Peripheral maps and synapse elimination in the cerebellum of the rat. I. Representation of peripheral inputs through the climbing fiber pathway in the posterior vermis of the normal adult rat

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.

Vertical bias in dendritic trees of non-pyramidal neocortical neurons expressing GAD67-GFP in vitro

The neocortical neuropil has a strong vertical (orthogonal to pia) orientation, constraining the intracortical flow of information and forming the basis for the functional parcellation of the cortex into semi-independent vertical columns or 'modules'. Apical dendrites of excitatory pyramidal neurons are a major component of this vertical neuropil, but the extent to which inhibitory, GABAergic neurons conform to this structural and functional design is less well documented. We used a gene gun to transfect organotypic slice cultures of mouse and rat neocortex with the enhanced green fluorescent protein (eGFP) gene driven by the promoter for glutamic acid decarboxylase 67 (GAD67), an enzyme expressed exclusively in GABAergic cells. Many GAD67-GFP expressing cells were highly fluorescent, and their dendritic morphologies and axonal patterns, revealed in minute detail, were characteristic of GABAergic neurons. We traced 150 GFP-expressing neurons from confocal image stacks, and estimated the degree of vertical bias in their dendritic trees using a novel computational metric. Over 70% of the neurons in our sample had dendritic trees with a highly significant vertical bias. We conclude that GABAergic neurons make an important contribution to the vertical neocortical neuropil, and are likely to integrate synaptic inputs from axons terminating within their own module.

Synapse elimination in the central nervous system: functional significance and cellular mechanisms

Recent research into the developmental elimination of supernumerary synapses has increased understanding of this process. In this review we discuss synapse elimination both at the neuromuscular junction and in the central nervous system, considering some possible underlying mechanisms suggested by recent studies. In addition a well-described example of central nervous system synapse elimination, the climbing fiber-Purkinje cell synapse of the cerebellum, is used to explore the functional significance of synaptic regression during brain development.

Abnormal ipsilateral functional vibrissae projection onto Purkinje cells multiply innervated by climbing fibers in the rat

We have previously shown that synapse elimination occurring in the climbing fiber (CF)-Purkinje cell (PC) relationships during normal postnatal development is likely involved in the refinement of vibrissae projections onto the cerebellar cortex. In normal adult rats, CF-mediated vibrissae projections onto cerebellar Purkinje cells of the vermis of lobule VII are strictly contralateral and located in a narrow microzone whereas they are widely distributed in rats whose PCs remained multiply innervated by CFs due to postnatal irradiation. Given the proximity of this microzone to the midline, the question arose as to whether this synapse elimination process could participate in the segregation of ipsilateral and contralateral projections. In the present study, we compared the topographical map of the ipsilateral and contralateral CF-mediated projections of the third row of vibrissae onto the vermal PCs of lobule VII in adult normal rats and in polyinnervated rats. Using intracellular electrophysiological recordings, we examined the responsiveness of PCs to mechanical stimulation of vibrissae, and positioned responsive cells on an averaged planar map of lobule VII. In normal rats no ipsilateral responses were found, while in irradiated rats ipsilateral responses were distributed evenly from the midline to 700 microns apart. These results suggest that synapse elimination participates in the segregation of ipsi and contralateral mystacial inputs to the vermis.

Enlargement of olivo-cerebellar microzones in the agranular cerebellum of adult rats

Previous work on normal adult rat showed that the vibrissae project, through the climbing fiber (CF) system, onto the Purkinje cells (PCs) of the contralateral cerebellar hemivermis of lobule VII. The highly elaborated CF projections from a given row of vibrissae delimit a narrow parasagittal zone which can be regarded as a functional olivo-cerebellar microzone. Interestingly, the adult one-to-one relationship between PCs and CFs is preceded by a transient phase during which each PC receives synaptic inputs from several CF collaterals which will be eliminated but one, when granule cells begin to establish synapses on PCs. Therefore, the question arose as to whether this synaptic elimination process could participate in the refinement of the topographical organization of CF projections and could contribute to the formation of such precise peripheral maps onto the cerebellum. In the present study, the topographical map of the CF-mediated projection of mystacial vibrissae onto the vermal PCs of lobule VII was determined in adult rats whose cerebellar PCs remain polyinnervated by olivary CFs due to degranulation by postnatal X-irradiation. Using intracellular recordings, we examined the responsiveness of PCs in lobule VII during mechanical stimulation of the 3rd row of contralateral vibrissae, and positioned cells responding to the stimulation on an averaged planar map of lobule VII. Comparison of the results to those obtained in our previous work on normal rats showed that the activated cells were more numerous and more diffusely distributed.(ABSTRACT TRUNCATED AT 250 WORDS)

Extent of multiple innervation of cerebellar Purkinje cells by climbing fibers in adult X-irradiated rats. Comparison of different schedules of irradiation during the first postnatal week

