The Cerebellum: Brain for an Implicit Self (17 page)

Insulin-like growth factor-1 (IGF-1).
This basic peptide is involved in cell growth and differentiation and it operates at diverse synaptic sites. Purkinje cells exhibit IGF-1 immunoreactivity localized in the rough endoplasmic reticulum (
Aguado et al., 1992
). In the human cerebellum, IGF-1 immunoreactivity is pronounced throughout Purkinje cells and their extrusions, and it is also observed in the IO (
Aguado et al., 1994
). Electrical stimulation of the IO significantly increases the IGF-1 level in the cerebellar cortex. Whether IGF-1 is actually involved in LTD induction is uncertain, but because IGF-1 stimulates DAG production (
Kojima et al., 1990
), it is possible that DAG, in turn, stimulates PKC, which is required for LTD induction. IGF-1 may be involved in the maintenance of dendritic spine morphology because an IGF-1 antisense oligonucleotide injected into the IO was shown to induce a significant reduction in the size of the dendritic spines on Purkinje cells. Antisense oligonucleotide was also shown to evoke a significant and reversible decrease in IGF-1 level in the contralateral cerebellum (
Nieto-Bona et al., 1997
).

Brain-derived neurotrophic factor (BDNF).
Evidence is scarce regarding the involvement of this factor in conjunctive LTD induction. Nevertheless, when quisqualate application induces LTD, a significant increase in the level of BDNF’s mRNA expression occurs in cerebellar tissues, with a peak 4 hours after the application (
Yuzaki et al., 1994
). Even though the major source of this expression level increase is its granule cell fraction, the Purkinje cell fraction also contains BDNF’s mRNA. If BDNF is co-induced with LTD, it might play a role in the later phases of LTD.

The basic properties and underlying signal transduction of conjunctive LTD have been analyzed in great detail. The complexity of this signal transduction should be a safeguard for its robust operation under variable conditions. The possibility that conjunctive LTD plays a crucial role in motor learning is examined in later chapters. The pharmacological and genetic tools required for analyzing such roles are now becoming available.

In addition to conjunctive LTD in Purkinje cells, which was discussed in
Chapter 7
, “
Conjunctive Long-Term Depression (LTD)
,” a rich variety of synaptic plasticity subtypes and underlying signal transductions has been revealed throughout cerebellar neuronal circuits. These subtypes should play their individual roles in cerebellar function. In this chapter, we address the above and ask critical questions about how conjunctive LTD persists and the extent to which it contributes in the formation of a memory trace.

Homosynaptic LTD in parallel fiber-Purkinje cell synapses.
LTD is induced in these synapses when a relatively large set of parallel fibers is stimulated repetitively (
Hartell, 1996
). Nevertheless, as long as parallel fibers are stimulated moderately, LTD occurs only after the conjunctive stimulation of parallel fibers and a climbing fiber. This ensures that conjunctive LTD is a prevailing physiological process in cerebellar networks and associated with a unique structure, that is, climbing fibers (
Chapter 7
). Homosynaptic LTD at parallel fiber-Purkinje cell synapses has been demonstrated to occur in the part of the fish cerebellum-like tissues that are devoid of climbing fibers (
Bell et al., 1997
) (see also
Chapter 15
, “
Internal Models for Voluntary Motor Control
,”
Section 8
).

Long-term potentiation (LTP) in parallel fiber-Purkinje cell synapses.
Presynaptic LTP occurs in parallel fiber-Purkinje cell synapses when parallel fibers are stimulated at 4–8 Hz, without being paired with climbing fiber activation. This
form of LTP requires the activation of protein kinase A (PKA) by cyclic AMP but not NO (
Salin et al., 1996
;
Lev-Ram et al., 2002
). The knockout of the gene encoding the active zone protein, RIM1α, blocks presynaptic LTP. It is rescued, however, by the presynaptic expression of RIM1α. These findings suggest that the PKA-mediated phosphorylation of RIM1α at a single N-terminal site triggers presynaptic LTP (
Lonart et al., 2003
). A similar LTP occurs at parallel fiber-stellate cell synapses (
Lachamp et al., 2009
).

