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T A Reader

Publications and source records attributed to T A Reader.

At least 109 records · Page 6Linked to original sources
PubMed1987

Alpha-1 and alpha-2 adrenoceptor binding in cerebral cortex: competition studies with [3H]prazosin and [3H]idazoxan.

The tritiated adrenergic antagonists prazosin ([3H]PRZ) and idazoxan ([3H]IDA, or RX-781094) bind specifically and with high affinity in membrane preparations from cerebral cortex to alpha-1- and alpha-2-adrenoceptors respectively. Saturation experiments, performed to determine the density of receptors (Bmax; maximum binding capacity) and the dissociation constant (Kd 25 degrees C), were analyzed by the methods of Eadie and Hofstee, iterative modelling, and the procedure of Hill. The pharmacologic properties and specificity of the labelling was verified by displacement experiments using alpha-adrenergic antagonists and agonists. The antagonist drugs showed the following order of potency to displace [3H]prazosin: prazosin much greater than phentolamine much greater than corynanthine greater than pyrextramine much greater than yohimbine much greater than piperoxan greater than benextramine greater than idazoxan; for the agonists: clonidine much greater than (-)-noradrenaline much greater than (-)-adrenaline much greater than phenylephrine, while other drugs, such as (-)-propranolol, dopamine, (-)-isoproterenol and serotonin only competed with the alpha-1-ligand at concentrations above 20 microM. The alpha 2-sites labelled by [3H]idazoxan were characterized by the antagonist displacement sequence idazoxan much greater than phentolamine greater than yohimbine = greater than piperoxan much greater than pyrextramine much greater than benextramine much greater than prazosin much greater than corynanthine. The agonists order of potency to compete with [3H]idazoxan was clonidine much greater than phenylephrine = greater than (-)-adrenaline greater than (-)-noradrenaline, and for other related drugs it was (-)-propranolol much greater than dopamine much greater than serotonin greater than (-)-isoproterenol. These competition experiments clearly showed two pharmacologically distinct sites, but question the relative specificity of some of the adrenergic drugs.

Adrenergic alpha-Agonists↗
PubMed1987

Stereospecific binding of a new benzazepine, [3H]SCH23390, in cortex and neostriatum.

The binding of the D1 antagonist SCH23390 to membrane preparations from rat cerebral cortex was examined using enantiomers of dopamine agonists and antagonists to compete with the bound [3H]SCH23390 at its Kd value. The competition curves were compared with those obtained with preparations from the neostriatum. The results demonstrate that specific [3H]SCH23390 binding in the cerebral cortex has the same pharmacological profile as in the neostriatum, so that this radioligand can be used to label dopamine D1 receptors in brain regions with a sparse dopaminergic innervation.

Animals↗
PubMed1986

Dopamine in the visual cortex of the cat.

The endogenous content of noradrenaline (NA) and dopamine (DA) was determined by radioenzymatic assays in three different areas of the occipital (visual) cortex, in normal cats as well as in DA-deafferented animals. The use of HPLC methodology enabled us to detect and measure in addition two metabolites of DA: 3, 4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), thus confirming the existence of a DA innervation in this cortical region.

Animals↗
PubMed1986

Alpha-1 adrenoceptors are decreased in human epileptic foci.

Cortical alpha-1 adrenoceptors were measured in tissues obtained from 10 patients immediately following temporal lobectomy for intractable partial epilepsy. At operation each patient exhibited spontaneous spiking restricted to either the anterior (n = 5) or posterior (n = 5) portion of the first two temporal gyri. Control samples were obtained from the nonspiking half of the same gyrus. Receptor-binding assays were performed on isolated cortical membranes using [3H]prazosin. There was a reduction (p less than 0.01) in the receptor density (beta max) of the sites in the epileptic foci without any change in affinity (mean +/- SEM): spiking--beta max, 160.5 +/- 11.3 fmol/mg protein; affinity, 0.17 +/- 0.04 nM; nonspiking--beta max, 218.8 +/- 15.6 fmol/mg protein; affinity 0.17 +/- 0.04 nM. This relative decrease in alpha-1 adrenoceptor density may be the substratum of a noradrenergic hyposensitivity that could contribute to a localized diminution in inhibitory mechanisms in epileptic foci.

Adult↗
PubMed1986

Alpha-1 and alpha-2 adrenoceptor binding in cerebral cortex: role of disulfide and sulfhydryl groups.

