The cerebellopontine system: an electrophysiological study in the rat

Collateralization of cerebellar output to functionally distinct brainstem areas. A retrograde, non-fluorescent tracing study in the rat

The organization of the cerebellum is characterized by a number of longitudinally organized connection patterns that consist of matching olivo-cortico-nuclear zones. These entities, referred to as modules, have been suggested to act as functional units. The various parts of the cerebellar nuclei (CN) constitute the output of these modules. We have studied to what extent divergent and convergent patterns in the output of the modules to four, functionally distinct brain areas can be recognized. Two retrograde tracers were injected in various combinations of the following nuclei: the red nucleus (RN), as a main premotor nucleus; the prerubral area, as a main supplier of afferents to the inferior olive (IO); the nucleus reticularis tegmenti pontis (NRTP), as a main source of cerebellar mossy fibers; and the IO, as the source of climbing fibers. For all six potential combinations three cases were examined. All nine cases with combinations that involved the IO did not, or hardly, resulted in double labeled neurons. In contrast, all other combinations resulted in at least 10% and up to 67% of double labeled neurons in cerebellar nuclear areas where both tracers were found. These results show that the cerebellar nuclear neurons that terminate within the studied areas represent basically two intermingled populations of projection cells. One population corresponds to the small nucleo-olivary neurons whereas the other consists of medium- to large-sized neurons which are likely to distribute their axons to several other areas. Despite some consistent differences between the output patterns of individual modules we propose that modular cerebellar output to premotor areas such as the RN provides simultaneous feedback to both the mossy fiber and the climbing fiber system and acts in concert with a designated GABAergic nucleo-olivary circuit. These features seem to form a basic characteristic of cerebellar operation.

Electrophysiological properties of rat pontine nuclei neurons In vitro II. Postsynaptic potentials

We investigated the postsynaptic responses of neurons of the rat pontine nuclei (PN) by performing intracellular recordings in parasagittal slices of the pontine brain stem. Postsynaptic potentials (PSPs) were evoked by brief (0.1 ms) negative current pulses (10-250 microA) applied to either the cerebral peduncle or the pontine tegmentum. First, excitatory postsynaptic potentials (EPSPs) could be evoked readily from peduncular stimulation sites. These EPSPs exhibited short latencies, a nonlinear increment in response to increased stimulation currents, and an unconventional dependency on the somatic membrane potential. Pharmacological blockade of the synaptic transmission using 6,7-dinitroquinoxaline-2, 3-dione and ,-2-amino-5-phosphonovaleric acid, selective antagonists of the alpha-amino-3-hydroxy-5-methyl-4-isoxazilepropionate- (AMPA) and the N-methyl--aspartate (NMDA)-type glutamate receptors, showed that these EPSPs were mediated exclusively by excitatory amino acids via both AMPA and NMDA receptors. Moreover, the pharmacological experiments indicated the existence of voltage-sensitive but NMDA receptor-independent amplification of EPSPs. Second, stimulations at peduncular and tegmental sites also elicited inhibitory postsynaptic potentials (IPSPs) in a substantial proportion of pontine neurons. The short latencies of all IPSPs argued against the participation of inhibitory interneurons. Their sensitivity to bicuculline and reversal potentials around -70 mV suggested that they were mediated by gamma-aminobutyric acid-A (GABAA) receptors. In addition to single PSPs, sequences consisting of two to four distinct EPSPs could be recorded after stimulation of the cerebral peduncle. Most remarkably, the onset latencies of the following EPSPs were multiples of the first one indicating the involvement of intercalated synapses. Finally, we used the classic paired-pulse paradigm to study whether the temporal structure of inputs influences the synaptic transmission onto pontine neurons. Pairs of electrical stimuli applied to the cerebral peduncle resulted in a marked enhancement of the amplitude of the second EPSP for interstimulus intervals of 10-100 ms. Delays >200 ms left the EPSP amplitude unaltered. These data provide evidence for a complex synaptic integration and an intrinsic connectivity within the PN too elaborate to support the previous notion that the PN are simply a relay station.

