Characterization of 3H-AVP binding sites in particulate preparations of rat brain

Specific binding sites for vasopressin (AVP) were located in subcellular particulate fractions of rat brain with tritiated vasopressin of high specific activity, 22.5 Ci/mmol. Rat brain tissue was dissected, placed in cold 0.32 M sucrose containing proteolytic inhibitors, homogenized and fractionated into a crude nuclear fraction (1K pellet), crude mitochondrial fractions (12K pellet), and plasma membranes and microsomes (100K pellet). Specific binding of vasopressin was found in the 12K and 100K pellets in the presence of a divalent metal ion with Ni greater than Co greater than Mg greater than Mn greater than no metal ion at pH 7.4 in 50 mM Tris-Maleate buffer. Maximum specific binding of 16 nM AVP was located in the 100K anterior cortex fraction which bound 350 fmoles/mg protein; striatum, midbrain/thalamus, cerebellum, and medulla oblongata and pons bound specifically about 200 fmoles/mg protein and frontal poles and parietal cortex about 100 fmoles/mg protein in the 100K pellet. In all of the brain regions studied, except hippocampus and septum, the 100K pellet bound specifically 2 to 4 times more 3H-AVP than the 12K pellet. In the hippocampus with 16 nM AVP, the 12K pellet bound specifically 150 fmoles/mg protein; the septum, 75 fmoles/mg protein. Little or no binding to the 100K pellet was present in these regions. Bound AVP could be dissociated rapidly from the membranes by the addition of EDTA. The 12K hippocampal pellet was further fractionated into myelin, mitochondria, and synaptosomes; purification was confirmed by marker enzyme assays.(ABSTRACT TRUNCATED AT 250 WORDS)     

alpha-Bungarotoxin binding sites in the CNS.

The neurotoxin α-bungarotoxin (BuTX) has been extensively used as a specific and irreversible ligand to study the nicotinic acetylcholine receptor (nAChR) in skeletal muscle and electric fish. More recently it has been utilized to investigate the possibility of nAChRs in the CNS. The BuTX binding sites in the CNS have biochemical properties similar to muscle, have a distribution similar to other cholinergic markers, and have been histologically demonstrated to bind to post-synaptic membranes. But studies of ganglionic neurons, cultured sympathetic ganglion cells, and the PC 12 cell culture line have raised questions regarding the use of BuTX as a nicotinic receptor ligand in the CNS. The evidence for and against BuTX as a nicotinic receptor ligand is reviewed and discussed.     

Neonatal exposure to vasopressin decreases vasopressin binding sites in the adult kidney

Previous experiments showed that rats injected with vasopressin (AVP) during the first seven days after birth were less sensitive as adults to the antidiuretic effects of AVP than were control rats. In the present experiment, binding sites for AVP were measured in the kidneys of similarly treated adult rats. Neonatal exposure to AVP significantly decreased the number of binding sites in the adults, but did not affect the binding affinity of the sites. It is concluded that neonatal exposure to AVP which produces a long-lasting decrease in responsiveness of the kidney to AVP is correlated with a reduction in the number of AVP binding sites in the tissue.     

Modulation of GABA binding sites by CNS depressants and CNS convulsants

[³H]Muscimol binding at 23°C and muscimol stimulated [³H]flunitrazepam binding at 37°C to membranes of rat cerebral cortex have been investigated. In washed membrane preparations, 2 apparent populations of [³H]muscimol binding sites can be observed. At 23°C [³H]muscimol binding is more sensitive to inhibition by NaCl and by other salts than at 0°C. The CNS depressants etazolate and pentobarbital reversibly enhance [³H]muscimol binding and they increase the affinity of muscimol as a stimulator of [³H]flunitrazepam binding. Conversely the CNS convulsants picrotoxin, picrotoxinin and isopropylbicyclophosphate (IPTBO) reversibly interfere with [³H]muscimol binding when NaCl is present and these drugs antagonize the effects of etazolate. In the presence of NaCl, picrotoxin, picrotoxinin and IPTBO also decrease the apparent affinity of muscimol or GABA as stimulator of [³H]flunitrazepam binding. Binding of [³H]muscimol to GABA recognition sites of rat cerebral cortex is enhanced by Ag⁺, Hg⁺ and Cu²⁺ in μM concentrations, Ag⁺ being most potent. The effects of 100 μM AgNO₃ persist after repeated washing of the membranes. When membranes are pretreated with AgNO₃ only one apparent population of [³H]muscimol binding sites with high affinity (K_d: 6–8 nM) is found. In AgNO₃ pretreated membranes, the affinity of muscimol as stimulator of [³H]flunitrazepam binding is increased 18 times (EC₅₀ 14 nM) when compared to control membranes, (EC₅₀ 253 nM). In AgNO₃ pretreated membranes, etazolate, pentobarbital and IPTBO fail to perturb either [³H]muscimol binding or baseline and muscimol stimulated [³H]flunitrazepam binding. The results demonstrate that the apparent sensitivity of GABA binding sites of the GABA-benzodiazepine-picrotoxin receptor complex can be increased by etazolate and pentobarbital and decreased by picrotoxin and IPTBO. These drugs have in common that they interfere with [³H]dihydropicrotoxinin binding.     

