Effects of neonatal antithyroid treatment on brain [3H]-imipramine binding sites.

The action of the antithyroid, sulphydryl reagent methimazole (MMI) on the specific binding of [3H]-imipramine in the cerebral cortex and corpus striatum of immature and mature rats has been examined. Chronic administration of MMI through the first 30 days of life decreased the number of imipramine binding sites in cortical but not striatal membranes, as assessed 48 h after the last injection of goitrogen. A similar treatment did not affect the binding profile of [3H]-imipramine in mature rats. Acute administration of MMI to 30 day-old rats increased the number of imipramine binding sites shortly after the injection, an effect no longer evident 48 h later. MMI in vitro increased the binding of [3H]-imipramine. It is concluded that maturational impairment of the hypothyroid cortex, rather than any alteration of membrane bound thiol groups, was a major cause for the diminished binding of [3H]-imipramine in MMI-treated, immature rats.     

Effect of chronic treatment with selective monoamine oxidase inhibitors and specific 5-hydroxytryptamine uptake inhibitors on [3H]paroxetine binding to cerebral cortical membranes of the rat

[3H]Paroxetine is a highly selective ligand for the 5-hydroxytryptamine transporter complex and the specific binding of this ligand to membrane fractions from cerebral cortex or hippocampus was studied in rats treated with specific inhibitors of the uptake of 5-hydroxytryptamine and monoamine oxidase inhibitors. The Kd and Bmax of the binding of [3H]paroxetine to cerebral cortical membranes of the rat was unaffected, compared to sham controls, by either acute or chronic administration with citalopram or chlorimipramine. Also, chronic treatment with chlorimipramine did not alter the parameters of the binding of [3H]paroxetine to hippocampal membranes from the rat compared to sham controls. Furthermore, chronic and acute treatments with clorgyline or deprenyl did not produce any significant changes in the Kd and Bmax of the binding of [3H]paroxetine to cerebral cortical membranes in the rat. These findings on the binding of [3H]paroxetine are discussed in light of previous equivocal results on the plasticity of neuronal binding sites for [3H]imipramine after various pharmacological treatments.     

Strain differences in changes in some parameters of cerebral cortical adrenergic system following chronic imipramine administration to rats.

The characteristics of [3H]prazosin binding sites in the membranes from cerebral cortex, the basal level of formation of cyclic AMP in cortical slices, and the responsiveness of the cyclic AMP generating system to noradrenaline and isoproterenol in this preparation were measured in Long-Evans, Wistar and Sprague-Dawley rats treated chronically with saline or imipramine. No differences between strains and treatments were observed regarding the Bmax and KD of [3H]prazosin binding sites. The basal levels of cyclic AMP formation were similar in control rats of all strains, but imipramine treatment augmented it significantly in Sprague-Dawley rats. The responses of the cyclic AMP generating system to noradrenaline were significantly lower in Long-Evans than in the remaining strains of rats. Only in Sprague-Dawley rats a significant downregulation of response to noradrenaline was observed after imipramine treatment. All three strains of rats differed significantly among themselves in their responsiveness to isoproterenol; only in Sprague-Dawley rats this response was down-regulated significantly (by 80%) by imipramine treatment.     

3H]mazindol binding associated with neuronal dopamine and norepinephrine uptake sites

[3H]Mazindol labels neuronal dopamine uptake sites in corpus striatum membranes (KD = 18 nM) and neuronal norepinephrine uptake sites in cerebral cortex and submaxillary/sublingual gland membranes (KD = 4 nM). The potencies of various inhibitors of biogenic amine uptake in reducing [3H]mazindol binding in striatal membranes correlate with their potencies for inhibition of neuronal [3H]dopamine accumulation, whereas their potencies in reducing [3H]mazindol binding to cortical and salivary gland membranes correlate with their potencies for inhibition of neuronal [3H]norepinephrine accumulation. Similar to the dopamine and norepinephrine uptake systems, [3H]mazindol binding in all three tissues is dependent upon sodium (with potassium, lithium, rubidium, and Tris being ineffective substitutes) and chloride (with sulfate and phosphate being ineffective substitutes). In membranes of the cerebral cortex and salivary gland, half-maximal stimulation is observed at 50-80 mM NaCl, whereas in membranes of the corpus striatum half-maximal stimulation occurs at 240 mM NaCl. In striatal membranes NaCl increases the affinity of [3H]mazindol binding with no effect on the maximal number of sites. The enhancement of affinity is due to a selective slowing of the dissociation of the ligand from its binding site. The association of [3H]mazindol binding sites with neuronal dopamine uptake sites in the corpus striatum is further supported by the reduction of [3H]mazindol binding sites in striatal membranes following destruction of dopaminergic neurons by 6-hydroxydopamine. Similarly, the association of [3H]mazindol binding sites with neuronal norepinephrine uptake sites in cerebral cortex is supported by the reduction of [3H]mazindol binding to cortical membranes following destruction of noradrenergic neurons by N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine.     

