Effects of intravenous kainic acid, N-methyl-D-aspartate, and (--)-nuciferine on the cat spinal cord.

Kainic acid (a rigid conformational analogue of glutamate), N-methyl-D-aspartate (the methylated derivative of aspartate), and (--)-nuciferine (an aporphine alkaloid with a depressant effect on glutamate-induced neuronal firing), which, so far, have been examined in microiontophoretic studies, were investigated in spinal cats for their effects on some spinal cord activities after intravenous injections. At low doses, kainic acid (0.3 mg kg-1) enhanced segmental monosynaptic but not polysynaptic ventral root reflexes and increased the excitability of motoneurones, whereas N-methyl-D-aspartate (3 mg kg-1) facilitated polysynaptic but not monosynaptic reflexes. Higher doses of the two amino acids depolarized motoneurones and primary afferent endings, enhanced monosynaptic reflexes and depressed polysynaptic reflexes. (--)-Nuciferine (1--10 mg kg-1) depressed monosynaptic but not polysynaptic ventral root reflexes in a dose-dependent manner and antagonized the effects of kainic acid but not of N-methyl-D-aspartate on the spinal cord. The results are consistent with the hypothetical excitatory transmitter role of glutamate in primary afferents and of aspartate in excitatory spinal cord interneurones; the findings also suggest that (--)-nuciferine may be used as a systemically effective, rather selective blocker of central glutamate receptors.