Distribution and characterisation of somatostatin receptor mRNA and binding sites in the brain and periphery.

The distribution and nature of (somatostatin) SRIF receptors and receptor mRNAs was studied in the brain and periphery of various laboratory animals using in situ hybridisation, autoradiography and radioligand binding. The messenger RNA (mRNA) expression of SRIF receptors msst1, msst2, msst3, msst4 and msst5 was studied in the adult mouse brain by in situ hybridisation histochemistry using specific oligonucleotide probes and compared to that of adult rats. As observed in rat brain, sst3 receptor mRNA is prominently expressed across the mouse brain, although equivalent binding has not yet been identified in situ. Sst1 and sst2 receptor mRNA expression, was prominent and again comparable to that observed in rat brain, whereas sst4 and especially sst5 receptor mRNA show comparatively low levels, although the former appears to be widely distributed while the latter could only be identified in a few nuclei. Altogether, the data are compatible with current knowledge, i.e. sst1 and sst2 receptor mRNA is prominent (both receptors have been functionally identified in the brain and for sst2 in the periphery), sst3 mRNA is highly expressed but in the absence of any functional correlate remains elusive. The expression of sst4 mRNA is comparatively low (especially when compared to what is seen in the lung, where high densities of sst4 receptors are present) and it remains to be seen whether sst5 receptor mRNA, which is confined to a few nuclei, will play a role in the brain, keeping in mind that high levels are found in the pituitary. Radioligand binding studies were performed in CCL39 cells expressing the five human recombinant receptors and compared to binding in membranes of rat cerebral cortex with [125I]Tyr11-SRIF14 which in the presence of 120 mM labels primarily sst1 receptor as suggested by the better correlation hsst1 and similar rank order of potency. The profile of [125I]Tyr3-octreotide labelled sites in rat cortex correlates better with recombinant sst2 than sst3 or sst5 binding profiles. Finally, [125I]LTT-SRIF28-labelled sites in rat lung express a sst4 receptor profile in agreement with previous findings. SRIF receptor autoradiography was performed in the brain and peripheral tissue of rat and/or guinea-pig using a number of ligands known to label recombinant SRIF receptors: [125I]LTT-SRIF28, [125I]CGP 23996, [125I]Tyr10-CST, or [125I]Tyr3-octreotide. Although, [125I]Tyr10-CST has been shown to label all five recombinant SRIF receptors, it is apparent that this radioligand is not useful for autoradiographic studies. By contrast, the other three ligands show good signal to noise ratios in rat or guinea-pig brain, rat lung, rat pancreas, or guinea-pig ileum. In most tissues, [125I]Tyr3-octreotide represents a prominent part of the binding (when compared to [125I]LTT-SRIF28 and [125I]CGP 23996), suggesting that sst2 receptors are strongly expressed in most tissues; it is only in rat lung that [125I]LTT-SRIF28 and [125I]CGP 23996 show marked binding, whereas [125I]Tyr3-octreotide does apparently label no sites, in agreement with the sole presence of sst4 receptors in this tissue.     