Regulation of α1-adrenoceptors in the cerebral cortex of the rat by thyroid hormones

The influence of thyroid hormones on the concentration and properties of alpha 1-adrenoceptors in a crude membrane fraction obtained from the rat cerebral cortex was investigated using the [3H]-WB 4101 binding assay. Animals were made hypothyroid by feeding 6-propyl-2-thiouracil for 8 weeks. Hyperthyroidism was induced by triiodothyronine injections (50 microgram/100 g body weight) for 9 days. 1. The binding of [3H]-WB 4101 was saturable and of high affinity in controls as well as in hyper- and hypothyroid animals. The maximal number of binding sites (Bmax), which amounted to 95 fmol/mg protein in control animals, was increased by 27% in cortical membranes from hyperthyroid rats and reduced by 23% in the hypothyroid group. 2. The reduction in [3H]-WB 4101 binding due to 6-propyl-2-thiouracil feeding was reversible by triiodothyronine treatment. 3. Dissociation constants (KD) calculated from saturation experiments (0.25 nM) or kinetic data (0.21 nM) remained unchanged in altered thyroid states. 4. Inhibition of [3H]-WB 4101 binding by adrenergic agonists and antagonists revealed no differences between euthyroid and hypothyroid animals. The higher affinity of prazosin to the binding sites compared with yohimbine indicated that [3H]-WB 4101 predominantly labeled alpha 1-adrenoceptors. It is concluded that thyroid hormones regulate the number of alpha 1-adrenoceptors in membranes of the rat cerebral cortex, leaving their affinities unchanged.     

Binding of [3H]SCH23390 to a non-dopaminergic site in bovine kidney.

Binding of the D1 dopamine receptor antagonist [3H]SCH23390 to bovine renal cortical membranes has been studied. Specific binding of [3H]SCH23390 was saturable and reversible and stereoisomers of SCH23390 competed stereoselectively. In contrast, competition with the isomers of butaclamol was not stereoselective and dopamine failed to compete for the [3H]SCH23390 binding site. The site is therefore not a D1 dopamine receptor. Competition studies with a very wide range of compounds failed to define the nature of the [3H]SCH23390 binding sites in renal cortex whereas in parallel studies the characteristics of [3H]SCH23390 binding in caudate nucleus were entirely consistent with those of D1 dopamine receptors. The nature of [3H]SCH23390 binding in preparations of tubular and glomerular membranes was found to be virtually identical to those of crude renal cortical membranes indicating lack of compartmentation of these sites. Autoradiographic studies of [3H]SCH23390 binding in bovine kidney showed significantly higher levels of binding sites in renal cortex compared with renal medulla and this was confirmed by direct ligand binding studies.     

Effect of chronic desmethylimipramine or electroconvulsive shock on selected brain and platelet neurotransmitter recognition sites

Rats were treated with electroconvulsive shock (ECS), desmethylimipramine (DMI), ECS plus DMI, or diazepam. In vitro analyses showed that chronic ECS produced an elevated density of recognition sites for [3H]imipramine (IMI) in platelet membranes, but had no effect on membrane preparations derived from cortical tissue. A similar elevation in receptor binding was seen exclusively in platelets after chronic ECS plus DMI, whereas no effect was observed with DMI alone. Equilibrium dissociation constant (KD) values for [3H]IMI were also increased in platelet membranes from rats given chronic ECS or ECS plus DMI treatment. Chronic ECS or DMI administration produced a decreased density of beta-adrenergic recognition sites in frontal cortex and cerebellum as assessed by [3H]dihydroalprenolol (DHA) binding. The combination of ECS plus DMI produced a similar decrease. In addition, chronic diazepam administration produced a down-regulation of the beta-adrenergic receptor only in the cerebellum. These data provide evidence for the differential regulation of brain and peripheral neurotransmitter recognition sites.     

