Characterization of the N-methyl-D-aspartate (NMDA) receptor in the embryonic chick brain

Relatively little is known about the properties of the NMDA receptor in avian species. In the present study, NMDA receptor pharmacology and function has been studied in the embryonic chick brain. The competitive antagonist ligand [3H]CGP 39653 bound to telencephali membranes from 17-day-old chick embryos with KD and Bmax values of 6.6 nM and 3.3 pmol/mg protein, respectively. The binding was inhibited by CGS 19755, L-glutamate, R-CPP and D-AP5 with pI50 values of 7.57, 7.49, 7.28 and 7.08, respectively. L-glycine, S-AMPA, kainate and MK-801 only weakly affected the binding. The inhibitory potency of NMDA (pI50 value 6.98), however, was greater than seen in other species, suggesting a species difference in receptor structure. Such a possibility was investigated by use of two NMDA-R1 antibodies, AB 1516 and 69921 A, which recognize the C-terminal region and an intracellular domain of the rat receptor, respectively. Immunoreactive signals (approximately 116 kDa) were found for a rat brain homogenate, whereas only 69921A gave an immunoreactive signal in the chick. The functional status of the NMDA receptors was investigated in serum-free cultures of neurons from 8-day-old chick embryos. Glutamate and NMDA were found concentration-dependently, and in a manner that could be antagonized by CGS 19755 and MK-801, to produce cell death. Thus, the present study indicates that chick embryonic brain expresses functional NMDA receptors that are pharmacologically and possibly structurally different from their rodent counterparts.