Evaluation of a Novel Non-Competitive Antagonist as a Radioligand for the N-Methyl-D-Aspartate Receptor-Channel Complex In Vivo

McGregor, Ailsa L (1998) Evaluation of a Novel Non-Competitive Antagonist as a Radioligand for the N-Methyl-D-Aspartate Receptor-Channel Complex In Vivo. PhD thesis, University of Glasgow.

Abstract

This thesis reports the synthesis and initial evaluation of the substituted guanidine N-(l- Napthyl)-N'-(2-iodophenyl)-N'-methylguanidine (CNS 1261) as a potential agent to image N-methyl-D-aspartate (NMDA) receptor activation in vivo. The pharmacology of unlabelled CNS 1261 was investigated in vitro using ligand binding assays and in vivo using autoradiographic procedures. Investigations were designed to examine the ability of CNS 1261 to inhibit radioligand binding to the NMDA receptor and to other neurotransmitter receptors. [14C]2-deoxyglucose autoradiography was used to measure local rates of glucose utilisation following systemic administration of CNS 1261. The effects of CNS 1261 on glucose utilisation were compared to those obtained in a previous study carried out by Kurumaji et al (1989) using MK801. Secondly, in order to assess the ability of [125I]CNS 1261 to measure levels of NMDA receptor activation in the brain, a modification of the [125I]MK801 in vivo binding technique described by McCulloch et al (1992) was used to investigate the uptake and retention of [125I]CNS 1261 in the normal rat brain. The pattern of[125I]CNS 1261 uptake was compared to that of [125I]MK801 in parallel experiments. Finally, autoradiographic techniques were used to assess the utility of [125I]CNS 1261 in imaging changes in NMDA receptor activation in vivo. NMDA receptor activation was manipulated via proton blockade and microinjection of the excitatory amino acid agonist NMDA into the cortex Pharmacology of CNS 1261 The ability of CNS 1261 to inhibit [3H]MK801 binding was investigated in crude synaptic rat brain membranes (Cambridge NeuroScience Inc.). CNS 1261 (10muM) binds to the NMDA receptor in vitro with high affinity (Ki = 25nM for displacing [3H]MK801 binding). The effects of unlabelled CNS 1261 (l0nM and 1muM) on the binding of ligands to the NMDA receptor and to other neurotransmitter receptors was more extensively investigated by submitting the compound for commercial screening (Novascreen). CNS 1261 (1?M) was extremely selective showing activity (99% inhibition) only in the [3H]MK801 binding assay. The effects of CNS 1261 (1, 3 and l0mg/kg) on local cerebral glucose use were examined in the conscious lightly restrained rat. The systemic administration of CNS 1261 resulted in overt behavioural responses and marked alterations in glucose use within the central nervous system. Administration of CNS 1261 produced highly circumscribed changes in glucose utilisaion that could be readily visualised on autoradiograms. Decreases in glucose use (approximately 30%) were observed in layer IV of the frontal, sensory-motor and auditory cortices and in the inferior colliculus. Distinct, significant elevations in glucose use were observed in the hippocampus molecular layer, dentate gyrus, limbic system (posterior cingulate cortex, caudal entorhinal cortex, mamillary body, anteroventral thalamus), retrosplenial cortex and the myelinated fiber tract of the fornix. Correlation analysis revealed that the overall pharmacological effects of CNS 1261 were extremely similar to the overall effects of MK801, correlation coefficient, r = 0.87. Subtle differences were observed in a number of regions. A greater number of regions displayed extreme sensitivity to CNS 1261 than to MK801, such as the anteroventral thalamus, mamillary body and inferior colliculus. In contrast, the entorhinal cortex was much less sensitive to CNS 1261 than to MK801 [125I]CNS 1261 uptake in the normal rat brain Radio-iodinated (125I or 123I)CNS 1261 was synthesised at high specific activity (>2000Ci/mmol) and chemical purity (>97%) at the West of Scotland Radionuclide Dispensary, Glasgow. Investigation of the in vivo uptake and retention of [125I]CNS 1261 was carried out in halothane anaesthetised rats. The initial uptake of [125I]CNS 1261 reflected cerebral blood flow. At time points beyond 30 minutes the uptake of [125I]CNS 1261 reflected the classical pattern of NMDA receptor distribution with the highest levels of binding in the hippocampus and low levels in the hypothalamus and cerebellum. At 120 minutes, [125I]CNS 1261 uptake within hippocampal regions relative to that in the cerebellum (a measure of non-specific binding) was 70-140% greater than that observed with [125I]|MK801. The mechanisms underlying the superior imaging capability of [125I]CNS 1261 was addressed by investigation of the lipophilicity and metabolism of this tracer. (Abstract shortened by ProQuest.).

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: James McCulloch
• Keywords: Neurosciences
• Date of Award: 1998
• Depositing User: Enlighten Team
• Unique ID: glathesis:1998-75936
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 17:15
• Last Modified: 19 Nov 2019 17:15
• URI: https://theses.gla.ac.uk/id/eprint/75936