Inescapable shock reduces [3H]Ro 5-4864 binding to "peripheral-type" benzodiazepine receptors in the rat

[3H]Ro 5-4864 binding to "peripheral-type" benzodiazepine receptors was examined in brain and peripheral tissues of rats subjected to inescapable tailshocks. Two hours after a session of 80 (five-second) inescapable tailshocks, a significant reduction in [3H]Ro 5-4864 (10 mM) binding was observed in membranes from kidney (31%), cerebral cortex (29%), heart (19%) and pituitary (17%) compared to tissues from naive animals. In contrast, inescapable shock did not effect [3H]Ro 5-4864 binding to hippocampal, lung, or adrenal membranes. Scatchard analyses of [3H]Ro 5-4864 binding to renal membranes demonstrated that this session of tailshock reduced the density (Bmax) of "peripheral-type" benzodiazepine receptors without effecting the apparent affinity (Kd) of the radioligand for these sites. The effects of graded stress on [3H]Ro 5-4864 binding to cerebral cortex and kidney were investigated using 5, 20, or 80 (five-second) inescapable shocks. In cerebral cortical membranes, sessions of either 5 or 20 shocks did not affect, while 80 shocks reduced (29%) [3H]Ro 5-4864 (10 mM) binding. In renal membranes, 5 shocks significantly increased (35%), 80 shocks significantly decreased [3H]Ro 5-4864 (10mM) binding (31%). These findings demonstrate that the density of "peripheral-type" benzodiazepine receptors in both peripheral tissues and the central nervous system can be rapidly modulated by stress.     

Solubilization and characterization of the 5-hydroxytryptamine transporter complex from rat cerebral cortical membranes

The 5-hydroxytryptamine transporter complex from rat cerebral cortical membranes was solubilized with digitonin. The affinity of the solubilized transporter complex for [3H]paroxetine, a very selective and potent inhibitor of 5-hydroxytryptamine uptake, was not affected and remained unchanged when compared with the parent membrane preparation. The solubilization yield of membrane-bound [3H]paroxetine binding sites was 42%. The pharmacological profile of the solubilized transporter complex was similar to that of the intact transporter in membranes of the cerebral cortex, with the exception of tryptamine, which exhibited a 10-fold loss in potency to inhibit [3H]paroxetine binding to the solubilized transporter when compared to membranes. The Stokes radius determined by gel filtration was 7.6 nm. This successful solubilization of the neuronal 5-hydroxytryptamine transporter complex is the starting point for purification of this macromolecular moiety.     

Characterization of [3H]atipamezole as a radioligand for alpha 2-adrenoceptors.

Atipamezole (MPV-1248, 4-(2-ethyl-2,3-dihydro-1H-inden-2-yl)-1H-imidazole), a potent alpha 2-adrenoceptor antagonist, was tritiated to high specific activity. We then compared [3H]atipamezole and [3H]rauwolscine as radioligands for alpha 2-adrenoceptors in rat cerebral cortex, neonatal rat lung, and human platelets. (-)-Noradrenaline and phentolamine were used to define specific alpha 2-adrenergic binding. Unlabelled atipamezole was used in a similar manner to define saturable, high-affinity non-adrenergic binding. [3H]Atipamezole binding to human platelets (Kd 1.3 nM) and rat brain membranes (Kd 0.5 nM) equilibrated rapidly and was displaced in the expected manner by alpha 2-adrenergic ligands. In contrast, [3H]atipamezole binding in neonatal rat lung membranes was only effectively inhibited by unlabelled atipamezole, and by high concentrations of idazoxan. The total density of binding sites for [3H]atipamezole was clearly in excess of the density of alpha 2-adrenoceptors in this tissue, as defined by [3H]rauwolscine binding. We conclude that [3H]atipamezole binds with high affinity to alpha 2-adrenoceptors in human platelets and rat cerebral cortex, and that the compound can be used to investigate alpha 2-adrenoceptor properties and drug actions in these tissues. In neonatal rat lung, [3H]atipamezole identified an additional population of binding sites, distinct from both classical alpha 2-adrenoceptors and idazoxan-defined imidazoline receptors. The pharmacological identity of these binding sites remains to be elucidated. This non-adrenergic component in the binding characteristics of [3H]atipamezole complicates its use as a general alpha 2-adrenoceptor radioligand.     

