Nicotinic receptor activation contrasts pathophysiological bursting and neurodegeneration evoked by glutamate uptake block on rat hypoglossal motoneurons

Abstract

Excitotoxicity is thought to be one of the early processes in the onset of amyotrophic lateral sclerosis (ALS) because high levels of glutamate have been detected in the cerebrospinal fluid of such patients due to dysfunctional uptake of this transmitter that gradually damages brainstem and spinal motoneurons. To explore potential mechanisms to arrest ALS onset, we used an established in vitro model of rat brainstem slice preparation in which excitotoxicity is induced by the glutamate uptake blocker dl-threo-β-benzyloxyaspartate (TBOA). Because certain brain neurons may be neuroprotected via activation of nicotinic acetylcholine receptors (nAChRs) by nicotine, we investigated if nicotine could arrest excitotoxic damage to highly ALS-vulnerable hypoglossal motoneurons (HMs). On 50% of patch-clamped HMs, TBOA induced intense network bursts that were inhibited by 1-10 μm nicotine, whereas nAChR antagonists facilitated burst emergence in non-burster cells. Furthermore, nicotine inhibited excitatory transmission and enhanced synaptic inhibition. Strong neuroprotection by nicotine prevented the HM loss observed after 4 h of TBOA exposure. This neuroprotective action was due to suppression of downstream effectors of neurotoxicity such as increased intracellular levels of reactive oxygen species, impaired energy metabolism and upregulated genes involved in endoplasmic reticulum (ER) stress. In addition, HMs surviving TBOA toxicity often expressed UDP-glucose glycoprotein glucosyltransferase, a key element in repair of misfolded proteins: this phenomenon was absent after nicotine application, indicative of ER stress prevention. Our results suggest nAChRs to be potential targets for inhibiting excitotoxic damage of motoneurons at an early stage of the neurodegenerative process.

Nicotine protects rat hypoglossal motoneurons from excitotoxic death via downregulation of connexin 36

“…As mitochondrial dysfunction and motoneuron death are observed after 4-h treatment with TBOA, 13 we evaluated Cx36 expression (Figures 3a and b; red) in relation to immunoreactivity of single HMs (identified with SMI 32 immunostaining; green). DAPI was used for nuclear staining (blue).…”

“…Even if the nucleus hypoglossus represents only a minor region of this tissue, this approach was earlier useful to detect changes in gene profiling, protein synthesis and expression, and mitochondrial energy production 13, 14 which, although cannot be directly attributed to HMs (or other brainstem motoneurons), yet they provide an index of the metabolic state of the network and of any ongoing neuroprotective process.…”

Impaired short‐ and long‐latency afferent inhibition in amyotrophic lateral sclerosis

“…Studies performed on hypoglossal motor neurons revealed that muscarinic ACh receptors not only inhibit glutamatergic transmission but also depress inhibitory neurotransmission on brainstem motor neurons . Nevertheless, it was shown that brainstem hypoglossal motor neuron excitotoxicity was suppressed by activation of neuronal nicotinic receptors with their conventional agonist, nicotine . Gamma aminobutyric acid (GABA) activity also modifies the cholinergic system at various levels of the central nervous system.…”

“…43 Nevertheless, it was shown that brainstem hypoglossal motor neuron excitotoxicity was suppressed by activation of neuronal nicotinic receptors with their conventional agonist, nicotine. 44 Gamma aminobutyric acid (GABA) activity also modifies the cholinergic system at various levels of the central nervous system. Giorgetti et al reported that, although the GABA-a agonist muscimol has no effect on cortical ACh release, the GABA-b agonist baclofen augments spontaneous ACh release in the rat cortex.…”

Revisiting nicotine’s role in the ageing brain and cognitive impairment

“…Nicotine has shown anti-excitotoxic effects through a calcium-dependent pathway that is mediated via α 7 nAChRs (Shimohama et al, 1998;Dajas-Bailador et al, 2000;Corsini et al, 2016). This may result from the modulatory effects of α 7 nAChRs on the glutamate-induced prevention of the PI3-K/Akt pathway (Cui et al, 2013).…”