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D R Burt

Publications and source records attributed to D R Burt.

At least 55 records · Page 3Linked to original sources
PubMed1983

Biochemical similarity of rat pituitary and CNS TRH receptors.

Chemical properties of receptor binding sites for thyrotropin-releasing hormone (TRH) in rat pituitary, retina, amygdala and hypothalamus were compared by examining the influence of sulfhydryl reagents on specific binding of [3H](3-Me-His2)-TRH ([3H]MeTRH). Dithiothreitol-induced reduction of disulfide bonds, alkylation of thiol residues by N-ethylmaleimide and their oxidation by 5,5-dithiobis(2-nitrobenzoic acid) (DTNB), all produced marked reduction of [3H]MeTRH binding, which could be prevented in part by preincubation with exogenous TRH. In all tissues, concentration-dependent loss of binding activity was observed following exposure to micromolar heavy metals and mono- and divalent cations, with apparent additive effects between cations and DTT and NEM. Most changes appeared to reflect only a decrease in receptor density (Bmax). The similar sensitivity of all tissues to these compounds complements existing evidence for a close resemblance of TRH receptors in the CNS and pituitary.

Amygdala↗
PubMed1983

Receptors for thyrotropin-releasing hormone (TRH) in rabbit spinal cord.

Specific binding of 3H-labeled [3-Me-His2]TRH ([3H]MeTRH) to membranes of rabbit spinal cord (thoracic) involved a homogeneous population of high-affinity sites (Kd = 2.7 +/- 0.17 (n = 5) nM, Bmax = 204 +/- 12(5) fmol/mg protein). TRH analogs competed for the binding in the following rank order of potency: MeTRH greater than TRH greater than TRH-Gly-NH2 greater than Ser-His-Pro-NH2 greater than Thr-His-Pro-NH2 greater than pGlu-His-Pro-NH-C2H5 greater than TRH-free acid. Competition experiments with rat amygdala, run in parallel with rabbit spinal cord, revealed a closely similar pattern of activity. These properties help identify binding sites in the rabbit spinal cord as physiological receptors for TRH. The binding sites resemble receptors previously demonstrated in pituitary and CNS tissues of other species. The densities of [3H]MeTRH binding sites in different segments were generally similar, although density in the thoracic segment appeared to be somewhat higher. In all segments, binding seemed to be enriched in the dorsal gray matter. Dorsal roots and their associated ganglia, however, displayed little or no binding.

Amygdala↗
PubMed1983

Rat brain TRH receptors: kinetics, pharmacology, distribution and ionic effects.

Optimal conditions for measuring receptor binding for thyrotropin-releasing hormone (TRH) in the rat central nervous system (CNS) have been determined using 3H-labelled [3-Me-His2]TRH [( 3H]MeTRH). Binding assays conducted at 0 degree C for 5-6 h using sodium phosphate- and/or Hepes-buffered tissue resuspensions, with subsequent filtration through Whatman GF/B filters, yielded the best results. Association and dissociation of [3H]MeTRH binding to amygdala membranes were time and temperature dependent. Dissociation kinetics appeared biphasic. Progressive reduction in receptor affinity and capacity and increased radioligand breakdown were observed at elevated temperatures. Bacitracin (25-1000 microM) prevented peptide degradation but inhibited receptor binding (8-37%). Detailed competition experiments using MeTRH and other drugs yielded a pharmacological profile similar to that observed previously in other tissues indicating TRH receptor identification. Highest density of TRH receptors was observed in the retina and numerous limbic areas. Monovalent and divalent cations modulated [3H]MeTRH binding by reducing apparent receptor number.

Amygdala↗
PubMed1983

Taeniarhynchaena micropalamae gen. et sp. n. (Cestoda, dilepididae) from the stilt sandpiper Micropalama himantopus (Bonaparte, 1826).

