The role of dopamine in modulating the structure and function of striatal circuits

Acute dopaminergic modulation of striatal MSN excitability

D₁ receptors are positively coupled to adenylyl cyclase (type V) through G_olf (Herve et al., 1995). Elevation in cytosolic cAMP levels leads to the activation of protein kinase A (PKA). PKA has a variety of intracellular targets that affect cellular excitability. For example, PKA can regulate glutamate receptor trafficking via the phosphoprotein DARPP-32, the tyrosine kinase Fyn or the protein phosphatase striatal-enriched tyrosine phosphatase (STEP) (Braithwaite et al., 2006; Hallett et al., 2006; Lee et al., 2002; Scott et al., 2006; Snyder et al., 2000). Although slightly less clear, D₁ receptor activation may also directly enhance NMDA receptor currents, via L-type voltage-gated calcium channels (Blank et al., 1997; Cepeda et al., 1993; Liu et al., 2004). In addition, D₁ receptor activation has several other consequences on the milieu of conductances being integrated during cellular activity, such as reducing Na⁺ channel (likely Nav1.1) conductivity and inhibiting N-type voltage-gated calcium channels (Carr et al., 2003; Kisilevsky et al., 2008; Scheuer and Catterall, 2006; Surmeier and Kitai, 1993). Such actions of D₁ receptors are consistent with the classical notion of D₁ receptor signalling as ‘excitatory’.

D₂ receptors couple to G_i/o proteins, leading to inhibition of adenylyl cyclase through Gα_i subunits (Stoof and Kebabian, 1984). In parallel, released Gbγ subunits are capable of reducing Cav2 Ca²⁺ channel opening and of stimulating phospholipase Cβ isoforms, generating diacylglycerol (DAG) andprotein kinase C (PKC) activation as well as inositol trisphosphate liberation and the mobilization of intracellular Ca²⁺ stores (Hernandez-Lopez et al., 2000; Nishi et al., 1997). D₂ receptors also are capable of transactivating tyrosine kinases (Kotecha et al., 2002).

Studies of voltage-dependent channels are largely consistent with the proposition that D₂ receptors act to reduce the excitability of striatopallidal neurons and their response to glutamatergic synaptic input. Activation of D₂ receptors decreases α-amino-3-hydroxy-5-methyl-4-isoxalone propionic acid (AMPA) receptor currents in MSNs (Cepeda et al., 1993; Hernandez-Echeagaray et al., 2004) and diminishes pre-synaptic glutamate release, although it is unclear if the latter involves pre- or post-synaptically situated D₂ receptors (Bamford et al., 2004; Yin and Lovinger, 2006). D₂ receptor activation has also been shown to negatively modulate Cav1.3 Ca²⁺ channels through a calcineurin-dependent mechanism (Hernandez-Lopez et al., 2000; Olson et al., 2005), reduce opening of voltage-dependent Na⁺ channels (presumably by a PKC-mediated enhancement of slow inactivation) (Surmeier and Kitai, 1993) and promote the opening of K⁺ channels (Greif et al., 1995). Such actions of D₂ receptors are consistent with the classical notion of D₂ receptor signalling as ‘inhibitory’.

Given the consequences DA has on MSN excitability, post-synaptic response to glutamate and pre-synaptic glutamatergic release, it is not a large conceptual leap to assume that it may play a role in corticostriatal synaptic plasticity. Indeed, the pioneering work of Calabresi and others (1992) utilized rodent tissue slices containing cortex and striatum to demonstrate long-term depression (LTD) in striatal MSNs and pointed to the importance of DA in governing its induction. We have recently made great progress in elucidating the role of DA in both LTD and long-term potentiation (LTP) induction in striatal MSNs. This work will be a focus of the remainder of this chapter.

