The ontogeny of associative cerebellar learning

A disynaptic basal ganglia connection to the inferior olive: potential for basal ganglia influence on cerebellar learning

Recent studies have shown that the cerebellum and the basal ganglia are interconnected at subcortical levels. However, a subcortical basal ganglia connection to the inferior olive (IO), being the source of the olivocerebellar climbing fiber system, is not known. We have used classical tracing with CTb, retrograde transneuronal infection with wildtype rabies virus, conditional tracing with genetically modified rabies virus, and examination of material made available by the Allen Brain Institute, to study potential basal ganglia connections to the inferior olive in rats and mice. We show in both species that parvalbumin-positive, and therefore GABAergic, neurons in the entopeduncular nucleus, representing the rodent equivalent of the internal part of the globus pallidus, innervate a group of cells that surrounds the fasciculus retroflexus and that are collectively known as the area parafascicularis prerubralis. As these neurons supply a direct excitatory input to large parts of the inferior olivary complex, we propose that the entopeduncular nucleus, as a main output station of the basal ganglia, provides an inhibitory influence on olivary excitability. As such, this connection may influence olivary involvement in cerebellar learning and/or could be involved in transmission of reward properties that have recently been established for olivocerebellar signaling.

Impaired cerebellar plasticity and eye-blink conditioning in calpain-1 knock-out mice

Calpain-1 and calpain-2 are involved in the regulation of several signaling pathways and neuronal functions in the brain. Our recent studies indicate that calpain-1 is required for hippocampal synaptic plasticity, including long-term depression (LTD) and long-term potentiation (LTP) in field CA1. However, little is known regarding the contributions of calpain-1 to cerebellar synaptic plasticity. Low frequency stimulation (LFS, 5 Hz, 5 min)-induced LTP at parallel fibers to Purkinje cell synapses was markedly impaired in cerebellar slices from calpain-1 knock-out (KO) mice. Application of a selective calpain-2 inhibitor enhanced LFS-induced LTP in both wild-type (WT) and calpain-1 KO mice. Three protocols were used to induce LTD at these synapses: LFS (1 Hz, 15 min), perfusion with high potassium and glutamate (K-Glu) or dihydroxyphenylglycine (DHPG), a mGluR1 agonist. All three forms of LTD were impaired in calpain-1 KO mice. DHPG application stimulated calpain-1 but not calpain-2 in cerebellar slices, and DHPG-induced LTD impairment was reversed by application of a protein phosphatase 2A (PP2A) inhibitor, okadaic acid. As in hippocampus, BDNF induced calpain-1 activation and PH domain and Leucine-rich repeat Protein Phosphatase 1/suprachiasmatic nucleus oscillatory protein (PHLPP1/SCOP) degradation followed by extracellular signal-regulated kinase (ERK) activation, as well as calpain-2 activation leading to degradation of phosphatase and tensin homolog deleted on chromosome ten (PTEN) in cerebellar slices. The role of calpain-1 in associative learning was evaluated in the delay eyeblink conditioning (EBC). Calpain-1 KO mice exhibited significant learning impairment in EBC during the first 2 days of acquisition training. However, after 5 days of training, the percentage of conditioned responses (CRs) between calpain-1 KO and WT mice was identical. Both calpain-1 KO and WT mice exhibited typical extinction patterns. Our results indicate that calpain-1 plays critical roles in multiple forms of synaptic plasticity and associative learning in both hippocampus and cerebellum.

Normal cognitive and social development require posterior cerebellar activity

Cognitive and social capacities require postnatal experience, yet the pathways by which experience guides development are unknown. Here we show that the normal development of motor and nonmotor capacities requires cerebellar activity. Using chemogenetic perturbation of molecular layer interneurons to attenuate cerebellar output in mice, we found that activity of posterior regions in juvenile life modulates adult expression of eyeblink conditioning (paravermal lobule VI, crus I), reversal learning (lobule VI), persistive behavior and novelty-seeking (lobule VII), and social preference (crus I/II). Perturbation in adult life altered only a subset of phenotypes. Both adult and juvenile disruption left gait metrics largely unaffected. Contributions to phenotypes increased with the amount of lobule inactivated. Using an anterograde transsynaptic tracer, we found that posterior cerebellum made strong connections with prelimbic, orbitofrontal, and anterior cingulate cortex. These findings provide anatomical substrates for the clinical observation that cerebellar injury increases the risk of autism.

