The Cerebellum: Brain for an Implicit Self (12 page)

Golgi cells are large neurons that extend their dendrites like a bouquet into the molecular layer and also extend their descending dendrites to the granular layer. Golgi cells have broadly branching axons in the granular layer (Color Plate X A). It
is now apparent that Golgi cells exert extensive control of spatio-temporal signal organization and information storage in the granular layer (see
D’Angelo, 2008
). It is also apparent that they play unique roles in computational aspects of cerebellar function (
Chapter 9
).

The molecular layer dendrites of each Golgi cell receive ~4,800 excitatory inputs from parallel fibers, and the same cell’s descending dendrites in the granular layer receive ~230 excitatory mossy fiber terminals (
Pellionisz and Szentágothai, 1973
). Through such inputs, each Golgi cell recorded in the C
₃
zone has a localized receptive field in the forelimb skin (
Ekerot and Jörntell, 2001a
,
b
). The major excitatory inputs from parallel fibers to Golgi cells are mediated by both AMPA and NMDA receptors (
Dieudonné, 1998
) and mGluR2 (
Watanabe et al., 2003
). Interestingly, parallel fiber-induced mGluR2 activation hyperpolarizes Golgi cells via G-protein-coupled inward K
⁺
channels, as contrasted to the depolarizing action mediated by mGluR1 on Purkinje cells. mGluR2 is expressed also in the somata and axons of Golgi cells whose axon terminals can be activated by glutamate released from mossy fiber terminals (
Ohishi et al., 1994
). Golgi cells also receive inhibitory synapses from Lugaro cells (see below) but not from Purkinje cells or other Golgi cells. Morphological evidence suggests that climbing fibers might make contact with Golgi cells (
Palay and Chan-Palay, 1974
;
Sugihara et al., 1999
) but its functional significance is unclear (see
Chapter 8
, “
Multiplicity and Persistency of Synaptic Plasticity
”).

A Golgi cell projects, in turn, a broadly branching axon and supplies GABA-mediated inhibitory synapses (
Chadderton et al., 2004
) to ~5,700 granule cells (in cats,
Palkovits et al., 1971
). Characteristically, Golgi cell-granule cell synapses produce significant cross-talk with non-postsynaptic cells. This is caused by the spillover of the inhibitory transmitter, GABA. This effect is manifested as a slowly rising and decaying, small-amplitude IPSC. GABA spillover within the mossy fiber glomerulus may be promoted both by the anatomical specialization of this glomerular synapse and by the presence of the high-affinity α6-subunit-containing GABA
_A
receptor in granule cells. GABA spillover may also play a role in regulating the number of active granule cells in the cerebellar cortex (
Rossi and Hamann, 1998
). A study has shown that a major proportion (~70%) of Golgi cell somata are immunopositive to both glycine and GABA, and these two immunoreactivities are co-localized in the same glomeruli and even in the same Golgi cell terminals, as confirmed by electron microscopy (
Ottersen et al., 1988
). Hence, Golgi cells may release not only GABA but also glycine as an inhibitory transmitter. The selective ablation of Golgi cells by an immunotoxin-mediated cell targeting technique was shown to impair motor function (
Watanabe et al., 1998
). This effect is presumably attributable to attenuated GABA/glycine-mediated inhibition, as well as to the
attenuated action of NMDA receptors on granule cells. A recent finding was that Golgi cells caused pure GABAergic inhibition of granule cells in contrast to pure glycinergic inhibition of unipolar brush cells (
Dugué et al., 2005
). This specialization resulted from the differential expression of GABA
_A
and glycine receptors by target cells and not by a segregation of GABA and glycine in presynaptic terminals. These results exemplify the postsynaptic selection of co-released transmitters, this being a mechanism that enables target-specific signaling in mixed inhibitory networks.