LTD at glutamatergic synapses on MSNs

The easiest form of synaptic plasticity to see at MSN glutamatergic synapses is LTD (Calabresi et al., 2000). Unlike the situation at many other synapses, striatal LTD induction requires pairing of post-synaptic depolarization with moderate to high-frequency afferent stimulation at physiological temperatures (Calabresi et al., 2000; Kreitzer and Malenka, 2005). Typically for the induction to be successful, post-synaptic L-type calcium channels and G_q-linked mGluR5 receptors need to be co-activated (Kreitzer and Malenka, 2005; Lovinger et al., 1993). Both L-type calcium channels and mGluR5 receptors are found near glutamatergic synapses on MSN spines, making them capable of responding to local synaptic events (Carter and Sabatini, 2004; Carter et al., 2007; Day et al., 2006; Testa et al., 1994). The interaction between these two membrane proteins in the process of LTD induction undoubtedly involves calcium. Recent work showing that prolonging the opening of L-type channels with an allosteric modulator eliminates the need to stimulate mGluR5 receptors (Adermark and Lovinger, 2007), points to shared regulation of dendritic calcium concentration. However, there is an asymmetry, as increasing mGluR5 activation by bath application of agonists does not eliminate the need for L-type calcium channel opening (Kreitzer and Malenka, 2005; Ronesi et al., 2004). This might reflect a requirement for calcium-induced calcium release (CICR) from intracellular stores in LTD induction. In many cell types, CICR depends upon calcium influx through voltage-gated calcium channels, including L-type channels (Nakamura et al., 2000). Activation of mGluR5 and the production of inositol-1,4,5-triphosphate (IP3) could serve to prime these dendritic calcium stores, boosting CICR evoked by activity-dependent calcium entry through L-type calcium channels and thus promoting LTD induction (Berridge, 1998; Taufiq Ur et al., 2009; Wang et al., 2000).

A key event in the induction of LTD is the post-synaptic generation of endocannabinoids (ECs). ECs diffuse retrogradely to activate pre-synaptic CB1 receptors and decrease glutamate release probability. Having both pre- and post-synaptic induction criteria confers synaptic specificity on LTD expression (Singla et al., 2007). The molecular identity of the metabolic pathway leading to EC production in MSNs is still uncertain. There are two abundant striatal ECs: anandamide and 2-arachidonylglycerol (2-AG). Although previous studies have underscored the neural regulation of anandamide synthesis in the striatum (Giuffrida et al., 1999), collateral support for it has been modest (Ade and Lovinger, 2007). Recent work has provided compelling support for the proposition that 2-AG and its synthetic enzyme diacylglycerol lipase α (DAGLα) are essential (Gao et al., 2010; Lerner et al., 2010; Tanimura et al., 2010). The door is still slightly open for anandamide however. In Lerner et al.'s elegant and focused study, they found that inhibition of DAGLα was effective in preventing LTD induction only in response to moderate frequency afferent stimulation, not to higher frequency stimulation (~100 Hz). Why this would be is unclear. Both DAGLα and phospholipase D (PLD) are calcium-stimulated enzymes (Brenowitz et al., 2006). It could be that PLD requires a greater elevation in post-synaptic calcium concentration that would come with higher frequency afferent stimulation.

One still unresolved question about the induction of striatal LTD is whether activation of D₂ receptors is necessary. Activation of D₂ receptors is a potent stimulus for anandamide production (Giuffrida et al., 1999). However, recent work showing the sufficiency of L-type channel opening in EC-dependent LTD (Adermark and Lovinger, 2007), makes it clear that D₂ receptors play a modulatory – not obligatory – role. The real issue is the role of D₂ receptors in LTD induction using synaptic stimulation. Attempts to address this question using BAC mice have consistently found that in D₂ receptor expressing striatopallidal MSNs, D₂ receptor activation seems to be necessary (Kreitzer and Malenka, 2007; Shen et al., 2008; Wang et al., 2006). This could be due to the need to suppress A2a adenosine receptor signalling that could impede efficient EC synthesis and LTD induction (Fuxe et al., 2007a, 2007b; Shen et al., 2008). Indeed, Lerner et al. demonstrate quite convincingly that antagonism of A2a receptors promotes EC-dependent LTD induction in striatopallidal MSNs (Lerner et al., 2010).

