Functional diversity and developmental changes in rat neuronal kainate receptors.

Whole-cell currents evoked by kainate and the GluR5-selective agonist (RS)-2-amino-3-(3-hydroxy-5-tertbutylisoxazol-4-yl)propanoic acid (ATPA) were used to compare the physiological properties of kainate receptors expressed by neurons from rat hippocampus, spinal cord and dorsal root ganglia. In contrast to kainate, which evoked desensitizing currents with similar decay rates and steady-state components in all three cell types, responses to ATPA were distinctly different in the three cell populations. Currents evoked by ATPA displayed a significant steady-state component in hippocampal neurons, but decayed rapidly to baseline in dorsal root ganglion (DRG) cells. ATPA failed to evoke current in many of the spinal neurons. ATPA caused steady-state desensitization in DRG cells with an IC50 of 41 nm. Recovery from desensitization of DRG cell receptors by ATPA was significantly slower than for any previously described agonist. In contrast, hippocampal kainate receptors recovered from desensitization by ATPA within a few seconds. Half-maximal activation of kainate receptors in hippocampal neurons required 938 nm ATPA. In DRG cells treated with concanavalin A the EC50 for ATPA was 341 nm. ATPA evoked current in embryonic hippocampal neurons but with lower amplitude relative to kainate than in cultured postnatal neurons. Collectively, these results highlight functional differences between neuronal kainate receptors that may reflect their distinct subunit composition and their diverse roles in synaptic transmission. Glutamate activates a trio of ionotropic receptors that are named for the agonists kainate, AMPA and NMDA. Recent work suggests that the kainate subtype can serve as postsynaptic receptors at a variety of CNS synapses (Vignes & Collingridge, 1997; Castillo et al. 1997; DeVries & Schwartz, 1999; Li et al. 1999) and may also function to modulate transmitter release from presynaptic terminals (Chittajallu et al. 1996; Clarke et al. 1997; Rodriguez-Moreno et al. 1997). Glutamate receptor (GluR) subunits that can contribute to kainate receptors include GluR5, 6 and 7, which are capable of forming functional homomeric ion channels (Bettler et al. 1990; Egebjerg et al. 1991; Schiffer et al. 1997), and KA1 and 2, which combine with GluR5-7 to produce heteromeric receptors (Werner et al. 1991; Herb et al. 1992). Analysis of receptors produced by expression of homomeric and heteromeric subunit combinations in HEK cells has demonstrated a rich diversity of physiological and pharmacological characteristics that depend on specific structural features of the individual subunits (Herb et al. 1992; Kohler et al. 1993). In addition, studies of the native expression patterns for these subunits (Wisden & Seeburg, 1993; Tolle et al. 1993; Bahn et al. 1994) have revealed distinct, but overlapping, distributions, suggesting that different cell types in the nervous system may express kainate receptors with unique functional properties. In order to isolate neuronal kainate receptors for detailed study it is necessary to block both NMDA and AMPA receptors because none of the readily available excitatory amino acids are entirely selective for the kainate subtype. Recent work on neuronal kainate receptors has taken advantage of antagonists for AMPA and NMDA receptors that eliminate contamination of currents mediated by the kainate receptor channels. In particular, the selective, non-competitive antagonism of AMPA receptors by GYKI 53655 (1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7,8-methylenedioxy-3,4-dihydro-5H-2,3-benzodiazepine) (Paternain et al. 1995; Wilding & Huettner, 1995; Bleakman et al. 1996) and SYM 2206 ((RS)-4-(4-aminophenyl)-1,2-dihydro-1-methyl-2-propylcarbamoyl-6,7-methylenedioxyphthalazine) (Pelletier et al. 1996; Li et al. 1999; Rodriguez-Moreno et al. 2000) has proven crucial to elucidating the function of kainate receptors. More recently, a number of compounds have been described that are selective within the kainate receptor subfamily. The AMPA analogue (RS)-2-amino-3-(3-hydroxy-5-tertbutylisoxazol-4-yl)propanoic acid (ATPA) (Lauridsen et al. 1985) has been shown to activate selectively homomeric receptors formed by the GluR5 subunit (Clarke et al. 1997) as well as heteromeric receptor combinations that include a GluR5 subunit (Bortolotto et al. 1999; Cui & Mayer, 1999; Paternain et al. 2000). In addition, the proprietary compounds LY 293558 and LY 382884 act as selective competitive antagonists of receptors that include a GluR5 subunit (Bortolotto et al. 1999). Although the selectivity of these drugs for GluR5 may not be absolute (Paternain et al. 2000), they have provided at least provisional evidence for involvement of GluR5 subunits in both pre- and postsynaptic kainate receptors (Clarke et al. 1997; Bortolotto et al. 1999). In the present study we have used kainate and ATPA to compare the properties of endogenous kainate receptors expressed by neurons from rat hippocampus, dorsal root ganglia and spinal cord. Although kainate and glutamate activate similar currents in all three cell types (Huettner, 1990; Wilding & Huettner, 1997; Paternain et al. 1998; Li et al. 1999), our results reveal significant functional differences in the receptors present on these three cell populations.