MRI analysis of mGluR5 and mGluR1 antagonists, MTEP and R214127 in the cerebral forebrain of awake, conscious rats

Metabotropic glutamate receptor 5 tracer [18F]-FPEB displays increased binding potential in postcentral gyrus and cerebellum of male individuals with autism: a pilot PET study

Background

Autism is a neurodevelopmental disorder that is first manifested during early childhood. Postmortem experiments have identified significantly elevated expression of metabotropic glutamate receptor 5 (mGluR5) in cerebellar vermis and prefrontal cortex of individuals with autism.

Methods

In the current study we employed the mGluR5 tracer [¹⁸F]-3-fluoro-5-[(pyridin-3-yl)ethynyl]benzonitrile ([¹⁸F]-FPEB) to quantify mGluR5 binding in vivo in adults with autism vs. healthy controls using positron emission tomography (PET).

Results

We identified significantly higher [¹⁸F]-FPEB binding potential in the postcentral gyrus and cerebellum of individuals with autism. There was a significant negative correlation between age and [¹⁸F]-FPEB binding potential in the cerebellum but not in the postcentral gyrus. In the precuneus, [¹⁸F]-FPEB binding potential correlated positively with the lethargy subscale score for the Aberrant Behavioral Checklist (ABC). In cerebellum, there were significant negative correlations between [¹⁸F]-FPEB binding potential and ABC total score, ABC hyperactivity subscale score, and the ABC inappropriate speech subscale score.

Conclusions

These novel findings demonstrate for the first time that mGluR5 binding is altered in critical brain areas of subjects with autism, suggesting abnormal glutamate signaling in these regions. Finally, the correlations between altered [¹⁸F]-FPEB binding potential in the cerebellum and precuneus suggest that some autistic symptoms may be influenced by abnormal glutamate signaling.

Functional modulation of G-protein coupled receptors during Parkinson disease-like neurodegeneration

G-protein coupled dopamine and metabotropic glutamate receptors (mGlu) can modulate neurotransmission during Parkinson's disease (PD)-like neurodegeneration. PET imaging studies in a unilateral dopamine denervation model (6-OHDA) showed a significant inverse correlation of presynaptic mGlu4 and postsynaptic mGlu5 expression in the striatum and rapidly declining mGlu4 and enhanced mGlu5 expression in the hippocampus during progressive degeneration over time. Immunohistochemical studies verified the decreased mGlu4 expression in the hippocampus on the lesion side but did not show difference in mGlu5 expression between lesion and control side. Pharmacological MRI studies showed enhanced hemodynamic response in several brain areas on the lesion side compared to the control side after challenge with mGlu4 positive allosteric modulator or mGlu5 negative allosteric modulator. However, mGlu4 response was biphasic having short enhancement followed by negative response on both sides of brain. Studies in mGlu4 expressing cells demonstrated that glutamate induces cooperative increase in binding of mGlu4 ligands - especially at high glutamate levels consistent with in vivo concentration. This suggests that mGlu allosteric modulators as drug candidates will be highly sensitive to changes in glutamate concentration and hence metabolic state. These experiments demonstrate the importance of the longitudinal imaging studies to investigate temporal changes in receptor functions to obtain individual response for experimental drugs.