Metabotropic glutamate receptor 4‐immunopositive terminals of medium‐sized spiny neurons selectively form synapses with cholinergic interneurons in the rat neostriatum

AbstractMetabotropic glutamate receptor 4 (mGluR4) is localized mainly to presynaptic membranes in the brain. Rat neostriatum has been reported to contain two types of mGluR4‐immunoreactive axon varicosities: small, weakly immunoreactive varicosities that were distributed randomly (type 1) and large, intensely immunoreactive ones that were often aligned linearly (type 2). In the present study, most type 1 terminals formed asymmetric synapses on dendritic spines, whereas type 2 terminals made symmetric synapses on dendritic shafts, showing immunoreactivity for GABAergic markers. After depletion of neostriatal neurons, type 2 but not type 1 varicosities were largely decreased in the damaged region. When medium‐sized spiny neurons (MSNs) were labeled with Sindbis virus expressing membrane‐targeted green fluorescent protein, mGluR4 immunoreactivity was observed on some varicosities of their axon collaterals in immunofluorescence and immunoelectron microscopies. Furthermore, type 2 varicosities were often positive for substance P but mostly negative for striatal interneuron markers and preproenkephalin. Thus, striatonigral/striato‐entopeduncular MSNs are likely to be the largest source of type 2 mGluR4‐immunopositive axon terminals in the neostriatum. Next, in the double‐immunofluorescence study, almost all choline acetyltransferase (ChAT)‐immunopositive and 41% of NK1 receptor‐positive dendrites were heavily associated with type 2 mGluR4‐immunoreactive varicosities. Neuronal nitric oxide synthase (nNOS)‐positive dendrites, in contrast, seemed associated with only a few type 2 varicosities. Conversely, almost all type 2 varicosities were closely apposed to NK1 receptor‐positive dendrites that were known to be derived from cholinergic and nNOS‐producing interneurons. These findings indicate that the mGluR4‐positive terminals of MSN axon collaterals selectively form synapses with neostriatal cholinergic interneurons. J. Comp. Neurol. 500:908–922, 2007. © 2006 Wiley‐Liss, Inc.