L-glutamate (GLU) and L-aspartate (ASP) are neurotransmitter candidates at primary afferent and intraspinal synapses in the mammalian spinal cord. Several receptor types exist for the excitatory amino acids based on selective activation by kainate (KA), quisqualate (QA) and N-methyl-D-aspartate (NMDA), whereas GLU can activate both NMDA and non-NMDA (i.e. either QA or KA) receptors. Recent evidence in culture suggests that a non-NMDA receptor mediates monosynaptic EPSPs formed between dorsal root ganglion and dorsal horn neurons as well as between spinal cord neurons. The purpose of the present study is to test the chemosensitivity of spinal cord neurons to the above mention amino acids during the first week in culture. The result indicate that at day 2-3, most spinal cord neurons are sensitive to QA (10 MuM) are small or absent. QA and GABA responses are associated with conductance increases while GLU responses result in little or no apparent conductance change. Under voltage clamp GLU-activated currents have a region of zero or negative slope conductance consistent with a mixed agonist action on both NMDA and non-NMDA receptors. Both QA and GLU-activated currents have reversal potentials near O mV. GABA responses reverse at -50 mV with KMeSO4 and at O mV with CsCl solutions in the patch electrode, consistent with a chloride conductance. By day 7, spinal cord neurons are highly sensitive to all agonists tested. These results suggest that both NMDA and non-NMDA receptors are present before detectable synaptic activity, but that sensitivity increases during a period of rapid synapse formation.
