Activation of metabotropic glutamate receptor 5 has direct excitatory effects and potentiates NMDA receptor currents in neurons of the subthalamic nucleus

Abstract

The subthalamic nucleus (STN) is a key nucleus in the basal ganglia motor circuit that provides the major glutamatergic excitatory input to the basal ganglia output nuclei. The STN plays an important role in normal motor function, as well as in pathological conditions such as Parkinson's disease (PD) and related disorders. Development of a complete understanding of the roles of the STN in motor control and the pathophysiological changes in STN that underlie PD will require a detailed understanding of the mechanisms involved in regulation of excitability of STN neurons. Here, we report that activation of group I metabotropic glutamate receptors (mGluRs) induces a direct excitation of STN neurons that is characterized by depolarization, increased firing frequency, and increased burst-firing activity. In addition, activation of group I mGluRs induces a selective potentiation of NMDA-evoked currents. Immunohistochemical studies at the light and electron microscopic levels indicate that both subtypes of group I mGluRs (mGluR1a and mGluR5) are localized postsynaptically in the dendrites of STN neurons. Interestingly, pharmacological studies suggest that each of the mGluR-mediated effects is attributable to activation of mGluR5, not mGluR1, despite the presence of both subtypes in STN neurons. These results suggest that mGluR5 may play an important role in the net excitatory drive to the STN from glutamatergic afferents. Furthermore, these studies raise the exciting possibility that selective ligands for mGluR5 may provide a novel approach for the treatment of a variety of movement disorders that involve changes in STN activity.

Fig. 1.

Group I mGluR-mediated depolarization of STN neurons. A, Representative current-clamp traces of membrane potential changes in response to DHPG (100 μm), LY354740 (100 nm), and l-AP-4 (1 mm) from a holding potential of −60 mV. B, Corresponding change in membrane input resistance accompanying the change in membrane potential. C, Mean data ± SEM of membrane potential changes, showing a significant depolarization by the group I-selective agonist DHPG (**p < 0.001).D, Dose–response curve of DHPG-mediated changes in membrane potential.

Fig. 2.

Ionic basis of DHPG-induced current.A, Leak-subtracted I–V plot in normal K⁺ conditions in the presence and absence of DHPG (100 μm). B, SubtractedI–V plot representing DHPG-induced current alone, showing reversal potential of approximately −80 mV. C, Representative leak-subtracted I–V plot in the presence of potassium channel block and cesium. D, SubtractedI–V plot representing DHPG-induced current alone, showing reversal potential of −30 mV. E, Mean data ± SEM of DHPG-induced inward current amplitude (picoamperes) in voltage-clamp mode in normal potassium conditions, potassium block and cesium, and in the presence of cadmium (100 μm) (**p < 0.01).

Fig. 3.

Postsynaptic effects of group I mGluR activation in STN neurons. A, Representative current-clamp traces of firing rate before drug application (at −50 mV) and dramatic increase in the presence of DHPG (100 μm) that is countered by current injection to return membrane potential to the predrug level. B, DHPG-mediated switch to burst-firing mode (from a holding potential of −60 mV), which is countered by hyperpolarizing current injection to maintain membrane potential at the predrug level. C, DHPG-mediated membrane oscillations in the presence of TTX are also countered by hyperpolarizing current injection. Action potentials are truncated in A andB. Scale bars in C also apply toB.

Fig. 4.

Activation of group I mGluRs potentiates NMDA-evoked currents in STN neurons but has no effect on kainate-evoked currents. A, Representative voltage-clamp traces of NMDA-evoked currents in predrug, agonist, and wash conditions. Only the group I-selective agonist caused a reversible potentiation of NMDA-evoked currents. The group II and III agonists had no effect on NMDA-evoked currents. B, Mean data ± SEM of percent potentiation of NMDA-evoked currents by DHPG over predrug current amplitude. DHPG caused a significant potentiation compared with vehicle (*p < 0.05). C, Mean data ± SEM of percent predrug kainate-evoked current amplitude showing no difference compared with vehicle. D, Mean data ± SEM of percent potentiation of NMDAR currents by DHPG in normal K⁺ at −60 and −80 mV, cesium and potassium channel block at −60 mV, and in the presence of Cd²⁺ (100 μm).

Fig. 5.

Immunostaining for mGluR1a in the STN. A, Low-power light micrograph of mGluR1a in the STN. B, C, High-power electron micrographs of mGluR1a-immunoreactive dendrites (Den) that form asymmetric synapses (arrowheads) with unlabeled terminals. Note that the dendrite in B contains vesicles (arrows). D, High-power electron micrograph of mGluR1a-immunoreactive terminal that forms an asymmetric synapse (arrowhead) with an immunoreactive dendrite.CP, Cerebral peduncle. Scale bars: A, 500 μm; B–D, 0.5 μm.

Fig. 6.

Immunostaining for mGluR5 in the STN. A, Low-power light micrograph of mGluR5 immunostaining in the STN. B, Low-power electron micrograph of mGluR5-immunoreactive dendrites (Den).C, High-power electron micrograph of a small mGluR5-immunoreactive dendrite that forms an asymmetric synapse (arrowhead) with the unlabeled terminal.D, High-power electron micrograph of a large mGluR5-immunoreactive dendrite that forms a symmetric synapse (arrow) with an unlabeled terminal. The open arrowhead points to a puncta adherentia. CP, Cerebral peduncle. Scale bars: A, 500 μm;B, 1 μm; C, D, 0.5 μm.

Fig. 7.

mGluR5 mediates group I mGluR-evoked depolarization of STN neurons. A, Membrane potential traces showing depolarization with DHPG (100 μm), which is blocked by the mGluR5-selective antagonist MPEP (10 μm). Membrane depolarization is not blocked by the mGluR1-selective antagonist CPCCOEt (100 μm).B, Mean data ± SEM of change in membrane potential showing a significant inhibition of DHPG-mediated depolarization of STN neurons by MPEP (10 μm) compared with DHPG alone (**p < 0.001). MPEP also significantly blocks depolarization mediated by the mGluR5-selective agonist CBPG (100 μm) (*p < 0.05).

Fig. 8.

mGluR5 mediates group I mGluR-induced potentiation of NMDA-evoked currents. A, Current traces of NMDA-evoked currents before, during, and after application of DHPG (100 μm). The potentiation is blocked by MPEP (10 μm) but not CPCCOEt (100 μm).B, Mean data ± SEM of percent potentiation of NMDA-evoked currents by DHPG over predrug conditions. MPEP (10 μm) significantly blocks potentiation of NMDA-evoked current compared with DHPG alone (*p < 0.05).