Research reportLocalization of dopamine D2 receptors on cholinergic interneurons of the dorsal striatum and nucleus accumbens of the rat
Introduction
This study focused on dopamine–acetylcholine interactions in the dorsal striatum and the nucleus accumbens of the rat brain. The dorsal striatum which integrates a variety of cortical and limbic information is involved primarily in the regulation of motor function. The nucleus accumbens (NAcc), which is a major area of the ventral striatum, has been an area of intensive investigation as central to the rewarding and locomotor effects of drugs of abuse and other naturally rewarding behaviors. The dopaminergic mesolimbic pathway, which originates in the ventral tegmental area (VTA) and provides extensive afferent projections to the dorsal striatum and NAcc, is targeted by several drugs of abuse including cocaine, amphetamine, heroin, morphine, nicotine and alcohol [4], [42]. These dopaminergic projections maintain distinct topography within the subdivisions of the dorsal striatum, and the two major divisions of the NAcc, the shell and core [18], [25], [48]. These subdivisions, therefore, have distinct structure, connectivity, and morphology and thus subserve separate reward and motor-related phenomena.
This study focused on cholinergic interneurons of the dorsal striatum and NAcc, which are amenable to dopaminergic mesolimbic influences through the activation of dopamine (DA) receptors that are located directly on these neurons. Cholinergic interneurons are important neuronal integrators and modulators of striatal function and dysfunction. These neurons possess key receptors that are linked to molecular signaling pathways critical for plasticity [10]. Striatal cholinergic interneurons also express long-term potentiation (LTP) [47] and play a unique role in reward mediated associative learning [3]. Furthermore, these local circuit neurons have been demonstrated to exert powerful influences on medium spiny output neurons (MSNs) and thus on overall striatal signaling [27]. More specifically, cholinergic interneurons have been reported to modulate corticostriatal NMDA receptor signaling onto MSNs and the LTP that is expressed by these striatal output neurons [1], [10].
The effects of DA actions on cholinergic interneurons are mediated through two types of G-protein coupled receptors: D1-class (D1, D5) and D2-class (D2, D3, D4). The cellular processes associated with these receptor subtypes have been linked directly to neural plasticity and learning as well as drug-seeking behavior [14], [36], [41], [51]. It has been shown that dopamine exerts mainly opponent action on D1- and D2-like dopamine receptors. Acetylcholine (ACh) release is increased by dopamine D1-like agonists [12], while dopamine D2 receptor activation has been shown in both in vivo and in vitro studies to inhibit ACh release [7], [13], [16], [33], [45]. In addition to their opposing effects, however, synergistic effects can also occur through the co-activation of dopamine D1 and D2 receptors [28], [29], [32].
The aim of the present study was to identify the distribution of dopamine D2 receptors on cholinergic interneurons of the dorsal striatum and NAcc in the rat brain, using light microscopy with immunocytochemical dual labeling procedures. This study provides evidence of a neuroanatomical substrate for possible reward and motor mediated DA influences onto dorsal striatal and accumbal cholinergic neuronal network systems. The findings from this work should lead to a better understanding of cholinergic circuitry, specifically that involving DA signaling onto cholinergic neurons and the effects of acetylcholine neurotransmission on overall striatal function and dysfunction.
Section snippets
Animals and tissue preparation
Twelve adult male Sprague–Dawley rats (Simonsen Labs) were used in this study. Experimental procedures conformed to National Institute of Health guidelines and were carried out under an institutionally reviewed and approved research protocol. Animals were administered an overdose of sodium pentobarbital (Nembutal; 100 mg/kg i.p.) and perfused transcardially with 60 ml of 0.1 M phosphate buffered saline (PBS), pH 7.4 followed by 200 ml of 4% paraformaldehyde/0.1% glutaraldehyde in PBS, pH 7.4.
Light microscopic dual immunolabeling: D2 (Vector SG) and ChAT (DAB)
Cholinergic interneurons of the dorsal striatum and shell and core compartments of the nucleus accumbens were identified by ChAT-IR visualized by the brown DAB immunoreaction product. ChAT immunoreactive neurons displayed the morphological features characteristic of large aspiny cholinergic interneurons [30], [38]. The somas of these neurons were oval, elongated, or multipolar, with a 30–50-μm soma diameter from which several, infrequently branching dendrites emerged, extending up to a
Discussion
The main finding of this study was the cellular localization of dopamine D2 receptors on cholinergic interneurons of the dorsal striatum and NAcc of the rat brain. Dopamine D2 receptors are targeted by several therapeutic drugs used in the treatment of such striatal-related disorders as Tourette Syndrome, Parkinson’s disease, and schizophrenia. Additionally, dopamine D2 receptors play a critical role in relapse and drug seeking behavior associated with several drugs of abuse [14], [41]. The
Acknowledgements
The authors gratefully acknowledge the support provided by grant AA13497-02 from the National Institute of Health/National Institute on Alcohol Abuse and Alcoholism (NIH/NIAAA) and Integrative Neuroscience Initiative on Alcoholism (INIA) (AAA), a Waggoner Center Faculty Research Fellowship (AAA), The University of Texas Faculty (AAA) and Undergraduate Student Research (VC and BEH) Fellowships, and NIH grant MH65728 (MLB) and the Morton Meyerson Family Foundation.
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Violeta Chen is presently at NIH/NIAAA in the Department of Biochemistry and Biophysics.