Basilar Pontine Nuclei Research Articles

A direct spino-cortical circuit bypassing the thalamus modulates nociception.

Nociceptive signals are usually transmitted to layer 4 neurons in somatosensory cortex via the spinothalamic-thalamocortical pathway. The layer 5 corticospinal neurons in sensorimotor cortex are reported to receive the output of neurons in superficial layers; and their descending axons innervate the spinal cord to regulate basic sensorimotor functions. Here, we show that a subset of layer 5 neurons receives spinal inputs through a direct spino-cortical circuit bypassing the thalamus, and thus define these neurons as spino-cortical recipient neurons (SCRNs). Morphological studies revealed that the branches from spinal ascending axons formed a kind of disciform structure with the descending axons from SCRNs in the basilar pontine nucleus (BPN). Electron microscopy and calcium imaging further confirmed that the axon terminals from spinal ascending neurons and SCRNs made functional synaptic contacts in the BPN, linking the ascending sensory pathway to the descending motor control pathway. Furthermore, behavioral tests indicated that the spino-cortical connection in the BPN was involved in nociceptive responses. In vivo calcium imaging showed that SCRNs responded to peripheral noxious stimuli faster than neighboring layer 4 cortical neurons in awake mice. Manipulating activities of SCRNs could modulate nociceptive behaviors. Therefore, this direct spino-cortical circuit represents a noncanonical pathway, allowing a fast sensory-motor transition of the brain in response to noxious stimuli.

Mesodiencephalic junction GABAergic inputs are processed separately from motor cortical inputs in the basilar pons

SummaryThe basilar pontine nuclei (bPN) are known to receive excitatory input from the entire neocortex and constitute the main source of mossy fibers to the cerebellum. Various potential inhibitory afferents have been described, but their origin, synaptic plasticity, and network function have remained elusive. Here we identify the mesodiencephalic junction (MDJ) as a prominent source of monosynaptic GABAergic inputs to the bPN. We found no evidence that these inputs converge with motor cortex (M1) inputs at the single neuron or at the local network level. Tracing the inputs to GABAergic MDJ neurons revealed inputs to these neurons from neocortical areas. Additionally, we observed little short-term synaptic facilitation or depression in afferents from the MDJ, enabling MDJ inputs to carry sign-inversed neocortical inputs. Thus, our results show a prominent source of GABAergic inhibition to the bPN that could enrich input to the cerebellar granule cell layer.

Efferent and afferent connections of supratrigeminal neurons conveying orofacial muscle proprioception in rats.

The supratrigeminal nucleus (Su5) is a key structure for controlling jaw movements; it receives proprioceptive sensation from jaw-closing muscle spindles (JCMSs) and sends projections to the trigeminal motor nucleus (Mo5). However, the central projections and regulation of JCMS proprioceptive sensation are not yet fully understood. Therefore, we aimed to reveal the efferent and afferent connections of the Su5 using neuronal tract tracings. Anterograde tracer injections into the Su5 revealed that the Su5 sends contralateral projections (or bilateral projections with a contralateral predominance) to the Su5, basilar pontine nuclei, pontine reticular nucleus, deep mesencephalic nucleus, superior colliculus, caudo-ventromedial edge of the ventral posteromedial thalamic nucleus, parafascicular thalamic nucleus, zona incerta, and lateral hypothalamus, and ipsilateral projections (or bilateral projections with an ipsilateral predominance) to the intertrigeminal region, trigeminal oral subnucleus, dorsal medullary reticular formation, and hypoglossal nucleus as well as the Mo5. Retrograde tracer injections into the Su5 demonstrated that the Su5 receives bilateral projections with a contralateral predominance (or contralateral projections) from the primary and secondary somatosensory cortices, granular insular cortex, and Su5, and ipsilateral projections (or bilateral projections with an ipsilateral predominance) from the dorsal peduncular cortex, bed nuclei of stria terminalis, central amygdaloid nucleus, lateral hypothalamus, parasubthalamic nucleus, trigeminal mesencephalic nucleus, parabrachial nucleus, juxtatrigeminal region, trigeminal oral and caudal subnuclei, and dorsal medullary reticular formation. These findings suggest that the Su5, which receives JCMS proprioception, has efferent and afferent connections with multiple brain regions that are involved in emotional and autonomic functions as well as orofacial motor functions.

