Reliable long-lasting depression interacts with variable short-term facilitation to determine corticostriatal paired-pulse plasticity in young rats

Abstract

Synaptic plasticity at corticostraital synapses is proposed to fine tune movment and improve motor skills. We found paired-pulse plasticity at corticostriatal synapses reflected variably expressed short-term facilitation blended with a consistent background of longer-lasting depression. Presynaptic modulation via neuotransmitter receptor activation was ruled out as a mechanism for long-lasting paired-pulse depression by examining the effect of selective receptor antagonists. EPSC amplitude and paired-pulse plasticity, however, was influenced by block of D2 dopamine receptors. Block of glutamate transport with l-transdicarboxylic acid (PDC) reduced EPSCs, possibly through a mechanism of AMPA receptor desensitization. Removal of AMPA receptor desensitization with cyclothiazide reduced the paired-pulse depression at long-duration interstimulus intervals (ISIs), indicating that AMPA receptor desensitization participates in corticostriatal paired-pulse plasticity. The low-affinity glutamate receptor antagonist cis-2,3-piperidine dicarboxylic acid (PDA) increased paired-pulse depression, suggesting that a presynaptic component also exists for long-lasting paired-pulse depression. Low Ca(2+)-high Mg(2+) or BAPTA-AM dramatically reduced the amplitude of corticostriatal EPSCs and both manipulations increased the expression of facilitation and, to a lesser extent, they reduced long-lasting paired-pulse depression. EGTA-AM produced a smaller reduction in EPSC amplitude and it did not alter paired-pulse facilitation, but in contrast to low Ca(2+) and BAPTA-AM, EGTA-AM increased long-lasting paired-pulse depression. These experiments suggest that facilitation and depression are sensitive to vesicle depletion, which is dependent upon changes in peak Ca(2+) (i.e. low Ca(2+)-high Mg(2+) or BAPTA-AM). In addition, the action of EGTA-AM suggests that basal Ca(2+) regulates the recovery from long-lasting paired-pulse depression, possibly thourgh a Ca(2+)-sensitive process of vesicle delivery.

Figure 1. Paired-pulse plasticity at corticostriatal synapses

A, superimposed paired responses for the ISIs indicated. Cells were voltage clamped at the resting membrane potential of −70 mV. B, paired-pulse ratio (EPSC₂/EPSC₁) is plotted as a function of ISI (the ISI is plotted on a logarithmic scale). Each time point reflects the average of 3 consecutive pairs separated by an inter-trial interval of 30 s. •, the average obtained from 25 cells. ⋄ and ○, individual examples plotted to illustrate the range of paired-pulse plasticity observed at ISIs shorter than 1 s. C, frequency histograms of paired-pulse ratio measured at ISIs of 50 and 500 ms. These two intervals were selected to represent peaks of facilitation (50 ms) and depression (500 ms) at corticostriatal synapses. The large number of cells in this sample comes from including the many ‘controls’ accumulated during each phase of pharmacological testing. The paired-pulse ratio bin size for the abscissa is 5%. The average paired-pulse ratio measured at 50 ms was 89.9% (range: 48.7–141.1%) and at 500 ms was 71.9% (range: 50.9–94.3%). D, frequency histograms separating cells into three categories of PPR at the 50 ms ISI: depression (PPR > 95%), no change (95% < PPR < 105%) and facilitation (PPR > 105%). Histograms are illustrated for all cells (n = 155) and then separated based upon postnatal age. P9–P11 (n = 29), P12–P15 (n = 65) and P16–P19 (n = 61).

Figure 2. Adenosine receptor activation does not contribute to the paired-pulse plasticity created by activation of corticostriatal synapses in vitro

A, representative current traces illustrating adenosine-mediated reductions of EPSC amplitude and increases in the paired-pulse ratio (PPR) measured at 50 and 500 ms ISIs. B, bar graph illustrating how adenosine (50 μm) reduced EPSC amplitudes to 26.9 ± 3.1% of control (P < 0.003, n = 6). PPR at an ISI of 50 ms changed from 91.6 ± 8.4% to 237 ± 28.2% of control (P < 0.002) and the PPR at an ISI of 500 ms changed from 71.8 ± 3.3% to 116.6 ± 10.4% of control (P < 0.02, n = 6). C, treatment induced change in EPSC amplitude and PPR (50 and 500 ms). Measures of EPSC amplitude and PPR for 50 and 500 ms were obtained before and after each adenosine-related treatment. Post-treatment measures were normalized to pretreatment measures and illustrated as the percentage change in each value created by the treatment (EPSC amplitude and PPR for 50 and 500 ms ISIs). Each cell was exposed to a single drug application condition (i.e. theophylline alone, adenosine + theophylline, etc.) As shown in B, adenosine had a clear effect on EPSC amplitude and the PPR at 50 and 500 ms. The adenosine effects were blocked with the non-selective antagonist theophylline (100 μm) and the selective A1 receptor antagonist DPCX (0.5 μm). Addition of either antagonist alone did not affect EPSC amplitude or paired-pulse plasticity.

