Table 1.

Expression of metabotropic glutamate receptors in the basal ganglia.

Group	Signaling pathway	Receptor	Basal ganglia localization	Synaptic localization	Antiparkinsonian effects in animal models	Neuroprotective effects
I	Coupled to Gq and PI hydrolysis	mGlu1	SNc, striatum, GPe, GPi/SNr, STN	Pre- and postsynaptic	Limited beneficial effects of antagonists	No data
		mGlu5	Striatum, GPe, GPi/SNr, STN	Predominately postsynaptic	Antagonists have moderate symptomatic effects and may be beneficial for LIDs	Yes
II	Coupled to Gi/o and inhibition of adenylyl cyclase	mGlu2/3	SNc, striatum, GPi/SNr	Predominately presynaptic	Agonists demonstrate antiparkinsonian effects in acute but not chronic PD models	Yes
III	Coupled to Gi/o and inhibition of adenylyl cyclase	mGlu4	SNc, striatum, GPe, GPi/SNr	Predominately presynaptic	Group III agonists, mGlu4- preferring agonists and mGlu4 PAMs have antiparkinsonian effects in several animal models of PD	Yes for group III agonists and mGlu4 PAMs
		mGlu6	Not expressed in basal ganglia	–	–	–
		mGlu7	Striatum, GPe, GPi/SNr	Predominately presynaptic	Agonists demonstrate antiparkinsonian effects in animal models of PD	No data
		mGlu8	Striatum, GPi/SNr	Predominately presynaptic	No antiparkinsonian effects reported	No data

GPe: Globus pallidus pars externa; GPi: Globus pallidus pars interna; LID: Levodopa-induced dyskinesias; mGlu: Metabotropic glutamate; PAM: Positive allosteric modulator; PD: Parkinson’s disease; PI: Phosphoinositide; SNc: Substantia nigra pars compacta; SNr: Substantia nigra pars reticulata; STN: Subthalamic nucleus.