Differential perikaryal localization in rats of D1 and D2 dopamine receptors on striatal projection neuron types identified by retrograde labeling Yun-Ping Deng a , Wan-Long Lei a,b , Anton Reiner a, * a Department of Anatomy & Neurobiology, University of Tennessee Health Science Center, 855 Monroe Ave., Memphis, TN 38163, United States b Department of Anatomy, Zhongshan Medical College, Sun Yat-Sen University, Guangzhou, China Received 23 February 2006; received in revised form 6 July 2006; accepted 7 July 2006 Available online 17 August 2006 Abstract The localization of D1 and D2 dopamine receptors to striatal projection neuron types has been controversial, with some data favoring segregation of D1 to direct pathway neurons (substance P-containing) and D2 to indirect pathway neurons (enkephalinergic), and others reporting significant colocalization of D1 and D2 on individual projection neuron types. In the present study, we used subtype-specific antibodies against D1 and D2 and confocal laser scanning microscopy to determine their perikaryal localization in striatum in general, and in direct and indirect pathway neuron perikarya defined by retrograde labeling in particular. We found that D1 in rat was detectable on 49.5% of NeuN-immunolabeled striatal perikarya, and D2 on 61.6% of NeuN-immunolabeled perikarya, implying that at least 15–20% of D1+ neurons must possess D2 and vice versa. Secondly, we retrogradely labeled neuronal perikarya from the external globus pallidus (GPe), internal globus pallidus (GPi) or substantia nigra with rhodamine dextran amine 3 kDa (RDA3k). We found that 92% of perikarya labeled from nigra and 96% of perikarya labeled from GPi immunolabeled for D1, but only 23% of perikarya labeled from GPe immunolabeled for D1. Since direct pathway neurons (striato-nigral and striato-GPi) have a collateral projection to GPe, it is possible that many of the D1+ striatal perikarya retrogradely labeled from GPe were direct pathway neurons. About 96% of perikarya retrogradely labeled from GPe were immunolabeled for D2, while about 40% of those retrogradely labeled from GPi and 44% of those retrogradely labeled from nigra immunolabeled for D2. These findings suggest that: (1) while many striato-GPi/ SN neurons possess D1 and D2, the majority mainly or exclusively possess D1 and (2) the vast majority of striato-GPe neurons mainly or exclusively possess D2. # 2006 Elsevier B.V. All rights reserved. Keywords: Striatum; Basal ganglia; Dopamine receptor; Localization; Immunohistochemistry 1. Introduction Five dopamine receptor genes (termed D1–D5) are expressed in brain, with the D1 and D5 genes coding for proteins with D1- type pharmacology and D2, D3 and D4 genes coding for proteins with D2-type pharmacology (Sibley, 1991; Sunahara et al., 1991; Van Tol et al., 1991; DeKeyser, 1993). Among these genes, D1 and D2 are the most heavily expressed in the striatal part of the basal ganglia and are the main dopamine receptors that mediate the responses of striatal neurons to the dopaminergic input from the midbrain (Bouthenet et al., 1991; Civelli et al., 1991; Van Tol et al., 1991). The cellular localization of D1 and D2 dopamine receptors in the striatum has been studied using in situ hybridization histochemistry (ISHH), single-cell reverse tran- scription-polymerase chain reaction (RT-PCR), and immuno- histochemistry (Mengod et al., 1989; Dearry et al., 1990; Gerfen et al., 1990; Le Moine et al., 1990, 1991; Weiner et al., 1990, 1991; Chen et al., 1991; Lester et al., 1993). By ISHH and immunolabeling, D1 has been found to be expressed in about half and D2 in slightly more than half of striatal neurons (Weiner et al., 1990, 1991; Meador-Woodruff et al., 1991; Huang et al., 1992; Levey et al., 1993; Hersch et al., 1995; Le Moine and Bloch, 1995; Yung et al., 1995; Ince et al., 1997; Lei et al., 2004). Consistent with this restriction to many but not all striatal neurons, several ISHH studies have reported, for both rats and primates, that the two dopamine receptors are largely localized to www.elsevier.com/locate/jchemneu Journal of Chemical Neuroanatomy 32 (2006) 101–116 * Corresponding author. Tel.: +1 901 448 8298; fax: +1 901 448 7193. E-mail address: areiner@utmem.edu (A. Reiner). 0891-0618/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jchemneu.2006.07.001