Taeniarhynchaena gen. n. differs from other genera in its long, cylindrical, proboscidiform rostellum that can be withdrawn and coiled within the rostellar sac. The distal end of the rostellum is expanded and bifid forming two lobes each bearing wrench-shaped hooks peripherally. Other generic characters are the regular alternation of genital apertures, genital ducts dorsal to excretory vessels, testes restricted to posterior moiety of the proglottis, and a persistent uterus. Type species: Taeniarhynchaena micropalamae gen. et sp. n. from the stilt sandpiper Micropalama himantopus (Bonaparte, 1826) taken near Churchill on Hudson Bay, Canada. It is 1.5 to 2.9 mm long with 11 to 16 proglottides. The rostellum 0.45 to 0.5 long and 0.023 to 0.036 mm in diameter has five hooks 0.023 mm long on each lobe. Testes 14 in posterior two-fifths of the proglottis.

Animals↗
PubMed1982

TRH receptor binding in retina and pituitary: major species variation.

Retinas and anterior pituitary glands of nine readily available mammalian and one avian species have been examined for their TRH-sensitive binding of [3H]-[3-Me-His2]TRH. Among mammals, major species variations in TRH receptor binding have been detected in both tissues, amounting to about 100-fold in the retina and 20-fold in the pituitary. In the retina, TRH receptor binding was very high in the rat, quite high in sheep and guinea pig, intermediate in rabbit, pig, and mouse, and low but detectable in chicken, cat, calf and dog. In the pituitary gland, binding was very high in sheep, quite high in rabbit, rat, pig, calf and guinea pig, intermediate in chicken, and fairly low in mouse, cat, and dog. A number of possible interfering variables, including affinity differences, albino vs. non-albino strains, barbiturate anesthesia, time after death, sex and estrous cycle, age, and history of light exposure, were considered and/or tested directly, with generally negative results.

Animals↗
PubMed1982

Species differences in the brain regional distribution of receptor binding for thyrotropin-releasing hormone.

A survey of the regional distribution of binding of 1 nM [3H](3-Me-His2)thyrotropin-releasing hormone ([3H]MeTRH) to TRH receptors in the brains of eight mammalian species revealed major species differences in both the absolute and relative values of TRH receptor binding in different brain regions. Several brain regions exhibited binding equal to or exceeding that in the anterior pituitary gland of the same species, including the amygdala in the guinea pig and rat, the hypothalamus in the guinea pig, the nucleus accumbens in the rabbit, and all these and other regions in the cat and dog, for which pituitary binding was exceptionally low. Species could be divided into two groups according to which brain region appeared highest in binding: rabbits, sheep, and cattle had highest binding in the nucleus accumbens/septal area, whereas guinea pigs, rats, dogs, cats, and pigs had highest binding in the amygdala/temporal cortex area. The nucleus accumbens consistently exceeded the caudate-putamen in receptor binding. For most brain regions, rabbits, rodents, and sheep tended to be higher than carnivores, cattle, or pigs. Further regions that exhibited appreciable binding in most species included the olfactory bulb and tubercle, hippocampus, and various cortical and brain stem areas. In fact, essentially all brain regions appeared to have detectable levels of TRH receptors in at least some species, but no rat peripheral tissues have yet shown detectable receptor binding. The species differences appeared to reflect largely if not entirely differences in receptor density, although this was not tested in every species.

Animals↗
PubMed1981

Properties of [3H](3-Me-His2)TRH binding to apparent TRH receptors in the sheep central nervous system.

[3H](3-methyl-His2)thyrotropin releasing hormone ([3H]MeTRH) binds to sites in the sheep central nervous system (CNS) whose properties closely resemble both those of CNS binding sites for [3H]TRH and those of pituitary binding sites for [3H]MeTRH. Detailed studies for binding of [3H]MeTRH in the sheep nucleus accumbens and retina have yielded equilibrium dissociation constants of about 4 nM and densities of binding sites of about 3 and 2 pmol/g wet weight, respectively. The binding affinity of [3H]MeTRH was 8- to 10-fold higher than that of [3H]TRH, resulting in much lower non-specific binding with the new ligand. The association reaction had a rate constant of about 2-3 x 10(7) M-1 min-1, while the biphasic dissociation reaction had rate constants of 8-9 x 10(-2) min-1 for the fast phase and 1-2 x 10(-2) min-1 for the slow phase. The regional distribution of binding in the sheep CNS was similar to that observed previously with [3H]TRH. Highest binding outside the pituitary was in the nucleus accumbens area and retina, with another peak in the amygdala-temporal cortex area. Binding was widely distributed, so that no CNS region appeared totally devoid of binding. Nineteen TRH analogs, ranging in potency over 6 orders of magnitude, showed nearly identical abilities to complete for binding of [3H]MeTRH in the CNS areas and in the sheep anterior pituitary gland in side-by-side experiments. These findings argue strongly for identification of [3H]MeTRH binding sites in the CNS as TRH receptors.

