Abab Receptors at Glutamatergic Synapses in the at Striatum

bstract—Although multiple effects of GABAB receptor actiation on synaptic transmission in the striatum have been escribed, the precise locations of the receptors mediating hese effects have not been determined. To address this ssue, we carried out pre-embedding immunogold electron icroscopy in the rat using antibodies against the GABAB eceptor subunits, GABAB1 and GABAB2. In addition, to inestigate the relationship between GABAB receptors and gluamatergic striatal afferents, we used antibodies against the esicular glutamate transporters, vesicular glutamate transorter 1 and vesicular glutamate transporter 2, as markers for lutamatergic terminals. Immunolabeling for GABAB1 and ABAB2 was widely and similarly distributed in the striatum, ith immunogold particles localized at both presynaptic and ostsynaptic sites. The most commonly labeled structures ere dendritic shafts and spines, as well as terminals formng asymmetric and symmetric synapses. In postsynaptic tructures, the majority of labeling associated with the lasma membrane was localized at extrasynaptic sites, alhough immunogold particles were also found at the postsynptic specialization of some symmetric, putative GABAergic ynapses. Labeling in axon terminals was located within, or t the edge of, the presynaptic active zone, as well as at xtrasynaptic sites. Double labeling for GABAB receptor subnits and vesicular glutamate transporters revealed that abeling for both GABAB1 and GABAB2 was localized on gluamatergic axon terminals that expressed either vesicular lutamate transporter 1 or vesicular glutamate transporter 2. he patterns of innervation of striatal neurons by the vesiclar glutamate transporter 1and vesicular glutamate transorter 2-positive terminals suggest that they are selective arkers of corticostriatal and thalamostriatal afferents, repectively. These results thus provide evidence that presynptic GABAB heteroreceptors are in a position to modulate he two major excitatory inputs to striatal spiny projection