In the
in vivo
cerebellar cortex, Golgi cells discharge tonically at ~5 Hz to decrease the firing rate of granule cells via GABA synapses (
Chadderton et al., 2004
). This should help stabilize parallel fiber discharges despite large changes in mossy fiber discharge (
Albus, 1971
). Golgi cell inhibition decreases noise in parallel fiber discharges in the presence of noisy mossy fiber inputs (
Philipona and Coenen, 2004
). A computer simulation suggested that feedback inhibition from Golgi to granule cells induced 10–50 Hz oscillations in spike discharges from the latter (
Maex and de Schutter, 2005
). This may account for the large-amplitude oscillations recorded in the granular layer of freely moving rats (
Hartman and Bower, 1998
) and monkeys (
Pellerin and Lamarre, 1997
;
Courtemanche et al., 2002
,
2005
). In rat cerebellar slices, Golgi cells display spontaneous firing at 1–10 Hz (at room temperature) or 2–20 Hz (at a bath temperature of 35°C–37°C). Because this firing persisted in the presence of various pharmacological blockers, Forti et al. (
2006
) proposed that Golgi cells behave under given conditions as pacemaker neurons with multiple ionic mechanisms. Moreover, the dendrites of Golgi cells are electrically coupled via gap junctions involving connexin-36. These gap junctions may explain how Golgi cells synchronize their firing in the absence of coincident synaptic input and how sparse, coincident mossy fiber input to Golgi cells triggers a mixture of excitation and inhibition in their discharge and its spike desynchronization (
Vervaeke et al., 2010
). The gap junctions may either promote network synchronization or trigger rapid network desynchronization, depending on the nature of the synaptic input.

The presence of Lugaro cells has long been known, but they have been characterized relatively recently as a unique type of inhibitory neuron located in the granular layer. Typically, their fusiform cell somata are located in or slightly below the Purkinje cell layer (
Aoki et al., 1986
;
Sahin and Hockfield, 1990
) (Color Plate X B). The categorization of Lugaro cells has recently been expanded to include cells with similar properties that are located throughout the granular layer (
Sahin and Hockfield, 1990
;
Laine and Axelrad, 2002
). Axons of Lugaro cells form a parasagittal
plexus, but they also extend transversely for ~2 mm, forming contacts with the apical dendrites of Golgi cells. It is approximated that the cerebellar cortex contains one Lugaro cell and one Golgi cell for every 15 Purkinje cells. Moreover, the axons of more than 10 Lugaro cells converge onto only one Golgi cell, whereas the axon of one Lugaro cell diverges onto ~150 Golgi cells (
Dieudonne and Dumoulin, 2000
). Lugaro cells can be distinguished immunohistochemically from Golgi cells; that is, the latter express mGluR2 and somatostatin but not calretinin, whereas Lugaro cells express calretinin but not mGluR2 or somatostatin (
Sahin and Hockfield, 1990
;
Geurts et al., 2001
).

A unique physiological feature of Lugaro cells is that in cerebellar slices, they are normally silent. They are, however, highly sensitive to serotonin, this being unlike basket cells, stellate cells, Purkinje cells, and Golgi cells. In the presence of serotonin, Lugaro cells discharge regularly at 5–15 Hz and induce IPSCs in Golgi cells (
Dieudonne and Dumoulin, 2000
). GABA-mediated inhibitory synapses between Lugaro cells and Golgi cells are unusually resistant to bicuculline, a commonly used GABA
_A
antagonist, probably caused by an unusually long GABA dwell time and/or a high GABA concentration in synaptic clefts (
Dean et al., 2003
). Double immunocytochemical staining in the rat cerebellum has demonstrated that Lugaro cell axonal varicosities co-express GABAergic and glycinergic markers. Indeed, serotonin-evoked activation of Lugaro cells induced IPSCs in Golgi cells, which were identified pharmacologically as mediated by both GABA and glycine. Golgi cells also exhibited spontaneously occurring IPSCs, which were purely GABAergic and were likely to arise via basket cell-Golgi cell synapses (
Dumoulin et al., 2001
).

Because of the large divergence from Lugaro cells to Golgi cells (1:150), it appears that when activated by serotonin, Lugaro cells influence numerous Golgi cells. An interesting possibility is that Lugaro cells play a role in synchronizing activity among Golgi cells situated along a parallel fiber beam, as observed in anesthetized rats (
Vos et al., 2000
). Lugaro cells may switch the operation of Golgi cells from the individual rhythmic mode to the synchronous mode of discharge.