Animals↗
PubMed1981

Guanine nucleotides modulate TRH-receptor binding in sheep anterior pituitary.

Binding of thyrotropin-releasing hormone (TRH, thyroliberin) to receptors in chilled membrane resuspensions from sheep anterior pituitary glands is reduced in affinity almost 2-fold by micromolar concentrations of guanosine 5'-triphosphate (GTP) and the hydrolysis-resistant analog guanyl-5'-yl imidodiphosphate (GppNHp) added to tissue during a 10-min preincubation at 37 degree C. Guanosine 5'-diphosphate (GDP) produced a similar effect at 1 mM, while GMP, ATP, ADP and AMP appeared relatively inactive. The number of TRH-binding sites was not affected by the nucleotide treatments. These results are consistent with reports of guanine nucleotide effects on other receptor types and with evidence suggesting an adenylate cyclase mechanism for some of TRH's effects in the anterior pituitary.

Adenine Nucleotides↗
PubMed1981

Muscarinic receptors in the posterior pituitary gland.

The characteristics of atropine-sensitive binding of l-[3H]-quinuclidinyl benzilate ([3H]-QNB) to membrane suspensions of sheep posterior pituitary indicate that the binding sites represent muscarinic cholinergic receptors. Scatchard plots of 6 saturation experiments showed a single class of binding sites, with an equilibrium dissociation constant of 16 +/- 2 pM and a density equivalent to 1.8 +/- 0.2 pmol/g wet weight of tissue. Kinetic analysis of 2 association and 3 dissociation curves yielded mean association and dissociation rate constants of 3.9 x 10(8) M-1 min-1 and 4.3 x 10(-3) min-1, respectively. The binding had a detailed pharmacology for 12 drugs consistent with muscarinic receptor identification. In rat neurointermediate lobes, superior cervical ganglionectomy had no demonstrable effect on [3H]-QNB binding. The location(s) and functional role(s) of neurohypophyseal muscarinic receptors remain to be elucidated.

Animals↗
PubMed1981

Preparation of 3H-[3-M3-His2]TRH as an improved ligand for TRH receptors.

[3H]-3-methyl-His2]thyrotropin releasing hormone ([3H]MeTRH) binds to sites in the sheep anterior pituitary gland which appear to be the same as those occupied by [3H]TRH and which can therefore be identified as TRH receptors. In competition experiments performed in parallel, both ligands gave the same number of binding sites, 15 pmol/g wet weight, and showed the same pharmacology for 19 TRH analogs ranging over more than 5 orders of magnitude in potency. The apparent dissociation constant of binding of [3H]MeTRH was about 3.5 nM compared to 29 nM for [3H]TRH. Kinetic experiments with [3H]MeTRH yielded a rate constant for association of 1.4 x 10(7) M-1 min-1 and rate constants for the biphasic dissociation of 5 x 10(-2) min-1 (fast phase) and 7 x 10(-3) min-1 (slow phase). Detailed methods are described for preparation of [3H]MeTRH by reduction of the dehydroproline precursor and its purification by ion exchange and antibody affinity chromatography. The major advantage of the new ligand is that its higher affinity of binding gives relatively less non-specific binding than is obtained with [3H]TRH, particularly in central nervous tissue.

Animals↗
PubMed1980

Pituitary cell cultures contain muscarinic receptors.

