Ž . Brain Research 851 1999 270–276 www.elsevier.comrlocaterbres Short communication Subcellular immunolocalization of NMDA receptor subunit NR-1 in the chinchilla vestibular periphery Gail Ishiyama a , Ivan Lopez b , Akira Ishiyama b, ) a Department of Neurology, UCLA School of Medicine, Los Angeles, CA USA b DiÕision of Head and Neck Surgery, CHS 62-132, UCLA School of Medicine, Los Angeles, CA USA Accepted 28 September 1999 Abstract Ž . The immunohistochemical localization of N-methyl-D-aspartate NMDA glutamate receptor subunit, NR-1 was investigated in the chinchilla cristae ampullaris and utricular maculae at the light and electron microscopy level with the use of specific antipeptide antibodies. The afferent calyces that innervate type I hair cells, and the basolateral type I vestibular hair cell is NR-1 immunoreactive. The afferent boutons innervating type II hair cells and the basal portion of type II hair cell are NR-1 non-immunoreactive. These findings are consistent with NMDA receptor mediation of afferent excitatory neurotransmission from type I, but not type II hair cells to the primary afferent vestibular nerve. The NMDA receptors on the type I hair cell are located in areas of synaptic specialization, and may play a role in autoregulation. The localization of the NMDA receptor subunit in type I but not type II hair cells is intriguing. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Chinchilla laniger ; NMDA R1 receptor; Vestibular periphery The neurochemical transmission at the cytoneural junc- tion from the vestibular hair cells to the primary afferent Ž nerve is generally believed to be glutamatergic review: w x. Ref. 12,25 Electrophysiological studies support gluta- matergic neurotransmission at the hair cell to afferent w x wx nerve synapse in the frog 1,35,37,38 and in the cat 6. Glutamate-like immunoreactivity, is found in both type I w x and type II hair cells cytoplasm of mammals 2,8,14,34 . While there is a general consensus that the vestibular hair cell transmitter is glutamate, there is controversy over Ž. which subtype s of glutamate receptor mediates this neu- rochemical transmission. Glutamate receptors are classified into three categories: ionic NMDA types, ionic non-NMDA types, which in- cludes AMPA and kainate types, and metabotropic. The NMDA receptor is a ligand gated ion channel character- ized by high permeability to calcium, long activation times, modulation by glycine and a voltage dependent magne- w x sium block 15 . Under normal synaptic activity, NMDA receptors are not likely to contribute to fast signal trans- mission because of the magnesium block. The kinetics of ) Corresponding author. Fax: q1-310-794-5089; e-mail: ishiyama@ucla.edu the NMDA receptor is slower, and longer lived compared to the AMPA receptor, which has rapid onset and offset. Thus, AMPA receptors rather than NMDA receptors are expected to mediate primary transduction of sensory infor- mation, since sensitivity to rapid changes in stimuli re- quires rapid kinetics. The properties of the NMDA recep- tor are ideal for synaptic learning via long term potentia- wx tion 4 . The NMDA receptor can be reconstituted from two subunit types, NR1 and NR2A-D. NR1 can self-assemble to form channels with NMDA receptor characteristics, however when assembled in combination with NR2 the whole-cell currents activated by NMDA increase by sev- eral orders of magnitude. NR2 subunits cannot self-assem- ble to form a functional NMDA receptor without NR1. The NR1 subunit is likely a constituent of all NMDA receptor subtypes, whereas the NR2A to D are each en- coded by a separate gene and have unique expression w x patterns in the brain 19,23 . Depending on which of the four NR2 subunits assembles with NR1, the strength of the magnesium block, glycine sensitivity, deactivation kinet- ics, and single-channel characteristics are altered. Previous immunohistochemical studies have demon- wx wx strated both AMPA subunits 9 and kainate subunits 7 at the post-synaptic membrane of the afferent vestibular ter- 0006-8993r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0006-8993 99 02171-X