Cyclic AMP and afferent activity govern bidirectional synaptic plasticity in striatopallidal neurons

Abstract

Recent experimental evidence suggests that the low dopamine conditions in Parkinson's disease (PD) cause motor impairment through aberrant motor learning. Those data, along with computational models, suggest that this aberrant learning results from maladaptive corticostriatal plasticity and learned motor inhibition. Dopaminergic modulation of both corticostriatal long-term depression (LTD) and long-term potentiation (LTP) is proposed to be critical for these processes; however, the regulatory mechanisms underlying bidirectional corticostriatal plasticity are not fully understood. Previously, we demonstrated a key role for cAMP signaling in corticostriatal LTD. In this study, mouse brain slices were used to perform a parametric experiment that tested the impact of varying both intracellular cAMP levels and the strength of excitatory inputs on corticostriatal plasticity. Using slice electrophysiology in the dorsolateral striatum, we demonstrate that both LTP and LTD can be sequentially induced in the same D2-expressing neuron and that LTP was strongest with high intracellular cAMP and LFS, whereas LTD required low intracellular cAMP and high-frequency stimulation. Our results provide a molecular and cellular basis for regulating bidirectional corticostriatal synaptic plasticity and may help to identify novel therapeutic targets for blocking or reversing the aberrant synaptic plasticity that likely contributes to motor deficits in PD.

Keywords: LTD; LTP; cAMP; dorsolateral striatum; motor learning; synaptic plasticity.

Figure 1.

Corticostriatal LTD in striatopallidal MSNs in dorsolateral striatum. A, EPSC amplitudes were normalized to baseline, averaged, and plotted versus the time of the recording. HFS induces LTD (n = 12 cells/slices, t₍₁₁₎ = 3.35, p = 0.006), Inset, Representative traces before (solid line) and after (dashed line) HFS. Scale bar, 100 pA, 10 ms. B, There is an increase in the PPR from cells that responded to HFS with a significant depression of EPSC amplitude after HFS (n = 8 cells/slices, t₍₇₎ = −2.55, p = 0.038). A subset of cells did not show LTD, and the PPR from these cells were not different after HFS (n = 4 cells/slices, t₍₃₎ = −0.86, p = 0.452). C, D, HFS induces no LTD in the presence of the D2 receptor antagonist sulpiride (10 μm; n = 6 cells/slices, t₍₅₎ = −0.09, p = 0.93; C) and the PPR was not different in sulpiride-pretreated neurons (t₍₅₎ = −1.11, p = 0.32; D). E, F, HFS results in consistent LTD in the presence of the D2 receptor agonist quinpirole (10 μm; n = 6 cells/slices, t₍₅₎ = 9.09, p = 0.0002; E), HFS-induced LTD in quinpirole-treated neurons is accompanied by an increase in PPR (n = 6 cells/slices, t₍₅₎ = −2.44, p = 0.058; F). G, HFS induces LTD when the cell is dialyzed with the PLC inhibitor U73122 (10 μm) in the presence of extracellular quinpirole (10 μm; n = 6 cells/slices, t₍₅₎ = −3.25, p = 0.023). H, This LTD is accompanied by a trend toward an increase in PPR in the subset of cells that show LTD (n = 5 cells/slices, t₍₄₎ = −2.45, p = 0.07). The LTD magnitude (t₍₁₁₎ = 1.87, p = 0.09) and prevalence (5/6 with quinpirole and the PLC inhibitor vs 6/6 with quinpirole alone, p = 0.3 by χ²) were not different from control recordings without the PLC inhibitor.

Figure 2.

cAMP modulation of corticostriatal LTD. A, HFS induces LTD when the cell is dialyzed with 2 μm Sp-cAMP (n = 6 cells/slices, t₍₅₎ = 2.77, p = 0.04). Inset, Representative traces before (solid line) and after HFS (dashed line). Scale bar, 100 pA, 10 ms. B, HFS induces no change in synaptic strength with 20 μm Sp-cAMP in the recording pipette (n = 5 cells/slices, t₍₄₎ = 0.89, p = 0.43). C, LTD is also inhibited by 500 μm Sp-cAMP in the recording pipette (n = 5 cells/slices, t₍₄₎ = 0.37, p = 0.73). D, On average, there is no change in synaptic transmission when 100 μm Rp-cAMP is included within the patch pipette (n = 8 cells/slices, t₍₇₎ = 0.63, p = 0.55).

