Anatomical, physiological and biochemical studies of the cerebellum from mutant mice. I. Electrophysiological analysis of cerebellar cortical neurons in the staggerer mouse.

An electrophysiological analysis of cerebellar cortical neurons was performed in staggerer mutant mice on postnatal days 18 to 22, and data were compared with cerebellar activities in normal mice of the same age. Bioelectrical activities were elicited through local (LOC) and juxtafastigial (JF) stimulation. In staggerer mice, parallel fibers (PF) exhibited no major alterations, i.e. they conducted impulses and they activated most of the cerebellar cortical units encountered, but some minor differences existed in comparison with normal mice, i.e. their excitability was reduced, as tested by their refractory period and conduction velocity. On the contrary, Purkinje cell (PC) responses presented marked abnormalities. (1) Depth profiles during antidromic invasion of PC and unitary antidromic responses revealed a lower safety factor than normal in the invasion of the somatodendritic region of PC. In particular, a marked IS-SD delay or block occurred in about 60% of unitary antidromic responses of the mutant mouse. (2) Spontaneous or evoked typical climbing fiber responses (CFR) were never recorded in staggerer mice, despite functional synapses between climbing fibers (CF) and PC disclosed with harmaline injections. Activation of PC via CF under harmaline consisted of rhythmical trains of simple spikes, Finally, most of the PC studies in the staggerer mice were certainly activated via PF but with a low efficacy. The results suggest that PC are directly affected by the mutation.