Mu and kappa opiate binding sites in the rabbit CNS

We have examined the ability of various opiates to compete with the binding of 3H-etorphine (0.5 nM) in membranes from the rabbit cerebellum and thalamus. Our data suggest that greater than 80% of 3H-etorphine binding occurs at mu receptor sites in cerebellum membranes. In thalamus membranes, D-Ala2, D-Leu5-enkephalin (DADL) resolves binding of 3H-etorphine into two components. The first component accounts for about 50% of binding and may represent interaction of the radioligand with mu receptor sites. The second component is unaffected in the presence of high (1-5 microM) concentrations of DADL. The ranking of potency for opiate inhibition of the second component is ethylketocyclazocine greater than naloxone much greater than morphine much greater than DADL, suggesting it represents binding of 3H-etorphine to a kappa-opiate binding site. In the rabbit brain, the kappa-opiate binding site is particularly abundant in the thalamus followed by frontal cortex and caudate nucleus.     

Changes in rat liver cyclo(His-Pro) binding sites upon castration and testosterone administration

Histidyl-proline diketopiperazine [cyclo(His-Pro)] binding was compared in livers from male and female rats. Cyclo(His-Pro) binding of female rat liver was very much lower than that of male rat liver. Scatchard analysis showed that the sex difference in cyclo(His-Pro) binding was due to different binding capacity. Cyclo(His-Pro) binding of castrated male rat liver was significantly decreased. Testosterone replacement raised the binding to the control level, and an excess of testosterone increased the specific binding beyond the control level. The testosterone-induced changes in cyclo(His-Pro) binding were also due to variation in the binding capacity. These findings indicate that testosterone is an important factor in the regulation of cyclo(His-Pro) binding in the rat liver.     

Changes in lectin binding sites during early human liver development

 In this study we investigated whether changes in glycosylation during liver morphogenesis correlate with the early development of individual structures in the human liver. Therefore, we localized the binding of the lectins from Sambucus nigra (SNA; specific for sialic acid), Triticum vulgare (WGA; specific for N-acetylglucosamine and sialic acid), Ricinus communis (RCA I; specific for β-galactose), Lotus tetragonolobus (LTA; specific for α-fucose) and Concanavalia ensiformis (Con A; specific for α-mannose) in the human liver between the 4th and the 12th gestational week (GW). Cell membranes of early hepatocytes (5th–6th GW) showed strong staining for RCA I, which decreased noticeably from the 8th–9th GW onward. Early intrahepatic capillaries (4th–5th GW) showed reactions only for WGA and RCA I. Reactions for SNA occurred later (6th–9th GW). At this time a fine granular staining for SNA was visible at the sinusoidal sides of hepatocytes. The hepatocytes of the outer limiting plate were specifically stained by WGA, Con A, and SNA in the 9th GW and the staining remained visible in developing bile ducts up to the 12th GW. The possible biological significance of the appearance or disappearance of carbohydrate moieties during early human liver development is discussed. Accepted: 21 October 1996     

Maternal deficiency of protein or protein and calories during lactation: effect upon CNS myelin subfraction formation in rat offspring.

The present study has examined the effects of maternal protein and protein-calorie deficiency during lactation on the development of CNS myelin subfractions in rat offspring. The offspring of both the protein and protein-calorie deficient rats had decreased brain and body weights, as well as delayed CNS myelination. Delayed active CNS myelination was demonstrated by the fact that 53-day-old nutritionally stressed pups incorporated significantly more [³H]leucine and [¹⁴C]glucose into all myelin subfractions than age-matched controls. Delayed myelination was also supported by the tremendous accretion of myelin proteins in the nutritionally deprived pups between 25 and 53 days of age. Despite the delayed active synthesis of myelin, the myelin deficit persisted in the offspring of protein deficient rats. These offspring had a deficiency of light + medium myelin throughout the study. At 17 days, both groups of nutritionally stressed rats had an excess of the high molecular weight proteins in heavy myelin. Heavy myelin from 17 day offspring of protein-calorie deficient rats had a deficiency of basic proteins, while that from the offspring of protein deficient rats had a deficiency of proteolipid protein. The protein composition of all myelin subfractions was normal at 53 days.     

