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M Spedding

Publications and source records attributed to M Spedding.

At least 55 records · Page 3Linked to original sources
PubMed1994

Clozapine inhibits serotoninergic transmission by an action at alpha 1-adrenoceptors not at 5-HT1A receptors.

This study examined the mechanism underlying the influence of clozapine upon serotoninergic transmission in the rat. In vitro, clozapine manifested weak affinity at 5-HT1A receptors (pKi = 6.5) as compared to the agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (9.0), but high affinity at alpha 1-adrenoceptors (8.2) as compared to the alpha 1-adrenoceptor antagonist, prazosin (9.7). Ex vivo, clozapine (inhibitory dose (ID)50 = 0.7 mg/kg s.c.) mimicked prazosin (0.5) in potently occupying central alpha 1-adrenoceptors whereas, as compared to 8-OH-DPAT (0.2), it failed to occupy 5-HT1A receptors (> 10.0). The firing of serotoninergic neurones in the dorsal raphe nucleus was abolished by 8-OH-DPAT, clozapine and prazosin with ID50 values of 0.006, 0.09 and 0.07 mg/kg i.v., respectively. At comparable doses, they reduced striatal turnover of 5-HT. While the 5-HT1A receptors antagonists, (-)-tertatolol (2.0 mg/kg i.v.) and spiperone (0.63 mg/kg i.v.), blocked the action of 8-OH-DPAT upon dorsal raphe nucleus firing, they failed to modify the effect of clozapine and prazosin. In contrast, the alpha 1-adrenoceptor agonist, cirazoline (0.005 mg/kg i.v.) prevented the actions of clozapine and prazosin, but not that of 8-OH-DPAT. It is concluded that clozapine only weakly interacts with 5-HT1A receptors and that its potent alpha 1-adrenoceptor antagonist properties underlie inhibition of serotoninergic transmission.

5-Hydroxytryptophan↗
PubMed1994

Differential effects of non-peptidic tachykinin receptor antagonists on Ca2+ channels.

The effects of a range of non-peptidic neurokinin receptor antagonists on dihydropyridine binding at voltage-dependent Ca2+ channels from rat skeletal muscle were studied. As previously reported, the binding studies on dihydropyridine binding sites revealed a temperature-dependency effect of the tachykinin NK1 receptor antagonist (+/-)-CP 96345 ((2S,3S)-cis-2-(diphenylmethyl)-N-((2-methoxyphenyl) methyl)-1-azabicyclo-[2.2.2.]-octan-3-amine) similar to d-cis-diltiazem. Its related homologue CP 99994 ((+)-2S,3S)-3-(2-methoxybenzylamino)-2-phenylpiperid ine) was devoid of such activity. However, RP 67580 (perhydroisoindol-4-one-(3aR,7aR)-7,7-diphenyl- 2[1-imino-2-(2-methoxyphenyl)ethyl]) and SR 48968 ((S)-N-methyl-N-[4-(4-acetylamino-4-phenylpiperidino)-2-(3,4-dichloro phenyl) butyl]benzamide) (tachykinin NK1 and NK2 receptor antagonists) were also potent inhibitors of [3H]PN 200-110 (Isradipine) binding without temperature dependency, indicating that actions on ion channels may contribute to their pharmacological effects. Furthermore, all the compounds had affinity for the D888 ((-)-devapamil) phenylalkylamine site, indicating that many neurokinin antagonists may have affinity for Ca2+ channels.

Animals↗
PubMed1994

Alpha 2-adrenoceptors: more subtypes but fewer functional differences.

The proliferation of receptor subtypes based on differences in amino acid sequence does not necessarily coincide with functional differences. The number of alpha 2-adrenoceptor subtypes, as defined by ligand-binding and molecular studies, has been increasing in the past few years, which suggests the possibility of distinct physiological and pathological pathways that could be targeted by new selective drugs. However, the evidence from functional studies has been less convincing. This could be due to the lack of sufficiently selective ligands or to the similarity between the activated state of receptor subtypes. Species differences and the local receptor environment are also important determinants of the pharmacological profile of a particular subtype. The pharmacology of the putative subtypes of alpha 2-adrenoceptors and their function are discussed in this review by Alison MacKinnon, Mike Spedding and Christine Brown.

Adrenergic alpha-Antagonists↗
PubMed1993

[3H]p-aminoclonidine and [3H]idazoxan label different populations of imidazoline sites on rat kidney.

