Are unconventional NMDA receptors involved in slowly adapting type I mechanoreceptor responses?
Specific immunohistochemical staining for NMDA receptor NR2A/B subunits was found in the outer root sheath layer of rat sinus hair (whisker) follicle. Co-localization with CK 20 confirmed that Merkel cells were stained. The NR2A/B staining seen on Merkel cells was pericellular. In addition it appeared that NF70-positive staining was in close proximity to, but did not colocalise with NR2A/B immunoreactivity, indicating that NR2A/B was only expressed by Merkel cells and not their adjacent nerve terminals. Merkel cells and the nerve terminals have previously been associated with electrophysiological recordings from slowly adapting type I (St I) mechanoreceptor unit activity. Pharmacological experiments with isolated sinus hairs using a wide range of ionotropic glutamate receptor antagonists found that only certain NMDA receptor blockers depressed St I unit responses to mechanical stimuli. AMPA/kainate receptor antagonists (CNQX and NBQX, 100 microM) had no effect, nor did classical competitive NMDA receptor antagonists, D-AP5 (600 microM) and R-CPP (100 microM), nor the NMDA glycine site antagonist 5,7-dichlorokynurenic acid (100 microM). The only effective NMDA receptor blockers were those selective for the polyamine site: ifenprodil (IC50 20 microM) and Ro 25-6981 (IC50 approximately 50 microM), and the associated ion channel: MK 801, ketamine and (+/-)-1-(1,2-diphenylethyl)piperidine (IC50 < 100 microM). The two enantiomers of MK 801 were equipotent. All effects were long lasting, consistent with their non-/uncompetitive actions. The most potent drug tested, ifenprodil, at an effective dose of 30 microM, had a mean recovery time of 74 min. A three-fold increase in drug concentration was required to depress St II units (associated with non-synaptic lanceolate endings). Changes in Zn2+ did not affect St I unit responses. These data suggest that unconventional NMDA receptors are involved in St I unit responses, but question the notion of a glutamatergic synapse between the Merkel cell and nerve terminal.
Cahusac
P M B
PM
Department of Psychology, University of Stirling, Stirling FK9 4LA, Scotland, UK. p.m.b.cahusac@stir.ac.uk
Senok
S S
SS
Hitchcock
I S
IS
Genever
P G
PG
Baumann
K I
KI
D000222:Adaptation, Physiological / Q000502:physiology; D000818:Animals; D018698:Glutamic Acid / Q000378:metabolism; D066298:In Vitro Techniques; D018862:Merkel Cells / Q000502:physiology* / Q000648:ultrastructure; D008854:Microscopy, Electron; D051381:Rats; D017208:Rats, Wistar; D018091:Receptors, AMPA / Q000378:metabolism; D018092:Kainic Acid Receptors / Q000378:metabolism; D016194:Receptors, N-Methyl-D-Aspartate / Q000378:metabolism*; D009435:Synaptic Transmission / Q000502:physiology; D014738:Vibrissae / Q000294:innervation*
D018698:Glutamic Acid; D018091:Receptors, AMPA; D018092:Kainic Acid Receptors; D016194:Receptors, N-Methyl-D-Aspartate; C409466:NR1 NMDA receptor; C120997:NR2A NMDA receptor; C121001:NR2B NMDA receptor