The entire trajectories of single pontocerebellar axons and their lobular and longitudinal terminal distribution patterns in multiple aldolase C-positive compartments of the rat cerebellar cortex.

The mammalian cerebellar cortex is compartmentalized, both anatomically and histochemically, into multiple parasagittal bands. To characterize the multiple zonal patterns of pontocerebellar mossy fiber projection, single neurons in the basilar pontine nucleus (BPN) were labeled by injecting biotinylated dextran amine into the BPN, and the entire axonal trajectory of single labeled neurons (n = 25) was reconstructed in relation to aldolase C compartments of Purkinje cells in rats. Single pontocerebellar axons, after passing through the contralateral middle cerebellar peduncle, ran transversely in the deep cerebellar white matter toward and often across the midline, and on their ways, gave rise to 2-10 primary collaterals at almost right angles in specific lobules only contralaterally or bilaterally with contralateral predominance. Each primary collateral further branched in a parasagittal plane to form a strip-shaped longitudinal termination zone with rosette-type swellings clustered in aldolase C-positive compartments in a single or multiple lobules, mainly in compartment 4+//5+, 5+//6+, and 6+//7+. Axons arising from the central, rostral, and lateral part of the BPN projected with multiple branches, mainly to simple lobule, crus II and paramedian lobule, to crus I and dorsal paraflocculus, and to ventral paraflocculus and lobule IXc, respectively. The results showed the pontocerebellar projection is closely related to lobular and compartmental organization of the cerebellum. A comparison of single axon morphologies of different mossy fiber systems indicates that the projection pattern of single pontocerebellar neurons with multiple collaterals innervating different longitudinal compartments arranged in a mediolateral direction represents a general feature of mossy fiber projection.

Divergent projections of single pontocerebellar axons to multiple cerebellar lobules in the mouse

The basilar pontine nucleus (PN) is the key relay point for the cerebrocerebellar link. However, the projection pattern of pontocerebellar mossy fiber axons, which is essential in determining the functional organization of the cerebellar cortex, has not been fully clarified. We reconstructed the entire trajectory of 25 single pontocerebellar mossy fiber axons labeled by localized injection of biotinylated dextran amine into various locations in the PN and mapped all their terminals in an unfolded scheme of the cerebellum in 10 mice. The majority of axons (20/25 axons) entered the cerebellum through the middle cerebellar peduncle contralateral to the origin, while others entered through the ipsilateral pathway. A small number of axons (1/25 axons) had collaterals terminating in the cerebellar nuclei. Axons projected mostly to a combination of lobules, often bilaterally, and terminated in multiple zebrin (aldolase C) stripes, more frequently in zebrin-positive stripes (83.9%) than in zebrin-negative stripes, with 66.5 mossy fiber terminals on the average. Axons originating from the rostral (plus medial and lateral), central and caudal PN mainly terminated in the paraflocculus, crus I and lobule VIb-c, in the simplex lobule, crus II and paramedian lobule, and in lobules II-VIa, VIII and copula pyramidis, respectively. The results suggest that the interlobular branching pattern of pontocerebellar axons determines the group of cerebellar lobules that are involved in a related functional localization of the cerebellum. In the hemisphere, crus I may be functionally distinct from neighboring lobules (simple lobule and crus II) in the mouse cerebellum based on the pontocerebellar axonal projection pattern.

Amygdala central nucleus modulation of cerebellar learning with a visual conditioned stimulus.

Previous studies found that reversible inactivation of the central amygdala (CeA) severely impairs acquisition and retention of cerebellum-dependent eye-blink conditioning (EBC) with an auditory conditioned stimulus (CS). A monosynaptic pathway between the CeA and basilar pontine nuclei (BPN) may be capable of facilitating cerebellar learning. However, given that the CeA projects to the medial auditory thalamus, a critical part of the auditory CS pathway in EBC, the CeA influence on cerebellar learning could be specific to auditory stimuli. Here we examined the generality of CeA facilitation of EBC acquisition and retention in rats using a visual CS. As in our previous studies using an auditory CS, inactivation of the CeA with muscimol severely impaired acquisition and retention of EBC with a visual CS. Extending training to 15 100-trial sessions resulted in acquisition of EBC, indicating that the CeA plays a modulatory role in cerebellar learning and is not part of the necessary neural circuitry for EBC. Tract-tracing experiments verified that axons from the CeA reach both the BPN and medial auditory thalamus (part of the necessary auditory CS pathway), but were not found in the ventral lateral geniculate (part of the necessary visual CS pathway). The neuroanatomical results suggest that the CeA most likely modulates cerebellar learning through its projection to the BPN. The findings of the current study are consistent with the hypothesis that the CeA modulates cerebellar learning by increasing CS-related sensory input to the cerebellar cortex and interpositus nucleus via the BPN. This increase in CS-related input is thought to constitute an increase in attention to the CS during EBC.