In the presence of RS-15385-197 to preclude binding to alpha 2-adrenoceptors, [3H]p-aminoclonidine labelled a low affinity high capacity site, (Kd = 127.6 +/- 19.7 nM, Bmax 978 +/- 172 fmol/mg protein) whereas [3H]idazoxan labelled a high affinity low capacity site (Kd = 1.66 +/- 0.28 nM, Bmax 45.3 +/- 11.4 fmol/mg protein). Clonidine and p-aminoclonidine showed moderate affinity for the site labelled by [3H]p-aminoclonidine, but low affinity for the site labelled by [3H]idazoxan, whereas idazoxan showed high affinity for [3H]idazoxan and low affinity for [3H]p-aminoclonidine binding. Naphazoline inhibited [3H]idazoxan in a biphasic manner suggesting that [3H]idazoxan may label an heterogeneous population of imidazoline sites. GTP inhibited [3H]idazoxan but not [3H]p-aminoclonidine binding. These results suggest that [3H]idazoxan labelled imidazoline I2 binding sites, whereas [3H]p-aminoclonidine labelled a novel subtype which showed marked differences to the imidazoline I1 binding site reported in bovine and human brainstem.

Adrenergic alpha-Antagonists↗
PubMed1993

Modulation of alpha 1-adrenoceptors in rat left ventricle by ischaemia and acyl carnitines: protection by ranolazine.

Myocardial alpha 1-adrenoceptor number has been reported to increase during ischaemia in myocytes consequent to an increase in acyl carnitine levels. We investigated whether this phenomenon occurs in vivo and whether the novel antiischaemic agent ranolazine will protect against it. Thirty-minute occlusion of the left anterior descending coronary artery (LAD) in anaesthetised rats produced an approximate doubling of the left ventricular (LV) alpha 1-adrenoceptor population. The carnitine palmitoyl transferase 1 (CPT1) inhibitor sodium 2-[5-(4-chlorophenyl)-pentylene]oxiran-2-carboxylate (POCA 100 micrograms/kg) reduced this ischaemia-induced increase when administered intraperitoneally (i.p.) 15 min before ischaemia and abolished the increase when administered intravenously (i.v.). The CPT1 inhibitor sodium 2-tetradecyl oxirane carboxylate dihydrate (TGDA) (500 micrograms/kg) inhibited the upregulation when administered i.p. and significantly decreased alpha 1-adrenoceptor density when administered i.v.; however, this agent, unlike POCA, reduced [3H]-prazosin binding directly. The alpha 1-adrenoceptor antagonist prazosin (100 micrograms/kg i.v.) did not prevent the increase. Direct addition of palmitoyl carnitine (10 microM) to membranes from nonischaemic myocardium caused a doubling in alpha 1-adrenoceptor number, and this effect was selective for heart membranes as compared with cerebral cortex; beta-adrenoceptor number was not modified. Ranolazine (500 micrograms/kg) inhibited upregulation when administered 15 min i.v. before ischaemia or after 3-day twice-daily (b.i.d.) i.p. pretreatment. This drug did not inhibit CPT1 directly.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetanilides↗
PubMed1993

Modulation of central noradrenergic function by RS-15385-197.

1. RS-15385-197, a highly potent and selective alpha 2-adrenoceptor antagonist, was examined in a variety of in vitro and in vivo functional tests to assess the selectivity of its interaction with central noradrenergic neurones in the rat. 2. In hypothalamic slices, RS-15385-197 was potent in augmenting K(+)-evoked release of [3H]-noradrenaline, with an EC50 of 9 nM. Idazoxan and yohimbine showed 100 fold less activity. This was due to its antagonist action at presynaptic alpha 2-adrenoceptors, as RS-15385-197 (10 microM), did not directly release [3H]-noradrenaline from cortical slices unlike reserpine (10 microM), and did not inhibit noradrenaline re-uptake into cortical synaptosomes. 3. In vivo, RS-15385-197 (0.5 mg kg-1, p.o.) increased levels of 3-methoxy-4-hydroxy-phenylglycol (MHPG) in the cerebral cortex without modifying levels of 5-hydroxyindoleacetic acid (5-HIAA). This dose, but not a lower dose (0.1 mg kg-1, p.o.) caused beta-adrenoceptor down-regulation in the cortex when administered once daily for 14 days whereas 5-HT2 receptor number was unaltered, indicating a selective effect on noradrenergic transmission. 4. Selective depletion of cortical 5-HT by administration of p-chlorophenylalanine (PCPA; 100 mg kg-1, i.p. for 14 days) or 5,7-dihydroxytryptamine (5,7-DHT; 150 micrograms i.c.v.) prevented the beta-adrenoceptor down-regulation caused by RS-15385-197, indicating that a tonic 5-hydroxytryptaminergic input was required for it to elicit beta-adrenoceptor down-regulation. It was not possible to prevent the loss of activity of RS-15385-197 in these 5-HT-depleted animals by co-administration with the 5-HT1A partial agonist, 8-hydroxy-n-dipropyl aminotetralin (8-OH-DPAT, 0.3 mg kg-1, i.p. twice daily for final 3 days).5. At a dose (1 mg kg-1, p.o.) which completely prevented the hypoactivity produced by clonidine(0.1 mgkg-1, p.o.), RS-15385-197 did not affect behavioural stereotypy induced by 8-OH-DPAT(0.3 mg kg-1, s.c.). Similarly, following chronic dosing with the racemate, RS-15385-196 (3 mg kg-1,p.o., once daily for 14 days), there was no effect on the behavioural and hypothermic response to 8-OH-DPAT (0.5 mg kg-1, s.c.). Therefore, RS-1 5385-197 was selective for central alpha2-adrenoceptors over 5-HT1A receptors in in vivo functional tests.6. Thus, RS-15385-197 was highly selective in interacting with central noradrenergic neurones in the rat in vitro and in vivo. It is therefore currently the agent of choice for investigations of the role of alpha 2-adrenoceptors in the CNS.