Figure 3. GABA_B receptor activation does not contribute to the paired-pulse plasticity created by activation of corticostriatal synapses in vitro

A, representative current traces illustrating baclofen-mediated reductions of EPSC amplitude and increases in the PPR measured at 50 and 500 ms ISIs. Baclofen (5 μm) reduced EPSC amplitude to 10.7 ± 1.7% of control (P < 0.002, n = 5). PPR at an ISI of 50 ms changed from 87.9 ± 8.6% to 186 ± 30.4% (P < 0.02) and the PPR at an ISI of 500 ms changed from 74.8 ± 3.6% to 132.0 ± 16.5% (P < 0.03, n = 5). B, GABA_B receptor-related treatment outcomes on EPSC amplitude and PPR at 50 and 500 ms ISIs are plotted as a percentage of these measures obtained prior to the indicated pharmacological treatment. Baclofen effects were partially blocked by the selective GABA_B receptor antagonists saclofen (500 μm) and CGP 35348 (500 μm). Addition of saclofen (P < 0.05, n = 5) or CGP 35348 (P < 0.05, n = 3) alone both reduced the amplitude of EPSCs, but they did not affect paired-pulse plasticity.

Figure 4. Analysis of mGluR modulation of corticostriatal EPSCs

Addition of the mGluR agonist t-ACPD (5 μm) reduced the amplitude of the EPSC (P < 0.01) and increased the PPR at 50 ms (P < 0.02) and 500 ms (P < 0.03) (n = 5). Addition of t-ACPD (5 μm) in combination with the mGluR antagonist MCPG (500 μm) partially blocked the effects of t-ACPD (n = 5). Addition of MCPG alone (500 μm) had no effect (n = 5). The selective mGluR2/3 receptor antagonist MSPG (100 μm) did not have an effect by itself and it was ineffective in blocking the action of t-ACPD.

Figure 5. D2 dopamine receptor contribution to the paired-pulse plasticity created by activation of corticostriatal synapses in vitro

A, bar graphs represent the percentage change in EPSC amplitude and PPR for 50 and 500 ms ISIs created by dopamine-related treatments. Control values for EPSC amplitude and PPR were obtained in each cell before and after each treatment and plotted as percentage of control (each treatment referenced to its own control). Dopamine (20 μm) reduced the EPSC amplitude (n = 5). Addition of the D2 receptor antagonist sulpiride in combination with dopamine did not fully block the effect dopamine had on EPSC amplitude (n = 5). Addition of sulpiride alone caused an increase in the 50 ms ISI PPR (P < 0.05, n = 5). A similar trend was seen with addition of the selective D2 receptor antagonist raclopride (3 μm), which reduced EPSC amplitude and increased the PPR at 50 and 500 ms (P < 0.05, paired t tests; n = 6). B, cumulative frequency histograms for sEPSC frequency. Ba, example of a raclopride (3 μm) induced increase in sEPSC frequency. Raclopride changed the average sEPSC frequency from 1.53 to 2.55 events s⁻¹. Bb, example of raclopride induced decrease in sEPSC frequency. Raclopride changed the average sEPSC frequency from 3.97 to 1.68 events s⁻¹. Bc, average cumulative histograms for sEPSC frequency before and after addition of raclopride (n = 5).

Figure 6. G-protein activation modulates corticostriatal paired-pulse plasticity

A, the G-protein activator NEM was added to brain slices. Representative current traces are shown for NEM-mediated increases in EPSC amplitude and decreases in the paired-pulse ratio (PPR) measured at 50 and 500 ms ISIs. B, NEM (200 μm) increased EPSC amplitude (P < 0.03) and reduced the PPR at 50 ms (P < 0.003) and 500 ms (P < 0.012) (n = 4). Pretreatment of slices with NEM was effective in blocking the change in EPSC created by adenosine. The effect of adenosine alone is shown for comparison. The adenosine + NEM responses are compared with those responses obtained first in NEM alone (adenosine + NEM/NEM).