Is EC-dependent LTD inducible in the other major population of MSNs that do not express D₂ receptors – the D₁ receptor dominated striatonigral MSNs? Kreitzer and Malenka (2007) reported that LTD was not inducible in these MSNs using a minimal local stimulation. This result was confirmed subsequently (Fig. 2) (Shen et al., 2008). However, using macroelectrode stimulation, EC-dependent LTD is readily inducible in identified D₁ MSNs (Wang et al., 2006), consistent with the high probability of MSN LTD induction seen in previous work (Calabresi et al., 2007). Thus, the stimulation paradigm seems critical to LTD induction in D₁ MSNs. Why? The problem with these induction protocols is that the type of axon and cell activated by the electrical stimulus is poorly controlled. With intra-striatal stimulation or with nominal white matter stimulation in coronal brain slices, glutamatergic afferent fibres, dopaminergic fibres and fibres intrinsic to the striatum are all activated, producing a mixture of neuromodulators that makes the interpretation of results less than straightforward. In Kreitzer and Malenka's case, minimal local stimulation of both dopaminergic and glutamatergic fibres appears to be critical to the LTD induction failure, as blocking D₁ receptors unmasked a robust EC-dependent LTD in D₁ MSNs (Shen et al., 2008), establishing a clear parallel to the A2a receptor phenomenon described by Lerner et al. (Lerner et al., 2010). This kind of complication also appears to be responsible for the apparent D₂ receptor dependence of LTD induction in D₁ MSNs using macroelectrodes that more effectively activate cholinergic interneuron axons (Wang et al., 2006).

The neuromodulator mixture created by non-specific electrical stimulation could also be a factor in slice studies implicating nitric oxide (NO) signalling in LTD induction (Calabresi et al., 1999). The principal sources of NO in the striatum are NO synthase expressing interneurons and endothelial cells. Experiments by Sergeeva et al. (2007) implicate both neuronal and endothelial sources of NO in LTD. Calcium entry and stimulation by NO in both cell types presumably occur in response to elevation in extracellular glutamate. In nitric oxide synthase (NOS) interneurons, N-methyl-D-aspartate (NMDA) receptors are necessary for NOS activation (Ondracek et al., 2008). However, this creates a problem in that LTD induction in the dorsal striatum of adult rodents is not NMDA receptor dependent. Recent work by Sergeeva's group has shown that there is a developmental dependence to the signalling mechanisms responsible for NO production and LTD, with engagement of NMDA receptor-stimulated NO production being necessary for EC-dependent LTD induction only in juvenile rodents (Chepkova et al., 2009). This suggests that endothelial cells play a more pivotal role in the adult dorsal striatum. Another interesting aspect of the NO story is where it is acting. MSNs express very high levels of NO-stimulated soluble guanylyl cyclase and protein kinase G (Ariano, 1983). But these cells appear not to be the target of NO in LTD. Rather, it appears that the site of NO action is downstream of CB1 receptor activation, in the pre-synaptic terminal (Sergeeva et al., 2007). How this relates to Lovinger's evidence implicating pre-synaptic gene expression in the expression of LTD is unclear.

The lack of specificity in activating inputs to MSNs during the induction of plasticity also raises questions about the type of glutamatergic synapse being affected by EC-dependent LTD. Studies using nominal white matter or cortical stimulation in a coronal brain slices typically assume that the glutamatergic fibres being stimulated are of cortical origin, but very few of these fibres are left intact in this preparation (Kawaguchi et al., 1989). The thalamic glutamatergic innervation of MSNs is similar in magnitude to that of the cerebral cortex, perhaps constituting as much as 40% of the total glutamatergic input to MSNs, terminating on both shafts and spines (Smith et al., 2009; Wilson, 2004). As a consequence, it is not really known whether EC-dependent LTD is present at corticostriatal or thalamostriatal synapses or both. The localization of CB1 receptors on corticostriatal terminals, but not thalamostriatal terminals (Uchigashima et al., 2007), is consistent with the hypothesis that LTD is a corticostriatal phenomenon, but more definitive studies are needed. Cutting brain slices in planes that preserve cortical and/or thalamic connectivity is one way to sort this out (Ding et al., 2008; Kawaguchi et al., 1989; Smeal et al., 2007). But these approaches have limitations given the highly convergent nature of the glutamatergic input to MSNs (Wilson, 2004). Optogenetic approaches offer a powerful alternative strategy (Zhang et al., 2006) that would allow glutamatergic inputs from various cortical and thalamic regions to be dissected.

LTP at glutamatergic synapses on MSNs

Less is known about the mechanisms controlling induction and expression of LTP at glutamatergic synapses. Most of the work describing LTP at glutamatergic synapses has been done with sharp electrodes (either in vivo or in vitro), not with patch-clamp electrodes in brain slices that afford greater experimental control and definition of the cellular and molecular determinants of induction. However, there have been a number of studies using these approaches in the last few years that have made progress in characterizing LTP mechanisms.

