SYNAPTIC CONNECTIONS BETWEEN SPINY NEURONS OF THE DIRECT AND INDIRECTPATHWAYS IN THE NEOSTRIATUM OF THE RAT - EVIDENCE FROM DOPAMINE-RECEPTOR AND NEUROPEPTIDE IMMUNOSTAINING

The flow of cortical information through the basal ganglia occurs thro ugh the so-called 'direct pathway' and 'indirect pathways'. The object of the present work was to attempt to determine whether spiny neurons in the neostriatum that give rise to the direct pathway (i.e. the str iatonigral/entopeduncular pathway) and those giving rise to the indire ct pathways (i.e. striatopallidal pathway) are synaptically interconne cted. The approach was to carry out double immunocytochemistry at the electron microscopic level using antibodies against peptides or dopami ne receptor subtypes that are selectively associated with the neurons that give rise to the direct (substance P or D-1 receptors) and indire ct pathways (enkephalin or D-2 receptors). Sections of perfuse-fixed r at neostriatum were immunostained to reveal both substance P immunorea ctivity and D-2 receptor immunoreactivity or enkephalin and D-1 recept or immunoreactivity, respectively. Double peroxidase methods were empl oyed using different chromogens that were distinguishable at both the light and electron microscopic levels. In the electron microscope subs tance P-immunoreactive terminals were seen in synaptic contact with de ndritic structures that displayed immunoreactivity for D-2 receptor. S imilarly, enkephalin-immunoreactive terminals were seen in synaptic co ntact with D-1-immunoreactive dendritic structures. Thus, axon collate rals of neurons giving rise to the direct pathway form synaptic contac ts with neurons that give rise to the indirect pathway and axon collat erals of neurons giving rise to the indirect pathway form synaptic con tact with neurons that give rise to the direct pathway. These results indicate that the two pathways of information flow through the basal g anglia are synaptically linked at the level of the neostriatum.