Thomas C. Watson

Back to front: cerebellar connections and interactions with the prefrontal cortex

Although recent neuroanatomical evidence has demonstrated closed-loop connectivity between prefrontal cortex and the cerebellum, the physiology of cerebello-cerebral circuits and the extent to which cerebellar output modulates neuronal activity in neocortex during behavior remain relatively unexplored. We show that electrical stimulation of the contralateral cerebellar fastigial nucleus (FN) in awake, behaving rats evokes distinct local field potential (LFP) responses (onset latency ~13 ms) in the prelimbic (PrL) subdivision of the medial prefrontal cortex. Trains of FN stimulation evoke heterogeneous patterns of response in putative pyramidal cells in frontal and prefrontal regions in both urethane-anesthetized and awake, behaving rats. However, the majority of cells showed decreased firing rates during stimulation and subsequent rebound increases; more than 90% of cells showed significant changes in response. Simultaneous recording of on-going LFP activity from FN and PrL while rats were at rest or actively exploring an open field arena revealed significant network coherence restricted to the theta frequency range (5–10 Hz). Granger causality analysis indicated that this coherence was significantly directed from cerebellum to PrL during active locomotion. Our results demonstrate the presence of a cerebello-prefrontal pathway in rat and reveal behaviorally dependent coordinated network activity between the two structures, which could facilitate transfer of sensorimotor information into ongoing neocortical processing during goal directed behaviors.

Electrophysiological Mapping of Novel Prefrontal – Cerebellar Pathways

Whilst the cerebellum is predominantly considered a sensorimotor control structure, accumulating evidence suggests that it may also subserve non-motor functions during cognition. However, this possibility is not universally accepted, not least because the nature and pattern of links between higher cortical structures and the cerebellum are poorly characterized. We have therefore used in vivo electrophysiological methods in anaesthetized rats to directly investigate connectivity between the medial prefrontal cortex (prelimbic subdivision, PrL) and the cerebellum. Stimulation of deep layers of PrL evoked distinct field potentials in the cerebellar cortex with a mean latency to peak of approximately 35 ms. These responses showed a well-defined topography, and were maximal in lobule VII of the contralateral vermis (a known oculomotor centre); they were not attenuated by local anaesthesia of the overlying M2 motor cortex, though M2 stimulation did evoke field potentials in lobule VII with a shorter latency (approximately 30 ms). Single unit recordings showed that prelimbic cortical stimulation elicits complex spikes in lobule VII Purkinje cells, indicating transmission via a previously undescribed cerebro-olivocerebellar pathway. Our results therefore establish a physiological basis for communication between PrL and the cerebellum. The role(s) of this pathway remain to be resolved, but presumably relate to control of eye movements and/or distributed networks associated with integrated prefrontal cortical functions.

Neural substrates underlying fear-evoked freezing: the periaqueductal grey-cerebellar link.

At the heart of the brain circuitry underlying fear behaviour is the periaqueductal grey (PAG). We address an important gap in understanding regarding the neural pathways and mechanisms that link the PAG to distinct patterns of motor response associated with survival behaviours. We identify a highly localised part of the cerebellum (lateral vermal lobule VIII, pyramis) as a key supraspinal node within a chain of connections that links the PAG to the spinal cord to elicit fear‐evoked freezing behaviour. Expression of fear‐evoked freezing behaviour, both conditioned and innate, is dependent on cerebellar pyramis neural input–output pathways. We also address an important controversy in the literature, namely whether or not ventrolateral PAG (vlPAG) increases muscle tone. We provide evidence that activation of the vlPAG causes an increase in α‐motoneurone excitability, consistent with a role in generating muscle tone associated with fear‐evoked freezing.

