Quantitative and qualitative changes in AMPA receptor expression during spinal cord development

An immunocytochemical study of the distribution of AMPA selective glutamate receptor subunits in the normal human motor system

Glutamate is the major mediator of fast excitatory neurotransmission in the mammalian central nervous system. Disturbances of this neurotransmitter system have been implicated in chronic degenerative neurological disease. Recently, major advances in our knowledge and understanding of the molecular biology of the glutamatergic receptor system have been made. It is now known that functional glutamate receptors consist of various combinations of some 20 identified subunits. A growing body of circumstantial evidence suggests that the non-N-methyl-D-aspartate subtype of glutamate receptors may mediate, at least in part, the selective motor neuron death seen in the human neurodegenerative disease amyotrophic lateral sclerosis. We have used subunit specific immunocytochemistry to study the distribution and potential subunit composition of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) selective glutamate receptors, (a subgroup of non-N-methyl-D-aspartate selective glutamate receptors formed by combinations of GluR1-4 subunits), in the human motor system. Motor neurons in the spinal cord, brainstem, and motor cortex were relatively strongly immunoreactive with the GluR2/3 subunit antibody, moderately so with the GluR4 subunit antibody, and showed relatively low levels of immunoreactivity with the GluR1 subunit antibody. This is the first detailed study of AMPA receptor subunit expression in the human motor system. Motor neurons express a distinct subunit profile when compared with other groups of neurons in the human nervous system. There were no significant differences in the pattern of relative AMPA subunit expression (GluR2/3 > or = GluR4 > GluR1) between groups of motor neurons typically affected (in the spinal cord and hypoglossal nucleus), or spared (oculomotor and Onufs nucleus) by the amyotrophic lateral sclerosis disease process. However, oculomotor motor neurons had higher levels of expression of all AMPA subunit proteins which may indicate greater AMPA mediated glutamatergic input in the normal function of this neuronal population. This study does not support a role for differential subunit composition of AMPA receptors in determining the selective vulnerability of motor neurons in amyotrophic lateral sclerosis. However, the overall density of receptors may be of importance.

Upregulation of spinal glutamate receptors in chronic pain

Recent studies indicate that glutamate binding to N-methyl-D-aspartate receptors in the spinal cord is involved in triggering the development of chronic pain However, the processes which directly underlie the increased pain remain unclear. Here we report that, following peripheral nerve injury (ligation of the sciatic nerve) in the rat, there is an increase in immunoreactive labelling of non-N-methyl-D-asparatate, AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate), glutamate receptors in the superficial laminae of the lumbar spinal cord ipsilateral to the ligation. The increase in AMPA receptor expression peaks 14 days after nerve ligation and decreases 35 days post-ligation, corresponding to the time-course of heightened sensitivity to mechanical and thermal noxious stimuli (hyperalgesia) induced by the ligation. Given evidence that AMPA receptors in the superficial laminae mediate fast nociceptive transmission in the spinal cord, our findings suggest that an upregulation of spinal AMPA receptors contributes to hyperalgesia following peripheral nerve injury.

Glycine effects on glutamate-receptor elicited acetylcholinesterase release from slices and synaptosomes of the spinal ventral horn

To study the mechanisms by which glutamate-elicited acetylcholinesterase release (GEAR) might play a part in the pathogenesis of excitotoxically triggered motor neurone disease, and to investigate the interaction of GEAR with spinal glycinergic mechanisms, we measured acetylcholinesterase (AChE) and cholinergic markers, after stimulating ventral horn slices and synaptosomes from the mouse spinal cord, with both glutamate- and glycine-receptor agonists. Glutamate (GLU), kainate and AMPA, as well as glycine (GLY) evoked dose-related, calcium-dependent liberation of soluble forms of AChE from both slices and synaptosomes. GLY-evoked AChE release showed remarkable age-related postnatal changes. In the immature slice of the ventral horn. GLY potentiated the GEAR response in the presence of strychnine, suggesting N-methyl-D-aspartate (NMDA) receptor involvement, and was also able to evoke a strychnine-sensitive AChE release in the absence of exogenous GLU. After the 28th postnatal day, nearly all the AChE secreted was released either after the activation of non-NMDA glutamate receptors or by strychnine-sensitive GLY-evoked AChE release mechanisms. Both GEAR and GLY-evoked AChE release might impair the negative feedback loop which modulates the overactivation of motor neurones, and cause prolonged extracellular rises of soluble AChE. These effects might augment the vulnerability of motor neurones to excitotoxic stress, promote fiber outgrowth, and eventually accelerate the metabolic exhaustion of lower motor neurones. It is possible that the mechanisms described are operative at the spinal cord of ALS/MND patients.