Amygdala Modulation of Cerebellar Learning.

The current study is the first to demonstrate that the amygdala modulates sensory-evoked and learning-related neuronal activity within the cerebellum during acquisition and retention of associative learning. The findings suggest a model of amygdala-cerebellum interactions in which the amygdala gates conditioned stimulus inputs to the cerebellum through a direct projection from the medial central nucleus to the basilar pontine nucleus. Amygdala gating of sensory input to the cerebellum may be an attention-like mechanism that facilitates cerebellar learning. In contrast to previous theories of amygdala-cerebellum interactions, the sensory gating hypothesis posits that the gating mechanism continues to be necessary for retrieval of cerebellar memory after learning is well established.

Structural basis of cerebellar microcircuits in the rat.

The topography of the cerebellar cortex is described by at least three different maps, with the basic units of each map termed "microzones," "patches," and "bands." These are defined, respectively, by different patterns of climbing fiber input, mossy fiber input, and Purkinje cell (PC) phenotype. Based on embryological development, the "one-map" hypothesis proposes that the basic units of each map align in the adult animal and the aim of the present study was to test this possibility. In barbiturate anesthetized adult rats, nanoinjections of bidirectional tracer (Retrobeads and biotinylated dextran amine) were made into somatotopically identified regions within the hindlimb C1 zone in copula pyramidis. Injection sites were mapped relative to PC bands defined by the molecular marker zebrin II and were correlated with the pattern of retrograde cell labeling within the inferior olive and in the basilar pontine nuclei to determine connectivity of microzones and patches, respectively, and also with the distributions of biotinylated dextran amine-labeled PC terminals in the cerebellar nuclei. Zebrin bands were found to be related to both climbing fiber and mossy fiber inputs and also to cortical representation of different parts of the ipsilateral hindpaw, indicating a precise spatial organization within cerebellar microcircuitry. This precise connectivity extends to PC terminal fields in the cerebellar nuclei and olivonuclear projections. These findings strongly support the one-map hypothesis and suggest that, at the microcircuit level of resolution, the cerebellar cortex has a common plan of spatial organization for major inputs, outputs, and PC phenotype.

Neuronal Correlates of Cross-Modal Transfer in the Cerebellum and Pontine Nuclei

Cross-modal transfer occurs when learning established with a stimulus from one sensory modality facilitates subsequent learning with a new stimulus from a different sensory modality. The current study examined neuronal correlates of cross-modal transfer of pavlovian eyeblink conditioning in rats. Neuronal activity was recorded from tetrodes within the anterior interpositus nucleus (IPN) of the cerebellum and basilar pontine nucleus (PN) during different phases of training. After stimulus preexposure and unpaired training sessions with a tone conditioned stimulus (CS), light CS, and periorbital stimulation unconditioned stimulus (US), rats received associative training with one of the CSs and the US (CS1-US). Training then continued on the same day with the other CS to assess cross-modal transfer (CS2-US). The final training session included associative training with both CSs on separate trials to establish stronger cross-modal transfer (CS1/CS2). Neurons in the IPN and PN showed primarily unimodal responses during pretraining sessions. Learning-related facilitation of activity correlated with the conditioned response (CR) developed in the IPN and PN during CS1-US training. Subsequent CS2-US training resulted in acquisition of CRs and learning-related neuronal activity in the IPN but substantially less little learning-related activity in the PN. Additional CS1/CS2 training increased CRs and learning-related activity in the IPN and PN during CS2-US trials. The findings suggest that cross-modal neuronal plasticity in the PN is driven by excitatory feedback from the IPN to the PN. Interacting plasticity mechanisms in the IPN and PN may underlie behavioral cross-modal transfer in eyeblink conditioning.