Cerebellar-dependent associative learning is impaired in very preterm born children and young adults

Preterm birth incorporates an increased risk for cerebellar developmental disorders likely contributing to motor and cognitive abnormalities. Experimental evidence of cerebellar dysfunction in preterm subjects, however, is sparse. In this study, classical eyeblink conditioning was used as a marker of cerebellar dysfunction. Standard delay conditioning was investigated in 20 adults and 32 preschool children born very preterm. Focal lesions were excluded based on structural magnetic resonance imaging. For comparison, an equal number of matched term born healthy peers were tested. Subgroups of children (12 preterm, 12 controls) were retested. Preterm subjects acquired significantly less conditioned responses (CR) compared to controls with slower learning rates. A likely explanation for these findings is that preterm birth impedes function of the cerebellum even in the absence of focal cerebellar lesions. The present findings are consistent with the assumption that prematurity results in long-term detrimental effects on the integrity of the cerebellum. It cannot be excluded, however, that extra-cerebellar pathology contributed to the present findings.

Spontaneous activity and functional connectivity in the developing cerebellorubral system

The development of the cerebellar system depends in part on the emergence of functional connectivity in its input and output pathways. Characterization of spontaneous activity within these pathways provides insight into their functional status in early development. In the present study we recorded extracellular activity from the interpositus nucleus (IP) and its primary downstream target, the red nucleus (RN), in unanesthetized rats at postnatal days 8 (P8) and P12, a period of dramatic change in cerebellar circuitry. The two structures exhibited state-dependent activity patterns and age-related changes in rhythmicity and overall firing rate. Importantly, sensory feedback (i.e., reafference) from myoclonic twitches (spontaneous, self-generated movements that are produced exclusively during active sleep) drove neural activity in the IP and RN at both ages. Additionally, anatomic tracing confirmed the presence of cerebellorubral connections as early as P8. Finally, inactivation of the IP and adjacent nuclei using the GABAA receptor agonist muscimol caused a substantial decrease in neural activity in the contralateral RN at both ages, as well as the disappearance of rhythmicity; twitch-related activity in the RN, however, was preserved after IP inactivation, indicating that twitch-related reafference activates the two structures in parallel. Overall, the present findings point to the contributions of sleep-related spontaneous activity to the development of cerebellar networks.

Retention of eyeblink conditioning in periweanling and adult rats

Eyeblink conditioning is a well-established model for studying the developmental neurobiology of associative learning and memory, though nothing is known regarding developmental differences in retention between periweanling and adult rats. The present study examined retention of eyeblink conditioning in periweanling (postnatal day 24 at the start of training) and adult rats 1, 7, or 28 days after acquisition. Retention was assessed by (1) a conditional stimulus (CS)-alone test session followed by (2) CS-unconditional stimulus (US) reacquisition tests. Conditional response (CR) levels at acquisition were comparable in most respects between ages, and robust CR levels were present at the start of retention tests for both ages in the 1 day group, with CR percentages at block 1 of reacquisition higher in periweanlings relative to adults. At the 7 day retention test there was a trend toward significance for higher CR percentages at the CS-alone test in adults relative to periweanlings, though there were no age differences at reacquisition testing. When testing occurred 28 days after acquisition, however, periweanlings showed fewer CRs relative to adults during reacquisition despite low CR levels in both ages throughout the CS-alone test. Furthermore, periweanlings in the 28 day group required more trials at reacquisition than all other groups to exceed CR levels from their first acquisition session. These findings are consistent with rapid forgetting in the young commonly referred to as "infantile amnesia." The well-characterized eyeblink preparation may be useful for future studies investigating neural mechanisms responsible for rapid forgetting in developing animals.

Embryological exposure to valproic acid disrupts morphology of the deep cerebellar nuclei in a sexually dimorphic way

Autism spectrum disorders (ASD) is diagnosed in males at a much higher rate than females. For this reason, the majority of autism research has used male subjects exclusively. However; more recent studies using genetic sex as a factor find that the development of the male and female brain is differentially affected by ASD. That is, the natural sex-specific differences that exist between male and female brains lead to sexually dimorphic expressions of autism. Here we investigate the putative sexual dimorphism that exists in the deep cerebellar nuclei of male and female rats exposed to valproic acid (VPA) on embryological day 12.5. We find natural sex-specific differences in adult nucleus area, length, and estimated cell populations. Therefore VPA exposure during embryology creates some sex-specific deficits such as higher cell counts in the VPA males and lower cell counts in the VPA females. At the same time, some effects of VPA exposure occur regardless of sex. That is, smaller nucleus area and length lead to truncated nuclei in both VPA males and females. These deficits are more pronounced in the VPA males suggesting that genetic sex could play a role in teratogenic susceptibility to VPA. Taken together our results suggests that VPA exposure induces sexually dimorphic aberrations in morphological development along a mediolateral gradient at a discrete region of the hindbrain approximate to rhombomere (R) 1 and 2. Sex-specific disruption of the local and long-range projections emanating from this locus of susceptibility could offer a parsimonious explanation for the brain-wide neuroanatomical variance reported in males and females with ASD.