Ultrastructural study of the GABAergic and cerebellar input to the nucleus reticularis tegmenti pontis

The nucleus reticularis tegmenti pontis is an intermediate of the cerebrocerebellar pathway and serves as a relay centre for sensorimotor and visual information. The central nuclei of the cerebellum provide a dense projection to the nucleus reticularis tegmenti pontis, but it is not known to what extent this projection is excitatory or inhibitory, and whether the terminals of this projection contact the neurons in the nucleus reticularis tegmenti pontis that give rise to the mossy fibre collaterals innervating the cerebellar nuclei. In the present study the nucleus reticularis tegmenti pontis of the cat was investigated at the ultrastructural level following anterograde and retrograde transport of wheat germ agglutinin coupled to horseradish peroxidase (WGA-HRP) from the cerebellar nuclei combined with postembedding GABA immunocytochemistry. The neuropil of this nucleus was found to contain many WGA-HRP labeled terminals, cell bodies and dendrites, but none of these pre- or postsynaptic structures was double labeled with GABA. The vast majority of the WGA-HRP labeled terminals contained clear spherical vesicles, showed asymmetric synapses, and contacted intermediate or distal dendrites. Many of the postsynaptic elements of the cerebellar afferents in the nucleus reticularis tegmenti pontis were retrogradely labeled with WGA-HRP, while relatively few were GABAergic. We conclude that all cerebellar terminals in the nucleus reticularis tegmenti pontis of the cat are nonGABAergic and excitatory, and that they contact predominantly neurons that project back to the cerebellum. Thus, the reciprocal circuit between the cerebellar nuclei and the nucleus reticularis tegmenti pontis appears to be well designed to function as an excitatory reverberating loop.

Olivary projecting neurons in the nucleus of Darkschewitsch in the cat receive excitatory monosynaptic input from the cerebellar nuclei

The direct projection from the cerebellar nuclei to the inferior olive is GABAergic. In the present study, we investigated the projection from the cerebellar nuclei to the mesodiencephalic junction which is known to provide an excitatory projection to the inferior olive. The mesodiencephalic junction was studied in cat following anterograde transport of wheatgerm agglutinated horseradish peroxidase from the cerebellar nuclei in combination with: (1) retrograde transport of gold-lectin conjugate from the inferior olive; and (2) postembedding GABA-immunocytochemistry. Light microscopic analysis revealed that overlap of the anterograde and retrograde labeling was especially prominent in the nucleus of Darkschewitsch. Electron microscopic examination of this area showed: (1) that many cerebellar terminals made synaptic contacts with neurons that project to the inferior olive; (2) that virtually all cerebellar terminals were non-GABAergic and displayed an excitatory morphology; and (3) olivary projecting neurons were non-GABAergic. It is concluded that the indirect cerebellar projection to the inferior olive via the nucleus of Darkschewitsch is disynaptic and excitatory.

Cerebellar and brainstem circuits involved in classical eyeblink conditioning

Model systems are one useful strategy for the investigation of the mechanisms of learning. Whereas mammalian model systems generally do not offer the ease of identifying circuitry and exploring cellular mechanisms of learning that is realized with invertebrate preparations /37,97/, research involving the rabbit classical eyeblink conditioning paradigm has now reached the state at which much of the basic conditioning neural circuit appears to have been identified /9,65,66,85,89,91/. Despite a dispute as to precisely where in the circuitry convergence of the associated stimuli may occur, there is substantial evidence identifying the stimulus input pathways and motor output pathway. The present summary of this research details these paths. In addition, the proposed sites of convergence of the conditioning stimuli are discussed. Finally, a hypothesized neural circuit responsible for classical eyeblink conditioning is presented along with some suggestions for future research directions.

Cerebellar contributions to instrumental learning

There is emerging evidence that the cerebellum is involved in spatial and nonspatial instrumental learning tasks. Cerebellar-lesioned animals have deficits in water maze learning tasks that may be explained by two-way interactions with higher order brain regions. There is suggestive evidence that cerebellar modulation extends to shock avoidance and discrimination learning. Although this evidence needs to be confirmed by a wider range of lesion methods and choice of learning tasks, it is in line with the hypothesis that the cerebellum affects cognitive processes and is not strictly concerned with motor control and the acquisition and retention of conditioned reflexes.