NMDA receptors in invertebrates

The concept of an NMDA receptor has evolved over some thirty years. The initial impetus was the need to better distinguish “glutamate preferring” from “aspartate preferring” receptors among neurons of the mammalian spinal cord (Curtis and Watkins, 1963; McCulloch et al., 1974). This search gave rise to substantial evidence for a diversity of ionotropic excitatory amino acid (EAA) receptors with quantitatively different sensitivities for various EAA analogues. The properties of these receptors are reviewed in Mayer and Westbrook (1987), Shinozaki (1988), Cotman et al. (1988), Sansom and Usher-wood (1990) and Watkins (1989). The vertebrate receptors are named after the preferred selective agonist. The most important distinction is between the NMDA and non-NMDA (quisqualate and kinate) receptors. Although both receptor types are highly sensitive to glutamate and mediate nonspecific cation conductances, the NMDA receptor is particularly significant because the ion channel is voltage dependent and highly permeable to Ca*+ (Mayer and Westbrook, 1987). To our knowledge there has never been a major effort to determine whether any invertebrate species possesses an NMDA receptor. Rather, the modest data available have been produced in the context of structure-activity studies to better characterize particular glutamate and/or aspartate receptors. Although there are some gross similarities between some vertebrate and invertebrate EAA receptors, significant differences are invariably found in the details. Altogether we have identified 21 studies in which NMDA action was tested on an invertebrate glutamate/aspartate receptor. Although the overwhelming majority of results are negative, a few positive indications emerge. These are summarized in Table 1. In the foregoing these results are considered relative to the vertebrate NMDA receptor and the EAA receptors of the invertebrate species tested. Operationally, the NMDA receptor is characterized by the effects of selective agonists and antagonists, the action of modulators (Mg’+, glycine), voltage dependence in the presence of Mg*+, a large single channel conductance and ionic permeability. When agonist affinities are measured by the capacity to displace selective antagonists from the NMDA receptor, L-glutamate is the most potent (Watkins, 1989; Oliverman and Watkins, 1989). NMDA, ibotenate and L-homocysteate are strong competitors for the NMDA binding site while L-quisqualate, L-kainate and AMPA are not. D-a-aminoadipate (Davies and Watkins, 1979) was the first competitive antagonist discovered, to exhibit a preference for the NMDA receptor. Subsequently, more potent blockers were developed in which a phosphonate group is substituted for the w-carboxyl group and additional methylene groups extend the molecule length (Evans et al., 1982). The most effective of these are 2-amino-5phosphonopentanoate (APV or D-AP5) and 2-amino-7-phosphonoheptanoate (D-AP7). Recently these have been superseded by higher affinity antagonists derived from piperdine (Olverman and Watkins, 1989). Activation of the NMDA receptor requires glytine (K, N 200 nM), a requirement normally satisfied in vivo by the interstitial fluid (Johnson and Ascher, 1987; Thomson et al., 1989). The receptor’s glycine binding site is insensitive to strychnine but is competitively blocked by kynurenate, 7-Cl-kynurenate and HA-966 (Ascher and Johnson, 1989). M$+(Kd = 72 nM) blocks current flow through the open NMDA receptor channel and imparts a voltage-dependence to channel activation (Davies and Watkins, 1977; Ault et al., 1980; Ascher and Nowak, 1988). At 1 .O mM Mg*+, membrane depolarizations to above 30 mV are sufficient to overcome the Mg*+ block. Co*+ and Mn*+ can mimic the action of Mg*+ but Ba*+, Ca*+ and Cd*+ do not. In physiological salines the single channel conductance is about 50 pS which is substantially larger than that of the non-NMDA receptor channels. Elevated external Ca*+ reduces the single channel conductance at negative membrane potentials from 56 pS (in 0 Caz+) to 15 pS at 100 mM Ca*+ (Ascher and Nowak, 1988). The NMDA channel is permeable to Na + , K + and Ca*+ and the Ca*+ permeability is unusually high; Pc,Z+/PN,+ * 1.0 (Ascher and Johnson, 1989). In normal physiological salines the reversal potential of the NMDA-elicited current is -0 mV. In many neurons, NMDA receptors are interspersed among non-NMDA receptors (Jahr and Stevens, 1987; Cull-Candy and Usowicz, 1987) and the voltage-sensitivity and slower kinetics of the NMDA activated channels are thought to have a