Adrenergic alpha-Antagonists↗
PubMed1993

Identification of a novel calcium antagonist binding site in rat brain by SR 33557.

1. In K(+)-depolarized taenia preparations from guinea-pig caecum SR 33557 was a potent antagonist of Ca(2+)-induced contractions and antagonized the effect of the calcium channel activator Bay K 8644. 2. SR 33557 displayed high affinity (pKi 9.54 +/- 0.04, nH 1.01) for the [3H]-(+/-)-PN 200-110 binding site in rat cerebral cortex membranes. In the presence of 5 mM Ca2+ this affinity was reduced (pKi 8.82 +/- 0.01, nH 1.05) whilst the affinity of nitrendipine was unaffected by this concentration of Ca2+. 3. Saturation binding experiments in rat cerebral cortex carried out in the absence and presence of SR 33557 (0.1-1.0 nM) indicated an apparently competitive interaction at the dihydropyridine site, in that SR 33557 reduced the KD of [3H]-(+/-)-PN 200-110 binding without any effect on Bmax. In kinetic experiments, the rate of dissociation of [3H]-(+/-)-PN 200-110 from rat cerebral cortex was unchanged in the presence of SR 33557 (5 nM). 4. D-cis-diltiazem fully reversed the inhibition [3H]-nitrendipine binding to rat cerebral cortex produced by SR 33557 indicating the site of action of SR 33557 to be distinct from the dihydropyridine (DHP) binding site. 5. Saturation analysis indicated that [3H]-SR 33557 (0.01-0.8 nM) labelled a single class of binding sites in rat cerebral cortex membranes with high affinity (KD 0.12 +/- 0.01, Bmax 222 +/- 20 fmol mg-1 protein), although kinetic data indicated the existence of negative cooperativity between the binding sites. 6.In competition studies, a variety of different calcium antagonists displayed similar affinity for [3H]-SR 33557 and [3H]-(+/-)-PN 200-110 sites. The [3H]-SR 33557 site was sensitive to the inhibitory effect of divalent cations. The affinity of Cd2+ was 0.026 +/- 0.015 mM and the rank order of affinity was Cd2+ >Ca2+ >Mn2+ >Mg2+ >Na+.7. We propose that SR 33557 labels a distinct site in rat cerebral cortex. The coupling between the SR 33557 and DHP site appears to be very close, resulting in apparently competitive interactions in some experimental protocols but can be revealed as negatively allosteric in other circumstances.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1993

The effect of lifarizine (RS-87476), a novel sodium and calcium channel modulator, on ischaemic dopamine depletion in the corpus striatum of the gerbil.

1. Unilateral ligation of the right common carotid artery in the anaesthetized gerbil for 3 h caused a 62.7% decrease in ipsilateral dopamine in the corpus striatum from 1.40 (+/- 0.13, n = 27) micrograms g-1 in the non-ischaemic hemisphere to 0.47 (+/- 0.07, n = 27) micrograms g-1 in the ischaemic hemisphere (all results are expressed as mean +/- s.e. mean). In sham-operated animals there were no differences in the dopamine levels (1.31 +/- 0.14 micrograms g-1, n = 11, left; 1.27 +/- 0.13 micrograms g-1, n = 11 in the right hemisphere). Animals with intact communicating arteries in the circulus arteriosus were excluded. 2. Lifarizine (RS-87476; 250, 500, but not 50, micrograms kg-1, i.p.) protected against this dopamine depletion showing only a 9.2% decrease at 250 micrograms kg-1, i.p. (P < 0.01) and no decrease at 500 micrograms kg-1, i.p. (P < 0.01). 3. Nicardipine (250 micrograms kg-1, p.o.) was effective when administered chronically once daily for 10 days (26.6% decrease, P < 0.05) but not when administered acutely at 50 micrograms kg-1, i.p.

