Electrophysiological analysis of the trigemino-tecto-olivo-cerebellar (crus II) projection in the rat

Modular output circuits of the fastigial nucleus for diverse motor and nonmotor functions of the cerebellar vermis

The cerebellar vermis, long associated with axial motor control, has been implicated in a surprising range of neuropsychiatric disorders and cognitive and affective functions. Remarkably little is known, however, about the specific cell types and neural circuits responsible for these diverse functions. Here, using single-cell gene expression profiling and anatomical circuit analyses of vermis output neurons in the mouse fastigial (medial cerebellar) nucleus, we identify five major classes of glutamatergic projection neurons distinguished by gene expression, morphology, distribution, and input-output connectivity. Each fastigial cell type is connected with a specific set of Purkinje cells and inferior olive neurons and in turn innervates a distinct collection of downstream targets. Transsynaptic tracing indicates extensive disynaptic links with cognitive, affective, and motor forebrain circuits. These results indicate that diverse cerebellar vermis functions could be mediated by modular synaptic connections of distinct fastigial cell types with posturomotor, oromotor, positional-autonomic, orienting, and vigilance circuits.

The anatomical pathway from the mesodiencephalic junction to the inferior olive relays perioral sensory signals to the cerebellum in the mouse

Key points

Perioral tactile signals are transmitted via the infraorbital nerve (ION) to trigeminal nuclei. Each cerebellar Purkinje cell (PC) receives this signal as complex spikes (CSs) via a climbing fibre (CF) emerging from the inferior olive (IO).

The anatomical pathway from trigeminal nuclei to the IO is not clearly identified.

In the present study, we examined candidate anatomical pathways for perioral sensory signalling by analysing CSs recorded from PCs in male mice by single unit recording.

CS generation by ION stimulation was inhibited by injection of a GABA_A receptor agonist, muscimol, into the contralateral mesodiencephalic junction, which is referred to as the area parafascicularis prerubralis (PfPr). The number of CSs evoked by mechanical whisker stimulation was also decreased by contralateral PfPr inhibition.

These results suggest the existence of a sensory signalling pathway to the IO via the PfPr in mice.

Abstract

Perioral tactile signals are transmitted via the infraorbital nerve (ION) to trigeminal nuclei. Each cerebellar Purkinje cell receives this signal as complex spikes (CSs) via a climbing fibre emerging from the inferior olive (IO). However, the anatomical pathway from the trigeminal nuclei to the IO is not clearly identified. In the present study, we recorded CSs from Purkinje cells in male mice by single unit recording, and examined the signal transduction pathway. CSs were evoked by electrical stimulation of the ipsilateral or contralateral ION with a latency of 20–70 ms. CS generation by ipsilateral ION stimulation was inhibited by injection of a GABA_A receptor agonist, muscimol, into the contralateral mesodiencephalic junction, ranging from around the fasciculus retroflexus to the interstitial nucleus of Cajal, which is referred to as the area parafascicularis prerubralis (PfPr). CSs evoked by contralateral ION stimulation were also suppressed by muscimol injection into the PfPr, although the effective area was more restricted. Furthermore, CSs evoked by mechanical stimulation around the whisker region were suppressed by PfPr inhibition. We also found that the primary motor cortex plays a role to suppress this signalling pathway. These results indicate the existence of an anatomical pathway for conducting perioral sensory signals to the IO via the PfPr.

Perioral tactile signals are transmitted via the infraorbital nerve (ION) to trigeminal nuclei. Each cerebellar Purkinje cell (PC) receives this signal as complex spikes (CSs) via a climbing fibre (CF) emerging from the inferior olive (IO).

The anatomical pathway from trigeminal nuclei to the IO is not clearly identified.

In the present study, we examined candidate anatomical pathways for perioral sensory signalling by analysing CSs recorded from PCs in male mice by single unit recording.