Quantification and localization of galactose-specific binding sites in rat liver during postnatal development

We investigated the number and distribution of galactose-specific binding sites in developing livers from suckling rats of various ages using Lac-BSA-Au5 (lactosylated bovine serum albumin adsorbed onto colloidal gold particles 5 nm in diameter) as electron-dense ligand, and performing transmission electron microscopy of the specimen. It has been reported that the number of galactose-specific binding sites increases rapidly during organ development post partum (p.p.) and this was ascribed to hepatocyte receptor increase only. We now have investigated in in situ and in vitro experiments whether the binding sites of identical sugar specificity but located on sinusoidal cells show the same increase in expression or are independently regulated. We therefore quantified the number of particles bound by isolated hepatocytes and liver macrophages and found a gradual increase of both binding activities with age, the binding levels of adult liver cells being reached at day 15 p.p. This was confirmed with experiments using in situ prefixed organs thus proving the validity of this finding also for the intact organ. In both sets of experiments--in vitro as well as in vivo--ligand was found binding statistically distributed as single particles on hepatocytes of all ages, whereas on liver macrophages the binding pattern changed during development. On liver macrophages from rats 15 days of age ligand binding occurs in the preclustered pattern described for macrophages from adult rat livers whereas liver macrophages of newborn rats express a different binding pattern: they bind the ligands mostly as single particles with only few and small microaggregates.(ABSTRACT TRUNCATED AT 250 WORDS)     

Decrease in triiodothyronine binding sites in chick embryo erythrocytes during early development

Specific thyroid hormone (TH) binding sites have been detected in nuclei of erythrocytes obtained from developing chick embryos. The binding characteristics and relative affinities for TH analogs were those expected of TH receptors. Nuclear triiodothyronine (T3) saturation analysis was carried out in vitro by incubating intact erythrocytes in M199 medium with 3-200 pM [125I]T3 for 1 hr at 37 degrees C or 20-24 hr at 21 degrees C. Nuclei were obtained by centrifugation after lysing the erythrocytes in a stabilizing buffer containing 0.3% saponin, followed by addition of Triton X-100 (final concentration 0.2%) to minimize the nonspecific binding. Scatchard analysis of equilibrium binding data suggested that the nuclei possess a single class of binding sites. The binding is reversible and the rate of dissociation is temperature dependent. T3 and T4 appear to bind to the same sites, but the affinity of T3 was 16 times greater. Among TH analogs tested, Triac had the highest affinity followed by L-T3, D-T3, Tetrac, L-T4, D-T4, T2, and rT3. Serial studies performed on different days of chick embryogenesis demonstrated a rapid and significant decrease of the erythrocyte nuclear T3 receptor. On Day 5, the number of T3 binding sites was maximal at 1600 +/- 100 per nucleus. The number declined steadily until, by Day 20, it had reached about 60 +/- 10 sites/nucleus. RBC from adult and baby chickens had less than 1% as many binding sites as those from Day 5 embryos. There was no significant change in the affinity of the sites (Kd approximately equal to 20 pM at 37 degrees C). The reason for the loss of T3 binding sites during embryogenesis is not known. Since the plasma level of the TH increases during embryogenesis, this may reflect down regulation. Another possibility is that the change in erythrocyte population which occurs during this period involves production of erythrocytes which contain fewer T3 binding sites.     

Autoradiographic localization of osteogenin binding sites in cartilage and bone during rat embryonic development

Osteogenin, a novel bone differentiation factor isolated from bone, has been recently purified and the amino acid sequence determined. Osteogenin in conjunction with a collagenous bone matrix substratum induces cartilage and bone formation in vivo. In order to understand the developmental role of osteogenin during cartilage and bone morphogenesis we examined the binding and distribution of iodinated osteogenin in developing rat embryos. Whole embryo tissue sections were made from 11, 12, 13, 15, 18, and 20 day fetuses. The specific binding of osteogenin at different stages of rat embryonic development was determined by autoradiography. Maximal binding was observed in mesodermal tissues such as cartilage, bone, perichondrium, and periosteum. During Days 11-15, peak binding was localized to perichondrium during limb and vertebral morphogenesis. By Day 18 periosteum exhibited the highest concentration of autoradiographic grains. Osteogenin was also localized in developing membranous bones of the calvarium and other craniofacial bones. Considerably less binding was observed, in decreasing order, in muscle, liver, spleen, skin, brain, heart, kidney, and intestine. The observed maximal binding during skeletal morphogenesis implies a developmental role for osteogenin.     