The presence of muscarinic receptors in 5-day dissociated cell cultures of rat anterior pituitary glands was detected by atropine-sensitive binding of 3H-quinuclidinyl benzilate (3H-QNB). Muscarinic receptor levels in cultures were compared to levels of receptors for dopamine and thyrotropin releasing hormone.

Animals↗
PubMed1980

The use of spinal cord cell cultures in the study of neurotoxicological agents.

Primary cell cultures of mouse embryo spinal cords were used to study the biological effects of exposure to organophosphate esters and neurotoxic amino acids. The effects of exposure were correlated with markers of cholinergic function. The purpose of this study was to examine the mechanisms whereby xenobiotics produce neurotoxicologic damage.

Animals↗
PubMed1980

Development of muscarinic receptor binding in spinal cord cell cultures and its reduction by glutamic and kainic acids.

The density of [3H]quinuclidinyl benzilate ([3H]QNB) binding sites in dissociated cell cultures of mouse spinal cord increased almost 4-fold between the 5th and 21st days of maturation in vitro. The mean specific binding of [3H]QNB in 21-day cultures was 340 pmol/g protein. Up to 79% of [3H]QNB binding sites were lost within a few days after exposure of the cultures to 1 mM glutamate. This neurotoxic action of glutamate showed a steep concentration dependence in the range 0.1-1 mM and did not vary with the duration of exposure of the cultures from 10 min to 24 h. Kainic acid was only moderately more potent than glutamate in reducing [3H]QNB binding. The susceptibility of 8-day cultures to glutamate-induced loss of [3H]QNB binding was less than that of 12- and 19-day cultures. These findings suggest that 80% or more of muscarinic binding sites in the spinal cord cell cultures are on neurons and that the sensitivity of these neurons to the toxic action of glutamate increases with their maturation in vitro.

Aging↗
PubMed1980

Muscarinic receptor binding in sheep anterior pituitary.

The existence of typical muscarinic receptors in membranes of sheep anterior pituitary was detected by binding of [3H]quinuclidinyl benzilate ([3H]QNB), a potent and specific muscarinic antagonist. [3H]QNB binding sites in anterior pituitary had an equilibrium dissociation constant of about 20-40 pM, a rate constant for association at 37 degrees C of about 2 x 10(8) M-1 min-1, a rate constant for dissociation at 37 degrees C of about 4 x 10(-3) min-1, and the expected specificities for a variety of cholinergic and other drugs. The concentration of [3H]QNB binding sites in anterior pituitary, about 4-6 pmol/g tissue, was at least twice that in posterior pituitary, but less than a quarter that in the hypothalamus. Since there is no established cholinergic or other innervation of the anterior pituitary, the presence there of typical muscarinic receptors, if we assume that they are functional, suggests that acetylcholine reaches the tissue through the hypophyseal portal circulation, and that it may thus have a role in the regulation of pituitary function.

Animals↗
PubMed1980

Binding sites for thyrotropin-releasing hormone in sheep nucleus accumbens resemble pituitary receptors.

TRH binds to sites in the nucleus accumbens-septal area of sheep brain. These sites appear to represent receptors for at least some of its behavioral effects in other species and are very similar to sheep pituitary receptors. All measurements were performed on ice to prevent peptide degradation. High affinity [3H]TRH binding in brain regions was distinguished from interfering low affinity binding by use of [3-Me-His2]TRH, a more potent and specific analog, in blank tubes at a 1-microM concentration. The nucleus accumbens-septal area, particularly the nucleus accumbens itself, showed the highest binding of any of a variety of brain regions surveyed. Binding sites in both nucleus accumbens and anterior pituitary had an equilibrium dissociation constant of about 20-40 nM, a rate constant for association of about 1-3 x 10(6) M-1 min-1, and a rate constant for dissociation of about 0.07 min-1. Seventeen TRH analogs showed closely similar potencies in competing for binding in the two tissues. Six weak analogs appeared to be more potent in the nucleus accumbens than in the pituitary, but this was an artifact of their relatively greater potency in competing for low affinity binding sites which are absent in pituitary. The only major difference between the high affinity binding sites in the two tissues was in their concentration, which was about 2- to 3-fold higher in the pituitary.

Animals↗