Expression of the striatal DARPP-32/ARPP-21 phenotype in GABAergic neurons requires neurotrophins in vivo and in vitro.

The medium spiny neuron (MSN) is the major output neuron of the caudate nucleus and uses GABA as its primary neurotransmitter. A majority of MSNs coexpress DARPP-32 and ARPP-21, two dopamine and cyclic AMP-regulated phosphoproteins, and most of the matrix neurons express calbindin. DARPP-32 is the most commonly used MSN marker, but previous attempts to express this gene in vitro have failed. In this study we found that DARPP-32 is expressed in <12% of E13- or E17-derived striatal neurons when they are grown in defined media at high or low density in serum, dopamine, or Neurobasal/N2 (Life Technologies), and ARPP-21 is expressed in <1%. The percentage increases to 25% for DARPP-32 and 10% for ARPP-21 when the same cells are grown in Neurobasal/B27 (Life Technologies) for 7 d. After growth in Neurobasal/B27 plus brain-derived neurotrophic factor (BDNF) for 7 d, E13-derived MSNs are 53.7% DARPP-32-positive and 29. 0% ARPP-21-positive; E17-derived MSNs are 66.8% DARPP-32-positive and 51.5% ARPP-21-positive. The percentage of calbindin-positive neurons also is increased under these conditions. Finally, ARPP-21 expression is reduced in mice with a targeted deletion of the BDNF gene. We conclude that BDNF is required for the maturation of a large subset of patch and matrix MSNs in vivo and in vitro. In addition, we introduce a culture system in which highly differentiated MSNs may be generated, maintained, and studied.