Pre- and post-synaptic structures in insect CNS: Intramembranous features and sites of α-bungarotoxin binding

The central neuropile of thoracic ganglia in the central nervous system (CNS) of the cockroach Periplaneta americana contains synapses with characteristic pre- and post-synaptic membrane specializations and associated structures. These include dense pre-synaptic T-bars surrounded by synaptic vesicles, together with post-synaptic densities of varying electron opacity. Exocytotic release of synaptic vesicles is observed only rarely near presynaptic densities, but coated pits are seen at variable distances from them, and may be involved in membrane retrieval. After freeze-fracture, paralinear arrays of intramembranous particles (IMPs) are detected on the P face of many presynaptic terminals, with associated dimples indicative of vesicular release. The E face of these membranes exhibits protuberances complementary to the P face dimples, as well as scattered larger IMPs. Post-synaptic membranes possess dense IMP aggregates on the P face, some of which may represent receptor molecules. Electrophysiological studies with biotinylated α-bungarotoxin reveal that biotinylation does not inhibit the pharmacological effectiveness of the toxin in blocking acetylcholine receptors on an identified motoneurone in the metathoracic ganglion. Preliminary thin section ultrastructural analysis of this tissue post-treated with avidin-HRP or avidin-ferritin indicates that α-bungarotoxin-binding sites are localized at certain synapses in these insect thoracic ganglia.     

Bone specific binding sites for 1,25(OH)2D3 in magnesium deficiency

It has been reported that some hypoparathyroid patients with magnesium deficiency showed altered responses to vitamin D treatment. In the same way, in vitro bone studies have demonstrated the existence of a decrease in the 1,25-dihydroxyvitamin D3-induced resorption in bone as a result of magnesium deficiency. These findings suggest some kind of alteration in the 1,25(OH)2D3 in bone in magnesium deficiency. In the present work, using a binding assay based on the 1,25(OH)2D3 and 3H-1,25(OH)2D3 competition for the hormone binding sites in rat calvaria homogenates, a significant decrease in the number of 1,25(OH)2D3 specific binding sites has been found in calvaria incubated in magnesium-deficient medium compared to magnesium-replete ones. Alterations in the hormone-receptor affinity were not found. These results suggest that an alteration in the 1,25(OH)2D3 action on magnesium-deficient bone could be due, at least in part, to a decrease in the number of available vitamin D receptors in bone cells.     

Pre- and post-synaptic structures in insect CNS: intramembranous features and sites of alpha-bungarotoxin binding

The central neuropile of thoracic ganglia in the central nervous system (CNS) of the cockroach Periplaneta americana contains synapses with characteristic pre- and post-synaptic membrane specializations and associated structures. These include dense pre-synaptic T-bars surrounded by synaptic vesicles, together with post-synaptic densities of varying electron opacity. Exocytotic release of synaptic vesicles is observed only rarely near presynaptic densities, but coated pits are seen at variable distances from them, and may be involved in membrane retrieval. After freeze-fracture, paralinear arrays of intramembranous articles (IMPs) are detected on the P face of many presynaptic terminals, with associated dimples indicative of vesicular release. The E face of these membranes exhibits protuberances complementary to the P face dimples, as well as scattered larger IMPs. Post-synaptic membranes possess dense IMP aggregates on the P face, some of which may represent receptor molecules. Electrophysiological studies with biotinylated alpha-bungarotoxin reveal that biotinylation does not inhibit the pharmacological effectiveness of the toxin in blocking acetylcholine receptors on an identified motoneurone in the metathoracic ganglion. Preliminary thin section ultrastructural analysis of this tissue post-treated with avidin-HRP or avidin-ferritin indicates that alpha-bungarotoxin-binding sites are localized at certain synapses in these insect thoracic ganglia.     

Effect of piracetam administration on 3H-N-methylscopolamine binding in cerebral cortex of young and old rats.

Piracetam, a nootropic drug, has been used for some time in Alzheimer's disease for its facilitatory effect on learning and memory. Rats treated with piracetam (500 mg/kg, p.o.) daily, during 1 and 2 weeks, showed a significant increase in muscarinic receptor number (Bmax) and in the dissociation constant values (Kd) in the cerebral motor cortex, in binding studies using 3H-NMS as ligand. The effect was observed not only in young rats (control- Bmax = 663.4 fmol/mg protein, Kd = 0.45 nM; treated- Bmax = 961.9 fmol/mg protein, Kd = 0.82 nM) but also in aged animals (control- Bmax = 628.0 fmol/mg protein, Kd = 0.47 nM; treated-Bmax = 747.6 fmol/mg protein, Kd = 0.84 nM). Since piracetam does not interact with muscarinic receptors, the reason for its effect expressed as the enhanced number of brain muscarinic receptors is not clear but could be the result of stimulation of phospholipid synthesis and thus would represent an indirect action of the drug.