Previous studies have argued that LTP induced in MSNs by pairing high-frequency stimulation of glutamatergic inputs, and post-synaptic depolarization depends upon co-activation of D₁ DA and NMDA receptors (Calabresi et al., 2007). The involvement of NMDA receptors in LTP induction is not controversial. What is controversial is the involvement of D₁ receptors. Robust expression of these receptors is only found in striatonigral MSNs, roughly half of the MSN population, making it difficult to understand how LTP induction could be universally dependent on them unless some rather complicated, indirect mechanism was involved. Again, the advent of BAC transgenic mice has provided a tool to sort this issue out. Using perforated patch recordings to preserve the intracellular milieu controlling the induction of synaptic plasticity, our group found that the induction of LTP at glutamatergic synapses was dependent on D₁ DA receptors only in striatonigral MSNs, not in D₂ receptor expressing striatopallidal MSNs (Fig. 2) (Flajolet et al., 2008; Shen et al., 2008). In D₂ MSNs, LTP induction required activation of A2a adenosine receptors. These receptors are robustly expressed in striatopallidal MSNs and have a very similar intracellular signalling linkage to that of D₁ receptors; that is, they positively couple to adenylyl cyclase and PKA. Acting through PKA, D₁ and A2a receptor activation leads to the phosphorylation of DARPP-32 and a variety of other signalling molecules, including MAPKs, linked to synaptic plasticity (Sweatt, 2004).

The nature of the co-operativity between NMDA receptors and D₁/A2a receptor signalling in the induction of LTP remains to be resolved. This interaction governs the timing dependence of spike timing-dependent plasticity (STDP) (Pawlak and Kerr, 2008; Shen et al., 2008). For example, when D₂ receptors in striatopallidal MSNs were blocked and A2a receptors were stimulated, pairing a short burst of post-synaptic spikes with an excitatory post synaptic potential (EPSP) 10 ms later led to LTP induction, whereas with normal G protein-coupled receptor stimulation this protocol invariably produced LTD. In contrast, in striatonigral MSNs, pairing post-synaptic spiking with a trailing pre-synaptic volley only produced LTD in the absence of D₁ receptor stimulation, suggesting that PKA signalling could abrogate LTD induction. Reversing the order of stimulation gave LTP only when D₁ receptors were stimulated and yielded LTD otherwise, arguing that PKA signalling not only could shut down LTD induction, but was also necessary for LTP induction. Conceptually similar results have been reported in other cell types (Seol et al., 2007; Tzounopoulos et al., 2007), leading to the notion that LTD and LTP induction are governed by ‘opponent processes’ that interact at synaptic sites to determine the sign of synaptic plasticity. Altered activation of these processes could be responsible for ‘anti-Hebbian’ plasticity reported in the striatum (Fino et al., 2005). How these opponent processes interact with one another and the cellular mechanisms underlying changes in synaptic strength remains to be determined. Given the evidence that PKA signalling can potentiate NMDA receptor currents (Blank et al., 1997; Colwell and Levine, 1995), it is tempting to think that A2a and D₁ receptors promote LTP induction in this way. Molecules like regulator of calmodulin signalling (RCS), whose affinity for calmodulin and negative regulation of calcium signalling is dramatically elevated by PKA phosphorylation, could also contribute to the opponent interaction (Xia and Storm, 2005). The proposition that there is an LTD ‘cancelling’ signal arising from D₁ or A2a receptors but which requires some measure of co-operativity from NMDA receptor signalling (and CaMKII) would appear to be an economical solution to the plasticity problem, as it makes little sense to allow both processes to proceed independently. Another potential mediator of this interaction is STEP (Braithwaite et al., 2006). Activation of STEP promotes the endocytosis of both NMDA and AMPA receptors and is inactivated by PKA phosphorylation (Tashev et al., 2009; Zhang et al., 2008). Calcium activation of STEP also shortens ERK1/2 and Fyn kinase signalling, establishing a connection to striatal LTP (Dunah et al., 2004; Flajolet et al., 2008; Nguyen et al., 2002; Paul et al., 2003; Pelkey et al., 2002).

