Facilitation of corticostriatal transmission following pharmacological inhibition of striatal phosphodiesterase 10A: role of nitric oxide-soluble guanylyl cyclase-cGMP signaling pathways

Abstract

The striatum contains a rich variety of cyclic nucleotide phosphodiesterases (PDEs), which play a critical role in the regulation of cAMP and cGMP signaling. The dual-substrate enzyme PDE10A is the most highly expressed PDE in striatal medium-sized spiny neurons (MSNs) with low micromolar affinity for both cyclic nucleotides. Previously, we have shown that systemic and local administration of the selective PDE10A inhibitor TP-10 potently increased the responsiveness of MSNs to cortical stimulation. However, the signaling mechanisms underlying PDE10A inhibitor-induced changes in corticostriatal transmission are only partially understood. The current studies assessed the respective roles of cAMP and cGMP in the above effects using soluble guanylyl cyclase (sGC) or adenylate cyclase (AC) specific inhibitors. Cortically evoked spike activity was monitored in urethane-anesthetized rats using in vivo extracellular recordings performed proximal to a microdialysis probe during local infusion of vehicle, the selective sGC inhibitor ODQ, or the selective AC inhibitor SQ 22536. Systemic administration of TP-10 (3.2 mg/kg) robustly increased cortically evoked spike activity in a manner that was blocked following intrastriatal infusion of ODQ (50 μm). The effects of TP-10 on evoked activity were due to accumulation of cGMP, rather than cAMP, as the AC inhibitor SQ was without effect. Consistent with these observations, studies in neuronal NO synthase (nNOS) knock-out (KO) mice confirmed that PDE10A operates downstream of nNOS to limit cGMP production and excitatory corticostriatal transmission. Thus, stimulation of PDE10A acts to attenuate corticostriatal transmission in a manner largely dependent on effects directed at the NO-sGC-cGMP signaling cascade.

Keywords: cGMP; medium-sized spiny neuron; nitric oxide; nitric oxide synthase; phosphodiesterase 10A; soluble guanylyl cyclase.

Figure 1.

Effect of systemic PDE10A inhibition on cortically evoked activity recorded in vivo: extracellular recordings proximal to a microdialysis probe. A, Representative traces of cortically evoked spike activity of a single-unit recorded during intrastriatal aCSF infusion. In each case, 10 superimposed traces of cortically evoked spike responses from a single striatal neuron recorded under control conditions are shown for each stimulus intensity (600, 800, and 1000 μA). Arrow indicates stimulus artifact. B, Corresponding peristimulus time histograms showing the response of the same striatal neuron to cortex stimulation delivered over 50 stimulation trials. The probability of evoking a spike increases, whereas the spike onset latency typically decreases, with increasing stimulus intensities. C, Systemic administration of TP-10 significantly increased the mean ± SEM probability of eliciting cortically evoked spike activity during aCSF infusion compared with vehicle-treated rats (*p < 0.05, **p < 0.01, ***p < 0.001, two-way ANOVA, Tukey post hoc test). A trend toward a decrease in the mean ± SEM onset latency of cortically evoked spike responses during aCSF infusion was observed following systemic treatment with TP-10 compared with vehicle-treated rats (^#p < 0.09). Results are based on n = 20–26 cells/8 or 9 rats per group.

Figure 2.