The tritiated adrenergic antagonists Prazosin ([3H]PRZ) and Idazoxan ([3H]IDA, or RX-781094) bind specifically and with high affinity to alpha 1 and alpha 2-adrenoceptors respectively, in membrane preparations from cerebral cortex. Saturation experiments performed to determine the density of receptors and the dissociation constant (Kd) were analyzed by the methods of Eadie Hofstee, iterative modelling, and the procedure of Hill, while the specificity of the labelling was verified by displacement experiments. Since receptors are proteins, we examined the role of disulfide (-SS-) bridges and sulfhydryl (-SH) groups in the specific combination of [3H]PRZ and [3H]IDA to the alpha 1 and alpha 2 adrenoceptors. Pretreatment of the membranes with the -SS- reactive DL-dithiothreitol (DTT) or the alkylating agent N-ethylmaleimide (NEM), alone or in combination, decreased specific binding of both ligands, with only minor changes in the non-specific counts. The [3H]IDA binding (alpha 2-sites) was more sensitive to both DTT and NEM than the [3H]PRZ sites (alpha 1-adrenoceptors), and the initial changes induced by alkylation of the alpha 2-site were due to an important decrease in the affinity for [3H]IDA, as judged by the increase in the Kd. This modulation in the affinity caused by alkylation of a thiol group could explain the higher potency of the blocking agent tetramine disulfide benextramine at the alpha 2-site. The results provide evidence for the participation of -SS- and -SH groups in the binding site of alpha 1- and alpha 2-adrenoceptors in the cerebral cortex.

Adrenergic alpha-Agonists↗
PubMed1986

Effects of p-chlorophenylalanine on cortical monoamines and on the activity of noradrenergic neurons.

The catecholamines noradrenaline, dopamine, adrenaline, the indoleamine 5-hydroxy-tryptamine (5-HT; serotonin), and some of their major metabolites were assayed, using high performance liquid chromatography (HPLC), in the neocortex of normal rats as well as in animals in which 5-HT synthesis had been inhibited with p-chlorophenylalanine. Besides important depletions in serotonin and in 5-hydroxyindole-3-acetic acid, noradrenaline levels were significantly reduced, but the content in 3-methoxy-4-hydroxyphenylglycol was increased, indicating an augmented utilization of this amine. The levels of dopamine and 3-methoxytyramine were also reduced, although homovanillic acid and 3,4-dihydroxyphenylacetic acid levels remained constant. The spontaneous unitary activity of identified noradrenergic neurons in the Locus coeruleus was increased, indicating an hyperactivity of this system. These results can be interpreted in relation to functional interactions between the catecholamines and serotonin; i.e.: a decrease in endogenous serotonin results in the loss of a negative feedback control of noradrenaline release.

Adrenergic Fibers↗
PubMed1986

Alpha-adrenoceptors and monoamine contents in the cerebral cortex of the rodent Jaculus orientalis: effects of acute cold exposure.

The tritiated adrenergic antagonists prazosin ([3H]PRZ) and idazoxan ([3H]IDA, or RX-781094) bind specifically and with high affinity to alpha 1- and alpha 2-adrenoceptors respectively, and were used to measure adrenoceptors in membrane preparations obtained from the cerebral cortex of Jaculus orientalis. Membrane preparations were also obtained from a group of cold exposed animals, to determine whether these adrenoceptors could be modified by a thermic stress. The density of receptors (Bmax; maximum binding capacity) and the dissociation constant (Kd 25 degrees C) were estimated by iterative modelling, and by using the procedure of Hill. After acute cold exposure (16 hr, 5 degrees C) there was a decrease in the affinity of the alpha 1-adrenoceptors, as judged by the Kd 25 degrees C for [3H]PRZ, with no changes in the Bmax. The alpha 2-sites did not show any significant changes, as revealed by [3H]IDA binding. Pretreatment of the membrane preparations from control animals with the disulfide and sulfhydryl reactives DL-dithiothreitol, 5,5'-dithiobis-(2-nitrobenzoic acid) and N-ethylmaleimide decreased specific [3H]PRZ and [3H]IDA binding, with minor changes in non-specific counts, indicating that the fixation of these ligands was to the receptor proteins. The endogenous cortical monoamine contents were also determined in the frontal cerebral cortex of these same animals, using high performance liquid chromatography with electrochemical detection. The catecholamine levels and their major metabolites were found to be stable in the cortex after the acute thermic stress, but there was a marked reduction in serotonin with a normal content in 5-hydroxyindole-3-acetic acid.

