DOPAMINE INNERVATION OF THE MONKEY MEDIODORSAL THALAMUS: LOCATION OF PROJECTION NEURONS AND ULTRASTRUCTURAL CHARACTERISTICS OF AXON TERMINALS D. S. MELCHITZKY, a,c S. L. ERICKSON b AND D. A. LEWIS a,b * a Department of Psychiatry, University of Pittsburgh, Pittsburgh, PA 15213, USA b Department of Neuroscience, University of Pittsburgh, Pittsburgh, PA 15213, USA c Department of Biology, Mercyhurst College, Erie, PA 16546, USA Abstract—Dopamine (DA) axons and receptors have recently been identified in the primate thalamus, including the me- diodorsal thalamic nucleus (MD). In order to determine whether the DA innervation of the primate MD shares the anatomical features of the mesocortical or nigrostriatal DA projections, we performed tract-tracing and immunocyto- chemistry studies in macaque monkeys (Macaca fascicularis) to identify the location of the DA neurons that project to MD and immuno-electron microscopy to determine the distribu- tion of the dopamine transporter (DAT) in axons within the MD. Similar to the mesocortical projection, retrogradely- labeled, tyrosine hydroxylase-containing neurons were present in dorsal tier ventral mesencephalic nuclei, such as the ventral tegmental area and the dorsal portion of the sub- stantia nigra pars compacta. In contrast, no dual-labeled neu- rons were present in the ventral tier nuclei, the primary origin of the nigrostriatal DA pathway. In addition, like the DA pro- jection to the prefrontal cortex, DAT immunoreactivity was predominantly localized to the pre-terminal portion of axons in the MD, and was infrequently found in association with synaptic vesicles, in contrast to nigrostriatal DA axons. These findings indicate that the DA projection to the MD shares anatomical features with the mesocortical DA system, suggesting that the functional properties of DA neurotrans- mission in the MD might be more similar to those in the cortex than in the striatum. © 2006 IBRO. Published by Elsevier Ltd. All rights reserved. Key words: dopamine transporter, tyrosine hydroxylase, pre- frontal cortex, ventral mesencephalon. Dopamine (DA) influences a range of brain functions, in- cluding cognitive processes such as working memory. Re- gions of the primate brain that mediate working memory, such as the prefrontal cortex (Goldman et al., 1971; Alex- ander and Goldman, 1978) and the mediodorsal thalamic nucleus (MD) (Isseroff et al., 1982; Aggleton and Mishkin, 1983), contain both DA axons (Lewis et al., 1987, 2001; Melchitzky and Lewis, 2001; Sánchez-González et al., 2005) and DA receptors (Suzuki et al., 1998; Gurevich and Joyce, 1999), indicating that DA neurotransmission in these brain regions might regulate working memory func- tioning. Indeed, either increases or decreases in the level of DA neurotransmission in the prefrontal cortex impair performance on working memory tasks (Brozoski et al., 1979; Roberts et al., 1994; Arnsten et al., 1994). Further- more, subjects with schizophrenia perform poorly on tests that involve working memory (Park and Holzman, 1992), and interestingly, abnormalities in the DA system have been reported in both the prefrontal cortex (Akil et al., 1999; Abi-Dargham et al., 2002) and the thalamus (Ya- suno et al., 2004; Talvik et al., 2003) of individuals with schizophrenia. The presence of DA axons in the macaque monkey MD has been reported in several studies (Freeman et al., 2001; Melchitzky and Lewis, 2001; Sánchez-González et al., 2005), with a pattern of innervation distinct from that of norepinephrine (Melchitzky and Lewis, 2001). For exam- ple, we found that axons immunoreactive for the dopamine transporter (DAT) were heterogeneously distributed within MD, with the highest density located in the parvocellular and multiform subdivisions. Consistent with other studies demonstrating that antibodies against tyrosine hydroxy- lase (TH) preferentially label DA-containing axons in the primate brain (Lewis et al., 1987; Noack and Lewis, 1989), the density and distribution of TH-immunoreactive (IR) axons were very similar to those of DAT-IR axons. In contrast, axons labeled for DA--hydroxylase, a specific marker of norepinephrine-containing structures, were present in higher density and were homogeneously distrib- uted throughout the entire MD. The DA projections to cortical or subcortical structures arise from neurons in the ventral mesencephalon, which in primates is composed of two main DA-containing cell groups, the dorsal and ventral tier neurons (Haber and Fudge, 1997). The striatum receives projections primarily from DA neurons in the ventral tier, comprised of cells in the substantia nigra pars compacta. In contrast, the cells of the dorsal tier, which consists of the dorsal portion of the substantia nigra pars compacta, the ventral tegmental area and the retrorubral area (Haber and Fudge, 1997), provide DA projections to cortical areas, including the prefrontal cortex. The DA projections to the striatum and prefrontal cortex also differ in the subcellular distribution of DAT. For example, in both monkeys (Lewis et al., 2001) and rodents (Sesack et al., 1998), DAT immunoreactivity is present in *Correspondence to: D. A. Lewis, Western Psychiatric Institute and Clinic, Biomedical Science Tower, W1651, 3811 O’Hara Street, Pittsburgh, PA 15213, USA. Tel: +1-412-624-3934; fax: +1-412-624-9910. E-mail address: lewisda@upmc.edu (D. A. Lewis). Abbreviations: COMT, catechol-O-methyltransferase; CTb, cholera toxin b subunit; DA, dopamine; DAT, dopamine transporter; IR, immu- noreactive; MD, mediodorsal thalamic nucleus; PB, phosphate buffer; TH, tyrosine hydroxylase. Neuroscience 143 (2006) 1021–1030 0306-4522/06$30.00+0.00 © 2006 IBRO. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.neuroscience.2006.08.056 1021