Colocalization of somatostatin receptors with DARPP-32 in cortex and striatum of rat brain

Somatostatin (SST)-positive medium-sized aspiny interneurons are selectively spared in excitotoxicity. The biological effects of SST are mediated via five different receptors, namely somatostatin receptor (SSTR)1-5; however, SSTR subtype spared in excitotoxicity and involved in neuroprotection is not known. Dopamine- and cAMP-regulated phosphoprotein (DARPP-32) is predominantly expressed in medium-sized projection neurons that are most vulnerable in excitotoxicity. In the present study, we determined the colocalization of SST and SSTRs with DARPP-32 in rat brain cortical and striatal regions using immunofluorescence immunohistochemistry. We also determined the expression of DARPP-32 in SSTR1-5 immunoprecipitate prepared from cortex and striatum. SST-positive neurons in cortex and striatum are devoid of colocalization with DARPP-32. However, in cortical and striatal brain regions, three different neuronal populations either expressing SSTRs and DARPP-32 alone or displaying colocalization were identified. Quantitative analysis reveals that in cortex and striatum, SSTR1 and 5 are most predominant receptor subtypes colocalized with DARPP-32 followed by SSTR4, 2, and 3 in cortex whereas SSTR2, 4, and 3 in striatum. Importantly, DARPP-32 is expressed in SSTR1-5 immunoprecipitate prepared from cortex and striatum. Taken together, these results provide the first evidence that the SSTR-positive neurons lacking colocalization with DARPP-32 might be spared in excitotoxicity.