CNS PATHOLOGY IN THE NEUROLOGICAL MUTANT RATS ZITTER, TREMOR AND ZITTER-TREMOR DOUBLE MUTANT (SPONTANEOUSLY EPILEPTIC RAT, SER) - EXAGGERATION OF CLINICAL AND NEUROPATHOLOGICAL PHENOTYPES IN SER

The pathological alterations in the central nervous system (CNS) were examined in three kinds of mutant rat; the zitter (zi/zi; Zi), the tremor rat (tm/tm; Tm) and the spontaneously epileptic rat (SER) which is a double mutant carrying both zitter and tremor genes. Two major alterations demonstrated in these mutants were hypomyelination and vacuolation or spongy degeneration. Hypomyelination was observed predominantly in SER and to a lesser extent in Zi, and was accompanied by a redundant or aberrant myelin sheath formation in addition to a decreased number of myelinated fibres. This appeared to be related to the occurrence of tremor. There was no abnormality in the structure of the myelin lamellae and oligodendrocytes or any destruction of myelin sheaths by phagocytic cells. The number of radial components in CNS myelin was increased almost equally in Zi, Tm and SER. Vacuolation was prominent in SER and Tm, especially in the brainstem and thalamus. Zi also developed mild vacuolation with advancing age. Vacuolation seemed to be related to the epileptic phenomena in SER and Tm. Vacuoles consisted mainly of swollen astrocytic processes and enlargement of the extracellular space, as well as occasional enlargement of periaxonal spaces. Thus both pathological findings-the hypomyelination derived from the zitter mutation with tremor, and the vacuolation from the tremor mutation with epileptic symptoms-were mutually exaggerated in SER. It is postulated that the two different genetic loci with zi and tm mutations interact and synergistically reinforce each other both clinically and pathologically in SER.