The nature of this interaction also has implications for the distal reward problem (Sutton and Barto, 1981). The change in DA release produced by the consequences of action selection occurs later in time than the pre- and post-synaptic activity that produced the action. In theoretical treatments of this issue, there are two strategies for dealing with this temporal delay or distal reward. One way is to have temporally co-incident pre- and post-synaptic activity create an eligibility trace (perhaps expressed as elevated CAMKII or calcineurin) that subsequently can be acted on by an outcome-dependent signal, in this case DA. However, if DA receptor signalling changes the impact of patterned synaptic stimulation on intracellular signalling cascades controlling the induction of plasticity, it is difficult to see how this could work. An alternative approach is to have repeating/reverberating activity or have the outcome event trigger a fictive replay of the action selection (Drew et al., 2006; Genovesio et al., 2006; Tsujimoto and Sawaguchi, 2004). As the corticostriatal pathway is the first leg of a multi-synaptic loop between the cortex, basal ganglia, thalamus and again cortex (Alexander and Crutcher, 1990), it is not hard to imagine how such an approach may work.

Dendritic excitability and synaptic plasticity

Although most of the induction protocols that have been used to study striatal plasticity are decidedly unphysiological, involving sustained, strong depolarization and/or high-frequency synaptic stimulation that induces dendritic depolarization, they do make the necessity of post-synaptic depolarization clear. In a physiological setting, what types of depolarization are likely to gate induction? One possibility is that spikes generated in the axon initial segment (AIS) propagate into dendritic regions where synapses are formed. Recent work has shown that STDP is present in MSNs (Fino et al., 2005; Pawlak and Kerr, 2008; Shen et al., 2008). But there are reasons to believe that this type of plasticity is relevant for only a subset of the synapses formed on MSNs. MSN dendrites are several hundred microns long, thin and modestly branched. Their initial 20–30 μm are largely devoid of spines and glutamatergic synapses. Glutamatergic synapse and spine density peak near 50 μm from the soma and then modestly decline with distance (Wilson, 2004). Because of their geometry and ion channel expression, AIS generated spikes rapidly decline in amplitude as they invade MSN dendrites (as judged by their ability to open voltage-dependent calcium channels), producing only a modest depolarization 80–100 μm from the soma. This is less than half the way to the dendritic tips (Day et al., 2008), arguing that a large portion of the synaptic surface area is not normally accessible to somatic feedback about the outcome of aggregate synaptic activity. High-frequency, repetitive somatic spiking improves dendritic invasion, but distal (>100 μm) synapses remain relatively inaccessible.

In the more distal dendritic regions, what controls plasticity? The situation in MSNs might be very similar to that found in deep layer pyramidal neurons where somatically generated bAPs do not invade the apical dendritic tuft (Golding et al., 2002). In this region, convergent synaptic stimulation is capable of producing a local calcium spike or plateau potential that produces a strong enough depolarization to open L-type calcium channels, to unblock NMDA receptors and promote plasticity. In vivo, convergent synaptic inputs to MSNs can trigger plateau potentials called up-states (Wilson and Kawaguchi, 1996). Although transitions from the resting down-state to the up-state have all the hallmarks of an active, regenerative process (e.g. stereotyped transition kinetics, a narrow range of up-state potentials), transitions are very difficult to manipulate with a sharp electrode impaling the somatic region (Wilson and Kawaguchi, 1996). This suggests that the site of up-state generation is in distal dendritic regions that cannot be easily manipulated. If this were the case, distal dendrites should have ionic conductances that could support a plateau. Calcium imaging using two-photon laser scanning microscopy (2PLSM) has shown that there is robust expression of both low-threshold Cav3 and Cav1 channels in MSN dendrites (Carter and Sabatini, 2004; Carter et al., 2007; Day et al., 2008), a result that has been confirmed using cell-type-specific gene profiling (Day et al., 2006) (unpublished observations). The rich investment of MSN dendrites with strongly rectifying Kir2 K⁺ channels also creates a favourable biophysical condition for plateau potential generation.