Central cannabinoid receptors modulate acquisition of eyeblink conditioning

Delay eyeblink conditioning is established by paired presentations of a conditioned stimulus (CS) such as a tone or light, and an unconditioned stimulus (US) that elicits the blink reflex. Conditioned stimulus information is projected from the basilar pontine nuclei to the cerebellar interpositus nucleus and cortex. The cerebellar cortex, particularly the molecular layer, contains a high density of cannabinoid receptors (CB1R). The CB1Rs are located on the axon terminals of parallel fibers, stellate cells, and basket cells where they inhibit neurotransmitter release. The present study examined the effects of a CB1R agonist WIN55,212-2 and antagonist SR141716A on the acquisition of delay eyeblink conditioning in rats. Rats were given subcutaneous administration of 1, 2, or 3 mg/kg of WIN55,212-2 or 1, 3, or 5 mg/kg of SR141716A before each day of acquisition training (10 sessions). Dose-dependent impairments in acquisition were found for WIN55,212-2 and SR141716A, with no effects on spontaneous or nonassociative blinking. However, the magnitude of impairment was greater for WIN55,212-2 than SR141716A. Dose-dependent impairments in conditioned blink response (CR) amplitude and timing were found with WIN55,212-2 but not with SR141716A. The findings support the hypothesis that CB1Rs in the cerebellar cortex play an important role in plasticity mechanisms underlying eyeblink conditioning.

Absence of the transcription factor Nfib delays the formation of the basilar pontine and other mossy fiber nuclei

Transcription factors of the Nuclear Factor I (Nfi) family are important for the development of specific neuronal and glial populations in the nervous system. One such population, the neurons of the basilar pontine nuclei, expresses high levels of Nfi proteins, and the pontine nuclei are greatly reduced in mice lacking a functional Nfib gene. Pontine neurons, along with other precerebellar neurons that populate the hindbrain, arise from precursors in the lower rhombic lip and migrate anteroventrally to reach their final location. Using immunohistochemistry, we find that NFI-B expression is specific for mossy fiber populations of the precerebellar system. Analysis of the Nfib(-/-) hindbrain indicates that the development of the basilar pontine nuclei is delayed, with pontine neurons migrating 1-2 days later than in control animals, and that significantly fewer pontine neurons are produced. While the mossy fiber nuclei of the caudal medulla do form, they also exhibit a developmental delay. Nfia and Nfix null mice exhibit no apparent pontine phenotype, implying specificity in the action of NFI family members. Collectively, these data demonstrate that Nfib plays an important role in the generation of precerebellar mossy fiber neurons, and may do so at least in part by regulating neurogenesis.

Medial auditory thalamus inactivation prevents acquisition and retention of eyeblink conditioning.

The auditory conditioned stimulus (CS) pathway that is necessary for delay eyeblink conditioning was investigated using reversible inactivation of the medial auditory thalamic nuclei (MATN) consisting of the medial division of the medial geniculate (MGm), suprageniculate (SG), and posterior intralaminar nucleus (PIN). Rats were given saline or muscimol infusions into the MATN contralateral to the trained eye before each of four conditioning sessions with an auditory CS. Rats were then given four additional sessions without infusions to assess savings from the initial training. All rats were then given a retention test with a muscimol infusion followed by a recovery session. Muscimol infusions through cannula placements within 0.5 mm of the MGm prevented acquisition of eyeblink conditioned responses (CRs) and also blocked CR retention. Cannula placements more than 0.5 mm from the MATN did not completely block CR acquisition and had a partial effect on CR retention. The primary and secondary effects of MATN inactivation were examined with 2-deoxy-glucose (2-DG) autoradiography. Differences in 2-DG uptake in the auditory thalamus were consistent with the cannula placements and behavioral results. Differences in 2-DG uptake were found between groups in the ipsilateral auditory cortex, basilar pontine nuclei, and inferior colliculus. Results from this experiment indicate that the MATN contralateral to the trained eye and its projection to the pontine nuclei are necessary for acquisition and retention of eyeblink CRs to an auditory CS.

Precise spatial relationships between mossy fibers and climbing fibers in rat cerebellar cortical zones.

