Addition of inhibition in the olivocerebellar system and the ontogeny of a motor memory

Abstract

The developmental emergence of learning has traditionally been attributed to the maturation of single brain regions necessary for learning in adults, rather than to the maturation of synaptic interactions within neural systems. Acquisition and retention of a simple form of motor learning, classical conditioning of the eyeblink reflex, depends on the cerebellum and interconnected brainstem structures, including the inferior olive. Here, we combined unit recordings from Purkinje cells in eye regions of the cerebellar cortex and quantitative electron microscopy of the inferior olive to show that the developmental emergence of eyeblink conditioning in rats is associated with the maturation of inhibitory feedback from the cerebellum to the inferior olive. The results are consistent with previous work in adult animals and indicate that the maturation of cerebellar inhibition within the inferior olive may be a critical factor for the formation and retention of learning-specific cerebellar plasticity and eyeblink conditioning.

Fig. 1

Learning-related inhibition of complex spikes parallels the development of eyeblink conditioning. (a) Eyeblink CRs for P17 and P24 rats that received paired or unpaired CS–US presentations. Inset, left, slow and fast sweep (asterisk in slow sweep) of raw US-elicited complex spike activity from a Purkinje cell in an eye region of lobule HVI (arrows indicate complex spikes); scale bars = 105 ms, 55 μV; 18 ms, 40 μV. Inset, right, averaged integrated EMG activity from CR trials for the P17 (gray) and P24 (black) rats from which complex spikes in b–e were recorded. (b–e) Histograms (left) and rasters (right) of single complex spikes from a P17 rat on trials without (b) and with (c) CRs and a P24 rat on trials without (d) and with (e) CRs.

Fig. 2

Learning-related changes in complex spike response magnitude, onset latency, peak latency and rhythmicity parallel the ontogeny of eyeblink conditioning. (a, b) Mean neuronal activity for all complex spikes recorded from Purkinje cells in lobule HVI in the P17 (a) and P24 (b) paired groups during the fifth session of conditioning on trials without (top) and with (bottom) CRs. Vertical lines indicate the onsets of the CS and US, respectively. (c) Mean onset and peak latencies for complex spikes recorded during the fifth conditioning session in the paired groups on trials with (black bars) and without (white bars) CRs. (d) Distribution of the proportions of complex spikes recorded during the fifth conditioning session in the paired groups that exhibited significant autocorrelations at each frequency (time lag⁻¹) during trials with (black bars) and without (white bars) CRs.

Fig. 3

Addition of inhibitory synapses parallels the development of eyeblink conditioning. (a) Excitatory axodendritic (top left) and axospinous (bottom left) synapses; inhibitory axodendritic (top right) and axospinous (bottom right) synapses. Scale bar, 0.2 μm. (b) Emergence of excitatory (left) and inhibitory (right) axodendritic synapses (asterisks) between the look-up (top) and reference (bottom) sections to form excitatory and inhibitory multiple synapse boutons. Scale bar, 0.3 μm. (c) Estimated mean total number (+s.e.m.) of excitatory axodendritic (D+) and axospinous (S+) synapses; inhibitory axodendritic (D−) and axospinous (S−) synapses; excitatory (MSB+) and inhibitory (MSB−) multiple synapse boutons; and multivesicular bodies (MVB). Inset, multivesicular body.