Animals↗
PubMed1985

Evidence for the participation of disulfide and sulfhydril groups in the specific binding of [3H]prazosin in cerebral cortex.

The tritiated alpha 1 antagonist prazosin [3H]PRZ binds specifically and with high affinity to postsynaptic adrenoceptors in membrane preparations from cerebral cortex. Since adrenoceptors are of protein nature, it was of interest to investigate the possible role of disulfide (--SS--) and sulfhydril (--SH) groups in the binding of [3H]PRZ. Pretreatment of the membranes with the disulfide and sulfhydryl reactives DL-Dithiothreitol, L-Dithiothreitol, Dithioerythritol or 5',5'-Dithiobis-(2-nitrobenzoic acid) (DTNB), alone or in combination with the alkylating agent N-Methylmaleimide (NMM), decreased specific [3H]PRZ binding, with minor changes in the non-specific counts. Saturation experiments revealed that all these reagents reduced the affinity of the binding site for [3H]PRZ, as judged by the Kd 25 degrees C, but only the alkylating agent NMM and the oxydizing reagent DTNB produced in addition to the increase in Kd, a decrease of the maximum binding capacity (Bmax). The present results provide evidence for a participation of --SS-- and/or --SH groups in the recognition site of the alpha 1-adrenoceptor of cerebral cortex.

Animals↗
PubMed1984

Catecholamines and serotonin in the rat central nervous system after 6-OHDA, 5-7-DHT and p-CPA.

The norepinephrine (NE), epinephrine (EPI), dopamine (DA) and serotonin (5-HT) contents were measured radioenzymatically in seven anatomically defined regions (frontal cerebral cortex, hippocampus, hypothalamus, midbrain, pons-medulla oblongata, cerebellum and spinal cord) in adult normal animals, after treatment with the tryptophan hydroxylase inhibitor p-chlorophenylalanine (p-CPA), and after the intraventricular administration of either 6-hydroxydopamine (6-OHDA) or 5,7-dihydroxytryptamine (5,7-DHT). The effects of p-CPA seemed not restricted to 5-HT, since reductions in catecholamine (CA) content were detected in several regions. After 5,7-DHT given under desimipramine (DMI) protection, comparable reductions in 5-HT levels were obtained but the changes in CA were less severe than after p-CPA. The neurotoxin 6-OHDA decreased the CA in all regions but also 5-HT content in hippocampus, hypothalamus and pons-medulla. The significance and the interpretation of these changes are discussed in relation to the specificity of the drugs employed, together with an assessment of the local monoamine turnover and the possible functional effects of monoamine interactions in the CNS.

5,7-Dihydroxytryptamine↗
PubMed1983

Long-term unilateral noradrenergic denervation: monoamine content and 3H-prazosin binding sites in rat neocortex.

Direct biochemical determinations of alpha 1 adrenoceptor sites were performed in the neocortex of rats subjected to a selective unilateral noradrenergic deafferentation, obtained by microinjecting 6-OHDA in the right dorsal noradrenergic bundle (DNB). After 3 months survival the alpha 1 sites were assayed using 3H-Prazosin (3H-PRZ), in both the denervated and the contralateral (control) cortex. The catecholamines dopamine (DA), epinephrine (EPI), norepinephrine (NE), as well as the indoleamine serotonin (5-HT) were measured using radioenzymatic assays in samples of the frontal cortex of these same animals, as well as in the septum and hippocampus in order to assess the extent and specificity of the deafferentation. The results document that unilateral NE-deafferentations of the cerebral cortex are feasable, the reduction in NE persists for at least three months, and there is an increased endogenous DA content. In the denervated cerebral cortex specific binding of 3H-PRZ showed an increase (+ 45%) in the density of receptor sites (Bmax) without any changes in the dissociation constant (KD 25 degrees C). The results have to be considered in relation both to plasticity changes in monoamine fibers and to denervation-induced alterations of the postysynaptic alpha-adrenoceptors.

Afferent Pathways↗
PubMed1983

Selective noradrenergic denervation and 3H-prazosin binding sites in rat neocortex.

Biochemical determinations of alpha 1-noradrenergic binding sites were performed in the neocortex of normal rats and of rats which had been subjected to a selective noradrenergic deafferentation obtained by microinjecting 6-OHDA in the dorsal noradrenergic bundle. The extent and the specificity of the deafferentation was assessed by measuring the catecholamines dopamine, epinephrine and norepinephrine, as well as the indoleamine serotonin using radioenzymatic assays. In the denervated animals (90% reduction in NE levels) specific binding of the alpha 1-noradrenergic antagonist prazosin revealed an increase only in the number of binding sites (Bmax) without changes in the dissociation constant (Kd).