Classically, mossy fiber and climbing fiber terminals are regarded as having very different spatial distributions in the cerebellar cortex. However, previous anatomical studies have not studied these two major cerebellar inputs with sufficient resolution to confirm this assumption. Here, we examine the detailed pattern of collateralization of both types of cerebellar afferent using small injections of the bidirectional tracer cholera toxin b subunit into the posterior cerebellum. The cortical and zonal location of these injections was characterized by mapping climbing fiber field potentials, the distribution of retrogradely labeled olivary neurons, and the intrinsic zebrin pattern of Purkinje cells. Labeled climbing fiber collaterals were distributed as longitudinal strips and were always accompanied by clusters of labeled mossy fiber rosettes in the subjacent granular layer. Two- and three-dimensional reconstructions and quantitative analysis showed that mossy fibers also collateralized to other stripe-like regions usually below Purkinje cells with the same zebrin-positive or zebrin-negative characteristics as that of the injection site and associated climbing fiber collaterals. The distribution of retrogradely labeled neurons in two major sources of mossy fibers, the lateral reticular and basilar pontine nuclei, revealed interlobular and some interzonal differences. These data indicate that nonadjacent cerebellar zones, sharing the same climbing fiber input and zebrin identity, also share a common mossy fiber input. Other cerebellar cortical regions that receive collaterals from the same mossy fibers usually also have the same zebrin signature. Together with the distribution of neurons in precerebellar centers, the findings suggest a revision of the modular hypothesis for information processing in the cerebellar cortex.

Axonal plasticity is associated with motor recovery following amphetamine treatment combined with rehabilitation after brain injury in the adult rat

Clinical and laboratory studies have suggested that amphetamine treatment when paired with rehabilitation results in improved recovery of function after stroke or traumatic brain injury. In the present study, we investigated whether new anatomical pathways developed in association with improved motor function after brain damage and amphetamine treatment linked with rehabilitation. Following a unilateral sensorimotor cortex lesion in the adult rat, amphetamine (2 mg/kg) was administered in conjunction with physiotherapy sessions on postoperative days two and five. Physiotherapy was continued twice daily for the first 3 weeks after injury, and then once daily until week six. Performance on skilled forelimb reaching and ladder rung walking was used to assess motor improvement. Our results show that animals with sensorimotor cortical lesions receiving amphetamine treatment linked with rehabilitation had significant improvement in both tasks. Neuroanatomical tracing of efferent pathways from the opposite, non-damaged cortex resulted in the novel finding that amphetamine treatment linked with rehabilitation, significantly increased axonal growth in the deafferented basilar pontine nuclei. These results support the notion that pharmacological interventions paired with rehabilitation can enhance neuronal plasticity and thereby improve functional recovery after CNS injury.

Fos and FRA protein expression in rat precerebellar structures during the Neurolab Space Mission

Changes in gene expression were examined in precerebellar structures during and after space flight. These structures included the inferior olive (IO), the source of climbing fibers, and the lateral reticular nucleus (LRt) and basilar pontine nuclei (PN), sources of mossy fibers. We examined two immediate early gene products with two different time courses of expression: Fos, which persists only for a few (6–8) h after activation and FRA expression, which lasts for longer periods of time, i.e. hours and/or days after activation. Gravity effects on Fos and FRA gene expression were evident in vestibular and visual areas of the IO, including the dorsomedial cell column, the beta subnucleus and the dorsal cap of Kooy of the medial nucleus (which projects to the flocculonodular lobe, i.e. to the vestibular area of the IO involved in the olivary control of the vestibulo-ocular reflex (VOR)). Gene expression also affected the subnuclei A, B, and C and the caudal part of the medial IO. These olivary regions do not receive vestibular afferents, but rather spinal afferents, and are particularly involved in the olivary control of the vestibulospinal reflex (VSR). Changes in Fos expression were also observed in the LRt and the PN. We suggest that sensory substitution, in which signals produced by a subject’s own activity replace activity normally provided by macular stimulation, contributes to the recovery of microgravity-related postural and motor deficits. While no consistent increases in FRA expression occurred in vestibular IO regions 24 h after launch, consistent increases in FRA expression occurred 24 h after landing. We hypothesize that this asymmetrical pattern of gene expression resulted from (i) tonic microgravity experienced after launch counteracting the effects of increased phasic gravitational forces experienced during launch, and (ii) the tonic gravitational field experienced after landing potentiating the effects of increased phasic gravitational forces experienced during landing. The specificity of these results is demonstrated by an absence of direct gravity-related changes in Fos expression in other precerebellar structures such as the external cuneate nucleus, group X, and the dorsal column nuclei that transmit exteroceptive and proprioceptive signals to thalamic nuclei and somatosensory areas of the cerebral cortex. The gravity-related Fos and FRA expression changes in the IO and the LRt seen here are of interest in view of the important role their projections play in adaptive gain changes of the VOR and VSR during sustained visuo-vestibular and neck-vestibular stimulation.

