Quantitative immunocytochemistry of DARPP-32-expressing neurons in the rat caudatoputamen.

DARPP-32, a dopamine and cAMP-regulated phosphoprotein with an apparent molecular weight of 32 kD, is enriched in dopaminoceptive brain regions. Chief among these regions is the caudatoputamen which contains a large number of DARPP-32-containing medium sized spiny neurons. Since medium spiny neurons are a heterogeneous population with respect to connections and chemical neuroanatomy, it seemed of interest to determine whether DARPP-32 is present in all medium-sized neurons, or only within a specific subpopulation. The present study used immunocytochemistry and quantitative analysis to address this issue. We demonstrate that DARPP-32 is contained in almost all medium-sized neurons (96.4%) and is not detected in large neurons. Taken together with previous observations that the DARPP-32-containing medium-sized neurons project heavily to all neostriatal targets, these data demonstrate that DARPP-32 is present in virtually all neostriatal output neurons. Thus, the DARPP-32 cascade represents a final common pathway through which convergent afferent fibers using a variety of neurotransmitter agents may modulate striatal outflow.