Effects of Paired and Unpaired Eye-Blink Conditioning on Purkinje Cell Morphology

Abstract

This experiment addressed (1) the importance of conjunctive stimulus presentation for morphological plasticity of cerebellar Purkinje cells and inhibitory interneurons and (2) whether plasticity is restricted to the spiny branches of Purkinje cells, which receive parallel fiber input. These issues were investigated in naive rabbits and in rabbits that received paired or unpaired presentations of the conditioned stimulus (CS) and unconditioned stimulus (US). To direct CS input to the cerebellar cortex, pontine stimulation served as the CS. Air puffs to the cornea served as the US. Paired condition rabbits received pontine stimulation for 350 msec paired with a coterminating 100-msec air puff. Unpaired condition rabbits received the same stimuli in a pseudorandom order at 1- to 32-sec intervals. Rabbits were trained for a mean of 12 days. Naive rabbits received no treatment. In Golgi-stained Purkinje neurons in lobule HVI, total dendritic length, main branch length, total spiny branch length, and number of spiny branch arbors were all greater in the naive group than in the paired and unpaired groups, which did not differ. No differences were found between the hemispheres ipsilateral and contralateral to the trained eye. The dendritic length and number of branches for inhibitory interneurons did not differ across groups. The Purkinje cell morphological changes detected with these methods do not appear to be uniquely related to the conjunctive activation of the CS and US in the paired condition.

Eyeblink conditioning leads to fewer synapses in the rabbit cerebellar cortex.

“…Although the majority of motor skills training experiments that have quantified pf–PC synapses have found synaptogenesis (Anderson et al, 1996; Black et al, 1990; Federmeier, Kleim, & Greenough, 2002), eyeblink research has suggested the opposite effect on pf–PC synapses. Eyeblink training has been shown to alter PC morphology as indicated by smaller dendritic length, shorter spiny branch length and a lower number of spiny branch arbors in paired and unpaired rabbits, compared with naïve rabbits (Anderson et al, 1999). Although this effect was not specific to the paired (conditioned) group, it shows that stimulation by means of the pfs and climbing fibers tends to diminish PC dendritic complexity and size.…”