activate numerous chemically diverse carcinogens. We evaluated the metabolism of the dietary heterocyclic amine carcinogen 2-amino-1-methyl-6- phenylimidazo[4,5-b]pyridine (PhIP) by microsomes from yeast expressing the human CYP1B1 protein. PhIP metabolites were analysed by HPLC with fluorescence and absorbance detection. We found that human CYP1B1 metabolizes PhIP to three products: N2-OH-PhIP, a mutagenic activation product; 4'-OH-PhIP, a detoxification product; and 2-OH-PhIP, the mutagenic potential of which is unknown. Metabolite identity was confirmed by co-elution with authentic standards and synchronous fluorescence spectroscopy. The identity of the 2-OH-PhIP standard was additionally confirmed by mass spectrometry. Kinetic studies of the formation of N2-OH-PhIP, 4'-OH-PhIP and 2-OH-PhIP by CYP1B1 indicated apparent Km values of 5.7 +/- 1.3, 2.2 +/- 0.5 and 1.3 +/- 0.2 microM, respectively. Apparent turnover rates were 0.40 +/- 0.03, 0.93 +/- 0.02 and 0.04 +/- 0.00 nmol product/min nmol P450, respectively. At saturating levels of substrate, CYP1B1-mediated formation of the non- mutagenic metabolite 4'-OH-PhIP was favored two-fold over that of the mutagenic metabolite, N2-OH-PhIP and >10-fold over that of 2-OH-PhIP. The formation of N2-OH-PhIP, a potent mutagen implicated in the etiology of human colon and breast cancer, indicates that CYP1B1 may play an important role in PhIP-mediated carcinogenesis.      

Exposure to 60 Hz magnetic fields and risk of lymphoma in PIM transgenic and TSG-p53 (p53 knockout) mice

The results of a number of epidemiology studies suggest that exposure to power frequency (50 and 60 Hz) magnetic fields may be a risk factor for hematopoietic neoplasia. To generate experimental data to test this hypothesis, the influence of magnetic field exposure on lymphoma induction was determined in two strains of mice that are genetically predisposed to the disease. PIM mice, which carry the pim-1 oncogene, are highly sensitive to lymphoma induction by N-ethyl-N-nitrosourea (ENU); ENU-treated PIM mice were studied as a 'high incidence' lymphoma model. TSG-p53 (p53 knockout) mice, in which the p53 tumor suppressor gene has been deleted from the germ line, develop lymphoma as an age- related change; hemizygous TSG-p53 mice were studied as a 'low incidence' lymphoma model. Beginning 1 day after a single i.p. injection of 25 mg ENU/kg body wt, groups of 30 PIM mice/sex were exposed for 18.5 h/day to pure, linearly polarized, transient-free 60 Hz magnetic fields at field strengths of 0 (sham control), 0.02, 2.0 or 10.0 Gauss (G). An additional group of 30 PIM mice/sex was exposed intermittently (1 h on, 1 h off) to 10.0 G fields. Groups of 30 TSG-p53 mice/sex were exposed continuously to magnetic field strengths of 0 (sham control) or 10.0 G; TSG-p53 mice received no ENU. Studies were terminated after 23 weeks of magnetic field exposure. Lymphoma incidence in male PIM mice exposed continuously to 10.0 G magnetic fields was significantly reduced from that seen in sex-matched sham controls; survival, lymphoma incidence and lymphoma latency in other groups of PIM mice did not differ from sham controls. Survival and lymphoma incidence in all groups of TSG-p53 mice was 7% or less, regardless of magnetic field exposure regimen. These data do not support the hypothesis that exposure to magnetic fields is a significant risk factor for lymphoid neoplasia in mice with a genetic predisposition to the disease.      

Plasma lecithin: cholesterol acyltransferase and plasma lipolytic activity in preterm infants given total parenteral nutrition with 10% or 20% Intralipid

The effect of 10% or 20% Intralipid on lipid clearing enzymes, plasma lipids and apoproteins was investigated during the first 5 days after birth in 37 premature infants maintained on total parenteral nutrition; 21 infants received 20% and 16 received 10% Intralipid, respectively. Lipid was infused over a 20-h period at rates of 1, 2 and 3 g/kg/day on consecutive days. Plasma lecithin: cholesterol acyltransferase (LCAT) activity was low and increased significantly (p < 0.05) only during infusion of 3 g/kg/day in both groups of infants. Plasma lipolytic activity was generally not affected by the regimen orpreparation(10% or 20%) of Intralipid infused, except for higher (p < 0.05) levels at 3 g/kg/ day of 20% compared with prelipid infusion. Plasma triglyceride concentrations were similar after 10% or 20% Intralipid, whereas plasma total cholesterol was significantly higher during infusion of 2 and 3 g/ kg/day of 10% compared with 20% Intralipid. The efficient clearing of 20% Intralipid might be related to the lower lecithin: triglyceride ratio which is compatible with the low LCAT activity of premature infants. Apoprotein A-J, apoprotein B, cholesterol, LCAT, plasma lipolytic activity, triglycerides