CS generation by ION stimulation was inhibited by injection of a GABA_A receptor agonist, muscimol, into the contralateral mesodiencephalic junction, which is referred to as the area parafascicularis prerubralis (PfPr). The number of CSs evoked by mechanical whisker stimulation was also decreased by contralateral PfPr inhibition.

These results suggest the existence of a sensory signalling pathway to the IO via the PfPr in mice.

Animal models for investigating benign essential blepharospasm

The focal dystonia benign essential blepharospasm (BEB) affects as many as 40,000 individuals in the United States. This dystonia is characterized by trigeminal hyperexcitability, photophobia, and most disabling of the symptoms, involuntary spasms of lid closure that can produce functional blindness. Like many focal dystonias, BEB appears to develop from the interaction between a predisposing condition and an environmental trigger. The primary treatment for blepharospasm is to weaken the eyelid-closing orbicularis oculi muscle to reduce lid spasms. There are several animal models of blepharospasm that recreate the spasms of lid closure in order to investigate pharmacological treatments to prevent spasms of lid closure. One animal model attempts to mimic the predisposing condition and environmental trigger that give rise to BEB. This model indicates that abnormal interactions among trigeminal blink circuits, basal ganglia, and the cerebellum are the neural basis for BEB.

An internal model architecture for novelty detection: implications for cerebellar and collicular roles in sensory processing

The cerebellum is thought to implement internal models for sensory prediction, but details of the underlying circuitry are currently obscure. We therefore investigated a specific example of internal-model based sensory prediction, namely detection of whisker contacts during whisking. Inputs from the vibrissae in rats can be affected by signals generated by whisker movement, a phenomenon also observable in whisking robots. Robot novelty-detection can be improved by adaptive noise-cancellation, in which an adaptive filter learns a forward model of the whisker plant that allows the sensory effects of whisking to be predicted and thus subtracted from the noisy sensory input. However, the forward model only uses information from an efference copy of the whisking commands. Here we show that the addition of sensory information from the whiskers allows the adaptive filter to learn a more complex internal model that performs more robustly than the forward model, particularly when the whisking-induced interference has a periodic structure. We then propose a neural equivalent of the circuitry required for adaptive novelty-detection in the robot, in which the role of the adaptive filter is carried out by the cerebellum, with the comparison of its output (an estimate of the self-induced interference) and the original vibrissal signal occurring in the superior colliculus, a structure noted for its central role in novelty detection. This proposal makes a specific prediction concerning the whisker-related functions of a region in cerebellar cortical zone A₂ that in rats receives climbing fibre input from the superior colliculus (via the inferior olive). This region has not been observed in non-whisking animals such as cats and primates, and its functional role in vibrissal processing has hitherto remained mysterious. Further investigation of this system may throw light on how cerebellar-based internal models could be used in broader sensory, motor and cognitive contexts.

Trigeminocerebellar and trigemino-olivary projections in rats

Retrograde and anterograde neuronal tracers (HRP, biocytin, biotinylated dextran-amine) were used to study the organisation of trigeminocerebellar and trigemino-olivary connections, focusing on the connectivity between trigeminal nuclear regions and the sagittal zones of the rat cerebellar cortex. Trigeminocerebellar projections were bilateral, but mostly ipsilateral. Direct trigeminocerebellar fibres originated mostly in the principal trigeminal nucleus (VP) and pars oralis (Vo), pars interpolaris (Vi), and to a lesser extent in pars caudalis (Vc) of the spinal trigeminal nucleus. Consistent projections were found from the Vc to cerebellar lobules IX and X. The trigeminal fibres terminated in the cerebellum in an organised fashion. The ventral part of the VP, Vo and Vi projected to the medial A zone and the C3 and D2 subzones, whereas the dorsal part of the nuclei projected to the lateral A zone and the C2, D0 and D1 subzones. In lobules IX and X, the organisation was different. The medial half of the VP, Vo, Vi and Vc projected to the lateral aspects of these lobules whereas their lateral part projected to their medial aspects. Trigeminal projections to the deep cerebellar nuclei were also present. Projections to a given sagittal zone concomitantly reached its corresponding nuclear target. Trigemino-olivary projections were principally contralateral. The Vo, Vi and Vc projected to the rostromedial dorsal accessory olive, the adjacent dorsal leaf and the dorsomedial part of the ventral leaf of the principal olive, which are known to project subzones C3, D0 and D1 of the rat cerebellar cortex.