Species-selective binding of [3H]-idazoxan to alpha 2-adrenoceptors and non-adrenoceptor, imidazoline binding sites in the central nervous system.

1. We have used the imidazoline derivative [3H]-idazoxan to define alpha 2-adrenoceptors and non-adrenoceptor, imidazoline binding sites in cerebral cortex membranes of calf, mouse, rat, guinea-pig and man. 2. Competition experiments using the selective alpha-adrenoceptor drugs, rauwolscine and corynanthine, indicated that [3H]-idazoxan bound to a single population of sites in the calf and mouse membranes. However, [3H]-idazoxan also labelled non-adrenoceptor, imidazoline binding sites in the rat (15%), guinea-pig (30%) and human (40%) cerebral cortex membranes. 3. Competition experiments with adrenaline and cirazoline in the guinea-pig cortex, verified [3H]-idazoxan binding to both alpha 2-adrenoceptors and to non-adrenoceptor, imidazoline binding sites. 4. It has been postulated by several groups that [3H]-idazoxan may possess partial agonist activity. To investigate this further, saturation experiments were performed in the cerebral cortex membranes of all five species in the absence and presence of 300 microM guanosine triphosphate (GTP). GTP had no effect on [3H]-idazoxan binding in guinea-pig cerebral cortex; in both rat and mouse membranes 300 microM GTP increased the dissociation constant for [3H]-idazoxan by 2-3 fold without significantly affecting the Bmax. GTP reduced the Bmax by approximately 30% and 60% in calf and human cerebral cortex membranes, respectively, without significantly altering the Kd. 5. Saturation experiments were performed in the calf cerebral cortex membranes in the absence and presence of 300 microM GTP with the selective alpha 2-adrenoceptor agonist [3H]-clonidine and the selective muscarinic antagonist [3H]-quinuclidinyl benzilate (QNB).(ABSTRACT TRUNCATED AT 250 WORDS)     

Differences in the binding of [3H][D-Ser2,Thr6]leucine-enkephalin and [3H][D-Pen2,D-Pen5]enkephalin to brain membranes of morphine tolerant-dependent rats.

The effect of morphine tolerance-dependence and abstinence on the characteristics of delta-opiate receptors was determined in male Sprague-Dawley rats. Two ligands used for characterizing the receptors were [3H][D-Ser2,Thr6]leucine-enkephalin ([3H]DSTLE) and [3H][D-Pen2,D-Pen5]enkephalin ([3H]DPDPE). Rats were implanted s.c. under light ether anesthesia with six morphine pellets (each containing 75 mg of morphine free base). Rats which served as controls were implanted similarly with placebo pellets. Two sets of rats were used. In one group of rats, the pellets were left intact (tolerant-dependent) at the time of sacrificing and in the other the pellets had been removed 18 h earlier (abstinent). The spinal cord and brain regions (amygdala, hippocampus, hypothalamus, corpus striatum, mid-brain, pons and medulla and cortex) were dissected for binding studies. The binding of [3H]DSTLE to membranes of cerebral cortex of morphine-tolerant-dependent rats was decreased in comparison to control rats, and was due to a decrease in Bmax rather than Kd value. The binding of [3H]DSTLE to other brain regions or spinal cord of morphine-tolerant-dependent and abstinent rats did not differ from their respective controls. On the other hand, the binding of [3H]DPDPE was unaffected in any brain region or the spinal cord of morphine-tolerant-dependent and abstinent rats when compared to their controls. The decrease in binding of [3H]DSTLE to cortical membranes of morphine-tolerant-dependent rats amounted to 15%. Since DSTLE also binds to mu-opiate receptors, which have earlier been shown to be decreased in cortex of morphine-tolerant-dependent rats, and the binding of a more selective delta-opiate ligand [3H]DPDPE was unaffected, it is concluded that central delta-opiate receptors do not play a role in the development of morphine-induced tolerance-dependence or abstinence processes in the rat.     