Lev-Ram et al. (
2002
,
2003
) demonstrated that LTP occurs postsynaptically in parallel fiber-Purkinje cell synapses after parallel fiber activation at 1 Hz unpaired with climbing fiber activation. Postsynaptic LTP does not require cyclic AMP or cyclic GMP acting on these synapses, but it does require NO activation, for example, when it is induced by bath application of an NO donor (NOR3) (
Lev-Ram et al., 2002
). Such NO-induced LTP requires soluble N-ethylmaleimide-sensitive fusion (NSF) protein attachment receptor (SNARE) proteins and GluR2-NSF interaction at the synapses (
Kakegawa and Yuzaki, 2005
). When postsynaptic LTD was saturated by repeated conjunction, a single bout of parallel fiber stimulation or application of NO reversed it and enabled conjunctive LTD to be re-induced (
Lev-Ram et al., 2003
). Conversely, when postsynaptic LTP was saturated one bout of conjunction revitalized fresh postsynaptic LTP. Such interactions did not occur between presynaptic LTP and conjunctive LTD. The NO-evoked postsynaptic LTP was shown to be caused by excessive incorporation of AMPA receptors into the postsynaptic membrane of parallel fiber-Purkinje cell synapses (
Yamaguchi 2009
).

Cannabinoid-receptor-mediated presynaptic LTD.
The brain as a whole contains endogenous cannabinoids and their receptors. There is a particularly high density of these receptors in the cerebellum, largely in the molecular layer. As a result, cannabinoid intoxication leads to the degradation of motor coordination. Parallel fiber-evoked EPSCs in Purkinje cells are strongly inhibited by the bath application of a cannabinoid receptor agonist, and this inhibition is completely blocked by a cannabinoid CB1 receptor antagonist (
Lévénès et al., 1998
;
Takahashi and Linden, 2000
). The retrograde regulation of the endocannabinoid-mediated presynaptic LTD is mediated by postsynaptic mGluR1s (
Maejima et al., 2001
). Either postsynaptic Ca
²⁺
concentration elevation or activation of Gq/11-coupled receptors induces the release of endocannabinoids (endogenous cannabinoids). When these two factors coincide, endocannabinoid release is markedly enhanced, this being attributed to the Ca
²⁺
dependence of phospholipase Cβ (PLCβ) (
Hashimotodani et al., 2007
). A released endocannabinoid acts on parallel fiber cannabinoid receptors to inhibit Ca
²⁺
influx into parallel fibers and thereby depresses the release of glutamate from parallel fibers (
Brown et al., 2004
).
Cannabinoid agonists impair LTD (
Levenes et al., 1998
) resulting, for example, in impaired eye-blink conditioning (Steinmetz and Freeman, 2010). In a recent study using gene-manipulated mice, the retrograde messenger was identified as 2-arachidonoylglycerol (2-AG), which is produced by synthesizing the enzyme diacylglycerol lipase α (DGLα) (
Tanimura et al., 2010
).

Homosynaptic LTD in climbing fiber-Purkinje cell synapses.
Climbing fiber-Purkinje cell synapses undergo a modest degree of postsynaptic LTD after brief 5 Hz stimulation of climbing fibers (Hansel and Linden, 2000). LTD at both climbing fiber- and parallel fiber-Purkinje cell synapses requires an elevated postsynaptic Ca
²⁺
concentration, and activation of mGluR1s and PKC.

Rebound potentiation.
A prolonged potentiation of basket/stellate cell-mediated IPSPs in Purkinje cells is induced by coactivation of climbing fibers. This rebound potentiation is input-nonspecific (
Kano et al., 1992
). It is caused by a Ca
²⁺
-dependent upregulation of postsynaptic GABA receptor function, and it involves the protein phosphatases, 1 and 2B, CamKII, PKA, DARPP-32 (a substrate of PKA and calcineurin, which inhibits protein phosphatase 1 when phosphorylated by PKA), and the GABAB receptor (
Kawaguchi and Hirano, 2002
). Rebound potentiation might cooperate with LTD in depressing the parallel fiber-mediated activation of Purkinje cells (
Yamamoto et al., 2002
).

Synaptic plasticity induced by fear conditioning.
The pairing of acoustic and nociceptive stimuli leads animals to express fear responses to otherwise neutral acoustic stimuli. In cerebellar slices obtained from fear-conditioned rats, both excitatory and inhibitory transmission to Purkinje cells were potentiated for up to 24 hours after conditioning (
Scelfo et al., 2008
). How these changes are induced is presently unknown. One possibility is that the neuromodulation is mediated by a hypothalamic neuropeptide (
Chapter 4
, “
Input and Output Pathways in the Cerebellar Cortex
”).

