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Title: 5HT receptors and α₂-adrenoceptors : classification, ligand binding, functional correlates and alterations in cerebral ischaemia
Author: Brown, Christine Mary
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1988
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This study investigates 5HT receptor and alpha2-adrenoceptor subtypes. The principal technique used is radioligand binding. Where possible, functional correlates for the binding sites were examined. Once characterised the 5HT binding assays were used to examine the effect of cerebral ischaemia on 5HT neuropharmacology. Finally, preliminary investigations of a possible interaction of 5HT with alpha2-adrenoceptors were undertaken. 1. Under carefully defined conditions, 5HT binding sites were identified on rat and gerbil brain membranes, using [3H]-5HT as radioligand. 5HT and 5HT binding sites were characterised by 8 compounds that showed selectivity for the two subtypes (5HT, 5CT, 8-OH-DPAT, RU24969, buspirone, spiperone, mianserin and pindolol). 5CT and RU24969 yielded triphasic concentration response curves in the rat but not the gerbil cortex, providing evidence that [3H]-5HT labels three distinct 5HT sites in these membranes. High affinity [3H]-5HT binding to an uptake site or 5HT 1D binding site, was absent in the rat cortex. However, preliminary studies indicated that a fourth, distinct [3H]-5HT binding site was present on bovine cortex. 2. More selective radioligands led to better definition of heterogeneity of 5HT binding sites. [3H]-8-OH-DPAT and [3H]-WB4101 were found to label 5HT binding sites, predominantly in the hippocampus, characterised by high affinity for 5HT, 8-OH-DPAT and buspirone. The binding of 3 [3H]-8-OH-DPAT was sensitive to GTP, suggesting the ligand may act as an agonist. [125I]-CYP (in the presence of 30uM isoprenaline) was found to label the 5HT 1B subtype showing high affinity for 5HT, cyanopindolol, and 1B RU24969. 3. The pharmacological profile of 5HT 1A sites in the rat and gerbil appeared to be similar. Unlike human or pig, a 5HT 1B site was present on gerbil brain membranes, which was similar to the rat. A 5HT binding site in the gerbil was not evident from these studies. 4. The 5HT 1A binding site in the hippocampus was significantly 1A down-regulated following chronic dosing with an antidepressant, which suggests a possible role for this subtype in the aetiology of depression. 5HT release studies suggest the autoreceptor is likely to be a 5HT subtype as showed by high affinity for 5HT and 5CT whereas the 5HT ligand mianserin and the 5HT ligand MDL 72222 were inactive. 5. In peripheral tissue, a high affinity, low capacity [3H]-5HT binding site was identified on rat parenchymal lung membranes, which showed high affinity for 5HT and 5CT. This binding site was distinct from the 5HT uptake site as it showed only 3 low affinity for imipramine and desipramine. No specific [3H]-5HT binding was found on guinea-pig or rabbit atrial membranes. 6. The kinetic and pharmacological characteristics of the binding of [3H]-ketanserin, [3H]-mianserin and [125I]-LSD to brain membranes from rat and/or gerbil were studied. All three ligands labelled sites with the characteristics of the 5HT binding site, showing high affinity for 5HT2 antagonists and low affinity for 5HT. 7. [3H]-LSD was found to label a 5HT receptor on human platelet membranes, the affinity of drugs to inhibit this binding correlated well with their ability to inhibit 5HT-induced platelet aggregation. 8. High affinity, saturable, reversible binding, which could be inhibited by 5HT 2 antagonists, was identified in homogenates of pig coronary and human umbilical arteries, labelled by [ 125I]-LSD. This binding site correlated well with the receptor mediating the response to 5HT in the isolated PCA. 9. A reliable, rapid and highly predictive method for selection of stroke-prone gerbils was developed, using a microscopic examination of the anastomosis between the anterior cerebral arteries in the circulus arteriosus. Isoelectric EEG recordings also proved useful for identifing stroke-prone animals. 10. Global unilateral ischaemia (up to 3h) and brief bilateral ischaemia with recovery was found to have pronounced effects upon the level of 5HT in two areas of the gerbil brain, frontal cortex and corpus striatum. 11. Unilateral cerebral ischaemia (3h) caused a significant ipsilateral decrease in the number of 5HT2 binding sites in the gerbil frontal cortex, labelled by [3H]-ketanserin, with no significant effect on the affinity. However, unlike 3h postmortem tissue, the remaining binding sites showed high affinity for antagonists, and therefore were still viable. 12. Bilateral ischaemia (5min) with 72h recovery also caused a significant decrease in the number of 5HT2 binding sites. Quipazine-induced head shakes, mediated via a 5HT2 receptor, were observed in the gerbil. However, no significant decrease in head shakes was seen in animals 24 or 72h following 5min bilateral ischaemia.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available