Down-regulation of brain and spinal cord K-opiate receptors in spontaneously hypertensive, Wistar-Kyoto normotensive, and Sprague-Dawley rats by chronic treatment with U-50, 488H.

The effects of chronic administration of U-50,488H, a K-opiate receptor agonist, on the binding of [3H]ethylketocyclazocine ([3H]EKC) to K-opiate receptors on the cerebral cortical and spinal cord membranes of spontaneously hypertensive (SHR), Wistar-Kyoto normotensive (WKY), and Sprague-Dawley (SD) rats were determined. Age-matched (10 weeks old) male rats of each strain were injected twice daily for 7 days with either U-50,488H (25 mg/kg, i.p.) or its vehicle. On day 8, the rats were killed. The cerebral cortex and the spinal cord were isolated for binding studies. The systolic blood pressure and heart rate of SD and WKY rats did not differ but the blood pressure of SHR rats were higher than that of SD and WKY rats. The receptor density (Bmax) and apparent dissociation constant (Kd) values of [3H]EKC binding to the spinal cord of WKY and SHR rats did not differ. However, the spinal cord of SD rats had higher Bmax and Kd values than WKY or SHR rats. The cortex of the SD rats had a lower Bmax value than the other two strains. Treatment with U-50,488H decreased the Bmax value of [3H]EKC in spinal cord of SD rats, increased the Kd value in SHR rats, and had no effect in WKY rats. Decreases in the Bmax value were produced in the cortex of all strains of rats, but a greater effect was observed in WKY and SHR rats than in SD rats.     

Triethyllead-induced peroxidative damage in various regions of the rat brain

Adult male Fisher 344 rats (8-10 wk old) were dosed ip with 1.75 mg/kg body weight of triethyllead chloride (TEL) for 5 consecutive days. Rats were sacrificed 1, 7, or 21 d after the last injection. The rate of lipid peroxidation was significantly elevated in frontal cortex at all three time points assayed (1, 7, or 21 d). However, hippocampal and cerebellar membranes showed no changes in peroxidative capacity at these time points. In order to determine whether cortical membrane damage was reflected in alteration of a restricted protein population, a series of high-affinity receptor binding sites was determined in cortical membranes derived from treated rats 7 d after the last injection of triethyllead. The rate of lipid peroxidation was significantly increased in the frontal cortex of triethyllead treated rats; however, no changes in the binding of [3H]spiroperidol, [3H]quinuclidinyl benzilate, and [3H]benzodiazepine were seen in animals exposed to triethyllead. The cortical wet weight, protein content, and cell number were also unchanged by TEL treatment, reflecting an absence of gross damage.     

Selective effects of thiol reagents on the binding sites for imipramine and neurotransmitter amines in the rat brain.

The action of the antithyroid drugs methimazole (MMI) and propylthiouracil (PTU) on the binding of [3H]-imipramine, [3H]-5-hydroxytryptamine [3H]-5-HT) (to 5-HT1-receptors) and [3H]-spiperone (to 5-HT2-, D2-receptors) of rat brain membranes has been examined. The synaptosomal uptake of [3H]-5-HT was also studied. Micromolar concentrations of the disulphide bond reducing agents MMI, PTU, dithiothreitol (DTT) and mercaptoethanol increased both the binding of [3H]-imipramine and the uptake of [3H]-5-HT. In contrast, they decreased the number of 5-HT1-receptors, and did not affect 5-HT2-and D2-sites. Reaction with membrane-bound sulphydryl (SH) groups by micromolar concentrations of N-ethylmaleimide (NEM), hydroxymercuribenzoic acid (PCMB), or Ellman's reagent (DTNB) decreased the binding of [3H]-imipramine, the number of 5-HT1-receptors, and the uptake of [3H]-5-HT. Millimolar concentrations of NEM were necessary in order to decrease partially 5-HT2- and D2-receptors. The effects of NEM on imipramine recognition sites and on the uptake of 5-HT could be prevented by DTT; protection was not obtained in other receptor systems. Three groups of receptors have been, thus, postulated, based upon their different sensitivity towards alterations in membrane [disulphide bridges in equilibrium SH] equilibrium: Group I, including imipramine recognition sites and the uptake system for 5-HT; Group II, including 5-HT1-receptors; Group III, including 5-HT2-and D2-receptors.     