Animals↗
PubMed1982

Altered neuronal responsiveness to biogenic amines in rat cerebral cortex after serotonin denervation or depletion.

To further investigate monoaminergic mechanisms in cerebral cortex, responsiveness of cortical neurons to microiontophoretic applications of serotonin (5-HT), dopamine (DA) or noradrenaline (NA) was examined in the frontoparietal region of control, 5,7-dihydroxytryptamine (5,7-DHT)- and p-chlorophenylalanine (PCPA)-treated rats anesthetized with urethane. As a rule, 100 nA applications of either one of these biogenic amines induced marked slowing or total interruption of 'spontaneous' firing overlasting the 30 s period of ejection. Given the large amounts of monoamines ejected, it could be inferred that such microiontophoretic applications produced a maximal activation of receptors. In control rats, the responses to 5-HT, DA and NA were of approximately equal duration (approximately equal to 5 min). Two to 4 weeks after denervation with 5,7-DHT, most neurons (75%) exhibited greatly prolonged responses to 5-HT (approximately equal to 14 min), and marked depressions of firing could be induced by small ejection currents (approximately equal to 2 nA) having little or no effect in the controls. In addition, 85% of the units supersensitive to 5-HT showed considerably shortened responses to DA and NA (approximately equal to 1 min). After 2-14 days of depletion with PCPA, there was no change in the responsiveness to 5-HT in spite of a 91% lowering of cortical 5-HT content equivalent to that measured after denervation. Nevertheless, responsiveness to DA and NA was again diminished in a majority (80%) of the units tested. In control or PCPA-treated rats, acute administration of the 5-HT re-uptake blocker fluoxetine increased the duration of depressions induced by 100 nA applications of 5-HT but did not enhance responsiveness to low ejection currents. This suggested that, after 5-HT denervation, the suppression of re-uptake was mainly responsible for the prolongation of 5-HT responses ('presynaptic' component of supersensitivity), whereas a modification of 5-HT receptors accounted for the greater efficacy of small doses of 5-HT ('postsynaptic' component). Responsiveness to the microiontophoretic application of phenylephrine (PHE), a noradrenergic a-agonist, was comparable with that to NA in PCPA- and 5,7-DHT-treated as well as in control rats. Therefore, the hyposensitivity to DA and NA appeared indicative of a desensitization of catecholamine receptors caused by the absence of 5-HT. Such a desensitization may be viewed as an adaptive change resulting from an increased release of endogenous DA and NA. This interpretation would in turn imply that, normally, 5-HT regulates catecholamine release in the neocortex.

Afferent Pathways↗
PubMed1982

The development of the host-response in juvenile Lymnaea palustris to invasion by Fasciola hepatica.

Laboratory studies indicate that miracidia of Fasciola hepatica can penetrate several species of lymnaeids in Europe, other than the natural host lymnaea truncatula. However, the production of cercariae from infections in abnormal hosts is unlimited to a small percentage of juvenile snails infected before a species-specific age. Juvenile Lymnaea palustris, of known ages, were exposed individually to F. hepatica miracidia and killed at 24 h intervals up to 14 days post exposure, and then processed for histological examination to ascertain reasons for the failure of F. hepatica infections to develop successfully in this abnormal host. The results indicate that the course of infection is rapidly halted by a cellular encapsulation response against the sporocyst. The response may be divided into two stages: first, the development of a cellular capsule and the concomitant degeneration of the sporocyst; second, the removal of the remains of the parasite and dispersal of the capsule. The efficiency of the response appears to increase with increasing age of the snail; statistical manipulation of the data obtained from histological investigations suggest that the defence mechanisms develop quickly in the first 6 days post hatching with little subsequent development during the experimental period. However, as sporocysts were encapsulated and killed regardless of age of the snail at exposure to infection, it is unlikely that in the host-parasite system used any of the infections would have survived.

Aging↗
PubMed1982

Catecholamines and serotonin in rat frontal cortex after PCPA and 6-OHDA: absolute amounts and ratios.