Survival promotion of rat cerebellar granule neurons by co-culture with pontine explant

Cultured cerebellar granule neuron (CGN) of the rat is the most frequently used model system for analysis of activity-dependent neuronal survival. CGNs do not survive longer than 2 weeks in a standard culture medium unless KCl (or other excitants such as glutamate) is added. It is assumed that KCl represents synaptic activity, but no tests have been made on whether the survival of CGNs really depends on the synaptic input. Here we co-cultured CGNs with an explant of the pons including the basilar pontine nucleus (BPN), which is one of the input sources of CGNs in vivo, to confirm if synaptic input is really a determinant for the survival of these cells. In this co-culture system, the viability of CGNs was significantly increased without the addition of KCl. The survival promotion was confined to the population of CGNs having contact with neurites of BPN and was cancelled by an application of tetrodotoxin or antagonists of glutamate receptors, indicating that the survival depended on synaptic activity. Explants of other glutamatergic tissues including the hippocampus failed to promote the survival, although neurites grew out from these explants as vigorously as from the BPN explants. Calcium and FM1-43 imaging examinations revealed that the CGNs had formed functional synapses with the BPN explant but not with the hippocampal explant. These results, confirming the assumption that synaptic activity determines neuronal survival, provide evidence for presynaptic contribution to the survival.

Role of the dorsolateral pontine nucleus in short-term adaptation of the horizontal vestibuloocular reflex.

The dorsolateral pontine nucleus (DLPN) is a major component of the cortico-ponto-cerebellar pathway that carries signals essential for smooth pursuit. This pathway also carries visual signals that could play a role in visually guided motor learning in the vestibular ocular reflex (VOR). However, there have been no previous studies that tested this possibility directly. The aim of this study was to determine the potential role of the DLPN in short-term VOR gain adaptation produced by viewing a scene through lenses placed in front of both eyes. In control experiments, adaptation of VOR gain was achieved by sinusoidal rotation (0.2 Hz, 30 degrees /s) for 2 h while the monkey viewed a stationary visual surround through either magnifying (x2) or minifying (x0.5) lenses. This led to increases (23-32%) or decreases (22-48%) of VOR gain as measured in complete darkness (VORd). We used injections of muscimol, a potent GABA(A) agonist (0.5 microl; 2%), to reversibly inactivate the DLPN, unilaterally, in three monkeys. After DLPN inactivation, initial acceleration of ipsilateral smooth-pursuit was reduced by 35-68%, and steady-state gain was reduced by 32-61%. Despite these significant deficits (P < 0.01) in ipsilesional smooth pursuit, the VOR during lens viewing was similar to that measured in preinjection control experiments. Similarly, after 2 h of adaptation, VORd gain was not significantly different (P > 0.61) from control adaptation values for either ipsi- or contralesional directions of head rotation. This was the case even though a stable ipsilesional smooth pursuit deficit persisted throughout the full adaptation period. Our results suggest that visual error signals for short-term adaptation of the VOR are derived from sources other than the DLPN perhaps including other basilar pontine nuclei and the accessory optic system.

Pontocerebellar Axon Guidance: Neuropilin-1- and Semaphorin 3A-Sensitivity Gradients across Basilar Pontine Nuclei and Semaphorin 3A Variation across Cerebellum

