Facilitation of distinct inhibitory synaptic inputs by chemical anoxia in neurons in the oculomotor, facial and hypoglossal motor nuclei of the rat
Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by the selective loss of motor neurons in the brainstem and spinal cord. Clinical studies have indicated that there is a distinct region-dependent difference in the vulnerability of motor neurons. For example, the motor neurons in the facial and hypoglossal nuclei are more susceptible to neuronal death than those in the oculomotor nucleus. To understand the mechanism underlying the differential susceptibility to cell death of the neurons in different motor nuclei, we compared the effects of chemical anoxia on the membrane currents and postsynaptic currents in different motor nuclei. The membrane currents were recorded from neurons in the oculomotor, facial and hypoglossal nuclei in brain slices of juvenile Wistar rats by using whole-cell recording in the presence of tetrodotoxin that prevents action potential-dependent synaptic transmission. NaCN consistently induced an inward current and a significant increase in the frequency of spontaneous synaptic inputs in neurons from these three nuclei. However, this increase in the synaptic input frequency was abolished by strychnine, a glycine receptor antagonist, but not by picrotoxin in neurons from the hypoglossal and facial nuclei, whereas that in neurons from the oculomotor nucleus was abolished by picrotoxin, but not by strychnine. Blocking ionotropic glutamate receptors did not significantly affect the NaCN-induced release facilitation in any of the three motor nuclei. These results suggest that anoxia selectively facilitates glycine release in the hypoglossal and facial nuclei and GABA release in the oculomotor nucleus. The region-dependent differences in the neurotransmitters involved in the anoxia-triggered release facilitation might provide a basis for the selective vulnerability of motor neurons in the neurodegeneration associated with ALS.