Figure 3.

cAMP modulation of corticostriatal potentiation. A, Schematic of LFS/depolarization protocol. B, 2 μm Sp-cAMP in the recording pipette paired with LFS results in a weak potentiation (n = 5 cells/slices, t₍₄₎ = −1.89, p = 0.13). Inset, Representative traces before (solid line) and after LFS (dashed line); Scale bar 100 pA, 10 ms. C, With 20 μm Sp-cAMP in the recording pipette, LFS induces a robust increase in the normalized peak amplitude (n = 7 cells/slices, t₍₆₎ = −5.85, p = 0.001). D, With 500 μm Sp-cAMP in the pipette, LFS induces a small potentiation (n = 5 cells/slices, t₍₄₎ = −1.77, p = 0.15). E, With the PKA inhibitor, Rp-cAMP (100 μm) in the recording pipette, LFS induces a potentiation of EPSC amplitudes (n = 6 cells/slices, t₍₅₎ = −4.06, p = 0.01).

Figure 4.

LFS and corticostriatal LTP. All neurons were dialyzed with 20 μm Sp-cAMP to promote potentiation, except in B. Arrow indicates the time of the conditioning stimulus. Inset, Representative traces before (solid line) and after conditioning stimulus (dashed line). In the case where no conditioning stimulus was given, the solid line represents the first 10 min and the dashed line represents the last 10 min of recording. Scale bar, 100 pA, 10 ms. A, LFS paired with depolarization induced robust LTP (n = 6 cells/slices, t₍₅₎ = −2.84, p = 0.04). B, In the absence of 20 μm Sp-cAMP, LFS/depolarization does not increase EPSC amplitude (n = 5 cells/slices, t₍₄₎ = −1.18, p = 0.30). C, In the absence of any stimulation, 20 μm Sp-cAMP in the internal solution did not induce a change in EPSC amplitude (n = 5 cells/slices, t₍₄₎ = −0.61, p = 0.58). D, Depolarization without LFS induced a weak trend toward LTP (n = 5 cells/slices, t₍₄₎ = −2.63, p = 0.06). E, Bath application of the NMDA antagonist d-AP5 (50 μm) inhibited LTP induction (n = 5 cells/slices, t₍₄₎ = 0.96, p = 0.39).

Figure 5.

cAMP modulation of HFS- and LFS-induced corticostriatal plasticity in the same D2-expressing MSNs. Shown is a single-cell example of EPSC amplitudes recorded after LFS and HFS with 2 μm (A) or 20 μm (B) Sp-cAMP administered via the recording pipette. Inset, Representative traces before (solid line) and after (dashed line) LFS/HFS. Scale bar, 100 pA, 10 ms. Note that LFS-induced potentiation data (C1, D1) are the same results presented in Figure 3, B and C. C1, LFS induces a weak potentiation with 2 μm Sp-cAMP in the recording pipette. C2, HFS stimulation, administered 10 min after the LFS, induces robust LTD (n = 5 cells/slices, t₍₄₎ = 6.66, p = 0.003). C3, Average AMPA/NMDA ratio 21 min from the start of the recording (t₍₄₎ = −3.56, p = 0.02). Inset, Representative traces of AMPA and NMDA EPSC. Scale bar 50 pA, 10 ms. Vertical dotted lines indicate the sampling window after the decay of the AMPA receptor-mediated EPSC, when the NMDA EPSC amplitude was assessed. D1, With 20 μm Sp-cAMP in the recording pipette, LFS induces a robust potentiation. D2, Subsequent HFS has no effect of EPSC amplitude (n = 7 cells/slices, t₍₆₎ = −0.03, p = 0.97). D3, This potentiation of EPSC amplitude is accompanied by an increase in the AMPA/NMDA ratio (21 min after the beginning of the recording; t₍₆₎ = −2.60 p = 0.04).