Extrastriatal circuits activated by intrastriatal muscimol: a [14C]2-deoxyglucose investigation

The quantitative autoradiographic [14C]2-deoxyglucose technique has been employed in conscious, lightly restrained rats to investigate the functional consequences (which are reflected as alterations in local rates of glucose utilization) of unilateral intrastriatal administration of the GABAergic agonist, muscimol. Intrastriatal injections of muscimol (500 ng) effect a widespread, homogeneous reduction (by 33%) in glucose utilization throughout the ipsilateral striatum. Increased glucose utilization following intrastriatal muscimol injection was observed in each region to which the principal striatal efferent projections are directed (ipsilateral globus pallidus increased by 29%; ipsilateral entopeduncular nucleus increased by 33%; ipsilateral pars reticulata increased by 46%). Significant focal alterations in glucose utilization occurred in a number of other regions ipsilateral to the injection, including lateral habenular nucleus (increased by 16%), pars compacta of the substantia nigra (increased by 28%), ventrolateral nucleus of the thalamus (decreased by 40%), sensory-motor cortex (decreased by 47%), deep layer of the superior colliculus (decreased by 18%), and subthalamic nucleus (decreased by 18%). In the overwhelming majority of brain regions examined, glucose utilization was unaltered by intrastriatal administration of muscimol. All regions which displayed significant alterations in glucose use with intrastriatal muscimol are known to have direct connections with the striatum (e.g. pallidus and nigra), or to be anatomical components in polysynaptic striatal outflow pathways (e.g. striatal-pallidal-ventral thalamic-neocortical circuit). The present results indicate that activity within striatal efferent circuits can be differentially modified by the effects of the GABAergic agonist, muscimol, within the caudate nucleus.