Detail, proportion, and foci among face receptive fields of climbing fiber responses in the cat cerebellum

This paper reports a theoretical analysis of the transformation from a tactile stimulus of the face to climbing fiber responses in three regions of the cat cerebellum. The database consisted of climbing fiber receptive fields on the face from 75 responses from the anterior lobe, 33 responses from the paramedian lobule (PML), and 52 responses from the crus IIp of the anesthetized cat. The receptive fields were similar in being composed of discrete areas on the skin, or skin compartments. The regional differences in the configurations of the receptive fields were reflected in which compartments most often combined to form receptive fields. Each region had a distinct pattern of the preponderance of skin compartments that combined to form receptive fields, and yet the preponderant compartments were all chosen from one composite set of compartments that applied to all three regions. The climbing fiber representation of the face differed over the three regions (1) in the parts of the face that were represented; (2) in the frequency with which certain areas were included in the receptive fields; and (3) in the details of the face that could be distinguished by differing ensembles of climbing fiber responses. The majority of the climbing fiber receptive fields from either the anterior lobe or the crus IIp were unique to the region, whereas the majority of the receptive fields of responses from the PML were encountered in one or both of the other regions. Overlapping all face receptive fields from each of the three regions revealed that the receptive fields were differentially focused on or around the cornea, nose, or chin. In the anterior lobe, the face receptive fields mainly included the chin (43%) and the glabrous tip of the nose (40%), but few included the cornea (1%). In the PML, the receptive fields included the cornea (24%) and the chin (58%), but none included the nose. In the crus IIp, the cornea was included in the greatest number (37%) of receptive fields, whereas the nose and the chin were included equally (24%). The different sets of climbing fiber receptive fields in each of the three regions afforded the regions differing abilities to distinguish among complex patterns of stimuli, depending on the portion of the face stimulated.

The tectorecipient zone in the inferior olivary nucleus in the rat

Tecto-olivary and olivocerebellar projections in the rat were investigated in order to identify the tectorecipient zone in the inferior olivary nucleus and to determine whether inferior olivary neurons projecting to the cerebellar tecto-olivo-recipient zones (lobule VII, crus II, lobulus simplex, and paramedian lobule) originate in the different regions within the tectorecipient zone. An electrophysiological method and an axonal transport technique of wheat-germ agglutinin-conjugated horseradish peroxidase were used. The tectorecipient zone was identified in the caudomedial region of the medial accessory olive. Neurons projecting to lobule VII originated in the caudomedial region of the tectorecipient zone, but those to crus II, lobulus simplex, and paramedian lobule originated in its rostrolateral region. These observations suggest that there are two independent tecto-olivo-cerebellar systems: 1) superior colliculus--the medial region of the tectorecipient zone--lobule VII--the caudomedial region of the fastigial nucleus; and 2) superior colliculus--the rostralateral region of the tectorecipient region--crus II, lobulus simplex, and paramedian lobule--the dorsolateral protuberance of the fastigial nucleus.

Electrophysiological analysis of the trigemino-olivo-cerebellar (crura I and II, lobulus simplex) projection in the rat

In albino rats the whisker area was electrically stimulated while climbing fiber responses were surveyed in the cerebellar hemisphere on the ipsilateral side. They were identified both deep in the intercrural sulcus, and in the posterior superior fissure. Histological examination has revealed that the response areas extend longitudinally from the dorsal surface of crus II to the ventral surface of crus I in the intercrural sulcus, and from the rostral surface of crus I to the caudal surface of lobulus simplex in the posterior superior fissure. These are supposed to be transmitted through direct trigemino-olivary projections.