Neural substrates underlying fear-evoked freezing

At the heart of the brain circuitry underlying fear behaviour is the periaqueductal grey (PAG). We address an important gap in understanding regarding the neural pathways and mechanisms that link the PAG to distinct patterns of motor response associated with survival behaviours. We identify a highly localised part of the cerebellum (lateral vermal lobule VIII, pyramis) as a key supraspinal node within a chain of connections that links the PAG to the spinal cord to elicit fear‐evoked freezing behaviour. Expression of fear‐evoked freezing behaviour, both conditioned and innate, is dependent on cerebellar pyramis neural input–output pathways. We also address an important controversy in the literature, namely whether or not ventrolateral PAG (vlPAG) increases muscle tone. We provide evidence that activation of the vlPAG causes an increase in α‐motoneurone excitability, consistent with a role in generating muscle tone associated with fear‐evoked freezing.

The olivo-cerebellar system and its relationship to survival circuits

How does the cerebellum, the brain’s largest sensorimotor structure, contribute to complex behaviors essential to survival? While we know much about the role of limbic and closely associated brainstem structures in relation to a variety of emotional, sensory, or motivational stimuli, we know very little about how these circuits interact with the cerebellum to generate appropriate patterns of behavioral response. Here we focus on evidence suggesting that the olivo-cerebellar system may link to survival networks via interactions with the midbrain periaqueductal gray, a structure with a well known role in expression of survival responses. As a result of this interaction we argue that, in addition to important roles in motor control, the inferior olive, and related olivo-cortico-nuclear circuits, should be considered part of a larger network of brain structures involved in coordinating survival behavior through the selective relaying of “teaching signals” arising from higher centers associated with emotional behaviors.

The periaqueductal gray orchestrates sensory and motor circuits at multiple levels of the neuraxis

The periaqueductal gray (PAG) coordinates behaviors essential to survival, including striking changes in movement and posture (e.g., escape behaviors in response to noxious stimuli vs freezing in response to fear-evoking stimuli). However, the neural circuits underlying the expression of these behaviors remain poorly understood. We demonstrate in vivo in rats that activation of the ventrolateral PAG (vlPAG) affects motor systems at multiple levels of the neuraxis through the following: (1) differential control of spinal neurons that forward sensory information to the cerebellum via spino-olivo-cerebellar pathways (nociceptive signals are reduced while proprioceptive signals are enhanced); (2) alterations in cerebellar nuclear output as revealed by changes in expression of Fos-like immunoreactivity; and (3) regulation of spinal reflex circuits, as shown by an increase in α-motoneuron excitability. The capacity to coordinate sensory and motor functions is demonstrated in awake, behaving rats, in which natural activation of the vlPAG in fear-conditioned animals reduced transmission in spino-olivo-cerebellar pathways during periods of freezing that were associated with increased muscle tone and thus motor outflow. The increase in spinal motor reflex excitability and reduction in transmission of ascending sensory signals via spino-olivo-cerebellar pathways occurred simultaneously. We suggest that the interactions revealed in the present study between the vlPAG and sensorimotor circuits could form the neural substrate for survival behaviors associated with vlPAG activation. SIGNIFICANCE STATEMENT Neural circuits that coordinate survival behaviors remain poorly understood. We demonstrate in rats that the periaqueductal gray (PAG) affects motor systems at the following multiple levels of the neuraxis: (1) through altering transmission in spino-olivary pathways that forward sensory signals to the cerebellum, reducing and enhancing transmission of nociceptive and proprioceptive information, respectively; (2) by alterations in cerebellar output; and (3) through enhancement of spinal motor reflex pathways. The sensory and motor effects occurred at the same time and were present in both anesthetized animals and behavioral experiments in which fear conditioning naturally activated the PAG. The results provide insights into the neural circuits that enable an animal to be ready and able to react to danger, thus assisting in survival.

