Increased mGluR1 mRNA Subunit Levels in Purkinje Cells in the Crus II Cerebellar Hemisphere but Not Vermis in Autism: An in Situ Hybridization Histochemical Study

Background:  In addition to activation of ligand-gated channels by the excitatory neurotransmitter glutamate, receptor activation also occurs via G-protein coupled metabotropic glutamate receptors (mGluRs) which signal to intracellular cascades.  mGluRs are homomeric receptors with eight subtypes in three groups. Human cerebellar mGluR1 mRNAs in Group I, are normally highly expressed in the granule cell layer and in Purkinje cells (PCs) and have a lower signal in the molecular layer.  Functionally, mGluR1s play an important role in synaptic signaling and modulating synaptic plasticity in the cerebellum.  Glutamate transmission via mGluR1s in PCs result from the production of a complex postsynaptic response consisting of a local dendritic calcium signal from IP-3 receptor-mediated calcium release from internal stores and, the production of a slow excitatory postsynaptic potential mediated by the novel transient receptor potential ion channel TRPC3. 

Objectives:  To determine whether there are alterations in PC mGluR1 mRNA levels in two distinct regions of the cerebellar cortex in autism cases relative to age- and postmortem-interval matched controls.  The hypothesis is that there will be an increase in mGluR1 levels in the Crus II region of the lateral hemisphere in autism cases but normal levels in the vermis (centered on lobule VI).  The rationale is that there have been more severe PC deficits and previously demonstrated GAD 67 mRNA changes in Crus II in autism cases.

Methods: In situ hybridization histochemistry and autoradiography were used to examine the cellular distribution of mGluR1 receptor subunit mRNA in the Crus II region and quantitative analysis of mGluR1 expression was obtained by measuring silver grain density in 120 Purkinje cell somata for each case from n=9 control and autism cases (Crus II) and n=8 control and autism cases (vermis). The age range was 16-39 years for autism and 17-43 years for controls.  Grain density corresponding to mGluR1 mRNA labeling was expressed as a mean number of pixels per surface area using NIH Image J analysis software and two-tailed unpaired t-tests were applied to determine significance at a p <0.05 level.

Results:  Statistical analysis of PCs showed a significant increase in mGluR1 mRNA levels in the autism group compared with control in the Crus II area (p = 0.0138), representing a 27% mean increase in mRNA levels in the autism group relative to controls. In contrast, there was no significant difference in vermal lobule VI (p = 0.2752) between autism cases and controls.

Conclusions: The Crus II region, which receives strong input from the frontal lobe via the pons, and implicated in high order cognitive processes, contains PCs with significantly increased mGluR1 mRNA expression levels, suggesting excess activation of synaptic signaling and altered modulation of synaptic activity in the autism brain. Combined with previous findings of decreased GAD 67 mRNA levels in Crus II PCs in autism, these data suggest that there are imbalances in the excitation:inhibition ratios in the cerebellum that potentially influence its functions in motor learning, timing and correction of movements and/or frontal lobe-related cognitive functions.