The GABAergic cerebello-olivary projection in the rat

Abstract

Immunocytochemical detection of glutamate decarboxylase (GAD), the predominant biosynthetic enzyme of gamma-aminobutyric acid (GABA), reveals the presence of a dense GABAergic innervation in all parts of the inferior olive. One brain center that provides a substantial projection to the inferior olive is the cerebellar nuclei, which contain many small GABAergic neurons. These neurons were tested as a source of GABAergic olivary afferents by combining retrograde tract tracing with GAD immunocytochemistry. As expected from previous studies, injections of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) into the inferior olive retrogradely label many small neurons in the interposed and lateral cerebellar nuclei and the dorsal part of the lateral vestibular nucleus, and fewer neurons in the ventro-lateral region of the medial cerebellar nucleus. These projections are predominantly crossed and are topographically arranged. The vast majority, if not all, of these projection neurons are also GAD-positive. The relative contribution of this projection to the GABAergic innervation of the inferior olive was tested by lesion of the cerebellar nuclei, or the superior cerebellar peduncle. Within 10 days the lesion eliminates most GAD-immunoreactive boutons in the principal olive, the rostral lamella of the medial accessory olive, the ventrolateral outgrowth, and the lateral part of the dorsal accessory olive ventral fold. Thus, the effectiveness of this depletion demonstrates that the cerebellar nuclei provide most of the GABAergic innervation to regions of the inferior olive known to receive a cerebellar projection. Moreover, when the lateral vestibular nucleus is damaged, the dorsal fold of the dorsal accessory olive is depleted of GABAergic boutons. The synaptic relations that boutons of the GABAergic cerebello-olivary projection share with olivary neurons were investigated at the electron microscopic level by GAD-immunocytochemistry, anterograde degeneration of the cerebellar axons or anterograde transport of WGA-HRP. All of these methods confirm that GABAergic, cerebello-olivary axon terminals contain pleomorphic vesicles, and synapse on various portions of olivary neurons, and especially on dendritic spines within glomeruli, often in very close proximity to the gap junctions that characteristically couple the dendritic profiles. These results demonstrate four major points: that virtually all of the GABAergic, and presumably inhibitory, neurons of the cerebellar and dorsal lateral vestibular nuclei are projection neurons; that a large portion of the inferior olive receives GABAergic afferents from the cerebellar nuclei; that a portion of the dorsal accessory olive receives GABAergic afferents from the dorsal lateral vestibular nucleus; and that cerebello-olivary fibers often synapse near gap junctions, and therefore could influence electrical coupling of olivary neurons.

Disruption of the olivo-cerebellar circuit by Purkinje neuron-specific ablation of BK channels

“…Our results provide clear evidence that the anatomical organization of the olivo-cerebellar circuit (17)(18)(19), which comprises the cerebellar cortex, the DCN, and the IO, indeed operates as a closed loop. This suggestion is in agreement with recent results of Marshal and Lang (32), demonstrating that experimentally increasing PN firing increases the firing rate and synchrony level of CS.…”

“…This circuit is a loop of interconnected structures where the cerebellar cortex innervates the inferior olivary nucleus (IO) via the DCN, and the olivary neurons via the climbing fibers are fed back into the cerebellar cortex. Whereas the olivary neurons exert an excitatory action on the PN at the cerebellar cortex, both the PN and DCN terminals exert an inhibitory action on their target cells (17)(18)(19). Thus, it is likely that this long-range feedback loop will be affected by the alteration in PN's activity observed in the BK −/− mouse.…”

Ionic Currents and Spontaneous Firing in Neurons Isolated from the Cerebellar Nuclei

“…Small neurons (5-10 m in soma diameter) also were released by the cell isolation procedure. These neurons are likely to be the inhibitory neurons that project to the inferior olive (Nelson and Mugnaini, 1989;Fredette and Mugnaini, 1991;Ruigrok, 1997). As described in Materials and Methods, the present experiments were limited to the large cerebellar nuclear neurons, which had similar electrophysiological properties.…”

“…The large cells (15-25 m in soma diameter) are likely to be the glutamatergic neurons that project to the red nucleus and ventral thalamus (Bentvoglio and Kuypers, 1982;Gonzalo-Ruiz et al, 1988;Teune et al, 1995Teune et al, , 1998Shinoda et al, 1997). The small cells (5-10 m in soma diameter) are likely to be inhibitory neurons that project to the inferior olive (Nelson and Mugnaini, 1989;Fredette and Mugnaini, 1991;Ruigrok, 1997). All cerebellar nuclear recordings were made from large neurons.…”

Editorial on the Honorary Cerebellum Issue for the Retirement of Enrico Mugnaini

“…More recently the acoustic system recurred in the paper with Enrique Saldaña on the striking tonotopic organization of the inferior colliculus [35]. GAD histochemistry identified GABAergic neurons in structures as diverse as the amygdala and olfactory bulb [1] and it allowed him to study the cerebellar nucleo-olivary pathway as a GABAergic system [36]. The cholinergic innervation and receptors of the cerebellum were studied with Dick Jaarsma [37], while the UBC appeared as a neglected interneuron in a paper with Alessandra Floris in 1994 [23], but developed in the best-known cerebellar interneuron in a large series of Enrico's papers.…”