Effects of intrastriatal infusion of the selective sGC inhibitor ODQ and the selective AC inhibitor SQ 22536 on TP-10-mediated facilitation of cortically evoked activity. All cells were recorded proximal to a microdialysis probe during aCSF, ODQ (50 μm), or SQ 22536 (100 μm) infusion. A, Top, Local ODQ infusion did not affect the mean ± SEM probability of eliciting spike activity during cortical stimulation in vehicle-treated rats (p > 0.05). However, the same pretreatment (≥20 min) of ODQ blocked the TP-10-mediated increase in the mean ± SEM probability of eliciting spike activity during cortical stimulation (*p < 0.05, ***p < 0.001, two-way ANOVA, Tukey post hoc test compared with the TP-10/vehicle-treated group). Bottom, No significant changes in the mean ± SEM spike onset latency were observed following ODQ infusion in vehicle- or TP-10-treated groups (p > 0.05). Results are based on n = 19–26 cells/8–12 rats per group. B, Top, Local SQ 22536 infusion did not affect the TP-10-mediated increase in the mean ± SEM probability of eliciting spike activity during cortical stimulation (^#p = 0.054, *p < 0.05, ** p < 0.01, two-way ANOVA, Tukey post hoc test). Bottom, No significant changes in the mean ± SEM spike onset latency were observed following SQ 22536 infusion in vehicle- or TP-10-treated groups (p > 0.05). Results are based on n = 20–26 cells/8 or 9 rats per group.

Figure 3.

The effect of PDE10A inhibition with TP-10 on striatal concentrations of cGMP is absent in nNOS KO mice, whereas the elevation of cAMP is unaffected. C57BL/6J WT and nNOS KO mice were injected with vehicle or TP-10 (3.2 mg/kg, s.c.). Brain tissue measures of cGMP accumulation were performed 30 min after vehicle/drug injection (Schmidt et al., 2008). A, Systemic TP-10 administration induced a robust increase in cGMP levels in the striatum of WT mice. *p < 0.05 (two-way ANOVA, Bonferroni post hoc test). Striatal concentrations of cGMP were significantly decreased in vehicle-treated nNOS KO animals compared with age-matched WT controls. ^#p < 0.05 (two-way ANOVA, Bonferroni post hoc test). Moreover, striatal cGMP levels were not elevated by TP-10 treatment in nNOS KO mice and were significantly reduced compared with TP-10 treated WT mice. ^¥p < 0.05 (two-way ANOVA, Bonferroni post hoc test). B, TP-10 administration induced a robust increase in cAMP levels in the striatum of both WT and nNOS KO mice. *p < 0.05 (two-way ANOVA, Bonferroni post hoc test). No influence of genotype was observed (p > 0.05). Results are based on n = 6 mice per group.

Figure 4.

Decreased cortically evoked and spontaneous firing activity in MSNs recorded from nNOS KO mice. A, Representative electrode placements in frontal cortex and dorsal striatum of the mouse. Large black arrows indicate the termination sites of the stimulating electrode implanted in the cortex and the recording electrode implanted in striatum. B, Representative traces of cortically evoked spike activity of a single-unit recorded in the mouse. Ten superimposed traces of cortically evoked spike responses from a single striatal MSN are shown. Arrow indicates stimulation artifact. C, Top, Representative record demonstrating spontaneous activity of an MSN that responded to cortical stimulation. Bottom, In addition to their irregular spiking pattern and stereotypical response to cortical stimulation, MSNs were identified by action potential duration (>1.0 ms) as documented in previous studies (Mallet et al., 2005). Although not frequently encountered, fast spiking interneurons (FSIs) were distinguished from MSNs by their short-latency response to cortical stimulation, rapid firing pattern, and short duration action potential (<0.95 ms). FSIs were not included in this study. D, Spike probability was titrated to ∼50% by the experimenter, and no differences in the probability of responses evoked by cortical stimulation were observed between WT and nNOS KO mice, indicating that currents were titrated accurately across groups (p > 0.05; t test). E, Striatal MSNs recorded in nNOS KO mice showed decreased responsiveness to cortical stimulation. Thus, an increase in the current intensity required to elicit spike activity ∼50% of the time was observed in nNOS KO mice. ***p < 0.001 (t test). F, No difference in the mean ± SEM onset latency of cortically evoked spikes was observed in MSNs recorded in WT and nNOS KO mice (p > 0.05; t test). D–F, Data are derived from n = 43 WT MSNs (9 mice) and n = 26 nNOS KO MSNs (8 mice). G, Percentage of cells that responded robustly to cortical stimulation that were firing or quiescent in WT and nNOS mice. The number of cells that are quiescent or firing is indicated within the percentage bars. An overall decrease in the population of spontaneously firing MSNs recorded was observed in the striatum of nNOS KO mice compared with WT controls. *p < 0.05 (χ² test). H, I, No difference in the mean ± SEM firing rate or coefficient of variance (CV) of the interspike interval (ISI) of spontaneously firing striatal MSNs was observed across groups (p > 0.05; t test). Data are derived from n = 42 WT and n = 25 nNOS KO MSNs (9 mice per group).