The multiple innervation of cerebellar Purkinje cells (PCs) by climbing fibers (CFs) that is transient in normal developing rats can be experimentally maintained in cerebella which have been degranulated by repetitive postnatal X-irradiation restricted to the first postnatal week. Since the involution of redundant CFs occurs essentially between postnatal days 5 and 10, and given that postirradiation effects last 2-3 days, the question arose to know whether it is possible to further delimit a 'critical period' of irradiation within the first week. An estimate of the extent of multiple innervation of PCs by CFs was made in adult rats that had been irradiated according to 5 different schedules: in two groups, rats received X-rays applied repetitively during the first postnatal week (PN0-7 groups); in the 3 other groups, X-rays were delivered either during the first part of the week (early group PN1-3) or during the last part of the week (late groups PN4-7). In addition, two daily doses were tested (150 and 200 r). The CF pathway was electrically stimulated in anesthetized rats at the level of the inferior olive or in the cerebellar white matter. Intracellular recordings of spontaneous and evoked CF responses in PCs allowed to estimate the number of afferent CFs and to calculate the mean value (m) per PC for each group. The majority of recorded cells was located in lobules VII and VIII and similar results were obtained in these two lobules.(ABSTRACT TRUNCATED AT 250 WORDS)

Somatosensory climbing fiber responses in the caudal posterior vermis of the cat cerebellum

Tactile stimulation of the body surface elicited climbing fiber responses in 5% of the 839 Purkinje cells recorded in the caudal posterior vermis (lobules VII-IX) of cats anesthetized with sodium pentobarbital. The cells responsive to tactile stimulation were mainly encountered around the prepyramidal fissure between lobules VIIb and VIIIa. This region was characterized by responses representing only selective areas of the ipsilateral face, forepaw, hindlimb, or proximal tail-pelvic territory. Climbing fiber responses representing the face and forepaw were encountered more laterally and those representing the proximal tail-pelvic area more medially. Tactile representations of the body surface are located within the same cortical domain as other climbing fiber inputs, which suggests that the caudal posterior vermis is a multifunctional region of the cerebellum.

Representation of vibrissae inputs through the climbing fiber pathway in lobule VII of the adult rat cerebellar vermis

The present study gives a detailed description of the functional characteristics and of the topographic distribution of Purkinje cell (PC) responses, mediated through the climbing fiber pathway and elicited by mechanical stimulation of two different rows (A and C) of vibrissae in a circumscribed region of the posterior vermis of the rat cerebellum. Experiments were carried out on normal adult rats under barbiturate anesthesia. PCs were recorded in an area extending 1500 microns laterally to the midline in the vermal part of lobule VII contralateral to the stimulation. Using micromapping techniques and computer analysis, we located the cells on the map of the unfolded PC layer. Taking into account the mean latency of the responses and the probability of discharge of PCs, restricted areas of projection were found. For each row, these areas formed two longitudinal patches located between midline and plane 1100 microns and separated by a non-responsive plane at 500 microns. Cells having the best characteristics of responses to the stimulation of row C were located mainly in plane 200 microns. Cells giving the best responses to the stimulation of row A were located mainly in the posterior part of plane 200 microns which was therefore a zone of convergence for both rows.

Peripheral maps and synapse elimination in the cerebellum of the rat. II. Representation of peripheral inputs through the climbing fiber pathway in the posterior vermis of X-irradiated adult rats

The present study gives a detailed description of the functional characteristics and of the topographic distribution of responses mediated through the climbing fiber (CF) pathway in the cerebellum of adult rats which have been repeatedly irradiated from birth so that the Purkinje cells (PCs) in the adult remained innervated by several climbing fibers instead of one. Experiments were carried out under urethane anaesthesia. After electrical stimulation of the contralateral snout, the ipsilateral and contralateral hindlimb or forepad, and the tail, CF-EPSPs were recorded from PCs in an area extending 1000 microns laterally to the midline in the vermal part of lobules VII and VIII. The stepwise variation of CF-EPSP amplitude demonstrated the multiple innervation of the PCs by CFs in these animals. The responses of a given PC through separate CFs were analyzed separately and, using precise micromapping techniques and computer analysis, cells were located on the map of the unfolded PC layer. Taking into account the mean latency of the responses and the probability of discharge of the PCs, restricted areas of projection were found for the snout, the forepads and the tail. Zones of short-latency responses formed compact patches of approximately 1 mm2. Their disposition was very similar to that found in normal rats. The responses evoked through the different CFs converging on a given PC were in general very homogeneous and very similar to those recorded in control rats. There was some overlap of projection zones from tail and snout and from forepads and snout. In this latter case, there was a convergence of several peripheral inputs on some of the PCs tested. No precise projection of the hindlimbs could be detected in the same lobules. These results suggest that, with the type of stimulation used, the representation of peripheral inputs through CF pathway is roughly conserved even though multiple innervation of PCs by CFs is maintained until adulthood.