In vivo imaging of Α7 nicotinic receptors as a novel method to monitor neuroinflammation after cerebral ischemia

Abstract

In vivo positron emission tomography (PET) imaging of nicotinic acetylcholine receptors (nAChRs) is a promising tool for the imaging evaluation of neurologic and neurodegenerative diseases. However, the role of α7 nAChRs after brain diseases such as cerebral ischemia and its involvement in inflammatory reaction is still largely unknown. In vivo and ex vivo evaluation of α7 nAChRs expression after transient middle cerebral artery occlusion (MCAO) was carried out using PET imaging with [ C]NS14492 and immunohistochemistry (IHC). Pharmacological activation of α7 receptors was evaluated with magnetic resonance imaging (MRI), [ F]DPA-714 PET, IHC, real time polymerase chain reaction (qPCR) and neurofunctional studies. In the ischemic territory, [ C]NS14492 signal and IHC showed an expression increase of α7 receptors in microglia and astrocytes after cerebral ischemia. The role played by α7 receptors on neuroinflammation was supported by the decrease of [ F]DPA-714 binding in ischemic rats treated with the α7 agonist PHA 568487 at day 7 after MCAO. Moreover, compared with non-treated MCAO rats, PHA-treated ischemic rats showed a significant reduction of the cerebral infarct volumes and an improvement of the neurologic outcome. PHA treatment significantly reduced the expression of leukocyte infiltration molecules in MCAO rats and in endothelial cells after in vitro ischemia. Despite that, the activation of α7 nAChR had no influence to the blood brain barrier (BBB) permeability measured by MRI. Taken together, these results suggest that the nicotinic α7 nAChRs play a key role in the inflammatory reaction and the leukocyte recruitment following cerebral ischemia in rats.

Inflammation in stroke: the role of cholinergic, purinergic and glutamatergic signaling

“…Nevertheless, a recent study showed that the daily treatment of PHA 568487 during the first week after transient cerebral ischemia in rats displayed a nonsignificant decrease of the expression marker values for both proinflammatory and anti-inflammatory microglia markers. 22 Moreover, this study observed fewer values of selectines, adhesion molecules and infiltrated T lymphocytes after treatment with PHA, suggesting a possible role of α7 nAChRs in the regulation of leukocyte infiltration into the ischemic tissue. 22 Likewise, the infiltration of leukocytes after stroke can also be influenced by the disruption of the BBB; however, activation of α7 receptors showed similar levels of BBB disruption after MCAO in rats.…”

“… 22 Moreover, this study observed fewer values of selectines, adhesion molecules and infiltrated T lymphocytes after treatment with PHA, suggesting a possible role of α7 nAChRs in the regulation of leukocyte infiltration into the ischemic tissue. 22 Likewise, the infiltration of leukocytes after stroke can also be influenced by the disruption of the BBB; however, activation of α7 receptors showed similar levels of BBB disruption after MCAO in rats. 22 In contrast, Zou and colleagues examined the effect of α7 nAChRs activation with PHA after cerebral ischemia in mice showing an improvement of the BBB integrity.…”

“…PET imaging with [ 11 C]NS14492 described an overexpression of these receptors as a response to the ischemic injury that was identified in microglia and infiltrated macrophages at day 7 after ischemia ( Figures 1 and 2 ). Finally, PET imaging of neuroinflammation with [ 18 F]DPA-714, a specific radioligand for the translocator protein (18KDa) (TSPO), 70 showed a reduction in the radioligand uptake as a result of treatment with PHA 568487 on day 7 after cerebral ischemia, supported by a reduction in activated microglia and macrophages 22 ( Figure 2 ). Hence, the anti-inflammatory activity exerted by the cholinergic system following stroke has been attributed to α7 nAChRs, although α4β2 nicotinic receptors have also been involved in the inflammatory reaction underlying cerebral ischemia in rats.…”

“… 14 Following cerebral ischemia, several preclinical studies have observed that both the local upregulation and the pharmacological activation of nicotinic receptors protects the brain against ischemic injury, suggesting the protective central cholinergic effect and the potential role of these receptors as promising inflammatory biomarkers. 15 – 23 …”

Cholinergic Modulation of Glial Function During Aging and Chronic Neuroinflammation

“…Expression levels of α7nAChR have been investigated in various inflammatory models. On one hand, an increase of α7nAChR expression in both microglia and astrocytes was reported in cerebral ischemia in rats, leading to microglial activation and pro-inflammatory cytokine production (Niranjan et al, 2012;Wu L. et al, 2014;Colás et al, 2018). This was counteracted by nicotine treatment, which reduced pro-inflammatory cytokine production as well as microglial activation, and also prevented neuronal death in the CA1 region of the hippocampus in rats (Guan et al, 2015).…”