Research reportLocalization of mGluR1a-like immunoreactivity and mGluR5-like immunoreactivity in identified populations of striatal neurons
Introduction
Excitatory amino acids (EAAs) are important neurotransmitters within the striatum, which receives a massive EAAergic input from the neocortex and allied structures 11, 16, 27, 41, 49and other probable EAAergic inputs from midline thalamic nuclei [12]. Metabotropic glutamate receptors (mGluRs) are G-protein coupled EAA receptors composed of a single polypeptide chain 32, 39. Intrastriatal injections of mGluR agonists produce behavioral changes 19, 35, 36, alter striatal neurotransmitter metabolism [36], and change the activity of both striatal neurons and neurons within other basal ganglia nuclei [19]. mGluR agonists stimulate phosphoinositide turnover in cultured striatal neurons [9], modulate striatal adenylyl cyclase activity in striatal slices 13, 50, inhibit corticostriate neurotransmission 5, 24, modulate striatal neuron N-methyl-d-aspartate receptors [7], modulate striatal neuron GABA release 5, 50, and mediate long-term depression of corticostriate neurotransmission [6]. Striatal mGluRs may also be involved in pathophysiological processes; mGluR active agents modulate N-methyl-d-aspartate receptor-mediated neurotoxicity within the striatum 8, 30, and mGluR antagonists are proposed as candidate agents for symptomatic therapy of Parkinson's disease [19].
Molecular cloning has established the existence of eight mGluR subtypes (mGluR1–8) with different patterns of expression within the central nervous system and retina 32, 39. Primary sequence, pharmacologic profiles and effector systems allow categorization of mGluR subtypes into three groups 32, 39. Studies with transfection systems show that Type 1 mGluRs (mGluR1 and mGluR5) are coupled to phosphoinositide hydrolysis, while both Type 2 mGluRs (mGluR2 and mGluR3) and Type 3 mGluRs (mGluR4, 6, 7, 8) are negatively coupled to adenylyl cyclase. mRNAs for all mGluR subtypes except mGluR6 and mGluR8 are known to be expressed within the striatum. In situ hybridization studies demonstrate expression of mGluR1, mGluR4, mGluR5 and mGluR7 mRNAs in many striatal neurons 1, 18, 28, 29, 37, 40, 47. mGluR2 mRNA is probably expressed by striatal cholinergic interneurons 28, 47and mGluR3 by both striatal neurons and glia 28, 47. Initial surveys of mGluR8 mRNA expression with in situ hybridization do not reveal much striatal signal but cannot exclude expression by a subpopulation of neurons [10].
Striatal neurons are divided into a number of subpopulations based on projection targets and expression of several neurochemical markers 15, 21. One basic division is into projection neurons whose axons arborize outside the striatum and interneurons with terminals restricted to the striatum. Subpopulations of both striatal projection neurons and interneurons are recognized. Striatal projection neurons are distinguished by expression of neuropeptides and target areas innervated. In rodents, three main subpopulations can be distinguished; striatal neurons expressing tachykinins and projecting to the substantia nigra pars compacta, a more numerous population of striatal neurons expressing tachykinins and projecting to the substantia nigra pars reticulata and the entopeduncular nucleus, and striatal neurons projecting to the globus pallidus. Three well-characterized populations of striatal interneurons are recognized [21]; a population of cholinergic interneurons, another population of interneurons co-expressing somatostatin, neuropeptide Y and nitric oxide synthase, and a third population expressing GABA and parvalbumin. Another population of GABAergic neurons containing calretinin is recognized also [21]. Considerable evidence indicates that striatal neuron subpopulations express neurotransmitter receptors differentially. Dopamine D1 receptors are expressed preferentially by striatonigral projection neurons, while dopamine D2 neurons and adenosine A2a receptors are expressed preferentially by striatopallidal projection neurons 14, 17, 38, 42. Striatal neuron subpopulations differ with respect to expression of muscarinic cholinergic receptors and the GluR1–4 subtypes of EAA receptors 4, 45, 46. We utilized immunofluorescence immunohistochemistry in conjunction with retrograde tract-tracing to define the distribution of some Type 1 mGluRs among striatal projection neuron and interneurons.
Section snippets
Fluoro-Gold labeling
Eleven male Sprague-Dawley rats (175–199 g; Spartan Labs, Hastings, MI) were anesthetized with ketamine/xylazine (10:3, 1 mg/kg i.p.) and placed in a Kopf stereotaxic frame. Fluoro-Gold (FG; Fluorochrome, Englewood, CO; 0.2 μl of a 2% solution in 0.9% saline) was pressure-injected into either the substantia nigra (n=5) or the globus pallidus (n=6) using the following coordinates from the atlas of Paxinos and Watson. For substantia nigra: AP −5.4, ML +1.8, DV −8.2. For pallidal injections, the
Fluoro-Gold tract tracing and mGluR1a receptor immunohistochemistry
Labeling for both striatonigral and striatopallidal neurons was seen throughout all quadrants of the striatum (Table 1; Fig. 1) following either intranigral or intrapallidal injection of FG. Within all quadrants approximately 60% to 70% of FG-labeled neurons were double-labeled for mGluR1a immunoreactivity.
Fluoro-Gold tract tracing and mGluR5 receptor immunohistochemistry
Labeling of both striatonigral and striatopallidal neurons was seen throughout all quadrants of the striatum (Table 1; Fig. 1) following either intranigral or intrapallidal injection of FG.
Discussion
Our previous studies have shown that Type 1 mGluR binding sites are expressed abundantly within the striatum and are expressed by striatonigral neurons 2, 44. We have extended these studies to demonstrate the association of some Type 1 mGluR subtypes, mGluR1a and mGluR5, with specific subpopulations of striatal neurons. Our approach does presume, however, that identification of receptor immunoreactivity within perikarya is an accurate predictor of receptor expression on dendrites, and cannot
Acknowledgements
This research was supported by The National Institute on Aging, The National Eye Institute, Research to Prevent Blindness, Inc., the Lucille P. Markey Trust, and the Geriatrics Research, Education, and Clinical Center of the Ann Arbor VAMC. Grant Numbers: AG08671, AG11335, EY02687.
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