The question is how the plateaus or up-states are normally generated. Based on the sparse connectivity between individual cortical axons and MSNs (Kincaid et al., 1998; Wilson, 2004), modelling studies have concluded that several hundred pyramidal neurons need to be near simultaneously active for a sufficient amount of current to be injected into dendrites for an up-state to be generated (Stern et al., 1997; Wilson, 2004). These studies have assumed that MSN dendrites are passive. However, if dendrites are not passive but active, then the convergence requirements could be dramatically different. Although glutamate uncaging experiments at proximal spines have not revealed regenerative processes (Carter et al., 2007), the situation could be different at more distal locations. If this is the case, spatial convergence of glutamatergic inputs onto a distal dendrite could induce a local plateau potential capable of pulling the rest of the cell into the up-state, fundamentally altering the impact of synaptic input on other dendrites. This is a way in which spatially convergent excitatory input to one dendrite could gate synaptic input to another. The lack of temporal correlation between up-state transitions and EPSP-driven spike generation is consistent with a scenario like this one (Stern et al., 1998). If this were how MSNs operated, it would fundamentally change our models of striatal information processing.

In vivo studies of striatal synaptic plasticity have provided an important counterpoint to the perspectives based on reduced in vitro preparations. The pioneering work of (Charpier and Deniau, 1997) demonstrated that with more intact input, LTP was readily inducible in MSNs, contrary to the prevailing model. More recently, Stoetzner et al. have shown that the sign of synaptic plasticity in MSNs is influenced by anaesthetic and presumably the degree of cortical synchronization in corticostriatal projections (Stoetzner et al., 2010). In particular, they show that in barbiturate anaesthetized rats, 5 Hz stimulation of motor cortex evokes LTP in the striatum, but that in awake animals the same stimulation induced LTD. A challenge facing the field is how to bridge these observations. Because glutamatergic connections are sparse, it is virtually impossible to reliably stimulate a collection of synapses onto a particular MSN dendrite with an electrode in a brain slice. Optogenetic techniques might provide a feasible alternative strategy. Another strategy would be to employ two-photon laser uncaging (2PLU) of glutamate at visualized synaptic sites (Carter and Sabatini, 2004). These tools are becoming more widely available and should allow the regenerative capacity of MSN dendrites to be tested soon. If it turns out to be the case that up-states are locally generated in dendrites, then it also becomes feasible to characterize their role in the induction of synaptic plasticity. Up-states could be sufficient, as in the apical tuft of pyramidal neurons, or they could simply be necessary by promoting back-propagation of spikes into the distal dendrites (Kerr and Plenz, 2002).

Homeostatic plasticity in PD models

Sorting out how DA regulates synaptic plasticity in striatal MSNs has obvious implications for disease states that are triggered by alterations in the function of dopaminergic neurons. Second in prominence among DA-dependent disorders only to drug abuse, PD is a common neurodegenerative disorder whose motor symptoms are attributable largely to the loss of dopaminergic neurons innervating the dorsal striatum. In the prevailing model, the excitability of the two major populations of MSNs shifts in opposite directions following DA depleting lesions, creating an ‘imbalance’ in the regulation of the motor thalamus favouring suppression of movement (Albin et al., 1989; Wichmann and DeLong, 1996). In particular, D₂ receptor expressing striatopallidal MSNs spike more, whereas D₁ receptor expressing striatonigral MSNs spike less in the PD state. The mechanisms underlying this shift were not known at the time the model was formulated, but have widely been assumed to reflect changes in intrinsic excitability that accompanied loss of inhibitory D₂ receptor signalling and excitatory D₁ receptor signalling. Indeed, studies by our group and others have found electrophysiological support for this view (Mallet et al., 2006; Surmeier et al., 2007).

What about synaptic remodelling? Several studies have suggested that in the absence of DA, synaptic plasticity is lost, essentially ‘freezing’ the striatal circuit in its pre-depleted state (Calabresi et al., 2007; Kreitzer and Malenka, 2007). However, recent studies of defined MSN populations have shown that although DA is necessary for plasticity to be bidirectional and Hebbian, it is not necessary for the induction of plasticity per se (Shen et al., 2008). Following DA depletion, pairing pre-synaptic and post-synaptic activity – regardless of which came first – induced LTP in D₂ MSNs and LTD in D₁ MSNs. This result adds a new dimension to the prevailing model by showing that activity-dependent changes in synaptic strength parallel those of intrinsic excitability following DA depletion. Work in vivo examining the responsiveness of anti-dromically identified MSNs to cortical stimulation following unilateral lesions of the striatal dopaminergic innervation is consistent with this broader model (Mallet et al., 2006).

