Olfactory learning abilities in staggerer mutant mice

Higher blood harmane (1-methyl-9H-pyrido[3,4-b]indole) concentrations correlate with lower olfactory scores in essential tremor

BACKGROUND

Harmane (1-methyl-9H-pyrido[3,4-b]indole), a neurotoxin, may be an environmental risk factor for essential tremor (ET). Harmane and related chemicals are toxic to the cerebellum. Whether it is through this mechanism (cerebellar toxicity) that harmane leads to ET is unknown. Impaired olfaction may be a feature of cerebellar disease.

OBJECTIVE

To determine whether blood harmane concentrations correlate with olfactory test scores in patients with ET.

METHODS

Blood harmane concentrations were quantified using high performance liquid chromatography. Odor identification testing was performed with the University of Pennsylvania Smell Identification Test (UPSIT).

RESULTS

In 83 ET cases, higher log blood harmane concentration was correlated with lower UPSIT score (rho=-0.46, p<0.001). 25/40 (62.5%) cases with high log blood harmane concentration (based on a median split) had low UPSIT scores (based on a median split) vs. 12/43 (27.9%) ET cases with low log blood harmane concentration (adjusted odd ratios (OR) 4.04, 95% confidence intervals (CI) 1.42-11.50, p=0.009). When compared with the low log blood harmane tertile, the odds of olfactory dysfunction were 2.64 times higher in cases in the middle tertile and 10.95 times higher in cases in the high tertile. In 69 control subjects, higher log blood harmane concentration was not correlated with lower UPSIT score (rho=0.12, p=0.32).

CONCLUSIONS

Blood harmane concentrations were correlated with UPSIT scores in ET cases but not controls. These analyses set the stage for postmortem studies to further explore the role of harmane as a cerebellar toxin in ET.

Cytochrome oxidase activity in the olfactory system of staggerer mutant mice

The staggerer mutation is characterized by the disruption of a single recessive gene encoding for the nuclear hormone receptor RORalpha. In addition to the well-studied gene expression in the cerebellum causing massive primary Purkinje cell loss, the RORalpha gene is also expressed in the thalamus and the olfactory bulb. A quantitative histochemical study of cytochrome oxidase activity was performed in staggerer mutants and their respective controls in order to determine whether olfactory bulb neuropathology leads to neuronal metabolic alterations in olfactory and related limbic regions. In the staggerer olfactory bulb, the core and the shell of the glomeruli had lower levels of cytochrome activity, whereas higher levels were found in the external plexiform and granular layers. Other olfactory and limbic regions were unchanged, except for a higher level in the accessory olfactory bulb and a lower level in the most ventral part of the medial orbital cortex. These results are discussed with regard to the olfactory deficits and changes in social interactions previously observed in this mutant.