Animals↗
PubMed1993

Sodium modulation of 3H-agonist and 3H-antagonist binding to alpha 2-adrenoceptor subtypes.

1. The alpha 2-adrenoceptors on human platelets and neonatal rat lung were characterized with the agonist and antagonist ligands [3H]-adrenaline and [3H]-RS-15385-197 respectively. A correlation of affinities for 3H-antagonist binding showed the receptors to be of the alpha 2A-(platelet) and alpha 2B-(neonatal rat lung) adrenoceptor subtypes, whereas a correlation of affinities for 3H-agonist binding showed the receptors to have similar characteristics (r = 0.88). 2. NaCl (100 mM) had no effect on [3H]-RS-15385-197 binding in the human platelet, but increased the density of sites labelled with [3H]-RS-15385-197 in neonatal rat lung by 52%. NaCl increased the density of sites labelled by [3H]-adrenaline in neonatal rat lung, but there was a consequent 3.5 fold decrease in affinity. In the human platelet, no specific [3H]-adrenaline binding was observed in the presence of 100 mM NaCl. 3. In the neonatal rat lung, NaCl had no significant effect on the affinity of prazosin for [3H]-RS-15385-197 binding; however, imiloxan affinity was increased 13 fold. The affinity of the catecholamines, adrenaline and noradrenaline was significantly decreased, whereas the imidazolines, oxymetazoline and UK-14,304 were much less affected. The affinity of prazosin and imiloxan for [3H]-adrenaline binding was significantly increased in the presence of 10 and 100 mM NaCl. Conversely, the affinity of adrenaline and noradrenaline was decreased in the presence of NaCl, and there was no change in the affinity of the imidazoline agonists. 4. In the human platelet, NaCl had no effect on the affinity of prazosin for [3H]-RS-15385-197 binding but the affinity of imiloxan was significantly increased. NaCl significantly decreased the affinity of the catecholamines adrenaline and noradrenaline, whereas the affinity of UK-14,304 and oxymatazoline was much less affected. Competition experiments with [3H]-adrenaline in the presence of NaCl in platelets were difficult to characterize as there was no specific binding under these conditions.5. The results show that both the alpha2A- and alpha2B-adrenoceptor subtypes are allosterically regulated by Na+, but only the alpha2B-subtype showed a significant increase in density. Interestingly, there is a differential regulation of imidazoline (unchanged) and catecholamine (decreased affinity) agonist interactions with these subtypes. Na+ may therefore critically affect receptor subtype selectivity of drugs. The implications for receptor subclassification are discussed.

Adrenergic alpha-Agonists↗
PubMed1993

Changes in [3H]-PK 11195 and [3H]-8-OH-DPAT binding following forebrain ischaemia in the gerbil.

1. A high density of [3H]-PK 11195 binding sites was present in gerbil cortical membranes (Bmax [3H]-PK 11195 1360 +/- 71 fmol mg-1 protein) in comparison to rat cortical membranes (254 +/- 21 fmol mg-1 protein). This effect was species-specific as similar findings were obtained with hippocampal membranes (Bmax 1430 +/- 111 fmol mg-1 protein in gerbil, compared to 196 +/- 31 in rat). 2. RO 5-4864, also a peripheral type benzodiazepine compound, displayed low affinity for the [3H]-PK 11195 site in the gerbil (pKi 6.57 +/- 0.02 and 6.70 +/- 0.12 in hippocampus and cortex respectively) compared to rat (pKi 8.16 +/- 0.07 and 8.48 +/- 0.02). Central benzodiazepine compounds, diazepam and flunitrazepam, also displayed this trend. 3. RO 5-4864 displaced [3H]-PK 11195 binding from gerbil and rat cortical membranes through a competitive interaction with Hill slopes close to unity. In both tissues, saturation isotherms of [3H]-PK 11195 binding indicated that the presence of RO 5-4864 caused changes in Kd without any effect on Bmax. In kinetic experiments, the presence of RO 5-4864 failed to modify the rate of dissociation of [3H]-PK 11195 from equilibrium in both rat and gerbil cortical membranes. 4. Forebrain ischaemia in the Mongolian gerbil (5 min bilateral carotid artery occlusion) with 7 days recovery caused a significant (P<0.05) decrease in the density of hippocampal 5-HTlA binding sites labelled by [3H]-8-OH-DPAT (Bmax control, 393 +/- 33 fmol mg-1 protein; ischaemic, 289 +/- 21 fmol mg protein)and an increase (P<0.01) in [3H]-PK 11195 binding sites (Bmax control, 1430 +/- 111 fmol mg-1 protein; ischaemic, 2160 +/- 170 fmol mg-1 protein). Ischaemia and recovery had no effect on the affinity of either ligand.5. Autoradiography experiments in gerbil brain sections revealed that the ischaemia-induced increase in[3H]-PK 11195 binding was consistent and significant in the CA, subfield on the hippocampus (control,152 +/- 42 fmol mg-1 tissue; ischaemic, 314 +/- 43 fmol mg-1 tissue).

