NMDA receptors in the dorsal horn play an important role in nociception and synaptic plasticity. In late embryonic and early postnatal life there is expression of NR1, NR2B, NR2D and NR3A subunit mRNA in the spinal cord, with no detectable signal for NR2A and NR2C. Recent reports describe NMDA receptors in the ventral horn of neonatal spinal cord with properties that do not correlate in all aspects with studies of cloned NMDA receptors or with native receptors elsewhere in the CNS. In particular, a large conductance (69 pS) NMDA receptor was described (Palecek et al. 1999).

In this study, 10-day-old rats were killed humanely and the single-channel properties of NMDA receptors in laminae I and II of the dorsal horn of 300 µm transverse spinal cord slices were studied in outside-out membrane patches when activated by NMDA (100 nM) and glycine (10 µM). Our aim was to determine the single-channel properties of neonatal dorsal horn NMDA receptors. Data are expressed as means ± S.E.M.

Five distinct conductance levels were observed of 19.9 ± 0.8, 32.9 ± 0.6, 42.2 ± 1.1, 53.0 ± 1.0 and 68.7 ± 1.5 pS (n = 6 patches). Direct transitions occurred between all levels. 28 % involved 69 pS openings, of which 19 % were between 69 and 53 pS. Transitions between 69 pS openings and 20, 33 and 42 pS openings were rare (< 3 %). There was no significant difference (unpaired t test, P > 0.05) in the frequency of direct transitions from 42 to 20 pS compared with 20 to 42 pS transitions. The K_b (0 mV) for Mg²⁺ block was 89 µM (n = 6 patches). The Mg²⁺ unblocking rate constant (1.5 Ω 10³ s^-1) was similar to other reported values but the Mg²⁺ blocking rate constant (1.6 Ω 10⁷ M^-1 s^-1) was larger, implying an unusually high Mg²⁺ sensitivity (e.g. Ascher & Nowak, 1988). The NR2B subunit-selective antagonist, ifenprodil (1 µM), had no significant effect on overall channel activity, but significantly decreased the mean open time of 53 pS openings from 1.56 ± 0.3 to 0.90 ± 0.2 ms (paired t test, P = 0.037, n = 8 patches).

Co-expression of NR1a, NR2A and NR2D subunits in Xenopus oocytes (Cheffings & Colquhoun, 2000) results in novel NMDA receptors with the expected conductances of 40/50 pS (NR1/NR2A) and 20/40 pS (NR1/NR2D) and an additional conductance level of 30 pS. These receptors show direct transitions between 20 and 50 pS conductance levels, which were also observed in our study but not in studies of native NMDA receptors elsewhere in the CNS (Momiyama et al. 1996) or in the adult dorsal horn (Momiyama, 2000). Our results therefore suggest neonatal laminae I and II NMDA receptors are not simply composed of NR1 and NR2B or NR1 and NR2D subunits, but may be due to an as yet uninvestigated combination of two NR2 subunits with the NR1 subunit or a combination of NR3A, NR2 and NR1 subunits.This work was upported by the MRC, Wellcome Trust and UCL Graduate School.

Ascher, P. & Nowak, L. (1988). J. Physiol. 399, 247-266. abstract

Cheffings, C.M. & Colquhoun, D. (2000). J. Physiol. 526, 481-491. abstract

Momiyama, A. (2000). J. Physiol. 523, 621-628. abstract

Momiyama, A., Feldmeyer, D. & Cull-Candy, S.G. (1996). J. Physiol. 494, 479-492. abstract

Palecek, J., Abdrachmanova, G., Vlachova, V. & Vicklicky, L. (1999). Eur. J. Neurosci. 11, 827-836.