But this poses a problem. Neurons are homeo-static; sustained perturbations in synaptic or intrinsic properties that make neurons spike more or less than their set-point engage homeostatic mechanisms that attempt to bring activity back to the desired level (Marder and Goaillard, 2006; Turrigiano, 1999). One of the most common mechanisms of homeostatic plasticity is to alter synaptic strength or to scale synapses. In striatopallidal MSNs, the elevation in activity following DA depletion triggers a dramatic down-regulation of glutamatergic synapses formed on spines (Fig. 3) (Day et al., 2006). This can be viewed as a form of homeostatic plasticity. Like scaling seen in other cell types, the synaptic modification depends upon calcium entry through voltage-dependent L-type calcium channels that presumably report activity levels.

In an attempt to better characterize the homeo-static mechanisms controlling synapse density in MSNs, striatum from transgenic mice expressing a D₂ receptor reporter construct was co-cultured with wild-type cerebral cortex. In these co-cultures, MSN dendrites develop nearly normal spine density with pre-synaptic glutamatergic terminals (Fig. 4) (Segal et al., 2003; Tian et al., 2010). Sustained (>3 h) depolarization induced a pruning of glutamatergic synapses and spines in striatopallidal MSNs. This pruning was antagonized by dihydropyridines, implicating L-type calcium channels as with DA depletion (Fig. 4) (Day et al., 2006; Neely et al., 2007; Segal et al., 2003). However, unlike the situation in vivo, L-type channels with a Cav1.3 pore-forming subunit were not necessary, but rather ones with a Cav1.2 subunit. It could be that this reflects some abnormality in the cultured MSNs. But it seems more likely that this difference is a reflection of local and global mechanisms underlying spine pruning. In vivo, low-threshold Cav1.3 channels are located near glutamatergic synapses where they are capable of being activated by synaptic depolarization. Their activation could be important to effective propagation of synaptic depolarization to the soma. Thus, eliminating or antagonizing Cav1.3 channels should attenuate the synaptic consequences of DA depletion, mitigating the homeostatic drive. High-threshold Cav1.2 channels appear to be largely somatic where they report spiking (or strong depolarization). In our experiments, by bath application of elevated potassium, the normal dendritic synaptic mechanisms were bypassed and somatic Cav1.2 channels directly activated. These channels have been implicated in other forms of homeostatic synaptic plasticity induced by global alterations in excitability or synaptic activity (Turrigiano, 1999).

In MSNs, calcium entry through Cav1.2 L-type calcium channels triggered a signalling cascade that led to a transcriptionally dependent spine pruning. The first step in this cascade was activation of the calcium-dependent protein phosphatase calcineurin. Calcineurin dephosphorylates myocyte enhancer factor 2 (MEF2) (Flavell et al., 2006), increasing its transcriptional activity. As in other neurons (Flavell et al., 2006; Shalizi et al., 2006), MEF2 up-regulation increased the expression of two genes linked to synaptic remodelling – Nur77 and Arc (Fig. 5). These experiments establish a translational framework within which adaptations in striatal synapses that are linked to the symptoms of PD can be explored.

There are other recently described network adaptations relevant to homeostatic plasticity in PD models. For example, although feed-forward inhibition through fast spiking GABAergic interneurons does not appear to be directly altered, low-threshold GABAergic interneurons do elevate their input to at least a subset of MSNs in PD models (Dehorter et al., 2009; Mallet et al., 2005). Recurrent collateral inhibition between MSNs, which is normally strongest between D₂ MSNs, is almost abolished following DA depletion (Taverna et al., 2008). These adaptations in conjunction with enhanced striatopallidal MSN excitability are likely to contribute to the transmission of beta band activity from the cortex through the striatum to the globus pallidus (Murer et al., 2002).

A major gap in the existing literature is a description of the intrinsic changes in MSN excitability following prolonged DA depletion. All the work with identified cell types has relied on short-term (~<1 week) DA depletions (e.g. Day et al., 2006; Kreitzer and Malenka, 2007), but there clearly are slower adaptations that take 3–4 weeks to stabilize. Given the robust differences in the anatomy and intrinsic physiology of striatonigral and striatopallidal MSNs that exist in the normal striatum (Gertler et al., 2008), it is easy to conjecture that these resting differences are due to differential regulation of basal excitability by DA. If that were true, losing DA could trigger homeostatic processes that make MSNs much more alike.