EMG ANALYSIS OF HARMALINE-INDUCED TREMOR IN NORMAL AND 3 STRAINS OF MUTANT MICE WITH PURKINJE-CELL DEGENERATION AND THE ROLE OF THE INFERIOR OLIVE

The effects of intraperitoneal injections of 10 mg/kg harmaline were t ested in normal mice and three strains of cerebellar mutant mice with Purkinje cell degeneration. Ten normal (wildtype) mice (+/+), as well as five lurcher (lc/+), six nervous (nr/nr), and eight Purkinje cell d egeneration (pcd/pcd) mutants were implanted with chronic electromyogr am (EMG) electrodes in the hamstring and quadriceps muscle groups of t he right hindlimb. 2. EMGs were recorded in each of the mice during sp ontaneous activity before and after intraperitoneal injections of 0.3 ml harmaline (10 mg/kg). Spectral analysis was used to quantify the am plitude and frequency of tremor found in the EMGs after harmaline admi nistration. Normal mice responded to harmaline with strong, continuous 11- to 14-Hz tremor. Mutants from the pcd/pcd strain also reacted wit h continuous tremor, but of lower amplitude and frequency. In contrast , nr/nr mutants exhibited intermittent paroxysmal tremor lasting for o nly a few seconds, and lc/+ mutants showed no evidence of tremor whats oever. 3. In order to detect covert tremor that was possibly not revea led by focal intramuscular EMG recordings, several mutant and normal m ice were also tested on a suspended platform to which an accelerometer was attached. The results confirmed the findings from EMG recording. 4. An incidental observation made during the course of this study was that harmaline tremor disappeared from the normal mouse during swimmin g and reappeared when the animal was withdrawn from the water. 5. Alth ough Purkinje cells appeared to increase both the depth of modulation and the frequency of tremor, the inhibitory action of the cerebellar c ortex does not seem to be essential for the generation of tremor. 6. P arasagittal cerebellar sections of the normal, wild-type mice and the three strains of cerebellar mutant mice of various ages were stained w ith cresyl violet and examined for Purkinje cell degeneration. Purkinj e cell degeneration was found to be complete in the pcd/pcd and lc/+ s trains. Although an initial examination of parasagittal sections of th e nr/nr strain failed to find any surviving Purkinje cells, further ex amination of sections cut in the coronal plane revealed small clusters of Purkinje cells in the vermal area of the posterior lobe. 7. The re trograde transport of wheat-germ-agglutinin-conjugated horseradish per oxidase (WGA-HRP) pressure-injected into the cerebellar cortex was use d to study the olivocerebellar projections in the wild-type mice and t he three strains of cerebellar mutant mice. 8. The volume of cerebella r cortex injected with WGA-HRP varied from similar to 50% in the least case to close to 90% in the greatest case, and some diffusion into th e cerebellar nuclei occurred in all four groups of mice. The degree of retrograde labeling differed markedly among the strains. In the wild- type mouse, all olivary nuclei appeared heavily labeled, whereas retro grade transport produced moderate labeling in the pcd/pcd strain and o nly very faint marking in the nr/nr strain. At low magnification, no l abeling was seen in the lc/+ mouse, although a few faintly marked cell s were identified at higher (x400) magnification. Together these obser vations suggest that differences in the functional integrity of inferi or olivary neurons among the three mutant strains is a plausible expla nation for the disparate reaction to the tremor-inducing drug harmalin e.