Dual Radiotracer Analysis of Cholinergic Neuronal Changes in Prediabetic Mouse Pancreas

Abstract

Background: Pancreatic neuronal changes associated with beta cell loss in type 1 diabetes mellitus are complex, involving, in part, parasympathetic mechanisms to compensate for preclinical hyperglycemia. The parasympathetic neurotransmitter acetylcholine (ACh) mediates insulin release via M3 muscarinic receptors on islet beta cells. The vesicular ACh transporter (VAChT) receptor has been shown to be a useful marker of cholinergic activity in vivo. The positron emission tomography (PET) radiotracer (ϩ)-4-[ 18 F]fluorobenzyltrozamicol ([ 18 F]FBT) binds to the VAChT receptor on presynaptic cholinergic neurons and can be quantified by PET. The compound 4-diphenylacetoxy-N-methylpiperidine (4-DAMP), available in a tritiated form, binds to M3 muscarinic receptors on beta cells and is a potential target for assessing pancreatic beta cell mass. In this study, we investigate the feasibility of dual radiotracer analysis in identifying neurofunctional changes that may signify type 1 diabetes mellitus in its early preclinical state. Methods: Ex vivo determinations of pancreatic uptake were performed in prediabetic nonobese diabetic mice and controls after intravenous injection of [ 18 F]FBT or 4-[ 3 H]DAMP. Beta cell loss in prediabetic mice was confirmed using immunohistochemical methods. Results: [ 18 F]FBT uptake was significantly higher in prediabetic pancreata than controls: 3.22 Ϯ 0.81 and 2.51 Ϯ 1.04, respectively (P Ͻ 0.03). 4-[ 3 H]DAMP uptake was significantly lower in prediabetic pancreata than controls: 0.612 Ϯ 0.161 and 0.968 Ϯ 0.364, respectively (P ϭ 0.01). Conclusions: These data suggest that a combination of radiotracer imaging agents that bind to neuronal elements intimately involved in insulin production may be an effective method of evaluating changes associated with early beta cell loss using PET.

Enhanced Cholinergic Response in Pancreata of Nonhuman Primates with Impaired Glucose Tolerance Shown on [18F]Fluorobenzyltrozamicol Positron Emission Tomography

“…[ 18 F]FBT was synthesized as previously described. 14,17 Approximately 5 mCi of [ 18 F]FBT was injected intravenously into each subject. Each PET scan was acquired in a dynamic fashion from time of injection through 90 min post-injection.…”

Molecular imaging of β-cells: diabetes and beyond

“…18 F-FBT was further used as a tool to investigate glucose tolerance in adult female monkeys [153]. Considering the reduced cholinergic innervation in T1DM and the role of the parasympathetic neurotransmitter ACh in insulin production, Clark and co-authors further tested the feasibility of dual radiotracer analysis in identifying neurofunctional changes in T1DM using 18 F-FBT and 4-3 H-DAMP, with the former probe targeting VAChT and the latter probe binding to M3 muscarinic receptors on β-cells [154]. The authors found that the combined use of these two tracers may efficiently assess insulin production ability, indicating that noninvasive synergistic imaging of pancreatic cholinergic activity and M3 muscarinic receptor density may be an effective method for evaluating BCM and β-cell function.…”

Imaging of β-Cell Mass and Function

“…Clark et al, using 18 F-4-fluorobenzyltrozamicol, imaged vesicular acetylcholine transporters on presynaptic neuronal vesicles innervating the pancreas, showing increased 18 F-4-fluorobenzyltrozamicol up-take in prediabetic rodent pancreata (9). …”