The variation in action of excitatory amino acids in relation to distance of iontophoretic application to spinal motoneurones

Uneven distribution of excitatory amino acid receptors on ventral horn neurones of newborn rat spinal cord

1. The distribution of excitatory amino acid receptors on ventral horn neurones was investigated using slices of newborn rat spinal cord. 2. The neurone and the tip of the pipette used to inject amino acids were visualized using Lucifer Yellow under a fluorescent microscope. The pipette was precisely located on the soma and dendrite of the neurone under visual control, and L-glutamate (Glu), L-aspartate (Asp), N-methyl-D-aspartate (NMDA), kainate (KA) and quisqualate (Quis) were ionophoretically applied with a short pulse. The potential changes were intracellularly recorded from the soma. 3. Sensitivity to Glu as tested with short pulses (1-2 ms) was almost the same at the soma and along dendrites. 4. The amplitude of the responses to NMDA produced at the soma and the proximal part of the dendrite was about the same as that of Glu, but smaller than that of Glu at the distal part of the dendrite. Suppression of the Glu potential by an NMDA receptor antagonist, 2-amino-5-phosphonovaleric acid (APV), was greater at the soma than at the dendrite, suggesting that the contribution of NMDA receptors to the Glu potential was greater at the soma. 5. Sensitivity to Asp was about one-half that to Glu sensitivity on the soma and even less on the dendrite. Sensitivity to KA was high at the soma and low at the dendrite. However, Quis responses were produced throughout the neurone. 6. The Quis response induced by the application of a short pulse showed two phases: a fast response followed by a very slow depolarization that lasted more than 10 s. 7. The fast Quis response was easily desensitized and insensitive to APV. The time course of the fast Quis potential was shorter than that of Glu. 8. The slow Quis response was more pronounced at the dendrites than at the soma and was reduced by the intracellular injection of EGTA, suggesting the contribution of Ca2+ in the cell, possibly mediated by a second messenger system. 9. Experimental results suggest that the distribution of excitatory amino acid receptors differs between the soma and the dendrites of spinal neurones.

NMDA Actions on Rat Abducens Motoneurons

Intracellular recordings were made from rat abducens motoneurons in vivo during local extracellular micro-ionophoretic application of N-methyl-d-aspartate (NMDA) and NMDA receptor antagonists. Typical NMDA responses, at a resting potential of -60 mV, consisted of a slow depolarization with an apparent increase in membrane resistance, bursts of action potentials followed by stable repetitive firing. Ionophoretic applications of aminophosphonovalerate (APV), kynurenate or MK801 reduced or blocked the NMDA-induced responses. The NMDA responses were voltage-dependent. NMDA responses induced by short (< 30 s) NMDA application pulses were blocked by hyperpolarizing the neuron. Long duration (> 30 s) NMDA applications induced rhythmic plateau potentials in hyperpolarized abducens motoneurons. The rhythmic depolarizations (15 - 30 mV) were modulated in both frequency and duration by current injection. They were abolished by further hyperpolarization or replaced by stable repetitive firing when hyperpolarization was removed. Our data show that NMDA receptors are present in rat abducens motoneurons and may be involved in the induction of rhythmic activities. The voltage-dependent blockade of somatic NMDA receptor-associated ion channels by cell hyperpolarization may be important for these oscillations. It is suggested that the rhythmic behaviour is due to the activation of dendritic NMDA receptors.

L-homocysteate is a potent neurotoxin on cultured cortical neurons

L-Homocysteate (L-HCA) neurotoxicity was quantitatively studied in dissociated cell cultures prepared from the fetal mouse neocortex. Five minute exposure to 3 microM-1 mM L-HCA was associated with neuronal cell loss, but not glial cell loss; the extent of neuronal damage was dependent on the concentration of L-HCA, with an ED50 of approximately 40 microM. The stereoisomer D-HCA was a somewhat weaker neurotoxin than L-HCA. Ion substitution experiments suggested that L-HCA neurotoxicity can be separated into two components on the basis of differences in time course and ionic dependence: an acute, sodium-dependent 'excitotoxic' component, marked by rapid early cell swelling; and a late, calcium-dependent component, marked by delayed cell degeneration. L-HCA neurotoxicity could be attenuated by 2-amino-5-phosphonovalerate (APV), ketamine, and kynurenate, but not by L-glutamate diethyl ester or gamma-D-glutamylaminomethyl sulfonate, consistent with a predominant involvement of N-methyl-D-aspartate receptors. APV and ketamine produced different effects on the L-HCA concentration-toxicity relation, the former drug consistent with a competitive, and the latter drug consistent with a non-competitive, mechanism of antagonism.