A recent report (Fagan & Cahusac, 2001) suggested the involvement of glutamate receptor-mediated transmission at a cutaneous slowly adapting mechanoreceptor. We tested the effect of the non-competitive NMDA ionotropic glutamate receptor antagonist MK-801 on responses of sinus hair type I (St I), and on sinus hair type II (St II) mechanoreceptors in isolated rat vibrissae.

Adult male Wistar rats (250-300 g body weight) were initially anaesthetized with urethane (20 % w/v, 6 mg kg^-1 I.P.), followed by intracardiac injection of a similar amount of urethane. Whisker pads were then excised from the killed rat. A single vibrissa dissected from the whisker pad was mounted in an organ bath superfused with synthetic interstitial fluid continuously bubbled with a gas mixture of 95 % O₂ and 5 % CO₂ to maintain a pH of 7.4. Fine nerve strands were teased from the deep vibrissal nerve attached to the hair follicle for single-unit recordings. St I and St II receptors were identified by their firing pattern in response to mechanical stimulation as previously described (Senok & Baumann, 1997). Evoked firing was induced using feedback-controlled displacement of the hair shaft in the most sensitive direction for 5 s (500 ms ramps and 4000 ms plateau) every 30 s. Spontaneous firing was continuously recorded during the intervals between stimuli.

Superfusion of MK-801 (100 µM) for 20 min depressed the responses of St I receptors by 45-60 % (n = 4). The effect was long lasting, recovering to about 80 % of control values in 90-120 min (see Fig. 1). Treatment with 300 µM MK-801 resulted in 90-100 % inhibition of responses with recovery to only about 15 % of control > 2 h after washout. MK-801 (100 µM) had no significant effect on the responses of St II receptors. However, 300 µM MK-801 treatment resulted in an initial increase in responses followed by a rapid and complete cessation of all responses. Some responsiveness returned in some receptors after > 60 min, but no complete recovery was recorded in the time period studied.The data suggest the involvement of the ionotropic NMDA glutamate receptor in the transduction of touch-pressure in the St I receptor. Other selective NMDA and non-NMDA receptor modulators are presently being tested.This work was funded by The Wellcome Trust, ref. 061023.

Figure 1. The effect of 100 µM MK-801 on the responses to mechanical stimulation of a St I and a St II receptor. Responses were taken as the number of spikes per 5 s stimulus period.

Fagan, B.M. & Cahusac, P.M.B. (2001). NeuroReport 12, 341-347.

Senok, S.S. & Baumann, K.I (1997). J Physiol. 500, 29-37.