Transmission of Predictable Sensory Signals to the Cerebellum via Climbing Fiber Pathways Is Gated during Exploratory Behavior

Pathways arising from the periphery that target the inferior olive [spino-olivocerebellar pathways (SOCPs)] are a vital source of information to the cerebellum and are modulated (gated) during active movements. This limits their ability to forward signals to climbing fibers in the cerebellar cortex. We tested the hypothesis that the temporal pattern of gating is related to the predictability of a sensory signal. Low-intensity electrical stimulation of the ipsilateral hindlimb in awake rats evoked field potentials in the C1 zone in the copula pyramidis of the cerebellar cortex. Responses had an onset latency of 12.5 ± 0.3 ms and were either short or long duration (8.7 ± 0.1 vs 31.2 ± 0.3 ms, respectively). Both types of response were shown to be mainly climbing fiber in origin and therefore evoked by transmission in hindlimb SOCPs. Changes in response size (area of field, millivolts per millisecond) were used to monitor differences in transmission during rest and three phases of rearing: phase 1, rearing up; phase 2, upright; and phase 3, rearing down. Responses evoked during phase 2 were similar in size to rest but were smaller during phases 1 and 3, i.e., transmission was reduced during active movement when self-generated (predictable) sensory signals from the hindlimbs are likely to occur. To test whether the pattern of gating was related to the predictability of the sensory signal, some animals received the hindlimb stimulation only during phase 2. Over ∼10 d, the responses became progressively smaller in size, consistent with gating-out transmission of predictable sensory signals relayed via SOCPs. SIGNIFICANCE STATEMENT A major route for peripheral information to gain access to the cerebellum is via ascending climbing fiber pathways. During active movements, gating of transmission in these pathways controls when climbing fiber signals can modify cerebellar activity. We investigated this phenomenon in rats during their exploratory behavior of rearing. During rearing up and down, transmission was reduced at a time when self-generated, behaviorally irrelevant (predictable) signals occur. However, during the upright phase of rearing, transmission was increased when behaviorally relevant (unpredictable) signals may occur. When the peripheral stimulation was delivered only during the upright phase, so its occurrence became predictable over time, transmission was reduced. Therefore, the results indicate that the gating is related to the level of predictability of a sensory signal.

Translational neurophysiology in sheep: Measuring sleep and neurological dysfunction in CLN5 affected Batten disease sheep

With their large brains and long lives, sheep offer advantages over rodents for studying progressive neurodegenerative disease in timeframes relevant to humans. Perentos et al. demonstrate the validity of an ovine model of Batten disease by revealing in vivo EEG abnormalities that resemble those in affected children.

Editor's Choice: Translational neurophysiology in sheep: measuring sleep and neurological dysfunction in CLN5 Batten disease affected sheep

With their large brains and long lives, sheep offer advantages over rodents for studying progressive neurodegenerative disease in timeframes relevant to humans. Perentos et al. demonstrate the validity of an ovine model of Batten disease by revealing in vivo EEG abnormalities that resemble those in affected children.

A13 Phenotyping neural networks in an ovine model of huntington's disease

A large animal transgenic model of Huntingtons disease (HD) has been recently developed in domestic sheep (Ovis aries L).1 Sheep are a long-lived species with large brains, complex basal ganglia and convoluted neocortex; compared to standard models in short-lived rodents with lissencephalic cortices, this ovine model may therefore be better-suited for electrophysiological characterisations of early disease stages and for the development of novel treatments. Using a sheep model will also reduce costs relative to non-human primates, and enables large-scale, long-term and multi-site electrophysiological recording during behavioural testing paradigms.2 Sleep is an essential and ubiquitous brain state that is known to deteriorate in humans suffering from HD. As a first step towards assessing the normal electrophysiological features of sheep, we aimed to characterise sleep architecture using electroencephalography in healthy sheep. In addition we performed some common behavioural manipulations known to modulate sleep architecture. These include sleep deprivation and learning prior to sleep. The method can be expanded to generate sensitive biomarkers of intra-cortical network activity and interactions between brain structures in the behaving animal. If deterioration of these network interactions precedes the appearance of overt symptoms, this could then be targeted for treatments that might delay symptom onset. We also present deep brain recordings (of local field potentials and single neuron spike activity) from an anaesthetised animal as a proof of concept. Sheep disease models offer unique opportunities for long term in vivo electrophysiological testing, with the potential lead to novel therapeutic treatments targeting early stages of HD progression. References 1. Jacobsen, et al. Hum Mol Genet 2010;19:1873–82. 2. Morton AJ, Avanzo L. Executive decision-making in the domestic sheep. PLoS One 2011;6:e15752.