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, 100μM) had no effect, nor did classical competitive NMDA receptor antagonists, D-AP5 (600μM) and R-CPP (100μM), nor the NMDA glycine site antagonist 5,7-dichlorokynurenic acid (100μM). The only effective NMDA receptor blockers were those selective for the polyamine site: ifenprodil (IC50 20μM) and Ro 25-6981 (IC50 ≈50μM), and the associated ion channel: MK 801, ketamine and (±)-1-(1,2-diphenylethyl)piperidine (IC50 <100μM). 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 30μM, 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.