Involvement of sulfhydryl groups in cerebral cortical [3H]clonidine binding in developing rats

The involvement of essential sulfhydryl groups in alpha 2-adrenoceptor function was investigated in the cerebral cortex of 7 and 70 day old rats. N-Ethylmaleimide (NEM) inhibited specific [3H]clonidine binding to cerebral cortical membranes in a concentration-dependent and biphasic manner in both infant and adult rats. The inhibitory effect of NEM was attenuated by simultaneous addition of dithiothreitol 30 microM and 1 mM, though dithiothreitol up to 1 mM did not affect the binding. p-Chloromercuriphenylsulfonic acid also caused a significant reduction in [3H]clonidine binding at both stages. Scatchard analysis of [3H]clonidine binding showed that NEM 10 and 100 microM caused a significant decrease in Bmax of high affinity binding sites without changing KD. Neither Bmax nor KD values were changed by NEM 10 and 100 microM in low affinity sites. The treatment with NEM 10 and 100 microM significantly reduced the increase of binding induced by Mn2+ 10 and 100 microM which was observed on day 70 but not on day 7. It is suggested that two distinct categories of essential sulfhydryl groups are involved in cerebral cortical alpha 2-adrenoceptors and are functionally mature by day 7. Furthermore, sulfhydryl groups are involved in the Mn2+-induced increase of alpha 2-receptors in adult rats.     

3H]N-methyl-carbamylcholine, a new radioligand specific for nicotinic acetylcholine receptors in brain

The present study characterized the binding of [3H]N-methyl-carbamylcholine ([3H]methyl-carbachol), a new radioligand, to rat cerebral cortex membranes and demonstrated the autoradiographic distribution of these sites in rat brain. With atropine used to block muscarinic acetylcholine sites and nicotine to define non-specific binding, [3H]methyl-carbachol bound specifically, saturably and with high affinity (Kd = 11.0 nM, Bmax = 118.4 fmol/mg protein and Hill coefficient = 0.92) to a population of presumably nicotinic sites in cerebral cortex membranes. When nicotine was used to block nicotinic acetylcholine sites and atropine to define non-specific binding there was no specific binding of [3H]methyl-carbachol (concentrations up to 45 nM) to possible muscarinic sites in cerebral cortex membranes. The binding parameters under non-selective conditions (without blockade of either muscarinic or nicotinic acetylcholine sites) had very similar values to those obtained under nicotinic conditions (Kd = 8.0 nM, Bmax = 125.0 fmol/mg protein and Hill coefficient = 0.98). [3H]Methyl-carbachol binding was potently inhibited by nicotinic agonists and antagonists but only poorly displaced by muscarinic agents. Autoradiographic studies evidenced high densities of [3H]methyl-carbachol binding sites in the interpeduncular nucleus, various thalamic nuclei, superior colliculus and layers III/IV of the cortex. Such a distribution was very similar to those previously reported for nicotinic acetylcholine sites and other radioligands. These results suggest that [3H]methyl-carbachol is a specific radioligand of the neuronal nicotinic receptor. Its stability and high selectivity constitute distinct advantages over previously used nicotinic radioligands such as acetylcholine and nicotine.     

3H]-RS-15385-197, a selective and high affinity radioligand for alpha 2-adrenoceptors: implications for receptor classification.