8-Hydroxy-2-(di-n-propylamino)tetralin↗
PubMed1993

Protective effects of ranolazine in guinea-pig hearts during low-flow ischaemia and their association with increases in active pyruvate dehydrogenase.

1. In isolated Langendorff-perfused, electrically-paced, hearts of guinea-pigs, global low-flow-ischaemia (LFI; at 0.7 ml min-1) resulted in marked increases in the rates of release of lactate, lactate dehydrogenase (LDH) and creatine kinase (CK) over a 30 min period. At the end of the LFI period, tissue ATP content was significantly reduced from a control value of 11.8 +/- 0.8 (5) to 5.6 +/- 0.8 (5) mumol g-1 dry weight. 2. The presence of ranolazine [(+/-)-N-(2,6-dimethyl-phenyl)-4[2-hydroxy-3-(2-methoxy-phenoxyl)- propyl] - l-piperazine acetamide dihydro-chloride; RS-43285-193] at 10 microM, from 20 min prior to and during LFI, resulted in significant reductions in the release of lactate, LDH and CK during the ischaemic period and a significant preservation of tissue ATP (9.0 +/- 1.1 (6) mumol g-1 dry wt.). Ranolazine did not prevent the reductions in creatine phosphate or glycogen observed in LFI, nor did it have any significant effects on any contractile parameters before or during the LFI period. 3. Neither ranolazine nor LFI affected the total amounts of tissue pyruvate dehydrogenase (PDH) activity; however, the significant reduction in the amount of active, non-phosphorylated PDH caused by LFI (from 88.2 +/- 5.5 to 44.2 +/- 3.2% of total activity) was partially but significantly prevented by ranolazine (67.2 +/- 6.8%). This effect of ranolazine on PDH may be part of the mechanism whereby the compound reduces lactate release and preserves tissue ATP during ischaemia.

Acetanilides↗
PubMed1992

Classification of calcium channels and calcium antagonists: progress report.

The molecular biology and electrophysiology of calcium channels is proceeding apace, partially driven by the clinical success of the drugs classed as "calcium antagonists." Indeed, there are now more than 70 drugs in development that have been claimed to be "calcium antagonists" of one type or another. In order to ensure that there is a logical nomenclature for the channels and the drugs acting at the channels, an international committee has met over the last 2 years to classify the channels and the drugs. This report consists of an overview of current thinking, which will be finalized in a document submitted to the IUPHAR (International Union of Pharmacologists) nomenclature committee.

Calcium Channel Blockers↗
PubMed1992

[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.This suggests that [3H]-RS-15385-197 labels both alpha2A- and alpha2B-adrenoceptor subtypes.4. Prazosin and methysergide inhibited the binding of [3H]-RS-15385-197 in the rat cerebral cortex in a simple manner consistent with an interaction at a single site. Although oxymetazoline inhibited [H]-RS- 15385-197 with a Hill slope significantly different from unity, the slope was increased to unity in the presence of Gpp(NH)p, suggesting an agonist-like interaction.5. The site labelled by [3H]-RS-15385-197 in the rat cortex shows high affinity for oxymetazoline and low affinity for prazosin which could be taken as evidence for classifying the site as an alpha2A-subtype.However, the affinities of yohimbine, rauwolscine and oxymetazoline at this site do not correspond to the population of sites in the human platelet. Yohimbine and rauwolscine were 20 fold selective for the platelet alph2A-subtype, whereas phentolamine was 2 fold and imiloxan was 10 fold selective for the cortex subtype. Indeed although the site showed some similarities with the alpha2A-subtype, the highest degree of homology was observed between this site and the rat submaxillary gland and the RG20 clone,tentatively called the alpha2D-adrenoceptor subtype. We propose that the alpha2-adrenoceptor in the rat cortex may therefore correspond to the putative alpha2D-subtype of the adrenoceptor.

Animals↗