Inactivation of cerebellar output axons impairs acquisition of conditioned eyeblinks

The cerebellum and anxiety

Although the cerebellum is traditionally known for its role in motor functions, recent evidence points toward the additional involvement of the cerebellum in an array of non-motor functions. One such non-motor function is anxiety behavior: a series of recent studies now implicate the cerebellum in anxiety. Here, we review evidence regarding the possible role of the cerebellum in anxiety-ranging from clinical studies to experimental manipulation of neural activity-that collectively points toward a role for the cerebellum, and possibly a specific topographical locus within the cerebellum, as one of the orchestrators of anxiety responses.

A trigeminal conditioned stimulus yields fast acquisition of cerebellum-dependent conditioned eyeblinks

Classical conditioning of the eyeblink response in the rabbit is a form of motor learning whereby the animal learns to respond to an initially irrelevant conditioned stimulus (CS). It is thought that acquired conditioned responses (CRs) are adaptive because they protect the eye in anticipation of potentially harmful events. This protective mechanism is surprisingly inefficient because the acquisition of CRs requires extensive training - a condition that is unlikely to occur in nature. We hypothesized that the rate of conditioning in rabbits could depend on CS modality and that stimulating mystacial vibrissae as the CS could produce CR acquisition faster than the traditional auditory or visual stimulation. We tested this hypothesis by conditioning naïve rabbits in the delay paradigm using a weak airpuff CS (vCS) directed to the ipsilateral mystacial vibrissae. We found that the trigeminal vCS yields significantly faster CR acquisition. We next examined if vCS-evoked CRs are dependent on the intermediate cerebellum in the same fashion as CRs evoked by the traditional auditory CS. We found that vibrissal CRs could be abolished by inactivating the cerebellar interposed nuclei (IN) with muscimol. In addition, injections of picrotoxin in the IN shortened the onset latency of vibrissal CRs. These findings suggest that the tone and vCS-evoked CRs share similar cerebellar dependency.

Inactivating the middle cerebellar peduncle abolishes the expression of short-latency conditioned eyeblinks

The interposed nuclei (IN) of the cerebellum play a crucial role in the classically conditioned eyeblink circuit. It has previously been shown in well-trained animals that injecting the IN with GABA(A) antagonists produces short-latency conditioned responses (SLRs). The mechanism underlying SLR generation is not clear. According to one concept, SLRs originate in cerebellar nuclei in response to direct inputs from collaterals of mossy fibers. An alternate explanation is that SLRs are produced by extra-cerebellar circuits that are excited by increased tonic activity in cerebellar nuclei or by the combined action of inputs to cerebellar nuclei from mossy fiber collaterals and incompletely blocked Purkinje cells. In the present study, we examined whether cerebellar afferent axons in the middle cerebellar peduncle (MCP) participate in SLR expression. We hypothesized that if SLRs are evoked by the sensory mossy fiber input to the IN and cerebellar cortex, then blocking the MCP should abolish these responses. Well-trained animals, which had been implanted with dual injection cannulae in the left IN and the left MCP, were injected with gabazine (GZ) into the IN to produce SLRs followed by an injection of the sodium channel blocker tetrodotoxin (TTX) into the MCP. TTX infusions in the MCP suppressed both CRs and SLRs. These findings suggest that the expression of SLRs depends on both direct and cerebellar cortex-mediated sensory information from the mossy fiber system.

The cerebellum and eye-blink conditioning: learning versus network performance hypotheses

Classical conditioning of the eye-blink reflex in the rabbit is a form of motor learning that is uniquely dependent on the cerebellum. The cerebellar learning hypothesis proposes that plasticity subserving eye-blink conditioning occurs in the cerebellum. The major evidence for this hypothesis originated from studies based on a telecommunications network metaphor of eye-blink circuits. These experiments inactivated parts of cerebellum-related networks during the acquisition and expression of classically conditioned eye blinks in order to determine sites at which the plasticity occurred. However, recent evidence revealed that these manipulations could be explained by a network performance hypothesis which attributes learning deficits to a non-specific tonic dysfunction of eye-blink networks. Since eye-blink conditioning is mediated by a spontaneously active, recurrent neuronal network with strong tonic interactions, differentiating between the cerebellar learning hypothesis and the network performance hypothesis represents a major experimental challenge. A possible solution to this problem is offered by several promising new approaches that minimize the effects of experimental interventions on spontaneous neuronal activity. Results from these studies indicate that plastic changes underlying eye-blink conditioning are distributed across several cerebellar and extra-cerebellar regions. Specific input interactions that induce these plastic changes as well as their cellular mechanisms remain unresolved.