LTP and LTD in parallel fiber-basket/stellate cell synapses.
Burst stimulation of a parallel fiber bundle unpaired with climbing fiber activity induces a long-lasting decrease in the size of the receptive fields of inhibitory interneurons. At the same time, it causes a large increase in the size of the receptive fields of Purkinje cells (
Jörntell and Ekerot, 2002
,
2003
). In contrast, parallel fiber stimulation paired with climbing fiber activity induces a long-lasting increase in the size of receptive fields in interneurons, whereas it causes a long-lasting decrease in the size of the receptive fields of Purkinje cells. These findings suggest that parallel
fiber synapses in inhibitory interneurons undergo LTP when parallel fiber synapses in Purkinje cells undergo LTD, and vice versa. Such a reciprocal pattern may have a synergistic effect, which augments the information storage capacity of cerebellar cortical networks. Alternatively, the parallel fiber-Purkinje cell synapses and parallel fiber-basket/stellate cell synapses may share different roles in the operation of cerebellar circuits. For example, a computer simulation study adopted such an assumption that synaptic plasticity first developed in parallel fiber-Purkinje cell synapses, and with additional training, this was transferred to parallel fiber-basket/stellate cell synapses for long-term memory storage (
Kenyon, 1997
).

LTP in mossy fiber-granule cell synapses.
High-frequency mossy fiber stimulation induces LTP in mossy fiber-granule cell synapses, providing postsynaptic NMDA receptors are activated. This LTP is accompanied by significant NO release in the granular layer, which is also dependent on NMDA receptor activation as well as NO synthase activation (
Maffei et al., 2003
). LTP is one of the processes that regulate the degree of divergence from a mossy fiber to granule cells (
D’Angelo et al., 1999
) in addition to the enhancement of intrinsic excitability (
Armano et al., 2000
) and Golgi cell inhibition (
Chadderton et al., 2004
).

Long-term changes in Golgi cell responses.
Effects of conjunctive stimulation of climbing fibers and peripheral afferents on Golgi cells have been examined (
Xu and Edgley, 2008
) because anatomical evidence has shown that Golgi cells are contacted by climbing fibers, and because climbing fiber signals may be transferred to Golgi cells by spillover of the transmitter or via synapses with NG2+ glial cells (
Chapter 5
, “
Inhibitory Interneurons and Glial Cells in the Cerebellar Cortex
”). After the conjunction, Golgi cell responses to peripheral afferent stimulation were significantly reduced. The reductions developed progressively over 20 minutes of conjunctive stimulation and were persistent for up to 84 minutes. These results are in keeping with the idea that cerebellar circuits contribute to learning mechanisms.

LTD in inhibitory synapses.
Inhibitory synapses of cerebellar nuclear neurons, presumably supplied by Purkinje cell axons, undergo postsynaptic LTD (
Morishita and Sastry, 1996
). This results from an input-nonspecific decrease in postsynaptic GABA sensitivity (not depending on the activation of GABA
_A
receptors), which is
caused by an increase in intracellular Ca
²⁺
concentration and activation of protein phosphatases. It may be worthy noting that this form of LTD explains the seemingly puzzling previous observation that destruction of the inferior olive depresses Purkinje cell inhibition on vestibular nuclear neurons remotely (
Ito et al., 1979
). Probably, in the absence of climbing fiber activity, Purkinje cells increased discharge (
Colin et al., 1980
) and this in turn caused LTD in the Purkinje cell inhibition of nuclear neurons.

LTP/LTD in excitatory synapses.
LTP has also been observed to occur in the interpositus and vestibular nuclear neurons of both acute and chronic rat
in vivo
preparations. This is seen when stimulating pulse trains are applied to the white matter at the entrance of inferior peduncle fibers to the cerebellum. The LTP lasted for at least 8 days in all but one of the preparations (
Racine et al., 1986
). However, a more recent study in rat
in vitro
preparations demonstrated that high-frequency burst stimulation of mossy fibers in the white matter, either alone or paired with postsynaptic depolarization, induced LTD of the mossy fiber-interpositus neuron synapse (
Zhang and Linden, 2006
). This form of LTD was blocked by infusion of a Ca
²⁺
chelator through a micropipette to the interpositus neuron and required activation of mGluR1 and protein translation.