To assess the role of semaphorin 3A (Sema3A) and its receptor component neuropilin-1 (Npn-1) in pontocerebellar axon guidance, we compared the distributions of Sema3A, Npn-1, and DiI-labeled pontocerebellar axons in neonatal mouse cerebellum. Between embryonic day 18 and birth there was a large increase in Npn-1 expression in the basilar pontine nuclei (BPN), the major source of pontocerebellar axons. Sema3A expression in cerebellum also increased at this time. In the BPN, Npn-1 and the response of axons to Sema3A were graded with high Npn-1 and Sema3A responsiveness rostrally and lower levels caudally. The Npn-1 gradient was not smooth and cells with higher and lower expression were interspersed. Between birth and postnatal day 5, pontocerebellar axons projected to lobules of the hemispheres, including those with low to moderate levels of Sema3A, but did not enter regions with high levels of Sema3A, including the flocculus and much of the vermis. These results suggest that varying neuropilin levels on BPN axons, which correlated with their varying responses to Sema3A, combined with varying Sema3A levels across cerebellum, may contribute to guiding subsets of BPN axons to their distinct target regions within cerebellum.

Projections of the basilar pontine nuclei and nucleus reticularis tegmenti pontis to the cerebellar nuclei of the rat.

This study showed the precise projection pattern of the basilar pontine nuclei (BPN) and the nucleus reticularis tegmenti pontis (NRTP) to the cerebellar nuclei (CN), as well as the different anatomic features of BPN and NRTP projections. The staining of BPN or NRTP with biotinylated dextran labeled projection fibers to complementary topographic areas in the CN. In fact, BPN principally project to a rostrocaudally oriented column of the nucleus lateralis (NL), which at the midcentral level shifts to the lateroventral part of the nucleus, as well as to the caudolateral part of the nucleus interpositus posterioris. The NRTP projects to a rostrocaudal column of the NL, which at the midcentral level shifts medially, as well as to the nucleus interpositalis and to the caudal part of the nucleus medialis. BPN axons in the CN usually branch into short collaterals of simple morphology that involve small terminal areas, whereas NRTP axons branch into longer collaterals of complex morphology involving terminal areas of different sizes. Each site of injection is at the origin of a set of terminal areas in the CN. The set of projections from different BPN or NRTP areas were partially, but never completely, overlapping. Thus, the set of terminal areas in the CN was specific for each area of both BPN and NRTP. Injection of tetramethyl-rhodamine-dextran-amine into the CN stained cell bodies of BPN and NRTP with different repartition on the two sides. The study showed that CN are innervated by the contralateral BPN and not very much by the ipsilateral BPN, whereas they are innervated by NRTP bilaterally, even if with a contralateral prevalence. In conclusion, this study supports the hypothesis that both BPN and NRTP are concerned in the central program for skilled movements, even if they are probably involved in different functional roles.

Nr-CAM and TAG-1 are expressed in distinct populations of developing precerebellar and cerebellar neurons

Nr-CAM and TAG-1 interact at the floor-plate during the formation of spinal cord commissural projections [Stoeckli, E.T., Landmesser, L.T., Sci. 274 (1995) 1123-1133; Fitzli, D., Stoeckli, E.T., Kunz, S., Siribour, K., Rader, C., Kunz, B., Kozlov, S.V., Buchstaller, A., Lane, R.P., Suter, D.M., Dreyer, W.J., Sonderegger, P., J. Cell. Biol. 149 (2000) 951-968]. We report here the spatio-temporal patterns of expression of these two adhesion molecules during the development of the lower brainstem (medulla and pons) and cerebellum. Nr-CAM and Tag-1 label distinct populations of precerebellar neurons at key steps of their development. Nr-CAM expression starts at E11.5–E12 in the floor-plate, that constitutes an intermediate target during axon outgrowth and nuclear migration of precerebellar neurons. At E13–E14, it is expressed in both floor-plate and inferior olivary nuclei (ION) neurons before being strictly restricted to ION neurons from E15 onwards. Furthermore Nr-CAM, which is widely expressed in the cerebellum during embryonic development, becomes strictly confined to Purkinje and Golgi cells in postnatal cerebellum, suggesting a possible role of Nr-CAM for the maturation or stabilization of the synaptic contacts, in particular between climbing fibers and Purkinje cells. On the other hand, Tag-1 is expressed by migrating neurons that will form the lateral reticular and basilar pontine nuclei. These results emphasize the possibility that TAG-1/Nr-CAM interactions are also involved in the development of the cerebellar system (precerebellar and cerebellar neurons). However, the pattern of cerebellar expression of TAG-1 – early migrating Purkinje cells up to E14 and external granular cells – prevents the implication of this adhesion molecule in the organization of extracerebellar projections.