Small globular cells with globular somata are located at various depths in the granular layer (
Laine and Axelrad, 2002
). They extend three to four long-radiating dendrites that course through the three layers of the cerebellar cortex. Their axons project into the molecular layer and expand a local plexus, with a pattern similar to that of Lugaro cells. The axons of several small globular neurons project an axon-collateral that courses for a long distance in the transverse direction, immediately
above Purkinje cell somata and parallel to parallel fibers. Cytochemical observations using mice expressing GFP-labeled glycinergic and GABAergic neurons have shown that small globular neurons are glycinergic/GABAergic inhibitory neurons lacking mGluR2 and neurogranin (
Simat et al., 2007
). Small globular neurons are characterized by their strong GABAergic inhibition received via axon collaterals of Purkinje cells and also by their activation to elicit spike discharges during the perfusion of monoamines, serotonin, or noradrenaline (Hirono et al., 2010). Small inhibitory neurons other than small globular neurons have not been examined well except for the findings that (1) they exhibit smaller and fewer spontaneous IPSCs and that (2) small fusiform Lugaro cells are only slightly responsive to noradrenaline.

Bergmann glial cells are astrocytes of unique morphology. Their cell somata are located among Purkinje cell somata, and their palisades (Bergmann fibers) extend to the molecular layer between Purkinje cell dendrites. A neuron-specific transmembrane protein, Delta/Notch-like EGF-related receptor (DNER), has been shown to play a role in the morphogenesis of Bergmann glia in the mouse cerebellum (
Eiraku et al., 2005
). In the developing cerebellum, DNER is highly expressed in Purkinje cell dendrites, which are closely associated with radial fibers of Bergmann glia expressing Notch. DNER deficiency retards the formation of radial fibers and results in an abnormal arrangement of Bergmann glia.

Glutamate released from parallel fibers onto spines of Purkinje cell dendrites enters the glutamate-glutamine cycle. First, it is transported to Bergmann fibers that enclose the synapses of Purkinje cells and interneurons in the molecular layer, where it is converted to glutamine by a glial enzyme, glutamine synthetase. Glutamine itself has no action on glutamate or other receptors. It is transferred back to Purkinje cells where it is converted to glutamate by phosphate-activated glutaminase. Two major types of glutamate transporter, GLT1 and GLAST, are located in the plasma membrane of Bergmann fibers. Their highest density is in glial processes surrounding synapses between parallel fibers and Purkinje cell spines (
Chaudhry et al., 1995
). A Purkinje-cell-specific glutamate transporter, EAAT4, is expressed at a high density in the membrane of postsynaptic spines surrounding a synaptic cleft. EAAT4 is responsible for the vast majority of glutamate uptake by Purkinje cells. It regulates mGluR1 transmission and LTD (
Auger and Attwell, 2000
;
Wadiche and Jahr, 2005
). EAAT4 activity can be detected by recording glutamate transporter current from Purkinje cells. The tetanic stimulation of climbing
fibers evokes a long-lasting potentiation of glutamate transporter current, probably implying an antiexcitotoxic adaptive response (
Shin and Linden, 2005
).

A rather unexpected recent finding was that Bergmann glial somata express a relatively high density of AMPA receptors (in rat, 17-fold more than Purkinje cell somata), which are distributed uniformly throughout the Bergmann glial membrane. Quantal events were also observed in Bergmann glial cells. Their kinetics were as fast as those of neurons (
Matsui and Jahr, 2003
;
Matsui et al., 2005
). Bergmann glial cells were characterized by their tightly encapsulating excitatory synapses on Purkinje cells (
Palay and Chan-Palay, 1974
;
Spacek, 1985
). This arrangement makes it possible to locate glutamate transporters near their sites of release and to show an increase in the diffusion distance between adjacent synapses and thereby keep synapses functionally isolated from each other (
Barbour and Hausser, 1997
;
Huang and Bergles, 2004
). Moreover, recent findings suggest that AMPA receptors in perisynaptic Bergmann glial processes are activated by ectopic fusion of synaptic vesicles outside synaptic active zones (
Matsui and Jahr, 2003
;
Matsui and Jahr, 2004
) and also by spill out of glutamate from the synaptic cleft (
Bergles et al., 1997
;
Dzubay and Jahr, 1999
). Other studies have shown that stimulation of parallel fibers induces three types of currents in Bergmann glial cells: a fast AMPA receptor current, a slow glutamate uptake current that peaks in 5–10 milliseconds, and a slow G-protein-coupled current (
Clark and Barbour, 1997
;
Bellamy and Ogden, 2005
). The fast AMPA receptor current signaled the ectopic release of glutamate from parallel fibers. This implies a direct and rapid mechanism of neuron-to-glia communication that does not rely on transmitter spillover from synaptic clefts (
Matsui and Jahr, 2004
).