1. RS-15385-197 is the most potent and selective alpha 2-adrenoceptor antagonist available. We have used [3H]-RS-15385-197 to define alpha 2-adrenoceptor subtypes. The binding of [3H]-RS-15385-197 to membranes of rat cerebral cortex, rat neonatal lung and human platelets was reversible, saturable and of high affinity. Saturation experiments indicated that [3H]-RS-15385-197 bound to a single population of sites in all 3 tissues with high affinity (0.08-0.14 nM). The density of sites labelled by [3H]-RS-15385-197 was greater in the cortex (275 fmol mg-1 protein) than in the neonate lung (174 fmol mg-1 protein) and human platelet (170 fmol mg-1 protein). The density of sites labelled with [3H]-RS-15385-197 in the cortex was significantly greater than that labelled with [3H]-yohimbine (121 fmol mg-1 protein). 2. The selective alpha 2-adrenoceptor antagonists, idazoxan, yohimbine, rauwolscine and WY 26703 displaced [3H]-RS-15385-197 binding to rat cerebral cortex in a simple manner with Hill slopes close to unity. The affinities derived for these antagonists against [3H]-RS-15385-197 were similar to the values obtained for the displacement of [3H]-yohimbine indicating the alpha 2-adrenoceptor nature of the binding site. 3. alpha 2A-Adrenoceptor selective compounds, oxymetazoline and BRL 44409, showed high affinity for [3H]-RS-15385-197 binding in the human platelet and lower affinity in the neonate lung, while the alpha 2B-selective compounds, prazosin and imiloxan, showed high affinity for [3H]-RS-15385-197 binding in the neonate lung.(ABSTRACT TRUNCATED AT 250 WORDS)     

Tricyclic antidepressants and calcium channel blockers: interactions at the (-)-desmethoxyverapamil binding site and the serotonin transporter

The activity of various calcium channel blockers at the serotonin transporter, as determined by their effects on imipramine binding and serotonin uptake, was investigated in rat brain and human platelets. In addition, the effect of tricyclic antidepressants on the binding of calcium channel blockers was evaluated. Verapamil was found to be a competitive inhibitor of both imipramine binding and serotonin uptake in brain and platelets. The inhibitory activity of verapamil was calcium-independent. Chronic verapamil treatment resulted in a significant decrease (28%) in [3H]imipramine binding in the rat hypothalamus but had no effect on [3H]imipramine binding to cerebral cortex membranes or on [3H]serotonin uptake in these two brain regions. Tricyclic antidepressants inhibited (-)-[3H]desmethoxyverapamil binding but did not affect [3H]nitrendipine binding to rat cerebral cortex membranes. Chronic imipramine treatment induced a non-significant increase (34%) in (-)-[3H]desmethoxyverapamil binding but did not alter [3H]nitrendipine binding in rat cerebral cortex. These interactions may be relevant to an understanding of the beneficial effects of verapamil in major affective disorders and may suggest an involvement of calcium channels in the pharmacological activity of tricyclic antidepressants.     

3H]tryptamine binding sites of rat cerebral cortex: pharmacological profile and plasticity

Equilibrium saturation analysis of the binding of [3H]tryptamine to membranes from the cerebral cortex of the rat at 0 degrees C indicated that [3H]tryptamine bound to a single class of high affinity binding sites (Kd = 1.29 +/- 0.13 nM). The binding of [3H]tryptamine was potently inhibited by tryptamine itself, beta-carboline, tetrahydro-beta-carboline and several beta-phenylethylamine derivatives. Structure-activity relationships of the beta-phenylethylamines tested showed that substitutions in para-position were the most potent with the following rank order of potency H less than OH less than Cl less than OCH3. Although chronic treatment with parachlorophenylalanine did not affect the parameters of the binding of [3H]tryptamine to cerebral cortical membranes of the rat, chronic treatment with clorgyline and deprenyl resulted in a 49% decrease in the density of binding sites for [3H]tryptamine, with no change in Kd. This modulation of the binding of [3H]tryptamine lends support to the proposal that binding sites for [3H]tryptamine could represent a specific class of receptors in the CNS. As such, the structure-activity relationships revealed within and between the various families of compounds tested provides useful information for the development of new chemical tools as potential agonists and/or antagonists at these sites.