The catecholamines dopamine, epinephrine and norepinephrine, as well as the indoleamine serotonin were measured using radioenzymatic assays, in the frontal cortex of normal rats and of rats in which serotonin synthesis had been inhibited with p-chlorophenylalanine. Besides serotonin, norepinephrine levels were significantly reduced, thus changing the monoamine ratios. To further evaluate these changes, two models of catecholaminergic deafferentation were analyzed, i.e., the massive catecholamine depletion induced by intraventricular 6-OHDA and the more selective norepinephrine deafferentation, obtained by microinjecting 6-OHDA in the dorsal noradrenergic bundle. The results are interpreted in relation to the functional interactions between the catecholamines and between the catecholamines and serotonin.

Animals↗
PubMed1982

Adrenomedullary secretory response to midbrain stimulation in rat: effects of depletion of brain catecholamines or serotonin.

Electrical stimulation of the periaqueductal gray substance (PAG) of the rostral midbrain of the rat produced biphasic or monophasic pressor responses depending on the duration of the stimulus train. Marked increases in plasma noradrenaline (NA) and adrenaline (A) levels accompanied the pressor responses, indicating the participation of the adrenal medulla. Depletion of central catecholamines (CA) by intraventricular administration of 6-hydroxydopamine (6-OHDA) did not affect the primary vasomotor component but markedly depleted adrenal CA levels and attenuated the adrenomedullary component of the response to brain stimulation. The intraperitoneal administration of p-chlorophenylalanine (pCPA) not only depleted brain serotonin (5-HT) levels but also reduced brain CA levels significantly. The adrenaline (A) levels were reduced in the adrenal glands of these rats and the adrenal secretory response to brain stimulation was attenuated. In contrast, the selective destruction of central 5-HT neurons by intraventricular injection of 5,7-dihydroxytryptamine (5,7-DHT) in rats pretreated with desmethylimipramine (DMI) did not influence either the pressor nor the plasma CA responses to brain stimulation. Furthermore, the adrenal glands of these rats were normal. The results suggest that: (i) the central catecholaminergic neurons play an important role in the regulation of the adrenal glands but are not essential for the activation of the sympathetic vasoconstrictor fiber system: (ii) the pressor and plasma CA responses to PAG stimulation are not dependent on the central serotonergic system.

5,7-Dihydroxytryptamine↗
PubMed1981

Distribution of catecholamines and serotonin in the rat cerebral cortex: absolute levels and relative proportions.

The rat cerebral cortex was dissected in five regions and analyzed for the catecholamines noradrenaline, adrenaline and dopamine, and for the indoleamine serotonin using sensitive radioenzymatic assay methods with thin-layer chromatography. The noradrenaline concentration was highest in the ventral cortex, lateral to the hypothalamus, had intermediate values for the prefrontal, frontal and parietal cortical areas and was lowest in the occipital cortex. Dopamine levels were also highest in the cortex lateral to the hypothalamus, and moderate in the prefrontal and frontal cortical areas, with the lowest values measured for the occipital cortex. The ratios dopamine/noradrenaline further supports the hypothesis that they are independent transmitters. Traces of adrenaline were measured in all regions examined. The serotonin distribution was found to be non-homogeneous, with the highest values for the prefrontal cortex and ventral cortex lateral to the hypothalamus. The functional significance of these amines and their ratios are discussed in relation to their role as putative modulators of cortical neuronal excitability.

Animals↗
PubMed1981

Responsiveness of cortical neurons to serotonin after 5,7-DHT denervation or PCPA depletion.

1. In adult rats under urethane anesthesia, a vast majority of spontaneously active neurons in the frontoparietal cortex undergo a prolonged depression of their firing rate upon microiontophoretic application of 5-HT. 2. In 5,7-DHT-deafferented cortex, this effect is of a longer duration (mean 14 min) than in controls (mean 5 min). Moreover, small ejection currents of 5-HT are sufficient to induce a prolonged depression of the firing rate. 3. In PCPA-pretreated rats, there are no changes in the responsiveness to 5-HT. 4. In control and PCPA-pretreated rats, blocking of the 5-HT reuptake with fluoxetine increases the duration of responses to 5-HT (mean 15 min), whereas small ejection currents remain without effect. 5. These data indicate that, in the cerebral cortex, denervation supersensitivity to 5-HT results primarily from the removal of 5-HT afferents, and not from depletion of their 5-HT content. The enhanced responsiveness to microiontophoresed 5-HT appears to be due to a suppression of reuptake mechanisms, mainly responsible for the prolongation of 5-HT effects, and to a modification of receptors on target cells, which accounts for their greater sensitivity to 5-HT.

5,7-Dihydroxytryptamine↗