Figure 7. Glutamate receptor desensitization contributes to paired-pulse depression at corticostriatal synapses

A, representative current traces before and after addition of the AMPA receptor desensitization reducing compound cyclothiazide (100 μm). Cyclothiazide increased corticostriatal EPSC duration, which is consistent with the ability of cyclothiazide to remove desensitization of glutamate receptors. Cyclothiazide also reduced the paired-pulse depression seen at an ISI of 500 ms (P < 0.05, n = 6). The second EPSC evoked at an ISI of 50 ms started from an elevated baseline due to the removal of desensitization during the first response of the pair. Control responses are superimposed for comparison. B, the glutamate uptake inhibitor PDC (100 μm) reduced the EPSC amplitude (P < 0.03), but it did not affect the PPR at 50 ms and 500 ms (n = 8).

Figure 8. Increased block of the 500 ms ISI paired EPSP by the low-affinity AMPA receptor antagonist PDA

A, bath application of 1 mm PDA decreased EPSC amplitude to 45.30 ± 4.96% (P < 0.02, n = 5) and the PPR evoked at an ISI of 500 ms from 73.91 ± 5.77% to 62.76 ± 6.06% (P < 0.003, n = 5). B, representative EPSCs evoked with the 500 ms ISI pairing before (a) and after (b) bath application of PDA. The first EPSC amplitudes are normalized in c to illustrate the increased block of the second EPSC by PDA. APV (50 μm) was included to eliminate NMDA receptor activation, since PDA is a weak NMDA agonist. The identical experiment performed with the high-affinity antagonist CNQX (1 μm) blocked the first and second EPSC of the pair equivalently and no change in paired-pulse plasticity was observed.

Figure 9. Ca²⁺ differentially regulates paired-pulse facilitation and depression

The Ca²⁺ sensitivity of paired-pulse facilitation and depression were examined by exposing corticostriatal synapses to a low Ca²⁺–high Mg²⁺ external solution, BAPTA-AM or EGTA-AM. All experiments were performed using a ‘paired statistical design’ where control and post-Ca²⁺ treatment measures were obtained sequentially from the same set of synapses. ISIs of 50 and 500 ms were tested to represent paired-pulse facilitation and depression, respectively. A, representative current traces for changes in corticostriatal paired-pulse plasticity occurring in response to changing the extracellular solution from 2.4 mm Ca²⁺–1.3 mm Mg²⁺ to 0.5 mm Ca²⁺–3.2 Mg²⁺ (maintains osmolarity), application of BAPTA-AM (50 μm), and application of EGTA-AM (50 μm). B, EPSC amplitude and paired-pulse plasticity measured after each Ca²⁺ manipulation are ‘normalized’ to the value measured prior to perfusion with the experimental solution. The low-Ca²⁺ solution reduced the corticostriatal EPSC to 33.9% of control (P < 0.015; paired t test, n = 5) and the PPR at an ISI of 50 ms increased from 92.6 ± 7.7% to 160 ± 10.1% (P < 0.015; paired t test, n = 5). By contrast, the paired-pulse plasticity evoked at an ISI of 500 ms was less sensitive to the change in extracellular Ca²⁺ with the PPR going from 80.1 ± 4.1% to 105.3 ± 10.1% (P = 0.076, paired t test, n = 5). BAPTA-AM (50 μm) reduced the EPSC amplitude to 32.4% of control (P < 0.00001, paired t test, n = 8) and the PPR measured at an ISI of 50 ms increased from 89.3% to 116.4% (P < 0.015, paired t test, n = 8) and the PPR measured at an ISI increased from 74% to 104% (P < 0.03, paired t test, n = 8). EGTA-AM (50 μm) reduced the amplitude of corticostriatal EPSCs to 77% of control (P < 0.04, paired t test, n = 8), but it did not cause a significant difference in the PPR measured at 50 ms where the PPR changed from 80 ± 3% to 88 ± 4% (P = 0.09, paired t test, n = 8). By contrast, EGTA-AM reduced the PPR measured at 500 ms from 78.1% to 61% (P < 0.002, paired t test, n = 8). The effect of EGTA at the 500 ms ISI was completely opposite to the increase in PPR produced by either BAPTA or low extracellular Ca²⁺.