Figure 5.

Genetic deletion of nNOS blocks cortically evoked spike activity elicited by PDE10 inhibitor administration: within-subjects studies. A, Spike probability was titrated to ∼50% by the experimenter, and no differences in the probability of responses evoked by cortical stimulation were observed between WT and nNOS KO mice, indicating that currents were titrated accurately across groups (p > 0.05; t test). B, Striatal MSNs recorded in nNOS KO mice exhibited decreased responsiveness to cortical stimulation. Thus, an increase in the current intensity required to elicit spike activity ∼50% of the time was observed in nNOS KO mice. *p < 0.05 (t test). C, Pairwise comparisons revealed that TP-10 administration induced a facilitatory effect on cortically evoked spike activity in the WT group as significant reductions in current intensity required to evoke spike activity 50% of the time were observed 40–60 min after drug (^#p < 0.05; **p < 0.01 compared with 10–20 min postdrug measures; two-way ANOVA, Tukey post hoc test). In nNOS KO mice, TP-10 did not change the current intensity required to evoke spike activity 50% of the time (p > 0.05, two-way repeated-measures ANOVA). Results are derived from n = 6 cells/6 mice per group.

Figure 6.

Genetic deletion of nNOS attenuates cortically evoked spike activity elicited by PDE10 inhibitor administration: between-subjects studies. A, Representative traces of cortically evoked spike activity of a single unit recorded in a WT (left) and nNOS KO (right) mouse after TP-10 administration. Ten superimposed traces of cortically evoked spike responses from a single striatal MSN are shown. Arrow indicates stimulus artifact. B, Spike probability was titrated to ∼50% by the experimenter, and no differences in the probability of responses evoked by cortical stimulation were observed between WT and nNOS KO mice, indicating that currents were titrated accurately across groups (p > 0.05; t test). C, Striatal MSNs recorded in nNOS KO mice in the presence of the PDE10A inhibitor exhibited decreased responsiveness to cortical stimulation compared with WT controls. Thus, an increase in the mean ± SEM current intensity required to elicit spike activity ∼50% of the time was observed in nNOS KO mice. ***p < 0.001 (t test). D, Striatal MSNs recorded in nNOS KO mice in the presence of the PDE10A inhibitor also exhibited an increase in the mean ± SEM onset latency of cortically evoked spikes compared with WT controls. *p < 0.05 (t test). B–D, Data are derived from n = 23 WT and n = 29 nNOS KO MSNs (8 mice per group). E, Percentage of cells that responded robustly to cortical stimulation that were firing or quiescent in the WT and nNOS mice (the number of cells for each condition is indicated within the percentage bars). There was no statistical difference in the population of spontaneously firing activity of cortically driven MSNs recorded in the striatum of WT and nNOS KO mice after TP-10 injection (p > 0.05; χ² test). F, A trend toward a decrease in the mean ± SEM firing rate of MSNs recorded in the striatum of nNOS KO mice was observed after TP-10 injection compared with WT controls. ^#p = 0.0827 (t test). G, No difference in the mean ± SEM coefficient of variance (CV) of the interspike interval (ISI) of spontaneously firing activity was observed between WT and nNOS KO mice after TP-10 injection (p > 0.05; t test).